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Volcanic Eruption Articles By Date
INDIA
October 20, 2013 | PTI
IMPHAL:
A suspected volcano-like eruption has been reported in a remote village
of Manipur near the India-Myanmar border which forced locals to
evacuate the area, official sources said on Sunday. According to
locals in Tusom village in Ukhrul district of Manipur, a deafening sound
was followed by the rolling down of a huge boulder from a nearby
hilltop which then released a lava-like liquid that charred trees and
plants on the hill slopes. Although the incident...
Dec 9, 2012 - NEW YORK: Volcanic activity in the Deccan Traps near modern-day Mumbai, and not an asteroid, may have killed the dinosaurs about ...
Dec 9, 2012 - NEW YORK: Volcanic activity in the Deccan Traps near modern-day Mumbai, and not an asteroid, may have killed the dinosaurs about ...
Coastal ecosystem responses to late stage Deccan Trap volcanism:
the post K–T boundary (Danian) palynofacies of Mumbai
(Bombay), west India
J.A. Cripps
a,
*
, M. Widdowson
b
, R.A. Spicer
b
, D.W. Jolley
c
a
School of Earth Sciences and Geography, Kingston University, Kingston-upon-Thames, KT1 2EE, United Kingdom
b
Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom
c
Centre for Palynology, University of Sheffield, Sheffield, S3 7HF, United Kingdom
Received 24 March 2004; received in revised form 23 August 2004; accepted 12 November 2004
Abstract
The Deccan Trap continental flood basalt eruptions of India occurred c. 67–63 Ma, thus spanning the Cretaceous–Tertiary
boundary (65 Ma). Deccan eruptions were coeval with an interval of profound global environmental and climatic changes and
widespread extinctions, and this timing has sparked controversy regarding the relative influence of Deccan volcanism upon end-
Cretaceous catastrophic events. If Deccan Trap activity was capable of affecting global ecosystems, evidence should be present
in proximal Indian sedimentary facies and their palaeontological contents. The impact of late stage Deccan volcanism upon
biota inhabiting Mumbai (Bombay) Island’s post K–T boundary lagoonal systems is documented here. Sediments (or
b
intertrappeans
Q
) which accumulated within these lagoons are preserved between Trap lavas that characterise the closing stages
of this flood basalt episode.
Mumbai Island Formation intertrappean faunal and floral communities are conspicuously distinct from those common to
many pre K–T boundary, late Maastrichtian intertrappeans across the Deccan province. The latter sedimentary intercalations
mostly developed in cognate semiarid, palustrine ecosystems; by contrast, those around Mumbai evolved in sheltered,
peripheral marine settings, within subsiding continental margin basins unique to this late Deccan stage, and under an
increasingly humid Danian climate. Geochemical analyses reveal that Mumbai sedimentation and diagenesis were intimately
related to local explosive volcanic and regional intrusive activity at c. 65–63 Ma. Although tectonic and igneous events
imprinted their signatures throughout these sedimentary formations, organisms usually sensitive to environmental perturbations,
including frogs and turtles, thrived. Critically, palynofacies data demonstrate that, whilst plant material deposition was
responsive to environmental shifts, there were no palpable declines in floral productivity following Mumbai pyroclastic
discharges. Therefore, it is implausible that this late stage explosive volcanism influenced major ecosystem collapses globally.
D
2004 Elsevier B.V. All rights reserved
Physical Volcanology of Large Igneous Provinces
Research paper
Correlation of the Deccan and Rajahmundry Trap lavas: Are these the longest and largest lava flows on Earth?
- a Volcano Dynamics Group, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK
- b Astrogeology Research Program, US Geological Survey, 2255 N. Gemini Dr. Flagstaff, AZ 86001, United States
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Abstract
We propose that the Rajahmundry Trap lavas, found near the east coast of peninsular India,
are remnants of the longest lava flows yet recognized on Earth
(∼ 1000 km long). These outlying Deccan-like lavas are shown to belong
to the main Deccan Traps. Several previous studies have already
suggested this correlation, but have not demonstrated it categorically.
The exposed Rajahmundry lavas are interpreted to be the distal parts of
two very-large-volume pāhoehoe flow fields, one each from the Ambenali
and Mahabaleshwar Formations of the Wai Sub-group in the Deccan Basalt
Group. Eruptive conditions required to emplace such long flows are met
by plausible values for cooling and eruption rates, and this is shown by
applying a model for the formation of inflated pāhoehoe sheet flow
lobes. The model predicts flow lobe thicknesses similar to those
observed in the Rajahmundry lavas. For the last 400 km of flow, the lava
flows were confined to the pre-existing Krishna valley drainage system
that existed in the basement beyond the edge of the gradually expanding
Deccan lava field, allowing the flows to extend across the subcontinent
to the eastern margin where they were emplaced into a littoral and/or
shallow marine environment. These lavas and other individual flow fields
in the Wai Sub-group may exceed eruptive volumes of 5000 km3, which would place them amongst the largest magnitude effusive eruptive units yet known.
We
suggest that the length of flood basalt lava flows on Earth is
restricted mainly by the size of land masses and topography. In the case
of the Rajahmundry lavas, the flows reached estuaries and the sea,
where their advance was perhaps effectively terminated by cooling and/or
disruption. However, it is only during large igneous province basaltic
volcanism that such huge volumes of lava are erupted in single events,
and when the magma supply rate is sufficiently high and maintained to
allow the formation of very long lava flows. The Rajahmundry lava fields
were emplaced around 65 Ma during the later times of Deccan volcanism,
probably just after the K/T environmental crisis. However, many
lava-forming eruptions of similar magnitude and style straddled the K/T
boundary.
Keywords
- basalt lava flows;
- pāhoehoe;
- Rajahmundry Traps;
- flood basalts;
- Deccan;
- long lava flows;
- large igneous provinces
Figures and tables from this article:
Fig. 1.
Sketch
map of main Deccan province on NW peninsula India and outlying related
lava-covered areas of the Deccan large igneous province (shaded), with
the location of the Rajahmundry Traps shown. Dots indicate towns.
Godavari and Krishna drainage systems and Western and Eastern Ghats also
shown; on-shore part of Godavari Basin is approximately area containing
buried lavas. Boxed area is that of Fig. 7.
Fig. 2.
Sketch
map of central to southern part of main Deccan province showing
inferred original extent of Deccan province and Wai Sub-group lava
flows, including mapped and inferred boundaries of Ambenali and
Mahabaleshwar Formation lavas (see text for discussion and source of
information). Exposure area of Rajahmundry lavas (RT) and outlines of
crustal structures along which developed drainage systems that lavas may
have followed are also shown.
Fig. 3.
View
of quarry at Gouripatnam (N 17° 2′ 3.8″; E 81° 36′ 14.2″) exposing
hackly jointed core of a sheet lobe of the Rajahmundry Trap lavas.
Uppermost quarry level shows deep weathering of the lava; second lower
level is formed of back-filled material. Trucks and people lower right
give scale. Photo courtesy of Matthias Raab (University of Melbourne).
Fig. 4.
Five logs of sections through lava flows around the Mahabaleshwar Plateau, Western Ghats (see location of Mahabaleshwar on Fig. 1), in Deccan volcanic province, that straddle the Chron 29R/29N paleomagnetic boundary (after Jay, 2005).
Names are road-cut traverses up the ghats along which the lava flows
were logged; elevations are metres above sea level. Column to right of
each log is palaeomagnetic polarity; white Chron 29R, black is Chron 29N
(grey shows parts of sections where no samples were obtained). Note
that Chron29R/29N transition falls in lower Mahabaleshwar Formation.
Logs show varying numbers of lava units between paleomagnetic reversal
horizon, along which logs are aligned, and top of Ambenali chemotype
lava flows.
Fig. 5.
Logged section of lavas in upper part of the Ambenali Ghat traverse, Mahabaleshwar Plateau (as for Fig. 4,
Ambenali log). Elevations (left of column) are metres above sea level.
Note that there are fewer good quality exposures of lava units towards
top due to deep weathering and topographic bench formation. Note also
occurrence of upper Mahabaleshwar chemotype lava flows, with similar
compositional character to the upper RT lava. Right column continues
from left.
Fig. 6.
Concentrations of Nb, Zr, and Ba in main series Deccan lava samples from (left column) Ambenali Ghat near Mahabaleshwar (see Fig. 5) after Jay (2005),
and (three other columns) from Rajahmundry Trap lavas, labelled C&J
Unpub (data from J. Cripps and A.E. Jay, unpublished data), Knight 2003
(data from Knight et al., 2003), and Baksi 2001 (data from Baksi, 2001).
Sample labels are: A is Ambenali Formation, AE is SE Deccan Ambenali
Formation; M is Mahabeleshwar Formation, Mu is upper Mahabeleshwar
chemotype, ME and MuE are SE Deccan Mahabaleshwar and upper
Mahabeleshwar chemotypes, respectively. Grey bar gives range of 16
samples from bottom to top of one Mahabeleshwar Formation sheet lobe ( Jay, 2005). U and L designate Upper and Lower Rajahmundry Trap lavas; KU is Kolhapur unit of Mahabeleshwar Formation discussed by Baksi (2001).
M'war and Amb averages are average values of these elements in Ambenali
and Mahabeleshwar Formation lavas from sample data set compiled by Widdowson et al. (2000b); M'war/Amb. discriminant value is that adopted for this study (see also Table 3).
Fig. 7.
Simplified
geological map of southeastern main Deccan province with outcrop areas
of Deccan formations shown, together with the Kagnar and Bhima rivers
which connect to the Krishna River. Only exposure of Bushe Fm is in NE
of area shown; laterite mapped is upper-level Tertiary laterite of Widdowson (1997).
Sample collection sites for Ambenali and Mahabaleshwar chemotype lavas
(see text) shown as squares. Towns are shown as dots; dark area on
Manjra River is a lake.
Fig. 8.
Plot
of total lava lobe thickness vs down-flow change in temperature from
thermal flow model with conditions appropriate for Deccan-Rajahmundry
lavas, see text. Maximum allowable temperature decrease of 0.05 °C/km,
permitting < 50 °C cooling over 1000 km is indicated.
Table 1.
Stratigraphic sequence of main sub-groups, assigned
volumes, paleomagnetic chrons, and typical thicknesses for the Deccan
Basalt Group
 - View Within Article
Table 2.
Major and trace element analyses by XRF (except ⁎) for main Deccan series lavas and Rajahmundry Trap lavas
- RT
lava: lower RT or upper RT flow as designated by author. Chemotype
designation as discussed in text; Amb = Ambenali type;
Mwar = Mahabaleshwar type; upper Mwar = upper Mahabaleshwar type.
(J&W, 2007) = Jay and Widdowson, 2007; (Cripps and Jay, unpublished) = AE Jay and J Cripps, unpublished data; (Baksi) = Baksi (2001); (Knight) = Knight et al. (2003), see text; Knight: ⁎ trace elements by IC-PMS, all others by XRF; ⁎⁎ total Fe as FeO. ⁎⁎⁎ Fe calculated as Fe2O3 for the normalised recalculation; LOI = loss on ignition; nr = not reported.
- View Within Article
Table 3.
Thicknesses and criteria used to distinguish the
geochemical formations (chemotypes) in Deccan Basalt Group in the
Western Ghats, India, after 71, 72 and 8, and this study ⁎
(Ambenali and Mahabaleshwar Formations only). No entry (dash) indicates
an element or ratio not useful for discrimination for that formation
- View Within Article
Copyright © 2007 Elsevier B.V. All rights reserved.
Deccan Volcano - John Seach
The Deccan Traps is located in central west India and dates from 66 million years ago.
The lava flows are some of the largest on earth covering 900 km and meet the coast at the Arabian Sea.
Deccan volcanism coincided with the decline of the dinosaurs raising the possibility
that the Indian volcanoes were involved with their decline.
Deccan lava meets the Arabian Sea at Goa. The lava flows cover 900 km throughout central and western India.
The Reunion mantle plume was responsible for the lava flows which covered 500 000 sq km.
Lava meets the sea at Goa, India
Lonar meteorite crater, Deccan volcano basalt, India
Deccan Volcano Eruptions
66 million years ago
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The
Deccan beyond the plume hypothesis
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Summary
The widely accepted mantle
plume model (e.g.,
Morgan, 1981; Richards et al., 1989; Campbell & Griffiths, 1990) postulates that
(i) the currently active Réunion Island, in the
Indian Ocean, is fed by the narrow “tail”
of a mantle plume that rises from the core-mantle boundary,
(ii) the Deccan continental flood basalt (CFB) province
of India originated from the “head” of the
same plume during its early eruptive phase near the
end of the Cretaceous, and (iii) the Lakshadweep-Chagos
Ridge, an important linear volcanic ridge in the Indian
Ocean, is a product of this plume. It is not generally
appreciated, however, that this so-called “classic”
case of a plume contradicts the plume model in many
ways. For example, there is little petrological evidence
as yet that the Deccan source was abnormally hot, and
the short (~ 1.0 – 0.5 Myr) duration claimed by
some for the eruption of the Deccan is in conflict with
recent Ar-Ar age data that suggest that the total duration
was at least ~ 8 Myr (Sheth
et al.,
2001a,b).
The Deccan CFB was associated with the breakup of the
Seychelles microcontinent from India (e.g.,
Mahoney,
1988). Geological and geophysical data from the Deccan
provide no support for the plume model and arguably
undermine it altogether (Sheth, 2005a,b).
The interplay of several intersecting continental rift
zones in India is apparently responsible for the roughly
circular outcrop of the Deccan. The Lakshadweep-Chagos
Ridge, and the islands of Mauritius and Réunion, are located along fracture zones, and the
systematic southerly age progression along the Ridge
(though questioned) may be a result of southward crack
propagation through the oceanic lithosphere. This idea
avoids the problem of a 10° palaeolatitude discrepancy
which the plume model can only solve with the ad hoc
inclusion of mantle roll. Published Ar-Ar age data for
the Lakshadweep-Chagos Ridge basalts have been seriously
questioned (Baksi, 1999, 2005),
and geochemical data suggest that they likely represent
post-shield volcanism (Sheth et al., 2003) and so are unsuitable for hotspot-based plate reconstructions.
“Enriched” isotopic ratios such as higher-than-N-MORB values of 87Sr/86Sr,
observed in basalts of the Ridge and the Mascarene Islands
may mark the involvement of delaminated enriched continental
mantle instead of a plume (Smith, 1993). High values of
the 3He/4He ratio also do not
represent a deep mantle component or plume (Anderson, 1998a;
1998b).
The three Mascarene Islands (Mauritius, Réunion,
and Rodrigues) are not related to the Deccan but reflect
the recent (post-10 Ma) tectonic-magmatic development
of the African Plate.
I relate CFB volcanism
to continental rifting, which often (but not always)
evolves into full-fledged sea-floor spreading (Sheth, 1999a, 2005a).
I ascribe the rifting itself not to mantle plume heads
but to large-scale plate dynamics, possibly aided by
long-term thermal insulation beneath a supercontinent
which may have surface effects similar to those predicted
for “plume incubation” models. Non-plume,
plate tectonic models are capable of explaining the
Deccan in all its greatness.
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Figure 1. The
1,200-m-thick exposed section through the Deccan
basalt pile at Mahabaleshwar, Sahyadri (Western
Ghats) region. Grand! Photo by Hetu Sheth.
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Since the rapid rise to
dominance of the plume-head/plume-tail model for flood
basalts ( Richards et al., 1989; Campbell
& Griffiths, 1990), hundreds of papers have
invoked, or supported, a plume head origin for the Deccan
Traps of India. These papers are in unanimous agreement
on two issues: (i) the Deccan originated from the ancestral
R éunion hotspot
which upwelled beneath India in the late Cretaceous,
and (ii) the hotspot, now located on the African plate,
is fed by a deep mantle plume. The overall appearance
of the Deccan, with its roughly circular outcrop, and
the linear Laccadives-Chagos (more correctly, Lakshadweep-Chagos)
Ridge to the south of India, looks very much like what
is expected for a spherical plume head and a narrow
plume tail (Figures 2 & 3). Nevertheless, the following
observations and deductions suggest that the plume model
is not valid for the Deccan ( Sheth, 1999a, b,
2005a).
Figure 2. Map showing
the approximate boundaries of the Precambrian cratons
making up the Indian shield (e.g., Pandey & Agrawal,
1999; Naqvi & Rogers, 1987), the granulite terrain,
the Precambrian structural trends (heavy broken lines),
rift zones crossing peninsular India (e.g., Biswas,
1987), and the present outcrop areas of the Deccan and
Rajmahal flood basalts. Inset shows the breakup of the
Seychelles microcontinent, situated along the northern
tip of the Mascarene Plateau (black), from India, soon
after the Deccan flood basalt episode (after Norton
and Sclater, 1979; Mahoney, 1988). The Koyna and Kuruduvadi
“rifts” have been proposed based on gravity
surveys and may represent humps of the granitic basement
rather than rifts.
Figure 3. Prominent
structural-tectonic features of southern Asia and the
Indian Ocean basin (based on Mahoney et al., 2002).
Abbreviations for localities are: Q, Quetta; Z, Zhob;
B, Barmer, M, Mundwara; D, Dhandhuka; B, Bombay; R,
Rajahmundry. WG is the Western Ghats region (ages from
Venkatesan et al., 1993 and others). ~ 64 Ma age for
Rajahmundry basalts is from Baksi (2001a). G, ~ 61 Ma
Goa dykes (Widdowson et al., 2000). KK, ~ 90-69 Ma Karnataka-Kerala
dykes (e.g., Radhakrishna et al., 1994; Anil Kumar et
al., 2001). SMI are the St. Mary's Islands volcanics
(85.5 Ma, Pande
et al., 2001), part of the Indo-Madagascar CFB which
in India is otherwise represented by the KK dykes. The
associated flood basalt lavas are not represented or
known in India; there are many Precambrian dyke swarms
throughout southern India as well. 72-73 Ma ages for
Quetta and Zhob rocks and 65 Ma age for Dhandhuka-Botad
lavas are from Mahoney et al. (2002), as also the modelled
hotspot track showing expected ages in Ma. Note the
rift zones underlying the Deccan, and the absence of
any triple junction. OFZ, Owen Fracture Zone; MFZ, Mauritius
Fracture Zone; VFZ, Vishnu Fracture Zone. Click
here for enlargment.
Abnormally hot mantle? There is no evidence for “abnormally hot”
mantle sources for the common and voluminous Deccan
basalts (Figure 4). Some picritic liquids are encountered
in boreholes in the northwestern Deccan and in the Narmada
region (Krishnamurthy et al., 2000). The borehole
lavas were conjectured by Campbell & Griffiths
(1990) to be high-temperature, high-melt-fraction liquids
from the plume axis. Peng & Mahoney (1995),
however, found that they are somewhat alkalic and could
be high-pressure, low-degree melts. The Deccan flood
basalt sequence is best developed in the Western Ghats
region with ~3 km of stratigraphic thickness (Figures
1-3), and picritic basalts are found there, but these
are enriched in cumulus olivine and clinopyroxene and
do not represent liquid compositions. The parental melts
of these picrites are estimated to have contained only
~ 9-10% MgO (Beane & Hooper, 1988; Sheth,
2005b).
Figure
4. (a) A plot of 624 samples of Deccan basalts of the
Western Ghats (data of Beane, 1988; courtesy J. J. Mahoney)
on the well-known TAS diagram. Note the complete absence
of compositions other than basalt and basaltic andesite,
and the nearly exclusive subalkalic (tholeiitic) nature.
Dividing lines between alkalic and subalkalic fields
proposed by Macdonald & Katsura (1964) and Irvine
& Baragar (1971) are also shown. (b) Plot of the
same samples on the familiar AFM diagram, showing the
Fe enrichment trend typical of tholeiitic basalts. Typical
tholeiite trend (Thingmuli, Iceland) and calc-alkaline
trend (Cascades) are also shown, along with boundaries
between the two fields proposed by Kuno (1968) and Irvine
& Baragar (1971). See Sheth
(2005b) for an extended petrological discussion.
Very short (1 – 0.5 Myr) eruptive duration? Very rapid emplacement
of the Deccan Traps is one of the key arguments for
a plume origin, though also not incompatible with plate-related
and stress-caused mechanisms. The duration of volcanism
has also been one of the most debated issues. Recent
40Ar-39Ar data for trachyte and
basalt flows from Bombay (Sheth et al., 2001a,b)
suggest the total duration to have been no less than
~ 8 – 9 Myr. There may have been a major, rapid,
short-duration eruptive phase in the Western Ghats,
estimated by some to have lasted only 1.0 – 0.5
Myr (e.g., Duncan & Pyle,
1988; Courtillot et al., 1988; Hofmann
et al., 2000), and by others 4 – 5 Myr (Venkatesan
et al., 1993; Pande, 2002). Also, the data
do not always justify the arguments advanced. Allègre
et al. (1999) report an Re-Os isochron age of 65.6
± 0.3 Ma (2σ) for several lava flow samples,
arguing for a very short duration for the volcanism.
That random, non-comagmatic samples collected across
an area 1000 km wide and and at various topographic-stratigraphic
levels should define an isochron is remarkable, but
the goodness-of-fit (F) value for the claimed
isochron, which was not reported, is 22 (Baksi,
2001b); the line is clearly an “errorchron”
(Faure, 1986).
Catastrophic eruption rates? Some authors have explained “the
extremely high lava eruption rates” in CFBs by
hot plume heads, though there is no direct and simple
relationship between melt production and melt eruption
(Th. Thordarson, pers. comm., 2005). A large proportion
of the Deccan basalts comprise pahoehoe compound lava
flows (e.g., Walker, 1970; Bondre et al.,
2004a,b). My own fieldwork at scores of places in the
Deccan, and on the Kilauea volcano, Hawaii (Sheth,
2003), shows that the size and scale of individual
flow units of many large Deccan compound flows are the
same as those of modern Kilauea lava flows (Figure 5).The
large volumes of the individual Deccan lava flows compared
to the Hawaiian flows may reflect in part the great
amount of decompression during India-Seychelles continental
breakup (Figure 2 inset), considerable lengths (40 –
50 km) of the fissure systems (Figure 6; see also Self
et al., 1997), excess source fertility (Sheth,
2005b), mantle volatiles such as CO2
(Presnall
& Gudfinnsson,
2005), and similar features.
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Figure 5. (a) The newborn
toe of a compound pahoehoe basalt flow that has emerged
from under the solidified lava crust, as a “breakout”.
The front is about 1 m from the camera and 0.5 m wide.
Ropes are forming in the frontal part and satellite
breakouts emerge at right and left (bright yellow portions).
Kilauea, Hawaii, May 2002. Photo by Hetu Sheth. This
is how I believe the compound pahoehoe flows of the
Deccan were emplaced. (b) Broader view of the actively
inflating pahoehoe compound flow containing the lobe
shown in (a). Note how numerous lobes are juxtaposed
laterally and vertically. I am seen opening with the
hammer the solidified roof of a lobe which yellow-hot
magma (at ~ 1200°C) is filling. Kilauea, May 2002.
Photo by Jyotiranjan Ray. This is how many compound
pahoehoe flows of the Deccan look. Compare with (c).
(c) Section across part of a compound pahoehoe lava
flow of the Deccan, showing the distribution of vesicles
and pipe vesicles. Some 17 flow units are seen. Modified
from Walker (1970). Compare with (a) and (b).
Figure 6. Map of the
Dediapada dyke swarm in the Narmada-Satpura region of
the Deccan (after Krishnamacharlu, 1970). This is one
of the large, spectacular oriented dyke swarms of the
Deccan Traps. Geochemical studies of these dykes are
currently underway. Elevations are in metres.
Internal age progression? None exists within the Deccan (Figure
3). Courtillot & Renne (2003) suggested that
the 60–61 Ma volcanic activity well within the
Deccan (e.g., at Bombay, Sheth et al.,
2001a,b),
was “minor”, and that the duration of Deccan
volcanism was indeed very short. However, (a) this activity
is not minor; large volumes of lava are emplaced in
the subsurface along the west coast, and there is a
scarcity of geochronological data. In comparison, the
Western Ghats section has been heavily sampled and dated.
(b) Whatever its magnitude, the late-persisting volcanism
must be still explained without ad hoc auxiliary
hypotheses. It is not. For example, according to prevalent
views the plume head was all consumed in a quick phase
around 66 – 65 Ma, and the predicted 60 Ma volcanic
basement to the south of India, on top of the Maldives
Ridge, should have formed from the narrow (100 –
200 km wide) plume tail. It is not clear how this plume
tail could produce basalt in Bombay, 1,000 km to the
north, at 60.5 Ma (Sheth et al., 2001b). Suggestions
such as northward dragging of the plume tail by the
plate are ad hoc, and such drag and tilting
would make impossible any systematic age progression
in the first place. Furthermore, if the ~ 69 Ma mafic
dykes reported from Kerala, southernmost India (Radhakrishna
et al., 1994) do represent early Deccan-related
magmatism, as Sheth
(1999b) considered likely, an entirely different,
non-plume, passive, continental-breakup-related model
for Deccan volcanism is even more attractive.
Enriched mantle: plume or continent? Smith (1993) proposed
that ocean-island volcanism is derived from enriched
continental mantle delaminated from a continent rifted
along an ancient suture (see also Lithospheric
delamination page). “Enriched” isotopic
ratios such as higher-than-N-MORB values of 87Sr/86Sr,
for example, are usually taken as plume signatures.
However such compositions may instead mark involvement
of shallow-level, enriched continental mantle. High
values of 3He/4He may also be
explained by shallow models (e.g., Anderson,
1998a, 1998b;
see also Helium fundamentals
page). The ~ 68.5 Ma alkalic complexes (Mundwara,
Barmer) in the northern part of the Deccan province,
related by Basu et al. (1993) to the Réunion
plume based on Sr and He isotopic ratios, could thus
be derived from the continental mantle.
The “enriched” plume model was never
required to explain continental intraplate volcanism,
given the abundance of “enriched” mantle
domains within the continental lithosphere itself. The
plume model was extrapolated to continental magmatism
from the ocean basins based on the world view that the
oceanic mantle was entirely ”depleted“,
MORB-like, convecting and homogeneous. The reasoning
was that anything “enriched” or anomalous
had thus to come from plumes (Anderson, 1996; Smith & Lewis, 1999).
However, if continental mantle is introduced into the
oceanic mantle, e.g., by delamination during continental breakup (e.g., Smith,
1993), enriched plumes are not required to explain either
continental or oceanic intraplate volcanism, and the
whole argument can be turned around. Rather than the
Deccan having formed from a deep mantle plume now located
under Réunion island, Réunion volcanism
may be in part sourced from delaminated Indian continental
mantle.
Mahoney et al. (2002) recently reported Réunion-like elemental
and isotopic compositions for mafic ophiolitic rocks,
dated by them at 72 – 73 Ma, and outcropping in
Pakistan. They opined that some of these may represent
pre-Deccan oceanic seamounts. The associated intrusions
were emplaced in continental shelf-and-slope-type marine
sediments along the northern margin of India. Mahoney
et al. (2002) considered the continental mantle
delamination model, but argued that it does not explain
Réunion-type volcanism occurring on the updrift
side of India at 72 – 73 Ma, and concluded that
the plume model is the most viable option. Notably,
they supported the plume-head-impact model
rather than the plume-head-incubation model, despite
the ~ 8 Myr age gap between the Pakistani rocks and
the 66 – 65 Ma voluminous basalt volcanism of
the Deccan.
Nevertheless, the analyzed intrusions are
located within the boundary of the Indian continental
mantle, and the true oceanic seamounts may not have
been far from the northern margin of India. Continental
mantle delaminated during the early stages of India-Seychelles
breakup could have migrated northward ahead of India
and fed the seamounts built on oceanic lithosphere.
The continent followed behind, and when it converged
upon Asia it simply overrode these seamounts. This is
a better explanation for the observations than the plume
model. If the lateral flow of continental mantle proposed
here seems ad hoc, note that the mechanism of long-distance lateral flow is required
even by the plume model, as for the Rodrigues Ridge
(Morgan, 1981; Figure 3) which is not located along the conjectured Deccan-Réunion
hotspot track and trends roughly E-W. The rocks analyzed
by Mahoney et al. (2002) and Basu et al.
(1993) are undersaturated and alkalic, and have ocean-island-basalt-type
characteristics (e.g., Sr isotopic ratios), but
rather than being melts from a hot plume, they may be
melts of carbonated lherzolite (see Keshav &
Gudfinnsson, 2004).
The “hotspot track”: plume under plate, or crack
propagation? The claimed southerly younging
age progression along the Chagos-Laccadive Ridge and
up to Réunion Island (though duly questioned
by Baksi, 1999, 2005)
does not require a lithospheric plate moving over a
fixed plume. It may be explained by southward crack
propagation through the oceanic lithosphere (see below).
The narrow “hotspot track” may represent
localized melting and magma focusing from a wider area
(the “transform-fault effect”, Langmuir
& Bender, 1984). In support of this, I note
that the Chagos-Laccadive Ridge lies along the Vishnu
Fracture Zone. The Ridge may mark the location of a
major Gondwanic transform (Reeves & de Wit,
2000; Reeves
et al.,
2004).
It is possible that the
current volcanism at Réunion Island may be unrelated
to the Deccan geodynamically, though it taps delaminated
Indian continental mantle brought beneath the African
plate by the ridge jump at ~ 30 Ma (Sheth, 2005a;
see also Burke, 1996). Burke (1996),
a plume proponent, argued that the Deccan plume died
out at 30 Ma and the Réunion plume is a different
plume.
The Cambay triple junction and other fiction. Originally included by
Burke & Dewey (1973) in their world-wide
list of plume-generated triple junctions, the Cambay
triple junction has been popularized by several subsequent
papers supporting the Réunion plume model for
the Deccan. However, the triple junction is not real
(Sheth, 1999b,
2005a;
Figures 2 & 3). Another unfortunate development
is proliferation of model-dependent interpretations
by which every geological and geophysical observation
from the Deccan is interpreted an effect of the Réunion
plume. For example, low-seismic velocity mantle underlying
the Cambay rift of the Deccan is interpreted as a remnant
of the plume (Kennett & Widiyantoro, 1999)
instead of warm, low-density upper mantle welling up
due to rift-related convection. This geophysical feature
may even be a recent (post-Deccan) development (Sheth,
2005a).
Pre-volcanic lithospheric uplift, or lack
thereof? Pre-volcanic lithospheric uplift of up to a few kilometres
is an essential prerequisite for all thermal models
such as the plume model. This is yet another issue on
which specialists of different flood basalt provinces
have come to diametrically opposed conclusions (e.g.,
Czamanske et al.,
1998; He et al., 2003; Tejada
et al., 2004; Saunders et al.,
2005; see also Dhanjori page).
Campbell & Griffiths (1990) cite the Deccan
as a good example of a flood basalt with pre-volcanic
uplift, but the Pachmarhi area mentioned by them as
evidence for this appears instead to show the very opposite
(recent uplift). Pachmarhi is on the Satpura horst between
the Tapi and Narmada rifts. The very youthful landscape
(e.g., kilometre-high escarpments in the basement
Gondwana sandstones) and several planation surfaces
(as high as 1,300 m above MSL) indicate very recent
uplift (Ollier & Pain, 2001).
The
same is true of the Deccan plateau region, where the
Deccan-basement contact is in the subsurface over vast
areas. Major rivers draining the Deccan plateau are
of the antecedent type, i.e., they were in
existence before the Western Ghats (Sahyadri Range)
rose in their way, and the popular dome-flank drainage
picture of the Indian drainage painted by Cox
(1989) is highly speculative. The uplift of the Western
Ghats is post-volcanic and recent (possibly Miocene
and younger), and not pre-volcanic uplift produced by
a plume ( Sheth,
2005a). There are two possible interpretations:
(1) Pre-volcanic lithospheric uplift occurred and then
completely decayed and was overprinted by post-volcanic
uplift. This is what plume proponents advocate. (2)
Pre-volcanic uplift never did take place and the plume
explanation is invalid. Option (2) is more plausible,
and there is in fact actual support for it in the form
of an uplifted, extensive planation surface below the
Deccan lavas in central India ( Dixey, 1970; see
Sheth,
2005a). Note that the Western Ghats rise much higher
in southern India (the region little or not affected
by Deccan volcanism) than they do in the Deccan plateau
region (Figure 7).
Figure 7. The main
elements of the physiography of the Indian peninsula.
The Western Ghats escarpment is shown by the heavy
broken line. Note the pronounced easterly drainage.
After Ollier & Powar (1985) and Sheth
(2005a).
Palaeolatitudes: true polar wander or crack
propagation? The Deccan lavas erupted at ~ 30°S latitude. Réunion
Island is at 21°S today (Figure 8a). To explain
this significant discrepancy in the framework of the
plume model, some workers have proposed true polar wander
(TPW) of the Earth's mantle (e.g., Vandamme & Courtillot,
1990). In this view, subsequent to the Deccan eruptions,
the Réunion plume remained fixed in the mantle,
while the mantle itself rolled like a ball, inside the
lithospheric shell, in a northerly direction. Such speculation
indicates well the extent of special pleading permitted
within the plume model. Burke (1996) has questioned
this postulated TPW.
I propose a
much simpler alternative to the TPW, illustrated by
the schematic diagram shown in Figure 8b. This is that
the systematically changing palaeolatitudes between
the Deccan and ODP Site 706 (33 Ma) indicate southward
crack propagation in a northward-moving plate with the
condition that the northward plate motion was faster
than the southward crack propagation. The situation
shown in Figure 8b is for a plate in the southern hemisphere.
At time T1, there is an active volcano 1 at the crack
tip at latitude 60°S. Between times T1 and T2, the crack tip has moved
southward by 10°, but because the plate itself has moved north by
20°, the new volcano 2 at the crack tip has the latitude
of 50°S. A similar progression occurs between times T2
and T3. Thus, although the crack tip propagates southward, the palaeolatitudes systematically become more
northerly.
I conclude,
based on diverse evidence, that the Réunion plume
model for the Deccan is wrong.
Figure 8: (a)
Palaeolatitude variation from the Deccan to Réunion
Island through the ODP Leg 115 sites (Vandamme &
Courtillot, 1990). (b) Schematic cartoon showing the
development of volcanism resulting from a crack propagating
more slowly southward than the plate moves northward,
in the Southern Hemisphere.
Additional thoughts
Deccan
volcanism was associated with the separation of the
Seychelles microcontinent from India (Figure 2, inset),
and this breakup itself is often ascribed to the Réunion
plume head impact. I propose instead that the breakup
occurred because of prolonged continental extension
and an eventual ridge jump.
One interesting question is whether eclogite, a mantle
rock more fusible than peridotite, could have been a
source in part for the Deccan lavas. The rifted western
continental margin of India follows the NNW-SSE Dharwar
structural trend of the Precambrian southern Indian
shield (e.g., Biswas, 1987). Also, the Narmada
zone that crosses India has been proposed as an ancient
suture between the southern (Dharwar) and northern (Aravalli)
protocontinents (e.g., Naqvi et al., 1974; Naqvi
& Rogers, 1987; Radhakrishna, 1989). Such an ancient
suture may have contained trapped, eclogitized oceanic
crust. Foulger
et al.
(2005) recently proposed such a model for Icelandic
volcanism, and Sheth
(2005b) explored it in considerable detail for the
Deccan. If eclogite constituted a major source for the
Deccan, mantle fertility and not high mantle temperatures
are implicated (see also Yaxley, 2000).
I
have already explained why, despite all problems and
anomalies, the whole Deccan province resembles so much
what a plume-head/plume-plume tail is expected to create.
The interplay of the Deccan rift zones is responsible
for this. Several older sedimentary rift basins underlie
the Deccan (Figures 2 & 3). These are the Cambay,
Kachchh, Narmada-Tapi and Godavari rifts. They are not
arranged radially, are much older than the Deccan, being
of at least Jurassic age, and certainly were not produced
by whatever produced the Deccan. The presence of several
such major rifts means that lithospheric control (Anderson,
1998c), and lithospheric extension (Sheth,
2000) were important in Deccan volcanism. The nearly circular outcrop of the Deccan proper
does not reflect a spherical plume head beneath, but
simply results from the confluence of numerous rift
zones and the continental margin in west-central India.
This is a likely explanation because elsewhere, in the
peripheral/outlier parts of the Deccan along individual
isolated rifts, the lava outcrop is linear or localized
(e.g., the Deccan outliers of the Kachchh and
Rajahmundry areas, the latter being on the Godavari
rift near the east coast of India). The “hotspot
track” on the oceanic crust, as argued above,
may be related to melting and magma focusing along a
southward-propagating fracture. The available seismic
data for Réunion Island support the idea that
its location is related to structural heterogeneity
of the underlying lithosphere (Charvis et al.,
1999; de Voogd & Pontoise, 1999; Hirn, 2002).
In conclusion, a non-plume,
plate-tectonic model involving continental breakup and
related mantle convection and decompression melting,
is suitable for the Deccan. If radial, focused flow
of the upper mantle occurs (instead of vertical flow
as in the plume model), a potentially unlimited volume
of the mantle is available for processing.
The plume model was proposed for the
Deccan more than 30 years ago, when little was known
about the Deccan and about the plume mode of convection.
Today we know a lot more about the Deccan, and the plume
model becomes less tenable as our knowledge grows. The
originators and champions of the plume model had little
or no personal knowledge of the Deccan, and their broad
generalizations should have been put to critical tests
by regional experts on the Deccan. The tendency has
been to assume a plume origin and infer the plume properties
and characteristics based on whatever is required by
the observations, but if volatiles, mantle fertility,
continental geology, and dynamic, evolving plates are
considered, one no longer needs mantle plumes (Sheth,
2005b). The new voices asking for new explanations
(e.g.,
http://www.mantleplumes.org/Deccan2.html)
are a good sign. The plume model for the Deccan has
been around for over thirty years, and has failed. A
new, scientifically tenable non-plume model for the
Deccan could be rapidly developed if only a few of the
many Deccan/flood basalt enthusiasts share the task.
We live in interesting times.
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Why the Deccan Traps are Important!
 The
timing of when the bulk of the eruptions occurred, 65 million years
ago, is interesting because this is at the same time as what is known as
the K-T boundary. Found in the rock at the K-T boundary is the presence
of enriched iridium, an element rare in Earth's crust but abundant in
meteorites. An impact event now identified at what is named Chicxulub Crater
is widely believed to have caused the extinction of the dinosaurs.
There is strong evidence though that this could not have been the only
factor though. So scientists are now looking at the Deccan Traps as a
contributing factor perhaps in the extinction of the dinosaurs.
The impact of the meteorite at Chicxulub Crater
would have caused massive damage around the Earth and triggered an
impact winter. The Deccan Traps would have contributed a further global
2°C drop in temperature and the realease of massive amounts of sulphuric
gasses into the atmosphere. There is data supporting a killing off of
foraminifera with the main eruptions at the Deccan Traps.
The
Deccan Volcanic Province:
Thoughts about its genesis
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S. Rajan, Anju
Tiwary & Dhananjai Pandey
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Click here to
go to Discussion of this page
 Click here to
download a PDF version of this webpage
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1.
Impetus for this
contribution
The Deccan volcanic
province (DVP) is one of the world's largest LIPs
and perhaps the best studied continental flood basalt
(CFB). However, its genesis and evolution are still
poorly understood.
Recently, Sheth
(1999) convincingly refuted the plume model as a basis
for the genesis of the DVP (see The
Deccan beyond the plume hypothesis). However,
his suggestion that the DVP and Laccadive-Chagos ridge
formed as a consequence of southward crack propagation
along the Vishnu fracture zone is not consistent with
data concerning the geomagmatic and tectonic history
of the Indian peninsular plate. The current status
of knowledge thus represents a shortfall of understanding.
The need to fill this gap in our knowledge, and to
establish a genetic model for the DVP based on the
vast volumes of existing scientific data, was the
impetus for the present contribution.
2.
Introduction
The DVP is one of the
Earth's giant continental flood basalts and has a
total exposed area of about half a million square
kilometers, between latitudes 16° -
24° N and longitudes 70° - 77° E.
In the northwestern, central and southern Indian peninsula,
the approximate volume of the DVP is about 2 x 106
km3 and its estimated age is 64-65 Ma.
It is generally believed that the DVP originated during
Gondwanaland breakup as part of the Seychelles-India
separation event. Another important belief concerning
this CFB is that it is the “head” of a
plume which is currently active as Reunion volcanism,
with the “tail” consisting of the rather
irregular chain of volcanic islands extending from
Reunion to and along the Laccadive-Maldive-Chagos
ridges. The latter model has been refuted convincingly,
as mentioned above. However, a viable alternative
model has not yet been proposed. In this contribution
we attempt to provide an alternative hypothesis based
on existing geological information about the DVP.
3.
The Deccan volcanic province:
existing scientific data
The DVP erupted on the
Archean-Proterozoic shield areas of south, north-west
and central India and the adjoining offshore area
off the west coast (Figure 1) (Devey & Stephens,
1991). The volcanics cover two cratonic areas –
the Dharwar craton of the south Indian shield and
the central Indian craton. Apart from this, the DVP
is associated with four major rift zones of peninsular
India (Figure 1). It is juxtaposed with the east-west-running
Narmada-Satpura-Tapi rift which is a horst-and-graben-type
rift zone that trends ENE-WSW for > 1600 km along
central India (Mishra, 1977). In northwest
India, the DVP is in contact with the Cambay, Kutch
and West Coast rifts.
Figure 1: Geological map showing
the location of the Deccan volcanic province and its
relationship with the geo-tectonic features of the
region. Modified after Sheth (2005).
To evaluate the causes
of DVP genesis, it is essential to understand the
geological context of its hosts. Therefore, to interpret
the data from the Deccan volcanics, we first review
the nature of the hosts.
Along the West Coast
rift, the south Indian shield witnessed several
prior phases of magmatism before it hosted the Deccan
volcanics. The first recorded event occurred at 678
Ma when gabbro, granophyre and anorthosite magmatism
occurred (Nair & Vidyadharan, 1982). This
was followed by granitic plutonism, almost 128 My
later, i.e., at 550 Ma (Soman et al.,
1983). Subsequently, the region hosted pegmatitic
intrusions at 460 Ma (Soman et al., 1982).
These pegmatites mark the end of the major phases
of the magmatic episode because there was then a complete
hiatus of magmatic events in the region until 93 Ma
(date averaged from six differing K-Ar dates), i.e.,
for more than 350 My! At 93 Ma, the West Coast region
again experienced magmatism, this time involving rhyolitic
and dacitic volcanics (Valsangkar et al., 1980).
This was followed by vast mafic volcanism and plutonism
which resulted in the DVP and associated dyke swarms
during the period 64-65 Ma, and covered the huge area
mentioned above.
Along the Narmada-Tapi
rift zone, prior to the Deccan episode, no major
magmatic event is reported. Instead, the horst (Satpura)
and grabens (Narmada and Tapi) paved the way for enormous
sedimentation, which gave rise to the Mahakoshal group
of rocks (Chanda & Bhattacharya, 1966).
These metasediments recorded several phases of shearing
implying that the Narmada – Tapi rift is associated
with intense shear deformation along an east-west
trend. These metasediments are followed by Jurassic-early
Cretaceous sediments on top of which the Deccan volcanics
lie (Chanda & Bhattacharya, op.cit).
The tectonics of
the northwest Indian peninsula and the offshore region
are inter-related. The summary of the tectonic
history of this region given here is based on the
studies of several workers (Glennie, 1932;
Qureshy, 1971; Owen, 1976; Kaila
et al., 1979; 1981; Biswas, 1982; 1987;
Harbison & Bassinger, 1973; Gupta et
al., 1998). As shown in Figure 1, four major rift
zones are in contact with the Deccan volcanics, the
Narmada-Tapi rift, the West Coast rift, the Cambay
rift and the Kutch rift. Kutch rifting occurred in
the late Triassic to early Jurrasic followed by early
Cretaceous Cambay rifting. The Narmada-Tapi and West
Coast rifts were reactivated in the late Cretaceous.
The Narmada-Tapi rift zone is believed to be extending
along its trend into the offshore area of the Indian
west coast.
Concerning geophysics
(Chandrasekharam, 1985), the Bouguer gravity
anomaly pattern and seismic profiles along the West
Coast rift indicate:
-
thinning
of the continental crust along the western coast.
This implies delamination of the lithosphere beneath
the west coast,
-
rifting
of the coast in a horst-and-graben pattern, and
-
shear displacement
of the West Coast fault.
Deep Seismic Sounding
(DSS) investigations have been carried out in the
Indian peninsula (Reddy et al., 1999). This
study indicates that the crust seems to become thinner
(24 km) towards the northern parts of the west coast,
that is north of 15°N. The west coast was also
characterized by upwarp of the Moho during the late
Cretaceous period.
Vertical crustal movements
are recognized along the West Coast rift, and shear
displacement along the Narmada-Tapi rift zone and
its extension into the offshore areas (Biswas,
1982).
4.
The new proposed hypothesis: an effort to bridge the
gap in knowledge
4.A:
The first phase of magmatism, 678 to 460 Ma: Of
the four rifts, only the West Coast rift exclusively
hosts magmatic rocks older than the Deccan volcanic
event at 64-65 Ma. These magmatic rocks, which formed
at 678-460 Ma, show a clear trend of fractional melting
(gabbro-rhyolite to pegmatites). This implies the
presence of a magma chamber beneath the Indian lithosphere
under the west coast. In this magma chamber, magma
could have remained stored and preserved its primary
chemistry. The primary magma could have risen from
its depth of segregation which was beneath or within
the west coast lithosphere, but certainly not deeper
than 200 km because the first magmatic event was gabbroic.
The hiatus in igneous activity from 460 Ma
to 93 Ma along the West Coast rift indicates that
during this period the magma pressure in the chamber
was less than lithospheric, and/or the temperature
was too low for magma to cross the liquidus of its
compositions. By 93 Ma, that is 350 My later, this
problem was removed.
4.B The
beginning of the second phase of igneous activity,
at 93 Ma: This phase, along the West Coast rift,
is marked by a magmatic event which was not plutonic,
like previous ones, but rather volcanic. Unlike the
previous episode, it began with magma of felsic composition
– the rhyolites. It is relevant to note that
Madagascar-Indian plate breakup took place at around
93 Ma. This breakup also occurred along the western
continental margin of India. Obviously, such a megascale
rifting event will leave its signature in the form
of volcanic, not plutonic activity, which can explain
the simultaneous rhyolitic volcanism. The composition,
felsic instead of mafic, probably indicates that the
temperature was too low to melt mafic components in
the magma chamber beneath the West Coast region, but
elevated enough to produce the rhyolitic melts, i.e.,
it was around 1000°C.
4.C The
Deccan volcanism, at 64-65 Ma: The second event
of this phase is the Deccan volcanism which occurred
after a hiatus of 30 Ma. Also, after exactly the same
hiatus, and at the same time of 64-65 Ma, the Indian
plate experienced breakup from yet another partner
at Gondwanaland time – the Seychelles. This
probably indicates that the second stage of magmatism,
which began at 93 Ma, was controlled by the breakup
events between Gondwanaland microplates and the Indian
plate. Also, it shows that the breakup process was
a gradual and progressive phenomenon, starting with
Madagascar-India separation and, after another 30
My, Seychelles-India separation. Since this process
controlled West Coast magmatism, which tectonism is
expected to do, we deduce that the magmatism was also
progressive. Thus, we conclude that (i) the rhyolitic
volcanism at 93 Ma resulted from Madagascar-India
breakup, and (ii) that this breakup event was a continuous
process which led to Seychelles-India breakup after
another 30 My and to Deccan volcanism at the same
time. The progressive chemical trend of the volcanics,
i.e., from rhyolite to basalt, indicates
gradual progressive increase in temperature and/or
gradual progressive lowering of the liquidus in the
magma chamber as a result of gradual progressive rifting/breakup
of the Indian plate with Madagascar and the Seychelles
respectively.
The shift from plutonism
during first phase to volcanism during the
second phase perhaps indicates the presence of direct,
uninterrupted conduits from the magma chamber to the
surface of the continental crust during the second
phase. This is what is expected during extensive rifting
events such as Madagascar-India and Seychelles-India
separation which did not occur during the first phase.
The large volume
of the Deccan volcanics and the high rate
of volcanism during the Deccan episode indicate:
-
higher rate of adiabatic
decompression due to continental scale rifting,
-
consequently, higher
rate of melting of magma in the chamber,
-
further, continental
delamination of the western continental crust due
to elevation in temperature and decrease in viscosity
caused by the presence of a heat source in the form
of a magma chamber at its base, and
-
a direct plumbing
system between the melt and the surface, during
eruption.
Considering that the
West Coast and Narmada-Tapi rift zones were reactivated
at the time of Deccan volcanism and the Cambay and Kutch
rifts were also available as direct conduits for the
upward movement of melt, we infer that the presence
of these four rifts and geophysical evidence of lithosphere
thinning beneath the westen coast explains the size,
volume and eruption rate of the Deccan volcanics.
The geochemical
variation within the Deccan volcanics, as mentioned
above, perhaps indicates differences in the chemistry
of the host rocks. For example, along the Narmada-Tapi
rift zone the magmatic melt must have interacted with
the host sediments, which are Mahakoshal Jurassics
along with Archean metamorphics. Similarly along the
West Coast, Cambay and Kutch rifts, the melt would
have interacted with Archean-Precambrian metamorphics.
Consequential changes in the chemistry would be reflected
in the geochemistry of the DVP volcanics.
4.D
Post Deccan
4.D.1: 61 Ma:
The rifting which had started at 93 Ma resulted in
the opening of the Carlsberg ridge at about 61 Ma.
Deccan volcanism also continued, as suggested by the
age of DVP rocks from the Bombay area (Sheth &
Ray, 2002). This is the time when the Laccadive
ridge also experienced Deccan volcanism.
4.D.2: 55-50 Ma:
Sea-floor spreading along the Carlsberg ridge resulted
in the emplacement of ocean floor between the ridge
and the continental margin of the Indian West Coast.
Deccan volcanism along the West Coast rift produced
the Maldive ridge at around 55 Ma and the Chagos ridge
at around 50 Ma. Clearly, the Maldive and Chagos ridges
formed by interaction of the West Coast rift magma
with Carlsberg ridge mantle magma because the Laccadive
ridge, at 61 Ma, is characterised by melt with the
same chemistry as melt from the magma chamber beneath
the western continental crust of India.
The locations of these
ridges mark the position of India at a given point
in time. At this time, the Indian plate was moving
northwards at a velocity of 18-19 cm/year. Thus, the
position of Indian plate was controlled by (a) its
own velocity in a northerly direction, and (b) the
speed of Carlsberg ridge propagation.
4.D.3: 45-35 Ma:
By this time, the northward movement of India had
slowed considerably because the Indian plate had collided
with the Eurasian plate. As a result, intraplate tectonics
were solely responsible for deformational events.
Along the West Coast rift, two types of forces were
important (Figure 2), the north-south-trending West
Coast and Cambay rift forces and the east-west-trending
Narmada-Tapi rift force. The net vector force of these
two combined resulted in shearing and stretching along
the West Coast rift zone (Figure 2). Simultaneously,
isostatic balancing forces resulting from the emplacement
of a huge volume of volcanics resulted in vertical
movements along the west coast. These conclusions
are based on the fact that the West Coast rift is
bounded by intersecting sets of faults and fractures
which extend up to the Laccadive ridge (Figure 2).
The fracture system north of 16°N formed during
the late Cretaceous whereas the systems to the south
of 16°N formed during the middle to late Tertiary.
This indicates that deformation along the coast had
started in the late Cretaceous and gradually progressed
southwards during the late Tertiary. Probably, the
combined outcome of these forces resulted in Laccadive
ridge separation (due to stretching), southward displacement
(due to shearing) and subsidence (due to isostatic
balancing) during the late Tertiary.
Figure 2: Diagrammatic representation
of the proposed hypothesis described herein for the
geological history of the Indian plate from 65 Ma
onwards. The major stress directions along western
continental margin of India 45-35 Ma are indicated
by red arrows.
Summary
We propose that
a magma chamber underlies the West Coast rift of the
Indian peninsular. In support of this idea, we point
to evidence for continuous magmatism along the west
coast at 678 – 460 Ma, which shows a continuous
trend of fractional melting. This was followed by
a lull in magmatism for almost 350 My. Magmatism started
again at around 93 Ma when Madagascar broke away from
the Indian plate. Since that rifting took place along
the west continental margin of India (i.e. today’s
western coast), we conclude that the associated magmatism
along the western continental margin of the Indian
plate was related to Madagascar-India break up. Rifting
led to decompression which in turn led to partial
melting in the magma chamber beneath the western continental
margin of India. This is supported by the fact that
magmatism during this phase started with felsic volcanism
and not mafic.
As rifting proceeded
along the western margin of the Indian plate, the
rate of partial melting in the magma chamber increased
proportionately leading to the mafic Deccan volcanism.
The high rate of Deccan volcanism was due to the fact
that along with the West Coast rift, three other deep
crustal rifts were activated simultaneously. This
could have been due to continuous rifting along the
west coast from 93 Ma onwards. We propose that the
high volume of Deccan volcanism was because:
-
The magma
chamber accumulated magma for almost 350 My, between
460 Ma and 93 Ma, during which time magma was added
to the chamber and crystallized, and
-
Delamination
of the Indian continental crust occurred above the
magma chamber.
We envisage the
magma chamber process to be as follows. A huge magma
chamber progressively accumulates melt, first from
the underlying mantle and eventually from both mantle
and continental crust (by delamination). The molten
material solidifies with time and remains in the chamber
during the period 460-93 Ma. When continental breakup
of greater India starts (including Madagascar rifting
away) the solified magma begins to melt. Since the
melting point of felsic components is lowest, these
are melted first and rhyolites typically comprise
the first phase of volcanics. Alkaline magma is not
initially formed because it has a higher melting point.
Following Deccan volcanism, the Carlsberg
ridge formed and the Indian plate continued to move
north. The systematic time progression of volcanism
between the Carlsberg ridge and the Indian plate is
due to sea floor spreading and not to plate
movement above hotspot.
When the Indian plate collided with
the Eurasian plate, its velocity decreased considerably
and intraplate forces started to play a lead role.
The main forces were then an EW force along the
Narmada-Tapi rift and a NS force along the West
Coast rift. The combined affect of these forces
led to rifting and shearing along the West Coast
rift and resulted in subsidence, pull-apart and
en echelon deformation along the Laccadive
ridge.
To reiterate, the proposition that
~ 106 km3 of Deccan basalts
was erupted from a magma chamber in which it was
stored is radical. However, there are several strong
points that support this suggestion:
-
The long history of magmatism
along the same zone which culminated in the
eruption of the Deccan Traps,
-
A lull in the magmatism
for 350 My and its subsequent reactivation,
-
DSS results which show
that the Moho upwarped beneath the west coast
during the late Cretaceous. This could be associated
with formation of the magma chamber,
-
Continental scale rifting
(Seychelles-India) and Deccan volcanism are
contemporaneous. The timing of the major phase
of Deccan volcanism is considered to be ~ 66-62
Ma (Courtillot et al., 1986; Venkatesan
et al., 1993) and the timing of Seychelles-India
breakup is also thought to be late Cretaceous
(Biswas, 1982). Note that the breakup
of Gondwanaland was a continuous process which
started (in the case of greater India) with
the separation of Madagascar (at ~ 93 Ma) and
by 63 Ma, Seychelles also separated from mainland
India (Biswas, 1982; Gombos et
al., 1995). The breakup of Seychelles-India
is thought to be the main rifting phase associated
with Deccan flood basalt eruption and the available
data suggest that they were contemporaneous.
This age data provides strong evidence in favor
of stored magma which underwent progressive
melting, in proportion to advancement in rifting
(and consequent lowering of liquidus of the
magma in the chamber), and resulted in volcanism
whose rate and volume corresponded to the rate
and scale of rifting.
-
The time of reactivation
of the deep crustal continental rifts coincided
with breakup events elsewhere in greater India,
-
The volcanism in the second
phase started out felsic and gradually became
mafic over a 30-My period, which points toward
progressive lowering of the liquidus, and not
a sudden increase in temperature, and
-
Geophysical evidence points
towards thinning of the continental crust north
of 15°N, that is exactly beneath the Deccan
volcanic province. This provides additional
support for the existence of delaminated continental
crust there.
We offer these ideas
for further work and discussion.
Acknowledgments
The authors appreciate the continuous
encouragement, valuable discussions and critical
comments of Dr. Harsh K. Gupta, Secretary, Department
of Ocean Development, Government of India and Dr.
P.C. Pandey, Director, NCAOR. The authors are thankful
to Prof. John J. Mahoney who very kindly provided
his critical comments which helped us to improve
the manuscript. We were inspired to work on the
Deccan by the thought-provoking arguments and iconoclastic
views of Dr. H. C. Sheth. We thank our colleagues
at NCAOR for their remarkable contribution all through
the brainstorming sessions and discussions of the
Deccan.
|
Dinosaur Deaths Outsourced to India?
Boulder, CO, USA - A series of monumental volcanic eruptions in India
may have killed the dinosaurs 65 million years ago, not a meteor impact
in the Gulf of Mexico. The eruptions, which created the gigantic Deccan
Traps lava beds of India, are now the prime suspect in the most famous
and persistent paleontological murder mystery, say scientists who have
conducted a slew of new investigations honing down eruption timing.
"It's the first time we can directly link the main phase of the
Deccan Traps to the mass extinction," said Princeton University
paleontologist Gerta Keller. The main phase of the Deccan eruptions
spewed 80 percent of the lava which spread out for hundreds of miles. It
is calculated to have released ten times more climate altering gases
into the atmosphere than the nearly concurrent Chicxulub meteor impact,
according to volcanologist Vincent Courtillot from the Physique du Globe
de Paris.
Keller's crucial link between the eruption and the mass extinction
comes in the form of microscopic marine fossils that are known to have
evolved immediately after the mysterious mass extinction event. The same
telltale fossilized planktonic foraminifera were found at Rajahmundry
near the Bay of Bengal, about 1000 kilometers from the center of the
Deccan Traps near Mumbai. At Rajahmundry there are two lava "traps"
containing four layers of lava each. Between the traps are about nine
meters of marine sediments. Those sediments just above the lower trap,
which was the mammoth main phase, contain the incriminating
microfossils.
Keller and her collaborator Thierry Adatte from the University of
Neuchatel, Switzerland, are scheduled to present the new findings on
Tuesday, 30 October, at the annual meeting of the Geological Society of
America in Denver. They will also display a poster on the matter at the
meeting on Wednesday, 31 October.
Previous work had first narrowed the Deccan eruption timing to within
800,000 years of the extinction event using paleomagnetic signatures of
Earth's changing magnetic field frozen in minerals that crystallized
from the cooling lava. Then radiometric dating of argon and potassium
isotopes in minerals narrowed the age to within 300,000 years of the
65-million-year-old Cretaceous-Tertiary (a.k.a. Cretaceous-Paleogene)
boundary, sometimes called the K-T boundary.
The microfossils are far more specific, however, because they
demonstrate directly that the biggest phase of the eruption ended right
when the aftermath of the mass extinction event began. That sort of
clear-cut timing has been a lot tougher to pin down with
Chicxulub-related sediments, which predate the mass extinction.
"Our results are consistent and mutually supportive with a number of
new studies, including Chenet, Courtillot and others (in press) and Jay
and Widdowson (in press), that reveal a very short time for the main
Deccan eruptions at or near the K-T boundary and the massive carbon
dioxide and sulfur dioxide output of each major eruption that dwarfs the
output of Chicxulub," explained Keller. "Our K-T age control combined
with these results strongly points to Deccan volcanism as the likely
leading contender in the K-T mass extinction." Keller's study was funded
by the National Science Foundation.
The Deccan Traps also provide an answer to a question on which
Chicxulub was silent: Why did it take about 300,000 years for marine
species to recover from the extinction event? The solution is in the
upper, later Deccan Traps eruptions.
"It's been an enigma," Keller said. "The very last one was Early
Danian, 280,000 years after the mass extinction, which coincides with
the delayed recovery."
Keller and her colleagues are planning to explore the onset of the
main phase of Deccan volcanism, that is, the rocks directly beneath the
main phase lavas at Rajahmundry. That will require drilling into the
Rajahmundry Traps, a project now slated for December-January 2007/2008.
WHEN & WHERE
Main Deccan Volcanism Phase Ends at K-T Mass Extinction: Evidence from the Krishna-Godavari Basin, SE India
Colorado Convention Center Room 506
Tuesday, October 30, 11:00 a.m. - 11:15 a.m.
[ view abstract ]
Paleoenvironment After Main Deccan Volcanism Ended at K-T Mass Extinction: Evidence From The Krishna-Godavari Basin, SE India.
Colorado Convention Center Room 407
Tuesday, October 30, 11:15 a.m. - 11:30 a.m.
[ view abstract ]
Age and Paleoenvironment of Deccan Volcanism and the K-T Mass Extinction
Colorado Convention Center Exhibit Hall E/F
Wednesday, October 31, 8:00 a.m. - 12:00 p.m.
[ view abstract ]
CONTACT INFORMATION
Gerta Keller
Professor, Dept. of Geosciences
Princeton University, Guyot Hall, Princeton, NJ 08544, USA
Email: gkeller@princeton.edu
Telephone: 609-258-4117
Thierry Adatte
Professor, Geological Institute
University of Neuchatel, Neuchatel, CH-2007, Switzerland.
Email: Thierry.Adatte@unine.ch
Telephone: 41 32 726-2617
Cell phone: 41 79 371-2715
Sunil Bajpai
Professor, Department of Earth Sciences
Indian Institute of Technology, Roorkee 247 667, Uttarakhand, India
Email: sunilbajpai2001@yahoo.com
For information and assistance during the GSA Annual Meeting, 27-31
October, contact Ann Cairns in the onsite newsroom, Colorado Convention
Center Room 604, +1-303-228-8486, acairns@geosociety.org.
ADDITIONAL SOURCES
1) Regarding the dating of 80 percent of the Deccan Traps
to within 300,000 years of the K-T boundary and the greenhouse gas
releases
Anne-Lise Chenet
Cambridge University, UK
Email: alc69@cam.ac.uk
Vincent Courtillot
Director, Institut de Physique du Globe de Paris
Université Paris 7, et Institut Universitaire de France, Paris.
Email: courtil@ipgp.jussieu.fr
Telephone: 0033(0)14427-3908
Frederic Fluteau
Professor, Institut de Physique du Globe de Paris
Université Paris 7, et Institut Universitaire de France, Paris.
Email: fluteau@ipgp.jussieu.fr
Media Guide 2014
History Carved Out of the Deccan Traps
Author Rasoul Sorkhabi, Ph.D.
Ancient cave temples carved out of the Deccan basalts are some
of the best places to view both the world-renowned Deccan Traps and the
Indian mythology narrated on these rocks.
This statue of Shiva
depicts four faces representing Mahadeva (the calm “great lord,” central
figure), Aghora (the frightful or destructive aspect of Shiva, on the
left), Uma (the beautiful feminine aspect, on the right), and Nandin
(the sacred bull as the mouth or doorkeeper of Shiva, not visible). This
sculpture is in Cave No. 1 on Elephanta Island. Photo: Rasoul Sorkhabi |
The Deccan Traps, one of the Earth’s largest igneous provinces, cover
over 500,000 km2 of west-central India. Erupted about 66 million years
during the extinction of the dinosaurs, these flood basalts, in
cooperation with the sea, rains and rivers, have shaped the landscape of
west-central India. Ancient cave temples have been carved out of the
Deccan basalts in many places and the Elephanta Caves located on a small
island offshore Mumbai (Bombay) is one such place.
Flood Basalts in Central India
The triangular peninsula of India is largely a Precambrian shield, with a
central flat area, the so-called Deccan Plateau, surrounded by the
mountain ranges of the Eastern and Western Ghats. The name Deccan is
derived from the Sanskrit word ‘dâkshin’, meaning “south.” The
west-central parts of the Indian peninsula are dominated by flood
basalts which form a prominent terraced landscape; this form of flood
basalt is called ‘trap’, after the Dutch-Swedish word ‘trappa’, meaning
‘stairs’.
A large number of geochronological data have been reported from the
Deccan Traps over the past four decades, and the data cluster between 69
and 63 Ma (corresponding to the magnetic polarity epochs of 31 Reverse
and 28 Normal) suggests that the main phase of eruption was at 66.9 ±0.2
Ma, shortly before the Cretaceous-Tertiary (K-T) boundary at 65.5 ±0.2
Ma. This age range is also consistent with paleontological data from the
interbedded sediments. Aside from terraces, the Deccan basalts also
form numerous dikes, some of which represent the youngest phase of the
volcanic activity. While some scientists support a several million year
duration, others have argued that the eruption occurred within a million
years at the K-T boundary.
The original extent of the Deccan Traps has been estimated as 1.5
million km3, but the latter is highly imprecise as erosion on land and
undersea subsidence on the western Indian margin have altered the rock
volume accessible to us. The Deccan Traps are thickest on the Western
Ghat Range (over 2,000 km thick) or in fault-bounded grabens in
west-central India, but become thinner (less than 100 m) close to the
margin of the trap province. Over 95% of these lavas are tholeiitic
basalts (tholeiite, named after Tholey, Germany is a type of basalt rich
in silica). Mantle xenoliths in the Deccan Traps have been reported
from a few places.
Most scientists believe that the Deccan Traps poured out as the Indian
plate, on its northward journey after the Gondwana breakup, passed over
the Reunion hotspot, a still active volcanic island located in the
south-west Indian Ocean. Coeval with (or probably as a result of) this
event, there was also a continental rift-drift between India and the
Seychelles Islands. Indeed, flood basalts of similar age also occur on
the Seychelles. (For Seychelles see the article “An Oil Prone Frontier
Basin,” GEO ExPro, Vol. 4, No. 3). The occurrence of petroleum
reservoirs below the Deccan Traps remains unexplored.
Distribution of the Deccan
Traps in India and their linkage in space and through time to the
Reunion Hotspot. Inset: A simple paleotectonic sketch map showing the
outpouring of the Deccan basalts at 66 Ma (K-T boundary) related to the
impingement of Reunion plume beneath the Indian continental plate, and
subsequent rifting between Seychelles and India. Image: Rasoul Sorkhabi |
A view of two of the caves on Elephanta Island. The Deccan basalts are prominently seen in the photo. Photo: Rasoul Sorkhabi |
Cave Temples in Deccan Traps
One can see exposures of the Deccan Traps in the Indian states of
Gujarat, Madhya Pradesh, and Maharashtra in India, but vegetation, soil
cover, and land development often mask these rocks. Cliffs of lavas on
the Western Ghats and hill caves in Maharashtra perhaps provide the best
outcrops to examine these formations. The hill caves are particularly
important as many of these are also ancient Hindu or Buddhist temples,
centuries old and portraying the Indian myths on rocks.
Some of the best known Deccan Trap caves are close to Mumbai (Bombay),
including Ajanta (perhaps the oldest one dating back to 200 B.C.),
Mandapesvara Caves, Kanheri Caves, Jogeshwari Caves, Mahakali Caves, and
of course, the Elephanta Caves, which are our subject here.
Cave No. 1 or the Great
Cave is the largest and most celebrated of all the Elephanta caves. This
cave temple (restored in the 1970s) contains many statutes and
sculptures of Lord Shiva and his life stories in Hindu mythology. Photo:
Rasoul Sorkhabi |
Elephanta Island
The Elephanta Caves are located on Elephanta Island, offshore Mumbai,
precisely 11 km north-west of Apollo Bunder near the Gateway of India,
where numerous ferries take visitors to the island daily. The entire
island, about 2.5 km long and 7 km in circumference, is made up of the
Deccan basalts, covered with trees and bushes. Three villages on the
island house a few thousand people engaged in farming, fishing, and
tourism.
Through centuries, the island has come under the rule of various Indian
dynasties. In 1534, the Portuguese occupied it. In 1661, when Charles II
of England married Catherine of Braganza, daughter of King John IV of
Portugal, Elephanta Island was given to the British royal court as a
marriage dowry, thus beginning British control of the island until 1947,
when India gained independence.
The native name for the island is “Gharapuri” – the “town of Ghari
priests (those priests belonging to the Shudra or laborer and artisan
class, and devoted to Lord Shiva). But the Portuguese called it Fontis
(Elephanta) after a huge elephant statute that once stood on the island.
There are seven temple caves. The first five, on the western part of the
island, are Hindu temples dedicated to Shiva, a deity which along with
Brahma (‘creator’) and Vishnu (‘preserver’) forms the supreme Hindu
pantheon. Shiva - literally the ‘Auspicious One’- is often translated as
the ‘lord of destruction’ but as one observes his various sculptures in
Elephanta Caves he plays a far more varied role in Hindu mythology. The
rock architecture of these Hindu caves has been dated between the 5th
and 8th centuries.
The other two caves are Buddhist temples dating back to the 3rd century
or even older and are not open to visitors. The Buddhist Stupa on the
eastern part of the island is the highest point of the island; it is
called the Stupa Hill and is about 173m in elevation.
The Elephanta Caves were originally colour-painted but today only traces
remain on the bare rock. Much damage has been done to the caves through
centuries of weathering but also by the Portuguese soldiers who fired
shots into the caves (to test the echo of their big guns), thus breaking
some sculptures and pillars. In 1909, the Elephanta Caves came under
the authority of the Archaeological Survey of India, and in 1987 UNESCO
included it in the World Heritage list.
A trip to Mumbai is not complete without a visit to the amazing
Elephanta Islands, where a portion of India’s ancient history and
mythology are preserved and displayed by the Deccan basalts – a fine
sight, especially for geologists.
The stepped nature of the
layered basalts of the Deccan Traps is clearly seen inland at Matheran,
90 km from Mumbai Photo: Nichalp, via Wikimedia Commons |
 Get This Document Now
Deccan plume, lithosphere rifting, and volcanism in Kutch, India
- a Florida International University, Miami, FL 33199, USA
- b Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA
- c Presidency College, Kolkata, India
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Abstract
Kutch
(northwest India) experienced lithospheric thinning due to rifting and
tholeiitic and alkalic volcanism related to the Deccan Traps K/T
boundary event. Alkalic lavas, containing mantle xenoliths, form
plug-like bodies that are aligned along broadly east–west rift faults.
The mantle xenoliths are dominantly spinel wehrlite with fewer spinel
lherzolite. Wehrlites are inferred to have formed by reaction between
transient carbonatite melts and lherzolite forming the lithosphere. The
alkalic lavas are primitive (Mg# = 64–72) relative to the tholeiites
(Mg# = 38–54), and are enriched in incompatible trace elements. Isotope
and trace element compositions of the tholeiites are similar to what are
believed to be the crustally contaminated Deccan tholeiites from
elsewhere in India. In terms of Hf, Nd, Sr, and Pb isotope ratios, all
except two alkalic basalts plot in a tight cluster that largely overlap
the Indian Ridge basalts and only slightly overlap the field of Reunion
lavas. This suggests that the alkalic magmas came largely from the
asthenosphere mixed with Reunion-like source that welled up beneath the
rifted lithosphere. The two alkalic outliers have an affinity toward
Group I kimberlites and may have come from an old enriched
(metasomatized) asthenosphere. We present a new model for the
metasomatism and rifting of the Kutch lithosphere, and magma generation
from a CO2-rich lherzolite mantle. In this model the earliest
melts are carbonatite, which locally metasomatized the lithosphere.
Further partial melting of CO2-rich lherzolite at about
2–2.5 GPa from a mixed source of asthenosphere and Reunion-like plume
material produced the alkalic melts. Such melts ascended along deep
lithospheric rift faults, while devolatilizing and exploding their way
up through the lithosphere. Tholeiites may have been generated from the
main plume head further south of Kutch.
Keywords
- Deccan Traps;
- mantle xenoliths;
- plume;
- volcanism;
- rifting;
- lithosphere;
- Kutch
Figures and tables from this article:
Fig. 1.
Simplified geological map of Kutch (bottom; Biswas 2005).
Our area of study is around Bhuj. There are several major roughly E–W
faults (KHF: Kutch Highland fault, KMF — Kutch Mainland fault, NPF —
Nagar Parker fault, IPF — Island Belt fault) that slice up the geology.
The xenolith-bearing alkalic bodies occur as plugs inside shallow marine
Mesozoic sediments at Bhuj and along a WNW trending belt south of the
KMF. The inset shows the location of Bhuj in India.
Fig. 2.
(a)
Composition of olivines in Kutch wehrlite and lherzolite xenoliths are
compared. Filled circles — this study; and unfilled circles — Krishnamurthy et al. (1989).
Also shown is a calculated equilibrium melting residue trend from a
hypothetical source (circle with cross). The field for olivine
phenocrysts in Deccan picrites from the northwest (Source: Krishnamurthy et al., 2000)
is shown for comparison purpose. This plot suggests that primitive
Deccan picrites, considered by many to be the parental magma to the
tholeiites, may have been derived by about 10% partial melting of
lherzolite. (b) Clinopyroxenes in the wehrlite and lherzolite xenoliths
from Kutch are compared (our data). Increased partial melting should
result in strong depletion in Na 2O with increasing Mg# (i.e.,
Mg/Mg + Fe) in the residual clinopyroxenes. The lack of such
correlation in Kutch cinopyroxenes suggests that these are not simple
products of partial fusion, and metasomatic enrichment in Na 2O is offered as an explanation. See text for further discussion.
Fig. 3.
Primitive mantle ( McDonough and Sun, 1995)
normalized trace element patterns in Kutch volcanics (our data). The
alkalic rocks show prominent negative Pb anomaly whereas tholeiites
clearly show a positive Pb spike that is similar to the basalts from
Saurashtra area, which is further south of our study area. The
Saurashtra data are from Melluso et al. (1995).
Fig. 4.
(a).
Initial Nd–Sr isotope compositions of Kutch basalts are compared with
Deccan basalts from northwestern India and some select formations
(Ambenali, Mahabaleshwar, Thakurvadi) from the Western Ghats ( 11, 30, 49 and 51).
Reunion hot spot generated lavas and Central Indian Ocean Ridge basalts
data are also shown for comparison (source of data: GEOROC). Initial ɛSr is calculated using present day bulk earth 87Sr/ 86Sr = 0.7047 and 87Rb/ 86Sr = 0.08168, and ɛNd using chondritic earth with 143Nd/ 144Nd = 0.512638 and 147Sm/ 144Nd = 0.1967. (b). Pb vs Sr isotope plot for the Kutch volcanics compared with other basalts. (c). Initial ɛHf– ɛNd
diagram comparing the Kutch volcanics with Indian MORB, Reunion and
Mauritius lavas, and Group 1 kimberlites. Kutch tholeiites plot over a
wide area that covers much of the Indian MORB field and Deccan basalts
from other areas (not shown). Two alkalic basalts plot inside the
kimberlite field and perhaps represent very small degrees of melts from
an enriched sublithospheric source. Other alkalic basalts plot between
Indian MORB and Reunion lavas, suggesting that they were derived from a
mixed source of depleted asthenosphere and the Reunion plume. Data
sources: Reunion: Bosch et al. (2008); Indian MORB: Meyzen et al. (2007) and the GEOROC database; Mauritius: Paul et al. (2005); G1 kimberlites: Nowell et al. (2004).
Fig. 5.
Schematic
block diagram showing inferred geological relationships between the
main structural elements of Kutch rift zone and Deccan volcanism (the
structural elements are mostly based on 4 and 5). The subsurface Deccan Trap ridge at the center of the diagram is based on gravity data interpretations ( Chandrasekhar and Mishra, 2002).
We consider the lithosphere to be about 90–100 km thick on either side
of this paleo-rift zone, and under such conditions it should have a
garnet peridotite lower layer. Such a layer is missing in beneath the
rift zone as evident from the absence of garnet peridotite xenoliths in
the alkalic basalts. Therefore, we suggest that the rift was already
extended and thinned during prior Late Triassic–Jurassic rifting.
Isotope data suggest that the alkalic basalts were produced from a mixed
asthenospheric (Indian MORB-like) and plume (Reunion-like) source (this
is shown as black “blobs”).
Fig. 6.
A geodynamic model of magma generation in Kutch is presented based on peridotite-CO 2 melting relations ( Presnall and Gudfinnsson, in press).
In all three figures the bottom diagram shows a schematic geological
cross-section (north is approximately to the right and south is to the
left), and the top shows magma production in pressure–temperature phase
diagrams. In all three figures volatile free (gray) and CO 2-saturated
(black) lherzolite solidi are shown as solid lines and geothermal
gradient is shown as a dashed curve. The position of the geotherm
changes in response to rifting and later on due to arrival of deeper,
hotter, Reunion-like bodies. Magma generation is shown in three stages
(I, II, and III) as initial rifting (stage I), arrival and melting of CO 2-rich
peridotitic blobs (stage II, alkalic melt production), and generation
of tholeiitic picrites from the main plume head (stage III).
Lithospheric thinning due to rifting causes CO 2-bearing
asthenosphere to rise and cross the volatile bearing peridotite solidus,
generating carbonatitic melt. As these melts rise through the
lithosphere they freeze, releasing CO 2-rich vapor. These
melts and associated vapor metasomatizes the lithosphere, converting
spinel lherzolite wall rock to spinel wehrlite along veins used by such
fluids. Stage II shows the arrival of Reunion-like bodies that break off
the leading edge of main Deccan plume head and begin to melt once their
solidus is crossed around ∼ 75–90 km. In this stage the geotherm rises
higher and produces alkalic magmas, which ascend along pathways created
by deep lithospheric rift faults and erupt to form small bodies
distributed along the strike of major east–west fault systems. In the
lowermost diagram the plume head is shown as thermally zoned with a hot
core that produces tholeiitic magmas and a cooler rim, which feeds the
alkalic magmas. The thick arrow shows direction of plate movement. Stage
III represents production of picritic tholeiite magmas from the hotter
core of the plume head as the volatile-free solidus is crossed by the
hot plume geotherm. It is speculated that the tholeiite magmas were not
generated at Kutch but the lavas arrived from elsewhere further south.
Table 1.
Trace elements in Kutch volcanics
- View Within Article
Table 2.
Isotopic composition of Kutch volcanic rocks
- Strontium,
Nd and Pb isotope compositions were determined on a Finnigan MAT 262
TIMS at FSU. Sr isotope ratios were corrected for fractionation using 86Sr/88Sr = 0.1194 and are reported against the measured value of the E&A standard: 87Sr/86Sr = 0.708000 ± 14 (2SD, n = 11). Nd isotope ratios are corrected for fractionation using 146Nd/144Nd = 0.7219, and are reported against the measured value of the La Jolla standard: 143Nd/144Nd = 0.511846 ± 11 (2SD, n = 8). The NBS-981 Pb standard was measured at 206Pb/204Pb = 16.90 ± 0.02, 207Pb/204Pb = 15.45 ± 0.02, 208Pb/204Pb = 36.60 ± 0.04 (n = 18) and the reported Pb isotope ratios are corrected for fractionation relative to the NBS-981 values reported by Todt et al. (1996). The JMC 475 Hf standard was measured at 176Hf/177Hf = 0.282185 ± 19 (2SD, n = 11) and the Hf isotope compositions are reported relative to the widely accepted JMC value of 176Hf/177Hf = 0.282160. Initial (in.) isotope ratios, and ɛNd and ɛHf
values are calculated at 65 Ma, using the Rb/Sr, Sm/Nd and Lu/Hf ratios
from the trace element data, and present day values for CHUR: 143Nd/144Nd = 0.512638, 147Sm/144Nd = 0.1967, 176Hf/177Hf = 0.282772, 176Lu/177Hf = 0.0332.
- View Within Article
Copyright © 2008 Elsevier B.V. All rights reserved.
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