Trace element geochemistry of Amba Dongar carbonatite complex, India: Evidence for fractional crystallization and silicate-carbonate melt immiscibility

Jyotiranjan S Ray∗ and P N Shukla
Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India.
∗e-mail: jsray@prl.ernet.in

Carbonatites are believed to have crystallized either from mantle-derived primary carbonate magmas
or from secondary melts derived from carbonated silicate magmas through liquid immiscibility
or from residual melts of fractional crystallization of silicate magmas. Although the observed coexistence
of carbonatites and alkaline silicate rocks in most complexes, their coeval emplacement in
many, and overlapping initial 87Sr/86Sr and 143Nd/144Nd ratios are supportive of their cogenesis;
there have been few efforts to devise a quantitative method to identify the magmatic processes. In
the present study we have made an attempt to accomplish this by modeling the trace element contents
of carbonatites and coeval alkaline silicate rocks of Amba Dongar complex, India. Trace element
data suggest that the carbonatites and alkaline silicate rocks of this complex are products of
fractional crystallization of two separate parental melts. Using the available silicate melt-carbonate
melt partition coefficients for various trace elements, and the observed data from carbonatites, we
have tried to simulate trace element distribution pattern for the parental silicate melt. The results
of the modeling not only support the hypothesis of silicate-carbonate melt immiscibility for the
evolution of Amba Dongar but also establish a procedure to test the above hypothesis in such
complexes.

A preliminary geochemical study of zircons and monazites from Deccan felsic dikes, Rajula, Gujarat, India: Implications for crustal melting

Nilanjan Chatterjee1 and Somdev Bhattacharji2
1Department of Earth, Atmospheric and Planetary Sciences, Room 54-1216, Massachusetts Institute
of Technology, Cambridge, Massachusetts 02139, U.S.A.
2Department of Geology, Brooklyn College and Graduate Center of the City University of New York,
Brooklyn, New York 11210, U.S.A.

Zircons of 10–100 µm size and monazites of up to 10 µm size are present in rhyolite and trachyte
dikes associated with Deccan basalts around Rajula in the southern Saurashtra Peninsula
of Gujarat. On the basis of structural conformity of the felsic and basaltic dikes, K-Ar ages and
trace element considerations, a previous study concluded that the felsic rocks are coeval with the
Deccan Volcanics and originated by crustal anatexis. The felsic rocks contain two populations of
zircons and monazites, one that crystallized from the felsic melt and the other that contains inherited
crustal material. Trace element variations in the rhyolites and trachytes indicate that zircons
and monazites crystallized from the felsic melts, but compositional analysis of a zircon indicates
the presence of a small core possibly inherited from the crust. Hf compositional zoning profile of
this zircon indicates that it grew from the host rhyolitic melt while the melt differentiated, and Y
and LREE contents suggest that this zircon crystallized from the host melt. Pb contents of some
monazites also suggest the presence of inherited crustal cores. Hence, any age determination by
the U-Th-Pb isotopic method should be interpreted with due consideration to crustal inheritance.
Temperatures estimated from zircon and monazite saturation thermometry indicate that the crust
around Rajula may have been heated to a maximum of approximately 900◦C by the intruding
Deccan magma. Crustal melting models of other workers indicate that a 1–2 million year emplacement
time for the Deccan Traps may be appropriate for crustal melting characteristics observed in
the Rajula area through the felsic dikes.


Petrogenesis of granitoid rocks at the northern margin of the Eastern Ghats Mobile Belt and evidence of syn-collisional magmatism

S Bhattacharya1∗, Rajib Kar2 and S Moitra1
1Indian Statistical Institute, 203 B.T. Road, Kolkata 700 108, India.
2Department of Geology, J.K. College, Purulia 723 101, India.
∗e-mail: samar@isical.ac.in

The northern margin of the Eastern Ghats Mobile belt against the Singhbhum craton exposes
granitic rocks with enclaves from both the high-grade and low-grade belts. A shear cleavage developed
in the boundary region is also observed in these granitoids. Field features and petrography
indicate syn-tectonic emplacement of these granitoids. Petrology-mineralogy and geochemistry indicate
that some of the granitoids are derived from the high-grade protoliths by dehydration melting.
Others could have been derived from low-grade protoliths. Moreover, microstructural signatures in
these granitoids attest to their syn-collisional emplacement.

Charnockitic magmatism in southern India

H M Rajesh1∗ and M Santosh2
1Department of Geographical Sciences and Planning, University of Queensland, St Lucia, 4072 Queensland,
Australia.
2Department of Natural Environmental Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan.
∗e-mail: rajeshuu@yahoo.com

Large charnockite massifs cover a substantial portion of the southern Indian granulite terrain. The
older (late Archaean to early Proterozoic) charnockites occur in the northern part and the younger
(late Proterozoic) charnockites occur in the southern part of this high-grade terrain. Among these,
the older Biligirirangan hill, Shevroy hill and Nilgiri hill massifs are intermediate charnockites,
with Pallavaram massif consisting dominantly of felsic charnockites. The charnockite massifs from
northern Kerala and Cardamom hill show spatial association of intermediate and felsic charnockites,
with the youngest Nagercoil massif consisting of felsic charnockites. Their igneous parentage
is evident from a combination of features including field relations, mineralogy, petrography, thermobarometry,
as well as distinct chemical features. The southern Indian charnockite massifs show
similarity with high-Ba–Sr granitoids, with the tonalitic intermediate charnockites showing similarity
with high-Ba–Sr granitoids with low K2O/Na2O ratios, and the felsic charnockites showing
similarity with high-Ba–Sr granitoids with high K2O/Na2O ratios. A two-stage model is suggested
for the formation of these charnockites. During the first stage there was a period of basalt underplating,
with the ponding of alkaline mafic magmas. Partial melting of this mafic lower crust formed
the charnockitic magmas. Here emplacement of basalt with low water content would lead to dehydration
melting of the lower crust forming intermediate charnockites. Conversely, emplacement of
hydrous basalt would result in melting at higher fH2O favoring production of more siliceous felsic
charnockites. This model is correlated with two crustal thickening phases in southern India, one
related to the accretion of the older crustal blocks on to the Archaean craton to the north and the
other probably related to the collision between crustal fragments of East and West Gondwana in
a supercontinent framework.


Chemical evolution, petrogenesis, and regional chemical correlations of the flood basalt sequence in the central Deccan Traps, India

L Melluso1∗, M Barbieri2 and L Beccaluva3
1Dipartimento di Scienze della Terra, Universit`a di Napoli Federico II, via Mezzocannone 8, 80134 Napoli, Italy.
2Dipartimento di Scienze della Terra, Universit`a di Roma “La Sapienza”, Italy.
3Dipartimento di Scienze Geologiche, Universit`a di Ferrara, Italy.
∗e-mail: melluso@unina.it

The lava sequence of the central-western Deccan Traps (from Jalgaon towards Mumbai) is formed
by basalts and basaltic andesites having a significant variation in TiO2 (from 1.2 to 3.3 wt%), Zr
(from 84 to 253 ppm), Nb (from 5 to 16 ppm) and Ba (from 63 to 407 ppm), at MgO ranging from
10 to 4.2 wt%. Most of these basalts follow a liquid line of descent dominated by low pressure fractionation
of clinopyroxene, plagioclase and olivine, starting from the most mafic compositions, in
a temperature range from 1220◦ to 1125◦C. These rocks resemble those belonging to the lowermost
formations of the Deccan Traps in the Western Ghats (Jawhar, Igatpuri and Thakurvadi)
as well as those of the Poladpur formation. Samples analyzed for 87Sr/86Sr give a range of initial
ratios from 0.70558 to 0.70621. A group of flows of the Dhule area has low TiO2 (1.2–1.5 wt%) and
Zr (84–105 ppm) at moderate MgO (5.2–6.2 wt%), matching the composition of low-Ti basalts of
Gujarat, low-Ti dykes of the Tapti swarm and Toranmal basalts, just north of the study area. This
allows chemical correlations between the lavas of central Deccan, the Tapti dykes and the northwestern
outcrops. The mildly enriched high field strength element contents of the samples with
TiO2 > 1.5 wt% make them products of mantle sources broadly similar to those which generated
the Ambenali basalts, but their high La/Nb and Ba/Nb, negative Nb anomalies in the mantle normalized
diagrams, and relatively high 87Sr/86Sr, make evident a crustal input with crustally derived
materials at less differentiated stages than those represented in this sample set, or even within the
sub-Indian lithospheric mantle.


High-Ti type N-MORB parentage of basalts from the south Andaman ophiolite suite, India

Rajesh K Srivastava1∗, R Chandra2, Anant Shastry1
1Igneous Petrology Laboratory, Department of Geology, Banaras Hindu University, Varanasi 221 005, India.
2Department of Geology, Bundelkhand University, Jhansi 284 128, India.
∗e-mail: rajeshgeolbhu@yahoo.com

A complete dismembered sequence of ophiolite is well exposed in the south Andaman region that
mainly comprises ultramafic cumulates, serpentinite mafic plutonic and dyke rocks, pillow lava,
radiolarian chert, and plagiogranite. Pillow lavas of basaltic composition occupy a major part of the
Andaman ophiolite suite (AOS). These basalts are well exposed all along the east coast of southern
part of the south AOS. Although these basalts are altered due to low-grade metamorphism and late
hydrothermal processes, their igneous textures are still preserved. These basalts are mostly either
aphyric or phyric in nature. Aphyric type exhibits intersertal or variolitic textures, whereas phyric
variety shows porphyritic or sub-ophitic textures. The content of alkalies and silica classify these
basalts as sub-alkaline basalts and alkaline basalts. A few samples show basaltic andesite, trachybasalt,
or basanitic chemical composition. High-field strength element (HFSE) geochemistry suggests
that studied basalt samples are probably derived from similar parental magmas. Al2O3/TiO2
and CaO/TiO2 ratios classify these basalts as high-Ti type basalt. On the basis of these ratios
and many discriminant functions and diagrams, it is suggested that the studied basalts, associated
with Andaman ophiolite suite, were derived from magma similar to N-MORB and emplaced in the
mid-oceanic ridge tectonic setting.


Relative contributions of crust and mantle to the origin of the Bijli Rhyolite in a palaeoproterozoic bimodal volcanic sequence (Dongargarh Group), central India

S Sensarma1,∗, S Hoernes2 and D Mukhopadhyay3
1Department of Geology, St. Anthony’s College, Shillong 793 001, India.
2Mineralalogisch-Petrologisches Institut, der Universit¨at Bonn, D- 53115 Bonn, Germany.
3Department of Geology, University of Calcutta, Calcutta 700 019, India.
∗e-mail: sensarma2002@yahoo.co.in

New mineralogical, bulk chemical and oxygen isotope data on the Palaeoproterozoic Bijli Rhyolite,
the basal unit of a bimodal volcanic sequence (Dongargarh Group) in central India, and one of
the most voluminous silicic volcanic expressions in the Indian Shield, are presented. The Bijli
Rhyolite can be recognized as a poorly sorted pyroclastic deposit, and comprises of phenocrystic
K-feldspar + albite ± anorthoclase set in fine-grained micro-fragmental matrix of quartz-feldsparsericite-
chlorite-iron-oxide ± calcite. The rocks are largely metaluminous with high SiO2, Na2O+
K2O, Fe/Mg, Ga/Al, Zr, Ta, Sn, Y, REE and low CaO, Ba, Sr contents; the composition points to
an ‘A-type granite’ melt. The rocks show negative Cs-, Sr-, Eu- and Ti- anomalies with incompatible
element concentrations 2–3 times more than the upper continental crust (UCC). LREE is high
(La/Yb ∼ 20) and HREE 20–30 times chondritic. δ18Owhole-rock varies between 4.4 and 7.8‰(mean
5.87 ± 1.26‰).
The Bijli melt is neither formed by fractionation of a basaltic magma, nor does it represent a
fractionated crustal melt. It is shown that the mantle-derived high temperature basaltic komatiitic
melts/high Mg basalts triggered crustal melting, and interacted predominantly with deep crust
compositionally similar to the Average Archaean Granulite (AAG), and a shallower crustal component
with low CaO and Al2O3 to give rise to the hybrid Bijli melts. Geochemical mass balance
suggests that ∼ 30% partial melting of AAG under anhydrous condition, instead of the upper continental
crust (UCC) including the Amgaon granitoid gneiss reported from the area, better matches
the trace element concentrations in the rocks. The similar Ta/Th of the rhyolites (0.060) and average
granulite (0.065) vs. UCC (0.13) also support a deep crustal protolith. Variable contributions
of crust and mantle, and action of hydrothermal fluid are attributed for the spread in δ18Owhole-rock
values. The fast eruption of high temperature (∼ 900◦C) rhyolitic melts suggests a rapid drop in
pressure of melting related to decompression in an extensional setting.


Late Archaean mantle metasomatism below eastern Indian craton: Evidence from trace elements, REE geochemistry and Sr{Nd{O isotope systematics of ultramac dykes

A Roy1, A Sarkar2, S Jeyakumar3, S K Aggrawal3, M Ebihara4 and H Satoh5
1Coal Wing, Geological Survey of India, DK-6, Sector-II, Salt Lake, Kolkata 700 091, India.
2Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, West Bengal 721 302, India.
3Fuel Chemistry Division, Bhaba Atomic Research Centre, Mumbai 400 085, India.
4Faculty of Science, Tokyo Metropolitan University, Tokyo 192-03, Japan.
5Geological Survey of Japan, Tsukuba 305-8567, Japan.
e-mail: a−roy119@yahoo.com a−roy119@hotma

Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out
on ultramac (harzburgite and lherzolite) dykes belonging to the newer dolerite dyke swarms of
eastern Indian craton. The dyke swarms were earlier considered to be the youngest mac magmatic
activity in this region having ages not older than middle to late Proterozoic. The study indicates
that the ultramac members of these swarms are in fact of late Archaean age (Rb-Sr isochron
age 2613  177 Ma, Sri  0:702  0:004) which attests that out of all the cratonic blocks of India,
eastern Indian craton experienced earliest stabilization event. Primitive mantle normalized trace
element plots of these dykes display enrichment in large ion lithophile elements (LILE), pronounced
Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE
plots exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; (HREE)N  2{3
times chondrite, (Gd/Yb)N  1). The "Nd(t) values vary from +1:23 to −3:27 whereas 18O values
vary from +3:16‰ to +5:29‰ (average +3:97‰0:75‰) which is lighter than the average mantle
value. Isotopic, trace and REE data together indicate that during 2.6 Ga the nearly primitive
mantle below the eastern Indian Craton was metasomatised by the fluid ( silicate melt) coming
out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted, variable
"Nd, low Sri(0:702) and low 18O bearing EMI type mantle. Magmatic blobs of this metasomatised
mantle were subsequently emplaced in deeper levels of the granitic crust which possibly originated
due to the same thermal pulse.


Geochemistry and petrogenesis of anorogenic basic volcanic-plutonic rocks of the Kundal area, Malani Igneous Suite, western Rajasthan, India

A Krishnakanta Singh and G Vallinayagam
Wadia Institute of Himalayan Geology, Northeast Unit, Vivek Vihar, Itanagar 791 113, India.
Department of Earth Sciences, Kurukshetra University, Kurukshetra 136 119, India.
e-mail: kk−luwang@redimail.com

The Kundal area of Malani Igneous Suite consists of volcano-plutonic rocks. Basalt flows and gab-
bro intrusives are associated with rhyolite. Both the basic rocks consist of similar mineralogy of
plagioclase, clinopyroxene as essential and Fe-Ti oxides as accessories. Basalt displays sub-ophitic
and glomeroporphyritic textures whereas gabbro exhibits sub-ophitic, porphyritic and intergrannu-
lar textures. They show comparable chemistry and are enriched in Fe, Ti and incompatible ele-
ments as compared to MORB/CFB. Samples are enriched in LREE and slightly depleted HREE
patterns with least signicant positive Eu anomalies. Petrographical study and petrogenetic mod-
eling of [Mg]-[Fe], trace and REE suggest cogenetic origin of these basic rocks and they probably
derived from Fe-enriched source with higher Fe/Mg ratio than primitive mantle source. Thus, it is
concluded that the basic volcano-plutonic rocks of Kundal area are the result of a low to moderate
degree (< 30%) partial melting of source similar to picrite/komatiitic composition. Within plate,
anorogenic setting for the basic rocks of Kundal area is suggested, which is in conformity with the
similar setting for Malani Igneous Suite.


Geochemistry and petrogenesis of early Cretaceous sub-alkaline mafic dykes from Swangkre-Rongmil, East Garo Hills, Shillong plateau, northeast India

Rajesh K Srivastava∗ and Anup K Sinha
Igneous Petrology Laboratory, Department of Geology, Banaras Hindu University, Varanasi 221 005, India.
∗e-mail: rajeshgeolbhu@yahoo.com

Numerous early Cretaceous mafic and alkaline dykes, mostly trending in N-S direction, are
emplaced in the Archaean gneissic complex of the Shillong plateau, northeastern India. These
dykes are spatially associated with the N-S trending deep-seated Nongchram fault and well exposed
around the Swangkre-Rongmil region. The petrological and geochemical characteristics of mafic
dykes from this area are presented. These mafic dykes show very sharp contact with the host
rocks and do not show any signature of assimilation with them. Petrographically these mafic
dykes vary from fine-grained basalt (samples from the dyke margin) to medium-grained dolerite
(samples from the middle of the dyke) having very similar chemical compositions, which may be
classified as basaltic-andesite/andesite. The geochemical characteristics of these mafic dykes suggest
that these are genetically related to each other and probably derived from the same parental
magma. Although, the high-field strength element (+rare-earth elements) compositions disallow
the possibility of any crustal involvement in the genesis of these rocks, but Nb/La, La/Ta, and
Ba/Ta ratios, and similarities of geochemical characteristics of present samples with the Elan Bank
basalts and Rajmahal (Group II) mafic dyke samples, suggest minor contamination by assimilation
with a small amount of upper crustal material. Chemistry, particularly REE, hints at an alkaline
basaltic nature of melt. Trace element modelling suggests that the melt responsible for these mafic
dykes had undergone extreme differentiation (∼ 50%) before its emplacement. The basaltic-andesite
nature of these rocks may be attributed to this differentiation. Chemistry of these rocks also indicates
∼ 10–15% melting of the mantle source. The mafic dyke samples of the present investigation
show very close geochemical similarities with the mafic rocks derived from the Kerguelen mantle
plume. Perhaps the Swangkre-Rongmil mafic dykes are also derived from the Kerguelen mantle
plume.


Geodynamic evolution and crustal growth of the central Indian Shield: Evidence from geochemistry of gneisses and granitoids

M Faruque Hussain1, M E A Mondal1∗ and T Ahmad2
1Department of Geology, Aligarh Muslim University, Aligarh 202 002, India.
2Department of Geology, University of Delhi, New Delhi 110 007, India.
∗e-mail: emondal@lycos.com

The rare earth element patterns of the gneisses of Bastar and Bundelkhand are marked by LREE
enrichment and HREE depletion with or without Eu anomaly. The spidergram patterns for the
gneisses are characterized by marked enrichment in LILE with negative anomalies for Ba, P and
Ti. The geochemical characteristics exhibited by the gneisses are generally interpreted as melts
generated by partial melting of a subducting slab. The style of subduction was flat subduction,
which was most common in the Archean. The rare earth patterns and the multi-element diagrams
with marked enrichment in LILE and negative anomalies for Ba, P and Ti of the granitoids of both
the cratons indicate interaction between slab derived melts and the mantle wedge. The subduction
angle was high in the Proterozoic. Considering the age of emplacement of the gneisses and granitoids
that differs by ∼ 1 Ga, it can be assumed that these are linked to two independent subduction
events: one during Archaean (flat subduction) that generated the precursor melts for the gneisses
and the other during the Proterozoic (high angle subduction) that produced the melts for the
granitoids. The high values of Mg #, Ni, Cr, Sr and low values of SiO2 in the granitoids of Bastar and
Bundelkhand cratons compared to the gneisses of both the cratons indicate melt-mantle interaction
in the generation of the granitoids. The low values of Mg#, Ni, Cr, Sr and high values of SiO2 in
the gneisses in turn overrules such melt-mantle interaction.

Petrology of the prehistoric lavas and dyke of the Barren Island, Andaman Sea, Indian Ocean

M A Alam∗1, D Chandrasekharam1, O Vaselli2, B Capaccioni3, P Manetti4 and
P B Santo3
1Department of Earth Sciences, Indian Institute of Technology, Bombay, Mumbai 400 076, India
2Department of Earth Sciences, University of Florence, Florence 50121, Italy
3Institute of Volcanology and Geochemistry, University of Urbino, Urbino 61029, Italy
4CNR-Institute of Geosciences and Earth Resources, Pisa 56124, Italy
∗e-mail: ayaz@iitb.ac.in

Although Barren Island (Andaman Sea, Indian Ocean) witnessed several volcanic eruptions during
historic times, the eruptions that led to the formation of this volcanic island occurred mainly during
prehistoric times. It is still active and currently in the fumarolic stage. Its volcanic evolution appears
to be characterized by a constructive phase with the piling up of lava flows and scoria deposits
and Strombolian activities, followed by a sudden collapse of the main cone. Deposits of a possible
caldera-forming eruption were not recognized earlier. After a period of peri-calderic hydromagmatic
activity, whose deposits presently mantle inner and outer caldera walls, a new phase of intracalderic
Vulcanian activities took place. A prominent dyke in the SE inner side of the caldera
wall was recognized. Petrographically the lava flows and dyke are similar but they differ in their
chemical composition (viz., SiO2, MgO, Ni, Cr) significantly. Similarity in major, minor and trace
element composition (viz., K/La, K/Nb, K/Rb, K/Ti ratios) of these rocks together with Chondrite
normalized trace element (Rb, Ba, Sr, P, Zr, Ti and Nb) and REE (La, Ce, Nd and Y) patterns of the
Barren Island prehistoric lava flows and dyke and low-K lavas of Sunda Arc indicates that Barren
Island must have evolved from a source similar to that of Sunda Arc lavas during the Quaternary
Period.


Silica-poor, mafic alkaline lavas from ocean islands and continents: Petrogenetic constraints from major elements

Shantanu Keshav∗ and Gudmundur H Gudfinnsson
Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA.
∗email: s.keshav@gl.ciw.edu

Strongly silica-poor (ne-normative), mafic alkaline lavas generally represented by olivine nephelinites,
nephelinites, melilitites, and olivine melilitites have erupted at various locations during Earth’s
history. On the basis of bulk-rock Mg#, high concentrations of Na2O, TiO2, and K2O, and trace
element geochemistry, it has been suggested that these lavas represent low-degree melts that have
undergone little crystal fractionation en route to the surface. Many of these lavas also carry highpressure
mantle material in the form of harzburgite, spinel lherzolite, and variants of websterite
xenoliths, and rare garnet-bearing xenoliths. However, phenocryst phases instead indicate that
these magmas cooled to variable extents during their passage. We note subtle, yet important, differences
in terms of CaO, Al2O3, CaO/Al2O3, and CaO/MgO. High-pressure experimental melting
studies in CMAS-CO2 (3–8GPa) and natural lherzolitic systems (3GPa) demonstrate that
at an isobar increasing F leads to a moderate decrease in CaO + MgO, whereas CaO/MgO and
CaO/Al2O3 sharply decrease. Relatively high CaO/Al2O3 indicates melting in the presence of garnet
(≥ 85 km). Studies also demonstrate that CO2-bearing lherzolitic systems, when compared with
anhydrous ones, also have higher CaO content in the coexisting melt at a given P and T. Comparison
of the bulk-rock major-element chemistry of silica-poor, mafic alkaline lavas with experimentally
determined high-pressure melts indicates that melting of anhydrous mantle lherzolite
or garnet pyroxenite is not able to explain many of the major element systematics of the lavas.
However, high-pressure partial melts of carbonated lherzolite have the right major element trends.
Among ocean islands, lavas from Samoa and Hawaii are perhaps the products of very low degree
of partial melting. Lavas from Gran Canaria and Polynesia represent products of more advanced
partial melting. On continents, lavas from South Africa and certain localities in Germany are the
products of a very low degree of partial melting, and those from Texas and certain other localities
in Germany are products of a slightly more advanced degree of partial melting of a carbonated
lherzolite. Lavas from Deccan, Czech Republic, and Freemans Cove are the products of even more
advanced degree of partial melting. The mere presence of mantle xenoliths in some of these lavas
does not necessarily mean that the erupted lavas represent direct mantle melts.


Tectono-thermal evolution of the India-Asia collision zone based on 40Ar-39Ar thermochronology in Ladakh, India

Rajneesh Bhutani1∗, Kanchan Pande2 and T R Venkatesan3
1Department of Earth Sciences, Pondicherry University, Pondicherry 605 014, India.
2Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai 400 076, India.
3A2, Anand flats, 40, 2nd main road, Gandhinagar, Adyar, Chennai 600 020, India.
∗e-mail: bhutani@vsnl.net

New 40Ar-39Ar thermochronological results from the Ladakh region in the India-Asia collision zone
provide a tectono-thermal evolutionary scenario. The characteristic granodiorite of the Ladakh
batholith near Leh yielded a plateau age of 46.3 ± 0.6Ma (2σ). Biotite from the same rock yielded
a plateau age of 44.6 ± 0.3Ma (2σ). The youngest phase of the Ladakh batholith, the leucogranite
near Himya, yielded a cooling pattern with a plateau-like age of ∼ 36 Ma. The plateau age of
muscovite from the same rock is 29.8 ± 0.2Ma (2σ). These ages indicate post-collision tectonothermal
activity, which may have been responsible for partial melting within the Ladakh batholith.
Two basalt samples from Sumdo Nala have also recorded the post-collision tectono-thermal event,
which lasted at least for 8MY in the suture zone since the collision, whereas in the western part
of the Indus Suture, pillow lava of Chiktan showed no effect of this event and yielded an age of
emplacement of 128.2 ± 2.6Ma (2σ). The available data indicate that post-collision deformation
led to the crustal thickening causing an increase in temperature, which may have caused partial
melting at the base of the thickened crust. The high thermal regime propagated away from the
suture with time.


40Ar-39Ar age of a lava flow from the Bhimashankar Formation, Giravali Ghat, Deccan Traps

Kanchan Pande1,2∗, S K Pattanayak1, K V Subbarao2, P Navaneethakrishnan2 and
T R Venkatesan1
1Planetary and Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India.
2Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai 400 076, India.
∗e-mail: kanchanpande@iitb.ac.in

We report here a 40Ar-39Ar age of 66.0±0.9Ma (2σ) for a reversely magnetised tholeiitic lava flow
from the Bhimashankar Formation (Fm.), Giravali Ghat, western Deccan province, India. This age
is consistent with the view that the 1.8–2km thick bottom part of the exposed basalt flow sequence
in the Western Ghats was extruded very close to 67.4 Ma.

Magmatic underplating beneath the Rajmahal Traps: Gravity signature and derived 3-D configuration

A P Singh∗, Niraj Kumar and Bijendra Singh
National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India
∗e-mail: apsingh−ngri@yahoo.com apsingh@ngri.res.in

The early Cretaceous thermal perturbation beneath the eastern continental margin of the Indian
shield resulted in the eruption of the Rajmahal Traps. To understand the impact of the magmatic
process that originated in the deep mantle on the lower crustal level of the eastern Indian shield
and adjoining Bengal basin the conspicuous gravity anomalies observed over the region have been
modelled integrating with available geophysical information. The 3-D gravity modelling has delineated
10–15km thick high-density (ρ = 3.02 g/cm3) accreted igneous layer at the base of the crust
beneath the Rajmahal Traps. Thickness of this layer varies from 16km to the west of the Rajmahal
towards north to about 12km near Kharagpur towards south and about 18km to the east of the
Raniganj in the central part of the region. The greater thickness of the magmatic body beneath
the central part of the region presents itself as the locus of the potential feeder channel for the
Rajmahal Traps. It is suggested that the crustal accretion is the imprint of the mantle thermal
perturbation, over which the eastern margin of the eastern Indian shield opened around 117Ma
ago. The nosing of the crustal accretion in the down south suggests the possible imprint of the
subsequent magmatic intrusion along the plume path.


Two- and three-dimensional gravity modeling along western continental margin and intraplate Narmada-Tapti rifts: Its relevance to Deccan flood basalt volcanism

Somdev Bhattacharji1, Rajesh Sharma1 and Nilanjan Chatterjee2
1Department of Geology, Brooklyn College and Graduate Center of the City University of New York,
Brooklyn, New York 11210, U.S.A.
2Department of Earth, Atmospheric and Planetary Sciences, Room 54-1216,
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, U.S.A.

The western continental margin and the intraplate Narmada-Tapti rifts are primarily covered
by Deccan flood basalts. Three-dimensional gravity modeling of +70 mgal Bouguer gravity highs
extending in the north-south direction along the western continental margin rift indicates the
presence of a subsurface high density, mafic-ultramafic type, elongated, roughly ellipsoidal body. It
is approximately 12.0 ± 1.2 km thick with its upper surface at an approximate depth of 6.0 ± 0.6km,
and its average density is 2935 kg/m3. Calculated dimension of the high density body in the upper
crust is 300 ± 30km in length and 25 ± 2.5 to 40 ± 4 km in width. Three-dimensional gravity
modeling of +10 mgal to −30 mgal Bouguer gravity highs along the intraplate Narmada-Tapti rift
indicates the presence of eight small isolated high density mafic bodies with an average density of
2961 kg/m3. These mafic bodies are convex upward and their top surface is estimated at an average
depth of 6.5 ± 0.6 (between 6 and 8 km). These isolated mafic bodies have an average length of
23.8 ± 2.4 km and width of 15.9 ± 1.5 km. Estimated average thickness of these mafic bodies is
12.4±1.2 km. The difference in shape, length and width of these high density mafic bodies along the
western continental margin and the intraplate Narmada-Tapti rifts suggests that the migration and
concentration of high density magma in the upper lithosphere was much more dominant along the
western continental margin rift. Based on the three-dimensional gravity modeling, it is conjectured
that the emplacement of large, ellipsoidal high density mafic bodies along the western continental
margin and small, isolated mafic bodies along the Narmada-Tapti rift are related to lineamentreactivation
and subsequent rifting due to interaction of hot mantle plume with the lithospheric
weaknesses (lineaments) along the path of Indian plate motion over the R´eunion hotspot. Mafic
bodies formed in the upper lithosphere as magma chambers along the western continental margin
and the intraplate Narmada-Tapti rifts at estimated depths between 6 and 8 km from the surface
(consistent with geological, petrological and geochemical models) appear to be the major reservoirs
for Deccan flood basalt volcanism at approximately 65 Ma.


Emplacement kinematics of nepheline syenites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt, west of Khariar, NW Orissa: Evidence from meso- and microstructures

T K Biswal∗, Harish Ahuja and Himansu Sekhar Sahu
Indian Institute of Technology Bombay, Powai, Mumbai, 400 076, India.
∗email: tkbiswal@geos.iitb.ac.in

Nepheline syenite plutons emplaced within the Terrane Boundary Shear Zone of the Eastern Ghats
Mobile Belt west of Khariar in northwestern Orissa are marked by a well-developed magmatic
fabric including magmatic foliation, mineral lineations, folds and S-C fabrics. The minerals in the
plutons, namely microcline, orthoclase, albite, nepheline, hornblende, biotite and aegirine show,
by and large, well-developed crystal faces and lack undulose extinction and dynamic recrystallization,
suggesting a magmatic origin. The magmatic fabric of the plutons is concordant with
a solid-state strain fabric of the surrounding mylonites that developed due to noncoaxial strain
along the Terrane Boundary Shear Zone during thrusting of the Eastern Ghats Mobile Belt over
the Bastar Craton. However, a small fraction of the minerals, more commonly from the periphery
of the plutons, is overprinted by a solid state strain fabric similar to that of the host rock.
This fabric is manifested by discrete shear fractures, along which the feldspars are deformed into
ribbons, have undergone dynamic recrystallization and show undulose extinction and myrmekitic
growth. The shear fractures and the magmatic foliations are mutually parallel to the C-fabric of
the host mylonites. Coexistence of concordant solid state strain fabric and magmatic fabric has
been interpreted as a transitional feature from magmatic state to subsolidus deformation of the
plutons, while the nepheline syenite magma was solidifying from a crystal-melt mush state under
a noncoaxial strain. This suggests the emplacement of the plutons synkinematic to thrusting along
the Terrane Boundary Shear Zone. The isotopic data by earlier workers suggest emplacement of
nepheline syenite at 1500 + 3/− 4Ma, lending support for thrusting of the mobile belt over the
craton around that time.


The Neoproterozoic Malani magmatism of the northwestern Indian shield: Implications
for crust-building processes

Kamal K Sharma
Department of Geology, Government Postgraduate College, Sirohi 307 001, India.
e-mail: sharmasirohi@yahoo.com

Malani is the largest event of anorogenic felsic magmatism (covering ∼ 50, 000km2) in India. This
magmatic activity took place at ∼ 750Ma post-dating the Erinpura granite (850 Ma) and ended
prior to Marwar Supergroup (680 Ma) sedimentation. Malani eruptions occurred mostly on land,
but locally sub-aqueous conditions are shown by the presence of conglomerate, grits and pillow
lava. The Malani rocks do not show any type of regional deformation effects. The Malanis are
characterised by bimodal volcanism with a dominant felsic component, followed by granitic plutonism
and a terminal dyke phase. An angular unconformity between Malani lavas and basement
is observed, with the presence of conglomerate at Sindreth, Diri, and Kankani. This indicates that
the crust was quite stable and peneplained prior to the Malani activity. Similarly, the absence of
any thrust zone, tectonic m´elange and tectonised contact of the Malanis with the basement goes
against a plate subduction setting for their genesis. After the closure of orogenic cycles in the
Aravalli craton of the northwestern shield, this anorogenic intraplate magmatic activity took place
in a cratonic rift setting under an extensional tectonic regime.


A brief comparison of lava flows from the Deccan Volcanic Province and the Columbia-Oregon Plateau Flood Basalts: Implications for models of flood basalt emplacement

Ninad R Bondre1∗, Raymond A Duraiswami2 and Gauri Dole2
1Department of Geology, Miami University, Oxford, Ohio 45056 USA.
2Department of Geology, University of Pune, Pune 411 007, India.
∗e-mail: bondren1@muohio.edu

The nature and style of emplacement of Continental Flood Basalt (CFB) lava flows has been a
matter of great interest as well as considerable controversy in the recent past. However, even a
cursory review of published literature reveals that the Columbia River Basalt Group (CRBG) and
Hawaiian volcanoes provide most of the data relevant to this topic. It is interesting to note, however,
that the CRBG lava flows and their palaeotopographic control is atypical of other CFB provinces
in the world. In this paper, we first present a short overview of important studies pertaining
to the emplacement of flood basalt flows. We then briefly review the morphology of lava flows
from the Deccan Volcanic Province (DVP) and the Columbia-Oregon Plateau flood basalts. The
review underscores the existence of significant variations in lava flow morphology between different
provinces, and even within the same province. It is quite likely that there were more than one
way of emplacing the voluminous and extensive CFB lava flows. We argue that the establishment
of general models of emplacement must be based on a comprehensive documentation of lava flow
morphology from all CFB provinces.


Possible lava tube system in a hummocky lava flow
at Daund, western Deccan Volcanic Province, India

Raymond A Duraiswami1∗, Ninad R Bondre2 and Gauri Dole1
1Department of Geology, University of Pune, Pune 411 007, India.
2Department of Geology, Miami University, Oxford, Ohio 45056.
∗e-mail: raymond−d@rediffmail.com

A hummocky flow characterised by the presence of toes, lobes, tumuli and possible lava tube system
is exposed near Daund, western Deccan Volcanic Province, India. The lava tube system is exposed
as several exhumed outcrops and is composed of complex branching and discontinuous segments.
The roof of the lava tube has collapsed but original lava tube walls and fragments of the tube roof
are seen at numerous places along the tube. At some places the tube walls exhibit a single layer of
lava lining, whereas, at other places it shows an additional layer characterised by smooth surface
and polygonal cracks. The presence of a branching and meandering lava tube system in the Daund
flow, which represents the terminal parts of Thakurwadi Formation, shows that the hummocky flow
developed at a low local volumetric flow rate. This tube system developed in the thinner parts of
the flow sequence; and tumuli developed in areas where the tube clogged temporarily in the sluggish
flow.


Cones and craters on Mount Pavagadh, Deccan Traps:
Rootless cones?

Hetu C Sheth∗, George Mathew, Kanchan Pande, Soumen Mallick and Balaram Jena
Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
∗e-mail: hcsheth@iitb.ac.in

Rootless cones, also (erroneously) called pseudocraters, form due to explosions that ensue when
a lava flow enters a surface water body, ice, or wet ground. They do not represent primary vents
connected by vertical conduits to a subsurface magma source. Rootless cones in Iceland are well
studied. Cones on Mars, morphologically very similar to Icelandic rootless cones, have also been
suggested to be rootless cones formed by explosive interaction between surface lava flows and
ground ice. We report here a group of gentle cones containing nearly circular craters from Mount
Pavagadh, Deccan volcanic province, and suggest that they are rootless cones. They are very similar
morphologically to the rootless cones of the type locality ofM´yvatn in northeastern Iceland. A group
of three phreatomagmatic craters was reported in 1998 from near Jabalpur in the northeastern
Deccan, and these were suggested to be eroded cinder cones. A recent geophysical study of the
Jabalpur craters does not support the possibility that they are located over volcanic vents. They
could also be rootless cones. Many more probably exist in the Deccan, and volcanological studies
of the Deccan are clearly of value in understanding planetary basaltic volcanism.