Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

M. I. Bhat ¹, T. Ahmad ¹

Affiliations
1 Wadia Institute of Himalayan Geology, 33, General Mahadeo Singh Road, Dehra Dun 248 001, IN

Abstract

The Late Palaeozoic Abor Volcanics of the eastern Himalayas have typical continental tholeiite chemistry in terms of rather high and variable Fe/Mg ratios and incompatible trace elements, except P, abundances. The compatible trace elements (Ni, Cr, Co) are low for their specific MgO levels. Variable abundances of incompatible elements at constant Mg numbers, lack of significant Eu anomalies and large range in the ratios of more incompatible to less incompatible elements (e.g. La/Yb) suggest that fractionation was not significant in their compositional evolution. Fe-Mg relations indicate that the melts were generated from a mantle source with enriched Fe/Mg ratio compared to pyrolitic mantle; melting was followed by separation of an immiscible sulphide phase which resulted in lowering of Ni and P contents, and some clinopyroxene fractionation. Model calculations on REE data suggest that the source was enriched by about l1x and 4x chondrite in light REE (La) and heavy REE (Yb) respectively. This enrichment of the source was probably due to an undersaturated silicate melt and a CO₂-rich fluid phase.

Keywords

Petrogenesis, Abor Volcanics, Geochemistry, Himalaya.

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Z. S. H. Abu-Hamatteh ¹, M. Raza ¹, T. Ahmad ²

Affiliations
1 Deptt. of Geology, Aligarh Muslim University, Aligarh-202002, IN
2 Wadia Insitute of Himalayan Geology, 33, Gen. Mahadeo Singh Road, Dehra Dun-248001, IN

Abstract

The deep water Jharol sediments of Aravalli Supergroup host sporadic bodies of mafic-ultramafic. rocks. Field evidence suggests that at least some of these rocks are extrusive in nature. Low to medium grade metamorphism and intense deformation has obliterated the original mineralogy and texture of these rocks. The ultramafic rocks are relatively more altered (serpentinized) with highly disturbed geochemical composition which cannot be used for petrogenetic interpretations. On the other hand, the mafic rocks are relatively less altered with some relict plagioclase feldpars. Geochemically, these mafic volcanics are Mgand Fe- rich tholeiites.

These tholeiites have nearly flat to slightly enriched REE patterns (about 30 to 40x % chondrite La and 20 to 25x % Chondrite Yb), with (LalYb)N ratio ranging between 1.29 and 1.54 and (GdIYb)N ratio ranging between 0.97 and 1.07. All samples show slight to strong positive Eu anomaly. Immobile incompatible-element geochemical patterns and other binary plots suggest that these tholeiites have enriched chemistry and that grobably they have been derived from enriched source(s) and resemble closely with within plate and initial rift tholeiites.

Trace elements, including REE based petrogenetic modelling indicate that (a) the source had about 3x% chondoritic REE abundances with nearly flat to slightly depleted pattern and (b) that these rocks were generated by about 8 to 12% partial melting of the modelled source followed by some olivine and clinopyroxene fractionation. Field as well as trace element characteristics indicate rift related (lithospheric extension) generation of these basalts with some influence of asthenosphere (deep mantle) on the lithospheric source(s) as indicated by positive Nb and Sr anomalies.

Keywords

Geochemistry, Jharol Mafics, Aravalli Supergroup, Proterozoic Volcanism, Rajasthan.

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T. Ahmad ¹, M. Deb ¹, J. Tarney ², M. Raza ³

Affiliations
1 Department of Geology, University of Delhi, Delhi - 110 007, IN
2 Department of Geology, University of Leicester, Leicester LE1 7RH, GB
3 Department of Geology, Aligarh Muslim University, Aligarh - 202 001, IN

Abstract

The Aravalli-Delhi orogen in Rajasthan and northern Gujarat, NW India, hosts extensive bodies of mafic metavolcanic rocks within the Archaean Banded Gneissic Complex (BGC) basement and in the Proterozoic supracrustal belts of Bhilwara, Aravalli, Jharol and Delhi. There is no significant occurrence of mafic dykes in these terrains. All metavolcanic rocks within the supracrustal belts show typical tholeiitic compositions except for the samples from the Basantgarh area of the Delhi belt, which show calc-alkaline nature. The Bhilwara belt and Aravalli belt samples show light rare earth elements (LREE) enrichment, the Jharol belt samples are predominantly LREE depleted, whereas the Delhi belt samples show wide variations from slightly depleted through flat to highly fractionated REE patterns. Multi-element patterns are consistent with the REE patterns in terms of enrichment or depletion for the large ion lithophile elements (LILE) and with respect to high field strength elements (HFSE). The incompatible trace elements ratios (e.g. Nb/Zr, Y/Zr, Ce/Nd, etc.) are similar for the metavolcanic rocks of the Bhilwara and Aravalli belts. Samples from the Jharol and Delhi belts differ amongst themselves as well as with those from the Bhilwara and Aravalli belts. The Delhi belt samples show large variations in incompatible trace element ratios. Comparison of multi-element patterns for the volcanic rocks from the Aravalli-Delhi orogen with those from various modern tectonic settings, potentially allows constraining their tectonic setting of eruption. The tholeiitic metavolcanic rocks of the Bhilwara belt erupted along the incipient divergent eastern margin of the Archaean BGC continent and the basal Aravalli and Jharol metavolcanic rocks along its rifted western margin. The Delhi belt metavolcanic rocks indicate the evolution of the Delhi island arc through eastward subduction and emplacement of the Basantgarh-Ajari and Ranakpur-Desuri metavolcanic rocks close to the Phulad-Jetgarh suture zone on the western fringe of the arc and those of the Ambaji-Deri region along back arc spreading zones to the east, where protracted rifting led to the emplacement of gabbro-syenite plutons.

Keywords

Geochemistry, Mafic Volcanism, Tectonics, Proterozoic, Delhi-Aravalli Orogen.

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M. E. A. Mondal ¹, R. Chandra ², T. Ahmad ³

Affiliations
1 Department of Geology, Aligarh Muslim University, Aligarh - 202 002, IN
2 Department of Geology, Bundelkhand University, Jhansi - 284 128, IN
3 Department of Geology, University of Delhi, Delhi - 110 007, IN

Abstract

Precambrian mafic magmatism of the Bundelkhand craton is represented by the mafic dyke swarms that have extensively traversed the gneisses and granitoids and metamorphosed mafic-ultramafic rocks associated with greenstone-gneiss, occurring as small linear isolated basins. The mafic dykes were emplaced (during ∼2.1 to ∼2.0 Ga) in three phases, two in NW-SE direction and the third in NE-SW direction, depicting episodic events subsequent to the stabilization of the craton (at ∼2.5 Ga). The former represent culmination of tectono-magmatic event in the Bundelkhand region. The mafic-ultramafic suite of rocks associated with greenstone-gneiss units, range in composition from basalt through basaltic-andesite to basaltic-komatiite. The basaltic-komatiites display remarkable geochemical similarity with boninites. The dykes are of varying compositions and are mostly low-Ti tholeiites and range from sub-alkaline basalt through basaltic andesite and dacite. They show enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE). Incompatible trace elemental ratios indicate that crustal contamination alone cannot be the dominant process for the evolution of the dykes. Ti/Zr and Ti/Y ratios of the dykes reflect near primary chemical characteristics of the mantle sources. Large variations observed in Nb/La and La/Sm ratios possibly reflect varying influence of the crustal material and also some LREE-LILE enrichment processes. To explain the observed geochemical characteristics, it is proposed that the mantle source region was contaminated by sediments/crustal components probably via subduction processes. Geochemical characteristics of the dykes swarm probably reflects enriched mantle source characteristics followed by fractional crystallisation of the magma. Our study indicates important role of subduction processes in the evolution of the Precambrian crust and subcontinental lithosphere in the Bundelkhand and neighbouring shield and Himalayan areas.

Keywords

Precambrian Mafic Dykes, Bundelkhand Craton, Enriched Mantle Sources, Crustal Contamination, Subduction Processes.

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