Petrology and geochemistry of late archaean granitoids in the northern part of Eastern Dharwar Craton,Southern India: Implications for transitional geodynamic setting |
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Authors: | B C Prabhakar M Jayananda Mohamed Shareef T Kano |
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Institution: | 1.Department of Geology,Bangalore University,Bangalore,India;2.Department of Earth Sciences,Yamaguchi University,Yamaguchi,Japan |
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Abstract: | The results of field, petrographic and geochemical work of the granitoids of Hutti-Gurgunta area in the northern part of Eastern
Dharwar Craton (EDC) is presented in this paper. This crustal section comprises polyphase banded to foliated TTG gneisses,
middle amphibolite facies Gurgunta schist belt and upper greenschist facies Hutti schist belt and abundant granite plutons.
The focus of the present study is mainly on basement TTG gneisses and a granite pluton (∼ 240 sq km areal extent), to discuss
crustal accretion processes including changing petrogenetic mechanism and geodynamic setting. The TTGs contain quartz, plagioclase,
lesser K-feldspar and hornblende with minor biotite while the granite contain quartz, plagioclase, K-feldspar and hornblende.
Late stage alteration (chloritisation, sericitisation and epidotisation) is wide spread in the entire area. A huge synplutonic
mafic body which is dioritic to meladioritic in composition injects the granite and displays all stages of progressive mixing
and hybridization. The studied TTGs and granite show distinct major and trace element patterns. The TTGs are characterized
by higher SiO2, high Al2O3, and Na2O, low TiO2, Mg#, CaO, K2O and LILE, and HFS elements compared to granite. TTGs define strong trondhjemite trend whilst granite shows calc-alkaline
trend. However, both TTGs and granite show characteristics of Phanerozoic high-silica adakites. The granite also shows characteristics
of transitional TTGs in its high LILE, and progressive increase in K2O with differentiation. Both TTGs and granite define linear to sub-linear trends on variation diagrams. The TTGs show moderate
total REE contents with fractionated REE patterns (La/YbN =17.73–61.73) and slight positive or without any significant Eu anomaly implying little amount of amphibole or plagioclase
in residual liquid. On the other hand, the granite displays poor to moderate fractionation of REE patterns (La/YbN = 9.06–67.21) without any significant Eu anomaly. The TTGs have been interpreted to be produced by low-K basaltic slab melting
at shallow depth, whereas the granite pluton has been formed by slab melting at depth and these melts interacted with peridotite
mantle wedge. Such changing petrogenetic mechanisms and geodynamic conditions explain increase in the contents of MgO, CaO,
Ni and Cr from 2700 Ma to 2500 Ma granitoids in the EDC. |
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