Crustal accretion and metamorphism of Mesoarchean granulites in Palghat-Cauvery Shear Zone,Southern India |
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| Authors: | A K Choudhary A K Jain Sandeep Singh Rm Manickavasagam K Chandra |
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| Institution: | (1) Dept. Geology, University of Rajasthan, 302004 Jaipur, India |
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| Abstract: | This work provides unequivocal evidence of the existence of Mesoarchean granulite facies metamorphic event in the Palghat-Cauvery
Shear Zone (PCSZ) of South India. Charnockite samples from two prominent hills at Kollaimalai (KM) and Pachchaimalai (PM)
as well as from two quarries within the Bhavani Shear Zone (BSZ) have been analyzed for their Sm-Nd and Rb-Sr ages to investigate
the existence or otherwise of the Archean granulite facies events within the PCSZ. The Rb-Sr whole-rock isochron ages for
massive charnockites from both the hills appear to be contemporaneous at 2.9 Ga with the initial Sr isotopic ratios of 0.7012
and 0.7014, respectively. However, the Rb-Sr data for whole-rock samples of basic granulites from one of the quarries within
the BSZ indicate open system behavior, while the charnockites from the other quarry have insufficient spread in 87Rb/86Sr ratios and do not yield any isochron. The Sm-Nd data, on the other hand, do not distinguish between the massive charnockite
and the lowland charnockite and yield Depleted Mantle model ages in the range 2.98±0.3 Ga for all of them. The ɛT
CHUR for all of these rocks are highly positive. Both the Sr isotopic ratios and positive ɛT
CHUR values for these rocks strongly suggest a mantle source for all of them. An upper age limit of ∼3.28 Ga may be assigned to
the crustal accretion of the protolith of all these rocks on the basis of their Nd model ages. The Rb-Sr isochron ages of
2.9 Ga for the two massifs could be the age of granulite facies metamorphism. Thus, the metamorphism in the KM and PM Hills
took place within ∼100 Ma of crustal accretion of these rocks and probably was part of the same geological event of crust
formation and metamorphism. The open system behavior with respect to Rb-Sr isotopes in the basic granulite from Bhavani is
possibly due to the migration of Sr isotopes, triggered during the later shearing of these rocks. |
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