Neoarchaean crustal evolution along the eastern flank of Nallamalai Shear Zone, southern India

ABSTRACT

The Southern Granulite Terrane (SGT) in India is composed of Archaean to Proterozoic crustal blocks and intervening shear/suture zones. The Nallamalai shear zone lies along the eastern flank of the Shevaroy Block. This study is focused on the eastern part of this shear zone where the basement rocks are dominantly composed of charnockite, quartzo-feldspathic gneiss and metagabbro, all of which preserve original magmatic texture. Zircon U-Pb LA-ICPMS analysis on the charnockite samples yielded an age of 2680 to 2500 Ma for the protolith emplacement followed by metamorphism at 2520 to 2450 Ma. Magmatic zircon grains in the quartzo-feldspathic gneiss and metagabbro yielded weighted mean ages of ca. 2557 Ma and ca. 2583 Ma with metamorphism at ca. 2518 Ma. Zircon Lu-Hf data suggest that the protolith of metagabbro was sourced mainly from the recycled Mesoarchaean crustal component. The Ɛ_Hf^(t) of charnockite (−0.4 to 4.1) and quartzo-feldspathic gneiss (0.5 to 6.0) represent a combined source of the mantle as well as recycling crust. All the rocks underwent amphibolite to granulite facies metamorphism and petrological studies and pseudosection modelling shows a similar P-T range of 7–8 kbar and 650–830°C. Results from the present study suggest that crustal evolution along the eastern flank of the Nallamalai Shear Zone involved melting of older crustal components (Mesoarchaean age) with significant juvenile input (Ɛ_Hf^(t) = ~+6.0) within a continental arc setting.

KEYWORDS:

• Neoarchaean crustal growth
• neoarchaean-paleoproterozoic metamorphism
• zircon geochronology
• Lu-Hf isotope
• P-T conditions
• crust–mantle interaction
• southern India

Acknowledgments

We thank Prof. Robert J. Stern, for his efficient editorial handling and the journal reviewers Kaushik Das, Pradip K. Singh and one anonymous for their constructive suggestions, which helped to improve the quality of the manuscript. The early version of the manuscript has benefited from the perceptive comments of D. Prakash, V. J. Rajesh and two anonymous reviewers. A special thanks to K. Sajeev for the discussions and helps provided through-out this work. We are also thankful to S. Kiran and N. Nishanth for their valuable support during whole-rock geochemical analysis at NCESS, Thiruvananthapuram and to C. Vandana for operational assistance in EPMA analysis from Advanced Facility for Microscopy and Micro Analysis (AFMM), IISc. We would like to extend our thanks to Sreehari Lakshmanan, P. Rajkumar, and P. G Athira for their support during field work and Biju Sekhar for giving constructive comments during preparation of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary Material

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Additional information

Funding

PVT acknowledges the financial support from DST-INSPIRE Fellowship (IF150424).