Mesoarchean crustal growth in the Coorg block and Mercara suture zone, southern India: evidence for reconstructing the Ur supercontinent

Bing Yu^1,2* , M. Santosh^1,3, Richard M. Palin², Cheng-Xue Yang¹

¹School of Earth Sciences and Resources, China University of Geosciences Beijing, No. 29 Xueyuan Road, Haidian District, Beijing 100083, China

²Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, United Kingdom

³Department of Earth Science, University of Adelaide, Adelaide SA 5005, Australia

*E-mails:eart0615@ox.ac.uk ; bing-yu@email.cugb.edu.cn

 

The existence of Earth's oldest supercontinent Ur is controversial due to be paucity of rock records related to early continent-building processes, particularly since juvenile felsic crust formed during the Early Archean (4.0–3.2 Ga) in many cases had been reworked or destroyed. The Paleo-Mesoarchean terranes are therefore of potential interest, the subduction-related arc magmatism and high-grade metamorphism in these regions can be used as important clues to trace the history of the assembly of Earth’s oldest supercontinent.

The Southern Granulite Terrane (SGT) in India is a collage of continental blocks ranging in age from Paleo-Mesoarchean through Neoarchean and Late Neoproterozoic-Cambrian, providing a near-complete record of the history of assembly and disruption of several supercontinents. The Coorg Block and the surrounding Mercara suture zone are important windows for tracing the history of the Ur supercontinent. We investigate the Coorg Block composed dominantly of charnockites and mafic granulites and the Mercara Suture Zone exposing extensive khondalites (granulite facies metapelites). The khondalite belt in the Mercara Suture Zone is currently the oldest known khondalite series, and zircon U-Pb data of detrital zircons indicate that the protoliths formed during the Paleoarchean (up to 3.5 Ga), with high-grade metamorphism at ca. 3.1 Ga, coinciding with the timing of assembly of supercontinent Ur. Phase equilibria modelling indicated a peak temperature of above ca. 900 °C and pressure up to 12 kbar. The charnockite suite (usually associated with mafic granulite enclaves) from the Coorg block ranges in age from Mesoarchean to late Neoarchean, with magmatic xenocrysts showing ages up to 3.5 Ga, this suggests that old crustal components are present in the crystalline basement. The peak of magmatic emplacement for the charnockite and gabbroic suites, both showing arc affinity, occurred at ~3.15 Ga and the arcs accreted onto the Dharwar craton to the north during the Neoarchean transition, building the ‘expanded Ur’. Mafic granulites formed by the underplating of basaltic magma derived through slab partial melting during subduction. The magmatic zircon core suggests Mesoarchean emplacement and the metamorphic zircons as well as monazite indicate collisional metamorphism at 3.0-3.1 Ga marking the assembly of the Ur supercontinent.   

We also combine our data with published results from other regions including Madagascar and East Antarctica, where remnants of the early crust are preserved in an attempt to reconstruct the Ur supercontinent. Our spatio-temporal analysis and model simulations suggest near-simultaneous assembly of the early crustal nucleic on the globe around ca. 3.1-3.0 Ga, although some parts of the supercontinent did not cratonize until the Late Mesoarchean.

Keywords: High-grade metamorphism; Zircon and monazite Geochronology; Ur supercontinent

Reference:
Yu et al., 2021. Gondwana Research 91: 129-151.
Yu et al., 2022. Precambrian Research 370: 106537.
Yang et al., 2023. Gondwana Research, 118, 1-36.