Mesoproterozoic granulites of the Shillong–Meghalaya Plateau: Evidence of westward continuation of the Prydz Bay Pan-African suture into Northeastern India

Abstract

Thermo-chronologic considerations in the Australo-Antarctic domains suggest that the Prydz Bay Pan-African suture of East Antarctica continues westward into India. However, the location of the suture within Eastern India has so far been uncertain because of a lack of adequate thermo-chronological information. In this study, electron microprobe (EPMA) monazite dates and mineral paragenesis of granulite facies metapelites are reported from two areas of the Shillong–Meghalaya gneissic complex (SMGC), a crustal block located in the extreme northeast of the Indian shield close to the Australo-Antarctic block in Neoproterozoic-Cambrian paleomagnetic reconstructions of the Rodinia supercontinent. In the Garo-Goalpara Hills region, a well constrained Mesoproterozoic age of 1596 ± 15 Ma ( n = 103) is correlated with a counterclockwise pressure–temperature path with near peak conditions of 7–8 kbar and 850 °C. Rare matrix monazite rims record younger ages (1032–1273 Ma). At Sonapahar region, 50 km ESE of Garo-Goalpara Hills, homogenous monazite grains in granulite facies metapelites yield EPMA dates tightly clustered at 500 ± 14 Ma ( n = 36) irrespective of their textural setting in a well-annealed mineral matrix. In a few zoned monazite grains, the cores yield older ages of 1078 ± 31 Ma ( n = 10) and 1472 ± 38 Ma ( n = 13). The 500 Ma date corresponds with the ca. 880–480 Ma Rb–Sr dates of porphyritic granites that predominantly intruded the east-central part of the SMGC. We propose that the progressively eastward dominance of Cambro-Ordovician ages in the SMGC indicates a Pan-African final amalgamation of the Indian plate with the Australo-Antarctic plate and a northward extension of the Prydz Bay suture through the SMGC, with the western boundary of the suture possibly located between the Garo-Goalpara Hills and Sonapahar areas.