Late Cambrian Reworking of Paleo-Mesoproterozoic Granulites in Shillong-Meghalaya Gneissic Complex (Northeast India): Evidence from PT Pseudosection Analysis and Monazite Chronology and Implications for East Gondwana Assembly

Abstract

Pressure-temperature (PT) pseudosection analysis of garnet-cordierite-sillimanite metapelites indicates that the granulites in the central part of the Shillong-Meghalaya Gneissic Complex (SMGC) in Northeast India evolved along a clockwise PT path with near isobaric heating (prograde) between 635° and 730°C at ∼5.7–5.5 kb, peak PT conditions of approximately 730°C/5.5 kb, and cooling between 650° and 595°C at 3.4–3.2 kb. Chemical dating indicates that an overwhelming majority of monazite in the metapelites and granites intrusive into the metapelites formed at Ma . The retrograde PT segment of the Late Cambrian metamorphic PT path in the SMGC is similar to those estimated in the Prydz Bay area of East Antarctica. It is suggested that the Late Cambrian metamorphism, felsic magmatism, and deformation in central SMGC may be correlated with the Pan-African collision between India and Australia-Antarctica during the assembly of East Gondwana, consistent with paleogeographic reconstructions based on paleomagnetic data. By contrast, an overwhelming majority of chemical dates from monazite cores in western SMGC (Garo-Goalpara) metapelites are Paleo-Mesoproterozoic ( Ma, ), with younger rims of some matrix monazite grains dated at 1141–946, , and 649–524 Ma. PT pseudosection analysis of the Garo-Goalpara metapelites indicates a counterclockwise path of metamorphism that probably occurred at ∼1.6 Ga, with the end phase of retrogression possibly in the Neoproterozoic/Early Paleozoic. The final PT conditions at Garo-Goalpara (∼5.8 kb/630°C) are similar to the prograde conditions of Late Cambrian metamorphism in central SMGC. The 820-Ma dates from western SMGC correspond to high-grade metamorphism anatexis in the 876–784-Ma N-trending Eastern Indian Tectonic Zone at the eastern margins of Eastern Indian Precambrian gneissic complexes located further south in a reconstructed pre-Cretaceous configuration of the Indian shield.