Paleoproterozoic emplacement and Cambrian ultrahigh-temperature metamorphism of a layered magmatic intrusion from the Central Madurai Block, southern India: From Columbia to Gondwana

摘要

The Madurai Block in the Southern Granulite Terrane (SGT) of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly. This block is composed of three sub-blocks: the Neoarchean Northern Madurai block, Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block. The margins of these blocks are well-known for the occurrence of ultrahigh-temperature (UHT) granulite facies rocks mostly represented by Mg-Al metasediments. Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block. The major rock types of the layered intrusion include spinel orthopyroxenite, garnet-bearing gabbro, gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture. The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel, cordierite and orthopyroxene. The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions. Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10?kbar pressure and a minimum temperature of 980?°C. We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases, which yield temperature above 1000?°C. FMAS petrogenetic grid, Al-in-orthopyroxene isopleth, conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature (P–T) path and near isothermal decompression. The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca. 2.0?Ga and the metamorphic overgrowths yield weighted206Pb/238U mean ages ca. 520?Ma. Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded206Pb/238U weighted mean ages of ca. 532?Ma and 523?Ma marking the timing of metamorphism. We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting, defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks. The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.