Petrology and zircon U–Pb geochronology of metagabbros from a mafic–ultramafic suite at Aniyapuram: Neoarchean to Early Paleoproterozoic convergent margin magmatism and Middle Neoproterozoic high-grade metamorphism in southern India

Abstract

Several mafic–ultramafic complexes occur within the Palghat-Cauvery Suture Zone (PCSZ) in southern India. The PCSZ is regarded in recent models as the zone along which crustal blocks were amalgamated during the Late Neoproterozoic–Cambrian (550–530Ma) Gondwana assembly. Here we report petrologic and zircon U–Pb geochronologic data from gabbros associated with the Aniyapuram mafic–ultramafic suite in the central domain of the PCSZ. Geothermobarometry and pseudosection approach in the system NCFMASHTO for the metagabbro (Grt+Cpx+Opx+Hbl+Pl+Qtz+Ilm+Rt) yield peak P–T condition of 9.8–10.6kbar and 730–790°C, which was followed by decompression to 6.5–8.0kbar and ca. 750°C as inferred from the formation of Opx+Pl symplectite around garnet, probably along a clockwise P–T path. Zircon U–Pb data analyzed by LA-ICP-MS plot along a well-defined discordia with upper and lower intercepts in the concordia at 2436±22Ma and 731±11Ma respectively, suggesting Neoarchean–Early Paleoproterozoic magmatic emplacement of the protolith and progressive Pb loss related to the Middle Neoproterozoic (Cryogenian) thermal event (or high-grade metamorphism). These results closely compare with the available Neoarchean magmatic ages of mafic–ultramafic complexes (e.g., Sittampundi, Devanur, Agali Hills, and Kanja Malai) and Middle Neoproterozoic magmatic event (e.g., Manamedu and Kadavur) in the PCSZ and adjacent granulite blocks. The 650Ma concordia ages obtained from unzoned zircons might indicate the timing of high-grade metamorphism or post-peak hydration event. The P–T conditions obtained from Aniyapuram are significantly lower than the high-pressure and ultrahigh-temperature conditions of the 550–530Ma final collisional event (P>14 kbar and T>950°C). The Middle Neoproterozoic (ca. 730Ma or 650Ma) high-grade metamorphism in Aniyapuram reported for the first time from the PCSZ is possibly associated with magmatism in arc tectonic setting.