A Rodinian suture in western India: New insights on India-Madagascar correlations

Abstract

We report detailed evidence for a new paleo-suture zone (the Kumta suture) on the western margin of southern India. The c. 15-km-wide, westward dipping suture zone contains garnet-biotite, fuchsite-haematite, chlorite-quartz, quartz-phengite schists, biotite augen gneiss, marble and amphibolite. The isochemical phase diagram estimations and the high-Si phengite composition of quartz-phengite schist suggest a near-peak condition of c. 18kbar at c. 550°C, followed by near-isothermal decompression. The detrital SHRIMP U–Pb zircon ages from quartz-phengite schist give four age populations ranging from 3280 to 2993Ma. Phengite from quartz-phengite schist and biotite from garnet-biotite schist have K–Ar metamorphic ages of ca. 1326 and ca. 1385Ma respectively. Electron microprobe-CHIME ages of in situ zircons in quartz-phengite schist (ca. 3750Ma and ca. 1697Ma) are consistent with the above results. The Bondla ultramafic-gabbro complex in the west of the Kumta suture compositionally represents an arc with K–Ar biotite ages from gabbro in the range 1644–1536Ma. On the eastern side of the suture are weakly deformed and unmetamorphosed shallow westward-dipping sedimentary rocks of the Sirsi shelf, which has the following upward stratigraphy: pebbly quartzite/sandstone, turbidite, magnetite iron formation, and limestone; farther east the lower lying quartzite has an unconformable contact with ca. 2571Ma quartzo-feldspathic gneisses of the Dharwar block with a ca. 1733Ma biotite cooling age. To the west of the suture is a c. 60-km-wide Karwar block mainly consisting of tonalite-trondhjemite-granodiorite (TTG) and amphibolite. The TTGs have U–Pb zircon magmatic ages of ca. 3200Ma with a rare inherited core age of ca. 3601Ma. The K–Ar biotite cooling age from the TTGs (1746Ma and 1796Ma) and amphibolite (ca. 1697Ma) represents late-stage uplift. Integration of geological, structural and geochronological data from western India and eastern Madagascar suggest diachronous ocean closure during the amalgamation of Rodinia; in the north at around ca. 1380Ma, and a progression toward the south until ca. 750Ma. Satellite imagery based regional structural lineaments suggests that the Betsimisaraka suture continues into western India as the Kumta suture and possibly farther south toward a suture in the Coorg area, representing in total a c. 1000km long Rodinian suture.