Spinel + quartz assemblage in granulites from the Achankovil Shear Zone, southern India: Implications for ultrahigh-temperature metamorphism

Abstract

We report the finding of equilibrium spinel + quartz assemblage enclosed within garnet in garnet–orthopyroxene–cordierite granulites from Pakkandom within the Achankovil Shear Zone, a region which is considered as the trace of an accretionary suture in recent tectonic models on southern India. The spinel + quartz bearing granulites are composed of poikiloblastic garnet and subidioblastic orthopyroxene in the matrix of quartz, plagioclase, biotite, cordierite, and Fe–Ti oxides. Garnet contains numerous inclusions of sillimanite and biotite as well as spinel and quartz. The spinel in direct contact with quartz has moderate X Mg (= Mg/(Fe 2+ + Mg) = 0.44–0.47), and is Zn and Fe 3+ poor ( X Zn = Zn/(Fe 2+ + Mg + Zn) = 0.027–0.036, Fe 3+/(Fe 2+ + Fe 3+) = 0.12–0.17). Spinel is also present in the matrix surrounded by magnetite, but the matrix spinel contains more Zn( X Zn = 0.067–0.072) and does not show any contact relationship with quartz. Such Zn- and Fe 3+-poor spinel in direct contact with quartz has been regarded as a diagnostic evidence of ultrahigh-temperature (UHT) metamorphism. The high-temperature stability of the spinel + quartz is also supported by the results of geothermobarometric calculation of garnet–orthopyroxene assemblages that provides robust evidence for peak UHT metamorphism at 920–980 °C and 8–10 kbar, which was further confirmed by Al-in-Opx and magnetite–ilmenite geothermometers (900–950 °C and ∼1000 °C, respectively). The peak UHT event was followed by decompression down to 4.0–4.2 kbar and 640–670 °C toward the stability of cordierite along a clockwise P–T path. Similar spinel + quartz assemblage enclosed in poikiloblastic garnet has also been reported from the Palghat–Cauvery Shear Zone system, the trace of a major suture zone within the Gondwana amalgam with evidence for prograde high-pressure ( P up to 20 kbar) metamorphism followed by UHT event. The fine-grained spinel + quartz may thus indicate decompression from higher pressure conditions, confirming the recent attributes of the Achankovil Shear Zone being a collisional suture with evidence for subduction–exhumation processes which accompanied the final collision and amalgamation of the Gondwana supercontinent during the latest Neoproterozoic.