Pulsed tectonic evolution in long-lived orogenic belts: An example from the Eastern Ghats Belt, India

Abstract

This study characterizes two metamorphic cycles from two samples of an aluminous granulite of the Eastern Ghats Belt, India using petrological, phase equilibria, geospeedometric, and, petrochronological data. The first metamorphic cycle ensued through dehydration melting of F-biotite along a shallow dP/dT prograde path reaching ultrahigh temperature (>950 °C) at 7–8 kbar pressure. The peak metamorphic assemblage is represented by sapphirine + spinel + Al-orthopyroxene + cordierite + ilmenite + plagioclase + quartz which constitute the coarse granoblastic assemblage. Reaction corona comprising orthopyroxene + sillimanite + garnet ± F-biotite over the peak phases imply near-isobaric cooling from the peak with the termination of the first cycle. The melt produced during the prograde stage has been lost to a large extent to stabilize the peak assemblage. The second metamorphic cycle ensued with the decomposition of cordierite by a skeletal intergrowth of orthopyroxene + sillimanite + quartz and a symplectic intergrowth of garnet + quartz between orthopyroxene and cordierite. A delicate symplectite comprising cordierite + quartz + K-feldspar ± plagioclase also formed at this stage within the leucocratic layers. These intergrowth textures resulted from reworking of the cooled granulite (increasing P-T) through incipient melting of F-biotite subsequent decompression, and melt-solid interaction during the terminal stage of the second metamorphic cycle. Geospeedometric data involving garnet and biotite suggest the nonlinear nature of cooling with an initial fast cooling from the peak condition of the first metamorphic cycle. Monazite petrochronological data yield 1002 ± 3 Ma and 944 ± 6 Ma ages that belong to the peaks of first (ca. 1030–990 Ma) and the second (ca. 950–900 Ma) metamorphic cycles respectively of the central crustal province of the belt. This two-cycle evolution is related to several alternate extension (pull) and compression (push) cycles in an accretionary tectonic setting during ca. 1030–900 Ma.