Petro-tectonic imprints in the sapphirine granulites from Anantagiri, Eastern Ghats mobile belt, India

Abstract

Sapphirine granulite occurring as lenses in charnockite at Anantagiri, Eastern Ghat, India, displays an array of minerals which developed under different P-T-X conditions. Reaction textures in conjunction with mineral chemical data attest to several Fe-Mg continuous reactions, such as spinel+rutile+quartz+MgFe₋₁=sapphirine+ilmenite cordierite=sapphirine+quartz+MgFe₋₁ sapphirine+quartz=orthopyroxene+sillimanite+MgFe₋₁ orthopyroxene+sapphirine+quartz=garnet+MgFe₋₁ orthopyroxene+sillimanite=garnet+quartz+MgFe₋₁ orthopyroxene+sillimanite+quartz+MgFe₋₁=cordierite. Calculated positions of the reaction curves in P-T space, together with discrete P-T points obtained through geothermobarometry in sapphirine granulite and the closely associated charnockite and mafic granulite, define an anticlockwise P-T trajectory. This comprises a high-T/P prograde metamorphic path which culminated in a pressure regime of 8.3 kb above 950°C, a nearly isobaric cooling (IBC) path (from 950°C, 8.3 kb, to 675°C, 7.5kb) and a terminal decompressive path (from 7.5 to 4.5 kb). Spinel, quartz, high-Mg cordierite, and sapphirine were stabilized during the prograde high-T/P metamorphism, followed by the development of orthopyroxene, sillimanite, and garnet during the IBC. Retrograde low-Mg cordierite appeared as a consequence of decompression in the sapphirine granulite. Deformational structures, reported from the Eastern Ghat granulites, and the available geochronological data indicate that prograde metamorphism could have occurred at 3000±100 and 2500±100 Ma during a compressive orogeny that was associated with high heat influx through mafic magmatism. IBC ensued from P_max and was thus a direct consequence of prograde metamorphism. However, in the absence of sufficient study on the spatial variation in P-T paths and the strain histories in relation to time, the linkage between IBC and isothermal decompression (ITD) has remained obscure. A prolonged IBC followed by ITD could be the consequence of one extensional mechanism which had an insufficient acceleration at the early stage, or ITD separately could be caused by an unrelated extensional tectonism. The complex cooled nearly isobarically from 2500 Ma. It suffered rapid decompression accompanied by anorthosite and alkaline magmatism at ~1400-1000 Ma.