Contrasting pressure-temperature-deformation history across a vestigial craton-mobile belt boundary: the western margin of the Eastern Ghats Belt at Deobhog, India

摘要

At Deobhog, migmatitic gneisses and granulites of the Eastern Ghats Belt are juxtaposed against a cratonic ensemble of banded augen gneiss, amphibolite and calcsilicate gneiss, intruded by late hornblende granite and dolerite. In the migmatitic gneiss unit, early isoclinal folds (syn-D-1M and D-2M) are reoriented along N-S-trending and E-dipping sheer planes (S-3M), with (S-1M-S-3M) intersection lineations having steep to moderate plunges. The near-peak P-T condition was syn-D-3M (greater than or equal to 900 degreesC, 9.5 kbar), as inferred from syn-D-3M Grt+Opx-bearing leucosomes in mafic granulites, and from thermobarometry on Grt (corona)-Opx/Cpx-P1-Qtz assemblages. The P-T values are consistent with the occurrence of Opx-Spr-Crd assemblages in spatially associated high-Mg-Al pelites. A subsequent period of cooling followed by isothermal decompression (800-850 degreesC, c. 7 kbar) is documented by the formation of coronal garnet and its decomposition to Opx+P1 symplectites in mafic granulites. Hydrous fluid infiltration accompanying the retrograde changes is manifested in biotite replacing Opx in some lithologies. The cratonic banded gneiss-granite unit also documents two phases of isoclinal folding (D-1B & D-2B), with the L-2B lineation girdle different from the lineation spread in the migmatitic gneiss unit. Calcsilicate gneiss (Hbl-P1-Cpx-Scap-Cal) and amphibolite (Hbl-P1+/-Grt+/-Cpx) within banded gneisses record syn-D-2B peak metamorphic conditions (c. 700 degreesC, 6.5 kbar), followed by cooling (to c. 500 degreesC) manifested in the stabilization of coronal clinozoisite-epidote. The D-3B shear deformation post-dates granite and dolerite intrusions and is characterized by top-to-the-west movement along N-S-trending, E-dipping shear planes. Deformation mechanisms of quartz and feldspar in granites and banded gneisses and amphibole-plagioclase thermometry within shear bands in dolerites document an inverted syn-D-3B thermal gradient with temperature increasing from 350 to 550 degreesC in the west to 2700 degreesC near the contact with the migmatitic gneiss unit. The thermal gradient is reflected in the stabilization of chlorite after hornblende in S-3B shears to the west, and post-D-2B neosome segregation along D-3B folds and shears to the east. The contrasting lithologies, early structures and peak metamorphic conditions in the two units indicate unconnected pre-D-3 P-T-deformation histories. The shared D-3 deformation in the two units, the syn-D-3 inverted thermal gradient preserved in the footwall cratonic rocks and the complementary cooling and hydration of the hanging wall granulites across the contact are attributed to westward thrusting of 'hot' Eastern Ghats granulites on 'cool' cratonic crust. It is suggested that the Eastern Ghats migmatitic gneiss unit is not a reworked part of the craton, but a para-autochthonous/allochthonous unit emplaced on and amalgamated to the craton. [References: 55]