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

At Deobhog, migmatitic gneisses and granulites of the Eastern Ghats Belt ar e juxtaposed against a cratonic ensemble of banded augen gneiss, amphibolit e 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 ne ar-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 gr anulites, and from thermobarometry on Grt (corona)-Opx/Cpx-P1-Qtz assemblag es. The P-T values are consistent with the occurrence of Opx-Spr-Crd assemb lages in spatially associated high-Mg-Al pelites. A subsequent period of co oling 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 accompanyi ng the retrograde changes is manifested in biotite replacing Opx in some li thologies. The cratonic banded gneiss-granite unit also documents two phases of isocli nal folding (D-1B & D-2B), with the L-2B lineation girdle different from th e lineation spread in the migmatitic gneiss unit. Calcsilicate gneiss (Hbl- P1-Cpx-Scap-Cal) and amphibolite (Hbl-P1+/-Grt+/-Cpx) within banded gneisse s 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 granit e and dolerite intrusions and is characterized by top-to-the-west movement along N-S-trending, E-dipping shear planes. Deformation mechanisms of quart z and feldspar in granites and banded gneisses and amphibole-plagioclase th ermometry within shear bands in dolerites document an inverted syn-D-3B the rmal gradient with temperature increasing from 350 to 550 degreesC in the w est to 2700 degreesC near the contact with the migmatitic gneiss unit. The thermal gradient is reflected in the stabilization of chlorite after hornbl ende 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 conditio ns 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 coo ling and hydration of the hanging wall granulites across the contact are at tributed to westward thrusting of 'hot' Eastern Ghats granulites on 'cool' cratonic crust. It is suggested that the Eastern Ghats migmatitic gneiss un it is not a reworked part of the craton, but a para-autochthonous/allochtho nous unit emplaced on and amalgamated to the craton.