THE TECTONICS OF SLOW-COOLED OROGENS CHARACTERIZED BY COUNTER-CLOCKWISE P–T–T PATHS AND UHT METAMORPHISM

There are many unresolved issues in UHT metamorphism, but particularly generating the extreme temperatures. As an example we take the Eastern Ghats Province (EGP) of India, part of a large Grenville-age orogen, which is characterized by a counter-clockwise (CCW) evolution, peak T of 1000–950°C and P of 0.7–0.8 GPa, and cooling at ~1°C/Ma (Korhonen et al., 2013a, PR; 2013b, JMG; 2014, EPSL). For crust that evolves along CCW P–T–tpaths, it is likely that temperature of 1,000°C at mid-crustal depth was attained only where mantle heat was involved, suggesting a model of extending lithosphere during heating, whereas the slow rate of cooling requires conductive thermal relaxation over vertical length scales greater than present-day crustal thicknesses.

Throughout the EGP, granite is interleaved with residual sill–grt or opx–grt granulites, for which zircon and monazite geochronology demonstrates contemporaneity. In the central part of the EGP the Nd and Sr isotope compositions of the granites may be matched by simple mixing among the various exposed granulites, suggesting they may be consanguineous. However, by including published geochemical data from an area to the north, it is clear that an increasingly important mass input from the mantle was involved in granite genesis from SW to NE in the EGP, which is confirmed by AFC modeling between an exemplar mantle-derived melt at 1,000 Ma and the range of residual crustal lithologies (Korhonen et al., in review). We postulate that the spatial variation in mantle input to the granites is related to changing feedback between rates of extension and flux of mantle-derived melt around the peak of metamorphism.

Thus, both the metamorphic evolution and the granite genesis require heat and mass input from the mantle, consistent with a model of extension and conductive cooling. Other factors affecting the specific evolution of the EGP include: high radiogenic heat production; drainage of melt from granulites to the overlying crust; latent heat associated with crystallization of melt in the source and in the overlying crust; and, subsidence and sedimentation due to thermal contraction after peak T. Consideration of these factors will: increase the ability of the crust to achieve UHT conditions; load the compacting granulites; generate HT–LP, CCW P–T–t paths; and, reduce the rate of cooling.