Fluid evolution and exhumation path of the Trivandrum Granulite Block, southern India

摘要

In charnockites and host garnet-biotite gneisses of the Trivandrum Granulite Block (TGB), southern India, fluid inclusions are mainly represented by CO_2-rich categories. Aqueous inclusions with low salt concentration (0.33-13.5 wt percent NaCl equiv.) occur subordinately. We recognize three generations of CO_2-rich inclusions, which have contrasting fluid densities that correlate with the different stages of metamorphic evolution established from mineral phase equilibria studies. Although some of these inclusions show variable depression in melting temperatures, laser Raman spectroscopic studies revealed no volatiles other than CO_2. We report here the occurrence of high-density CO_2-rich inclusions for the first time from this region with homogenization temperatures in the range of -21.9 to-15.6 deg C (rho=1.041-1.012 g/cm~3). The marked secondary character of these high-density inclusions and their textural association with late brittle deformational features indicate their entrapment at low temperature conditions (250-300 deg C), post-dating charnockite formation. Regional metamorphism in TGB (stage M1) took place at ca. 700 deg C and 5.36 kbar, although higher temperatures prevailed in the northern part of the terrain. A subsequent period of decompression (of the order of 6-7 km) was accompanied by partial recrystallization of host rocks and intensive processes of charnockite formation (M2) that extended to subisobaric cooling from 785-805 to 680-700 deg C. The relatively small population of aqueous inclusions in charnockites and their low salinity preclude any major role for metasomatic processes involving increase of potassium activity, and confirm the important role of CO_2-rich fluids in stabilizing low water activities. During the final phase of exhumation of the region (ca. 9km in depth), additional heat input resulted in mineralogical alteractions correlated to stage M3. While the source of heat and fluids involved in charnockite formation is postulated to be magmas derived from sublithospheric sources, the late heat and fluid transfer are correlated to late Pan-African ultrabasic and alkaline intrusives emplaced at shallow depths.