A. Mohan et Bf. Windley, CRUSTAL TRAJECTORY OF SAPPHIRINE-BEARING GRANULITES FROM GANGUVARPATTI, SOUTH-INDIA - EVIDENCE FOR AN ISOTHERMAL DECOMPRESSION PATH, Journal of metamorphic geology, 11(6), 1993, pp. 867-878
Ganguvarpatti is part of a Precambrian terrane characterized by granul
ite facies rocks, including charnockites, mafic granulites, sapphirine
-bearing granulites, leptynites and gneisses. A sequence of reactions
deduced from the multiphase reaction textures provide information on t
he metamorphic history of this area, as they formed in response to dec
ompression during uplift. Geothermobarometry and constraints from reac
tion textures define a segment of a P-T path traversed by the granulit
es of Ganguvarpatti. Near-peak metamorphic conditions of c.800 degrees
C and 8 kbar were succeeded by a symplectitic stage at a significantl
y lower pressure (c.700 degrees C and 4.5 kbar), documenting a nearly
isothermal decompression P-T path and rapid uplift (c.12km) followed b
y cooling. The presence of many fluid inclusions of extremely low dens
ity in the charnockites is consistent with a nearly isothermal uplift
path. Attainment of a maximum pressure of c.8 kbar indicates c.27km de
pth of burial during metamorphism. This would imply a total crustal th
ickness of c.65-70km at 2.6-2.5 Ga. Such a profound crustal thickness
and a clockwise decompressive P-T path is interpreted as a consequence
of tectonic thickening of crust, accomplished by collision tectonics
of the southern granulite terrane against the Dharwar craton along the
Palghat-Cauvery shear zone via northward subduction.