Origin and Ductile-Shearing of the Microgranitoid Enclaves in the Granulite-Granitoid Terrane Around Devariya-Bandanwara Area, Central Rajasthan

摘要

Two distinct types of enclaves, namely, gneissic enclaves and microgranitoid enclaves occur abundantly in the granitoid rocks belonging to the granulite-granitoid terrane of the Banded Gneissic Complex. Field relationships, and petrographic and microstructural studies indicate that the gneissic enclaves are the xenoliths of the older rocks, but the microgranitoid enclaves are formed by the process of magma mixing and magma mingling. Both microgranitoid enclaves and the host granitoids are cut by numerous normal fault type and strike-slip type of ductile shear zones. Stress analysis by the fault-slip method and, strain analysis using the feldspar megacrysts as strain markers, reveal that the normal fault type of shear zones are the extensional structures formed by non-coaxial and flattening type of deformation. A critical comparison of the concentrations of major and trace elements, in the unsheared and the sheared microgranitoid enclaves, implies the addition of SiO_2, Al_2O_3, Na_2O, K_2O, Pb, Rb and Sr, and loss of Cr and Cu during the ductile shearing. The slopes of isocon lines, passing through the plots of the immobile elements, such as Y and TiO_2 indicate that about 18-64% gain in volume has occurred during the ductile shearing of the microgranitoid enclaves.