The Origin and Tectonic Setting of Precambrian Greywacke of Ribandar-Chimbel,Goa,India: Petrological and Geochemical Evidence
Glancia Q FERNANDES;Sridhar D.IYER;Mahender KOTHA;Department of Earth Science,Goa University;Department of Civil Engineering,Don Bosco College of Engineering;CSIR-National Institute of Oceanography;
The Precambrian greywacke of Ribandar-Chimbel belonging to the Sanvordem Formation of the Goa Group,India,has been studied for petrography and analyzed for major trace elements. The greywacke is characterized by angular to sub-round grains of quartz,feldspar,biotite,chlorite and clay minerals. The abundance of clay in the matrix seems to have influenced the Al_2O_3 content and the K_2O/Al_2O_3 ratio. The variation diagrams indicate a decreasing trend of TiO_2,Al_2O_3,Fe_2O_3 and Mg O; whereas Na_2O and CaO exhibit a scatter which could be a result of the variable presence of feldspar within the sediments. The immobile elements,vanadium(25 to 144 ppm),nickel(up to 107 ppm) and chromium(up to 184 ppm),reflect abundance of clay minerals. The greywacke shows strongly fractionated REE patterns with LaN/YbN = 8 to 26 and with higher total REE abundances(up to 245 ppm). The low REE enrichment and depletion in heavier REE with prominent negative Eu anomaly(Eu/Eu*= 0.54 to 0.79) suggest a derivation of the greywacke from an old upper continental crust composed chiefly of felsic components. Petrological evidence and geochemical data suggest that the deposition of the greywacke largely took place in a deep to shallow basin that progressively changed from that of a continental island arc to an active continental setting.
【CateGory Index】： P534.1;P542