Petrogenesis of the Bondla layered mafic-ultramafic complex, Usgaon, Goa

Abstract

The Bondla mafic-ultramafic complex is a layered intrusion that consists predominantly of peridotites and gabbronorites. A chromitite-pyroxenite-troctolite horizon serves as a marker to subdivide the intrusion into two zones. The Lower Zone displays gravity stratified layers of chromite that alternate with those of olivine, which up-section are followed by olivine+pyroxene-chromite cumulates. The Upper Zone comprises gabbroic rocks that exhibit uniform layering. On the basis of modal and cryptic variation exhibited by the minerals this zone can be subdivided in to several lithohorizons starting from the troctolites at the base to gabbronorites and leucogabbros at the top. The junction between the two zones is marked by the distinct reversal in cryptic variation exhibited by the chromites and pyroxenes. The peridotite chromites contain higher Al(2)O(3) and lower Cr(2)O(3) than those from the chromitite above. Similarly clinopyroxenes from pyroxenite and troctolites are more magnesian that those from the peridotites stratigraphically below them. The complex in general is characterized by a gabbroic mineral assemblage in which both Ca-rich and Capoor pyroxenes coexist and displays a Fe-enrichment trend providing evidence of evolution from a contaminated tholeiitic magma. The rocks are characterized by low-TiO(2); Ni, Cr and V, show negative correlation with Zr whereas the large ion lithophile elements (LILE) are positively correlated and the Nb/La ratio varies from 0.4-0.6. These characteristics are consistent with a low-TiO(2) sub-alkaline tholeiitic magma that may have been modified by fractional crystallization and successive injections of more primitive melts in the magma chamber. The complex evolved in a periodically replenished magma chamber that consisted of two separate but interconnected sub-chambers.