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.

Paleoproterozoic/Mesoproterozoic tectonism in the northern fringe of the Western Dharwar Craton (India): Its relevance to Gondwanaland and Columbia supercontinent reconstructions

“…Metamorphic garnets overgrowing the tectonic fabric in granitoids are common in the well‐foliated/gneissose varieties in the south but are absent in the weakly foliated granitoids northwards within the corridor. The lithologies in the corridor are in sharp contrast to those in the Goa Schist Belt (WDC) dominated by NNW‐trending bands of greenschist facies metagreywacke (chlorite‐muscovite ± calcite ± biotite schist), Banded Iron Formation, amphibolite (actinolite‐chlorite schist), polymict and quartz conglomerate and quartzite, and mafic‐ultramafic complexes comprising gabbro, troctolite, wehrlite, dunite, peridotite, pyroxenite, and chromitite, and chromite‐bearing serpentinite [ Jena , , ; Geological Survey of India , ; Dessai et al ., ; Ishwar‐Kumar et al ., ; Rekha et al ., ]. The Quepem granitoid pluton intrusive into the schists is deformed along with the supracrustals [ Rekha et al ., ].…”

The Geology of Goa Group: Revisited

“…The Chandranath granite gneiss was considered to be syntectonic with the first phase of folding, the Tamdi felspathic gneiss and the Dudhsagar granite were considered to be synkinematic with the second phase, whereas the Canacona granite was considered to be post-tectonic (Gokul et al 1985). In the light of the recent work (Dhoundial et al 1987;Devaraju et al 2007Devaraju et al , 2010Dessai et al 2009) especially on the granitic gneisses, greenstones and the intrusive granites from representative sections, it is proposed to revisit the classification of the greenstones from this area. The Goa Group of Gokul et al (1985) needs to be integrated (see also Devaraju et al 2010) with the contiguous Dharwar Supergroup of WDC, as otherwise it would give an erroneous impression of forming a separate stratigraphic entity within the Dharwar craton.…”

“…Jena, 1980Jena, , 1985 which were later identified to form a layered peridotites-gabbro complex (e.g. Balakrishnan et al 1992;Dessai et al 2009). Dessai and Peshwa (1982) have shown that the complex is intruded along a major shear zone that runs NW-SE.…”

“…The ultramafic zone consists of cumulates of chromitite layers alternating with those of olivine (Dessai et al 1995). The mafic zone shows uniform layering (Dessai et al 2009). These layered intrusives were placed stratigraphically higher than the 'intrusive' Chandranath granitic gneisses (Gokul et al 1985).…”

“…Geological map of Goa (revised after GSI, 1996, by integrating arguments based on the field, petrological, geochemical and isotope data following the works of Dhoundial et al 1987;Devaraju et al 2007;2010;Dessai et al 2009). …”