Mineral chemistry-perspective of the Kadavur gabbro-anorthosite Complex, Southern Granulite Terrane of the Indian shield: Implications to its petrogenesis in a ridge-subduction setting

Mineral chemistry data of constituent silicate and oxide phases are often regarded as a useful tool for assessing the petrogenesis of mafic rocks. In view of this, the chemistry of minerals from the Kadavur gabbro-anorthosite Complex (10°35′N: 78°11′E), a magmatic intrusion in the Southern Granulite Terrane of the Indian shield, has been evaluated using several thermo-barometric methods and tectonic discrimination diagrams. The Complex represents a magmatic intrusion that consists of a highly deformed schistose gabbro-anorthosite type and an undeformed layered gabbro-anorthosite type, with local patches of pegmatoidal gabbro-anorthosite bodies. The constituent minerals in the Complex include clinopyroxene, orthopyroxene, plagioclase, amphibole, and subordinate amounts of ilmenite and magnetite. Pyroxene thermometry (clinopyroxene thermometry, orthopyroxene thermometry and two pyroxene thermometry) gives mean temperatures of ∼1060 °C for the layered gabbro- anorthosite bodies, and ∼ 1124 °C for the pegmatoidal bodies; and ∼ 1130 °C for the schistose gabbro-anorthosite. Hornblende-plagioclase and amphibole thermometry give temperatures of ∼1000–1190 °C. The co-existing oxide (magnetite-ilmenite) thermometry gives a lower temperature of ∼420 °C. The mean clinopyroxene pressure value is 17 kbar for the schistose gabbro-anorthosite, 9 kbar for the layered gabbro-anorthosite, and 11 kbar for the pegmatoid bodies. During the earlier phase of deformation associated with magmatic crystallization (responsible for schistose type), the ambient temperature and pressure of crystallization of the Complex were 955 to 1285°C at ∼17 kbar. The magma equilibrated at shallow to intermediate levels allowing for localized input of H₂O. Consideration of whole-rock geochemical data (especially immobile trace elements) indicates that parent magma was of tholeiitic and alkaline composition and later underwent fractionation. The rare earth element (REE) distribution in the schistose gabbro-anorthosite is broadly similar to that of N-MORB, while the geochemical characteristics of the layered gabbro-anorthosite resemble island arc basalt (IAB). The pegmatoidal gabbro-anorthosite bodies have a transitional affinity between both the N-MORB and IAB. The mineral chemistry and whole-rock geochemistry data suggest that the schistose gabbro-anorthosite mostly corresponds to “non orogenic” and “MORB-type” while layered gabbro-anorthosite and pegmatoid bodies correspond to an ‘orogenic’ and ‘island-arc’ setting, suggesting a clear shift from a MORB setting to an arc-setting suggesting a ridge-subduction event. The Kadavur gabbro-anorthosite Complex is analogous to other well-known Archaean gabbro-anorthosite Complexes around the world.