Abstract:
This paper reports new petrological, geochemical and isotopic data for Carlsberg Ridge Basalts (CRB) of northwest Indian Ocean and evaluates their petrogenetic aspects in the context of the geochemical and tectonic evolution of the Indian Ocean mantle. The CRB samples exhibit tholeiitic to transitional composition of precursor melts derived by high degree, shallow level partial melting of a spinel peridotite mantle source. CRB reflects distinct E-MORB affinity with selective enrichment in incompatible trace elements. Higher values of Zr/Hf (33.8-47.3) and Zr/Sm (24.9-36.4) in conjunction with lower Nb/Ta (1.7-7.3) ratio corroborate their origin from an enriched mantle source. Negative Nb anomalies with lower Nb/Y (0.04-0.11) and Zr/Y (2.5-3.5) conform to a non-plume origin of these basalts. Higher Zr/Nb (25.5-71.5) and Th/Nb (0.6-0.42) compared to OIB substantiate contributions from recycled subductio-processed components in the source mantle. Lower Nb/U (6.2-37.9) values with higher Ba/Nb (6.1-21.9), and Ba/Th (27.7-147.5), Zr/Nb (25.5-71.5) and Th/Nb (0.6-0.42) compared to OIB and N-MORB attest to role of a metasomatized oceanic lithosphere that recycled into the depleted upper mantle attributing to the source heterogeneity. Sr-Nd isotopic signatures (sup(87) Sr/ sup(86) Sr: 0.702668 to 0.702841 and sup(143) Nd/ sup(144) Nd: 0.512972 to 0.513068) of CRB suggest a HIMU source component preserved in the northwest Indian Ocean Mid Oceanic Ridge mantle. The compositional diversity of the Indian Ocean mantle can be translated in terms of periodic refertilization of depleted N-MORB type mantle through delamination and recycling of oceanic (HIMU component) and continental lithosphere (EM I component) concurrent with Neoproterozoic-Palaeozoic amalgamation and Jurassic dispersal of Gondwana Supercontinent respectively. This study complies with the derivation of CRB from a geochemically heterogeneous Indian Ocean mantle that experienced a protracted residence beneath the Gondwana Supercontinent prior to the opening of Indian Ocean and trapped recycled metasomatized oceanic lithosphere genetically linked with multiple stages of paleo-ocean closure and continental convergence during Gondwana assembly.
