Abstract:
The tectonic and magmatic processes contributing to the heterogeneous nature of the upper mantle posit important constraints on the composition and differentiation of the Earth at various scales. Mid-oceanic ridges of intermediate, slow and very-slow spreading rates across the world's oceans are characterized by different crustal configurations portraying the compositional diversity of the upper mantle. The two viable processes that are attributed to chemical and isotopic heterogeneity of the mantle are: (i) multiple episodes of melt replenishment and melt-rock interaction in open magma systems; and (ii) recycling of oceanic and continental crustal materials and components of sub-continental lithospheric mantle. Here we present petrological, geochemical and zircon U-Pb geochronology data for the lower crustal oceanic gabbros from the Central Indian Ridge (CIR) of the Indian Ocean. This study presents continental (2525 Ma-173 Ma) zircons from the lower oceanic crust gabbros of the Central Indian Ridge and invokes Indian Ocean MOR mantle heterogeneity through episodic entrainment of ancient continental lithosphere of Madagascan and Gondwana origin prior to Indian Ocean opening. The geochemical features show a marked deviation from typical depleted N-MORB compositions and conform to an E-MORB affinity, which might suggest enriched lithospheric input into the depleted asthenospheric mantle. The transitional depleted to enriched mantle signature substantiates the role of lithosphere-asthenosphere interaction contributing towards accretion of lower oceanic crust gabbros beneath the CIR. The HFSE and REE compositions can be translated in terms of melt extraction by shallow level melting of a chemically heterogeneous upper mantle carrying depleted asthenospheric and recycled lithospheric components. Zircon U-Pb geochronology reveals a wide spectrum of ages ranging from 2525 Ma to 173 Ma suggesting multiple episodes of recycling of older crust into asthenospheric mantle. The continental inheritance of the dated zircon grains can be interpreted to represent trapped relics of older continental lithosphere into the Indian Ocean MOR mantle. Our results envisage delamination and recycling of older continental lithosphere into mantle asthenosphere (i) during the Mozambique ocean closure and Gondwana amalgamation at around 750 Ma, and (ii) in response to the dispersal of Gondwanaland at approx. 167 Ma ensued by opening of the Indian Ocean.
