Articles written in Journal of Earth System Science

    • Paleoproterozoic ($\sim 1.88–1.89 \rm{Ga}$) ultramafic–mafic sills, Cuddapah basin, India—revisited: Implications for interaction between mantle plume and metasomatized subcontinental lithospheric mantle


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      A number of mafic–ultramafic sills, which are supposed to be part of the widespread $\sim 1.88–1.89 \rm{Ga}$ Hampi–Bastanar Large Igneous Province of the Indian shield, are reported to intrude into sedimentary/ meta-sedimentary rocks of the Proterozoic Cuddapah basin. Mineral chemistry of chrome-spinel, olivine and pyroxene from the cumulate ultramafic rocks of the Pulivendla sills, emplaced at the base of Tadpatri Formation of the Cuddpaha basin, is presented for better understanding on their nature and genesis. The intermediate Cr#, low Mg#, high $\rm{Fe^{3+}}$# and Ti (apfu) coupled with relatively low Al (apfu) content of studied spinel indicate their Alaskan-type nature. Moreover, the Ca-rich clinopyroxene together with classic variations of $\rm{Al_{2}O_{3}}$, $\rm{TiO_{2}}$ and $\rm{SiO_{2}}$ follow the trend of arc type of intrusions. The calculated $\rm{Al_{2}O_{3}}$ content of parental melt of the studied rocks also suggests arc-type characteristics. Whole-rock geochemistry show fractionated chondrite-normalized REE patterns, higher Ba/Nb and low Nb/La that suggest contribution from an enriched mantle source, whereas higher Th/Yb and negative Ta–Nb–Ti anomaly on the primitive mantle-normalized multi-element spidergrams emphasize the involvement of a subducted component in the lithospheric mantle source. Although, the mineral chemistry and geochemistry is akin to Alaskan-type intrusion, lack of concentric zoning of lithologies (olivine-rich ultramafic rocks in centre surrounded by mafic rocks), rarity of primary hornblende and abundance of orthopyroxene contradict it to be an Alaskan-type intrusion. It is suggested that the mantle source region of the Pulivendla ultramafic–mafic rocks was modified by a fluid-induced metasomatism during an ancient subduction event. Further, Al-in-olivine thermometry suggests crystallization temperature between $1410^{\circ}$ and $1484^{\circ}\rm{C}$, which is $260^{\circ}\rm{C}$ higher than the average temperature of MORB. Similarly, the estimated mantle potential temperature is also significantly higher (> $1600^{\circ}\rm{C}$) than the different secular cooling models of the earth at $\sim 1.88–1.89 \rm{Ga}$ and consistent with the thermal regime of a mantle plume. The existence of mantle plume during 1.88–1.90 Ga, which has played an important role in the genesis of mafic–ultramafic sills of the Cuddapah basin, is also well supported by a radiating mafic dyke swarm, domal uplift and magmatic underplating.

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