Isotopic and geochemical studies of ancient-modern subduction processes and the Deccan plume volcanism

This study examines geochemical characteristics of rocks from different geographic and plate-tectonic settings that include an inferred Archean subduction zone in the Eastern Indian Craton, the Mesozoic Franciscan Subduction Complex from coastal California, the Quaternary subduction in the Kamchatka peninsula, Russia, and the Cretaceous-Tertiary Deccan flood basalt province in the Indian peninsula.; Trace element geochemical characteristics and Nd-isotopic studies of stratigraphically-controlled, little metamorphosed sandstones from the Eastern Indian Archean craton document that these sandstones formed by bimodal mixing of amphibolites and tonalites, the oldest rocks (∼3.3 Ga) in the craton. The amphibolites and tonalites originated by subduction processes in the Archean, which combined with the sandstones' depleted mantle Nd model ages of 3.6--4.0 Ga indicate the presence of older crust of these ages in this craton.; Sr-, Nd-, Pb-isotopic ratios and major and trace element studies of the high-grade tectonic blocks of the Franciscan Subduction Complex indicate that the protoliths of these rocks were are tholeiites without any detrital continental crustal component. The tectonic model developed implies that these arc tholeiites had formed in a subduction zone different from the east-dipping Franciscan subduction and were the first to be subducted when the Franciscan subduction was initiated.; Os-, Nd-, and Pb-isotopic signatures of sub-arc mantle xenoliths from Kamchatka suggest that this mantle wedge was metasomatized by fluids from the subducting slab. Os was scavenged from the mantle by slab-devolatilization leaving the Os-imprint of the slab-fluids on the mantle xenoliths. Nd- and Pb-isotopes of the xenoliths are similar to those of the Pacific MORB. The Nd-isotopes are possibly the original signature of the mantle wedge while the fluid from the subducting slab has affected the Pb-isotopes.; Study of major and trace elements and the Sr-, Nd-, Pb-, and Hf-isotopes of the Deccan flood basalts show that the lower lava sequences of the Deccan was formed by high-degree partial melting of subducted ∼2.6 Ga oceanic crust that was incorporated in the ascending plume. In contrast, the stratigraphically higher part of the Deccan lava suites were formed by lower-degree partial melting of the lower-mantle derived plume which was possibly contaminated by the source-rocks of the lower subgroups.