Constraints on the mantle sources of the Deccan traps from the petrology and geochemistry of the basalts of Gujarat state (Western India)

Abstract

The late Cretaceous-early Tertiary flood basalts in the Gujarat area of the northwestern Deccan Traps (Kathiawar peninsula, Pavagadh hills and Rajpipla) exhibit a wide range of compositions, from picrite basalts to rhyolites; moreover, the basaltic rocks have clearly distinct TiO₂ contents at any given degree of differentiation and strongly resemble the low-titanium and hightitanium basalts found in most of the Gondwana continental flood basalt (CFB) suites. Four magma groups are petrologically and geochemically distinguished: (1) A low-Ti group, characterized by rocks with varying SiO₂ saturation, and with TiO₂ <1.8 wt%, extremely low incompatible trace element abundances, low Zr/γ (av- 3.8), Ti/ V (av. 27), and a very slight large ion lithophile element (LJLE) enrichment over high field strength elements (HFSE). These rocks share some features with the Bushe Formation of the Western Ghats farther south, but have distinct geochemical characters, in particular the strong depletion in most incompatible trace elements. (2) A high-Ti group, characterized by a more K-rich character than the low-Ti rocks, and with a strong enrichment in incompatible elements, similar to average ocean island basalt (OIB), e.g. high TiO₂ (>1.8 wt% in picrites), Nb (>19 p.p.m.) Zr/γ (av. 6.5) and Tt/V (av. 47). (3) An intermediate-Ti group, with TiO₂ contents slightly lower than the high-Ti rocks at the same degree of evolution, and with correspondingly lower incompatible trace element contents and ratios, in particular K₂O, Nb, Ba and Zr/Y (av. 5.2). (4) A potassium-rich group (KT), broadly similar in geochemical character to the high-Ti group but showing more extreme K, Rb and Ba enrichment (av. K₂0/Na₂0~l; Ba/Y~20). The most primitive low-Ti and high-Ti picrites, when corrected for low-pressure olivine fractionation, show distinct major (and trace) element geochemistry, in particular for CaO/AI₂O₃, CaO/TiO2 and Al₂O₃/TiO₂, and moderate but significant variations in their SiO₂ and Fe₂O_st contents; these characteristics strongly suggest the involvement of different mantle sources, more depleted for the low-Ti picrites, and richer in cpxfor the high-Ti picrites, but with broadly the same pressures of equilibration (27-14 kbar). This, in turn, suggests a strong lateral heterogeneity in the Gujarat Trap mantle. Low-Ti picrites and related differentiates in Kathiawar are reported systematically for the first time here, and suggest the existence of HFSE-depleted mantle in the northwestern Deccan Traps, with extension at least to the Seychelles Islands and to the area of the Bushe Formation near Bombay in the pre-drift position, before the development of the Carlsberg Ridge. The absence of correlations between LILE/HFSE ratios and SiO₂ argues against crustal contamination processes acting on the low-Ti picrites, possibly owing to their probably rapid uprise to the surface. Consequently, the mantle region of this rock group was probably re-enriched by small amounts of ULE-rich materials. The substantially higher, trace element enrichment of the least differentiated high-Ti picrites, relative to the basalts of the Ambe-noli and Mahableshwar Formations of the Western Ghats, testifies also to the presence of more incompatible element rich, OIB4ike mantle sources in northern and northwestern Gujarat. These sources were geochemicaily similar to the present-day Reunion mantle sources.