CONSTRAINTS ON THE MANTLE SOURCES OF THE DECCAN-TRAPS FROM THE PETROLOGY AND GEOCHEMISTRY OF THE BASALTS OF GUJARAT STATE (WESTERN INDIA)

The bate Cretaceous-early Tertiary flood basalts in the Gujarat area o f the northwestern Deccan Traps (Kathiawar peninsula, Pavagadh hills a nd Rajpipla) exhibit a wide range of compositions, from picrite basalt s to rhyolites; moreover, the basaltic rocks have clearly distinct TiO 2 contents at any given degree of differentiation and strongly resembl e the low-titanium and high-titanium basalts found in most of the Gond wana continental flood basalt (CFB) suites Four magma groups are petro logically and geochemically distinguished: (1) A low-Ti group, charact erized by rocks with varying SiO2 saturation, and with TiO2 <1.8 wt %, extremely low incompatible trace element abundances, low Zr/Y (av. 3. 8), Ti/V (av. 27), and a very slight large ion lithophile element (LIL E) enrichment over high field strength elements (HFSE). These rocks sh are some features with the Bushe Formation of the Western Ghats farthe r south, but have distinct geochemical characters, in particular the s trong depletion in most incompatible trace elements. (2) A high-Ti gro up, characterized by a more K-rich character than the low-Ti rocks, an d with a strong enrichment in incompatible elements, similar to averag e ocean island basalt (OIB), e.g. high TiO2 (>1.8 wt % in picrites), N b (>19 p.p.m.) (av. 6.5) and Ti/V (au. 47). (3) An intermediate-Ti gro up, with TiO2 contents slightly lower than the high-Ti rocks at the sa me degree of evolution, and with correspondingly lower incompatible tr ace element contents and ratios, in particular K2O, Nb, Ba and Zr/Y (a v. 5.2). (4) A potassium-rich group (KT), broadly similar in geochemic al character to the high-Ti group but showing more extreme K, Rb and B a enrichment (au. K2O/Na2O similar to 1; Ba/Y similar to 20). The most primitive low-Ti and high-Ti picrites, when corrected for low-pressur e olivine fractionation, show distinct major (and trace) element geoch emistry, in particular for CaO/Al2O3, CaO/TiO2 and Al2O3/TiO2, and mod erate but significant variations in their SiO2 and Fe2O3t contents; th ese characteristics strongly suggest the involvement of different mant le sources more depleted for the low-Ti picrites, and richer in cpx fo r the high-Ti picrites, but with broadly the same pressures of equilib ration (27-14 kbar). This, in turn, suggests a strong lateral heteroge neity in the Gujarat Trap mantle. Low-Ti picrites and related differen tiates in Kathiawar are reported systematically for the first time her e, and suggest the existence of HFSE-depleted mantle in the northweste rn Deccan Traps, with extension at least to the Seychelles Islands and to the area of the Bushe Formation near Bombay in the pre-drift posit ion, before the development of the Carlsberg Ridge. The absence of cor relations between LILE/HFSE ratios and SiO2 argues against crustal con tamination processes acting on the low-Ti picrites, possibly owing to their probably rapid uprise to the surface. Consequently, the mantle r egion of this rock group was probably re-enriched by small amounts of LILE-rich materials. The substantially higher trace element enrichment of the least differentiated high-Ti picrites, relative to the basalts of the Ambenali and Mahableshwar Formations of the Western Ghats, tes tifies also to the presence of more incompatible element rich, GIB-lik e mantle sources in northern and northwestern Gujarat. These sources w ere geochemically similar to the present-day Reunion mantle sources.