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VOLCANISM AND INTRUSIONS OF THE DECCAN TRAPS, INDIA: GEOCHEMISTRY AND GEOCHRONOLOGY OF THE MAGMATIC ROCKS AND PALEOENVIRONMENTAL CONSEQUENCES

The Deccan Traps are one of the most important Large Igneous Provinces (LIP) in the world, they are mainly constituted by tholeiitic lava flows, now covering almost one sixth of the Indian continent, and reaching the maximum exposed thickness of 1300 m. The emplacement of such quantities of magma and its timing (ca. 66Ma) close to the Cretaceous-Paleogene (K-Pg) boundary, have led several authors to propose a causal link between the formation of the province and the K-Pg boundary mass extinction . Moreover, a distinctive feature of the province is the presence, beside the tholeiites, of several alkaline bodies, associated with the main fault zones of India. 
These two important aspects of the Deccan Traps have been investigated in the northern portion of the province by sampling both alkaline and tholeiitic rocks, in order to provide precise timing of emplacement and to constrain the relationship between them through the definition of their mantle source. 
40Ar/39Ar step-heating analyses provided two different age peaks that straddle the K-Pg boundary, one with an age comparable with the main phase of Deccan volcanism (ca 66.5 Ma), the other slightly younger (ca. 65.2 Ma), thus confirming the synchrony between the emplacement of the province and the end-Cretaceous mass extinction. Therefore, the input in the atmosphere of huge quantities of gas, produced by the emplacement of the Deccan Traps, could have contributed to the extinction. In particular a crucial role may be provided in this sense by the alkaline magmas, which are likely enriched in volatile elements. 
Whole rock analyses showed that the samples span a wide compositional range varying from little-evolved compositions such as picro-basalts to fairly evolved ones such as rhyolite; the large variation is observed in the total alkali content (Na2O+K2O) as well, ranging from the subalakaline basalt field, up to strongly alkaline samples like phonolites. Alkaline rocks appear enriched in the most incompatible elements and present higher La/Yb ratios (19.5-68.8). They are also characterized by negative K anomaly (possibly suggesting the presence of a K-rich residual mantle mineral), and Pb spikes; on average they present patterns similar to those displayed by other alkaline rocks of the Deccan Traps, thus suggesting analogous origin and source. 
Sr-Nd isotopes define a large spectrum of compositions, departing from a similar depleted end-member (eNdt ca. +3 and 87Sr/86Srt ca. 0.705) and trending towards low eNdt and relatively low 87Sr/86Srt (-12.96 and 0.71061, respectively), and toward very high 87Sr/86Srt (0.72788) and low eNdt (-12.50). The trend with higher 87Sr/86Srt has been interpreted as the result of Assimilation- Fractional Crystallization (AFC) process, starting from a magma similar to Reunion OIBs or Central Indian Ridge basalts progressively contaminated by rocks from the Indian cratons (Dharwar and Aravalli cratons). The trend towards low eNdt and relatively low 87Sr/86Srt is defined mainly by mafic sulbalkaline rocks from the Phenai Mata intrusion. These rocks are characterized also by very high 207Pb/204Pbt and 208Pb/204Pbt ratios. Given the preliminary 187Os/188Ost compositions (0.1584 to 0.2457), mafic subalkaline rocks assimilated only negligible amounts of crust. Their enriched Sr-Nd-Pb isotopic composition is thus best interpreted as resulting from recycling of ancient sediments in their mantle source. On the contrary, alkaline samples present quite homogeneous isotopic compositions, slightly more enriched than that of nearby carbonatite complexes (e.g. Amba Dongar) and substantially more enriched than Reunion basalts. Therefore, the alkaline rocks are unlikey a product of the Reunion mantle plume, but more probably of the subcontinental lithospheric mantle, as is consistent also with their incompatible trace element contents and patterns.