Trace element and Sr-Nd-Hf-Pb isotope evidence for a multi-magma chamber system beneath the Deccan Volcanic Province, India

Abstract

With a size of over 500,000 km2, the Deccan Volcanic Province (DVP) covers a large area of the Indian subcontinent. Stratigraphically continuous lava flows piled up to 3000 m record about five million years of volcanic activity. The peak activity of one million years coincides with the migration of the Indian subcontinent across the Réunion plume. Previous studies inferred an isotopic “common signature” for most DVP basalts that is best explained by a mixture of early Réunion plume-related melts and melts from the depleted upper mantle. Further chemical variability of the DVP sequence was explained by the incorporation of material from the lithospheric mantle and continental crust possibly within a single magma chamber system. Yet, indications for a complex crustal magmatic system have also been brought forward by earlier studies and find recently more support in explaining the magmatic plumbing system of the DVP and Large Igneous Provinces in general.This study presents new major and trace element data, including high-precision concentration measurements of high-field strength elements by isotope dilution and Sr-Nd-Hf-Pb isotope compositions for basaltic samples that cover nearly the complete DVP main section. Based on significant variations in Nb/Th (and Th/Ta) over a wide range of MgO concentrations we identified three distinct magma differentiation trends. The distinct differentiation trends result from three types of parental melts that are best explained by different proportions of lower and upper mantle melts indicated by diluted Réunion plume-like trace element and isotope compositions. During their ascent, these separate melts assimilated variable amounts of metasomatized lithospheric mantle and crustal material. Most likely the assimilation occurred before any major differentiation events, thus fixing the Nb/Th (and Th/Ta) of each group regardless of the degree of differentiation. The chemical trends can also be replicated using a statistical numerical approach, confirming that their geochemical signature truly reflects individual differentiation of three distinct parental melts. Field relations require that lavas related to these three unique differentiation trends erupted contemporaneously. This is in contrast to the idea of a gradually evolving single magma chamber system and the concept of a general chemostratigraphy for the DVP main plateau. Rather, our study reveals fast-changing contributions of different mantle reservoirs and provides new insight into the magma-plumbing systems of Large Igneous Provinces that may be more complex.