Age and geochemistry of carbonatites and associated silicate rocks in Sarnu-Dandali alkaline complex, Deccan Large Igneous Province: Evidence for melt immiscibility and plume-lithosphere interaction

Abstract

Carbonate–silicate liquid immiscibility is the most widely accepted petrogenetic model for explaining the origin of carbonatites, accounting for approximately three-quarters of global carbonatite occurrences. Many, however, attribute the coexistence of carbonatites and alkaline silicate rocks to the coincidental emplacement of two independent parental magmas through a single crustal conduit. Thus, the exact cause of the carbonatite-alkaline silicate rock association remains equivocal. Here, we present the results of ⁴⁰Ar/³⁹Ar dating, geochemical, and C-O-Sr–Nd-Pb isotopic investigations of the Sarnu-Dandali carbonatite-alkaline complex, part of the Deccan Large Igneous Province (LIP), to shed light on this coexistence. Additionally, we explore the roles of the Deccan-Reunion mantle plume and the Indian continental lithosphere in generating the carbonatites of the complex. ⁴⁰Ar/³⁹Ar age data reveal that, although the complex underwent multiple cycles of alkaline magmatism, the activity at ~ 68.8 Ma was synchronous with the carbonatite intrusion. Interlaced spatial association of carbonatites and alkaline silicate rocks, including melt inclusions of the former in the latter, their complementary trace element patterns (and δ¹³C and δ¹⁸O), and overlapping initial Sr–Nd-Pb isotopic ratios indicate their co-genesis through carbonate–silicate liquid immiscibility. Trace element and isotopic ratio modeling suggests that the parental melanephelinitic magma for the complex evolved through concurrent crustal assimilation (up to 6%), fractional crystallization of silicates, and immiscible separation of carbonate melt, with the latter occurring before phonolite crystallization. The least contaminated isotopic ratios suggest that the parental carbonated-silicate magma derived from a mantle source with a mixed signature of the Reunion plume and metasomatized continental lithosphere.