Evidence and timing of metasomatism of the lithospheric mantle before large-scale Deccan magmatism: Insights from the phlogopite-spinel-wehrlite xenoliths from Sarnu-Dandali alkaline igneous complex, Rajasthan, northwestern India

Wehrlite xenoliths recovered from lamprophyre dykes from Sarnu-Dandali alkaline igneous complex record three distinct events of modification in the mantle below northwest India. The first event marks mantle metasomatism by carbonate-rich fluids from the upwelling mantle, which formed secondary olivine and clinopyroxene at the expense of orthopyroxene, in mantle peridotite with little or no orthopyroxene. The phlogopite veining characterizes the second event in the mantle during rifting. These veins are formed by the crystallization of upwelling silica undersaturated alkaline fluids along the fractures and grain boundaries in the carbonated metasomatized mantle. Petrographic observations reveal separate phlogopite veins and carbonate infiltrations where carbonate activity preceded the phlogopite veining. The final event is an eruption of lamprophyres and mela-nephelinites in the study area at the later phase of the Deccan eruption, transporting wehrlite xenoliths to the surface. Estimated temperatures range from 911 degrees to 940 degrees C and pressure from 15.6 to 19.3 kbar with an average of 18.5 kbar, showing their derivation from a shallower depth of subcontinental lithospheric mantle (56-67 km). Separation of Greater-Seychelles from India at c. 68.5 Ma under extensional tectonic settings led to the lithosphere thinning and formation of wehrlite xenoliths in the rift margins.

Automated summary

Wehrlite xenoliths from northwest India record three events of modification in the mantle, including mantle metasomatism, phlogopite veining during rifting, and eruption of lamprophyres and mela-nephelinites. These events are related to the separation of Greater-Seychelles from India.