Plagioclase megacrysts from the shallow crystalline mush of the Deccan Traps

We analyzed the mineral composition of Giant Plagioclase Basalt flows from the Deccan large igneous province. In these flows typically occurring at the transition between geochemically distinct volcanic formations, plagioclase occurs as mm-to cm-seized crystals and crystal clots, which sometimes show evidence of high-temperature deformation, as constrained by EBSD analyses. Plagioclase have anorthite contents generally in the range An65-An60, sometimes showing relatively high-An cores (ca. An75). Clinopyroxene has mainly augitic and rarely pigeonitic composition and generally crystallized at shallow depth. Minor and trace element compositions of plagioclase crystals are correlative to those of their host-rocks. Most core-rim profiles of rapidly diffusing elements (e.g., Mg) are flat, suggesting diffusive re-equilibration. On the contrary, slower diffusing elements like K and Fe are zoned in some samples. In particular, Fe shows a marked increase at the crystal rims. Sr isotopic compositions of plagioclase cores are slightly but significantly different from those of the crystal rims and of the surrounding matrix in several samples.

These geochemical and textural characteristics of the analyzed plagioclase suggest that they derived from a crystalline mush crystallized and stored for variably long periods (years to centuries) in the shallow magmatic plumbing system, which was repeatedly flushed and partially deformed by magma rising from deeper levels. Finally, plagioclase-rich magmas became cargos of Fe-rich, dense magmas possibly mobilized by increased CO2 contents heralding the arrival of e mantle-derived magmas.