Cretaceous potassic igneous activity in the Raniganj Basin, eastern India: Compositional variations in mica and apatite as recorders of lamproite magma evolution

Abstract

Two early cretaceous potassic sill intrusions from ramnagore and rajpura in the raniganj gondwana sedimentary basin of eastern india consist of macrocrysts and microcrysts of olivine, together with phenocrysts and microphenocrysts of mica, apatite and diopside set in a groundmass of mica, diopside, k-feldspar, apatite and rutile. Titanian magnesian aluminous chromite occurs as a groundmass phase in the ramnagore rocks. The compositional characteristics of olivine are similar to those of peridotite-derived mantle xenocrysts in terms of their high forsterite and ni contents coupled with low mn, ca, al and ti. Mineralogically, the sill intrusions are classified as phlogopite–diopside–k-feldspar–apatite olivine lamproites. Mica and apatite crystals commonly show core–overgrowth rim textures with up to three distinct compositional zones. The mica zones are m-i core: high ti, low-to-high cr and moderate ba; m-ii inner rim: moderate ti, low cr and high ba; and m-iii outer rim: low ti, cr and ba. Rajpura mica show a fourth zone (m-iv), which occurs as patches at the crystal margins, characterized by higher fet and lower k compared to the other three zones. M-i phlogopite is interpreted to be “antecrystic”, whereas m-ii and m-iii mica represent magmatic growth zones. M-iv mica has probably developed during hydrothermal alteration. Both the rajpura and ramnagore mica exhibit a trend of decreasing al2o3 with increasing feot, which is typical for bona fide lamproites. Rajpura apatite crystals show three distinct compositional zones, viz. Ap-i core: high-sr–low-lree; ap-ii inner rim: high-sr–high-lree; and ap-iii microporous outer rim: moderate-sr–low-lree and considerable quantities of ba and na. The ap-i and ap-ii generations represent two distinct magmatic growth zones, whereas ap-iii developed during hydrothermal alteration. Ramnagore apatite exhibit oscillatory zoning with fluctuations in sr and lree contents. Diopside has al–na-poor compositions, typical of lamproitic clinopyroxene. The study demonstrates how the mineralogy of lamproites records crystallization from multiple pulses of mantle-derived k-rich magmas, with protracted magmatic differentiation and late-stage hydrothermal evolution. © 2022 elsevier b.v.