Compositional Variations in Mica and Apatite as Recorder of Lamproite Magma Evolution: Evidence from Eastern India

Mineralogical study of two early Cretaceous potassic intrusions from Ramnagore and Rajpura in the Raniganj Gondwana sedimentary basin in eastern India reveals that macrocrysts and microcrysts of olivine, phenocrysts and microphenocrysts of mica, apatite and diopside are 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. Compositional characteristics of olivine are comparable to those of mantle peridotite-derived xenocrysts in terms of their high forsterite and Ni contents coupled with low Mn, Ca, Al and Ti contents. Mineralogically, the intrusions are classified as phlogopite-apatite-diopside-K-feldspar lamproites. Micas and apatites 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 micas show a fourth zone M-IV, which occurs as patches at the grain margin with high FeT and low K compared to the preceding zones. M-I phlogopite is interpreted to be "antecrystic", while M-II and M-III micas are magmatic growth zones. M-IV mica has developed by hydrothermal alteration. Both Rajpura and Ramnagore micas exhibit a trend of decreasing Al2O3 with increasing FeOT which is typical of micas in bona fide lamproites. Rajpura apatites show three distinct compositional zones, viz. Ap-I core: moderate Sr and low LREE; Ap-II inner rim: high Sr and LREE; and Ap-III microporous outer rim: low Sr, negligible LREE and considerable Ba and Na. The Ap-I and Ap-II apatites are two magmatic growth zones, while Ap-III has resulted from hydrothermal alteration. Ramnagore apatites exhibit oscillatory zoning with fluctuations in Sr and LREE contents. Diopsides from both the intrusions have Al-Na-poor compositions, typical of lamproitic clinopyroxenes. The study demonstrates how the final mineral contents and compositions in lamproites is the result of contributions from multiple pulses of magma originating at mantle depths, crystallization of minerals at different stages of magmatic differentiation, and late- to post-magmatic hydrothermal alteration.