Orthopyroxene megacrysts from the Chilka Lake anorthosite massif, Eastern Ghats, India: a clue to magmatic evolution

The present study reports the occurrence of orthopyroxene megacrysts from the Chilka Lake anorthosite massif, Eastern Ghats, India. An insight into the mineral chemistry of different phases, coupled with detailed field and petrographic evidences from this study, shed light on a long debate on the origin of orthopyroxene megacrysts in anorthosite massifs. The megacrysts contain exsolved lamellae of plagioclase and opaque oxides (ilmenite, rutile) oriented along orthopyroxene cleavage planes. The trace element distribution patterns of the megacryst and matrix plagioclase are mirror reflections of each other and mutually complementary. The calculated compositions of melts in equilibrium with these two phases show comparable patterns for LREE (light rare earth elements, La-Sm), but differ markedly in terms of HREE (heavy rare earth elements, Eu-Lu), suggesting that the megacrysts and matrix plagioclases did not crystallize simultaneously. We infer that the orthopyroxene megacrysts have a longer crystallization history, initially as a low-Ca non-quad member of the pyroxene group at pressure >= 10 kbar, incorporating some amount of Ca, Al and Ti in their structure. Subsequently, they have been carried by a plagioclase crystal mush to mid-crustal levels at pressure similar to 4-6 kbar following a near-isothermal decompression that may be linked to the emplacement of the anorthosite massif, giving rise to the exsolution lamellae of plagioclase and opaque oxides.