An overview on geochemistry of Proterozoic massif-type anorthosites and associated rocks

Abstract

A critical study of 311 published WR chemical analyses, isotopic and mineral chemistry of anorthosites and associated rocks from eight Proterozoic massif anorthosite complexes of India, North America and Norway indicates marked similarities in mineralogy and chemistry among similar rock types. The anorthosite and mafic-leucomafic rocks (e.g., leuconorite, leucogabbro, leucotroctolite, anorthositic gabbro, gabbroic anorthosite, etc.) constituting the major part of the massifs are characterized by higher Na2O + K2O, Al2O3, SiO2, Mg# and Sr contents, low in plagioclase incompatible elements and REE with positive Eu anomalies. Their δ18O%(5.7-7.5), initial 87Sr/86Sr (0.7034-0.7066) and εNd values (+1.14 to +5.5) suggest a depleted mantle origin. The Fe-rich dioritic rocks occurring at the margin of massifs have isotopic, chemical and mineral composition more close to anorthosite-mafic-leucomafic rocks. However, there is a gradual decrease in plagioclase content, An content of plagioclase and XMg of orthopyroxene, and an increase in mafic silicates, oxide minerals content, plagioclase incompatible elements and REE from anorthosite- mafic-leucomafic rocks to Fe-rich dioritic rocks. The Fe-rich dioritic rocks are interpreted asresidual melt from mantle derived high-Al gabbro melt, which produced the anorthosite andmafic-leucomafic rocks. Mineralogically and chemically, the K-rich felsic rocks are distinct fromanorthosite-mafic-leucomafic-Fe-rich dioritic suite. They have higher δ18O values (6.8-10.8%) and initial 87Sr/86Sr (0.7067-0.7104). By contrast, the K-rich felsic suites are products of melting of crustal precursors. © Indian Academy of Sciences.