Role of magma injection and mixing in the formation of chromitite in Archean anorthosites: Evidence from the Sittampundi Complex, southern India

Abstract

Chromitite occurs in many Archean anorthosite complexes. The Neoarchean Sittampundi Anorthosite Complex (SAC) in southern India is composed of mainly (amphibole-rich) anorthosite, minor gabbro, two pyroxene granulite, and many anorthosite-related chromitite layers and lenses. High-Mg# orthopyroxene megacrysts are present in the chromitite but absent in the anorthosite. These megacrysts are disturbed and/or corroded by Cr-spinel and interstitial amphibole into fragment, but are still largely optically continuous. The absence of orthopyroxene in the anorthosite indicates that the mineral was formed in an earlier stage in the deep magma chamber(s), and then partially remelted when it was entered into the extant magma chamber. Two types of amphiboles are recognized based on textural and trace elemental features. Amphibole-1 in the chromitites is coarse-grained, and commonly surrounded by Cr-spinel and amphibole-2, which forms resorption texture along amphibole-1 rims. Chondrite-normalized REE patterns of the amphibole-1 in chromitite and that in anorthosite have similarly positive Eu* anomalies (1.53–3.04) and low total REE contents (ΣREE = 2.90–4.42 ppm). This implies that amphibole-1 in the chromitite was captured from the anorthosite. Meanwhile, amphibole-2 is fine-grained and occurs as interstitial infill among Cr-spinel, plagioclase, and amphibole-1. Amphibole-2 shows negative to weakly positive Eu anomalies (Eu/Eu* = 0.67–1.17), and slightly elevated total REE contents (ΣREE = 5.42–14.20 ppm). The resorption textures on orthopyroxene, amphibole-1, and plagioclase, and the close paragenetic relations between Cr-spinel and amphibole-2, suggest that Cr-spinel was saturated through the mixing of the anorthositic mush with a replenished, more primitive magma. The amphibole-2 REE patterns indicate that the injected magma had partially corroded the pre-existing plagioclase and amphibole-1, and mixed with the interstitial melt in the mush. Addition of Cr2O3 from the replenished magma and SiO2 from the anorthositic crystal mush had likely pushed the melt toward the Cr-spinel saturation and triggered chromitite formation. The high Al2O3 content of Cr-spinel is likely led by plagioclase remelting, which increased the Al3+ content in the mixed magma. Comparable Cr-spinel compositions of the Archean anorthosites worldwide suggest that their chromitites were possibly formed via similar magma injection and mixing processes.