Genesis of aluminous and intermediate granulites: A case study in the eastern Sierras Pampeanas, Argentina

Abstract

We report petrologic and geochemical features of anhydrous granulites from the eastern Sierras Pampeanas, Argentina. Aluminous Grt–Crd-bearing granulite is the most abundant rock type in the study region, and hosts intermediate Opx–Grt bearing granulites as interlayered lenses. Both granulite types crystallized under similar P– T conditions of T = 850–900 °C and P c. 7.5 kbar. The formation of Grt–Crd granulites by subsolidus dehydration conflicts with the absence of K-feldspar in them. Moreover, the aluminous granulites have an REE pattern that precludes postulating they derived by close-system metamorphism of an extensively weathered protoliths. Comparing mineral and rock chemistry with experimental results in simple systems (ACFM, AKF, and AFM) permits the mineralogical changes and the bulk composition evolution experienced by granulites during melting to be identified. This work demonstrates that orthopyroxene is a primary phase in Opx–Grt granulites, however never crystallized within aluminous granulites. The aluminous granulites have both mineral and chemical compositions matching those expected for a residuum derived from the partial melting of a greywacke-like sedimentary precursor. Compared with the composition of the average continental crust, aluminous granulites are depleted in most lithophile trace elements and LREE, have somewhat low K / Rb ratios, and high contents of elements having high partition coefficients for garnet and/or ilmenite (HREE—Y, Cr, Ni). However, aluminous granulites have initial Sr isotope ratios (< 0.708) significantly lower than those measured in the rest of the metasedimentary rocks of the region (> 0.713). We discuss the possibility that an intense but short-lived melting event led to isotopic disequilibrium during melting of biotite-bearing protoliths, leaving a granulitic residuum with lower initial Sr isotope ratios. In contrast, Opx–Grt granulites have trace element features (e.g., high Sr contents and REE patterns with marked positive Eu anomalies) that do not support the idea that they are simply residues left after biotite-dehydration melting and melt extraction. Rather, these rocks display several petrological, major and trace element and isotopic features that suggest they formed through the interaction of mafic magmas and aluminous granulites.