Charnockites: Petrologic and phase equilibria characterization of some global examples

Charnockites (orthopyroxene-bearing granitoids) constitute one of the major components in many Precambrian high-grade metamorphic terranes of the world. They are broadly classified into two types based on the nature of occurrence as massive charnockite that forms large (magmatic) batholiths, and incipient charnockite which occurs as patches, veins and lenses of mesoscopic scale developed through local dehydration reactions within felsic (both igneous and sedimentary) protoliths. Here we present petrological data on representative charnockites of different ages and tectonic settings from various parts of the world and evaluate their formation conditions based on phase equilibrium modeling in the system NCKFMASHTO. Massive charnockites ranging from Neoarchean to Paleoproterozoic age from the Salem and Nagercoil Blocks (southern India), Napier Complex (Antarctica), and the Limpopo Complex (Zimbabwe) record P-T conditions that are broadly equivalent to the regional P-T conditions of high-grade metamorphism recorded in these terranes, ranging from normal granultes to ultrahigh-temperature metamorphic rocks. Typical unmetamorphosed magmatic charnockite in a late Paleoproterozoic post-collisional extensional setting and carrying K-feldspar phenocrysts from the North China Craton yield a wide P-T stability range. Incipient charnockite from southern India records P-T conditions that are slightly lower than the peak P-T regimes estimated for the region. Regardless of the discrepancies in P-T conditions estimated from the phase equilibria approach, the data confirm that these rocks, in all the cases considered in this study, were generated under low aH2O conditions. The available fluid inclusion data from both massive and incipient charnockites confirm that the water activity was buffered to low levels, possibly through the presence of CO2-rich fluids, to stabilize the index mineral orthopyroxene in these rocks. Charnockites formed in different plate tectonic settings; in most cases as arc magmas within subduction-collision settings and some cases, post-collisional extension during different periods in Earth history, and serve as potential archives of melt and fluid processes in the continental crust.