Orthopyroxene-bearing granitoids of both magmatic and metamorphic origin occur in various terranes of the world, although their petrogenetic distinction remains equivocal. Here we investigate an example of unmetamorphosed magmatic charnockites (enderbite-charnockite) of the Late Paleoproterozoic Luyashan pluton in the North China Craton, where both charnockitic (granitic) and enderbitic (granodioritic) granitoids occurs in the pluton. In a thin section scale, charnockitic domains (~1–2 cm) are present within the outcrop scale enderbitic granitoid, that are composed of orthopyroxene, biotite, plagioclase, K-feldspar, quartz, ilmenite, magnetite, and pyrrhotite. Whereas the enderbite domain shows amphibole, clinopyroxene and apatite additionally. Mineral textures and orthopyroxene inclusions in amphibole and biotite preserved early stage orthopyroxene from both domains, where they carry pure endmember ilmenite and magnetite as the major oxide phases with similar compositions. Log (aH2O) - Temperature pseudosection modeling at log fO2 = −15 and pressure 800 MPa shows that orthopyroxene might be crystallized at ~900 °C. Subsequently, the mixed system achieved a higher water activity due to CO2 escape during cooling. During cooling, both the enderbitic and charnockitic domain are stable at a water content of >1 wt% or water activity (aH2O) of 0.4 or above. Our results suggest that mixing of immiscible granodioritic and granitic magma caused the formation of the enderbitic and charnockitic domains at similar water activity. Orthopyroxene inclusions in amphibole and biotite, lack of dehydration and oxidation textures, and absence of pyrite and apatite with monazite precipitates in the Luyashan igneous charnockites suggest that these rocks are significantly different from their metamorphic counterparts. Our study suggests that orthopyroxene crystallization in igneous charnockites is mainly dictated by the temperature and pressure at which magma starts crystallizing, rather than water activity. However, in the case of metamorphic charnockites, in the absence of partial melting, metasomatic orthopyroxene formation depends on low water activity of fluids and their dissolution capacity.
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© 2020 Elsevier B.V.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology