Quantitative P–T–X constraints on orthopyroxene-bearing high-pressure granulites in felsic–metapelitic rocks: evidence from the Huangtuling granulite, Dabieshan Orogen

High-pressure granulites are generally characterized by the absence of orthopyroxene. However, orthopyroxene is reported in a few high-pressure, felsic–metapelitic granulites, such as the Huangtuling felsic high-pressure granulite in the North Dabie metamorphic core complex in east-central China, which rarely preserves the high-pressure granulite facies assemblage of garnet + orthopyroxene + biotite + plagioclase + K-feldspar + quartz. To investigate the effects of bulk-rock composition on the stability of orthopyroxene-bearing, high-pressure granulite facies assemblages in the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) system, a series of P–T–X pseudosections based on the melt-reintegrated composition of the Huangtuling felsic high-pressure granulite were constructed. Calculations demonstrate that the orthopyroxene-bearing, high-pressure granulite facies assemblages are restricted to low XAl [Al2O3/(Na2O + CaO + K2O + FeO + MgO + Al2O3) 0.85] felsic–metapelitic rock types. This study also reveals that the XAl values in the residual felsic–metapelitic, high-pressure granulites could be significantly reduced by a high proportion of melt loss. We suggest that orthopyroxene-bearing, high-pressure granulites occur in residual overthickened crustal basement under continental subduction–collision zones and arc–continent collision belts.