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

Abstract

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 (Na 2O-CaO-K 2O-FeO-MgO-Al 2 O 3-SiO 2-H 2O-TiO 2- Fe 2O 3) 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 X Al [Al 2O 3/(Na 2O + CaO + K 2O + FeO + MgO + Al 2O 3) < 0.35, mole proportion] or high X Mg [MgO/(MgO + FeO) > 0.85] felsic-metapelitic rock types. This study also reveals that the X Al 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. © 2007 Blackwell Publishing Ltd.