Origin of the granulite enclaves in Indo-Sinian peraluminous granites, South China and its implication for crustal anatexis

Metasedimentary granulite enclaves hosted in the Indo-Sinian Darongshan–Shiwandashan granite belt from the southeastern Guangxi province, South China, experienced a three-stage metamorphic evolution: (1) the early prograde metamorphism in the stability field of sillimanite at ~800°C; (2) the peak metamorphism with a mineral assemblage of spinel+quartz at 7.5–8.0kbar and 950–1000°C (260–250Ma); and (3) a near-isothermal decompression stage with orthopyroxene+plagioclase and orthopyroxene+cordierite symplectic textures at 3.2–3.7kbar and 790–820°C (240–230Ma). The clockwise P–T–t path suggested that the granulite enclaves have undergone initial crustal thickening and subsequent rapid exhumation and cooling history. The granulite enclaves show peraluminous affinities (A/CNK=1.11–3.14, A/CNK=Al2O3/(CaO+Na2O+K2O) in molecular ratio) with Al2O3 up to 25.31%; chondrite-normalized REE patterns similar to those of upper continental crust; relative Rb, Ba, Th, LREE, K, Zr and Hf enrichment and Sr, Nb, Ta, P and Ti depletion in primitive mantle-normalized spidergrams; and highly radiogenic Sr (87Sr/86Sr(i)=0.71295–0.74273 ) and nonradiogenic Nd (εNd(t)=−14.8 to −11.2). Systematic elemental variation trends from the granulite enclaves to the host granites, and similar Sr–Nd isotope compositions in the enclaves and the host granites, indicate that the two have a coherent origin — the granulites were restites of the host granites. The granulite enclaves can also be subdivided into two groups: high-87Sr/86Sr(i) (>0.7303) and low-87Sr/86Sr(i) groups (