Ultrahigh‐Temperature Anatexis of Metapelites Generates Near‐Anhydrous Garnet Granitoids

Abstract

AbstractIt remains unresolved whether extensive granitoids can originate from ultrahigh‐temperature crustal anatexis beyond the stability of fusible hydrous minerals and, if any, how they are generated and characterized. Here we investigate near‐anhydrous garnet granitoids in the Jining Complex, North China, using bulk‐rock and mineral analyses combined with phase equilibria modeling. The Jining garnet granitoids are parautochthonously emplaced within granulite terranes and diagnostically contain perthite, abundant garnet but minor biotite. The rocks exhibit high maficity (FeOT + MgO) of 4–12 wt.%, minimal H2O contents below ∼0.16 wt.% and limited aluminum saturation indexes within 1.1–1.7. Their magmas are scarcely fractionated and have ultrahigh temperatures of 1000–1100°C as revealed by K‐feldspar rich in anorthite (10–13 mol.%) and Ti (175−276 ppm), and garnet rims depleted in heavy rare‐earth elements that crystallize from melts. Besides, these magmas have deficient H2O contents (<∼0.19 wt.%) and high densities and viscosities. The garnet granitoids are generated by metapelite anatexis at ultrahigh‐temperature conditions likely around ∼1100°C, with selective entrainment of 5–40 vol.% residua, and underwent some melt loss during magma crystallization. The melting temperatures are far beyond biotite stability and close to the crustal dry solidus. Garnet‐involved congruent melting reactions are required to produce the maficity‐rich and H2O‐deficient melts with high capability for entraining residua. The ultrahigh‐temperature garnet granitoids differ from ordinary low‐ and high‐temperature garnetiferous granites in both petrologic features and petrogenesis, and represent a new subcategory of S‐type granitoids. They can indicate mantle‐actuated extreme crustal thermal perturbations.