Partial Melts of Intermediate–Felsic Sources in a Wedged Thickened Crust: Insights from Granites in the Sulu Orogen

Abstract

Abstract High-pressure (>15 kbar) melts of intermediate–felsic materials have been well studied by experiments, whereas their existence in nature, especially in orogenic belts, is rarely examined. With the aim of identifying and characterizing high-pressure partial melts of intermediate–felsic continental crusts, this study presents comprehensive geochemical and geochronological data for 47 Jurassic granites (166∼157 Ma) from the Sulu orogen. These Sulu Jurassic granites (SJG) consist of quartz, K-feldspar and plagioclase with minor mineral assemblages of biotite ± muscovite ± garnet ± epidote ± allanite. Their low mafic mineral abundance, high SiO2 and Al2O3, and low FeOt + MgO contents show leucogranite-like affinities. They have low Mg#, low Rb/Sr, and mildly peraluminous features, collectively suggesting an intermediate–felsic orthogneissic source. Whole-rock Zr saturation thermometry and Ti-in-zircon thermometry together suggest initial magma temperatures between 695 ± 32 °C and 751 ± 27 °C (1 standard deviation), indicating derivation from water-present melting. The SJG notably feature high Sr contents (average 792 ppm), high Sr/CaO ratios (average 476) as well as inter-correlated low REE concentrations (average ΣREE 87 ppm), low Th concentrations (average 5·1 ppm) and positive Eu anomalies (Eu/Eu* up to 2·94). These characteristics are best explained by partial melting of intermediate–felsic sources under high pressure (>15 kbar), leaving residuum where feldspar is sparse or absent and allanite is present. Inherited zircon age spectra and Sr–Nd–Pb isotopic compositions suggest that their source components could be mainly the Triassic orthogneisses whose protoliths are from the northern margin of the South China Block, probably in a wedge structure where the exhumed felsic slabs were wedged into the crust of the North China Block in the middle–late Jurassic and formed a stacked thickened crust. The wedge structure was most probably driven by synchronous large-scale strike-slip of the Tanlu fault, as a far-field effect of the oblique subduction of the paleo-Pacific plate. The characteristic chemical features observed in this study may be applied to identifying partial melts with similar petrogenesis elsewhere.