Syn-tectonic emplacement of the Late Mesozoic Laojunshan granite pluton in the eastern Qinling, central China: An integrated fabric and geochronologic study

摘要

The porphyritic monzogranite of Laojunshan (eastern Qinling in central China) forms a W-E elongated pluton characterized by a varying abundance of cm-scale potassium-feldspar (Kfs) megacrysts (with or without plagioclase mantles), mafic microgranular enclaves (MMEs) and associated mafic dykes. The measurements of the shape preferred orientation of Kfs megacrysts and the tiling of megacrystic pairs throughout the pluton suggest that these megacrysts constitute magmatic foliations subparallel to the pluton's margin, possibly resulting from eastward non-coaxial magmatic flow. Felsic-mafic magma mixing and mingling were important during magma emplacement, as indicated by abundant MMEs, coeval mafic dykes and disequilibrium textures within Kfs megacrysts. Quantitative analyses of the crystal size distribution and the in-situ chemistry of Kfs megacrysts revealed that the temperature fluctuation in the deep magma chamber and the resultant textural coarsening are the most likely important mechanisms for the growth of Kfs megacrysts. The syn-tectonic and two-main-stage emplacement of the pluton in a sinistral transtensional regime is inferred by the mechanical coupling between magmatic-submagmatic-solid-state fabrics in the pluton and wall-rock deformations, as well as zircon U-Pb dating results, which produce eastward younging emplacement ages spanning from ～122 Ma to ～119 Ma. Combined with other studies, our work supports the idea that the Qinling-Dabie orogen was in a sinistral transtensional regime during the Early Cretaceous, which may correspond to the eastward extrusion tectonics in eastern China.