Early Silurian granitic rocks and associated enclaves as evidence of rapid cooling in a cognate magma system; the case of the Xuehuading-Panshanchong Pluton, South China Block

The study of enclaves in granitic plutons provides fundamental information on the petrogenesis of their host rocks. Here we combine U-Pb zircon ages, petrography, geochemistry and Nd-Hf isotope composition to investigate the origin of dioritic-granodioritic enclaves and their host granodiorites and biotite granites in the Xuehuading-Panshanchong area, which is a pivotal site to study the Palaeozoic intracontinental orogenic processes of the South China Block. Obtained ages indicate that the host rocks were formed in early Silurian time (c. 432 Ma). The enclaves are fine grained, but with mineral assemblages similar to their hosts and contain amphibole, biotite and plagioclase. All rocks have fractionated rare earth element patterns ((La/Yb)N = 2.86-8.16), except for one biotite granite that has a concave rare earth element pattern ((La/Yb)N = 1.50). Most rocks are depleted in Ta-Nb-Ti, and have negative Eu anomalies and epsilon Nd(t) (-8.86 to -5.75) and zircon epsilon Hf(t) (-13.30 to -4.11, except for one, -39.08). We interpret that the enclaves were formed at the borders of magma-ascending conduits, where the mafic mineral crystallization was enhanced by rapid cooling. Conversely, the biotite granites were produced by fractional crystallization from a related granodiorite magma. The sample with a concave rare earth element pattern may have been influenced by hydrothermal fluid-melt interaction. Geochemical modelling suggests that the granodiorites were likely generated by disequilibrium melting of heterogeneous amphibolites in the middle-lower crust. Considering the geological data for the Palaeozoic magmatic rocks in the South China Block, we propose that the Xuehuading-Panshanchong magmatism was likely triggered by piecemeal removal of the thickened lithospheric root and subsequent thermal upwelling of mantle, without a mantle-derived magma contribution to the granites.