Petrogenesis of a Late Jurassic Peraluminous Volcanic Complex and its High-Mg, Potassic, Quenched Enclaves at Xiangshan, Southeast China

Abstract

A late Mesozoic belt of volcanic–intrusive complexes occurs in SE China. Volcanic activity at Xiangshan in the NW of the belt took place mainly in the Late Jurassic (158–135Ma). The volcanic rocks from the Xiangshan volcanic complex include rhyolitic crystal tuffs, welded tuffs, rhyolite lavas, porphyritic lavas, and associated subvolcanic rocks. Mineral assemblages in these magmatic rocks include K-feldspar, plagioclase, quartz, Fe-rich biotite and minor amphibole, orthopyroxene and almandine. Mineral geothermometry indicates a high crystallization temperature (>850 C) for the Xiangshan magmas. The volcanic rocks are generally peraluminous; SiO2 contents are between 65 4% and 76 8% and the samples have high alkalis, rare earth elements (REE), high field strength elements and Ga contents and high Ga/Al ratios, but are depleted in Ba, Sr and transition metals. Trace element geochemistry and Sr–Nd–O isotope systematics imply that the Xiangshan magmas were probably derived from partial melting of Middle Proterozoic metamorphic lower-crustal rocks that had been dehydrated during an earlier thermal event. These features suggest an A-type affinity. Quenched mafic enclaves, hosted by the subvolcanic rocks, consist mainly of alkali feldspar, plagioclase, clinopyroxene, phlogopite and amphibole. Geothermometry calculations indicate that the primary magmas that chilled to form the quenched enclaves had anomalously high temperatures (>1200 C). The quenched enclaves have boninitic affinities; for example, intermediate SiO2 contents, high MgO and low TiO2 contents, high Mg-numbers and high concentrations of Sc, Ni, Co and V. However, they also have shoshonitic characteristics, e.g. enrichment in alkalis, high K2O contents with high K2O/Na2O ratios, high light REE and large ion lithophile element contents, low initial eNd values ( 4 2) and high initial Sr/Sr ratios (0 7081). We suggest a phlogopite-bearing spinel harzburgitic lithospheric mantle source for these high-Mg potassic magmas. Underplating of such anomalously high-temperature magmas could have induced granulite-facies lower-crustal rocks to partially melt and generate the Xiangshan A-type volcanic suite. A back-arc extensional setting, related to subduction of the Palaeo-Pacific plate, is favoured to explain the petrogenesis of the Xiangshan volcanic complex and quenched enclaves.