Differentiation mechanism and evolution of high-level magma chamber at Xiangshan, China

Abstract

The calc-alkaline volcanic magmas, which formed the Mesozoic uraniferous volcanic complex of Xiangshan, resulted from partial melting of the mixture of lower crust and enriched mantle with a high mixing proportion in a specific tectonic setting such as active continental margin or ocean-continent collision zone. The preliminary concentrations of U and Th occur in low-degree partial melts. Only small part of these melts was rapidly extracted and erupted and most intruded into the high-level magma chamber (depth: 12–13 km) of the compressed upper lithosphere, in which occurred a strong differentiation which would resulted in strong preconcentrations of the high-hygromagmaphile elements U and Th associated with strong depletion of the 3-d transition elements Ti, Sc, Co, Zr, etc. At the final stage of subduction of the West-Pacific-Kula plate towards the Asian continental plate, the regional tectonic environment was transformed from a compressive into a tensional setting. The strongly differentiated, U (and Th)-enriched silicic alkalic magmas in high level magma chamber extensively erupted, extruded and intruded. The hydrothermal fluids released as a result of late volcano-degassing and dewatering during crystallization-solidification of magmas, resulted in the remobilization, leaching, migration and reconcentration of uranium, which had been preconcentrated in volcanic rocks. Therefore, specific regional petrogeochemical criteria are expected for the uraniferous volcanic series.