Mesozoic magmatism and granitic dome in the Wugongshan Massif, Jiangxi province and their genetical relationship to the tectonic events in southeast China

摘要

In SE China, a Mesozoic granitic dome coeval to extensional tectonics is exposed in the Wugongshan massif. Its center is occupied by Mesozoic granitoids and granitic gneisses, and the E–W-trending Late-Paleozoic–Mesozoic Pingxiang and Anfu basins are located along its northern and southern sides, respectively. In this paper, the petrological, geochemical (major, incompatible, and rare earth elements) and isotopic characteristics of the granitoids and granitic gneisses of this dome allow us to constrain their geodynamic evolution. The Wugongshan Mesozoic K-feldspar-enriched granitoids consist of six granitic bodies and lie within a Paleozoic metamorphic basement. The Caledonian Shanzhuang granodiorite, containing some dioritic enclaves, occurs in the study area and is a calc-alkaline type granodiorite characterised by lower SiO2 and composed of andesine, biotite, quartz, hornblende, magnetite and sphene. The Mesozoic granitoids, either Indosinian plutons or Yanshanian bodies, show a geochemical affinity of peraluminous S-type granite, having higher SiO2, K2O contents, Al>K+Na+2Ca, enriched in Rb, Th and depleted in Eu. They also exhibit LREE-enriched patterns and marked negative Eu anomalies. The Yanshanian plutons also contain characteristic minerals such as sillimanite, garnet and monazite. The Wugongshan Mesozoic granitoids have higher ISr values (0.70981–0.72885) and lower var epsilonNd(T) (−10.6 to −14.7; an average of −12.6) than those of the early Paleozoic Shanzhuang granodiorite. The Mesozoic granitoids were likely derived from ancient sedimentary rocks by partial melting. The Wugongshan Mesozoic granitoids contain three petrological zones, that is, an Early Cretaceous massive granitic zone in the core (Zone I), a foliated granitic zone (Zone II) and a Triassic granitic gneissic zone in the outer-rim (Zone III). The petrological and geochemical features of massive granitoids (Zone I) are similar to those of foliated granites (Zone II) and of granitic gneisses (Zone III) in this Mesozoic dome. Moreover, geometric and kinematic features within the ductile parts (Units 2 and 3) are also similar to those within the brittle part (Unit 1) of the dome. All these features show that the geologic evolution of the Wugongshan doming is a continuous process lasting from Triassic to Early Cretaceous time. Published 40Ar/39Ar and K–Ar ages in the Wugongshan Mesozoic granitic rocks fall into two major age fields, corresponding to two geodynamic events of intracontinental deformation and magmatic activity. The first tectonothermal event, dated at 226–259 Ma on the granitic gneisses in the outer-rim of dome, can be linked to Indosinian collision between the South China plate and the North China plate during Triassic time. The second event took place during the Early Yanshanian, dated at 180–120 Ma on the granites of the core of dome, and are responsible for final doming of the Wugongshan plutons. This event may be related to a westward subduction of the Paleo-Pacific oceanic plate under the SE-China continent during Late Jurassic–Early Cretaceous time.