Melting of the Meso-Neoproterozoic juvenile crust for the origin of the Late Triassic Mo mineralization in South Qinling, central China: Evidence from geochronology and geochemistry of the Yangmugou deposit

Abstract

The Yangmugou deposit is a representative Late Triassic Mo deposit in South Qinling, central China. The geochronology and geochemistry of the ore-forming (porphyritic) monzogranite and barren dacite in the Yangmugou deposit are analyzed to infer the petrogenesis of the magmatic rocks and metallogenic dynamics. The dacite has a zircon U–Pb age of 211.7 ± 2.1 Ma, in accordance with the ages of the (porphyritic) monzogranite and Mo mineralization, suggesting that Mo mineralization and magmatic activity occurred in the Late Triassic. The ore-forming (porphyritic) monzogranite exhibits low Mg# and Sr/Y values, LREE enrichment, HREE depletion, and significantly negative Eu anomalies. Additionally, the (87Sr/86Sr)i ratios of monzogranite range from 0.7053 to 0.7059 and εNd(t) values of −3.73 to −3.6. The (206Pb/204Pb)i, (207Pb/204Pb)i and (208Pb/204Pb)i values of monzogranite are 17.143–17.517, 15.453–15.781 and 37.318–37.981, respectively. These geochemical characteristics show that the monzogranite is a highly fractionated I-type granite that originated from the melting of Meso-Neoproterozoic juvenile crust. The barren dacite exhibits enriched LREEs, and features high Sr contents and Sr/Y ratios, indicating that the rock can be classified as adakite. Furthermore, the (87Sr/86Sr)i values of dacite range from 0.7052 to 0.7055, εNd(t) values of −2.25 to −2.49, and εHf(t) values of −1.0 to 5.0. The Sr–Nd–Hf isotopic features and high Mg# values, Cr contents and Th/Ce ratios indicate that the dacite was generated by the interaction between subducted continental crustal-derived adakitic melts and the mantle wedge.The regional geological features indicate that the closure of the Paleo-Tethys Ocean caused the collision between the North China Plate and the Yangtze Block in Late Triassic. This event resulted in crustal thickening in South Qinling and caused partial melting of the subducted continental lower crust of the Yangtze Block, forming a primitive adakitic melt. The ascending primitive adakitic melt reacted with the overlying mantle, forming the high-Mg# adakitic dacite. Furthermore, the asthenospheric upwelling caused by the Late Triassic breakoff of the oceanic slab triggered the partial melting of Meso-Neoproterozoic juvenile crust, forming the low-Mg#, highly fractionated Mo-bearing (porphyritic) monzogranite. In addition, the Late Triassic Yangmugou ore-forming (porphyritic) monzogranite and barren dacite formed under local stress relaxation or in a local extensional environment during the collision between the North China Plate and the Yangtze Block.