Petrogenesis of the Middle Jurassic Yinshan volcanic-intrusive complex, SE China: Implications for tectonic evolution and Cu-Au mineralization

Volcanic–subvolcanic rocks and associated Cu-Au deposits in Yinshan are located in the eastern part of the Neoproterozoic Jiangnan orogenic belt between the Yangtze and Cathaysia blocks, constituting a large metallogenic district in southeastern China. This paper presents new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb dating, element and Sr-Nd-Hf isotopic data for Yinshan igneous rocks in the Dexing region. Our results show that the Yinshan igneous rocks were produced during the Middle Jurassic (ca. 176–166 Ma). Geochemically, the Yinshan igneous rocks are characterized by calc-alkaline and show some adakitic affinities with high Sr contents (up to 903 ppm), Sr/Y (up to 93.2) and LaN/YbN ratios (12.4 to 65.1) and low Y (2.53 to 15.8 ppm) and Yb contents (0.22 to 1.39 ppm). The absence of a negative Eu anomaly, extreme depletion in Y and Yb, relatively low MgO contents (0.2 to 2.9 wt.%), and relatively high 207Pb/204Pb ratio (15.555–15.575), indicate that Yinshan igneous rocks were likely derived from the partial melting of thickened lower continental crust. In addition, the Yinshan igneous rocks show similar element characteristics to arc-derived igneous rocks with bulk Earth-like εNd (t) values (− 1.1 to + 1.8) and initial 87Sr/86Sr (0.7036–0.7090), relatively depleted εHf (t) values (− 1.5 to + 5.8), and Hf model ages of 0.8–1.2 Ga. However, there is no arc-related evidence in the study area in the Middle Jurassic, which indicates that these arc-related elemental and isotopic compositions are possibly inherited from Meso- to Neo-Proterozoic juvenile continental crust formed by the Neoproterozoic oceanic crust subduction along the Jiangnan Orogen. The geodynamic environment responsible for the development of the Middle Jurassic Yinshan igneous rocks is probably a localized intra-continental extensional environment along the NE Jiangxi Deep Fault, as a tectonic response to far-field stress at the margins of South China Plate during the early stage of the paleo-Pacific plate subduction. Regarding the Cu-Au mineralization, we infer that Cu-Au were released from subducted oceanic slab and reserved in the juvenile crust during Neoproterozoic subduction along the eastern Jiangnan Orogen. Partial melting of Cu-Au rich Neoproterozoic juvenile crust during the Middle Jurassic time in the Yinshan area caused the formation of adakitic rocks and large-scale Cu-Au deposits. Our model provides a new insight into genesis of adakitic rocks and related Cu-Au deposits in a non-arc setting.

► Yinshan volcanic–subvolcanic complex were located in the Dexing region, South China.
► They were formed in Middle Jurassic at the intra-continental extensional setting.
► These adakitic rocks were derived from partial melting of the thickened lower crust.
► The magma sources were Neoproterozoic subduction-related juvenile crustal materials.
► Yinshan Cu-Au mineralization was controlled by Neoproterozoic juvenile crust.