Neoproterozoic and Early Paleozoic magmatic records from the Chalukou ore district, northern Great Xing'an Range, NE China: Implications for tectonic evolution and Mesozoic Mo mineralization

摘要

To constrain the pre-mineralization tectonic evolutionary history and the nature of the magma source of the Chalukou Mo deposit, which is located on the western margin of the Xing’an block, we recognized and studied a period of Neoproterozoic magmatism and a suite of Early Paleozoic volcanic rocks in this ore district. The magmatic zircons of quartz chlorite schist collected from the Wolegen Group yielded a concordant206Pb/238U age of 685.1 ± 4.5 Ma (MSWD = 0.21), and they show very high εHf(t) values (+15.5 to +17.3) and young Hf modal ages (573–664 Ma) close to the formation age, indicating that the protolith was generated during the Neoproterozoic, and was most likely derived from a depleted mantle region. This Neoproterozoic magmatism might represent an important crustal growth event within the Xing’an Block. The host wallrocks in the Chalukou deposit mainly consist of rhyolite, dacite, andesite and volcanic tuff which have undergone slight deformation and metamorphism. The results of zircon LA-ICP-MS U-Pb dating indicated that they were generated during 472–475 Ma. The results of whole-rock geochemical analysis indicate that they have arc-affinity features characterized by LREE enrichment with moderate to strong fractionation between LREE and HREE (LaN/YbN = 7.7–13.6), intense negative Eu anomalies (δEu = 0.36–0.71), pronounced positive Ba, Th, U, Pb and La anomalies and negative Nb-Ta, P and Ti anomalies. Hf isotopes show various εHf (t) values ranging from −2.0 to +5.9 and slightly old two-stage model ages ranging from 1070 Ma to 1573 Ma. Combined with discrimination diagrams, these data indicate that they were derived from the partial melting of a slightly older lower crust with the addition of a juvenile crust component in an active continental margin setting related to the subduction of the Paleo-Asian Ocean between the Xing’an and Songliao blocks. The existence of Neoproterozoic magmatic activity and Mesoproterozoic lower crustal materials obtained from the Hf model ages of the Paleozoic volcanic rocks in the Chalukou ore district demonstrated that Precambrian basement exists in the Xing’an Block, but its distribution area may be limited. As the largest porphyry Mo deposit in China, the ore-forming materials might have multiple sources, considering the long-term and multiple magmatic events that occurred under different tectonic settings in the ore district. The previous modification and refertilization of the lower crust and/or lithospheric mantle by the subduction of the Paleo-Asian Ocean might be important in the generation of the fertile lower crust, which is the direct source of the ore-related porphyries.