Tectonic Settings and Geological Implications of Neoproterozoic Liujiaping VMS Deposit, Northwestern Yangtze Block, China

摘要

Liujiaping VMS (volcanic massive sulfide) deposit contains mainly copper and zinc, which is located at the Longmenshan orogenic belt of the northwestern margin of Yangtze block. The deposit is hosted in Neoproterozoic Datan terrane (composed of Datan granitoids and Liujiaping group) and is a typical, and the biggest, VMS deposit in this area. The Datan granitoids and Liujiaping group are contemporary and both parental magmas have the same genesis. The tectonic evolution history of Northwestern Yangtze is complicated. Chronology, isotope and geochemistry of the Liujiaping VMS ores and wall rocks (especially the Datan granitoids) are analyzed to restrict the tectonic progress. High-precision secondary ion mass spectrometry (SIMS) analysis of the Datan granitoids resulted in two concordant ages, 815.5 +/- 3.2Ma and 835.5 +/- 2.6Ma, which are contemporary with the Liujiaping Cu-Zn ore and volcanics. The wall rocks are characterized by enrichment in LREE and with a weak negative anomaly of Eu. The Pb isotope data of sulfide and volcanics from the Liujiaping deposit indicate that the material source is lower crust. Together with variable negative anomalies of high strength field elements HFSE (Th, Nb, Ta, Zr, Hf, P and Ti), positive epsilon(Nd) (825Ma) values (+1.8 to +3.1) and the Nd model age T-2DM=1.2-1.3Ga, it shows that the Liujiaping deposit and wall rocks were formed by partial melting of Mesoproterozoic lower crust. Geological and geochemical characteristics of Liujiaping deposit indicate that this deposit was formed during subduction of the oceanic crust. This study clarified that that the Liujiaping deposit and the northwestern margin of the Yangtze block were part of an arc setting at similar to 820Ma rather than intra-continental rift.