Geochronology, geochemistry and Sr-Nd-Pb isotopic constraints on the origin of the Qian'echong porphyry Mo deposit, Dabie orogen, east China

摘要

The giant Qian'echong porphyry molybdenum deposit is located in the Dabie orogen, east China. The molybdenum mineralization mainly occurs as molybdenite-bearing quartz veins hosted by the Devonian Nanwan Formation in the external contact zone of the Qian'echong stock. The Qian'echong stock comprises an earlier formed monzogranite and a later formed granite porphyry. Sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb dating constrains the timing of crystallization of the monzogranite and granite porphyry to 130 ± 2 and 129 ± 2 Ma, respectively. The Re-Os model ages of six molybdenite samples range from 127.9 ± 1.9 to 129.7 ± 1.9 Ma with an isochron age of 129.4 ±1.5 Ma, which are all consistent within errors with the zircon U-Pb ages, indicating an Early Cretaceous magmatic and mineralization event. The Qian'echong granites have moderate negative Eu anomalies and are relatively enriched in light rare earth elements (REE), but depleted in heavy REE, Y, and high field strength elements (HFSE; e.g., Nb, Ta, and Ti). The Qian'echong granites are I-type rather than A- or S-type, and they have high (~(87)Sr/~(86)Sr)i (0.706771-0.710326) and low ε_(Nd)(t) (-25.5 to -16.8). Two-stage Nd model ages (T_(2dm)) vary between 2.29 and 2.99 Ga. Sr-Nd-Pb isotopic data suggest that the Qian'echong granites were not derived from the North China Craton (NCC), but rather were generated from the Yangtze Craton (YC) lower crust. Paleoproterozoic inherited zircon age and whole-rock chemical and Sr-Nd-Pb isotopic data suggest that the Qian'echong granites were derived mainly from partial melting of ultrahigh pressure eclogites, with incorporation of some Paleoproterozoic to Archean YC crustal materials at lower crustal levels. Delamination or foundering of eclogitic lower crust, which extensively occurred in the Dabie orogen during the Early Cretaceous, had not taken place beneath the Qian'echong deposit when it formed. The Qian'echong molybdenum deposit formed in an extensional setting in the Dabie orogen, and may have been associated with a change in the subduction direction of the lzanagi (or Paleo-Pacific) Plate after 135 Ma.