Study On The Early Cretaceous Magmatism And Tectonic Dynamic Setting Of The Duolong Cu Mining District In The Bangong-Nujiang Metallogenic Belt,Tibet

The Duolong Cu mining district(DCMD),which is located on the western edge of the Qiangtang terrane(QT)to the northwest of the Bangong–Nujiang metallogenic belt,is closely associated with the first known high-sulfidation epithermal deposit(the Rongna Cu–Au deposit)in Tibet and has the potential to be a world-class deposit.Based on the previous researches,this study aims to resolve the problems relating with geochronology,geodynamic setting,origin and petrogenesis of magmatic rocks and to suggest a geodynamic setting model in the Beiya deposit.1.Dongwodong deposit belongs to a significant deposit in the eastern of the DCMD.Zircon grains of ore-bearing intrusions collected from Sena deposit yield 118.1±1.4Ma and ziron grains of ore-bearing intrusions collected from Dongwodong deposit ranging from 121.0Ma to 121.5 Ma,indicating their zircons are similar to the diagenesis and mineralization age(115 to 125Ma)of other deposits in the DCMD.2.Mineralized intrusions in the DCMD belong to medium-Calc alkaline to Shoshonite series and metaluminous granite.They show marked enrichments in light REEs and LILEs,but depletions in HFSEs and negative Nb-Ta-Ti anomalies in primitive-mantle-normalized multi-element patterns and display right deviation gentle type chondrite-normalized REE patterns with medium nagetive to discernible positive Eu anomalies,which are consistent with those of arc-type magmas wordwide.Zircons from mineralized intrusions in the DMPD show positive and large variations in their ?Hf(t)values(+0.0 to +15.4),implying that they originated by partial melting of juvenile basaltic crust with mantle-derived substances in the magmatic source region northward subduction of the Bangongco–Nujiang oceanic lithosphere(BNOL).This inference is further supported by their relatively high initial(87Sr/86Sr)i value of 0.0502 to 0.7111,varying ?Nd(t)ranging from-7.7 to +3.3.3.Zircon grains from the MF rhyolite yield 206Pb/238 U weighted mean ages of 109.3 ± 2.2 Ma,and a A similar age was also obtained for zircon grains from the MF andesite,which yielded an age of 108.2 ± 2.6 Ma.Recent geochronological studies have reported large volumes of coeval magmatic rocks with arc-type affinities to the south of the Bangong–Nujiang suture zone,and this is thought to have formed in relation to bidirectional subduction of the Bangongco–Nujiang ocean and the slab breakoff of BNOL.4.The MF volcanic rocks belong to high-K calc-alkaline to shoshonite series and displaymarked enrichments in light REEs and LILEs,but depletions in HFSEs and negative Nb-Ta-Ti anomalies in primitive-mantle-normalized multi-element patterns with medium nagetive Eu anomalies(0.65~0.80).They share the limited variation in Sr and Nd isotopic compositions(87Sr/86Sr)i values of 0.7050~0.7053,?Nd(t)values ranging from-1.51~1.29,possibly indicates minor crustal contamination.The MF volcanic share ?Hf(t)values ranging from +3.4 to +15.5 and TDM C ranging from 180 Ma to 956 Ma,indicating they contain a great proportion of mantle-derived material with depleted Hf isotopic characteristics.In light of the geochemical characteristics and field relationships,we propose that the MF volcanic rocks were all generated by partial melting of mantle wedges replaced by the fluid which is formed the dehydration of the oceanic crust subduction plate,and its rock-forming resulted from the mixing between the juvenile lower crust-derived melts and mantle-derived mafic melts under the background of BNOL subducting northward to QT.5.The fact that the mineralized intrusions and the MF volcanic rocks in the DCMD formed in a continental marginal arc setting indicating that the Bangong–Nujiang oceanic basin had not completely closed,and the collision between QT and Lhasa terrane should occur later than the Early Cretaceous.The MF volcanic rocks are located to the north of the BNSZ and yield zircon U–Pb ages ranging from 108.2 to 109.3 Ma,clearly meaning that the Bangong–Nujiang oceanic crust experienced northward subduction at that time and that the oceanic lithosphere finally subducted northward beneath the QT at a high angle.