Tectonomagmatic Evolution And Metallogenesis Of Bangongco Metallogenic Belt(Northern Part)

Among the three world-class copper-polymetallic metallogenic belts in Tibet,the Bangongco metallogenic belt is the latest discovered,least understood and least studied.This belt covers the west of South Qiangtang terrane,west segment of Bangong-Nujiang suture zone,and northwest of Lhasa terrane.Due to the extremely harsh natural and geographical conditions,the tectono-magmatic evolution and mineralization of the Bangongco metallogenic belt have not been systematically studied.Some major geological questions are yet on debate,such as the closure timing and process of the Bangong-Nujiang suture zone,and the basement nature of the South Qiangtang terrane.In this work,on basis of previous data and research achievements,we restrained the Late Mesozoic tectonic evolution of Bangong-Nujiang suture zone and crustal architecture of west South Qiangtang terrane by using scanning electron microscope(SEM),electron probe micro-analyzer(EPMA),hydrothermal titanite U-Pb dating,pyrite Rb-Sr dating,zircon U-Pb dating,Hf isotopic analyses,and whole-rock major&trace element and Sr-Nd isotopic analyses.We also explore the petrogenesis of Late Mesozoic granitoids and the tempo-spatial variations of mineralization as well as the control factors in the west of South Qiangtang Terrane1.With the Fuye-Mopanshan-Qingcaoshan Faults as a boundary,the late Mesozoic(ca.140-130 Ma)granitoid magmatic belt in the west of South Qiangtang terrane can be subdivided into the southern magmatic belt(SMB)and the northern magmatic belt(NMB).The granitoids in the NMB are dominated by I-type granitoids formed by partial melting of the ancient lower crust.On the contrary,the granitoids in the SMB consist of adakitic rocks,normal calc-alkaline I-type granitoids,and bajaitic rocks,in which the former two types originated from the remelting of the juvenile lower crust and the latter was generated by partial melting of metasomatized mantle-wedge peridotites.2.The Hf isotopic cross-section and the study of inherited zircons fully confirm the presence of Archean to Early Paleoproterozoic crystalline basements in the South Qiangtang terrane.With the Fuye-Mopanshan-Qingcaoshan Faults as a boundary,the crustal nature and evolution were significantly inconsistent between the north and south sides during the late Mesozoic.To the south,the margin area of South Qiangtang terrane has experienced crustal growth in Jurassic and finally evolved into a relatively juvenile lower crust.On the contrary,the interior area to the north always show ancient Precambrian crystalline basement.3.The Bangong-Nujiang suture zone had already entered a stage of soft-collisional evolution during the Early Cretaceous.The new data obtained here and the existing regional stratigraphy show the margin of South Qiangtang terrane,the Bangong-Nujiang suture zone,and the Northern Lhasa terrane all developed deep-water flysch deposits at mid-Early Cretaceous,indicating the presence of land-locked deep-sea basin crossing the north and south sides of the Bangong-Nujiang suture zone.4.Based on the sedimentary and tectonomagmatic data,we propose a late Mesozoic dynamic model for the evolution of northern part of Bangongco metallogenic belt.Specifically,the Bangong-Nujiang Meso-Tethys ocean was under the stage of double-sided subduction during the Jurassic-Berriasian(ca.170-140 Ma),and since ca.160 Ma,the angle of north-dipping oceanic slab gradually decreased(i.e.,flat-slab subduction).During the Valanginian(ca.140-132 Ma),the South Qiangtang and Lhasa terranes were amalgamated through soft collision,which resulted in the limited uplift and the formation of a regional unconformity between the Shamuluo Formation shallow-marine sedimentary rocks and their underlying ophiolitic melange.During the Hauterivian-Aptian(ca.132-113 Ma),the oceanic slab beneath the hinge zone sank vertically and pulled down the overlying strata,thereby causing a regional subsidence and the formation of land-locked deep-sea basin.Meanwhile,the break-off and rollback of the oceanic slab beneath the South Qiangtang terrane resulted in the emergence of regional extensional regime and a southward migrating magmatic system.During the Albian(ca.113-100 Ma),the complete detachment of the oceanic slab led to a strong isostatic rebound(uplift)of the entire hinge zone,thus resulting in the emergence of the large-scale terrestrial sedimentation(e,g.,the Qushenla,Abushan,and Jingzhushan formations).5.Pyrite Rb-Sr geochronological data indicate the Gaerqin porphyry Cu-Au deposits were formed at 121.7±1.9 Ma(MSWD of concordance=1.1).Hydrothermal titanite U-Pb geochronological data show the Xianqian skarn Fe deposits were formed at 106.4±1.1 Ma(MSWD of concordance=1.5).Together with the available data of metallogenic age in this region,we propose that the mineralization of the northern part of Bangongco metallogenic belt mainly occurred in early Cretaceous.6.We discover magmatic sulphide inclusions in the mafic microgranular enclaves(MME)within the Early Cretaceous granitoids from Duolong ore concentration area.This discovery suggests the parental magma of MMEs is a sulfide-saturated mafic magma.Thus,we think the sulfide-saturated mafic magma played an important role in the formation of Duolong Cu-Au deposits.7.The late Mesozoic granitoids at the north and south sides of Fuye-Mopanshan-Qingcaoshan Faults are significantly different in terms of metallogenic types and scales.The factors favorable for the formation of Cu-Au deposits in SMB granitoids include(1)the metal and sulfur-rich juvenile lower-crust formed in the margin of South Qiangtang terrane during the Jurassic;(2)At early Cretaceous,the break-off and rollback of the Bangong-Nujiang oceanic slab promoted the remelting of the metal and sulfur-rich juvenile lower-crust,which provided abundant metals and sulfur for the ore-forming magmatic-hydrothermal systems.(3)At early Cretaceous,the injection of water-rich and sulfide-saturated mafic magmas provided abundant water and certain amounts of metals and sulfur for the ore-forming magma chambers.On the contrary,in the NMB granitoids,only relatively sulfur-poor skam Fe deposits were formed.8.With the prolonging of time,the ore deposits hosted by SMB granitoids gradually transited from porphyry-type Cu-Au deposits to epithermal Cu-Au deposits.The geodynamic mechanism for the transformation of types of ore deposits is as follows:the Bangong-Nujiang oceanic slab detached from the overlying South Qiangtang terrane at ca.118-113 Ma,resulting in strong isostatic uplift.As a result,the depths of ore-forming magmatic-hydrothermal systems gradually decreased.