Timing and Tectonic Setting of the Gaoaobei TungstenMolybdenum Deposit in Nanling Range, South China

The Gaoaobei tungsten-molybdenum deposit is a newly discovered large-scale quartzvein-type deposit in the Nanling metallogenic belt in South China.The ore bodies are hosted in the Indosinian granites and the Cambrian Xiangnan Group slates and are controlled by NWW-oriented faults,which are obviously different from the“five-story building”model in southern Jiangxi Province.The magmatic rocks in the study area are dominated by medium-to coarse-grained biotite monzogranite,with a few NW-oriented fine-grained granite dykes.The medium-to coarse-grained biotite monzogranite and fine-grained granite dykes have zircon U-Pb ages of 229.4±1.9 Ma(MSWD=1.5)and 164.9±3.3 Ma(MSWD=0.75),respectively,corresponding to the Indosinian and Yanshanian magmatism.The monzogranites have higher contents of FeO,CaO,K2O,P2O5,and TiO2,while the granite dykes have slightly higher contents of SiO2,Al2O3,MnO,and Na2O.Their A/CNK values are 1.11-1.75 and 1.19-2.25,and the contents of CIPW normative corundum are 1.71%-6.66%and 2.41%-9.50%,suggesting both the monzogranites and granite dykes are S-type granite.The total amount of rare earth elements in the monzogranites(from 84.7 ppm to 129 ppm)is slightly lower than that in the granite dykes(from 128 ppm to 133 ppm).The Eu/Eu*values range from 0.12 to 0.30 in monzogranites and from 0.0011 to 0.0013 in granite dykes,indicating the fine-grained granites underwent more intense fractional crystallization.The monzogranite and granite dykes have high 87Sr/86Sri values of 0.7169-0.7193 and 0.72825-0.72880,lowεNd(t)values ranging from-10.2 to-9.6 and from-11.5 to-11.4,and TDM2 ages of 1835-1785 and 1957-1946 Ma,respectively.These isotope data indicate their origin from the remelting of the Paleoproterozoic crustal materials.Combined with regional geology,it is concluded that the medium-to coarse-grained biotite monzogranite was formed in a post-collisional extensional environment.In addition,40Ar-39Ar dating of the greisen type tungsten-molybdenum ore gave consistent plateau age of 164.0±1.2 Ma,isochronal age of 162.0±2.4 Ma and anti-isochronal age of 161.4±1.8 Ma.Combined with the published molybdenite Re-Os age,the Gaoaobei tungsten-molybdenum deposit was formed at~164 Ma,which is inferred to be genetically related to the contemporaneous finegrained granite dykes(165 Ma).The deposit was likely formed during the large-scale magmatism and mineralization event in the early Yanshanian of the Nanling Range in an intra-continental extensional environment caused by the subduction of the paleo-Pacific plate.The late and small granite dykes within the large granite plutons thus require further attention during mineral prospecting in the regions.

华夏板块发现冥古宙捕掳锆石

由于缺少冥古宙岩石的证据,对地球冥古宙地壳演化模式仍缺乏深入认识.锆石作为火成岩中常见的副矿物,由于其独特的物理化学稳定性,在研究岩石成因以及地壳演化等方面发挥着重要作用.因此,本文对华夏板块汤湖岩体中新发现的冥古宙捕掳锆石开展了一系列U-Pb年代学和Lu-Hf同位素地球化学研究.结果表明,该锆石具有明显的核幔结构,核部年龄为4102±12 Ma且具有较高的Th/U比(0.48)和ε_(Hf)(t)值(0.07),证实了该捕掳锆石属岩浆成因且形成于冥古宙.本文提出了两种假说且均可解释该锆石的成因:(1)其由4200Ma的亏损地幔派生出的岩石在4100 Ma经历再造;和(2)其由原始地幔在4100 Ma派生出.但是不管是哪种模式都指示了地球在冥古宙存在新生地壳的生长事件.