青海都兰县阿斯哈金矿区花岗斑岩岩石地球化学、锆石U-Pb年代学与Hf同位素研究

阿斯哈金矿区钻孔中新发现的花岗斑岩,呈岩脉、岩株或舌状分布。岩石呈灰-灰白色,细-中粒斑状结构,块状构造。主要矿物为石英、斜长石、钾长石、黑云母及少量角闪石,副矿物有磁铁矿、钛铁矿、锆石、磷灰石和榍石等。全岩高硅(SiO_2=70.03%~72.83%),高钾(K_2O=3.96%~5.21%),低P_2O_5(0.09%~0.10%),低FeO~T/MgO(1.76~3.91),A/CNK值(1.04~1.29)多数小于1.1,为高钾钙碱性系列弱过铝质-过铝质花岗岩类岩石,具有I型花岗岩特征。样品具有轻稀土富集和中等负Eu异常(δEu=0.71~0.78)特征,且富集大离子亲石元素(LILE:K、Rb、Th、U等)、亏损高场强元素(HFSE:Nb、Ta、Ti等)。LA-ICP-MS锆石U-Pb定年结果显示,其结晶年龄为222.1±3.9 Ma(MSWD=5.4,n=15),形成于晚三叠世。锆石Hf同位素组成(ε_(Hf)(t)=-4.3~-1.4)和二阶段Hf模式年龄(t_(DM2)=1.34~1.53 Ga),表明新发现的花岗斑岩源于古老地壳物质的深熔或重熔,并可能有幔源物质的加入。上述特征表明,阿斯哈金矿区花岗斑岩形成于东昆仑晚古生代-早中生代构造-岩浆旋回的碰撞晚期-后碰撞阶段,可能是晚三叠世幔源岩浆底侵下地壳岩石部分熔融的产物,并在形成过程中有幔源物质的加入。此外,研究区花岗斑岩的侵位与沟里地区(包括阿斯哈)金多金属矿床的形成具有内在的联系。

Geochemistry,Zircon U-Pb Dating and Hf Isotopic Compositions of the Granite-porphyry in Asiha Gold Ore District,Dulan County,Qinghai Province

The newly discovered granite-porphyry dike in the Asiha Au ore district consist mainly of quartz, plagioclase, K-feldspar, biotite and a small quantity of hornblende, with magnetite, ilmenite, zircon, apatite and titanite as accessory minerals. This granite-porphyry is characterized by high SiO2 (SiO2=70.03%?72.83%), high K2O (K2O=3.96%?5.21%), low P2O5 (0.09%?0.10%) and low FeOT/MgO (1.76?3.91). Most A/CNK (1.04?1.29) values are less than 1.1, with I-type granite features. This indicates that it belongs to high-K calc-alkaline and weak peraluminous-peraluminous granite. The rocks are enriched in large ion lithophile elements (LILE; e.g., K, Rb, Th and U) and LREE with moderate negative Eu anomalies (δEu=0.71?0.78),but depleted in high field strength elements (HFSE; e.g., Nb, Ta, and Ti). LA-ICP-MS zircon U-Pb dating yielded an age of 222.1±3.9 Ma (MSWD=5.4,n=15).εHf(t) (from?4.3to?1.4) andtDM2 (1.34?1.53 Ga) values of zircons indicate that the porphyry was derived from ancient crust sources andmixed probably with juvenile mantle material. Therefore, we propose that the newly discovered porphyry formed during the collision to post-collision stage of Late Paleozoic to Early Mesozoic tectonic-magmatic cycle in the East Kunlun region. It is a production of partial melting of crustal rocks intrigued by mantle underplating in Late Triassic. In addition, there is a close genetic relationship between the porphyry and Au polymetallic deposits in the Gouli area (includes Asiha).