东昆仑小圆山铁多金属矿区斜长花岗斑岩锆石U-Pb测年、岩石地球化学及找矿意义

Zircon U-Pb dating and geochemical characteristics of the plagiogranite porphyry from the Xiaoyuanshan iron-polymetallic ore district in East Kunlun Mountains

东昆仑祁漫塔格地区是青海省重要的矽卡岩型铁多金属成矿带,小圆山矽卡岩型铁多金属矿位于青海祁漫塔格东段,其成矿作用与斜长花岗斑岩关系密切,矿体产于斜长花岗斑岩的外接触带。利用LA-MC-ICP-MS锆石U-Pb定年技术,获得斜长花岗斑岩加权平均年龄为(216.9±1.9)Ma(n=16,MSWD=2.9),厘定其形成时代为晚三叠世。岩石地球化学研究表明,斜长花岗斑岩为过铝质高钾钙碱性系列岩石。岩石明显富集大离子亲石元素(Rb、K)、LREE和活泼的不相容元素(如U、Th),相对亏损高场强元素(如Nb、Ta、P、Ti),显示了轻稀土元素强烈富集的右倾式稀土配分型式,呈明显负Eu异常。综合青海祁漫塔格地区已有年代学资料和区域地质构造演化特征,认为小圆山斜长花岗斑岩可能形成于后碰撞构造阶段,为区域东昆仑造山带晚古生代—早中生代构造旋回的产物。

Qimantag in East Kunlun Mountains is an important skarn-type iron-polymetallic metallogenic belt in Qinghai Province,and the Xiaoyuanshan iron- polymetallic ore district is located in the eastern Qimantag, Qinghai. The mineralization has close relationship with plagiogranite porphyry, and the orebody occurs in the outer contact zone of plagiogranite porphyry. LA-MC-ICP-MS zircon U- Pb dating shows that the weighted mean age of plagiogranite porphyry is 216.9 ± 1.9 Ma（n=16,MSWD=2.9）,suggesting Late Triassic. Detailed petrogeochemical studies show that the plagiogranite porphyry belongs to the peraluminous highK calc- alkaline series. This suite of rocks are enriched in LILE（such as Rb, K）, LREE and more mobile incompatible elements（such as U, Th） and relatively depleted in HFSE（such as Nb, Ta, Ti, P）. The plagiogranite porphyry is rich in LREE and poor in HREE, with obviously negative Eu anomalies. Based on a combined study of geochronology and evolutionary characteristics of regional structures, the authors consider that the plagiogranite porphyry was formed at the post-collisional development stage of the orogen, corresponding to the late stage of the Late Paleozoic-Early Mesozoic tectonic event in eastern Kunlun orogenic belt.