西藏冈底斯西段帮布勒Pb-Zn-Cu矿床晚白垩世岩浆岩成因及意义

Petrogenesis and Geological Implications of Late Cretaceous Intrusion from Bangbule Pb-Zn-Cu Deposit, Western Gangdese, Tibet

帮布勒矿床位于念青唐古拉多金属成矿带西段,为该成矿带已知新发现的最西端大型矽卡岩型Pb-Zn-Cu矿床.矿区内岩浆活动相对单一,主要发育灰色-浅灰色石英斑岩,呈岩株、岩脉状产出,在空间和时间上均与矿化密切相关.为研究石英斑岩的成因及其形成环境,在详实的野外基础地质工作与室内显微观察基础上,对其进行岩石地球化学、锆石U-Pb定年、全岩Sr-Nd-Pb及锆石Hf同位素分析.结果显示:石英斑岩的LA-ICP-MS U-Pb年龄为77.2±0.8 Ma和77.3±0.7 Ma,形成于晚白垩世;石英斑岩SiO2含量变化于72.78%~77.12%,K2O含量为3.80%~5.55%,铝饱和指数A/CNK为0.88~1.18,显示高钾钙碱性、偏铝质、高分异I型花岗岩特征;稀土总量变化介于146.89×10^-6~247.89×10^-6,具轻稀土相对富集,重稀土亏损的特点,同时具有明显的Sr、Eu、Nb、Ta、P等异常,暗示了其岩浆形成中经历了重要的结晶分异过程.石英斑岩的εHf(t)集中于-7.92^-5.73,对应的地壳模式年龄为1 651~1 121 Ma;全岩(87Sr/86Sr)i比值为(0.714 8~0.725 8);εNd(t)值(-9.01^-7.32),其二阶段Nd模式年龄为1 612~1 477 Ma;铅同位素显示206Pb/204Pb、207Pb/204Pb、208Pb/204Pb分别介于18.686~18.781、15.699~15.762和39.131~39.344.帮布勒石英斑岩可能形成于班公湖-怒江洋南向俯冲结束后的后碰撞伸展环境,与拉萨地块中元古代古老基底部分熔融有关;其发现说明了冈底斯带并不存在绝对意义上的喷发静宁期(80~70 Ma),暗示了区域岩浆活动的连续性.

The Bangbule deposit is located in the western part of the Nyainqing Tanggula Pb-Zn-Ag polymetallic metallogenic belt, which is a newly discovered large skarn Pb-Zn-Cu polymetallic deposit in 2010. The magmatic activity in the mining area is relatively simple, with the development of gray-light gray quartz porphyry, which is mainly composed of stock and dyke, and is closely related to the skarn Pb-Zn-Cu mineralization in space and time. In this paper, based on the detailed field basic geological field work and microscopic observation, we study the geochemistry, zircon U-Pb dating, whole rock Sr-Nd-Pb and zircon Hf isotopes of quartz porphyry. The results showed that the LA-ICP-MS U-Pb age of quartz porphyry is 77.2 ±0.8 Ma and 77.3 ± 0.7 Ma, which occurred in the Late Cretaceous. The contents of SiO2 in quartz porphyry vary from 72.78% to 77.12%, the contents of K2O are 3.80%~5.55%, and the A/CNK is 0.88-1.18, which showing high-K calc-alkaline, metaluminous and high differentiation I-type granite characteristics. Total REE contents are between 146.89 × 10^-6 and 247.89 × 10^-6, which is characterized by relative enrichment of light rare earth and depletion of heavy rare earth. It has obvious anomalies such as Sr, Eu,Nb, Ta and P, suggesting that the magma experienced an important crystallization differentiation process. The εHf(t) of quartz porphyry is concentrated in the range of -7.92 to -5.73, and the corresponding crustal model is 1 651-1 121 Ma. The (87Sr/86Sr)iand the εNd(t) are 0.714 8-0.725 8 and -9.01 to -7.32, respectively, with the corresponding second Nd model ages (tDM 2) of 1 612-1 477 Ma. The lead isotopes show that 206Pb /204Pb、207Pb /204Pb、208Pb/204Pb are 18.686-18.781, 15.699-15.762 and 39.131-39.344, respectively. In summary, the Bangbule quartz porphyry may have been formed by the magma derived from partial melting of the Mesoproterozoic Lhasa block during the extension environment in the post collision after Bangong -Nujiang south subduction. The discovery of the Late Cretaceous quartz porphyry in the Bangbule deposit shows that there is no absolute eruption in the Gangdese belt (80-70 Ma), which suggesting the continuity of the regional magmatic activity.