冈底斯斑岩铜矿与南部青藏高原隆升之关系——来自含矿斑岩中多阶段锆石的证据

Relationship between Gangdese porphyry copper deposits and uplifting of southern Tibet plateau: Evidence from multistage zircon of ore-bearing porphyries

文章通过阴极发光研究结合SHRIMPU_Pb法精确定年,发现青藏高原南部冈底斯斑岩铜矿带含矿斑岩中的锆石由残留、变质、岩浆3种类型组成,其年龄分别为(51.1±4.8)Ma、(21.1±2.6)Ma和(14.47±0.5)Ma。LA_ICP_MS分析表明残留锆石的特点是Y(1121×10-6)、HREE(641×10-6)和MREE(182×10-6)含量高,U(207×10-6)、Th(171×10-6)和Hf(0.96%)含量低。与残留锆石和岩浆锆石相比,变质锆石Th/U比值明显降低(0.54)。在3种类型的锆石中都具有明显的负Eu异常和正Ce异常,但岩浆锆石以Ce异常变化大为特征。冈底斯铜矿带含矿斑岩中识别出的这3个锆石阶段与冈底斯碰撞造山带演化中的3个重要构造事件相对应。作为印度与亚洲大陆碰撞拼贴的主缝合带,这种一致性允许我们提出这样一个构造模式:50～60Ma之前印度与亚洲大陆碰撞期间发生地幔镁铁质岩浆底侵,形成了含矿斑岩的源区;约21Ma前,由于软流圈物质上涌,同时造成了底侵镁铁质岩石部分熔融形成含矿岩浆和地壳快速隆升;约15Ma前,伴随着高原南部地壳隆升后的伸展塌陷,含矿岩浆侵位,形成了冈底斯斑岩铜矿带。

Cathodoluminescence imaging combined with SHRIMP U-Pb dating demonstrates that zircons from orebearing porphyries of the Gangdese porphyry copper belt on the southern Tibetan plateau are composed of inherited, metamorphic and magmatic zircons with ages of （51.1 ± 4.8） Ma, （21.1 ± 2.6） Ma and （ 14.47 ± 0.5 ） Ma, respectively. LA-ICP-MS analysis shows that the inherited zircons are characterized by high concentrations of Y （1121 ×10^-6）, HREE （641 ×10^-6） and MREE （182 ×10^-6）, and low abundances of U （207 × 10^-6）,Th （171 ×10^-6） and Hf （0.96%）. Compared with the inherited and magmatic zircons, the metamorphic zircons have markedly lower Th/U ratios （0.54 on average）. Although the three types of zircons all have obvious negative Eu anomalies and positive Ce anomalies, the magrnatic zircons show much larger variation in Ce anomaly. What is of significance is that these zircon types correspond to three important tectonic events in the evolution of the Gangdese collisional orogen, and this makes the authors advance the following tectonic model. During the India-Asia collision （50-60 Ma）, mantle-derived marie magmas were underplated, leading to the formation of the source of ore-bearing porphyries. Some 21 Ma ago asthenospheric upwelling resulted in partial melting of the underplated marie rocks under the conditions of metamorphic garnet-amphibolite facies and rapid uplifting of the southern plateau. Concomitantly with the extensional crustal collapse after the uplift at about 15 Ma, the orebearing magmas were emplaced, forming porphyry copper deposits.