滇西北红牛—红山铜矿床斑岩锆石U-Pb年龄、Hf同位素及地球化学特征

Zircon U-Pb Geochronology,Hf isotopic compositions and geochemical characteristics of porphyry in Hongniu-Hongshan copper deposit,northwest Yunnan

滇西北格咱铜多金属矿集区是西南三江特提斯构造域重要的Cu—Mo多金属成矿带之一,燕山期伴随着构造—岩浆—热液作用形成了一系列的斑岩—矽卡岩型Cu(Mo)多金属矿床。其中红牛—红山铜矿床是区内代表性矿床。笔者等在系统的野外工作基础上对红牛—红山铜矿花岗斑岩—石英二长斑岩复式岩体进行了LA-ICP-MS锆石U-Pb定年、岩石地球化学和锆石Lu—Hf同位素分析。结果表明,斑岩富硅(SiO_(2)=59.19%~72.20%)、富碱(K_(2)O+Na_(2)O=6.65%~12.33%),具有较高的稀土元素含量(ΣREE=163×10^(-6)~588×10^(-6),平均值为272×10^(-6)),轻重稀土元素分馏程度较高(LREE/HREE=17.2~31.7),负铕异常(δEu=0.64~0.84),相对富集Rb、Th、U、K等大离子亲石元素(LILE),而相对亏损Nb、Ti、Ta、P等高场强元素(HFSE),为一套准铝质—过铝质的高钾钙碱性—钾玄岩系列高分异I型花岗岩,有向A型花岗岩演化趋势,具高锶低钇特征。3件花岗斑岩样品锆石LA-ICP-MS U-Pb年龄分别为80.11±0.63 Ma、76.59±0.53 Ma、76.49±0.63 Ma,2件石英二长斑岩锆石LA-ICP-MS U-Pb年龄为77.27±0.70 Ma、76.99±0.75 Ma,均属于燕山晚期。锆石ε_(Hf)(t)值为-10.3~-4.6,两阶段模式年龄T_(DM2)为1.277~1.582 Ga,峰值为1.35~1.40 Ga,揭示了红牛—红山花岗斑岩—石英二长斑岩源于中元古代地壳基底的深熔作用。综合认为红牛—红山花岗斑岩—石英二长斑岩的形成与早期形成的印支期格咱岛弧加厚地壳部分熔融密切相关。

The Gezan Copper Polymetallic Ore Concentration Area is one of the important Cu—Mo polymetallic mineralisation zones in the Southwest Sanjiang Tethys tectonic domain.A series of porphyry—skarn type Cu(Mo)polymetallic deposits were formed during the Yanshan period along with tectonic—magmatic—hydrothermal interaction.Among them,the Hongniu—Hongshan copper deposit is a representative deposit in the region.This paper carried out LA-ICP-MS zircon U-Pb dating,rock geochemistry and zircon Lu—Hf isotope analysis on the granite porphyry—quartz monzonite porphyry complex in Hongniu—Hongshan copper deposit based on systematic field work.The results show that the porphyry is Si-rich(SiO_(2)=59.19%~72.20%)and alkali-rich(K_(2)O+Na_(2)O=6.65%~12.33%),has a high content of rare earth elements(ΣREE=163×10^(-6)~588×10^(-6),average value is 272×10^(-6)),a high fractionation degree of light and heavy rare earth elements(LREE/HREE=17.17~31.71),negative Eu anomaly(δEu=0.64~0.84),relatively enriched in large ion-loving elements such as Rb,Th,U and K(LILE),and relatively depleted in high field strength elements such as Nb,Ti,Ta and P(HFSE).The Hongniu—Hongshan copper deposit complex pluton is a set of quasi-aluminous—peraluminous high-potassium calcalkaline-shoshonite series of highly differentiated I-type granites,which has the trend of evolving to A-type granites,with high strontium and low yttrium characteristics.The zircon LA-ICP-MS U-Pb ages of the three granitic porphyry samples are 80.11±0.63 Ma,76.59±0.53 Ma and 76.49±0.63 Ma,respectively,and the zircon LA-ICP-MS UPb ages of the two quartz-monzonite porphyries are 77.27±0.70 Ma and 76.99±0.75 Ma.All belong to the Late Yanshanian.The zirconε_(Hf)(t)values range from-10.3 to-4.6,and T_(DM2)ages range from 1.277 to 1.582 Ga,with peak between 1.35 and 1.40 Ga,which indicates that the granite porphyry—quartz monzonite porphyry in Hongniu—Hongshan copper deposit originated from deep melting at the base of the Middle Proterozoic crust.Combined with previous studies,the formation of the granitic porphyry—quartz diorite in Hongniu—Hongshan copper deposit is closely related to the partial melting of the thickened crust of the Geza island arc formed in the early Indosinian period.