西藏冈底斯厅宫铜矿侵入岩成因:LA-ICP-MS锆石U-Pb年龄、地球化学及Sr—Nd—Pb同位素证据

Petrogenesis of Intrusions in Tinggong Copper Deposit,Gangdese,Xizang( Tibet):Evidence from LA-ICP-MS Zircon U-Pb Dating,Petrogeochemical,and Sr—Nd—Pb Isotopic Composition

西藏厅宫斑岩铜钼矿床是冈底斯成矿带上最早开发的斑岩铜矿之一,矿区大面积分布的黑云母花岗闪长岩、二长花岗岩、钾长花岗岩均为容矿岩体,因此详细开展这几套岩体的评价工作,对于今后的找矿勘探工作具有重要的指导意义。本文通过分析上述三类岩体LA-ICP-MS锆石U-Pb测年,全岩主微量、稀土元素,Rb-Sr、Sm-Nd、Pb同位素,结果显示三类岩体锆石结晶年龄分别为50.32 Ma、48.34 Ma、49.41 Ma,早于成矿事件,与成矿事件无关。三类岩体均属高钾钙碱性,弱过铝质I型花岗岩,稀土元素呈轻稀土富集的右倾V型模式图,重稀土分馏不明显,明显负δEu,δEu随SiO2含量的升高而降低,推测与岩浆结晶分异的程度有关,微量元素均明显亏损高场强元素Ba、Nb、Sr、Ti,富大离子亲石元素Th、U、K、Cs、Rb。Pb同位素结果表明初始n（^206Pb）/n（^204Pb）、n（^207Pb）/n（^204Pb）、n（^208Pb）/n（^204Pb）计算值较为分散,分别为18.414～18.5137、15.616～15.7039、38.6394～39.0029,放射成因Pb含量较高。初始n（^87Sr）/n（^86Sr）同位素较为均一,主要介于0.70320～0.70477之间。初始n（^143Nd）/n（^144Nd）同位素介于0.51260～0.51265之间,计算得出的εNd（t）为0.51～1.44,均大于0。上述分析结果表明,厅宫矿区大面积分布的容矿始新世岩体并非为成矿岩体,上述岩体具有同源复式侵入特征,岩浆起源与特提斯洋发生的板片前端的拖拽作用造成板片的回旋、断离有关,岩浆起源于新生下地壳/地幔,并在岩浆侵位过程中经历过AFC过程。

The Tinggong porphyry Cu—Mo deposit is one of the first explored deposits in Tibet. Ore-bearing biotite granodiorite,coarse grain monzogranite and fine grain K-feldspar grantie are spread within this area and detailed studies of above major granites are benefited for future exploration. LA-ICP-MS zircon U-Pb dating,major and trace elements,Rb-Sr,Sm-Nd,Pb isotope composition were test by this study. The results show that the age of above granites are 50. 32 Ma,48. 34 Ma,and 49. 41 Ma,respectively,which are apparently older than that of mineralization. Above stocks belong to high-K calc-alkaline,weakly peraluminous,I-type granites. The REE patterns indicate a LREE rich,with weak / no HREE fractionation V-type model and strong depleted δEu,which are reverse with SiO2 content. All granites are rich in Th,U,K,Cs,and Rb,depleted in Ba,Nb,Sr,Ti. Whole rock Pb isotopic compositions show that calculated n（^206Pb） / n（^204Pb） n（^207Pb） / n（^204Pb）,n（^208Pb） / n（^204Pb）are 18. 414 ～ 18. 5137,15. 616 ～ 15. 7039,38. 6394 ～ 39. 0029,respectively,indicating a high radio-Pb isotope composition. Calculated n（^87Sr） / n（^86Sr） and n（^143Nd） / n（^144Nd） restrict in narrow range： 0. 70320 ～ 0. 70477,0. 51260 ～ 0. 51265,respectively,with calculated positive εNd（ t）（ 0. 51 ～ 1. 44）. The above results indicate that spread magna are not related to the mineralization and derived from new lower crust / depleted mantle during the roll-back and break-off of northward subducted ocean crust,undergoing AFC process during emplace.