西藏厅宫矿区始新世斑岩的厘定及其地质意义

Determination of Eocene porphyritic monzogranite intrusions in Tinggong ore district of Tibet and its geological significance

厅宫铜矿的勘查工作一直是围绕英云闪长斑岩、斑状二长花岗岩等几套斑岩进行的,而未对这几期岩浆岩的年代及成因做过详细评价,并且一直认为这几套斑岩为中新世岩浆岩。笔者通过研究,发现在矿区大面积出露的斑状二长花岗岩并非中新世岩浆岩,而是始新世岩浆岩〔(48.88±0.53)Ma〕,为冈底斯岩基的重要组成部分。文章通过精确的SIMS锆石微区O同位素和LA-MC-ICPMS锆石Hf同位素分析,获得斑状二长花岗岩锆石εHf(t)值变化于4.4～8.4之间,δ18O值变化于5.81‰～6.62‰之间,显示出亏损的地球化学特征,表明该岩体可能起源于新生地壳的部分熔融或者楔形地幔部分熔融形成的岩浆发生分异结晶后的产物。通过对比研究还发现,厅宫矿区岩浆岩组合特征与矿集区内冲江、白容及岗讲矿床类似。因此,在进行找矿勘查工作部署时,应充分考虑这套斑状二长花岗岩与矿化的关系,对缩小勘查范围具有重要的现实意义。

The exploration of the Tinggong porphyry copper deposit（PCD） has been always focused on a suite of porphyritic intrusions,which have been thought to be the Miocene in age.But there is no precise dating work on them,and the genesis of these intrusions is still enigmatic.Zircon U-Pb SIMS dating of porphyritic monzogranite in this study indicates that the intrusions intensively exposed in the Tinggong ore district are Eocene 〔（48.88±0.53） Ma〕 instead of Miocene in age,and constitute a part of the Gangdise batholiths.High-precision in-situ ion microprobe analysis of zircon Hf-O isotopes from porphyritic monzogranite shows relatively stable ranges：εHf（t）=4.4～8.4,and δ18O=5.81‰～6.62‰,close to data of the member of depleted mantle（DM） in the plot of δ18O versus εHf（t）.Such isotopic features indicate that abundant mantle-derived materials（up to 70%～90%） were involved in the formation of porphyritic monzogranite.Further studies indicate that such mineralization-unrelated porphyritic intrusions occur not only in Tinggong but also in such ore deposits as Chongjiang,Bairong and Gangjiang in the ore concentration area.Thus,the relationship between the porphyritic intrusions and Cu-Mo mineralization should be adequately considered in the upcoming exploration in this area.