冈底斯中段达布埃达克质含矿斑岩:增厚下地壳熔融与斑岩铜钼矿成因

Ore-Bearing Adakitic Porphyry in the Middle of Gangdese:Thickened Lower Crustal Melting and the Genesis of Porphyry Cu-Mo Deposit

冈底斯中段达布斑岩铜钼矿床发育在印度-欧亚大陆后碰撞地壳伸展环境,含矿斑岩为一套高钾钙碱性岩系,其地球化学组成与典型的埃达克岩的地球化学组成非常类似,如高w(Sr)(401×10-6～1060×10-6)与w(Sr)/w(Y)、[w(La)/w(Yb)]N比值,亏损重稀土元素Yb与Y,无Eu异常、具正Sr异常等。在微量元素原始地幔蛛网图中,明显富集大离子亲石元素Rb、Ba、Th、Sr,亏损高场强元素Nb、Ta、P、Ti。认为达布埃达克质含矿斑岩起源于富钾增厚下地壳的部分熔融,是在中新世后碰撞构造环境中在板片断离作用下受到软流圈热源上涌影响而触发形成的。埃达克质岩浆在上升的过程中可能交代同时期形成的超钾质岩浆,使二者产生混合交换反应,一方面导致埃达克质岩浆的w(MgO)增高,一方面埃达克质岩浆萃取超钾质岩浆中的金属元素,有利于成矿。达布Cu、Mo成矿作用与高Mg#埃达克质岩关系密切,表明在冈底斯地块上寻找与高Mg#埃达克质岩有关的Cu多金属矿床是一个新的找矿思路。

The Dabu porphyry Cu-Mo deposit in the Gangdese in southern Tibet was formed during the post-collision crustal extension. The ore-bearing porphyry which belongs to the high-alkaline series are geochemically similar to the typical adakites,e, g,high Sr contents（401×10^-6～1060×10^-6） and Sr/Y and （La/Yb）N ratios,depletion of Y and Yb,without Eu anomalies and positive Sr anomalies. The analysis of the trace elements indicates that they are rich in large ion lithiphile elements Rb.Ba.Th.Sr, and the depletion in the high field strength elements Nb,Ta,P,Ti. We suggest that ore-bearing porphyry adakitic magmas were derived by partial melting of the relatively K-rich thickened mafic lower crust beneath the Gangdese block, which may relate to the heat from asthenosphere upwelling after slab break-off , in the postcollision in Miocene. When the Dabu adakitic magmas went through the magmas channel,the interactions between the adakitic magmas and ultrapotassic magmas by producing in the same time possibly took place： the MgO content of the adakitic magmas was likely elevated; on the other hand ,the adakitic magmas extracted Cu and Mo in ultrapotassic magmas and easily led to the formation of porphyry Cu-Mo deposit. Meanwhile,Cu-Mo mineralization in Dabu deposit had very close relationship with the high-Mg^# adakitic magmatism,indicating the Cu ore deposits which relate to adakitics can be a new ore-prospecting strategy in the Gangdese block.