滇西北休瓦促晚白垩世岩浆-成矿作用动力学机制探讨

Discussion on the dynamic mechanism of the Late Cretaceous magmatism-metallogenesis in Xiuwacu,northwestern Yunnan Province

休瓦促W-Mo矿床位于滇西北义敦—格咱岛弧中部,为一中-大型斑岩岩浆热液矿床。由于自然环境恶劣,对该区晚白垩世构造-岩浆活动的研究工作起步较晚,且主要集中于富碱岩浆成岩-成矿年代学、岩石成因、动力学背景、流体特征和成矿物质来源研究等方面,但对岩浆-成矿作用构造应力场和动力学机制的研究还比较薄弱。文中对休瓦促W-Mo矿床东、西矿区重点坑道和剖面开展构造-岩相蚀变特征解析研究,并结合锆石U-Pb、Lu-Hf和O同位素特征,认为休瓦促花岗岩体为一个两期复式岩体,以近S-N向F4断层为界可分为东、西矿区。东矿区以晚三叠世(212～201 Ma)似斑状黑云母花岗岩为主;西矿区以晚白垩世(85.6～84.4 Ma)似斑状石英二长花岗岩和二长花岗斑岩为主。西矿区W-Mo矿床寄主石英二长花岗岩锆石原位U-Pb、Lu-Hf和O同位素显示其形成时代为(80.18±0.80)Ma(LA-ICP-MC U-Pb),ε_(Hf)(t)和δ^(18) O值分别为-4.49～-8.07和5.98‰～7.45‰,Hf同位素T_(DM2)模式年龄分别为1 432～1 239 Ma,结合前人地球化学特征资料,推知晚白垩世石英二长花岗岩物质来源于加厚下地壳部分融熔作用。对矿区135个晚白垩世石英二长花岗岩岩脉和白钨矿-辉钼矿石英脉产状数据进行区域构造解析研究,可判断义敦—格咱地区晚白垩世区域构造应力场特征为NE-SW向伸展。综上所述,义敦—格咱地区晚白垩世花岗岩可能形成于NW-SE向延伸的新特提斯洋盆俯冲作用致使先存古特提斯—中特提斯碰撞造山加厚下地壳减压部分熔融的动力学背景。

The Xiuwacu W-Mo deposit is a middle-large porphyry magmatic hydrothermal deposit located at the central section of the Yidun-Geza orogenic belt in northwestern Yunnan Province.Adverse natural environment in this region has delayed studies on Late Cretaceous tectono-magmatic activities in Late Yanshan,with focuses mainly on geochronology,petrogenesis,geodynamic background,fluid characteristics and source of ore-forming materials related to the Late Cretaceous alkali-rich granite,etc.However,discussion on structural stress field and dynamic mechanism is still rare.In this paper,we investigated in detail the structures and alteration-lithofacies of key sections and gallery in the east and west districts,Xiuwacu W-Mo deposits.Taking into account the zircon U-Pb,Lu-Hf and O isotopic characteristics,we believe that the Xiuwacu granitic pluton is a two phase complex pluton divided into east and west ore districts with a near S-N F4 fault as the boundary.The east district is dominated by Late Triassic(212-201 Ma)porphyritic biotite granite,while mainly Late Cretaceou porphyritic quartz monzogranite and monzogranitic porphyry are in the west district.The situ-zircon U-Pb,Lu-Hf and O isotopic analysis on the porphyritic quartz monzogranite suggests that the monzogranite formation age is 80.18±0.80 Ma(LA-ICP-MC U-Pb)(negative zirconεHf(t)=-4.49--8.07,variableδ18 O ranged 5.98‰-7.45‰).Hf isotopic TDM2 model age is 1432-1239 Ma.Together with known geochemical characteristics,they show that the magmatic-mineralization material source of Late Cretaceous porphyritic quartz monzogranite was derived from the partial melting of thickened lower crust.Furthermore,Late Cretaceous regional tectonic stress field is characterized by NE-SW extension in the Yidun-Geza Area by regional structural analysis on 135 typical Late Cretaceous quartz monzogranite dykes and scheelite-molybdenite quartz veins.In summary,we believe that Late Cretaceous magmatic metallogenesis in this region is formed by the NW-SE extension of Neo-Tethys oceanic subduction resulting in partial melting of pre-existing thickened crust decompression.