吉林省兰家金矿南泉眼闪长岩U-Pb年代学和地球化学特征

Zircon U-Pb Age and Geochemistry of Nanquanyan Diorite in the Lanjia Gold Deposit, Jilin Province

南泉眼闪长岩是兰家矽卡岩型金矿的成矿岩体,研究程度极低.通过岩石地球化学和LA-ICP-MSU-Pb年代学研究,讨论了岩石成因、来源和构造背景.研究表明,南泉眼闪长岩为准铝质的钙碱性岩石;岩石明显富集大离子亲石元素(如Rb、K、Sr)、活泼的不相容元素(如Th、U)和轻稀土元素,相对亏损高场强元素(如Nb、Ta、Ti、P)和重稀土元素,具有明显的负Eu异常,Nb/Ta、La/Nb、Th/La比值显示岩石具壳源特征,Al2O3+MgO+FeOt图解显示岩浆具弧型活动大陆边缘特征.闪长岩岩浆锆石LA-ICP-MSU-Pb加权平均年龄为170.21±0.73Ma(MSWD=0.14);谐和年龄值为170.21±0.39Ma(MSWD=0.018),为中侏罗世.成岩构造背景为燕山早期太平洋板块俯冲引起大陆弧伸展和岩石圈减薄,软流圈物质上涌、底侵提供热动力,不成熟下地壳物质部分熔融形成南泉眼闪长岩.

Nanquanyan diorite, hardly well-studied, is the metallogenic intrusion of the Lanjia skarn type gold deposit. Based on the studies of geochemistry, zircon LA-ICP-MS U-Pb data, this paper mainly discusses petrogenesis, magma source, tectonics setting of the diorite. Studies show that Nanquanyan diorite belongs to calc-alkaline and metaluminous series. The trace elements of diorite show enrichment in LILE （such as K, Rb, Sr）, more mobile incompatible elements （such as Th, U） and LREE, relative depletment of HFSE （such as Nb, Ta, Ti, P） and HREE, and show obvious negative Eu anomalies （3Eu= 0. 19-0. 25）. The ratios of Nb/Ta, La/Nb and Th/La indicate that the diorite mainly comes from the crust. In the Al2O3 ＋ MgO＋Fe2O3 diagram, the characteristics of the diorite are similar to those of arc-type active continental margin magmatic rocks. LA-ICP-MS U-Pb zircon data of the diorite give weighted average ages of 170. 21±0. 73 Ma（MSWD=0. 14） and concordant age of 170. 21±0. 39 Ma （MSWD = 0. 018）, which are in the middle Jurassic period. It is suggested that during the early Yanshan, subduction of Pacific plate led to extension of continental arc and lithospheric thinning, and asthenosphere mantle magma underplated subsequently and provided thermopower, finally partial melting of the immature lower crurst formed Nanquanyan diorite.