北秦岭桃官坪和火燎沟小岩体年代学、地球化学及岩浆性质

Geochronology,geochemistry,and magmatic properties of small plutons for Taoguanping and Huoliaogou,Northern Qinling

对桃官坪和火燎沟小岩体开展岩相学、岩石地球化学、锆石U-Pb年代学和Hf同位素等研究,明确它们的形成时代、岩石成因、岩浆来源和性质。锆石U-Pb定年结果显示,桃官坪二长花岗岩和火燎沟花岗斑岩分别形成于148.1±0.9 Ma和152.2±1.2 Ma,二者均属于高钾钙碱性–钾玄岩系列,显示准铝质–过铝质的特征,相对富集轻稀土元素和大离子亲石元素(如Rb和Ba),亏损高场强元素(如Nb和Ta)。对比两种岩体:①桃官坪二长花岗岩具有比火燎沟花岗斑岩更高的分异性,显示分异的I型花岗岩特征;②桃官坪二长花岗岩和火燎沟花岗斑岩以古老的壳源岩浆为主,均形成于碰撞后的挤压–伸展转换背景;③桃官坪二长花岗岩的形成温度与火燎沟花岗斑岩的形成温度相近,但前者具有相对较高的氧逸度。结合前人在华北克拉通南缘的研究成果,认为北秦岭地区与成矿相关的桃官坪二长花岗岩是具有分异特征和高氧逸度特征的I型花岗岩,是该地区形成斑岩型Mo矿床的有利因素,可作为本地区的找矿标志之一。

To clarify the formation age,petrogenesis,magmatic source,and properties of the Taoguanping and Huoliaogou small plutons,petrography,zircon U-Pb geochronology,petrogeochemistry,zircon Hf isotope,and other research methods were used in this study.Zircon U-Pb dating indicates that the Taoguanping monzogranite and Huoliaogou granite porphyries formed at 148.1±0.9 Ma and 152.2±1.2 Ma,respectively.In whole-rock geochemical characteristics,they belonged to the high-K calc-alkaline to shoshonite series,showed metaluminous to peraluminous characteristics,were relatively enriched in light rare earth elements(LREE)and large-ion lithophile elements(LILE,e.g.,Rb and Ba),and were depleted in high field strength elements(HFSE,e.g.,Nb and Ta).By comparing the two plutons,this study concludes that:(1)The Taoguanping monzogranite has higher differentiation than the Huoliaogou granite porphyry,showing I-type granite with fractionated characteristics;(2)The Taoguanping monzogranite and Huoliaogou granite porphyry were mainly sourced from the ancient crust-derived magma,which was formed in a post-collisional compression-extension transitional setting;and(3)the formation temperature of the Taoguanping monzogranite is similar to the Huoliaogou granite porphyry;however,the former has relatively high oxygen fugacity.Based on the previous research results in the southern margin of the North China Craton,it is preliminary recognized that the ore-related Taoguanping monzogranite in the North Qinling area is characterized by I-type granite with fractionated characteristics and high oxygen fugacity,which is a favorable factor for porphyry molybdenum deposit formation in this area and can be used as a prospecting indicator in this area.