西秦岭碌础坝岩体的锆石U-Pb年龄、岩石成因及其地质意义

Zircon U-Pb Ages and Petrogenesis of the Luchuba Pluton in West Qinling,and Its Geological Significance

碌础坝岩体位于秦岭构造带的西段,自内向外依次由含斑电气石黑云母二长花岗岩、含斑细粒黑云母二长花岗岩、似斑状黑云母二长花岗岩、中粗粒黑云母二长花岗岩和中粒黑云母石英闪长岩组成,岩体中岩浆暗色包体发育。LA-ICPMS锆石U-Pb定年显示,碌础坝岩体形成于早中生代,其岩浆演化可划分为中三叠世(235Ma)和晚三叠世(218~209Ma)两期。早期为中粒黑云母石英闪长岩,该期岩石SiO2含量较低,富碱,属于准铝质、高钾钙碱性系列,具有I型花岗岩的特征;晚期为黑云母二长花岗岩,具有高硅、富碱的特征,属于准铝质-弱过铝质、钾玄岩-高钾钙碱性I型花岗岩。从早到晚,岩石的稀土元素总量具有由高到低的变化趋势,两期岩石的稀土元素配分曲线均为轻稀土相对富集的右倾型,晚期黑云母二长花岗岩更亏损HREE,早期闪长岩负铕异常不明显,晚期花岗岩的负铕异常较为显著。两期岩石均富集K、Rb、Ba等大离子亲石元素,而相对亏损P、Nb、Ta、Ti等高场强元素。岩体中发育的岩浆暗色包体与寄主岩石的主要氧化物具有相关性,微量和稀土元素特征相似。岩体中黑云母二长花岗岩和中粒黑云母石英闪长岩的εNd(t)值分别为-8.0和-7.0,相应的tDM为1.43Ga和1.40Ga。早期中粒黑云母石英闪长岩的εHf(t)为-4.47~0.53,集中于-3^-1之间,tDM C主要为1.6~1.3Ga;晚期黑云母二长花岗岩的εHf(t)变化于-11.64~1.16,集中于-7^-2范围内,tDM C为1.7~1.4Ga。岩石地球化学和Nd-Hf同位素组成特征表明,碌础坝岩体的源区物质是以中元古代壳源物质为主,有少量年轻幔源组分的参与。该岩体与其北边的中川岩体在年代学、岩石地球化学和同位素组成上具有一定的相似性,因此,从岩浆作用角度考虑,该岩体的外围也可能与中川岩体的一样,具有金的成矿潜力。

The Luchuba pluton is located in the western segment of the Qinling Orogen.It consists of porphyritic-bearing tourmaline biotite monzogranites, porphyritic-bearing and fine-grained biotite monzogranites,porhyritic biotite monzogranites,mid-coarse-grained biotite monzogranites and mediumgrained biotite quartz diorites from its center to margin and contains mafic magma enclaves.LA-ICPMS zircon U-Pb dating suggests that the Luchuba pluton formed in early Mesozoic,and can been divided into two stages,Middle Triassic（235 Ma）and Late Triassic（218~209 Ma）.The early stage is mediumgrained biotite quartz diorites,characterized by low in silica and high in alkali,showing metaluminous and high K cal-alkaline series with I type features.The late biotite monzogranites are rich in silica and alkali,being metaluminous to weak peraluminous and shoshonitic to high K cal-alkaline series with I type features.From early to late stage,the REE content is decreased,and the two-stage granitoids are characterized by enrichment in LREE,but the late biotite monzogranites deplete in HREE.The early biotite quartz diorites have slightly negative Eu anomalies,and the late biotite monzogranites show obvious negative Eu anomalies.All the rocks are enriched in large ion lithophile elements（K,Rb,Ba）,and depleted in high-field strength elements（P,Nb,Ta,Ti）.TheεNd（t）of biotite monzogranite and biotite quartz diorites are-8.0and-7.0,respectively,corresponding Nd model ages from 1.43 to 1.40 Ga.TheεHf（t）values of the early stage biotite quartz diorites are-4.47 to 0.53,mainly between-3and-1,and tDM Cof 1.6to 1.3Ga;while theεHf（t）values of late stage biotite monzogranite are-11.64 to 1.16,mainly from-7to-2,and tDM Cof 1.7to 1.4Ga.The host granitoids and mafic magma enclaves show linear relationship in most major elements and similar trace elements.Geochemistry and Nd-Hf isotopic compositions suggest that the host granitoids derived mainly from the Mesoproterozoic continental crust,with involvement of juvenile component.This pluton is similar to the Zhongchuan pluton,adjacent to its north,in chronology,petro-geochemistry and isotopic compositions,It may imply that there are also potential for gold mineralization surround the Luchuba pluton as that of the Zhongchuan pluton from the view of magmatism.