广西六万山印支早期花岗岩地球化学特征及锆石U-Pb年龄

Zircon U-Pb ages and geochemical characteristics of Early Indosinian Liuwanshan granites,Guangxi

广西六万山印支早期花岗岩的岩性主要为堇青黑云二长花岗岩,根据岩石结构特征及侵入接触关系可划分为4个侵入单元。利用LA-ICP-MS U-Pb同位素测定技术,4个侵入单元的锆石^(206)Pb/^(238)U年龄加权平均值分别为252.8±1.9Ma(MSWD=0.92)、249.2±1.9Ma(MSWD=1.2)、248.3±3.2Ma(MSWD=2.1)、246.8±2.8Ma(MSWD=2.2)。地球化学分析结果表明,花岗岩在岩石化学上富集大离子亲石元素Rb、K等,相对亏损高场强元素Ta、Nb、Ti等,反映出与俯冲带有关的岛弧岩浆岩相似的地球化学特征。稀土元素球粒陨石标准化配分曲线呈右倾型,具明显的Eu负异常,相对富集重稀土元素,与典型壳源花岗岩配分曲线一致。部分熔融与分离结晶模型(La/Sm-La)显示,花岗岩主要由部分熔融作用形成,演化过程经历了一定程度的分离结晶作用。结合花岗岩岩石化学特征、微量元素地球化学特征及花岗岩构造环境判别图解,认为六万山印支早期花岗岩的形成经历了早期洋-陆俯冲阶段到晚期陆-陆碰撞阶段的构造演化过程。

The Liuwanshan granites consist mainly of cordierite-bearing biotite monzogranite, which can be divided into four emplacement units according to rock structure and the emplacement relationship. LA-ICP-MS zircon U-Pb dating shows that the^(206)Pb/^(238)U weighted average ages of zircons from these emplaced units are 252.8±1.9Ma(MSWD=0.92), 249.2±1.9Ma(MSWD=1.2), 248.3±3.2Ma(MSWD=2.1), and 246.8±2.8Ma(MSWD=2.2), respectively. Geochemical data reveal that these granites are characterized by enrichment of large ion lithophile elements(LILE, such as Rb and K) and relative depletion of high field strength elements(HFSE, such as Nb, Ta and Ti), suggesting geochemical characteristics of typical subduction-related arc magmatic rocks. REE distribution patterns show right-inclined shape with obvious Eu negative anomaly and relative enrichment of HREE, which is consis-tent with crust-source granites distribution patterns. La-La/Sm diagram indicates that the Liuwanshan granites were mainly formed by partial melting and experienced a certain degree of fractional crystallization. Combined with petrochemical characteristics, trace elements geochemeical characteristics and tectonic setting discrimination diagrams, the authors have reached the conclusion that Liuwanshan granites were formed in a tectonic setting of early ocean-continent subduction and later continental collision.