赣南苗云岩体U-Pb年代学、地球化学特征及其成因

位于武夷—云开造山带中部的广东始兴—江西全南一带,是研究华南早古生代构造属性的重要区域;以盆地内苗云岩体二云母花岗岩为研究对象,通过系统的年代学和地球化学研究,分析其成因,对早古生代赣南地区构造属性特征进行补充。结果表明:苗云岩体二云母花岗岩具有高硅、高碱、富铝等特征,且相对富钾贫钠,属于高钾钙碱性系列过铝质S型花岗岩类;岩石富集大离子亲石元素Rb、K和Th,亏损Ba、Nb、Ta、Sr等元素,具有岛弧岩浆岩的特征;稀土元素总量较低,富集轻稀土,轻重稀土分馏明显,具有中等Eu负异常,暗示其主要来自于地壳熔融;构造判别图显示其形成于同碰撞的挤压环境,表明苗云岩体可能形成于陆陆碰撞所导致的陆壳加厚环境。SHRIMP锆石U-Pb年龄分析结果显示,其加权平均年龄为(442. 7±3. 2) Ma,为晚奥陶末期—早志留初期岩浆产物;结合区域地质特征,表明赣南地区在晚奥陶末期—早志留初期已由俯冲环境转换到陆陆碰撞环境。

U-Pb geochronology, geochimistry and genesis of Miaoyun pluton in South Jiangxi Province

Systemic chronology and petro-geochemistry analysis of Miaoyun pluton was conducted to analyze its genesis, Which makes up understanding of tectonic attributes of the south Jiangxi province in Eary Paleozoic time.Whole rock major and trace elements analyses indicate that the characteristics of rich silicon, high alkali and rich aluminum, and belong chemically to peraluminous series of S type. The two-mica granites are strongly enriched in Rb, K, Th, and depleted in Ba, Ta, Nb and Sr. Such geochemical features are typical of Subduction-related magmatic rocks. The total amount of REE is relatively low, showing distinct rich LREE, negative Eu anomaly is obvious, suggesting that they were derived from the partial melting of the lower crus. Based on trace element ratios and related discrimination diagrams, the two-mica granites are believed to have been formed in a syn-collisional tectonic setting and maybe formed in the thickened crust caused by the collision between continents. U-Pb dating on zircon by SHRIMP in this study suggests that the two-mica granites were emplaced at 4442.7±3.2Ma, indicating a Late Ordovician-Early Silurian magmatic event. Combined with constructional environment, it showed that it was transformed from plate subduction to continents collision in the south Jiangxi province at Late Ordovician-Early Silurian.