广东河源地区早白垩世径尾岩体的年代学和地球化学特征及其地质意义

Geochronology and geochemistry of Jingwei pluton in Heyuan area of Guangdong Province and their geological significance

华南晚中生代花岗岩及其形成的构造背景是目前研究的热点之一。笔者等报道了分布于华南板块东南部河源地区径尾岩体的岩石学、地球化学和年代学特征,以探讨该岩体的形成时代、岩石成因及其构造环境。岩石学研究表明,径尾岩体岩性较为单一,主要为黑云母二长花岗岩。LA-ICP-MS锆石U-Pb定年结果显示,径尾黑云母二长花岗岩的年龄在143.2±1.3~145.4±1.1 Ma之间,表明其形成时代为早白垩世。岩石学及地球化学特征显示,径尾黑云母二长花岗岩属于准铝质—弱过铝质系列,具有富SiO_(2)(71.15%~76.11%),富K_(2)O(4.68%~5.89%),贫Al_(2)O_(3)(12.58%~14.03%)的特点,富集Rb、U、Th等大离子亲石元素,亏损P、Ti、Ba、Sr等元素,稀土配分型式呈燕式分布,δEu值为0.11~0.26,负Eu异常明显,Y/Nb>1.2,锆石饱和温度较高,为791~829℃,属于高分异的A2型花岗岩。黑云母二长花岗岩中锆石具有低的εHf(t)值(-7.3~-0.4),对应的Hf同位素二阶段模式年龄(TDM2)为1198~1632 Ma,指示其可能是在伸展构造环境中及低压、高温条件下由古老壳源基底物质部分熔融形成,并伴有少量幔源岩浆的加入,随后在岩浆侵位过程中发生了一定程度的分离结晶。结合区域地质资料,认为径尾黑云母二长花岗岩的形成可能与俯冲古太平洋板块的后撤所导致的岩石圈伸展过程有关。

Objectives and Methods:The study of the late Mesozoic granites in South China and its tectonic setting is a hotspot issue.In this study,we report new zircon U-Pb dating,petrogeochemistry,and zircon Lu—Hf isotopic analysis of biotite monzogranites from the Heyuan area,with the aim of determining its age and petrogenesis,important for understanding the late Mesozoic tectonic evolution of the South China Block.Resluts:The Petrography feature suggest that Jingwei pluton mainly consist of biotite monzogranites.Zircon LA-ICP-MS dating results show that biotite monzogranites were formed at the Early Cretaceous(143.2±1.3~145.4±1.1 Ma).The Jingwei granites is characterized by weak peraluminous and metaluminous,high content of SiO_(2)(71.15%~76.11%),K_(2)O(4.68%~5.89%),but low content of Al_(2)O_(3)(12.58%~14.03%),depletion in P,Ti,Ba,Sr,and enrichment in Rb,U,Th.The chondrite-normalized patterns of REE are in seagull forms,with strong negative Eu anomalies,with 0.11 to 0.26 ofδEu,Y/Nb>1.2,and higher zircon saturation temperatures(791~829℃),which indicates that the Jingwei pluton is similar to A2-type granite.The zircons in the granites have lowεHf(t)values(-7.3~-0.4)and ancient two-stage Hf model ages(TDM2)(1198~1632 Ma),indicating it has probably formed by partial melting of the predominantly ancient crustal rocks,being mixed with little mantle derived mafic malts,under a low-pressure,high-temperature condition at an extensional tectonic setting,and subsequently the magmas fractionated biotite,plagioclase,K-fedspar,apatite and zircon during emplacement.Conclusion:Combined with regional geological background,we infer that the Jingwei biotite monzogranites was probably related to the roll-back of the subducted Paleo-Pacific plate.