摘要:
桂东南灵山-藤县断裂带内基性侵入岩的演化背景,是认识钦杭结合带西南段构造演化的关键。本文采用LAICP-MS技术测得三门滩岩体角闪苏长岩和东胜岩体石英角闪苏长岩的锆石U-Pb年龄分别为230.0Ma±0.9Ma和236.5Ma±1.3Ma。岩石地球化学特征显示出低镁、富钛铁的特征;稀土含量低且变化大,ΣREE(43~189)×10-6,δEu值0.79~1.63,以具Eu正异常为特征;大离子亲石元素(LILE)Rb、Ba、Th强烈富集,伴有Ce、P和Sm弱富集的"三隆起"特征,高场强元素(HFSE)Ta、Nb和Zr、Hf在弱富集的背景上显示为"谷"形,总体显示为大陆性火山弧钙碱性玄武岩的地球化学特征。研究认为,藤县基性侵入岩形成于中三叠世,属大陆性火山弧钙碱性玄武岩,形成于活动大陆边缘弧环境,为桂东南地区印支期大陆边缘造山带的地质记录。
Abstract:
The basic intrusive rocks in the Tengxian region, southeastern Guangxi occur as N-E-orientated beads along the Lingshan-Tengxian fault, including the Sanmentan, Dongsheng, Longcun, Tangcun and Hanchi intrusive rock masses, and consist dominantly of norite, gabbro norite, gabbro and quartz gabbro diorite. The LA-ICP-MS zircon U-Pb ages are dated at 230.0±0.9 Ma for hornblende norite from the Sanmentan rock masses and 236.5±1.3 Ma for quartz hornblende norite from the Dongsheng rock masses. Geochemically, these intrusive rocks are characterized by low Mg and high Ti and Fe contents. The total REE contents are relatively low and highly variable, with ΣREE amounts of (43-189)×10-6,δEu values from 0.79 to 1.63 showing positive Eu anomalies. The primitive mantle-normalized trace element distribution patterns display the geochemical signatures of continental volcanic arc calc-alkaline basalts characteristic of the highly enrichment of the large-ion lithophile elements(LILE) Rb, Ba and Th and slightly enrichment of the elements Ce, P and Sm, indicating "three uplifts" features and the "valley" shape distribution of the high field strength elements(HFSE)Ta, Nb, Zr and Hf against the background of weak enrichment. The results of research have reflected that the basic intrusive rocks in the Tengxian region were formed during the Middle Triassic, and are assigned to the continental volcanic arc calc-alkaline basalts in the active epicontinental arc environments, and represent the geological records of the Indosinian epicontinental orogenic belts in southeastern Guangxi.
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