桂东南晚中生代陆川岩体的成因及其构造意义

桂东南晚中生代岩浆活动与古太平洋板块俯冲密切相关。本文对桂东南陆川岩体开展了LA-ICP-MS锆石U-Pb定年、岩石地球化学和Sr-Nd-Hf 同位素研究，获得陆川岩体黑云母二长花岗岩（主体）和花岗细晶岩的形成年龄分别为104.8±0.8Ma 和103.9±1.0Ma，属于早白垩世。地球化学特征上，二者均属于准-弱过铝质、高钾钙碱性岩石系列，富集轻稀土、大离子亲石元素（如Rb、Ba、Th）而亏损高场强元素（如Nb、Ta、P），具较明显的Eu 负异常、高的(87Sr/86Sr) i (0.708600～0.709158) 和低εNd (t)(-7.46~-7.05)等特征。二者εHf (t)(-7.46~-7.05)分别为-4.8~＋0.7（平均-2.36）和-3.8~＋1.2（平均-2.45），对应的Hf 同位素两阶段模式年龄（TDM2）分别为0.99~1.30 Ga和0.96~1.24 Ga，与Nd同位素模式年龄（TDM2）（1.29~1.30 Ga和1.27~1.28 Ga）基本一致。元素及同位素结果表明，岩石源区主要为中元古代晚期的壳源变质杂砂岩，可能有少量地幔物质的加入。结合区域地质资料，认为陆川岩体形成于早白垩世晚期古太平洋板块向华南板块俯冲/后撤导致的弧后伸展背景，华南地区同期（110~90 Ma）岩浆活动的西北边界大致位于北东向博白-岑溪断裂带一线。

Petrogenesis and Its Tectonic Significance of Late Mesozoic Luchuan Intrusion in Southeast Guangxi

Late Mesozoic magmatic activity in southeast Guangxi was closely related to subduction of the Paleo-Pacific plate. We carried out zircon LA-ICP-MS U-Pb dating, petrogeochemistry and Sr-Nd-Hf isotopic studies for the Luchuan intrusion in the southeast Guangxi province. The biotite monzogranite (main body of Luchuan pluton) and granitic aplite have yielded weighted mean ages of 104.8±0.8 Ma and 103.9±1.0 Ma, respectively. They belong to the series of peraluminous high-K calc-alkaline rocks and are enriched in LREE, large ion lithophile elements (such as Rb, Ba, Th) and depletion of high field strength elements (such as Nb, Ta, P), which are characterized by obvious Eu negative anormaly, high (87Sr/86Sr) i value and low εNd (t) value (mean= -7.28). Their 者εHf (t) values are -4.8 to +0.7(mean =-2.36) and -3.8 to +1.2(mean=-2.45), respectively. The corresponding Hf model ages(TDM2) range from 0.99 to 1.30 Ga and 0.96 to 1.24 Ga, which is consistent with the Nd model ages(TDM2) (1.29-1.30 Ga and 1.27-1.28 Ga, respectively). It is suggested that the source rocks are mainly crustal-derived graywacke in the late Mesoproterozoic, with a small amount of mantle material. Combined with regional geological data, we proposed that the Luchuan pluton was formed in a backarc extension setting caused by the subduction or rollback of the Paleo-Pacific plate into the South China plate in the late Early Cretaceous. During the same period (110-90 Ma), the line of NE trending Bobai-Cenxi fault was the northwestern boundary of magmatic activity in South China.