桂东南新地-安平地区辉长岩的年代学、地球化学特征及其地质意义

Geochronology,geochemistry and geological significance of gabbros from Xindi-Anping area,southeastern Guangxi

桂东南新地-安平地区的辉长岩体主要沿着岑溪-梧州深大断裂带两侧分布,与早古生代地层或震旦纪地层呈侵入接触关系,并被白垩纪地层覆盖。岩相学研究表明,其主体岩性以角闪辉长岩和紫苏角闪辉长岩为主,还有少量的含长辉石岩、辉长闪长岩和辉绿（玢）岩。LA-MC-ICP-MS锆石U-Pb定年结果显示,角闪辉长岩的形成年龄为249.1±2.8 Ma（MSWD=0.44）,相当于早三叠世。地球化学特征研究表明,这些辉长岩体具有较低MgO（2.13%-3.59%）和较高Fe2O3T（16.42%-33.53%）、P2O5（0.58%-1.18%）和TiO2（1.52%-2.31%）的特征;岩体相对富集大离子亲石元素（LILE,如Ba、Rb和U）,而Nb-Ta、Zr-Hf和Ti等高场强元素相对亏损,显示出与俯冲带有关的岛弧岩浆岩相似的地球化学特征,表明其可能形成于与俯冲-消减作用有关的活动大陆边缘（大陆弧）环境。结合区域地质特征分析认为,扬子板块和华夏板块结合带（称之为钦-杭结合带）西南段有古生代洋盆的存在,该洋盆一直延续至中三叠世的印支运动才最终闭合,本区广泛发育的大陆弧型辉长岩应为该洋盆发生俯冲-消减作用的地质记录。

The gabbroic intrusions are situated along the Cenxi-Wuzhou fault zone in Xindi-Anping area, southeastern Guangxi. Petrographic studies indicate that these rock bodies intruded into the Lower Paleozoic or Sinian strata, and were covered by Cretaceous. The gabbroic intrusions are mainly composed of amphibolite gabbro, hypersthene-amphibolite gabbro and minor plagioclase-bearing pyroxenite, gabbro-diorite and diabase （-porphyry）. LA-MC-ICP-MS zircon U-Pb dating results yielded a weighted mean 206pb/238U age of 249.1 ± 2.8 Ma （MSWD= 0.44） for the amphibolite gabbro, suggesting an emplacement of Early Triassic. Geochemical analy sis shows that the gabbroic intrusions are characterized by relatively low MgO （2.13 % -3.59 % ） values and rel- atively high FeaO3T（16.42% --33.53% ）, P205（0.58% -- 1.18% ） and TiO2 （1.52% --2.31% ） values, and that the intrusions are relatively enriched in large ion lithophile elements （LILE： Ba, Rb and U） and relatively depleted in high field strength elements （HFSE： Nb-Ta, Zr-Hf and Ti）. Such signatures display geochemical characteristics of typical subduction-related arc volcanic rocks, which suggests that the gabbros were formed in an active continental margin （continental arc） setting. In combination with the regional tectonic evolution, the authors consider that there existed a Paleozoic paleo-ocean basin in the southwestern segment of joint belt be- tween Yangtze and Cathaysia plates （so-called Qinzhou-Hangzhou joint belt）, and the oceanic basin had not been closed until the Indosinian orogeny in middle Triassic. The continental arc gabbro in the region was an important record of this oceanic lithosphere subduction during the Paleozoic period.