Geochemistry of Nanhuan-Cambrian Sandstones in Southeastern Hunan, and Its Constraints on Neoproterozoic-Early Paleozoic Tectonic Setting of South China
Geochemistry of Nanhuan-Cambrian Sandstones in Southeastern Hunan, and Its Constraints on Neoproterozoic-Early Paleozoic Tectonic Setting of South China
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发表时间：
2007
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通讯作者：
Ma Tieqiu
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作者：
Ma Tieqiu
湘东南地区位于扬子陆块与华夏陆块的过渡地带。本区在南环-寒武纪形成了巨厚的复理石建造。沉积物砂岩总体上以中等SiO2含量、高K2 O/Na 2 O和Al 2 O3/（Na 2 O +CaO）比值、明显高Fe 2 O3 *+MgO和CaO含量为特征。稀土元素分布模式与典型的后太古代页岩和上陆壳相似，轻稀土富集，重稀土富集，Eu负异常明显。常量和微量元素特征反映了被动大陆边缘、活动大陆边缘和大陆弧几种不同构造环境的信息。根据原岩的蚀变、风化条件和沉积物的搬运沉降过程，大陆弧和活动大陆边缘形成的岩石应具有与被动大陆边缘明显不同的地球化学特征，而被动大陆边缘形成的岩石可包含大陆弧和活动大陆边缘的某些地球化学信息。因此，湘东南地区的南华-寒武系砂岩形成于被动大陆边缘构造环境。这一认识与形成于大陆弧和活动大陆边缘的古-中元古代岩石是南环-寒武系砂岩的主要原岩，强烈断陷沉积盆地具有一定活动性的客观事实相吻合。板溪群与冷家溪群之间至少40~60 Ma没有沉积物，江南造山带新元古代岩浆活动与碰撞造山有关，南环-早古生代沉积物中缺乏岛弧火山物质，中元古代连续存在，（甚至更早）湘东南地区的结晶基底表明，扬子陆块与华夏陆块之间的早古生代沉积盆地不是洋盆而是陆内裂谷盆地。根据湘东南砂岩地球化学特征对扬子-华夏陆块之间新元古代-早古生代沉积盆地形成的构造背景的认识与根据区域资料的认识是一致的。
Southeastern Hunan lies in the transition area of the Yangtze and Cathaysia block. Thick flysch formation formed during the Nanhuan—Cambrian in the area. The sandstones of the sediments on the whole are characterized by intermediate SiO2 contents, high K2O/Na2O and Al2O3/ (Na2O+CaO) ratios, clearly high Fe2O3*+MgO and CaO contents. The REE distribution patterns are uniform and similar to typical post-Archean shales and the upper continental crust, with LREE enrichment, falt HREE, and significant negative Eu-anomalies. Characteristics of major and trace elements show information of several different tectonic settings of passive continental margin, active continental margin and continental arc. According to the corroded protoliths, efflorescent conditions and carry-subside processes for sediment, the rocks formed in continental arc and active continental margin should possess geochemical characteristics different observably from those in passive continental margin, while the rocks formed in passive continental margin can embrace some geochemical information of continental arc and active continental margin. Therefore the Nanhuan—Cambrian sandstones from southeastern Hunan were formed in a tectonic setting of passive continental margin. The idea coincide with the objective fact that Paleo-Mesoproterozoic rocks formed in continental arc and active continental margin were major protoliths of the Nanhuan—Cambrian sandstones, and that the violently down-faulted sedimentary basin took on some activity. Some evidences such as at least 40～60 Ma absence of sediment between the Banxi Group and Lengjiaxi Group, Neoproterozoic magmatism related with collisional orogeny in Jiangnan orogens, the absence of island-arc volcanic material in the Nanhuan—Early Paleozoic sediments and the continuous Mesoproterozoic (even earlier) crystalline basement in southeastern Hunan indicate that the Neoproterozoic—Early Paleozoic sedimentary basin between the Yangtze and Cathaysia block was not an oceanic but intracontinental rift basin. The idea about the tectonic setting of the Neoproterozoic—Early Paleozoic sedimentary basin between the Yangtze and Cathaysia block based on the geochemistry of the sandstones from southeast Hunan correspond with that based on regional materials.