甜水海地块寒武纪安山岩的地球化学和年代学研究:对西昆仑-喀喇昆仑造山带原特提斯洋演化的启示

甜水海地块作为西昆仑-喀喇昆仑造山带重要的组成部分，夹持于麻扎-康西瓦板块缝合带和洪山湖-乔尔天山板块缝合带之间，其构造属性的探讨对认知西昆仑地区早古生代地质演化具有重要的科学意义。笔者在甜水海地块中部麻扎地区通过野外填图新发现并厘定出一套火山岩-沉积岩组合，主要由安山岩、英安岩、流纹岩及其火山碎屑岩和少量沉积岩组成；与安山岩互层产出的英安岩和流纹岩锆石U-Pb年龄为519~513Ma，表明这套火山-沉积地层形成于中寒武世。该火山岩套的地球化学特征为：安山岩SiO₂含量为57.0%~67.2%，具有高MgO（1.91%~7.17%）含量和Mg^#值（39.1~65.3，平均值为55.0），较高Cr（31.0×10^-6~190×10^-6）和Ni（12.2×10^-6~121×10^-6）含量，低FeO^T/MgO比值（1.12~3.26），低Al₂O₃（13.7%~17.3%）和K₂O（1.12%~5.52%）含量的特征，属于低铁钙碱系列高镁安山岩-镁安山岩；英安岩较安山岩SiO₂含量高（63.8%~71.8%），具有相似的Al₂O₃（13.4%~15.4%）含量，低MgO（0.28%~1.19%）含量和Mg^#值（9.98~36.5，平均值为25.3），低Cr（11.5×10^-6~25.2×10^-6）和Ni（4.33×10^-6~11.8×10^-6）含量，高FeO^T/MgO比值（3.64~18.9）和K₂O（4.84%~9.27%）含量的特征，属于高钾钙碱性系列。安山岩和英安岩总体表现出右倾轻稀土富集分配模式，富集Rb、Ba、K、Th等大离子亲石元素，亏损Nb、Ta、Ti高场强元素，具有典型的岛弧岩浆岩特征。野外调查表明，安山岩与英安岩在空间上相邻，呈互层产出，且它们都出露于中寒武系地层中，暗示了二者在成因上紧密相联。岩石地球化学特征指示了安山岩和英安岩可能源于富集地幔楔不同比例的部分熔融作用，所产生的岩浆在上升过程中又遭受了不同比例的地壳混染。推测麻扎地区寒武纪火山岩形成于原特提斯洋向南俯冲消减机制下的岛弧构造环境，同时结合区域资料，认为在寒武纪甜水海地块存在两期岩浆作用，在早寒武系末期（~520Ma）原特提斯洋发生双向俯冲，甜水海地块北缘转化为活动大陆边缘。

Geochronology and geochemical study of the Cambrian andesite in Tianshuihai Terrane: Implications for the evolution of the Proto-Tethys Ocean in the West Kunlun-Karakoram Orogenic Belt

As an important component of the West Kunlun-Karakoram Orogenic Belt, the Tianshuihai Terrane is located between the Mazha-Kangxiwa and the Hongshanhu-Qiaoertianshan suture zones. It is of important scientific significance for the exploring and understanding of the Early Paleozoic geological evolution of the orogenic belt. Through field geological mapping, we newly discovered a volcanic-sedimentary rock assemblage in the Mazha area of the central part of the Tianshuihai Terrane, which is mainly composed of andesite, dacite, rhyolite, volcanic clastic rocks and a small number of sedimentary rocks. The zircon U-Pb ages of the dacites and rhyolites that were interbedded with andesites were in a range of 519~513Ma, indicating that these volcanic-sedimentary rocks were formed in the Middle Cambrian. The SiO₂ contents of andesites vary within 57.0%~67.2%, and andesites have high MgO contents (1.91%~7.17%) and Mg^# values (39.1~65.3, averaged in 55.0), higher Cr (31.0×10^-6~190×10^-6) and Ni (12.2×10^-6~121×10^-6) contents, low FeO^T/MgO ratios (1.12~3.26), low Al₂O₃ (13.7%~17.3%) and K₂O (1.12%~5.52%) contents. These rocks belong to a type of high-magnesium andesite-magnesium andesite of the low-iron calcium-alkaline series. Compared to the andesites, the dacites have high SiO₂ contents (63.8%~71.8%), similar Al₂O₃ contents (13.4%~15.4%), low MgO contents (0.28%~1.19%) and Mg^# values (9.98~36.5, averaged in 25.3), low Cr (11.5×10^-6~25.2×10^-6) and Ni (4.33×10^-6~11.8×10^-6) contents, high FeO^T/MgO ratios (3.64~18.9) and K₂O contents (4.84%~9.27%), indicating that they belong to the high potassium calc-alkaline series. The andesites and dacites generally exhibit a right-dip REE distribution pattern, enriched in Rb, Ba, K, Th and other large ion lithophile elements, and depleted in Nb, Ta, Ti and other high field strength elements, with typical island arc igneous rock characteristics. Field surveys show that these andesites and dacites are spatially distributed in adjacent area and vertically interbedded, and both of them are located in the Middle Cambrian strata, suggesting that they are closely related in genesis. The geochemical characteristics indicate that these andesites and dacites may be derived from the partial melting of the enriched mantle wedge in different proportions, and the magma produced was subjected to different proportions of crustal contamination during its ascending process. We speculate that the Cambrian volcanic rocks in the Mazha area were formed in an island arc tectonic setting under the southward subduction of the Proto-Tethys Ocean. Meanwhile, combined with the regional geological data, it is inferred that there are two stages of magmatism occurred in the Tianshuihai Terrane in the Cambrian, and double subductions occurred in the Proto-Tethys ocean at the end of the Early Cambrian (~520Ma), during which the northern margin of the Tianshuihai Terrane was transformed into an active continental margin.