西秦岭中—晚三叠世构造属性:来自将其那梁侵入杂岩体的年代学及地球化学证据

西秦岭造山带印支早期的构造环境仍存在较多争论,选择西秦岭将其那梁杂岩体进行详细的年代学、岩石学及地球化学分析,以期对该科学问题进行深入探讨。将其那梁杂岩体由石英闪长岩和花岗斑岩组成,石英闪长岩LA-ICP-MS锆石U-Pb年龄为(240.0±1.5) Ma,形成时代为中三叠世,属于早印支期。将其那梁杂岩体具有富钾(K₂O=3.09%~3.54%)、富碱(K₂O+Na₂O=6.44%~7.20%)和过铝质(A/CNK=1.05~1.56)特征,Mg^#值(54~67)较高,属于过铝质高钾钙碱性岩类。将其那梁杂岩体石英闪长岩和花岗斑岩具有相似的微量元素及稀土元素组成,轻重稀土元素分馏明显(LREE/HREE=8.19~14.63),呈右倾特征,显示无或弱负Eu异常(δEu=0.87~1.03),具有亏损Nb、Ta、Zr等高场强元素和富集Ba、Rb、Sr等大离子亲石元素的地球化学特征。岩石地球化学特征指示,将其那梁杂岩体主要源于下地壳高钾变基性岩的部分熔融,且有幔源物质参与其中。结合区域地质背景,认为将其那梁杂岩体形成于火山弧构造环境,可能与中—晚三叠世阿尼玛卿—勉略洋向北俯冲有关,反映了中—晚三叠世西秦岭地区具有活动大陆边缘的属性。

Middle-Late Triassic Tectonic Attribute in West Qinling: Evidence from Geochronology and Geochemistry of the Jiangqinaliang Intrusive Complex

There are still many controversies about the early Indosinian tectonic setting of the West Qinling orogenic belt. To deepen our understanding on this issue, a study of the petrology, geochronology, and geochemistry on the Jiangqinaliang intrusive complex in West Qinling were reported. The Jiangqinaliang intrusive complex is composed of quartz diorite and granite porphyry. LA-ICP-MS zircon U-Pb age of the quartz diorite is (240.0±1.5)Ma, representing the formation age is Middle Triassic, which belongs to the early Indosinian. The Jiang-qinaliang intrusion complex is characterized by high potassium (K₂O=3.09%-3.54%), rich alkali (K₂O+Na₂O=6.44%-7.20%) and peraluminous (A/CNK=1.05-1.56), as well as high Mg^# values (54-67), belonging to the high-potassium calcium-alkaline peraluminous rocks. Quartz diorite and granite porphyry have similar trace element and rare earth element composition and show that light and heavy rare earth elements clearly fractionated (LREE/HREE=8.19-14.63), with right-leaning characteristics. In addition, they exhibit no or weak negative Eu anomaly (δEu=0.87-1.03), depleted in the high-field elements, such as Nb, Ta, and Zr, and enriched in the large ion lithophile elements, such as Ba, Rb, and Sr. These characteristics indicate that the Jiangqinaliang intrusion complex is mainly derived from partial melting of high-potassium metabasic rocks in lower crust, and involved mantle materials. Combined with regional geological background, it is suggested that the Jiangqinaliang intrusion complex was formed in a volcanic arc tectonic setting and may be related to the northward subduction of the Animaqin-Mianlue ocean during the Middle-Late Triassic, showing that the West Qinling area has the attribute of an active continental margin in the Middle-Late Triassic.