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大柴旦地区是柴北缘古生代超高压带的重要组成部分,与超高压岩石相伴的花岗岩十分发育。这些花岗岩具有两类不同的岩石地球化学特征,Ⅰ类以 Na_2O/K_2O 比值小于1、明显的负 Eu 异常和低 Sr、高 Y 为特征,具有 S-型花岗岩的属性,Ⅱ类以 Na_2O/K_2O 比值大于1、弱负 Eu 异常到正 Eu 异常和高 Sr、低 Y 为特征,具有Ⅰ-型花岗岩的属性,反映了它们的源岩及成因上的差异。锆石 SHRIMP U—Ph 定年结果表明,大柴旦地区花岗岩的年龄可分为三组,第一组年龄为446.3±3.9Ma,第二组年龄分别为408.6±4.4Ma、403.3±3.8Ma、401.8±3Ma,第三组年龄分别为374.5±1.6Ma、372±2.1Ma。结合区域地质特征,我们认为,第一组年龄可能反映了柴达木陆块与中南祁连板块碰撞的时代,第二组年龄可能反映了深俯冲地下的板块由于拆沉而折返的时代,第三组年龄可能反映了碰撞隆起后造山带上不同块体之间的伸展、滑塌的时代。
The Dachaichaan area is an important part of the Paleozoic EHP belt along the north margin of the Qaidam basin. Granites associated with ultra-high pressure rocks are well developed. These granites have two types of petrogeochemical characteristics. Class I is characterized by a Na2O / K2O ratio of less than 1, significant negative Eu anomalies and low Sr and high Y, with properties of S-type granites. Class II is characterized by Na2O / K2O Ratio greater than 1, weak Eu anomaly to normal Eu anomaly and high Sr, low Y are characterized by the properties of I-type granites, reflecting their source rock and genetic differences. The results of zircon SHRIMP U-Ph dating show that the age of granite in Dachaidan area can be divided into three groups, the first group is 446.3 ± 3.9Ma, the second group is 408.6 ± 4.4Ma, 403.3 ± 3.8Ma and 401.8 ± 3Ma, the third group of age were 374.5 ± 1.6Ma, 372 ± 2.1Ma. In combination with regional geological features, we believe that the first group of ages may reflect the era of the collision between the Qaidam block and the Central-South Qilian plate. The second group of age may reflect the times when the subducted plate below the deep subduction relics due to delamination. The three groups of ages may reflect the era of extension and collapse between different blocks on the orogenic belt after the collision uplift.