秦岭岩群(杂岩)侵入体的年代学、地球化学特征及其对古生代构造运动的启示

Geochronology and Geochemistry of Intrusions in the Qinling Complex: Implications for the Paleozoic Tectonic Evolution

秦岭岩群(杂岩)是北秦岭构造带主要的构造岩石单元之一,对研究华北板块和扬子板块之间的构造演化具有重要意义。本文对秦岭岩群(杂岩)中的4个侵入体(包括片麻状变辉长岩、糜棱岩化花岗岩、片麻状石英闪长岩和片麻状花岗岩)进行了地球化学、锆石U-Pb年代学和Lu-Hf同位素分析,以探讨北秦岭古生代岩浆作用。结果表明4个样品富集LREE,亏损HREE;片麻状变辉长岩的锆石年龄集中在481~471Ma,糜棱岩化花岗岩锆石年龄具有486Ma的强峰,代表早古生代岩浆热事件;片麻状石英闪长岩和片麻状花岗岩的锆石年龄分别在417~384Ma之间和400~388Ma之间,代表秦岭岩群(杂岩)在晚古生代早期经历的岩浆活动事件。片麻状变辉长岩的εHf(t)值在-5.64^-3.32之间,代表其原岩来自古老地壳;片麻状石英闪长岩和片麻状花岗岩的εHf(t)值均为正值,分别在7.62~11.84和7.50~10.70之间变化,表明其原岩来自亏损地幔。

The Qinling Complex is one of the main tectonic rock units in the north Qinling tectonic belt,which has great significance to study of the tectonic evolution between the North China and Yangtze blocks. This paper presents analyses of geochemical data,zircon U-Pb ages and Lu-Hf isotope features of four intrusions in the Qinling Complex,which include gneissic meta-gabbro,mylonitic granite,gneissic quartz diorite and gneissic granites,and discussion on the Palaeozoic magmatism in the north Qinling. The results show that the four samples are all enriched in LREE and depleted in HREE. The zircon ages of gneissic meta-gabbro cluster in 481 Ma^471 Ma,and those of mylonitic granite have a strong peak in 486 Ma,implying a magmatic event in Early Palaeozoic. Besides,the zircon ages of gneissic quartz diorite and gneissic granites are between 417 Ma^384 Ma and 400 Ma^388 Ma,indicating the Qinling Complex experienced a magmatic event in Late Paleozoic. The ε Hf ( t )values of gneissic meta-gabbro range from -5.64 to-3.32,which means that the protolith originated from the ancient crust. Correspondingly,the ε Hf ( t )values of gneissic quartz diorite and gneissic granites are respectively between 7.62~11.84 and 7.50~10.70,indicating that the protolith was from the depleted mantle.