摘要:席家坝花岗片麻岩、斜长角闪岩和金盆村花岗片麻岩出露于扬子西北缘汉南岩体的东北部。这些侵入体中的锆石具有较高Th/U比值，且在阴极发光图像中显示清晰的岩浆振荡环带，具有典型的岩浆成因锆石特征。我们对这3个侵入体分别开展了LA-ICP-MS锆石U-Pb定年工作，结果显示其形成于846±3 Ma、800±3 Ma和815±3 Ma，对应的εHf（t）为-25～-13、-1～+12、+6～+10。席家坝花岗片麻岩的二阶段模式年龄TDM2为3.43～2.56 Ga、席家坝斜长角闪岩的一阶段模式年龄TDM1为1.41～0.88 Ga、金盆村花岗片麻岩的二阶段模式年龄TDM2为1.31～1.11 Ga。席家坝斜长角闪岩和金盆村花岗片麻岩亏损的Hf同位素组成显示，汉南地区在新元古代经历了一期重要的地壳生长事件。锆石的Ti温度计算出这些花岗片麻岩侵入体的岩浆锆石结晶温度为610 ℃～790 ℃。结合区域资料，全岩Zr饱和温度计算结果显示扬子西北缘新元古代花岗质岩浆岩的形成温度随时间有规律性变化，可以识别出3期高温岩浆岩事件。第一期为897～887 Ma，以西乡流纹岩和米仓山喜神坝黑云母花岗岩为代表，对应汉南同期柳树店洋岛玄武岩，可能与～900 Ma地幔柱岩浆活动有关。第二期为817～760 Ma，以米仓山红庙镇钾长花岗岩和铁船山流纹岩为代表，对应汉南820～750 Ma基性侵入岩记录，可能与板块俯冲形成的弧后盆地伸展相关。第三期为～707 Ma，以西乡正长花岗岩为代表，对应同时期扬子西北缘基性侵入岩记录，可能与米仓山、汉南地区弧—陆后碰撞伸展有关。
关键词: 扬子西北缘/
汉南岩体/
花岗岩形成温度/
地壳生长/
构造演化
Abstract:The granitic gneiss and amphibolite are well exposed in the northeastern Hannan massif, such as the Xijiaba and Jinpen areas. Zircons from these rocks have high Th/U ratios and show oscillatory zoning in the CL images, indicating a typical magmatic origin. Here, we present LA-ICP-MS zircon U-Pb age data for the three plutons, which show crystallization ages of 846±3 Ma, 800±3 Ma and 815±3 Ma, respectively. The situ zircon Hf isotopic analysis demonstrates that the three plutons have significantly variable Hf isotopic compositions with respective εHf（t）ranges at -23～-13, -1～+12 and +6～+10. The zircons from the Xijiaba granitic pluton show Archean Hf two stages model ages（TDM2）at 3.43～2.56 Ga. In contrast, the zircons from the Xijiaba amphibolitic pluton have much younger zircon TDM1 ages between 1.41 Ga and 0.88 Ga. For the Jinpen granitic pluton, the zircon grains also give young TDM2 ages at 1.31～1.11 Ga. The depleted Hf isotopic compositions of zircons from the Xijiaba and Jinpen granitic plutons indicate a significant Neoproterozoic crustal growth developed in this region. Based on Ti-in-zircon thermometer, the crystallization temperature of zircons from the studied granitic plutons ranges from 610 ℃ to 790 ℃. Together with the published data in this region, whole-rock Zr saturation temperatures of granitic magmatic rocks indicate that the formation temperatures of these rocks show a regular variation after time. Three high-temperature magmatic events can be distinguished from this variation. The first event occurred at 897～887 Ma and was recorded by the Xixiang rhyolite and the Xishenba granitic pluton. It was likely related to the underplating of the coeval Liushudian ocean island basalt, which was possibly produced by a ca. 900 Ma mantle plume. The second event happened at 817～760 Ma and was recorded by the Hongmiao granitic pluton and Tiechuanshan rhyolite. This high-temperature magmatic event was likely corresponding to the 820～750 Ma basic intrusions in the Hannan area. It was most likely formed by stretching of a new back arc basin. The third event took placed at ca.707 Ma and verified by the Xixiang syenogranite. The contemporaneous basic intrusions were also exposed at the northwestern margin of Yangtze block. They were all generated in an extension setting after arc-continent collision in the Micangshan and Hannan regions.
Key words:Northwestern margin of the Yangtze block/
Hannan plutons/
Formation temperature of granite/
Crustal growth/
Tectonic evolution
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