Petrology and Isotopic Chronology of Tagong Granite in Eastern Songpan-Ganzi Fold Belt and Its Tectonic Significance
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摘要: 松潘-甘孜造山带东缘的三叠纪-侏罗纪中酸性花岗岩，如塔公岩体，为晚三叠世的碰撞造山以及燕山期的构造演化提供了重要信息.区域地质调查填图表明，塔公岩体为石英闪长岩、花岗闪长岩、黑云母二长花岗岩.其地球化学特征表明岩石主体属高钾钙碱性系列、准铝质-过铝质过渡系列，稀土配分曲线显示其轻稀土富集、重稀土亏损，具有壳源岩浆特征，稀土-微量元素投点结果表明花岗岩形成于造山时期后碰撞阶段.对塔公花岗岩体采集的4件同位素样品的年龄数据质量较高，为214±1 Ma、214±1 Ma、215±1 Ma、216±1 Ma；该组年龄可以作为限制印支晚期扬子、华北和昌都板块之间碰撞时限的证据.基于塔公花岗岩类的岩石地球化学特征、同位素年代学特征，结合区域研究资料，认为松潘-甘孜造山带东缘类似塔公岩体的晚三叠世花岗岩类形成于昌都、扬子和华北地块后碰撞阶段的岩石圈拆沉作用.Abstract: The Triassic-Jurassic intermediate-acid granites in the eastern Songpan-Ganzi fold belt, such as the Tagong granite, provide important information for the collisional orogeny of the Late Triassic and the tectonic evolution in Yanshanian. The regional geological survey map shows that the Tagong granite consists of quartz diorite, granodiorite, biotite monzonitic granite. Its geochemical characteristics indicate that the main body of the rock belongs to high-potassium-calcium-alkaline series and quasi-aluminum-over-aluminum transition. The rare earth element pattern shows that it is enriched in light rare earth and depleted in heavy rare earth. It has the characteristics of crust-derived magma. The rare earth-trace elements dosing results indicate that granite was formed in the post-collision stage of the orogenic period. The isotope samples collected from the Tagong granite obtained high-quality isotopic age, namely 214±1 Ma, 214±1 Ma, 215±1 Ma, 216±1 Ma, which can be used as evidences to limit the collision time among the Yangtze, North China and Changdu plates in the Triassic. Based on the geochemical characteristics and isotopic chronology of the Tagong granitoids, combined with the regional research data, it is considered that the Late Triassic granitoids similar to the Tagong granite in the eastern Songpan-Ganzi fold belt were formed due to the lithospheric delamination in the post-collision stage of Changdu, Yangtze and the North China plates.
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Key words:
- Songpan-Ganzi fold belt /
- Tagong granite /
- granite /
- geochemistry /
- isotopic chronology /
- post-collision stage /
- delamination
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图 1 塔公岩体大地构造位置及采样位置
Fig. 1. Tectonic location and sampling position of the Tagong granite
图 2 塔公岩体及围岩的地质图
Fig. 2. Schematic geological map of the Tagong granite and its surrounding rocks
图 4 塔公花岗岩镜下特征
a. 石英闪长岩；b. 花岗闪长岩；c、d. 黑云母二长花岗岩
Fig. 4. The microscope characteristics of Tagong granite
图 5 塔公花岗岩体主量元素图解
Fig. 5. Graphical diagrams of the main elements of the Tagong granite
图 7 塔公岩体锆石阴极发光图像
Fig. 7. Cathodoluminescence (CL) images for zircons of Tagong granite
图 8 塔公花岗岩类锆石U-Pb谐和图和加权平均年龄
Fig. 8. Zircon U-Pb harmonic diagrams and weighted mean ages of Tagong granite
图 9 塔公花岗岩体的构造环境判别图解
Fig. 9. Diagrams of tectonic environment discrimination of Tagong granite
表 1 样品信息
Table 1. Sample information
样品编号 样品用途 采样高程(m) 经度 纬度 岩石名称 PM17-14-1 主量、微量、稀土元素分析 4 171 101°35'21"E 30°12'16"N 灰色细粒石英闪长岩 PM17-14-2 主量、微量、稀土元素分析 4 171 101°35'21"E 30°12'16"N 灰色细粒石英闪长岩 PM17-19-1 主量、微量、稀土元素分析；锆石U-Pb同位素 4 209 101°35'12"E 30°12'30"N 灰色细-中粒花岗闪长岩 PM17-29-1 主量、微量、稀土元素分析；锆石U-Pb同位素 4 143 101°34'46"E 30°13'28"N 灰色细-中粒黑云母花岗闪长岩 PM17-30-1 主量、微量、稀土元素分析 4 152 101°34'37"E 30°13'31"N 灰白色中粒黑云母二长花岗岩 PM17-34-1 主量、微量、稀土元素分析 4 124 101°34'21"E 30°13'47"N 灰色中细粒黑云母花二长岗岩 PM17-43-1 主量、微量、稀土元素分析；锆石U-Pb同位素 4 024 101°34'00"E 30°14'46"N 灰白色细粒黑云母二长花岗岩 PM17-63-1 主量、微量、稀土元素分析；锆石U-Pb同位素 3 984 101°32'33"E 30°16'50"N 灰白色细粒花岗闪长岩 PM17-66-1 主量、微量、稀土元素分析 3 948 101°32'32"E 30°17'13"N 灰白色细粒花岗闪长岩 
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