北秦岭西段吕梁期构造岩浆事件的年代学及其构造意义
Geochronology and significance of the early Mesoproterozoic tectono-magmatic event in the western segment of the North Qinling Mountains
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摘要: 北秦岭造山带西段宝鸡地区首次识别出中元古代早期的花岗岩体——陕西太白岩基巩坚沟变形侵入体和宝鸡岩基胡店变形侵入体，原岩均为中细粒二长花岗岩。依据锆石CL影像特征，结合锆石成因分析，进行锆石微区U-Pb同位素LA-ICPMS（激光剥蚀等离子体质谱）测定，分别获得1741Ma±12Ma和1770 Ma±13Ma的岩浆结晶年龄。岩石地球化学资料均显示变形侵入体形成于板块边缘的俯冲-碰撞构造环境，岩浆源于部分熔融的地壳物质，应代表北秦岭造山带西段中元古代早期一次强烈的构造岩浆事件。太白巩坚沟和宝鸡胡店变形侵入体的测年结果和陕西凤县红花铺英云闪长岩中捕获锆石的年龄信息（1765 Ma±8Ma），为较精确地限定北秦岭西段吕梁期构造岩浆事件的时限提供了依据。在分析北秦岭造山带西段早期构造属性的基础上，提出近1.8Ga的构造岩浆事件可能是Columbia 超大陆汇聚在北秦岭造山带西段的响应。Abstract: Two early Mesoproterozoic granite intrusions—the Gongjiangou deformation intrusion in the Taibai batholith and Hudian deformation intrusion in the BaoJi batholith—are for the first time identified in Baoji in the western segment of the North Qinling orogenic belt. Their protoliths are both medium- and fine-grained monzogranite. The authors studied the features of CL images of zircons and analyzed their origin. U-Pb LA-ICPMS dating of zircons yielded magmatic crystalline ages of 1741±12 and 1770±13) Ma. Both petrological and geochemical data show that: the deformation intrusions formed in a subduction-collision environment at plate margins and the magma originated by partial melting, which shows that there occurred a strong tectono-magmatic event in the western segment of the North Qinliing orogenic belt in the early Mesoproterozoic. The LA-ICPMS U-Pb dating zircon age (450.5±1.8 Ma) of the Gongjiangou deformation intrusion in Taibai and Hudian deformation intrusion in BaoJi and the age (1765 Ma±8 Ma) of zircons captured in the Honghuapu tonalite in Fengxian, Shaanxi, provide a basis for defining the time range of the early Mesoproterozoic Lüliangian tectono-magmatic event in the western segment of the North Qinling (~1.8 Ga). Based on an analysis of the tectonic attribute of the western segment of the North Qinling orogenic belt, the authors suggest that the nearly 1.8 Ga tectono-magmatic event might be the response of the convergence of the supercontinent Columbia in the western segment of the North Qinling orogenic belt.
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