陕西秦岭梁岩体LA-ICP-MS锆石U-Pb年龄及其地质意义

The LA-ICP-MS Zircon U-Pb Age of the Qinlingliang Intrusion,Shaanxi Province and Its Geological Significance

三叠纪花岗岩在秦岭造山带广泛发育,直接记录了秦岭造山带演化过程中的重要信息。秦岭梁岩体位于秦岭造山带商丹缝合带北侧,岩性主要为石英二长斑岩,斑晶为钾长石,部分斑晶具有白色斜长石环边;基质主要由石英（25%）、斜长石（45%）、钾长石（10%）、黑云母（15%）、角闪石（5%）组成。秦岭梁岩体中的锆石自形程度好,具环带结构,显示岩浆锆石的特征,两个样品的LA-ICP-MS锆石U-Pb同位素年龄分别为（216.5±2.1）Ma和（219.4±2.6）Ma,二者在误差范围内一致,表明该岩体的岩浆结晶年龄为晚三叠纪。秦岭梁岩体的岩浆结晶年龄与勉略洋壳向北俯冲的时间（248~200 Ma）一致,表明岩体的形成与勉略洋壳向北俯冲有密切联系;同时秦岭梁石英二长斑岩的地球化学特征显示其是洋壳俯冲时弧岩浆活动的产物。因此北秦岭地区勉略洋闭合时间和陆陆碰撞拼合时间应不早于216~219 Ma。

Triassic granite are widespread in the Qinling orogenic belt,which directly records the important information of the evolution process of the Qinling orogenic belt. The Qinlingliang intrusion is located in the north of the Shang-Dan suture belt,mainly consisting of the quartz monzonite porphyry,with the K-feldspar as phenocryst（ some phenocrysts have a white rim of plagioclase） and mainly quartz（ 25%）,plagioclase（ 45%）,Kfeldspar（ 10%）,biotite（ 15%） and amphibole（ 5%） as matrix. The zircons in the Qinlingliang intrusion show the characteristics of magmatic zircon with fine euhedral crystal and zonation. The LA-ICP-MS zircon U-Pb ages of two samples are（ 216. 5 ± 2. 1） Ma and（ 219. 4 ± 2. 6） Ma,indicating that the Qinlingliang intruded in the Late Triassic. The age is consistent with the time of the northern subduction of the Mian-Lüe oceanic crust（ 248- 200Ma）,indicating that they have close relationship. The geochemical characteristics of the Qinlingliang quartz monzonite prophyry intrusion suggest that it was the production of the arc magmatic activity under the oceanic crust subduction setting. Comprehensive discussions make it clear that the closing time of the Mian-Lüe ocean and the beginning time of full-scale collision between Yangtze and North China plates should be later than（ 216- 219） Ma.