黄陵隆起片麻岩和花岗岩的^(40)Ar/^(39)Ar年龄及其构造意义

^(40)Ar/^(39)Ar Ages of Gneiss and Granite from Huangling Uplift and Their Tectonic Significance

黄陵花岗岩是研究扬子陆块构造演化和岩浆活动的关键对象,高精度的^(40)Ar/^(39)Ar定年可以为确定黄陵花岗岩的构造演化提供准确的年代学依据.采用^(40)Ar/^(39)Ar阶段加热技术分别测定了黄陵隆起核部变质岩中角闪石、黄陵庙花岗岩中白云母和黑云母单矿物的年龄.角闪石^(40)Ar/^(39)Ar坪年龄(835.1±0.8)Ma,白云母坪年龄为(830.0±1.7)Ma,黑云母获得上凸型年龄谱,没有形成明显的年龄坪.分析认为,角闪石年龄指示围岩受到花岗岩侵入烘烤重置的时间,白云母年龄指示黄陵花岗岩冷却至350℃的年龄,岩体侵入时间不晚于835 Ma,在835~830 Ma经历快速冷却事件,冷却速率为50℃/Ma.黑云母低温阶段获得的年轻年龄216 Ma,记录了在晚三叠世黄陵地区缓慢差异隆升过程中发生的一期热事件.

The Huangling uplift,located in the front of the northeast corner of the Sichuan Basin,is composed mainly of Huangling granite.The granite is the geological record of the Jinning orogeny and the Rodinia supercontinent cracking in the Yangtze block,and it is critical for understanding the tectonic evolution and magmatic activity of the Yangtze block.To determine the formation time of the Huangling granite,abundant chronological data were published using isotope chronology methods such as U-Pb,K-Ar,and Rb-Sr.However,as a new chronology technique,^(40)Ar/^(39)Ar is rarely used in studying Huangling granite.In this study,the age of amphibole in metamorphic rocks and muscovite and biotite in Huanglingmiao granite has been determined by ^(40)Ar/^(39)Ar stage heating technique.The plateau age of amphibole was(835.1±0.8)Ma,and the plateau age of muscovite was(830.0±1.7)Ma.The biotite sample yielded an age spectrum with an upward convex shape,and no obvious flat section was formed.It was concluded that the age of amphibole is much lower than the formation age and metamorphic age of metamorphic rocks,but slightly higher than the muscovite age of nearby granite,indicating the time when the surrounding rock was reset by granite baking and then cooled to 500℃.The muscovite age indicates the time Huangling granite cooled to 350℃.Both of them restrict the formation time of Huangling granite.Although no plateau age was obtained,the biotite sample gave a meaningful age of 216 Ma in the low-temperature steps,which probably indicates the occurrence time of thermal disturbance in the later stage.The collision and merging of the North China and Yangtze plates caused the slow uplift of the Huangling,and activity of the NNWtrending faults in the Huangling area,and caused the thermal effect enough to affect the Ar closure of the biotite.The high precision ^(40)Ar/^(39)Ar age provides accurate chronological support for determining the tectonic evolution history of Huangling granite.