南阿尔金山清水泉镁铁-超镁铁质侵入体LA-ICP-MS锆石U-Pb同位素定年及其意义

Zircon LA-ICP-MS U-Pb isotopic dating for Qingshuiquan layered mafic-ulmafic intrusion southern Altun orogen,in northwestern China and its implication

清水泉镁铁-超镁铁质层状侵入体位于南阿尔金山阿帕—茫崖构造带中-北部的清水泉地区,其北部与南阿尔金山超高压变质带紧邻。该岩体包含有3～4个由辉石橄榄岩-角闪辉长岩构成的岩浆旋回,与元古宙变质沉积岩系呈侵入接触关系。对其中的角闪辉长岩进行了LA-ICP-MS锆石U-Pb同位素测年,其206Pb/238U表面年龄为(461±2)Ma～(471±2)Ma,其加权平均值为(467.4±1.4)Ma(n=21,MSWD=2.5),所测锆石具有明显的岩浆振荡环带,Th/U比值为0.32～1.16(平均0.64)。测年结果显示,清水泉镁铁-超镁铁质岩体的形成时代晚于南阿尔金山超高压变质岩的峰期变质时代(504～487 Ma),而老于该地区A型花岗岩的时代(425 Ma左右)。分析认为,伴随着同时代的"双峰式"岩浆侵入作用及广泛的变质热事件,大约465 Ma时的南阿尔金山已经由前期的陆-陆碰撞造山阶段转入到了碰撞后的裂谷伸展作用阶段。

Located in Qingshuiquan area neighboring the southern Altun UHP metamorphic belt,the Qingshuiquan mafic-ultramafic intrusion consists of three to four magmatic cycles and exhibits the features of layered mafic-ultramafic intrusion.LA-ICP-MS U-Pb isotopic dating of zircon from hornblende gabbro in the intrusion with Th/U ratios of 0.32～1.16（0.64 on average）and magmatic oscillatory bands was conducted,and the results show that its age is 467.4±1.4 Ma（MSWD=2.5）,which is younger than the age of UHP metamorphic rocks（504～487 Ma） and older than the age of A-type granites（～425 Ma） in southern Altun orogen.Meanwhile,the distribution of bimodal magma intrusion at ～465 Ma and contemporary metamorphic thermal events in southern Altun area suggest that the southern Altun orogen was in the tectonic stage of post-collision rifting or extending at ～465Ma.