Late Palaeozoic 40Ar/39Ar ages of the HP-LT metamorphic rocks from the Kekesu Valley, Chinese southwestern Tianshan: new constraints on exhumation tectonics

Although numerous ages have been obtained for the Chinese southwestern Tianshan high pressure/ultrahigh pressure-low temperature (HP/UHP-LT) metamorphic belt in the past two decades, its exhumation history is still controversial. The poor age constraint was related to the appealing low metamorphic temperatures and excess Ar commonly present under HP/UHP conditions. This study aims to provide new age constraints on the orogen’s exhumation by obtaining 40Ar/39Ar mica ages using the conventional step-heating technique, with emphasis on the avoidance of excess Ar contamination. From a cross section along the Kekesu Valley, four samples, three from the HP-LT metamorphic belt (TK050, TK051, and TK081) and one from the southern margin of the low pressure metamorphic belt (TK097), were selected for 40Ar/39Ar dating. Phengites from garnet glaucophane schist TK050 and the surrounding rock garnet phengite schist TK051 yield comparable plateau ages of 321.4 ± 1.6 and 318.6 ± 1.6 Ma, respectively, while epidote mica schist TK081 gives a younger plateau age of 293.3 ± 1.5 Ma. Considering the chemical compositions of phengites, mineral assemblages, and microstructures in the thin slices, we suppose that the former represents the time the HP rocks retrograded from the peak stage (eclogite facies) to the (epidote)-blueschist facies, whereas the latter reflects greenschist facies overprinting. Biotite and muscovite from two-mica quartzite TK097 give similar plateau ages of 253.0 ± 1.3 and 247.1 ± 1.2 Ma, interpreted to date movement on the post collisional transcrustal South Nalati ductile shear zone. By combining our new ages with published data, a two-stage exhumation model is suggested for the Chinese southwestern Tianshan HP/UHP-LT metamorphic belt: initial fast exhumation to a depth of about 30–35 km by ~320 Ma was followed by relatively slow (~1 mm year–1) uplift to ~10 km by ~293 Ma.

尽管近二十年来已针对中国西南天山高压-超高压低温（HP/UHP-LT）变质带获取了诸多年代学数据，但其折返历史仍存在诸多争议。该带年代学约束薄弱的原因，在于高压-超高压条件下普遍存在的过剩氩（excess Ar）以及颇具迷惑性的低变质温度。本研究采用常规阶段加热法获取云母的40Ar/39Ar年龄，旨在为该造山带的折返历史提供新的年代学约束，研究重点为规避过剩氩污染。研究团队从克克苏河谷剖面选取了4件样品：3件采自高压低温变质带（TK050、TK051与TK081），1件采自低压变质带南缘（TK097），并对其开展40Ar/39Ar定年测试。石榴子石蓝闪片岩TK050中的多硅白云母，以及围岩石榴子石多硅白云母片岩TK051中的多硅白云母，分别获得了321.4 ± 1.6 Ma与318.6 ± 1.6 Ma的近似坪年龄；而绿帘云母片岩TK081的坪年龄更年轻，为293.3 ± 1.5 Ma。结合多硅白云母的化学成分、矿物组合及薄片中的微观结构，我们认为前两组年龄代表高压岩石从峰期阶段（榴辉岩相）退变质至（绿帘）蓝片岩相的时间，而后一组年龄则反映了绿片岩相叠加改造的时代。二云石英岩TK097中的黑云母与白云母，分别获得了253.0 ± 1.3 Ma与247.1 ± 1.2 Ma的相似坪年龄，该年龄被解释为碰撞后跨地壳南那拉提韧性剪切带活动的年代。综合本次新获取的年代数据与已发表资料，我们提出中国西南天山高压-超高压低温变质带的两阶段折返模型：约320 Ma时快速折返至约30–35 km深度，随后以约1 mm·a^-1的相对缓慢速率持续抬升，至约293 Ma时达到约10 km深度。