柴北缘鱼卡超高压变质岩外来40Ar和冷却历史研究

柴北缘鱼卡高压-超高压变质岩中白云母受外来40Ar污染，导致常规阶段加热Ar/Ar定年常获得复杂的年龄图谱和偏老的定年结果，无法有效约束其冷却历史。目前我国针对白云母外来40Ar的研究还比较薄弱，尤其在微区原位和数值模拟方面。本项目拟通过XRF/EPMA分析和白云母单颗粒激光熔融Ar/Ar和Rb-Sr等时线定年，探讨白云母外来40Ar与岩石/矿物成分之间的耦合关系。通过白云母激光微区原位Ar/Ar定年和DiffArgP软件Ar扩散数值模拟，查明白云母中Ar溶度分布和超高压条件下的累积-扩散特征，厘清外来40Ar来源和赋存状态，探讨流体的丰贫和活动期次。在此基础上，基于白云母Ar/Ar和Rb/Sr等时线定年数据，精确厘定鱼卡高压-高超高压变质岩的退变质年代、冷却和折返速率，建立P-T-t-（D）曲线。研究成果将助于约束柴北缘高压-超高压造山带大陆深俯冲结束的时代和建立超高压变质岩的折返机制。

Muscovite and the high pressure variety, phengite, can be formed during the prograde, peak and retrograde stages along the P-T path over a protracted period. It is one of the most routinely-used mineral, which has been widely applied for 40Ar/39Ar dating for constraining metamorphic cooling and exhumation rates underpin many tectonic models. However, muscovite or phengite from high-ultrahigh pressure (UHP) metamorphic rocks is commonly contaminated with extraneous 40Ar and subsequently yielded suspected anomalously old ages, due to its argon Ar diffusion-accumulation model is no long completely follow with the classical closed temperature concept since under the special formation environment. The traditional multi-grain stepwise heating 40Ar/39Ar dating technique cannot provide a clear interpretation of either the source of the extraneous Ar or its spatial distribution of Ar in a mica grain. Here, in an alternative approach, in situ UV-laser microprobe, single-grain fusion Ar/Ar, Rb/Sr isochron dating and numerical modeling via soft DiffArgP are performed on phengite from the Yuka UHP metamorphic rocks, North Qaidam. The key advantages of these methods are that they can assess the solubility spatial distribution of Ar, the Ar diffusion profiles, as well as the origin and occurrence of extraneous 40Ar in mica. On this basis, the age of retrogression, cooling and exhumation rates of these UHP metamorphic rocks from mantle depths to upper crustal levels can be precisely constrained. Moreover, our new ages could constrain the end time of the deep continental subduction and better understanding the geodynamics of continental exhumation and uplift of the north Qaidam Orogen.