北秦岭高压-超高压变质岩退变质作用40Ar/39Ar年代学研究

The ultra-high pressure metamorphic (UHP) metamorphic eclogites in the North Qinling terrane, central China, containing coesites and microdiamonds, have been widely retrograded into amphibolites. Previous geochronological studies on these UHP rocks focused on the timing of the peak eclogite facies metamorphism. Study on timing the retrograde metamorphism and exhumation history, however, is rather rare. As a result, the exhumation mechanism of these UHP metamorphic rocks is still a controversial issue. Systematic field geology, mineralogy, petrology and geochemistry, and 40Ar/39Ar geochronology study will be promoted in this project for the eclogites and their retrogressed equivalent rocks, i.e., garnet-amphibolite and amphibolite, as well as host rocks gneisses of the Guanpo-Shuanghuaishu area, north Qinling terrane. In a comprehensive analysis of the petrographic and 40Ar/39Ar data, we aim to: (1) constrain the age of the amphibolite facies retrograde metamorphism, and (2) provide insight into the exhumation rates, to subsequently afford a better resolved P-T-t path of the post UHP evolution of this retrograde eclogite and its host rock. Moreover, applying combined laser stepwise heating, crushing in vacuo and powder stepwise heating 40Ar/39Ar techniques on these amphibole samples, not only provides information to directly constrain the timing of fluid activity and the age of amphibolite-facies retrogression, but also opens a new way to systematic to investigate the Ar composition and decipher its occurrence characteristic and origin of extraneous 40Ar.

北秦岭含金刚石-柯石英超高压榴辉岩广泛受到角闪岩相退变质作用叠加改造。区内已有的年代学工作主要集中在探讨榴辉岩相变质作用的时代。角闪岩相退变质作用时代，仍然缺乏精确的年代学工作，致使超高压变质岩的折返时限和机制仍不明确。本项目以北秦岭官坡-双槐树地区榴辉岩-片麻岩为研究对象，从榴辉岩及其退变质产物榴闪岩和围岩片麻岩着手，以40Ar/39Ar定年为主要技术手段，结合野外考察、室内矿物学、岩石学和地球化学研究，来探讨榴辉岩与围岩的变质P-T条件，限制榴辉岩角闪岩相退变质作用以及后期热构造事件叠加发生的时代，为原有的超高压变质岩折返机制提供可靠的年代学数据支撑。同时通过激光阶段加热、真空击碎提取流体包裹体和粉末阶段加热40Ar/39Ar定年技术的综合使用，探讨超高变质过程中形成的角闪石外来40Ar赋存机制及可能来源，为含外来40Ar的退变质角闪石矿物40Ar/39Ar年代学研究开辟新途径。

北秦岭地体广泛出露含柯石英和微粒金刚石的高压-超高压榴辉岩。这些岩石在折返过程中普遍经历了强烈的退变质作用叠加。目前针对区内超高压变质岩已经开展了大量的锆石U-Pb定年工作，有效的限定了其超高压变质作用峰期时代，但关于超高压变质岩在折返过程中的退变质演化、热构造叠加历史等问题，还缺乏系统的年代学工作。本项目以北秦岭官坡-双槐树超高压变质岩为主要研究对象，以40Ar/39Ar定年为主要技术手段，结合系统的岩石温-压计算和精细的变质矿物40Ar/39Ar年代学研究结果，有效的限定了北秦岭大陆深俯冲结束的时代，建立了超高压变质岩的折返机制。利用传统矿物对温压计获得北秦岭超高压变质岩两期角闪岩相退变质作用温压条件分别为：T = 600-680°C，P=1.2-1.4 GPa和T = 500-570°C，P=0.6-0.9 GPa。基于电子探针分析和40Ar/39Ar定年，发现颗粒最大且具有较高Si原子数的多硅白云母给出了最老的年代，而颗粒最细且Si原子数最低样品则给出了最年轻的年龄，暗示多硅白云母是岩石是折返过程中在上地壳不同深度所形成。通过角闪石激光阶段加热40Ar/39Ar定年，厘定了超高压榴辉岩早期高角闪岩相退变质作用的时代不晚于484 Ma，晚期绿帘角闪岩相退变质作用的时代为462 Ma。基于前人的锆石U-Pb定年结果和本研究40Ar/39Ar定年数据，我们绘制出北秦岭高压-超高压变质岩的折返P-T-t轨迹。结果显示，区内超高压变质岩在浮力驱动下早期具有较快的折返速率（~8.7 mm/yr），而后期在伸展和剥蚀作用的导致的地壳减薄驱动力之下折返速率显著降低（~0.3 mm/yr）。通过角闪石激光阶段加热和真空击碎提取流体包裹体40Ar/39Ar定年结果对比，查明了角闪石中的过剩40Ar主要赋存于角闪石的次生包裹体中，原生包裹体和角闪石矿物晶格中都基本不含过剩40Ar。