Abstract Coesite- and microdiamond- bearing ultra-high pressure (UHP) eclogites in the North Qinling terrane have been widely retrogressed to amphibolites. Previous geochronological studies on these UHP rocks mainly focused on the timing of peak eclogite facies metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing eclogite occurrences in the North Qinling terrane. In this study, mafic amphibolites and host schists from this domain were collected for 40Ar/39Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and 40Ar/39Ar ages. A first generation of amphibole from garnet amphibolites yielded irregularly-shaped age spectra with anomalously old apparent ages. Isochron ages of 484–473 Ma and initial 40Ar/36Ar ratios of 3695–774 are obtained from this generation of amphibole, indicating incorporation of excess argon. Second generation amphibole occurs in epidote amphibolites yielded flat age spectra with plateau ages of 464–462 Ma without evidence for excess argon. These ages suggest that the amphibolite-facies metamorphism has taken place as early as 484 Ma and lasted until 462 Ma for the North Qinling UHP metamorphic rocks. Phengite from the country-rock schists yielded 40Ar/39Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new 40Ar/39Ar ages and previous zircon U Pb geochronological data, we construct a new detailed pressure-temperature-time (P-T-t) path illustrating the retrograde metamorphism and exhumation rate of the North Qinling eclogites and host schists. The P-T-t path suggests that these UHP metamorphic rocks experienced initial medium-to-high exhumation rates (ca. 8.7 mm/yr) during the Early Ordovician (489–484 Ma), which was mainly derived from buoyancy forces. Subsequently, the exhumation rate decreased gradually from ~0.8 to 0.3 mm/yr from 484 to 426 Ma, which was probably governed by extension and/or erosion.

中文翻译：

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华中北秦岭超高压榴辉岩逆行变质作用的约束，来自角闪石和菱镁矿的40Ar/39Ar定年

摘要 北秦岭地体中含柯石英和微金刚石的超高压榴辉岩已广泛退化为角闪岩。以前对这些超高压岩石的年代学研究主要集中在榴辉岩相变质作用的高峰时间。坎峰沟超高压变质域是北秦岭地体中保存最完好的含柯石英榴辉岩矿点之一。在这项研究中，收集了来自该领域的镁铁质角闪岩和寄主片岩的 40Ar/39Ar 定年，以限制它们的逆行演化。两代角闪石根据其矿物共生和 40Ar/39Ar 年龄被识别。来自石榴石角闪石的第一代角闪石产生了形状不规则的年龄谱，具有异常古老的表观年龄。从这一代角闪石中获得了 484-473 Ma 的等时线年龄和 3695-774 的初始 40Ar/36Ar 比值，表明掺入了过量的氩气。第二代闪石出现在绿帘石闪石中，产生平坦的年龄光谱，平台年龄为 464-462 Ma，没有过量氩的证据。这些年代表明，北秦岭超高压变质岩的角闪岩相变质作用早在484 Ma，一直持续到462 Ma。来自乡间岩片岩的 Phengite 产生了 40Ar/39Ar 高原年龄为 426-396 Ma，较高的 Phengite Si 含量与较老的高原年龄有关。基于我们新的 40Ar/39Ar 年龄和以前的锆石 U Pb 年代学数据，我们构建了一条新的详细的压力-温度-时间 (PTt) 路径，说明了北秦岭榴辉岩和寄主片岩的逆行变质作用和折返率。PTt 路径表明，这些 UHP 变质岩在早奥陶世（489-484 Ma）期间经历了初始中高折返率（约 8.7 毫米/年），这主要来自浮力。随后，剥脱率从 484 到 426 Ma 从~0.8 到 0.3 毫米/年逐渐下降，这可能是由伸展和/或侵蚀控制的。