Constraints on retrograde metamorphism of UHP eclogites in North Qinling, Central China, from 40Ar/39Ar dating of amphibole and phengite
Constraints on retrograde metamorphism of UHP eclogites in North Qinling, Central China, from 40Ar/39Ar dating of amphibole and phengite
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角闪石和多白硅白云石40Ar/39Ar测年对北秦岭超高压榴辉岩逆变质作用的制约
DOI：
10.1016/j.gr.2020.06.003
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发表时间：
2020-07
影响因子：
6.1
通讯作者：
Hua-Ning Qiu
中科院分区：
文献类型：
--
作者：
Rong-Guo Hu;Jan R. Wijbrans;Fraukje M. Brouwer;Xiu-Juan Bai;Hua-Ning Qiu
北秦岭含柯石英和微金刚石超高压榴辉岩已普遍退化为斜长角闪岩。以往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高峰期的时间上。坎峰沟超高压变质域是北秦岭保存最完好的含柯石英榴辉岩产出地之一。本研究对该区域的镁铁质斜长角闪岩和寄主片岩进行了40 Ar/39 Ar年龄测定，以确定它们的退变质演化。根据矿物共生关系和40 Ar/39 Ar年龄，将角闪石划分为两个世代。第一代角闪石的石榴角闪岩产生了不规则形状的年龄谱，具有异常古老的视年龄。该世代角闪石的等时线年龄为484-473 Ma，初始~（40）Ar/~（36）Ar比值为3695-774，表明有过量Ar掺入。第二代角闪石产于绿帘石斜长角闪岩中，其年龄谱平坦，坪年龄为464-462 Ma，没有过量氩的证据。这些年龄表明，北秦岭超高压变质岩石的角闪岩相变质作用发生在484 Ma，并持续到462 Ma。多硅白云母的40 Ar/39 Ar坪年龄为426-396 Ma，多硅白云母的Si含量越高，坪年龄越大。根据我们新的40 Ar/39 Ar年龄和以往的锆石U-Pb年代学资料，建立了一个新的详细的压力-温度-时间（P-T-t）轨迹，说明了北秦岭榴辉岩和寄主片岩的退变质和折返速率。P-T-t轨迹表明，早奥陶世（489-484 Ma），这些超高压变质岩经历了一个中-高折返速率（约8.7 mm/yr），其折返速率主要来自浮力。随后，折返速率逐渐下降，从~0.8至0.3毫米/年，从484至426 Ma，这可能是由伸展和/或侵蚀。
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 for40Ar/39Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and40Ar/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 initial40Ar/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 yielded40Ar/39Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new40Ar/39Ar ages and previous zircon Usingle bondPb 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. TheP-T-tpath 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.
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