Constraints on retrograde metamorphism of UHP eclogites in North Qinling, Central China, from 40Ar/39Ar dating of amphibole and phengite

Coesite- and miaudiamond- bearing ultra-high pressure (UHP) edogites 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 fades metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing edogite 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. lsochron ages of 484-473 Ma and initial 40Ar/39Ar 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 li-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 edogites 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. (C) 2020 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.