Tectonic and metamorphic evolution of the North Qaidam ultrahigh-pressure metamorphic terrane, western China

Ultra-high pressure (UHP) metamorphism is defined by the presence of coesite, a dense form of SiO2. Although a volumetrically insignificant component of orogens, rocks that experienced UHP conditions are important because they represent continental materials that have been subducted to depths greater than 80 km and returned to the surface. My thesis integrates field mapping in the North Qaidam UHP metamorphic belt with detailed petrologic, geochronologic, and thermochronologic studies to gain new insight into UHP metamorphism and exhumation within a well-defined structural and geologic framework. This dissertation describes results of these studies, which utilize the Luliang Shan UHP lithologies to: (1) contribute to the understanding of the tectonic setting of the early Paleozoic North Qaidam by assessing how the formation and timing of UHP rocks fits into the overall evolution of the region, (2) evaluate the metamorphic history recorded in the mafic eclogites and host gneisses, (3) examine the origin of excess argon in mica, a widespread problem in UHP rocks worldwide.; This work has resulted in the identification and characterization of the North Qaidam ophiolite complex in the Luliang Shan; and trace-element geochemistry has identified the ophiolite as a supra-subduction zone type which formed at a site of subduction; this previously unknown site of subduction and suture is also interpreted to have facilitated the UHP metamorphism of the North Qaidam rocks. Petrologic and geochronologic results constrain the exhumation to the surface indicating that the North Qaidam UHP terrane was exhumed in a two-stage, two-rate exhumation process: the first stage was fast (3.6 km/My) from the mantle to the lower crust; the second stage was slow from the lower to middle crust (0.12-0.4 km/My). Each locality has a subtly different exhumation history due to the spatially varying age of UHP metamorphism in the North Qaidam metamorphic belt; decreasing from north-west to south-east over an 80 My span (505 to 425 Ma) (Altyn Shan → Luliang Shan → Xitie Shan → Dulan). This implies diachronous subduction of the North Qaidam terrane.; Age data from 40Ar/39Ar thermochronology of mica can provide important constraints on pressure-temperature-time (P-T-t) path and the tectono-thermal evolution of orogens; however, application to white mica from UHP and granulite terranes commonly results in ages that are impossibly old, despite relatively flat plateau over most of the 39 Ar release spectrum. 40Ar/39Ar analyses on phengite from eclogite and muscovite from the enclosing orthogneiss in the Luliang Shan revealed that mica from eclogite had yielded geologically unreasonable apparent ages, whereas the mica from the felsic gneiss yielded geologically reasonable ages.