Petrochronological study of zircon from felsic dikes associated with eclogites in the North Qinling orogen: Constraint on rapid exhumation of HP-UHP terrane

The North Qinling (NQ) orogen is a typical high-pressure (HP) and ultrahigh-pressure (UHP) terrane involved in multiple orogenies, whose exhumation processes have yet been well constrained. Six plagioclase-bearing felsic dikes were collected for zircon petrochronological study to place precise age constraints on the end of the UHP metamorphism. The felsic dikes show no to strong deformation and consist mainly of K-feldspar, plagioclase, and amphibole, with minor biotite and muscovite. Zircons from these dikes have euhedral shape and exhibit weak, planar, or oscillatory zoning. They possess very low Th/U ratios, steep heavy rare earth element enriched pat-terns, and variable Eu anomalies, with Ti-in-zircon temperatures between ca. 600 and 660 degrees C. Therefore, the zircon crystals precipitated from the felsic dikes under relatively low pressure and high temperature conditions. These felsic dikes jointly yielded a weighted mean age of 490 +/- 2 Ma (MSWD = 0.39). This age is indistin-guishable from the peak HP-UHP metamorphic age of 492 +/- 4 Ma within analytical errors for the NQ unit. The resultant exhumation rate is averaged at about 3.2 cm/yr, and the minimum is about 0.8 cm/yr from the peak UHP metamorphic to at least the middle crust depth. The NQ terrane thus is a fast exhumation HP-UHP terrane. Considering the regional tectonic evolution, the contemporary jump of ocean subduction slab is likely triggering the rapid exhumation. Our results show that zircon petrochronology of felsic dikes is a powerful tool for con-straining the early exhumation of HP-UHP rocks involving in complex metamorphic processes.