Coherence of the Dabie Shan UHPM terrane investigated by Lu-Hf and 40Ar/39Ar dating of eclogites

The Central China Orogenic Belt is the largest known ultrahigh-pressure metamorphic (UHPM) belt. Currently exposed UHP metamorphic rocks reflect subduction of massive swathes of continental crust to depths exceeding 100 km. Subsequent uplift exposed the voluminous sequence more or less intact. Deciphering responsible exhumation processes requires well constrained P-T-time paths. Most workers accept Triassic (~240 and 220-200 Ma) peak UHP metamorphism on the basis of zircon U-Pb ages in Central and Eastern Dabie Shan, while Western Dabie Shan, Qinling, North Qaidam and Altyn Tagh exhibit Ordovician (420-500 Ma) UHPM. However, contrasting reports of Carboniferous and Ordovian UHPM in Eastern Dabie Shan (Jian et al. 2001; Qiu & Wijbrans, 2006, 2008), and Ordovician, Carboniferous and Triassic (U)HPM in Western Dabie Shan (Wu et al. 2009) question this simple East-West gradient. Here, we investigate PTt-paths for localities throughout Dabie Shan to determine how far west the Triassic UHP event is documented, and how far east the Carboniferous and Ordovician events can be traced. Based on this distribution we aim to establish whether the Dabie Shan terrane is an amalgam of blocks that underwent UHPM at different times rather than a single coherent terrane. Eclogite samples are investigated for thermobarometry, Lu-Hf Grt-Cpx geochronology, and 40Ar/39Ar thermochronology. For fresh eclogites Thermocalc was used to establish equilibration conditions of the UHPM assemblage, Grt and Cpx of which were subsequently used for Lu-Hf isotope analysis. In addition, retrogressed eclogites, two fresh eclogites and two orthogneisses were analysed for 40Ar/39Ar isotope distributions in Phg, Bt, Amp and Kfs. Four fresh eclogites, all collected at reported UHP-localities confirm established PT-estimates for peak-metamorphism above the Coe-in reaction at 450-680 °C, with higher T for eastern Dabie. In one sample this is confirmed by the presence of a Coe inclusion in Cpx. Lu-Hf Grt-Cpx isochrons yield consistent Triassic ages in marked contrast to eclogitic zircon from one of the localities recording vestiges of earlier metamorphism (Wu et al. 2009). This suggests that the Grt-Cpx Lu-Hf system was reset by the Triassic UHPM event, and that all four samples were located within a single tectonic unit in the Triassic. The 40Ar/39Ar data reveal a wide scatter from 495-118 Ma, without clear spatial relationships. We attribute >300 Ma Amp and Phg ages to extraneous Ar, while 214-118 Ma ages of Amp, Bt, Phg and Kfs reflect cooling after Triassic UHPM and Cretaceous granitic magmatism. Due to the lack of spatial coherence further thermobarometric work is required to interpret these results in terms of tectonic coherence. The existing data indicate that pre-Triassic metamorphism can not be traced by Lu-Hf grt-cpx geochronology, and that the entire Dabie Shan Terrane was subjected to Triassic UHP metamorphism, which erased previous signatures of some, but not all isotopic systems.