A coherent lithostratigraphic unit in the coesite-eclogite complex of Dabie Shan, China: geologic and petrologic evidence
As a contribution to the discussion about the "in situ" vs. "exotic" origin of the ultrahigh-pressure metamorphic (UHPM) rocks, and to the real size of the exhumed tectonic units, the Wuhe-Pailou Unit (WPU) in eastern Dabie Shan was studied. The WPU typically consists of two micas—epidote gneiss, marble with minor eclogite, and jadeite-bearing granofels with rare garnetite and eclogite nodules; these lithologies are enclosed within leucocratic granitic gneiss. Jadeite-bearing granofels is the most peculiar lithology of the WPU, being the only felsic rock in Dabie Shan still preserving extensive relics of the UHPM mineral assemblage. The UHP peak assemblage is coesite+jadeite+garnet±epidote±phengite±kyanite, with accessory rutile, apatite, and zircon. All eclogites consist of garnet, omphacite, quartz/coesite, rutile or high-Al titanite±phengite±clinozoisite±kyanite. Two micas—epidote gneiss associated with the jadeite-bearing granofels typically consists of epidote, muscovite, biotite, plagioclase, K-feldspar, rare garnet, and accessory rutile, titanite, apatite, and zircon. Locally, the WPU paragneiss contains garnet with inclusions of phengite, kyanite, rutile, and amphibole+Na plagioclase symplectite after omphacite. A clockwise P- T path was inferred for the WPU, which consists of an early prograde part up to coesite-eclogite facies conditions ( P∼3.3 GPa and T∼720 °C), an early decompression stage and a late part characterised by a near-adiabatic decompression down to medium pressure ( P∼1.1 GPa, T∼670 °C) and to low-pressure amphibolite-facies conditions, followed by a final cooling to greenschist-facies conditions. Both field and laboratory data consistently suggest that the WPU, characterised by the close association of paragneiss, marble, eclogite, and granofels, is a supracrustal metasedimentary sequence, which underwent subduction, UHPM, and exhumation as a coherent tectonic unit. Therefore, within collisional orogens, large coherent slices of continental crust may be subducted at depths greater than 100 km.