Two fresh types of eclogites in the Dabie-Sulu UHP metamorphic belt, China: Implications for the deep subduction and earliest stages of exhumation of the continental crust

Two fresh types of eclogites, namely the massive eclogite and foliated eclogite, are discernible in large eclogite bodies surrounded by country rock gneisses from the Dabie (大别)-Sulu (苏鲁) UHP metamorphic zone. They are different in mineral assemblage, texture and structure at various scales. The massive eclogite has a massive appearance with a metamorphic inequigranular and granoblastic texture, which consists mainly of nominally anhydrous minerals such as garnet, omphacite, rutile with inclusions of coesite and rare microdiamond. Massive eclogites which formed at the peak UHP metamorphic conditions (∼3.1–4.0 GPa, 800±50 °C) within the coesite to diamond stability field recorded the deep continental subduction to mantle depths greater than 100 km during the Triassic (∼250–230 Ma). The diagnostic UHP minerals, mineral assemblages and absence of notable macroscopic deformation indicate the peak metamorphic’ forbidden-zone’ P-T conditions, an extremely low geothermal gradient (≤7 °C·km−1) and low differential stress. The foliated eclogite is composed of garnet+omphacite+rutile+phengite+kyanite+zoisite+talc+nyböite±coesite/quartz pseudomorphs after coesite. It is quite clear that the foliated eclogite bears relatively abundant hydrous mineral, and shows well-developed penetrative foliation carrying mineral and stretching lineation reflecting intense plastic deformation or flow of eclogite minerals. The foliated eclogite occurred at mantle levels and recorded the earliest stages of exhumation of UHP metamorphic rocks. At a map scale, the foliated eclogites define UHP eclogite-facies shear zones or high-strain zones. Asymmetric structures are abundant in the zones, implying bulk plane strain or dominant non-coaxial deformation within the coesite stability field. The earliest stages of exhumation, from mantle depths to the Moho or mantle-crust boundary layering, were characterized by a sub-vertical tectonic wedge extrusion, which occurred around 230-210 Ma. The three-dimensional relationship between the massive and foliated eclogites is well displayed a typical ‘block-in-matrix’ rheological fabric pattern indicating the partitioning of deformation and metamorphism in the UHP petrotectonic unit. The existing data support the now widely accepted concept of deep continental subduction/collision and subsequent exhumation between the Yangtze and Sino-Korean cratons. The pressure is a constitutive geological variable. The influence of tectonic overpresure on UHP metamorphism is rather limited.