New data regarding hotly debated topics concerning UHP metamorphism of theDabie-Sulu belt, east-central China

Critical but controversial problems in the study of UHP metamorphic rocks f rom the Dabie-Sulu region include: (1) the possible existence of ophiolitic melange; (2) the "in situ" versus "foreign" origin of UHP eclogites and th eir enclosing gneisses; (3) the possible presence and role of fluids during ultrahigh-pressure (UHP) recrystallization; (4) the timing of collision be tween the Yangtze and Sino-Korean continental blocks; (5) the polarity of s yncollisional subduction; and (6) a single-versus multistage exhumation sce nario fur the UHP rocks. These questions are discussed in light of new geol ogical, geochemical, and isotopic constraints. Our conclusions for the Dabie-Sulu belt are as follows: (1) Mafic-ultramafi c blocks are of two distinct origins: one group samples lithosphere of the suprasubduction-zone mantle wedge, whereas the second group represents post -collisional magmatic intrusions. Neither lithologic group repre sents true oceanic crust. (2) Quartzofeldspathic gneisses enveloping the eclogites ar e of two types-metasedimentary "in situ" anti igneous "foreign." The paragn eisses contain UHP garnets + white micas, and are uniformly older (235 +/- 5 Ma) than the orthogneisses (210 +/- 5 Ma), which are devoid of UHP Ininer alogic indicators. (3) Fluids were active under UHP conditions and allowed the formation of UHP hydrous phases such as phengite and zoisite. However, the aqueous fluids may have been restricted to certain channels/pathways du ring exhumation. External fluids were absent until ascent of the UHP rocks to middle-crustal levels. (4) The Yangtze and Sino-Korean continental block s collided during 230 to 240 Ma, when supracontinental rocks experienced UH P metamorphism. The I-IF metamorphic event dated as >400 Ma might record a subduction of oceanic crust during the Paleozoic. (5) An ancient mantle wed ge is revealed by geochemical characteristics of Mesozoic magmatic rocks de veloped on the southern margin of the Sino-Korean craton, the hanging wall of the UHP-rock-bearing unit. Seismic tomography images reveal that the Yan gtze block extends beneath the Dabie-Sulu orogenic belt. This indicates tha t both oceanic and continental crust had a northward subduction polarity. ( 6) Taking petrologic and geochronological data into account, we prefer a mu ltistage exhumation model. The UHP rocks were exhumed rapidly during the fi rst stage (230 to 200 Ma), perhaps reflecting a corner-flow mechanism. Then , buoyancy and mantle upwelling brought the UHP rocks up to middle-crustal levels during the second stage (200 to 170 Ma). Extension and thermal uplif t, as well as erosion, eventually exposed the UHP rocks to the surface in t he third stage (170 to 120 Ma).