Search

Critical but controversial problems in the study of UHP metamorphic rocks from the Dabie-Sulu region include: (1) the possible existence of ophiolitic melange; (2) the “in situ” versus “foreign” origin of UHP eclogites and their enclosing gneisses; (3) the possible presence and role of fluids during ultrahigh-pressure (UHP) recrystallization; (4) the timing of collision between the Yangtze and Sino-Korean continental blocks; (5) the polarity of syncollisional subduction; and (6) a single-versus multistage exhumation scenario for the UHP rocks. These questions are discussed in light of new geological, geochemical, and isotopic constraints. Our conclusions for the Dabie-Sulu belt are as follows: (1) Mafic-ultramafic blocks are of two distinct origins: one group samples lithosphere of the suprasubductionzone mantle wedge, whereas the second group represents postcollisional magmatic intrusions. Neither lithologic group represents true oceanic crust. (2) Quartzofeldspathic gneisses enveloping the eclogites are...

Four main faults outline the metamorphic terrain in the Dabie Mountains, central China. The difference of metamorphic grade in the northern, central and southern parts is well-established. This study suggests that these metamorphic rocks belong to three individual geological-tectonic units, which are the northern Andean-type magmatic arc, the central UHPM rock-bearing melange zone and the mobilized sedimentary cover and basement of Yangtze southern continental margin. An old ductile sheared zone through Wuhe and the south of Yuexi-Shuihou divides the Dabie metamorphic complex into northern and central units. The northern unit is predominantly occupied by tonalitic-trondhjemitic grey gneisses and amphibolites facies metasediments. Several approximately E-W-trending ductile-sheared mylonitic belts cut through the grey gneisses, and are especially developed in the northern and southern margins of the unit (high-strain zones). There is a series of lenses of basic granulites, serpentinites and Grt-pyroxenites in the high-strain zones. They do not have the petrological and geochemical characteristics of ophiolitic rocks. However, they may represent the root of an arc combined with grey gneisses. Our study also demonstrates that all rocks from the northern block did not undergo eclogite facies metamorphism. Mesozoic post-collisional granites occur extensively in the Dabie terrain, especially in the northern unit. Coesite and diamond-bearing eclogites in the central unit indicate a regional ultra-high-pressure metamorphism of pressures of > 30 kbar and temperature of 600–800°C. Except for eclogites, various ultra-high-pressure metamorphic (UHPM) rocks include ultramafic rocks, marbles and other metamorphosed sedimentary rocks. They, as large-scale blocks, are mixed with lower P-T encompassing host rock orthogneisses. The central block is a UHPM rock-bearing melange zone. The south block is a metasedimentary sequence of continental margin of the Yangtze plate. The boundary between the central and southern blocks is located in the north from Huangzhen, extending eastwards through the dam of Huangliangting reservoir. In addition, the Foziling Group, north of the Dabieshan terrain was possibly deposited in an extensional basin on an extensional shear zone, comparable to the basin in the Variscan of France and the Caledonides of S. Norway (Dewey, Ryan and Anderson, Geological Society Special Publication No. 76, pp. 325–343, 1993). The authors hope that this information will provide more valid evidence of regional tectonic evolution.

Based on petrological, structural, geological and geochronological research, the authors summarize the progress of ultra-high pressure (UHP) metamorphic rock study since 1989 by Chinese geoscientists and foreign geoscientists in the Dabie-Su-Lu region. The authors introduce and discuss a two-stage exhumation process for the UHP metamorphic rocks that have various lithologies; eclogite, ultramafics, jadeitic quartzite, gneiss, schist and marble. The metamorphic history of UHP metamorphic rocks is divided into three stages, that is, the pre-eclogite stage, coesite eclogite stage, and retrograde stage. Prior to UHP metamorphism, the ultramafics had a high temperature environment assemblage of mantle and others had blueschist facies assemblages. The granulite facies assemblages, which have recorded a temperature increase event with decompression, have developed locally in the Weihai basaltic rocks. Isotopic ages show a long range from > 700 Ma to 200 Ma. The diversity in protoliths of UHP metamorphic rocks may be related to the variation of isotopic ages older than 400 Ma. The Sm-Nd dating of ~ 220 Ma could reflect the initial exhumation stage after the peak UHP metamorphism in relation to the collision between the Sino-Korean and Yangtze blocks and subsequent events. Petrological and structural evidence imply a two-stage exhumation process. During the initial exhumation, the UHP metamorphic rocks were sheared and squeezed up in a high P/T regime. In the second exhumation stage the UHP metamorphic rocks were uplifted and eventually exposed with middle crustal rocks.

Coesite in a granulite host-rock is reported from Weihai City, northeastern Shandong Province, China. Coesite occurs as inclusions in garnet, which is separated from the fine-grained matrix of quartz + clinopyroxene + homblende by a zoned corona of plagioclase + clinopyroxene + orthopyroxene + hornblende. Petrological evidence shows that the coesite-bearing granulite is retrograded from eclogite. Three stages in the metamorphic history of the granulite have been recognized. (i) Coesite-eclogite stage: The assemblage reconstructed from the inclusions in garnet is Mg-rich garnet + omphacite + coesite + rutile. (ii) During the granulite stage, these minerals reacted to form quartz + clinopyroxene + hornblende (in the matrix), clinopyroxene + albite (omphacite pseudomorphs) and plagioclase + clinopyroxene + orthopyroxene + hornblende (as corona around garnet). (iii) Amphibolite stage: Earlier minerals were replaced by biotite, epidote and hornblende. The P-T conditions estimated with various geothermobarometers show a T-increasing and P-decreasing path from the eclogite stage (T ∼ 720°C, P > 28 kbar) to the granulite stage (T ∼ 850°C, P ∼ 10 kbar). Thus the exhumation of the ultra-high-pressure rocks in Weihai proceeded under increasing temperature, and may not have been as fast as it was thought before