Zhai Mingguo

Implications of Palaeozoic ophiolites from Western Junggar, NW China, for the tectonics of central Asia

Western Junggar, NW China, contains fragments of several Palaeozoic island arcs, accretionary prisms and ophiolites which were accreted to each other, and possibly to the southern margin of Eurasia, by the end of the Palaeozoic. The ophiolites of Western Junggar are important in reconstructions of these events. Four ophiolite belts are recognized, from south to north these are: Tangbale, Mayila, Dalabute and Barleik/Honggulueleng/Hebukesair. They record the production and destruction of oceanic tracts in this region from the Cambrian to the Carboniferous. The following characteristics are representative of them all. (1) The ophiolitic successions are strongly deformed. (2) Sheeted-dyke swarms are not preserved. (3) Basic igneous rocks show a range of chemical characteristics, from depleted, island arc tholeiite to ocean-island basalt. Altered N-type mid-ocean basalts may be represented. (4) Large-scale thrusts, typically directed towards the southeast, lie at the base of many individual ophiolitic bodies. (5) The ophiolites rarely mark the position of possible sutures between arcs; they are normally highly deformed and imbricated with other lithologies. Accretion of Palaeozoic arcs and obduction of ophiolites was complete by the end of the Carboniferous, but the timing and sequence of events remain obscure. No continental collisions occurred during this period of assembly; crustal growth occurred by the amalgamation of non-continental blocks.

Petrological-tectonic units in the coesite-bearing metamorphic terrain of the Dabie Mountains, central China and their geotectonic implications

Abstract 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.

Recent Advances in the Study of Granulites from the North China Craton

The North China craton is one of the most important Archean cratons in the world. Nearly all Archean rocks in this craton were subjected to high-grade granulite-upper amphibolite-facies metamorphism. Protoliths include metamorphosed supracrustal rocks, orthogneisses, and basic intrusive rocks. Most granulites are low to moderate-pressure and high-temperature metamorphic rocks. Isotopic data show that granulite-upper amphibolite-facies rocks represent Archean formations, and greenschist-low amphibolite-facies rocks are early to middle Proterozoic formations. The major granulite-facies metamorphism occurred in four metamorphic episodes of >3000 Ma, ∼2900 to 2700 Ma, ∼2600 to 2450 Ma, and ∼1750 to 1900 Ma. The 2600-2450-Ma granulite-facies recrystallization is the most important latest Archean metamorphic event in the North China craton. Geochemical data indicate that the Archean moderate to low-pressure granulite terrain possibly represents a section of the middle to lower continental crust rather than lowe...