Zengqiu Zhong

Geochemical and Sr-Nd-Pb isotopic compositions of Cretaceous granitoids: constraints on tectonic framework and crustal structure of the Dabieshan ultrahigh-pressure metamorphic belt, China

Abstract The Dabie orogenic belt can be divided into four tectonic and lithological units. They are, from north to south, the North Huaiyang (NHY) unit, the Northern Dabie complex (NDC) unit, the ultrahigh-pressure (UHP) metamorphic unit, and high-pressure (HP) metamorphic unit. Cretaceous granitoids from the four tectonic units, irrespective of tectonic unit and lithology, show surprisingly similar major and trace element and particularly Sr–Nd–Pb isotopic compositions, indicating a similar crustal source for their magma genesis. They have initial ( 87 Sr/ 86 Sr) t values of 0.7068–0.7091 and Nd-depleted mantle model ages of 1.6–2.4 Ga and e Nd ( t ) values at the time of their emplacement (120 Ma) ranging from −13 to −26. Their present-day Pb isotopic compositions are 206 Pb/ 204 Pb=15.62–17.06, 207 Pb/ 204 Pb=15.21–15.44, and 208 Pb/ 204 Pb=36.59–37.81. With comparison to the Sr–Nd–Pb isotopic compositions of the basement rocks in the Dabie orogenic belt, the isotopic compositions of the Cretaceous granitoids indicate a source that is similar to the Northern Dabie complex but distinct from the ultrahigh-pressure metamorphic unit. This strongly suggests that the exposed Northern Dabie complex extends in the deep crust southward beneath the UHP/HP metamorphic units and northward beneath the North Huaiyang unit. The Northern Dabie complex is best interpreted as the core of a dome within the Dabie orogenic belt. The UHP/HP metamorphic unit is only a thin-skinned slab confined in shallower crustal levels over the Northern Dabie complex. Present tectonic framework for the UHP/HP terrain was formed by extensional tectonics and post the UHP/HP metamorphism. The Northern Dabie complex has an affinity to the Yangtze craton. The similar source for the Cretaceous granitoids also suggests that the suture between the North China and the Yangtze cratons is marked by the Xiaotian–Mozitang Fault separating the North Huaiyang unit and the Northern Dabie complex rather than the previously thought Wuhe–Shuiko Fault, which separates the Northern Dabie complex and the UHP unit.

U-Pb zircon age of the foliated garnet-bearing granites in western Dabie Mountains, Central China

U-Pb zircon dating on two foliated garnet-bearing granite samples in the western Dabie ultra-high-pressure (UHP) metamorphic unit yields concordant ages of (234±4) Ma and (227±5) Ma, respectively. These ages, following the UHP peak metamorphism, represent the magma emplacement ages for the foliated garnet-bearing granites. This, for the first time, shows that there are the Triassic granites in the Dabie Mountains. The foliated garnet-bearing granites resemble A-type granite in geochemical characteristics, indicating that they were formed in extensional geodynamic setting. The magma formation reflects a reheating event in the Dabie orogenic belt and it enhances the transfer of tectonic regime from collision into extension and promotes the rapid exhumation into lower crust for the UHP metamorphic rocks.

Age of granulite from Huangtuling, Dabie Mountain: Pb-Pb dating of garnet by a stepwise dissolution technique

A new stepwise dissolution scheme, involving acids with different properties, enables the selective recovery of radiogenic and common Pb from a mineral, and makes single-mineral Pb-Pb dating possible. Garnet from a granulite sample from Huangtuling, northern Dabie Mountain yields a Pb-Pb isochron age of (1998 ±35) Ma, which is interpreted as the timing of peak metamorphism of granulite facies.

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.

Two Discrete UHP and HP Metamorphic Belts in the Central Orogenic Belt, China

ABSTRACT An attempt is made to confirm the existence of the two discrete UHP and HP metamorphic belts in the central orogenic belt in China. Detailed geological mapping and structural and petrological analyses of the Kanfenggou ( ) and Xiangfanggou ( ) slices exposed in the eastern Qinling ( ) orogen indicate that they experienced ultrahigh pressure and high pressure metamorphism, respectively. The former, situated in northern Qinling, contains a large volume of fine–grained coesite and quartz pseudomorphs after coesite- and microdiamond-bearing eclogite lenses, whereas the latter, located in southern Qinling, preserves the relicts of a high pressure metamorphic mineral assemblage. Based on extensive fieldwork together with compilations at the scale of the orogenic belt, and a comparison of Pb isotopic compositions between the UHP metamorphic rocks from Kanfenggou slice and the Dabie UHP metamorphic belt, we propose that there are at least two discrete ultrahigh pressure metamorphic belts with different ages and tectonic evolution within the central orogenic belt in China. The first is the South Altun-North Qaidam-North Qinling ultrahigh pressure metamorphic belt of Early Paleozoic age (∼500–400 Ma). The Kanfenggou ultrahigh pressure slab is located at its eastern segment. The second is the well constrained Dabie ( )-Sulu ( ) ultrahigh/high pressure metamorphic belt of Triassic age (∼250–220 Ma). The Xiangfanggou high pressure metamorphic slab is a westward extension of the Dabie-Sulu ultrahigh/high pressure metamorphic belt. The Pb isotopic compositions of the UHP metamorphic rocks from Kanfenggou UHP fragment in East Qinling are different from those of the UHP rocks in Dabie UHP metamorphic belt, but are consistent with those of the rocks from the Qinling rock group and Erlangping ( ) rock group. The East Qinling UHP metamorphic belt does not appear to link with the Dabie-Sulu UHP metamorphic belt. These two ultrahigh metamorphic belts are separated by the suture-like Shangnan ( ) Danfeng ( ) fault system or Guishan ( )-Meishan ( ) fault, and a series of fault-bounded tectonic slices with different ages and rock assemblages. The Early Paleozoic ultrahigh metamorphic belt towards the east cannot be linked to the Triassic Dabie-Sulu ultra-high/high pressure metamorphic belt, and so does not form a huge ultrahigh pressure metamorphic belt extending more than 4 000 km in the central orogenic belt. In addition, the South Altun-North Qaidam-North Qinling ultrahigh metamorphic belt probably represents an intercontinental deep subduction/collision belt between the Sino-Korean and Yangtze cratons, occurring during the Early Paleozoic. The Dabie-Sulu ultrahigh/high pressure belt may have been formed by an intracontinental deep subduction/collision process occurring in the northern part of the Yangtze craton during the Triassic. It is suggested that the two phase crustal subduction/collision processes, with an interval of ∼200–300 Ma, responsible for the formation of the two ultrahigh/high pressure metamorphic belts occurred along spatially different belts within the central orogenic belt, China.