Progress and controversy in the study of HP-UHP metamorphic terranes in the West and Middle Central China orogen

Abstract

During the past ten years, various types of HP-UHP metamorphic rocks have been discovered in the South Altyn Tagh, the North Qaidam and the North Qinling (秦岭) in the West and Middle Central China orogen. The UHP rocks, as lentoid bodies in regional gneisses, include eclogite (garnet-bearing pyroxenite), garnet peridotite and various pelitic or felsic gneisses. There are many records of minerals and microstructures of exsolution indicate the UHP metamorphism, such as coesite (or its pseudomorph), diamond, exsolution of clinopyroxene/amphibole/+rutile or rutile+quartz+apatite in garnet, exsolution of quartz in omphacite and exsolution of kyanite+spinel in precursor stishovite. The discovery of microstructure evidence for the presence of precursor stishovite in typical Alrich gneiss from the South Altyn Tagh reveals continental subduction and exhumation to and from a depth of more than 350 km. It is the petrological record of the deepest subduction and exhumation of continental rock in the world. The in situ zircon U-Pb dating using LA-ICP-MS or SHRIMP methods shows that the metamorphic ages of the HP-UHP rocks in the South Altyn Tagh, the North Qaidam and the North Qinling are 475-509, 420-457, and 485-514 Ma, respectively. The metamorphic ages of HP-UHP rocks in the North Qaidam are 20-80 Ma younger than those in the South Altyn Tagh and the North Qinling, and the metamorphic ages do not systematically increase or decrease from the South Altyn Tagh through the North Qaidam to the North Qinling. The absence of time transgressive variety of the metamorphism in the three regions does not support the hypothesis that the HP-UHP rocks in these regions form the same HP-UHP metamorphic zone. And the HP-UHP rocks in these regions can not be simply correlated to the collision between the North China plate and the South China plate. At present, the study of the HP-UHP rocks in the West and Middle Central China orogen faces several key issues or challenges, such as: (1) the continental subduction to the mantle depth of stishovite stability field (>9 GPa) is occasional or universal; (2) the mechanism of exhumation for the continental rocks subducted to the depth of stishovite stability field (>300 km); (3) the tectonic setting and geodynamical mechanism of producing the HP-UHP metamorphic zones in the South Altyn Tagh, the North Qaidam and the North Qinling. Further studies aiming at these problems will make important progress not only in metamorphism of the HP-UHP rocks in the West and Middle Central China orogen, but also in continental deep subduction and exhumation in solid earth science. It will also contribute to the establishment of the theory of continental deep subduction. KEY WORDS: HP-UHP metamorphic zone, continental deep subduction, zircon LA-ICP-MS and SHRIMP dating, South Altyn Tagh, North Qaidam, North Qinling.

Granulites record the tectonic evolution from collisional thickening to extensional thinning of the Tongbai orogen in central China

“…Cheng et al () considered ages of 494 ± 3 Ma for eclogite facies metamorphism based on whole‐rock+garnet+omphacite Lu‐Hf isochron dating of two eclogite samples. Several other studies have also suggested Late Cambrian to Early Ordovician ages for the HP to UHP eclogite facies metamorphism due to the continental subduction (Li et al, ; Liao et al, ; Liu, Yang, et al, ; Ratschbacher et al, ; Wang et al, ; Yu et al, ). The Cambrian metamorphic ages for the Tongbai orogen suggest that this orogen shares the same episode of continental collision with the Qinling orogen.…”

“…This is consistent with the northward subduction of the North Qinling microcontinent beneath the Erlangping arc terrane in the Cambrian. The synchronous UHP eclogite facies metamorphism in the Qinling orogen provides robust evidence for continental deep subduction to the subarc depths (Liao et al, ; Liu et al, ; Liu, Yang, et al, ; Wang, Wu, Gao, Liu, et al, ; Wang, Wu, Gao, Zhang, et al, ; Yu et al, ). It is known that continental subduction for collisional orogeny leads to crustal thickening, which is evident in the Rocky Mountains, the Central Andes, and the Himalaya (e.g.…”

Orogenic, Ophiolitic, and Abyssal Peridotites

“…Examples of HT metamorphism are observed in the country rocks of the Beni Bousera (northern Morocco) and Ronda (southern Spain) massifs (Loomis, 1972), as well as in the limestones hosting the Pyrenean lherzolites (Goldberg and Leyreloup, 1990). However, several orogenic peridotites are associated with HP to UHP terranes, notably in the Central Alps (Alpe Arami, Cima di Gagnone, and Ulten peridotites), China (Dabie-Sulu, Qinling, and North Qaidam-Altyn-Tagh UHP metamorphic belts), the Western Gneiss Region (WGR) in Scandinavian Caledonides, and the Bohemian Massif in Central Europe Ernst, 1981;Ernst and Liou, 2008;Kotkova et al, 2011;Liou et al, 2002aLiou et al, ,b, 2009Liu et al, 2010;Reverdatto and Selyatitskii, 2005;Schmaedicke et al, 2010;Terry et al, 2000;van Roermund, 2009b;Zhang et al, 2005).…”

Silurian high-pressure granulites from Central Qiangtang, Tibet: Constraints on early Paleozoic collision along the northeastern margin of Gondwana

“…Considering the paleogeographic framework of eastern Gondwana, early Paleozoic metamorphic rocks, especially the ultrahigh/high- Chen et al, 2007Chen et al, , 2009Zhang et al, 2008;Song et al, 2006;Yu et al, 2013), the South Altyn (501-485 Ma; Zhang et al, 2005;Liu et al, 2012;Cao et al, 2013), the North Qinling orogen (ca. 511-480 Ma, Liu et al, 2010;Zhang et al, 2011;Wang et al, 2011;Cheng et al, 2011Cheng et al, , 2012ca. 440-424 Ma, Zhang et al, 2011), Sibumasu (ca.…”

“…501-485 Ma; Zhang et al, 2005;Liu et al, 2012;Cao et al, 2013), the North Qinling orogen (ca. 511-480 Ma; Liu et al, 2010;Zhang et al, 2011;Cheng et al, 2011Cheng et al, , 2012Wang et al, 2011;ca. 440-424 Ma, Zhang et al, 2011), Sibumasu (ca.…”