Determination of the early Paleozoic accretionary complex in Southwestern Yunnan, China: Implications for the tectonic evolution of the Proto-Tethys Ocean

Accretionary complex study provides important knowledge on the subduction and the geodynamic processes of the oceanic plate,which represents the ancient ocean basin extinction location.Nevertheless,there exist many disputes on the age,material source,and tectonic attribute of the Lancang Group,located in Southwest Yunnan,China.In this paper,the LA-ICP-MS detrital zircon U‒Pb chronology of nine metamorphic rocks in the Lancang Group was carried out.The U‒Pb ages of the three detrital zircons mainly range from 590-550 Ma,980-910 Ma,and 1150-1490 Ma,with the youngest detrital zircons having a peak age of about 560 Ma.The U‒Pb ages of the six detrital zircons mainly range from 440-460 Ma and 980-910 Ma,and the youngest detrital zircon has a peak age of about 445 Ma.In the Lancang Group,metamorphic acidic volcanic rocks,basic volcanic rocks,intermediate-acid intrusive rocks,and high-pressure metamorphic rocks are exposed in the form of tectonic lens in schist,rendering typical melange structural characteristics of“block+matrix”.Considering regional deformation and chronology,material composition characteristics,and the previous data,this study thinks the Lancang Group may be an early Paleozoic tectonic accretionary complex formed by the eastward subduction of the Changning-Menglian Proto-Tethys Ocean,which provides an important constraint for the Tethys evolution.

How many sutures in the southern Central Asian Orogenic Belt:Insights from East Xinjiang-West Gansu(NW China)?

How ophiolitic mèlanges can be defined as sutures is controversial with regard to accretionary orogenesis and continental growth.The Chinese Altay,East junggar,Tianshan,and Beishan belts of the southern Central Asian Orogenic Belt（CAOB） in Northwest China,offer a special natural laboratory to resolve this puzzle.In the Chinese Altay,the Erqis unit consists of ophiolitic melanges and coherent assemblages,forming a Paleozoic accretionary complex.At least two ophiolitic melanges（Armantai,and Kelameili） in East Junggar,characterized by imbricated ophiolitic melanges,Nb-enriched basalts,adakitic rocks and volcanic rocks,belong to a Devonian-Carboniferous intra-oceanic island arc with some Paleozoic ophiolites,superimposed by Permian arc volcanism.In the Tianshan,ophiolitic melanges like Kanggurtag,North Tianshan,and South Tianshan occur as part of some Paleozoic accretionary complexes related to amalgamation of arc terranes.In the Beishan there are also several ophiolitic melanges,including the Hongshishan,Xingxingxia-Shibangjing,Hongliuhe-Xichangjing,and Liuyuan ophiolitic units.Most ophiolitic melanges in the study area are characterized by ultramafic,mafic and other components,which are juxtaposed,or even emplaced as lenses and knockers in a matrix of some coherent units.The tectonic settings of various components are different,and some adjacent units in the same melange show contrasting different tectonic settings.The formation ages of these various components are in a wide spectrum,varying from Neoproterozoic to Permian.Therefore we cannot assume that these ophiolitic melanges always form in linear sutures as a result of the closure of specific oceans.Often the ophiolitic components formed either as the substrate of intra-oceanic arcs,or were accreted as lenses or knockers in subduction-accretion complexes.Using published age and paleogeographic constraints,we propose the presence of （1） a major early Paleozoic tectonic boundary that separates the Chinese Altay-East Junggar multiple subduction systems of the southern Siberian active margin from those of the northern Tarim;and （2） a major Permian suture zone that separates the Tianshan-Beishan from the northern active margin of the Tarim Craton.These new observations and interpretations have broad implications for the architecture and crustal growth of central Asia and other ancient orogens as well.

Rock Assemblages and Formation Ages of the Baishuijiang Group in the Southwest Qinling Orogenic Belt, Northwest China

The Baishuijiang Group, located in the southwest Qinling orogenic belt, is divided into three belts according to the characteristic of the matrix and rock blocks based on the large scale geological mapping. The north belt and south belt are composed of abyssal mudstone and siltstone, and limestone, chert and basic and ultrabasic rock blocks. The middle belt consists of a few limestone blocks and turbidites, which were formed in the trench environment. At present, in the Baishuijiang Group, many fossils were found in matrix and rock blocks, the fossils contain the Cambrian small shell fossils(Xiao, 1992;Tao et al., 1992), Silurian chitinozoas, scolecodonts and spores, and Ordovician graptolites, and middle Devonian Coral and conodonts in limestone and chert blocks(Wang et al., 2011a), and Permian radiolarians in the matrix(Wang et al., 2007). The volcanic rock blocks have undergone different degree of metamorphism. Their geochemical characteristics indicate that the rocks are similar to oceanic island arc and seamount(Wang et al., 2009), and SHRIMP U-Pb dating yielded ages from Neoproterozoic to early Paleozoic(Yan et al., 2007;Wang et al., 2009, 2011b). Therefore, comprehensive analysis of regional data, the Baishuijiang group is an accretionary complex which was consisted of matrix and blocks, and was finally formed during Permian-Triassic.

Accretionary complex:Geological records from oceanic subduction to continental deep subduction

Accretionary complex was usually formed by offscraping of the subducting crustal material over the trench and thus often referred to as subduction zone mélange.The structure,composition and forming process of accretionary wedges can provide important insights into the evolution history of ocean basin,ocean-continent material cycle,continental accretion and thus contribute to understanding of the origin of plates and the growth of continents.Accretionary complex is characterized by a block-in-matrix structure associated with imbricate thrusts and isoclinal folds,diversified metamorphic types and intense water-rock interactions,which are distinct to the traditional stratigraphy.Since the proposal of the concept of accretionary wedge over a hundred years ago,great progress has been made in a variety of research focuses,such as the identification of the distribution of accretionary complexes,their compositions and formation mechanisms,the affinities of the matrix and igneous rocks,the recognition of the Ocean Plate Stratigraphy(OPS),the reconstruction of oceanic basin,the dynamic background of the tectonic evolution,the relationship between subduction zone and orogenic belt and,in particular,the accretionary complexes in continental subduction zones.These studies have significantly improved our understanding of the plate tectonic theory.Challenges remain in the identification of ancient accretionary complexes,the detailed analysis of accretionary complex zones,the accretion characteristics during continental collision,and the geochemical tracing of water-rock interaction during the accretion.China contains representative orogenic belts and accretionary complex zones in the world,and its geological records provide the best opportunity to make new breakthroughs in understanding of the plate tectonics.

Metamorphic Characteristics and Tectonic Implications of the Kadui Blueschist in the Central Yarlung Zangbo Suture Zone,Southern Tibet

The Kadui blueschist is located in the central section of Yarlung Zangbo suture zone （YZSZ）, southern Tibet, and has been subjected to the subduction of the Neo-Tethyan Ocean below the Asian Plate and provides important clues for better understanding the evolution of the India-Asia convergence zone. In this paper, the systematical petrographic and mineral chemical studies were carried out on the Kadui blueschist, which reveal a mineral assemblage of sodic amphibole, chlorite, epidote, albite and quartz with accessory minerals of titanite, calcite and zircon. Electron microprobe analyses demonstrate that amphibole shows zoned from actinolite core to ferrowinchite/riebeckite rim composition indicating that the sodic amphibole has formed during a prograde metamorphic event. The protolith of the blueschist is an intermediate-basic pyroclastic rock. The calculated pseudosection indicates a clockwise P-T path and constrains peak metamorphic conditions of about 5.9 kbar at 345 ℃. This condition is transitional between pumpellyite-actinolite, greenschist and blueschist facies with a burial depth of 20-22 km and a thermal gradient of 15-16 ℃/km. This thermal gradient belongs to high pressure intermediate P/T facies series and is possibly related to a warm subduction setting of young oceanic slabs. Our new findings indicate that the Kadui blueschist in the central part of YZSZ experienced a rapid subduction and exhumation process as a response to a northward subduction of the Neo-Tethyan oceanic lithosphere during the initial India-Asia collision stage.

Imbricate structure of the Otakeyama area, Okayama Permian Yoshii Group in the Prefecture, southwest Japan

The Yoshii Group of the Akiyoshi terrane is distributed over the Otakeyama area, Okayama Prefecture, southwest Japan. The Yoshii Group is composed of siliceous rocks and clastics, comprising chert-clastic sequences. The Yoshii Group is divided into four structural units （Units Ot 1, Ot 2, Ot 3, and Ot 4） in structurally ascending order. Lithological similarities and radiolarian age suggest that these units exhibit almost identical ocean plate stratigraphy. These units are structurally-repeated to form an imbricate structure. This structure was possibly formed by off-scrape accretion at a toe of an accretionary complex. Additionally, a review of previous studies indicates that an imbricate structure is common in the Akiyoshi terrane. The presence of an imbricate structure in the Akiyoshi terrane, Permian accretionary complex, indicates the possibility that a specific horizon of Permian pelagic sequences acted as a drcollement zone.