Palaeozoic collisional tectonics and magmatism of the Chinese Tien Shan, central Asia

The Chinese Tien Shan range is a Palaeozoic orogenic belt which contains two collision zones. The older, southern collision accreted a north-facing passive continental margin on the north side of the Tarim Block to an active continental margin on the south side of an elongate continental tract, the Central Tien Shan. Collision occurred along the Qinbulak-Qawabulak Fault (Southern Tien Shan suture). The time of the collision is poorly constrained, but was probably in in the Late Devonian-Early Carboniferous. We propose this age because of a major disconformity at this time along the north side of the Tarim Block, and because the Youshugou ophiolite is imbricated with Middle Devonian sediments. A younger, probably Late Carboniferous-Early Permian collision along the North Tien Shan Fault (Northern Tien Shan suture) accreted the northern side of the Central Tien Shan to an island arc which lay to its north, the North Tien Shan arc. This collision is bracketed by the Middle Carboniferous termination of arc magmatism and the appearance of Late Carboniferous or Early Permian elastics in a foreland basin developed over the extinct arc. Thrust sheets generated by the collision are proposed as the tectonic load responsible for the subsidence of this basin. Post-collisional, but Palaeozoic, dextral shear occurred along the northern suture zone, this was accompanied by the intrusion of basic and acidic magmas in the Central Tien Shan. Late Palaeozoic basic igneous rocks from all three lithospheric blocks represented in the Tien Shan possess chemical characteristics associated with generation in supra-subduction zone environments, even though many post-date one or both collisions. Rocks from each block also possess distinctive trace element chemistries, which supports the three-fold structural division of the orogenic belt. It is unclear whether the chemical differences represent different source characteristics, or are due to different episodes of magmatism being juxtaposed by later dextral strike-slip fault motions. Because the southern collision zone in the Tien Shan is the older of the two, the Tarim Block sensu stricto collided not with the Eurasian landmass, but with a continental block which was itself separated from Eurasia by at least one ocean. The destruction of this ocean in Late Carboniferous-Early Permian times represented the final elimination of all oceanic basins from this part of central Asia.     

Kornerupine-quartz symplectites in paragneiss, Lac Ste-Marie, Quebec, Canada: pseudomorphic replacement of a hexagonal phase, probably osumilite

Summary Kornerupine has been reported from several localities in the Grenville Province. It is most abundant and accessible within quartz - sillimanite - K-feldspar - tourmaline -Fe/Ti-oxides - garnet paragneiss, forming a 300m by 1500m lens in diopside- and chondrodite-bearing marble at Hinck's bridge along the Gatineau river near Lac Ste-Marie. Biotite-rich zones within the lens contain six-sided prisms, up to 7 cm in diameter and 30 cm long. These prisms consist of symmetrically zoned (coarse rim and finer grained centre) symplectitic intergrowths of kornerupine-quartz overgrowing the biotite foliation, with minor sillimanite (as fibrolitic inclusions in kornerupine), K-feldspar, and scattered trails of ilmenite.The kornerupine-quartz symplectite in the six-sided prism appears to be a replacement of an early single hexagonal phase. Habit and bulk composition of the prism point to osumilite as the precursor. Boron-bearing fluids during amphibolite- to granulite-grade metamorphism stabilised tourmaline-quartz intergrowths which crystallized by pseudomorphic replacement of osumilite. Numerous dark green tourmalines are found as relict inclusions in quartz (rare in kornerupine) in core and margin of a prism. In a later stage, kornerupine-quartz symplectites were formed by replacement of the tourmaline with loss of alkalis. On the margin of the prisms kornerupine is in turn partially in contact with (replaced by) fine-grained, light green, euhedral tourmaline. The inferred miner4l assemblages may reveal an early portion of the P-T-t path for the region. Kornerupine — quartz assemblages are not often described, whereas granite pegmatites commonly contain tourmaline — quartz symplectitic intergrowths.

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Kornerupine - Quarz Symplektite in Paragneis, Lac Ste-Marie, Quebec, Kanada: pseudomorpher Ersatz einer hexagonalen Phase, möglicherweise Osumilith

Zusammenfassung Kornerupin ist in der Grenville Province an mehreren Orten gefunden worden. An der Hinck's-Brücke am Gatineau Fluß, nahe dem Ste-Marie See, kommt Kornerupin in hoher Konzentration in einer 300 m x 1500 m Paragneis-Linse mit Quarz, Sillimanit, Kalifeldspat, Turmalin, Fe/Ti-Oxid und Granat vor, die von Diopsid- und Chondroditführendem Marmor eingeschlossen ist. In dieser Linse treten in Biotit-reichen Zonen sechseckige Prismen mit Durchmessern bis 7 cm und Längen bis 30 cm auf. Die Prismen sind symmetrisch vom Rand (grobkörnig) zum Zentrum (feinkörnig) zoniert und bestehen aus symplektitischen Kornerupin-Quarz Verwachsungen mit geringen Mengen an feinkörnigem Sillimanit (fibrolitische Einschlüsse in Kornerupin), Kalifeldspat und Ilmenit-Ketten.Die Kornerupin-Quarz-Verwachsung im sechseckigen Prisma ist an Stelle einer älteren hexagonalen Phase entstanden. Habitus und Gesamtchemismus des Prismas weist auf Osumilith als erste frühe Phase hin. Bor-führende Fluide stabilisierten während einer amphibolit- bis granulit-faziellen Metamorphose die Turmalin-Quarz-Verwachsung, die pseudomorph Osumilith ersetzte. Viele dunkelgrüne Turmaline sind als Relikteinschlüsse im Quarz und selten auch im Kornerupin im Zentrum und am Rand des Prismas zu finden. In einem späteren Stadium wurde Kornerupin-Quarz Symplektit durch Verdrängung von Turmalin mit Abgabe von Alkalien gebildet. Am Rand des Prismas ist Kornerupin teilweise in Kontakt mit (oder ersetzt durch) feinkörnigem, hellgrünen, idiomorphen Turmalin. Die abgeleiteten Mineralvergesellschaftungen entsprechen einem frühen Teil des P-T t Pfades für diese Region. Kornerupin-Quarz Paragenesen sind selten, wogegen in Granit-Pegmatiten Turmalin-Quarz Verwachsungen häufig auftreten.[/ p]

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With 4 Figures     

Tectonic facies and the archipelago-accretion process of the West Kunlun,China

A tectonic facies investigation carried out in the West Kunlun, China allows us to have worked out a tectonic model of orogen. The tectonic facies, from the north to the south, are composed of the following: 1. Southern Tarim tectonic realm; 2. North Kudi magmatic arc; 3. Kudi mélange; 4. Kudi micro-continent; 5. main shear zone; 6. Xianan Bridge calc alkaline complex; 7. Mazar-Kangxiwar mélange-accretion complex; and 8. Tianshuihai foreland fold-thrust belt. The tectonic facies 1»5 recorded the history of the northward subduction of the Prototethys and southward accretion of Eurasia in the Late Proterozoic-Early Paleozoic time, while the tectonic facies 6»8 recorded the history of the northward subduction of the Paleotethys and southward accretion of Eurasia in the Late Paleozoic-Early Mesozoic time, that of the tectonic evolution of the passive margin of the Qiangtang block, and that of the docking, and the final amalgamation of the Qiangtang block to the Eurasian continent. The tectonic facies investigation has indicated that a complicated archipelago-accretion orogenesis took place in the West Kunlun orogen, which was the important character of southward growth of the Eurasian continent.     

Accretionary processes and metallogenesis of the Central Asian Orogenic Belt: Advances and perspectives

As one of the largest Phanerozoic orogens in the world,the Central Asian Orogenic Belt (CAOB) is a natural laboratory for studies of continental dynamics and metallogenesis.This paper summarizes the research progresses of the accretionary processes and metallogenesis of the CAOB since the Peopled Republic of China was founded,and puts forward the prospect for future research.During the early period (1950s-1970s),several geological theories were applied to explain the geological evolution of Central Asia.In the early period of China's reform and opening-up,the plate tectonics theory was applied to explain the evolution of the northern Xinjiang and Xingmeng regions,and the opinion of subduction-collision between Siberian Kazakhstan and China-North Korea-Tarim plates was proposed.The idea of the Solonker-Yanbian suture zone was established.In the 1990s,the study of the CAOB entered a period of rapid development.One school of scholars including geologists from the former Soviet Union proposed a multi-block collision model for the assemblage of the CAOB.In contrast,another school of scholars,led by a Turkish geologist,Celal Sengor,proposed that the Altaids was formed through the growth and strike-slip duplicates of a single island arc,and pointed out that the Altaids is a special type of collisional orogen.During this period,Chinese geologists carried out a lot of pioneering researches on ophiolites and high-pressure metamorphic rocks in northern China,and confirmed the main suture zones accordingly.In 1999,the concept of"Central Asian metallogenic domain"was proposed,and it became one of the three major metallogenic domains in the world.Since the 21st century,given the importance for understanding continental accretion and metallogenic mechanism,the CAOB has become the international academic forefront.China has laid out a series of scientific research projects in Central Asia.A large number of important scientific research achievements have been spawned,including the tectonic attribution of micro-continents,timing and tectonic settings of ophiolites,magmatic arcs,identification and anatomy of accretionary wedges,regional metamorphism-deformation,(ultra)high-pressure metamorphism,ridge subduction plume-plate interaction archipelagic paleogeography and spatio-temporal framework of multiple accretionary orogeny,continental growth accretionary metallogenesis,structural superposition and transformation etc.These achievements have made important international influences.There still exist the following aspects that need further study:(1) Early evolution history and subduction initiation of the Paleo-Asian Ocean;(2) The accretionary mechanism of the extroversion Paleo-Asian Ocean;(3) The properties of the mantle of the Paleo-Asian Ocean and their spatiotemporal distribution;(4) The interaction between the Paleo-Asian Ocean and the Tethys Ocean;(5) Phanerozoic continental growth mechanism and its global comparison;(6) Accretionary metallogenic mechanism of the Central Asian metallogenic domain;and (7) Continental transformation mechanism.     

High-temperature,high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian,NW Scotland: Crustal-scale subduction in the Neoarchaean

Eclogites and associated high-pressure (HP) rocks in collisional and accretionary orogenic belts preserve a record of subduction and exhumation, and provide a key constraint on the tectonic evolution of the continents. Most eclogites that formed at high pressures but low temperatures at > 10–11 kbar and 450–650 °C can be interpreted as a result of subduction of cold oceanic lithosphere. A new class of high-temperature (HT) eclogites that formed above 900 °C and at 14 to 30 kbar occurs in the deep continental crust, but their geodynamic significance and processes of formation are poorly understood. Here we show that Neoarchaean mafic–ultramafic complexes in the central granulite facies region of the Lewisian in NW Scotland contain HP/HT garnet-bearing granulites (retrogressed eclogites), gabbros, lherzolites, and websterites, and that the HP granulites have garnets that contain inclusions of omphacite. From thermodynamic modeling and compositional isopleths we calculate that peak eclogite-facies metamorphism took place at 24–22 kbar and 1060–1040 °C. The geochemical signature of one (G-21) of the samples shows a strong depletion of Eu indicating magma fractionation at a crustal level. The Sm–Nd isochron ages of HP phases record different cooling ages of ca. 2480 and 2330 Ma. We suggest that the layered mafic–ultramafic complexes, which may have formed in an oceanic environment, were subducted to eclogite depths, and exhumed as HP garnet-bearing orogenic peridotites. The layered complexes were engulfed by widespread orthogneisses of tonalite–trondhjemite–granodiorite (TTG) composition with granulite facies assemblages. We propose two possible tectonic models: (1) the fact that the relicts of eclogitic complexes are so widespread in the Scourian can be taken as evidence that a > 90 km × 40 km-size slab of continental crust containing mafic–ultramafic complexes was subducted to at least 70 km depth in the late Archaean. During exhumation the gneiss protoliths were retrogressed to granulite facies assemblages, but the mafic–ultramafic rocks resisted retrogression. (2) The layered complexes of mafic and ultramafic rocks were subducted to eclogite-facies depths and during exhumation under crustal conditions they were intruded by the orthogneiss protoliths (TTG) that were metamorphosed in the granulite facies. Apart from poorly defined UHP metamorphic rocks in Norway, the retrogressed eclogites in the central granulite/retrogressed eclogite facies Lewisian region, NW Scotland have the highest crustal pressures so far reported for Archaean rocks, and demonstrate that lithospheric subduction was transporting crustal rocks to HP depths in the Neoarchaean.