Tarim Craton Research Articles

Origin of terranes in the eastern Central Asian Orogenic Belt, NE China: U–Pb ages of detrital zircons from Ordovician–Devonian sandstones, North Da Xing'an Mts.

Our new data provide evidence for the depositional age and tectonic setting of Ordovician–Devonian sandstones in the northern Da Xing'an Mountains, the eastern segment of the Central Asian Orogenic Belt (CAOB). The depositional age of the Duobaoshan Formation is younger than Middle Ordovician (462 ± 5 Ma), that of the Woduhe and Huolongmen formations is younger than Late Silurian with the youngest detrital zircon age of 417 ± 8 Ma, and 419 ± 5 Ma, respectively. The depositional age of the Niqiuhe Formation is younger than Early Silurian with the youngest detrital zircon age of 432 ± 6 Ma. All the detrital zircon ages of Ordovician–Devonian sandstones represent similar distribution characteristics in probability diagrams showing four dominant populations of ~ 500 Ma, ~ 800 Ma, ~ 1800 Ma and ~ 2500 Ma. These age groups are widely found in Devonian or pre-Devonian clastic sediments and metamorphic complexes of NE China and Mongolia. Combined with new paleomagnetic data from the Baydaric and Tuva–Mongolian composite blocks in Central Mongolia and from the Lesser Karatau block of Central Kazakhstan, and the age information from adjacent cratons and continental microplates, we suggest that the basement blocks of NE China have their origin in the Tarim craton. These basement blocks include the Erguna and Jiamusi–Khanka–Bureya blocks which rifted from the Tarim craton at ~ 800 Ma and moved close to the Siberian craton at (or earlier than) ~ 500 Ma with other microcontinents within the CAOB.

Early Paleozoic (ca. 465 Ma) eclogites from Beishan (NW China) and their bearing on the tectonic evolution of the southern Central Asian Orogenic Belt

The Beishan orogen is situated in the southern margin of the Central Asian Orogenic Belt (CAOB). It is connected with the Tianshan orogen to the west and the Mongolia-Xing’anling orogen to the east. This orogen exhibits a well-preserved Neoproterozoic to Late Paleozoic sequence containing ophiolites, accretionary complexes and eclogites. In this paper we provide new geochronological data for eclogites and their country paragneiss to constrain the tectonic evolution of the Beishan orogen. U–Pb zircon dating of eclogites yielded two concordant ages, 886 ± 4 Ma for the cores and 465 ± 10 Ma for the rims. The zircon cores display bright luminescence with homogeneous or planar/sector zoning, intermediate Th/U ratios (0.17–0.47), and HREE-enriched patterns with negative Eu anomalies (Eu/Eu * = 0.16–0.58). These features, coupled with their formation temperatures of 680–710 °C, suggest that the core age represents the time of an earlier upper amphibolite facies metamorphic episode. By contrast, the zircon rims have dark luminescence with homogeneous or fir-tree zoning, low Th/U ratios (<0.02), and flat HREE distributions with small or no negative Eu anomalies (Eu/Eu * = 0.55–1.16). Thus, the rim age reflects the time of the eclogite facies metamorphism. The occurrence of tiny inclusions of garnet, omphacite and rutile in the zircon rims is consistent with the above age interpretation. In addition, the mineral inclusions and host zircons yielded P–T conditions of 660 °C and >12 kbar for the eclogite facies metamorphism. On the other hand, zircon dating of a paragneiss near the eclogite produced two major groups of ages, ca. 1450 Ma for a magmatic event and ca. 900 Ma for a metamorphic event. It seems that eclogites and their country rocks have suffered the same earlier metamorphic episode. Based on the available age data obtained for eclogites, ophiolites and granitoids from the Beishan area, a tectonic scenario can be constructed as follows. The ca. 1000 Ma oceanic crust representing by protoliths of the eclogites was involved in Early Neoproterozoic orogenesis to have formed a microcontinental block. During the Early Paleozoic, island arcs and microcontinental blocks were progressively accreted to the northern margin of the Tarim craton (Dunhuang block). This was followed by the closure of Paleoasian Ocean and final amalgamation between the Tarim craton and Kazakhstan block in the Late Paleozoic.

Detrital and xenocrystic zircon ages from Neoproterozoic to Palaeozoic arc terranes of Mongolia: Significance for the origin of crustal fragments in the Central Asian Orogenic Belt

The Central Asian Orogenic Belt contains many Precambrian crustal fragments whose origin is unknown, and previous speculations suggested these to be derived from either Siberia, Tarim or northern Gondwana. We present an age pattern for detrital and xenocrystic zircons from Neoproterozoic to Palaeozoic arc and microcontinental terranes in Mongolia and compare this with patterns for Precambrian rocks in southern Siberia, the North China craton, the Tarim craton and northeastern Gondwana in order to define the most likely source region for the Mongolian zircons. Our data were obtained by SHRIMP II, LA-ICP-MS and single zircon evaporation and predominantly represent arc-related low-grade volcanic rocks and clastic sediments but also accretionary wedges and ophiolitic environments. The Mongolian pattern is dominated by zircons in the age range ca. 350–600 and 700–1020 Ma as well as minor peaks between ca. 1240 and 2570 Ma. The youngest group reflects cannibalistic reworking of the Palaeozoic arc terranes, whereas the Neoproterozoic to late Mesoproterozoic peak reflects both reworking of the arc terranes as well as Neoproterozoic rifting and a Grenville-age crust-formation event. The 700–1020 Ma peak does not exist in the age spectra of the Siberian and North China cratons and thus effectively rules out these basement blocks as potential source areas for the Mongolian zircons. The best agreement is with the Tarim craton where a major Grenville-age orogenic event and early Neoproterozoic rifting have been identified. The age spectra also do not entirely exclude northeastern Gondwana as a source for the Mongolian zircons, but here the Neoproterozoic age peak is related to the Pan-African orogeny, and a minor Grenville-age peak may reflect a controversial orogenic event in NW India. Our Mongolian detrital and xenocrystic age spectrum suggests that the Tarim craton was the main source, and we favour a tectonic scenario similar to the present southwestern Pacific where fragments of Australia are rifted off and become incorporated into the Indonesian arc and microcontinent amalgamation that will evolve into a future orogenic belt.

Subducted Precambrian oceanic crust: geochemical and Sr–Nd isotopic evidence from metabasalts of the Aksu blueschist, NW China

Abstract: The Aksu Proterozoic blueschist terrane in northwestern China is regarded as one of the oldest well-substantiated Precambrian blueschist terranes in the world. Previous work has focused on the high-pressure metamorphism: both the age and P – T conditions of the metamorphic event have been well discussed. However, little attention has been paid to protolith identification of the blueschist terrane. In this paper, we present geochemical and Sr–Nd analyses of metabasalt samples from the blueschist terrane. The results show that the protoliths of these mafic schists were enriched mid-ocean ridge basalt (E-MORB). Crystal fractionation has led to differentiation of trace element concentrations. This provides the opportunity to obtain a protolith Sm–Nd isochron age of 890 ± 23 Ma (MSWD = 0.68). We conclude that the Aksu blueschist terrane represents part of an ocean crust at the NW edge of Rodinia around Tarim at c . 890 Ma. It experienced subduction and exhumation followed by accretion to the Tarim craton. If the north margin of present Tarim faced outboard of Rodinia during subduction, it might form one section of the arcuate subduction zone around the supercontinent. Alternatively, the subduction zone may have been located between Tarim and Australia and would later be closed.

Zircon ages for a felsic volcanic rock and arc-related early Palaeozoic sediments on the margin of the Baydrag microcontinent, central Asian orogenic belt, Mongolia

Magmatic zircons from the basal part of an arc terrane in the accretionary complex NE of the Baydrag microcontinental block in central Mongolia were dated at 544 ± 7 Ma and reflect arc magmatism at the Ediacaran/Cambrian boundary. Detrital zircon ages for clastic metasediments of the adjacent Dzag zone suggest a depositional age <445 ± 6 Ma and suggest that the zircons were not derived from the arc terrane which was probably not exposed in the Late Ordovician. The detrital age pattern is similar to the overall pattern for Mongolian detrital and xenocrystic zircons and suggests significant input from a Grenville-age source that could either be the Tarim craton or microcontinental blocks within the central Asian collage of arcs and microcontinents.

Archean crustal evolution of the northern Tarim craton, NW China: Zircon U–Pb and Hf isotopic constraints

Tarim craton is one of the major cratons in Central Asia. Basement rocks of the craton, such as TTG gneisses, are dominantly exposed in the northern (Kuluketage area) and the Eastern (Altyn Tagh Mountain). Recent years, only a few reliable Archean ages have been obtained for the basement rocks in the Altyn Tagh Mountain. Geochronological and geochemical studies have been conducted on the orthogneisses exposed in the Kuluketage area in order to unravel complex history of the Tarim craton. Zircon U–Pb dating of the rocks yielded three weighted mean 207Pb/ 206Pb ages of 2516 ± 6, 2575 ± 13 and 2460 ± 3 Ma, indicating that their protoliths were formed in the late Neoarchean to early Paleoproterozoic. The late Neoarchean orthogneisses exhibit low Sr/Y ratios (4–19) and Mg # values (35–44), with depleted Sr contents, negative Nb, Ta and Ti anomalies and strongly fractionated REE patterns ((La/Yb) N = 13–62), displaying typical geochemical features of arc igneous rocks. In contrast, the early Paleoproterozoic orthogneisses are characterized by high-SiO 2 contents, Sr/Y ratios (37–67) and Mg # values (46–67), with variably fractionated REE patterns ((La/Yb) N = 9–89), slight Sr enrichment and positive Eu anomalies, resembling high-SiO 2 adakites derived from subducted basaltic slab-melts. The late Neoarchean TTGs have low ɛ Hf( t) values (–5 to +1) and initial Hf compositions (0.280987–0.281160) with Mesoarchean two-stage model ages ( T DM2 = 2.9–3.3 Ga), suggesting that the crustal materials of the basement rocks in this area were initially extracted from a depleted mantle in the late Paleo- to Mesoarchean and were reworked in the late Neoarchean. The history of crustal growth is different from that of the North China and Yangtze cratons and thus implies relatively younger cratonization than the North China and Yangtze cratons. Zircons in the early Paleoproterozoic orthogneisses have high ɛ Hf( t) values (+4 to +10) and young Neoarchean two-stage model ages (2.5–2.7 Ga), revealing a juvenile crustal growth event in the late Neoarchean. Because there is no record of coeval juvenile crustal growth in the other two cratons, we suggest a separate crustal evolution for the northern Tarim craton which was stayed far from the other two cratons before the early Paleoproterozoic.

Thermotectonic evolution of Precambrian basement rocks of the Kuruktag uplift, NE Tarim craton, China: evidence from apatite fission-track data

The Kuruktag uplift is located directly northeast of the Tarim craton in northwestern China. Neoarchaean-to-Neoproterozoic metamorphic rocks and intrusive rocks crop out widely in the uplift; thus, it is especially suited for a more complete understanding of the thermal evolution of the Tarim craton. Apatite fission-track (AFT) methods were used to study the exhumation history and cooling of these Precambrian crystalline rocks. Nine apatite-bearing samples were collected from both sides of the Xingdi fault transecting the Kuruktag uplift. Pooled ages range from 146.0 ± 13.4 to 67.6 ± 6.7 Ma, with mean track lengths between 11.79 ± 0.14 and 12.48 ± 0.10 μm. These samples can be divided into three groups based on age and structural position. Group A consists of five samples with AFT apparent ages of about 100–110 Ma and is generally associated with undeformed areas. Group B comprises three specimens with AFT apparent ages lower than 80 Ma and is mostly associated with hanging wall environments close to faults. Group C is a single apatite sample with the oldest relative apparent age, 146.0 ± 13.4 Ma. The modelled thermal history indicates four periods of exhumation in the Kuruktag uplift: late-Early Jurassic (180 Ma); Late Jurassic–Early Cretaceous (144–118 Ma); early-Late Cretaceous (94–82 Ma); and late Cenozoic (about 10 Ma). These cooling events, identified by AFT data, are assumed to reflect far-field effects from multi-stage collisions and accretions of terranes along the south Asian continental margin.

End-Permian to mid-Triassic termination of the accretionary processes of the southern Altaids: implications for the geodynamic evolution, Phanerozoic continental growth, and metallogeny of Central Asia

The Altaids is one of the largest accretionary orogenic collages in the world with the highest rate of Phanerozoic continental growth and significant metallogenic importance. It is widely accepted that subduction-related orogenesis of the Altaids started in the late Precambrian and gradually migrated southward (present coordinates). However, it is uncertain when and how the building of the Altaids was finally completed. Based on structural geology, geochemical, geochronological, and paleomagnetic data, this paper presents late Paleozoic to early Mesozoic accretionary tectonics of two key areas, North Xinjiang in the west and Inner Mongolia in the east, together with neighboring Mongolia. The late Paleozoic tectonics of North Xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of Siberia and its final amalgamation with the passive margin of Tarim, which may have lasted to the end-Permian to early/mid-Triassic. In contrast, in Inner Mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of Siberia and the northern active margin of the North China craton, which may have lasted to the mid-Triassic. The final products of the long-lived accretionary processes in the southern Altaids include late Paleozoic to Permian arcs, late Paleozoic to mid-Triassic accretionary wedges composed of radiolarian cherts, pillow lavas, and ophiolitic fragments, and high-pressure/ultrahigh-pressure metamorphic rocks. Permian Alaskan-type zoned mafic-ultramafic complexes intruded along some major faults of the Tien Shan. We define a new Tarim suture zone immediately north of the Tarim craton that is probably now buried below the Tien Shan as a result of northward subduction of the Tarim block in the Cenozoic. The docking of the Tarim and North China cratons against the southern active margin of Siberia in the end-Permian to mid-Triassic resulted in the final closure of the Paleoasian Ocean and terminated the accretionary orogenesis of the southern Altaids in this part of Central Asia. This complex geodynamic evolution led to formation of giant metal deposits in Central Asia and to substantial continental growth.

Age and origin of high Ba–Sr appinite–granites at the northwestern margin of the Tibet Plateau: Implications for early Paleozoic tectonic evolution of the Western Kunlun orogenic belt

The Buya appinite–granite is a typical high Ba–Sr granite emplaced at the northern West Kunlun orogenic belt along the northwestern margin of the Tibetan Plateau. The granite is dated at ca. 430 Ma using the SHRIMP U–Pb zircon method. It consists of alkaline feldspar granites with coeval appinite enclaves. The granite possesses high SiO 2 (69.77–72.69%), K 2O (4.44–5.10%) and total alkalinity (K 2O + Na 2O = 8.80–9.92%), Sr (655–1100 ppm), Ba (1036–1433 ppm) and LREE, and low HREE and HFSE contents and insignificant negative Eu anomalies. Consequently, the samples have very high Sr/Y (74–141) and (La/Yb) N (37–96) ratios. On the other hand, they have low MgO (or Mg #), Cr and Ni contents and low radiogenic Nd isotopes ( ɛNd( T) = − 8.4 to − 10.4). The high Ba–Sr and other geochemical signatures of the granite also appear in the appinite enclaves except that the appinite enclaves have relatively higher abundances in these elements and higher ɛNd( T) values (− 5.7 to − 6.7). Elemental and isotope compositions suggest that the appinites were derived from partial melting of an enriched lithospheric mantle source probably induced by upwelling of the asthenosphere due to the delamination of a subducted slab. The granite was likely derived from partial melting of the mafic lower crust (with residual garnet), associated with involvement of minor LILE-enriched appinitic magma, followed by crystal fractionation of hornblende, biotite, apatite and allanite. In combination with previous investigations on the evolution of the Western Kunlun, we suggest that the Buya high Ba–Sr plutons represent the end of an early Paleozoic crust thickening event after a terrane accretion on southern Tarim craton, and the beginning of a post-orogenic collapse phase in the Paleozoic West Kunlun orogenic belt.

Three-dimensional thermal structure of the Chinese continental crust and upper mantle

We invert S-wave velocities for the 3D upper-mantle temperatures, in which the position with a temperature crossing the 1300°C adiabat is corresponding to the top of the seismic low velocity zone. The temperatures down to the depth of 80 km are then calculated by solving steady-state thermal conduction equation with the constraints of the inverted upper-mantle temperatures and the surface temperatures, and then surface heat flows are calculated from the crustal temperatures. The misfit between the calculated and observed surface heat flow is smaller than 20% for most regions. The result shows that, at a depth of 25 km, the crustal temperature of eastern China (500–600°C) is higher than that of western China (<500°C). At a depth of 100 km, temperatures beneath eastern and southeastern China are higher than the adiabatic temperature of 1300°C, while that beneath west China is lower. The Tarim craton and the Sichuan basin show generally low temperature. At a depth of 150 km, temperatures beneath south China, eastern Yangtze craton, North China craton and around the Qiangtang terrane are higher than the adiabatic temperature of 1300°C, but is the lowest beneath the Sichuan basin and the regions near the Indian-Eurasian collision zone. At a depth of 200 km, very low temperature occurs beneath the Qinghai-Tibet Plateau and the south to the Tarim craton.

Precambrian metamorphic basement and sedimentary cover of the North China Craton: A review

The North China Craton is the largest and oldest of the three cratons in China (i.e. North China, South China and Tarim Cratons). It consists of an Archean and Paleoproterozoic metamorphic basement and overlying Mesoproterozoic unmetamorphosed sedimentary cover. A recently proposed model for the evolution of the North China Craton envisages discrete Eastern and Western Blocks that developed independently during the Archean and collided along the Trans-North China Orogen during a Paleoproterozoic collisional event, conventionally named the Lüliangian Orogeny. The Lüliangian Orogeny is one of the most important events in the Precambrian evolution of the North China Craton and is considered to be part of the global-scale 2.0–1.8 Ga collisional events that led to the assembly of a Paleo-Mesoproterozoic Columbia supercontinent. Voluminous and thick-layered clastic rocks and carbonates were deposited in the interior or on the margins of the North China Craton following its final amalgamation at ∼1.85 Ga. In addition, a series of rifting events took place in the North China Craton between 1.8 and 1.6 Ga, which may have been related to the dispersal of the supercontinent Columbia. Platformal carbonates with abundant stromatolites are predominant in the Mesoproterozoic to Cambrian strata of the craton, but no glacial deposits similar to the Nantuo Tillite of the Yangtze Craton were developed in the North China Craton in the Neoproterozoic. The North China Craton also did not undergo extensive Neoproterozoic thermo-tectonic events as those in the Tarim and Yangtze Cratons, suggesting that the North China Craton was not involved in the same tectonic processes during the assembly and break-up of the Rodinia supercontinent.

Lithospheric thickness of the Chinese continent

We invert for the upper-mantle temperatures of the Chinese continent in the depth range of 70–240 km from a recent S-velocity model. The depth where temperatures intersect a mantle adiabat with a potential temperature of ∼1300 °C is in close correspondence with the top of the seismic low velocity zone for most regions. This correspondence implies that seismic lithosphere estimated from short-time scale seismic information may be equivalent to the long-time scale geodynamical lithosphere. Defining the 1300 °C adiabat as coinciding with the lithospheric base, we estimate the seismic-thermal lithosphere thickness. The estimated thickness shows obvious dependence on the tectonic settings. Beneath eastern China, which mainly belongs to the circum-Pacific tectonic domain, it has a thickness of ∼100 km; and beneath the Qinghai–Tibet plateau and south to the Tarim craton, which belong to the Tethyan tectonic domain it has a thickness of ∼160–220 km. The lithospheric thicknesses of the three large para-platforms/cratons range from ∼170 km for the western Yangtze, ∼140 km for Tarim, and ∼100 km for Sino-Korean. The three cratons may have been reshaped by Phanerozoic tectonic activities and are thinner than most cratons in other continents.

Types, geological features and geodynamic significances of gold-copper deposits in the Kanggurtag metallogenic belt, eastern Tianshan, NW China

Occupying the middle of the central Asia Paleozoic accretionary and collisional orogenic belt, the eastern Tianshan area has a great economic potential due to Au-Cu mineralization during syn- and post- orogenic events. In the Kanggurtag Au-Cu metallogenic belt, three major types of gold deposits have been recognized: ductile-shear-zone-hosted gold deposits, magmatic hydrothermal gold deposits, and epithermal gold deposits. Four kinds of copper deposits have also been identified recently: the porphyry-type, the skarn-type, the magmatic type, and volcanic/sedimentary-type. Tectonically, the development of these late Paleozoic gold and copper deposits was closely associated with the subduction and collision of the ancient Tianshan ocean that intervened between the Tarim craton and the Siberian block. In the early to mid-Carboniferous, N-dipping subduction beneath the Dananhu arc generated magmatic intrusions, leading to formation of the porphyry Cu deposits. The magmatic front migrated southward to form the Yamansu arc upon the Kanggurtag accretionary wedge. In the latest Carboniferous to early Permian, during the closure of the ancient Tianshan ocean, large mafic-ultramafic complexes were emplaced, resulting in several magmatic copper-nickel deposits. Gold deposits of the shear-zone-type are controlled by the Kanggurtag ductile shear zone, which is related to collisional orogenesis. The epithermal gold deposits are associated with extensional tectonics and post-tectonic volcanic activity. The tectonic settings, geological features, and temporal and spatial distributions of these different types of gold and copper deposits reflect, to a great extent, the accretionary and collisional tectonics that occurred between the northern margin of the Tarim block and the southern margin of the Siberian block.

Major Types and Characteristics of Late Paleozoic Ore Deposits, East Tianshan, Northwest China

One of the largest and most important metallogenic provinces in China is the East Tianshan, where seven major types of Late Paleozoic metal deposits have been recognized: (1) porphyry-type Cu-Mo-(Au) ore; (2) volcanic Fe-Cu; (3) orogenic lode gold; (4) magmatic Cu-Ni sulfide; (5) epithermal gold; (6) volcanic hydrothermal Cu; and (7) skarn Cu-Ag. Tectonically, the development of these Late Paleozoic mineral deposits was closely associated with subduction and closure of the ancient Tianshan ocean, lying between the Tarim craton and the Junggar-Kazakhstan composite block. In Late Devonian to Early Carboniferous time, the northern edge of the Tarim craton existed as a passive continental margin, whereas the ancient Tianshan oceanic crust was subducted beneath the southern margin of the Junggar-Kazakhstan block, resulting in the formation of the Dananhu-Tousuquan magmatic arc and associated porphyry-type Cu-Mo-(Au) deposits. In Middle Carboniferous time, the ancient Tianshan oceanic crust began to subduct beneath the northern margin of the Tarim craton, leading to the formation of the Aqishan-Yamansu magmatic arc and associated volcanic Fe-Cu deposits. In the Late Carboniferous, the ancient Tianshan ocean closed, and a continent-arc collision occurred, leading to the formation of the Tianshan orogen. Following collision, an extensional event, associated with the emplacement of voluminous ultramafic-mafic complexes and formation of a number of large- to medium-scale magmatic copper-nickel ore deposits, occurred along the Kangger suture zone. In Early Permian time, East Tianshan entered into a post-collision stage associated with widespread emplacement of granitoid bodies and within-plate volcanism, which led to the formation of hydrothermal copper deposits, skarn-type Cu-Ag deposits, post-orogenic gold deposits, and epithermal gold deposits.

ORDOVICIAN—PERMIAN PALAEOGEOGRAPHY OF CENTRAL EURASIA: DEVELOPMENT OF PALAEOZOIC PETROLEUM‐BEARING BASINS

In this paper, we discuss three petroleum‐bearing basins of Palaeozoic age in Central Eurasia—the Precaspian, Tarim and Chu‐Sarysu Basins. We make use of recently‐published palaeogeographic maps of the Central Eurasian region, six of which are presented here (Late Ordovician, Early‐Middle Devonian, Late Devonian, Early Carboniferous, Early Permian and Late Permian). The maps illustrate the development through the Palaeozoic of the Palaeoasian and Palaeotethys Oceans; of the East European, Siberian and Tarim cratons; and of the Kazakhstan and other microcontinental blocks.The Kazakhstan block formed during the Late Ordovician and is a collage of Precambrian and Early Palaeozoic microcontinents and island arcs. It is surrounded by collisional foldbelts (Ob‐Zaisan, Ural‐Tianshan and Junggar‐Balkhash) which formed in the Late Carboniferous — Permian. We believe that the formation of a stable Kazakhstan block is not consistent with the existence of the previously‐identified “Kipchak arc” within the Palaeoasian ocean, or (as has previously been proposed) with activity on this arc up to the end of the Palaeozoic.The oil and gas potential of the Precaspian, Tarim and Chu‐Sarysu Basins depends to a large extent on their tectonic stability during the Palaeozoic and subsequent time. The Precaspian Basin has been stable since the Cadomian orogeny (Early Cambrian) and is known to have major hydrocarbon potential. The Tarim Basin (NW China) has somewhat lower potential because the margins of the Tarim continental block have been affected by a series of collisional events; that margin with the Palaeotethys Ocean, for example, was active during the Late Palaeozoic. The Chu‐Sarysu Basin on the Kazakhstan block is the least stable of the three and contains only minor gas accumulations.

Timing and history of marine hydrocarbon accumulation in Tarim craton

Based on the analyses of hydrocarbon inclusions, K-Ar dating of authigenic illites and oil/gas-water contact retrospection as well as other methods, the marine primary reservoirs in the cratonic region of the Tarim Basin are found to have been formed in the late Caledonian to early Hercynian, late Hercynian, and Himalayan, and oil and gas adjusted and redistributed in the Yanshanian (Jurassic to Cretaceous) and Himalayan (Tertiary). The analyses also show that the remaining primary oil accumulations are mostly formed in the late Hercynian, and the secondary oil reservoirs resulted from the adjustment and redistribution of the earlier accumulations during the Yanshanian and Himalayan (especially late Himalayan), of which the Himalayan is also the major stage of gas accumulation. The primary oil reservoirs formed in the late Hercynian mostly occur within the Paleozoic formation, whereas the secondary reservoirs formed in the Himalayan in the Mesozoic. The late adjustment and redistribution of reservoirs in the craton region of the Tarim Basin is ubiquitous because of the intensified tectonic movements since the Mesozoic, especially Cenozoic and intrusion of the high-to overmature gas. Furthermore, the lower amplitude of reservoirs in the craton region also makes them easier to be adjusted and redistributed. Therefore, the remaining paleouplift and palaeoslope developed in relatively stable tectonic regions are the main potential areas for the middlelarge marine reservoirs in the Tarim Basin.

OIL & GAS MAJORS SIGN CHINESE AGREEMENT

The Sawayaerdun deposit, located in the southern Tianshan orogenic belt in Xinjiang, NW China, is one of many orogenic gold deposits in the Central Asian Orogenic Belt (CAOB). The deposit is controlled by reverse faults and hosted in highly deformed carbonaceous turbidites that have undergone pyritic, arsenopyritic, silicification, carbonate and sericitic alteration. Gold occurs mainly in the form of electrum and native gold, and is also present in pyrite and arsenopyrite as “invisible gold”. Three generations of auriferous pyrite have been recognized through detailed petrographic studies and EMPA analyses. The pyrite with framboidal texture (Py0) is disseminated in the host rock, and is locally enriched in gold. The anhedral pyrite (Py1), associated with silicification and quartz veins that experienced strong deformation, has an average Au content of 0.021 wt.%. The euhedral–subhedral pyrite (Py2), showing octahedron cube and pyritohedron habits, is associated with least deformed quartz veins that are best developed in the main orebodies, and has an average Au content of 0.023 wt.%. The Py0 is interpreted to have formed contemporaneously with the ore-bearing rocks, with a maximum depositional age of 355 ± 7.3 Ma based on U–Pb dating of detrital zircons. Re–Os isotopic analyses of four Py1 samples yielded an isochron age of 324 ± 4.8 Ma, and those of five Py2 samples yielded an isochron age of 282 ± 12 Ma. Py1 is interpreted to have formed in the syn-tectonic stage, during the collision between the Tarim craton and the Central Tianshan terrane in Late Carboniferous, whereas Py2 was formed in a later mineralization event, during the late- to post-tectonic stage in Early Permian. These study results suggest that multiple stages of gold mineralization have developed in the Sawayaerdun deposit, and similar mineralization processes may have taken place in other parts of the Western Tianshan Orogen.

OIL & GAS MAJORS SIGN CHINESE FRAMEWORK AGREEMENT

The Sawayaerdun deposit, located in the southern Tianshan orogenic belt in Xinjiang, NW China, is one of many orogenic gold deposits in the Central Asian Orogenic Belt (CAOB). The deposit is controlled by reverse faults and hosted in highly deformed carbonaceous turbidites that have undergone pyritic, arsenopyritic, silicification, carbonate and sericitic alteration. Gold occurs mainly in the form of electrum and native gold, and is also present in pyrite and arsenopyrite as “invisible gold”. Three generations of auriferous pyrite have been recognized through detailed petrographic studies and EMPA analyses. The pyrite with framboidal texture (Py0) is disseminated in the host rock, and is locally enriched in gold. The anhedral pyrite (Py1), associated with silicification and quartz veins that experienced strong deformation, has an average Au content of 0.021 wt.%. The euhedral–subhedral pyrite (Py2), showing octahedron cube and pyritohedron habits, is associated with least deformed quartz veins that are best developed in the main orebodies, and has an average Au content of 0.023 wt.%. The Py0 is interpreted to have formed contemporaneously with the ore-bearing rocks, with a maximum depositional age of 355 ± 7.3 Ma based on U–Pb dating of detrital zircons. Re–Os isotopic analyses of four Py1 samples yielded an isochron age of 324 ± 4.8 Ma, and those of five Py2 samples yielded an isochron age of 282 ± 12 Ma. Py1 is interpreted to have formed in the syn-tectonic stage, during the collision between the Tarim craton and the Central Tianshan terrane in Late Carboniferous, whereas Py2 was formed in a later mineralization event, during the late- to post-tectonic stage in Early Permian. These study results suggest that multiple stages of gold mineralization have developed in the Sawayaerdun deposit, and similar mineralization processes may have taken place in other parts of the Western Tianshan Orogen.

Geology and Geochemistry of Se-Bearing Formations in Central China

It is widely believed that selenium, a typical dispersed element, does not form independent deposits. However, this has been challenged recently by discoveries and investigations of Se-bearing formations in China. In this paper we present our findings with a systematic discussion on the genesis of these Se-bearing formations. The Se-bearing formations typically are developed in tensile tectonic settings, mainly along the margins of the Yangtze and the Tarim cratons, or within the Yangtze craton. They are mostly Cambrian, but a few are Permian in age. Lithologically, they largely consist of carbonaceous cherts and carbonaceous shales (slates) that contain abundant organic matter. In general, the Se-bearing formations are characterized by exceptional associations of elements. In particular, they are characterized by elements with multiple oxidation states, such as V, U, P, Ni, Mo, etc., which are typical for the black shale series, and by elements like As, Ba, and Sb, which are considered as indicative elements for hydrothermal sedimentation. In some cases, they also contain Au and platinum-group elements (PGE), which are quite unusual for ordinary depositions. Organic geochemical studies and biomarker investigations revealed that the Corg in rocks are relatively high in these Se-bearing formations, ranging from 1.3% to 17.8%. The fraction of soluble organic matter extracted from the samples is commonly less than 50 ppm. Gas chromatographic (GC) analyses suggest that saturated hydrocarbons in these formations consist mainly of low-carbon-number alkane, with the highest peak of C15-C18 and with low ratios of Pr/Ph (0.62-1.03). Using gas chromatographic-mass spectrometric (GC-MS) analyses, many biomarkers have been identified, including the relatively complete series of long-chain tricyclic terpanes and hopanes in m/z 191 mass chromatogram and the steroids in m/z 217 mass chromatogram. Evidence suggests that the source or precursors of the organic matter might have been some sort of bacteria and/or algae. The reducing sedimentary environments that hosted these bacteria and algae favored the growth and preservation of such organisms. It was these organisms that acted as the carrier of Se, leading to Se enrichment and deposition. Finally, some evidence also indicates that the source of the Se was also closely related to the formation of hydrothermal sedimentation of cherts.

Tectonics of Precambrian basement of the Tarim craton

The Altyn Tagh Mountain is the main area where the Precambrian basements of Tarim craton are exposed. There are two ophiolitic belts in Altyn Tagh: one belt is exposed in the northern margin of Altyn Tagh whose formation age is about (829±60) Ma, the other is situated along the southern margin of Altyn Tagh and has a formation age of about (1449±270) Ma. This paper proposes a simple tectonic model for the Precambrian basement of Tarim craton established from ophiolites in Altyn Tagh area. The south Tarim block had amalgamated with Qaidam block during about 1400—1500 Ma along the present Altyn fault, while the south Tarim-Qaidam united block was still separated from the north Tarim block by an ocean. The united block of south Tarim and Qaidam collided with north Tarim block along the zone of high positive anomaly of central Tarim, Hongliugou and Lapeiquan in about 800 Ma. So since the Sinian (beginning at 800 Ma) there has been an integrated basement for Tarim craton.