Tarim Craton Research Articles

FORMATION AND EVOLUTION OF THE DUNHUANG BLOCK

Dunhang Block is located between the North China and the Tarim Cratons (Figure). It is bounded by the Beishan Orogenic Belt to the north and Altyn Tagh Orogenic Belt to the south, respectively; in the west the Qiemo-Xingxingxia fault separates the block from Tarim Craton, and in the east the Altyn Tagh Fault separates it from the Alxa block of western part of the North China Craton. Although Archean-Paleoproterozoic basement rocks, which are referred to as Milan Complex, exposed along the Northern Altyn Tagh Orogenic Belt, some researchers suggested that their rock associations, metamorphisms and evolutionary history present obviously different with those of the Dunhuang Complex in Dunhuang region, Gansu Provence, thus the Milan Complex should be excluded from the Dunhuang Block, and is considered as basement rocks of the southwestern Tarim Craton.

CO-EVOLUTION OF PROTEROZOIC CRATONIC FRAGMENTS IN WESTERN AND NORTHERN MONGOLIA

CAOB occupies a vast area that extends from the Urals to the Far East Asia and from the Siberian craton to the North China and Tarim cratons (Fig. 1, A). In order to better constrain Precambrian tectonic evolution of the CAOB, it is important to revisit Precambrian terranes of Mongolia as outlined in Badarch et al. [2002] that contain Archean to Proterozoic metamorphic basement and Neoproterozoic metasedimentary and volcanic rocks.

NO EXCESSIVE CRUSTAL GROWTH IN THE CENTRAL ASIAN OROGENIC BELT: FURTHER EVIDENCE FROM FIELD RELATIONSHIPS AND ISOTOPIC DATA

Abstract We provide new field observations and isotopic data for key areas of the Central Asian Orogenic Belt (CAOB), reiterating our previous assessment that no excessive crustal growth occurred during its ca. 800 Ma long orogenic evolution. Many Precambrian blocks (microcontinents) identified in the belt are exotic and are most likely derived from the northern margin of Gondwana, including the Tarim craton. Ocean opening in the Palaeo-Asian Ocean, arc formation and accretionary processes began in the latest Mesoproterozoic along the southern margin of the Siberian craton and continued into the Neoproterozoic, giving rise to tectono-metamorphic terranes distinct from the exotic microcontinents in that they include tectonically mixed ancient crust as well as juvenile, mantle-derived igneous rocks. Several previous assessments of crustal growth based on the distribution of oceanic and island arc complexes and on Nd isotopic data for post-accretion igneous rocks are questionable, and we show that such data, in combination with the occurrence of old zircon xenocrysts, frequently signify tectonic mixing of juvenile and ancient crustal components. The only truly juvenile terranes, including oceanic crust and intra-oceanic arcs, seem to occur in northeastern Kazakhstan, in the Altai-Sayan region of Siberia and in the Lake and Trans-Altai zones of Mongolia. The largest area of pre-CAOB continental crust forms a broad belt from northwestern Kazakhstan via the Kyrgyz North and Middle Tianshan to the Yili Block and Chinese Central Tianshan in NW China. Most arcs in the CAOB formed on older continental crust, or with substantial addition of old crustal material via sediment recycling, similar to the situation in the present Southwest Pacific in southern Indonesia, and we suspect that the volume of old material in the lower crust of the CAOB is considerable but largely unaccounted for because of lack of geophysical data. Comparing the lithospheric mantle domains as revealed by Os model ages, with ancient crust at least Mesoproterozoic in age and predating formation of the CAOB significantly reduces the volume of new juvenile crust generated during the orogeny. We conclude that the volume of truly juvenile crustal material in the CAOB is about 20%, similar to that in other accretionary orogens through Earth history, and considering the ca. 800 Ma history of the belt this is not anomalous.

Neoproterozoic paleogeography of the Tarim Block: An extended or alternative “missing-link” model for Rodinia?

Recent reconstructions of the Rodinia supercontinent and its breakup incorporate South China as a “missing link” between Australia and Laurentia, and place the Tarim craton adjacent to northwestern Australia on the supercontinent's periphery. However, subsequent kinematic evolution toward Gondwana amalgamation requires complex geometric shuffling between South China and Tarim, which cannot be easily resolved with the stratigraphic records of those blocks. Here we present new paleomagnetic data from early Ediacaran strata of northwest Tarim, and document large-scale rotation at near-constant paleolatitudes during Cryogenian time. The rotation is coeval with Rodinia breakup, and Tarim's paleolatitudes are compatible with its placement between Australia and Laurentia, either by itself as an alternative “missing link” or joined with South China in that role. At the same time, indications of subduction-related magmatism in Tarim's Neoproterozoic record suggest that Rodinia breakup was dynamically linked to subduction retreat along its northern margin. Such a model is akin to early stages of Jurassic fragmentation within southern Gondwana, and implies more complicated subduction-related dynamics of supercontinent breakup than superplume impingement alone.

Multiple-stage mineralization in the Sawayaerdun orogenic gold deposit, western Tianshan, Xinjiang: Constraints from paragenesis, EMPA analyses, Re–Os dating of pyrite (arsenopyrite) and U–Pb dating of zircon from the host rocks

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 0.021wt.%. 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 0.023wt.%. The Py0 is interpreted to have formed contemporaneously with the ore-bearing rocks, with a maximum depositional age of 355±7.3Ma based on U–Pb dating of detrital zircons. Re–Os isotopic analyses of four Py1 samples yielded an isochron age of 324±4.8Ma, and those of five Py2 samples yielded an isochron age of 282±12Ma. 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.

Paleoproterozoic subduction-related magmatism and crustal evolution of the Dunhuang Block, NW China

The Precambrian basement rocks of the Tarim Craton, NW China, occur mostly along the margins of the craton. These rocks have been metamorphosed under amphibolite-facies or granulite-facies, particularly in the Dunhuang and Kuluketage Blocks, whose crustal evolution and paleographic relationship are still unclear. This study presents in-situ zircon U-Pb ages and Lu-Hf isotopes and whole-rock elemental compositions for the Paleoproterozoic meta-felsic and mafic rocks in the NE Altyn Tagh area in order to constrain the Paleoproterozoic tectono-thermal events of the Dunhuang Block and to further investigate its relation to the Kuluketage Block. The meta-felsic rocks have a crystallization age of ∼1.96Ga. Their zircon εHf(t) values vary from −0.5 to +7.2, with TDM1 model ages from 2.4 to 2.1Ga, suggesting that their parental magmas were mainly derived from juvenile sources, with a relatively small contribution from pre-existing ancient materials. The meta-felsic rocks have geochemical affinities with typical high-Al TTG generated by slab melting except for the extremely low Cr/Ni ratios. The steeply negative slope of their REE patterns indicates that garnet and/or hornblende extraction was involved in the magma generation process. The ∼2.06 meta-mafic rocks have slightly variable positive εHf(t) values (0 to +3.4), corresponding to fairly uniform TDM1 model ages of 2.4–2.3Ga. These results indicate that their parental magma was sourced mainly from juvenile components with minor old materials. The meta-mafic rocks have nearly flat REE patterns that bear a remarkably close resemblance to E-MORB, but the depletion in Nb, Ta, Zr, Hf and Th is typical of arc rocks. The wide range of zircon εHf(t) values from the meta-felsic and mafic rocks indicates that the Dunhuang Block experienced both crustal growth and crustal reworking during the Paleoproterozoic. In both the Dunhuang and Kuluketage Blocks, plenty of early Precambrian basement rocks experienced comparable magmatism and metamorphism. However, the Dunhuang Block has a crust-formation age peaked at ∼2.83Ga, and it experienced intensive tectono-thermal events in Paleoproterozoic. These events are nearly absent in the Kuluketage Block. The discrepancy implies that the Dunhuang and Kuluketage Blocks were probably unrelated terranes before Paleoproterozoic.

Sources and provenance of the Neoproterozoic placer deposits of the Northern Kazakhstan: Implication for continental growth of the western Central Asian Orogenic Belt

We present results of in situ analysis of zircon U-Pb ages, Hf isotopic compositions for detrital zircons combined with whole-rock Nd isotopic data for the Neoproterozoic quartzites and zircon-rutile placer deposits in quartzite-schist sequences of the Kokchetav, Ishkeol'mes and Erementau-Niyaz terranes of the Northern Kazakhstan and discuss possible provenances and position of these terranes in Mesoproterozoic. Concordia ages of detrital zircons from the Northern Kazakhstan part of the Central Asian Orogenic Belt (CAOB) demonstrate age peaks in intervals ca. 1.13–1.46, 1.65–1.78, 1.86–1.92, 2.70 and 2.84 Ga. Obtained data indicate that Mesoproterozoic, latest Paleoproterozoic (Staterian), late Paleoproterozoic '(Orosinian) and Neoarchean rocks were the sources of these metasediments. Hf-in-zircon and Nd whole rock data show that the Mesoproterozoic and latest Paleoproterozoic rock complexes were originated from mainly short-lived juvenile and mixed crustal-juvenile sources that imply their derivation in subduction-related oceanic and continental arc setting or from mafic underplating. Rocks with ages of ca 1.65–1.78, 1.86–1.92, 2.70 and 2.84Ga have not been recognised previously in the western CAOB. It is possible that these rocks were completely eroded or overlapped by younger sequences and composed the buried basement of Precambrian terranes of the western CAOB. Comparison of Meso-Paleoproterozoic age patterns for the Northern Kazakhstan, Tian Shan and Tarim craton demonstrate that the crustal basement of the Northern Kazakhstan is similar to the Chinese Central Tian Shan terrane and different from the Archean basement formation of the Tarim Craton. Age peaks at ca. 1.92–1.86, 1.78–1.65 and 1.46–1.13Ga, identified for detrital zircons from Neoproterozoic zircon-rutile placer deposits of the Northern Kazakhstan correspond to the formation and breakup of the Columbia/Nuna supercontinent and assembly of the Rodinia supercontinent or large continental domain.

Proterozoic–Mesozoic history of the Central Asian orogenic belt in the Tajik and southwestern Kyrgyz Tian Shan: U-Pb,40Ar/39Ar, and fission-track geochronology and geochemistry of granitoids

Multimethod geochronology (U-Pb zircon; 40 Ar/ 39 Ar hornblende, biotite, feldspar; apatite fission track) on granitoids, gneisses, and Cenozoic intramontane basin clastics of the Gissar-Alai ranges, South Tian Shan collisional belt, west of the Talas-Fergana fault, elucidates a history of Neoproterozoic magmatism, late Paleozoic magmatism and metamorphism, and Mesozoic−Cenozoic thermal reactivation. Zircon-core and grain-interior U-Pb ages of ca. 2.7−2.4, 2.2−1.7, 1.1−0.85, and 0.85−0.74 Ga tie the early evolution of the Gissar-Alai ranges to that of the Tarim craton. At least part of the Gissar range crystalline basement—the Garm massif—shows U-Pb zircon crystallization ages of ca. 661‒552 Ma (median ca. 609 Ma), again suggesting a Tarim craton connection. Tarim collided with the Middle Tian Shan block at ca. 310‒305 Ma, completing the protracted formation of the South Tian Shan collisional belt. The massive Gissar range granitoids intruded later (ca. 305‒270 Ma), contemporaneous with peak Barrovian-type metamorphism in the Garm massif rocks. Major- and trace-element compositions suggest that the Gissar granitoid melts have continental arc affinity. Zircon e Hf and whole-rock e Nd values of −2.1 to −6.9 and −2.7 to −7.2, respectively. and Hf-isotope crustal model and Nd-isotope depleted mantle model ages of ca. 1.0‒1.2 and ca. 1.1‒2.2 Ga, respectively, suggest significant input of Precambrian crust in the Gissar granitoid and Garm orthogneiss melts, consistent with the U-Pb ages of inherited and detrital zircons. The distinct ca. 661‒552 Ma Garm gneiss crystallization ages and the ca. 1.0−2.2 Ga model ages (and the lack of 2.4−3.4 Ga model ages) tie the Garm gneisses and the reworked crust of the Gissar range to the northern rim—the Kuqa and Kolar sections—of the Tarim craton, suggesting a united Karakum-Tarim craton. Although about contemporaneous with widespread postcollisional magmatism in the entire Tian Shan, the large volume and short duration of the Gissar range magmatism, including crustal thickening and prograde metamorphism during Tarim craton‒Middle Tian Shan block collision, and formation and closure of an oceanic back-arc basin (the Gissar basin), indicate its origin in a distinct setting. Combined, this likely resulted in midcrustal melting and upper-crustal batholith emplacement. Mafic dikes and pipes intruded at ca. 256−238 Ma (median ca. 241 Ma); the source region of the parental melts was within the asthenospheric mantle. The simplest interpretation for these basanites is that they were part of the Tarim flood basalt province; this would extend this province westward from the Tarim craton into the southwestern Tian Shan and imply that the relatively short-lived flood basalt event (ca. 290‒270 Ma) was followed by much less voluminous but longer-lasting hotspot magmatism. The 40 Ar/ 39 Ar and detrital apatite fission-track dates outline post−Gissar-Alai range granitoid emplacement cooling, Cimmerian collision events at the southern margin of Asia, Late Cretaceous crustal extension and local magmatism, and early Cenozoic shortening and burial in the far field of the India-Asia collision.

Petrology and Geochemistry of the Banded Iron Formation of the Kuluketage Block, Xinjiang, NW China: Implication for BIF Depositional Setting

AbstractThe Kuluketage block, located in the northeast Tarim craton, is one of the largest Precambrian blocks in the Xinjiang province. Recently, many banded iron formation (BIF)‐type (Superior‐type) deposits have been discovered in the western part of the Kuluketage block. These deposits occurred in the Paleoproterozoic Shayiti Formation, Xingditage Group, which has a nearly E–W distribution in the southern Xinger and Xingdi faults. Tremolite biotite schist and quartzite are the main wall rocks. The geochemical characteristics of schist indicate that the BIFs occurred in a passive continental margin environment. The LA–ICP–MS zircon 206Pb/238U ages of BIF and late syenite are 1945 ± 10 Ma(MSWD = 0.77) (weighted average age) and 1974 ± 27 Ma(MSWD = 1.05) (upper intercept age), respectively, indicating that the BIFs occurred in the Paleoproterozoic. In addition, the approximately 1.9 Ga magmatic and metamorphic events are consistent with the global‐scale 2.1–1.8 Ga collisional orogen events which are associated with the assembly of the Columbia supercontinent. The geochemical characteristics show that magnetite and quartz are dominant components (total content, 91.65–98.22 wt.%), and the Zr(Nb) and TiO2, Zr(Nb) and Al2O3 and Zr and Y/Ho display strongly positive correlations, illustrating the addition of crustal materials into the chemical precipitate of the original BIFs. The higher Zr, Nb and Al2O3 contents and a lower Y/Ho ratio of the Kuluketage BIFs indicate a higher terrigenous detrital component contaminant compared to BIFs of North China Craton (NCC). The rare earth and yttrium (REY) distribution patterns show a slight LREE enrichment and weak Eu positive anomaly features, indicating that the source of Fe and Si of the Kuluketage BIFs is mainly from the contribution of low‐temperature hydrothermal alteration of the oceanic crust. In addition, along with the decreasing BIF depositional age, the declining of Eu anomaly values reflects the increasing importance of low‐temperature hydrothermal solutions relative to high‐temperature hydrothermal solutions. Moreover, no Ce anomalies in studied BIFs, NCC and Xinyu BIFs are attributed to relative reducing environmental condition when the original BIFs precipitated.

Early Paleozoic subduction processes of the Paleo-Asian Ocean: Insights from geochronology and geochemistry of Paleozoic plutons in the Alxa Terrane

The Alxa Terrane is situated in a key area between the North China and Tarim cratons. Paleozoic magmatic records in this terrane place important constraints on the subduction processes of the southern Paleo-Asian Ocean. New data of zircon U-Pb ages and whole-rock elemental and isotopic data reveal two groups of intermediate to felsic plutons in the Alxa Terrane. One group consists of diorites and granitoids that were emplaced at ca. 460–440Ma and characterized by lower Al2O3/TiO2 ratios and higher TiO2 contents, implying high temperature–low pressure crystallization conditions and a shallow source region. The second group is dominated by granitoids aged at ca. 420–407Ma and displays high Sr and Ba, low Y and high rare earth elements, with very high Sr/Y ratios and mostly positive Eu anomalies. These characteristics imply low temperature–high pressure crystallization conditions and source regions at deep crustal levels where garnet is stable in the residual phase. Both of the two groups are mostly calc-alkaline to high-K calc-alkaline, depleted in Nb, Ta and Ti and enriched in Ba, K and Sr, indicative of an arc affinity most likely related to the southward subduction of the Paleo-Asian Ocean. Zircon εHf(t) and whole-rock εNd(t) values of these magmatic rocks decrease from 458Ma to 440Ma and increase from 417Ma to 407Ma, whereas whole-rock initial 87Sr/86Sr ratios display an opposite trend. Such an isotopic change suggests a tectonic switch from an advancing to a retreating subduction regime at ~407Ma. Synthesized data from this and previous studies suggest that the 460–400Ma magmatic arc in the Alxa Terrane represented the western extension of the Paleozoic arc belt on the northern margin of the North China Craton.

An analysis of crustal magnetic anomaly and Curie surface in west Himalayan syntaxis and adjacent area

In this paper, the magnetic model NGDC-720-V3 is used to investigate the distribution of crustal magnetic anomaly and the depth of Curie surface in west Himalayan syntaxis and its adjacent area. The Curie surface is compared with regional heat flow, the Moho, and seismicity. The results show that the magnetic anomaly and Curie surface are both consistent with the regional tectonic setting. Tarim craton, Tadjik Basin, and Indian Plate have positive magnetic anomaly and a Curie surface deeper than 36 km, corresponding to low heat flow there. In contrast, orogenic belts such as west Himalayan syntaxis, Tianshan Mountain, Afghanistan, and Tibetan Plateau have negative anomaly and a Curie surface shallower than 32 km, corresponding to high heat flow. The north of the syntaxis presents a positive anomaly on the surface, resulting from Ferghana Basin, while the negative anomaly in large depth is probably associated with the subduction of the slab and the resulting heat. The depth of the Curie surface has an inverse relationship to the Moho depth, which is attributed to temperature and isostatic balance. The distribution of earthquakes with epicenter deeper than 100 km clearly suggests the subduction of the Indian plate deep down to the asthenosphere and the deep earthquakes were caused by shear motion within the subducting slab.

Tectonic evolution and paleokarstification of carbonate rocks in the Paleozoic Tarim Basin

Thick carbonate rocks were developed in the depression of the Tarim craton during the Cambrian–Middle Ordovician periods. The compressional tectonic movement during the Middle Caledonian–Hercynian created the paleouplifts, which became the base for the paleokarst in the Ordovician carbonate rocks. Based on the large quantity of seismic, drilling, and geological outcrop data, this study analyzed the paleokarst development in relation to the multi-stage tectonic movements in the Paleozoic Era and different stages of karstification and hypothesized paleogeomorphology and paleokarst water system of those stages. Fractures from the tectonic movements in the carbonate and non-carbonate rocks were essential for water cycle, and therefore, the karst development in deep carbonate rocks. Paleokarsts in the Tarim Basin can be classified into four major types based on the paleogeomorphology, degree of karstification, and the layering, i.e., Tahe type, gentle hill type, high steep hill type, and covered-semi-open type. Relatively, the Tahe type was mostly on hill slopes and had the strongest karstification, the gentle hill type often located in the plain areas or basin bottoms and had least karstification, the high steep hill type was controlled by faults and had medium karstification, the semi-open type was controlled by precipitation and hydraulic gradient, and fracture passages and karst caves were mostly developed along major fractures. Overall, the paleokarsts of the Ordovician carbonate rocks in the Tarim Basin can be characterized by long geologic history, multiple development stages, deep burial depth, and various karst types.

Detrital zircons from Neoproterozoic sedimentary rocks in the Yili Block: Constraints on the affinity of microcontinents in the southern Central Asian Orogenic Belt

The Yili Block is one of the Precambrian microcontinents dispersed in the Central Asian Orogenic Belt (CAOB). Detrital zircon U–Pb ages and Hf isotopic data of Neoproterozoic meta-sedimentary rocks (the Wenquan Group) are presented to constrain the tectonic affinity and early history of the Yili Block. The dating of detrital zircons indicates that both the lower and upper Wenquan Groups have two major populations with ages at 950–880Ma and 1600–1370Ma. Moreover, the upper Wenquan Group has two minor populations at ~1100Ma and 1850–1720Ma. According to the youngest age peaks of meta-sedimentary rocks and the ages of related granitoids, the lower Wenquan Group is considered to have been deposited during the early Neoproterozoic (900–845Ma), whereas the upper Wenquan Group was deposited at 880–857Ma. The zircon εHf (t) values suggest that the 1.85–1.72Ga source rocks for the upper Wenquan Group were dominated by juvenile crustal material, whereas those for the lower Wenquan Group involved more ancient crustal material. For the 1.60–1.37Ga source rocks, however, juvenile material was a significant input into both the upper and lower Wenquan Groups. Therefore, two synchronous crustal growth and reworking events were identified in the northern Yili Block at ca. 1.8–1.7Ga and 1.6–1.3Ga, respectively. After the last growth and reworking event, continuous crustal reworking took place in the northern Yili Block until the early Neoproterozoic. Comparing the age patterns and Hf isotopic compositions of detrital zircons from the Yili Block and the surrounding tectonic units indicates that the Yili Block has a close tectonic affinity to the Chinese Central Tianshan Block in the Precambrian. The Precambrian crustal evolution of the Yili Block is distinct from that of the Siberian, North China and Tarim Cratons. Such difference therefore suggests that the Yili Block and the Chinese Central Tianshan Block may have been united in an isolated Precambrian microcontinent within the CAOB rather than representing two different blocks rifted from old cratons on both sides of the Paleo-Asian Ocean.

Geochronological, Petrological, and Geochemical Constraints on Ni-Cu Sulfide Mineralization in the Poyi Ultramafic-Troctolitic Intrusion in the Northeast Rim of the Tarim Craton, Western China

The Poyi magmatic Ni-Cu sulfide deposit is situated in the Beishan fold belt in the northeastern rim of the Tarim craton. Many Permian magmatic Ni-Cu sulfide deposits, such as those in East Tianshan, are present in the adjacent Central Asian orogenic belt to the north. The Poyi deposit is hosted in a small dike-like ultramafic-troctolitic body that was emplaced into a much larger gabbroic intrusion. Our new zircon U-Pb isotope data reveal that these two intrusions formed ~6 Ma apart. The ultramafic-troctolitic intrusion was emplaced at 269.9 ± 1.7 Ma, whereas the gabbroic intrusion was emplaced at 276.1 ± 1.9 Ma. The results show that the Poyi deposit is the youngest among the major magmatic Ni-Cu sulfide deposits (≥0.2 Mt Ni) of Permian ages in the Beishan-Tianshan region. Sulfide mineralization in the Poyi deposit occurs as steeply dipping disseminated sulfide lenses mostly associated with wehrlites in the center of the dike. Olivine from the Poyi ultramafic rocks has Fo content up to 91 mol %, which is similar to the lower limit of mantle olivine and the most primitive within the Permian Beishan-Tianshan nickel belt. Like other magmatic sulfide deposits in this belt, olivine from the Poyi deposit is depleted in Ca (<1,000 ppm). The estimated parental magma for the Poyi most primitive ultramafic rocks contains 15 ± 2 wt % MgO. Cotectic olivine-sulfide segregation from the Poyi magma is inferred from systematic variation of Fo-Ni contents in olivine from some sulfide-barren ultramafic rock samples, and supported by the occurrence of sulfide droplets as small inclusions in olivine (Fo90 mol %) in these rocks. The involvement of multiple pulses of sulfide-charged magma with different compositions is indicated by the abrupt change of olivine Fo content with depth and the presence of olivine with similar Fo contents but dramatically different Ni abundances in the different parts of the deposit. The (87Sr/86Sr)i ratios and ɛ Nd(total) of the Poyi ultramafic rocks and troctolites range from 0.7042 to 0.7052 and from 4.9 to 6.0, respectively, which are close to the isotope compositions of depleted mantle and within the ranges of major magmatic Ni-Cu sulfide deposits of Permian ages in East Tianshan. No more than 5 wt % of bulk crustal contamination is required to explain the variations of Sr-Nd isotopes in the Poyi ultramafic-troctolitic intrusion. The abundances of incompatible trace elements in whole rocks and clinopyroxene crystals indicate very weak light REE enrichments coupled by significant negative Nb-Ta anomalies in the parental melts. Bulk sulfides in the Poyi deposit are characterized by positive correlations between any pair of platinum-group elements (PGE), indicating that PGE tenor variations in the deposit are mainly controlled by variable R-factors (magma/sulfide mass ratios). The estimated initial concentrations of PGE in the parental magma for the Poyi deposit are almost two orders of magnitude lower than the abundances of PGE in some continental picrites. Given that the parental magma for the Poyi deposit is as primitive as a primary mantle-derived magma, the depletion of PGE in the Poyi deposit is most likely due to previous sulfide segregation at depth. Based on these observations, we conclude that sulfide saturation in the Poyi PGE-depleted, Mg-rich magma was triggered by addition of crustal sulfur during magma ascent and that the Poyi deposit was a dynamic conduit used by multiple pulses of olivine- and sulfide-charged magma.

Middle to Late Ordovician arc system in the Kyrgyz Middle Tianshan: From arc-continent collision to subsequent evolution of a Palaeozoic continental margin

New geological, geochronological and isotopic data reveal a previously unknown arc system that evolved south of the Kyrgyz Middle Tianshan (MTS) microcontinent during the Middle and Late Ordovician, 467–444Ma ago. The two fragments of this magmatic arc are located within the Bozbutau Mountains and the northern Atbashi Range, and a marginal part of the arc, with mixed volcanic and sedimentary rocks, extends north to the Semizsai metamorphic unit of the southern Chatkal Range. A continental basement of the arc, indicated by predominantly felsic volcanic rocks in Bozbutau and Atbashi, is supported by whole-rock Nd- and Hf-in-zircon isotopic data. εNd(t) of +0.9 to −2.6 and εHf(t) of +1.8 to −6.0 imply melting of Neo- to Mesoproterozoic continental sources with Nd model ages of ca. 0.9 to 1.2Ga and Hf crustal model ages of ca. 1.2 to 1.7Ga. In the north, the arc was separated from the MTS microcontinent by an oceanic back-arc basin, represented by the Karaterek ophiolite belt. Our inference of a long-lived Early Palaeozoic arc in the southwestern MTS suggests an oceanic domain between the MTS microcontinent and the Tarim craton in the Middle Ordovician.The time of arc-continent collision is constrained as Late Ordovician at ca. 450Ma, based on cessation of sedimentation on the MTS microcontinent, the age of an angular unconformity within the Karaterek suture zone, and the age of syncollisional metamorphism and magmatism in the Kassan Metamorphic Complex of the southern Chatkal Range. High-grade amphibolite-facies metamorphism and associated crustal melting in the Kassan Metamorphic Complex restricts the main tectonic activity in the collisional belt to ca. 450Ma. This interpretation is based on the age of a synkinematic amphibolite-facies granite, intruded into paragneiss during peak metamorphism. A second episode of greenschist- to kyanite–staurolite-facies metamorphism is dated between 450 and 420Ma, based on the ages of granitoid rocks, subsequently affected or not affected by this metamorphism. The latest episode is recorded by greenschist-facies metamorphism in Silurian sandstones and granodiorites and by retrogression of the older, higher-grade rocks. This may have occurred at the Silurian to Devonian transition and reflects reorganization of a Middle Palaeozoic convergent margin.Late Ordovician collision was followed by initiation of a new continental arc in the southern MTS. This arc was active in the Early Silurian, latest Silurian to Middle Devonian, and Late Carboniferous, whereas during the Givetian through Mississippian (ca. 385–325Ma) this area was a passive continental margin. These arcs, previously well constrained west of the Talas-Ferghana Fault, continued eastwards into the Naryn and Atbashi areas and probably extended into the Chinese Central Tianshan. The disappearance of a major crustal block with transitional facies on the continental margin and too short a distance between the arc and accretionary complex suggest that plate convergence in the Atbashi sector of the MTS was accompanied by subduction erosion in the Devonian or Early Pennsylvanian. This led to a minimum of 50–70km of crustal loss and removal of the Ordovician arc as well as the Silurian and Devonian forearcs in the areas east of the Talas-Ferghana Fault.

Granitoid magmatism in the Qinling orogen, central China and its bearing on orogenic evolution

The Qinling orogen is a typical composite orogen for understanding multi-stages of magmatism and orogenic processes. Many studies have been carried out on the magmatic rocks in the Qinling orogen but their petrogenesis is still controversial. This paper presents a review of all granitoid rocks based on previous and new studies of geochronology and geochemistry. Four distinct periods of granitoid magmatism, Neoproterozoic (979–711 Ma), Paleozoic (507–400 Ma), Early Mesozoic (250–185 Ma) and Late Mesozoic (160–100 Ma), have been recognized from the Qinling orogen according to zircon U-Pb ages, intrusion associations and deformation, as well as regional geology. The Neoproterozoic granitic rocks consist of three stages at 979–911, 894–815 and 759–711 Ma, respectively, corresponding to strongly deformed S-type, weakly deformed I-type and A-type granitoids. They can be interpreted as magmatic occurrences in syn-collisional, post-collisional and extensional settings, respectively, in response to old continental terranes of the Neoproterozoic tectonomagmatic events in the old continents of China, such as South China and Tarim cratons. Although this continental terrane would be involved in the Phanerozoic Qinling orogeny, the Neoproterozoic magmatic rocks are not the products of the Qinling orogenic processes. The Paleozoic magmatic rocks can be classified into three stages at 507–470, 460–422 and 415–400 Ma, respectively. The first-stage magmatism is temporally associated with ultra-high pressure metamorphism in the North Qinling terrane. These magmatic rocks are interpreted as magmatic occurrences in subductional, syn-collisional and post-collisional settings, respectively. The Early Mesozoic magmatic rocks occur in two stages at 252–185 and 225–200 Ma, respectively. The first-stage granitoids are mainly represented by I-type quartz diorites and granodiorites, and the second stage by granodiorites and monzogranites with the I-to A-type characteristics and some with rapakivi textures. Their emplacement ages and geochemical parameters such as A/CNK, K2O/Na2O ratios and eNd(t) values do not show any polarity change in perpendicular to subduction/collision zone. Therefore, all these Early Mesozoic granitoids are unlikely the product of continental subduction as some researchers suggested. Instead, they are plausibly related to the subduction of the Mianlue Ocean and the subsequent collision between the South China Craton and the South Qinling terrane. The Late Mesozoic granitoids were emplaced mainly at two stages of 160–130 and 120–100 Ma, and characterized by the evolution from I- to I-A- and A-type granitoids. These characteristics are consistent with the granitoid magmatic evolution from contractional to extensional settings during the Jurassic/Cretaceous in eastern China. Accordingly, the Late Mesozoic granitoid rocks in the Qinling orogen probably have a similar petrogenetic mechanism to those of the huge magmatic belt along the western Pacific margin, i.e., intra-continent magmatism related to a far-field effect of the subduction of Paleo-Pacific plate.

Synchronous crustal growth and reworking recorded in late Paleoproterozoic granitoids in the northern Tarim craton: In situ zircon U-Pb-Hf-O isotopic and geochemical constraints and tectonic implications

Identifying the relative contribution of various crustal and mantle materials in the source of granitoids is crucial for the study of granite petrogenesis and crustal growth. Extensive and diverse late Paleoproterozoic metamorphosed granitoids are exposed in the western Kuruktag block, northern Tarim craton, marking an important tectonothermal event. Here, we report sensitive high-resolution ion microprobe (SHRIMP) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U-Pb ages, in situ zircon Hf-O isotopic data, and whole-rock geochemical data for a variety of granitoids, including monzogranite, quartz diorite/quartz monzonite, garnet-bearing granodiorite, tonalite, and trondhjemite. Geochronological data show that all these granitoids were emplaced in a relatively short period at ca. 1.93–1.94 Ga and were immediately metamorphosed at ca. 1.91–1.92 Ga. In situ zircon Hf-O isotopic data suggest that both mantle-derived juvenile materials and supracrustal rocks (i.e., the Xingditag Group) were involved in magma genesis. The sodic trondhjemite and tonalite were generated by partial melting of newly underplated mafic lower crust originating from both depleted and relatively enriched mantle, with some assimilation of sedimentary materials. In contrast, the other (mostly high-K) granitoids (monzogranite, quartz diorite/quartz monzonite, and garnet-bearing granodiorite) were produced by partial melting of metasedimentary rocks due to intrusion of, and mixing with, variable amounts of mantle-derived mafic magmas, suggesting that these granitoids may also have recorded substantial crustal growth. These petrogenetic interpretations imply that most granitoids in the study area were a result of synchronous crustal growth and reworking, which must be carefully considered in models of continental growth. The adakitic trondhjemite and tonalite have high Sr/Y, La/Yb, Gd/Yb, Nb/Ta, and Zr/Sm ratios, suggesting partial melting with a rutile eclogite residue and implying significant crustal thickening (>50 km). This tectonothermal event most likely occurred in an Andean-type continental arc and was followed by continental collision at ca. 1.85 Ga along the newly recognized late Paleoproterozoic North Tarim orogen. Such an accretion-to-collisional orogen implies an interior position of this area within the Columbia (or Nuna) supercontinent. Based on geological correlations, a Tarim–North China connection is suggested for Columbia reconstruction.

Late Paleozoic metallogenesis and evolution of the East Tianshan Orogenic Belt (NW China, Central Asia Orogenic Belt)

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

Rapakivi granites within Phanerozoic collisional orogens as a possible consequence of continental subduction and following exhumation of the Precambrian crust: Evidences from the Permian Jangart rapakivi in South Tien Shan collisional belt, Eastern Kyrgyzstan

All worldwide known rare occurrences of Phanerozoic A-type granites with typical rapakivi texture do not usually have any significant volume of contemporaneous basic rocks in magmatic association and occur only in those collisional orogens, which border ancient cratons and contain HP–UHP metamorphic terranes, suggesting connection of their petrogenesis with a deep continental subduction and subsequent exhumation of the ancient crust. Early Permian Jangart rapakivis and genetically related leucogranites and Li-mica granites widely spread within the Saryjaz syntaxis, which is distinguished by the shortest distance between the Tarim craton and Caledonian Kazakh continent, collided in the Late Carboniferous with the possibly deepest continental subduction of the Tarim under the South Tien Shan collisional belt. Rapakivi plutons occur within the remarkable huge (20×120km) mega-mélange (mega-breccia) terrane of diapiric (not tectonic) origin, which was identified by the latest geological data and deciphering of satellite imagery. According to geophysical data the most of subducted Tarim continental crust was trapped within lithospheric mantle and never exhumed. But in the Saryjaz syntaxis, where the collisional suture has been subjected to flecsure-like oroclinal bending due to sinistral strike-slip tectonics during oblique collision between the Tarim and Kazakh plates, the subduction channel was weakened and exhumation occurred. The suggested P–T–t path of exhumed slices of the subducted Tarim continental crust presumes a relatively long (around 20Ma) residence time on a mantle depth as well as additional internal radiogenic heating and rapid (around 2Ma) isothermal exhumation, which were favorable for extensive melting and subsequent rapakivi formation. Phanerozoic rapakivis originated from the same ancient lower crust granulite source rocks as their abundant Proterozoic counterparts but mechanism of heating and melting of those source rocks was radically different.

The Paleoproterozoic magmatic–metamorphic events and cover sediments of the Tiekelik Belt and their tectonic implications for the southern margin of the Tarim Craton, northwestern China

The Tiekelik Belt in the southwestern part of the Tarim Craton, NW China, consists of Archean–Paleoproterozoic orthogneisses, and granitoids, and younger sedimentary successions. U–Pb dating of Paleoproterozoic rocks in the Tiekelik Belt has provided new information on the southwestern margin of the Tarim Craton. Two orthogneisses from the Heluositan Complex yielded crystallization ages in the interval 2310–2260Ma and metamorphic ages of 2040–2007Ma and 1830Ma, respectively. Two syenogranites from the Buweituwei pluton yielded crystallization ages of 1900 and 1795Ma. Detrital zircon analyses from sedimentary rocks nonconformably overlying the Buweituwei and Akazi plutons indicate maximum depositional ages between 1950Ma and 1780Ma. Major detrital inputs are from the basement in the region and form prominent peaks at 1900–1800Ma and 2300–2200Ma. Our new data, combined with previous results, allow us to summarize the Archean–Paleoproterozoic tectonic evolution of the Tiekelik Belt as follows: (1) Emplacement of Archean-aged TTG magmas during the 3.14–2.76Ga interval; (2) A series of 2.41–2.26Ga magmatic events that represent an important period of crustal reworking within the Tarim craton; (3) 2.03–1.80Ga magmatic and metamorphic events that include the emplacement of a 1.9Ga mafic dyke suite into the Heluositan Complex along with several episodes of intraplate magmatism. The late magmatic events are coeval with metamorphism and may support an orogenic extensional model for the Tiekelik Belt during the 1.9–1.8Ga interval; (4) post-1.8Ga subsidence and formation of a Changcheng sequence in the Tiekelik Belt. The Paleoproterozoic ages of major tectonothermal events within the Tiekelik, North Altyn–Dunhuang block and Alashan block are almost identical and it is argued that they were connected at that time as North China and Tarim were part of the larger Columbia supercontinent.