Southern Central Asian Orogenic Belt Research Articles

Late Paleozoic Tectonic Evolution of the Paleo‐Asian Ocean in the Northern Alxa Block (NW China)

AbstractThe northern Alxa Block occupies a key position in the southern margin of the Central Asian Orogenic Belt (CAOB) and records late Paleozoic subduction and closure processes of the Paleo‐Asian Ocean (PAO). However, there are still controversies regarding the timing and location of the final closure of the PAO. This study presents structural deformation data and geochronological and geochemical data for Permian volcanic rocks, as well as detrital zircon provenance analysis of Permian sedimentary rocks along the Nuoergong‐Langshan Zone (NLZ) in the northern Alxa Block. During the early to middle Permian, the PAO lithospheric slab subducted beneath the northern Alxa Block, rendering a continental volcanic arc in the NLZ and also giving rise to extensive folding, thrusting, and crustal thickening. The crustal thickening in the NLZ may trigger the early Permian magmatic flare‐up that constructed much of the continental arc plutons. In the middle to late Permian, a retroarc foreland basin was formed, within which the Dahongshan Formation deposited. Significant active tectonic movements led to regional unconformity occurring during this period. A large‐scale dextral ductile shear deformation in the NLZ resulting from the lateral extrusion of the thickened crust after the terminal amalgamation of the southern CAOB was constrained between 272 and 249 Ma, suggesting a late Permian tectonic transition from compression to transpression. Combining these findings with published data, we suggest that the final consumption of the PAO occurred in the late Permian, probably along the Enger Us suture zone in the northern margin Alxa Block.

Provenance and tectonic setting of late Paleozoic sedimentary rocks from the Alxa Tectonic Belt (NW China): Implications for accretionary tectonics of the southern Central Asian Orogenic Belt

AbstractThe Central Asian Orogenic Belt has long been considered the largest Phanerozoic accretionary orogen in the world; it developed through the subduction and final closure of the Paleo–Asian Ocean. However, the architecture and duration of the accretionary orogenesis of the Central Asian Orogenic Belt are still controversial despite decades of investigation. In this study, we present field, compositional, and stratigraphically controlled detrital zircon geochronological data for late Paleozoic sedimentary rocks from the Alxa Tectonic Belt to constrain their provenance, tectonic setting, and the overall tectonic configuration of the southern Central Asian Orogenic Belt. A Devonian sample yields a unimodal age peak (ca. 424 Ma) and broad late Mesoproterozoic ages. A Carboniferous sample has Early Silurian (ca. 438 Ma) and Late Devonian (ca. 382 Ma) peaks along with Neoproterozoic to Archean ages. The Permian samples are dominated by Ordovician–Devonian and Carboniferous–Permian ages. They yield maximum depositional ages ranging from ca. 291 Ma to 248 Ma and contain abundant zircon ages that are close to their depositional ages. These data reveal Ordovician–Silurian and Carboniferous–Permian magmatic flare-ups separated by a Devonian magmatic lull in the southern Central Asian Orogenic Belt. The arc terranes in southern Mongolia, central Beishan, and northern Alxa provided major detritus for the late Paleozoic sediments. An abrupt shift of zircon εHf(t) values at ca. 400 Ma reveals significant late Paleozoic crustal growth and excludes southern Alxa as a source. Oceanic basins prevented detritus from southern Alxa from reaching northern Alxa during Permian–Early Triassic time. A geological and provenance comparison of Permian basins in the southern Central Asian Orogenic Belt reveals the existence of two separate forearcs ascribed to bipolar subduction of the Paleo–Asian Ocean. Combined with recent paleomagnetic data, this leads us to advocate for an archipelago-style accretionary process induced by subduction retreat for the late Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt, which continued into Late Permian–Early Triassic.

Early–Middle Permian post-collisional granitoids in the northern Beishan orogen, northwestern China: evidence from U–Pb ages and Sr–Nd–Hf isotopes

The geochemistry and Sr–Nd isotope, zircon U–Pb, and zircon Hf isotope compositions are reported for monzogranites and granodiorites from the Hazhu area in the northern Beishan orogen, northwestern China. Zircon U–Pb dating yielded ages of 270.1 ± 1.1 and 277.4 ± 1.2 Ma for the monzogranites and 263.6 ± 1.2 and 262.2 ± 1.1 Ma for the granodiorites. These monzogranites and granodiorites are metaluminous to weakly peraluminous I-type and belong to mid-K calc-alkaline and high-K calc-alkaline series. They exhibit high Mg# values and moderate degrees of differentiation (D.I. = 70.7–88.1). They are enriched in large-ion lithophile elements and light rare earth elements and depleted in high field strength elements. They show high (87Sr/86Sr)i ratios of 0.6995–0.7070 and high εNd(t) values of 4.37–5.70 with Nd model ages (TDM) of 522–789 Ma, suggesting a juvenile crustal origin. Furthermore, their εHf(t) values are all positive, and Hf isotopic crustal model ages ([Formula: see text] = 394–1097 Ma) also indicate a juvenile crustal origin. According to the data obtained in this study and other regional geological data acquired recently, the Hazhu granitoids were derived from common sources of melting from the Neoproterozoic to late Paleozoic juvenile crusts. The younger intrusions (granodiorites) are more basic, likely as a result of more juvenile lower crust being melted along with asthenospheric upwelling, which led to the addition of more basic components. These granitoids formed in a post-collisional setting. The tectonic regime transformed from an arc-related compressional setting to post-collisional extension, likely as a result of lithospheric extension and thinning in response to oceanic lithospheric delamination. These granitoids in the northern Beishan orogen were probably emplaced in a post-collisional extensional setting and suggest vertical continental crustal growth in the southern Central Asian Orogenic Belt.

Early Permian to Late Triassic tectonics of the southern Central Asian Orogenic Belt: geochronological and geochemical constraints from gabbros and granites in the northern Alxa area, NW China

AbstractSituated between the North China Craton to the east and the Tarim Craton to the west, the northern Alxa area in westernmost Inner Mongolia in China occupies a key location for interpreting the late-stage tectonic evolution of the southern Central Asian Orogenic Belt. New LA-ICP-MS zircon U–Pb dating results reveal 282.2 ± 3.9 Ma gabbros and 216.3 ± 3.2 Ma granites from the Yagan metamorphic core complex in northern Alxa, NW China. The gabbros are characterized by low contents of Si, Na, K, Ti and P and high contents of Mg, Ca, Al and Fe. These gabbros have arc geochemical signatures with relative enrichments in large ion lithophile elements and depletions in high field strength elements, as well as negative εNd(t) (−0.91 to −0.54) and positive εHf(t) (2.59 to 6.37) values. These features indicate that a depleted mantle magma source metasomatized by subduction fluids/melts and contaminated by crustal materials was involved in the processes of magma migration and emplacement. The granites show high-K calc-alkaline and metaluminous to weakly peraluminous affinities, similar to A-type granites. They have positive εNd(t) (1.55 to 1.99) and εHf(t) (5.03 to 7.64) values. These features suggest that the granites were derived from the mixing of mantle and crustal sources and formed in a postcollisional tectonic setting. Considering previous studies, we infer that the final closure of the Palaeo-Asian Ocean in the central part of the southern Central Asian Orogenic Belt occurred in late Permian to Early–Middle Triassic times.

Detrital zircon constraints on late Paleozoic tectonism of the Bogda region (NW China) in the southern Central Asian Orogenic Belt

The Chinese North Tianshan (CNTS) in the southern part of the Central Asian Orogenic Belt (CAOB) has undergone multistage accretion-collision processes during Paleozoic time, which remain controversial. This study addresses this issue by tracing the provenance of Late Paleozoic sedimentary successions from the Bogda Mountain in the eastern CNTS through U–Pb dating and Lu–Hf isotopic analyses of detrital zircons. New detrital zircon U–Pb ages (N ​= ​519) from seven samples range from 261 ​± ​4 ​Ma to 2827 ​± ​32 ​Ma. The most prominent age peak is at 313 ​Ma and subordinate ages vary from 441 ​Ma to 601 ​Ma, with some Precambrian detrital zircon ages (~7%) lasting from 694 ​Ma to 1024 ​Ma. The youngest age components in each sample yielded weighted mean ages ranging from 272 ​± ​9 ​Ma to 288 ​± ​5 ​Ma, representing the maximum depositional ages. These and literature data indicate that some previously-assumed “Carboniferous” strata in the Bogda area were deposited in the Early Permian, including the Qijiaojing, Julideneng, Shaleisaierke, Yangbulake, Shamaershayi, Liushugou, Qijiagou, and Aoertu formations. The low maturity of the sandstones, zircon morphology and provenance analyses indicate a proximal sedimentation probably sourced from the East Junggar Arc and the Harlik-Dananhu Arc in the CNTS. The minor Precambrian detrital zircons are interpreted as recycled materials from the older strata in the Harlik-Dananhu Arc. Zircon ɛHf(t) values have increased since ~408 ​Ma, probably reflecting a tectonic transition from regional compression to extension. This event might correspond to the opening of the Bogda intra-arc/back arc rift basin, possibly resulting from a slab rollback during the northward subduction of the North Tianshan Ocean. A decrease of zircon ɛHf(t) values at ~300 ​Ma was likely caused by the cessation of oceanic subduction and subsequent collision, which implies that the North Tianshan Ocean closed at the end of the Late Carboniferous.

Geochronology and geochemistry of granodiorite at Jinwozi Au deposit: Tectonomagmatic evolution for Palaeozoic Beishan Orogen (Central Asian Orogenic Belt) in NW China

The Beishan Orogen (southern Central Asian Orogenic Belt) contains many Palaeozoic gold ore‐hosting intrusions, whose age, petrogenesis and tectonic setting are yet to be well understood. In the Mazongshan arc of the Beishan Orogen, the Jinwozi granodiorite contains high K2O (2.71–5.69 wt%), A/CNK (1.10–1.49) and 10000Ga/Al (mostly >2.6), and has high Nb (15–21 ppm), Zr (337–451 ppm) and light rare earth element contents (∑LREE = 207–257 ppm), but low Sr (92–208 ppm) and negative Eu anomalies (δEu = 0.55–0.70), with calculated Zr saturation temperature of 910–860°C. They exhibit typical features of peraluminous A‐type granites formed in subduction‐related settings. The rocks have high large‐ion lithophile element and Hf contents, but low contents in certain high‐field‐strength elements. In addition, the zircon εHf(t) values (−3.6 to −0.1) indicate partial melting of the crust, while the two‐stage model ages (TDM2 = 1385 to 1611 Ma) and several Palaeozoic magmatic rocks with Mesoproterozoic‐inherited zircon suggest the occurrence of Mesoproterozoic crustal accretion. The ore‐hosting granodiorite pluton at the Jinwozi Au deposit was LA‐ICP‐MS zircon U–Pb dated to be Silurian (ca. 428–422 Ma), older than the previously suggested Late Palaeozoic ages and before the interpreted amalgamation timing of the Beishan Orogen. We propose that the Jinwozi granodiorite is derived from pre‐existing crustal material recycling.

Tectonic evolution of the Chinese Tianshan Orogen from subduction to arc-continent collision: Insight from polyphase deformation along the Gangou section, Central Asia

AbstractThe Central Asian Orogenic Belt, as the largest accretionary orogen on Earth, is an ideal candidate to study the geodynamics of convergent plate boundaries through a prolonged period. The evolution of this orogen has been explained by different tectonic models, which incorporated one, or a combination, of the following mechanisms: lateral stacking of arc systems along major shear zones, arc amalgamation, oroclinal bending, and trench migration. Here we elucidate major mechanisms responsible for the tectonic evolution of the Central Asian Orogenic Belt, focusing on the Chinese Tianshan Orogen in the southern Central Asian Orogenic Belt. Structural observations from the ∼50-km-long Gangou section show evidence of polyphase deformation. The earliest episode of orogen-parallel sinistral shearing, constrained to the Early Devonian (ca. 399 Ma) by syn-deformational intrusions, was possibly controlled by oblique subduction. This was followed by an episode of ∼NE–SW contractional deformation, dated at ca. 356 Ma (40Ar/39Ar age of syn-deformational hornblende), and likely associated with an episode of trench advance. The third stages of deformation during the latest Carboniferous and Permian involved ∼NE-SW contraction, orogen-parallel extension, and dextral transpression. Our new geochronological data constrain the timing of orogen-parallel extension to ca. 303–293 Ma, and confirm that dextral activation along shear zones occurred during the Permian. The results highlight the role of trench migration, oblique tectonics, and syn-collisional orogen-parallel extension in the build-up of the Central Asian Orogenic Belt, and contribute to the pre-collisional reconstruction of this orogenic system.

Late Carboniferous crustal evolution of the Chinese Central Tianshan microcontinent: Insights from zircon U–Pb and Hf isotopes of granites

The Central Tianshan microcontinent (CTM) in the southern Central Asian Orogenic Belt is characterized by Precambrian crustal basements and widespread Palaeozoic granitic rocks. However, the Palaeozoic tectonic process and associated crustal evolution of the CTM have not been well constrained. In this paper, we report whole‐rock geochemistry, zircon U–Pb ages, Hf isotopes, and trace elements for the Late Carboniferous granodiorite and monzogranite from the Hongliujingbei area of the eastern CTM. These data were further integrated with previously published whole‐rock geochemistry and zircon Hf isotopic data of the intensive Late Carboniferous (327–300 Ma) magmatic rocks in the CTM in order to gain more insights into the crustal and tectonic evolution of the CTM during the Late Carboniferous. LA‐ICP‐MS zircon U–Pb dating results indicate that the granodiorite and monzogranite from the Hongliujingbei area were emplaced at ~320 Ma. Their highly variable zircon εHf(t) values (−0.1 to +12.9) suggest that the magmas of these rocks were formed by the mixing of mantle‐ and crustal‐derived magmas. The Late Carboniferous magmatic rocks from the CTM, as a whole, show a wide range of zircon εHf(t) values (−3.7 to +13.4) and geochemical characteristics typical for magma evolution in an active continental margin setting. Therefore, we suggest that the Late Carboniferous magmatic rocks in the CTM were probably generated in a continental arc formed by the southward subduction of the North Tianshan Ocean Plate beneath the CTM, and their mostly depleted zircon Hf isotopic compositions indicate that juvenile crustal growth was a dominant process in the CTM during the Late Carboniferous.

Constraints on the tectonic evolution of the southern central Asian orogenic belt from early Permian–middle Triassic granitoids from the central Dunhuang orogenic belt, NW China

The Dunhuang orogenic belt (DOB), located in the southern Central Asian Orogenic Belt (CAOB), exposes Precambrian and Paleozoic intrusive-metamorphic rocks. To better understand its evolution, we carried out detailed petrological and geochronological investigations on the major lithologies of the central DOB. The central DOB can be divided into two terranes, the north and the south, with contrasting geological history. The north is characterized by Precambrian crystalline rocks (ca. 3.1–1.6 Ga) overprinted by Devonian tectono-thermal events. The Precambrian rocks show evidence for major juvenile crust growth with additions of older crustal materials at 2.7–2.5 Ga, a major crustal reworking event at 2.0–1.9 Ga, and juvenile additions with minor crustal reworking at 1.8–1.7 Ga. In contrast, the south is characterized by early Devonian (ca. 406 Ma) TTG-gneiss and amphibolite gneiss, and some Late Paleozoic–early Mesozoic granitoid rocks (ca. 284–238 Ma). The Late Paleozoic–early Mesozoic granitoid rocks from the south were emplaced in two phases: early Permian (ca. 284–275 Ma) and late Permian–middle Triassic (ca. 255–238 Ma). They belong to the high-K calc-alkaline I-type series which were generated from mixing juvenile materials with Mesoproterozoic continental crust. The early Permian and the late Permian–middle Triassic granitoid rocks are considered as arc/subduction-related and post-collisional rocks, respectively. These suggest that the final assembly of the DOB with the Beishan or final assembly within the DOB happened in the middle Permian (ca. 275–255 Ma). The DOB formed by accretion/collision of multiple terranes/microcontinents, similar to other areas in the CAOB.

Early Paleozoic tectonic evolution and magmatism in the Eastern Tianshan, NW China: Evidence from geochronology and geochemistry of volcanic rocks

The Eastern Tianshan provides significant insights in relation to unraveling the history of Early Paleozoic magmatism, ocean-continent transition, and tectonic evolution of the southern Central Asian Orogenic Belt. This investigation represents a detailed geochronological and geochemical study of the Early Paleozoic volcanic rocks from the Harlik and Dananhu terranes in the Eastern Tianshan. UPb zircon ages indicate periods of volcanism in Harlik Early Silurian (441–434 Ma) and Dananhu Late Ordovician (448–445 Ma). The Harlik Early Silurian volcanic rocks unconformably overlie Ordovician sediments, and consist of alkaline (434 Ma) and tholeiitic (440 Ma) basalts. The alkaline basalts originated from low degree partial melting of previously subduction-modified lithosphere and subsequent interaction with upwelling asthenosphere. The tholeiitic basaltic magmas were derived from partial melting of a spinel facies lithospheric mantle source metasomatized by early slab-derived fluids, which then underwent crystal fractionation of clinopyroxene and olivine. In contrast, the Dananhu Late Ordovician high-Mg andesites were generated by partial melting of a hydrous, depleted mantle wedge and these melts experienced a low degree of crystal fractionation. Combined with results from previous geological studies, it is proposed that the Harlik and Dananhu terranes had different volcanism and tectonic histories. The Harlik tholeiitic basalts were generated in a post-collision setting, which may be associated with the closure of the Armantai Ocean. Contrarily, the alkaline basalts were formed in an intraplate setting during the initial stages of intracontinental extension that gave rise to the Karamaili Ocean. The Dananhu Late Ordovician andesites represent the initial phase of arc-related volcanism due to the subduction of the North Tianshan Ocean, where the Dananhu arc may have temporally evolved from an Andean- to oceanic island-type arc in Early Paleozoic.

Geology of the Gobi and Mongol Altai junction enhanced by gravity analysis: a key for understanding of the Mongolian Altaides

ABSTRACT The geological map L-47-V at a scale 1:500,000 covers part of Mongolian Altaids with ophiolite fragments in southern Central Asian Orogenic Belt in SW Mongolia. This region has a basin and range topography with Neoproterozoic and Palaeozoic units exposed at NW–SE trending ranges rising along major intracontinental faults and with intermontane basins filled by Mesozoic and Cenozoic sediments in between. The map shows clear N–S tectonic zonation featuring the northerly Precambrian Baidrag microcontinent, the lower Palaeozoic Lake Zone in the centre and the southerly Palaeozoic Gobi-Altai and Trans-Altai zones. Gravity highs are located in the SW part of the map and low to intermediate Bouguer anomalies in the NE part. NW–SE trends of gravity anomalies correlate well with the contact between the Trans-Altai and the Gobi-Altai zones but the important first-order geological boundary between the Lake and Gobi-Altai zones cannot be delineated by the gravity gradients.

Plastic deformation and rheology of the sinistral strike‐slip ductile shear zone in the Bayan Nuru area, Alxa Zuoqi, NW China: Product of subduction of Paleo‐Pacific Plate

The Bayan Nuru ductile shear zone is located in the Yabulai–Bayan Nuru tectonic belt of the middle part of the southern Central Asian Orogenic Belt (CAOB) and east of the Langshan tectonic belt. This article is the first report on the Bayan Nuru ductile shear zone, and provides dynamic, kinematic, and geochronological data of the shear zone. Accurately determining the shear orientation and estimating the shearing age of the Bayan Nuruductile shear zone, has important reference value and significance for understanding the tectonic evolution in Alxa during “Yanshan movement” period. All shearing signatures, including asymmetrical folds, rotated porphyroclasts, “domino” structures, “mica fishes,” S–C fabrics, and asymmetric stress shadows, developed in the shear zone, indicate sinistral shear. The temperature environment at the time of formation of the shear zone was determined by the mineral deformation thermometer and EBSD quartz C‐axis fabric analysis. The temperature was 350–550°C. The differential stress (σ) was estimated to be 44.31–97.94 MPa and the strain rate(έ) to be 10−14 s−1(350°C) and 10−10s−1(550 °C). The average kinematic vorticity of the Bayan Nuru NNE‐trending ductile shear zone is 0.93, indicating that the shear zone was dominated by simple shear. The thickness of the ductile shear zone was reduced by 7%–22%. Combined with the age of granitic mylonite (289.53 ± 0.74 Ma) and the NNE‐trending spread feature, the Bayan Nuru ductile shear zone in the western part of the Langshan fault zone may have been formed in the context of continuous compressional intraplate deformation, caused by the subduction of the Paleo‐Pacific Plate at 170–150 Ma.

Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen, southern Central Asian Orogenic Belt: Constraints from P-T path and zircon U-Pb dating

Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen, which connects the Tianshan Orogen to the west and the Mongolia-Xing’anling Orogen to the east. The study of metamorphism in Beishan area is of great significance to explain the tectonic evolution of Beishan orogen. According to the microstructures, mineral relationships, and geothermobarometry, we identified four stages of mineral assemblages from the garnet amphibolite sample: (1) a pre-peak stage, which is recorded by the cores of garnet together with core-inclusions of plagioclase (Pl1); (2) a peak stage, which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase (Pl2) ​+ ​amphibole (Amp1) ​+ ​Ilmenite (Ilm1) ​+ ​biotite (Bt1), developed at temperature-pressure (P-T) conditions of 818.9–836.5 ​°C and 7.3–9.2 ​kbar; (3) a retrograde stage, which is recorded by garnet rims ​+ ​plagioclase (Pl3) ​+ ​amphibole (Amp2) ​+ ​orthopyroxene (Opx1) ​+ ​biotite (Bt2) ​+ ​Ilmenite (Ilm2), developed at P-T conditions of 796.1–836.9 ​°C and 5.6–7.5 ​kbar; (4) a symplectitic stage, which is recorded by plagioclase (Pl4) ​+ ​orthopyroxene (Opx2) ​+ ​amphibole (Amp3) ​+ ​biotite (Bt3) symplectites, developed at P-T conditions of 732 ​± ​59.6 ​°C and 6.1 ​± ​0.6 ​kbar. Moreover, the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9 ​± ​4.7 ​Ma for the protolith and 281.4 ​± ​8.5 ​Ma for the peak metamorphic age. Therefore, the mineral assemblage, P-T conditions, and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t (Pressure-Temperature-time) path, indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian, then the southern Beishan area underwent a process of thinning of the continental crust.

Carboniferous Highly Fractionated I‐type Granites from the Kalamaili Fault Zone, Eastern Xinjiang, NW China: Petrogenesis and Tectonic Implications

Carboniferous magmatism is one of the most important tectonothermal events in the Central Asian Orogenic Belt (CAOB). However, the final closure time of the Kalamaili Ocean between East Junggar and Harlik Mountain is still debated. Early Carboniferous (332 Ma) and late Carboniferous (307–298 Ma) granitic magmatism from Kalamaili fault zone have been recognized by LA‐ICP‐MS zircon U‐Pb dating. They are both metaluminous highly fractionated I‐type and belong to the high‐K calc‐alkaline. The granitoids for early Carboniferous have zircon εHf(t) values of –5.1 to +8.5 with Hf model ages (TDM2) of 1.78–0.83Ga, suggesting a mixed magma source of juvenile material with old continental crust. Furthermore, those for late Carboniferous have much younger heterogeneous zircon εHf(t) values (+5.1 to +13.6) with Hf model ages (TDM2=1.03–0.45 Ga) that are also indicative of juvenile components with a small involvement of old continental crust. Based on whole‐rock geochemical and zircon isotopic features, these high‐K granitoids were derived from melting of heterogeneous crustal sources or through mixing of old continental crust with juvenile components and minor AFC (assimilation and fractional crystallization). The juvenile components probably originated from underplated basaltic magmas in response to asthenospheric upwelling. These Carboniferous highly fractionated granites in the Kalamaili fault zone were probably emplaced in a post‐collisional extensional setting and suggested vertical continental crustal growth in the southern CAOB, which is the same or like most granitoids in CAOB. This study provides new evidence for determining the post‐accretionary evolution of the southern CAOB. In combination with data from other granitoids in these two terranes, the Early Carboniferous Heiguniangshan pluton represents the initial record of post‐collisional environment, suggesting that the final collision between the East Junggar and Harlik Mountain might have occurred before 332 Ma.

Association of Permian gabbro and granite in the Langshan, southern Central Asian Orogenic Belt: Age, origin, and tectonic implications

The Permian magmatism in the Alxa region provides critical geological evidence to constrain the geodynamic processes in the southern Central Asian Orogenic Belt. The Shouji batholith is located in the southern Langshan, eastern Alxa region, and contains monzogranites, K-feldspar granites, and gabbros. New zircon UPb age data show that the Shouji batholith formed at 282–268 Ma. The Shouji granites have high (Na2O + K2O), Nb, and Zr contents, and high 10000∗Ga/Al, FeOT/(FeOT + MgO), Yb/Nb, and Yb/Ta ratios, similar to the values of A2-type granites. The Shouij granites exhibit enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE); depletion in Nb, Ti, and Sr; and enriched isotopic compositions. These Shouji granites were generated by partial melting of dehydrated tonalite–trondhjemite–granodiorite rocks of the Diebusige Complex in the lower crust. The Shouji gabbros are sodium-rich and low-K calc-alkaline rocks. They are enriched in LREE and LILE, depleted in high field strength elements, and have enriched isotopic compositions. These Shouji gabbros were generated by partial melting of a lithospheric mantle source metasomatized by melts derived from an ancient continental crust in an active continental margin. Roll-back of the Enger Us subducting slab accounts for generations of the Shouji A2-type granites and gabbros. The zircon UPb ages of Carboniferous–Permian magmatism show a younging trend toward the northwest in the southern Alxa region. Evidence from late Paleozoic magmatism and arc-derived sediments indicates that the southern Alxa switched from an active continental margin to an intra-oceanic arc during the early Carboniferous–Late Permian as a result of retreat of the subducting Enger Us slab.

Early Carboniferous High Ba‐Sr Granitoid in Southern Langshan of Northeastern Alxa: Implications for Accretionary Tectonics along the Southern Central Asian Orogenic Belt

Voluminous granitoids are widely distributed in the Langshan region, northeast of the Alxa block, and record the evolutionary processes of the southern Central Asian Orogenic Belt. The Dabashan pluton was emplaced into the Paleoproterozoic Diebusige complex. Early Carboniferous zircon LA‐ICP MS U‐Pb ages were from 327 Ma to 346 Ma. The Dabashan pluton can be classified as monzogranite and syenogranite, and exhibits high K2O contents and K2O/Na2O ratios, which reveal a high‐K calc‐alkaline nature. The samples display strongly fractionated REE patterns, and are enriched in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). The Dabashan plutons display unusually high Ba (823–2817 ppm) and Sr (166–520 ppm) contents and K/Rb ratios (315–627), but low Rb/Ba ratios (0.02–0.14), and exhibit fertile zircon Hf isotopic compositions [εHf(t)=–14 to –20], which are comparable to those of typical high Ba–Sr granitoids. Based on the geochemical compositions of the samples, we suggest that subducted sediments and ancient crustal materials both played important roles in their generation. Basaltic melts were derived from partial melting of subcontinental lithophile mantle metasomatized by subducted sediment‐related melts with residual garnet in the source, which caused partial melting of ancient lower crust. Magmas derived from underplating ascended and emplaced in the middle–upper crust at different depths. The resultant magmas experienced some degree of fractional crystallization during their ascent. Given these geochemical characteristics, together with regional tectonic, magmatic, and structure analysis data, an active continental margin environment is proposed for the generation of these rocks.

Crustal maturation through chemical weathering and crustal recycling revealed by Hf–O–B isotopes

Juvenile continental crust is dominantly formed at intra-oceanic arcs via subduction zone magmatism. However, it remains unclear how basaltic oceanic arcs convert to granodioritic continental compositions. Here we present zircon U–Pb and Hf–O isotope data, whole-rock B isotope compositions, combined with a synthesis of over 1100 geochemical analyses from magmatic rocks that span a wide range of emplacement ages (∼490–270 Ma), from the Junggar intra-oceanic arc of the southern Central Asian Orogenic Belt (CAOB). Geochemical data show that the Junggar evolved from a juvenile oceanic arc composition (low SiO2, K2O and Rb, and high MgO contents) to one closer to continental crust (high SiO2, K2O and Rb, and low MgO contents) at ca. 300 Ma. All samples show very high and uniform Hf isotope ratios with εHf(t) values from +10.6 to +14.4. Zircon δ18O values and whole-rock δ11B values, however, are highly variable. The Silurian – Carboniferous (pre-300 Ma) rocks have distinctly lower zircon δ18O values ranging from 5.16 to 6.72‰ with an average of 5.78‰, and high δ11B values (−7.5–+12.2‰), suggesting that they were derived from the asthenospheric mantle wedge, which supports the genesis of primitive intra-oceanic crust during that time interval. In contrast, the Early Permian (post-300 Ma) rocks display much higher zircon δ18O values (8.24 to 10.29‰) and lower δ11B values (−9.0 to −12.2‰), combined with the presence of Carboniferous inherited zircons, which requires a source component comprising young, weathered, volcanogenic sediments from the Carboniferous Junggar intra-oceanic arc. The Early Permian rocks were produced by low degrees of partial melting of all sources (ca. 10%) and element and Hf–O–B isotope mixing calculations indicate a contribution of more than ∼50% from weathered sediments in those sources. Thus, the evolution of the Junggar segment from a primitive basaltic intra-oceanic arc toward a young continent was related to the recycling of crustal residues of chemical weathering and the younger magmatism was generated by crustal melting of these weathering products after they were buried to lower crustal depths. Our study highlights that an intra-oceanic arc's own chemical weathering history promotes its transformation into continental crust during collisional events and clarifies the relationship between continental crust formation and intra-oceanic arcs through time.

Early Palaeozoic arc-related gabbro-diorite suite in East Junggar, southern Central Asian Orogenic Belt: petrogenesis and tectonic implications

ABSTRACT The Armantai and Kalamaili oceans were both important branches of the Palaeo-Asian Ocean between the Altai microcontinent and the Junggar juvenile crust; however, their geological evolution is not completely clear. This paper reports detailed petrological, zircon geochronological, whole-rock elemental and Sr-Nd-Pb isotopic data for a newly discovered gabbro-diorite suite (Akputo pluton) in East Junggar to constrain this issue. Quartz diorite and hornblende gabbro yield intrusion 206Pb/238U ages of ca. 444–440 Ma, which indicates that mafic-intermediate magmatism occurred in the earliest Silurian. The gabbroic-dioritic rocks have tholeiite to calc-alkaline affinities and are characterised by moderately fractionated rare earth element patterns with flat heavy rare earth elements, enrichment in Rb, Ba, K, Sr, and depletion in Nb, Ta, Ti. Low initial Sr and Pb isotopic compositions and positive εNd(t) indicate a derivation from the isotopically depleted mantle sources. The geochemical compositions and melting calculations suggest that the gabbro-diorite suite originated from partial melting of spinel-bearing mantle sources that had been metasomatised by subduction-derived fluids and sediments. The Akputo intrusions formed in an arc setting related to the southward subduction of the Armantai Ocean. Taking the regional angular unconformity between the Ordovician sediments and Silurian volcanics into account, we further suggest that the Early to Middle Silurian was a transition period of the closure of Armantai Ocean and the initial spreading of Kalamaili Ocean.

Geochronology, petrology and Sr-Nd-Hf-S isotope geochemistry of the newly-discovered Qixin magmatic Ni-Cu sulfide prospect, southern Central Asian Orogenic Belt, NW China

The Qixin mafic-ultramafic complex is present in the southern margin of the Central Asian Orogenic Belt in northern Xinjiang where many magmatic Ni-Cu sulfide deposits occur. To determine the temporal association of the newly-discovered Ni-Cu prospect associated with the Qixin complex with the known major magmatic Ni-Cu sulfide deposits in the region, as well as the origin of sulfide mineralization in the Qixin complex, we have carried out geochronological, petrological and geochemical studies of this complex. Gabbro, olivine gabbro, norite, olivine gabbronorite, troctolite, websterite and lherzolite are the major rock types of this complex. Two drill cores completed to date show two types of sulfide mineralization associated with the complex: (1) disseminated sulfides that occur near the base of bodies of lherzolite and troctolite; (2) blotchy-massive sulfides in the hornfels country rocks. The disseminated sulfides are composed of pyrrhotite, pentlandite and chalcopyrite, whereas the blotchy-massive sulfides in the country rocks are composed of pentlandite, violarite, pyrrhotite and chalcopyrite. Zircon crystals from the Qixin olivine gabbronorite yield a U-Pb age of 286.5 ± 2.5 Ma, which is among the oldest post-subduction magmatic Ni-Cu sulfide deposits in the region. The TiO2/Al2O3 ratios of Cr-spinel and clinopyroxene from the Qixin complex are similar to those from the older, subduction-related mafic-ultramafic intrusions in the region. This and other arc-like geochemical characteristics such as whole-rock negative Nb-Ta anomalies coupled with low initial 87Sr/86Sr (0.7036–0.7057), positive εNd(t) (+2.3 to +6.0) and high εHf(t) of zircon (+8.3 to +15), indicate that the parental magma of the Qixin complex was derived from a mantle source modified by pre-Permian metasomatic processes, similar to other contemporaneous mafic-ultramafic intrusions in the region. Mixing calculations using Sr-Nd-Hf isotopes indicate that the parental magma for the Qixin mafic-ultramafic rocks experienced no more than 15 wt% contamination with siliceous country rocks. The δ34S values of different sulfide assemblages of the Qixin Ni-Cu prospect are similar and vary from 1.1 to 1.7‰, which are within the range of typical mantle values as well as that of the country rocks, providing no direct evidence for addition of external sulfur. However, the role of external sulfur in sulfide mineralization at Qixin still cannot be ruled out, as forward modelling indicates that fractional crystallization plus crustal contamination involving siliceous country rocks free of S could not have induced sulfide saturation in the Qixin magma. This may also be true for the contemporaneous magmatic Ni-Cu sulfide deposits in the region. Based on the results from this study, we suggest that fractures in the solid country rocks nearby the Qixin complex and the ultramafic bodies of the complex are preferred targets if exploration continues.

Geochronology and geochemistry of Late Carboniferous dykes in the Aqishan–Yamansu belt, eastern Tianshan: Evidence for a post-collisional slab breakoff

With aim of providing constraints on the Late Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt (CAOB), an integrated study was conducted on the geochronological and geochemical data for dioritic, granitic and diabase dykes from the Aqishan–Yamansu belt in the eastern Tianshan, NW China. Zircon U-Pb dating indicates that the dioritic and granitic dykes were both emplaced in the Late Carboniferous (∼311 Ma and ∼315 Ma). The dioritic dykes show adakitic characteristics and have high Na2O and positive εHf(t) values (+12 to +17), which suggest an origin from partial melts of a subducted oceanic slab. The granitic dykes have high SiO2 and K2O contents and are characterized by enriched light rare earth elements (LREE) and slightly flat heavy rare earth elements (HREE), with negative Eu and Nb–Ta–Ti anomalies. These dykes are alkali-calcic and show geochemical features of highly fractionated I-type granites. Their positive εHf(t) values (+16 to +17) suggest that they were derived from a juvenile accreted oceanic crustal source. The coeval diabase dykes have low SiO2 and K2O contents but high TiO2, MgO and Mg# (54–59). They are enriched in LREE and show characteristics of enriched mid-ocean ridge basalts (E-MORB). The relatively high Ba/Th, slightly low Th/Ta ratios, and negative Nb-Ta anomalies imply a mantle source metasomatised by slab-derived fluids. Thus, these basic dykes were generated likely by partial melting of the upwelling asthenosphere mantle with a slight influence of slab-derived fluids. Therefore, we suggest that the formation of these Late Carboniferous dykes were triggered by a post-collisional slab breakoff and the Aqishan–Yamansu belt was a continental arc formed by south-dipping subduction of the Kangguer oceanic plate.