Xiaoran Zhang

Paleozoic accretionary orogenesis in the Paleo‐Asian Ocean: Insights from detrital zircons from Silurian to Carboniferous strata at the northwestern margin of the Tarim Craton

A detrital zircon U-Pb and Lu-Hf isotopic study was carried out in the Middle Silurian to Late Carboniferous sedimentary strata of the northwestern Tarim Craton in order to understand accretionary processes in the southern part of the Central Asian Orogenic Belt. Detrital zircons from these strata yielded U-Pb ages clustering around 2.8–2.3 Ga, 2.0–1.7 Ga, 1.3–0.9 Ga, 880–600 Ma, and 500–400 Ma, with age populations and Hf isotopic signatures matching those of magmatic rocks in the Tarim Craton and the Central Tianshan Block. Abundant 500–400 Ma detrital zircons most likely reflect deposition in a retroarc foreland basin inboard of an Andean-type magmatic arc to the north, supporting the northern Tarim-Central Tianshan connection during early Paleozoic time. The absence of 380–310 Ma zircon population in the Carboniferous siliciclastic rocks suggests that the Central Tianshan Block may have been separated from the Tarim Craton in the Early Devonian, caused by the interarc/back-arc opening of the South Tianshan Ocean. We propose an accretionary orogenic model switching from advancing to retreating mode during Paleozoic time in the southwestern part of the Paleo-Asian Ocean. This transition most likely occurred coevally with the rifting of Southeast Asian blocks from the northeastern margin of Gondwana.

Tarim and North China cratons linked to northern Gondwana through switching accretionary tectonics and collisional orogenesis

Accretionary and collisional orogenesis in the Tarim and North China cratons during Paleozoic time can be correlated with events associated with the assembly and subsequent incipient dispersal of Gondwana. Zircon U-Pb and Hf isotopic data from the northern margins of the two cratons and neighbors have revealed comparable eHf(t)-time patterns. Zircons with magmatic ages of 500–400 Ma display a large spread and decreasing eHf(t) values with time, whereas 400–310 Ma zircons have dominantly positive eHf(t) values and an overall increasing trend. The marked shift of the zircon Hf array at ca. 400 Ma was most likely related to a major tectonic switch from advancing to retreating accretionary orogenesis, corresponding to the development of regional extension. The commencement of subduction at 500 Ma and establishment of an advancing accretionary orogen along the preexisting passive margin was synchronous with early Paleozoic continental collisional events along the southern margins of the two cratons. The temporal agreement of these events, and their accordance with collision and/or accretion events during Gondwana assembly, suggest that the Tarim and North China cratons likely collided with the northern Australia margin of East Gondwana at ca. 500 Ma. They subsequently dispersed from Gondwana in the Early Devonian, coinciding with switching accretionary tectonics along the northern margins of the two cratons that were possibly induced by the slab rollback of the subducting paleo–Asian Ocean plate.

Tectonic evolution from subduction to arc-continent collision of the Junggar ocean: Constraints from U-Pb dating and Hf isotopes of detrital zircons from the North Tianshan belt, NW China

The Junggar ocean, once situated north (present coordinate) of the Yili and Central Tianshan blocks during early Paleozoic to late Carboniferous time, was a major southern branch of the Paleo–Asian Ocean, the opening, expansion, and final closure of which led to the development of the Central Asian Orogenic Belt between Eastern Europe–Siberia and Tarim–North China. However, the detailed evolution of the Junggar ocean has not been well constrained. This paper reports U-Pb and Lu-Hf isotopic data of detrital zircons from sandstones in the North Tianshan belt, which can provide new insights into understanding the Paleozoic development of the Junggar ocean. Most detrital zircons exhibit oscillatory zoning and high Th/U ratios, typical of igneous origin. The predominant Paleozoic zircons yield major age populations at ca. 294, 313–327, 338–375, 440–455, and 474–502 Ma and are interpreted to have been derived from the long-lived volcanic- and island-arc systems formed by the southward subduction of the Junggar ocean and subsequent collisional and postcollisional magmatism. The minor Precambrian zircons yield ages scattering at ca. 550, 680–765, 890, 970–990, 1160–1250, 1500, 1690–1750, 1840–1970, 2440–2500, and 2615–2700 Ma, which are nearly, but not fully, congruent with those from the adjacent Beishan and Kuluketage Precambrian terranes. Therefore, our results indicate that the Central Tianshan block was once part of the Tarim block during Precambrian time. Most of the 541–440 Ma zircons possess low negative eHf(t) values, while the <440–300 Ma zircons exhibit dominantly positive eHf(t) values, which can be linked to the Junggar oceanic slab rollback since ca. 440 Ma. This event, subsequently, gave rise to the opening of the South Tianshan back-arc basin/ocean between the Central Tianshan and Tarim blocks, exhumation of high-pressure granulites, and formation of a series of island arcs in the Junggar ocean. Combined with previous studies, we suggest that the Junggar ocean was probably closed at ca. 300 Ma in association with arc-continent collision, followed by postcollisional magmatism. It deserves mentioning that the ca. 0.5 and 1.4–1.5 Ga (detrital) zircons and contemporaneous magmatic rocks only occur in the Central Tianshan block, not in the Tarim block. Therefore, more detailed investigations are needed to better elucidate the origin and Precambrian evolution of the Central Tianshan block.

Late Ordovician adakitic rocks in the Central Tianshan block, NW China: Partial melting of lower continental arc crust during back-arc basin opening

Late Ordovician (ca. 451 Ma) calc-alkaline adakitic granodiorite and granite, associated with coeval S-type granites, were discovered in the Central Tianshan block, NW China. They are characterized by high Sr (526−620 ppm), but low Y (6.8−8.6 ppm) and heavy rare earth element (HREE) contents (e.g., Yb = 0.7−0.8 ppm), and thus have high Sr/Y (66−86) ratios. The granodiorite has SiO 2 of 69.3 wt%, MgO of 1.4 wt%, and Mg# of 48, exhibiting light (L) REE−enriched patterns ([La/Yb] N = 15.82, [Dy/Yb] N = 1.25) with strong positive Eu (δEu = 1.65) but weak negative Nb, Ta, and Ti anomalies. Its low Cr (9.0 ppm) and Ni (6.2 ppm) concentrations, low negative e Hf ( t ) and e Nd ( t ) values (−11.3 to −5.0 and −4.8, respectively), old one-stage Nd model age (ca. 1.4 Ga), and high initial 87 Sr/ 86 Sr ratio (0.7129) are characteristics of derivation from the partial melting of thickened ancient lower crust, most likely in the eclogite facies. The adakitic granite samples contain slightly higher SiO 2 (72.4−73.0 wt%), but lower MgO (0.6−0.7 wt%) and Mg# (37−40), with low Cr (7.9−12.1 ppm) and Ni (5.6−7.4 ppm), displaying concave-upward REE patterns ([La/Yb] N = 19.25−30.01, [Dy/Yb] N = 0.85−1.01) with negligible Eu (δEu = 0.87−1.03) and pronounced negative Nb, Ta, and Ti anomalies. Their positive e Hf ( t ) (+4.3 to +7.1) and e Nd ( t ) (+1.6 to +3.2) values, young one-stage Nd model ages (0.9−0.8 Ga), and low initial 87 Sr/ 86 Sr ratios (0.7055−0.7060) indicate generation by the partial melting of relatively juvenile lower crust, probably in the garnet-bearing amphibolite facies. Combined with previous investigations, we suggest that the Central Tianshan developed as a continental arc along northern Tarim, facing the southward subduction of the Junggar Ocean during Ordovician time. Lower-crustal melting was likely induced by asthenosphere upwelling in response to rollback of the Junggar oceanic slab, accompanied by crustal extension and incipient opening of the South Tianshan back-arc basin along northern Tarim and separation of the Central Tianshan as a microcontinent. The heat brought by the ascending adakitic magmas possibly caused anatexis of the basement rocks, forming the coeval pelite-derived S-type granites. The eastern segment of the South Tianshan Ocean was most probably closed prior to ca. 380 Ma. This study provides ample evidence that adakitic rocks can be produced by partial melting of (thickened) lower crust during incipient opening of a back-arc basin.

Varying contents of sources affect tectonic-setting discrimination of sediments: a case study from permian sandstones in the eastern Tianshan, Northwestern China

This article reports new geochemical data from Permian sandstones, most probably deposited during a period of postcollisional extension, from the Yamansu, Shaquanzi, and Xingxingxia areas in the Eastern Tianshan, northwestern China, to determine and characterize their provenance and source nature. The medium- to coarse-grained sandstones are composed mainly of angular to subangular quartz and volcanic fragments, with minor plagioclase and/or microcline, suggesting proximal deposition. Although the samples show relatively large variations in SiO2 (59.7–70.8 wt%), Al2O3 (9.0–14.9 wt%), and Fe2O3T + MgO (2.3–5.2 wt%) contents, they yield low chemical index of alteration values (41–55) but high index of compositional variability values (1.06–1.78), suggestive of their derivation mainly from compositionally immature sources with weak chemical weathering. Their relatively low Zr concentrations (99–225 ppm) and GdN/YbN ratios (1.20–1.53) manifest insignificant zircon and monazite enrichments. Most of the samples are characterized by upper continental crust–like rare earth element (REE) patterns with δEu values of 0.63–0.99, reflecting dominant contributions from intermediate to felsic rocks. In particular, the Xingxingxia and Shaquanzi samples have relatively low SiO2/Al2O3 (4.51–5.82) and Th/U (2.67–3.77) ratios but high ∑REE (total REE) contents (71.8–141.1 ppm), indicative of a low degree of sedimentary sorting/recycling, whereas the Yamansu samples have higher SiO2/Al2O3 (6.30–6.57) and Th/U (3.93–5.16) ratios but low ∑REE contents (77.1–86.8 ppm), probably indicative of a higher degree of sedimentary recycling or more involvement of recycled materials. Tectonic-setting discrimination using geochemical data from the samples yielded inconsistent results, with the Xingxingxia and Shaquanzi samples dominantly plotted in or adjacent to the active continental margin or continental island arc field, while the Yamansu samples mostly plotted in or adjacent to the passive margin field. Combining our findings with those of previous detrital zircon studies, we suggest that the inconsistent tectonic discriminations of the Permian sandstones most likely resulted from their receiving different proportions of source rocks that have distinct geochemical compositions. This study provides ample evidence that the geochemical compositions of sedimentary rocks within a continuous sequence or in the same depositional tectonic setting can vary significantly as a result of changes in contents of different source materials and thus that the use of tectonic-setting discrimination diagrams requires great caution.

Geochronology and Geochemistry of Paleozoic to Mesozoic Granitoids in Western Inner Mongolia, China: Implications for the Tectonic Evolution of the Southern Central Asian Orogenic Belt

Situated between the South Tianshan suture zone to the west and the Solonker suture zone to the east, the Yagan and Zhusileng-Hangwula arcs (YZHAs) in western Inner Mongolia in China occupy a critical place to investigate the tectonic history of the middle segment of the southern Central Asian Orogenic Belt (CAOB). In this work, field-based petrological studies and zircon U-Pb dating results reveal several episodes of granitic magmatism from 400 to 230 Ma in the YZHAs. Whole-rock geochemical and zircon Lu-Hf isotopic data indicate that all the 400–230 Ma granitoids underwent intensive fractional crystallization and were generated by magma mixing involving different proportions of mantle- and crust-derived materials. The ∼400 Ma monzogranites show (high-K) calc-alkaline affinities, akin to S-type granitoids. They were most likely generated in a postcollisional setting, corresponding to the assembly of the YZHAs before the Early Devonian. The 298–290 Ma granitoids belong to transitional I/S-type to A-type, whereas the 280–277 Ma granitoids are typical I-type. These Permian granitoids show increasingly evolved zircon εHf(t) values and formed from crust-mantle magma mixing, suggesting an advancing subduction setting. The ∼230 Ma monzogranites exhibiting fairly positive zircon εHf(t) values (+6.26 to +10.49) and high contents of mafic compositions and transition elements probably formed in a postcollisional setting after the assembly of the YZHAs and the Alxa Terrane. We infer that the final assembly of the middle segment of the southern CAOB probably occurred in the Early–Middle Permian.