Discovery of the early Jurassic Gajia mélange in the Bangong–Nujiang suture zone: Southward subduction of the Bangong–Nujiang Ocean?

International Journal of Earth Sciences - Mélange records a series of geological processes associated with oceanic subduction and continental collision. This paper reports for the first time...     

Reconstructing in space and time the closure of the middle and western segments of the Bangong–Nujiang Tethyan Ocean in the Tibetan Plateau

International Journal of Earth Sciences - When and how the Bangong–Nujiang Tethyan Ocean closed is a highly controversial subject. In this paper, we present a detailed study and review of the...     

The Middle Triassic evolution of the Bangong–Nujiang Tethyan Ocean: evidence from analyses of OIB-type basalts and OIB-derived phonolites in northern Tibet

International Journal of Earth Sciences - In this paper, we present new major and trace element chemical data for the basalts and phonolites of the Nare ocean island fragment (NaOI), as well as...     

Zircon U–Pb age and Sr–Nd–Hf isotopic constraints on the age and origin of Triassic mafic dikes,Dalian area,Northeast China

Post-orogenic mafic rocks from Northeast China consist of swarms of dolerite dikes. We report a new U–Pb zircon age, as well as whole-rock geochemical and Sr–Nd–Hf isotopic data. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon analysis yielded an age of 210.3 ± 1.5 million years (i.e. Triassic) for these mafic dikes. Most Dalian mafic rocks exhibit low K₂O + Na₂O contents, and span the border between alkaline and calc-alkaline rock associations in the total alkali–silica diagram. The investigated dikes are also characterized by relatively high (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7061–0.7067) and negative ?_Nd (t) (?4.7 to??4.3) and ?_Hf (t) values (?4.1 to??1.1), implying that they were derived from an enriched lithospheric mantle source. The mafic dikes are characterized by relatively low MgO (4.65–5.44 wt.%), Mg^# (41–44), and compatible element content [such as Cr (89.9–125 ppm) and Ni (56.7–72.2 ppm)], which are the features of an evolved mafic magma. No evidence supports the idea that the mafic rocks were affected by significant assimilation or crustal contamination during emplacement. We conclude that the dolerites formed in a post-orogenic extensional setting, related to lithospheric delamination or ‘collapse’ of the Central Asian Orogenic Belt (CAOB), also termed the Xingmeng Orogenic Belt in China.     

Zircon U–Pb age,Hf isotopic compositions and geochemistry of the Silurian Fengdingshan I-type granite Pluton and Taoyuan mafic–felsic Complex at the southeastern margin of the Yangtze Block

This work presents an integrated study of zircon U–Pb ages and Hf isotope along with whole-rock geochemistry on Silurian Fengdingshan I-type granites and Taoyuan mafic–felsic intrusive Complex located at the southeastern margin of the Yangtze Block, filling in a gap in understanding of Paleozoic I-type granites and mafic-intermediate igneous rocks in the eastern South China Craton (SCC). The Fengdingshan granite and Taoyuan hornblende gabbro are dated at 436 ± 5 Ma and 409 ± 2 Ma, respectively. The Fengdingshan granites display characteristics of calc-alkaline I-type granite with high initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7093–0.7127, low εNd(t) values ranging from −5.6 to −5.4 and corresponding Nd model ages (T_2DM) of 1.6 Ga. Their zircon grains have εHf(t) values ranging from −2.7 to 2.6 and model ages of 951–1164 Ma. The Taoyuan mafic rocks exhibit typical arc-like geochemistry, with enrichment in Rb, Th, U and Pb and depletion in Nb, Ta. They have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7053–0.7058, εNd(t) values of 0.2–1.6 and corresponding T_2DM of 1.0–1.1 Ga. Their zircon grains have εHf(t) values ranging from 3.2 to 6.1 and model ages of 774–911 Ma. Diorite and granodiorite from the Taoyuan Complex have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7065–0.7117, εNd(t) values from −5.7 to −1.9 and Nd model ages of 1.3–1.6 Ga. The petrographic and geochemical characteristics indicate that the Fengdingshan granites probably formed by reworking of Neoproterozoic basalts with very little of juvenile mantle-derived magma. The Taoyuan Complex formed by magma mixing and mingling, in which the mafic member originated from a metasomatized lithospheric mantle. Both the Fengdingshan and Taoyuan Plutons formed in a post-orogenic collapse stage in an intracontinental tectonic regime. Besides the Paleozoic Fengdingshan granites and Taoyuan hornblende gabbro, other Neoproterozoic and Indosinian igneous rocks located along the southeastern and western margin of the Yangtze Block also exhibit decoupled Nd–Hf isotopic systemics, which may be a fingerprint of a previous late Mesoproterozoic to early Neoproterozoic oceanic subduction.     

Zircon U–Pb ages and Hf isotopic compositions from the Sin Quyen Formation: the Precambrian crustal evolution of northwest Vietnam

The location of the suture zone between the South China and Indochina blocks in northwest Vietnam has been under debate for decades. Generally, the boundary between these blocks has been placed along (1) the Ailaoshan–Red River zone or (2) the Song Ma zone. The Sin Quyen Formation, lying between these zones, was previously regarded as a Palaeo- and Mesoproterozoic sequence. It comprises its provenance and tectonic affinity. We analysed detrital zircons from two paragneisses and one migmatite of the Sin Quyen Formation employing laser ablation inductively coupled plasma mass spectrometry U–Pb dating techniques. U–Pb ages of these zircons show three main periods of zircon formation: ～2.7–3.0, ～2.2–2.5, and ～1.8 Ga, suggesting that Sin Quyen rocks were mainly derived from Palaeoproterozoic and Archaean basement units. Inasmuch as the South China basement comprises rocks of similar ages, we conclude that the Sin Quyen Formation belongs to that block. Our new data strengthen the view that the suture between the South China and Indochina blocks is located within the Song Ma zone. In addition, zircons with U–Pb ages >3.0 thousand million years represent the oldest minerals reported in northwest Vietnam so far, indicating the existence of Mesoarchaean crustal remnants in this region.     

U–Pb Zircon Ages of Early Cretaceous Volcanic Rocks in the Tethyan Himalaya at Yangzuoyong Co Lake,Nagarze, Southern Tibet,and Implications for the Jurassic/Cretaceous Boundary

Upper Jurassic–Lower Cretaceous transitional successions are widely distributed in the Tethyan Himalaya, southeast of Yangzuoyong Co Lake, southern Tibet. In ascending order, these include the Weimei (J₃, Tithonian), Sangxiu/Jiabula formations (K₁, Berriasian). The J/K boundary is located between the Weimei Formation and Sangxiu/Jiabula Formations. Ammonites found in J/K boundary sections in the research area have been classified into three assemblages: Valanginites–Phyllopachyceras assemblage zone (Valanginian), Spiticeras–Thurmanniceras assemblage zone (Berriasian) and Haplophylloceras–Blanfordiceras–Himalayites assemblage zone (Tithonian). Six nannofossil zones: Calcicalathina oblongata assemblage zone, Speetonia colligate zone, N. st. steinmannii zone, N. st. minor zone, P. beckmanni–N. st. minor interval zone, Conusphaera–Polycostella–Nannoconus–Watznaueria assemblage zone were recognized as well.On the basis of lithology, biostratigraphy and geochronology of the J/K transitional deposition succession, this study suggests that the J/K boundary, in southern Tibet, is located on the bottom of P. beckmanni–N. st. minor interval zone, which is further definited as and disappear of Polycostella beckmanni. To address the paucity of previously reported reliable ages for the J/K boundary, this study reports four U–Pb zircon ages (140–142 Ma) obtained with Secondary Ion Mass Spectrometry (SIMS) from the volcanic rocks interbedded in the lower Sangxiu Formation, which is expected to provides a direct date reference for the J/K boundary in the Tethyan Himalaya, southern Tibet. From integration of our new (SIMS) U–Pb zircon ages with calcareous nannofossils and ammonites, the age of the N. st. minor zone (NK-D) directly above the P. beckmanni-N. st. minor interval zone (NJK-C) of the basal Berriasian in the Tethyan realm is estimated to be 141–142 Ma. This research is not only helpful to improve the isotopic determination of absolute age for the J/K boundary, but also implies that the Tethyan Himalaya of southern Tibet may be an ideal location in which to explore the J/K boundary in both biostratigraphy and geochronology in future.     

Sedimentary record of Jurassic northward subduction of the Bangong–Nujiang Ocean: insights from detrital zircons

The subduction polarity and related arc–magmatic evolutional history of the Bangong–Nujiang Ocean, which separated the South Qiangtang terrane to the north from the North Lhasa terrane to the south during the Mesozoic, remain debated. This study tries to reconstruct the subduction and evolution of the Bangong–Nujiang Ocean on the basis of U–Pb and Hf isotopic analyses of detrital zircons in samples from sedimentary rocks of the middle-western section of the Bangong–Nujiang suture zone in Gerze County, central Tibet. The Middle Jurassic Muggargangri Group in the Bangong–Nujiang suture zone was deposited in a deep-sea basin setting on an active continental margin. The Late Jurassic strata, such as the Sewa Formation, are widely distributed in the South Qiangtang terrane and represent deposition on a shelf. The Early Cretaceous Shamuluo Formation in the Bangong–Nujiang suture zone unconformably overlies the Muggargangri Group and was probably deposited in a residual marine basin setting. The detrital zircons of the Muggargangri Group contain seven U–Pb age populations: 2.6–2.4 Ga, 1.95–1.75 Ga, 950–900 Ma, 850–800 Ma, 650–550 Ma, 480–420 Ma, and 350–250 Ma, which is similar to the age populations in sedimentary rocks of the South Qiangtang terrane. In addition, the age spectra of the Shamuluo Formation are similar to those of the Muggargangri Group, indicating that both had a northern terrane provenance, which is conformed by the north-to-south palaeocurrent. This provenance indicates northward subduction of the Bangong–Nujiang oceanic crust. In contrast, two samples from the Sewa Formation yield variable age distributions: the lower sample has age populations similar to those of the South Qiangtang terrane, whereas the upper possesses only one age cluster with a peak at ca. 156 Ma. Moreover, the majority of the late Mesozoic detrital zircons are characterized by weakly positive εHf(t) values that are similar to those of magmatic zircons from arc magmatic rocks in the South Qiangtang terrane. The findings, together with information from the record of magmatism, indicate that the earliest prevalent arc magmatism occurred during the Early Jurassic (ca. 185 Ma) and that the principal arc–magmatic stage occurred during the Middle–Late Jurassic (ca. 170–150 Ma). The magmatic gap and scarcity of detrital zircons at ca. 140–130 Ma likely indicate collision between the Qiangtang and Lhasa terranes. The late Early Cretaceous (ca. 125–100 Ma) magmatism on both sides of the Bangong–Nujiang suture zone was probably related to slab break-off or lithospheric delamination after closure of the Bangong–Nujiang Ocean.     

Evolution of granitoids in the Catalina metamorphic core complex, southeastern Arizona: U–Pb, Nd, and Hf isotopic constraints

The Santa Catalina Mountains, SE Arizona, was one of the first metamorphic core complexes to be described. Despite its status as a type example, relatively little is known about precise ages and origins of the intrusive rocks that make up most of the crystalline core. U–Pb and Hf isotopic data by laser ablation–inductively coupled plasma–mass spectrometry from zircons and Nd isotopic results from whole rocks were obtained for 12 granitoids ranging from 1,440 to 26 Ma. Results confirm that the 1.44-Ga Oracle Granite extends through the Catalina Range as variably mylonitic granite and banded gneiss. Laramide intrusions (67–73 Ma) display initial ε_Nd values ?5 to ?8 and ε_Hf from ?7.5 to ?9. Magmatic ages for the prominent white granite sills of the Wilderness suite are 46–57 Ma, in agreement with Terrien (2012), and these granites have initial ε_Nd values ?8 to ?10 and ε_Hf from ?7 to ?14. Lastly, the undeformed Catalina Granite has an age of 26 Ma, with an initial ε_Nd and ε_Hf of ?6 and ?8, respectively. Our Nd results agree with limited results from Farmer and DePaolo (89:10141–10160, 1984). Although the Catalina Granite seems to have a significant juvenile component based on Nd and Hf, most of the Laramide and Wilderness intrusions contain Nd and Hf compositions lying close to the evolution of 1.44-Ga Oracle Granites, a fact that is confirmed by the U–Pb data, which show both 1.7- and 1.4-Ga zircon cores in these samples, with 1.4 Ga as the dominant core age. In order to become the dominant source of most of the 72–45-Ma magmas, the Oracle pluton must not only extend across the whole Catalina region, but also have abundant deep-seated equivalents to provide magma sources.     

Petrology,geochemistry, and geochronology of boninitic dikes from the Kangqiong ophiolite: implications for the Early Cretaceous evolution of Bangong–Nujiang Neo-Tethys Ocean in Tibet

The Kangqiong ophiolite is exposed in the central–western part of the Bangong–Nujiang suture zone (BNSZ) of central Tibet. This study reports new data for boninitic dikes with the aim of reconstructing the geodynamic and petrogenetic evolution of the Kangqiong ophiolite. Ten samples of boninitic dikes that cross-cut the mafic cumulates have very low TiO₂ (0.34–0.42%) contents and high MgO (6.65–8.25%) contents. LA-ICP-MS U–Pb analyses of zircon from the boninitic dikes yield an age of 115 Ma. They are characterized by positive εHf(t) values varying from +13.1 to +15.0. Taking into account the geochemical characteristics of the mantle section, the Kangqiong ophiolite should be generated in a fore-arc spreading setting resulting from intra-oceanic subduction. Based on our data and previous studies, we propose that the BNSZ represents the major suture and records the Early Cretaceous intra-oceanic subduction of the Bangong–Nujiang Neo-Tethys Ocean, and the Shiquan River–Yongzhu–Jiali ophiolitic mélange belt represents a back-arc basin. These two belts, together with the northern Lhasa subterrane should, represent an Early Cretaceous intra-oceanic subduction system and back-arc basin in central Tibet that is similar to present-day active intra-oceanic subduction systems in the western Pacific Ocean. The final closure of the Bangong–Nujiang Neo-Tethys Ocean might have taken place later than the Early Cretaceous.     

Early Cretaceous continental arc-related volcanic rocks in the Duobuzha area,northern Tibet: implications for evolution history of the Bangong–Nujiang Ocean

New whole-rock major and trace elements data, zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages, and zircon Hf isotope compositions were analysed for Early Cretaceous volcanic rocks, also called Meiriqieco Formation (MF) in the Duobuzha area of the Southern Qiangtang–Baoshan Block (SQBB), northern Tibet. Our aim is to clarify their petrogenesis and tectonic setting, and constrain the evolution process on the northern margin of Bangong–Nujiang suture zone (BNSZ) during Early Cretaceous time. The MF volcanic rocks are mainly composed of andesites with subordinate basalts and rhyolites with high-K calc-alkaline affinity. Zircon LA-ICP-MS U–Pb dating for two andesite and one rhyolite samples give uniform ages within error of ca.113, 114, and 118 Ma, respectively, indicating they were erupted on the Early Cretaceous. The MF andesites have variable zircon ε_Hf(t) values (+0.5 to +10.5), which is different from those of MF rhyolites (+7.9 to +10.7). All the MF rocks are enriched in large ion lithophile elements, and depleted in high field strength elements, yielding the affinity of arc rocks. The MF basalts were most likely derived from the mantle wedge that was metasomatized by fluids released from subducting slab with the involvement of subducted sediments. The MF rhyolites were generated by partial melting of the juvenile mafic lower crust. The MF andesites are interpreted to have formed by mixing of the magmas that parental of the MF basalts and the MF rhyolites. In addition, a couple of distinctly magmatic sources are identified in the SQBB, and this may be related to mantle components injected into the continental crust. Combined with published geological data in the BNSZ and SQBB, we consider that the MF volcanic rocks are formed in a continental arc setting, suggesting that BNO were subducting during the Early Cretaceous time in the Duobuzha area.     

Constraints from zircon U–Pb ages, O and Hf isotopic compositions on the origin of Neoproterozoic peraluminous granitoids from the Jiangnan Fold Belt, South China

Zircon textures and micro-chemical compositions precisely record the origin and petrogenesis of granitoids, which are crucial for evaluating crustal growth and reworking, thermal and geodynamic evolution. Zircons in peraluminous granitoids from the three largest 820 Ma complexes (Guibei, Yueyang and Jiuling) in the Jiangnan Fold Belt in South China are used to constrain their sources and petrogenetic processes. Zircons in the Guibei granitoids have complex internal structures. Nearly all magmatic and inherited zircons have similar εHf (?6.8 to +5.6) and δ¹⁸O values (8.8–11.6 ‰) and dominantly lie between εHf evolution vectors for a crust created between 1.7 and 2.1 Ga, suggesting that the Guibei granitoids were produced by partial melting of recycled heterogeneous supracrustal material. However, the Yueyang granitoids contain zircons with high εHf (?0.5 to +9.7) and relatively low δ¹⁸O values (5.9–8.4 ‰) and two-stage model ages of 1.1–1.8 Ga, and thus may have been formed by melting of mafic rocks from the lower crust. The Jiuling granitoids and their enclaves contain more complex zircons with more variable εHf (?7.2 to +9.7) and δ¹⁸O values (7.0–10.6 ‰), and lie along the mixing trend between the above-proposed infracrustal and supracrustal granitoids. Therefore, the Neoproterozoic peraluminous granitoids in the Jiangnan Fold Belt were produced by melting and mixing of continental crust. Compared with extremely low (≤4 ‰) and negative δ¹⁸O values of Neoproterozoic igneous zircons formed in its northern active continental margin, the high δ¹⁸O peraluminous granitoids in the southeastern Yangtze Block are considered to have been formed by melting of hydrothermally unaltered continental crust triggered by asthenosphere upwelling in the Nanhua back-arc basin.     

Zircon U–Pb dating and the petrological and geochemical constraints on Lincang granite in Western Yunnan,China: Implications for the closure of the Paleo-Tethys Ocean

Lincang granite is a batholith located in the Sanjiang region and is an important research subject for understanding subduction and collision during the Paleo-Tethyan period. It is widely exposed in the Lincang Terrane and extends south into Burma. Based on various petrological and geochemical investigations performed from south to north across the Lincang granite, a new set of data, which includes zircon chronological and Hf isotopic data, is presented to discuss the origin of the Lincang granite and its tectonic significance. The Lincang granite is a peraluminous, high-K calc-alkaline body with sub-parallel REE patterns and a strong negative Eu anomaly. This anomaly is characteristic of a post-collision peraluminous S-type granitic batholith. The 200–230 Ma formation age of the Lincang granite was determined using LA-ICP-MS zircon U–Pb dating. Thus, it has been confirmed that the granite formed during the late Triassic period, and the formation process lasted for approximately 30 Ma. Geochemical and isotopic compositions indicate that the primary magma of Lincang granite most likely originated from a crustal source, and possibly underwent an assimilation–fractionation crystallization (AFC) process during its emplacement. The Lincang granite formed during the continental collision between the Baoshan–Gengma Terrane and the Lanping–Simao Terrane after the northeast subduction of the Paleo-Tethyan Oceanic Plate. Therefore, the late Triassic Lincang granite is important evidence for the closure of the Paleo-Tethyan Ocean.     

Zircon U–Pb geochronology of the Mesozoic metamorphic rocks and granitoids in the coastal tectonic zone of SE China: Constraints on the timing of Late Mesozoic orogeny

The coastal Changle-Nan’ao tectonic zone of SE China contains important geological records of the Late Mesozoic orogeny and post-orogenic extension in this part of the Asian continent. The folded and metamorphosed T₃–J₁ sedimentary rocks are unconformably overlain by Early Cretaceous volcanic rocks or occur as amphibolite facies enclaves in late Jurassic to early Cretaceous gneissic granites. Moreover, all the metamorphic and/or deformed rocks are intruded by Cretaceous fine-grained granitic plutons or dykes. In order to understand the orogenic development, we undertook a comprehensive zircon U–Pb geochronology on a variety of rock types, including paragneiss, migmatitic gneiss, gneissic granite, leucogranite, and fine-grained granitoids. Zircon U–Pb dating on gneissic granites, migmatitic gneisses, and leucogranite dyke yielded a similar age range of 147–135 Ma. Meanwhile, protoliths of some gneissic granites and migmatitic gneisses are found to be late Jurassic magmatic rocks (ca. 165–150 Ma). The little deformed and unmetamorphosed Cretaceous plutons or dykes were dated at 132–117 Ma. These new age data indicate that the orogeny lasted from late Jurassic (ca. 165 Ma) to early Cretaceous (ca. 135 Ma). The tectonic transition from the syn-kinematic magmatism and migmatization (147–136 Ma) to the post-kinematic plutonism (132–117 Ma) occurred at 136–132 Ma.     

Zircon U–Pb and Hf isotopic study of Mesozoic felsic rocks from eastern Zhejiang,South China: Geochemical contrast between the Yangtze and Cathaysia blocks

The Jiangshan–Shaoxing Fault Zone (JSFZ) in Zhejiang Province has been proposed to represent a suture between the Yangtze and Cathaysia blocks in South China. In this study, in-situ zircon U–Pb and Hf isotopic analysis and whole-rock major- and trace-element measurement of early to middle Cretaceous felsic rocks across the fault zone were conducted to constrain the nature of the fault zone. Twelve Cretaceous granitoid bodies were sampled from the NW and SE sides of the fault zone, respectively, with composition ranging from diorite to granite (SiO₂ = 56.2–76.6 wt.%). These granitoids yielded U–Pb ages ranging from 135–100 Ma, with a systematic variation in zircon Hf isotopic compositions (ε_Hf(t) = + 6.9 to –7.0 in the NW side vs. + 1.9 to ? 12.9 in the SE side). The T_DM2 values for the granitoids from the NW side are 0.34 to 1.33 Ga, with two peaks at ca. 876 and 1170 Ma respectively, whereas those from the SE side are 0.70 to 1.62 Ga, with a single peak at ca. 1126 Ma. The Hf isotopic disparity for the two sides may indicate a fundamental difference in the lower crustal compositions of the Yangtze and Cathaysia blocks, supporting that the JSFZ is possibly a suture zone between the two blocks. Our results together with the available geological data suggest that the Mesoproterozoic materials are important for both the Yangtze and Cathaysia basement and the Neoproterozoic magmatic activities were important in the Yangtze Block, possibly related to the break-up of the Rodinia supercontinent, but less significant in the Cathaysia Block. This may imply that the two blocks have not completely juxtaposed in the Neoproterozoic.     

Permian–Carboniferous arc magmatism in southern Mexico: U–Pb dating,trace element and Hf isotopic evidence on zircons of earliest subduction beneath the western margin of Gondwana

Undeformed felsic to mafic igneous rocks, dated by U–Pb zircon geochronology between 311 and 255 Ma, intrude different units of the Oaxacan and Acatlán metamorphic complexes in southwestern Mexico. Rare earth element concentrations on zircons from most of these magmatic rocks have a typical igneous character, with fractionated heavy rare earths and negative Eu anomalies. Only inherited Precambrian zircons are depleted in heavy rare earth elements, which suggest contemporaneous crystallization in equilibrium with metamorphic garnet during granulite facies metamorphism. Hf isotopic signatures are, however, different among these magmatic units. For example, zircons from two of these magmatic units (Cuanana pluton and Honduras batholith) have positive εHf values (+3.8–+8.5) and depleted mantle model ages (using a mean crustal value of ¹⁷⁶Lu/¹⁷⁷Hf = 0.015) (T _DMC) ranging between 756 and 1,057 Ma, whereas zircons from the rest of the magmatic units (Etla granite, Zaniza batholith, Carbonera stock and Sosola rhyolite) have negative εHf values (?1 to ?14) and model ages between 1,330 and 2,160 Ma. This suggests either recycling of different crustal sources or, more likely, different extents of crustal contamination of arc-related mafic magmas in which the Oaxacan Complex acted as the main contaminant. These plutons thus represent the magmatic expression of the initial stages of eastward subduction of the Pacific plate beneath the western margin of Gondwana, and confirm the existence of a Late Carboniferous–Permian magmatic arc that extended from southern North America to Central America.     

Jurassic Subduction of the Paleo-Pacific Ocean in NE China: Zircon U–Pb and Sr-Nd-Hf isotopic Evidence from the Huashan and Taili Monzogranites

The Qilian orogen along the NE edge of the Tibet‐Qinghai Plateau records the evolution of Proto‐Tethyan Ocean that closed through subduction along the southern margin of the North China block during the Early Paleozoic. The South Qilian belt is the southern unit of this orogen and dominated by Cambrian‐Ordovician volcano‐sedimentary rocks and Neoproteozoic Hualong complex that contains similar rock assemblages of the Central Qilian block. Our recent geological mapping and petrologic results demonstrate that volcano‐sedimentary rocks show typical rock assembles of a Cambrian‐early Ordovician arc‐trench system in Lajishan Mts. along the northern margin of the Hualong Complex. Island arc rocks including basalt, andesite, dacite, rhyolite, and breccia is in fault contact with ophiolite complex consisting of mantle peridotite, serpentinite, gabbro, dolerite, plagiogranite, and basalt. Accretionary complexes are tectonically separated from the ophiolite‐arc rocks, with various rock assemblages spatially. They consist of pillow basalt, basalt breccia, tuff, chert, and limestone blocks with a seamount origin within the scaly shale in Dingmaoshan and Donggoumeikuang areas, and basalt, chert, and sandstone blocks within muddy shale matrix and mélange at Lajishankou area. Abundant radiolarians occur in red chert, and trilobite, brachiopod, and coral fossils occur within Dingmaoshan limestone blocks. Although partial basalt or chert blocks are highly disrupted, duplex, thrust fault, rootless intrafolial fold, tight fold, and penetrative foliation are well‐developed at Donggoumeikuang area. Spatially, accretionary complexes lie structurally beneath ophiolite complex and above the turbidites of the Central Qilian block. Ophiolite and accretionary complexes are also overlapped by late Ordovician molasse deposits sourced from Cambrian arc‐trench system and the Central Qilian block. These observations demonstrate that a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt formed during the early Paleozoic southward subduction of the South Qilian Ocean collided with the Central Qilian block prior to the late Ordovician.