Paleoproterozoic orogenic event in the western North China Craton: Insights from zircon U-Pb-Lu-Hf isotopes and geochemistry of meta-supracrustal rocks in the Beidashan complex, Alxa block

<p>     The tectonic evolution and affinity of the Alxa Block has long been controversial. The NW-SE trending Longshoushan Belt is in the southwestern margin of the Alxa Block, separated the Qilian Block. In this study, we present zircon U-Pb and Hf-isotope data of the middle and eastern Longshoushan, which could constrain the provenance and formation age of the Longshoushan Belt, and further constrain the tectonic evolution and affinity of the Alxa Block. The U-Pb ages of the detrital zircons from the amphibolite-facies metamorphosed volcanic-sedimentary rocks of the middle Longshoushan range from 3006 to 1981 Ma (peak at 2010 Ma), which were consistent with the Alxa Block and the western North China Craton, indicating that the middle Longshoushan was deposited in the Palaeoproterozoic, not in the Archean, and had tectonic affinity with the Alxa Block and the western North China Carton. Combined with the identical crustal growth events at 2.4-2.5 Ga of the middle Longshoushan, the Alxa Block and the western North China Craton, the Alxa Block was an integrated part of the Western Block of the North China Craton. The U-Pb ages of the detrital zircons from the greenschist-facies metamorphosed volcanic-sedimentary rocks of the eastern Longshoushan range from 3389 to 529 Ma (peak at 2.5 Ga and 1.0 Ga), which were highly consistent with Hexi Corridor, indicating that the eastern Longshoushan was deposited in the Cambrian, and had an affinity with the Hexi Corridor. In the Early Palaeozoic, the North Qilian Ocean subducted the Alxa Block and formed a typical trench-arc-basin system. With the closure of the North Qilian Ocean, the Central Qilian Block collided with the Alxa Block, formed the eastern Longshoushan, which was a foreland basin in the Hexi Corridor.</p>

The Alxa Block in northern China has been traditionally considered as the westernmost part of the Archean North China Craton (NCC). However, recent studies revealed that there are few Archean rocks exposed in the Alxa Block, and the Paleoproterozoic geology of this block is different from that of the western part of the NCC. Thus, the tectonic affinity of the Alxa Block to the NCC and/or other Precambrian blocks needs further investigations. In this study, we carry out integrated analyses of <i>in situ</i> zircon U–Pb age and Hf–O isotopes as well as whole-rock geochemistry and Nd isotopes for the Neoproterozoic Dabusushan and Naimumaodao granites from central Alxa Block. Secondary ion mass spectrometry (SIMS) U–Pb zircon dating results indicate that the Naimumaodao and Dabusushan granite plutons were formed at <i>ca.</i> 930 Ma and <i>ca.</i> 910 Ma, respectively. These granites are peraluminous (A/CNK value &gt;1.0), and contain peraluminous minerals such as muscovite and tourmaline, similar to those of S-type granites. They are characterized by high zircon δ¹⁸O values of <i>ca.</i> 8.2 to 12.1 permil, corresponding to a calculated magmatic δ¹⁸O value of <i>ca.</i> 10.5 to 14.3 permil, variable zircon ε_Hf(t) values of −6.2 to +3.8 (corresponding to Hf model ages of 2.2 to 1.6 Ga) and whole-rock ε_Nd(t) values of −10.1 to −4.5 (corresponding to Nd model ages of 2.4-1.9 Ga). The petrological and Nd–Hf–O isotopic study indicated that these granites were most probably generated by remelting of dominant (meta)sedimentary rocks in an orogenesis-related compressional environment. There is a clear contrast in the Precambrian geological evolution, including basement rock age data, Precambrian magmatism and detrital zircon age patterns, between the Alxa Block and the NCC. Furthermore, the new <i>in-situ</i> detrital zircon ages on Neoproterozoic (meta)sedimentary rock suggest that Alxa Block is likely related to the Cathaysia Block of South China during the Neoproterozoic, and amalgamated with the NCC since the Early Paleozoic. Thus, our new data suggest that the Alxa Block is most likely a separated Precambrian terrane from the Western Block of the NCC.

Phanerozoic amalgamation of the Alxa Block and North China Craton: Evidence from Paleozoic granitoids, U–Pb geochronology and Sr–Nd–Pb–Hf–O isotope geochemistry

Timing of amalgamation of the Alxa Block and the North China Block: Constraints based on detrital zircon U–Pb ages and sedimentologic and structural evidence

The tectonic relationship between the Alxa Block and the North China Craton has long been controversial. The Helanshan area lies at the western margin of the Ordos Block and to the east of the Alxa Block, and it contains Eidacaran Zhengmuguan Formation and Tuerkeng Formation. The Zhengmuguan Formation is made up of tillites, limestone and dolomine, and the Tuerkeng Formation consists of silty slate. A parallel unconformity marks the boundary between the Tuerkeng Formation and the Early Cambrian Suyukou Formation, which is composed mainly of phosphatic pebbly sandstone and sandstone. U–Pb dating ages of detrital zircons in the Zhengmuguan and Suyukou Formations reveal two Neoproterozoic age peaks of 818±4Ma (n=88) and 905±8Ma (n=20). Neoproterozoic magmatic events have yet been reported in the basement of the Ordos Block, while some Neoproterozoic igneous rocks have been found in the basement of the Alxa Block. Moreover, the two Neoproterozoic age peaks correspond well with the ages of Neoproterozoic igneous rocks in Alxa Block. The Hf isotopic characteristics of a part of Neoproterozoic zircons from the Ediacaran Zhengmuguan Formation in the Helanshan area (εHf(t)=–7.812 to 3.274, TDMC=2211–1578Ma, n=10) are similar to those Neoproterozoic igneous zircons from the Langshan area (εHf(t)=–1.105 to 5.928, TDMC=1.75–1.38Ga, n=23) and Bayinnuoergong area in the Alxa Block. The parental magmas had a long residency time in the crust. It seems, therefore, that Alxa Block provenance provided Neoproterozoic clastics for Helanshan area in Ediacaran, and Alxa Block had already been a part of the North China Craton by the late Neoproterozoic, even earlier.

LA-ICP-MS zircon U–Pb dating of the Langshan Group in the northeast margin of the Alxa block, with tectonic implications

The affinity between the North China Craton and the microcontinent Alxa Block during the Paleo-Mesoproterozoic: From a perspective of the stratigraphy and provenance analysis

Paleogeographic affinity of the Alxa Block across the Archean–Proterozoic: Insights from metamorphosed Archean basement

Petrogenesis and geochronology of Paleoproterozoic magmatic rocks in the Kongling complex: Evidence for a collisional orogenic event in the Yangtze craton

Poly-phase structural evolution of the northeastern Alxa Block, China: Constraining the Paleozoic-Recent history of the southern central Asian Orogenic belt

The Carboniferous-Permian tectonic setting for the southernmost Central Asian Orogenic Belt: Constraint from magmatic and sedimentary records in the Alxa area, NW China

The Jinchuan deposit, which is located in the Longshoushan uplift (LU) on the southwest margin of the Alxa Block (AB), is the third largest Ni‐Cu deposit in the world. There are igneous rocks in the LU that widely developed in ~830 Ma, which include ore‐bearing ultramafic rock and ore‐poor mafic‐ultramafic rocks. These rocks were produced in an intracontinental extension‐rift environment, which is likely related to the Rodinia supercontinent (RS) breakup. However, contemporary magmatic rocks are absent in the AB, except the ~830 Ma igneous rocks in the LU. In this study, we present two types of metamorphic mafic‐ultramafic rocks at the border of Jinchuan II and IV mineral areas, which is inside the F17 fault zone. One type is a red‐coloured altered rock formed by ultramafic rock oxidation; there are a small number of zircons with multiple ages in the rock, which are considered to be inherited and metamorphic (hydrothermal) zircons. The other type is a chlorite tremolite schist formed by the metamorphism of fault clastics; there are a large number of zircons developed in this rock, which are mostly magmatic zircons with ages between 900 and 800 Ma and a few hydrothermal zircons. Analysis of the trace elements, the 900–800 Ma zircons, have high (Sm/La)N, low Ce/Ce*, and high Th/U values, which fit the characteristics of typical magmatic zircons. There are also high Y, U, HREE, Nb, and Ta values, which indicates that the host rocks were mostly larvikite and dolerite, and high Y, Ce, Yb, Th, Gd, and Er values indicate a within‐plate basaltic origin. All these 900–800 Ma zircons have a εHf (t) in the range of −2 to −7, which is similar to the Jinchuan ore‐bearing ultramafic rock with a partial melting origin of enriched mantle. The absence of detrital zircon age peaks in the Neoproterozoic indicates that the AB has been separated from the North China Craton since the Neoproterozoic to Paleozoic. The detrital zircons in the Yangtze Block (YB) strata with Neoproterozoic ages are slightly younger than those in the AB, with more depleted εHf (t) values, which is similar to the zircons from the Jiangnan Orogen igneous rocks. All this evidence indicates that the AB may have experienced at least 100 Myr of intracontinental extension‐rift environment from 900 to 800 Ma during the Neoproterozoic and participated in the breakup of the RS. However, the location of the AB during the Rodinia may not have been beside the YB.

Proterozoic evolution of the Alxa block in western China: A wandering terrane during supercontinent cycles

Provenance and paleogeography of Carboniferous–Permian strata in the Bayanhot Basin: Constraints from sedimentary records and detrital zircon geochronology

As an important tectonic unit in the middle part of the southern Central Asian Orogeny, the Alxa Block was affected by multiple deformational events occurring in the Central Asian tectonic regime during the Paleozoic and Mesozoic. The record of deformation in the Alxa Block represents the entire evolutionary process from a continental margin to an intraplate setting. In the Langshan region of the northeastern Alxa Block, four important Paleozoic‐early Mesozoic deformation events have been distinguished: (1) nearly north‐south‐striking ductile thrusting along the eastern Alxa Block in the Late Devonian; (2) nearly east‐west‐trending brittle top‐to‐the‐south thrusting in the Mid‐Late Permian; (3) nearly east‐west‐trending initially ductile then brittle dextral transtension with 30–40 km of displacement in the Late Permian; and (4) northeast‐trending ductile sinistral shearing with 120–125 km of displacement in the Mid‐Late Triassic. The ductile thrusting in the Late Devonian may have resulted from the interaction between the North China Craton and the Alxa Black, which was a peri‐eastern Gondwana block. Parallel east‐west‐trending thrusts and related folds formed in the Neoproterozoic Langshan Group due to the closure of the Paleo‐Asian Ocean in the Mid‐Late Permian. The Late Permian east‐west‐trending and dextrally ductile transtension may have resulted from intraplate adjustment after the Central Asian Orogeny and the formation of the Ural Orogenic Belt to the west; this shear zone may have been part of a huge ductile tectonic belt developed along the whole southern Central Asian Orogenic Belt (CAOB). During the Early Triassic, the subduction of oceanic crust along the southern central CAOB terminated then the eastern Alxa Block was affected by the collision between the North China Craton and the Yangtze Craton to the south. Since the late Mesozoic, the Langshan region has experienced another five important deformation events, which were all far‐field tectonic effects related to plate margin tectonic activities (e.g., the closure of the Mongolian‐Okhotsk Ocean, the collision between the Qiangtang and Lhasa blocks and the India‐Eurasia collision). The large‐scale brittle and ductile strike‐slip faults in the northeastern Alxa Block all formed in intraplate settings since the late Paleozoic in response to the collisional orogenies occurring along the plate margins or the intraplate adjustment following the closure of the Paleo‐Asian Ocean.