The Liuyuan Volcanic Belt in NW China revisited: evidence for Permian rifting associated with the assembly of continental blocks in the Central Asian Orogenic Belt

Abstract

The basaltic pillow lavas in the Liuyuan region of NW China, considered to be part of an ophiolitic suite, have been central to the models on tectonic setting, evolution and timing of the final closure of the Palaeo-Asian Ocean. New field evidence on the sedimentary units associated with the basalts reveals comparable sequences in the northern and southern flanks of the Liuyuan Volcanic Belt with coarse to fine sediments from periphery to the centre. The dacites and rhyolites formed coevally with the pillow basalts. The pillow basalts are interlayered with lacustrine sandstone, claystone and clayey lake deposits. Detrital zircons from these sediments yield zircon U–Pb ages of 291–285 Ma. Andesites, dacites and rhyolites from the basaltic sequence yield U–Pb ages of 280–277 Ma, similar to the 282–280 Ma ages of gabbros that intrude the pillow lavas. All these rocks cover the 460–440 Ma granite and greenschist basement and have been intruded by gabbros of c. 272 Ma age, with subsequent (230–227 Ma) north–south contractional thrusting and folding. The data from our study are incompatible with the existing models that consider the basalts as part of an ophiolitic suite. Along the northern continental margin of China from west to east, the Tarim, Dunhuang-Alxa and North China cratonic areas all show evidence for regional extension through rifting during early–middle Permian time. These rift features and basaltic eruptions occurred coevally with the assembly of various microcontinental blocks against the Siberian craton at c. 300–250 Ma, synchronous with amalgamation of the Central Asian Orogenic Belt (CAOB) on the northern side of the Liuyuan Rift. These events were also broadly synchronous with formation of the global supercontinent Pangea.

An Andean‐type arc transferred into a Japanese‐type arc at final closure stage of the Palaeo‐Asian Ocean in the southernmost of Altaïds

“…They further proposed that the Liuyuan Complex formed as an ophiolite in a forearc in Carboniferous–Permian time in the light of (a) all these basalts are imbricated against Permian tuffaceous sediments and limestones, and (b) geochemistry feature. In the pillow basalts, many other intruded gabbros were also discovered and were dated by Wang et al () with ages of 270 ± 1 Ma (marked as 24 in Figure and Table ) and 277 ± 3 Ma (marked as 26 in Figure and Table 1) in two different locations, respectively. Occasionally, layers of dacites and rhyolites occurred within the pillow basalts; dacites (marked as 29 and 30 in Figure and Table ) and rhyolites (marked as 28 in Figure 3 and Table 1) from the basaltic sequence yield U–Pb ages of 280–277 Ma, similar to or a little older than the 270–277 Ma ages of gabbros that intrude into the pillow lavas (Wang et al, ).…”

“…The oldest stratum is marked as Ohn a by the previous 1 to 200,000 geological mapping (Gansu Bureau of Geology and Mineral Resources [GBGMR], 1967) and consists of gneisses, porphyritic rocks, meta‐sandstones, and basalts, with no fossils in it. A granitic vein (marked as 5 in Figure and Table ) intruded into the Ohn a stratum, which was dated by Wang et al () obtaining an age of 422 ± 5 Ma. In the north of the Ohn a stratum, large areas of Ohn b stratum (unit) crop out (Figure ).…”

“…Style of Palaeozoic accretionary orogenesis, type of continental growth, and termination time of the BOC are three hotly debated issues which attracted many geologists to pursue the final evolution of the Palaeo‐Asian Ocean. Generally, there are two major opinions advocated by previous studies in the BOC (Figure b): (a) Before the Devonian era, the Palaeo‐Asian Ocean have already closed between the south Dunhuang Block and the Southern Mongolian accretionary system which is located in the southernmost part of the Siberian Craton (Figure a), and after this time, the BOC underwent intraplate deformation, after which this orogen underwent rifting in the Carboniferous to Permian (Zuo et al, ; He, Ren, Yao, & Fu, ; Gong, Liu, Li, Liang, & Dai, ; Su et al, ; Zhang, Dostal, Zhao, Liu, & Guo, ; Chen et al, ; Wang, Luo, Santosh, Wang, & Wang, ;); (b) the Palaeo‐Asian Ocean in the BOC region had a long survival from Precambrian to Late Palaeozoic, closed at the end‐Permian, and the last terminal orogenesis was in the Early Triassic (Guo et al, ; Mao et al, ; Tian et al, ; Tian et al, ; Tian, Xiao, Zhang, & Lin, ; Wan et al, ; Xiao et al, ). For instance, based on the geochemistry and structural feature of basalts, Permian tuffaceous sediments, and limestones in the Liuyuan Basin, Mao et al () proposed that these rocks formed as an ophiolite in a forearc at the Carboniferous–Permian era.…”

“…Based on the petrological and geochemical data from these two types of Permian turbidites, Guo et al () further advocated that an arc‐continent collision took place in the Early Permian, and this is in agreement with the Permian termination of the southern Altaïds. However, after a new field and geochronology study to the sedimentary units and its associated volcanic rocks in the Liuyuan Basin, Wang et al () suggested that they formed in a local rift tectonic setting according to their compositions and intrusive relationships (Figure b). The authors also concluded that there was no widespread extension during the formation of the Permian Liuyuan rift; conversely, the tectonic setting involved the continuous assembly of continental blocks that were accreted to the north, towards the Siberian Craton.…”

“…(a) Tectonic model presented by Guo et al (2014); the Liuyuan Ocean is located between the Shibanshan Andean‐type arc to the south and the Huniushan island arc to the north. (b) Rift basin model presented by Wang et al (), the Liuyuan pillow lava formed in a rift basin (a lake) after the Permian extension event [Colour figure can be viewed at wileyonlinelibrary.com]…”