Article

Magmatism of collision and post-orogenic period in northern Alaxa Region in Inner Mongolia

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Abstract

The studied region in this paper involves northern Alaxa region, Inner Mongolia, China. Geotectonically it belongs to the Tarim and North China plates. This paper chiefly has studied the magmatism of collisional and post-orogenic period between the Tarim and North China plates. The thermal event occurred in 251.3 Ma ± is Cs-type magmatism of collisional period, it only is distributed in northern part of suture, and their products no are typical S-type granites. The gneissosity in body and penetrative cleavage in country rock are orogenic structural features of syn-collision. The thermal event takes place in 228 Ma ± is A-type magmatism of post-orogenic period, and their sources belongs to matter of lower crust. According to difference of their sources, the studied A-type granites can be divided into As type and Ai type. The former strongly are contaminated by sediments in continental crust, magmas had undergone fractional crystallisation. The latter strongly undergoes mixing with mantle matter.

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... Three fault zones occur in the Alxa Block from north to south: the Ya-Gan Fault, the Engger Us Ophiolite Belt, and the Qagan Qulu Ophiolite Belt (Wu and He 1993;Wang et al. 1994Wang et al. , 1998aFig. 1b). ...
... The northern margin of the Alxa Block developed an island arc at the end of the early Paleozoic and southward subduction occurred in the late Paleozoic, which was accompanied by intense volcanic activity (Batkhishig 2010;Blight 2010;Li 2006;Wang et al. 1992;Wang et al. 1998a;Wu and He 1992;Wu et al. 1993;Zhang et al. 2013). In the late Paleozoic, the Huhetaoergai arc zone southward subducted under the Zhusileng zone and the Paleo-Asian Ocean, where Ya-Gan structural belt located, gradually closed. ...
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To better constrain the petrogenesis and tectonic setting of the Chaheilingashun pluton in the southern Ya-Gan Fault, North Alxa Block, NW China, this study presents new zircon U–Pb ages, whole-rock geochemistry, and Hf isotopes of diorites from the Chaheilingashun pluton. The Chaheilingashun pluton is composed of medium- to coarse-grained monzonitic diorite, with moderate to high SiO2 contents (58.22–62.77 wt.%), moderate total alkali contents (Na2O + K2O, 5.72–7.66 wt.%), and high Al2O3 contents (14.26–17.69 wt.%), which indicate that the diorite exhibits calc-alkaline to alkaline and metaluminous to peraluminous series. The correlations of SiO2 with oxides and the right-dipping oblique trends of rare earth elements (REEs) demonstrate that the magma experienced fractional crystallization. In particularly, the correlations of Rb with Ba and Sr, and the weak negative Eu anomalies indicate that the plagioclase has undergone fractional crystallization. In addition, this paper has shown that both fractional crystallization and partial melting also contributed to the magmatic process through the Eu anomalies and La/Sm-La diagram. The positive εHf(t) values (2.2–4.6) and low Mg#, Cr, and Ni indicate that diorite was generated from the partial melting of juvenile mafic crust. Together with previous results, the diorites are inferred to have formed in an post-collisional extensional environment. The zircon U–Pb age of diorite from the Chaheilingashun pluton is dated at 289.3 Ma, suggesting that the final closure time of the Paleo-Asian Ocean was might before 289.3 ± 2.3 Ma.
... The study area is located in the middle part of the Alxa block, and is surrounded by the Siberian Plate to the north, the Tarim Plate to the west, and the North China Plate to the south (Xiao & Santosh, 2014; Figure 1). Following Wang et al. (1998), herein the Alxa tectonic unit FIGURE 1 (a) Geological sketch map of the Central Asian Orogenic Belt (modified after Sengör, Natal'in, & Burtman, 1993;Jahn, Wu, & Chen, 2000); (b) Tectonic sketch of the northern Alxa block (Wang, Wang, Wang, & Wu, 1998); (c) Sketch tectonic map of the Bayan Nuru ductile shear zone. Abbreviations: ZSCA, Zongnaishan-Shalazhashan; CHBB, Chaganchulu-Huoersen tectonic zone; YBCA, Yabulai-Bayan Nuru tectonic zone [Colour figure can be viewed at wileyonlinelibrary.com] is divided into two subunits (Figure 1b) with the Engeerwusu ophiolitic mélange belt as the boundary. ...
... The study area is located in the middle part of the Alxa block, and is surrounded by the Siberian Plate to the north, the Tarim Plate to the west, and the North China Plate to the south (Xiao & Santosh, 2014; Figure 1). Following Wang et al. (1998), herein the Alxa tectonic unit FIGURE 1 (a) Geological sketch map of the Central Asian Orogenic Belt (modified after Sengör, Natal'in, & Burtman, 1993;Jahn, Wu, & Chen, 2000); (b) Tectonic sketch of the northern Alxa block (Wang, Wang, Wang, & Wu, 1998); (c) Sketch tectonic map of the Bayan Nuru ductile shear zone. Abbreviations: ZSCA, Zongnaishan-Shalazhashan; CHBB, Chaganchulu-Huoersen tectonic zone; YBCA, Yabulai-Bayan Nuru tectonic zone [Colour figure can be viewed at wileyonlinelibrary.com] is divided into two subunits (Figure 1b) with the Engeerwusu ophiolitic mélange belt as the boundary. ...
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The time of termination of orogenesis for the southern Altaids has been controversial. Systematic investigations of field geology, geochronology and geochemistry on newly discriminated mafic-ultramafic rocks from northern Alxa in the southern Altaids were conducted to address the termination problem. The mafic-ultramafic rocks are located in the Bijiertai, Honggueryulin, and Qinggele areas, stretching from west to east for about 100 km. All rocks occur high-grade gneisses as tectonic lenses that are composed of peridotite, pyroxenite, gabbro, and serpentinite, most of which have undergone pronounced alteration, i.e., serpentinization and chloritization. Geochemically, the rocks are characterized by uniform compositional trends, i.e., with low SiO2-contents (42.51-52.21 wt.%) and alkalinity (Na2O + K2O) (0.01-5.45 wt.%, mostly less than 0.8 wt.%), and enrichments in MgO (7.37-43.36 wt.%), with Mg# = 52.75-91.87. As the rocks have been strongly altered and have a wide range of loss-on-ignition (LOI: 0.44-14.07 wt.%) values, they may have been subjected to considerable alteration by either seawater or metamorphic fluids. The REE and trace element patterns show a relatively fractionated trend with LILE enrichment and HFSE depletion, similar to that of T-MORB between N-MORB and E-MORB, indicating that the parental melt resulted from the partial melting of oceanic lithospheric mantle overprinted by fluid alteration of island-arc origin. The ultramafic rocks are relics derived from the magma after a large degree of partial melting of oceanic lithospheric mantle with superposed island arc processes under the influence of mid-ocean-ridge magmatism. LA-ICP MS U-Pb zircon ages of gabbros from three spots are 274 ± 3 Ma (MSWD = 0.35), 306 ± 3 Ma (MSWD = 0.49), 262 ± 5 Ma (MSWD = 1.2), respectively, representing the formation ages of the mafic-ultramafic rocks. Therefore, considering other previously published data, we suggest that the mafic-ultramafic rocks were products of south-dipping subduction, most probably with a slab window caused by ridge subduction, of the Paleo-Asian Ocean plate beneath the Alxa block in the Late Carboniferous to Late Permian before the Ocean completely closed. This sheds light on the controversial tectonic history of the southern Altaids and supports the concept that the termination of orogenesis was in the end-Permian to Triassic.
Article
The Wuliji pluton in the Northern Alxa Region, Inner Mongolia, is the principal part of Shalazhashan Mountain. It belongs to the Zongnaishan-Shalazhashan Arc Zone, northwestern North China Plate, whose north is Engger Us Ophiolite Belt and south is Qagan Qulu Ophiolite Belt. The pluton was emplaced into Upper Carboniferous-Lower Permian Amushan Formation. According to the research about the original Carboniferous Amushan Formation, the lower and middle sections of the Carboniferous Amushan Formation consist of volcanic, clastic, and carbonate rocks, interpreted to represent the sedimentary association of a volcanic arc and back-arc basin; the upper section of the Amushan Formation is a molasse composed of silty shale, sandstone, gravel-bearing sandstone, and conglomerate. The Wuliji pluton consists mainly of biotite monzonitic granite, amphibole-bearing biotite monzonitic granite, and monzonitic granite. Geochemical analyses show that the pluton has both metaluminous and peraluminous characteristics, and on average has SiO2 > 70 wt%, Al2O3 > 14 wt%, and high contents of Na2O+K2O (8.5 wt%), which define a calc-alkaline series. In addition, REE patterns show enrichment of LREE and weak negative Eu anomalies (δ Eu = 0.3–1). Altogether, the samples are depleted in Nb, Ta, Ti, P, Sr, and Ba, and enriched in Rb, Th, and K. These geochemical traits are interpreted to reflect an arc component. A secondary ion mass spectrometry (SIMS) U-Pb zircon age of the biotite monzonitic Wuliji pluton in the Northern Alxa Region, Inner Mongolia, is 250.8 ± 2.0 Ma (1σ). Samples have ɛ Nd(t) values between −0.1 and 1.3, which suggests that the granites were derived from mixing between the crust and mantle. Based on the SIMS age and geochemical characteristics, Wuliji granite is interpreted to be a post-collisional granite, the result of mantle-derived melt and assimilated juvenile arc crust. However, according to the newest international stratigraphic classification standard, the upper section of the Amushan Formation is Lower Permian in age, indicating that the back-arc basin had already closed in Early Permian. We conclude that the Paleo-Asian Ocean represented by the Engger Us Ophiolite Belt subducted southward in Late Carboniferous, at the same time that the trench-arc-basin system formed in the Northern Alxa Region. The Paleo-Asian Ocean was closed in Early Permian and the Northern Alxa Region entered a post-collisional period in the Late Permian, as indicated by the Wuliji granites. This suggests that the genesis of the Wuliji granites is consistent with the pluton emplacement at the upper crust, which occurred widely in the northern margin of the North China Plate in Late Carboniferous to Triassic.
Article
The Yagan area of the southernmost Sino–Mongolian border is characterized by an extensional structure where a large metamorphic core complex (Yagan–Onch Hayrhan) and voluminous granitoids are exposed. New isotopic age data indicate that the granitoids, which were previously regarded as Paleozoic in age, were emplaced in early and late Mesozoic times. The early Mesozoic granitoids have 228±7 Ma U–Pb zircon age, and consist of linear mylonitic quartz monzonites and biotite monzogranites. Their chemical compositions are similar to those of potassic granites and shoshonitic series, and show an intraplate and post-collisional environment in tectonic discrimination diagrams. Their fabrics reveal that they experienced syn-emplacement extensional deformation. All these characteristics suggest that the adjustment, thinning and extensional deformation at middle to lower crustal levels might have occurred in the early Mesozoic. The late Mesozoic granitoids have a U–Pb zircon age of 135±2 Ma, and are made up of large elliptical granitic plutons. They are high-K calc-alkaline, and were forcefully emplaced in the dome extensional setting. Both the early and late Mesozoic granitoids have εNd (t) values of −2.3 to +5, in strong contrast with the negative εNd (t) values (−11) of the Precambrian host rocks. This suggests that juvenile mantle-derived components were involved in the formation of the granitoids. The similar situation is omnipresent in Central Asia. This study demonstrates that tectonic extension, magmatism and crustal growth are closely related, and that post-collisional and intraplate magmatism was probably a significant process for continental growth in the Phanerozoic.
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