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The Solonker-Xar Moron-Changchun-Yanji Suture (SXCYS) is regarded as a tectonic boundary between the Central Asian Orogenic Belt and the North China Craton. Formed during the closure of Paleo-Asian Ocean and modified by the subsequent magmatic activities, this suture might have recorded complex processes of accretionary orogeny and crustal recycling. Thus, seismic reflection profiling was conducted along a length of 160 km from Naiman to Ar Horqin Banner, Inner Mongolia, and the lithospheric structures of central SXCYS have been examined. A prominent feature is a group of strong reflections from the middle crust to the topmost mantle beneath the center of the profile, which consists of massive south dipping reflections and a few middle crustal reflections with a opposite vergence. This group of strong reflections is interpreted to be the subduction relict formed during the final closure of the Paleo-Asian Ocean. The estimated lateral extent of the suture zone is ca. 100 km across strike, and its subduction polarity is southward with an apparent dip angle of 15 to 20°. The upper crust along the whole profile is characterized by weak reflectivity, which also extends to the middle crust. This absence of reflective structures may be attributed to the magmatic activities during or after the continental convergence. Two strong reflection clusters extending from the bottom of upper crust to the lower crust are also identified, which may be caused by distinct magmatic intrusions. Overprinted on the dipping reflections, the Moho is identified at a depth of 33 to 36 km, which is inferred to be formed at an extensional background contemporary with the subsurface Mesozoic sediments and extensional faults. A detailed model of the evolution processes of the central SXCYS is proposed, which includes the oceanic subduction, the final collision, and the modification by subsequent magmatic activities. • Lithospheric structures of the central SXCYS are obtained by deep seismic profiling. • The location and polarity of a Palerzoic subduction relict within northern HHS is confirmed. • Syn-collision and post-collision magmatic activities are recognized on seismic profile. • A detailed model of the formation of central SXCYS is proposed. [ABSTRACT FROM AUTHOR]

Three large-scale thrusts developed in the Daqingshan orogen, Inner Mongolia, with a northeast-east trending from Louhuashan to Suletu are described in this paper. These northwest-directed thrusts, together with the four tectonic belts (or tectonic sheets) separated by these faults, formed a typical thrusting system. The originally defined Hohhot metamorphic core complex may be a tectonic sheet, making the uppermost tectonic unit of the thrusting system and truncated by the Daqingshan detachment fault on top. The existing tectono-chronological studies showed that the thrusting of this region possibly happened at the same time as the extension along the top detachment fault. This presents that the contraction and extension in Daqingshan orogen were probably simultaneous. The structural pattern of the Daqingshan orogen can be correlated to the Himalayan orogen, and models for simultaneous thrusting and detachment in Himalaya may give a hint for explaining the development of the Daqingshan orogen. [ABSTRACT FROM AUTHOR]

In the Datan area, located 60 km northeast to Hohhot, an undeformed plutonic complex has intruded into the Panyangshan thrust system on the northern margin of the North China Craton (NCC). The plutonic complex consists of the predominant earlier Datan pluton and the scattered later Hanqinggou plutons. The Datan pluton has relatively high ε Hf (t) (−7.4 to 0.2), low (87Sr/86Sr) i (0.704280 to 0.706508), and negative ε Nd (t) (−14.9 to −10.6), and the core phase with the lowest SiO 2 content has features of C-type Adakite-like granite (high Sr/Y, (La/Yb) N , and Sr; low Y and Yb; no Eu anomaly; and HREE depletion). This suggests that the Datan pluton was probably derived from the metamorphic mafic rocks of the thickened continental lower crust that was heated and injected with juvenile mantle-derived melt. The Hanqinggou plutons which have similar geochemical features but are more depleted in Ba, Ti, and P may represent the residual phase of the Datan pluton. Crystal fractionation predominated by feldspar, biotite, apatite, zircon, titanite, ilmenite, apatite, allanite, titanite, and zircon took place during the magmatic evolution. The dating results suggested that the zircon U Pb ages for the undeformed Datan pluton intruding into the Panyangshan thrust system ranged from 252 ± 1 Ma to 248 ± 1 Ma. Additionally, the youngest detrital zircon U Pb age of the autochthon recorded in the literature is 274 ± 4 Ma. These indicate that the Panyangshan thrust system representing the crustal thickening occurred from ca. 274 Ma to ca. 252 Ma and may be due to the subduction of the Paleo-Asian Ocean and the collision between the Mongolian arcs and the NCC. This contraction was followed by asthenospheric upwelling and initial extension represented by Datan complex, indicating a significant tectonic conversion in ca. 250 Ma. • Tectonic conversion on the northern margin of the NCC took place in ca. 250 Ma. • The Panyangshan thrust reflects syn-collision regime in Middle-Late Permian • The Datan plutonic complex indicates the beginning of post-collision stage. [ABSTRACT FROM AUTHOR]

• The Zhaojinggou Nb-Ta deposit was formed by the high evolution of peraluminous magma in the Early Cretaceous. • A Nb-Ta-riched brine-melt phase segregated from the crystal mush in the latest stage of crystallization. • Fluorine played a key role in the magma differentiation and mineralization. The large Zhaojinggou Nb-Ta deposit is located in the northern margin of the North China Craton (NCC), Inner Mongolia, China. The deposit is mainly hosted in the mineralized dikes around the Hutoushan pluton and the residual pegmatite shell at the top of the pluton. Samples from the pluton and the mineralized dikes are characterized by high contents of SiO 2 (73.3%–84.3%), total alkalis (5.24%–9.60%) and Al 2 O 3 (7.94%–13.8%), and moderate A/CNK values (with an average of 1.04) indicating a weakly peraluminous affinity. The trace element patterns are characterized by Rb, Nb, Ta, Zr, and Hf enrichment and markedly negative Ba, Sr, P, Eu, and Ti anomalies. Zircon U-Pb dating suggests that the emplacement of the pluton and the Nb-Ta mineralization occurred coevally in the Early Cretaceous (126 ± 1 Ma and 127 ± 2 Ma, respectively). Variations in both bulk-rock geochemistry (K/Rb, Zr/Hf, Nb/Ta) and mineral composition (mica and columbite) consistently unveil a magmatic evolutionary trend from the Hutoushan alkali-feldspar granite, through the Hutoushan pegmatite shell, to the mineralized dikes. Typical A-type characteristics of the granite and ancient Hf isotopic features [εHf(t) ranging from −6.80 to −12.95, and crust model ages from 1.61 to 2.00 Ga] indicate that the parental magma was derived from the partial melting of Palaeoproterozoic crust with insignificant mantle material involved. Considering the significantly higher bulk contents of Nb, Ta, and F in the pegmatite shell and the mineralized dikes than that in the alkali-feldspar granite, as well as the similar composition in the biotite from the pegmatite shell and the alkali-feldspar granite, we propose that a residual Nb-Ta-enriched brine-melt phase segregated from the crystal mush at the late stage of crystallization. The progressive concentration of F in the melt was conducive to the solubility of Nb and Ta in granitic melts and decreased the viscosity by depolymerizing the melt, and thus contributed to the high magma differentiation and Nb-Ta concentration. The post-magmatic fluid triggered the fluid-rock interaction, resulting in the extensive albitization, M-type "tetrad effect", extremely low Nb/Ta ratio (<5), and further mineralization in the mineralized dikes. [ABSTRACT FROM AUTHOR]