Ondor Sum Research Articles

Heavy mineral indicators of ridge subduction beneath the Bainaimiao arc in the southeastern Central Asian Orogenic Belt

As one of the major subduction-accretionary systems in the southeastern Central Asian Orogenic Belt, the Bainaimiao arc presents a valuable opportunity for investigating the subduction–accretion process of accretionary orogens. It is located adjacent to the northern margin of the North China Craton and developed arc-related deposits during the Silurian–Devonian period. In this paper, we employed heavy mineral geochemistry and U–Pb ages of zircon and rutile, to determine the provenance of the Silurian–Devonian deposits and reconstruct the paleogeography of the source area. The stratigraphic units involved include the Xuniwusu Formation of Early Silurian and the Xibiehe Formation of Early Devonian.Both the Xuniwusu and Xibiehe formations exhibit heavy mineral assemblages dominated by zircons and tourmalines. Nevertheless, the Xibiehe Formation contains Cr-spinels and garnets which are absent in the Xuniwusu Formation. Geochemical analyses of the detrital Cr-spinels exhibit characteristics of a supra-subduction zone ophiolite. The garnets originated from middle- to high-grade metamorphic rocks, as well as mafic and felsic rocks. The tourmalines in both formations primarily indicate a metasedimentary rock source, with a small percentage of Li-poor granitoid species in the Xuniwusu Formation. The U–Pb age spectra of both detrital zircon and rutile in the Xuniwusu Formation show multiple peaks. The age spectra consist of Precambrian ages, indicative of a recycled origin, as well as Paleozoic ages, suggesting a magmatic origin. In contrast, the U–Pb age spectra of zircon and rutile in the Xibiehe Formation display a unimodal age peak at ca. 455 and 490 Ma, respectively. These differences suggest a provenance shift from the Xuniwusu to Xibiehe formations. The Xuniwusu Formation was sourced from the Bainaimiao arc itself, whereas the Xibiehe Formation has a mixed origin of the Bainaimiao arc and Ondor Sum accretionary complex. The provenance change correlates with the unconformity between the two formations, suggesting a forearc uplift event. In combination with the previous research on the Tulinkai ophiolite, it can be inferred that the forearc uplift was triggered by ridge subduction of the Paleo-Asian Ocean plate at ca. 450–430 Ma. This research provided a new perspective on the ridge subduction of the oceanic slab in fossil orogens.

Formation of the Permo-Triassic Ondor Sum thrust system and its constraint on Solonker Ocean closure

ABSTRACT A previously unidentified overthrust system has recently been discovered in the Ondor Sum area at the boundary between the Solonker – Xar Moron Suture Zone (SXSZ) and the Early to Mid – Paleozoic Subduction – accretion Belt to the south. The overthrusting occurred during the middle Permian and was overprinted by late Permian to Middle Triassic dextral shearing. The overthrusting (D1) was characterized by the development of WSW – ENE-trending folds and reverse faults, implying NNW – SSE shortening. The later deformation (D2) involved WSW – ENE dextral strike-slip shearing with a minor reverse component, as indicated by slightly plunging striae on slickensides. A comparison of geochronological and structural data for ophiolites in the SXSZ suggests that the overthrust system formed in response to the closure of the Solonker Ocean and the collision along the SXSZ. The earlier NNW – SSE shortening and later WSW – ENE dextral shearing indicate that a tectonic transformation took place along the SXSZ from the middle Permian to the Late Triassic. Dextral and sinistral strike-slip faults are, respectively, distributed to the south and northwest of the Songliao – Xilinhot Block to the north of the SXSZ, implying eastward extrusion of the Songliao – Xilinhot Block after the closure of Solonker Ocean. The tectonic transformation and extrusion might have been a part of the intracontinental tectonic transformation of the eastern segment of the southern Central Asian Orogenic Belt.

Two Phases of Metamorphism in the High-Pressure Schists in Central Inner Mongolia, China: Implications for the Tectonic Transition From Terminal Subduction of the Paleo-Asian Ocean to Continental Collision

High-pressure (HP) rocks exhumed from subduction zones usually record much warmer geotherms than numerical modelling results, as their peak mineral assemblages are always modified during the exhumation process. The decompressional metamorphic evolution of HP rocks should be considered carefully if using their P–T records to constrain the thermal structure of a subduction zone. The Ondor Sum Group, known as mélanges containing various high P/T metamorphic rocks in central Inner Mongolia, represents a fragment of subducted oceanic crust in the Early Paleozoic, However, the thermal structure of the subduction zone is unavailable due to an absence of exhumed HP rocks with P–T estimates. In this study, the HP schists were newly discovered in the Ondor Sum Group at Airgin Sum in the central Inner Mongolia. The HP schists include a garnet–phengite schist (sample EL01) and a garnet–amphibole schist (sample EL08). The petrography of these rocks and phase equilibrium modelling using THERMOCALC suggest two phases of metamorphism, both of which are characterized by clockwise P–T evolutions involving pre-peak prograde stage, peak-stage, and post-peak decompression stage. The peak P–T conditions of the early-phase metamorphism are constrained by P–T pseudosections to be ∼18 kbar/∼535°C for sample EL01 and 18 kbar/∼500°C for sample EL08. The peak P–T conditions of the late-phase metamorphism are constrained by P–T pseudosections to be ∼8 kbar/532°C in EL01 and ∼7.0 kbar/495°C in EL08. Available U–Pb data of zircons from the HP schists and a granodiorite vein using LA–ICP–MS constrain the timing of early-phase HP metamorphism in the early Paleozoic and the late-phase metamorphism supposed to be in the Devonian based on the previous reported ages. The peak P–T conditions for the early-phase metamorphism were high-P/T conditions with a thermal gradient of ∼8°C/km, pointing to a warm oceanic subduction. The overprinting late-phase metamorphism represents medium-P/T conditions with a geothermal gradient of 22–25°C/km, which we attribute to a collisional thickening process. As a result, we suggest that the HP schists in the Ondor Sum Group represent the terminal stage of subduction of the Paleo-Asian oceanic plate, and that the schists were involved in the continental collision after the closure of the Paleo-Asian Ocean in the Devonian.

Permo-Carboniferous provenance shifts at the northern margin of the North China Craton and their tectonic implications: Detrital zircon U–Pb–Hf records from central Inner Mongolia

Permo-Carboniferous provenance shifts at the northern margin of the North China Craton (NCC) were reconstructed. The reconstruction was based on the detrital zircon U–Pb–Hf data from Late Paleozoic sandstones from Ondor Sum and Wude in the Southern Orogenic Belt (SOB) of the Southeast Central Asian Orogenic Belt (SE CAOB). Sandstone samples from the Lower Devonian Naqing Formation, Upper Carboniferous Hailasiamu Formation, and the first and second members of the Upper Carboniferous Amushan Formation in Ondor Sum present a nearly unimodal detrital zircon age grouping (c. 380–520 Ma), and they have similar zircon Hf isotopic compositions with igneous rocks in the SOB, implying that their provenance was from the SOB region. In contrast, samples in the same area from the third to fifth members of the Lower Permian Amushan Formation and the Lower Permian Sanmianjing Formation contain multimodal age groupings, with the ages mainly concentrated at the Late Carboniferous–Early Permian, Neoarchean, and Paleoproterozoic. The coexistence of Neoarchean and Paleoproterozoic grains suggests that the NCC could be a reliable source. In addition, the Hf isotopic composition of the Late Carboniferous–Early Permian zircons overlap with those from the NCC, and therefore, it is concluded that there were Permo-Carboniferous provenance shifts from the SOB to the NCC for the sandstones from the Upper Paleozoic in the Ondor Sum area. Late Carboniferous and Early Devonian sandstone samples from Wude had sediment contributions from both the SOB and the NCC, with detrital zircon grains from the SOB constituting the majority. Whereas, the Early Permian samples from this area contained sediment contributions predominantly from the NCC. We suggest that the Permo-Carboniferous provenance shifts were the result of significant local subsidence of the SOB, and the uplift of the Inner Mongolia Paleo-Uplift after the Early Devonian collision between the Bainaimiao arc belt and the NCC.

Ore genesis and tectonic setting of the Bieluwutu Cu-Pb-Zn volcanogenic massive sulfide deposit in Xing’an-Mongolia orogenic belt, China

The Bieluwutu Cu-Pb-Zn deposit is located in the Ondor Sum region of the western Xing’an-Mongolia orogenic belt, North China. The host rocks of the deposit consist of metasandstone, tuffaceous slate, andesitic tuff and fragmented felsic volcanic rocks, which have previously been regarded as the Early Carboniferous Amushan Group. Zircon U-Pb ages for two tuffaceous units are 435 ± 2 Ma and 430 ± 2 Ma, showing that volcanism occurred in the Early Silurian. The tuffaceous rocks are enriched in large ion lithophile elements (LILE; e.g., Rb, Ba, K) and light REE, and depleted in high field strength elements (HFSE; e.g., Nb, Ta and Ti) with depleted isotopic components (εNd(t) = 0.14–0.84), indicating that they were derived from partial melting of lower continental crust with the involvement of mafic magma.Bieluwutu is intended to be a Kuroko-type VHMS deposit formed in an interarc rift basin in the Early Silurian. Three types of ores have been identified at Bieluwutu, including massive Pb-Zn, brecciated Cu-Zn and disseminated Cu ores. Results of sulfide laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses indicate that Se and Sn are enriched in chalcopyrite from the disseminated and brecciated ores with high Se/Te ratios. Tellurium, Co, Pb and Bi are enriched in chalcopyrite, sphalerite and pyrite from the brecciated ores, whereas Mn, Ni and Tl concentrations are higher in pyrite from the massive ores. These differences suggest a decreasing temperature and increasing f(O2) trend from the disseminated to brecciated and massive ores. High contents of Mn, Tl, Ni in massive ores are typical for metals derived from surrounding felsic volcanic rocks. Strong enrichment of trace elements, such as Bi, Te, Se and Sn, is consistent with a magmatic contribution to the ore-forming fluids. The underlying subvolcanic intrusion not only served as a heat engine but also supplied volatiles and metals.

Tectono-sedimentary analysis of the Lower Cretaceous succession in the sags of the Ondor Sum uplift, Southern Erlian Basin, NE China

The tectonics of continental rift basins and the accompanying deposition processes are areas of increasing research interest. The Ondor Sum uplift is located in the southern Erlian Basin, NE China. It is a promising site for petroleum exploration. In this study, we used 2D and 3D seismic, well-log and core data to conduct a tectono-sedimentary analysis of the sags of the Ondor Sum uplift.Three types of depositional systems were identified, including fan-delta, braided-delta and lake depositional systems. Of these, the fan-delta and lake deposits are highly developed, while the distribution of braided-delta deposits is limited. The rift sags in the area consist of a SE-faulting and NW-overlapping type, a NW-faulting and SE-overlapping single-faulted dustpan-like rift sag and a double-faulted rift sag. The tectonic evolution models of the area were of two types: an inheritably-developed type, applicable to the single-faulted dustpan-like rift sag and double-faulted rift sag, and a structurally migrated type.The early-to medium-term-activated as well as the long-term-activated faults controlled both the sediment filling of the Wulanhua and Chazhong sags and the distribution of their sand bodies. Both differential and balanced uplifting during the Late Yanshanian period determined the stratigraphic development and configuration of the sags. Given the similarities the sags of the Ondor Sum uplift shares with the petroliferous sags of the inner Erlian Basin, combining with the petroleum discovery in the Wulanhua and the Chazhong sags and relatively shallow burial depth of the target layers, the sags in this area are likely to contain hydrocarbons and should thus be explored for their resource potential.

The Zhangjiatun igneous complex in the southeastern margin of the Central Asian Orogenic Belt, NE China: Evidence for an Early Paleozoic intra-oceanic arc

The subduction-accretionary evolution of the southeastern margin of the Central Asian Orogenic Belt (CAOB), consisting of the northern North China Craton, Bainaimiao arc, and Ondor Sum accretionary complex from south to north, is still not well understood. In order to clarify the Early Paleozoic subduction process in the southeastern margin of the CAOB, we conducted geochronological, geochemical, and Sr-Nd-Hf isotope analyses of the Zhangjiatun igneous complex rocks. The Zhangjiatun complex is composed of hornblende gabbro, quartz diorite, and tonalite. Zircon U-Pb dating of the complex yields ages ranging from 486 Ma to 440 Ma. We interpret this result as indicating Early Paleozoic arc magmatism along the southeastern margin of the CAOB. Late Cambrian to Early Ordovician mafic-intermediate intrusions have low-K tholeiitic compositions characterized by low (La/Yb)N (0.4–1.2) and initial 87Sr/86Sr (0.70439–0.70447) ratios, positive εNd(t) (+6.5 to +7.6), and εHf(t) (+13.5 to +19.0) values. The geochemical values indicate the rocks to be of N-MORB affinities, likely generated by melt differentiation from a depleted mantle. An Early Silurian tonalite exhibits oceanic plagiogranite signatures, expressed by high Na2O, and low K2O, Rb, and total REE contents. The tonalite also yields high (La/Yb)N values, low Sr/Y and initial 87Sr/86Sr (0.70345) ratios, and positive εNd(t) (+3.3) and εHf(t) (+8.0 to +12.5) values. Geochemical modelling suggests that the tonalite may have originated from hydrous melting of hornblende gabbro with a minor contribution from melting of oceanic sediments. The Late Cambrian to Early Ordovician mafic-intermediate intrusions with MORB characteristics likely resulted from initial intra-oceanic subduction along the southern margin of the Paleo-Asian Ocean. We interpret the Early Silurian tonalite to have been generated as the arc became fully developed. We further suggest that the southeastern margin of the CAOB has involved two subduction systems during the Early Paleozoic: a northern south-dipping intra-oceanic arc and a southern north-dipping continental arc.

Detrital Zircon U‐Pb Ages and Geochemistry of the Silurian to Permian Sedimentary Rocks in Central Inner Mongolia, China: Implications for Closure of the Paleo‐Asian Ocean

The Solonker suture zone has long been considered to mark the location of the final disappearance of the Paleo‐Asian Ocean in the eastern Central Asian Orogenic Belt (CAOB). However, the time of final suturing is still controversial with two main different proposals of late Permian to early Triassic, and late Devonian. This study reports integrated whole‐rock geochemistry and LA‐ICP‐MS zircon U‐Pb ages of sedimentary rocks from the Silurian Xuniwusu Formation, the Devonian Xilingol Complex and the Permian Zhesi Formation in the Hegenshan‐Xilinhot‐Linxi area in central Inner Mongolia, China. The depositional environment, provenance and tectonic setting of the Silurian‐Devonian and the Permian sediments are compared to constrain the tectonic evolution of the Solonker suture zone and its neighboring zones. The protoliths of the silty slates from the Xuniwusu Formation in the Baolidao zone belong to wacke and were derived from felsic igneous rocks with steady‐state weathering, poor sorting and compositional immaturity. The protoliths of metasedimentary rocks from the Xilingol Complex were wackes and litharenites and were sourced from predominantly felsic igneous rocks with variable weathering conditions and moderate sorting. The Xuniwusu Formation and Xilingol Complex samples both have two groups of detrital zircon that peak at ca. 0.9–1.0 Ga and ca. 420–440 Ma, with maximum deposition ages of late Silurian and middle Devonian age, respectively. Considering the ca. 484–383 Ma volcanic arc in the Baolidao zone, the Xuxiniwu Formation represents an oceanic trench sediment and is covered by the sedimentary rocks in the Xilingol Complex that represents a continental slope sediment in front of the arc. The middle Permian Zhesi Formation metasandstones were derived from predominantly felsic igneous rocks and are texturally immature with very low degrees of rounding and sorting, indicating short transport and rapid burial. The Zhesi Formation in the Hegenshan zone has a main zircon age peak of 302 Ma and a subordinate peak of 423 Ma and was deposited in a back‐arc basin with an early marine transgression during extension and a late marine regression during contraction. The formation also crops out locally in the Baolidao zone with a main zircon age peak of 467 Ma and a minor peak of 359 Ma, and suggests it formed as a marine transgression sedimentary sequence in a restricted extensional basin and followed by a marine regressive event. Two obvious zircon age peaks of 444 Ma and 280 Ma in the Solonker zone and 435 Ma and 274 Ma in Ondor Sum are retrieved from the Zhesi Formation. This suggests as a result of the gradual closure of the Paleo‐Asian Ocean a narrow ocean sedimentary environment with marine regressive sedimentary sequences occupied the Solonker and Ondor Sum zones during the middle Permian. A restricted ocean is suggested by the Permian strata in the Bainaimiao zone. Early Paleozoic subduction until ca. 381 Ma and renewed subduction during ca. 310–254 Ma accompanied by the opening and closure of a back‐arc basin during ca. 298–269 Ma occurred in the northern accretionary zone. In contrast, the southern accretionary zone documented early Paleozoic subduction until ca. 400 Ma and a renewed subduction during ca. 298–246 Ma. The final closure of the Paleo‐Asian ocean therefore lasted at least until the early Triassic and ended with the formation of the Solonker suture zone.

Multiple metamorphic events recorded in the metamorphic terranes in central Inner Mongolia, Northern China: Implication for the tectonic evolution of the Xing’an-Inner Mongolia Orogenic Belt

The tectonic evolution of the Xing’an-Inner Mongolia Orogenic Belt (XIMOB) in the southeastern segment of the Central Asian Orogenic Belt is not well understood. It is arguable whether the Paleo-Asian Ocean was closed in Devonian or Early Mesozoic and whether the tectonic regimes during Carboniferous and Permian were operated by continuous subduction-arc systems or extension after the orogenic process. Comprehensive studies of metamorphism for four metamorphic terranes including the Ondor Sum Group, Baoyintu Group, Xilingol Complex, and early Triassic metamorphic sequences in central Inner Mongolia provide good constraints on the arguable issues about the XIMOB. According to the zircon U-Pb dating analyses in this study, quartz schists of the Ondor Sum Group in the northern and southern orogenic belts (NOB and SOB) have a broad detrital zircon age range from late Archean to early Paleozoic with a similar youngest deposition age of 434–430 Ma, but have different zircon age populations with a ∼650 Ma peak for the NOB but not for the SOB. Biotite gneisses of the Xilingol Complex have age populations and the youngest deposition age of 436 Ma similar to the Ondor Sum Group in the NOB. Amphibolites in the complex have a metamorphic zircon age of 321 ± 3 Ma and hornblende 40Ar/39Ar age of 309 ± 2 Ma. The early Triassic metamorphic sequences, metabasites in Wulangou and mica-schists in Shuangjing both have broad zircon age ranges from the late Archean to late Paleozoic. Their protoliths are constrained to have formed later than 258 Ma and 261 Ma respectively and the metamorphism occurred soon afterwards in the Early Triassic constrained by an actinolite 40Ar-39Ar age of 241 ± 19 Ma in metabasite and a muscovite 40Ar-39Ar age of 240 ± 2 Ma in mica-schist. Combined with the results previously published, this study identified four phases of metamorphism in central Inner Mongolia since Paleozoic. The Silurian (430–426 Ma) high P/T metamorphism of the Ondor Sum Group was related to bidirectional subduction processes. The Devonian (∼400 Ma) medium P/T metamorphism of the Baoyintu Group suggests a crustal thickening orogeny. The Carboniferous (337–321 Ma) low P/T metamorphism of the Xilingol Complex is indicative of an extension process of a previous orogen. The Early Triassic (∼240 Ma) extensive medium-low P/T metamorphism along the Solonker suture zone was attributed to the closure of limited ocean basins. Thus, an alternative tectonic scenario for the metamorphic terranes in central Inner Mongolia involves: (i) the early Paleozoic trench–arc system (500–425 Ma); (ii) the Devonian collisional orogeny due to closure of the Paleo-Asian-Ocean (400–360 Ma); (iii) the Carboniferous and Permian extension after the collision (350–250 Ma), and (iv) the Early Triassic within-plate orogeny due to the closure of limited ocean basins (∼240 Ma).

Geochronological data of igneous and metamorphic rocks from the Xing’an-Mongolia Orogenic Belt of the eastern Central Asian Orogenic Belt: implications for the final closure of the Paleo-Asian Ocean

In order to better constrain the evolution of the Xing’an-Mongolia Orogenic Belt and the resulting closure of the Paleo-Asian Ocean, we conducted an integrated investigation involving U-Pb dating of igneous and detrital zircon and a synthesis of existing work across the Erdaojing and Ondor Sum complexes in the eastern segment of the Central Asian Orogenic Belt. Geochronological analysis revealed a different tectonic setting between the Ondor Sum complex (Tulinkai area) and the Erdaojing accretion complex, as detrital zircon U-Pb analysis identified different major age provenances: (1) the Precambrian basement of the North China Craton and Paleozoic Bainaimiao Arc along the northern margin of North China to the south are the provenance rocks of the schists in the Tulinkai area, and (2) the Precambrian basement of the Xilinhot complex and Paleozoic Baolidao arc to the north are the provenance rocks of the schists in the Erdaojing area. We infer that the existence of the ocean between the above two regions in the late of Early Paleozoic in accordance with the result of this study. Furthermore, a comparison of the youngest age population in the intruded-diorite rock and metasedimentary rocks with U-Pb ages implies that the final closure of the Paleo- Asian Ocean along the Solonker Suture Zone most likely occurred in the period between 239 and 222 Ma (Late Paleozoic-Triassic).

Age of the Ondor Sum Group in western Inner Mongolia and its position in the Central Asia Orogenic Belt.

Age of the Ondor Sum Group in western Inner Mongolia and its position in the Central Asia Orogenic Belt.

Permian magmatic sequences of the Bilihe gold deposit in central Inner Mongolia, China: Petrogenesis and tectonic significance

The Bilihe gold deposit is located in the eastern section of the Ondor Sum–Yanji Suture at the southern margin of the Xing'an–Mongolian Orogenic Belt (XMOB) and the northern margin of the North China Craton (NCC), central Inner Mongolia. The magmatic rocks in the ore district are generally high-K calc-alkaline, enriched in LREE, Zr, and Hf, and depleted in HREE, Nb, Ta, and P. The magmatic evolution sequences are norite gabbro→granodiorite porphyry→granite or norite gabbro→andesite→dacite porphyry→granodiorite, which show a trend of decreasing TiO2, FeO, MgO, CaO, and P2O5 with increasing SiO2. In the Bilihe ore district, hydrothermal processes were coeval with granitic magmatism for a period of ~17Myr (272–255Ma). The ages of the granite, granodiorite porphyry, granodiorite, and dacite porphyry are 271.5–264.1Ma, 269.8–255.8Ma, 268.3Ma, and 268.6–259.4Ma, respectively. The magmatic rocks contain magmatic, hydrothermal, and magmatic–hydrothermal zircons. The magmatic zircons have δCe>4, La<3ppm, and SmN/LaN>2.5; the hydrothermal zircons have δCe<4, La>3ppm, and SmN/LaN<2.5. The Nb/Ta and Zr/Hf ratios of granodiorite are 12.7–14.99 and 40.2–46.56, respectively. The Zr/Hf ratios successively increase in the sequence of granite (27.4–29.02)→granodiorite porphyry (29.19–32.18)→dacite porphyry (33.54–38.5)→norite gabbro (36.75–38.37), and their Nb/Ta ratios are 9.09–12.38. Zircons in granodiorite yield ε Hf (t) values of −0.29 to −56 (n=13) and 2.07–7.62 (n=5), and they give a Hf two-stage model age (tDM2) of 807–4765Ma. The ε Hf (t) values of the zircons in granite, granodiorite porphyry, and dacite porphyry are −0.46 to 8.03, 3.17 to 10.32, and −0.78 to 6.58, respectively, and their Hf tDM2 ages are 787–1324Ma, 638–1091Ma, and 868–1343Ma, respectively. Dehydration partial melting of subducted oceanic crust resulted in the formation of dacite porphyry; partial melting of depleted mantle resulted in the formation of norite gabbro; mixing of depleted mantle and lower crust resulted in the formation of granodiorite porphyry; partial melting of lower crust resulted in the formation of granite; and mixing of lower crust and old upper crust resulted in the formation of granodiorite. Magmatic rocks in the ore district with ages of 272–255Ma were formed during the late stages of closure of the Paleoasian Ocean; i.e., during the transformation from a collisional to extensional setting.

Geological Features and Ore Genesis of the Ondor Sum Deposit

Acta Geologica Sinica - English EditionVolume 88, Issue s2 p. 1081-1082 Meeting Abstracts Geological Features and Ore Genesis of the Ondor Sum Deposit Kerui WU, Corresponding Author Kerui WU Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 ChinaCorresponding author. E-mail: nfjj@mx.cei.gov.cnSearch for more papers by this authorFengjun NIE, Fengjun NIE Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this author Kerui WU, Corresponding Author Kerui WU Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 ChinaCorresponding author. E-mail: nfjj@mx.cei.gov.cnSearch for more papers by this authorFengjun NIE, Fengjun NIE Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this author First published: 29 December 2014 https://doi.org/10.1111/1755-6724.12379_10Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article.Citing Literature Volume88, Issues2Special Issue: Meeting Abstracts: The 14th Quadrennial International Association on the Genesis of Ore Deposits Symposium. August 19–22, 2014, Kunming, ChinaDecember 2014Pages 1081-1082 RelatedInformation

The pre-Devonian tectonic framework of Xing’an–Mongolia orogenic belt (XMOB) in north China

A new tectonic division of the Xing’an–Mongolia orogenic belt (XMOB) in north China has been presented according to our research and a lot of new data of tectonics, geochronology and geochemistry. Four blocks and four sutures have been recognized in the XMOB, including the Erguna (EB), Xing’an–Airgin Sum (XAB), Songliao–Hunshandake (SHB), and Jiamusi (JB), and Xinlin–Xiguitu (XXS), Xilinhot–Heihe (XHS), Mudanjiang (MS) and Ondor Sum–Yongji sutures (OYS). The framework of the XMOB is characterized by a tectonic collage of the blocks and orogenic belts between them. Different Precambrian basements have been found in the blocks, including the Neoproterozoic metamorphic rocks and plutons in the EB, the Neoproterozoic metamorphic rocks in western and eastern of segments of the XAB, Mesoproterozoic and Neoproterozoic metamorphic rocks in middle segments of the XAB, respectively, the Neoproterozoic metamorphic rocks and Mesoproterozoic volcanic rocks and plutons in the SHB, and Neoproterozoic metamorphic rocks in the JB. The XXS resulted from a northwestward subduction of the XAB beneath the EB during the Cambrian, which was followed by the forming of the XHS and OYS in the northwest and south margins of the SHB in the Silurian, respectively. The MS was caused by a westward subduction of the JB beneath the east margin of the SHB during the middle Devonian. The three Cambrian, Silurian and middle Devonian events indicate that the XMOB belongs to a pre-middle Devonian multiple orogenic belt in the Central Asian Orogenic Belt (CAOB). Forming of the XMOB suggests that the southeast part of the Paleo Asian Ocean closed before the middle Devonian.

Blueschist metamorphism and its tectonic implication of Late Paleozoic–Early Mesozoic metabasites in the mélange zones, central Inner Mongolia, China

Blueschists in central Inner Mongolia are distributed as layers and blocks in mélanges including the southern zone in Ondor Sum area and the northern zone in Manghete and Naomuhunni areas. They have been attributed to the subduction of Early Paleozoic oceanic crust. Blueschists from Ondor Sum and Naomuhunni are characterized by occurrence of sodic amphibole coexisting with epidote, albite, chlorite, calcic amphibole (in Ondor Sum) and muscovite (in Naomuhunni). Blueschists in Manghete contain porphyroblastic albite with inclusions of garnet and epidote in a matrix dominated by calcic–sodic amphibole, epidote, chlorite, albite and muscovite. Phase equilibria modeling for three blueschist samples using pseudosection suggest that the AlM2 contents in sodic amphibole can be used as a good barometer in the limited assemblage involving sodic amphibole+actinolite+epidote+chlorite+albite+quartz under pressures <4–6kbar, while this barometer is largely influenced by temperature and bulk Fe2O3 contents in the actinolite-absent assemblage sodic amphibole+epidote+chlorite+albite+quartz of higher pressure and the AlM2 contents are not pressure-controlled in the albite-absent assemblage sodic amphibole+epidote+chlorite+quartz under pressures>7–10 kbar. In the sodic amphibole-bearing assemblages, the NaM4 contents in sodic amphibole mainly decrease as temperature rises, being a potential thermometry. The calculated pseudosections constrain the P–T conditions of blueschists to be 3.2–4.2kbar/355–415°C in Ondor Sum, 8.2–9.0kbar/455°C–495°C in Manghete and 6.6–8.1kbar/420–470°C in Naomuhunni. These P–T estimates indicate a rather high geothermal gradient of 18–25°C/km for the blueschist metamorphism, being of intermediate P/T facies series. Available zircon U–Pb age data suggests that the protoliths of blueschists were formed later than Late Paleozoic–Early Mesozoic and metamorphosed soon afterwards. An alternative interpretation for the tectonic implication of blueschists in central Inner Mongolia is that they may be a new type attributed to closure of limited ocean basins and do not represent a tectonic regime occurred in conventional subduction setting.

Early Paleozoic to Middle Triassic bivergent accretion in the Central Asian Orogenic Belt: insights from zircon U-Pb dating of ductile shear zones in central Inner Mongolia, China

The Solonker suture zone records the termination of convergence in the southern Central Asian Orogenic Belt (CAOB). The timing of termination of the Solonker suture and the tectonic style of the southern CAOB have long been controversial. The Baolidao arc–accretion complex and the Ondor Sum subduction–accretion complex are located immediately to the north and south of the Solonker suture zone respectively. However, the tectonic settings and formation ages of these two complexes are debated. In this paper, we present a systematic study of LA-ICPMS U-Pb zircon ages of multiple units and lithologies from both the complexes to constrain the timing of final collision between the South Mongolian microcontinent and the North China craton. The geochronological results indicate that the northward subduction of the Paleoasian Oceanic lithosphere beneath the South Mongolian microcontinent at ~ 452 Ma induced extensive melting of overriding continental crust. Continuous subduction led to accretionary wedge-continent collision at the southern margin of the South Mongolian microcontinent, represented by the 416 ± 3 Ma granitic activity in the southern margin of the Baolidao arc–accretion complex. After Early Paleozoic collision, widespread magmatism occurred at 316–302 Ma in response to transient extension along the southern margin of the South Mongolian microcontinent. There was a deep marine basin during Permian times to the south of the microcontinent. The closure of this oceanic basin led to Late Paleozoic–Early Triassic north-dipping subduction and final suturing of the CAOB during Late Permian to Middle Triassic times. Proximal forearc sediments composed of arc volcanic and plutonic materials with crystallization ages varying from 299 ± 3 Ma to 267 ± 3 Ma, and some intrusions at 274 ± 2 Ma caused by the melting of continental crust were produced along the northern margin of the Solonker suture zone. Accretion and collision occurred at ~ 263 Ma. The southward subduction of the Paleoasian Oceanic lithosphere beneath the North China craton at ~ 451 Ma induced high-grade metamorphism and partial melting in the south-dipping subduction zone. An arc/forearc-related ocean at the northern margin of the North China craton existed during the southward subduction (324-300 Ma) and was filled during transient regional extension (299-290 Ma) in Linxi area to form the protolith of the Shuangjing Schist, accompanied with widespread Late Carboniferous to Early Permian arc volcanic and plutonic activity in the CAOB. Initial subduction of the arc/forearc-related oceanic basin occurred during 290–280 Ma, and was followed by the convergence of the North China craton and the South Mongolian microcontinent until ~ 230 Ma. The eastern section of the CAOB formed by bivergent Early Paleozoic to Middle Triassic accretion until ~ 263–230 Ma that ended in the closure of the Paleoasian Ocean.

Triassic sedimentation and postaccretionary crustal evolution along the Solonker suture zone in Inner Mongolia, China

Detrital zircon U-Pb dating of the Xingfuzhilu Formation in southern Inner Mongolia yields a maximum depositional age of around 220 Ma. The predominantly Permian and Triassic zircons are characterized by oscillatory zoning and euhedral shapes, with mostly positive zircon eHf(t) values (+2.0 to +16.4), indicating that they were derived from a proximal magmatic source. Early-Middle Paleozoic zircons have variable zircon eHf(t) values from −6.2 to +11.2 and are characterized by weak oscillatory zoning and subhedral-subrounded shapes, suggesting that the sources are a proximal magmatic arc, possibly mixed with components of the Ondor Sum magmatic arc and the magmatic arc at the northern margin of the North China Craton. The remnants of Precambrian blocks in the southeastern Central Asian Orogenic Belt (CAOB), and the North China Craton may also have been a minor source region for the Xingfuzhilu succession. These results, combined with regional data, indicate that a closing remnant ocean basin or narrow seaway possibly existed in the Middle Permian (Guadalupian) immediately prior to final collision of the CAOB and closure of the Paleo-Asian Ocean. Subsequent collision resulted in the crustal uplift and thickening along the Solonker suture zone, accompanied by possible slab break-off and lithospheric delamination during the Latest Permian to Middle Triassic. The resultant orogen in the Late Triassic underwent exhumation and denudation of rocks in response to the postorogenic collapse and regional extension. Vertical crustal growth in the Triassic is documented by detrital zircons from the Xingfuzhilu Formation and appears to have been widespread across entire eastern CAOB.

A new interpretation of the tectonic setting and age of meta-basic volcanics in the Ondor Sum Group, Inner Mongolia

The Ondor Sum Group in the central Inner Mongolia is mainly composed of meta-basic volcanics intercalated with ferruginous quartzite and quartz schist, and has been interpreted as slices of oceanic crust or an ophiolite suite of the Early Paleozoic or much older ages. This paper presents new LA-ICP-MS zircon U-Pb dating and geochemical data for the meta-basic volcanics. The results show that zircons in the meta-basic volcanics were derived from complicated sources, most of which may be captured by basic magma from the country rocks or other sources. They yield a large age range from the Late Archean to Early Mesozoic with the youngest age group between 246 and 261 Ma, constraining the protolith of the meta-basic volcanics formed in the Late Permian to Early Triassic. The meta-basic volcanics have an affinity to E-MORB in geochemistry, and also a similarity toward OIB, representing a tectonic setting of limited intra-continental ocean basin. This limited basin might have been related to the continuous extension of the area since the Early Permian and finally closed in the Early Mesozoic.

Crustal structure of the northern margin of the North China Craton and adjacent region from SinoProbe02 North China seismic WAR/R experiment

Abstract The SinoProbe02 North China seismic experiment consisted of three seismic recording efforts along a 450 km long profile. The profile extended from northwest of Beijing, across the northern margin of the North China Craton (NCC), and across the southeastern portion of the Central Asian Orogenic Belt (CAOB) to the Solonker suture zone. We processed, analyzed, and modeled the data collected during a wide-angle reflection and refraction effort. The recording of seismic waves from 8 explosions was conducted in 4 deployments of 300 Texan recorders with station spacings of 1–1.5 km. The P-wave field provided good quality data for most of the profile. Arrivals of refracted and reflected waves from sediments and basement (Pg), intracrustal phases (PcP, PlP), and the Moho (PmP, Pn) were typically observed. Travel time modeling was done layer-by-layer using the top to bottom approach and was constrained by a coincident deep reflection seismic profile. The initial velocity model was altered by trial and error, and the forward model was updated by damped least-squares inversion. In our modeling, calculated travel times fit observed arrivals for all 1439 traces with an average RMS error of 0.113 s. The final 2-D P-wave velocity model contains many features of tectonic significance: (1) the velocity structure varies significantly from the NCC to the CAOB; (2) thicker crust appears beneath the Yinshan–Yanshan belt and was probably generated by compression in the early Jurassic and modified during the craton's destabilization and extension; the flat and relatively shallow Moho in the CAOB may be attributed to the extension; (3) strong velocity variations beneath the Bainaimiao arc and Ondor Sum subduction accretion complex indicate that multiple pulses of magmatism occurred during the complex tectonic evolution of this area.

Structural and kinematic analysis of the Early Paleozoic Ondor Sum-Hongqi mélange belt, eastern part of the Altaids (CAOB) in Inner Mongolia, China

We present a structural and kinematic study of an Early Paleozoic subduction mélange and a magmatic arc that form the main elements of the Southern Orogen Belt of Inner Mongolia, which lies in the eastearn part of the Altaids or Central Asia Orogenic Belt. The structural analysis of the mélange conducted in the Hongqi and Ondor Sum areas (western Inner Mongolia) shows two phrases of ductile deformation. The D1 event is responsible for the pervasive S1 foliation, NW–SE striking L1 stretching lineation and F1 intrafolial folds. These microstructures are coeval with a greenschist facies metamorphism. During D2, NW-verging F2 folds associated with a S2 axial planar cleavage deformed S1 and L1. The D1 kinematic criteria indicate a top-to-the-NW sense of shear. D1 and D2 developed before the unconformable deposition of the Early Devonian shallow water sandstone. A lithosphere scale geodynamic model involving an Early Paleozoic southeast-directed subduction beneath the North China Craton and late Silurian collision of the North China Craton with an hypothetical microcontinent is proposed to account for the microstructural evolution.