Closure Of Paleo-Asian Ocean Research Articles

Petrogenesis of Permo-Triassic intrusive rocks in Northern Liaoning Province, NE China: implications for the closure of the eastern Paleo-Asian Ocean

ABSTRACT The Changchun-Yanji belt recorded widespread Permo-Triassic magmatism, but their origins remain unclear, inhibiting a comprehensive understanding of the magmatic response to the final closure of the Paleo-Asian ocean in the eastern Central Asian Orogenic Belt (CAOB). Here, we present new geochronological, geochemical, and Hf isotopic data for the Permo-Triassic plutons from Northern Liaoning province, NE China. Combined the published ages with our new data, the Permo-Triassic magmatism in the eastern CAOB can be divided into five episodes: early Permian (293–274 Ma), middle–late Permian (270–257 Ma), latest late Permian–Middle Triassic (255–242 Ma), Late Triassic (240–215 Ma), and latest Late Triassic (209–200 Ma). The middle Permian and Late Triassic mafic plutons (i.e. ~266 Ma Mengjiagou gabbro–diorite, ~240 Ma Jiancaicun gabbro and ~224 Ma Shudetun gabbro-diorite) contain relatively high TFe2O3, MgO, Cr and Ni contents with positive εHf(t) values (+1.2 to +7.2), suggesting a depleted mantle origin. These mafic rocks together with the coeval granitoids make up typical bimodal associations, suggesting that they were formed under an extensional environment. The conclusions are also supported by occurrence of A-type granites during 270–257 Ma and 240–215 Ma. By contrast, the granitoids of 255–242 Ma in the eastern CAOB, including the Jianshanzi (~251 Ma) and Daganhe (~242 Ma) monzogranites, show typical geochemical features of adakitic granites, with high Sr/Y ratios and negative εHf(t) values (–8.6 to – 22.0), suggesting that the magmas were generated through partial melting of thickened ancient lower crust. Combined with previous studies, a four-stage tectonic evolution scenario was proposed: (1) active continental margin stage during 293–274 Ma; (2) continuing subduction resulted in the initiation collision, moderate crustal thickening, and slab break-off during 270–257 Ma; (3) final closure of the Paleo-Asian Ocean associated with continued crustal thickening occurred during 255–242 Ma; (4) lithospheric delamination in a post-collisional extensional environment occurred during 240–215 Ma.

Mineralogical features and petrogenetic significance of the clinopyroxene and hornblende of the Wuhaolai mafic complex in northern North China Craton, Inner Mongolia

The Wuhaolai mafic complex is located in the north margin of the North China Craton (NCC), Inner Mongolia. To discuss the mineralogical features, magma evolution process, and tectonic setting of the complex, we analyzed the geochemical compositions of clinopyroxene and hornblende using an electron probe. The results revealed that the parental magma of this complex belonged to the intraplate alkaline basalt series. The normal zoning texture and the relation between Mg# and FeO, Al2O3, CaO, Na2O, SiO2 and Cr2O3 suggested that the clinopyroxenes of pyroxenite and gabbro crystallized from the same parental magma. The similar CaO content of clinopyroxenes indicated that the parental magma of the Wuhaolai complex may have suffered crustal contamination. Furthermore, the characteristics of hornblende demonstrated that the magma source was modified by fluids derived from subducted slab. Based on the value of Kdcpx (0.23–0.27), the equilibrium melt with clinopyroxene exhibited a relatively low Mg# (43–53), indicating that the parental magma was derived from the lithospheric mantle and underwent crystal fractionation. The gabbro crystallization temperature and pressure was found to be lower than that of pyroxenite, indicating that gabbro was formed at a lower depth than that of pyroxenite. Combining the tectonic setting discrimination diagram of clinopyroxene with the results of previous studies on the late Paleozoic intrusions near the research area, we proposed that the Wuhaolai complex was formed in an intraplate environment. The magma source was modified by fluids derived from the subducted slab during the subduction of the Paleo-Asian Ocean (PAO). After the PAO closure, the parental magma of the Wuhaolai complex was produced by the partial melting of the enriched lithospheric mantle

The Late Permian highly fractionated I-type granites from Sishijia pluton in southestern Inner Mongolia, North China: A post-collisional magmatism record and its implication for the closure of Paleo-Asian Ocean

Whole-rock major and trace elements, zircon UPb geochronology and Hf isotopic data have been implemented for the felsic rocks from the Sishijia pluton in southeastern Inner Mongolia, North China, in order to determine their petrogenesis and futher constrain the termination of Paleoasian Oceanic subduction in southeastern Central Asian Orogenic Belt. Petrographic observation indicates that Sishijia granitoids mainly consist of syenogranites and porphyritic granites, with the major mineral compositions of alkaline feldspar, quartz, plagioclase and biotite, without any alkaline melanocratic or peraluminous minerals. Miarolitic structures are widely developed in these granitoids, indicating for the characteristics of shallow emplacement depth and rapid condensation. Zircon LA-ICP-MS UPb dating yields weighted mean ages of 256.2 ± 1.4 and 250.9 ± 1.2 Ma for syenogranites, and 251.6 ± 1.4 Ma for porphyritic granites, representing a Late Permian magmatism record. These granitoids exhibit weakly peraluminous nature (A/CNK = 1.03–1.08) with high SiO2 (76.50–81.92 wt%) and K2O (3.64–5.56 wt%) concentrations and differentiation index values (D.I. = 93.8–97.2). Chondrite-normalized REE patterns of these samples display enrichment of LREE with high ratios of (La/Yb)N (12.29–30.34) and significant negative Eu anomalies (δEu = 0.15–0.53). In mantle-normalized multielement variation diagrams, they are all characterized by typical depletions of Ba, Nb, Ta, Sr, P, and Ti and enrichments of Rb, Th, U, and K. Petrological and geochemical features of the pluton indicate a highly fractionated I-type affinity. Zircon Hf isotope analyses show variably negative εHf(t) values ranging from −13.7 to −4.0 and corresponding two-stage model ages varying of 2148–1540 Ma, indicating that they were derived from an Paleo- to Mesoproterozoic crustal source with minor juvenile materials. In combination with previous geological evidences, it is reasonable to propose that the Sishijia granitoids emplaced in a post-collisional extension regime during the transformation from pre-collision to post-orogenic, which symbolize a Late Permian final closure of Paleo-Asian Ocean in southeastern segment of the Central Asian Orogenic Belt.

Unconformity-bounded Upper Paleozoic megasequences in the Beishan Region (NW China) and implications for the timing of the Paleo-Asian Ocean closure

The Upper Paleozoic sedimentary sequence in the Beishan Region, northwest China, provides a complete record of the complex amalgamation and extension history of the Central Asian Orogenic Belt (CAOB) for this part of China. Integrating newly obtained stratigraphic, paleontological and radiometric data, this paper reviews the Upper Paleozoic stratigraphic sequences of the Beishan Region. The Upper Paleozoic strata here are divided into three megasequences (MS). They are, in ascending order: Devonian, Mississippian–Guadalupian and Lopingian megasequences (MS1, MS2, and MS3), separated by two major unconformities each indicative of a huge sedimentary hiatus. MS1 is made up of continental and epicontinental marine deposits in the pre-rift stage; MS2 of transgressive marine and volcanic strata in the syn-rift stage; and MS3 of continental sediments in the inversion stage. The major unconformity between MS1 and MS2 is interpreted to represent the regional uplift and erosion caused by the final amalgamation of CAOB in the Beishan Region. The syn-rift MS2 represented by transgressive marine and volcanic strata in the Beishan Region can be well-correlated with Pennsylvanian–Guadalupian megasequences in Inner Mongolia, where they rest on a major interregional unconformity. This correlation suggests the closure of the Paleo-Asian Ocean in Beishan and Inner Mongolia before the Carboniferous, followed by the development of a synchronous marine rift megasequence in the southern part of the CAOB.

Mesoproterozoic and Paleozoic hydrothermal metasomatism in the giant Bayan Obo REE-Nb-Fe deposit: Constrains from trace elements and Sr-Nd isotope of fluorite and preliminary thermodynamic calculation

Fluorite of various occurrences could portray the hydrothermal metasomatism occurred in the giant Bayan Obo REE-Nb-Fe ore deposit. Two Mesoproterozoic and Paleozoic hydrothermal events with distinguished fluid origin and geochemical properties have been studied. In Mesoproterozoic, discrete fluorite from the dolomite-dominating REE-Nb ores and banded fluorite aggregates from the banded REE-Nb-Fe ores are extremely enriched in LREE and depleted in HFSE compared with the primitive mantle. In Paleozoic, fluorite from veined-breccia REE-Nb ores varies significantly in trace element components, ranging from LREE-enriched HFSE-depleted fluorite to fluorite with “flat” REE pattern and extremely depletion of Zr. The veined fluorites have variable and higher radiogenic Sr isotopic components (initial 87Sr/86Sr ratios: 0.703464–0.708208) than the discrete and banded fluorite (0.703183–0.705047), indicating crustal contamination in the Paleozoic hydrothermal fluids from Precambrian basement rocks and Bayan Obo Group. Fluorites of different occurrences and associated bastnäsite and monazite have similar Nd isotopic components and Nd evolution history, which is mostly consistent with whole rock of the ore-hosting dolomite. The Sm-Nd system of some fluorite and bastnäsite were reset during the Paleozoic alteration triggered by the closure of Paleo-Asian Ocean. In such geological setting, according to the δ34S of the dominating sulfur-bearing mineral, Paleozoic veined sulfides (∼0 ± 3‰), the prime sulfur source was oceanic crust/MORB. Relatively high δ34S of barite (∼13–14‰) was caused by fractionation of sulfur isotopes at an equilibrium temperature of 400–430 °C, rather than contribution from ancient seawater. The trace element, Sr-Nd isotope of fluorite and sulfur isotope characters of sulfides indicate carbonatitic origin of the Mesoproterozoic hydrothermal fluids, while the Paleozoic fluids, derived from subducted Paleo-Asian oceanic crust, carried sulfur components from MORB, scavenged high radiogenic Sr components from Precambrian basement rocks or Bayan Obo group, may leached a deep-seated carbonatite stock and finally altered the Mesoproterozoic mineralized ore-hosting dolomite.Both the Mesoproterozoic and Paleozoic hydrothermal fluids had initial temperature of ∼400–500 °C, and gangue minerals (fluorite, sulfide and sulfate) precipitated at the temperature of ∼320–440 °C. Preliminary thermodynamic calculations of the REE-F-C-Ca system constrained that the pressure of both fluids would not exceed 2 kbar. The drop of temperature and increase of pH are two critical factors to REE mineralization. Paragenesis of Fe-bearing minerals indicates an increase of fO2 in the Mesoproterozoic hydrothermal fluids and a drop of fS2 in the Paleozoic fluids in the Fe-O-S system. Furthermore, in the Ca-F-C-O system, the Mesoproterozoic hydrothermal fluids evolved with increasing aCa2+ and decreasing aF−, while the Paleozoic fluids present characteristic of mixing origin again with significantly variable aCa2+ and aF−.

Transformation from Paleo-Asian Ocean closure to Paleo-Pacific subduction: New constraints from granitoids in the eastern Jilin–Heilongjiang Belt, NE China

The eastern Jilin–Heilongjiang Belt (EJHB) of NE China is a unique orogen that underwent two stages of evolution within the tectonic regimes of the Paleo-Asian and Paleo-Pacific oceans. 158 available zircon U–Pb ages, including 26 ages obtained during the present study and 132 ages from the literature, were compiled and analyzed for the Mesozoic and Cenozoic granitoids from the EJHB and the adjacent Russian Sikhote–Alin Orogenic Belt (SAOB), to examine the temporal–spatial distribution of the granitoids and to constrain the tectonic evolution of the East Asian continental margin. Five stages of granitic magmatism can be identified: Early Triassic (251–240Ma), Late Triassic (228–215Ma), latest Triassic to Middle Jurassic (213–158Ma), Early Cretaceous (131–105Ma), and Late Cretaceous to Paleocene (95–56Ma). The Early Triassic granitoids are restricted to the Yanbian region along the Changchun–Yanji Suture, and show geochemical characteristics of magmas from a thickened lower crust source, probably due to the final collision of the combined NE China blocks with the North China Craton. The Late Triassic granitoids, with features of A-type granites, represent post-collisional magmatic activities that were related to post-orogenic extension, marking the end of the tectonic evolution of the Paleo-Asian Ocean. The latest Triassic to Paleocene granitoids with calc-alkaline characteristics were NE-trending emplaced along the EJHB and SAOB and young towards the coastal region, and represent continental marginal arc magmas that were associated with the northwestwards subduction of the Paleo-Pacific Plate. Two periods of magmatic quiescence (158–131 and 105–95Ma) correspond to changes in the subduction direction of the Paleo-Pacific Plate from oblique relative to the continental margin to subparallel. Taking all this into account, we conclude that: (1) the final closure of the Paleo-Asian Ocean occurred along the Changchun–Yanji Suture during the Early Triassic; (2) the onset of the subduction of the Paleo-Pacific Plate beneath the East Asian continental margin probably took place during the latest Triassic (ca. 215Ma); (3) changes in the drifting direction of the Paleo-Pacific Plate were responsible for the intermittent magmatic activities; and (4) roll-back of the subducted plate resulted in the oceanwards migration of the magmatic arc and large-scale back-arc extension throughout NE China during the Early Cretaceous.

A review of the Paleozoic tectonics in the eastern part of Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is the largest accretionary orogen in the world, which is responsible for considerable Phanerozoic juvenile crustal growth. The NE China and its adjacent areas compose the eastern segment of the CAOB, which is a key area for providing important evidence of the CAOB evolution and understanding the NE Asian tectonics. The eastern segment of the CAOB is composed tectonically of four micro-blocks and four sutures, i.e. Erguna block (EB), Xing'an block (XB), Songliao-Xilinhot block (SXB), Jiamusi block (JB), Xinlin-Xiguitu suture (XXS), Heihe-Hegenshan suture (HHS), Mudanjiang-Yilan suture (MYS) and Solonker-Xar Moron-Changchun-Yanji suture (SXCYS). The EB and XB were amalgamated by westward subduction, oceanic island accretions and final collision in ca. 500Ma. The XB and SXB were amalgamated by subduction-related Early Paleozoic marginal arc, Late Paleozoic marginal arc and final collision in the late Early Carboniferous to early Late Carboniferous. The JB probably had been attached to the SXB in the Early Paleozoic, but broken apart from the SXB in the Triassic and collided back in the Jurassic. The closure of Paleo-Asian Ocean had experienced a long continue/episodic subduction–accretion processes on margins of the NCC to the south and the SXB to the north from the Early to Late Paleozoic. The final closure happened along the SXCYS, from west Solonker, Sonid Youqi, Kedanshan (Keshenketengqi), Xar Moron River through Songliao Basin via Kailu, Tongliao, Horqin Zuoyizhongqi, Changchun, to the east Panshi, Huadian, Dunhua, Yanji, with a scissors style closure in time from the Late Permian-Early Triassic in the west to the Late Permian-Middle Triassic in the east. The amalgamated blocks should compose a united micro-continent, named as Jiamusi-Mongolia Block (JMB) after Early Carboniferous, which bounded by Mongo-Okhotsk suture to the northwest, Solonker-Xar Moron-Changchun suture to the south and the eastern margin of JB to the east.

Mantle upwelling during Permian to Triassic in the northern margin of the North China Craton: Constraints from southern Inner Mongolia

The nothern margin of the North China Craton (NCC) preseves the tectonic imprints of early Paleozoic to Triassic tectonic processes. Here we investigate the geochronology, geochemistry, zircon hafnium and whole rock Nd isotope data on a suite of magmatic rocks from the Ningcheng pyroxenite–gabbro–diorite complex and the Jiwangyingzi Formation in the Inner Mongolia region of the northern NCC. The zircon U–Pb dating identifies multiple stages of mafic to intermediate magmatism from Early Permian (298±4Ma) to Middle Triassic (219±1Ma) in this area and their hafnium isotopes display diverse mantle sources. An integration of field observations, geochronology and elemental and isotopic geochemistry indicates a complex petrogenetic history for the magmatic suite. The Ningcheng pyroxenite–gabbro–diorite complex is a product of multiple pulses of magmas with crystal fractionation, hybridization and assimilation, with the involvement of melts derived from lithospheric mantle and even the asthenosphere. Our data reveal an increasing involvement of mantle component from Late Permian to Triassic. The features of progressive mantle upwelling correlate with tectonic processes associated with the closure of Paleo-Asian Ocean and subsequent post collisional extension, resulting in active magmatism from Carboniferous to Triassic in this region.

The Jiguanshan porphyry Mo deposit in the Xilamulun metallogenic belt, northern margin of the North China Craton, U–Pb geochronology, isotope systematics, geochemistry and fluid inclusion studies: Implications for a genetic model

The Jiguanshan porphyry Mo deposit is located in the southern part of Xilamulun metallogenic belt at the northern margin of the North China Craton (NCC). In the Jiguanshan mining district, two stages of granitoids intrusions have been recognized: a pre-ore granite porphyry with stockworks and veins of Mo mineralization, and a granite porphyry with disseminated Mo mineralization. Zircon U–Pb data and Hf isotope analyses show that the dissemination-mineralized granite porphyry yielded a weighted mean 206Pb/238U age of 156.0±1.3Ma, with a crustal εHf(t) values from −5.6 to +0.2, and that the main group of magmatic zircons from the pre-ore granite porphyry have a weighted mean 206Pb/238U age of 167.7±1.7Ma with εHf(t) values from −3.2 to +1.0. Combined with groundmass Ar–Ar age data of the granite porphyry and molybdenite Re–Os age, it is suggested that the Mo mineralization of Jiguanshan deposit was formed in the late Jurassic (153~155Ma) during tectonic and magmatic events that affected northeast China. The Mo mineralization was a little bit later than the host granite porphyry. Besides disseminated in the host granite porphyry, Mo mineralization also presents in middle Jurassic pre-ore granite porphyry, Jurassic fine-grained diabase, Triassic quartz porphyry, and in rhyolitic volcanic rocks as well as syenite of Devonian age.The Jiguanshan mining district was affected by the tectonic events associated with the Paleo-Asian Ocean closure, and later by far-field tectonism, related to subduction of the Paleo-Pacific plate (Izanagi) in the Jurassic-Cretaceous. The tectonic and thermal events linked with the latter are commonly referred to as Yanshanian tectono-thermal event, and consists of a series of geodynamic, magmatic and ore-forming processes, which in the mining district area included the intrusion of the pre-ore granite porphyry, the host granite porphyry, Mo mineralization, and fine-grained diabase. Major and trace element analyses show that the host granite porphyry is characterized by high silica abundances (SiO2=77.16 to 77.51%), high Rb/Sr ratios (13.57 to 14.83), high oxidation (Fe2O3/FeO=34.25 to 62.00) and high alkalies (Na2O+K2O=8.21 to 8.38%). Petrographic and microthermometry studies of the fluid inclusions from Mo mineralized veins, characterized by plenty of daughter mineral-bearing inclusions, showed that the predominant homogenization temperatures range from 250 to 440°C. Combined with Laser Raman analysis of the fluid inclusions, it is indicated that Mo mineralization is related to a high-temperature, hypersaline and high-oxygen fugacity H2O–NaCl fluid system, with high F contents.Based on geology, geochronology, isotope systematics, geochemistry and fluid inclusion studies as well as regional geology, we propose, for the first time, a genetic model for the Jiguanshan porphyry Mo deposit. During the Jurassic geodynamic evolution of northeast China, high silicic, high oxidized and alkaline-rich granitic magma probably derived from partial melting of the lower crust, episodically intruded along faults into the country rocks. This fluid system, fractionating from the highly differentiated granitic magma and bearing Mo with minor Cu metals, migrated upwards and interacted with the older wall rocks and associated fractures, in which the ore minerals precipitated, resulting in the development of what we refer to as the “Jiguanshan-type” porphyry Mo deposit.

Early Paleozoic (ca. 465 Ma) eclogites from Beishan (NW China) and their bearing on the tectonic evolution of the southern Central Asian Orogenic Belt

The Beishan orogen is situated in the southern margin of the Central Asian Orogenic Belt (CAOB). It is connected with the Tianshan orogen to the west and the Mongolia-Xing’anling orogen to the east. This orogen exhibits a well-preserved Neoproterozoic to Late Paleozoic sequence containing ophiolites, accretionary complexes and eclogites. In this paper we provide new geochronological data for eclogites and their country paragneiss to constrain the tectonic evolution of the Beishan orogen. U–Pb zircon dating of eclogites yielded two concordant ages, 886 ± 4 Ma for the cores and 465 ± 10 Ma for the rims. The zircon cores display bright luminescence with homogeneous or planar/sector zoning, intermediate Th/U ratios (0.17–0.47), and HREE-enriched patterns with negative Eu anomalies (Eu/Eu * = 0.16–0.58). These features, coupled with their formation temperatures of 680–710 °C, suggest that the core age represents the time of an earlier upper amphibolite facies metamorphic episode. By contrast, the zircon rims have dark luminescence with homogeneous or fir-tree zoning, low Th/U ratios (<0.02), and flat HREE distributions with small or no negative Eu anomalies (Eu/Eu * = 0.55–1.16). Thus, the rim age reflects the time of the eclogite facies metamorphism. The occurrence of tiny inclusions of garnet, omphacite and rutile in the zircon rims is consistent with the above age interpretation. In addition, the mineral inclusions and host zircons yielded P–T conditions of 660 °C and >12 kbar for the eclogite facies metamorphism. On the other hand, zircon dating of a paragneiss near the eclogite produced two major groups of ages, ca. 1450 Ma for a magmatic event and ca. 900 Ma for a metamorphic event. It seems that eclogites and their country rocks have suffered the same earlier metamorphic episode. Based on the available age data obtained for eclogites, ophiolites and granitoids from the Beishan area, a tectonic scenario can be constructed as follows. The ca. 1000 Ma oceanic crust representing by protoliths of the eclogites was involved in Early Neoproterozoic orogenesis to have formed a microcontinental block. During the Early Paleozoic, island arcs and microcontinental blocks were progressively accreted to the northern margin of the Tarim craton (Dunhuang block). This was followed by the closure of Paleoasian Ocean and final amalgamation between the Tarim craton and Kazakhstan block in the Late Paleozoic.