Elsevier

Lithos

Volumes 376–377, 1 December 2020, 105795
Lithos

Research Article
Middle Paleozoic intermediate-mafic rocks of the Tsoroidog Uul’ accretionary complex, Central Mongolia: Petrogenesis and tectonic implications

https://doi.org/10.1016/j.lithos.2020.105795Get rights and content

Highlights

  • Four groups of Middle Paleozoic intermediate-mafic rocks were identified.

  • They are classified into MORB, OIB, CAB and VAT, respectively.

  • The intermediate-mafic rocks were generated from heterogenous mantle sources.

  • The accretionary complex was formed by subduction of oceanic lithosphere.

Abstract

The Tsoroidog Uul’ accretionary complex is hosted by the Tsetserleg terrane in the southwestern Khangay-Khentey orogenic system (Central Mongolia), which represents the segment of the Central Asian Orogenic Belt and has significant regional implications for its tectonic evolution. This paper reports the results of field investigations and petrography, bulk-rock major and trace element geochemical, as well as Smsingle bondNd isotopes of Middle Paleozoic intermediate-mafic rocks from the Tsoroidog Uul’ accretionary complex. We investigate a wide range of rock types which can be divided into 4 groups on the basis of their TiO2 and REE characteristics. Group 1 is characterized by moderate TiO2, relatively flat chondrite-normalized REE patterns (La/Smn = 1.0; Gd/Ybn = 1.2). These basalts are divided into two subgroups: (1) Nb/Thpm = 3.6, Nb/Lapm = 0.8, Zr/Nb = 24.6, and Ce/Ybpm = 0.8 (N-MORB type); (2) Nb/Thpm = 1.3, Nb/Lapm = 1.1, Zr/Nb = 11.9, and Ce/Ybpm = 1.6 (E-MORB type). Group 2 shows high TiO2 and LREE (La/Smn = 3.0), differentiated HREE (Gd/Ybn = 2.5), positive Nb anomalies shown in primitive mantle-normalized multi-element patterns (Nb/Thpm = 1.2; Nb/Lapm = 1.1), and low Zr/Nbav. ratios (~6). Group 3 displays low TiO2, high LREE (La/Smn = 3.6), Zr/Nbav. = 24.4, and low Nb (Nb/Lapm = 0.2). Group 4 exhibits moderate TiO2, flat REE patterns (La/Smn = 0.8; Gd/Ybn = 1.1), negative Nb anomalies (Nb/Thpm = 0.3; Nb/Lapm = 0.6) and Zr/Nbav. = 33. The εNd(t) values are positive for Group 1 and Group 2, but negative for Group 3. Based on their petrological and geochemical features, we suggest that the Group 1 and 2 mafic volcanic rocks were formed in an oceanic environment, and represent mid-oceanic ridge basalt (MORB) and oceanic-island basalt (OIB), respectively. In contrast, Group 3 intermediate dikes probably have supra-subduction origin with calc-alkaline features, whereas Group 4 represents arc tholeiite basalt including remnants of the continental volcanic arc. Overall, the Middle Paleozoic intermediate-mafic rocks of the Tsoroidog Uul’ accretionary complex were probably generated from heterogenous mantle sources. Thus, we propose that spatial and temporal changes of the Paleo-Pacific Oceanic lithosphere, which subducted under the continental margin of the Siberian Craton, resulted in the variable composition of the intermediate-mafic rocks of this complex. The accretionary complex of the Tsetserleg terrane, which extends into Ulaanbaatar terrane, was formed by subduction of the Paleo-Pacific Oceanic lithosphere or Mongol-Okhotsk Ocean.

Introduction

The Central Asian Orogenic Belt (CAOB) (Fig. 1a) is one of the largest orogenic systems on Earth and extends from the Ural Mountain to the Pacific Ocean and from Siberian craton to the Tarim and Sino-Korean Cratons (Zorin, 1999; Badarch et al., 2002; Khain et al., 2002; Jahn, 2004; Xiao et al., 2004; Windley et al., 2007).This orogenic belt was formed by the accretion of island arcs, ophiolites, oceanic islands, seamounts, accretionary wedges, oceanic plateaus and microcontinents. However, the tectonic evolution of the CAOB remains debated with three different models including: (1) strike-slip duplication and oroclinal bending of a giant magmatic arc (Şengör et al., 1993); (2) successive accretion of oceanic and continental terranes to the Siberian craton (Badarch et al., 2002; Windley et al., 2007), and (3) two stages of evolution involving the Pacific-type accretion during the Devonian-Carboniferous, followed by the Tethysian-type oroclinal bending and collisional shortening during the Permian to Jurassic (Lehmann et al., 2010; Schulmann and Paterson, 2011). Nonetheless, recent studies regarding the stratigraphy, petrology, structural geology, geochemistry, and geochronology of diverse units from the CAOB have greatly developed our understanding of the processes that control its global tectonic and continental growth (e.g., Jahn et al., 2000a, Jahn et al., 2000b, Jahn, 2004; Xiao et al., 2003, Xiao et al., 2004; Kovalenko et al., 2004; Windley et al., 2007; Yarmolyuk et al., 2008; Safonova et al., 2009; Wilhem et al., 2012; Yarmolyuk et al., 2012; Kovach et al., 2013; Safonova and Santosh, 2014).

The Mongolia region (Fig. 1a) occupies a large part of the CAOB and has been divided into two main domains known as Northern Mongolian and Southern Mongolian (Badarch et al., 2002; Tomurtogoo, 2003, Tomurtogoo, 2012). The Southern Mongolian domain is located within the northern orogenic belt of the North China and Tarim Cratons, which is dominated by Neoproterozoic to Paleozoic sedimentary rocks, arc-related volcanic and volcaniclastic rocks with ophiolite fragments. These basement rocks are covered by Middle Paleozoic carbonate rocks and a variety of post-Paleozoic volcanic and sedimentary rocks (Badarch et al., 2002; Tomurtogoo, 2003). In contrast, the Northern Mongolian domain belongs to the southern orogenic belt of Siberian craton, which consists of Archean–Proterozoic cratonic blocks, Neoproterozoic to Lower Paleozoic metamorphic rocks and ophiolites, and Paleozoic volcanic and sedimentary rocks (Badarch et al., 2002). They are widely intruded by Paleozoic to Mesozoic granitic rocks. The Khangay-Khentey orogenic system (also known as Khangai-Khentei and Khangai-Khantey in some literatures) is located in the central part of the Northern Domain (Fig. 1b; Tomurtogoo, 2005, Tomurtogoo, 2012). This orogenic system consists of the Khangay and Khentey mountain ranges, and is about 300 km wide and 1200 km long (Tomurtogoo, 2003). Several different tectonic settings have been proposed to explain the formation of the Khangay-Khentey orogenic system, which include: (1) miogeosyncline basin corresponding to a Devonian–Carboniferous thick turbidite basin probably deposited on a hidden Archean–Neoproterozoic basement (Badarch et al., 2002; Badarch, 2005), (2) oceanic turbidite terranes (Tomurtogoo, 2003), and (3) accretionary complex (Şengör and Natal'in, 1996; Zorin, 1999; Kurihara et al., 2009; Safonova et al., 2009; Hara et al., 2013; Tomurtogoo, 2005, Tomurtogoo, 2012; Tsukada et al., 2013; Safonova and Santosh, 2014). The Khangay-Khentey orogenic system comprises diverse terranes such as the Zag-Kharaa turbidite terrane and the Asraltkhayrkhan, Kharkhorin, Tsetserleg, Ulaanbaatar, and Onon accretionary terranes (Tomurtogoo, 2005, Tomurtogoo, 2012). Based on detrital zircon geochronological and micropaleontological studies of the Paleozoic sedimentary rocks from the Tsetserleg and Ulaanbaatar terranes, numerous researchers have identified their provenance and depositional age, and proposed a geodynamic model for the Khangay-Khentey orogenic system (Kelty et al., 2008; Kurihara et al., 2009; Bussien et al., 2011; Purevjav and Roser, 2012, Purevjav and Roser, 2013; Suzuki et al., 2012; Takeuchi et al., 2012; Hara et al., 2013; Erdenechimeg et al., 2018). However, the tectonic evolution of this orogenic system is still unresolved.

The Tsetserleg terrane is located in the southwestern part of the Khangay-Khentey orogenic system (Fig. 1b), which mainly consists of Devonian and Carboniferous siliceous sedimentary rocks (Purevjav and Roser, 2012) and oceanic volcanic rocks (Erdenesaikhan et al., 2013; Tsukada et al., 2013) (Fig. 1c). Previous studies were mainly focused on the geochemistry and geochronology of the Paleozoic sedimentary rocks from the Tsoroidog Uul’ accretionary complex (TUAC), which is considered to be the most important complex of the Tsetserleg terrane (Oyunchimeg et al., 2018). In contrast, the geochemical compositions of the oceanic volcanic rocks from this complex have been briefly reported in the literature (Oyunchimeg et al., 2017). Since basalts that originate from oceanic environments are extremely important for understanding the reconstruction of different geodynamic settings (Safonova et al., 2008, Safonova et al., 2011a, Safonova et al., 2011b, Safonova et al., 2012; Saccani and Principi, 2016, Saccani et al., 2018; Safonova et al., 2020), more research should be addressed on such rock types of the TUAC. Thus, we present the first detailed petrographic, geochemical and Smsingle bondNd isotopic studies of the intermediate-mafic rocks from the Tsetserleg terrane. All these data are used to constrain the petrogenesis, mantle sources, and tectonic settings of the studied rocks. These new data further allow us to unravel the tectonic evolution of the Khangay-Khentey orogenic system.

Section snippets

Geological background

The TUAC is located in the southeastern part of the Tsetserleg terrane (Fig. 1b), which has a length of 30 km and a width of 25 km. The Tsetserleg terrane is bordered by the Galuut fault along the southern side with the Zag terrane and the Baidrag uplift, the Kharkhorin fault along the northeastern side with the Kharkhorin terrane, and the Mid Mongolian Tectonic fault along the southeastern side with the Southern Mongolian domain (Badarch et al., 2002; Bussien et al., 2011; Kelty et al., 2008;

Sampling strategy

Over 80 volcanic and subvolcanic rock samples (mainly mafic volcanic and intermediate-mafic subvolcanic rocks (Fig. 6a–h), were collected from the chert-basalt sequences of the Erdentsogt Formation around the Mount Oyut, Mount Tsoroidog, and Tuya village. The sample descriptions are summarized in Table 1 and their lithostratigraphic positions are shown in Fig. 2b.

Petrographic analysis and sample preparation for geochemical studies were performed at the Institute of Geology, Mongolian Academy of

Petrography

Most of the studied rock samples were affected by secondary alteration as shown by replacement of primary minerals. For instance, plagioclase is rarely replaced by albite, whereas clinopyroxene is occasionally pseudomorphosed either by chlorite or actinolitic amphibole. Nevertheless, it should be noted that primary igneous textures in these rocks are well preserved. The volcanic rocks are generally composed of aphyric, porphyritic, olivine basalts and weakly metamorphosed basalts, whereas the

Petrogenesis and mantle sources

Overall, the TUAC incorporates a wide range of intermediate-mafic rock types as shown in the previous sections. The geochemical characteristics of these rocks can be used for determining the nature and petrogenesis of the magmatic events which occurred in the Tsetserleg terrane from the Khangay-Khentey orogenic system. In this study, we discuss the petrogenesis of volcanic and subvolcanic rocks based on the less mobile incompatible elements and stable isotopic values of these rocks. Some

Conclusions

1. Four groups of intermediate-mafic rocks from the Middle Silurian to Upper Devonian Erdentsogt Formation in the Tsoroidog Uul’ accretionary complex were identified. They are associated with OPS sediments such as dark-brownish metacherts, brown-reddish cherts, limestones, whitish-gray siliceous siltstones/shales and turbidite clastic rocks.

2. Group 1 oceanic basalts formed at a mid-oceanic ridge setting with a N-MORB and E-MORB signatures. Group 2 basaltic rocks formed in a plume-related

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This investigation is a result of a basic research project named “Geodynamic setting of Khangay-Khentey orogenic system and structural criteria of the gold mineralization” performed by Branch of Regional Geology and Tectonic, Institute of Geology, Mongolian Academy of Sciences from 2016 to 2018. We would like to state that this contribution is dedicated to Academician Dr. Tomurtogoo Onongo, who was a leading geologist of Mongolia and devoted his entire life to the Mongolian geology. We are

References (91)

  • T.K. Kelty et al.

    Detrital-zircon geochronology of Paleozoic sedimentary rocks in the Hangay-Hentey basin, north-central Mongolia: implications for the tectonic evolution of the Mongol- Okhotsk Ocean in Central Asia

    Tectonophysics

    (2008)
  • E.V. Khain et al.

    The most ancient ophiolite of the Central Asian fold belt: U-Pb and Pb-Pb zircon ages for the Dunzhugur complex, Eastern Sayan, Siberia, and geodynamic implications

    Earth Planet. Sci. Lett.

    (2002)
  • V. Kovach et al.

    Zircon ages and Hf isotopic constraints on sources of clastic metasediments of the Slyudyansky high-grde complex, southeastern Siberia: implication for continental growth and evolution of the Central Asian Orogenic Belt

    J. Asian Earth Sci.

    (2013)
  • V.I. Kovalenko et al.

    Isotope provinces, mechanisms of generation and sources of the continental crust in the Central Asian mobile belt: geological and isotopic evidence

    J. Asian Earth Sci.

    (2004)
  • T. Kurihara et al.

    Upper Silurian and Devonian pelagic deep-water radiolarian chert from the Khangai-Khentei belt of Central Mongolia: evidence for Middle Paleozoic subduction-accretion activity in the Central Asian Orogenic Belt

    J. Asian Earth Sci.

    (2009)
  • H. Li et al.

    Geochemistry of volcanic rocks at Zhaokalong iron-copper-polymetallic ore deposit, Qinghai Province, China: implications for the tectonic background

    Proc. Earth Planet. Sci.

    (2013)
  • S. Maruyama et al.

    Superplume, supercontinent, and post-perovskite: Mantle dynamics and anti-plate tectonics on the core-mantle boundary

    Gondwana Res.

    (2007)
  • D. Orolmaa et al.

    Permian-Triassic granitoid magmatism and metallogeny of the Hangayn (Central Mongolia)

    Russ. Geol. Geophys.

    (2008)
  • S. Osozawa et al.

    Structural evolution of the Bayanhongor region, west-Central Mongolia

    J. Asian Earth Sci.

    (2008)
  • J.A. Pearce

    Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust

    Lithos

    (2008)
  • N. Purevjav et al.

    Geochemistry of Silurian- Carboniferous sedimentary rocks of the Ulaanbaatar terrane, Hangay-Hentey belt, Central Mongolia: Provenance, paleoweathering, tectonic setting, and relationship with the neighbouring Tsetserleg terrane

    Chem. Erde

    (2013)
  • E. Saccani et al.

    Petrogenesis and tectono-magmatic significance of basalts and mantle peridotites from the Albanian–Greek ophiolites and sub-ophiolitic mélanges. New constraints for the Triassic–Jurassic evolution of the Neo Tethys in the Dinaride sector

    Lithos

    (2011)
  • E. Saccani et al.

    New insights into the geodynamics of Neo-Tethys in the Makran area: evidence from age and petrology of ophiolites from the Coloured Melange complex (SE Iran)

    Gondwana Res.

    (2018)
  • I. Safonova et al.

    Accretionary complexes in the Asia-Pacific region: Tracing archives of ocean plate stratigraphy and tracking mantle plumes

    Gondwana Res.

    (2014)
  • I. Safonova et al.

    Neoproterozoic basalts of the Paleo-Asian Ocean (Kurai accretion zone, Gorny Altai, Russia): geochemistry, petrogenesis, geodynamics

    Russ. Geol. Geophys.

    (2008)
  • I. Safonova et al.

    Pacific superplume-related oceanic basalts hosted by accretionary complexes of Central Asia, Russian Far East and Japan

    Gondwana Res.

    (2009)
  • I. Safonova et al.

    Geochemistry, petrogenesis and geodynamic origin of basalts from the Katun' accretionary complex of Gorny Altai (southwestern Siberia)

    Russ. Geol. Geophys.

    (2011)
  • I. Safonova et al.

    Geochemical diversity in oceanic basalts hosted by the Zasur’ya accretionary complex, NW Russian Altai, Central Asia: implications from trace elements and Nd isotopes

    J. Asian Earth Sci.

    (2011)
  • I. Safonova et al.

    Late Paleozoic oceanic basalts hosted by the Char-suture-shear zone, East Kazakhstsan: Geological position, geochemistry, petrogenesis and tetonic setting

    J. Asian Earth Sci.

    (2012)
  • I. Safonova et al.

    Recognizing OIB and MORB in accretionary complexes: a new approach based on ocean plate stratigraphy, petrology and geochemistry

    Gondwana Res.

    (2016)
  • I. Safonova et al.

    The itmurundy pacific-type orogenic belt in northern Balkash, central Kazakhstan: revisited plus first U-Pb age, geochemical and Nd isotope data from igneous rocks

    Gondwana Res.

    (2020)
  • Y. Tatsumi

    Origin of the high-magnesian andesites in the Setouchi volcanic belt, Southwest Japan. Melting phase relations at high pressures

    Earth Planet. Sci. Lett.

    (1982)
  • B.L. Weaver

    The origin of ocean island basalts and member compositions: trace element and isotopic constrains

    Earth Planet. Sci. Lett.

    (1991)
  • C. Wilhem et al.

    The Altaids of Central Asia: a tectonic and evolutionary innovative review

    Earth-Sci. Rev.

    (2012)
  • R.K. Workman et al.

    Major and trace element composition of the depleted MORB mantle (DMM)

    Earth Planet. Sci. Lett.

    (2005)
  • L. Wu et al.

    Apparent polar wander paths of the major Chinese blocks since the late Paleozoic: toward restoring the amalgamation history of east Eurasia

    Earth-Sci. Rev.

    (2017)
  • J.H. Wu et al.

    Geochemistry and geochronology of the mafic dikes in the Taipusi area, northern margin of North China Craton: Implications for Silurian tectonic evolution of the Central Asian Orogen

    J. Earth Syst. Sci.

    (2017)
  • Y.A. Zorin

    Geodynamics of the western part of the Mongolia-Okhotsk collisional belt, Trans-Baikal region (Russia) and Mongolia

    Tectonophysics

    (1999)
  • G. Badarch

    Tectonic overview of Mongolia

    Mongol. Geoscient.

    (2005)
  • B. Buchan et al.

    Structural and lithological characteristics of the Bayankhongor Ophiolite Zone, Central Mongolia

    J. Geol. Soc. Lond.

    (2001)
  • D. Erdenechimeg et al.

    Geochemistry and geochronology of the Paleozoic sedimentary rocks in the Shar khutul area, Central Mongolia

    Proc. Mongolian Acad. Sci.

    (2018)
  • G. Erdenesaikhan et al.

    Middle Paleozoic greenstones of the Hangay region, central Mongolia: Remnants of an accreted oceanic plateau and forearc magmatism

    J. Mineral. Petrol. Sci.

    (2013)
  • I.V. Gordienko

    Geodynamic evolution of late Baikalides and Paleozoids in the folded periphery of the Siberian craton

    Russ. Geol. Geophys.

    (2006)
  • C. Herzberg

    Partial melting below the Ontong Java Plateau

  • Y. Isozaki

    Jurassic accretion tectonics of Japan

    Island Arc

    (1997)
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