Research ArticleMiddle Paleozoic intermediate-mafic rocks of the Tsoroidog Uul’ accretionary complex, Central Mongolia: Petrogenesis and tectonic implications
Graphical abstract
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 SmNd 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
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