Geochronology and geochemistry of basaltic rocks from the Sartuohai ophiolitic mélange, NW China: Implications for a Devonian mantle plume within the Junggar Ocean
Graphical abstract
Highlights
► New geochemical and geochronological data on basaltic rocks in the Sartuohai ophiolitic mélange. ► Tholeiitic rocks derived from 2% to 10% spinel lherzolite partial melting in a suprasubduction zone environment. ► Alkali basalts characterized with OIB affinity. ► A Devonian mantle plume-related magmatism within the Junggar Ocean is proposed.
Introduction
The Central Asian Orogenic Belt (CAOB), also named as the Altaids, is one of the largest accretionary orogens in the world (Fig. 1a; Şengör et al., 1993, Şengör and Natal’in, 1996, Khain et al., 2002, Jahn et al., 2004, Xiao et al., 2004, Xiao et al., 2009, Xiao et al., 2010, Xiao et al., 2012, Windley et al., 2007, Rojas-Agramonte et al., 2011, Choulet et al., 2011, Choulet et al., 2012a, Choulet et al., 2012b). It is widely accepted that the CAOB was built through prolonged and complex accretion-collision processes of Precambrian micro-continents, island arcs, seamounts, accretionary complexes and ophiolites during the evolution of the Paleo-Asian Ocean from Late Mesoproterozoic to Mesozoic (e.g. Coleman, 1989, Jahn, 2004, Windley et al., 2007, Kröner et al., 2008, Xiao et al., 2008, Xiao et al., 2010, Xiao and Kusky, 2009, Wong et al., 2010). Many ophiolitic mélanges have recently been reported in areas around of the West Junggar, such as Kalamaili ophiolitic mélange in East Junggar (Jian et al., 2005), Kuerti and Armantai ophiolitic mélange in Chinese Altai (Zhang et al., 2003, Xiao et al., 2006), Hegenshan ophiolitic mélange in Inner Mongolia (Zhang and Zhou, 2001) and Bayingou ophiolitic mélange in Tienshan range (Xu et al., 2005). The ages of the ophiolites in Chinese Altai and Tienshan range from 540 Ma to 325 Ma, and no ophiolite younger than 320 Ma has ever been documented.
As part of CAOB, the West Junggar is located at the southern margin of the CAOB (Fig. 1b), and is a key area for understanding the Paleozoic tectonic evolution of the CAOB. Several recent studies have reported new data and models on the Paleozoic tectonic framework and evolution, and associated mineral deposits of the West Junggar and adjacent regions (e.g. Coleman, 1989, Buckman and Aitchison, 2004, Xiao et al., 2008, Shen et al., 2009, Shen et al., 2012, Zhang et al., 2010, Ma et al., 2012, Xu et al., 2012). The West Junggar consists of arcs and accretionary complexes and preserves crucial evidence for Early Paleozoic intra-oceanic subduction and terrane amalgamation (Feng et al., 1989, Zhang et al., 1993, Wang et al., 2003, Buckman and Aitchison, 2004, Xiao et al., 2008, Xiao et al., 2010), followed by the emplacement of voluminous Late Carboniferous–Permian post-collisional granitoids (e.g. Chen and Arakawa, 2005, Chen and Jahn, 2004, Han et al., 2006, Su et al., 2006, Zhou et al., 2008, Chen et al., 2010). During the progressive accretion, several ophiolitic mélanges were formed and preserved in the accretionary complexes, including the Barleik, Mayle, Tangbale, Durbut, Karamay and Sartuohai (Fig. 1b) (Feng et al., 1989, Zhang et al., 1993, Zhang et al., 1995).
Although a number of studies have addressed the ophiolitic mélanges from the West Junggar, the Paleozoic tectonic setting of the this block is still disputed (Feng et al., 1989, Zhang et al., 1993, Wang et al., 2003, Buckman and Aitchison, 2004, Xiao et al., 2008), as illustrated by diverse models such as a single subduction zone (Şengör et al., 1993, Wang et al., 2003) and archipelagic tectonics (Feng et al., 1989, Zhang et al., 1995, Buckman and Aitchison, 2004). The main reason for the different interpretations of the tectonic setting and geodynamic significance of these ophiolitic mélanges is their complex structure and chaotic mixture of different units. The Sartuohai ophiolitic mélange (SOM) in the eastern segment of the West Junggar (BGMRXUAR, 1993, Zhou et al., 2001; Fig. 1c), which is thought to be a part of the Darbut ophiolitic mélange. Mafic rocks in the DOM display OIB, N-MORB or E-MORB affinities (Zhang et al., 1993, Wang et al., 2003, Buckman and Aitchison, 2004, Gu et al., 2009, Liu et al., 2009). Furthermore, the age of the DOM is uncertain since its matrix remains undated (Zhang et al., 1993, Gu et al., 2009, Liu et al., 2009).
The Sartuohai ophiolitic mélange contains alkali basalt and bears mantle information on the tectonic regime of this region during the Paleozoic. Moreover, the discovery of plume related rocks in the Sartuohai ophiolitic mélange brings important constraints of Altaids continental crust growth. This paper presents result of zircon U–Pb LA–ICP–MS dating and geochemical data for basalts and gabbros from the Sartuohai ophiolitic mélange, assesses the possible presence of a Devonian mantle plume in the region, and evaluates the petrogenesis and implications for understanding of the Paleozoic continental accretion of CAOB.
Section snippets
Geological setting
The West Junggar is divided into northern and southern parts by the sinistral strike-slip Xiemisitai Fault. Geologically, the northern West Junggar region is characterized by EW-trending brittle faults and fault-bounded blocks. This is in contrast to the southern West Junggar region, where major faults are mainly NE–SW oriented (Fig. 1b). These faults represent major Paleozoic structures, which are possibly unit boundaries. Although detailed investigations are rare in the West Junggar, several
Petrography
Based on optical microscopy, the harzburgite is dark green in color, with an assemblage of olivine (50–80%), orthopyroxene (10–20%), clinopyroxene (<5%) and reddish chromian spinel (1–5%). Olivine and pyroxene are completely replaced by serpentine and brucite, with a net-like texture of chrysotile and serpophite, accompanied by a few bastite pseudomorphs after enstatite (Fig. 3a).
The dunite is dark green and consists of olivine (90–95%), clinopyroxene (<5%), and spinel (1–5%). Olivine is
Sample description
After petrographic characterization of the rock types, twenty basalt and eight gabbro samples with the least amount of alteration were selected for whole-rock geochemical analyses and zircon U–Pb dating. To leach out the alteration minerals, the samples were crushed into small chips and soaked in cold 6 N hydrochloric acid before grinding into powder.
LA–ICP–MS zircon U–Pb dating
Concentration of the zircon crystals was achieved by means of a Wilfley table, a magnetic separator and heavy liquids. The zircons were separated
Zircon U–Pb geochronology
A representative alkali basalt (Sxo4) and associated gabbro (Sgb5) were chosen for zircon U–Pb dating, and the results are presented in Table 3.
Zircons in sample Sxo4 (45°54′00′′N, 84°46′05′′E) are light brown or colorless, and occur as subhedral, stubby to prismatic crystals, generally 100–200 μm in length, with length to width ratio of around 2.0. The CL images show that all of the grains possess good oscillatory zoning in the absence of any inherited cores or overgrowths (Fig. 4a). All the
Effects of alteration
The tholeiitic and alkali basalts, as well as the gabbros show high loss on ignition values (LOI = 1.1–3.71 wt.%; Table 4), which probably resulted from post-magmatic fluid–rock interaction forming low temperature minerals and carbonate veins as evident in thin section and even in hand specimen. Such alteration may have mobilized most major elements and some LIL elements such as Ba, Rb, Th, Sr, U and K as reflected in the wide variation of these elements (Table 4). However, the HFS elements such
Conclusions
- 1.
Zircons from the basalts and gabbros of the Sartuohai ophiolitic mélange yield weighted mean ages of 375 ± 2 Ma and 368 ± 11 Ma, respectively.
- 2.
Geochemically, the Sartuohai ophiolitic mélange includes at least two distinct magmatic units: (1) a Late Devonian fragmented ophiolite, the rock units in which were produced by ca. 2–10% spinel lherzolite partial melting in arc-related setting; and (2) contemporary alkali lavas, which were derived from 5% to 10% garnet + minor spinel lherzolite partial melting
Acknowledgements
We wish to thank Dr. Xiaoming Liu at Northwest University helped us with LA–ICP–MS zircon U–Pb analyses and Dr. Mingwu Liu for his help in carrying out the chemical analysis at Chang’an University. We are very grateful to Dr. Flavien Choulet and the other two anonymous reviewers, Editor-in-Chief Prof. Bor-ming Jahn and Editor Miss Irene Yao of the Journal of Asian Earth Sciences for their critical reviews and constructive comments that significantly improved the manuscript. We also thank Prof.
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