Elsevier

Journal of Asian Earth Sciences

Volume 59, 1 October 2012, Pages 141-155
Journal of Asian Earth Sciences

Geochronology and geochemistry of basaltic rocks from the Sartuohai ophiolitic mélange, NW China: Implications for a Devonian mantle plume within the Junggar Ocean

https://doi.org/10.1016/j.jseaes.2012.07.020Get rights and content

Abstract

The West Junggar domain in NW China is a distinct tectonic unit of the Central Asian Orogenic Belt (CAOB). It is composed of Paleozoic ophiolitic mélanges, arcs and accretionary complexes. The Sartuohai ophiolitic mélange in the eastern West Junggar forms the northeastern part of the Darbut ophiolitic mélange, which contains serpentinized harzburgite, pyroxenite, dunite, cumulate, pillow lava, abyssal radiolarian chert and podiform chromite, overlain by the Early Carboniferous volcano-sedimentary rocks. In this paper we report new geochronological and geochemical data from basaltic and gabbroic blocks embedded within the Sartuohai ophiolitic mélange, to assess the possible presence of a Devonian mantle plume in the West Junggar, and evaluate the petrogenesis and implications for understanding of the Paleozoic continental accretion of CAOB. Zircon U–Pb analyses from the alkali basalt and gabbro by laser ablation inductively coupled plasma mass spectrometry yielded weighted mean ages of 375 ± 2 Ma and 368 ± 11 Ma. Geochemically, the Sartuohai ophiolitic mélange includes at least two distinct magmatic units: (1) a Late Devonian fragmented ophiolite, 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 in an oceanic plateau or a seamount. Based on detailed zircon U–Pb dating and geochemical data for basalts and gabbros from the Sartuohai ophiolitic mélange, in combination with previous work, indicate a complex evolution by subduction–accretion processes from the Devonian to the Carboniferous. Furthermore, the alkali basalts from the Sartuohai ophiolitic mélange might be correlated to a Devonian mantle plume-related magmatism within the Junggar Ocean. If the plume model as proposed here is correct, it would suggest that mantle plume activity significantly contributed to the crustal growth in the CAOB.

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.

References (109)

  • H.Y. Geng et al.

    Geochemical, Sr–Nd and zircon U–Pb–Hf isotopic studies of Late Carboniferous magmatism in the West Junggar, Xinjiang: implications for ridge subduction?

    Chemical Geology

    (2009)
  • H.Y. Geng et al.

    Geochemical and geochronological study of early Carboniferous volcanic rocks from the West Junggar: petrogenesis and tectonic implications

    Journal of Asian Earth Sciences

    (2011)
  • M.C. Göncüoglu et al.

    Oceanization of the northern Neotethys: geochemical evidence from ophiolitic mélange basalts within the Izmir-Ankara suture belt, NW Turkey

    Lithos

    (2010)
  • B.M. Jahn et al.

    Sources of Phanerozoic granitoids in the transect Bayanhongor–Ulaan Baatar, Mongolia: geochemical and Nd isotopic evidence, and implications for Phanerozoic crustal growth

    Journal of Asian Earth Sciences

    (2004)
  • P. Jian et al.

    Zircon ages of the Bayankhongor ophiolite mélange and associated rocks: time constraints on Neoproterozoic to Cambrian accretionary and collisional orogenesis in Central Mongolia

    Precambrian Research

    (2010)
  • A.C. Kerr et al.

    Oceanic plateaus: problematic plumes, potential paradigms

    Chemical Geology

    (2007)
  • A.C. Kerr et al.

    The internal structure of oceanic plateaus: inferences from obducted Cretaceous terranes in western Colombia and the Caribbean

    Tectonophysics

    (1998)
  • 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 and Planetary Science Letters

    (2002)
  • A. Kröner et al.

    Paleozoic arc magmatisim in the Central Asian Orogenic Belt of Kazakhstan: SHRIMP zircon ages and whole-rock Nd isotopic systematics

    Journal of Asian Earth Sciences

    (2008)
  • G. Maheo et al.

    The South Ladakh ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implication for the closure of the Neo-Tethys

    Chemical Geology

    (2004)
  • M. Meschede

    A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with the Nb–Zr–Y diagram

    Chemical Geology

    (1986)
  • J. Meyer et al.

    Off-ridge alkaline magmatism and seamount volcanoes in the Masirah island ophiolite, Oman

    Tectonophysics

    (1996)
  • J.A. Pearce

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

    Lithos

    (2008)
  • J. Pearce et al.

    Tectonic setting of basic volcanic rocks determined using trace element analyses

    Earth and Planetary Science Letters

    (1973)
  • Y. Rojas-Agramonte et al.

    Detrital and xenocrystic zircon ages from Neoproterozoic to Palaeozoic arc terranes of Mongolia: significance for the origin of crustal fragments in the Central Asian Orogenic Belt

    Gondwana Research

    (2011)
  • P. Shen et al.

    Geochemical signature of porphyries in the Baogutu porphyry copper belt, western Junggar, NW China

    Gondwana Research

    (2009)
  • P. Shen et al.

    Methane-rich fluid evolution of the Baogutu porphyry Cu–Mo–Au deposit, Xinjiang, NW China

    Chemical Geology

    (2010)
  • G.J. Tang et al.

    Ridge subduction and crustal growth in the Central Asian Orogenic Belt: evidence from Late Carboniferous adakites and high-Mg diorites in the western Junggar region, northern Xinjiang (west China)

    Chemical Geology

    (2010)
  • G.J. Tang et al.

    Recycling oceanic crust for continental crustal growth: Sr–Nd–Hf isotope evidence from granitoids in the western Junggar region, NW China

    Lithos

    (2012)
  • B.L. Weaver

    The origin of ocean island basalt end-member compositions: trace element and isotopic constraints

    Earth and Planetary Science Letters

    (1991)
  • J.A. Winchester et al.

    Geochemical discrimination of different magma series and their differentiation products using immobile elements

    Chemical Geology

    (1977)
  • K. Wong et al.

    Geochemical and geochronological studies of the Alegedayi Ophiolitic Complex and its implication for the evolution of the Chinese Altai

    Gondwana Research

    (2010)
  • B. Xia et al.

    Seamount volcanism associated with the Xigaze ophiolite, Southern Tibet

    Journal of Asian Earth Sciences

    (2008)
  • W.J. Xiao et al.

    Geodynamic processes and metallogenesis of the Central Asian and related orogenic belts: introduction

    Gondwana Research

    (2009)
  • W.J. Xiao et al.

    Middle Cambrian to Permian subduction-related accretionary orogenesis of Northern Xinjiang, NW China: implications for the tectonic evolution of central Asia

    Journal of Asian Earth Sciences

    (2008)
  • W.J. Xiao et al.

    A review of the western part of the Altaids: a key to understanding the architecture of accretionary orogens

    Gondwana Research

    (2010)
  • Z. Xu et al.

    Ultramafic-mafic mélange, island arc and post-collisional intrusions in the Mayile Mountain, West Junggar, China: implications for Paleozoic intra-oceanic subduction–accretion process

    Lithos

    (2012)
  • T. Yellappa et al.

    A Neoarchean dismembered ophiolite complex from southern India: geochemical and geochronological constraints on its suprasubduction origin

    Gondwana Research

    (2012)
  • J.Y. Yin et al.

    Late Carboniferous high-Mg dioritic dikes in Western Junggar, NW China: geochemical features, petrogenesis and tectonic implications

    Gondwana Research

    (2010)
  • Z.C. Zhang et al.

    Late Paleozoic volcanic record of the Eastern Junggar terrane, Xinjiang, Northwestern China: major and trace element characteristics, Sr–Nd isotopic systematics and implications for tectonic evolution

    Gondwana Research

    (2009)
  • E. Aldanmaz

    Mantle source characteristics of alkali basalts and basanites in an extensional intracontinental plate setting, western Anatolia, Turkey: implication for multi-stage melting

    International Geology Review

    (2002)
  • E. Aldanmaz et al.

    Geochemical characteristics of mafic lavas from the Neotethyan ophiolites in western Turkey: implications for heterogeneous source contribution during variable stages of ocean crust generation

    Geological Magazine

    (2008)
  • F. An et al.

    SHRIMP U–Pb zircon ages of tuff in Baogutu formation and their geological significances

    Acta Petrologica Sinica

    (2009)
  • W.J. Bai et al.

    Tectonic evolution of different dating ophiolites in the Western Junggar, Xinjiang

    Acta Petrologica Sinica

    (1995)
  • P. Bao et al.

    Chromite Deposits of China

    (1999)
  • BGMRXUAR (Bureau of Geology and Mineral Resources of Xinjiang Uygur Autonomous Region), 1993. Regional Geology of...
  • Buckman, S., Aitchison, J.C., 2004. Tectonic evolution of Paleozoic terranes in West Junggar, Xinjiang, NW China. In:...
  • S. Chen et al.

    Time constraints, tectonic setting of Dalabute ophiolitic complex and its significance for Late Paleozoic tectonic evolution in West Junggar

    Acta Petrologica Sinica

    (2010)
  • B. Chen et al.

    Petrology, geochemistry and zircon U–Pb chronology of gabbro in Darbut ophiolitic mélange, Xinjiang

    Acta Petrologica Sinica

    (2011)
  • S. Chen et al.

    Late Paleozoic peperites in West Junggar, China, and how they constrain regional tectonic and palaeoenvironmental setting

    Gondwana Research

    (2012)
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