Post-collisional plutons in the Balikun area, East Chinese Tianshan: Evolving magmatism in response to extension and slab break-off
Research Highlights
► C2-P1 intrusions at Balikun were studied to understand amalgamation of the CAOB. ► 300 Ma gabbroic pluton reflects upwelling of deep asthenospheric mantle. ► 285 Ma granitic plutons indicate crustal melting induced by extensive underplating. ► Relative low TZr of the granites excludes involvement of a mantle plume. ► Magmatism at Balikun reflects a slab break-off regime after arc–continent collision.
Introduction
The Central Asian Orogenic Belt (CAOB) covers a wide area of the Asian continent and is the largest accretionary orogen in the world (Zonenshain et al., 1990, Sengör et al., 1993, Sengör and Natal'in, 1996, Windley et al., 2007). During the orogeny, large amounts of juvenile material were incorporated into the crust, which represents the most important crustal growth event in the Phanerozoic (Sengör et al., 1993, Jahn, 2004). Its long-lasting (ca. 800 Ma) evolution resulted in many world-class mineral resources, making the CAOB one of the most resource-rich areas of the world (Yakubchuk, 2004, Seltmann and Porter, 2005). For these reasons, considerable effort has been made to unravel the complex tectonic history and metallogenesis of the CAOB (e.g. Coleman, 1989, Han et al., 1997, Windley et al., 1990, Badarch et al., 2002, Nikishin et al., 2002, Collins et al., 2003, Kovalenko et al., 2004, Xiao et al., 2004a, Xiao et al., 2004b, Greene et al., 2005, Windley et al., 2007, Mao et al., 2008, Kröner et al., 2008, Pirajno et al., 2008). However, no consensus has so far been reached, and the proposed models include multiple opening and closing of the same ocean (Chen et al., 1997a), continuous accretion along a roll back subduction zone (Sengör et al., 1993, Sengör and Natal'in, 1996), or an archipelago similar to Indonesia (Kröner et al., 2007, Windley et al., 2007).
During the evolution of the CAOB, the Late Paleozoic period is of special significance because: (1) the final closure of the Palaeo-Asian Ocean occurred in the Late Carboniferous to Permian (Wartes et al., 2002, Xiao et al., 2003, Charvet et al., 2007, Kröner et al., 2007, Windley et al., 2007); (2) widespread mineral deposits were formed (Li et al., 1998, Rui et al., 2002, Yakubchuk, 2004, Seltmann and Porter, 2005, Mao et al., 2008, Zhang et al., 2008), and (3) Early Permian intraplate basalt, dykes and zoned ultramafic/mafic intrusions contributed significantly to vertical crustal growth (Chen et al., 1997a, Litvinovsky et al., 2002a, Solomovich and Trifonov, 2002, Han et al., 2004, Zhou et al., 2004, Pirajno et al., 2008). A late Paleozoic to early Mesozoic superplume (Nikishin et al., 2002, Yakubchuk, 2004; Vladimirov et al., 2005, Mao et al., 2008) and a ridge-subduction regime (Windley et al., 2007) have been proposed to explain the episodic occurrence of within-plate magmatism. Because of the different tectonothermal consequences, these regimes generally result in distinct spatial and temporal distribution of magmatism (Ernst and Buchan, 2003, Sisson et al., 2003), which is an important clue to understanding the geodynamic background of this critical period in the history of the CAOB.
As “the backbone of the Central Asia,” the Tianshan (or Tien Shan in some literature) belt is one of the main mountain ranges of the CAOB and extends more than 2500 km from Uzbekistan to SW Mongolia. It consists mainly of microcontinents, ophiolites, island arcs, seamounts, oceanic plateaus, mélanges and flysch sedimentary deposits accreted between the Neoproterozoic and Late Paleozoic (Windley et al., 1990, Sengör and Natal'in, 1996, Volkova and Budanov, 1999, Xiao et al., 2004b). The study area of Balikun is an intramontane basin in between Harlik Shan and Bogda Shan, a key location connecting the eastern Chinese Tianshan to eastern Junggar and Altay Shan (Fig. 1). In contrast to the western Tianshan that experienced a complex tectonic history (Windley et al., 1990), the Bogda Shan-Harlik Shan area mainly experienced two pulses of magmatism, i.e. Early Permian post-collisional granites imposed on a Carboniferous island arc (Xiao et al., 2004b, Shu et al., 2005). The relatively simple evolutionary history provides a rare opportunity for understanding the tectonic evolution during this period. Mafic and granitic intrusions are common in the area, but few have been investigated in detail (Cunningham et al., 2003). In this paper, we report geochronological and geochemical data for gabbroic and granitic intrusions and, by revealing their source and petrogenesis, we attempt to unravel the complex geodynamic evolution of the eastern Tianshan in the Late Paleozoic.
Section snippets
Geological background
The Chinese Tianshan is a ca. 300 km wide orogenic collage separating the Tarim Block to the south from the Junggar Basin to the north (Fig. 1). It encloses several microcontinents, and experienced a complex evolutionary history, involving Paleozoic accretion and collision, Mesozoic thermal subsidence and Cenozoic thrusting and uplift (Windley et al., 1990, Allen et al., 1991). Post-collisional strike-slip and transcurrent movement modified the shape of the orogen and made it more difficult to
Intrusions
Plutonic intrusions, widely distributed in the Balikun area (Fig. 2), were emplaced into Ordovician to Carboniferous strata. They are dominated by granitic plutons, with a few dioritic and gabbroic bodies. One gabbroic intrusion (Shiquanzi) and three granitic intrusions (Dajiashan, Barkol Tagh and Daliugou) were selected for study (Fig. 2).
Analytical methods
Zircon grains were separated using conventional heavy liquid and magnetic techniques. Representative zircon grains were hand-picked under a binocular microscope and were cast in an epoxy mount and polished to half their thickness for analysis. Cathodoluminescence (CL) images of zircons were obtained using a JXA-8100 Electron Probe Microanalyzer with Mono CL3 Cathodoluminescence System for high resolution imaging and spectroscopy at the Guangzhou Institute of Geochemistry, Chinese Academy of
Shiquanzi gabbro
Zircon grains from sample K18 are mostly dark brown in color, and stubby to prismatic in shape (Fig. 3). The zircons generally have low transparency probably because of their strikingly high U (2325–10,001 ppm) and Th (2699–20,451 ppm) contents (Table 1). Although not all the zircons exhibit concentric zoning, their well developed crystal shape, internal banding and high Th/U ratios (1.07–2.73) are consistent with a mafic igneous origin (Fig. 3; Table 1). Sixteen grains were analyzed. Except for
Gabbros
A suite of 9 samples were collected from the Shiquanzi gabbro for geochemical analysis. Major oxide compositions, along with data for trace elements and Nd–Sr isotopes, are listed in Table 2. The gabbro is characterized by high TiO2 contents (2.3–3.1 wt.%) and shows a wide compositional range. The samples can be subdivided into high-Si (HS) (SiO2 = 48.5–51.8 wt.%) and low-Si (LS) (SiO2 = 45.1–46.7 wt.%) groups, corresponding to the fine-grained and coarse-grained rocks, respectively. The HS group
Source and petrogenesis of the gabbros
The gabbro samples have high TiO2 and HFSE contents and their Ta/Yb ratios (0.2–0.6) are significantly higher than E-MORB (0.2), suggesting a rather fertile mantle source (Thirlwall et al., 1994, Pearce et al., 2005). An enriched component in basalt has been ascribed to OIB involvement (e.g. Márquez et al., 1999), asthenosphere upwelling or infiltration (Shinjo et al., 1999, Ferrari, 2004), or enriched blebs or streaks within the depleted mantle domain, which resulted from the addition of
Summary and conclusions
Late Carboniferous to Early Permian mafic and felsic intrusions of Balikun in the eastern Tianshan have been dated in this study and record the following ages: Shiquanzi gabbro (301 ± 6 Ma), Dajiashan Pluton (287 ± 2 Ma), Barkol Tagh Batholith (284 ± 5 Ma) and Daliugou Pluton (288 ± 3 Ma). The gabbro consists of high-Si (HS) and low-Si (LS) varieties and contains relatively elevated HFSEs (Nb > 9 ppm), high εNd(t) (+ 7.17–+ 8.74) and low initial 87Sr/86Sr ratios (0.7030–0.7041), showing variable subduction
Acknowledgements
We thank Mrs. Liu Ying and Mrs. Chen Linli and Mr. Li Chaofeng for their kind help with analyses. CY benefited much from discussions with Professors A. Kröner, Zheng-Xiang Li and Chunming Wu. We are indebted to Robert Kerrich and an anonymous reviewer, whose insightful and constructive reviews have greatly improved the paper. We thank Nelson Eby for his kind and careful editorial help, and Bernard Bonin and Mehmet Keskin for their critical and constructive comments on an early version of the
References (135)
- et al.
A new terrane subdivision for Mongolia: implications for the Phanerozoic crustal growth of Central Asia
Journal of Asian Earth Sciences
(2002) A-type granites and related rocks: evolution of a concept, problems and prospects
Lithos
(2007)- et al.
Early Paleozoic paleomagnetism of East Kazakhstan: implications for paleolatitudinal drift of tectonic elements within the Ural–Mongolian belt
Tectonophysics
(2003) - et al.
Geochemistry and Sr, Nd, Pb isotopic composition of the Central Atlantic Magmatic Province (CAMP) in Guyana and Guinea
Lithos
(2005) - et al.
Origin of enriched ocean ridge basalts and implications for mantle dynamics
Earth and Planetary Science Letters
(2004) The A-type granitoids: a review of their occurrence and chemical characteristics and speculations on their petrogenesis
Lithos
(1990)- et al.
Petrogenesis of the Gross Spitzkoppe topaz granite, central western Namibia: a geochemical and Nd–Sr–Pb isotope study
Chemical Geology
(2004) - et al.
Thermomechanical modelling of slab detachment
Earth and Planetary Science Letters
(2004) - et al.
Petrological and geochemical evolution of the Kymi stock, a topaz granite cupola within the Wiborg rapakivi batholith, Finland
Lithos
(2005) - et al.
Depleted-mantle source for the Ulungur River A-type granites from North Xinjiang, China: geochemistry and Nd–Sr isotopic evidence, and implications for Phanerozoic crustal growth
Chemical Geology
(1997)