Lithos 140–141 (2012) 152–165

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Tracing a subducted ridge–transform system in a late Carboniferous accretionaryprism of the southern Altaids: Orthogonal sanukitoid dyke swarms in WesternJunggar, NW China

Chong Ma a,b,c,1, Wenjiao Xiao b,c,⁎, Brian F. Windley d, Guiping Zhao a, Chunming Han b,c, Ji'en Zhang b,c,Jun Luo b,c, Chao Li b,c

a Laboratory of Computational Geodynamics, College of Earth Science, Graduate University of the Chinese Academy of Sciences, Beijing 100049, Chinab State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, Chinac Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, Chinad Department of Geology, University of Leicester, Leicester LE1 7RH, UK

⁎ Corresponding author at: State Key Laboratory of Liof Geology and Geophysics, Chinese Academy of SciTel.: +86 10 82998524; fax: +86 10 62010846.

E-mail address: wj-xiao@mail.igcas.ac.cn (W. Xiao).1 Current address: 241 Williamson Hall, Department of

of Florida, Gainesville, Florida 32611-2120, USA.

0024-4937/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.lithos.2012.02.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 October 2011Accepted 4 February 2012Available online 12 February 2012

Keywords:Dyke swarmsSanukitoidsPaleostress inversionRidge subductionWestern JunggarAltaids

Two sets of Late Paleozoic, orthogonal, dioritic dyke swarms are defined in the eastern part of the WesternJunggar accretionary collage in the southern Altaids. Dioritic dykes (hornblende+plagioclase) are character-ized by relatively high SiO2, elevated MgO, Na2O, K2O, Mg#, Ni, Si and Cr contents, La/Yb ratios, very low Ycontents, and enrichments in large ion lithophile elements (LILE) and light rare earth elements (LREE),which are all comparable with those of sanukitoids. These dioritic dykes have low (87Sr/86Sr) ratios (mostlybetween 0.70366 and 0.70381) and high positive εNd (t) (+6.6 to +8.4) values, suggesting they weresourced from a depleted mantle. Accordingly, these dioritic dykes were probably derived by mixing of deplet-ed mantle components that originated from upwelling of asthenosphere through a slab window with meltsthat were derived by dehydration of a subducted slab. Detailed mapping and structural data of the dykeswarms indicate that the NW/SE-trending set was earlier than the NE/SW-trending dykes. Paleostress anal-ysis and inversion of the dyke swarms indicate that an early NE/SW-extension was associated with a NW/SE-trending slab window, and was followed by a slightly later NW/SE-extension that was associated with aNE/SW-trending slab window. Field relationships, together with published age constraints, suggest that thesetwo dyke swarms were intruded in the Late Carboniferous. Therefore two orthogonal slab windows wereopened, one (NE-SW) after the other (NW-SE) during this time period. A subducted ridge–transform systemcould well account for, and is consistent with, these relationships. We propose that in the Late Carboniferous aNW/SE-trending mid-oceanic ridge was subducted beneath the Darbut trench generating large-scale, closelyspaced NW/SE-trending dykes that were intruded through a slab window. That event was followed by subduc-tion of a NE/SW-trending transform fault (fracture zone) that connected with the earlier NW/SE-trending ridge,producing widely spaced NE/SW-trending dykes that were intruded through a NE/SW-trending vertical slabwindow. This new approach to the relationships between dyke swarms, adakitic/sanukitoid magmatism, ridgesubduction and slabwindows has broad implications for tectonic reconstruction of ancient accretionary orogens.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Ridge–trench interaction is one of the prominent mechanisms ofCircum-Pacific subduction systems in, for example, northern Cordilleran(Thorkelson et al., 2011), western California (Cole and Stewart, 2009;McCrory et al., 2009; Wilson et al., 2005), Chile (Lagabrielle et al.,

thospheric Evolution, Instituteences, Beijing 100029, China.

Geological Sciences, University

rights reserved.

2004), Japan (Brown, 1998; Maruyama, 1997), and southern Alaska(Cole et al., 2006; Sisson et al., 2003). Following a uniformitarian ap-proach (Windley, 1993), all subduction zones will eventually interactwith a spreading ridge (Sisson et al., 2003), and the effects should be ob-served in long-lived and very extensive orogenic belts such as the Altaids(Şengör et al., 1993) or Central Asian Orogenic Belt (Windley et al., 2007;Zhang et al., 2011a, 2011b). The geometrical configuration of a sub-ducted ridge–transform system is a leading factor controlling ridge–trench interaction in any given subduction zone, as in southern Chile(Cande and Leslie, 1986). In particular, pre-subduction ridge–transform–trench geometry exerts the principal control on the shapeand size of a slab window (Thorkelson, 1996) and, different incident an-gles of a ridge with a trench would induce different triple junction

153C. Ma et al. / Lithos 140–141 (2012) 152–165

migration rates along the trench (Sisson et al., 2003), and would evencause a triple junction to move back and forth in some cases (e.g.,Osozawa, 1994). Collectively, thiswould further temporally and spatiallyinfluence the subsequent magmatism, metamorphism (e.g. different

Balkha

K

Ura

lides Baikalides

Tethysides

(a)

Fig.1bFig.1b

Cent ra l Asian Orogenic Belt

Fig.1b

83°40´E

45°2

0´N

83°40´E

46°0

5´N

Darbut

Fig. 2bFig. 2b

BaogutuBaogutu

MaliyaMaliya

Miaoergou plutonMiaoergou pluton

Akbastao plutonAkbastao pluton

Miaoergou pluton

Baogutu

Fig. 2b

Maliya

Darbut

Triassic-Cretaceous

strike-slip fault

fold axis

Late Permian conglomerates

Carboniferousvolcaniclastics & OPS

Quaternary

ophiolitic rocksgranitic rocks

dyke

thrust /

100 km

KaramayKaramayKaramay

faultfaultfault

andandand

Fig. 1. (a) Simplified tectonic map of the Central Asian Orogenic Belt (Altaids) (Şengör et arounding Western Junggar (Chen et al., 2010; Coleman, 1989; Feng et al., 1989; Xiao et a1966; XBGMR, 1983; Zhang et al., 2011b). Positions of Fig. 2a and b are marked. ‘OPS’ in th

angles between ridge–transform and trench would result in distinguish-able metamorphic P–T paths for subduction complex rocks,Wakabayashi, 2004), ophiolite emplacement, and deformation of a con-vergent margin. Therefore, figuring out the configuration of a ridge

MONGOLIA

100 km

Thrust fault

Border line

Strike-slip fault

Junggar BasinYili

Tianshan Turpan

Bogda

sh

Tarim Basin

Eastern Junggar

Chinese Altay

Boshchekul-Chingiz

Zharma-Saur

Irtysh-Zaysan

Irtysh-Zaysan

RUSSIAAZAKHSTAN

(b)

Fig.1cWestern Junggar

Western Junggar

Western Junggar

Xiemisitai FaultXiemisitai FaultXiemisitai FaultIrtysh-Zaysan

Junggar Basin

46°0

5´N

85°20´E

85°20´E

45°2

0´N

(c)

(Paleo-trench)

(Paleo-trench)

beltbelt

KaramayKaramay

Fig. 2aFig. 2a

Hongshan plutonHongshan plutonHongshan pluton

Fig. 2a

Karamay

Baijiantan

Guai Shi

(Paleo-trench)

belt

pluton pluton pluton

ophiolite

ophiolite

ophiolite

l., 1993; Xiao et al., 2009). Panel b is outlined. (b) Major structures of the regions sur-l., 2010). Panel c is marked. (c) Geological map of the study area (Qi, 1993; XBGMR,e legend box refers to Ocean Plate Stratigraphy.

Fig. 3. Outcrop photos of dykes and their host granodiorites. (a) Thin dykes intruded into the Karamay pluton. (b) Thick dykes intruded in the Baogutu pluton. Person for scale.(c) A NE–SW dyke cuts across a NW–SE dyke in the Karamay pluton. Notebook for scale. (d) Intrusive contact of a dyke and the Karamay pluton.

5 km

Ndykes

fault

1 km

N

(a)

(b)

Carboniferous rocks

5

5

6

6

7

7

3

4

2

1 MAC10-17 MAC10-24

MAC10-34

MAC10-35

MAC10-18

MAC10-19

MAC10-21

43

12

NW-SE dykesamples

sample locations of Yin et al.(2009)

NE-SW dykesamples

granitic rocks

Fig. 2. (a) Distribution of dykes in the Karamay pluton area, and (b) Baogutu area. Positions indicated in Fig. 1c. Note the open and solid circles for the sample locations.

154 C. Ma et al. / Lithos 140–141 (2012) 152–165

155C. Ma et al. / Lithos 140–141 (2012) 152–165

subduction system and identifying a triple junction could help outlinethe framework of the physical, chemical and thermal characteristics ofa region and its geology.Modern seismological techniquesmake it easierto define the geometrical configuration of ongoing ridge subductions asaround the present-day Circum-Pacific (Thorkelson et al., 2011). Howev-er, because of the lack of extant oceans in pre-Mesozoic times, the geom-etry of ancient ridge–transform systems has been rarely identified.

Western Junggar (NW China) in the southern Altaids is ideal forinvestigating the presence and configuration of an ancient ridge–transform system because: (1) an increasing number of geochemical,Sr–Nd–Hf isotopic and field studies of Late Paleozoic rocks in WesternJunggar have produced robust evidence that can be best explained byridge subduction (Geng et al., 2009; Liu et al., 2009; Tang et al., 2010;Yin et al., 2010; Zhang et al., 2010, 2011a, 2011b); (2) there was no sig-nificant post-Paleozoic destructive deformation (Choulet et al., 2011);and (3) there are many unmetamorphosed and undeformed mafic-intermediate dykes, which provide appropriate structural markers.

Dyke swarms can be used as a powerful tool in rebuilding paleo-geographic tectonic regimes, such as the determination of subductionazimuth and dip (Lefort et al., 2005a; Lefort et al., 2005b). Crucially,mantle-derived dyke swarms can provide information on extensionalprocesses in the lithosphere. If the direction of extension is known, itis possible to decipher the shortening direction (Lefort et al., 2005a).Together with other corresponding geological and geochemical evi-dence, some specific geological events can be identified like obliquesubduction, as in Patagonia, Chile (Lefort et al., 2005b). In this studywe selected two critical areas in order to map over 220 dykes intwo major swarms from which seven samples were collected for

Fig. 4. (a) Photomicrograph in single-polarized light, showing the details of the contact betwdyke, in cross-polarized light. Hbl = hornblende, Pl = plagioclase, Px = pyroxene, and Chl

geochemical and isotopic analyses. Accordingly, in this paper weaim to use structural, geochemical and isotopic data to demonstratethat the dyke swarms in Western Junggar were formed by LateCarboniferous ridge subduction, and to discuss the role of dykeswarms in reconstructing the configuration of ancient subduction–accretion systems.

2. Tectonic background and regional geology

Western Junggar, a key NE-trending segment of the southernAltaids (Fig. 1a, b), is situated in the middle of the Kazakhstan oro-cline (Şengör et al., 1993; Xiao et al., 2010; Zhang et al., 1995). Tothe south is the northern Yili terrane, which is separated from West-ern Junggar by the Alakol–Junggar fault (Coleman, 1989; Feng et al.,1989). To the north is the Irtysh–Zaisan accretionary wedge that be-longs to the southernmost margin of the Paleozoic subduction–accretion system of the Siberian Craton (Xiao et al., 2009). To theeast is the Junggar basin filled withMesozoic and Cenozoic sediments.The main tectonic units in Western Junggar include the easternmostpart of the Zharma–Saur arc and the Boshchekul–Chingiz arc to thenorth of the Xiemisitai fault (Fig. 1b), and an accretionary collage tothe south of the Xiemisitai fault (Chen et al., 2010; Windley et al.,2007; Xiao et al., 2010).

The Western Junggar accretionary collage is characterized by wide-spread Paleozoic ophiolitic mélanges, island arc-related volcanic rocks,granitoids and oceanic sediments (Buckman and Aitchison, 2004;Feng et al., 1989; Zhang et al., 1993, 1995). The area in this study is lo-cated in the eastern part of the Western Junggar accretionary collage

een a dyke and its host granodiorite. (b), (c) and (d) Mineral assemblages of a dioritic= chlorite.

156 C. Ma et al. / Lithos 140–141 (2012) 152–165

(Fig. 1c). Crossing the center of the area is the NE-trending Darbut fault,along which there are many ophiolitic mélanges, formerly consideredto be remnants of a Devonian–Carboniferous oceanic plate, which wassubducted either northwestward or doubly with southeastward andnorthwestward polarities (Geng et al., 2009; Tang et al., 2010; Yang etal., in press; Zhang et al., 2011a, 2011b). However, the detailed configu-ration is controversial. In order to address this problem, based on a for-mer preliminary study of some sanukitoid dykes that were viablyinterpreted as the expression of a slab window generated by ridge sub-duction (e.g. Kamei et al., 2004),we selected two key areas based on theKaramay and Baogutu plutons that are noted for their abundance ofdykes (Fig. 1c).

Late Paleozoic granitoid batholiths, along with several small intru-sions, crop out on both sides of the Darbut fault. Devonian rocks occursparsely to the northwest of the fault. The two detailed study areas,on the SE side of the fault, are dominated by Carboniferous imbricatedaccretionary prisms containing red/gray bedded cherts, shales, tuffa-ceous sandstones, mélanges with blocks of limestones, agglomerates,massive mafic lavas, pillow basalts, and andesites, all within a shalematrix (Hao et al., 1993; Wu and Pan, 1990; XBGMR, 1966, 1983).The metamorphic grade of these accretionary rocks mainly rangesfrom prehnite–pumpellyite to low greenschist (Coleman, 1989).Many intermediate-mafic dykes are common to the southeast of theDarbut fault, and they intrude both the undeformed granitoid plutonsand their deformed accretionary host rocks.

3. Field occurrence

Both the Karamay and Baogutu areas (Fig. 1c) contain granitic plu-tons that are intruded by two sets of dioritic dykes (Fig. 2). Dykethicknesses range from 0.5 m to 15 m, mostly between 1 m and 3 m(Fig. 3a, b), and lengths vary from 100 m to 15 km (Fig. 2). The dis-tances between any two dykes vary from 0.5 m (Fig. 3a) to 1 km.

The well-preserved, crosscutting relationships between the twodyke swarms demonstrate that the NE/SW-trending dykes cut theNW/SE-trending dykes (Fig. 3c). The dyke swarms have not been pre-cisely radiometrically dated, but several lines of evidence constraintheir relative age. Many dykes of both swarms have no chilled mar-gins against the granitic rocks (Figs. 3d and 4a). This would imply acloser temperature of intrusion of the dykes and host pluton thanwould the presence of chilled dyke margins. Thus the period of timebetween them may have been short, perhaps within one millionyears, following the reasoning of Sano et al. (2002) who concludedthat the solidification of most minerals and the formation of plutonictextures of the Takidani granodiorite in Japan were accomplishedwithin one million years because the initial magma temperature ofgranitic rocks was ~1200 °C and the magma cooling rate was~1500 °C/Ma. In some places dioritic dykes cut the granitic rocks,but in others granitic veins cut the dykes (Kang et al., 2009). Thedykes and granitic rocks show strong evidence of interaction andinter-mixing (Zhang et al., 2009). The granodiorites of the Karamay(Geng et al., 2009; Kang et al., 2009) and Baogutu plutons containmany lensoid intermediate-mafic enclaves that typically range incomposition from dioritic to gabbroic. There is considerable evidencethat the granitic magma has progressively assimilated the enclaves,with the result that different enclaves demonstrate different stagesin the assimilation process from those with sharp margins (Fig. 3d)to those that have ghost-like, blurred and transitional boundaries(Kang et al., 2009). Some enclaves contain alkali feldspar phenocrystsand cm-scale patches or lenses of granodiorite (Zhang et al., 2009),suggesting that the enclaves were saturated with the assimilatinggranitic fluid. The petrology, geochemistry and age of the dioritic en-claves are all very similar to those of the dioritic dykes (see later), andthus the enclaves and dykes were considered by Kang et al. (2009) tobe derived from the same source.

4. Petrology and geochemistry of the dykes

Our samples of the dioritic dykes used for geochemical analyseswere taken from within the Karamay and Baogutu plutons (Fig. 2).The NE/SW-trending and NW/SE-trending dykes have similar mineralassemblages and textures. They are both massive, medium/fine-grained diorite or gabbroic diorite that mainly consist of plagioclase(45–50%), hornblende (25%), pyroxene (5–10%), quartz (5–10%), bio-tite (5%) and accessory minerals of opaque oxides and zircon. Thephenocrysts are mostly plagioclase and hornblende, or just one orthe other (Fig. 4b, c), and, occasionally, hornblende phenocrysts cutthrough pyroxenes (Fig. 4d). Chloritization is common.

4.1. Analytical methods

Whole-rock major elements were determined on fused glass disksusing a Phillips PW1500 X-ray fluorescence spectrometer at the Insti-tute of Geology and Geophysics, Chinese Academy of Sciences(IGGCAS), Beijing. Analytical precision is better than ±5%, estimatedfrom repeated analyses of the Chinese whole-rock andesite standardGSR-2. Trace element abundances were measured on a FinniganMAT Element mass spectrometer (ICP-MS) at IGGCAS after completedissolution. Powders (~40 mg) were dissolved in distilled HF+HNO3

in 15 ml Savillex Teflon screw-cap beakers at 200 °C for five days,dried and then diluted to 50 ml for analysis. Indium was used as aninternal standard to correct for matrix effects and instrument drift.Precision for all trace elements is estimated to be ±5%, and accuracyis better than ±5% for most elements by analyses of the GSR-2standard.

For Sr and Nd determinations, rock powders were weighted into7 ml Savillex TM Teflon screw-top capsules and added with mixed87Rb–84Sr and 149Sm–150Nd isotopically enriched tracers and, then gent-ly evaporated to dryness. After dissolution in Teflon capsules with HF–HNO3–HClO4 (2 ml–0.5 ml–0.2 ml) at about 120 °C for seven days, thesamples were separated by standard cation exchange techniques. Isoto-picmeasurements were performed on a FinniganMAT 262 thermal ion-izationmass spectrometer at the IGGCAS. Procedural blanks were lowerthan 300 pg for Rb and Sr and 100 pg for Sm andNd. 143Nd/144Nd valueswere corrected for mass fractionation by normalization to 146Nd/144Nd=0.7219, and 87Sr/86Sr ratios normalized to 86Sr/88Sr=0.1194.Typical within-run precision (2σ) for Sr and Nd was estimated to be ±0.000015. The measured values for the Jndi-1 Nd standard andNBS987 Sr standard were 143Nd/144Nd=0.512114±11 and 87Sr/86Sr=0.710260±8 respectively, during the period of data acquisition.

4.2. Results: dyke chemistry

Major and trace element compositions of representative dioriticsamples are given in Table 1. These samples contain intermediate SiO2

(52.3–60.6 wt.%) and high Al2O3 (15.8–18.0 wt.%). MgO contents main-ly range from 4.27 to 6.26 wt.%, and Mg# values are quite high (65–78,Fig. 5c). The rocks are enriched in sodium, with high Na2O/K2O ratios(2.7–4.6). The contents of Ni (54–156 ppm) and Cr (70–509 ppm) arehighly variable, mostly higher than 100 ppm and 190 ppm (Fig. 5c),respectively. The remarkably high Sr (582–1088 ppm) and low Y(6.9–13.2 ppm) signatures result in high Sr/Y ratios (48–131)(Table 1). The rare earth elements (REE) are significantly fractionated,characterized by low heavy REE (HREE) abundances (Yb≤1.33 ppm)and high (La/Yb)N ratios (4.1–14.0). However, the rocks exhibit onlymoderately fractionated HREE, as indicated by (Gd/Yb)N ratios (mostly1.7–2.5) and flat HREE patterns (Fig. 5a). In addition, there is nomarkedEu anomaly, but strong depletions of Nb and Ta, and remarkably posi-tive Ba, Pb and Sr anomalies (Fig. 5b). As a whole, the samples showstrong enrichments of LILE relative to high field strength elements(HFSE) and REE. Notably, the NE/SW-trending and NW/SE-trendingdykes have almost the same REE patterns and spider diagrams

Table 1Representative bulk-rock major and trace elements of the dioritic dykes in Western Junggar.

Sample MAC10-17 MAC10-18 MAC10-19 MAC10-21 MAC10-24 MAC10-34 MAC10-35

Latitude N45°30.565′ N45°30.880′ N45°30.880′ N45°30.880′ N45°39.432′ N45°41.527′ N45°41.460′

Longitude E84°19.300′ E84°19.017′ E84°19.017′ E84°19.017′ E84°41.105′ E84°45.123′ E84°45.128′

Major elements (wt.%)SiO2 58.4 60.6 55.3 52.3 57.0 55.3 56.8TiO2 0.71 0.57 0.81 0.98 0.85 0.74 0.66Al2O3 16.5 16.0 18.0 16.1 16.7 15.8 17.4Fe2O3 5.47 5.14 7.44 6.82 7.22 6.65 6.92MnO 0.06 0.08 0.07 0.09 0.10 0.10 0.10MgO 5.05 5.07 4.27 6.22 4.77 6.26 4.66CaO 6.41 5.49 7.14 7.00 5.76 7.25 6.89Na2O 4.61 4.19 3.84 4.03 3.85 3.63 3.24K2O 1.00 1.06 1.09 1.44 1.43 1.19 1.03P2O5 0.18 0.15 0.19 0.20 0.21 0.22 0.12LOI 1.04 1.10 1.44 4.18 2.34 2.64 2.02Total 99.5 99.4 99.5 99.4 100.2 99.7 99.9FeO 3.39 2.54 4.10 4.31 4.16 3.09 3.30Mg# 73 78 65 72 67 69 72Na2O/K2O 4.6 4.0 3.5 2.8 2.7 3.1 3.2

Trace elements (ppm)Li 8 11.7 10.2 19.5 25.8 19.9 29.7Be 0.7 0.65 0.56 0.75 0.67 0.56 0.55Sc 14.6 13.0 23.0 19.9 16.7 18.3 19.9V 146 124 165 155 137 156 164Cr 191 197 509 236 93 266 70Co 25 21.0 32.6 31.3 20.7 27.4 25.9Ni 135 135 139 138 54 156 72Cu 38.6 4.6 35.2 14.7 61.0 58.9 77.8Zn 29.1 41.7 47.5 45.5 63.9 72.1 64.0Ga 22.7 20.8 19.8 19.5 16.5 19.1 20.3Rb 24 23.0 17.1 22.9 31.6 23.9 28.8Sr 1088 798 745 699 582 678 751Y 8.3 6.9 7.8 13.2 12.0 9.9 8.1Zr 55.0 59.8 58.2 88.2 96.6 74.2 52.0Nb 1.50 1.78 1.66 2.56 2.89 2.24 1.60Cs 1.52 1.71 0.64 0.86 1.54 0.72 1.16Ba 365 429 339 295 434 547 449La 6.6 6.7 7.1 7.5 9.2 19.2 4.6Ce 15.0 14.4 15.8 18.5 20.0 38.5 10.3Pr 2.32 2.10 2.34 2.83 2.84 5.12 1.54Nd 10.5 9.3 11.2 13.3 12.2 20.6 7.2Sm 2.46 2.08 2.50 3.18 2.91 3.93 1.86Eu 0.75 0.71 0.80 1.10 0.88 1.17 0.68Gd 1.98 1.69 1.97 2.91 2.55 3.02 1.74Tb 0.30 0.24 0.29 0.46 0.40 0.41 0.27Dy 1.65 1.22 1.64 2.64 2.33 2.11 1.67Ho 0.33 0.26 0.30 0.54 0.47 0.39 0.35Er 0.86 0.68 0.77 1.38 1.29 1.07 0.90Tm 0.13 0.12 0.11 0.21 0.20 0.15 0.13Yb 0.84 0.76 0.74 1.33 1.25 0.98 0.80Lu 0.13 0.12 0.11 0.20 0.19 0.15 0.12Hf 2.00 1.83 1.75 2.31 2.57 2.10 1.60Ta 0.10 0.12 0.10 0.16 0.19 0.13 0.11Tl 0.10 0.09 0.07 0.09 0.15 0.12 0.14Pb 1.86 2.36 1.74 1.70 3.20 3.03 3.83Bi 0.01 0.01 0.03 0.01 0.02 0.02 0.06Th 0.62 1.06 0.65 0.58 1.60 2.44 0.82U 0.65 0.40 0.30 0.27 0.57 0.65 0.42Sr/Y 131 115 95 53 48 68 93(La/Yb)N 5.7 6.3 6.9 4.1 5.3 14.0 4.1(Gd/Yb)N 2.0 1.8 2.2 1.8 1.7 2.5 1.8

Sr-Nd isotopic compositionsSm (ppm) 2.20 1.81 2.37 3.14 3.44 4.29 1.90Nd (ppm) 9.7 8.4 10.7 13.2 15.1 23.4 7.5147Sm/144Nd 0.1369 0.1306 0.1344 0.1438 0.1376 0.1110 0.1543143Nd/144Nd 0.512914 0.512873 0.512886 0.512919 0.512901 0.512810 0.5129832σ 0.000013 0.000015 0.000015 0.000015 0.000013 0.000011 0.000012εNd (t) 7.7 7.1 7.2 7.5 7.4 6.6 8.4TDM (Ma) 470 508 507 504 499 506 430fSm/Nd -0.3 -0.3 -0.3 -0.3 -0.3 -0.4 -0.2Rb (ppm) 21.8 21.6 16.1 21.9 39.0 22.5 26.2Sr (ppm) 1052 777 746 708 692 694 74187Rb/86Sr 0.0600 0.0803 0.0623 0.0896 0.1633 0.0939 0.102587Sr/86Sr 0.703917 0.704061 0.703964 0.704195 0.704942 0.704092 0.7042382σ 0.000011 0.000016 0.000012 0.000015 0.000012 0.00001 0.000011(87Sr/86Sr)i 0.70366 0.70372 0.70370 0.70381 0.70424 0.70369 0.70380

Mg#=Mg/(Mg+Fe) in molecular.

157C. Ma et al. / Lithos 140–141 (2012) 152–165

1

10

NE-SW dykes

NW-SE dykes

(a)

(c)

(d)

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Roc

k/C

hond

rite

(b)

Ba U Ta Ce Pr Nd Hf Eu Tb Y Er Yb

0.1

1

10

100

1000

Rb Th Nb La Pb Sr Zr Sm Gd Dy Ho Tm Lu

Roc

k/P

rimiti

veM

antle

NE-SW dykes

NW-SE dykes

0

100

200

300

400

500

600

60 65 70

Cr

Ni

Mg#

ppm

75 80

DM

ε Nd(

t)

-8

-6

-4

-2

0

2

4

6

8

10

12

0.7022 0.7032 0.7042 0.7052 0.7062 0.7072

This studyTang et al., 2010

Xu et al., 2008

Qi, 1993

(87Sr/86Sr)i

Fig. 5. (a) Rare earth element patterns, and (b) spider trace element variation diagrams of the NE–SW dykes and NW–SE dykes. Chondrite and primitive mantle normalizing valuesafter Sun and McDonough (1989). (c) Plot of Mg# versus Cr and Ni and (d) plot of (87Sr/86Sr)i versus εNd(t) for the dioritic dykes in the Karamay pluton area and Baogutu area. Thedata from Xu et al. (2008) were obtained from similar dykes within the Karamay pluton, while the data from Qi (1993) and Tang et al. (2010) were obtained from similar dykes thatintrude accretionary strata in the Baogutu area. DM = depleted mantle, the data of which are from Rehkämper and Hofmann (1997).

158 C. Ma et al. / Lithos 140–141 (2012) 152–165

(Fig. 5a, b), and similar major element compositions (Table 1). The(87Sr/86Sr)i and εNd(t) have been calculated at 300 Ma on the basis ofour analyses and published data (see Section 6.2). As shown in a plotof (87Sr/86Sr)i versus εNd(t) (Fig. 5d) and Table 1, the samples display afairly restricted range of isotopic compositions with (87Sr/86Sr)i ratiosof 0.70366 to 0.70424 (mostly 0.70366–0.70381) and high positiveεNd(t) values of +6.6 to +8.4.

5. Paleostress inversion of the dyke swarms

It is necessary to investigate the paleostress when the dyke swarmsformed, because the stress field is a reflection of regional tectonics.Since dykes preserve the trajectory of the regional paleostress field atthe time of their intrusion (Féraud et al., 1987), it is useful for stressanalysis to be applied to dyke swarms. Paleostress inversion requiresseparating data into different subsets firstly in the field. A subset isdefined as a group of fractures, into which the dykes were injected,which moved during or were produced by a distinct tectonic event(Delvaux and Sperner, 2003). Based on the crosscutting relationships ofthe dyke swarms, we divided all the data into two subsets, belongingto the early NW/SE-trending dykes and the later NE/SW-trending dykes.

5.1. Methods

To study the Late Paleozoic tectonic stress field of the eastern partof the Western Junggar accretionary collage, we used the TENSORprogram (Delvaux and Sperner, 2003). Most methods of stress tensor

inversion (Angelier, 1989, 1994; Delvaux et al., 1995, 1997) are basedon two assumptions: that the state of stress is spatially and temporal-ly homogeneous (Keiding et al., 2009), and that the slip on a plane oc-curs in the direction of maximum shear stress (the Wallace–Botthypothesis, Bott, 1959). Regarding tensional fractures, into whichmagmatic dykes are injected, the slip magnitude on the plane is al-most zero, but still follows these principles. The inversion calculatesfour parameters of the reduced stress tensor: the principal stressaxes σ1 (maximum compression), σ2 (intermediate compression),σ3 (minimum compression), and the stress ratio R=(σ2−σ3)/(σ1

−σ3), which expresses the magnitude of σ2 relative to the magnitudeof σ1 and σ3. These parameters, in conjunction with the classical RightDihedron method and the iterative Rotational Optimization method(Delvaux and Barth, 2010; Delvaux and Sperner, 2003), have been in-corporated in the TENSOR program (available online at: http://www.damiendelvaux.be/Tensor/WinTensor/win-tensor.html). Accordingto the accuracy of the stress inversion methods, including that ofthe TENSOR program, numerous numerical modeling studies showthat the uncertainties in stress inversion due to geological and me-chanical factors generally fall within the range of measurement errors(Dupin et al., 1993; Pollard et al., 1993).

The stress regime is used to define the types of stress tensor andcan be expressed numerically by an index R′ that ranges from 0.0 to3.0. When σ1 is vertical, R′=R (extensional stress regime); when σ2

is vertical, R′=2−R (strike-slip stress regime); and when σ3 is verti-cal, R′=2+R (compressional stress regime) (Delvaux and Sperner,2003; Delvaux et al., 1997).

N

N N

0.99

N

R:0.5 F5:18.99QRw: C QRt: D

Schmidt Lower n/nt: 34/34

Schmidt Lower n/nt: 60/70

Schmidt Lower n/nt: 47/48

Schmidt Lower n/nt: 63/65

S1: 78/085

S3: 05/206

S2: 09/297

R:0.99 F5:33.25QRw: C QRt: E

S1: 14/111

S2: 74/313

S3: 05/202

R:0.99 F5:26.57QRw: C QRt: E

S1: 76/307

S2: 05/061

S3: 12/152

R:0.96 F5:33.77QRw: C QRt: E

S1: 68/010

S2: 18/221

S3: 10/127

(a) (b)

(c) (d)

Fig. 6. (a), (b), (c): Lower hemisphere equal area projections of dykes in the Karamay pluton area and (d) Baogutu area. Stress inversion results are represented by the orientation of the 3principal stress axes (a solid dot surrounded by a circle forσ1, a triangle forσ2 and a square forσ3). The related horizontal principal stress axes (SHmax) and horizontalminimumstress axes(Shmin) are marked by large arrows outside the stereogram. Their type, length and color indicate the horizontal deviatoric stress magnitude relative to the isotropic stress (σi) and are afunction of the stress regime and the stress ratio R=σ2−σ3/σ1−σ3. White arrows when σ3 is sub-horizontal (always Shmin), gray arrows when σ2 is sub-horizontal (either Shmin orSHmax), and black arrows when σ1 is sub-horizontal (always SHmax). Outward arrows indicate extensional deviatoric stress (bσi) and inward arrows indicate compressional deviatoricstress (>σi). The vertical stress (σv) is expressed in the small circle with stress arrows in the upper left corner of the figures by a solid circle for an extensional regime (σ1≈σv), andan open circle for a strike-slip regime (σ2≈σv).

159C. Ma et al. / Lithos 140–141 (2012) 152–165

5.2. Results

Using204measurements froma total of 217 [each is an average valueof at least three measurements of a given dyke; the other 13 averagemeasurements are excluded based on a misfit parameter that iscalculated for each dyke as a function of the model parameters (σ1, σ2,σ3 and the stress ratio R) that best fit the entire data set], four paleostresstensors were obtained from the western part of the Karamay pluton(Fig. 6a), the eastern part of the Karamay pluton (NW/SE-trendingdykes, Fig. 6b; NE/SW-trending dykes, Fig. 6c), and the Baogutu pluton(Fig. 6d). The dips and dip directions of the dykes were measured andcompiled to determine the paleostress tensors according to the methoddescribed above. Fig. 6 and Table 2 show the stress inversion results.

The NW/SE-trending dykes in the western Karamay pluton define atensorwith sub-horizontalσ2 axes andσ3 axes and sub-verticalσ1 axes.Themean orientations of the principal stress axes are σ1 at 78°/085°, σ2

at 09°/297° and σ3 at 05°/206°; the stress index R′=0.5 that indicates apurely extensional stress regime (Fig. 6a, Table 2). The NW/SE-trendingdykes in the eastern Karamay pluton reflect a tensor with sub-horizontal σ1 axes and σ3 axes and sub-vertical σ2 axes. The meanorientations of the principal stress axes are σ1 at 14°/111°, σ2 at 74°/313° and σ3 at 05°/202°; the stress index R′=1.01, indicating a trans-tensional stress regime (Fig. 6b, Table 2). The NE/SW-trending dykes

in the eastern Karamay pluton denote a tensor with sub-horizontal σ2

axes and σ3 axes and sub-vertical σ1 axes. The mean orientations ofthe principal stress axes are σ1 at 68°/010°, σ2 at 18°/221° and σ3 at10°/127°; the stress index R′=0.99, indicating a transtensional stress re-gime (Fig. 6c, Table 2). The NE/SW-trending dykes in the Baogutu plutonreflect a tensor with sub-horizontal σ2 axes andσ3 axes and sub-verticalσ1 axes. The mean orientations of the principal stress axes are σ1 at76°/307°, σ2 at 05°/061° and σ3 at 12°/152°; the stress index R′=0.96,also indicating a transtensional stress regime (Fig. 6d, Table 2).

6. Discussion

6.1. Petrogenesis and timing of the dioritic dykes

Both the NW/SE-trending and the NE/SW-trending dioritic dykeshave intermediate SiO2 (52.3–60.6 wt.%), elevated Al2O3 (15.8–18.0 wt.%), Na2O/K2O ratios (2.7–4.6), Cr (70–509 ppm), Ni(54–156 ppm) and Sr/Y ratios (48–131), relatively high MgO (mostly4.27–6.26 wt.%), remarkably high Mg# (65–78, Fig. 5c) and Sr(582–1088 ppm), and are characterized by significantly fractionatedREE and no marked Eu anomaly (Fig. 5a), strong depletions of Nb andTa, and significant positive Ba, Pb and Sr anomalies (Fig. 5b). These geo-chemical signatures are similar to those of the Setouchi sanukitoids in

160 C. Ma et al. / Lithos 140–141 (2012) 152–165

Japan, which have relatively high SiO2, elevated MgO, Na2O, K2O, Ni, Crcontents, and high Mg# and La/Yb ratios enriched in LILE, LREE (Kameiet al., 2004; Martin et al., 2005; Shirey and Hanson, 1984; Tatsumi andIshizaka, 1982; Tatsumi et al., 2003; Tiepolo and Tribuzio, 2008). There-fore, we propose that these dioritic dykes have the composition of high-Mg sanukitoids (see also, Yin et al. (2010) whose sample locations aremarked in Fig. 2).

High-Mg andesites (HMAs) including sanukitoids may be pro-duced either by subduction processes in convergent plate margins(Rapp et al., 1999, 2010; Tatsumi, 1982; Tatsumi and Ishizaka, 1982;Tatsumi et al., 2003), or in intra-continental settings (Gao et al.,2004; Huang et al., 2008; Qian and Hermann, 2010). In Western Jung-gar a Late Paleozoic intra-oceanic subduction system (Xiao et al.,2008) is indicated by the presence of widespread Cambrian toCarboniferous ophiolitic mélanges (Buckman and Aitchison, 2004;Feng et al., 1989; Xu et al., 2006; Zhang and Huang, 1992; Zhang etal., 1993, 1995, 2011a, 2011b), Carboniferous island arc-related volca-nic rocks, deep ocean sediments (Feng et al., 1989; Hao et al., 1993;Wu and Pan, 1990; XBGMR, 1966), and by depleted granitic rockswith high εNd(t=300) (mostly between +5 and +9) and low (87Sr/86Sr)i,t=300 (less than 0.7045) in the Miaoergou, Akbastao, Karamay,and Hongshan plutons, which indicate a juvenile lower crust-like sig-nature (Chen and Arakawa, 2005; Chen and Jahn, 2004; Gao et al.,2006; Geng et al., 2009; Su et al., 2006; Zhang et al., 2004).

Two petrogenetic models have been proposed to explainsubduction-related processes for the generation of sanukitoids: (a) par-tial melting of an enriched mantle source that has beenmetasomatizedby slab-derived melts, mainly in subduction zones (Tatsumi, 1982;Tatsumi and Ishizaka, 1982; Tatsumi et al., 2003), and (b) interactionof mantle peridotite and slab-derived melts in a mantle wedge(Moyen et al., 2003; Rapp et al., 1999, 2010). According to our Sr–Ndisotopic data, the dioritic dykes of Western Junggar show strongly de-pleted signatures (Fig. 5d), with high εNd(t) (+6.6 to +8.4) and low(87Sr/86Sr)i (0.70366–0.70381), consistent with the results of previousstudies (Qi, 1993; Tang et al., 2010; Xu et al., 2008). These data suggestthat model (a) can be ruled out.

Experimental data indicate that a high Mg# (>40) can only beobtained from the partial melting of a mantle component (Rapp andWatson, 1995). The high Mg# (>65) values and Cr and Ni contents(Fig. 5c) of the Western Junggar dioritic dykes suggest a strong man-tle affinity. On the other hand, the relatively high contents of horn-blende and biotite indicate an elevated water fugacity in themagma. Therefore, taking into account all the geochemical signatures,we propose that the sanukitoid diorite dykes in Western Junggarwere derived by the interaction of slab-derived melts with depletedmantle components during subduction of a hot, young oceanic slab,and these conditions can be best met by the thermal input of a sub-ducted, hot, mid-oceanic ridge (Sisson et al., 2003).

In order to further constrain the petrogenesis of the dykes, we needto figure out the links between the dykes and their host Baogutu andKaramay plutons. The Baogutu adakitic pluton is characterized by highSr concentrations (346–841 ppm) and Sr/Y ratios (31–67), and stronglyfractionated REE patterns with a negligible Eu anomaly (Tang et al.,2010), indicating melting at a high pressure in equilibrium with garnet(Stern, 2002), with little if any plagioclase. Moreover, they display very

Table 2Paleostress tensors from the dykes in Western Junggar.

Site n nt σ1 σ2

A 34 34 78/085 09/297B 63 65 14/111 74/313C 47 48 68/010 18/221D 60 70 76/307 05/061

Site A refers to NW–SE-trending dykes in the western Karamay pluton; site B refers to NW–Sin the eastern Karamay pluton; site D refers to NE–SW-trending dykes in the Baogutu area. nσ1, σ2, σ3: plunge and azimuth of the principal stress axes; R: stress ratio, (σ2−σ3)/(σ1−σ

depleted isotope compositions (εNd(t)=+5.8–+8.3, εHf(t)=+13.1–+15.7, and (87Sr/86Sr)I=0.7033–0.7054), which all suggest a MORB-like affinity (Tang et al., 2010). Therefore, the Baogutu pluton couldhave formed by partial melting of the edge of slab window (Tang etal., 2010), mixed with minor accretionary material during its ascent.

Comparedwith the Baogutu pluton, theKaramaypluton is character-ized by relative lower Sr concentrations (290–508 ppm) and Sr/Y ratios(17–26), and moderate to pronounced Eu negative anomalies (Tang etal., 2012), indicatingmelting at a lower pressure in equilibriumwith pla-gioclase (Stern, 2002). Furthermore, the Karamay pluton also displaysvery depleted isotope compositions (εNd(t)=+6–+9, εHf(t)=+12–+16, (87Sr/86Sr)I=0.7030–0.7045), which also indicate aMORB-like af-finity (Tang et al., 2012). Therefore, the Karamay pluton could have beengenerated largely by mixing of the melts of an accretionary prism thatwere triggered by heat from a slab window, with melts from dehydra-tion of a subducted slab, and somemelts of asthenosphere that upwelledthrough the slab window.

As regards the dykes in this study, besides their high Mg#, Cr, Ni,strong negative Nb and Ta anomalies, and the common presence ofhornblende and biotite, they also have much higher Sr concentrations(582–1088 ppm) and Sr/Y ratios (48–131) than the Baogutu and Kar-amay plutons, and no Eu anomaly, indicating melting at a higherpressure in equilibrium with garnet (Stern, 2002) rather than plagio-clase. The dykes also show very depleted isotope compositions(εNd(t)=+6.6–+8.4, (87Sr/86Sr)I=0.7037–0.7042). Therefore, wededuce that the melts that produced the dykes originated from a dee-per depth than the Baogutu and Karamay plutons, and wereemplaced subsequently, when the Baogutu and Karamay plutonswere not completely consolidated. Most likely, these melts couldhave formed mainly by mixing of the depleted mantle componentsthat originated from upwelling of the asthenosphere through a slabwindow, with melts that were derived by dehydration of a subductedslab.

With regard to the timing of the dyke swarms, we presented evi-dence above that suggests that the dykes and their host granodioritesunderwent plastic interactions that indicate the granodiorites werestill semi-crystalline, even hot, when the dykes were emplaced. Also,the dioritic enclaves that have extremely similar chemistry to the diorit-ic dykes,were progressively assimilated by the granodiorites suggestingsimultaneous conditions of crystallization. Therefore, from the field re-lations we conclude that themagmas of the dykes, the enclaves and thegranodioriteswere essentially coeval. According to recent studies, thesehost plutons have an age range of ca. 315–296 Ma, e.g. granodiorites ofthe Karamay pluton at 314–296 Ma (Tang et al., 2010), 315±2 Ma and306±5 Ma (Geng et al., 2009), and 296±4 Ma (Su et al., 2006), and theBaogutu pluton at 305±4 Ma (Chen and Arakawa, 2005), and315–310 Ma (Tang et al., 2010). Therefore we conclude, based ontheir field relations and isotopic data, that the dyke swarms wereemplaced in the Late Carboniferous.

6.2. Paleostress of the Karamay–Baogutu crust

The dyke swarms in Western Junggar are excellent candidates forpaleostress inversion and tectonic analysis. Paleomagnetic studies re-veal that there was remarkably little movement between Western

σ3 R R′ Stress regime

05/206 0.5 0.5 Purely extensional05/202 0.99 1.01 Transtensional10/127 0.96 0.96 Transtensional12/152 0.99 0.99 Transtensional

E-trending dykes in the eastern Karamay pluton; site C refers to NE–SW-trending dykes: number of dykes used for paleostress inversion; nt: total number of dykes measured;3); and R′: stress index.

161C. Ma et al. / Lithos 140–141 (2012) 152–165

Junggar, North Kazakhstan and Siberia after the Late Permian(Choulet et al., 2011). Furthermore, no metamorphism and deforma-tion affected the dyke swarms inWestern Junggar. Together, these re-lations mean that the dykes used for the paleostress inversion areprimary dykes and that they likely represent the original tectonicstress field. Careful examination of their crosscutting relationshipsleads us to conclude that they were produced in different periods (theNW/SE-trending earlier than the NE/SW-trending dykes, Fig. 3c), andthus represent different generations of fractures, which rules out thepossibility that they were conjugate. The Karamay–Baogutu area filledwith numerous dykes is more than 2000 km2, which is large enoughto reflect regional tectonic events.

According to the paleostress inversion results, the NW/SE-trendingdykes in the western Karamay pluton (Fig. 6a), the NE/SW-trendingdykes in the eastern Karamay pluton (Fig. 6c) and the NE/SW-trendingdykes in the Baogutu pluton (Fig. 6d) consistently indicate a σ1 sub-vertical, and σ2 and σ3 sub-horizontal regime, while the NW/SE-trending dykes in the eastern Karamay pluton (Fig. 6b) denote a σ2

sub-vertical, and σ1 and σ3 sub-horizontal regime. Moreover, thepaleostress inversion data of the NW/SE-trending dykes in the westernKaramay pluton reflect a purely extensional stress regime, but the otherthree dyke swarms reveal transtensional stress regimes (Table 2). Thereason why the directions of the three principal stress axes deducedfrom the NW/SE-trending dykes in the eastern Karamay pluton are dis-cordant with those of the other three areas could be due to a distur-bance by the already-existing NW/SE-trending dykes on the later NE/SW-trending dykes, because the later dykes cut the earlier dykesmostlyin the eastern part of the Karamay pluton. In short, these dyke swarmswere generated in purely extensional to transtensional stress regimesthat were characteristic of sub-vertical σ1 and sub-horizontal σ2 andσ3. According to the principles of emplacement of linear dyke swarms,the vertical and/or sub-vertical principal stress axes σ1 are producedwhen magma upwells vertically (Hoek and Seitz, 1995). Becausemuch of the accretionarymaterial did not undergo diagenetic processesandwere relatively physicallyweak, the upwelling ofmagma sheets be-neath them could have pushed up the overlying crust to generate frac-tures, into which the ascending melts injected to form the dykes.Therefore, the dyke swarms in the study area were likely due to anearly sheet of upwelling magma with a NW–SE trend and a later sheetof upwelling magma with a NE–SW trend.

6.3. Tectonic implications

There could be other tectonic scenarios that might form depletedintrusive rocks in an accretionary prism. Beside ridge subduction,the most possible model could be trench rollback, in which a subduct-ing slab rolls backwards to produce a strong trench suction effect,which enables upwelling of asthenosphere and extensive melting be-neath the accretionary prism, which further leads to an extensionalstress regime in the accretionary prism (Stern and Bloomer, 1992).Although the trench rollback model may seem feasible, it would in-duce a much broader and stronger magmatic province, as in theIzu–Bonin–Mariana subduction system, and it would give birth totholeiitic and/or boninitic magmas (Stern and Bloomer, 1992). Obvi-ously, this does not apply to the study area, which does not have abroad magmatic province or tholeiitic and/or boninitic rocks. Further-more, it is harder to generate two orthogonal sanukitoid dyke swarmsby trench rollback than by ridge subduction.

6.3.1. Ridge subductionRecent studies in the Western Junggar accretionary collage pro-

vide considerable, multi-disciplinary data, which point to a ridgesubduction model. The Baogutu area (Fig. 1c) contains many dio-rite–granodiorite porphyry stocks, which exhibit calc-alkaline com-positions and are characterized by depleted HFSE (Nb, Ta and Ti)and HREE (Yb and Y), high Sr contents, high positive εNd(t) and

εHf(t) values, and low (87Sr/86Sr)i and (206Pb/204Pb) ratios (Shenet al., 2009; Tang et al., 2010; Zhang et al., 2006; Zhao et al., 2006).All these characteristics are similar to those of modern adakites,which are typically generated by melting of hot subducted oceaniccrust plus reaction with mantle components that contribute themantle-derived geochemical signatures such as high Mg, Ni and Cr(Defant and Drummond, 1990; Yogodzinski et al., 1995). Fromtheir detailed chemical studies Tang et al. (2010) concluded thatthe 315–310 Ma Baogutu adakitic porphyries originated from themelted edge of a slab window that was generated by ridge subduc-tion. However, the Baogutu pluton was dated as young as 305±4 Ma (Chen and Arakawa, 2005), which may suggest that the slabwindow associated with that pluton could have been as young asLatest Carboniferous, although this needs confirmation. Further-more, different mafic rocks of the Darbut ophiolite belt (Fig. 1c) dis-play normal mid-ocean ridge basalt (N-MORB), enriched mid-oceanridge basalt (E-MORB) and ocean island basalt (OIB) signatures, anda 302±2 Ma (U–Pb zircon), E-MORB-type leucocratic gabbro thatprobably represents a forearc seamount above a slab window (Liuet al., 2009). E-MORB, N-MORB and OIB signatures in basaltic rocksof the Maliya ophiolitic mélange (Fig. 1c) formed in the Late Carbon-iferous and were subsequently incorporated into the forearc accre-tionary prism (Zhang et al., 2010). Comparing these rocks withthose formed during mid-ocean ridge subduction in Chile, Zhanget al. (2010) suggested that the Maliya ophiolitic mélange has all thecorrect diagnostic signatures of ridge subduction. Moreover, high-temperature charnockites, which form the magmatic margin of theMiaoergou pluton and have a U–Pb crystallization age of 305±3 Ma,have depleted characteristics (weak negative Eu anomaly, pronouncednegative Nb–Ta anomalies, high positive εNd(t) and εHf(t) values, andlow (87Sr/86Sr)i indicating derivation from melted, hot, juvenile lowercrust (Geng et al., 2009). The area from the Darbut fault to Guai Shi(Fig. 1c) is mainly characterized by an accretionary prism (Zhang et al.,2011b), which formed in an intra-oceanic subduction system in theLate Paleozoic (Xiao et al., 2008; Zhang et al., 2011a). From palaeomag-netic data Choulet et al. (2011) suggested that Western Junggar under-went a major anticlockwise rotation between the Late Carboniferous–Early Permian and the Late Permian, synchronous with the timing ofridge subduction and dyke emplacement.

In conclusion, we emphasize that many coeval rocks, the sanuki-toid dioritic dyke swarms, the Baogutu adakitic porphyries and theirdioritic enclaves, ophiolitic rocks with E-MORB, N-MORB and OIB sig-natures, high-temperature Miaoergou charnockites, and the Karamaygranitoids, are all situated in an accretionary prism that formed bysubduction–accretion processes. Taken together, these features canbe best explained by ridge subduction, which provided the necessaryhigh temperatures for partial melting of and reaction with the mantleand of subducted oceanic crust with MORB-like isotopic characteris-tics, which are necessary for production of high Mg–Cr–Ni sanuki-toids and adakites, as well as the necessary heat for the formation ofthe charnockites. These processes dominated the geological evolutionof the eastern part of the Western Junggar accretionary collage in theLate Carboniferous to earliest Permian.

6.3.2. Configuration of the ridge subductionRelevant conclusions made above are: the sanukitoid dyke

swarms in Western Junggar were generated by mixing of depletedmantle components that originated from upwelling of asthenospherethrough a slab window, and melts that were derived by dehydrationof a subducted slab; the paleostress study suggests that an earlysheet of upwelled NW/SE-trending magma was coupled with a latersheet of upwelled NE/SW-trending magma; field relationships sug-gest that these two dyke swarms were intruded between a paleo-trench and an arc with a similar age as their host Karamay and Bao-gutu plutons. Only a subducted ridge–transform system can providean appropriate, viable and robust model for such an active tectonic

162 C. Ma et al. / Lithos 140–141 (2012) 152–165

setting. The case here is considered to be similar to the Alaskan andChilean ridge subduction systems, where a trailing oceanic platewas subducted together with a plate trenchward of the spreadingcenter (Sisson et al., 2003 and references therein).

The subduction of a ridge–transform fault system provides an ap-propriate geometry for formation of two nearly orthogonal dykeswarms with a depleted mantle source. During subduction both aridge and a transform fault can be split apart to form a slab window,but their shapes are quite different (Thorkelson, 1996). It is wellknown that mid-ocean ridges and transform faults subduct oneafter the other (most notably at the South Chile Ridge, Lagabrielle etal., 2004). With regard to the two dyke swarms in Western Junggar,an appropriate question is: which one represents the mid-oceanridge, and which one the transform fault? Relevant facts are: (1) theNW/SE-trending dyke swarm is about 12 km wide, whereas the NE/SW-trending dyke swarm is about 30 kmwide; (2) the NW/SE-trend-ing dykes have a more concentrated outcrop, but the NE/SW-trendingdykes are more scattered; and (3) the NW/SE-trending dykes arelong, some extending for as much as 15 km, whereas the NE/SW-trending dykes are much shorter, the longest being about 2 to 3 km.Theoretically, a slabwindow that forms by lateral opening of a subductedmid-ocean ridge should be wide and would allow more magma to up-well through it than a fracture zone that forms by vertical opening of asubducted transform fault. Furthermore, a wide slab window with

slab window

BBlock Diagram (a)

(b)

mid-ocean ridgemid-ocean ridge

N

Darbut fault a

nd ophiolite belt (

Paleo-trench

)

Darbut fault a

nd ophiolite belt (

Paleo-trench

)to overriding plateto overriding plate

motion direction relativemotion direction relative

transform faulttransform fault

mid-ocean ridge

trench

asthenosphericmantle

hhdowngoing plate

downgoing plate

downgoing plate

directiondirectionrelative motion

direction

transform fault

slab windowslab window

Plan View

transform fault

Baijiantan

motion direction relativeto overriding plate

Darbut fault a

nd ophiolite belt (

Paleo-trench

)N

dyke swarm NW-SE

Karamay

Guai Shi

mid-ocean ridge

mm

accretionary

prism}

Baogutu

wslab windowdd w wslab windowb w wwb ddol ddslab windowwowoverriding plate

m

Fig. 7. (a) Schematic block diagram of a wide slab window beneath an accretionary prism fotrending dyke swarm. More details are presented in (b). (b) Plan view of a tectonic configswarm, corresponding to (a). The configuration of the ridge–transform fault is shown andThe positions of the fold axes, faults, Karamay, Baijiantan, Guai Shi and Baogutu are as in Fdot on the trench marks the position of the triple junction. (c) Schematic block diagram ofduction, illustrating the emplacement of the later NE/SW-trending dyke swarm. More detalater NE/SW-trending dyke swarm, corresponding to (c). The opening of the narrow slab winslab windows shown in (b) and (d), the overriding plate is illustrated as transparent. The r

considerable upwelling magma is potentially a more forceful structurethan a narrow fracture zone with sparse upwelling magma. Therefore,we suggest that the early NW/SE-trending dyke swarm represents theresponse to a subducted mid-ocean ridge and the later NE/SW-trendingdyke swarm was emplaced above a subducted transform fault.

The Darbut fault zone is regarded as the location of a fossil trench,and Guai Shi the position of a volcanic arc (Zhang et al., 2011b).According to the regional geology, the slab windows should be locat-ed between the fossil trench and the volcanic arc, and beneath theLate Carboniferous accretionary prism. Considering the possibilitythat the downgoing plate moved southeastward, as proposed byZhang et al. (2011b), the segment of ridge subduction could havegiven rise to a wide slab window and to the upwelling of a thicksheet of asthenosphere, which created the extensional stress regimein the accretionary prism and led to the emplacement of the earlierNW/SE-trending dyke swarm (Fig. 7a, b), a situation very similar tothe ridge subduction that occurred in the Alaskan (and other) accre-tionary orogens (Brown, 1998; Cole and Stewart, 2009; Kusky et al.,2003; Madsen et al., 2006). When the final segment or end of a mid-ocean ridge enters a trench, the transform fault segment that connectedwith the former ridge segment may be subducted to give rise to a nar-row slab window through which a thin sheet of asthenosphere can up-well and give rise to an extensional stress regime in the accretionaryprism; such a situation most likely led to the emplacement of the later

slab window

lock Diagram (c)

(d)

dyke swarmdyke swarm

NW-SENW-SE

NE-SWNE-SW

dyke swarmdyke swarm

N

Darbut fault a

nd ophiolite belt (

Paleo-trench

)

Darbut fault a

nd ophiolite belt (

Paleo-trench

)motion direction relativemotion direction relativeto overriding plateto overriding plate

htrenc

asthenosphericmantle

downgoing plate

accretionary

prismrelative motionrelative motion

directiondirectionrelative motion

direction

Plan View

Baijiantan

motion direction relativeto overriding plate

Darbut fault a

nd ophiolite belt (

Paleo-trench

)N

NW-SEdyke swarm

NE-SWdyke swarm

yBaogutuBaogutuadakitesadakitesBaogutu adakites

i ShGuai Shi

mid-ocean ridge

id-oceanid-ocean ridge ridge

}

slab windowslab windowl b ind windowwb w wwwslab windoslab window

e

overriding plat

id-ocean ridge

Karama

rmed by mid-ocean ridge subduction, illustrating the emplacement of the early NW/SE-uration of the study area at the time of formation of the early NW/SE-trending dykethe opening of the wide slab window is attributed to the mid-ocean ridge subduction.ig. 1c. The strike of the trench is the same as the Darbut ophiolite belt in Fig. 1c. Thea narrow slab window beneath an accretionary prism, formed by transform fault sub-ils are presented in (d). (d) Plan view of the study area at the time of formation of thedow is attributed to the transform fault subduction. In order to display the details of theidge outlined by dashed lines is extrapolated.

163C. Ma et al. / Lithos 140–141 (2012) 152–165

NE/SW-trending dyke swarm (Fig. 7c, d). In addition, since themarginsof a slab window are able to undergo partial melting to form adakites,the Baogutu adakites (Fig. 7d) can be used to further constrain the con-figuration of the subducted ridge, the position of which indicates a left-stepping ridge.

Numerically, according to the results of the paleostress inversion(Fig. 6), the direction of the sub-horizontal σ3 of the NW/SE-trendingdyke swarm was 202° to 206° (Fig. 6a, b), thus the direction of themid-ocean ridge should have been 112° to 116°, orthogonal to theσ3. Similarly, the direction of the sub-horizontal σ3 of the NE/SW-trending dyke swarm was 127° to 152° (Fig. 6a, b), and the directionof the transform fault should have been 037° to 062°. Moreover, thedirection of the fossil trench should be about 050°, because this isalso suggested by the direction of the Darbut fault zone, whichtoday strikes roughly 050°. With the presently available data it isnot possible to determine conclusively the precise directions of theridge and transform fault. However, the results of this study go along way to provide the relative positions of the Western Junggarridge and transform fault, as well as the numerical range of their di-rections with respect to the paleo-trench.

7. Conclusions

(1) The dioritic dykes (both NW/SE-trending and NE/SW-trending)in Western Junggar are characterized by relatively high SiO2, el-evated MgO, Na2O, K2O, Mg#, Ni, Si and Cr, La/Yb ratio, LILE andLREE, and very low Y contents, and accordingly they can beregarded as sanukitoids. They were probably derived by mixingof the depletedmantle components that originated fromupwell-ing of asthenosphere through a slab window with melts thatwere derived by dehydration of a subducted slab during ridgesubduction in the Late Carboniferous.

(2) The NW/SE-trending dyke swarm was derived from an earlysheet of upwelling magma with a NW–SE trend that resultedfrom subduction of a mid-oceanic ridge, and the NE/SW-trend-ing dyke swarm represents a later sheet of upwelling magmawith a NE–SW trend that intruded fractures related to the sub-duction of a transform fault.

(3) The paleostress inversion and structural analyses indicate thatthe direction of the mid-ocean ridge was NW–SE (possibly112° to 116°), and the direction of the transform fault wasNE–SW (may be 037° to 062°).

(4) This paper presents a case study of dyke swarms that indicate theconfiguration of a Late Paleozoic ridge–transform subductionsystem. With regard to ancient arcs, although their originalridge–transform fault systems and related magmatic stripes ofthe seafloor have vanished, it is still possible to reconstruct theconfiguration of a ridge-subduction system, and to trace the loca-tion of the triple junction by applying indirect methods. Bystudying the petrological and geochemical features of dykeswarms, their physical and chemical characters can be obtained;by analyzing their structural relations, the tectonic stress andspatial relationships can be unraveled. These two aspects, com-binedwith geological age constraints, can provide uswith a com-prehensive picture of a ridge–transform fault subduction system.This new approach to dyke swarms, which can trace the config-uration of a vanished ridge–transform system, has broad impli-cations for the tectonic reconstruction of ancient accretionaryorogens.

Acknowledgments

Professor D. Delvaux is thanked for help with the TENSOR pro-gram. We sincerely appreciate insightful and constructive commentsof Nelson Eby, John Wakabayashi and an anonymous reviewer. This

study was supported by funds from the Major State Basic ResearchDevelopment Program of China (2007CB411307), the Innovative Pro-gram of the Chinese Academy of Sciences (KZCX2-YW-Q04-08), andthe National Natural Science Foundation of China (40725009,40772136).

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