40 ar– 39 ar age and geochemistry of subduction-related...
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40Ar–39Ar age and geochemistry of subduction-relatedmafic dikes in northern Tibet, China: petrogenesis andtectonic implicationsBin Liuab, Chang-Qian Maab, Jin-Yang Zhangc, Fu-Hao Xiongab, Jian Huanga & Hong-An Jianga
a State Key Laboratory of Geological Processes and Mineral Resources, China University ofGeosciences, Wuhan, Chinab Faculty of Earth Sciences, China University of Geosciences, Wuhan, Chinac Faculty of Earth Resources, China University of Geosciences, Wuhan, ChinaPublished online: 17 Jul 2013.
To cite this article: Bin Liu, Chang-Qian Ma, Jin-Yang Zhang, Fu-Hao Xiong, Jian Huang & Hong-An Jiang (2014) 40Ar–39Arage and geochemistry of subduction-related mafic dikes in northern Tibet, China: petrogenesis and tectonic implications,International Geology Review, 56:1, 57-73, DOI: 10.1080/00206814.2013.818804
To link to this article: http://dx.doi.org/10.1080/00206814.2013.818804
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International Geology Review, 2014Vol. 56, No. 1, 57–73, http://dx.doi.org/10.1080/00206814.2013.818804
40Ar–39Ar age and geochemistry of subduction-related mafic dikes in northern Tibet, China:petrogenesis and tectonic implications
Bin Liua,b , Chang-Qian Maa,b*, Jin-Yang Zhangc , Fu-Hao Xionga,b , Jian Huanga and Hong-An Jianga
aState Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China; bFaculty ofEarth Sciences, China University of Geosciences, Wuhan, China; cFaculty of Earth Resources, China University of Geosciences, Wuhan,
China
(Accepted 19 June 2013)
The early Permian Xiaomiao mafic dike swarm in the East Kunlun orogenic belt (EKOB) provides an excellent opportu-nity to study the petrogenesis of such swarms developed in supra-subduction zone environments, and to investigate theearly plate tectonic history of the Palaeo-Tethyan Ocean. Hornblende 40Ar–39Ar dating results indicate that the mafic dikesformed in the early Permian (277.76 ± 2.72 Ma). The Xiaomiao mafic hypabyssals have the following compositional range:SiO2 = 46.55–55.75%, MgO = 2.80–7.38%, Mg# = 36–61, and (Na2O + K2O) = 2.87–4.95%. Chemically, they dis-play calc-alkali affinities, ranging in composition from gabbro to gabbroic diorite. All analysed dikes are enriched in lightrare earth elements and large-ion lithophile elements (e.g. Rb and Ba), but are depleted in heavy rare earth elements andhigh field strength elements (e.g. Nb, Ta, and Ti). Their ISr and εNd(t) values range from 0.707 to 0.715 and –2.60 to+2.91, respectively. They are geochemically similar to subduction-related basaltic rocks (e.g. island arc basalt), but differfrom E-MORB and N-MORB. Petrographic and major element data reveal that fractional crystallizations of clinopyroxene,olivine, hornblende, and Fe–Ti oxides may have occurred during magma evolution, but that crustal contamination was minor.Based on geochemical and Sr–Nd isotopic bulk-rock compositions, we suggest that the mafic dikes were likely generatedby 10–20% partial melting of a spinel + minor garnet lherzolite mantle source metasomatized by subducted, slab-derivedfluids, and minor sediments. Based on our results, we propose that the early evolution of the Palaeo-Tethyan Ocean involvedthe spreading and initial subduction of the Carboniferous to early Permian ocean basin followed by late Permian subduction,which generated the magmatic arc.
Keywords: mafic dikes; geochemistry; early Permian; Palaeo-Tethyan Ocean; East Kunlun orogenic belt
Introduction
Mafic dike swarms are key elements for understanding geo-dynamic processes in the history of the Earth (Srivastava2011). Moreover, mafic dike swarms are effectively used toexplore the magma conduit system, the evolution of mantlesources, ancient structures, and the timing of crustal evo-lution (e.g. Zhao and McCulloch 1993; Hoek and Seitz1995; Williams et al. 2001; Hanski et al. 2006; Ma et al.2012). In the past 30 years, numerous reports have beenpublished on the large-scale mafic dike swarms developedalong volcanic rifted margins or within stable cratons (e.g.Srivastava and Singh 2004; Zhao et al. 2010; Peng et al.2011). However, few studies have focused on the mafic dikeswarms developed in orogenic belts, especially concerningtheir origins in supra-subduction zone environments (e.g.Scarrow et al. 1998; Allen 2000).
Mafic dike swarms, granitoid intrusions, and vol-canic rocks are widespread in the East Kunlun orogenicbelt (EKOB) owing to the N-directed subduction of the
*Corresponding author. Email: [email protected]
A’nyemaqen Palaeo-Tethyan Ocean and the subsequentcontinent–continent collision (Figure 1B; e.g. Yang et al.2005; Luo et al. 2008; Xiong et al. 2011, 2012; Zhang et al.2012). Many studies have been undertaken on the grani-toid intrusions and volcanic rocks in the EKOB, but fewhave focused on the petrogenesis and tectonic settings ofthe mafic dike swarms (e.g. Xiong et al. 2011). Previousstudies of precise zircon U–Pb analyses in the EKOB sug-gest that the ages of granitoid intrusions and volcanicsrange from 263 to 213 Ma (Liu et al. 2004; Ding et al.2011; Xiong et al. 2012; Zhang et al. 2012). The ages ofthe Buqingshan and Dur’ngoi ophiolite (the remnants ofthe A’nymaqen Palaeo-Tethyan oceanic lithosphere) rangefrom 345 to 308 Ma (Chen et al. 2001; Yang et al. 2009;Liu et al. 2011). Due to the absence of late Carboniferousto late Permian (308–260 Ma) magmatic rocks and thepresence of Carboniferous to Permian stable marine sed-iments indicating unchanging palaeogeographic systems(Chen et al. 2008, 2010; Zhu et al. 2009), it is very difficult
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Figure 1. Tectonic location (A) and geological map (B) of the eastern part of the East Kunlun orogenic belt.
to trace the whole evolutionary history (nearly 48 millionyears) of the A’nyemaqen Palaeo-Tethyan Ocean. Someauthors have even suggested that the initiation of the N-directed subduction started at 260 Ma (e.g. Yang et al.2009).
In this study, we report the 40Ar–39Ar age andgeochemical data of the Xiaomiao mafic dike swarm inthe EKOB, which has important implications for reveal-ing the petrogenesis of mafic dike swarms that developedin supra-subduction zone environments and the early tec-tonic evolutionary Carboniferous to Permian (C-P) historyof the Palaeo-Tethyan Ocean in the East Kunlun region.
Geological backgrounds
The EKOB, located in the northern Tibet–Qinghai plateau,lies between the Qaidam Basin in the north and the BayanHar–Songpan Ganzi terrane in the south (Figure 1A). TheEKOB experienced the subductions of the Proto-TethysOcean and the Palaeo-Tethys Ocean and the subsequentcontinent–continent collision during which the CentralKunlun suture zone (Nuomuhong–Qingshuiquan suturezone) and the South Kunlun suture zone (A’nyemaqensuture zone) were successively formed (e.g. Yang et al.1996; Bian et al. 2004).
The EKOB is composed of two suture zones and threetectonic units, i.e. the north terrane of the East Kunlun(NEKL), the south terrane of the East Kunlun (SEKL), andthe Bayan Har–Songpan Ganzi terrane (Figures 1A and 1B;Xu et al. 2006). The main characteristics of the NEKL andthe SEKL and the south Kunlun suture zone are describedbelow. The other tectonic units are described in detail byZhang et al. (2012).
The NEKL is characterized by a widespreadPrecambrian metamorphic basement (thePalaeoproterozoic Jinshuikou Group) comprising thelower Baishahe and the upper Xiaomiao formations.The lower Baishahe Formation consists of marbles,gneisses, migmatites, and amphibolites. Their protolithscomprise limestones, marine shaly sandy clastic rocksand intermediate to basic volcanic rocks with ages of2.0–1.9 Ga (Wang et al. 2007). The upper XiaomiaoFormation consists of quartzite, marbles, gneisses, andschists. Their protoliths comprise shallow marine clasticrocks and carbonates with ages of 1.7–1.6 Ga (Chenet al. 2011b). The terrane underwent a Silurian collisionwith uplifting and Devonian post-collision extensionevents (Liu et al. 2012). During the Carboniferous andPermian, stable sedimentation occurred in the NEKLaccompanied by abundant marine clastic rocks andlimestones.
The SEKL lies between the central Kunlun suture zone(Nuomuhong–Qingshuiquan suture zone) and the southKunlun suture zone (A’nyemaqen suture zone). The mainpart of the Precambrian metamorphic basement comprisesthe Kuhai complex, which differs from the JinshuikouGroup of the NEKL. The Kuhai complex is composed ofvarious types of gneisses, schists, migmatites, and amphi-bolites, and their protoliths include clastic rocks, interme-diate to basic volcanic and intrusive rocks, and carbonates.The ages of the formations range from 2.3 to 1.1 Ga (Wanget al. 2007).
The Ordovician to the Silurian Nachitai Group con-sists of carbonates, clastic rocks, and volcanic rocks sim-ilar to the sediments of passive continental margins (Niet al. 2010). During the Carboniferous to Permian, marine
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sediments appeared in succession. The upper Permianbasal conglomerates unconformably overlie the middle tolower Permian or upper Carboniferous strata, which isinterpreted as the mark of continental collision or the ini-tial subduction of the A’nyemaqen Palaeo-Tethyan Ocean(Wang et al. 2003a; Li et al. 2012). The presence of latePermian to Middle Triassic calc-alkaline magmatism withsubduction-related signatures (e.g. Zhang et al. 2012) andupper Permian to Middle Triassic marine carbonates andclastic rocks (Cai et al. 2004; Cai et al. 2008) indicatethat the A’nyemaqen Palaeo-Tethyan Ocean had not closeduntil the Middle Triassic. The Upper Triassic BabaoshanFormation consists of marine-continental and continen-tal sediments and conglomerates unconformably overlyingMiddle Triassic marine rocks, and marks the closure ofthe A’nyemaqen Palaeo-Tethyan Ocean. The South KunlunSuture Zone (A’nyemaqen suture zone) is composedmainly of ophiolites, flysch, bioclastic limestone, chert andred abyssal clay, sandstone, and sandy slate (e.g. Bian et al.2004; Yang et al. 2009). The ophiolites spread from westto east along the South Kunlun fault zone, including theBuqingshan, Majixueshan, and Dur’ngoi ophiolites. Theseconsist of serpentinized harzburgites, cumulate gabbrosand pyroxenite, sheeted dikes, basalts, and radiolarian sili-calites (Bian et al. 2004; Yang et al. 2009). Basaltic rocksfrom the ophiolites display MORB-like and minor OIB-like
chemical compositions (Bian et al. 2004; Guo et al. 2006;Yang et al. 2009), with ages ranging from 345 to 308 Ma(Chen et al. 2001; Yang et al. 2009; Liu et al. 2011).
As a result of the N-directed subduction of theA’nyemaqen Palaeo-Tethyan Ocean and subsequentcontinent–continent collision, a large number of latePermian to Late Triassic magmatic rocks, includingabundant granites, mafic intrusive, and volcanic rocks,developed in the north and the south East Kunlun terranes,but more frequently in the north (Figure 1B, e.g. Xionget al. 2012; Zhang et al. 2012). It has been suggestedthat the late Permian to Middle Triassic (263–241 Ma)magmatisms were closely related to the subduction ofthe A’nyemaqen Palaeo-Tethyan Ocean, while magmaticepisodes at the end of the Middle Triassic (231 Ma) andthe Late Triassic (230–213 Ma) were related to subsequentcollision and post-collisional extension events, respectively(e.g. Yang et al. 2009; Ni 2010; Ding et al. 2011; Xionget al. 2012; Zhang et al. 2012).
Xiaomiao mafic dikes
In this study, we investigated more than 40 mafic dikeslocated in the north East Kunlun terrane (Figures 2A and2B). The dikes intruded the Palaeoproterozoic JinshuikouGroup (Figure 1B) as swarms striking 30◦ N–50◦ W
Figure 2. Field photographs (A and B) and photomicrographs of the medium-grained type (A) and the fine-grained type (B) for theXiaomiao mafic dikes.
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and dipping SW, with thicknesses ranging from 1 to2 m (Figures 2A and 2B). Thirteen samples were col-lected from the Xiaomiao mafic dikes (location: 36◦03.233′ N, 96◦ 26.382′ N). All samples were analysedfor geochemistry and Sr–Nd isotopes and one sample wasselected for hornblende 40Ar–39Ar dating. Ren et al. (2010)reported a weighted LA-ICP-MS zircon 206Pb/238U ageof 734 ± 7 Ma for the Xiaomiao mafic dikes and sug-gested that they were related to the break-up of the Rodiniasupercontinent. However, this age is debatable due to thedifferent ages of the inherited and metamorphic zircons.
On the basis of their textures, the mafic dikes could begrouped into two types of hornblende dolerites (Figures 2Cand 2D). The first type displays medium-grained textureswith plagioclases and hornblendes as the common minerals(Figure 2C). The plagioclases show abundant polysynthetictwinning and some zoned texture and some grains arestrongly altered and replaced by clay minerals and epidotes.The hornblendes are brown and sometimes show twinnedtexture. Some hornblendes are replaced by biotite. Rutiles,calcites, and opaque oxides are present in trace amounts.The dolerites of the second type have fine-grained texturescontaining plagioclase and hornblende with minor biotiteand opaque oxides (Figure 2D).
Analytical methods
The hornblende chosen for 40Ar–39Ar age dating was takenfrom a fresh outcrop. The fresh hornblendes were first sep-arated with a Frantz magnetic separator and subsequentlyselected by hand under the microscope. Argon isotope anal-yses were carried out on a GV-5400 mass spectrometer(GV Instruments, Manchester, UK) and a COHERENT-50 W CO2 laser heater (Lightstar Laser Technology CoLtd, Shenzhen, China) at the Guangzhou Institute ofGeochemistry, China Academy of Sciences. The selectedhornblendes and a monitor standard ZBH-25 biotite withan assumed age of 132.5 Ma were irradiated in the Beijing49–2 reactor for 54 h. The details of the experimental pro-cedures and analytical instruments are given in Qiu andJiang (2007). The results of the Argon isotope analysiswere calculated and plotted by the ArArCALC softwareversion 2.40 by Koppers (2002).
Major element abundances were obtained with an ana-lytical precision better than 5% using a 3080E1-type XRFspectrometer (PANalytical B.V., Almelo, The Netherlands)at at the Analytical Institute of the Bureau of Geology andMineral Resources (BGMRHP), Hubei Province, China.Trace elements, including rare earth element (REE), weredetermined by an Agilent 7500a ICP-MS (Musashino-shi, Nakacho, Tokyo, Japan) at the State Key Laboratoryof Geological Processes and Mineral Resources (GPMR),China University of Geosciences (Wuhan). Before test-ing, the samples were digested by HF + HNO3 in teflonbombs. The detailed sample-digesting procedures for ICP-MS analyses and analytical precision and accuracy for trace
elements and the analytical instruments are the same asdescribed by Liu et al. (2008). Analyses for Sr/Nd isotopicratios were conducted on a Finnigan Triton thermo-ionmass isotope spectrometer (Bremen, Germany) at GPMR,China University of Geosciences (Wuhan). The details ofthe instruments, experimental procedures and analyticalprecision have been discussed in some detail by Gao et al.(2004).
Results40Ar–39Ar age
The results of argon isotope analysis for hornblendesfrom the Xiaomiao mafic dikes are given in Table 1 andthe age spectrum is shown in Figure 3. The amount of40Ar released increased with the increase of the laserenergy (Table 1). When the export percentage of thelaser energy varied from 6.2% to 16.0%, the percent-age of 40Ar released ranged from 81.29% to 92.74%.The apparent ages were concentrated between 278.49 and276.35 Ma and yielded a flat plateau as shown in Figure 3.The plateau age is 277.76 ± 2.72 Ma, similar to theisochron age (277.59 ± 3.9 Ma) and the inverse isochronage (277.62 ± 3.91 Ma), except for the existence ofexcess argon. In addition, no evidence exists that thehornblendes were formed by the alteration of the otherminerals (e.g. pyroxenes). Consequently, the plateau age(277.76 ± 2.72 Ma) can be interpreted as the age ofhornblende crystallization and can be used to constrain thetime of emplacement of the Xiaomiao mafic dikes.
Major and trace elements
The data for major and trace elements are listed in Table 2.The mafic dikes have the following composi-
tion: SiO2 = 46.55– 55.75%, TiO2 = 0.71–1.63%,MnO = 0.14–0.18%, P2O5 = 0.11–0.28%,K2O = 0.37–1.91%, Na2O = 2.16–4.30%,(Na2O + K2O) = 2.87–4.95%, CaO = 6.28–11.56%,FeOt = 7.91–12.35%, and MgO = 2.80–7.38% withMg# = 36–61. The sample JS01-17 has contained highlevels of CO2 (2.61%) and H2O (2.69%) because ofalteration and some calcite filling. On a total alkali versussilica plot, all of the samples are plotted in the gabbroand gabbroic diorite fields and exhibit typical sub-alkalinecompositions (Figure 4A). In the FeOt–(Na2O + K2O)–MgO (AFM) diagram, the samples display no ironenrichment trend (Figure 4B). TiO2 and FeOt have positivecorrelations with relatively low Mg# values (e.g. 36–45),but no correlation with higher Mg# values (e.g. 45–61)(Figure 7). This further supports the idea that the samplesbelong to a calc-alkalic series.
The chondrite-normalized REE patterns of the maficdikes uniformly exhibit enrichment of light rare earthelements (LREEs) relative to heavy rare earth elements(HREEs) with (La/Yb)N ratios of 1.93–5.08, and slightly
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Table 1. 40Ar–39Ar age of selected hornblende from the Xiaomiao mafic dikes.
Incremental 40Ar(r) 39Ar(k)heating (%) 36Ar(a) 37Ar(ca) 38Ar(cl) 39Ar(k) 40Ar(r) Age(Ma) ±2σ (%) (%) K/Ca ±2σ
11G2028B 4.5 0.000079 0.001584 0.000022 0.000394 0.015151 435.50 ±19.49 39.39 0.82 0.107 ±0.01211G2028C 5.0 0.000064 0.006521 0.000048 0.000879 0.021888 294.00 ±8.73 53.68 1.84 0.058 ±0.00611G2028D 5.4 0.000047 0.016380 0.000127 0.001416 0.038480 318.60 ±2.85 73.39 2.96 0.037 ±0.00411G2028F 5.8 0.000046 0.019287 0.000153 0.001876 0.047517 298.67 ±4.37 77.62 3.92 0.042 ±0.00411G2028G 6.2 0.000072 0.047588 0.000415 0.003961 0.092737 277.73 ±4.64 81.29 8.27 0.036 ±0.00411G2028H 6.6 0.000057 0.042292 0.000337 0.003514 0.082178 277.45 ±5.13 82.99 7.34 0.036 ±0.00411G2028I 7.0 0.000043 0.038573 0.000294 0.003239 0.076103 278.66 ±5.45 85.61 6.76 0.036 ±0.00411G2028J 7.4 0.000038 0.045453 0.000373 0.003440 0.080591 277.86 ±4.21 87.66 7.18 0.033 ±0.00311G2028K 7.8 0.000010 0.012108 0.000109 0.000925 0.021621 277.43 ±3.42 87.49 1.93 0.033 ±0.00311G2028M 8.4 0.000029 0.050295 0.000391 0.003728 0.087299 277.79 ±3.05 91.12 7.78 0.032 ±0.00311G2028N 9.0 0.000057 0.090812 0.000762 0.007269 0.170696 278.49 ±2.21 90.97 15.18 0.034 ±0.00411G2028O 9.6 0.000043 0.091110 0.000720 0.006895 0.161134 277.24 ±2.22 92.74 14.40 0.033 ±0.00311G2028P 10.2 0.000033 0.061427 0.000474 0.004551 0.106513 277.65 ±2.30 91.56 9.50 0.032 ±0.00311G2028Q 11.0 0.000028 0.041294 0.000323 0.003175 0.074327 277.67 ±2.03 90.12 6.63 0.033 ±0.00311G2028R 12.5 0.000016 0.025636 0.000195 0.002012 0.047159 277.96 ±3.51 90.68 4.20 0.034 ±0.00411G2028T 16.0 0.000006 0.008293 0.000057 0.000612 0.014249 276.35 ±8.72 89.73 1.28 0.032 ±0.003
0.000669 0.598653 0.004801 0.047887 1.137645
Figure 3. Age spectrum of hornblendes from the Xiaomiao mafic dikes.
positive Eu anomalies (Eu/Eu∗ = 0.93–1.40) (Figure 5A).In the primitive mantle-normalized trace element spider-gram (Figure 5B), most of the mafic dikes are characterizedby enrichment of LILEs – such as Rb and Ba – and LREE,and the depletion of HFSE, such as Nb, Ta, and Ti. This issimilar to subduction-related basaltic rocks (e.g. Island arcbasalt (IAB)), but different from E-MORB and N-MORB(Figure 5B).
Sr–Nd isotope compositions
Data for Sr–Nd isotope compositions are given in Table 2with initial ratios calculated at an age of 278 Ma. The maficdikes have variable initial Sr ratios (ISr = 0.707–0.715),εNd(t) = –2.26 to +2.91, and TDM = 1.2–1.6 Ga.On the basis of the initial Sr ratios and εNd(t) values,the Xiaomiao mafic dikes can be subdivided into two
groups, one with relatively low initial Sr ratios and highεNd(t) values (ISr = 0.707–0.710, εNd(t) = 0.40–2.91),and the other with high Sr ratios and low εNd(t) values(ISr = 0.709–0.715, εNd(t) = –2.60 to –0.28). Figure 6shows that the Xiaomiao mafic dikes have lower εNd(t) val-ues than the Carboniferous A’nyemaqen MORB, but highervalues than the late Permian Bairiqili mafic dikes (251 Ma),the late Permian to Middle Triassic calc-alkali magmaticrocks and the Precambrian basement (Figure 6; Bian et al.2004; Yu et al. 2005; Xiong et al. 2011; Zhang et al. 2012).
Discussion
Fractional crystallization and crustal contamination
All of the samples from the Xiaomiao mafic dikes couldhave experienced different degrees of fractional crystal-lization prior to their emplacement due to their lower Mg#
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Tabl
e2.
Maj
oran
dtr
ace
elem
ents
and
Sr–
Nd
isot
opic
com
posi
tion
sof
sele
cted
sam
ples
from
the
Xia
omia
om
afic
dike
s.
Sam
ple
NM
06-1
NM
06-2
NM
06-3
AN
M06
-3B
JS01
-1JS
01-3
JS01
-12
JS01
-13
JS01
-17
JS01
-20
JS01
-9JS
01-1
9JS
01-2
112
JS02
XM
122a
XM
22a
XM
23a
XM
24a
XM
26a
XM
27a
SiO
251
.44
46.5
548
.53
55.7
548
.61
50.6
849
.97
48.8
951
.33
50.1
954
.93
52.7
349
.58
5249
.21
48.0
154
.54
51.9
453
.551
.63
TiO
21.
211.
331.
250.
711.
21.
631.
371.
081.
181.
270.
911.
171.
421.
221.
591.
640.
791.
231.
241.
23A
l 2O
316
.57
17.2
219
.17
18.1
717
.97
17.6
218
.62
1917
.63
18.7
818
.17
18.3
918
.34
16.7
418
.13
18.0
619
.33
16.9
117
.89
16.9
6Fe
2O
31.
083.
252.
642.
813.
173.
643.
116.
541.
891.
312.
421.
573.
591.
6811
.82
12.3
59.
038.
9210
.20
8.87
FeO
6.94
7.63
7.02
5.38
6.6
7.15
7.4
2.87
7.1
7.55
6.1
7.3
6.6
6.25
FeO
t7.
9110
.55
9.40
7.91
9.45
10.4
310
.20
8.75
8.80
8.73
8.28
8.71
9.83
7.76
10.6
411
.11
8.13
8.03
9.18
7.98
MnO
0.14
0.18
0.16
0.16
0.16
0.18
0.18
0.16
0.14
0.15
0.16
0.15
0.17
0.14
0.18
0.17
0.15
0.18
0.18
0.14
MgO
6.84
7.38
5.74
2.8
6.49
4.06
5.19
3.86
3.24
4.57
2.77
3.89
3.95
6.73
4.81
5.25
2.87
6.7
3.26
6.49
CaO
9.15
10.4
38.
786.
429.
537.
846.
2811
.56
7.36
8.35
6.28
6.92
8.07
8.9
8.16
7.96
7.16
9.38
6.94
9.24
Na 2
O2.
892.
353.
174.
32.
163.
473.
632.
53.
552.
723.
693.
583.
292.
792.
982.
963.
842.
633.
522.
8K
2O
0.54
0.92
0.72
0.65
1.8
0.76
0.64
0.37
0.79
1.91
1.39
1.2
1.02
0.98
1.19
1.23
0.72
0.31
1.49
0.5
P2
O5
0.13
0.12
0.11
0.23
0.14
0.26
0.21
0.13
0.28
0.18
0.24
0.16
0.11
0.14
0.26
0.24
0.22
0.16
0.21
0.15
CO
20.
740.
180.
110.
590.
280.
620.
10.
142.
610.
550.
90.
451.
250.
82H
2O
+2.
172.
262.
391.
841.
721.
893.
142.
712.
692.
281.
842.
32.
41.
43L
OI
1.29
1.69
2.39
2.3
4.42
1.91
22.
072.
851.
861.
911.
81.
51.
791.
591.
88To
tal
99.8
499
.80
99.7
999
.81
101.
1210
1.49
102.
2310
2.11
104.
2110
1.72
101.
8010
1.88
102.
6410
1.68
100.
2499
.67
100.
1510
0.15
100.
0299
.89
Na 2
O+
K2
O3.
433.
273.
894.
953.
964.
234.
272.
874.
344.
635.
084.
784.
313.
774.
174.
194.
562.
945.
013.
30
Mg#
60.6
455
.48
52.1
338
.69
55.0
340
.97
47.5
644
.01
39.6
248
.27
37.3
644
.31
41.7
361
.00
44.6
345
.71
38.6
359
.81
38.7
759
.17
CaO
/A
l 2O
30.
550.
610.
460.
350.
530.
440.
340.
610.
420.
440.
350.
380.
440.
530.
450.
440.
370.
550.
390.
54B
e1.
10.
70.
50.
90.
71.
10.
80.
51.
10.
61.
10.
90.
71.
31.
01.
00.
91.
21.
01.
1S
c30
.438
.824
.98.
030
.320
.519
.620
.917
.422
.013
.119
.318
.829
.422
.925
.311
.528
.814
.127
.6V
183
331
203
5425
420
417
617
413
818
282
172
201
172
230
268
7417
980
173
Cr
248.
212
8.2
85.9
4.4
38.5
0.8
1.7
37.7
2.5
0.8
225.
920
.534
.32.
522
2.7
3.9
209.
1C
o38
.135
.331
.814
.938
.920
.432
.723
.419
.326
.741
.123
.924
.338
.828
.227
.316
.035
.511
.334
.7N
i77
.726
.617
.60.
811
.90.
42.
028
.20.
32.
00.
00.
275
.94.
97.
81.
371
.82.
567
.3C
u49
.324
.418
.77.
217
.19.
08.
610
.75.
89.
84.
28.
89.
442
.3Z
n71
.796
.167
.167
.581
.971
.084
.354
.583
.166
.583
.477
.963
.367
.3G
a17
.918
.016
.417
.517
.516
.716
.518
.018
.916
.919
.218
.415
.417
.216
.616
.817
.717
.515
.117
.0R
b34
.750
.923
.421
.510
5.1
25.9
34.7
15.5
40.5
104.
041
.127
.634
.683
.239
.346
.729
.016
.058
.335
.2S
r22
036
047
950
939
854
047
469
555
741
551
646
751
321
756
356
356
622
653
218
6Y
28.7
17.8
15.8
20.2
15.7
22.4
17.7
16.8
22.0
18.3
22.6
22.3
20.3
29.7
21.8
21.2
20.4
29.7
27.8
33.7
Zr
125
8163
117
6312
187
7212
177
136
129
7412
110
496
109
123
125
120
Nb
4.0
3.1
2.4
4.4
2.5
6.3
3.8
2.7
5.9
3.5
6.7
5.4
2.9
4.0
5.8
5.3
4.1
4.0
5.0
3.8
Cs
4.32
4.47
1.46
1.35
11.6
72.
692.
340.
734.
959.
172.
743.
442.
817.
672.
914.
062.
461.
672.
512.
82B
a92
205
229
254
175
225
165
9238
638
455
546
422
798
380
390
289
9540
474
La
8.1
8.4
6.7
11.1
7.4
15.6
8.9
7.4
12.9
6.4
12.4
10.2
8.1
8.2
14.9
13.8
11.4
8.7
14.1
8.2
Ce
20.9
18.6
15.1
27.8
15.0
34.3
18.1
15.0
27.4
13.6
24.8
21.2
16.7
20.7
29.5
27.4
24.4
21.1
29.0
19.9
Pr
3.02
2.49
2.08
3.48
2.28
4.24
2.82
2.43
3.99
2.24
3.81
3.32
2.70
2.85
3.51
3.25
3.09
2.81
3.75
2.68
Nd
14.0
11.1
9.7
15.2
10.6
18.0
12.8
11.2
17.3
10.5
16.7
14.9
12.4
14.0
16.3
15.5
14.9
14.1
17.7
13.4
Sm
4.07
2.87
2.63
3.74
2.60
4.20
3.06
2.67
3.86
2.67
3.84
3.42
3.11
3.93
3.90
3.70
3.58
3.95
4.46
3.85
Eu
1.36
1.11
1.24
1.66
1.25
1.59
1.36
1.32
1.69
1.30
1.68
1.52
1.41
1.35
1.46
1.42
1.57
1.30
1.68
1.28
(Con
tinu
ed)
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nloa
ded
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eå U
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rsity
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rary
] at
07:
03 1
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International Geology Review 63
Tabl
e2.
(Con
tinu
ed).
Sam
ple
NM
06-1
NM
06-2
NM
06-3
AN
M06
-3B
JS01
-1JS
01-3
JS01
-12
JS01
-13
JS01
-17
JS01
-20
JS01
-9JS
01-1
9JS
01-2
112
JS02
XM
122a
XM
22a
XM
23a
XM
24a
XM
26a
XM
27a
Gd
4.83
3.21
2.96
3.83
2.90
4.41
3.35
3.08
4.19
3.20
4.11
3.92
3.56
4.60
3.95
3.79
3.49
4.50
4.65
4.34
Tb
0.81
0.52
0.46
0.59
0.45
0.65
0.52
0.48
0.63
0.51
0.63
0.62
0.57
0.83
0.63
0.59
0.55
0.78
0.75
0.76
Dy
5.07
3.09
2.90
3.51
2.95
4.10
3.05
2.97
3.90
3.25
3.96
3.91
3.66
5.20
3.73
3.58
3.31
4.90
4.60
4.77
Ho
1.11
0.70
0.62
0.78
0.57
0.81
0.65
0.61
0.78
0.66
0.82
0.81
0.74
1.02
0.80
0.77
0.72
1.07
1.00
1.04
Er
3.06
1.94
1.63
2.12
1.60
2.29
1.81
1.68
2.26
1.89
2.26
2.26
2.04
3.01
2.15
2.08
1.95
2.91
2.75
2.84
Tm
0.45
0.28
0.24
0.32
0.23
0.33
0.28
0.25
0.32
0.28
0.32
0.34
0.31
0.42
0.31
0.30
0.29
0.42
0.40
0.41
Yb
2.85
1.83
1.63
2.12
1.48
2.07
1.69
1.54
2.12
1.70
2.22
2.14
1.88
2.77
2.04
1.91
1.95
2.74
2.58
2.69
Lu
0.42
0.25
0.23
0.33
0.23
0.32
0.27
0.25
0.35
0.29
0.38
0.36
0.30
0.40
0.31
0.30
0.31
0.41
0.40
0.40
Hf
3.07
1.97
1.65
2.86
1.71
2.87
2.15
1.92
2.89
1.98
3.30
3.26
1.98
3.03
2.50
2.33
2.73
3.06
3.07
2.93
Ta0.
360.
200.
170.
300.
170.
420.
270.
270.
680.
250.
501.
380.
200.
340.
360.
330.
270.
350.
310.
33P
b11
.515
.311
.515
.113
.810
.18.
720
.319
.218
4.2
10.7
12.6
7.4
7.6
8.3
6.4
5.4
9.4
6.0
6.6
Th
3.22
1.44
1.04
1.88
1.37
4.00
1.36
1.09
2.63
0.87
1.51
1.42
1.32
3.11
3.63
3.21
1.70
3.14
2.59
2.91
U1.
260.
430.
350.
590.
391.
080.
420.
401.
020.
540.
500.
450.
421.
181.
020.
900.
551.
370.
801.
07(L
a/Y
b)N
1.93
3.10
2.77
3.55
3.39
5.08
3.55
3.23
4.12
2.56
3.78
3.22
2.90
2.01
4.92
4.88
3.94
2.15
3.69
2.06
Eu/
Eu∗
0.93
1.12
1.36
1.33
1.38
1.12
1.29
1.40
1.28
1.36
1.28
1.26
1.29
0.97
1.13
1.15
1.34
0.94
1.12
0.95
Nb/
La
0.50
0.37
0.36
0.40
0.34
0.41
0.43
0.36
0.46
0.55
0.54
0.53
0.36
0.49
0.39
0.38
0.36
0.45
0.36
0.47
Nb/
Ce
0.19
0.17
0.16
0.16
0.17
0.18
0.21
0.18
0.22
0.26
0.27
0.25
0.18
0.19
0.20
0.19
0.17
0.19
0.17
0.19
Nb/
Ta11
.19
15.8
314
.37
14.8
214
.41
14.8
814
.12
9.92
8.68
14.2
013
.27
3.89
15.0
511
.71
16.0
316
.00
15.1
511
.31
16.2
611
.58
Zr/
Hf
40.6
541
.18
38.2
840
.84
36.7
042
.00
40.6
237
.40
42.0
138
.61
41.1
839
.57
37.5
439
.91
41.7
841
.41
39.9
140
.22
40.6
440
.82
Ce/
Y0.
731.
040.
961.
380.
961.
531.
020.
901.
250.
741.
100.
950.
820.
701.
361.
291.
190.
711.
040.
59S
m/Y
b1.
431.
571.
611.
761.
762.
031.
811.
731.
821.
571.
731.
601.
651.
421.
911.
941.
841.
441.
731.
43L
a/Y
b2.
854.
594.
105.
255.
017.
525.
254.
786.
093.
795.
594.
774.
292.
987.
287.
225.
843.
185.
473.
04L
a/N
b2.
022.
682.
752.
522.
952.
472.
332.
782.
171.
821.
851.
902.
752.
052.
572.
612.
782.
202.
802.
14L
a/B
a0.
090.
040.
030.
040.
040.
070.
050.
080.
030.
020.
020.
020.
040.
080.
040.
040.
040.
090.
030.
11T
h/Y
b1.
130.
790.
630.
890.
931.
930.
810.
711.
240.
510.
680.
670.
701.
131.
781.
680.
871.
151.
001.
08B
a/T
h28
.57
142.
5022
1.25
134.
7012
8.01
56.2
412
0.83
84.8
214
6.47
443.
9436
6.71
325.
7217
1.62
31.3
510
4.74
121.
5516
9.71
30.1
615
6.16
25.3
5T
h/N
b0.
800.
460.
430.
430.
550.
630.
360.
410.
440.
250.
230.
270.
450.
770.
630.
610.
420.
790.
510.
76B
a/L
a11
.31
24.3
534
.23
22.7
823
.63
14.4
618
.60
12.5
229
.89
59.7
344
.76
45.4
628
.08
11.8
425
.60
28.2
925
.35
10.8
628
.69
9.01
Sr/
Nd
15.7
532
.33
49.2
133
.40
37.4
929
.97
37.1
062
.03
32.1
839
.55
30.8
431
.39
41.4
415
.50
34.5
936
.40
37.8
916
.08
30.1
213
.80
Nb/
Y0.
140.
180.
150.
220.
160.
280.
220.
160.
270.
190.
300.
240.
150.
140.
270.
250.
200.
130.
180.
11T
i/Y
252.
6944
6.77
475.
4521
0.77
458.
6743
5.72
464.
4038
5.98
321.
7941
5.99
240.
8931
4.01
419.
7724
6.47
438.
1446
3.87
231.
6524
8.13
267.
4321
8.56
87R
b/86
Sr
0.45
630.
4087
0.14
130.
1224
0.76
460.
1389
0.21
190.
0648
0.21
050.
7251
0.17
1387
Sr/
86S
r0.
7113
680.
7108
350.
7078
430.
7080
430.
7101
460.
7101
740.
7110
930.
7086
360.
7156
380.
7135
310.
7116
89
(87
Sr/
86S
r)I
(IS
r)0.
710
0.70
90.
707
0.70
80.
707
0.71
00.
710
0.70
80.
715
0.71
10.
711
147
Sm
/14
4N
d0.
1757
0.15
580.
1636
0.14
840.
1480
0.14
100.
1447
0.14
430.
1348
0.15
360.
1391
(143
Nd/
144
Nd)
i0.
5127
490.
5125
540.
5126
360.
5125
890.
5125
880.
5124
760.
5125
640.
5126
150.
5123
920.
5125
450.
5124
63εN
d(t
=27
8M
a)2.
9−0
.21.
10.
80.
8−1
.20.
41.
4−2
.6−0
.3−1
.4T
DM
(Ga)
1.6
1.6
1.6
1.3
1.3
1.4
1.3
1.2
1.5
1.5
1.4
Not
es:F
eOt=
All
Feca
lcul
ated
asFe
O;M
g#=
100∗
(Mg2+
/(M
g2++ F
e t2+
);B
lank
-und
etec
ted
orun
calc
ulat
ed.
a Dat
afr
omR
enet
al.(
2010
).
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64 B. Liu et al.
Figure 4. Total alkali versus silica (TAS) diagram (A); after Middlemost, 1994) and FeOt–(Na2O + K2O)–MgO (AFM) diagram (B);after Irvine and Barager (1971) of the Xiaomiao mafic dikes.
Figure 5. Chondrite-normalized REE distribution patterns (A) and primitive mantle-normalized trace element spider diagrams (B) ofthe Xiaomiao mafic dikes.
Note: Data for chondrite are from Taylor and Mclennan (1985); data for primitive mantle, N-MORB, and E-MORB are from Sun andMcDonough (1989); data for IAB are from Niu and O’Hara (2003); data for the late Permian mafic dike swarm are from Xiong et al.(2011); data for mafic dikes from the supra-subduction zone environment are from Creixell et al. (2009) and Scarrow et al. (1998).
Figure 6. Initial Sr ratios (ISr) versus εNd(t) diagram for theXiaomiao mafic dikes.
Note: Data for the Precambrian basement are from Yu et al.(2005); data for MORB are from Bian et al. (2004); data for latePermian mafic dikes swarms are from Xiong et al. (2011).
(36–61), Cr (0.79–248.20 ppm), Co (11.29–41.06 ppm),and Ni (0.04–77.65) compared with those of typicalprimitive basaltic melts (Frey et al. 1978; Tatsumi andEggins 1995). Various binary diagrams taking Mg# asthe abscissa should clearly reveal the process of frac-tional crystallization. In the binary diagrams (Figure 7), theXiaomiao mafic dikes show positive correlations betweenCaO, Cr, Ni, CaO/Al2O3, and Sc/Y versus Mg#, indicat-ing fractional crystallization of clinopyroxene and olivine(e.g. Graham et al. 1995; Morra et al. 1997; Naumann andGeist 1999). The negative correlation between P2O5 andMg# values (Figure 8) indicates no significant fractiona-tion of apatite. TiO2 and FeOt have positive correlationswith relatively low Mg# values (e.g. 36–45) but show nocorrelations with higher Mg# values (e.g. 45–61), suggest-ing that the fractional crystallization of Fe–Ti oxides mightonly occur in the late stage of mafic magma evolution.In the primitive mantle-normalized trace element spiderdiagram, most samples show depletion of Ba relative toRb (Figure 5). This suggests that hornblende fractionationoccurred during the evolution of the mafic magma because
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International Geology Review 65
Figure 7. Variation diagrams of TiO2, FeOt, CaO, P2O5, Cr, Ni, CaO/Al2O3, Sc/Y and Nb/La versus Mg# for the Xiaomiao mafic dikes.
Figure 8. εNd(t) and ISr versus SiO2 diagram, (A) and (B), for the Xiaomiao mafic dikes.
the Rb partition coefficient is higher than that of Ba inhornblende (Green 1994). This is also consistent with thepresence of hornblende as the dominant mafic phenocrystin the Xiaomiao mafic dikes. Overall, the Xiaomiao maficdikes could have undergone fractional crystallization ofclinopyroxene, olivine, and hornblende as well as someFe–Ti oxides.
Assimilation and fractional crystallization (AFC) hasbeen recognized as an important process during magmaticevolution, which can modify the elemental and isotopiccompositions of the initial magma (DePaolo 1981; Halamaet al. 2004; Mir et al. 2011). AFC does not readily takeplace in the shallow crust, as the cool shallow crust cannot
provide enough energy for the process and is difficultto consume (Scarrow et al. 1998). The Xiaomiao maficdikes were emplaced in the Precambrian basement in alate stage, indicating insignificant contamination of thebasement. In addition, crustal contamination increasesLILEs, K2O, and Na2O, but decreases P2O5, TiO2, andNb/La (e.g. Zhao and Zhou 2007; Mir et al. 2011). TheXiaomiao mafic dikes exhibit an obvious negative correla-tion between P2O5 versus Mg# and no correlation betweenNb/La versus Mg# (Figures 7A and 7I), which suggestsinsignificant crustal contamination. The Nb/La and Nb/Ceratios (0.36–0.55 and 0.16–0.27, respectively) are lowerthan those of the primitive mantle, average bulk crust,
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and average lower crust (1.02, 0.4; 0.69, 0.33; 0.83, and0.39; after Taylor and McLennan 1985), further suggestingthat crustal contamination was insignificant. The Nb/Ta(most >13, Table 2) and Zr/Hf values (36.70–42.00) areclose to those of MORB (17.7 and 36.1, respectively; afterSun and McDonough 1989), but differ from those of thecontinental crust (11 and 33, respectively; after Taylorand McLennan 1985), suggesting minimal crustal contam-ination of the mafic dikes. In addition, the absence ofcorrelations between ISr, εNd(t), and SiO2 (Figures 8A and8B) also suggests that crustal contamination was absent(Girardi et al. 2012).
Source characteristics and evidence forsubduction-related enrichment
Mafic dikes are usually derived from a lithospheric or anasthenospheric mantle (e.g. Sklyarov et al. 2003; Zhaoand Zhou 2007). In the primitive mantle-normalized traceelement spidergram, samples from a lithospheric mantleusually exhibit LILE and LREE enrichment and HFSEdepletion (e.g. Nb, Ta, and Ti), similar to subduction-related igneous rocks (Zhao et al. 2010). In contrast,samples from an asthenospheric mantle (e.g. OIB) usu-ally show LILE and HFSE enrichment (Zou et al. 2000)and have similar Sr–Nd isotope compositions as a depletedmantle (Chen et al. 2011a). The Xiaomiao mafic dikes arecharacterized by relatively low TiO2 (0.71–1.64%), enrich-ment of LILE and LREE and depletion of HFSE such asNb, Ta, and Ti relative to the primitive mantle (Figure 5)and resembles a lithospheric mantle.
REE ratios and abundances (e.g. Ce/Y, Sm/Yb, andLa/Yb and Sm) are widely used to measure the originof mafic magmas and the degree and variation of mantlemelting (e.g. Mckenzie and Bickle 1988; Mckenzie andO’Nions 1991; Aldanmaz et al. 2000; Su et al. 2012). Therelatively low Ce/Y content (0.59–1.53, Table 2) of themafic dikes suggests that they could be generated within
a spinel-garnet stability field (e.g. Mckenzie and Bickle1988). Moreover, in the Sm/Yb versus Sm and La/Ybdiagrams, the Xiaomiao mafic dikes plot near or belowthe spinel + garnet lherzolite melting curves with prim-itive mantle starting compositions. Their Sm/Yb ratiosare lower than the garnet lherzolite melting curves buthigher than the spinel lherzolite melting curves (Figures 9Aand 9B). This suggests that the parental magma might bederived from a mantle source consisting of spinel + minorgarnet lherzolite. Additionally, an approximately 10–20%partial melting of the lherzolites is required (Figures 9Aand 9B).
The Xiaomiao mafic dikes have two distinct groups ofSr–Nd isotope compositions: one with relatively low ini-tial Sr ratios and high εNd(t) values (ISr = 0.707–0.710,εNd(t) = 0.40–2.91) and the other with high initial Sr ratiosand low εNd(t) values (ISr = 0.709–0.715, εNd(t) = –2.60 to–0.28) (Figure 6). The two groups may represent two dif-ferent mantle sources, one depleted and one enriched.However, most of the Xiaomiao mafic dikes have thesame geographic occurrence and a similar geochemistry.They exhibit the features of comagmatic evolution in aHarker diagram, which is inconsistent with different man-tle sources. As discussed above (see previous section),crustal contamination is insignificant in the generation ofthe Xiaomiao mafic dikes. Consequently, the Xiaomiaomafic dikes may be derived from a heterogeneous mantlemodified by the fluid derived from the subducted-slab.
The Xiaomiao mafic dikes have a relatively high Th/Ybratio, and all of the project points fall outside the MORB-OIB array (Figure 10A), suggesting slab subduction-related enrichment (Pearce and Peate 1995). All of thesamples exhibit relatively high La/Nb (1.82–2.95) andlow La/Ba (0.02–0.11), which is typical of a subduction-modified continental lithospheric mantle (CLM) source(Saunders et al. 1992). This introduces another question.How does slab subduction modify the composition of themantle source: via interaction between a depleted mantle
Figure 9. Sm/Yb versus Sm (A) and Sm/Yb versus La/Yb (B) diagrams for the mafic dikes. Melting curves are after Aldanmaz et al.(2000) and Zhao and Zhou (2007, 2009). Dashed and solid curves are the melting trends from DM (depleted MORB) and PM (primitivemantle).
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Figure 10. Th/Yb–Nb/Yb, TiO2/Yb–Nb/Yb, Ba/Th–Th/Nb and Ba/La–Th/Yb plots for the Xiaomiao mafic dikes (after Pearce 2008).(A) and (B) are after Pearce (2008); (C) and (D) are after Hanyu et al. (2006) and Woodhead et al. (2001), respectively; data for thebasaltic rocks from oceanic arcs (including Tonga, Kermadec, Boinin, Mariana, and Aleutian arcs) are from the data base PetDB; data forthe basaltic rocks from the continental arc (Andean volcanic arc) are from the data base GEOROC.
and the fluids or melts released from altered oceanic crust,or from subducted sediments? Basaltic magmas formed bypartial melting of the mantle peridotite that have interactedwith slab melts usually have relatively high Na2O, P2O5,and TiO2, positive to weakly negative Nb anomalies, andnon-negative Ti anomalies relative to the primitive mantle(e.g. Sajona et al. 2000; Wang et al. 2003b).
All of the Xiaomiao mafic dikes display relativelylow TiO2 and P2O5 contents and notable negativeNb and Ti anomalies relative to the primitive mantle(Figure 5b), thus precluding the interaction between adepleted mantle and slab melts. Trace element ratios(e.g. Ba/Th, Th/Nb, Ba/La, and Th/Yb) are widelyused to identify metasomatic agents (aqueous fluids orsediments). The Xiaomiao mafic dikes have variableBa/Th (25.35–443.94) and Ba/La (9.01–59.78) ratiosbut relatively constant Th/Nb (0.23–0.80) and Th/Yb(0.51–1.93) ratios (Figures 10C and 10D). This resultcan be explained by the addition of aqueous fluidand minor sediment components into the mantle source(Woodhead et al. 2001; Hanyu et al. 2006; Tian et al.2011). The LREE-enrichment and positive Eu anoma-lies (Figure 5A) also indicate the addition of aque-ous fluids (Bau and Knittel 1993). The highly variable
Sr/Nd (13.80–62.03) ratios and relatively low Th/Yb(0.51–1.93) ratios of the Xiaomiao mafic dikes further sug-gest that the fluids could be derived from an altered oceaniccrust, rather than from subducted sediments (Woodheadet al. 1998).
In summary, we propose that the Xiaomiao mafic dikeswere most likely generated by 10–20% partial melting ofa spinel + minor garnet lherzolite mantle source meta-somatized by slab-derived fluids and minor subductedsediments.
Tectonic implications
Previous studies of the late Permian to Late Triassic evo-lution (263–213 Ma) of the A’nymaqen Palaeo-TethyanOcean in the East Kunlun region have proposed a modelinvolving a late Permian to Middle Triassic subduction,a collision at the end of the Middle Triassic, followedby a Late Triassic post-collision extension (e.g. Yanget al. 2009; Zhang et al. 2012). However, we knownothing about the early evolution (308–260 Ma) of theA’nymaqen Palaeo-Tethyan Ocean due to the absenceof late Carboniferous to late Permian (308–260 Ma)magmatic rocks and the presence of Carboniferous to
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Permian stable marine sediments with identical palaeo-geographical systems (Chen et al. 2008, 2010; Zhu et al.2009). Therefore, the early Permian Xiaomiao mafic dikes(∼278 Ma) reported in this article could provide someimportant clues for tracing the early tectonic evolution(C-P) of the Palaeo-Tethyan Ocean.
By using multiple discrimination diagrams based onimmobile elements (e.g. Ti, Zr, Y, Nb, Th, etc.), we wereable to effectively fingerprint basalts from different tec-tonic settings (e.g. Pearce and Cann 1973; Wood 1980;Pearce 1982). In the Ti versus Zr diagram, all of the sam-ples from the Xiaomiao mafic dikes plot in the volcanicarc basalt (VAB) field or in the area between VAB andMORB (Figure 11A). In the Cr versus Y and Hf/3–Th–Tadiagrams, most samples plot in the VAB field, while onlyscattered samples plot in the within-plate basalt (WPB)or MORB field (Figures 11B and 11C). In addition, theXiaomiao mafic dikes display enrichment of LILE andLREE and depletion of HFSE such as Nb, Ta, and Ti, whichfits to VAB and mafic dikes from the supra-subductionzone environment, but are distinct from MORB (Figure 5).Evidence also exists that the mantle source of the Xiaomiao
mafic dikes had been modified by slab subduction (see pre-vious section). Consequently, the Xiaomiao mafic dikesprobably formed in a supra-subduction zone setting,which indicates arc, arc front, and back-arc environ-ments (e.g. Scarrow et al. 1998; Allen 2000; Khan et al.2007).
Most samples of the Xiaomiao mafic dikes have higherFeOt/MgO and lower TiO2 than back-arc rocks, which dif-fer significantly from back-arc basalts and samples of back-arc mafic dike swarms, but are similar to samples of maficdike swarms from continental margin arcs (Figure 11D).The Xiaomiao mafic dikes have higher Nb/Yb, Th/Yb,and TiO2 than most oceanic arc basalts (Figures 10A and10B) and emplaced into the Precambrian basement in thelate stage, further suggesting a continental margin arc set-ting. However, the formation of the Xiaomiao mafic dikesis approximately 15 million years earlier than that of themagmatic arc in the EKOB, suggesting that the Xiaomiaomafic dikes could have formed in a special tectonic settingwhich is different from that of a continental margin arc.
The subduction infancy model (Stern and Bloomer1992) and the subduction initiation rule (Whattam and
Figure 11. Ti –Zr, Ti/Y–Nb/Y, Cr–Y, and FeOt/MgO–TiO2 diagrams for the Xiaomiao mafic dikes. (A) and (B) are after Pearce (1982);(C) is after Wood (1980); (D) the discrimination of continental arc-front and back-arc rocks is from Allen (2000); data for Andes back-arcbasalts are from Kay et al. (1994); data for mafic dike swarms from continental margin arc environment are from Scarrow et al. (1998);data for mafic dike swarms from back-arc environment are from Khan et al. (2007) and Allen (2000). Symbols as in Figure 5.
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Figure 12. Cartoons showing the early plate tectonics of the Palaeo-Tethyan evolution in the East Kunlun region.
Stern 2011) indicate that MORB/forearc basalt (FAB)tholeiites were first formed by decompression melting ofthe asthenospheric mantle at the beginning of the sinking ofthe oceanic lithosphere and the early proto-forearc spread-ing along the margin of the adjacent plate. VAB/Island arctholeiitic basalt (IAT) ± boninites then formed by par-tial melting of a depleted mantle strongly modified byslab-derived fluids. The magmatic arc was finally formedas the true subduction began and trench rollback slowedand stabilized. The basaltic lavas (346–308 Ma) fromthe A’nyemaqen ophiolites display a MORB and minorOIB-like chemical composition (Bian et al. 2004; Guoet al. 2006; Yang et al. 2009) and are considered to bederived from a depleted mantle (Figure 6). This suggeststhat they might be formed in the spreading period of
the A’nyemaqen Palaeo-Tethyan Ocean basin (Yang et al.2009) or at the beginning of the sinking of the oceaniclithosphere and early subduction initiation (Figure 12A;e.g. Metcalf and Shervais 2008; Dilek and Furnes 2009;Whattam and Stern 2011).
As discussed above, the Xiaomiao mafic dikes haveVAB/IAT-like chemical compositions, exhibit some fea-tures of the transition from a depleted mantle to an enrichedmantle (Figure 6), and are generated by partial meltingof the mantle source metasomatized by slab-derived flu-ids and minor subducted sediments. This indicates that theXiaomiao mafic dikes formed during the late subductioninitiation prior to the true subduction that generated themagmatic arc in the East Kunlun region (Figure 12B).Furthermore, the presence of the large-scale late Permian
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to Middle Triassic calc-alkaline magmatism, which hasobvious subduction-related features (e.g. Xiong et al. 2012;Zhang et al. 2012), might mark a gradual generation of amagmatic arc and the maturity of the subduction zone inthe East Kunlun region (Figure 12C).
The late Permian (∼250 Ma) mafic dike swarm showsenrichment of LILE, obvious depletion of HFSE (e.g. Nb,Ta, and Ti), an enriched mantle source, and relatively highFeOt/MgO and lower TiO2 (Figures 5 and 11C). Thisfurther supports slab subduction and the generation of con-tinental margin arc in the late Permian (Figure 12C). Fromthe Carboniferous to the late Permian, the εNd(t) values ofmafic rocks are gradually reduced and the ISr values grad-ually increased (Figure 6), indicating that the subductionof the A’nyemaqen Palaeo-Tethyan Ocean substantiallystrengthened at that time.
In conclusion, we suggest that the early plate tec-tonics (C-P) of the Palaeo-Tethyan Ocean includes theCarboniferous to early Permian spreading of the oceanbasin and initiation of subduction, and a late Permian truesubduction resulting in the generation of a magmatic arc(Figure 12). We infer that the initiation of the A’nyemaqenPalaeo-Tethyan N-directed subduction commenced no laterthan 278 Ma.
Conclusions
Based on the bulk-rock geochemistry, 40Ar–39Ar ages, andSr–Nd isotopic studies of the early Permian mafic dikeswarm in the EKOB, our main conclusions are as follows.
(1) We interpret the 40Ar–39Ar age of277.76 ± 2.72 Ma as the age of hornblendecrystallization (i.e. the Xiaomiao mafic dikesformed in the early Permian).
(2) Fractional crystallization of clinopyroxene, olivine,and hornblende ± Fe–Ti oxides was responsible forthe chemical variation of these mafic dikes.
(3) Geochemical and Sr–Nd isotopic data suggestthat the mafic dikes were probably generated by10–20% partial melting of a spinel and minorgarnet lherzolite mantle source, metasomatized byslab-derived fluids and minor subducted sediments.
(4) The early tectonic evolution of the Palaeo-TethyanOcean involved Carboniferous to early Permianocean basin spreading and a transition to latePermian subduction.
AcknowledgementsThis work was financially supported by the National NatureScience Foundation of China (Grant No. 41272079) andthe China Geological Survey (No. 1212010918002 andNo. 1212011121270). We thank Huaning Qiu, YongshengLiu, Haihong Chen, and Keqing Zong for their help with theanalytical work. Special thanks go to Professor Carl Eherls andProfessor Wenliang Xu for their meticulous language editing andconstructive reviews of this article.
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