Transitional time of oceanic to continental subduction in the Dabie orogen:Constraints from U –Pb,Lu –Hf,Sm –Nd and Ar –Ar multichronometric dating
Hao Cheng a ,b ,⁎,Robert L.King c ,Eizo Nakamura b ,Jeffrey D.Vervoort c ,Yong-Fei Zheng d ,Tsutomu Ota b ,Yuan-Bao Wu e ,Katsura Kobayashi b ,Zu-Yi Zhou a
a
State Key Laboratory of Marine Geology,Tongji University,Shanghai 200092,China
b
Institute for Study of the Earth's Interior,Okayama University at Misasa,Tottori 682-0193,Japan c
School of Earth and Environmental Sciences,Washington State University,Pullman,Washington 99164,USA d
CAS Key Laboratory of Crust-Mantle Materials and Environments,School of Earth and Space Sciences,University of Science and Technology of China,Hefei 230026,China e
State Key Laboratory of Geological Process and Mineral Resources,Faculty of Earth Sciences,China University of Geosciences,Wuhan 430074,China
a b s t r a c t
a r t i c l e i n f o Article history:
Received 22August 2008Accepted 9January 2009
Available online 8February 2009Keywords:
因噎废食什么意思Continental subduction Dabie Eclogite
Geochronology
Oceanic subduction Tectonic transition
We investigated the oceanic-type Xiongdian high-pressure eclogites in the western part of the Dabie orogen with combined U –Pb,Lu –Hf,Sm –Nd and Ar –Ar geochronology.Three groups of weighted-mean 206Pb/238U ages at 315±5,373±4and 422±7Ma are largely consistent with previous dates.In
contrast,Lu –Hf and Sm –Nd isochron dates yield identical ages of 268.9±6.9and 271.3±5.3Ma.Phengite and amphibole Ar –Ar total fusion analys give Neoproterozoic apparent ages,which are geologically meaningless due to the prence of excess 40Ar.Plagiocla inclusions in zircon cores suggest that the Silurian ages likely reprent protolith ages,whereas the Carboniferous ages correspond to prograde metamorphism,bad on the compositions of garnet inclusions.Despite weakly-prerved prograde major-and trace element zoning in garnet,a combined textural and compositional study reveals that the consistent Lu –Hf and Sm –Nd ages of ca.270Ma record a later event of garnet growth and thus mark the termination of high-pressure eclogite –facies metamorphism.The new U –Pb,Lu –Hf and Sm –Nd ages suggest a model of continuous process from oceanic to continental subduction,pointing to the ont of prograde metamorphism prior to ca.315Ma for the subduction of oceanic crust,while the peak eclogite –facies metamorphic episode is constrained to between ca.315and 270Ma.Thus,the initiation of continental subduction is not earlier than ca.270Ma.
©2009Elvier B.V.All rights rerved.
1.Introduction
Subduction zones are esntial to the dynamic evolution of the earth's surface due to plate tectonics.Subduction of oceanic and continental crust eventually leads to closure of backarc basins and arc-continent and continent-continent collisions (O'Brien,2001;Ernst,2005;Zheng et al.,2008),forming various types of high-pressure (HP)and ultrahigh-pressure (UHP)metamorphic rocks.Subduction of oceanic lithosphere caus a complex continuum of diagenetic and metamorphic reactions;many kilometres of oceanic lithosphere are ultimately consumed prior to the subquent continental slab subduction and collision.Subducted continental slabs that detach from the oceanic lithosphere that was dragging them into the mantle are expected to rapidly ri to Moho depths becau of their positive buoyancy.Thus,studying subducted oceanic crust in subduction zones can provide clues to the incorporation rate of supercrustal material
into the mantle and can shed light on the initiation of successive continental subduction.Determining a geochronological framework for determining the quence and duration of oceanic to continental subduction and HP and UHP metamorphism plays an esntial role in this respect.
Zircon has long been recognized as a promising geochronometer of the U –Pb decay system becau of its refractory nature,commonly prerved growth zones and mineral inclusions within a single grain.Recent developments in analytical techniques allow us to unravel a wealth of informatio
n contained in zircons with respect to their growth history and thus the prograde and retrograde metamorphic evolution of the host rock (Gebauer,1996;Wu et al.,2006;Zheng et al.,2007).The Lu –Hf garnet technique has been applied to constrain the prograde and high-temperature histories of metamorphic belts (e.g.,Duchêne et al.,1997;Blichert-Toft and Frei,2001;Anczkiewicz et al.,2004,2007;Lagos et al.,2007;Kylander-Clark et al.,2007;Cheng et al.,2008a )becau of its high closure temperature (Dodson,1973;Scherer et al.,2000)and the fact that garnet strongly partitions Lu over Hf,resulting in a high parent/daughter ratio (Otamendi et al.,2002).Combined with Sm –Nd age determination,the Lu –Hf garnet geochronometer can potentially be ud to estimate the duration of
Lithos 110(2009)327–342
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⁎Corresponding author.State Key Laboratory of Marine Geology,Tongji University,Shanghai 200092,China.Tel.:+862165982358;fax:+862165984906.
E-mail address:chenghao@tongji.edu (H.
Cheng).0024-4937/$–e front matter ©2009Elvier B.V.All rights rerved.doi:
10.1016/j.lithos.2009.01.013
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Lithos
j ou r n a l h o m e pa g e :ww w.e l s ev i e r.c o m/l o c a t e /l i t h o s
Fig.1.Simpli fied geologic map of the Huwan mélange area (b)in southern Dabie orogen (a),modi fied after Ye et al.(1993)and Liu et al.(2004b),showing the sample localities for the Xiongdian eclogite.References:asterisk,this study;[1],Ratschbacher et al.(2006);[2],Jahn et al.(2005);[3],Liu et al.(2004a);[4],Eide et al.(1994);[5],Webb et al.(1999);[6],Xu et al.(2000);[7],Ye et al.(1993);[8],Sun et al.(2002);[9],Jian et al.(1997);[10],Jian et al.(2000);[11],Gao et al.(2002);[12],Li et al.(2001);[13],Wu et al.(2008).amp —amphibole;brs —barroisite;phen —phengite;zrn —zircon.
328H.Cheng et al./Lithos 110(2009)327–342
garnet growth,which either reflects early prograde metamorphism (Lapen et al.,2003),exhumation(Cheng et al.,2009)or a particular garnet growth stage(Skora et al.,2006).Dating the exhumation of high-pressure(HP)and ultra-high-pressure(UHP)metamorphic rocks by conventional step-heating Ar–Ar technique was largely hampered and discredited due to the prence of excess/inherited argon(Li et al.,1994;Kelley,2002).However,the Ar–Ar geochron-ometer remains irreplaceable in constraining the exhumation of HP/ UHP metamorphic rocks becau of its intermediate closure tempera-ture.Nevertheless,timing must be integrated with textures and petrology in order to quantify the dynamics of geological process, whichever geochronological method is ud.
During the past two decades,considerable progress has been made in constraining the prograde metamorphism and exhumation of HP/ UHP metamorphism of the Dabie–Sulu orogen by a variety of geochronological methods,indicating a Triassic collision between the South China and North China ,Eide et al.,1994;Ames et al., 1996;Rowley et al.,1997;Hacker et al.,1998;Li et al.,2000,2004; Zheng et al.,2004).The initiation of continental subduction is pinned to ca.245Ma(Hacker et al.,2006;Liu et al.,2006a;Wu et al.,2006; Cheng et al.,2008a),but the exact time is poorly constrained.On the other hand,thefingerprints of early continental subduction may not be prerved in continental-type metamorphic rocks due to the succes-sive high-temperature prograde and retrograde overprints.Alterna-tively,the timing of initiation of continental subduction subquent to the termination of oceanic subduction may be registered in the HP/ UHP eclogites,who protoliths are of oceanic origin.Currently,the only outcropping candidate is the Xiongdian HP eclogite in the western part of the Dabie orogen(Li et al.,2001;Fu et al.,2002).However,U–Pb zircon ages ranging from216±4to449±14Ma have been obtained for the Xiongdian eclogite(Jian et al.,1997;Sun et al.,2002;Gao et al., 2002);the geological significance of this age spread is controversial. Efforts to clarify the geochronological evolution of the Xiongdian eclogite were hampered by a much older Sm–Nd garnet-whole-rock isochron of533±13Ma(Ye et al.,1993)and the fact that further Sm–Nd and Rb–Sr analys failed to produce mineral isochrons(Li et al., 2001;Jahn et al.,2005),although oxygen isotopic equilibrium was largely attained(Jahn et al.,2005).
Here,we prent a combined U–Pb,Lu–Hf,Sm–Nd,Ar–Ar and oxygen multi-isotopic and mineral chemical study of the Xiongdian eclogite.The differences in the systems,in conjunction with chemical profiles in garnet porphyroblasts and zircons,provide a window into the time-scales of the oceanic subduction and sub-quent exhumation.
2.Geochronological background and sample descriptions
The Qinling–Dabie–Sulu orogen in east-central China marks the junction between the North and South China Blocks(Cong,1996; Zheng et al.,2005).The western part of the Dabie orogen,usually termed the West Dabie and sometimes the Hong'an terrane,is parated from the Tongbaishan in the west by the Dawu Fault and from the East Dabie by the Shangma fault in the east(Fig.1a).It contains a progressive quence of metamorphic zones characterized by increasing metamorphic grade,from transitional blueschist–greenschist in the south,through epidote–amphibolite and quartz eclogite,to coesite eclogite in the ,Zhou et al.,1993;Hacker et al.,1998;Liu et al.,2004b,2006b).The Xiongdian eclogites crop out in the northwestern corner of the West Dabie,in the Xiongdian mélange within the Huwan mélange after the definition of Ratschba-cher et al.(2006),in analogy to the terms of the Sujiahe mélange(Jian et al.,1997)and Huwan shear zone(Sun et al.,2002).The Huwan mélange consists of eclogite,gabbro,amphibolite,marble,and quartzite.The e
clogitic metamorphic peak for the Xiongdian eclogite is estimated at600–730°C,1.4–1.9GPa(Fu et al.,2002),550–570°C,∼2.1GPa(Liu et al.,2004b)and540–600°C,∼2.0GPa(Ratschbacher et al.,2006),followed by retrogression at530–685°C and∼0.6GPa (Fu et al.,2002).
Except for the Xiongdian eclogite,consistent Triassic metamorphic ages have been obtained for other eclogites across the West Dabie (Webb et al.,1999;Sun et al.,2002;Liu et al.,2004a;Wu et al.,2008). This indicates that West Dabie is largely a coherent part of an HP–UHP belt elwhere in the Dabie–Sulu orogenic belt.Geochronological debate is limited to the Xiongdian eclogite(Fig.1b).U–Pb zircon ages ranging from ca.216to ca.449Ma have been obtained for the Xiongdian eclogite.Jian et al.(1997)reported ca.400,ca.373and 301±0.6Ma ages by ID–TIMS method.Weighted-mean SHRIMP ages range from335±2to424±5Ma(Jian et al.,2000).The Silurian U–Pb zircon ages were interpreted as the age of the protolith,while the Carboniferous ages mark high-pressure metamorphism(Jian et al., 1997,2000).Weighted-mean206Pb/238U SHRIMP U–Pb zircons ages of 433±9,367±10and398±5Ma were interpreted as the protolith age,while323±7and312±5Ma likely date the high-pressure metamorphism(Sun et al.,2002).A Triassic age of216±4Ma together with449±14and307±14Ma weighted-mean206Pb/238U SHRIMP U–Pb zircon ages appear to argue for the involvement of the Triassic subduction in the Xiongdian eclogite(
Gao et al.,2002).A garnet-whole-rock Sm–Nd isochron of533±13Ma(Ye et al.,1993)was interpreted to reflect the high-pressure metamorphism age.Several Table1
Chemical compositions of the Xiongdian eclogite from the western Dabie.龙的五行属性
3月2日复工
Sample number DB17DB18
(Major oxides in%)
SiO254.5452.45 TiO20.370.43 Al2O314.6212.35 Fe2O38.7710.15 MnO0.150.16 MgO 6.669.91 CaO10.3510.26 Na2O 2.88 2.65 K2O0.600.28 P2O50.060.05 Cr2O3⁎6601118 NiO⁎137247 L.O.I0.87 1.28 Total99.95100.11 (Trace elements in ppm)
Li27.627.0 Be0.560.47 Rb9.7813.8 Sr178130
Y12.612.7 Cs0.89 3.67 Ba86552.4 La 2.21 1.77 Ce 5.97 5.12 Pr0.880.80 Nd 4.35 4.10 Sm 1.25 1.26 Eu0.470.39 Gd 1.53 1.52 Tb0.280.29 Dy 1.83 1.91 Ho0.410.42 Er 1.14 1.19 Tm0.190.19 Yb 1.31 1.34 Lu0.200.20 Pb 6.44 1.85 Th0.050.07 U0.110.06 Zr28.828.2 Nb 1.19 1.77 Hf0.870.88 Ta0.050.08
开洋冬瓜⁎In ppm.
329
H.Cheng et al./Lithos110(2009)327–342
Sm –Nd and Rb –Sr analys failed to produces isochrons (Li et al.,2001;Jahn et al.,2005),which was believed to be due to unequilibrated isotopic systems despite the fact that oxygen isotopic equilibrium was largely attained (Jahn et al.,2005).Phengite 40Ar/39Ar ages of ca.430–
生津利咽350Ma have been explained as the retrograde metamorphic age (Xu et al.,2000).The 310±3Ma phengite 40Ar/39Ar age (Webb et al.,1999)is likely geologically meaningless due to the concave-upward age spectrum,indicating the prence of excess argon.Collectively,existing geochronology provides an apparently con flicting picture for the Xiongdian eclogites.The timing of the oceanic crust subduction and exhumation esntially remains to be resolved.
The two eclogites examined in this study were lected bad on their mineral asmblages,inclusion types and geological context (Fig.1).The one (DB17)from the east bank of the river to the east of Xiongdian village is a coar-grained and strongly foliated banded eclogite,compod mainly of garnet,omphacite and phengite.A cond (DB18)eclogite was sampled about 50m to the north of DB17and is strongly foliated with a similar mineralogy asmblage but sm
aller garnet grains.3.Methods
Sample preparation,mineral paration and chemical procedures for isotope analysis,instrumentation and standard reference materials ud to determine whole rock and bulk mineral compositions,in situ major and trace element analys (Institute for Study of the Earth's Interior,Okayama University at Misasa,Japan),zircon U –Pb isotope and trace element analys (China University of Geosciences in Wuhan),Lu –Hf and Sm –Nd isotope analys (Washington State University),Ar –Ar isotope analys (Guangzhou Institute of Geo-chemistry,Chine Academy of Sciences)and oxygen isotope analys (University of Science and Technology of China)are described in the Appendix .
4.Results
4.1.Bulk chemical composition
The Xiongdian eclogites are mainly of basaltic composition,but they show a wide range of major and trace element abundances.Despite the high SiO 2(52–58%)and low TiO 2(0.32–0.43%)
contents,
Fig.2.Whole rock chemical analysis data.(a)Chondrite-normalized REE distribution patterns of the Xiongdian eclogites.(b)Primitive-mantle-normalized spidergrams of the Xiongdian
eclogites.
Fig.3.Backscattered-electron images and rim-to-rim major-element compositional zoning pro files of reprentative garnets in the matrix and as inclusions in zircon.Amp —amphibole;Ap —apatite;Cal —calcite;Cpx —clinopyroxene;Zo —zoisite;Phen —phengite;Omp —omphacite;Qtz —quartz;Zrn —zircon.
330H.Cheng et al./Lithos 110(2009)327–342
they have MgO=5.1–9.9%,Cr=430–1118ppm,Ni=88–247ppm (Table 1;Li et al.,2001;Fu et al.,2002;Jahn et al.,2005).In contrast to existing LREE-enriched chondritic REE patterns,our samples have rather flat REE patterns around ten times more chondritic abundances with small,both negative and positive Eu anomalies (Fig.2a).Rubidium is depleted and Sr displays enrichment with respect to Ce.Both negative and no Nb anomalies relative to La were obrved (Fig.2b).The N-MORB-normalized value of Th is around 0.5,lower than previous reported values of up to 25(Li et al.,2001).4.2.Petrography and mineral composition
The Xiongdian eclogites occur as thin layers intercalated with dolomite –plagiocla gneiss and phengite –quartz schist (Fu et al.,2002),mainly consisting of garnet,omphacite,epidote (clinozoisite),
phengite and minor amphibole,quartz and kyanite (Fig.3).Zircons were obrved both as inclusions in garnet porphyroblasts and in the matrix.The samples have similar mineral asmblages,but differ in modal compositions.Omphacite (X Jd =0.46–0.48)is unzoned.Phengite has 3.30–3.32Si apfu and ∼0.4wt.%TiO 2.Garnets range in size from 0.5to 5mm in diameter,either as porphyroblasts or as coalesced polycrystals,mostly with idioblastic shapes with inclusions of quartz,calcite,apatite and omphacite (Fig.3).Garnet is largely homogeneous (Prp 24–25Alm 49–50Grs 24–25Sps 1.5–1.9),but shows a slightly Mn-enriched core (Fig.3d;Table 2).HREEs in large garnet porphyroblasts,such as Yb and Lu,display weak but continuous decreas in concentration from core to rim (Fig.4a),mimicking the MnO zoning pattern,which could be explained by their high af finity for garnet and likely aris from an overall Rayleigh distillation process during early garnet growth (Hollister,1966;Otamendi et al.,2002).
However,the limited variation in MREE concentrations,such as Sm and Nd,in garnet with respect to the weak zoning in HREE (Fig.4a)might be explained by growth in an environment where MREEs are not limited and continuously supplied by the breakdown of other phas.Hafnium has a fairly flat pro file (Table 3),re flecting its incompatible character in garnet and abnce of Hf-competing reactions involved in garnet growth.Two distinct domains can be de fined in the large garnet porphyr
oblasts bad on the chemical zoning and the abundance of inclusions.The zones are an inclusion-rich core with richer Mn and HREE and an inclusion-free rim with poorer Mn and HREE (Fig.3d).The inclusion-free rim for individual garnet has a rather similar width of 200–250μm (Fig.3).Although concentrations of Nd (0.22–0.41ppm)and Sm (0.33–0.48ppm)vary within single garnet grains,the Sm/Nd ratios (0.8–2.2)are consistent
Table 2
Reprentative major-element data of the garnets,omphacites,phengites,amphiboles and zoisites.
(wt.%)
Grt Omp Rim
Core Inclusions-in-zircon Rim Core SiO 238.6838.6438.6638.5338.6538.6637.8637.7555.9356.1256.1356.20TiO 20.050.060.050.050.050.050.050.080.120.110.110.11Al 2O 321.9221.9422.0721.9921.9921.8421.6821.8611.2611.2211.3311.26FeO ⁎22.9823.0523.0623.1623.0523.1124.4224.33 4.25 4.23 4.32 4.27MnO 0.680.720.790.880.750.6
80.990.930.030.020.030.02MgO 6.37 6.38 6.28 6.31 6.36 6.35 4.23 4.748.158.027.968.13CaO 9.108.949.028.929.038.9910.579.5013.2213.3613.3213.34Na 2O 0.030.030.030.030.030.030.020.01 6.65 6.41 6.39 6.42K 2O 0.000.000.000.000.000.000.000.000.000.000.000.00Total 99.8099.7799.9699.8799.9199.7199.8299.2199.6099.6099.7099.87O.N.12121212121212126666Si 2.986 2.984 2.981 2.975 2.980 2.988 2.958 2.962 1.996 2.010 2.010 2.007Al 1.994 1.997 2.006 2.001 1.999 1.990 1.997 2.0210.4740.4730.4780.474Ti 0.0030.0030.0030.0030.0030.0030.0030.0050.0030.0030.0030.003Fe 2+ 1.486 1.491 1.489 1.499 1.489 1.496 1.596 1.5990.1270.1270.1290.128Mn 0.0440.0470.0520.0580.0490.0440.0660.0620.0010.0010.0010.001Mg 0.7330.7350.7220.7260.7310.7320.4930.5540.4340.4280.4250.433Ca 0.7530.7400.7450.7380.7460.7440.8850.7980.5060.5130.5110.511Na 0.0040.0050.0050.0050.0050.0050.0030.0020.4600.4450.4430.445K感光
0.0000.000
0.000
0.000
0.0000.000
0.000
0.000
0.0000.000
阴历七月十五0.000
0.000
Phn Amp Zo Rim
Core Rim Core Mantle Core SiO 248.8649.0949.3349.0147.0847.0746.7246.7539.0538.9239.0239.02TiO 20.400.410.410.400.220.220.220.220.130.130.130.12Al 2O 329.0328.6829.0129.1912.6612.8112.5812.6228.5528.2128.7328.62FeO ⁎ 1.99 1.99 2.00 1.9711.6011.4811.4611.36 6.01 6.01 6.03 6.07MnO 0.000.000.000.010.100.090.090.090.050.050.060.05MgO 2.79 2.77 2.78 2.8012.2012.4712.441
2.300.070.060.070.07CaO 0.010.010.010.019.9710.0910.0710.1024.1023.8624.1324.14Na 2O 0.930.920.920.91 2.79 2.77 2.82 2.830.000.000.000.00K 2O 10.009.919.819.780.480.470.470.470.000.000.000.00Total 94.0293.7894.2894.0997.0997.4996.8896.7697.9697.2498.1698.09O.N.111111112323232312.512.512.512.5Si 3.302 3.323 3.318 3.304 6.831 6.800 6.799 6.809 3.008 3.019 3.000 3.003Al 2.313 2.288 2.300 2.319 2.164 2.182 2.158 2.167 2.592 2.579 2.603 2.596Ti 0.0200.0210.0210.0200.0240.0240.0240.0240.0070.0070.0070.007Fe 2+0.1120.1130.1130.111 1.407 1.387 1.394 1.3830.3870.3900.3880.390Mn 0.0000.0000.0000.0000.0120.0120.0120.0120.0040.0040.0040.004Mg 0.2820.2800.2790.282 2.639 2.686 2.699 2.6700.0070.0070.0070.008Ca 0.0010.0010.0010.001 1.550 1.562 1.570 1.577 1.989 1.983 1.988 1.990Na 0.1220.1210.1200.1190.7840.7770.7950.7980.0000.0000.0000.000K
0.862
0.855
0.842
0.841
0.089
0.087
0.088
0.088
0.000
0.000
0.000
0.000
⁎Total iron;concentrations reported as wt.%.
331
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