Article history:Received 21 September 2012Received in revised form 11 November 2012Accepted 14 November 2012Available online 27 November 2012
Keywords:U–Pb ageHf isotopeDetrital zirconsGondwana marginIndochinaTruong Son Belt
To constrain the paleoposition of Indochina within Gondwana during the Early Paleozoic, we performed in-situU–Pb and Hf isotope analyses on detrital zircons from three river sediment samples in the Truong Son Belt ofthe Indochina block. The age distributions yield dominant Neoarchean (~2.5 Ga), Mesoproterozoic (1.7–1.4 Ga), Grenvillian (~0.95 Ga), and Pan-African (0.65–0.5 Ga) age groups and minor Paleo- to Meso-archeanzircons. Hf isotope compositions of zircons for each age group exhibit large ranges of εHf(T), suggesting thatthe zircon host rocks have diverse sources. The oldest Hf model ages for zircons of Neoarchean, Grenvillian,and Pan-African age group yield ~3.7 Ga or older, while those of Mesoproterozoic age group show ~3.3 Ga.The remarkable similarity of age distribution and Hf isotope compositions among detrital zircons of Indochinaand those of Tethyan Himalaya, western Cathaysia, and Qiangtang suggests that Indochina was located outboardof Qiangtang and south of South China in the Indianmargin of Gondwana during the Early Paleozoic. Our resultsare consistent with the paleontological correlations of east Gondwana margin during the Early Paleozoic.
The Indochina block is one of the largest continental blocks in SE Asiaand occupiesmost of the Indochinapeninsula. During the Early Paleozoicthe Indochina block is considered to have been proximal to Gondwana,as well as other continental blocks in SE Asia (Metcalfe, 1998, 2006,2011a, 2011b). Constraining the paleopositions of these blocks duringthe Early Paleozoic is critical for understanding the geodynamic(e.g., rifting, orogenesis) development of pericratonic Gondwanamargins (e.g., Cawood et al., 2007; Metcalfe, 1998, 2011a, 2011b;Wang et al., 2010; Zhu et al., 2011, in press). In previous studies,Early Paleozoic reconstructions of these blocks are primarily based onpa-leomagnetic data, paleobiogeography, and stratigraphic correlations(e.g.,Metcalfe, 1998). The Early Paleozoic paleopositions of the Indochinablock differ widely which is partially related to the different methodsused for the reconstructions. For example, Torsvik and Cocks (2009) con-siders that both Indochina and South China were not a part of theGondwana margin and existed as isolated blocks during the Early Paleo-zoic. Metcalfe (1998, 2006, 2011a, 2011b) maintains a long-standing in-terpretation placing these terranes on the India-Australian margin ofGondwana. This inconsistency mainly comes from paucity of constraintson the paleopositions of SEAsian terranes. Especially, for Indochina, thereis no reliable paleomagnetic data (Li et al., 2004) and there are few pale-ontologic indicators (Metcalfe, 1998) in Early Paleozoic. Oneway to over-come this difficulty is by introducing a new approach.
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Recently, an integrated method of U–Pb ages and Hf isotope compo-sitions for detrital zircons was successfully applied to constrain thepaleopositions of SE Asian continental blocks such as South China,Lhasa, and Qiangtang (Duan et al., 2011; Yu et al., 2008; Zhu et al.,2011). Of these studies, Zhu et al. (2011) showed that the presence orabsence of younger (~950 Ma) or older (~1170 Ma) Grenvillian detritalzircons is useful to discriminate whether the blocks were derived fromthe Indian or the Australian margins. Therefore we can expect that theapplication of detrital zircon studies to the Indochina block may beable to shed light on the true paleoposition of Indochina. This study pre-sents new in-situU–Pb age andHf isotope data of detrital zircons collect-ed from river sediments in the Truong Son Belt (TSB) of the Indochinablock in order to constrain the paleoposition relative to the other Gond-wana derived terranes.
2. Geological background of the sample sites
The Indochina block occupies the largest portion of the Indochinapeninsula (Fig. 1A). It is divided from South China by the SongMa Suturein the north and from the Sibumasu by the Sukhothai Arc and thePaleo-Tethys Suture Zone (Metcalfe, 2011a; Sone and Metcalfe, 2008) inthe west. The eastern margin of the Indochina (North-Central Vietnamand eastern Laos) is composed of the Truong Son Belt (TSB) and Kontummassif. The TSB is mainly composed of Precambrian to Mesozoic sedi-mentary rocks intruded by plutonic rocks (Tran and Nguyen, 1989;Tran et al., 2008, Fig. 1B). These rocksweremetamorphosed alongnarrowNW- to W-trending shear zones (Lepvrier et al., 1997). The Kontummassif ismainly composed of high grademetamorphic rocks and plutonic
Fig. 1. A: Tectonic framework of SE Asia. Heavy lines indicate main sutures or faults (after Sone and Metcalfe, 2008; Metcalfe, 2011a). The boundary between Yangtze and Cathaysiablocks is adapted from Xu et al. (2007). B: Simplified geological map of the Truong Son Belt (TSB) (Tran and Nguyen, 1989; Tran et al., 2008), central Vietnam, showing samplelocalities of this study. PTSZ: Paleo-Tethys Suture Zone, KM: Kontum massif, EC: Eastern Cathaysia, WC: Western Cathaysia.
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rocks. Recent geochronological studies using zircon U–Pb methodsrevealed that metamorphic and plutonic rocks in the TSB and Kontummassif were formed in both ~450 Ma and ~250 Ma events (Carter et al.,2001; Lan et al., 2003; Nagy et al., 2001; Nam et al., 2001; Roger et al.,2007; Usuki et al., 2009).
Zircons in three modern river sediment samples from the TSB wereobtained for this study. Two samples, M03-21B (N18°26′5″ E105°13′15″) and M03-24B (N18°30′37″ E105°24′4″) (Fig. 1B), were collectedfrom a tributary of the Song Ca (Ca River). In the Song Ca area, UpperOrdovician–Silurian formations are widely distributed (Fig. 1B), whichare characterized by a sequence of terrigenous flysch, cherty shale con-taining radiolarians, and black shale containing graptoliteswith interbedsof limestone. The catchment area of the Song Ca is restricted in the east ofthe Truong Son range which is the border with Laos. Therefore, samplesM03-24B and M03-21B are most likely derived from outcrops along theSong Ca in Vietnamese territory. The Kim Cuong granite body occursfurther upstream from samples M03-21B and M03-24B along theVietnam–Laos border. The granitic pluton consists mainly of biotitegranodiorite and biotite granite from where Devonian to Late PermianK–Ar biotite ages and a Late Permian TIMS U–Pb zircon age are reported(Tran and Vu, 2011). A third sample, VNTS1 (N18°12′28″ E106°7′1″,Fig. 1B), was collected from a stream southeast of Ha Tinh city.Upper Ordovician–Silurian and Triassic sedimentary rocks exposeupstream of the sampling site. Small granitic bodies intrude these UpperOrdovician–Silurian and Triassic sedimentary rocks.
3. Analytical procedure
3.1. Sample preparation
Several kg of river-sediment was panned to collect heavy mineralconcentrates. Zircons were separated from the heavy mineral concen-trates using conventional heavy liquid and magnetic techniques andextracted by hand-picking under a binocularmicroscope. Representativezircon grainsweremounted in epoxy and polished to approximately halfthe mean grain thickness. The internal structures of the zircons and thepresence of inclusions were checked by using transmitted and reflectedoptical microscopy. Cathodoluminescence (CL) images of zircons weretaken using a panchromatic CL imaging system (GatanMini-CL) attachedto a scanning electron microscope (JEOL JSM-6360LV) at the Institute ofEarth Sciences, Academia Sinica, Taipei, for examining the internal struc-tures of zircon grains and selecting optimum spot locations for in-situU–Pb dating and Hf isotopic analysis. Representative CL images of ana-lyzed zircons are shown in Fig. 2.
3.2. U–Pb dating
In-situ zircon U–Pb isotopic analyses were carried out at the Depart-ment of Geosciences, National Taiwan University, Taipei, using laser abla-tion inductively coupled plasma mass spectrometry (LA-ICP-MS). AnAgilent 7500 s quadrupole ICP–MS equipped with a New Wave UP213
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laser ablation system is used. A detail description of the instrumental per-formance and analytical procedures are documentedbyChiu et al. (2009).Laser diameter was 30 μm. The 207Pb/206Pb, 206Pb/238U, 207Pb/235Uand 208Pb/232Th ratios were calculated using the GLITTER 4.0 software(Macquarie University), and were then corrected using the GJ-1(Jackson et al., 2004) as external calibrant. The Harvard zircon 91500(207Pb/206Pb age at 1065.4±0.6 Ma (2σ) and 206Pb/238U age at1062.4±0.8 Ma (2σ), Wiedenbeck et al., 1995) and Mud Tank (MT)zircon (U–Pb concordia age of 732±5 Ma, Black and Gulson, 1978)are used as reference zircons and were measured periodically. Duringthe analysis of this study, Harvard and Mud Tank zircons yielded theconcordia age of 1069.4±2.8 Ma (2σ, n=40) and 732.7±2.9 Ma(2σ, n=39), respectively. Common lead corrections were carried outusing the procedure of Andersen (2002). We use 207Pb/206Pb agesand 206Pb/238U ages for >1000 Ma and b1000 Ma zircons, respectively(Griffin et al., 2004). Concordia plots and age probability plots weremade using ISOPLOT (ver. 3.0) (Ludwig, 2003).
3.3. Hf isotope analyses
Hf isotopes were analyzed using a Nu Plasma multi-collector ICP–MS attached to a NewWave UP213 laser-ablation microprobe housedat the Institute of Earth Science, Academia Sinica, Taipei, documentedin Lan et al. (2009). The analytical procedure follows that described inGriffin et al. (2000, 2004). The Hf isotope was measured on the datedspots of individual zircons to minimize zoning effect but the laser ab-lation size is 55 μm, slightly larger than that of preexisting spots by the
A: VNTS1-70
29
Younger (<0.5 Ga) zircons
100um
440
C: M03-21B-30
246
B: M03-21B-49
J: M03-21B-31
2521
1
H: M0
E: M0D: M03-21B-20
531
Fig. 2. Representative cathodoluminescence images of zircons of this study (A–C: zircons wiU–Pb dating.
U–Pb dating. Data were normalized to 179Hf/177Hf=0.7325, using anexponential correction for mass bias. 176Hf/177Hf results of Mud Tankand Harvard zircon standards during analysis of this study are 0.282530±0.000050 (2σ, n=63) and 0.282314±0.000088 (2σ, n=22),respectively. Epsilon Hf values and model ages used in the figureswere calculated using the decay constant (1.865×10−11 per year) pro-posed by Scherer et al. (2001). Calculated model ages (TDMC ) are basedon two-stage model assuming 176Lu/177Hf of average continental crustis 0.015 (Griffin et al., 2004).
4. Results
4.1. Zircon U–Pb dating
One hundred and eighty four zircons from three samples were se-lected for U–Pb dating. The results of U–Pb dating are given in Table 1and concordia plots are shown in Fig. 3. Most analyses of the zirconsfrom the three samples fall on or near the concordia line with a verywide range from the Archean (3226 Ma) to Cenozoic (25 Ma). Amongthem, 20–50% of analyzed zircons are younger than 500 Ma (Fig. 3).The younger zircons (mostly ~450 Ma, ~250 Ma, and ~30 Ma) are col-orless with euhedral to subhedral, and sometimes angular morphology(Fig. 2A–C), and show clear oscillatory zoning in CL imaging. Th/Uratio of these zircons is high (>0.3, Table 1), suggesting magmatic ori-gins (Hoskin and Schaltegger, 2003). All three samples consistentlyhave ~250 Ma and ~450 Ma zircons (Fig. 3A–C), although M03-24Band M03-21B contain more ~450 Ma zircons than those of VNTS1. The
Older (>0.5 Ga) zircons
L: M03-24B-10
3226100um
589
3-24B-35
K: VNTS1-49
2526
F: VNTS1-40
975994
G: VNTS1-42
586
3-24B-47
I: M03-21B-18
1608
th b0.5 Ga ages, D–L: zircons with >0.5 Ga ages). Circles indicate the spot positions for
Table 1Summary of U–Pb–Th analyses.
Ratios Ages (Ma) Disc % Color in optical Shape Analyzed domainTh (ppm) U (ppm) Th/U
Fig. 3. Concordia diagrams (A: M03-24B, B: M03-21B, and C: VNTS1) and age histograms with the probability diagram (D: M03-24B, E: M03-21B, and F: VNTS1) of U–Pb zirconsanalyses. Error ellipses of each data point in concordia diagrams are 2σ. The upper and lower intercept ages and their discordia lines (broken lines) are shown for discordia zirconsin A and C. Enlargement of the younger part of concordia diagrams is also shown in A, B and C. Data for the older zircons (>500 Ma) are only shown in histograms (D, E, and F). Inthese plots, 207Pb/206Pb ages with discordant degree b20% were used except b1 Ga zircons where 206Pb/238U ages were used.
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Cenozoic zircons (~30 Ma) occur only in VNTS1. Based on zircon mor-phologies, internal texture, Th/U ratio with comparison of geology ofthe catchment area as described before, it is very likely that the zirconsoriginated from intrusive rocks of the TSB.
The main purpose of this study is to reveal age distribution ofdetrital zircons derived from Early Paleozoic sedimentary rocks in Indo-china. In contrast to the younger zircons, the older zircons are occasion-ally pink or purple in color and show roundedmorphology and their CLimages are typically dark or unclear (Fig. 2D–L). The Th/U ratio of theolder zircons are also >0.3 (Table 1). Some discordant zircons fromM03-24B define a discordia line with an upper intercept age of2466±14 Ma (Fig. 3A). Several Proterozoic zircons of VNTS1 showdiscordia lines with upper intercept ages of 2416±50 Ma and 2518±10 Ma (Fig. 3C). Age probability plots of the older zircons (>0.5 Ga)from M03-24B, M03-21B and VNTS1 consistently show Neoarchean
(~2.5 Ga), Mesoproterozoic (1.7–1.4 Ga), Grenvillian (~0.95 Ga), andPan-African (0.65–0.5 Ga) age peaks (Fig. 3D–F), although theMesoproterozoic peak age (~1.7 Ga) of VNTS1 is smaller than those ofthe other samples. The similarity of peak patterns between M03-24BandM03-21B confirms that the age distribution patterns are representa-tive of zircons from Song Ca (Ca River). M03-24B contains concordantPaleoarchean (3226±14 Ma) andMesoarchean (3066±15 Ma) detritalzircons and represents the oldest ages yet found in Indochina.
4.2. Zircon Hf isotopes
Hf isotopic data for the older (>0.5 Ga) zircons (n=105) areshown in Table 2 and a U–Pb age vs epsilon Hf diagram is shown inFig. 4. The oldest two Paleo- to Meso-archean zircons from sampleM03-24B show slightly negative and positive εHf(T) values (−1.1
Table 2Summary of Hf isotope analyses for the older (>0.5 Ga) zircons.
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and +3.6) yielding Hf crustal model ages (TDMC ) of 3.7 and 3.2 Ga,respectively. The Neoarchean, Mesoproterozoic, Grenvillian, andPan-African age groups from the three samples exhibit large ranges ofεHf(T), suggesting that the host magma of zircons from these groupshave diverse sources. The Neoarchean group yields varying εHf(T) values−22.4 to+4.5, which indicates both significant reworking of older crustand juvenile crust formation. The existence of very low negative εHf(T)indicates that the source magma includes reworked Eoarchean (TDMC =4.4–3.7 Ga) crustal materials. Grenvillian and Pan-African groupsshow large ranges of εHf(T) from strong negative values to positive,i.e.,−29.3 to +5.1 and−34.4 to +4.6, respectively. The strong neg-ative εHf(T) values from both the groups indicate reworked oldercrust (~3.7 Ga) in addition to juvenile input. The Mesoproterozoicgroup also ranges from negative (−15.1) to positive (+5.9), suggestingcrustal reworking and juvenile input, but the oldest Hf model age ofthis group indicates ~3.3 Ga, slightly younger than those of Neoarchean,Grenvillian, and Pan-African groups.
5. Discussion
5.1. Exotic origin of the detrital zircons
Analyzed zircons in this studywere collected frommodern river sed-iments in the TSB and not directly from outcrop of Early Paleozoic sedi-mentary rocks. Therefore,we need to discuss about the relation betweenthe sources for our zircons and the geology of catchment areas. As de-scribed before, the sampling sites for M03-24B and M03-21B and theirupstream area are mostly underlain by Upper Ordovician–Lower Siluri-an sedimentary rocks (Fig. 1B). They are also intruded by several largeand small plutonic bodies (Tran et al., 2008). So, zircons in samplesM03-24B and M03-21B are probably derived from Upper Ordovician–Lower Silurian sedimentary rocks and their intrusions. Although theseintrusions have not been studied in detail yet (Tran et al., 2008), someDevonian to Late Permian K–Ar biotite ages are reported in addition toa Late Permian TIMSU–Pb zircon age (Tran andVu, 2011). The Paleozoic
-30
-20
-10
0
10
20
epsi
lon
Hf
Western Cathaysia
Qiangtang
Tethyan Himalaya
Indochina (VNTS-1)
Indochina (M03-21B)
Indochina (M03-24B)
DM
CHUR
3.7 Ga3.3 Ga176 Lu/177 Hf = 0.015
0.95
Ga
2.5
Ga
500 1000 1500 2000 2500 3000 3500 4000
Age (Ma)
Fig. 4. Age vs epsilon Hf plot of zircons from samples M03-24B, M03-21B and VNTS1. For comparison, detrital zircons data from western Cathaysia, Qiangtang, and TethyanHimalaya are also plotted. Data sources: western Cathaysia: Yu et al. (2008, 2010), Qiangtang: Zhu et al. (2011), Dong et al. (2011), Tethyan Himalaya: Zhu et al. (2011). DM: De-pleted mantle, CHUR: Chondritic uniform Reservoir. The declining parallel lines indicate Hf crustal model ages (T C
DM) based on assuming 176Lu/177Hf of average continental crust is0.015 (Griffin et al., 2004).
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K–Ar ages can be interpreted asminimumages, as shown an Ar–Ar geo-chronological study for Early Paleozoic granulites overprinted by theIndosinian orogeny in the Kontum massif (Maluski et al., 2005). It iswell known that magmatic activities in the TSB mainly occurred inEarly-Middle Paleozoic and Permo-Triassic (Tran and Vu, 2011). There-fore, ~450 Ma and ~250 Ma euhedral zircons in our samples may comefrom intrusives in the catchment areas. On the other hand, the older zir-cons (>0.5 Ga) are most likely derived from Upper Ordovician–LowerSilurian sedimentary rocks. In the upstream of VNTS1, Triassic sedimen-tary rocks are also distributed in addition to Upper Ordovician–Siluriansedimentary rocks. However, the similarity of age distribution and Hfisotope compositions with those of M03-24B and M03-21B suggeststhat detrital zircons in VNTS1 may be derived mainly from the UpperOrdovician–Silurian sedimentary rocks or their recycled sources. There-fore, we interpret that age distributions and Hf isotope data of the olderzircons represent those of Early Paleozoic sedimentary rocks in the TSB.
The detrital zircons (>0.5 Ga) from the TSB showmajor age groupsof ~2.5, 1.7–1.4, ~0.95, and 0.65–0.5 Ga in addition to the two Paleo- toMeso-archean zircons which are the oldest materials ever recordedfrom Indochina. The subrounded or rounded shapes of these zirconssuggest strong ablation during long-distance transport. Counterpartigneous bodies of these ages have not been recognized in Indochina,although high quality age data is still limited. Instead, the Kontummassif,representing the largest basement exposure of Indochina (Hutchison,1989), is now regarded as exposures of deep crustal levels of thePermo-Triassic orogeny in contact with Paleozoic mid-crustal rocks(Nagy et al., 2001). Although Archean to Early Paleozoic zircon U–Pbages (concordant or near concordant ages by SHRIMPandupper interceptages by TIMS method) have been reported from several localities in theKontum massif and TSB (Carter et al., 2001; Lan et al., 2003; Nagy et al.,2001; Nam et al., 2001; Roger et al., 2007; Usuki et al., 2009), most ofthem could be interpreted as detrital ages based on occurrences of awide variety of ages even in one sample (Carter et al., 2001; Lan et al.,2003; Nagy et al., 2001; Usuki et al., 2009) and internal texture of thezircons (Usuki et al., 2009). This evidence suggests that the older zirconsof this studymostly represent exotic input from other adjacent continen-tal terranes previously linked with Indochina. The existence of >3 Gazirconsmay suggest that the source provenancewas linkedwith cratonicareas such as India, East Antarctica, orWestern Australia (Yu et al., 2008).
Similar interpretations are derived from recent detrital zircon studies inother SE Asian blocks, such as South China, Qiangtang, and Lhasa, andtheir age distribution is used to reconstruct their paleopositions on theGondwana margin (Duan et al., 2011; Wang et al., 2010; Yu et al., 2008;Zhu et al., 2011).
5.2. Comparison with Gondwana derived terranes
Yu et al. (2008) showed that detrital zircons from late Neoproterozoicmetasediments inwestern Cathaysia (Nanling–Yunkai area) are charac-terized by the existence of significant amounts of younger Grenvillianages (~1000–930 Ma) rather than typical Grenvillian ages (1300–1050 Ma). Recently, Zhu et al. (2011) showed that the source prove-nance for sediments of Qiangtang and Lhasa can be distinguished byusing Grenvillian detrital zircons. Qiangtang is dominated by ‘younger’Grenvillian (~950 Ma) ageswhich originated from the TethyanHimalaya.In contrast detrital zircons from Lhasa are dominantly ‘older’ Grenvillian(~1170 Ma) and derived from the Albany–Fraser belt in southwestAustralia (Zhu et al., 2011). As previously described, Indochina is clearlydominated by ‘younger’ Grenvillian ages (~950 Ma). Therefore, here wecompare age spectrum of Indochina with those from the westernCathaysia, Qiangtang and Tethyan Himalaya (Fig. 5). The zircon age prob-ability plots from lateNeoproterozoicmetasediments and river sedimentsfrom western Cathaysia (Wan et al., 2010; Wang et al., 2007; Xu et al.,2007; Yu et al., 2008, 2010), Ordovician quartzite from Qiangtang (Donget al., 2011; Zhu et al., 2011), and Neoproterozoic to Lower Permian sed-imentary rocks from Tethyan Himalaya (McQuarrie et al., 2008; Myrowet al., 2009, 2010; Zhu et al., 2011) showmajor age peaks in Neoarchean(~2500 Ma), Mesoproterozoic (1700–1400 Ma), Grenvillian (~950 Ma),and Pan-African (650–500 Ma), which are similar to those of Indochina.In comparison to the age spectra of Indochina, there is remarkable corre-lation with Tethyan Himalaya. The peak ages for spectra of Indochina;2467, 1733, 1606, 1481, 1147, 958, 817, 637, and 531 Ma (Fig. 5A) arecommon with those of Tethyan Himalaya, which also have the peaks at2466, 1726, 1617, 1453, 1153, 968, 817, 618, and 530 Ma (Fig. 5D). Thedata indicate that detrital zircons from Indochina, western Cathaysia,Qiangtang, and Tethyan Himalaya shared a common source provenance.
Hf isotopes of the detrital zircons also support our interpretation.Fig. 4 compares εHf(T) of detrital zircons from Indochina with those
A
B
C
D
Indochina(n=109, N=3)
Western Cathaysia(n=435, N=15)
Qiangtang(n=229, N=2)
Tethyan Himalaya(n=474, N=9)
500 1000 1500 2000 2500 3000 3500 4000
Age (Ma)
2467
1606
958
637
531
1453 16171726
968
817
17331481
952
2486
1147
1118
814
800
528
644
940
2472
24668171153
530618
1180
17201502
18001658
Fig. 5. Relative probability plot for zircon U–Pb ages from this study and for other Gondwana derived terranes. In these plots, 207Pb/206Pb ages with discordant degree b20% wereused except b1 Ga zircons where 206Pb/238U ages were used. A: river sediments from Indochina (This study), B: late Neoproterozoic metasediments and river sediments fromwest-ern Cathaysia (Wan et al., 2010; Wang et al., 2007; Xu et al., 2007; Yu et al., 2008, 2010), C: Ordovician quartzites from Qiangtang (Dong et al., 2011; Zhu et al., 2011), D: LowerPermian, Ordovician, Cambrian and Neoproterozoic sedimentary rocks from Tethyan Himalaya (McQuarrie et al., 2008; Myrow et al., 2009, 2010; Zhu et al., 2011). N: number ofsamples, n: number of grains analyzed.
156 T. Usuki et al. / Tectonophysics 586 (2013) 145–159
from western Cathaysia, Qiangtang, and Tethyan Himalaya. Each agegroup of Neoarchean, Mesoproterozoic, Grenvillian, and Pan-African ofIndochina have large ranges of εHf(T) and overlaps with those ofwestern Cathaysia, Qiangtang, and Tethyan Himalaya. For example, theNeoarchean group for Indochina is characterized by a large range of epsi-lon Hf containing strong negative values with Hf model ages of >3.7 Ga.The same pattern is observed in the Neoarchean zircons from westernCathaysia, Qiangtang and TethyanHimalaya. The epsilonHf of Grenvillianand Pan-African group from Indochina show large range from positive tostrongly negative with model ages of ~3.7 Ga. Mesoproterozoic zirconsfrom Indochina show a restricted epsilon Hf range and the oldest Hfmodel age of this group is slightly younger (~3.3 Ga) than that of theother group, which is also consistent with those fromwestern Cathaysia,Qiangtang and Tethyan Himalaya.
According to Yu et al. (2008), the detrital zircons in lateNeoproterozoic sediments from western Cathaysia were derived fromIndia and east Antarctica. Zhu et al. (2011) showed that the detritalzircons in Ordovician quartzite and metasediments from Qiangtangwere derived from Tethyan Himalaya. Cambrian–Ordovician sedimentsin theHimalayan terraneswere derived from India, Antarctica, east Africa,and Australia (Myrow et al., 2010). The interpretations and similaritiesof age distribution and Hf isotopic compositions of Indochina indicatethat detrital zircons from Indochina are likely derived from the India–Antarctica region of east Gondwana.
5.3. Paleoposition of Indochina in Early Paleozoic
In previous studies, the paleogeographic reconfigurations of Indochinaduring Early Paleozoic have been difficult to be constrained due to limitedEarly Paleozoic paleomagnetic data from Indochina (Li et al., 2004) andshortage of paleontological indicators for the Early Paleozoic (Metcalfe,1998). However, our U–Pb ages and Hf isotope data from Indochinawith comparison to those from other terranes, especially South China
andQiangtang,make it possible to constrain the paleoposition of Indochi-na in the Gondwanamargin. The paleoposition of Qiangtang in the Indianmargin is widely accepted (e.g., Allègre et al., 1984; Metcalfe, 2011a; Yinand Harrison, 2000). As described before, data from Zhu et al. (2011)also support this position. The origin of South China is more controversial(e.g., Metcalfe, 1998, 2011a, 2011b; Torsvik and Cocks, 2009). Torsvik andCocks (2009) consider that South China was not part of the Gondwanamargin during the Early Paleozoic but many studies place it along theIndian margin of Gondwana (e.g., Duan et al., 2011; Jiang et al., 2003; Liet al., 2004; Metcalfe, 1998, 2011a, 2011b; Yu et al., 2008; Zhu et al.,2011). According to Li et al. (2004), the paleolatitude of South China dur-ing the Ordovician was somewhere between of 9° to 24° south, which isconsistent with the paleoposition of South China proposed by Metcalfe(1998) and with reconstructions based on age spectra of detrital zircons(Duan et al., 2011; Yu et al., 2008). Jiang et al. (2003) also place SouthChina along the Indianmargin during late Neoproterozoic by comparisonof strata between northern India and South China, although he suggestedthat South China drifted to Australian margin during the earliestCambrian.
The detrital zircons from Indochina are dominated by ages of~0.95 Ga, which is characteristic of the Indian margin of Gondwanaaccording to Zhu et al. (2011). Moreover, as described before, agespectra of >0.5 Ga zircons from Indochina show remarkable similar-ity to Qiangtang and Tethyan Himalaya, indicating that Indochina wasconnected to the Indian Gondwana margin (Fig. 6). Next, we considerthe relative position to South China. Yu et al. (2008) proposed thatthe Cathaysia component of South China was linked to the Gondwanamargin based on the characteristics of U–Pb ages and Hf isotope compo-sitions of detrital zircons from late Neoproterozoic metasediments inCathaysia. Wang et al. (2010) also considered the similar configurationof South China based on their detrital zircon ages, stratigraphy andpaleocurrent data of Cambrian to Silurian sandstones in South China. Inaddition, detrital zircon data from western Cathaysia shows a similar
GISC
IC
QT
S
L
H
YG
AFB & MB(1300-1050 Ma)
Antarctica
Australia
India
Ordovician
WC
EC
TSB
PB
30°S
Meso-Neoproterozoic orogenic belts (1300-900 Ma)
Paleozoic aged orogenic belts (600-500 Ma)
Archean and Paleoproterozoic cratons
nPCMs(990-900 Ma)
EG (990-900 Ma)
NC
T
30°N
0°
SWB
Fig. 6. Paleogeography of east Gondwana in the Ordovician. Note the position of Indochina outboard of Qiangtang and south of South China based on our study. Lhasa is placed in theAustralian margin, according to Zhu et al. (2011). The configuration of other blocks and continents are after Metcalfe (2011a). The position and ages of Grenvillian and Pan-Africanorogenic belts are after Boger et al. (2001). Arrows indicate transportation path of detritus in the Indian and Australian margins, suggested by Zhu et al. (2011). IC: Indochina, SC:South China, S: Sibumasu, QT: Qiangtang, L: Lhasa, H: Himalayan terranes including Tethyan Himalaya and High Himalaya, GI: Greater India, NC: North China, T: Tarim, SWB: SWBorneo, TSB: Truong Son Belt, WC: western Cathaysia, EC: eastern Cathaysia, YG: Pilbra/Yilgarn craton, AFB: Albany–Fraser belt, MB: Musgrave block, EG: Eastern Ghats, nPCMs:northern Prince Charles Mountains, PB: Perth Basin.
157T. Usuki et al. / Tectonophysics 586 (2013) 145–159
age distribution (Yu et al., 2008) to those of Indochina, whereas those ofeastern Cathaysia (Wuyishan area), which has no significant ~0.95 Gaage group (Yu et al., 2010), is different from those of Indochina. Therefore,we interpret that Indochina was adjacent to western Cathaysia. The con-straints indicate that Indochina was located outboard of Qiangtang andsouth of South China along the Indian Gondwana margin (Fig. 6). Thepaleoposition of Indochina is also consistent with paleontologic data.According to Forty and Cocks (1998), Ordovician trilobites fromSibumasu, whichwas located along the Australianmargin near the Equa-tor, are tropical-type whereas those from Indochina are cool-water type.The diversity of trilobites suggests that Indochina was located at higherlatitudes than the Australian margin although during the Silurian,Sino-Australian brachiopods occurred within Indochina as well asSouth China and east Australia (e.g., Metcalfe, 1998). The presence of
Sino-Australian brachiopods can be explained by a significant clockwiserotation of Gondwana from the Ordovician to Silurian (e.g., Metcalfe,2011a). By this rotation, the position of Indochina in the Indian marginmight move to lower latitude and became similar with paleolatitude ofSouth China and Australia.
6. Conclusions
This study reports a new data set of in-situ zircon U–Pb ages andHf isotopes of detrital zircons interpreted as sourced from UpperOrdovician–lower Silurian sedimentary rocks in the TSB. In comparisonto detrital zircons from other blocks in SE Asia, we make the followingconclusions.
158 T. Usuki et al. / Tectonophysics 586 (2013) 145–159
1. The age distributions yield dominant Neoarchean (~2.5 Ga),Mesoproterozoic (1.7–1.4 Ga), Grenvillian (~0.95 Ga), and Pan-African (0.65–0.5 Ga) age groups and minor Paleo- to Meso-archean zircons.
2. Zircons of each age group exhibit wide ranges of εHf(T) from strongnegative to positive values. The oldest Hf model ages for zircons ofNeoarchean, Grenvillian, and Pan-African age group yield ~3.7 Gaor older, while those of Mesoproterozoic group contain ~3.3 Ga.
3. The remarkable similarity of age spectra and Hf isotope composi-tions of zircons from Indochina with those from Tethyan Himalaya,western Cathaysia, and Qiangtang suggests that Indochina waslocated outboard of Qiangtang and south of South China along theIndian margin of Gondwana during the Early Paleozoic.
Acknowledgments
This studywas supported byAcademia Sinica and theNational ScienceCouncil, Taiwan, ROC, under grants NSC100-2116-M-001-031 and NSC92-2116-M-001-007. This paper is the contribution IESAS 1734 of theInstitute of Earth Sciences, Academia Sinica. The authors would like toexpress their sincere thanks to T.Y. Lee andM.W. Yeh for sharing samples.Tuan Anh Tran assisted with fieldwork in central Vietnam.We thank ourcolleagues Y. Iizuka for assisting in cathodoluminescence images of zir-cons and S.L. Chung for providing the zircon dating facilities. The manu-script was improved by syntax polish of J.G. Shellnutt. Constructivecomments on the manuscript by I. Metcalfe and an anonymous reviewerare gratefully acknowledged.
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