Geochemical and Sr±Nd isotopic characteristics of granitic rocks
from northern Vietnam
Ching-Ying Lan
a,*, Sun-Lin Chung
b, Jason Jiun-San Shen
a, Ching-Hua Lo
b,
Pei-Ling Wang
a, Tran Trong Hoa
c, Hoang Huu Thanh
c, Stanley A. Mertzman
daInstitute of Earth Sciences, Academia Sinica, Taipei, Taiwan
bDepartment of Geology, National Taiwan University, Taipei, Taiwan
cInstitute of Geological Sciences, National Center for Natural Sciences and Technology, Hanoi, Viet Nam
dDepartment of Geosciences, Franklin and Marshall College, PA, USA
Received 30 June 1998; accepted 1 April 1999
Abstract
Five major felsic igneous suites from northern Vietnam, with ages from mid-Proterozoic to early Cenozoic, were studied. Representative granitic rocks from the Posen Complex (mid-Proterozoic) and the Dienbien Complex (late Permian to early Triassic) show geochemical characteristics similar to those of calc-alkaline to high-K calc-alkaline I-type granites. However, the former, located in the South China block, has signi®cantly higher initial Nd isotopic ratios [eNd(T)=+0.7 to +1.5] and older
Nd isotopic model ages (TDM01.7 Ga) than the latter [eNd(T)=ÿ4.7 to ÿ9.7; TDM01.3±1.5 Ga] which were emplaced south of
the Song Ma Suture and thus in the Indochina block. The generation of both complexes may be attributed to subduction-related processes that occurred in two distinct crustal provenances with dierent degrees of mantle inputs. On the other hand, Jurassic to Cretaceous granitic rocks from the Phusaphin Complex, contemporaneous rhyolites from the Tule Basin, and late Paleogene granitic rocks from the Yeyensun Complex, all exposed in the South China block between the Ailao Shan±Red River shear zone and the Song Ma Suture, display geochemical features similar to those of A-type granites with intermediate eNd(T) values (+0.6
to ÿ2.8) and younger TDM ages (0.6±1.1 Ga). These magmas are suggested to have been generated as a consequence of
intraplate extension in the western part of the South China block (Yunnan), and to have been transported to their present position by mid-Tertiary continental extrusion along the Ailao Shan±Red River shear zone related to the India±Asia collision. Overall, the isotopic and model age data, reported in this study indicate that in northern Vietnam, the most important crust formation episode took place in the Proterozoic. Likewise, repeated mantle inputs have played a role in the petrogenesis of Phanerozoic granitic rocks. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
Southeast Asia comprises several allochthonous con-tinental blocks derived from Gondwanaland. These blocks eventually amalgamated to form the Southeast Asian continent during Paleozoic and Mesozoic time, with amalgamation processes playing a key role in the evolution of the eastern Tethys and surrounding regions (cf. Metcalfe, 1988, 1990). Being one of the
major geological discontinuities in Southeast Asia, the Ailao Shan±Red River (ASRR) shear zone extends for over 1000 km and consists of four narrow, high-grade metamorphic gneiss ranges, namely, from southeast to northwest, the Day Nui Con Voi in northern Vietnam, the Ailao Shan, the Diancang Shan and the Xuelong Shan in Yunnan, western China (Fig. 1). The Ailao Shan belt, the longest of these ranges, is fringed to the south by a strip of low-grade schists in which dismem-bered ma®c and ultrama®c bodies, generally regarded as remnants of obducted Tethyan oceanic crust and mantle, have been reported (Zhang et al., 1994). There-fore, the ASRR shear zone along which the mid-Ter-tiary continental extrusion occurred (cf. Tapponnier et
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* Corresponding author. Tel.: +886-2-27839910, ext. 614; Fax: +886-2-27839871.
Fig. 1. Sample locality map of granitic rocks from northern Vietnam. The geologic map is modi®ed from the Bureau of Geology and Mineral Resources of Yunnan (1990) and Geologic Society of Vietnam (1988). Inset marks the major Cenozoic fault system in Asia (Tapponnier et al., 1990).
al., 1982, 1990) has often been viewed as a suture between the South China and Indochina blocks (e.g., Leloup et al., 1995). Dierent suturing ages have been proposed, ranging from Sinian (Precambrian) or Paleozoic (e.g., Metcalfe, 1992; Thanh et al., 1996) to Indosinian (early Triassic) or even late Triassic (e.g., Hutchison, 1989a,b). Based on magmatic and geologi-cal correlations, Chung et al. (1997) recently argued that the ASRR shear zone was propagating on the South China continental margin and does not corre-spond to a suture as alleged. Thus, the plate boundary between South China and Indochina should be located further south and most likely extends along the Song Ma ophiolite belt (Fig. 1).
It is widely accepted that the geochemical systema-tics of granitic rocks can provide important infor-mation regarding the crustal evolution of the region. The geochemistry of granitic rocks in northern Viet-nam, however, is poorly known. Some studies so far available are written in Vietnamese and are hardly accessible to the international community. This short-coming is one of the key reasons preventing a better understanding of the geotectonic evolution of this region. Nguyen et al. (1995) has shown that both I-and A-type granites may have been emplaced in the Tule Basin during Mesozoic±Cenozoic time. Neverthe-less, their petrogenesis and their relations to broadly synchronous granitic rocks from the nearby southeast-ern and southwestsoutheast-ern China continental margins remain uncertain.
In this paper, we report major and trace element compositions and Sr±Nd isotopic data for representa-tive granitic rocks of dierent ages that were collected between the ASRR shear zone and the Song Ma belt in northern Vietnam. Major goals to be addressed include: What are the chemical characteristics of these Vietnamese granitic rocks? Which kind of tectonic set-ting may have been involved in the magma generation? Are these granitic rocks geochemically comparable to magmas emplaced in mainland China and if so, what would be the regional tectonic implications? Moreover, this reconnaissance study could form the fundament for future detailed research in this particular region of Southeast Asia.
2. Geologic background
Two tectonic events are generally considered most in¯uential in the geological evolution of northern Viet-nam. One is the large scale (0600 km) sinistral displa-cement along the ASRR shear zone active during 027±22 Ma (Chung et al., 1997), the other is the suturing between Indochina and South China along the Song Ma belt. The Song Ma belt is characterized by the occurrence of metamorphosed ma®c and
ultra-ma®c masses, which are widespread as lower Paleozoic greenschists of deep sea origin and unconformably covered in many localities by Devonian redbeds (cf. Hutchison, 1989a,b). These ma®c±ultrama®c rocks, therefore, have been widely interpreted as ophiolitic fragments, derived from the Paleo-Tethys, owing to the collision of Indochina with South China. More-over, the Song Ma ophiolite can be correlated with the Shuanggou ophiolite cropping out in the south of the Ailao Shan range (Zhang et al., 1994) and together they delineate the plate boundary between Indochina and South China (Fig. 1). The collision time had pre-viously been thought to be Silurian based on a greens-chist metamorphic age of 0455 Ma obtained by the K±Ar method (Tran et al., 1979). However, based on a detailed Ar±Ar dating study, Lepvrier et al. (1997) have shown that all rock members of the Song Ma ophiolite share the same metamorphic age of 0245 Ma. This implies that the suturing between Indochina and South China took place in the earliest Triassic, thus causing the early phase of the Indosinian orogeny, that resulted in regional metamorphism and magma-tism (Hutchison, 1989a,b).
In this study, granitic rocks were collected from ®ve major igneous complexes in northern Vietnam, namely, the Posen Complex, Dienbien Complex, Phusaphin Complex, Yeyensun Complex, and volcanics (rhyolites) from the Tule basin, which are located between 2185' to 22830'N and 10385' to 104820'E. The sample lo-calities are shown in Fig. 1. The Dienbien Complex occurs in the Indochina block (i.e. south of the Song Ma Suture), whereas all other complexes are located between the ASRR shear zone and the Song Ma belt in the southern margin of the South China block (Fig. 1).
The Posen Complex is composed of granodiorite± granite migmatite associations. It belongs to Protero-zoic magmatism based on the correlated Banngam and Xomgiau complexes having mid-Proterozoic K±Ar ages of 1350±1386 Ma (Phan et al., 1991; Table 11). The Dienbien Complex consists of a diorite±granodior-ite±granite association and is late Permian-early Trias-sic in age. The reported U±Pb ages range 272±386 Ma and K±Ar ages range 221±253 Ma (Phan et al., 1991; Table 11). Our unpublished Ar±Ar amphibole dating gives an age of 0240.422.8 Ma. The Phusaphin Com-plex consists of alkaline granite associations and is late Jurassic to early Cretaceous in age. K±Ar ages of 79± 105 Ma have been documented (Phan et al., 1991; Table 12). Jurassic to Cretaceous rhyolites and the intrusives of the Phusaphin Complex can be regarded as a volcano±plutonic association in northwestern Vietnam. The Yeyensun Complex presents a late Cre-taceous to Paleogene granodiorite±monzonite±quartz biotite hornblende granite±granite granophyre±grano-syenite association. A K±Ar age as young as 42 Ma has been reported for this complex (Phan et al., 1991;
Table 12). Representative samples of these ®ve associ-ations were collected. In Table 1, the general infor-mation on the age, modal composition and rock type of the samples studied is given.
The granitic rocks are medium to ®ne grained pluto-nic rocks and porphyritic volcapluto-nic rocks. Rocks range in color from grey to red. K-feldspar is the most abun-dant mineral forming tabular crystals. Quartz is nor-mally interstitial or is intergrown with feldspar forming graphic or myrmekitic textures. Plagioclase is albite to oligoclase. Biotite is the most common ma®c mineral and pleochroic from light brown to dark brown. Microprobe analysis shows that it is Fe-biotite. Amphibole occurs in one sample and is pleochroic from green, brownish green to brown. The alteration minerals are white mica, epidote, chlorite and carbon-ate. The accessory minerals are magnetite, ilmenite, sphene, rutile, pyrite, apatite and zircon. Opaque min-erals are mainly magnetite, showing euheral to anhe-dral crystals. Ilmenite usually coexists with sphene. 3. Analytical techniques
3.1. Whole rock chemistry
Major- and fourteen trace-element (Rb, Ba, Sr, Th, U, Zr, Nb, Y, Cr, V, Ni, Cu, Zn, Ga) concentrations were determined in the Department of Geosciences, Franklin and Marshall College, USA, using X-ray ¯u-orescence (XRF) techniques on fused glass disks and pressed powder briquettes, respectively. Working curves were constructed using at least ®fty analyzed geochemical rock standards (Abbey, 1983; Govindar-aju, 1994). The amount of ferrous Fe was determined using a modi®ed Reichen and Fahey (1962) method. Concentrations of two of the trace elements (Sc, Co) were determined using ICP±AES spectrometer, also at Franklin and Marshall College. Analytical uncertain-ties range from 1 to 5% for major elements and from 2 to 10% for minor elements. Details of the analytical procedures can be found in Lan et al. (1995).
The concentrations of eight rare earth elements (REE), Hf and Ta were analyzed at the Institute of Geology, National Taiwan University, Taipei, by the instrumental neutron activation analysis (INAA). The reproducibility is better than 10% (Chung et al., 1989). 3.2. Sr±Nd isotopic data
Samples were analyzed for Sr and Nd isotopic com-position as well as Sm and Nd contents using a VG354 mass spectrometer for Sr and MAT262 for Sm and Nd at the Institute of Earth Sciences, Academia Sinica, Taipei. The analytical procedure used is described in Lan et al. (1986) and Shen et al. (1993).
Ta ble 1 Age ,modal comp osition and rock type of the gran itic roc ks from northe rn Vietn am Age Pr oterozoic Perm ian ±Tria ssic Jura ±Cretac eous Ju rassic ±Cretac eous Cretaceo us ±Paleo gene C omplex Pose n Dienb ien Phusap hin (r hyolite) Yeyensu n Sam ple no. RR2 8A RR2 8B RR2 9 H233 H28 0 V249 H182 V 188 T9 62 T929 T9 85 RR30 RR3 1A RR3 1B RR3 2 Qua rtz 30 30 25 38 30 31 15 15 16 10 15 20 30 45 20 K-f eldspar 40 36 50 40 40 54 60 75 14 38 62 60 48 65 Pl agiocla se 10 8 7 5 15 6 5 2 10 3 3 3 5 Am phibo le 10 Bio tite 10 8 5 10 5 10 5 10 2 2 3 W hite mica 3 3 3 3 3 3 2 2 Ep idote 3 10 5 1 22 1 C hlorite 1 1 1 2 3 1 Sp hene 3 3 1 1 2 1 1 C arbonat e 2 1 1 5 4 5 9 1 Ot hers a 1 1 2 2 2 2 62 81 30 1 3 1 3 Roc k type b GD GD G G GD G G G G G G G G G G aOthers: magnet ite, ilme nite, pyrit e, apat ite, rutil and zircon for plu tonic roc ks and groundmas s for volc anic roc ks. bGD, granodio rite; G, granite based on Fig. 2.
Ta ble 2 M ajor (wt%) and trace (pp m) ele ment co ncent ration of granit ic rocks from northern Vietnam C omplex Pose n Dienbien Phusap hin J± K rhy olite Yeyen sun Sam ple RR2 8A RR2 8B RR2 9 H233 H28 0 V249 H182 V188 T962 T929 T985 RR3 0 RR31A RR3 1B RR32 SiO 2 66.08 68.05 68.73 72.59 69.68 73.90 72.90 74.50 77.23 77.06 74.68 65.64 74.06 83.96 73.94 TiO 2 0.54 0.49 0.48 0.28 0.28 0.13 0.33 0.34 0.22 0.25 0.23 0.79 0.33 0.18 0.19 Al2 O3 15.88 15.44 14.94 13.54 15.14 14.58 12.77 11.88 11.02 11.21 12.33 14.03 12.44 7.72 13.51 Fe2 O3 2.29 1.74 1.27 0.43 1.05 0.21 1.19 0.97 1.22 0.79 1.09 3.64 2.57 0.95 1.06 Fe O 1.67 1.54 1.38 1.78 1.84 0.86 2.32 1.78 1.07 2.30 1.96 2.46 0.97 0.42 0.80 M nO 0.06 0.05 0.03 0.03 0.07 0.04 0.08 0.12 0.05 0.12 0.05 0.21 0.08 0.02 0.02 M gO 1.25 1.05 1.18 0.49 1.31 0.38 0.45 0.13 0.24 0.08 0.20 0.59 0.07 0.07 0.14 C aO 3.09 2.81 1.88 1.07 3.11 1.07 1.11 0.07 0.36 0.21 0.17 1.65 0.43 0.24 0.81 Na 2 O 4.35 4.28 4.17 2.69 3.81 3.31 3.65 2.63 2.63 1.10 3.07 4.85 3.24 1.49 4.13 K2 O 2.71 2.87 3.77 5.32 2.18 5.34 4.21 6.27 4.12 5.32 4.78 4.67 5.51 4.31 4.10 P2 O5 0.18 0.14 0.14 0.05 0.11 0.09 0.02 0.01 0.00 0.01 0.00 0.14 0.01 0.00 0.01 L.O .I 1.25 1.13 1.90 1.26 1.57 0.61 1.58 1.66 1.24 2.14 0.81 1.16 0.41 0.33 0.40 To tal 99.35 99.59 99.87 99.53 100.1 5 100.5 2 100.6 1 100.3 6 99.40 100.5 9 99.37 99.83 100.1 2 99.69 99.11 La 54.1 37.8 36.5 47.35 28.8 9.81 120.7 166.7 88.7 207.6 185.7 104.4 197.7 364.4 75.1 C e 102.2 76.4 73.2 75.74 55.7 21.3 248.9 327.8 190.6 419.8 321.8 218.4 414.0 725.0 128.0 Nd 54.3 28.6 47.3 31.8 35.0 8.00 134.9 160.6 121.8 243.3 209.4 131.3 135.4 248.8 71.7 Sm 6.20 5.36 5.00 6.28 4.48 2.04 20.4 21.4 16.9 28.8 29.7 15.7 20.6 34.2 7.77 Eu 1.23 1.07 1.08 0.92 1.08 0.77 2.07 1.20 0.76 0.72 1.67 3.21 1.12 1.85 0.96 Tb 0.87 0.67 0.68 1.08 0.78 0.46 2.78 3.60 2.44 3.17 3.76 1.84 1.85 2.42 1.07 Yb 2.26 1.70 1.44 2.41 1.53 1.41 12.2 13.2 11.0 12.0 9.29 6.54 5.21 4.15 3.31 Lu 0.25 0.28 0.28 0.43 0.23 0.19 1.33 1.74 1.52 1.43 1.43 1.06 0.90 0.63 0.53 Rb 67 75 84 156 82 158 108 202 209 176 97 123 164 161 114 Ba 1115 958 1246 760 367 740 277 91 69 43 211 936 362 203 1145 Sr 548 433 429 146 522 153 26 7 13 15 18 123 200 123 462 Th 5.7 6.7 6.0 23.0 13.6 5.5 24.6 38.3 23.4 29.0 28.4 14.4 17.5 31.4 10.2 U 1.3 1.7 2.4 2.9 4.1 1.8 4.3 7.7 5.8 5.0 5.3 2.3 3.1 4.7 3.2 Zr 196 163 169 158 114 58 877 1187 1002 1038 975 514 665 215 380 Hf 6.60 5.82 6.00 5.95 2.84 1.47 24.9 31.5 27.8 25.6 23.9 18.4 20.0 7.72 10.90 Nb 11.1 9.4 9.8 13.8 10.5 9.1 106.2 133.6 116.9 112.1 125.4 87.8 95.1 47.9 30.5 Ta 0.77 0.76 0.70 1 0.71 1.01 7.58 12.1 10.5 8.13 8.99 5.46 5.85 3.35 3.05 Y 19 15 16 25 15 13 100 124 101 107 156 63 44 38 29 C r 35 12 32 14 19 25 23 4 8 23 11 4 6 < 2 7 V 62 41 46 16 48 8 <2 6 5 <2 3 24 10 < 2 12 Sc 9.51 11.0 9.97 0.00 0.00 3.77 0.00 0.00 3.41 0.00 0.00 N i 5 3 43 53 4 4 4 3 64 3 2 2 C o 6 5 5 2.3 11 2 < 1 1 4 2 < 1 2 < 1 < 1 < 1 C u 9 10 2 5 4 1 2 16 1 3 1 5 332 Zn 44 35 20 41 41 30 54 295 42 180 170 175 91 11 29 Ga 19.0 17.6 16.1 15.8 17.3 13.1 28.3 21.4 34.1 23.1 30.5 29.5 28.2 15.7 17.7 A/ CNK 1.01 1.01 1.04 1.12 1.06 1.11 1.01 1.06 1.17 1.41 1.17 0.87 1.03 1.02 1.06
Brie¯y, aliquots of the ®ne powder weighing 10±55 mg were spiked for isotope dilution measurements prior to digestion using a mixture of HNO3 and HF acids in
tightly closed te¯on jars maintained at a temperature of 1208C for two overnights. The sample solution was subsequently evaporated to dryness and then converted to chloride. This procedure was repeated until careful observation of the sample solution under microscope con®rmed that total dissolution was achieved. Sr and REE were separated from major cations using AG50W-X8, 100±200-mesh cation-exchange resin col-umn in a HCl medium. Subsequently, Sm and Nd were separated from other REE using a AG50W-X4, 200±400 mesh cation-exchange resin column with 2-MLA (2-methyllactic acid) medium buered at pH 4.44 under a pressure of 0.2 kg cmÿ2. Aqua regia was ®rst
used to digest the organic matter in the collected Sm and Nd fractions. It is followed by 30 min UV light exposure after adding one drop of 4 N HNO3in each
of the collected Sm and Nd fractions before loading them onto individual ®laments.
Sr was loaded on a Ta single ®lament while Nd and Sm were loaded on a Re single ®lament and analyzed as mono-oxide ions. Samples were oxidized at 1.5 amp in air for 1 min. Oxygen was introduced to the source
chamber to enhance oxide emission. The isotopic com-positions were measured in jumping multi-collection mode. The isotopic ratios were corrected for mass frac-tionation by normalizing to 86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219, respectively. Values for the
NBS987 Sr standard yielded 87Sr/86Sr=0.710240 with
a long-term reproducibility of 0.000038 (95% con®-dence level) and for the La Jolla (UCSD) Nd standard, yielded143Nd/144Nd=0.511867 with a long-term
repro-ducibility of 0.000028. 4. Results
4.1. Whole rock chemistry
Major, rare earth and other trace element data for the granitic rocks from northern Vietnam are listed in Table 2. Most granitic rocks are subalkaline with molar ratios of Al2O3/(Na2O+K2O) ranging from 1.07
to 1.57 and peraluminous with aluminium index (A/ CNK=molar ratio of Al2O3/(CaO+Na2O+K2O))
ranging from 1.01 to 1.41, except RR30 which is meta-luminous. Except for RR30, most granitic rocks are corundum-normative (0.1±3.3%). On an An±Ab±Or
Fig. 2. Classi®cation of granitic rocks from northern Vietnam based on the composition of normative feldspars Ab±Or±An (O'Connor, 1965). G, granite; GD, granodiorite: QM, quartz monzonite; TM, trondhjemite; TN, tonalite.
diagram (Fig. 2) of O'Connor (1965), the majority (80%) of granitic rocks fall in the ®eld of granite. The exceptions are RR28A, RR28B and H280 being grano-diorites.
The analyzed granitic samples comprise a diverse range of compositions, with SiO2 from 65.6 to 84.0
wt%. Based on their chemical characteristics, they can be divided into two groups in the FeO/MgO and
(K2O+Na2O)/CaO versus (Zr+Nb+Ce+Y)
discrimi-nation diagrams of Whalen et al. (1987). Granitic rocks of the Posen Complex and the Dienbien Com-plex, both older than the Jurassic in age, belong to unfractionated M-, I- and S-type granites (OGT, Fig. 3) and fractionated granite (FG, Fig. 3) according to the discrimination diagrams of Whalen et al. (1987). Most of these rocks have Na2O > K2O. They show
LREE enriched chondrite-normalized REE patterns (Fig. 4a and b) with LaN=31±172. No signi®cant
negative Eu anomalies are observed. Overall, they have similar patterns in the ORG (Ocean Ridge Gran-ite, Pearce et al., 1984)-normalized trace-element plots (Fig. 5a and b), marked by variable enrichments in K,
Rb, Ba, Th and Ce and depletions in Ta and Nb. The low value of Y and Yb relative to the normalizing com-position is signi®cant. Such characteristic patterns are typical for volcanic arc granites of the I-type. In the tectonic classi®cation diagram of Pearce et al. (1984), the granitic rocks ®t into the ®eld of the volcanic arc granite (VAG, Fig. 6). They are classi®ed as calc-alka-line to high-K calc-alkacalc-alka-line I-type granites on the K2O
versus SiO2diagram of Peccerillo and Taylor (1976).
The granitic rocks of the Phusaphin Complex, Juras-sic±Cretaceous rhyolites and the Yeyensun Complex, all younger than Jurassic, plot within the A-type gran-ites ®eld in the discrimination diagrams (Fig. 3). They are rich in the alkalis, with (K2O+Na2O)=5.8±9.5
wt%, and K2O predominating or nearly equal to
Na2O contents. Their chondrite-normalized REE
pat-terns are generally highly LREE enriched (Fig. 4c and d) with LaN=238±1156 and pronounced negative Eu
anomalies. These A-type granitic rocks usually have higher REE concentrations than the older I-type grani-tic rocks. Their ORG-normalized trace-element pat-terns (Fig. 5c and d) also show generally similar shapes, most displaying enrichments in Rb and Th and depletions in Ba. In addition, from Hf to Yb the pat-terns are relatively ¯at and close to that of ORG. ORG-normalized (Ce/Nb)N ratios are close to 1 and
thus no signi®cant Ta±Nb anomaly is observed except for two rocks (RR31A and B) from the Yeyensun Complex, which do show negative spikes for Ta±Nb and Hf±Zr. These A-type granitic rocks usually have higher values of Hf, Zr, Sm, Y and Yb relative to the older I-type granitic rocks and hence have a less steep slope than the older I-type granitic rocks. The A-type granitic rocks plot in the within plate granite (WPG) ®eld (Fig. 6) in the tectonic classi®cation diagram of Pearce et al. (1984).
4.2. Sr±Nd isotopic data
The Sr and Sm±Nd data are listed in Table 3. The initial isotopic ratios of the granitic rocks (calculated for dierent ages as shown in the footnote of Table 3) vary with eNd(T) between +1.5 and ÿ9.7 and eSr(T)
between ÿ41 and +572, with dierent complexes hav-ing characteristic ranges of ratios (Fig. 7a). The Posen Complex has the highest eNd(T) (+1.5 to +0.7),
fol-lowed by decreasing eNd(T) in the Phusaphin Complex
and J±K rhyolite (+0.6 to ÿ0.7), the Yeyensun Com-plex (ÿ1.7 to ÿ2.8) and Dienbien ComCom-plex (ÿ4.7 to ÿ9.7). Conversely, in terms of Sr isotopic composition, the Posen Complex shows the lowest eSr(T) of ÿ41 to
+3, followed in ascending order by the Yeyensun Complex (+33 to +133) and Dienbien Complex (+86 to +317). The Phusaphin Complex and J±K rhyolite have a large variation of eSr(T) from +139 to +572.
As shown in Table 1, the J±K granitic rocks contain
Fig. 3. (a) FeO/MgO and (b) (K
2O+Na2O)/CaO vs
(Zr+Nb+Ce+Y) discrimination diagrams (Whalen et al., 1987) for granitic rocks from northern Vietnam. FG, fractionated felsic gran-ite; OGT, unfractionated M-, I- and S-type granite.
Fig. 4. Chondrite-normalized (Masuda et al., 1973) REE distribution pattern for granitic rocks from northern Vietnam.
signi®cant amounts of carbonate as a secondary altera-tion mineral. The Sr isotopic ratios of carbonate, indi-cated by seawater composition, vary from 0.7067 to 0.7090 (eSr(T) 0 +30 to +63) which are much lower
than those of J±K granitic rocks. Thus Sr system dis-turbed during secondary alteration was probably mini-mal. The large variation in Nd and Sr isotopic ratios is indicative of the heterogeneous nature for the proto-liths of the granitic rocks. Most 147Sm/144Nd ratios
vary from 0.0716 to 0.1183 except for V249 (0.1542). The granitic rocks of the Posen Complex and the Dienbien Complex, both older than the Jurassic in age, show mid-Proterozoic crustal residence ages of 1.3±1.8 Ga. In contrast, the granitic rocks of the Phu-saphin Complex, the J±K rhyolites and Yeyensun Complex, all younger than Jurassic, have late Protero-zoic TDMor crustal residence ages of 0.6±1.1 Ga.
5. Discussion and conclusions
5.1. Source characteristics of the granitic rocks
The Sm±Nd isotopic compositions of granitic rocks investigated in this study are displayed in Fig. 7b. It shows that the granitic rocks may be subdivided into
three groups in accordance with their ages. Group I consisting of A-type granitic rocks of the Phusaphin Complex, the J±K rhyolites and Yeyensun Complex, all younger than Jurassic, has the youngest, late Pro-terozoic TDM ages (0.6±1.1 Ga) and highest eNd(0)
values (ÿ3.4 to ÿ1.4). Group II consisting of the Dien-bien Complex shows young mid-Proterozoic TDM ages
(1.3±1.5 Ga) and intermediate eNd(0) values (ÿ7.1 to
ÿ11.0). Group III, consisting of the Posen Complex, presents old mid-Proterozoic TDM ages (01.7 Ga) and
low eNd(0) values (ÿ15.7 to ÿ17.0). The relatively
depleted nature of the Nd isotopic composition and the younger Nd model ages for Phanerozoic rocks relative to the Proterozoic rocks indicate that signi®-cant proportions of newly formed juvenile material must have been involved in the petrogenesis of Phaner-ozoic granitic rocks of northern Vietnam. In Fig. 7b, the granitic rocks of the Phusaphin Complex, the J±K rhyolites and the Yeyensun Complex (Group I) plot above those of the Posen Complex and toward the left, while those of the Dienbien Complex (Group II) plot toward the upper right in the direction of the direct mixing line with depleted mantle (DM) material. Intra-oceanic-arc material can have a Nd isotopic com-position similar to DM but have much lower Sm/Nd. Thus, dierent mantle material must have been
Fig. 6. (a) Nb vs Y and (b) Rb vs Y+Nb discriminate diagrams for granitic rocks from northern Vietnam showing the tectonic classi®cation suggested by Pearce et al. (1984). ORG, ocean ridge granite; Syn-COLG, syn-collision granite; VAG, volcanic arc granite; WPG, within plate granite.
Ta ble 3 Sr and Sm ±N d data for gran itic rocks from northern Vietn am Sam ple 87Sr/ 86Sr 2 2sm Sm (pp m) Nd (pp m) 147 Sm/ 144 Nd 143 Nd / 144 Nd 2 2sm eNd (0) a TDM b(Ga) eSr (T) c eNd (T) c Pose n co mplex RR2 8A 0.710 15 4 5.58 35.85 0.094 1 0.511 768 28 ÿ 16.97 1.76 +2.93 +0.78 RR2 8B 0.711 28 4 4.71 29.96 0.095 1 0.511 809 28 ÿ 16.17 1.72 ÿ 21.57 +1.40 RR2 9 0.711 16 4 5.28 32.87 0.097 1 0.511 829 28 ÿ 15.78 1.72 ÿ 41.28 +1.45 Die nbien compl ex H23 3 0.726 82 4 5.19 27.03 0.116 1 0.512 272 28 ÿ 7.14 1.37 +16 7.24 ÿ 4.76 H28 0 0.712 10 4 3.93 20.08 0.118 3 0.512 190 28 ÿ 8.74 1.53 +86.10 ÿ 6.34 V 249 0.736 98 4 2.04 d 8.00 d 0.154 2 0.512 074 28 ÿ 11.00 +31 6.49 ÿ 9.71 Phu saphin co mplex H18 2 0.739 19 4 20.4 d 134.9 d 0.091 4 0.512 542 28 ÿ 1.87 0.76 +13 9.32 +0.08 V 188 0.910 09 4 21.4 d 160.6 d 0.080 6 0.512 494 28 ÿ 2.81 0.75 +47 3.88 ÿ 0.66 J± K rhy olite T9 62 0.815 56 4 16.9 d 121.8 d 0.083 9 0.512 562 28 ÿ 1.48 0.69 +21 4.01 +0.61 T9 29 0.814 86 4 28.8 d 243.3 d 0.071 6 0.512 529 28 ÿ 2.13 0.67 +57 1.72 +0.18 T9 85 0.748 19 4 29.7 d 209.4 d 0.085 8 0.512 551 28 ÿ 1.70 0.71 +16 2.51 +0.36 Yey ensun comp lex RR3 0 0.715 75 4 15.7 d 131.3 d 0.072 3 0.512 518 28 ÿ 2.34 0.68 +13 3.04 ÿ 1.71 RR3 1A 0.711 38 4 20.6 d 135.4 d 0.092 0 0.512 504 28 ÿ 2.61 0.81 +75.23 ÿ 2.08 RR3 1B 0.711 65 4 34.2 d 248.8 d 0.083 1 0.512 463 28 ÿ 3.41 0.80 +67.65 ÿ 2.83 RR3 2 0.708 46 4 6.54 34.39 0.115 5 0.512 463 ÿ 3.41 1.07 +33.89 ÿ 1.91 a e Nd (0)=[( 143 Nd/ 144 Nd) sample /0.512 638 ÿ 1] 10 4 . b Crusta l residence mo del age assuming de rivation from a de pleted mantle sourc e with pre sent eNd of +10 , TDM =1/ l ln{1+ [( 143 Nd/ 144 Nd) sam ple ÿ 0.513 15]/[( 147 Sm/ 144 Nd) sample ÿ 0.2137, l=0.0 0654 Ga ÿ 1 ,sample s with 147 Sm/ 144 Nd > 0.15 having me aningle ss TDM ages and sho wn by . c Initial isoto pic ratio s calculated assuming die rent age s: 1350 M a for Pose n Com plex; 240 M a for Dienb ien Complex; 145 Ma for Phusap hin Complex and J± K rhyolite ;4 0 M a for Yey ensun C omplex, eNd (T)= [INd (T)/ ( 143 Nd/ 144 Nd) CHUR (T) ÿ 1] 10 4I Nd (T)= ( 143 Nd / 144 Nd) sample ÿ ( 147 Sm / 144 Nd) sample [exp lT ÿ 1] ( 143 Nd/ 144 Nd )CHUR (T)= 0.512638 ÿ 0.196 7[exp lT ÿ 1]. dSm and Nd conc entrations obt ained by INA A m ethod w ith ana lytical errors of 0 10% ,others ob tained by ID method with un certainties of 0.5%.
involved in the sources of Group I and Group II rocks. As discussed by Jahn et al. (1990), the model ages of young granitic rocks, regardless of the input of recent mantle components, are likely to re¯ect their approximate crustal residence times.
5.2. Correlation with South China and its tectonic implications
The I-type granitic rocks of the mid-Proterozoic Posen Complex are tentatively thought to be
geneti-Fig. 7. (a) Initial eNd(T) vs eSr(T) for granitic rocks from northern Vietnam, and (b) present Sm±Nd isotopic composition of granitic rocks from
northern Vietnam. Model ages, assuming depleted mantle sources (DM) can be calculated from the slope of the tie line connecting DM and any individual data point. Four reference lines corresponding to model ages of 0.6, 1.0, 1.5 and 2.0 Ga are shown.
cally linked with the assemblage of the Rodinia Super-continent (Li et al., 1996). If we adopt the ®rst Proter-ozoic crust-forming event of western South China (Yangtze block) to start at 02.1 Ga (Chen and Jahn, 1998), the Nd isotopic compositions of the granitic rocks of the Posen Complex fall roughly on the Nd isotope evolution trends of the known Proterozoic rocks (Fig. 8). Thus, remelting of Paleo- to Meso-Pro-terozoic crustal rocks, without signi®cant input of mantle-derived material, accounts for their isotopic sig-nature. Discrepancy occurs for the granitic rocks of the Posen Complex, being S-type on the basis of Nd isotopic compositions and I-type on the basis of trace element characteristics. Another alternative petroge-netic model for the Posen Complex is that the granitic rocks may be derived by melting of older crustal sources but with a signi®cant input of mantle com-ponent during magma genesis. Older crustal source rocks are waiting to be found to justify this alternative. Zircon geochronology is in progress to ®nd the oldest rock in northern Vietnam.
The I-type granitic rocks of the late Permian to early Triassic Dienbien Complex are located south of the Song Ma ophiolite zone and thus in the Indochina block. The Song Ma ophiolite belt represents the ocea-nic crust and mantle of the eastern Paleo-Tethys and
can be correlated with the Shuanggou ophiolite in Yunnan (Chung et al., 1998b). The I-type granitic rocks of the Dienbien Complex represent a part of the magmatic arc along the Indochina continental margin. Their petrogenesis is tentatively attributed to subduc-tion-related processes in relation with the closure of the Paleo-Tethys and/or suturing between South China and Indochina. A detailed study of these rocks is in progress. Due to the limited documentation at present, we can not identify the counterparts in western Yunnan that may correlate with the A-type granitic rocks of the Phusaphin Complex and the J±K rhyolites in northern Vietnam. The granitic rocks of western Yunnan are mainly S-type in nature for Jurassic to Cretaceous granitic rocks (Zhang and Xie, 1995). In contrast, abundant Jurassic to Cretaceous granitic rocks have been documented in the eastern part of South China. The latter vary from S- to I- and A-type granites, with the late Jurassic peraluminous S-type granites being dominant throughout the interior of South China (Li, 2000). The A-type granites, however, occur mainly in the coastal region of South China, along some major fault belts. They were emplaced through the earliest Cretaceous (0140 Ma) to late Cretaceous (090 Ma) as a result of the lithospheric extension (Li, 2000). However,
based on the similarity of lack of depletions in the high ®eld strength elements (Nb, Ta and Ti), we speculate that Permian±Triassic Emeishan ¯ood basalts in western Yunnan (Chung and Jahn, 1995; Chung et al., 1998a) may serve as the source ma-terial for a generation of Jurassic to Cretaceous A-type granitic rocks of the Phusaphin Complex and J±K rhyolites in northern Vietnam. Detailed study of Jurassic to Cretaceous extension related rocks are in progress.
The extension-related A-type granitic rocks of the Yeyensun Complex in northern Vietnam are con-sidered to have a close relationship to those in wes-tern Yunnan (in the weswes-tern part of South China; Zhang and Xie, 1995), but not to be related to those of the eastern part of South China where no Paleogene granites occur. This observation is com-patible with the results of Chung et al. (1997) and supports a sinistral oset of 0600 km for northern Vietnam along the ASRR shear zone after 030 Ma. Similar to the Paleogene high-potassic ma®c mag-mas in northwestern Yunnan and northwestern Viet-nam (Chung et al., 1997), the A-type granitic rocks of the Yeyensun Complex could have been formed by intraplate extension (Chung et al., 1997) in con-junction with the SE Asian tectonic history after the India±Asia collision, which probably began as early as 60 My ago (Lee and Lawver, 1995).
The granitic rocks of the Phusaphin Complex, the Jurassic±Cretaceous rhyolites and the Yeyensun Com-plex, having anomalously high Sm (7.7±34.2 ppm) and Nd (71.7±248.8 ppm) concentrations, show many com-mon characteristics with the enriched Sm±Nd Meso-zoic granites from SE China studied by Gilder et al. (1996). They have high eNd(T) values (+0.6 to ÿ2.8),
variable eSr(T) values (+33 to +572), relatively young
TDM ages (0.6±1.1 Ga) and are A-type granitic rocks.
These granitic rocks, having similar source provenance (Fig. 7b), outcrop along the extensional Mesozoic Tule basin. Hence, the model of Gilder et al. (1996) on iso-topic evolution and basin formation may also apply to the Mesozoic to Cenozoic granitic rocks in northern Vietnam.
5.3. Crustal evolution of northern Vietnam
The TDM data shown in Table 3 and Fig. 7b
indi-cate that the bulk of northern Vietnam was formed in the Proterozoic. This observation reinforces the idea that northern Vietnam belongs to the southern exten-sion of South China. Chen and Jahn (1998) reported that South China shows little vestige of Archean crus-tal history except at the northern margin of the wes-tern South China (Yangtze block) and northeaswes-tern part of eastern South China (Cathaysian block). Hence, northern Vietnam, similar to most of South
China, was formed during the Proterozoic. However, mixing of Archean and younger protoliths could also produce Proterozoic model ages. Zircon geochronology in progress will help to verify the presence of Archean rocks in northern Vietnam.
The crustal evolutionary history of this part of northern Vietnam is tentatively illustrated in Fig. 8. Northern Vietnam began its crustal evolution since the Precambrian. Around 240 Ma, the earliest injection of mantle input was admixed into the recycled sediments to produce granitic rocks. The fresh mantle input in the form of oceanic crust and mantle of the eastern Paleo-Tethys resulted from the suturing between South China and Indochina. Later at around 145 Ma, the extension following the suturing of South China and Indochina brought a second mantle input to mix with the sediments and produce the granitic rocks. Finally at around 40 Ma, the extension resulting from the India±Asia collision caused another mantle input to mix with the recycled sediments generating granitic rocks. For the extension-related events, the fresh man-tle input was in the form of intra-oceanic manman-tle ma-terial. Thus, mantle inputs made an important contribution to the petrogenesis of Phanerozoic grani-tic rocks in northern Vietnam.
Acknowledgements
Special thanks for administrative help are due to Drs Nguyen Trong Yem and Dinh Van Toan from the National Center for Natural Sciences and Technology (NCNST), Vietnam. Drs Tung-Yi Lee (National Tai-wan Normal University), Jian-Cheng Lee (Academia Sinica), Vuvan Van, Bui An Nien, Ngo Thi Phuong and others (Magmatic Department, NCNST) are thanked for their participation in collecting samples. The authors thank Miss Victoria W.H. Lee of Acade-mia Sinica for drawing the ®gures and typing the tables. S.A. Mertzman thanks the NSF for support through equipment grant EAR-9217945 to purchase a new Philips 2404 XRF vacuum spectrometer and Miss Karen R. Mertzman for her meticulous help in the lab. We also thank Drs Ulrich Knittel (Aachen), Fritz Finger (Universitat Salzburg) and Kevin Burke (Uni-versity Houston) for reviews that improved the manu-script. This research was supported by Academia Sinica and the National Science Council of the Repub-lic of China under grants NSC86-2116-M-001-023 and NSC87-2116-M-001-021 to C.Y. Lan. This paper is Contribution IESEP1999-012 of the Institute of Earth Sciences, Academia Sinica.
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