Geochemical and Sr–Nd isotopic constraints from the
Kontum massif, central Vietnam on the crustal
evolution of the Indochina block
Ching-Ying Lan
a,∗, Sun-Lin Chung
b, Trinh Van Long
c, Ching-Hua Lo
b,
Tung-Yi Lee
d, Stanley A. Mertzman
e, Jason Jiun-San Shen
aaInstitute of Earth Sciences, Academia Sinica, Nankang, P.O. Box 1-55, Taipei 11529, Taiwan, ROC bDepartment of Geosciences, National Taiwan University, Taipei 106, Taiwan, ROC
cSouth Vietnam Geological Mapping Division, Department of Geology and Minerals of Vietnam, Ho Chi Minh City, Viet Nam dDepartment of Earth Sciences, National Taiwan Normal University, Taipei 117, Taiwan, ROC
eDepartment of Geosciences, Franklin and Marshall College, Lancaster, PA 17604-3003, USA
Received 19 December 2001; received in revised form 20 April 2002; accepted 7 July 2002
Abstract
The Kontum massif, central Vietnam, consists mainly of high-grade (amphibolite to granulite facies) metamorphic rocks and represents the largest basement exposure (core complex) of the Indochina block. To explore the crustal evolution of Indochina, Sr and Nd isotopic and geochemical data for various rock types from the massif are reported. The basement rocks show a wide range of present dayεNdvalues from−22 (gneiss) to +15 (amphibolite), yielding depleted-mantle model ages (TDM) from 1.2
to 2.4 Ga along with an “exceptionally” old TDM of 2.7 Ga for a granulite. These data indicate that crustal formation in the
Indochina block took place principally during the Paleoproterozoic and Mesoproterozoic, and do not support the conventional notion that the Kontum core complex is composed of Archean rocks. Geochemical data indicate that the gneisses and schists have heterogeneous compositions characterized by a calc-alkaline nature, whereas most of the amphibolites are tholeiitic basalts with intraplate magmatic signatures. Therefore, the former may be interpreted as products from pre-existing Proterozoic crustal materials and the latter as resulting from the Paleozoic rifting event that disintegrated the Indochina block from Gondwanaland. During its accretion with other SE Asian continental blocks in Permo-Triassic time, the Indochina core complex was subjected to the Indosinian orogeny, characterized by a high-temperature, granulite facies metamorphism in the lower crust with associated charnockite magmatism and subsequent regional exhumation.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: Kontum massif; Sr–Nd isotope; Geochemistry; Indochina block; Vietnam
∗Corresponding author. Tel.:+886-2-27839910x614;
fax:+886-2-27839871.
E-mail address: kyanite@earth.sinica.edu.tw (C.-Y. Lan).
1. Introduction
Southeast Asia consists of allochthonous continen-tal blocks disintegrated from the northern margin of Gondwanaland. These include the South China, In-dochina, Sibumasu and West Burma blocks (see inset ofFig. 1), which amalgamated to form the Southeast 0301-9268/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
Fig. 1. Simplified geological map of the northern Kontum massif, central Vietnam (modified fromNam, 1998) showing the basement rocks and sample localities of this study. Samples KT8049/1 and KT8049 are located further south at 13◦21.8N and are not shown in the map. “Archean” complex is composed mainly of Kannack and Song Ba complexes while other formations and complexes belong to Proterozoic complex. Inset map showing the distribution of principal continental terranes of East and Southeast Asia (Metcalfe, 1998) and the location of the simplified geological map. Stars denote the locations of the Archean rocks in the South China block.
Asian continent during Paleozoic to Mesozoic peri-ods (Metcalfe, 1996, for review). The Song Ma belt, characterized by the occurrence of metamorphic mafic and ultramafic masses of ophiolitic fragments, repre-sents the plate boundary between Indochina and South China blocks. The suturing may have taken place in the early Triassic and caused the early phase of In-dosinian orogeny (Lepvrier et al., 1997; Chung et al., 1999), although the fossil fish record favors a close geographic association of the two blocks during De-vonian time (Thanh et al., 1996). Thus, Vietnam is composed of two continental blocks. The part north of the Song Ma suture belongs to South China block and the part south of the suture belongs to Indochina block.
Amphibolite and granulite facies metamorphic rocks of the Kontum massif in central Vietnam have been traditionally regarded as an exposed “Archean” core complex of the Indochina block (Hutchison,
1989; Tien, 1991; Bao et al., 1994). However, the inferred “Archean” ages were basically based on the petrological correlations that link the Kontum mas-sif with the classic Archean granulites terranes in Gondwanaland, such as those in East Antarctica, In-dia, Sri Lanka and Australia (Katz, 1993). Without any analytical details, a Pb isochron age of 2300 Ma and K–Ar age of 1650–1810 Ma were given for the Kontum massif byHutchison (1989). Recently, radio-metric ages have been reported for the metamorphic rocks from the Kontum massif using various meth-ods including K–Ar (Nam, 1998), 40Ar/39Ar (Lo et al., 1999), SHRIMP U–Pb zircon (Carter et al., 2001; Nam et al., 2001), composite 40Ar/39Ar and U–Pb (Nagy et al., 2001) techniques. The results show Permo-Triassic ages (∼250 Ma) for 10 sam-ples and mid- to late-Paleozoic ages (∼450 Ma) for four samples. The presumed Precambrian age of Kontum massif has been obtained in a zoned zircon
core from a gneiss yielding a concordant age of 1403± 34 Ma (Nam et al., 2001), in addition to some upper intercept ages for zircons from 1.1 to 2.7 Ga (Nagy et al., 2001). Geochronological studies have also been performed for rocks north of the Kon-tum massif and south of the Song Ma suture using 40Ar/39Ar (Lepvrier et al., 1997; Jolivet et al., 1999), K–Ar (Nam, 1998), composite Rb–Sr and U–Pb (Nagy et al., 2000) and SHRIMP U–Pb zircon (Carter et al., 2001) methods. The results yielded a wide age range including Oligocene-Miocene (19.6 ± 0.5 to 36.1 ± 1 Ma), Cretaceous (82 ± 10 to 130 ± 3 Ma), Permo-Triassic (∼250 Ma), mid- to late-Paleozoic (∼450 Ma) and Neoproterozoic (800 to 901 ± 26 Ma). Precambrian ages were registered in inherited zircon cores ranging from 800 to 2541±69 Ma (Carter et al., 2001) and upper intercept discordant zircons ages of 881± 26 to 901 ± 26 Ma (Nagy et al., 2000). The interpretation of these ages will be discussed in latter sections.
High-grade metamorphic rocks have resided at deep crustal levels for substantial periods of time. Thus, high-grade terrains provide important clues to the composition, tectonic setting and evolution of the lower continental crust. However, the high-grade metamorphic rocks of central Vietnam have been rarely subjected to detailed geochemical and isotopic studies. This paper presents the first systematic study of major and trace element as well as Sr and Nd iso-topic compositions for amphibolite to granulite facies metamorphic basement rocks from the Kontum mas-sif in order to constrain the crustal evolution of the Indosinian block.
2. Geological background
In the Indochina block, the Kontum massif repre-sents the largest continuous exposure of Precambrian basement rocks and has been viewed as a “stable” ex-posure of high-grade metamorphic rocks. They can be divided into an “Archean” complex and a Proterozoic complex (Fig. 1), which are further grouped into dif-ferent units under local names (Bao et al., 1994). These are metamorphic units of the Kannack complex, the Tak Po formation and the Kham Duc formation and magmatic complexes including the Song Ba, Song Re and Chu Lai complexes (Table 1).
The Kannack complex, distributed in the central part of Kontum massif, is the major constituent of the “Archean” complex. It is composed mainly of two-pyroxene-bearing granulites, in association with plutons of orthopyroxene-bearing granite (charnock-ite and enderb(charnock-ite) and cordier(charnock-ite-silliman(charnock-ite-bearing gneisses (khondalites). Mineral assemblages of the Kannack complex are typical of granulite facies metamorphism, with the peak T–P conditions of 800–850◦C and 8± 1 kbar (Department of Geology and Minerals of Vietnam—DGMVN, 1989). Protero-zoic formations are widely distributed in the Kontum massif and are composed of gneiss, amphibolite, schist, migmatite and lenses of marble. Mineral as-semblages of these Proterozoic rocks suggest an amphibolite facies metamorphism (DGMVN, 1989).
The Kannack complex and the intrusive Song Ba complex were speculated to be “Archean” (>2.6 Ga, Tien, 1991; Bao et al., 1994) based solely on their petrological similarities with the Archean granulites in other parts of the world. However, the recent geochro-nological studies for gneisses, migmatites and char-nockites (Nam, 1998; Nam et al., 2001; Carter et al., 2001; Nagy et al., 2001) from such an “Archean” Kan-nack complex showed Permo-Triassic ages (240.6 ± 5.0 to 258±6 Ma). Moreover, both Permo-Triassic and mid- to late-Paleozoic ages (407± 11 to 451 ± 3 Ma) were obtained for the gneiss, granodiorite and amphi-bolite samples from the surrounding Proterozoic com-plex (Carter et al., 2001; Nagy et al., 2001). SHRIMP U–Pb ages (Chung et al., unpublished data) were ob-tained on zircons in two of the samples in this study (KT289, a charnockite from the “Archean” Song Ba complex, and KT318, a gneiss from the Proterozoic Song Re complex). Zircons from KT289 give a con-cordant age of 260± 16 Ma. The majority of zircons from KT318 give concordant ages of 436± 10 Ma but three zircons give ages of 2541± 55, 1455 ± 24 and 869±57 Ma, which we interpreted to be inherited ages. The Paleozoic age registered in the Proterozoic complex has been interpreted as indicative of a rift-ing event (Hutchison, 1989; Carter et al., 2001; Nagy et al., 2001) that disintegrated the Indochina block from Gondwanaland. The Indosinian age registered in both the “Archean” and Proterozoic complexes has been widely considered to record the granulite facies to amphibolite facies metamorphism (Lepvrier et al., 1997; Nam et al., 2001; Carter et al., 2001).
Table 1
Major and trace element concentration for basement rocks from central Vietnam
Sample no. KT321/1E KT321/1 KT326 QN1275 KT318 KT308 KT196 KT205
Latitute 15◦7.3 15◦7.3 15◦0 14◦50 14◦46.5 14◦43 14◦46 14◦45 Longitude 108◦36.9 108◦36.9 108◦30.5 108◦32 108◦31.7 107◦43 107◦53.5 108◦31.5 Complex Tu Mo Rong Tu Mo Rong Tu Mo Rong Song Re Song Re Tak Po Tak Po Kham Duc Rock typea Mafic Gneiss Mgm Gneiss Mgm Gneiss Bt Gneiss Bt Gneiss Bt Gneiss Bt Gneiss Bt Gneiss Major element (%) by XRF SiO2 53.32 63.00 71.82 63.45 68.65 62.32 71.71 64.29 TiO2 0.37 0.42 0.23 0.92 0.40 0.83 0.29 0.76 Al2O3 11.46 13.90 15.27 15.63 15.04 17.64 14.49 14.87 Fe2O3 1.69 1.02 0.47 1.30 0.79 0.83 0.14 1.50 FeO 7.63 4.30 1.62 3.91 2.65 4.24 2.57 4.39 MnO 0.24 0.11 0.07 0.08 0.08 0.07 0.06 0.10 MgO 11.07 5.61 0.59 1.77 1.04 1.76 0.66 2.23 CaO 9.71 5.62 3.24 4.42 3.23 5.02 2.09 4.77 Na2O 1.07 1.41 3.95 2.56 2.63 4.02 2.71 2.14 K2O 1.47 2.58 1.99 4.08 4.49 2.07 3.91 2.47 P2O5 0.08 0.15 0.13 0.40 0.16 0.22 0.08 0.13 LOI 2.00 1.73 0.76 1.24 0.76 1.05 0.81 1.90 Total 100.11 99.85 100.14 99.76 99.92 100.07 99.52 99.55
Trace element (ppm) by XRF+ ICP-AES
Rb 125.0 233.0 96.4 121.9 155.3 89.9 141.2 104.9 Ba 139 259 313 1319 718 495 492 865 Sr 142 252 136 254 210 319 134 307 Zr 53 122 189 366 241 382 150 151 Y 28.6 19.3 21.9 34.6 42.3 28.2 35.3 28.2 Cr 1101 349 11 28 21 21 23 36 V 203 132 24 112 64 67 39 133 Sc 35 25 7 18 14 15 9.3 20.5 Ni 226 71 <0.5 12 4 6 4 11 Co 35 17 3 10 7 10 5 12 Cu 5 5 4 16 7 9 7 15 Zn 94 57 36 67 49 60 41 78 Ga 10.8 9.7 9.2 12.1 10.4 18.7 15.2 15.6
Trace element (ppm) by ICP-MS
La 11.6 30.8 34.5 43.7 38.0 55.4 32.7 53.1 Ce 25.5 59.8 62.7 89.5 76.0 106.8 66.1 97.5 Pr 3.0 6.9 6.9 11.0 8.1 10.6 8.0 11.3 Nd 12.1 26.6 24.4 42.2 30.2 39.3 29.4 40.5 Sm 3.0 4.7 4.7 8.3 6.2 6.7 5.8 7.7 Eu 0.79 0.85 0.82 2.22 1.00 1.30 1.12 1.68 Gd 2.92 4.49 4.64 8.17 5.42 5.05 5.79 6.72 Tb 0.55 0.66 0.79 1.16 0.97 0.77 0.98 1.08 Dy 3.63 3.78 4.87 6.39 5.70 4.38 5.93 6.19 Ho 0.75 0.78 1.06 1.27 1.14 0.80 1.24 1.21 Er 2.04 2.19 2.96 3.25 3.20 2.03 3.67 3.35 Tm 0.32 0.35 0.46 0.44 0.53 0.31 0.58 0.47 Yb 2.13 2.20 3.12 2.58 3.31 1.80 3.75 2.91 Lu 0.32 0.34 0.50 0.38 0.49 0.27 0.56 0.43 Th 3.61 12.23 16.54 10.97 27.68 16.14 12.27 14.03 U 1.00 2.49 3.27 2.01 4.41 0.92 1.39 2.05 Hf 1.5 3.9 5.6 8.6 6.1 7.5 3.8 5.7 Nb 4.6 4.0 7.5 17.8 12.1 14.9 7.9 12.9 Ta 0.36 0.50 0.65 0.88 0.81 0.33 0.42 0.91 Y 26 21 29 33 36 26 36 33 Cr 1140 325 16 31 30 26 42 61 V 197 130 21 109 60 69 32 118 Ni 281 75 46 13 9 11 8 Co 42 23 4 13 8 11 4 15
Table 1 (Continued )
Sample no. KT294/1 KT333/6 KT220 KT8049/1 KT8049 KT8122 TS25
Latitute 14◦12 14◦14 14◦16 13◦21.8 13◦21.8 15◦5 15◦26.72 Longitude 108◦32 108◦54.9 108◦32.5 108◦25 108◦25 108◦24.6 108◦35.24 Complex Kannack Song Ba Kham Duc Song Re Song Re Tak Po Dieng Bong Rock typea Gt Cd Gneiss Mgm Gneiss Bt Gneiss Felsic Gneiss Am Schist Bt Schist Am Schist Major element (%) by XRF SiO2 58.36 61.94 71.67 72.14 57.41 59.92 61.83 TiO2 1.62 0.60 0.30 0.17 0.76 1.01 0.49 Al2O3 18.49 20.94 14.37 15.16 13.65 17.93 18.08 Fe2O3 0.63 1.07 0.72 0.26 1.21 2.57 2.08 FeO 6.06 1.57 1.38 1.52 7.29 4.19 2.53 MnO 0.07 0.04 0.02 0.03 0.20 0.09 0.07 MgO 2.41 1.01 0.51 0.54 6.09 2.18 2.56 CaO 6.28 1.42 2.38 2.80 7.70 6.05 5.16 Na2O 2.42 4.37 2.14 4.04 1.73 1.92 4.67 K2O 1.42 4.47 5.08 2.20 1.35 2.23 1.24 P2O5 0.62 0.08 0.08 0.06 0.10 0.42 0.21 LOI 1.42 2.08 0.64 1.43 2.59 1.68 1.12 Total 99.80 99.59 99.29 100.35 100.08 100.19 100.04
Trace element (ppm) by XRF+ ICP-AES
Rb 93.3 116.6 150.2 121.8 78.0 86.7 34.9 Ba 593 1183 1420 378 261 462 473 Sr 244 237 212 279 114 719 848 Zr 655 306 192 106 134 137 83 Y 17 9.9 14.8 11.8 33 13.8 10.6 Cr 5 101 15 8 301 7 51 V 116 59 32 14 227 81 118 Sc 12 3 7.6 5 36 10 10 Ni 3 11 3 <0.5 61 5 18 Co 10 5 2 4 30 13 13 Cu 14 21 8 9 23 60 5 Zn 119 47 31 29 100 93 64 Ga 23.5 18.4 12.8 11.2 12.7 16.5 20.9
Trace element (ppm) by ICP-MS
La 45.7 72.1 58.4 21.0 25.4 36.9 19.1 Ce 106.6 118.8 101.9 38.6 49.6 78.8 39.4 Pr 13.5 13.0 11.1 3.9 6.3 10.6 4.9 Nd 53.6 44.7 37.6 13.0 24.1 42.6 17.6 Sm 9.8 6.4 5.5 2.1 5.1 8.5 2.9 Eu 1.63 2.52 1.74 0.59 0.99 2.17 0.91 Gd 7.70 5.06 4.22 1.63 5.14 7.19 2.61 Tb 0.89 0.58 0.61 0.26 0.89 0.97 0.37 Dy 3.74 2.23 3.22 1.52 5.82 4.61 2.09 Ho 0.64 0.38 0.60 0.31 1.23 0.77 0.40 Er 1.41 1.02 1.71 0.85 3.51 1.93 1.09 Tm 0.17 0.15 0.24 0.15 0.52 0.24 0.16 Yb 1.03 1.00 1.40 1.03 3.24 1.35 1.01 Lu 0.16 0.18 0.22 0.16 0.50 0.19 0.16 Th 6.62 28.91 15.77 7.28 12.77 5.69 4.15 U 0.95 2.98 1.57 1.18 3.64 2.55 0.85 Hf 14.9 10.1 6.7 2.4 2.1 6.1 2.5 Nb 19.9 15.6 7.6 8.4 8.9 13.5 4.4 Ta 1.11 1.50 0.60 0.55 0.88 0.85 0.39 Y 16 11 17 10 37 21 12 Cr 5 92 43 12 256 37 62 V 94 51 28 210 76 95 Ni 4 19 10 10 63 7 22 Co 11 6 4 3 36 16 12
Table 1 (Continued )
Sample no. KT298 TS22A KT235 KT8112 KT8116 KT291 KT289 KT293
Latitude 14◦15.2 14◦22.31 14◦19 15◦23 15◦13 14◦10.36 14◦15.7 14◦14.3 Longitude 108◦45.2 109◦6.42 109◦3 108◦16 108◦25.6 108◦34.37 108◦30.2 108◦30.6 Complex Kannack Phu My Phu My Kham Duc Kham Duc Song Ba Song Ba Kannack Rock typea Amphibolite Amphibolite Amphibolite Amphibolite Amphibolite Charnockite Charnockite Granulite Major element (%) by XRF SiO2 48.22 45.44 50.22 49.23 54.36 45.33 57.61 49.46 TiO2 1.60 1.85 1.07 2.00 0.49 1.85 1.05 1.12 Al2O3 12.93 15.37 14.74 13.81 13.11 20.47 17.81 15.39 Fe2O3 1.86 4.10 1.82 2.49 2.16 0.75 1.66 0.41 FeO 8.94 13.46 10.87 10.36 10.90 9.88 5.18 11.06 MnO 0.34 0.21 0.21 0.22 0.20 0.18 0.12 0.20 MgO 8.82 5.64 6.46 6.05 5.64 4.50 3.45 7.54 CaO 10.85 9.68 9.70 10.06 9.36 11.32 6.60 12.64 Na2O 0.36 2.42 1.56 3.51 2.02 1.53 3.81 0.82 K2O 2.14 0.21 0.88 0.31 0.29 1.24 1.16 0.24 P2O5 0.16 0.05 0.07 0.22 0.06 0.70 0.47 0.10 LOI 3.25 1.79 1.92 1.80 1.89 1.83 1.09 0.99 Total 99.47 100.22 99.52 100.06 100.48 99.58 100.01 99.97
Trace element (ppm) by XRF+ ICP-AES
Rb 105.8 6.4 23.0 9.9 8.4 62.7 20.5 11.5 Ba 196 23 144 36 27 434 180 32 Sr 120 200 96 134 50 435 473 115 Zr 95 38 59 143 23 412 191 47 Y 33.9 11.1 24.5 42.6 18.8 68 18.4 23.0 Cr 328 29 133 156 90 6 55 133 V 308 779 387 368 317 173 140 324 Sc 48 32 43.9 44 52 37 15 43 Ni 55 12 84 50 57 4 17 75 Co 36 88 44 39 45 21 15 44 Cu 155 359 127 71 45 20 18 37 Zn 120 105 103 113 101 124 98 101 Ga 13.0 22.6 15.3 15.3 9.1 22.1 19.6 14.0
Trace element (ppm) by ICP-MS
La 8.9 1.5 6.0 7.1 0.7 33.7 30.1 8.7 Ce 24.5 9.3 17.4 18.3 1.9 93.6 69.0 22.0 Pr 3.8 1.5 2.7 3.1 0.3 13.7 8.7 3.2 Nd 17.6 6.2 10.5 15.2 1.9 61.4 35.7 14.3 Sm 4.7 1.8 2.6 4.7 0.8 13.0 6.5 3.6 Eu 2.20 0.67 0.94 1.60 0.36 2.31 1.28 0.88 Gd 5.36 1.87 3.62 6.04 1.59 12.30 5.26 3.93 Tb 0.93 0.32 0.63 1.05 0.34 1.96 0.69 0.63 Dy 6.25 2.04 4.14 7.41 2.72 11.85 3.52 3.96 Ho 1.28 0.44 0.87 1.57 0.65 2.39 0.61 0.80 Er 3.62 1.18 2.55 4.43 2.03 6.57 1.48 2.11 Tm 0.53 0.19 0.39 0.64 0.32 0.98 0.21 0.33 Yb 3.39 1.18 2.44 4.04 2.14 6.56 1.30 2.11 Lu 0.51 0.18 0.38 0.62 0.35 0.96 0.17 0.31 Th 0.95 0.24 1.37 0.61 0.06 1.48 0.41 0.57 U 0.84 0.18 0.59 0.20 0.02 0.59 0.24 0.11 Hf 1.03 0.98 1.83 1.26 0.46 10.2 4.5 1.7 Nb 12.12 1.38 3.58 5.77 0.82 20.9 9.3 6.0 Ta 0.73 0.18 0.35 0.37 0.06 0.89 0.44 0.44 Y 37 12 24 46 18 65 18 21 Cr 289 33 152 136 71 6 60 144 V 295 708 358 359 308 158 124 301 Ni 71 12 95 54 66 6 100 Co 47 74 50 47 56 23 19 49
3. Samples and petrography
Samples of the Kontum massif were collected from the “Archean” and Proterozoic complexes (Fig. 1). They include different rock types (gneiss and schist, amphibolite, charnockite and granulite) from differ-ent formations and plutons (Table 1). The gneisses are composed of quartz, K-feldspar, plagioclase, ±muscovite, ±brown biotite, ±greenish-brown am-phibole, opaque, ±apatite, ±epidote, ±chlorite, and ±garnet. The schists consist of quartz, feldspar, ±brown biotite, ±greenish-brown amphibole, epidote, and opaque. The amphibolites contain plagioclase, greenish-brown amphibole, ±brown biotite, ±garnet and minor opaque, sphene,±epidote, ±chlorite. Both charnockite and granulite consist of pale pink to green pleochroic orthopyroxene, clinopyroxene, plagioclase, ±brown biotite, ±greenish brown amphibole and minor opaque (ilmenite, pyrite), apatite,±epidote.
4. Analytical techniques
4.1. Whole rock chemistry
Major and 11 trace element (Rb, Ba, Sr, Zr, Y, Cr, V, Ni, Cu, Zn and Ga) concentrations were determined in the Department of Geosciences, Franklin and Marshall College, USA, using X-ray fluorescence (XRF) tech-niques on fused glass disks and pressed powder bri-quettes, respectively. Working curves were constructed using at least 50 analyzed geochemical rock standards (Abbey, 1983; Govindaraju, 1994). The amount of fer-rous Fe was titrated using a modified Reichen and Fahey (1962)method, and loss on ignition was deter-mined by heating an exact aliquot of the sample at 950◦C for 1 h. Concentrations of two of the trace el-ements (Sc and Co) were determined using ICP-AES spectrometer, also at Franklin and Marshall College. Analytical uncertainties range from 1 to 5% for ma-jor elements and from 2 to 10% for minor elements. Details of the analytical procedures can be found in Boyd and Mertzman (1987)andMertzman (2000).
The concentrations of 14 rare earth elements (REE) and 10 trace element (Th, U, Hf, Nb, Ta, Y, Cr, V, Ni and Co) concentrations were analyzed at the Guangzhou Institute of Geochemistry, Chi-nese Academy of Sciences, Guangzhou, using a
Perkin-Elmer Sciex ELAN 6000 inductively-coupled plasma mass spectrometer (ICP-MS). The analyt-ical procedures and accuracy were reported in Li (1997). The uncertainties for all elements are less than 5%.
4.2. Sr and Nd isotopic data
Samples were analyzed for Sr and Nd isotopic com-position as well as Sm and Nd concentrations using a VG354 mass spectrometer for Sr and MAT262 for Sm and Nd at the Institute of Earth Sciences, Academia Sinica, Taipei. Sr, Sm and Nd separation was achieved using conventional cation-exchange chromatography (Lan et al., 1986; Shen et al., 1993). Sr was loaded on a single Ta filament while Sm and Nd were loaded on a single Re filament and analyzed as mono-oxide ions. The isotopic compositions were measured in multi-collectors with dynamic mode. The isotopic ra-tios were corrected for mass fractionation by nor-malizing to 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. Values for the NBS987 Sr stan-dard yielded87Sr/86Sr = 0.710240 with a long-term reproducibility of 0.000028 (95% confidence level) and for the La Jolla (UCSD) Nd standard, yielded 143Nd/144Nd = 0.511866 with a long-term repro-ducibility of 0.000026.
5. Results
Major and trace element concentrations of the ana-lyzed samples are presented inTable 1and their iso-topic data are presented inTable 2. It should be noted that all the rocks have experienced high-grade re-gional metamorphism and recrystallization. K, Na and low-field-strength elements (LFSE: Rb, Sr, Ba) were likely mobilized during amphibolite facies metamor-phism (e.g. Humphris and Thompson, 1978). Thus, these elements are not used for the petrogenetic in-terpretations. Diagrams of Zr/TiO2 versus Nb/Y of Winchester and Floyd (1977) and FeO∗/MgO versus SiO2 of Miyashiro (1974) are applied in Fig. 2 for the classification and distinction of calc-alkaline from tholeiitic for the basement rocks. Sm–Nd model ages (depleted mantle (DM) model age −TDM) or crustal residence ages are calculated assuming a linear evo-lution of DM fromεNd = 0 at 4.56 Ga to εNd= +10
Table 2
Sr and Sm–Nd isotopic compositions for basement rocks from central Vietnam
Sample no. 87Sr/86Sra ±2σ Smb (ppm) Ndb(ppm) 147Sm/144Nd 143Nd/144Ndc ±2σ ε Nd(0)d εNd(T)e TDMf (Ga) Gneiss KT321/1E 0.72035 1 4.73b 26.60b 0.1075 0.512213 12 −8.3 −3.2 1.3 KT321/1 0.72112 1 4.43 24.02 0.1115 0.512025 10 −12.0 −7.1 1.7 KT326 0.71966 1 5.50 28.74 0.1157 0.512233 12 −7.9 −3.3 1.4 QN1275 0.72257 2 9.34 45.14 0.1251 0.512020 13 −12.1 −8.0 1.9g KT318 0.72592 2 8.01 36.97 0.1310 0.511975 12 −12.9 −9.2 2.2g KT308 0.71491 1 6.69b 39.34b 0.1028 0.512189 13 −8.8 −3.4 1.3 KT196 0.79198 2 7.36 34.19 0.1302 0.511978 5 −12.9 −9.1 2.1 KT205 0.71742 2 7.09 38.56 0.1112 0.511919 6 −14.0 −9.1 1.8 KT294/1 0.74511 2 11.63 59.78 0.1176 0.511858 11 −15.2 −12.7 2.0g KT333/6 0.78673 2 6.50 44.71 0.0879 0.511521 13 −21.8 −18.3 2.0g KT220 0.72583 2 5.57 37.72 0.0893 0.511865 11 −15.1 −11.7 1.6 KT8049/1 0.73706 2 2.10b 12.96b 0.0980 0.511817 15 −16.0 −10.4 1.8g Schist KT8049 0.75360 2 5.11b 24.05b 0.1285 0.511832 12 −15.7 −11.8 2.4g KT8122 0.70892 1 8.52b 42.64b 0.1208 0.512397 14 −4.7 −0.3 1.2 TS25 0.70621 1 2.89 15.73 0.1111 0.512323 19 −6.1 −1.2 1.2 Amphibolite KT298 0.76486 1 4.67b 17.60b 0.1604 0.512610 15 −0.6 +1.5 TS22A 0.70589 2 1.34 4.51 0.1796 0.512675 16 +0.7 +1.7 KT235 0.71839 2 2.61b 10.46b 0.1509 0.512478 15 −3.1 −0.5 KT8112 0.70636 1 4.99 15.80 0.1910 0.512910 14 +5.3 +5.6 KT8116 0.70889 1 0.93 2.04 0.2756 0.513397 12 +14.8 +10.3 Charnockite KT291 0.71595 1 13.04b 61.38b 0.1285 0.512062 16 −11.2 −9.1 1.9g KT289 0.71069 1 6.54b 35.68b 0.1108 0.512136 16 −9.8 −7.1 1.5g Granulite KT293 0.72209 2 3.75 14.51 0.1563 0.512128 14 −10.0 −8.7 2.7g
a Our long-term measured ratio for the NBS987 Sr standard being 0.710240 ± 0.000028.
b Concentration obtained by ICP-MS method, error±5%; others obtained by ID method using TIMS, uncertainty ±0.5%. c Our long-term measured ratio for the La Jolla Nd standard being 0.511866 ± 0.000026.
d ε
Nd(0) = [(143Nd/144Nd)sample/0.512638− 1] × 104. e ε
Nd(T) = [(143Nd/144Nd)sample(T)/(143Nd/144Nd)CHUR(T)−1]×104, (143Nd/144Nd)sample(T) = (143Nd/144Nd)sample−(147Sm/144Nd)sample
(expλT − 1), (143Nd/144Nd)
CHUR(T) = 0.512638–0.1967 (exp λT − 1), λ = 0.00654 Ga−1, T = 250 Ma for gneiss: KT294/1, KT333/6,
KT220, charnockite and granulite;T = 450 Ma for others.
f Crustal residence model age assuming derivation from a depleted mantle source with present ε
Nd of +10, TDM = 1/λ × ln{1 +
[(143Nd/144Nd)sample− 0.51315]/[(147Sm/144Nd)sample− 0.2137]}.
g Model ages represented the first crust-forming stage, others represented the second stage.
at the present time (Goldstein et al., 1984). We have limited samples with147Sm/144Nd close to the aver-age crustal value and less than 0.16, such that mean-ingful Sm–Nd model ages can be obtained to trace the crustal evolutionary history. Most amphibolites have 147Sm/144Nd higher than 0.16, so that their model ages are geologically meaningless. The chondrite val-ues used for REE normalization are fromMasuda et al. (1973)divided by 1.2 and the primitive mantle values
used for spidergram construction are from Sun and McDonough (1989).
5.1. Geochemical characteristics of the basement rocks
5.1.1. Gneiss and schist
The gneisses have a wide range in SiO2 (58.3– 72.1%). They all show high Al2O3(13.9–20.9%) and
Fig. 2. (a) Zr/TiO2vs. Nb/Y diagram ofWinchester and Floyd (1977)for the classification of different rock types and (b) FeO∗/MgO vs.
SiO2 diagram ofMiyashiro (1974)for the distinction of calc-alkaline from tholeiitic rocks for basement rocks from the Kontum massif,
high ASI (aluminum saturation index = molecular proportion of Al2O3/(CaO+ Na2O+ K2O), 0.9–1.2 mostly) except for the enclave (a mafic gneiss KT321/1E) which possesses lower SiO2(53.3%) and Al2O3 (11.5%). The schists have intermediate com-positions with SiO2 ranging from 57.4 to 61.8%. In Fig. 2a, it can be seen that most gneisses belong to the rhyodacite/dacite and trachyandesite fields while schists possess low Zr/TiO2 and belong to the an-desite/basalt to alkali basalt fields. Both gneisses and schists plot in the calc-alkaline field (Fig. 2b) except for gneiss KT294/1 and schist TS25.
The REE distribution patterns for the gneisses and schists can be divided into two groups. The first group is represented by the samples of eight gneisses (Fig. 3a), two schists (KT8122, TS25) from the Pro-terozoic complex north of the “Archean” complex, and one gneiss (KT8049/1) and one schist (KT8049) from the Proterozoic complex south of the “Archean” complex (Fig. 3b). This group shows moderately fractionated REE patterns (5.7(La/Yb)N20.3, ex-cept KT321/1E of 3.6) with low light REE (LREE) (60LaN176, except KT321/1E of 36.7), high heavy REE (HREE) (8.6YbN18, except KT8049/1 and TS25 of 4.9 and KT8122 of 6.5) and marked negative Eu anomalies (Eu/Eu∗ = 0.5–0.8, except KT8049/1 and TS25 of 0.9). Such characteristics are often ob-served in crust-derived, anatectic peraluminous gran-ites (Rogers and Greenberg, 1990; Holtz and Barrey, 1991) as well as in post-Archean sediments (Haskin et al., 1966; André et al., 1986). The second group is represented by three gneisses (KT294/1, KT333/6 and KT220) from the “Archean” complex (Fig. 3b). They show strongly fractionated (27.5(La/Yb)N47.6) REE patterns with high LREE (145LaN229) and low HREE (4.8YbN6.7) contents and positive Eu anomalies (1.0Eu/Eu∗1.3, except KT294/1 of 0.6). The strongly fractionated REE patterns, low Yb contents and no significant Eu anomalies for these gneisses are similar to those of Archean tonalite-trondhjemite-granodiorite (TTG) (usually (La/Yb)N> 20, 0.3YbN8.5;Martin, 1994). In the spidergrams (Fig. 4a and b), both groups display simi-lar Nb, Ta, (Sr), P and (Ti) negative anomalies, but the second group differs in having lower Y and HREEs. In addition, the mafic gneiss (KT321/1E) contains the highest Cr and Ni and lowest (La/Yb)N ratio (3.6) among all the analyzed samples. The migmatitic
gneiss (KT333/6) having the lowest εNd(0) contains low Fe and Mg, and the highest Al2O3(20.9%), ASI (1.4) and LREE contents.
In Fig. 5a and b, all the gneisses and schists are plotted in (La/Yb)N versus YbN and (Sr/Y) versus Y spaces. Five gneisses (four gneisses of Figs. 3b and 4bplus KT308) and two schists (except KT8049 in Figs. 3b and 4b) lie in the field of Archean TTG and modern adakite of Martin (1986) (Fig. 5a), but most samples fall in the overlap field between Archean and post-Archean rocks. Two gneisses (KT333/6 and KT8049/1 inFigs. 3b and 4b) and one schist (TS25 inFigs. 3b and 4b) plot in the field of Archean TTG and modern adakite ofDrummond and Defant (1990) (Fig. 5b). The remaining gneisses and schists lie in classical calc-alkaline island arc field. Thus, the geo-chemical characteristics of most analyzed gneisses and schists from the Kontum massif are similar to those of post-Archean granitoid and island arc material, de-spite a few samples which reveal geochemical sim-ilarities with Archean TTGs. All of our rocks show depletions in the high field strength elements (HFSE; e.g. Nb, Ta and Ti) and can be interpreted as derived from pre-existing crustal materials.
5.1.2. Amphibolites
The five amphibolites have low SiO2(45.4–54.4%) and variable mg-values (atomic 100Mg/(Mg+ Fe2+)) of 42–63. Although the amphibolites are tholeiitic in composition (Fig. 2a and b), they can be sep-arated into two types based on their TiO2, CaO and trace element contents. The first type has 1.07–2.00% TiO2, 9.68–10.85% CaO, 96–200 ppm Sr, 38–143 ppm Zr, 0.98–1.83 ppm Hf, 1.38–12.12 ppm Nb, 0.18–0.73 ppm Ta, 71–359 ppm Cu, 103–120 ppm Zn, 13.0–22.6 ppm Ga (Table 1) and moderately LREE-enriched patterns (Fig. 3c). They all show convex chondrite-normalized patterns in the LREE. Both positive (Eu/Eu∗= 1.1–1.4) and subtle negative (Eu/Eu∗ = 0.93–0.96) Eu anomalies are observed. The second type is represented by sample KT8116 only and is characterized by lower TiO2 (0.49%), CaO (9.36%), Sr (50 ppm), Zr (23), Hf (0.46), Nb (0.82), Ta (0.06), Cu (45), Zn (101) and Ga (9.1) con-tents and a strong depletion in LREE without negative Eu anomaly (Fig. 3c). Such differences indicate that the basaltic protoliths originated from two separate sources. In the spidergrams, the amphibolites (Fig. 4c)
Fig. 3. Chondrite-normalized REE distribution patterns for (a) and (b) gneiss and schist, (c) amphibolite and (d) charnockite and granulite from the Kontum massif, central Vietnam. VN357 and VN343 shown as dotted lines in (d) are fromNagy et al. (2001).
lack apparent anomalies in Nb and Ta, which suggest an intraplate tectonic setting for the petrogenesis of their protoliths.
5.1.3. Charnockite and granulite
In the Zr/TiO2versus Nb/Y plot ofFig. 2a, the two charnockites in this study plot in the tholeiite field and
not in the alkali basalt field. However, in a FeO∗/MgO versus SiO2 plot (Fig. 2b) one sample plots in the calc-alkaline field (KT289 with 57.6% SiO2). The two charnockites appear to be relatively evolved based on mg-values of 44 and 55 and LREE-enriched patterns (Fig. 3d) with negative Eu anomalies (Eu/Eu∗ = 0.5 and 0.7). The calc-alkaline sample (KT289,
Fig. 4. Primitive mantle-normalized spidergrams for (a) and (b) gneiss and schist, (c) amphibolite and (d) charnockite and granulite from the Kontum massif, central Vietnam. VN357 and VN343 shown as dotted lines in (d) are fromNagy et al. (2001).
(La/Yb)N = 15.2) displays a more fractionated REE pattern than the tholeiitic one (KT291, (La/Yb)N = 3.6), although the former has lower REE abundances than the latter. In the spidergram (Fig. 4d), they have generally similar patterns, although abundances of most elements are higher for the tholeiitic one than the calc-alkaline one. They are enriched in Rb, K and LREEs, and depleted in Th, Nb and Ta. A charnockite (VN357) reported byNagy et al. (2001) has a composition which plots in the calc-alkaline
field (Fig. 2b) and shows comparable LREE enrich-ment (Figs. 3d and 4d) similar to KT289.Nagy et al. (2001) reported another charnockite (VN343) with significantly higher silica content (73.2%; Fig. 2b). It exhibits a higher LREE content and an apparently more fractionated REE pattern ((La/Yb)N= 47) than the other charnockites (Fig. 3d). This sample shows depletions in Nb, P and Ti and strong enrichments in Th and K (Fig. 4d). The Vietnamese charnockites thus appear to be very heterogeneous.
Fig. 5. (a) (La/Yb)N vs. YbN and (b) (Sr/Y) vs. Y diagrams of the gneisses and schists from the Kontum massif, central Vietnam. Also
shown are the fields of Archean TTG and modern adakites (solid line) and post-Archean juvenile granitoids and classical calc-alkaline island arcs (dotted line) (Martin, 1986; Drummond and Defant, 1990).
The granulite sample (KT293) appears to have a tholeiitic composition (Fig. 2a and b). It also exhibits LREE-enriched pattern and negative Eu anomaly (Fig. 3d), but it has lower abundances of most trace elements and higher abundances of Cr, V, Sc, Ni, Co and Cu than those of the tholeiitic charnockites (KT291). Trace element concentrations of the gran-ulite are similar to those of estimates of the average lower continental crust (c.f.McLennan, 2001). In the spidergram (Fig. 4d), it can be seen that the granulite is depleted in Ba, U, Sr, P and Zr, contrasting with the pattern of the tholeiitic charnockites.
5.2. Isotopic compositions of the basement rocks The present day Nd isotopic ratios, εNd(0), of the studied samples show a wide range from+14.8 to −21.8. The gneisses and schists present a large range of negative εNd(0) from −4.7 to −21.8. A migmatitic gneiss (KT333/6), having the highest ASI, exhibits the lowest εNd(0). These rocks have crustal147Sm/144Nd value of 0.088–0.131, whereas, the amphibolites of the first type have slightly neg-ative to positive εNd(0) values (−3.1 to +5.3) and higher147Sm/144Nd of 0.151 to 0.191. The amphibo-lite of the second type (KT8116) exhibits the highest εNd(0) value (+14.8) and147Sm/144Nd (0.276). The charnockites and granulite show negativeεNd(0) from −9.8 to −11.2 coupled with medium147Sm/144Nd of 0.111–0.156. Overall, the analyzed basement rocks
show Paleoproterozoic to Mesoproterozoic TDMages (2.4–1.2 Ga). The only exception is granulite KT293 which has a late Archean TDM age (2.7 Ga). Among these basement rocks, two rock units previously be-lieved to be Archean (the Kannack and the Song Ba complexes), in fact define a Proterozoic TDM ages restricted between 2.0 and 1.5 Ga. This suggests the Kontum core complex is unlikely to be of Archean age but began its crustal evolution in the Paleopro-terozoic and MesoproPaleopro-terozoic. On the other hand, the present day Sr isotopic ratios of different rock types show an overlapped range from 0.7058 to 0.7920.
6. Discussion and conclusion
6.1. Proterozoic crustal formation of the Indochina block
The Sm–Nd isotopic compositions of the “Archean” and Proterozoic basement rocks and the Permo-Triassic charnockites analyzed in this study are displayed in Fig. 6. Also shown are the Oligo-Miocene granitoids from Bu Khang complex of central Vietnam (Nagy et al., 2000), Permo-Triassic (230–265 Ma) granitic plutons from the East Coast Province batholiths of Peninsular Malaysia (Liew and McCulloch, 1985), the late Archean basement rocks from the South China block (i.e. the Cavinh complex of northern Vietnam (Lan et al., 2001) and the Kongling complex
Fig. 6. Present day Sm–Nd isotopic data of Indochina block showing the “Archean” and Proterozoic basement rocks and Permo-Triassic charnockite of the Kontum massif, central Vietnam. Oligo-Miocene granitoids of Bu Khang complex (Nagy et al., 2000) in central Vietnam, Permo-Triassic granitic plutons of East Coast Province, Peninsular Malaysia (TDM ages are recalculated, Liew and McCulloch, 1985)
and basement rocks of South China block are shown for comparison. Data sources for South China block: Archean: Cavinh complex of northern Vietnam (Lan et al., 2001) and Kongling complex of China (Gao et al., 1999; Qiu et al., 2000); Proterozoic: Ailao Shan—Red River shear zone (Zhai et al., 1990; Zou et al., 1997; Zhang and Schärer, 1999; Lan et al., 2001). Nd model ages, assuming depleted mantle sources (DM), can be calculated from the slope of the tie lines connecting DM and any individual data point. Seven reference lines corresponding to model ages of 0.6, 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 Ga are shown.
of northern Yangtze (Gao et al., 1999; Qiu et al., 2000)), and Proterozoic rocks from the Ailao Shan— Red River shear zone (Zhai et al., 1990; Zou et al., 1997; Zhang and Schärer, 1999; Lan et al., 2001). All the rocks can be divided into two groups. Group I consists of rocks from central Vietnam and includes all the basement rocks and charnockites from this study and Oligo-Miocene granitoids of the Bu Khang complex, Permo-Triassic granitic rocks of Malaysia and Proterozoic basement rocks of the South China block. Group I is characterized by highεNd(0) values (−21.8 to +0.7) and essentially Proterozoic TDMages (1.0–2.4 Ga, except one 2.7 Ga). Group II, consisting of the Archean basement rocks of the South China block, shows very lowεNd(0) values (−50 to −29.6) and middle to late Archean TDM ages (2.7–3.5 Ga). In this regard, the basement rocks from central Viet-nam are remarkably different from the Archean rocks
from the South China block. So far no U–Pb dates older than 2.7 Ga have been reported for rocks from Vietnam, except some latest Archean ages for inher-ited zircon separates from the Kontum massif (2.7 Ga, Nagy et al., 2001; 2541±55 Ma, Chung et al., unpub-lished data) and the Bu Khang area (2541± 69 Ma, Carter et al., 2001). Hence, the crust in central Viet-nam appears to contain only insignificant amounts of recycled Archean rocks.
Proterozoic TDMages are also observed in the East Coast Province batholiths of the Peninsular Malaysia (Liew and McCulloch, 1985), located in the south-western part of the Indochina block (Metcalfe, 2000). As shown inFig. 6, the TDMages (mostly 1.0–1.6 Ga) for plutons from the East Coast Province of Malaysia are similar to the young TDM ages characteristic of some of the rocks from central Vietnam in Group I. Zircon inheritance ages of the same province in
Malaysia also show younger ages (800 and 1350 Ma; Liew and McCulloch, 1985) than those of rocks of central Vietnam. Thus, the consistent and dominant Proterozoic TDMages of Indochina block suggest that the role of Archean rocks in the crustal evolution of Indochina block is limited. The main crust formation in the Indochina block was in the Proterozoic.
The Nd isotopic evolution in the constituent rocks of the Indochina block is illustrated in Fig. 7. Ac-cording to theεNd(0) values and TDMages, two main crust-forming stages can be delineated. The first stage (solid lines inFig. 7) is represented by the formation of the protoliths of the Kannack, Song Ba and Song Re complexes of Kontum massif. The ultimate sources of these rocks may have Paleoproterozoic ages, prob-ably between 2.4 and 1.8 Ga. The second stage (dotted lines inFig. 7) is manifested by the formation of the protoliths of the basement rocks of Kham Duc, Tu Mo Rong, Tak Po and Dieng Bong complexes of the Kon-tum massif during Paleoproterozoic to Mesoprotero-zoic time (2.1–1.2 Ga). The PhaneroMesoprotero-zoic (PaleoMesoprotero-zoic, Indosinian and Tertiary) magmatic rocks, consisting
Fig. 7. Nd isotopic evolution diagram of igneous and metasedimentary rocks from the Indochina block. The solid lines represent the first crust-forming stage of basement rocks from Kannack, Song Ba and Song Re complexes. The dotted lines represent the second crust-forming stage of basement rocks from Kham Duc, Tu Mo Rong, Tak Po and Dieng Bong complexes. Data of granitoids (open symbols) are taken fromLiew and McCulloch (1985)andNagy et al. (2000). Others (solid symbols) are from this study.
of intermediate to acidic lithologies, are plotted in the evolutionary trend defined by the two suits of base-ment rocks. This suggests that the Paleoproterozoic to Mesoproterozoic crusts were important sources for the Phanerozoic rocks.
The first stage rocks defined by the evolutionary trend (Fig. 7) were probably produced by remelting of the Paleoproterozoic (and/or late Archean) crustal materials. In this sense, mixing of the old crustal rocks with a substantial mantle input during Paleopro-terozoic to MesoproPaleopro-terozoic generated rocks of the second stage. The mantle input was most significant at ∼450 Ma (Carter et al., 2001; Nagy et al., 2001), when rift-related magmatism occurred and thus caused mafic underplating in the lower crust, and eventually became the protolith of the amphibolites exposed during the Indosinian orogeny. In the meantime, the amphibo-lites served as the magma source for the charnockites, which were contaminated by crustal materials during ascent. The Indosinian orogeny furthermore resulted in a rapid, regional exhumation (Nam et al., 2001; Nagy et al., 2001) that brought the entire package
of the Kontum basement rocks to the surface. By analogy, a significant mantle input has also been con-sidered for generating the granitoids of the East Coast Province batholiths of Peninsular Malaysia (Liew and McCulloch, 1985) associated with the Indosinian orogeny along the western part of the Indochina block between 230 and 265 Ma.
6.2. Phanerozoic charnockites in the Indochina block
The origins (igneous versus metamorphic) of charnockites, a term first applied to hypersthene-bearing granites in India (Holland, 1900), have been the focus of much debate (Kilpatrick and Ellis, 1992). Therefore, it is important to determine whether the charnockites from the Kontum massif are of a meta-morphic or igneous origin. These charnockites have basic to intermediate composition (Table 1). Two charnockites of this study, together with two charnock-ites fromNagy et al. (2001), show decreasing TiO2,
Fig. 8. Geochemical variation diagrams of the charnockites from the Kontum massif, central Vietnam. Solid symbols are of this study while open symbols are ofNagy et al. (2001). Also shown are the fields for magmatic and metamorphic charnockites ofKilpatrick and Ellis (1992).
MgO, CaO and P2O5 with increasing SiO2, which we interpret to be a fractionation trend. Fractionation may indicate crystallization of Ti-rich (e.g. ilmenite) and P-rich (e.g. apatite) phases. This inference is sup-ported by the petrographic evidence for ilmenite and apatite. The decrease in Nb and Y with increasing SiO2can be interpreted by combined fractionation of ilmenite and apatite (Zhao et al., 1997). The develop-ment of small negative Eu anomalies with increasing SiO2 may also result from apatite crystallization in addition to feldspar (Zhao et al., 1997). In addition, at a given SiO2 content, these Vietnamese charnockites show relatively higher TiO2 and P2O5, lower CaO (Fig. 8) and medium mg values (25–40) than those of metamorphic charnockites (Kilpatrick and Ellis, 1992). Thus, the geochemical and mineralogical fea-tures suggest a magmatic, rather than metamorphic, origin for the charnockites from the Kontum mas-sif. Geochronological studies (Carter et al., 2001; Nam et al., 2001; Nagy et al., 2001) have shown that the charnockites formed during Indosinian time at
Fig. 9. Initial Nd and Sr isotopic composition for charnockites of the Kontum massif, central Vietnam at 250 Ma. Two-component mixing model was demonstrated with mantle (εNd= +1.5,87Sr/86Sr= 0.7035) and crust (εNd= −18,87Sr/86Sr= 0.780) as two end members. Two
different mixing arrays are shown with varying concentration, for curve (A): mantle: [Sr]= 200 ppm, [Nd] = 6 ppm, crust: [Sr] = 240 ppm, [Nd]= 45 ppm; for curve (B): mantle: [Sr] = 650 ppm, [Nd] = 25 ppm, crust: [Sr] = 150 ppm, [Nd] = 60 ppm which corresponding to the curvature [(Sr/Nd)mantle/(Sr/Nd)crust] of 6.3 and 9.6 for the hyperbola A and B, respectively. The concentrations of mantle and crust
members for curve A are similar to those of TS22A and KT333/6, respectively, while those for curve B are similar to those of ocean island basalt (Wilson, 1988) and metapelite from high-grade metamorphic terranes (Prame and Pohl, 1994; Villaseca et al., 1998), respectively.
249±2 to 258±6 Ma. Thus, they represent one of the youngest (Phanerozoic) igneous charnockite terranes in the world (Nam et al., 2001).
The source rock composition for charnockites is controlled by numerous variables. The initialεNd val-ues i.e.εNd(250 Ma), for the Kontum charnockites are −7.1 and −9.1, and can be interpreted as binary mix-ing (Fig. 9) between mantle-derived and crust-derived melts. The crustal end-member is proposed to have a composition similar to the gneiss KT333/6, which has the lowestεNd(250 Ma) value (−18.3) and highest Sr isotopic ratio (0.7817). The mantle component is as-sumed to have a composition similar to the amphibo-lite TS22A, which has an εNd(250 Ma) of +1.3 and chemical similarities to intraplate basalts. For simplic-ity,εNd(250 Ma) value and initial Sr isotopic ratio for the crust end member are respectively chosen as−18 and 0.780, while those for the mantle are +1.5 and
0.7035. Two different mixing arrays (curves A and B), with different concentrations of Sr and Nd for the two end-members, are presented inFig. 9. The curvatures of the two mixing lines, A and B, are represented by (Sr/Nd)mantle/(Sr/Nd)crust = 6.3 and 9.6, respectively. The two charnockites are bounded by the two assumed mixing curves. Although the Vietnamese charnockites are heterogeneous, the observed εNd(250 Ma) range of the charnockites from the Kontum massif can be explained in terms of the mixing of mantle-derived (80–60%) and crust-derived (20–40%) melts using this simple calculation.
6.3. Crustal evolution of the Indochina block TDM age provides a constraint for the Precambrian crustal record of central Vietnam. The crustal history of Indochina block as revealed from central Vietnam
Fig. 10. Summary of major tectono-thermal events in Indochina block as reveal from central Vietnam and Peninsular Malaysia. Two samples of this study have been subjected to U–Pb zircon dating using SHRIMP which give five stages as 2541± 55, 1455 ± 24, 869 ± 57, 436± 10 and 260 ± 16 Ma (Chung et al., unpublished data).
and Peninsular Malaysia is summarized inFig. 10. Nd model ages and U–Pb relict zircon or inherited zir-con core ages (Carter et al., 2001; Nagy et al., 2001) suggest that continental crust of late Archean age (2.7–2.5 Ga) was separated from a depleted mantle and served as the source material for rocks of central Vietnam. At least two major crust formation stages have been delineated from TDM ages (Fig. 7) for the
Indochina block (Paleoproterozoic (2.4–1.8 Ga) and Mesoproterozoic (2.1–1.2 Ga)). So far, no reliable Paleoproterozoic dates have been obtained by U–Pb method. The Mesoproterozoic crust formation event has been registered in the zircon core of granulite sample of Kannack complex in the Kontum massif at 1404± 34 Ma using SHRIMP U–Pb dating (Nam et al., 2001) and U–Pb zircon upper intercept age of
∼1350 Ma for adamellite of East Coast Province of Peninsular Malaysia (Liew and McCulloch, 1985). A Neoproterozoic event is presented by the upper intercept age of 881± 26 to 901 ± 26 Ma for zircon, allanite and monazite regression line from granitoid of Dai Loc pluton in central Vietnam (Nagy et al., 2000) and ∼800 Ma for zircon from granodiorite of East Coast Province of Peninsular Malaysia (Liew and McCulloch, 1985). The TDM ages in this study (which cover the rocks in the eastern part of Indochina block) and East Coast Province batholiths of Penin-sular Malaysia (Liew and McCulloch, 1985) (which cover the rocks of the western part of Indochina block) are Mesoproterozoic instead of Neoproterozoic. We thus interpret the Mesoproterozoic event as the major crust formation event in the Indochina block, and the Neoproterozoic event is the magmatism for the for-mation of the protolith of Dai Loc complex in central Vietnam and the granodiorite of East Coast Province of Peninsular Malaysia. An early Paleozoic event has been identified from SHRIMP U–Pb zircon concordia ages of gneisses from the Dai Loc and Kontum mas-sifs (407± 11 to 444 ± 17 Ma; Carter et al., 2001) and U–Pb zircon concordia ages of a granodiorite of Kontum massif (451± 3 Ma;Nagy et al., 2001). This event is most likely related to an extensional event that disintegrated the Indochina block from Gond-wanaland (Carter et al., 2001). The magmatism pro-duced the intraplate tholeiites, now represented by the amphibolites. Similar four age stages as 2541± 55, 1455± 24, 869 ± 57 and 436 ± 10 Ma (Chung et al., unpublished data) for sample KT318, reinforce the establishment of the Pre-Mesozoic crustal history.
The identification of 234 ± 5 to 260 ± 16 Ma high-grade metamorphism and charnockitic magma-tism in central Vietnam (Lepvrier et al., 1997; Nam, 1998; Lo et al., 1999; Carter et al., 2001; Nam et al., 2001; Nagy et al., 2001; Chung et al., unpublished data) and K–Ar biotite age of 264 ± 7 Ma, Rb–Sr whole rock age of 261± 9 Ma (Bignell and Snelling, 1977) and U–Pb zircon lower intercept age of 257± 4 and 264± 2 Ma (Liew and McCulloch, 1985) for the granitic plutons in East Coast Province of Penin-sular Malaysia suggest the Indosinian orogeny has affected the whole Indochina block. The fast cool-ing (∼45◦C/Ma) and rapid exhumation (Nam et al., 2001) that occurred during this time exposed the high-grade basement rocks and charnockite of the
Kontum massif. Using an intermediate value for the geothermal gradient (35◦C/km), this cooling rate suggests ∼15 km of exhumation during a period of 12 Myr, corresponding to an average exhumation rate of ∼1.3 mm per year. The Indosinian orogeny is a result of collision between the Indochina block and the South China block (Lepvrier et al., 1997; Chung et al., 1999; Lan et al., 2000, 2001). It caused a strong thermal overprinting (240–160 Ma, Lo et al., 1999) and exhumation of the Kontum massif. After sutur-ing, both the Indochina and South China blocks may have experienced similar tectonothermal histories. Vestiges of the late Jurassic to Cretaceous Yanshanian “orogeny” and the mid-Tertiary Himalayan orogeny frequently found in the South China block have also left their traces in central Vietnam. The former is registered in the Truong Son belt, central Vietnam— a granite with age of 90.3 ± 0.7 Ma and mylonitic gneisses with increasing age spectra from 82± 10 to 130± 3 Ma using Ar–Ar muscovite dating (Lepvrier et al., 1997). The latter is recorded in U–Pb lower intercept dates (22.5 ± 1.7 to 26.0 ± 0.2 Ma) and Rb–Sr mineral dates (19.6 ± 0.5 to 21.2 ± 0.5 Ma) of granite (Nagy et al., 2000) and Ar–Ar mineral dating (21.4 ± 0.4 to 36.1 ± 1 Ma) of various metamorphic rocks (Lepvrier et al., 1997; Jolivet et al., 1999) from Bu Khang massif. However, there is no clear indica-tion of thermal overprinting caused by the Cenozoic extrusion tectonics in the Kontum massif (Lo et al., 1999).
Acknowledgements
Dr. Nguyen Xuan Bao of Geological Survey Divi-sion No. 6, Vietnam is specially thanked for help with sample collection. We thank Y. S. Chia for preparing the figures, W. Y. Hsu and C. H. Chiu for analyzing the Sr, Sm and Nd isotopes and Y. Iizuka for mineral identification using probe analysis. The manuscript was improved by the comments of Kent C. Condie, Jian-xin Zhao and Zheng-Xiang Li and syntax pol-ish of Cin-Ty Lee. This research was supported by Academia Sinica and the National Science Council of the Republic of China under grants NSC88-2116-M-001-034 and NSC89-2116-M-001-025 to C.Y. Lan. This paper is Contribution IESAS794 of the Institute of Earth Sciences, Academia Sinica.
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