DSpace at VNU: Early Paleozoic medium-pressure metamorphism in central Vietnam: evidence from SHRIMP U-Pb zircon ages

Geosciences Journal Vol 13, No 3, p 245 − 256, September 2009 DOI 10.1007/s12303-009-0024-2 ⓒ The Association of Korean Geoscience Societies and Springer 2009 Early Paleozoic medium-pressure metamorphism in central Vietnam: evidence from SHRIMP U−Pb zircon ages Tadashi Usuki* Ching-Ying Lan Tzen-Fu Yui Yoshiyuki Iizuka Van Tich Vu Tuan Anh Tran Kazuaki Okamoto† Joseph L Wooden Juhn G Liou } Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan 115, Republic of China Faculty of Geology, Hanoi University of Science, Hanoi, Vietnam Institute of Geological Sciences, National Center for Natural Sciences and Technology, Hanoi, Vietnam Department of Earth, Planetary and Space Sciences, Faculty of Education, Saitama University, Saitama 338-8570, Japan } Department of Geological and Environmental Sciences, Stanford University, CA 94305-2115, USA ABSTRACT: To constrain the timing of collisional event in the Indochina block, SHRIMP U–Pb dating and REE analyses of zircon were carried out for two paragneiss samples of the Kham Duc Complex, central Vietnam Both samples contain kyanite, staurolite, and zoisite as relics from an early metamorphic stage (M1), and biotite and sillimanite as major minerals constituting the foliation formed during the late metamorphic stage (M2) The change in mineral assemblages indicates a clockwise P-T path composed of a high- or medium-P + low-T stage (M1) and a subsequent lowP + high-T stage (M2) The U−Pb concordia ages of zircon rims are 447 ± Ma and 452 ± Ma for the two samples, respectively These results are distinctly different from the available Ar–Ar mineral ages of 254–225 Ma Following the clockwise P-T path and phase equilibrium analyses of the Complex, we suggest that the zircon rims were formed near peak temperatures during the decompression The ~450 Ma zircon rim thus gives the minimum age constraint for a possible crustal thickening event during Early Paleozoic, whereas the reported Permo-Triassic Ar–Ar ages would result from an Indosinian overprint This Early Paleozoic event is most likely related to a collisional orogeny between the Indochina and South China blocks Late Neoproterozoic to Neoarchean ages are recorded from detrital zircon cores of the Kham Duc Complex, the Kontum Massif and Truong Son Belt, suggesting that their protoliths might have derived from sediments at the Gondwana margin Key words: zircon, SHRIMP analyses, Kham Duc Complex, Vietnam, Indochina block INTRODUCTION Southeast Asia has been formed by amalgamation of several continental blocks, including the Indochina, South China, and Sibumasu blocks (Fig 1a) However, the timing of amalgamations, in particular, that between the Indochina *Corresponding author: usu@earth.sinica.edu.tw Also at Division of Natural Sciences, The United Graduate School of Education, Tokyo Gakugei University, Tokyo 184-8501, Japan † and South China blocks, has been a subject of debate for decades (e.g., Hutchison, 1989; Findlay and Phan Trong Trinh, 1997; Metcalfe, 1999; Carter et al., 2001; Carter and Clift, 2008; Lepvrier et al., 2008; Nakano et al., 2008; Osanai et al., 2008), although recent geochronological studies revealed that two separate magmatic or tectonic events occurred respectively during the Ordo-Silurian (Nagy et al., 2001; Carter et al., 2001; Lan et al., 2003; Roger et al., 2007) and the Permo-Triassic (Carter et al., 2001; Tran Ngoc Nam et al., 2001; Lan et al., 2003; Lepvrier et al., 2004; Maluski et al., 2005; Nakano et al., 2006, 2008) (Fig 1b) On the basis of recent discovery of high-P metamorphic rocks from the Kontum massif and the Song Ma suture zone, a PermoTriassic collision between the Indochina and South China was proposed (e.g., Nakano et al., 2008; Osanai et al., 2008) On the other hand, based on regional seismic and thermochronological evidence combined with Early Mesozoic paleogeographic constraint, Carter and Clift (2008) proposed a reactivation event in the Indochina block instead of the Indochina-South China collision in Triassic Findlay and Phan Trong Trinh (1997), however, proposed a SiluroDevonian docking event of an island arc/forearc terrain to the South China block, based on a review of geological and chronological data of the Song Ma suture zone Lepvrier et al (2008) further argued that two possible collisional events occurred at Lower Triassic and Ordo-Silurian, respectively One reason for this apparent discrepancy comes from the fact that the pre-Indosinian tectonics in Indochina is not at all clear For example, the Ordo-Silurian event was suggested to be related with an arc magmatism (Nagy et al., 2001) or an early extension prior to Gondwana breakup (Carter et al., 2001) In previous studies, Ordo-Silurian ages were retrieved only from magmatic or low-P metamorphic rocks To address whether high- or medium-P metamorphic rocks existed at 246 Tadashi Usuki et al Fig a: Simplified tectonic terrane map of East and Southeast Asia showing major blocks discussed in this study (after Metcalfe, 1999) Box corresponds to Figure 1b The boundary between the Yangtze craton and the Cathaysia fold belt is after Xu et al (2007) RRSZ denotes Red River shear zone b: The Indochina block in relation to the South China block showing shear zones around Vietnam (Lepvrier et al., 2004) Box corresponds to Figure Solid squares denote the sample localities of published U−Pb zircon and monazite ages obtained by the SHRIMP or TIMS method in nearby areas Data from Roger et al (2000; 2007), Carter et al (2001), Tran Ngoc Nam et al (2001), Nagy et al (2001), Lan et al (2003) and Nakano et al (2006) Star marks the location of Archean crustal fragment of the Yangtze block (Lan et al., 2001; Tran Ngoc Nam et al., 2003) the Ordo-Silurian time is a key to understand the tectonics in the Indochina block The Kham Duc Complex (Fig 2) in central Vietnam consists primarily of medium-P metamorphic rocks (Phan Cu Tien, 1991), which is regarded as the product of a collisional event between the Indochina and South China blocks (e.g., Osanai et al., 2008) Despite recent petrologic studies on the P-T evolution of Kham Duc Complex (e.g., Usuki et al., 2004; Nakano et al., 2007; Osanai et al., 2008), the timing of metamorphism of this complex is poorly constrained Several Permo-Triassic Ar–Ar amphibole, biotite, and muscovite ages (254−225 Ma) were reported from the Kham Duc Complex and were regarded as the timing of collision between the Indochina and South China blocks (Fig 2; Lepvrier et al., 2004) However, Roger et al (2007) and Carter and Clift (2008) argued that these Ar–Ar ages could be alternatively interpreted as a result of complete resetting during a superimposed tectonic event Obviously, more robust age data are prerequisite to tackle the issue Having the above question in mind, in-situ zircon U−Pb Early Paleozoic medium-pressure metamorphism in central Vietnam 247 Fig Simplified geological map around the Kham Duc Complex (modified after Lepvrier et al., 2004) Sample localities (VNTS178 and VNTS18-2) and their U−Pb zircon ages (this study) are indicated by bold characters Also shown are Ar−Ar ages (Amp: amphibole, Mus: muscovite, Bt: biotite) of the Phuoc Son, Tien Phuoc and Tra Bong areas, reported by Lepvrier et al (2004) TPSZ: TamkyPhuoc Son Shear Zone, TBSZ: Tra Bong Shear Zone chronology and REE analyses were carried out for two gneiss samples from the Kham Duc Complex in order to constrain the timing of the medium-P metamorphism We also discuss the possible source provenance for the protoliths of the Kham Duc Complex GEOLOGICAL SETTING The Indochina block in Southeast Asia is bounded to the north by the NW-trending Song Ma suture zone with the South China block (Fig 1) The Truong Son Belt and Kontum Massif in north-central Vietnam belong to the Indochina block and are affected by the collisional event between the Indochina and South China blocks (e.g., Lepvrier et al., 2004; 2008; Osanai et al., 2008) In this collision zone, a series of NW to W-trending subparallel shear zones developed (Lepvrier et al., 2004) The Kham Duc Complex (Fig 2) in the southern part of the collision zone is exposed ~70 km along the east-west direction between Truong Son Belt and Kontum Massif To the north, the Complex is unconformably covered by unmetamorphosed sediments, and to the south it is bounded by the Tra Bong shear zone next to the Kontum Massif The lithology of the Complex is mainly composed of metapelites, amphibolites, and granodioritic orthogneisses Along the Tamky-Phuoc Son and Tra Bong shear zones (Fig 2) within the Complex, minor amounts of serpentinite and gabbro also occur Along the Ho Chi Minh road in the Phuoc Son area (Fig 2), a typical cross-section of the Kham Duc Complex crops out, where metapelites and amphibolites are intercalated on a centimeter to meter scale in thickness These metamorphic rocks have an EW-trending schistosity with a steep dip to N or S Usuki et al (2004) studied metapelites in this area and estimated a clockwise P-T path from the early higher-P and lower-T metamorphism (M1) to the later lowerP and higher-T metamorphic stage (M2) (Fig 3), based on the occurrences of relict kyanite, staurolite and zoisite (M1) and foliation-forming sillimanite and biotite (M2) Usuki et al (2004) estimated the P-T conditions of M1 and M2 as 11.8 kbar/464 oC and 6.8−7.0 kbar/711−722 oC, respectively, on the basis of garnet-biotite (Bhattacharya et al., 1992) and garnet-chlorite (Dickenson and Hewitt, 1986) geothermometers and garnet-plagioclase-aluminosilicate-quartz geobarometric calculation using a MS-Excel spread sheet by D Waters (http://www.earth.ox.ac.uk/~davewa/pt/th_tools.html) From the same area and the Hiep Duc area (~10 km SW of Tan An town, Fig 2), similar clockwise P-T paths have been also proposed (Fig 3, Nakano et al., 2007; Osanai et al., 2008) Ar–Ar ages of the late Permian to middle Triassic period (254−225 Ma) for amphibole, muscovite, and biotite were reported from the mylonitized schist, kyanitebearing schist, sillimanite-bearing schist, mylonitic dioritic gneiss, granodioritic orthogneiss, and post-tectonic granite (Lepvrier et al., 2004) (Fig 2) SAMPLE DESCRIPTIONS We chose two paragneiss samples from the Phuoc Son area (Fig and Table 1) in the Kham Duc Complex for the zircon SHRIMP analysis Both gneisses contain kyanite and 248 Tadashi Usuki et al morphism Based on the modal compositions (Table 1), these two samples were named as biotite gneiss (VNTS17-8) and garnet-biotite gneiss (VNTS18-2), respectively 3.1 Sample VNTS17-8 (Biotite Gneiss) This sample was collected from a meter-scale biotite gneiss layer hosted within a gneiss-amphibolite outcrop along Ho Chi Minh Road, 6.5 km northeast of Kham Duc town (Fig 2) The gneiss is mainly composed of biotite (31 vol%), quartz, plagioclase, muscovite, sillimanite, with minor or trace amounts of garnet, kyanite, staurolite, zoisite, K-feldspar, allanite, zircon, apatite, monazite, xenotime, rutile and ilmenite (Table 1) Garnet occurs as subhedral fine grains (~0.5−1 mm in diameter) and shows high XFe [Fe/(Fe + Mg) = 0.82] (Table 1, Usuki et al., 2004) Biotite and sillimanite define the major foliation (Fig 4a) In addition, kyanite (Fig 4b), staurolite (Fig 4c), zoisite and rutile were found as inclusions in plagioclase (XAn = 0.33) These occurrences demonstrate that staurolite, kyanite and zoisite are relict minerals from an early higher-P condition (M1), in contrast to sillimanite and biotite formed subsequently at a lower-P and higher-T condition (M2) Zircon, allanite, apatite, monazite, rutile and ilmenite occur in the matrix Some grains of zircon and apatite in the matrix are surrounded by monazite or xenotime Fig Clockwise P-T paths proposed for medium-pressure metamorphic rocks of the Kham Duc Complex Thick solid arrow is the P-T path of metapelites, and M1 and M2 indicate early higher-P/ lower-T and later lower-P/higher-T metamorphic conditions, respectively (Usuki et al., 2004) Dot-dashed and dashed lines indicate P-T paths reported by Nakano et al (2007) and Osanai et al (2008), respectively The breakdown reactions of staurolite, zoisite and margarite are adapted from Spear (1993) Light gray area indicates a melt-present field in association with garnet, biotite, sillimanite, albite, K-feldspar and quartz in the NaKFMASH model system Thin solid lines in light gray area indicate isopleths of Fe/ (Fe + Mg) in garnet (adapted from Spear et al., 1999) In this divariant field, garnet would grow along the P-T path that crosses the isopleths during decompressional heating and be consumed along the path during decompressional cooling Formation of metamorphic zircon (~450 Ma) during garnet breakdown and in the presence of a melt phase probably occurred during the decompression near the peak metamorphism (M2) Abbreviations are the same as those in Table 1, except for Ab = albite, An = anorthite, And = andalusite, As = aluminosilicate, Mrg = margarite, and L = liquid 3.2 Sample VNTS18-2 (Garnet-Biotite Gneiss) staurolite as relics, and abundant zircon crystals showing wide metamorphic rims as described later These samples are suitable for studying the timing of the medium-P meta- This sample was collected along Ho Chi Minh Road about 0.5 km east of sample VNTS17-8 (Fig 2) The gneiss outcrop consists of alternating lenses of 5−30 cm thick biotite-rich and felsic layers The sample was collected from a 15 cm-thick biotite-rich layer, and is composed of biotite (51 vol%), plagioclase, garnet, quartz and sillimanite with minor or trace amounts of kyanite, staurolite, chlorite, muscovite, K-feldspar, zircon, apatite, monazite, rutile and ilmenite Garnet grains, a few mm to 20 mm in size, are partially replaced by biotite, sillimanite and plagioclase (XAn = 0.41) (Fig 4d) Large garnet porphyroblasts are homogeneous (XFe = ~0.78) apart from a strongly retrograded rim (XFe = 0.82−0.83) (Table 1) Similar to sample VNTS17-8, Table GPS positions, mode of minerals, XFe (= Fe/(Fe + Mg)) of garnet and XAn (= Ca/(Ca + Na + K)) of plagioclase for the analyzed samples Sample Lat Long mode of minerals* Grt St Zo Bt Chl Mus Ky Sil Qtz Pl Kfs Aln Zrn Ap Mnz Xno Rt Ilm Biotite gneiss 15°29'24"N 107°49'33"E + + (VNTS17-8) Garnet-biotite gneiss 15°29'21"N 107°49'49"E 16 + (VNTS18-2) + 31 + + 29 24 + + 51 + + + 22 + + + + + + + + + + + + + XFe of Grt XAn of Pl 0.82 0.33 0.78 at core, 0.41 0.82−0.83 at rim *numbers indicate % + means less than 1% Abbreviations are Grt = garnet, St = staurolite, Zo = zoisite, Bt = biotite, Chl = chlorite, Mus = muscovite, Ky = kyanite, Sil = sillimanite, Qtz = quartz, Pl = plagioclase, Kfs = K-feldspar, Aln = allanite, Zrn = zircon, Ap = apatite, Mnz = monazite, Xno = xenotime, Rt = rutile, and Ilm = ilmenite Early Paleozoic medium-pressure metamorphism in central Vietnam 249 Fig Photomicrographs of samples VNTS17-8 and VNTS18-2 a: Sillimanite (Sil) and biotite (Bt) constituting the major foliation in VNTS17-8 b: Kyanite (Ky) relic surrounded by plagioclase (Pl) in VNTS17-8 c: Staurolite (St) relic enclosed in plagioclase (Pl) in VNTS17-8 d: Garnet (Grt) partially decomposed to biotite (Bt), sillimanite (Sil) and plagioclase (Pl) in VNTS18-2 kyanite, staurolite and rutile are included in plagioclase These observations clearly indicate that sample VNTS18-2 experienced a decompressional heating from the early higherP condition (M1) Coarse-grained quartz-plagioclase melt patches, which are strongly deformed, occur in the biotiterich matrix Zircon, allanite, apatite, monazite and ilmenite occur in the matrix Trace amounts of apatite and allanite are included in garnet ANALYTICAL METHODS Zircon U−Pb dating and REE analyses were conducted on a SHRIMP-RG (reverse geometry) ion microprobe housed at Stanford-U.S Geological Survey Mass Analysis Center (SUMAC) Zircon crystals from two gneiss samples were separated by the conventional heavy liquid technique, and were mounted in an epoxy disc with a 25-mm diameter and a 4-mm thickness All the grains were imaged, using a petrographic microscope under the transmitted and reflected lights Cathodoluminescence (CL) and back-scattered electron images were taken using a JEOL 5600 scanning electron microscope (SEM) to identify internal structures, inclusions and physical defects The mounted zircon crystals were washed with a saturated EDTA solution, thoroughly rinsed in distilled water, dried in a vacuum oven, and coated with Au Mounts were then placed in a loading chamber at high vacuum pressure (10-7 torr) for several hours to remove possible hydride effect Secondary ions were generated from the target spot with an O2- primary ion beam of 4−6 nA The primary ion beam typically produced a spot with a diameter of ~25 mm and a depth of 1−2 mm for 9−12 minutes The basic acquisition routine was described in Mattinson et al (2006) Selected sets of REEs were also measured immediately after the age determination In general, the number of scans through the mass sequence and the counting time on each peak varied on the basis of the sample ages and U−Th concentrations to improve counting statistics and age precision Concentration data for U, Th and REEs were calibrated against the well-characterized homogeneous zircon standard MAD-green (4196 ppm U) (Mazdab and Wooden, personal communication) Age data are standardized against R33 250 Tadashi Usuki et al (419 Ma; Black et al., 2004), which is analyzed repeatedly throughout the analytical session Data reduction for geochronology followed the methods described by Williams (1998) and Ireland and Williams (2003), and used the Squid and Isoplot programs of Ludwig (2001, 2003) Common Pb corrections were made using the 207Pb method for ages younger than 1000 Ma and 204Pb method for the older ages (Williams, 1998) Concordia ages were calculated using the Isoplot (Ludwig, 2003) Data reduction for trace elements was done with a MS Excel spreadsheet provided by SUMAC Averaged counting rate of each element of interest was ratioed to the appropriate high mass normalizing species to account for any primary current drift The derived ratios for the unknowns were compared to the averages of those for the standards to determine concentrations Spot to spot precisions (as measured on the standards) vary owing to elemental ionization efficiency and concentration For the MADgreen zircon, precisions are generally in the range of ±5% for HREE, ±10−15% for MREE, and up to ±40% for La (all values at 2σ) RESULTS The U−Pb isotopic compositions and REE concentrations of zircons are listed in Tables and 3, respectively Figure shows the CL images of analyzed zircons Figures and are the Tera-Wasserburg plots of metamorphic zircon domains and the chondrite-normalized (McDonough and Sun, 1995) REE patterns of zircons, respectively Zircon grains from two samples are ovoid in shape and 70−250 μm in long dimension Each grain can be clearly divided into a detrital core and a metamorphic rim in the CL image However, metamorphic rims of two samples show different internal structures; those of VNTS17-8 are dark (5−30 μm in thickness) and almost structureless (Fig 5a), whereas those of VNTS18-2 occasionally show weak oscillatory zoning (Fig 5b) Some zircons from both samples have a very thin, bright outermost rim (1−5 μm in thickness) (Fig 5) The U−Pb isotopes and REE contents of zircon were measured from metamorphic rims and detrital cores only, because the bright outermost rim is too thin for SHRIMP Table SHRIMP U−Pb−Th analytical data of zircon U Th (ppm) (ppm) Bt gneiss (VNTS17-8) 199 74 510 15 181 351 552 189 65 496 373 10 487 11 459 12 235 321 Grt-Bt gneiss (VNTS18-2) 253 22 286 158 73 51 196 30 300 40 21 659 355 23 183 53 24 440 129 25 195 51 26 248 141 30 244 48 31 250 67 32 152 21 33 136 53 34 244 43 Spot a 206 b 238 206 207 206 238 207 206 Pb* (ppm) 0.37 0.03 1.94 0.01 0.35 0.01 0.02 0.01 0.01 1.36 68 31 14 34 14 31 23 30 28 19 1.47 0.27