DSpace at VNU: U Pb and Sm Nd dating on ophiolitic rocks of the Song Ma suture zone (northern Vietnam): Evidence for upper paleozoic paleotethyan lithospheric remnants

Journal of Geodynamics 69 (2013) 140–147 Contents lists available at ScienceDirect Journal of Geodynamics journal homepage: http://www.elsevier.com/locate/jog U/Pb and Sm/Nd dating on ophiolitic rocks of the Song Ma suture zone (northern Vietnam): Evidence for upper paleozoic paleotethyan lithospheric remnants Nguye˜ˆ n V Vuo ng a,∗ , Bent T Hansen b , Klaus Wemmer b , Claude Lepvrier c , ˜ V Tích a , Ta Tro.ng Tha˘´ ng a,† Vu a b c VNU, University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam Geowissenschaftliches Zentrum der Universität Göttingen, Goldschmidtstr 1-3, 37077 Göttingen, Germany Institute of Earth Sciences of Paris, UMR 7193, UPMC-CNRS, Université P&M Curie, place Jussieu, 75252 Paris cedex 05, T45-46 E2, France a r t i c l e i n f o Article history: Received 12 August 2010 Received in revised form 27 March 2012 Accepted April 2012 Available online 16 April 2012 Keywords: U/Pb Sm/Nd ages Ophiolite Song Ma suture Paleotethys Vietnam a b s t r a c t The Sm/Nd isochron mineral dating technique, applied on lenses of ophiolitic rocks of the Song Ma suture zone, reveals crystallization ages of 387–313 Ma for titanites extracted from the mafic components (metagabbros, metabasalts) of the ophiolite suite These ages correspond to a large time interval within the Carboniferous period Such results mean that these blocks are lithospheric relics of an eastern branch of the Paleotethys They however not exclude that an older early Paleozoic ocean has previously existed in the area The overprinted metamorphism that affect these rocks and the metasedimentary host rocks, including locally HP granulite and eclogite facies conditions (Nakano et al., 2006, 2008, 2010), took place during the Triassic Indosinian orogeny after closure of the ocean, continental subduction and collision, leading to the suturing of the Indochina and South China blocks U/Pb and Ar/Ar data reveal that peak metamorphic conditions were attained 266–265 Ma ago, being then followed by cooling at 250–245 Ma © 2012 Elsevier Ltd All rights reserved Introduction The NW-trending Song Ma zone in northwestern Vietnam (Fig 1), is tectonically intercalated between the Sam Nua zone (mainly in the Laos territory), made of continental assemblages of Triassic sediments and mostly felsic volcanics and the Song Da zone, interpreted as a continental rift and characterized by Late Paleozoic and Mesozoic Formations, including Permian plume-related volcanics (Hanski et al., 2004) The Song Ma zone is known to include tectonic lenses of ultramafic and mafic rocks As early as the first half of the 20th century, this zone has been documented by French geologists (Deprat, 1914; Jacob, 1921; Fromaget, 1927, 1941, 1952) and has been described as the “Thanh Hoa cicatrice”; later the zone has been recognized as an oceanic suture between the Indochina block and the South China block (Hutchison, 1975, 1989; Bach et al., 1982; Bach and Thang, 1995; Sëngor et al., 1988; Chuong, 1995) Geochemical studies on the ultramafic rocks (serpentinites) of the Song Ma complex support an oceanic signature (Trung et al., 2006): relict chromian spinels found in the Hon Vang serpentinite body and in Bo Xing ultramafics (Thanh et al., 2011), in ∗ Corresponding author Tel.: +84 8585097; fax: +84 8583061 E-mail addresses: vuongnv@vnu.edu.vn, vuongnv@gmail.com (N V Vuo ng) † Deceased 0264-3707/$ – see front matter © 2012 Elsevier Ltd All rights reserved http://dx.doi.org/10.1016/j.jog.2012.04.003 chromitite, account for an original peridotite of spinel-bearing lherzolitic harzburgite type; fine-grained gabbros also indicate MORB affinity Although the geochemical signature of the Hon Vang and Bo Xing complex is clearly oceanic, indicating that the Song Ma suture represents relics of a paleo-oceanic lithosphere, the protolith age of the magmatic rocks, which compose the ophiolitic suite of Song Ma, is not well constrained Either the rocks are remnants of a branch of the eastern Paleotethys ocean floor, which closed during the Triassic Indosinian Orogeny, or they represent remobilized, disrupted fragments of an older ocean The suturing time is thought to have occurred in the Paleozoic, during the Silurian-Early Devonian (Hutchison, 1989; Findlay and Trinh, 1997; Findlay, 1998), a model supported by paleontological evidences (Janvier et al., 1994, 1997; Thanh et al., 1996) Most authors however favoured a collision of the Indochina and South China blocks that took place during the Triassic Indosinian event, prior to the Upper Triassic unconformity (Tri, 1977; Sëngor et al., 1988; Chung et al., 1999; Lepvrier et al., 2004, 2008; Trung et al., 2006), a model supported by Ar–Ar Triassic ages for the thermotectonic event recorded in the Truong Son belt and the Song Ma zone itself (Lepvrier et al., 1997; Maluski et al., 1999) According to Carter et al (2001), Carter and Clift (2008), the Indosinian event along Song Ma is not related to the Indochina-South China collision but linked to a Triassic reactivation of a preexisting suture by collision of the Sibumasu block with the Indochina-South China one Recently, Cai and Zhang (2009) have N V Vuo ng et al / Journal of Geodynamics 69 (2013) 140–147 141 Fig Simplified geological map of the Song Ma-Song Da area (Vietnamese and Laos territories), adapted and simplified from [Lepvrier et al., 2008] (1) Metamorphic formations of the Bu Kha.ng-Phu Hoa.t Tertiary dome; (2) Permo-Triassic granitic complexes (Song Ma, Dien Bien and other massifs); (3) Late Proterozoic to Early Paleozoic Nam Co and Song Ma Formations which include, as a tectonic mélange, various ophiolitic elements (ultrabasites, metabasalts, amphibolites, metagabbros and plagiogranites of the Chieng Khuong Complex); (4) Upper Silurian and Lower Devonian Formations; (5) Middle Devonian to Carboniferous Formations; (6) Permian volcanics including komatiites; (7) Lower Triassic terrigenous to Middle Triassic mainly carbonate sediments; (8) Unconformable terrigenous and coal-bearing Norian-Rhetian sediments (Suoi Bang Formation); (9) Upper Cretaceous red beds of the Yen Chau Formation put forward a new interpretation of the zone with South China and Hainan island correlations The studies on the Song Ma suture zone mainly focused on petrologic, metamorphic and tectonic aspects (Lien, 1980; Bach and Thang, 1995; Chuong, 1995; Findlay and Trinh, 1997; Findlay, 1997, 1998; Lepvrier et al., 1997, 2004, 2008; Nakano et al., 2008, 2010) Most of the isotopic ages have been obtained on metamorphic rocks collected in the Nam Co Formation through the 40 Ar/39 Ar method and all are grouped in the range of 250–245 Ma (Maluski et al., 1999; Lepvrier et al., 1997, 2004, 2008) Up to now, rare reliable data have been obtained and published on the crystallization age of the fragments of oceanic rocks found along the suture Previous studies provided distinct age groups of 455 and 425 Ma through K–Ar dating and an age of 541 ± 72 Ma (Thang and Truong, 1999; Lien, 1980) was obtained by the Rb–Sr technique More recently, Pham et al (2008), using LA-ICP-MS, have reported U–Pb zircon data on samples collected from the Huoi Hao metabasalt, Chieng Khuong plagiogranite and Song Ma granite; most zircon grains yield mostly concordant Late Permian age: 251 Ma and 253 Ma for the metabasalt, 258 ± and 260 ± Ma for Song Ma granite and 263 ± Ma for the Chieng Khuong plagiogranite A similar U–Pb zircon age of 271 Ma has been obtained on a sample of quartz-diorite of Chieng Khuong (Liu et al., in press) Clarifying the time of formation of the ophiolitic suite and the time of suturing along Song Ma are critical points for understanding the geodynamic evolution of the Indochina and South China continental blocks This paper presents the results of U/Pb and Sm/Nd datings from mainly metabasites samples collected along the Song Ma suture zone and a discussion of their geological significances Tectonic setting The NW-trending Song Ma deformation zone, in northwestern Vietnam and northeastern Laos (Fig 1), parallels to the South the Cenozoic Red River (Song Hong) shear zone It marks the northern 142 N V Vuo ng et al / Journal of Geodynamics 69 (2013) 140–147 edge of the Truong Son belt of central Vietnam, also characterized by a series of NW-trending Triassic shear zones, formed during the Indosinian Orogeny (Lepvrier et al., 1997) The zone extends over 300 km from the coastal zone of the Thanh Hoa province, following the course of the Song Ma River in Vietnam and Laos, before being deflected clockwise by the Lai Chau-Dien Bien Phu Fault zone (DBF), as a result of post-triassic dextral movements, which took place along this fault The Song Ma Zone (SMZ) sensu lato comprises several units: The Song Ma unit, in the axial part, is regarded as an oceanic suture zone between the Indochina and South China continental blocks (Hutchison, 1975, 1989; Findlay and Trinh, 1997; Findlay, 1997) It is marked by the occurrence of stretched lenses of different rock-types belonging to an ophiolitic complex (Sëngor et al., 1988; Hutchison, 1989), which are tectonically intercalated within a metasedimentary material of Neoproterozoic to Lower Paleozoic age (Nam Ty and Huoi Hao Formations) The ophiolitic suite consists in ultramafics of the Pac Nam Formation represented by small blocks of serpentinite, except in the Thanh Hoa area where they formed the huge serpentinized chromite-bearing peridotite of the Nui Nua massif Mafic components of the ophiolitic complex, grouped in the Bo Xinh Formation, consist in small lens-shape bodies of metagabbro, gabbrodiabase and diabase, metabasalt and dyke The complex also includes gneissified, foliated and elongated bodies of plagiogranite such as the Chieng Khuong massif and Ban Phung massif (Fig 1, DGMV, 2005a) All these magmatic rocks, as well as the sedimentary country rocks, are metamorphosed into greenschist to lower amphibolite metamorphic facies (DGMV, 2005a) Greenschist facies rocks include: quartz–sericite–chlorite, feldspath–quartz–biotite mineral assemblages in metasedimentary rocks, actinolite–epidote–chlorite and plagioclase–hornblende–epidote assemblages in metabasites (metabasalts and gabbroamphibolites) Accessory minerals consist mainly in titanite (sphene), rarely in apatite The ultramafic rocks are often strongly serpentinized with development of actinolite and talc minerals The Nam Co unit, folded in a broad antiform and described as the Song Ma anticlinorium (Tri, 1977; Findlay, 1997), is in faulted ductile contact, with the Song Ma suture zone The Nam Co antiform and the Song Da Zone (SDZ) are bounded by the Son Da Fault (SDF) The rocks of the Nam Co Formation, attributed to the Proterozoic and Lower-Middle Paleozoic, display metamorphic assemblages corresponding to the amphibolite facies They consist in pelitic schists, quartz-sericite schists, sericitic quartzites, phyllites, garnet-bearing micaschists, locally containing kyanite and staurolite (Lien, 1980; Findlay, 1997) Pieces of amphibolite are intercalated in the series More to the east, along Song Ma upstream and Dien Bien Phu area, in the Nam Su Lu and Nam Co Formations (DGMV, 2005a), high-pressure metamorphic rocks (Fig 1) have been discovered: high-pressure granulite (Nakano et al., 2008) and low-temperature eclogite (Nakano et al., 2010) Pelitic schists, containing garnet and phengite, with the presence of chloritoid, kyanite and staurolite in more aluminous schists, enclose blocks or lenses of eclogite and amphibolite and have been affected themselves by eclogite facies metamorphism Eclogite and amphibolite show a MORB-type affinity The eclogites record an estimate peak-pressure metamorphic conditions of at least 2.1–2.2 GPa and 600–620 ◦ C, prior to a clockwise P–T evolution and a subsequent cooling; similar high-pressure conditions of 1.9–2.3 GPa at 580–600 ◦ C have affected the host pelitic rocks (Nakano et al., 2008, 2010) The Song Ma granitic complex, to the South, extending over 80 km, lies along the Song Ma Fault, which separates the Sam Nua and Song Ma zones It is composed mainly of undeformed porphyritic granite and granodiorite (DGMV, 2005a) This calcalkaline complex is genetically linked with the volcanosedimentary material of the Anisian Dong Trau Formation Isotopic Rb/Sr ages of 232 ± 11 Ma have been reported from this material (DGMV, 2005b) Other data have been obtained giving U/Pb ages of 258 ± and 260 ± Ma (Pham et al., 2008) More to the WNW, but in a similar tectonic position, the Dien Dien Phu undeformed massif is a composite calcalkaline granitic complex made of several intrusive phases of Permian-Triassic ages, (DGMV, 2005b) U/Pb zircon ages of 229 ± and 202 ± Ma have been recently obtained on quartz–monzonite and porphyritic granodiorite, respectively (Liu et al., in press) The coal-bearing Suoi Bang Upper Triassic Formation unconformably covers the Song Ma and Dien Bien Phu intrusive massifs The metamorphic rocks of the Song Ma zone have suffered a complicated multi-stage deformation, including the superimposed development of planar and linear structures and the existence of several phases of folding with initial isoclinal folds and sheath folds and later upright folds associated with steep crenulation cleavage and kink-bands development (Findlay, 1997, 1998; Findlay and Trinh, 1997; Lepvrier et al., 1997; Hai, 2009) Such overprinting and refolding have obviously affected the initial geometry of the suture and explained the rather broad spatial distribution of the ophiolitic rocks fragments (Hai, 2009) Along Song Ma River, in the axial part of the zone, the Song Ma Formation displays a steep to vertical foliation coupled with high angle fold axes The ophiolitic fragments are transformed into elongated lenses NW-trending mylonites and ultramylonites bear a stretching lineation with angle of plunge varying from 5◦ to 20◦ eastward Kinematic criteria, including the indication given by sigmoidal porphyroclasts, reveal a dextral sense of shearing (Lepvrier et al., 1997, 2008) In the Nam Co Formation, as a result of a final stage of upright folding, the original flat-lying foliation has given rise to a succession of antiforms and synforms (Song Ma anticlinorium, Tri, 1977), examplified, along the cross-section from Mai Son to Chieng Khuong, by the large NW-trending Nam Co antiform In the area of Co Ma, the Lower Devonian limestones, transformed into marbles exhibit tight isoclinal folding; equivalent marbles, along the road from Tuan Giao to Dien Bien Phu (Muong Ang) area, are affected by a boudinage in the northern direction Samples and dating techniques 3.1 Samples positions and petrographic descriptions In the present study, several rock samples have been collected and dated by Sm/Nd and U/Pb dating techniques (Table 1) VN34-00 is a metagabbro-amphibolite collected in the southern part of the Nam Co anticline, near by Chieng Khuong VN18-00 is a gneissified granitic rock in the Chieng Khuong body VN19-00 is an amphibolite Other samples were gathered along the Song Ma River: a fine-grained mafic sample VN5-00 near by Quan Hoa and several pieces of metagabbro (VN21-00, VN22-00 and VN32-00) near by Song Ma town The metagabbro comprise amphibole minerals with grain-size ranging from 0.3 to mm, plagioclase crystals containing 70–80% albite and 20–30% anorthite, abundant and corroded clinopyroxen prisms Titanite was found as the main associated mineral, the other, less frequent minerals were rutile and calcite 3.2 Mineral separation Mineral separation, enrichment and analysis were carried out at the laboratory of isotope geology at the University of Goettingen, Germany, following the techniques described in Krogh (1973) Amphibole and titanite minerals were separated from VN5-00, VN21-00, VN22-00 and VN34-00 rock samples and analysed for N V Vuo ng et al / Journal of Geodynamics 69 (2013) 140–147 143 Table Samples position, rock types, dating methods and ages Position in Fig Sample number GPS coordinates Type of rock Dating method and mineral fraction VN 5-00 Metagabbro VN 18-00 VN 19-00 VN 21-00 20◦ 23 24.4 103◦ 58 32.2 20◦ 58 23.2 103◦ 59 39.4 20◦ 56 06.3 104◦ 00 14.5 21◦ 12 14.3 103◦ 33 34.0 VN 22-00 21◦ 12 02.0 103◦ 37 26.5 Garnet bearing Metagabbro VN 32-00 Gabbro VN 34-00 21◦ 02 14.0 103◦ 50 04.4 20◦ 55 44.3 103◦ 59 39.4 Sm–Nd Titanite U–Pb Zircon U–Pb Zircon Sm–Nd Amphibole, Clinozoisite Sm–Nd Amphibole, Pyroxene Sm–Nd Amphibole Sm–Nd Titanite, Amphibole U–Pb Titanite Chieng Khuong Granite Amphibolite Metagabbro Amphibolite Sm–Nd dating The 1.5 kg weighted sample was pulverized to about 0.25 mm grain size The mineral grains with the size less than 0.25 mm, which were mainly plagioclase and amphibole, were separated by means of a WilfleyTM table Strong magnetic amphibole was enriched with an isodynamic FrantzTM separator Weak magnetic green amphiboles were collected under a microscope Titanite was chosen as well-formed crackless single crystals Single crystals of amphibole and titanite were then cleaned ultrasonically in weak HNO3 solution for half an hour and then re-cleaned with distilled water Zircons were extracted from sample VN 34-00 and VN 18-00 for U–Pb dating 3.3 Analytical procedure Neodymium and samarium concentrations and isotopic compositions were performed on representative sample powder by conventional isotope dilution technique The samples were weighted into Teflon vials and spiked with suitable amounts of mixed 150 Nd–149 Sm spike solutions prior to dissolution in a mixture of ml HF and HNO3 (3:2) with a PicoTrace© digestion system The solutions were processed by standard cation-exchange techniques for purification of the Sm and Nd fractions For the determination of isotopic compositions Sm and Nd were loaded with 2.5 N HCl on pre-conditioned double Re filaments Measurements of isotopic ratios were performed on a Finnigan MAT 262 RPQ+© mass spectrometer in static mode (GZG Göttingen, Dept of Isotope Geology) Repeated measurement of the La Jolla Nd standard yielded a 143 Nd/144 Nd ratio of 0.511840 ± (n = 13, 2␦) over the course of this study The obtained Nd isotopic ratios of the samples were normalized to a 146 Nd/144 Nd ratio of 0.7219 Total procedure blanks were Age (Ma) 313 ± 32 222 ± 241.3 ± 5.3 387 ± 56 338 ± 24 322 ± 45 315 ± 92 265 ± consistently below 150 pg for Sm and Nd All 143 Nd/144 Nd ratios are reported with their 2␦ internal precision plus the uncertainties resulting from the spike correction For further details see Tütken et al (2002) Samples for U/Pb analyses were crushed in a jaw-crusher and subsequently milled before sieving (180 100–80 100–80 80–60