Chromian-spinel compositions from the Bo Xinh ultramafics, Northern Vietnam: Implications on tectonic evolution of the Indochina block

Journal of Asian Earth Sciences 42 (2011) 258–267

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Chromian-spinel compositions from the Bo Xinh ultramafics, Northern Vietnam: Implications on tectonic evolution of the Indochina block Ngo XuanThanh a,d, Mai Trong Tu b, Tetsumaru Itaya c, Sanghoon Kwon a,⇑ a

Department of Earth System Sciences, Yonsei University, Seoul 120-749, South Korea Department of Sciences – Technology and International Cooperation, Vietnam Institute of Geosciences and Mineral Resources, KM9, Nguyen Trai Street, Thanh Xuan, Hanoi, Viet Nam c Research Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700-0005, Japan d Department of Geology, Hanoi University of Mining and Geology, Dong Ngac, TuLiem, Hanoi, Viet Nam b

a r t i c l e

i n f o

Article history: Available online 19 February 2011 Keywords: Indochina block South China block Supra-subduction zone Serpentinized ultramafics Cr-spinel

a b s t r a c t The Bo Xinh ultramafics, north of the Song Ma fault zone are parts of the isolated bodies that have long been considered as remnants of Paleotethyan oceanic lithosphere between the Indochina and South China blocks. The core compositions of the early igneous stage Cr-spinels are used to deduce the petrogenesis and tectonic environments for the Bo Xinh ultramafics. These spinels are characterized by medium Cr2O3content (37.03–39.41 wt.%), high contents of Al2O3 (29.08–32.01 wt.%), FeO (15.08–17.80 wt.%) and MgO (14.09–16.13 wt.%), and very low TiO2 content (<0.12 wt.%) with medium Cr# (0.44–0.47) and high Mg# (0.62–0.71). They also show low Fe3+# and high Fe2+/Fe3+ ratios of 0.018–0.040 and 5.52–11.24, respectively. The Cr-spinel compositions along with forsterite number (Fo88–92) suggest that the parental magma was lherzolite to harzburgite in composition, indicating the forearc tectonic environment for the Bo Xinh ultramafics. Thus, the supra-subduction zone nature of ophiolites in the Bo Xinh ultramafics, combined with available information on magmatism, metamorphism and sedimentary environment, suggest the presence of a southward subduction zone since Cambrian and subsequent collision of the Indochina and South China blocks in Late-Silurian to Early-Devonian. This further indicates that Permian to Triassic magmatic and metamorphic events, recorded widely within the Indochina block, should be linked to the northward subduction of the Paleo-oceanic plate beneath the Indochina block, following by Middle to Late-Triassic collision between the Sibumasu and Indochina blocks. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Crustal growth through successive subduction and accretion generates well-defined suture zones marked by ophiolites in a supra-subduction zone setting (e.g., Hara et al., 2009; Maruyama et al., 2009; Santosh et al., 2009; Ali et al., 2010a; Braid et al., 2010; Isozaki et al., 2010; Saumer et al., 2010; Sajeev et al., 2010; Xiao et al., 2010; Zhang et al., 2010). The Indochina block is a Gondwana-derived terrane that separated by the opening of different branches of the Paleotethyan Ocean, and is marked by the presence of various suture zones (e.g., Tri, 1979; Lepvrier et al., 2004; Metcalfe, 2011). In the northwest, the Indochina block is separated from the Sibumasu block along the Nan-Uttaradit Suture, which is resulted from a Carboniferous to Early-Triassic northward subduction and subsequent collision between the Sibumasu-Qiangtang to Indochina blocks in Middle- to Late-Triassic (Wakita and Metcalfe, 2005). However, its suture boundary with Yangtze part of the South China block is still debated along either ⇑ Corresponding author. E-mail address: [email protected] (S. Kwon). 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.02.004

Song Ma or Song Da or Red River fault zones (Fig. 1; Tri, 1979; Hutchison, 1989; Lan et al., 2003; Lepvrier et al., 2004). In addition, the age of ocean closure along these possible sutures are also debated in terms of Silurian to Early-Devonian (Wakita and Metcalfe, 2005; Carter and Clift, 2008) or much later in Triassic (Lan et al., 2003; Li et al., 2006; Lepvrier et al., 2008). Regarding the subduction polarity between the Indochina and South China blocks, the occurrence of a series of Permian to Triassic subduction-related calc-alkaline granitoids in the southern side of the Song Ma fault zone suggest southward subduction of the Paleotethyan oceanic lithosphere beneath the proto-Indochina block, and their final amalgamation during Late-Triassic in age (Hutchison, 1989; Lan et al., 2003). In contrast, Tri (1979) proposed a northwest subduction of Indochina block beneath the South China block to the south and the Khorat microplate to the west. Lepvrier et al. (2004, 2008) support the model by Tri (1979), and that the Permian basalts in the Song Da, north of the Song Ma fault zone, were emplaced in a backarc setting. They further suggest that a southward subduction model is unlikely because most of the intrusions are post-collisional rather than subduction-related granitoids in the southern Song Ma. The Red-river fault/shear zone is interpreted as a result

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Fig. 1. Simplified geological map of the Song Ma area, Northern Vietnam showing the study area (modified after Bao and Luong (1981) and Lepvrier et al. (2008)).

of the India-Asia collision during Late-Paleocene and Early-Eocene (e.g., Patriat and Achache, 1984; Tapponnier et al., 1990; Thanh et al., 2010). In the northern Song Ma fault zone, several isolated serpentinized ultramafic bodies occur and most of them suffered various degrees of serpentinization, deformation and metamorphism. These bodies have been considered as remnants of MORB-like peridotites of Paleo-oceanic lithosphere between the Indochina and South China blocks (Findlay and Trinh, 1997; Findlay, 1998; Trung et al., 2006). However, due to the strong serpentinization, alteration and metamorphism, these ultramafic rocks preserve rare original petrologic information. In this paper, we present the mineral chemistry of chromian-spinels (Cr-spinels hereafter) from the serpentinized ultramafics within the Bo Xinh area (Bo Xinh ultramafics), north of the Song Ma fault zone that can serve as a

potential petrogenetic indicator due to its ability to survive by later alteration and metamorphism. In addition, combining with available magmatic, metamorphic and sedimentary information from this region, we also attempt to provide insights into the tectonic history of the Indochina block in Southeast Asia. 2. Geological setting The Bo Xinh area, north of the Song Ma fault zone consists of low- to high-grade metamorphic rocks, subduction and post-collision related granitoids, and sedimentary rocks (Fig. 1). The metamorphic rocks are low to upper amphibolite facies metapelites including mica schists, garnet-, garnet–hornblende-bearing schists, amphibolites and garnet amphibolites. Biotite-muscovite, garnet– phengite, garnet–biotite–phengite, garnet–andalusite–biotite–

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phengite schists as well as chloritoid-, kyanite- and staurolitebearing schists are common (Nakano et al., 2008a, 2008b, 2010). Amphibolite and garnet amphibolites typically occur as centimeter- to kilometer-scale lenses and/or blocks associated with the pelitic schists. Eclogites have been reported from the southwestern Song Ma suture zone, suggesting peak metamorphic temperature ranging from 620 to 680 °C at 2.1–2.3 GPa with monazite U–Pb ages of ca. 243 Ma (Nakano et al., 2010). A few monazite inclusions within garnets and the cores of some zoned monazites in garnet– phengite schists record 424 ± 15 Ma (Nakano et al., 2010). In the northern Song Ma fault, metamorphic rocks recorded several metamorphic events from Silurian to Late-Triassic, and were intruded by Devonian granitoids (Tri, 1979; Findlay and Trinh, 1997; Carter and Clift, 2008). Tri (1979) described the stratigraphy of the region and suggested that they are consistent with subduction complex. The subduction complex, northern Song Ma fault zone, contains pelitic schist, quartz-sericite schist, micaschist, quartzite, sericitoschist, greywacke and various lithologies related to the ophiolitic complex such as serpentinized ultramafics, metabasalt, diabase, amphibolite, gabbro, plagiogranite and eclogite (Lepvrier et al., 2008; Nakano et al., 2010). Although multiple metamorphism and deformation have obscured the original stratigraphic relationships, the metamorphic rocks in the ophiolitic complex are compatible with well known island-arc subduction complex such as early Paleozoic island-arc sequences in Tasmania, northern Victoria Land and NW Nelson in New Zealand (Findlay and Trinh, 1997). Based on this correlation, it is interpreted as metamorphosed relicts of forearc and island-arc complex (Findlay and Trinh, 1997). The serpentinite bodies extend in the NW–SE direction that is almost parallel to the Song Ma fault zone. These bodies have been interpreted as relicts of the former Paleotethyan oceanic lithosphere between the South China and Indochina blocks (e.g. Hutchison, 1975; Trung et al., 2006). MORB-like geochemical features are reported from the gabbros, parts of the Song Ma ophiolite, in the Honvang area (Trung et al., 2006). The post-Triassic, probably Cenozoic, Song Ma fault zone juxtaposed the Indochina with the block between the Song Ma and Red River Faults (Fig. 1). The collision between the Indian and Eurasian plates reactivate the faults within this area occur during Paleocene to Eocene (Findlay and Trinh, 1997; Thanh et al., 2010 and references therein). In the southern Song Ma fault, Cambrian to Triassic subduction-related granitoids are reported (Bao and Luong, 1981; Lan et al., 2003; Trung et al., 2007; Lepvrier et al., 2008). The high-Al granitoids (e.g. syenites) within Vietnam are reported in Middle Devonian to Silurian and Late-Triassic to Jurassic (e.g. Bao and Luong, 1981; Lepvrier et al., 2008). Permian calc-alkaline granitoids are reported from the northern side of Song Ma fault zone (Lepvrier et al., 2004; Lan et al., 2003). Some Late-Permian to Early-Triassic mafic– ultramafic rocks including komatiite-basalt in the Song Da fault zone have documented as the results of continental backarc rifting (Hanski et al., 2003; Lepvrier et al., 2004). However, some recent researches suggest that these komatiite-basalts have formed in relation to the Emeishan plume that reported widely in the northwestern China (e.g. Chung et al., 1997; Hoa, 2002; Ali et al., 2010b).

Fig. 2. Representative photomicrographs of the serpentinized ultramafics. (a) Cross-polarized photomicrograph showing the texture of serpentine and olivine pseudomorphs after serpentinization. (b) Open polarized photomicrograph of a spinel grain showing the primary igneous red-color core rimed by the secondary dark-color overgrowth. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

dotite olivines and orthopyroxenes (Fig. 2a). All the serpentine grains have been brecciated (Fig. 2a). Fine to very fine talc and carbonate veins are filled along their granular fractures. Cr-spinels are anhedral to subhedral in shape with relatively coarse grain (0.5– 1.5 mm) and fine grain (0.05–0.1 mm) sizes. All the grains have been metamorphosed into ferritchromite and Cr-magnetites in their rims and along the granular fractures. Some Cr-spinel grains contain inclusions of serpentine. These inclusions might be metamorphosed from the olivine and/or orthopyroxene minerals of the original peridotites. The red-brown to dark-brown primary igneous Cr-spinels (Fig. 2b) are preserved in some core parts of the coarse-grained ones. 4. Electron microprobe analyses Electron microprobe analyses of minerals were carried out at Okayama University of Science using a JEOL JXA-8900R. The quantitative analyses on mineral chemistry were performed with 15 kV accelerating voltage, 12 nA beam current and 3 lm beam size. Natural and synthetic silicates and oxides were used for calibration. The ZAF method (oxide basis) was employed for matrix corrections. Estimating Fe2+ and Fe3+ contents from total FeO is based on the charge balance using stoichiometic criteria (Droop, 1987). The representative data are shown in Tables 1 and 2. Elemental mapping (Cr, Fe, Mn and Mg) analyses were carried out to check how post metamorphism/magma affected to the alteration of Crspinel grains. Fig. 3 shows a representative Cr-spinel grain, which is rich Fe, Mn and low Cr, Mg and Al components in the rim parts, while the core parts show relatively higher Cr, Mg, Al and lower Fe, Mn than the rim parts, their contributions are homogeneous in the core part of the Cr-spinels. 4.1. Serpentine

3. Samples description Serpentinite samples were collected from the Bo Xinh area, where it is occurs with meta-gabbros and basalts. The samples are strongly serpentinized and metamorphosed so that no other primary peridotite minerals are survived except for some Cr-spinel grains. Most of the serpentinites are medium- to fine-grained. Some large serpentinite grains are probably pseudomorphs of peri-

The serpentinized ultramafic rocks consist mainly of antigorite and lizardite. Their chemical compositions are presented in Table 1. The serpentinites compose of SiO2 (37.45–44.19 wt.%), MgO (36.8–41.66 wt.%), FeO (1.12–4.46 wt.%), Al2O3 (0.01–1.53 wt.%) and CaO (0–0.16). The Si contents of some serpentines show slightly higher values (>4) than commonly reported that can be explained by the occurrence of minor antigorite (D’Antonio and Kristensen, 2004).

Table 1 Representative chemical analyses of serpentines of the Bo Xinh serpentinite. BXU2

BXU4

BXU3

BXU4

BXU5

BXU4

BXU5

BXU6

BXU5

BXU6

BXU6

BXU6

BXU10

BXU11

BXU12

BXU13

BXU12

BXU3

BXU11

BXU12

BXU13

SiO2 TiO2 Al2O3 Cr2O3 FeO MgO CaO Na2O K2O Total

41.32 0.04 0.51 0.00 3.12 39.60 0.14 0.01 0.00 84.73

41.57 0.06 0.99 0.04 2.86 38.98 0.15 0.00 0.00 84.64

41.81 0.03 0.78 0.02 2.55 39.99 0.05 0.02 0.00 85.25

40.36 0.02 1.28 0.00 3.91 38.59 0.11 0.01 0.00 84.28

40.98 0.02 0.53 0.05 1.82 40.39 0.05 0.00 0.00 83.84

38.01 0.00 1.02 0.15 4.46 37.22 0.13 0.00 0.01 81.01

39.15 0.04 1.05 0.17 3.71 37.92 0.08 0.00 0.00 82.12

40.31 0.03 0.08 0.01 2.05 39.99 0.05 0.00 0.00 82.52

39.95 0.02 0.18 0.05 2.04 39.46 0.05 0.00 0.00 81.76

42.94 0.02 0.36 0.00 1.59 38.75 0.02 0.00 0.00 83.69

41.33 0.01 0.18 0.00 1.66 40.33 0.02 0.00 0.01 83.54

39.35 0.02 0.33 0.00 2.08 41.66 0.02 0.00 0.00 83.45

41.46 0.05 0.26 0.04 1.91 41.07 0.04 0.00 0.01 84.83

42.62 0.01 0.07 0.01 2.01 40.07 0.01 0.03 0.05 84.87

41.59 0.00 0.18 0.00 3.87 38.79 0.04 0.00 0.02 84.49

42.21 0.00 0.26 0.00 1.12 41.26 0.00 0.00 0.00 84.85

40.32 0.01 0.46 0.00 3.00 39.69 0.08 0.01 0.00 83.57

38.92 0.01 0.39 0.00 1.95 41.56 0.04 0.01 0.01 82.89

41.11 0.02 0.22 0.03 2.22 39.90 0.02 0.03 0.00 83.55

44.19 0.06 1.53 0.25 4.46 41.66 0.15 0.05 0.05 92.38

38.01 0.00 0.00 0.00 1.12 37.22 0.00 0.00 0.00 76.36

O = 14 Si Ti Al Cr Fe Mn Mg Ca Na K

3.97 0.00 0.06 0.00 0.25 0.00 5.68 0.01 0.00 0.00

3.99 0.00 0.11 0.00 0.23 0.00 5.58 0.02 0.00 0.00

3.98 0.00 0.09 0.00 0.20 0.00 5.68 0.01 0.00 0.00

3.92 0.00 0.15 0.00 0.32 0.00 5.59 0.01 0.00 0.00

3.96 0.00 0.06 0.00 0.15 0.00 5.82 0.01 0.00 0.00

3.87 0.00 0.12 0.01 0.38 0.00 5.65 0.01 0.00 0.00

3.91 0.00 0.12 0.01 0.31 0.00 5.64 0.01 0.00 0.00

3.97 0.00 0.01 0.00 0.17 0.00 5.87 0.00 0.00 0.00

3.97 0.00 0.02 0.00 0.17 0.00 5.84 0.01 0.00 0.00

4.12 0.00 0.04 0.00 0.13 0.00 5.55 0.00 0.00 0.00

4.00 0.00 0.02 0.00 0.13 0.00 5.82 0.00 0.00 0.00

3.84 0.00 0.04 0.00 0.17 0.00 6.07 0.00 0.00 0.00

3.96 0.00 0.03 0.00 0.15 0.00 5.85 0.00 0.00 0.00

4.05 0.00 0.01 0.00 0.16 0.00 5.68 0.00 0.01 0.01

4.02 0.00 0.02 0.00 0.31 0.00 5.59 0.00 0.00 0.00

4.01 0.00 0.03 0.00 0.09 0.00 5.84 0.00 0.00 0.00

3.93 0.00 0.05 0.00 0.24 0.00 5.77 0.01 0.00 0.00

3.83 0.00 0.05 0.00 0.16 0.00 6.09 0.00 0.00 0.00

3.99 0.00 0.02 0.00 0.18 0.00 5.78 0.00 0.01 0.00

3.92 0.00 0.16 0.02 0.33 0.00 5.51 0.01 0.01 0.01

4.02 0.00 0.00 0.00 0.10 0.00 5.86 0.00 0.00 0.00

Table 2 Representative chemical analyses of Cr-spinel cores and altered rims of the Bo Xinh serpentinite. Domain Sample no.

Core Core BXU2 BXU4

SiO2 TiO2 Al2O3 Cr2O3 FeO MgO Total

0.03 0.06 0.02 0.01 0.03 0.05 29.92 31.44 30.14 37.68 37.59 38.77 17.02 16.35 15.66 14.90 15.00 15.36 99.56 100.47 100.00

FeO Fe2O3 Cr Ti Al Fe3+ Fe2+ Mg

13.92 3.44 0.88 0.00 1.04 0.08 0.34 0.66

14.29 2.29 0.87 0.00 1.08 0.05 0.35 0.65

0.66

0.65

0.46 4.49 0.04 16.19 16.28 1.00

0.44 6.93 0.03 15.90 16.28 1.05

Core Core BXU4 BXU5

Core Core BXU4 BXU5

Core Core Core Core Core BXU6 BXU5 BXU6 BXU6 BXU6

Core Core Core Core Core Core BXU10 BXU11 BXU12 BXU13 BXU12 BXU3

0.00 0.16 0.01 0.00 30.08 29.22 37.82 38.66 16.01 15.89 15.53 16.13 99.45 100.05

0.05 0.02 0.04 0.04 30.34 29.93 37.34 38.84 16.01 16.19 15.49 15.61 99.26 100.62

0.04 0.01 29.92 38.38 15.72 15.18 99.23

0.02 0.02 30.15 38.46 15.91 15.27 99.82

0.03 0.03 30.48 37.83 15.91 15.69 99.97

13.47 13.47 2.65 2.71 0.89 0.89 0.00 0.00 1.05 1.05 0.06 0.06 0.33 0.33 0.67 0.67

12.95 3.29 0.87 0.00 1.05 0.07 0.32 0.68

13.85 2.38 0.90 0.00 1.04 0.05 0.34 0.66

13.73 2.43 0.89 0.00 1.05 0.05 0.34 0.67

13.79 2.57 0.89 0.00 1.05 0.06 0.34 0.66

13.52 3.05 0.89 0.00 1.04 0.07 0.33 0.67

0.67

0.68

0.66

0.67

0.66

0.67

0.46 0.46 5.64 5.52 0.03 0.03 16.22 16.26 16.28 16.28 0.94 0.95

0.45 4.37 0.04 16.11 16.28 0.89

0.46 6.46 0.03 16.32 16.28 0.99

0.46 6.28 0.03 16.21 16.28 0.97

0.46 5.97 0.03 16.18 16.28 0.98

0.46 4.92 0.03 16.31 16.28 0.94

13.42 12.94 2.49 3.41 0.90 0.88 0.00 0.00 1.04 1.04 0.05 0.08 0.33 0.32 0.67 0.68 0.67

0.68

0.46 0.46 5.99 4.22 0.03 0.04 16.30 16.18 16.28 16.28 0.94 0.89

12.04 13.01 4.28 3.34 0.90 0.87 0.00 0.00 1.01 1.05 0.09 0.07 0.30 0.32 0.70 0.68 0.70

0.68

0.47 0.45 3.13 4.33 0.05 0.04 16.45 16.06 16.28 16.28 0.78 0.90

0.00 0.07 30.34 38.47 15.70 15.31 99.87

0.07 0.05 29.94 38.04 16.67 14.89 99.66

0.07 0.07 0.04 0.03 29.99 30.47 38.54 38.58 15.67 15.86 15.41 15.43 99.72 100.44

13.20 13.36 13.50 13.98 13.20 3.32 2.61 2.44 2.99 2.74 0.90 0.90 0.89 0.89 0.90 0.00 0.00 0.00 0.00 0.00 1.03 1.04 1.05 1.04 1.04 0.07 0.06 0.05 0.07 0.06 0.32 0.33 0.33 0.35 0.32 0.68 0.67 0.67 0.66 0.68 0.68

0.67

0.67

0.65

0.68

0.47 0.46 0.46 0.46 0.46 4.41 5.68 6.16 5.20 5.35 0.04 0.03 0.03 0.03 0.03 16.35 16.29 16.22 16.24 16.30 16.28 16.28 16.28 16.28 16.28 0.90 0.94 0.95 1.00 0.92

0.67

Core Core Core Core Core Rim Rim Rim Rim BXU10 BXU10 BXU11 BXU12 BXU13 BXU2 BXU3 BXU4 BXU5

0.00 0.01 0.00 0.00 0.00 0.03 0.05 0.07 0.04 0.04 30.11 30.57 30.45 30.07 30.71 38.88 38.63 38.29 38.70 38.56 16.00 15.92 16.11 16.27 15.71 15.08 15.29 15.18 15.38 15.34 100.11 100.47 100.09 100.45 100.36

0.16 0.06 28.57 38.22 15.56 14.59 97.16

0.05 0.08 29.69 38.57 16.08 15.41 99.87

13.64 13.39 2.30 2.42 0.89 0.92 0.00 0.00 1.06 1.02 0.05 0.06 0.33 0.34 0.67 0.66

13.24 3.15 0.90 0.00 1.03 0.07 0.33 0.68

0.67

0.66

0.67

0.46 0.47 6.59 6.16 0.03 0.03 16.17 16.51 16.28 16.28 0.96 0.97

0.47 4.67 0.03 16.35 16.28 0.91

0.00 0.00 30.12 38.64 16.06 15.47 100.30

0.01 0.02 30.03 38.61 15.40 15.03 99.10

13.30 13.57 3.06 2.03 0.89 0.90 0.00 0.00 1.04 1.05 0.07 0.05 0.33 0.34 0.67 0.66 0.67

8.54 0.24 0.87 26.1 49.99 9.34 95.08

0.66 0 7.53 0.17 0.36 0.31 20.71 1.48 0.48 34.81 31.25 25.92 36.61 61.9 55.41 4.2 0.21 8.05 97.15 95.2 97.68

14.49 28.11 31.36 17.66 39.45 9.44 33.94 41.95 0.8 0.93 0.94 0.77 0.01 0 0.01 0.01 0.04 0.82 0.07 0.02 1.15 0.24 0.97 1.19 0.47 0.79 1 0.56 0.54 0.21 0.01 0.45

0.66

0.53

0.21

0.01

0.45

0.46 0.46 4.83 7.43 0.03 0.02 16.29 16.30 16.28 16.28 0.92 0.98

0.95 0.41 0.58

0.53 3.31 0.12

0.93 1.03 0.49

0.97 0.47 0.6

261

Mg/ (Mg + Fe2+) Cr/(Cr + Al) Fe2+/Fe3+ Fe3+# F melt Al2O3 in melt FeO/MgO in melt

Core BXU3

N. XuanThanh et al. / Journal of Asian Earth Sciences 42 (2011) 258–267

Sample no.

262

N. XuanThanh et al. / Journal of Asian Earth Sciences 42 (2011) 258–267

Fig. 3. Elemental mapping (Cr, Fe, Mn and Mg) of a Cr-spinel shows primary igneous core and the secondary rim domains.

4.2. Cr-spinel The representative compositions of the Cr-spinels are listed in Table 2, and their core compositions are characterized by medium in Cr2O3 content (37.03–39.41 wt.%), high contents of Al2O3 (29.08–32.01 wt.%), FeO (15.08–17.80 wt.%) and MgO (14.09– 16.13 wt.%), and very low TiO2 content (0.00–0.12 wt.%). The Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)] have minor ranges from 0.44 to 0.47 and from 0.62 to 0.71, respectively. The Fe3+# [Fe3+/ (Fe3++Al + Cr)] are very low (0.018–0.040), while the Fe2+/Fe3+ ratios are high (5.52–11.24). These cores are rimed by magnetite and ferritchromite components. These rims are characterized by extremely high FeO content (49.99–65.31 wt.%) and very low contents of Cr2O3 (22.79–27.26 wt.%) and MgO (0.33–9.34 wt.%) at the outermost rims. This rim is transitionally zoned by a medium FeO (15.80–31.00 wt.%) and Cr2O3 (28.68–36.3 wt.%) contents, respectively. The altered Cr-spinel data normally have total major elements less than 99 wt.% that is due to containing more or less water component.

tions are unstable, under the effects of post-magmatic and/or metamorphic processes, they start to develop alteration products (e.g. Kimball, 1990; Burkhard, 1993; Barnes, 2000; Mellini et al., 2005). Depending on the degree of alteration, Cr-magnetite and/ or ferrian-chromite will start to form. These two secondary phases are usually attributed to the effects of low to medium grade metamorphism up to lower amphibolite facies (Thalhammer et al., 1990; McElduff and Stumpfl, 1991). The Cr-spinel compositions are plotted on a diagram proposed by Barnes and Roeder (2001) and Roeder (1994) in order to reexamine their primary igneous characteristics (Fig. 4). All the core compositions of the Cr-spinels are concentrated on Field I (Fig. 4), which represents Cr-spinels in primitive basalt, mantle peridotite and chromitite. The rim compositions are plotted close or within the fields of metamorphic Cr-spinel (Fig. 4). We have focused on these primary igneous Cr-spinel cores, and their compositions will be used for discussing the magma genesis and tectonic significance.

5.1. Magma petrogenesis 5. Discussions Cr-spinel [(Mg, Fe2+)(Cr, Al, Fe3+)O4] is a ubiquitous accessory mineral in basalts and peridotites, and its composition reflects geotectonic environments, magma chemistry, degree of partial melting, fractional crystallization, temperature dependent partitioning, and variations in oxygen fugacity (Dick and Bullen, 1984; Kimball, 1990; Burkhard, 1993; Barnes, 2000). Primitive Cr-Spinels are generally altered by serpentinization and/or metamorphism, and later magmatism (e.g. Kimball, 1990; Burkhard, 1993; Barnes, 2000; Mellini et al., 2005). Because primitive Cr-spinel composi-

The primary Cr-spinel core compositions from the Bo Xinh ultramafics are characterized by medium Cr# (0.44–0.47) and high Mg# (0.62–0.71), which are different from spinels Alaskan-type mafic–ultramafic complexes. These are comparable with those from lherzolite- to harzburgite-type peridotite that formed in an environment with a moderate degree of melt depletion (Arai, 1994; Dick, 1989). Equilibrium forsterite number of olivine calculated from Cr-spinel compositions shows a high Fo number (Fo89–92; Fig. 5), which is comparable with those from lherzolite to harzburgite peridotites in some other forearcs (e.g. Mariana

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Fig. 4. Mg/(Mg + Fe2+) vs. Cr/(Cr + Al) variation diagram: field I represents Crspinels in primitive basalt, mantle peridotites and chromitites, field II represents magnetite from metamorphic rocks, field III is magnetites from un-metamorphosed igneous rocks (fields are from Roeder (1994) and Mondal et al. (2001)).

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Fig. 6. Cr# vs. Mg# diagram for the chromian spinel cores. The field boundaries are from Dick and Bullen (1984), Bloomer et al. (1995) and Ohara et al. (2002). The equilibrium fosterite-number lines are from Kamenetsky et al. (2001).

Fig. 7. Al2O3 vs. Fe2+/Fe3+ diagram showing the fields of supra-subduction zone (SSZ) and mid oceanic ridge (MOR) peridotite after Kamenetsky et al. (2001).

Fig. 5. Plot of chromian spinels on Al2O3 vs. Cr2O3 diagram (after Franz and Wirth (2000)).

forearc serpentinite reported by Murata et al., 2009). The chemical core compositions of Cr-spinels are plotted in various discrimination diagrams. They plot within the mantle array field on Al2O3 vs. Cr2O3 diagram (Fig. 6), indicating for mantle origin. In the Mg# vs. Cr# diagram (Fig. 5), the primary Cr-spinel compositions plot within the forearc field. On the Al2O3 vs. Fe2+/Fe3+ diagram (Fig. 7) most of the Cr-spinels compositions plot on the supra-subduction zone (SSZ) peridotite field. These indicate that the Cr-spinels are formed in a supra-subduction zone tectonic setting (Kamenetsky et al., 2001), instead of MORB-peridotite (Pearce, 2003). Ti content in Cr-spinel is minor, but very important factor for tectonic discrimination of parental magma. Cr-spinels from

oceanic plateau and backarc have distinctive TiO2 and Fe3+# relationship; the intraplate basalts have the highest TiO2 content, while the arc magmas are the lowest (Arai, 1992). The Cr-spinel core compositions from the Bo Xinh ultramafics have extremely low TiO2 contents ranging from 0% (not able to detect) to 0.10%, and have no distinctive relation with Fe3+#. Herbert (1982) also suggested that Cr-spinels from oceanic ultramafic tectonites have the TiO2 contents that are not exceeding 0.3%. These clearly indicate that the Cr-spinels from the Bo Xinh ultramafics are derived from a forearc tectonic setting. The degree of partial melting of parental melt is calculated using the equation of Hellebrand et al. (2001), and the representative values are shown in Table 2. The calculated data suggest that the Bo Xinh ultramafics are a mantle restite, which experienced low degree of partial melting (ca. 15.8– 16.6%). They are within the ranges from high partial melting MORB to lower partial melting SSZ (Table. 2). The high-Al contents in Crspinel also indicates for low degree of partial melting of parental melt (Rollinson, 2008). Very low Fe3+ contents in the Cr-spinels

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indicate relatively low oxygen fugacity conditions of their primary source (Murck and Campbell, 1986). Further insights into the chemistry of the parental melts for the studied Cr-spinel can be obtained using equation from Maurel and Maurel (1982) to calculate Al2O3 contents of parental melts. The FeO/MgO ratio of the parental melt is determined by using the equation from Maurel and Maurel (1984). Representative calculation of Al2O3 values and FeO/MgO ratios are given in Table 2. These values from different tectonic setting are also shown to compare with our data. The Al2O3 parent melt content was estimated between 15.44 and 16.42 wt.%. This value is similar to those from Oman harzburgites, which is ranged from 11.9 to 16.3 (Auge, 1987), and is different from those in midoceanic ridge (Wilson, 1989). Estimated FeO/MgO ratio from parental melt yields 0.88–1.16 within the range of magmas in forearc setting (0.7–1.4) (Wilson, 1989). The Ca-depleted nature of serpentines is clear on the Al2O3–MgO–CaO diagram of Coleman (1977), and they plot within the field of metamorphic peridotites associated with ophiolites. The very low CaO (<0.14 wt.%) and Al2O3 (0.3–1.2 wt.%) contents in serpentine suggests very depleted mantle peridotites (Bonatti and Michael, 1989) (Fig. 8a). This further supports an interpretation that their protoliths were poor in clinopyroxene. On the Al2O3 vs. CaO diagram the serpentine compositions plot within the field that is similar to harzburgites recovered from modern forearcs (Ishii et al., 1992) (Fig. 8b). In summary, the primary igneous Cr-spinel compositions from the Bo Xinh serpentinized ultramafics have characteristics of those derived from mantle origin that experience moderate degree of partial melting in a forearc tectonic environment, and are different from the parageneses in Alaskan-type and arc-related mafic–ultramafic complexes (e.g. Eyuboglu et al., 2010, 2011a,b). The features that we reported in this study suggest partial melting with the involvement of subduction-related and MORB mantle of magma components that might form in initial stage of a forearc environment (Stern and Bloomer, 1992). In the initial subduction, due to the down going of the subducted slab, an extension zone will occur shortly in the front of the subducting site (Stern and Bloomer, 1992). Mantle derived magmas will be generated at this extension zone to form new oceanic crust in a forearc setting (Stern and Bloomer, 1992). Magmas including lherzolite–harzburgite, layered and some isotropic gabbros and volcanic rocks typically show low-K and low partial melting degree. These rocks generally display distinct geochemical signatures of MORB-like magmas (Dilek and Polat, 2008). Serpentinization occurs in matured subduction stage by the involvement of fluids released from a subducted slab including the injection of wet sediments into the mantle wedge in a forearc tectonic environment.

5.2. Tectonic implications The mafic and ultramafic rocks from the northern Song Ma fault have been studied from the western Thanh Hoa to the northern Dien Bien in Vietnam (Fromaget, 1941; Hutchison, 1975, 1989; Tri, 1979). They have been regarded as fragments of ophiolite suite representing the remnant of MORB crust of the Paleotethyan Ocean between the Indochina and South China blocks (Hutchison, 1975; Tri, 1979). Trung and Itaya (2004), Trung et al. (2006) reported Cr-spinel mineral chemistry as well as whole rock geochemistry of gabbros from Honvang area of the Song Ma ophiolite, suggesting that the serpentinite bodies with associated MORB-like gabbros in the Song Ma suture zone represent a remnant of Paleo-oceanic lithosphere. The low to very low TiO2 content along with medium Cr# and high Fe2+/Fe3+ ratio of the Cr-spinel from both Honvang (Trung and Itaya, 2004) and Bo Xinh areas suggest a supra-subduction zone tectonic setting (Dick and Bullen, 1984; Kamenetsky et al., 2001). In addition, MORB-like magmas are reported to be formed in nascent spreading centers in a backarc or forearc basin (e.g., Mirdita ophiolite; Dilek and Polat, 2008). This indicates that these serpentinized peridotites may be derived from the same tectonic environment. In the northern Song Ma fault zone, the occurrence of ophiolite complex that consists of serpentinite, amphibolite, gabbro, and mafic dike, together with metagreywacke has also been proposed for forearc affinity (Findlay and Trinh, 1997). Fossils discovered from the forearc complex have been reported to Cambrian–Ordovician ages (Tri, 1979; Bao and Luong, 1981; Huu, 2008). This suggests that the related subduction began during Cambrian (Fig. 9a), and is further supported by the U–Pb ages (440–514 Ma) of zircons from the ophiolite in Hainan Island (Xu et al., 2007). Within the Song Ma forearc ophiolite complex, the greywacke normally contains fragments of basalts, basaltic tuffs and andesitic basalts, which have been suggested as Cambrian to Ordovician ages with arc affinity (Bao and Luong, 1981). During Ordovician to Silurian (ca. 450–418 Ma), volcanics and plutons occurred widely in the central parts of Vietnam from southern Song Ma to Kontum areas such as Long Dai volcanic and Song Chay, Dai Loc plutons. Magmatism during this period is characterized by the calc-alkaline series (Bao and Luong, 1981; Carter et al., 2010). Note that this magmatism occurs mainly at the northern part of the Indochina, while no such magmatic event is found in the southern part such as Thailand area (Charusiri et al., 2010). This clearly indicates the existence of a southward subduction during Cambrian to Early-Silurian along the northern margin of the Indochina block (Fig. 9b). In the Late-Silurian to Devonian, magmatism became

Fig. 8. (a) Al2O3–MgO–CaO diagram of Coleman (1977). (b) Al2O3 vs. CaO diagram of Ishii et al. (1992).

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Fig. 9. Tectonic model showing the evolution of the Indochina block during EarlyPaleozoic to Mesozoic.

high-Al component such as 2-mica and/or amphibole-bearing granite, binary granite, syenite associated with pegmatite and aplite (e.g. Phia Phuong volcanics, Muong Lat, Truong Son plutons in Vietnam) that typical post-collision related magmatism (Bao and Luong, 1981; Chappell and White, 1974; Sylvester, 1989; Martin, 2006). During this time, flysch and molasse also occur widely in the southern Song Ma to central of Vietnam (such as Song Ca, Long Dai and Dai Giang formations), indicating their formation in a convergent boundary at the stage of continental collision. These characteristics in magmatism and sedimentation during this time clearly indicate that the amalgamation of Indochina and South China block occurred during Late-Silurian to Early-Devonian (Fig. 9c). Wakita and Metcalfe (2005) and Carter et al. (2010) support this Silurian to Devonian collision ages that are preserved across the southern China and most of the Vietnam. Recent discovery of fresh water fish fossils from central Vietnam also suggest that Indochina and South China blocks were linked before Middle Devonian time (Thanh et al., 1996). A series of Early-Permian to Early-Triassic subduction-related calc-alkaline granitoids (e.g. Dac Lin, Cam Thuy, Co Noi volcanic and Que Son, Van Canh, Dien Bien, Song Ma, Nui Chua plutonics) are widely exposed in the southern Song Ma area, while their occurrence is restricted within the ophiolitic complex in the northern Song Ma area (Fig. 1). These subduction-related granitoids in the southern Song Ma area had led many geologists to suggest a southward subduction of the Paleo-oceanic crust during Early-

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Permian to Late-Triassic along the Song Ma suture zone (e.g. Li et al., 2006; Lepvrier et al., 2008). However, Early-Permian to Early-Triassic granitoids also occurred widely in Thailand, Laos and Hainan Island (Carter and Clift, 2008). Within the Song Ma ophiolite, some subduction-related plutons are also occurred (Fig. 1) (Lan et al., 2000). This further suggests that Early-Permian to Middle-Triassic subduction-related plutonic rocks preserved within the Indochina blocks may not relate to the Indochina-South China subduction system. Permian to Triassic plutons and forearc complex, reported widely in Thailand and Laos, represented a northward subduction of Paleo-oceanic lithosphere of the Sibamasu-Qiangtang block underneath the Indochina block (Singharajwarapan and Berry, 2000; Metcalfe, 2001; Wakita and Metcalfe, 2005; Charusiri et al., 2010). Thus, the Permian to Triassic subduction-related magmatism within the Indochina block including Vietnam area should be explained by this subduction system (Fig. 9d–f). A series of alkaline plutonic rocks have been reported from Indochina (Pia Oac, Hai Van, Ban Chieng, etc., plutonics). The final collision between the Sibamasu-Qiangtang block and the Indochina block occurred in the Late-Triassic (Singharajwarapan and Berry, 2000; Wakita and Metcalfe, 2005). This collisional event has caused the regional metamorphism and reactivation of pre-existing structures within the Indochina (Carter and Clift, 2008). In summary, the primary Cr-spinels from the Bo Xinh serpentinized ultramafics, located in the north of the Song Ma fault zone, provide compelling evidences to support a forearc tectonic setting for the Song Ma supra-subduction zone ophiolites. This, together with the magmatic evolution of Indochina block, is marked by a southward subduction of the South China block beneath the Indochina block during Cambrian to Silurian, forming a series of subduction-related plutons within the Indochina blocks. The Silurian to Devonian post-collisional granitoids and high-grade metamorphism with syn-deposition of orogenic sediments indicate Middle Paleozoic amalgamation between the Indochina and South China blocks. In contrast, Permian to Triassic subduction-related plutons widespread in the Indochina block indicate a northward subduction along the southern margin of the Indochina block during Carboniferous to Early-Triassic and subsequent Middle- to LateTriassic collision of the Sibamasu-Qiangtang block to the Indochina block. This collisional event might have caused the regional metamorphism and reactivation of the pre-existing structures within Indochina. The Indochina region thus preserves important records for the breakup of Gondwana. Acknowledgements We acknowledge the valuable review comments from Dr. Eyuboglu and an anonymous reviewer. We are also thankful to Drs. M. Santosh and T.R.K. Chetty for the constructive review and editorial handling. This work was supported by Grant in-Aid NRF 2010-0011102 to S. Kwon. Ngo Xuan Thanh acknowledges the support from the second stage of Brain Korea (BK) 21 Project. References Ali, K.A., Azer, M.K., Gahlan, H.A., Wilde, S.A., Samuel, M.D., Stern, R.J., 2010a. Age constraints on the formation and emplacement of Neoproterozoic ophiolites along the Allaqi–Heiani Suture, South Eastern Desert of Egypt. Gondwana Research 18, 583–595. Ali, J.R., Fitton, J.G., Herzberg, C., 2010b. Emeishan large igneous province (SW China) and the mantle-plume up-doming hypothesis. Journal of the Geological Society 167, 953–959. Arai, S., 1992. Chemistry of chromian spinel in volcanic rocks as a potential guide to magma chemistry. Mineralogical Magazine 56, 173–184. Arai, S., 1994. Characterization of spinel peridotites by olivine–spinel compositional relationships: review and interpretation. Chemical Geology 113, 191–204. Auge, T., 1987. Chromite deposits in the northern Oman ophiolite: mineralogical constraints. Mineralium Deposita 22, 1–10.

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