The tectonics of the Indochina Peninsula consists of a collage of continental blocks, e.g., Sibumasu, Indochina and South China blocks, which are separated by the Changning-Menglian- Chiang Mai-Bentong-Raub suture, the Jinshajiang-Ailaoshan- Song Ma suture (Fig. 1a; Faure et al., 2014; Roger et al., 2014; Metcalfe, 2013, 2011; Liu et al., 2012; Sone and Metcalfe, 2008; Hutchison, 1989). The spatial-temporal relationships of widely developed Paleozoic to Mesozoic volcanic belts are significant to the Paleotethyan tectonic evolution research, such as the Loei belt, Luang Prabang-Xaignabouli belt, Phetchabun belt, Truong Son belt and Nan-Sa Kaeo suture in the Indochina Block. The present study mainly focuses on the Nan suture, Loei and Phetchabun volcanic belt (Thassanapak et al., 2017; Wang et al., 2017; Yang et al., 2016; Salam et al., 2014; Zaw et al., 2014; Panjasawatwong et al., 2006; Intasopa and Dunn, 1994; Intasopa, 1993), while there are few reports about the Petro-stratigraphic and tectonic settings of northwestern Laos (Qian et al., 2016a, b, 2015; Rossignol et al., 2016; Blanchard et al., 2013; Stokes et al., 1996) due to the lack of detailed geological survey work and large-scale geological maps.
The Loei belt is constrained by the western Nan-sa Kaeo suture, and may extend from western Cambodia up through Sa Kaeo, Phetchabun and the Loei Province in Thailand into NW Laos. The Loei belt contains multiple generations of successive arc-related magmatic events, including Silurian rhyolite (U-Pb zircon: ca. 434–428 Ma; Zaw and Meffre, 2007), Devonian– Carboniferous arc basalt/andesite (Rb/Sr age: 374–361 Ma; Panjasawatwong et al., 2006; Intasopa and Dunn, 1994), mid- Carboniferous volcanogenic sedimentary sequence (U-Pb zircon: 327±7 Ma; Zaw and Meffre, 2007), Late Carboniferous intrusive rocks (U-Pb zircon: 310±8 Ma; Salam et al., 2014; Zaw and Meffre, 2007), Late Permian–Earliest Triassic and Middle Triassic arc basaltic-rhyolitic rocks (U-Pb zircon: ca. 254–241 and 228 Ma; Kamvong et al., 2014; Salam et al., 2014; our unpublished data). In Laos, from Luang Prabang to Xaignabouli and Pak Lay belt, the existing data show that the Triassic (especially Late Triassic) basaltic-andesitic and volcanoclastic rocks are extensively developed (K-Ar: 210–227 Ma; U-Pb zircon: 215–250 Ma; Qian et al., 2016b; Rossignol et al., 2016; Blanchard et al., 2013; Stokes et al., 1996), the Carboniferous mafic, andesitic, rhyolitic rocks have also been reported in Luang Prabang and Muang Feuang (southeast of Xaignabouli) area (U-Pb zircon: 304–350 Ma; Qian et al., 2016a, 2015). However, the relationships between the Luang Prabang-Xaignabouli belt and the Loei and Phetchabun belt are poorly defined.
Based on the latest 1 : 200 000 geological mapping results finished by Chengdu Center of China Geological Survey, combined with geochronology, petrochemistry and zircon Hf isotopic analysts of the volcanic rocks from the Nam Hang Formation and overlying (?) Muang-Nan Formation, this paper aims at probing the connection of Luang Prabang-Xaignabouli belt and Loei-Phetchabun belt, and to improve our understanding of the tectonic evolution of northwestern Laos.1 GEOLOGICAL SETTING AND PETROGRAPHY
According to our latest geological survey results, sixteen lithostratigraphic units are established in the Xaignabouli area, including the pre-Carboniferous felsic high greenschist-low amphibolite facies schist; Lower Carboniferous Silang Formation lithic feldspathic sandstone; Upper Carboniferous Nam Hang Formation basalt, basaltic andesite, volcanic breccia, and basaltic-andesitic tuff display a significant characteristic of eruption rhythms (mapped as Permian–Early Triassic sequences on the 1 : 1 000 000 geological map; DGM, 1990); and Muang-Nan Formation siltstone, mudstone with basaltic- andesitic tuff interlayers, silicalite; Late Carboniferous to Permian fossils-rich limestone; Triassic marine-terrigenous detrital sequence with limestone layers, Jurassic–Cretaceous lacustrine sediments and Neogene coal-bearing lacustrine-limnetic sediments.
The Nam Hang Formation and the Muang-Nan Formation formed the Muang-Nan anticlinorium in the Xaignabouli area, no bottom exposed and conformable contact is clear between the two formations. The tuffaceous siltstones from the Upper Muang-Nan Formation contain brachiopods Crurithyris sp. and Neochonetes sp. Meanwhile, the fossil Profusulinella sp. is collected at the bottom of the Upper Carboniferous–Lower Permian Don Kaeo Formation limestone, which is one of the index fossils of the Late Carboniferous, where the limestone conformably overlies on the Muang-Nan Formation in Ban Hang of the Xaignabouli District. Accordingly, both Nam Hang Formation and Muang-Nan Formation are assigned to the Late Carboniferous (Fig. 2; Wu et al., 2017). However, Qian et al. (2016b) reported a zircon U-Pb age of 237.7 Ma of the basaltic- andesite from the riverside of the Mekong River near the Xaignabouli City. To strictly constrain the eruption age of the volcanic rocks and tectonic setting, two samples (JD03-N1, JD04-N1) from the Nam Hang Formation and one sample (JD21-N1) from the Muang-Nan Formation are collected for zircon U-Pb dating and Lu-Hf isotopic analyses respectively and relevant eight fresh samples for major elements, trace elements and REE analyses.
Sample JD03-N1 and sample JD04-N1 are taken from Nam Hang Formation on the western side of Mekong Bridge along the road from Xaignabouli to B.Muang-Nan (Fig. 1b; Figs. 3a, 3b). On this section, the outcrops are composed of the dark-grey olivine-phyric basalt, augite-phyric basalt, amygdaloidal basalt, basaltic-andesitic tuff and basaltic-andesite interlayers. JD03-N1 (19°25′23″N, 101°50′41″E) is the basaltic- andesitic tuff, mainly composed of breccia and tuffaceous matrix (Figs. 3a, 3d). The breccia content (ca. 20%–25%) includes angular andesite, basalt, and amygdaloidal basalt debris and clinopyroxene phenocrysts, with 2–5 mm in long axis, and the tuffaceous matrix consists of the phenocrysts of plagioclase and clinopyroxene, and the lithic debris (75%–80%) of andesite, basalts. Plagioclases are replaced by sericite and prehnite, clinopyxenes are partly replaced by chlorite, and some of them enclosed magnetite, zircon and apatite. JD04-N1 (19°25′33″N, 101°50′50″E) is basaltic-andesitic volcanic breccia (Figs. 3b, 3e), mainly composed of angular amygdaloidal andesitic- basaltic breccia (80%–85%) and tuff (15%–20%). The breccia mostly is 20–40 mm in long axis. Some calcite veins are developed along the lithic fissures.
Sample JD21-N1 (19°18′37″N, 101°49′41″E) is taken from the Muang-Nan Formation along the road from Xaignabouli to the Thai Hydropower Station (Fig. 1b), and here the conformable contact between the Nam Hang Formation and Muang-Nan Formation is clear (Fig. 3c). JD21-N1 is a cataclastic andesitic breccia-bearing tuff, outcrops as interlayers with sandy siltstone and siliceous siltstone. The lithology is mainly andesitic, amygdaloidal andesitic breccia (25%–30%, 2–5 mm in long axis) and plagioclase phenocrysts (approximately 3%–5%, 0.1–0.5 mm in long axis) and andesitic lithic- debris (65%–70%, 1–2 mm in long axis) (Fig. 3f), plagioclase is replaced by sericites and mineral lineation is partly developed of sericite.2 ANALYTICAL METHODS
Zircons were separated and cast in an epoxy mount, and polished to select the grains for analysis. Zircons were documented with transmitted and reflected light micrographs and cathodoluminescence (CL) images, using scanning electron microscope of FEI Quanta 400 FEG in the Hebei Institute of Regional Geological Survey, China. Zircon U-Pb dating was carried out by LA-ICP-MS at Beijing Geo Analysis Technology Co., Ltd. The laser beam spot diameter is 32 μm. Helium was used as the carrier gas, zircon GJ1 as an external standard, and the analysis method and instrument parameters were described by Hou et al. (2009). The isotope ratio of analyzed spots, individual U-Pb age, and U-Th-Pb contents were calculated using ICPMS DataCal (Liu et al., 2010). All analyzed zircon grains show the low signal of common 204Pb and high 206Pb/204Pb ratios, so common Pb correction was not necessary. The Isoplot/Ex_ver3 was used for the weighted mean age calculation and concordia diagrams making.
Zircon in situ Lu-Hf isotopes analysis was performed on an ESI NWR193 laser ablation microprobe instrument linked with a Neptune plus multi-collector ICP-MS at Beijing Geo Analysis Technology Co., Ltd. The analysis method was similar to that of Wu et al. (2006). The laser beam spot diameter is 40 μm. The isobaric interference correction was undertaken to calculate the 176Lu/177Hf and 176Hf/177Hf ratios using 176Lu/175Lu=0.026 58 and 176Yb/173Yb=0.796 218 (Chu et al, 2002). The reference standards for our normal analyses used zircon GJ1, which has a weighted mean 176Hf/177Hf ratio of 0.282 007±0.000 007 (2σ, n=36). In order to correct the instrumental quality deviation, the Yb isotope ratios were normalized to 172Yb/173Yb=1.352 74 and the Hf isotope ratios to 179Hf/177Hf=0.732 5 using an exponential law (Chu et al., 2002).
Eight samples were crushed below 200 mesh under non-pollution condition for major and trace elements analysis. Both major and trace elements analysis was completed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan. The main elements were analyzed by X-ray fluorescence spectrometry (XRF) and the trace elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The analysis accuracy of major elements is better than 5%, and analysis of trace elements is better than 8%.3 RESULTS 3.1 Zircon U-Pb Dating and Lu-Hf Isotopic Composition
CL images show that the zircon grains from the basaltic- andesitic tuff (JD03-N1) are euhedral, transparent and display igneous overgrowth (Fig. 4a). Twenty-seven analyses give Th content of 69–2 838 ppm and U content of 84–3 850 ppm, with Th/U=0.1–1.6. Eight analyses had a large error which wasn't calculated here. Twelve analyses yielded a weighted mean age of 235±2.6 Ma with MSWD=1.7 (Fig. 4b, Table 1). This age is interpreted as the crystallization age of the basaltic-andesitic tuff. One grain gives a relatively younger age (167 Ma), while the remaining six analyses give older 206Pb/238U ages of 264–797 Ma, which are interpreted as xenocrysts. A total of twenty-eight analyses were performed on twenty-eight zircon grains from sample JD04-N1 (basaltic-andesitic volcanic breccia). The CL images show that most zircons have concentric oscillatory zoning on a patchy zoned core, some display sector zoning (Fig. 4c). The dated grains give the Th and U contents of 147–685 ppm and 695–860 ppm respectively, with the Th/U ratio of 0.21–0.92. The 24 analyses gain a weighted mean age of 232±1.4 Ma with MSWD=0.54 (Fig. 4d, Table 1), which probably represents the volcano eruption time. Other 4 analyses have distinct older ages from 266 to 927 Ma, which are explained as capture zircons. A total of 14 dated spots of 28 zircon grains from JD04-N1 (basaltic-andesitic volcanic breccia) yield 176Yb/177Hf and 176Lu/177Hf ratios of 0.019 603– 0.032 234 and 0.000 844–0.001 371 respectively. In addition, the initial 176Hf/177Hf ratios range from 0.282 932 to 0.283 082, with εHf(t) values from +10.5 to +15.9 and single-stage depleted mantle Hf model ages (TDM) varying from 242 to 454 Ma (Fig. 4g; Table 2).
Twenty analyses were carried for sample JD21-N1 (andesitic breccia-bearing tuff), and the CL images show that there are many typical magmatic oscillatory zonings and no inherited cores (Fig. 4e). Six analyses show large error and lower concordance which have not been taken into calculating. The remaining fourteen analyses give the Th and U contents of 22 ppm–212 ppm and 54 ppm–419 ppm respectively, yielding Th/U values varying from 0.24 to 1.25. The concordant 14 data yielded a weighted mean age of 278±2.8 Ma (MSWD=1.3, Fig. 4f; Table 1), which is interpreted as the crystallization age of the andesitic breccia-bearing tuff. Ten grains were selected for Lu-Hf isotopic analysis and yield 176Yb/177Hf ratios ranging from 0.018 169 to 0.070 96, 176Lu/177Hf ratios from 0.000 859 to 0.002 929 and 176Hf/177Hf ratios from 0.282 856 to 0.282 982, with the fLu/Hf= -0.91 to -0.97. The εHf(t) values vary from +8.7 to +13.4 and the Hf-depleted model ages (TDM) from 388 to 570 Ma (Fig. 4g).3.2 Major and Trace Elements
Major and trace elements contents of 8 samples of volcanic rocks from the Nam Hang Formation and Muang-Nan Formation are listed in Table 3. The results of the analyzed samples give high LOI contents of 4.1–6.7, suggesting that there is some weathering-alteration. Therefore, major oxides normalization is needed (volatile-free), and the following diagrams and descriptions will use the recalculated data. Eight samples have normalized SiO2 contents ranging from 47.18 wt.% to 61.13 wt.%, with a wide range of MgO from 2.63 wt.% to 19.92 wt.% (Mg#=36–78) and Al2O3 from 8.5 wt.% to 20.77 wt.%, and characterized by low TiO2 from 0.44 wt.% to 1.03 wt.%. Considering the instability of the alkaline elements, the Zr/TiO2-Nb/Y (Fig. 5a; Winchester and Floyd, 1977) and Th-Co (Fig. 5b; Hastie et al., 2007) classification diagrams are selected for petrology classification, the samples are mainly defined as calc-alkaline or subalkaline andesite/basalt and basaltic andesite.
Volcanic rocks from the Nam Hang Formation and Muang-Nan Formation generally have similar REE patterns displaying LREE-enrichment, moderately fractionated HREE relative to LREE, with LREE/HREE=3.87–6.3, LaN/YbN= 3.38–8.28, δEu=0.9–1.1, without an Eu anomaly (Fig. 5c). In the primitive mantle-normalized spidergram (Fig. 5d), all the basalt and andesitic-basaltic tuff samples characterized by strong depletions in Nb, Ta, Ti and have high LILE/HFSE ratios, consistent with the arc volcanic rocks (Sun and McDonough, 1989).4 DISCUSSION 4.1 Age of Volcanic Rocks in the Xaignabouli Area
Our data show that the basaltic-andesitic tuff and volcanic breccia from the Nam Hang Formation in the Xaignabouli area have weighted mean ages of 235±2.6 and 232±1.4 Ma respectively, which is consistent with the basaltic-andesite age of 237.7±1.7 Ma (Qian et al., 2016b), marked as the Middle Triassic rather than the Permian–Early Triassic as mapped on the 1 : 1 000 000 geological map (DGM, 1990) or the Upper Carboniferous on the 1 : 200 000 geological map (Wu et al., 2017). Meanwhile, the andesitic tuff from the Muang-Nan Formation yielded a weighted mean age of 278±2.8 Ma, belonging to the Early Permian rather than the Upper Carboniferous as mapped on the 1 : 200 000 geological map (Wu et al., 2017). According to the latest 1 : 200 000 geological survey, no fossils were collected from the Nam Hang Formation, brachiopods fossils (Crurithyris sp. and Neochonetes sp.) were collected from the tuffaceous siltstones of the Muang-Nan Formation. Generally speaking, the fossils Crurithyris sp. appear in the Devonian to the Induan strata (Late Triassic), while the fossils Neochonetes sp. occur in the Carboniferous to Permian strata. So there lacks adequate evidence to define the eruption age of the volcanic rocks that are developed in the Xaignabouli area. We suggest that this set of volcanic rocks should be defined as a unique volcanic unit instead of stratigraphic layer.4.2 Petrogenesis and Tectonic Setting of Volcanic Rocks in the Xaignabouli Area
As described in the previous section, the geochemical samples have been altered to some degree. Hence, we are here just using the incompatible element ratios and Hf isotopic data to discuss the petrogenesis and tectonic setting of the volcanic rocks.
Volcanic rocks from the Nam Hang Formation and Muang-Nan Formation have Mg-numbers of 36–78, and Th content (0.94 ppm–2.01 ppm) is significantly higher than Ta content (0.06 ppm–0.3 ppm) with the Th/Ta ratio of 5–24 (much higher than that of the primitive mantle (1.6), indicative of the arc-like source (Taylor and McLeannan, 1985; Wang et al., 2001). All samples show variable Ba/La (2.63–69.29), Ba/Nb (20.44–170.06) and Ba/Th (14.88–510.35) ratios, and constant (La/Sm)N (1.58–2.15) (Figs. 6a, and 6b) (Su et al., 2012), which supports significant enrichment of slab-derived fluid in the source and negligible involvement of sediments.
Minor crustal contamination might produce negative Nb-Ta anomalies relative to the LILE and LREE but would result in positive Zr-Hf anomalies due to enrichment of these elements in crustal materials (Zhao et al., 2010). In the primitive mantle-normalized spidergram, all the samples display depletion in Nb-Ta, and the negative Zr-Hf anomalies displayed (Fig. 5d) indicate that little or no crustal contamination occurred. Fourteen zircon grains from the Nam Hang Formation basaltic-andesitic volcanic breccia and ten zircon grains from the Muang-Nan Formation andesitic tuff give a constant 176Hf/177Hf ratio of 0.282 932–0.283 082 and 0.282 856– 0.282 982, respectively, which means the Hf isotopic have a uniform distribution in the sampling zircons indicating a homogeneous magmatic origin. The positive and wide range εHf(t) values of 8.7–15.9 indicate that their source is mainly depleted mantle or partial melting of subducting oceanic slab (e.g., Pearce and Peate, 1995). Furthermore, all samples have relative higher Ba/Nb (20.44–170.06), La/Nb (2.13–7.97) and lower Nb/Th (0.73–3.43) ratios, supporting the magma from partial melting of subducting oceanic slab, rather than from depleted mantle (Li, 1994).
Considering the arc-like petrochemical characteristics of the volcanic rocks from the Xaignabouli area, we exclude the possibility of a back-arc basin or an oceanic ridge tectonic setting. In the Th/Yb-Nb/Yb plot (Fig. 7a), the samples fall into the field of oceanic island-arc or continental marginal arc setting. In the Hf/3-Th-Ta diagram (Fig. 7b), meanwhile, all the samples fall in the field of an island cal-alkali arc setting. Our samples have the Th/Hf ratio of 0.55–1.47, Ta/Hf ratio of 0.06–0.18, and the Th·Ta/Hf2 ratio of 0.06–0.18 which is higher than the primitive mantle (Th·Ta/Hf2=0.035) (Wang et al., 2001), so the magma might generate from the continental marginal arc setting.4.3 Tectonic Implications
Due to a low degree of large-scale geological survey and high-precision geochronology and geochemical data, the tectonic correlation and evolution are poorly defined in the northwestern Indochina Block, especially the extension of the Nan suture and the Loei belt (Wang et al., 2017; Qian et al., 2016b; Yang et al., 2016; Kamvong et al., 2014; Zaw et al., 2014; Metcalfe, 2013, 2011, 2006; Sone et al., 2012; Sone and Metcalfe, 2008; Hutchison, 1989). The Nan suture, also named the Nan-Uttaradit belt, characterized by a discontinuous and disrupted ophiolite sequences, has been considered as a remnant after the closure of the Paleotethyan Ocean (Yang et al., 2016, 2009; Hada et al., 1999; Metcalfe, 1999; Hutchison, 1989) or a back-arc basin (Metcalfe, 2013; Sone and Metcalfe, 2008; Ueno and Hisada, 2001) separating the Sukhothai arc from the Indochina Block which existed from the Carboniferous to the Late Triassic. There are two different opinions about the northern extension of the Nan suture, one supports that the Nan suture extends to the Jinghong suture zone (Song et al., 2018; Wang et al., 2017; Shi et al., 2015; Metcalfe, 2013, 2011; Barr and Charusiri, 2011; Sone and Metcalfe, 2008), and the other one supports that this suture through the Luang Prabang belt is linked to the Jinshajiang-Ailaoshan suture (Qian et al., 2016a; Yang et al., 2016; Feng et al., 2005). For its southern extension, it is generally accepted that it connected to the Sa Kaeo suture zone in Southeastern Thailand and to the Cambodia (Wang et al., 2017; Zaw et al., 2014; Sone and Metcalfe, 2008).
The Loei fold/volcanic belt, mainly develop the andesitic- rhyolitic volcanic rocks which abound in porphyry-related copper-gold and epithermal gold mineralization, have a wide age-spectrum with a main cluster of 305–350 and 216–252 Ma (Fig. 1; Qian et al., 2016a, b, 2015; Rossignol et al., 2016; Salam et al., 2014; Zaw et al., 2014; Blanchard et al., 2013; Zaw and Meffre, 2007; and our unpublished data). Qian et al. (2016a) got the zircon U-Pb ages of 336 and 305 Ma for diabase and basalt from Luang Prabang, according to the petrochemical characteristics and positive εNd(t) and εHf(t) values, they proposed that the Carboniferous magma generates from the continental back-arc basin setting, and the Luang Prabang belt extends north to the Ailaoshan suture zone and the south is linked to the Nan suture during the Late Paleozoic. However, during our latest geological survey in the Xaignabouli and M.kenthao districts, we found large-scale gabbro and chert in M.kenthao and M.Pakbeng areas just along the up-stream direction of Nan River, which need ongoing research. Spatially from Lincang in China, to Sukhothai and Chanthaburi in Thailand and the East Malysia volcanic belt display a tectonic affinity of the Permian–Early Triassic continental arc (Peng et al., 2013; Wang et al., 2010; Hennig et al., 2009), and the Permian– Triassic intermediate-mafic volcanic rocks in the Xaignabouli area are geochemically familiar with the volcanic rocks in the Loei and Phetchabun areas (Qian et al., 2016b). Based on the spatial relationship, it is more reasonable that the Nan-Uttaradit suture zone through the M.kenthao-M.Pakbeng in NW Laos is connected to the Jinghong suture in China, and the Xaignabouli- Luang Prabang volcanic belt is linked to the Loei belt. The Permian–Triassic volcanic rocks in the Xaignabouli-Luang Prabang belt might be a partial melting product of eastward subducting of the Nan back-arc basin.5 CONCLUSIONS
(1) The basaltic-andesitic tuff and volcanic breccia from the Nam Hang Formation in the Xaignabouli area present a Late Triassic origin (235±2.6 and 232±1.4 Ma, respectively), and the andesitic tuff from the Muang-Nan Formation formed in Early Permian (278±2.8 Ma), rather than the Upper Carboniferous as mapped on the 1: 200 000 geological map.
(2) The arc-like characteristics of the volcanic rocks from the Nam Hang Formation and Muang-Nan Formation, combined with the positive zircon εHf(t) values (8.7–15.9) indicate they might generate from partial melting of the subducting oceanic slab in continental margin arc setting.
(3) Synthesizing existing data, we suggest the Permian– Triassic Xaignabouli-Luang Prabang volcanic belt probably extends to the Loei belt in Thailand, in which the volcanic rocks might be a product of Nan back-arc basin eastward subduction.ACKNOWLEDGMENTS
We appreciate the helpful comments from two anonymous referees to improve an early version of this paper. This study was financially supported by the China Geological Survey (No. 121201010000150013). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0863-8.
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