Mei-feng Shi, Bo-i Xu, Zhen-o Wu, Shu-sheng Liu, Gerrit Pieter Goosen, Zhi-min Peng,Fei Nie, Hui-min Ling

a Chengdu Center, China Geological Survey, Ministry of Natural Resource, Chengdu 610081, China

b Department of Earth and Environmental Sciences, University of Waterloo, Waterloo N2L3G1, Canada

c Department of Geosciences, Nelson Mandela University, Port Elizabeth 6045, South Africa

Keywords:

Zircon U-Pb age

Geochronology

Petrogeochemistry

Tethyan evolution

Nan Suture

Sukhothai Arc

Geological survey engineering

Laos

A B S T R A C T

The Nan Suture and Sukhothai Arc Terrane are products of the eastward subduction of the Paleotethyan Ocean during the Late Carboniferous to Triassic. However, their footprints in northwestern Laos are poorly constrained. New geochronological and geochemical data presented in this study demonstrate a Late Permian origin for the andesitic rocks in the B.Xiengnou area rather than Late Triassic. The brecciabearing andesitic tuff in the B.On ultramafic complex yield a zircon U-Pb age of 260 ± 1.4 Ma,geochemically displaying a MORB-like signature. The andesitic tuff in the B.Kiophoulan-B.Houayhak belt gave the U-Pb age of 254 ± 1.3 Ma, with arc-like geochemical affinity. By combining geochronological and geochemical data from the Nan Suture and Sukhothai Arc Terrane, the authors suggest that the andesitic rocks in the B.On ultramafic complex formed in a back-arc basin background,which connected the Jinghong and Nan back-arc basin during the Permian; while the andesitic tuff in the B.Kiophoulan-B.Houayhak belt erupted in the Sukhothai continental arc setting.

1. Introduction

The Indochina Block that composes the main part of the Southeast Asia Peninsula, is separated from the South China Block by the Jinshajiang-Ailaoshan-Song Ma suture in the northeast and from the Sibumasu Block by the Changning-Menglian-Chiang Mai-Inthanon main Paleo-Tethyan suture in the west (Hutchison CS, 1975; Feng QL et al., 2005; Sone M and Metcalfe I, 2008; Peng TP et al., 2008; Hennig D et al.,2009; Metcalfe I, 2013; Khin Z et al., 2014; Shi MF et al.,2019; Wang YJ et al., 2018, 2020; Wang XY et al., 2018,2020). The tectonic nature and extension in different countries along the northwestern margin of the Indochina Block are poorly defined, especially the Nan Suture zone, which hinders the understanding of multi-terrane amalgamation processes in the interior of the Indochina Block.

The Nan Suture, or Nan-Uttaradit Suture, delineates the Sukhothai Permian-Triassic island arc system (or Sukhothai fold belt) with the western Indochina Terrane (Ueno K, 1999;Sone M and Metcalfe I, 2008; Wang YJ et al., 2018, 2020). It was deciphered as the remnants of the Paleotethyan Ocean(Hutchison CS, 1975; Bunopas S and Vella P, 1983; Barr SM and Macdonald AS, 1987; Hada S et al., 1999). Recent geochronological, petro-geochemical and paleontological evidence has demonstrated that the Nan Suture was a back-arc basin that existed during the Carboniferous to Middle Triassic(Ueno K, 1999; Sone M and Metcalfe I, 2008; Yang WQ et al., 2016; Hara H et al., 2017; Moonpa K and Montanated K,2018; Minezaki T et al., 2019; Wang YJ et al., 2020). To the south, it is generally accepted that the Nan Suture extends to the Uttaradit Suture and Sra Kaeo Suture in southeastern Thailand and to Pailin in western Cambodia (Udchachon M et al., 2018; Ito T et al., 2020; Hara H et al., 2020). However,there is controversy about its north extension in Laos and China, as well as the consumption time of the Nan back-arc basin. Sone M and Metcalfe I (2008), Hennig D et al. (2009)and Metcalfe I et al. (2017) postulate that the Nan Suture extends northerly into northwestern Laos and links to the Southern Lancangjiang belt (the Jinghong suture zone),constrained by the Permian radiolarian cherts and Permian-Triassic north-south extending volcanic rocks in China and Thailand. Qian X et al. (2016a) and Yang WQ et al. (2016)suggest that it extends to the Luang Prabang belt in Laos and connects with the Ailaoshan Paleo-Tethyan suture in southwest China, which supported by the synchronous Carboniferous mafic-ultramafic rocks (336-305 Ma) with MORB-like geochemical affinities. Wang YJ et al. (2020)further identified the Permian mafic rocks (294-265 Ma) from the Nan and Luang Prabang belts which exhibit both MORB-and arc-like geochemical signatures; and they proposed that the Nan Carboniferous-Permian back-arc basin northerly divided into two tracts, e.g., the Luang Prabang back-arc basin, and the Banpo-Nanlinshan (same as the Jinghong suture zone) back-arc basin.

One of the main obstacles to understanding the north extension and tectonic nature of the Nan Suture zone is the rare age and geochemical data in northwestern Laos.Benefiting from the geological mapping project in virgin areas in northwestern Laos, which cooperated between the China Geological Survey and the Department of Geology and Minerals of Lao PDR, this paper presents new geochronological and petro-geochemical data for volcaniclastic rocks from the B.Xiengnou area in northwestern Laos between the Jinghong and Nan suture zones (Fig. 1), to provide evidence for the north extension of the Nan Suture, and to link the magmatism to the evolution of the Paleotethyan ocean.

Fig. 1. Tectonic subdivision of Indochina block and distribution of the volcanic belts in NW Laos and eastern Thailand (modified from Wu ZB et al., 2017; Shi MF et al., 2019; Hara H et al., 2020). 1-Sukhothai Arc Terrane; 2-Indochina Block; 3-Ultramafic rocks; 4-Volcanic rocks;5-Strike-slip fault: RRF, Red River Fault; DBPF, Dien Bien Phu Fault; MPF, Mae Ping Fault; 6-Sutures:① Changning-Menglian-Chiang Mai-Inthanon suture, ② Jinshajiang-Ailaoshan-Song Ma suture, ③ Tam Ky suture; 7-Tarrane boundary; 8-Speculated Tarrane boundary.

2. Geological background and petrography

The study area locates in the B.Xiengnou sheet of the latest 1∶200000 geological maps, tectonically in the northwestern margin of the Indochina Block between the Jinghong suture and Nan suture (Fig. 1). The stratigraphy in this area is comprised of the Carboniferous-Permian association of low-grade metamorphic argillaceous siltstones(phyllite) and carbonate rocks, which includes the Upper Carboniferous B.Muang-Nan Formation, Upper Carboniferous-Lower Permian Don Kaeo Formation and Middle Permian Phanda Formation; unconformable overlaying Upper Permian basalts; the Middle-Upper Triassic shallow-marine to marine-terrigenous facies slates, sandstones and mudstone, including the Middle Triassic Pakhoung Formation, the Upper Triassic Ban Hang, Namtouan and Namliap formations; and the Jurassic red beds (Fig. 2).

Fig. 2. Simplified geological map of the B.Xiengnou area in NW Laos (modified from Wu ZB et al., 2017). F1-Khop fault, F2-B.Houaygneng fault, F3-B.Pakkhop fault, F4-B.On fault, F5-Pakpet-Nua fault, F6-Mokkalangkou fault.

The volcanic rocks are limited in the Middle Permian Phanda Formation, the Upper Permian basalts and the Late Triassic Namtouan Formation. The volcanic rock in the Phanda Formation is dominated by intermediate-mafic tuffacesous rocks, which interbedded with bioclastic limestone. The Upper Permian basalts outcrop along the Mekong River and are controlled by the NS-trending B.Pakkhop fault (F3). In the west of the B.Xiengnou, from the B.Kiophoulan to B.Houayhak zone, there developed a layer of volcanic conglomerate and andesitic tuff, which is defined as the bottom part of the Upper Triassic Namtouan Formation.No geochronological evidence has reported so far. Samples were collected from this layer for zircon U-Pb dating and geochemical analysis (Fig. 2).

Five ultrabasic intrusive rock bodies are identified in the B.Xiengnou area, e.g., Ph.Mokkalangkou, B.Kiophoulan,M.Pakbeng, B.On and Nam Pouy, along the Mokkalangkou Fault (F6), B.Pakkhop fault (F3) and B.On fault (F4) (Fig. 2).The biggest one, with a width of up to 800 m, the Ph.Mokkalangkou mafic-ultramafic complex, consists of pyroxenites and peridotites, which is highly deformed and sandwiched in the NW-SE faults. It was interpreted as the north extension of the Nan Suture (Wu ZB et al., 2017), but so far, no precise age data or isotopic data to support the interpretation. The authors tried to perform the U-Pb dating for the peridotites from the Ph.Mokkalangkou complex,however, few zircons were collected from the sample. The B.On ultramafic rock outcrops in the B.On village, northeast of B.Xiengnou. The outcrop only covers 0.5 km2and its contact relationship with the Late Triassic Namtouan Formation is unclear. The lithology is basically composed of strong altered serpentinites. A breccia-bearing andesitic tuff layer has been identified in the complex during our field investigation, with NE delineation of the breccia and cataclastic deformation. Four samples have been taken from this layer for zircon U-Pb dating and geochemical analysis.

Sample JD13-N1 (N19°35 ′35.6 ″, E100°41 ′2.12 ″) was collected from the volcanic breccia and tuff layer in the B.Kiophoulan-B.Houayhak belt (Figs. 3a, b), which displays unconformity contact with the underlying Upper Triassic Ban Hang Formation in the field and shows volcanic eruption rhythms. Sample JD13-N1 is andesitic tuff, consisting of tuffaceous and sedimentary materials on the microscopic images (Fig. 3e). The tuffaceous composition includes angular crystal and lithic fragments in random distribution, mostly with a grain size of 0.5-1 mm in the long axis. The crystal fragments (10-15 vol%) are mainly plagioclases and a few quartzes, and accessary minerals of magnetite, zircons and apatite. The lithic fragments (ca. 65 vol%) are andesite and a few rhyolites. The sedimentary composition consists of sandy debris and groundmass. The sandy debris (15-20 vol%)includes sub-rounded plagioclases, quartzes and lithic fragments (cherts, siltstone, andesite and rhyolite in composition), and spotted throughout the groundmass. The groundmass (5-10 vol%) is scale-like crypto-fine crystalline clay.

Fig. 3. Field and microscopy photographs of volcanic rocks in the B.Xiengnou area. a, b, e -andesitic tuff in the B.Kiophoulan-B.Houayhak zone; c, d and f -breccia-bearing andesitic tuff in the B.On village. Pl-plagioclase, Qtz-quartz.

Sample JD14-N1 (19°40 ′50.22 ″, 100°49 ′53.06 ″) is breccia-bearing andesitic tuff in the B.On ultramafic complex(Figs. 3c, d). It consists of volcanic breccia, tuffaceous fragments and a sedimentary matrix (Fig. 3f). The breccia(65-70 vol%) is mainly angular altered lithic andesite with random distribution, some 2-3 mm in grain size. The tuffaceous fragments (10 vol%-15 vol%) spotted in the sediments, and are generally composed of andesite and minor rhyolite. The sedimentary matrix (20 vol%-25 vol%) is subrounded andesitic debris, with a grain size of 1-2 mm. The accessary and secondary minerals, such as ilmenite, zircon,apatite, calcite, chlorite and opaque minerals, fill in the cataclastic fissures.

3. Analytical methods

Zircon grains were sorted using magnetic methods,mounted in epoxy resin and polished for analysis. The zircon’s internal texture was obtained by cathodoluminescence (CL)images using a scanning electron microscope of FEI Quanta 400 FEG at the Hebei Institute of Regional Geological Survey, China. Zircon U-Th-Pb composition analyses were conducted at the Beijing Geo-Analysis Technology Co., Ltd.Laser sampling was performed using an ESI NWR 193 nm laser ablation system. An AnlyitikJena PQMS Elite ICP-MS instrument was used to acquire ion-signal intensities. Zircon GJ1 was used as an external reference standard that was determined twice every five-sample spots. The analyzed spot diameter was 32 μm with helium as the carrier gas. Each analyses incorporated a background acquisition of approximately 15 s (gas blank) followed by 45 s data acquisition from the sample. Off-line raw data selection and integration of background and analyte signals, time-drift correction and quantitative calibration for U-Pb dating was conducted by ICPMS DataCal (Liu YS et al., 2010).Concordia diagrams were created and weighted-mean ages were calculated with Isoplot/Ex_ver3 (Ludwig KR, 2003).Individual analyses in table 1 and Concordia plots are shown with 1σ error, and uncertainties in ages are quoted at the 95% confidence level.

Table 1. LA-ICP-MS zircon U-Pb data of volcanic rocks in the B.Xiengnou area.

Table 1. (Continued)

Six samples were crushed and pulverized below a 200-mesh size in a steel mortar for whole rock major and trace elements analysis. Major oxide concentrations were determined using traditional X-ray fluorescence (XRF)methods, and trace elements were analyzed by inductively coupled plasma mass spectrometry (Aglient 7500a ICP-MS)at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan, China. The analytical precision of major elements is better than 5%. Analytical uncertainty is 5% for those with concentrations higher than 10×10-6and 8% for those with concentrations lower than 10×10-6.

4. Analytical results

4.1. Zircon U-Pb ages

Representative zircon CL images and dating results from two samples are displayed in Table 1 and Fig. 4. Numerous zircons were selected from sample JD13-N1. They are brown and euhedral, with clear oscillatory zoning, up to 50-200 µm long with length-to-width ratios of 2∶1 to 3∶1. No core-rim textures were observed. A total of 36 analyses were performed to give variable U (75×10-6-1020×10-6) and Th(60×10-6-1048×10-6) contents. The Th/U ratios range from 0.33-1.25, indicative of magmatic origin. Two analyses (spot No. 1 and 25) have low concordance and have been precluded in the calculations. Five analyses (spots No. 2, 13, 20, 21 and 34) gave significant older apparent206Pb/238U ages of 355-1719 Ma, which are interpreted as inherited ages. The remaining 29 analyses were calculated to have a weighted mean206Pb/238U age of 254 ± 1.3 Ma, with a mean standard weighted deviation (MSWD) of 0.62. This age is interpreted as the eruption time of the andesitic tuff in the B.Kiophoulan-B.Houayhak belt.

Fig. 4. Representative CL images (a, b) and LA-ICP-MS zircon U-Pb concordia diagrams (c, d) of volcanic rocks in the B.Xiengnou area.

Zircons from the breccia-bearing andesitic tuff (JD14-N1)are euhedral and commonly display oscillatory zoning, with a long axis of up to 50-100 μm and length-to-width ratios of 1∶1 to 2∶1. Forty-two analyses were determined, giving variable abundances of Th (40×10-6-1324×10-6) and U (72×10-6-1035×10-6), with Th/U ratios of 0.3-2.02. Thirteen spots of the zircon xenocrysts yielded significant older206Pb/238U age of 288-2829 Ma, and some of them have lower concordance, which were precluded in the calculation. Three spots (Nos. 16, 21 and 40) gave younger206Pb/238U ages but with large error or lower concordance. The remaining 26 analyses yielded a weighted mean206Pb/238U age of 260 ±1.4 Ma (MSWD=0.85). It is interpreted as the eruption age of the breccia-bearing andesitic tuff in the B.On ultramafic complex.

4.2. Petro-geochemical characteristics

Major and trace elements analytical results for three andesitic tuff samples from the B.Kiophoulan-B.Houayhak belt and three breccia-bearing andesitic tuff samples from the B.On ultramafic complex are shown in Table 2 (major oxides are volatile-free). The samples show high LOI (loss on ignition) values, 2.91-3.06 for andesitic tuff and 11.33-12.14 for breccia-bearing andesitic tuff. Thus, the high field strength elements (Nb, Ta, Zr, Hf, P, Ti, Y) and transitional elements(Ni, Cr, V, Sc) will be mainly used for the determination of their geochemical affinities.

The andesitic tuff in the B.Kiophoulan-B.Houayhak belt give normalized SiO2contents of 67.65%-67.89%, high Al2O3contents of 16.37%-16.54%, low TiO2(0.98%-1.01%)and MgO (1.43%-1.46%) with Mg-numbers (Mg#) of 30-31,showing sub-alkaline affinities (Na2O ＞ K2O). They are defined as the andesite or its equivalents in the Zr/TiO2vs.Nb/Y discrimination and calc-alkaline acidic series in the Thvs. Co discrimination (Fig. 5; Winchester JA and Floyd PA,1977; Hastie AR et al., 2007). These samples have low concentrations of Ni, Cr, V and Sc (Ni=6.53×10-6-6.81×10-6,Cr=22.2×10-6-24.5×10-6, V=120×10-6-123×10-6, Sc=18.2×10-6-18.7×10-6) respectively. The light rare earth elements(LREE) are moderately enriched relative to the heavy rare earth elements (HREE), with a (La/Yb)Nrange of 4.85-5.6,and no Eu anomalies (Eu/*Eu=1.01-1.03). All samples present positive Th-U and negative Nb-Ta anomalies on the primitive mantle-normalized trace element spidergrams (Fig. 6).

Table 2. Major, REE and trace elements composition of volcanic rocks in the B.Xiengnou area.

Fig. 5. Nb/Y vs. Zr/TiO2 (a) (Winchester JA and Floyd PA, 1977) and Th vs. Co classifications (b) (Hastie AR et al., 2007) of volcanic rocks in the B.Xiengnou area. CA-calc-alkaline; H-K-high-K calc-alcaline; SHO-shoshonite; IAT-island-arc tholeiite; B-basalt; BA/A-basaltic andesite and andesite; D/R*-dacite and rhyolite (* indicates that latites and trachytes also fall in the D/R fields).

Fig. 6. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element spidergrams (b) of volcanic rocks in the B.Xiengnou area. The OIB, E-MORB, N-MORB, normalized values for chondrite and primitive mantle are from Sun WD and McDonough WF(1989).

Three breccia-bearing andesitic tuff samples from the B.On ultramafic complex have normalized contents of SiO2(51.98%-53.33%), Al2O3(16.16%-17.02%), TiO2(0.77%-1.04%) and MgO (7.11%-7.64%) with Mg-numbers (Mg#) of 57-58, Na2O＞K2O, plotting into the andesitic or basaltic field in the Zr/TiO2vs. Nb/Y discrimination and calc-alkaline and island-arc tholeiitic series in the Th vs. Co discrimination(Fig. 5). They exhibit lower ΣREE contents (24×10-6-42×10-6)and relatively flat REE patterns, with a (La/Yb)Nof 0.62-1.17, and Ni, Cr, V and Sc contents of 79×10-6-88×10-6,301×10-6-375×10-6, 193×10-6-229×10-6and 38×10-6-41×10-6(Fig. 6). On the primitive mantle-normalized trace element spidergram, all the samples show negative anomalies of Th-U, Nb-Ta and positive anomalies of Sr.

5. Discussions

5.1. Age of volcanic rocks in the B.Xiengnou area

Our new zircon U-Pb dating results show that the crystallization ages of the andesitic rocks in the B.Xiengnou area are 254 ± 1.3 Ma and 260 ± 1.4 Ma, belonging to the Late Permian, rather than the Late Triassic as defined on the geological map at a scale of 1: 200000 (Wu ZB et al., 2017).According to the description in the geological survey reports,the Upper Permian basalts and ultramafic rock bodies are mainly controlled and sporadically distributed along the NS-or NE-trending faults. The studied andesitic tuff layer from the B.Kiophoulan -B.Houayhak belt displays unconformity or fault contact with the underlying Upper Triassic slates.Although the contact relationship between the B.On ultramafic complex and the mudstone of Upper Triassic Namtouan Formation is unclear because of the thick covering,it should be a tectonic (fault) contact based on the NE delineation of the breccia and cataclastic deformation of the breccia-bearing andesitic tuff layer in the B.On village. The tectonic contact suggested a thrust structure in depth. Both samples from the B.Kiophoulan-B.Houayhak belt and B.On ultramafic complex contain xenocrysts with apparent206Pb/238U age spectrums of 355-1719 Ma and 288-2829 Ma respectively (Table 1), which supports that the older continental basement existed before eruption of the andesitic magma.

5.2. Petrogenesis

The andesitic tuff in the B.Kiophoulan-B.Houayhak belt have high Al2O3and low TiO2, MgO, Ni, and Cr contents,with Mg-numbers of 30-31 and much higher Th/Ta ratios(11-11.4) than that (ca. 1.6) of the primitive mantle,suggesting an arc-like origin (Wang YJ et al., 2020). In addition, all the samples display consistent chondritenormalized REE patterns and primitive mantle normalized trace element spidergrams (Fig. 6, Sun WD and McDonough WF, 1989), e.g., slight enrichment in LREE relative to HREE,negative Nb-Ta anomalies and positive Th-U anomalies,indicative of insignificant crustal contamination. Three samples give lower Sm/Th (0.84-1.09) and higher Th/Y(0.16-0.21) ratios than the normal mid-ocean ridge basalts(N-MORB), indicative of an enriched mantle source (Su YP et al., 2012). The andesitic tuff sample contain xenocryst grains with ancient apparent ages, along with their high Al2O3contents (16.37%-16.54%), high Th/Ce (0.13-0.15) and Th/Nb (0.74-0.78) ratios, low Sr/Ce (8.16-9.45) and Ce/Pb(3.99-4.2) ratios, suggesting the probability of recycled sedimentary components. The La/Nb ratios range from 2.76 to 2.86 (＞1.4), V/Ti=47-48; three andesitic tuff samples fell into the calc-alkaline volcanic arc setting on the Ti/100 vs. Zrvs.Y×3 and TiO2vs. Zr diagrams (Fig. 7, Pearce JA and Cann JR, 1973; Winchester JA and Floyd PA, 1977). Those geochemical signatures synthetically suggest a continental arc environment for the generation of the andesitic tuff in the B.Kiophoulan-B.Houayhak belt.

Fig. 7. Ti/100 vs. Zr vs. Y×3 (a) (Pearce JA and Cann JR, 1973) and Zr vs. TiO2 (b) (Winchester JA and Floyd PA, 1977) tectonic discramination plots of volcanic rocks in the B.Xiengnou area. MORB-mid-ocean ridge basalt; VAB-volcanic-arc basalt; WPB-within-plate basalt.

Three breccia-bearing andesitic tuff in the B.On ultramafic complex have relatively higher Fe2O3(10.52%-11.13%), CaO (6.04%-6.5%), and MgO (7.11%-7.64%), with an Mg-number of 57-58, compared to the andesitic tuff in the B.Kiophoulan-B.Houayhak belt. These samples show poorly correlation between LOI and Zr, Th, REEs, Na2O, K2O, and positive correlation between Zr and Nb, Hf, La, Sm, Nd, and Y (no shown). Such signatures, along with the consistency of the primitive mantle normalized trace element spidergram(Fig. 6), indicate that no obvious low-temperature alteration occurred. The analyzed samples get much lower Th/Ce(0.01-0.08) and Th/La (0.03-0.21) than those of the continental crust (ca. 0.15 and 0.3 respectively), indicative of insignificant involvement of crustal materials in the parental magma. Their chondrite-normalized REE patterns resemble to E-MORB (enriched mid-oceanic ridge basalt) pattern (Fig. 6),slightly depleted in LREE concentrations relative to HREE concentrations (LaN/YbN=0.62-1.17), with no obvious Eu anomalies (Eu/*Eu=0.87-1.03). They show negative Th, Nb-Ta and positive Sr anomalies on the spidergrams (Fig. 6). The negative Nb-Ta anomalies did not align with the Zr-Hf positive anomalies, reflective of insignificant crustal assimilation. The positive Sr anomalies reveal that plagioclase occurred in the source. Three samples have La/Nb ratios of 2.67-3.29, V/Ti values of 21-25, and fall into the ocean floor or MORB field on the Ti/100 vs. Zr vs. Y×3 and TiO2vs. Zr discrimination diagrams (Fig. 7). In addition, zircons from the breccia-bearing andesitic tuff contain xenocrysts with apparent ages of 288-2 829 Ma, which demonstrates that the oceanic crust developed on a continental lithosphere rather than oceanic lithosphere. When combining those with the regional geological conditions, the andesitic tuff in the B.On ultramafic complex most likely formed in a back-arc basin environment.

5.3. Tectonic implication

Our study area locates within the tectonic line between the Simao-Indochina Block and Sukhothai Arc Terrane, which is delineated by the Jinghong suture in China and Nan suture in Thailand (Peng TP et al., 2008; Hennig D et al., 2009; Sone M and Metcalfe I, 2008; Yang WQ et al., 2016; Wang YJ et al., 2018, 2020). Available U-Pb ages for the Sukhothai Arc in northern Thailand show that volcanic rocks mainly formed in the Middle to Late Triassic (242-220 Ma)(Barr SM et al.,2000, 2006; Srichan W et al., 2009; Qian X et al., 2013,2016b, 2017), few record during the Permian (Hara H et al.,2017). Along the Jinghong suture, Hennig D et al. (2009)reported the Early Permian (292±1.4 Ma) basaltic andesite from the Nanlianshan volcano-plutonic complex, which is marked by MORB affinity and has been interpreted as oceanic sequences. Jian P et al. (2004, 2009) obtained the zircon SHRIMP U-Pb ages of 288-264 Ma from the Shuangjiang ophiolite and Banpo gabbro in south of Lincang, China. In northern Thailand, Yang WQ et al. (2016) and Wang YJ et al.(2020) also identified the Carboniferous-Permian (316-279 Ma)mafic-ultramafic rocks from the Nan ophiolitic complex,geochemically marked by both MORB- and arc-like signatures, and are interpreted as a back-arc basin product.Wang YJ et al. (2020) proposed that the Nan Carboniferous-Permian back-arc basin northerly divided into two tracts, e.g.,the Luang Prabang back-arc basin, and the Banpo-Nanlinshan(same as the Jinghong suture zone) back-arc basin. In northwestern Laos, there is no evidence to support the connection between the Jinghong and Nan sutures. Wu ZB et al. (2017) postulated their connection through the Ph.Mokkalangkou ultramafic complex in the Pakbeng district,but until now no precise geochronological data has been reported. Our U-Pb age and geochemical data from the B.On ultramafic complex have a consistent Late Permian oceanic tectonic setting with those of the mafic-ultramafic rocks from the Jinghong and Nan suture zones (Hennig D et al., 2009;Wang YJ et al., 2020). The Nan back-arc basin was in a pelagic environment from the Middle Permian to Middle Triassic, which is also supported by the radiolarian cherts(Saesaengseerung D et al., 2008; Sone M and Metcalfe I,2008; Ferrari OM et al., 2008; Yang WQ et al., 2009;Metcalfe I, 2013; Metcalfe I et al., 2017; Hara H et al., 2017).Therefore, by integrating the above evidence, the authors conclude that the Nan and Jinghong suture zones was probably a back-arc basin connected through the B.On-Ph.Mokkalangkou ultramafic rock belt in northwestern Laos during the Permian.

Both the Nan back-arc basin and the Sukhothai Arc are products of the supra-subduction of the main Paleotethyan oceanic lithosphere beneath the western margin of the Indochina Block. The Lincang igneous belt in China, the Chanthaburi igneous belt in Thailand and the East Malaysia igneous belt are considered as the north and the south extension of the Sukhothai arc (Barr SM and Macdonald AS,2006; Sone M and Metcalfe I, 2008; Hennig D et al., 2009;Wang YJ et al., 2010; Peng TP et al., 2013). Hara H et al.(2017) elucidated that the Sukhothai Arc started during the Late Carboniferous to Early Permian; was in an inactive state in the Middle Permian; and in an active condition from the Late Permian to Late Triassic. This is supported by the detrital zircon U-Pb dating and provenance analysis of the quartzose and lithic sandstones from the Sukhothai Arc in northern Thailand. The igneous rocks from the Lincang and Sukhothai belts have the age spectrum of 266-252 Ma, 249-238 Ma and 234-200 Ma (Table 3), interpreted as the arc, syn-collisional and post-collisional magmatism (Hennig D et al., 2009; Peng TP et al., 2013; Gardiner NG et al., 2016; Qian X et al.,2017). The Late Permian andesitic tuff presented in this study,in the B.Kiophoulan-B.Houayhak belt, geochemically identical to the calc-alkaline andesite, dacites and rhyolites in the Lincang and Sukhothai arc, which characterized by the enrichment in LILEs and LREEs and depletion in HFSEs,positive εNd(t) (+2.0 to +4.3) and zircon εHf(t) (+2.8 to +13.6)values, displays an arc-like affinity, and likely erupted in the Sukhothai continental arc.

Table 3. Zircon U-Pb data of ignous rocks in the Jinghong-Nan suture zones and the Lincang-Sukhothai-Chanthaburi terranes.

6. Conclusions

(i) Two andesitic tuff samples in the B.Xiengnou area yielded Late Permian zircon U-Pb ages, 254±1.3 Ma and 260±1.4 Ma respectively, rather than a Late Triassic origin as defined on the 1∶200000 geological map.

(ii) Breccia-bearing andesitic tuff in the B.On ultramafic complex show slight depletion in LREE concentration relative to HREE, negative Th, Nb-Ta and positive Sr anomalies,resembling to E-MORB affinity. However, the andesitic tuff in the B.Kiophoulan-B.Houayhak belt exhibits an arc-like origin, slightly enrichment in LREE compared to HREE,negative Nb-Ta anomalies and positive Th-U anomalies, and high La/Nb ratios.

(iii) Our data provide a clue to suggest the connection of the Jinghong and Nan suture zones, as a back-arc basin, in northwestern Laos through the B.On-Pakbeng ultramafic rock belt during the Permian.

(iv) The andesitic tuff from the B.Kiophoulan-B.Houayhak belt is probably a product of the Late Permian to Early Triassic volcanic activity in the Sukhothai continental arc.

CRediT authorship contribution statement

Mei-feng Shi, Zhen-bo Wu, Zhi-min Peng and Fei Nie conducted the field investigation. Mei-feng Shi conceived the experiments and wrote the manuscript under supervision of Bao-ci Xu. Gerrit Pieter Goosen helped the English polishing of the manuscript. Shu-sheng Liu worked as the project administration. Hui-min Liang helped CL image taking and formal analysis.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgement

This study is co-supported by the National Natural Science Foundation of China (41702087), the China Geological Survey (DD2019444) and the China Scholarship Council (201908575026). The authors appreciate the anonymous reviewers and editors for their constructive comments on this manuscript. The authors also want to express their gratitude to Sengkeo Sengkhamyong, Souksan Phomsylalai from the Department of Geology and Minerals of Laos for their help during the fieldwork.