J. Cent. South Univ. (2019) 26: 3502-3515

DOI: https://doi.org/10.1007/s11771-019-4269-2

Petrogenesis of Middle Triassic andesite in Sayaburi area, Laos: Constraints from whole-rock geochemistry, zircon U-Pb geochronology, and Sr-Nd isotopes

OUYANG Yuan(欧阳渊)^{1, 2}, LIU Hong(刘洪)², NIE Fei(聂飞)², CONG Feng(丛峰)²,ZHANG Jian-long(张建农)²,

ZHANG Jing-hua(张景华)², HUANG Han-xiao(黄瀚霄)²,LIU Shu-sheng(刘书生)², LEI Chuan-yang(雷传扬)³

1. College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China;

2. Chengdu Center, China Geological Survey, Chengdu 610081, China;

3. Sichuan Geological Survey, Chengdu 610081, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: Despite the presence of a large area of andesite in the Sayaburi Province of Laos, it has received very little attention. Based on a combination of detailed field investigations, geochronology and geochemical analysis, this study aims to explore the geochemical, Sr-Nd isotopic, and source rock characteristics, as well as the genesis and tectonic setting of the andesite in this region. In the Sayaburi Province, the andesite zircon U-Pb age is (241.2±1.2) Ma. The andesite rock is classified in the metaluminous-weak peraluminous calc-alkaline series. The light rare-earth elements (LREEs) are enriched and characterized by clear fractionation, whereas heavy rare-earth elements (HREEs) are relatively depleted and have no signs of fractionation. The average δEu is 0.96 with weak-or-no Eu anomalies. It is enriched in large ion lithophile elements such as Rb and K, while depleted in high field-strength elements such as Nb, Ta, P and Ti. For andesites in the Sayaburi Province, the (⁸⁷Rb/⁸⁶Sr)_t value ranges in 0.702849-0.704687, the εNd(t) value is between 3.53 and 4.77, the t_DM(t) value ranges in 633-835 Ma, and the t_DM2(t) ranges in 625–724 Ma. The results based on the synthesis of petrology, geochemistry, and regional tectonic background studies show that 1) the andesitic magma source in the study area is an enriched mantle, which is modified by subduction zone fluids; 2) the geotectonic background environment of the andesite in Sayaburi area is the continental island arc environment and related to the tectonic evolution of Jinghong–Nan–Uttaradit back-arc basin, which reflects that the magmatic source is enriched with a mantle wedge component modified by a subduction zone fluid (or melt).

Key words: Laos; Sayaburi area; zircon U-Pb geochronology; geochemistry

Cite this article as: OUYANG Yuan, LIU Hong, NIE Fei, CONG Feng, ZHANG Jian-long, ZHANG Jing-hua, HUANG Han-xiao, LIU Shu-sheng, LEI Chuan-yang. Petrogenesis of Middle Triassic andesite in the Sayaburi area, Laos: Constraints from whole-rock geochemistry, zircon U-Pb geochronology, and Sr-Nd isotopes [J]. Journal of Central South University, 2019, 26(12): 3502-3515. DOI: https://doi.org/10.1007/s11771-019-4269-2.

1 Introduction

The Sayaburi Province, which is in the northwestern part of Laos, is located in the eastern section of the Paleo-Tethys tectonic domain at the junction of the Sukhothai Terrane (SKT) and the Indochina Terrane (ICT). A large area of andesitic volcanic rocks is exposed in Sayaburi Province, Laos. These volcanic rocks from the Triassic period are in the Mojiang-Loei-Roviengcheung volcanic arc-zone in Simao Terrane (SMT), and previous studies show that this volcanic arc-zone was formed because of the subduction, collision, and orogenesis of the Jinghong-Nan-Uttaradit Suture Zone (JNS, see Figure 1) [1-11]. The abovementioned suture zone is a product of the Changning–Menglian- Chiangmai Paleo-Tethyan ocean closure. At 300–290 Ma, i.e., late Carboniferous-early Permian, owing to the eastward subduction of the Changning-Menglian-Chiangmai Paleo-Tethyan ocean, a post-arc crack occurred in the Jinghong- Nan-Uttaradit area; the Sukhothai Terrane was separated from the Indosinian block; and finally, the Jinghong-Nan-Uttaradit back-arc basin was formed (early Permian) [12-16]. In the early Permian, the Jinghong-Nan-Uttaradit back-arc basin started to subduct eastward [17-19], and until the middle Triassic, the ocean basin closed and the Sukhothai Terrane collided with the Indochina block, forming the late Triassic syn-collision S-type calc-alkaline granites and the post-collision and high-potassium calcified alkaline volcanic rocks [20].

Previous studies on the East Tethys have focused on the Sanjiang area in southwestern China, Thailand, Malaysia and Vietnam. Owing to the lack of geological and geochemical data support from Northwest Laos, division of the Tethys tectonic domain in Southeast Asia is not uniform. Especially, it is unclear whether the Jinghong-Nan-Uttaradit (South Lancang River) Paleo-Tethys ocean basin exists in Northwest Laos. The geochronology and petrogeochemical characteristics are important bases for determining the tectonic setting of volcanic rocks [21]. However, this region has received scarce attention; thus, there is lack of high-quality geochronology and geochemical data, leading to unanswered questions regarding the petrogenesis and geodynamic background of this region. The geochronology and geochemical characteristics of the andesite may reflect, to some extent, the genesis and geodynamic background of the andesitic magma source. This study aims to discuss the geochemical, Sr–Nd isotopic, and source-rock characteristics as well as the genesis and tectonic setting of the andesite in the Sayaburi Province, Laos, using field investigations combined with high-precision data analysis and it provides a basis for elucidating the evolutionary history of the ancient Tethys ocean in the Sayaburi region of Northwestern Laos.

2 Geological settings

The study area is in central and southern of the Simao Terrane and is under the administration of the Sayaburi Province, Laos, which is a country in the Indochina Peninsula, near Thailand. The SMT, which is in the western margin of Indochina Block,runs in a S-N direction, borders China to the north and Thailand to the south, and is parallel to the Jiangshajiang–Ailaoshan–Srepok suture zone (JAS) to the east and the JNS to the west [12]. The strata exposed throughout the zone, from the youngest to the oldest, is primarily the Middle Permian (which contains deposits of coal-bearing terrigenous clastic rock with volcanic rock) and the Devonian and Carboniferous systems (which contain a series of metamorphic rocks). Andesites are characterized by developed magmatic activities, wherein the shape of the rock mass is closely related to the zone’s structure and is generally distributed in a NE-SW direction. The volcanic rocks are primarily Late Carboniferous-Middle Triassic in age and are an island-type calc-alkaline volcanic rock series from basalt and andesite to rhyolite, and the intrusive rocks are characterized as island arc I-type granite in the form of stocks, batholiths, and dykes. These intrusions are in contact with the Permian-Triassic strata. Tectonic activity is relatively intense and characterized by the development of fault structures with no fold structures and small locally developed folds. The large main fault runs in a NE–SW direction, with smaller secondary faults in the E–W and S–N directions. Three groups of faults control the direction of the regional strata as well as the form and distribution of the rock mass, which provides the channel and space for the ascending magma and its emplacement.

Figure 1 Tectonic maps of Laos-Vietnam (modified after [4-9]) (a) and Ainyabuli(modified after [4])(b)(1-Quaternary; 2-Lower Jurassic A group; 3-Upper Triassic C group; 4-Upper Triassic B group; 5-Middle Triassic A group; 6-Middle Permian A group; 7-Upper Carboniferous V group; 8-Angle unconformity; 9-Parallel unconformity; 10-Fault; 11-Shear zone; 12-Samples; SCB-Southern China block; JAL-Jinshajiang-Ailaoshan-Laocai suture zone; SMS-Song Ma suture zone; ICT-Indochina Terrane; ADL-Ailaoshan-Dian Bien Phu-Loei suture zone; SMT-Simao Terrane; JNS: Jinghong-Nan-Uttaradit suture zone; SKT-Sukhothai Terrane)

Twelve samples are taken from middle triassic volcanic strata on the western of Sayaburi Area (Figure 1(b)). The composition of volcanic rocks is very complex in this strata. They include basalt, basalt andesite, basalt trachyte andesite, andesite, trachyte andesite, trachyte, rhyolite, etc. The samples in this paper were selected from fresh basalt trachyte andesite, andesite, and trachyte andesite. The samples are claret-colored, have a porphyritic and massive texture, and have a phenocryst content of approximately 30%–40%. They mainly comprise plagioclase and amphibole and contain small amounts of pyroxene, biotite, and quartz. The plagioclase has a long idiomorphic- subhedral columnar shape, a platy structure, and is characterized by local chloritization and sericitization. The plagioclase has the largest grain size (i.e., 0.1-1 mm, predominantly 0.5-0.7 mm). The amphibole has a long columnar shape and granular structure with a grain size of 0.3-0.5 mm. It has a jade-green color under plane-polarized light, clear pleochroism, and dark edges. The matrix has a hyalopilitic texture and mainly comprises microcrystalline plagioclase and andesitic glass (Figure 2), with auxiliary minerals such as apatite and magnetite.

Figure 2 Petrographic Characteristics of Andesite in Aainyabuli(Qz-Quartz; Pl-Plagioclase; Px-Pyroxene; Hb-Hornblende; Am-Amphibole; Bi-Biotite)

3 Samples and methods

In this study, samples were collected from the andesitic volcanic rock area in the northeastern region of the Sayaburi Province (Figure 1(b)), where there are several large quarries that are characterized by fresh bedrock outcrops. All the twelve samples collected were analyzed for their major and trace element concentrations. Of the twelve samples, two of them were selected for U-Pb geochronology, and 8 were selected for Sr-Nd isotope analysis.

Major and trace element analyses were conducted using X-ray fluorescence (XRF) and LA-ICP-MS at the Key Laboratory of Crust-Mantle Materials and Settings, University of Science and Technology of China, Chinese Academy of Sciences. We removed weathered surfaces and then chipped and powdered fresh surfaces and rock portions to an approximate 200 mesh size using a tungsten carbide ball mill. Major oxides were determined using XRF (SHIMADZU XRF-1800), with an analytical precision and accuracy that were generally better than 5%. Trace elements were analyzed by ICP-MS (Thermo Scientific X Series 2). The analytical uncertainties for trace element measurements were 1%-5% for abundances >10^-6 and 5%-10% for abundances <10^-6. The United States geological survey standard, GSP-2, and Chinese national standards, GSR-1 and GSR-2, were used to calibrate the elemental concentrations. Zircon crystals were handpicked from crushed whole-rock samples using techniques such as heavy liquid, magnet, and handpicking under a binocular microscope and were then mounted on epoxy resin disks.

We performed zircon cathodoluminescence (CL) imaging at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China, using an analytical scanning electron microscope (JSM-IT100) connected to a GATAN MINICL system. Analysis conditions included a 10.0-13.0 kV electric field voltage and an 80-85 μA current on the tungsten filament. We simultaneously performed the U-Pb dating and trace element analysis on zircon crystals with LA- ICP-MS at the Key Laboratory of Crust-Mantle Materials and Settings, University of Science and Technology of China, Chinese Academy of Sciences. A GeolasPro laser ablation system that consists of a COMPexPro 102 ArF excimer laser (193 nm wavelength and a 200 mJ maximum energy level) and a MicroLas optical system was used for laser sampling. An Agilent 7700e ICP-MS instrument was used to acquire ion-signal intensities. A T-connector was used to mix the carrier gas (He) and the make-up gas (Ar) before entering the ICP. The laser ablation system was equipped with a “wire” signal smoothing device. In this work, we used a spot size and laser frequency of 32 μm and 6 Hz, respectively. Zircon 91500 and glass NIST610 were used as external standards for U-Pb dating and trace element calibration, respectively. Each analysis involved a background acquisition of approximately 20-30 s followed by 50 s of data acquisition for each sample. ICPMSDataCal, an Excel-based software, was used to perform offline data selection, the integration of background and analyzed signals, time-drift corrections, and quantitative calibration for trace element analysis and U-Pb dating. Concordia diagrams and weighted mean calculations were made using the Isoplot/Ex_ver 4.15 software. The analysis process can be referred to Ref. [22].

The Sr-Nd isotopic ratios were analyzed using a thermal ionization mass spectrometer (ISOPROBE-T) at the Key Laboratory of Crust- Mantle Materials and Settings, University of Science and Technology of China, Chinese Academy of Sciences. Samples were dissolved in HF+HClO₄. Measured ⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios were normalized to ⁴³Nd/¹⁴⁴Nd=0.7219 and ⁸⁷Sr/⁸⁶Sr=0.1194, respectively, following exponential law corrections.

4 Results

4.1 Whole-rock major and trace element compositions

Table 1 lists the whole-rock geochemical data from the Middle Triassic andesite in the Sayaburi area. The andesite is characterized by low SiO₂ (58.74 wt%-64.32 wt%), high K₂O+Na₂O (6.09-8.45 wt%), high Al₂O₃ (15.96 wt%-19.19 wt%), MgO (0.97 wt%-3.36 wt%), Mg# (23-51), w(Na₂O)/w(K₂O) (1.49-3.28), and a Rittmann index (σ₄₃) of 2.16-3.76. The A/CNK values (the molar ratio of Al₂O₃/[CaO+K₂O+Na₂O]) range from 0.77 to 1.46, with an average of 1.08, which indicates that these rocks belong to the metaluminous-weakly peraluminous calc-alkaline series (Figures 3(a) and (b)) [23-25]. The chondrite-normalized REE patterns are characterized by significant light-REE (LREE) enrichment (La_N/Yb_N=6.19-11.1) and relatively flat heavy-REE (HREE) patterns, with no clear Eu anomalies (δEu=0.890-1.04) (Figure 3(d)) [26]. The primitive mantle-normalized trace-element spider diagrams (Figure 3(e)) [27] clearly indicate that these rocks are relatively enriched in the large ion lithophile elements (LILEs) such as Rb, K and Ce and depleted of the high field strength elements (HFSEs) such as Nb, Ta, P and Ti.

Table 1 Major and trace element contents of andesites in Sayaboury Province, Laos

Continued

4.2 Zircon U-Pb ages

In this study, we performed U-Pb dating on zircon from two andesite samples using LA-ICP-MS techniques (see results in Table 2). Zircon crystals show signs of characteristic magmatic zircons, especially based on their Th/U ratios of 0.23-1.52, with a mean of 0.79. A total of 44 zircon ablation spots were measured on the two andesite samples. Sample D017N1 had 17 ablation spots, excluding a small portion of an ablation spot where the ²⁰⁶Pb/²³⁸U age was older, which was possibly related to ablation spot disharmony due to broken zircons or the loss of common lead caused by mineral-bearing inclusions in the zircon. In addition, the age obtained from 6 ablation spots, e.g., D017N1-001, were older (>307±4 Ma) or younger (<223±3 Ma) than other values, which was possibly related to zircons in the early magma that were captured during magmatic processes or the zircons that formed during late magmatic stages. These results indicate the occurrence of multistage magmatism. Ages from the ²⁰⁶Pb/²³⁸U ratios for the 9 remaining ablation spots are identical, i.e., (241±3) to (245±5) Ma, with a weighted mean of (242.2±1.9) Ma (MSWD=0.11) (Figure 3(a)). For sample D017N2, we measured 27 ablation spots whose ²⁰⁶Pb/²³⁸U ages are identical, i.e., (239±6) to (244±3) Ma, with a weighted mean of (241.2±1.2) Ma (MSWD=0.13)(Figure 3(b)). ²⁰⁶Pb/²³⁸U ages for the two samples are nearly identical, which may represent the andesite’s age of formation.

4.3 Sr-Nd isotopic compositions

Table 3 lists the Rb-Sr and Sm-Nd isotopic compositions. The ⁸⁷Rb/⁸⁶Sr ratios range from 0.198321 to 0.909110, the ⁸⁷Sr/⁸⁶Sr ratios range from 0.7106 to 0.7180, the ¹⁴⁷Sm/¹⁴⁴Nd ratios range from 0.119212 to 0.127403, and the ¹⁴³Nd/¹⁴⁴Nd ratios range from 0.512723 to 0.512762. Analytical results show that these mineral grains have homogenous Rb-Sr and Sm-Nd isotopic compositions. The (⁸⁷Sr/⁸⁶Sr)_t ratios range from 0.702849 to 0.704034, (¹⁴³Nd/¹⁴⁴Nd)_t ratios range from 0.512511 to 0.512574, ε_Nd(t) ranges from 3.53 to 4.77, and t_DM2(t) ranges from 625 to 724 Ma.

Figure 3 TAS plot of Andesite in Aainyabuli (basemap modified after Ref. [23]) (a), K₂O vs SiO₂ plot of Andesite in Aainyabuli (basemap modified after Refs. [24, 25]) (b), chondrite-normalised REE patterns(The normalizing values for REE are from Ref. [26]) (c) and mantle-normalised multi-element diagrams (d) (normalizing values for trace elements are from Ref. [27])

Table 2 Zircons U-Pb dating of andesites in Sayaboury Province, Laos

Continued

Table 3 Sr and Nd isotopeeom position of andesites in Sayaboury province, Laos

Continued

Figure 4 Concordia diagrams of zircon in andesite

5 Discussion

5.1 Magmatic source

Andesite is rich in incompatible elements (e.g., Rb, Th, U, K) as well as in LREEs and depleted in HFSEs, namely, Nb, Ta, P, and Ti and TiO₂. It has high Al₂O₃ content (greater than 15%), low MgO content (less than 5%), and a FeO^T/MgO ratio of 3.07-5.92 (all greater than 1.5). These geochemical characteristics are similar to those of typical arc-volcanic rocks. In addition, the Mg# of the rocks in the study is less than 45 (except for the sample P16B), and the rocks contain plagioclase, amphibole, pyroxene and biotite. The major element compositions indicate that the rocks belong to the calc-alkaline series. Therefore, the andesite in this study is an important component of the orogenic belt (such as the volcanic island arc and the continental arc).

Considering the intermediate SiO₂ contents and the poor transition metal (Mg, Ni, Cr, etc.) abundances of the rock samples, these andesites cannot be issued from a pure crustal- or mantle- derived magma without subsequent differentiation and hybridization processes. The La/Sm ratios vary slightly (3.9-55.64), and the La/Nb ratio and La/Sm ratios show clear positive correlations (Figure 5(a)). Furthermore, the (⁸⁷Sr/⁸⁶Sr)t values were found to be correlated to the SiO₂ content (Figure 5(b)).

Figure 5 Crustal contamination plots for andesites in Sayaboury Province, Laos

These characteristics indicate the occurrence of crustal contamination in magma evolution processes.

The (⁸⁷Sr/⁸⁶Sr)t value is lower (mean=0.70378) and the ε_Nd(t) is positive (mean=4.14). In the (⁸⁷Sr/⁸⁶Sr)_t-ε_Nd(t) diagram (Figure 6), samples are on the mantle-derived magma evolution curve, and they exhibit contamination with lower crust material. These geochemical and isotopic characteristics indicate that the andesite has a mantle source provenance [29]. In addition, the geochemical characteristics such as low SiO₂ content and median Mg# values indicate that andesite provenance has the trace of a mantle source [30].

All samples show variable Ba/Th vs (La/Sm)_N, Ba/Nb vs Ba/La, and Th/Yb vs Ba/La ratios, suggesting significant enrichment of slab-derived fluid in the source and negligible involvement of sediments (Figures 7(a)-(d)). In Figure 7(c), the Zr/Yb and Nb/Yb show that the andesite occurs in the enriched mantle area. The andesite has high LREE/HREE and LILE/HFSE ratios, which reflects that the magmatic source is enriched with a mantle wedge component that is modified by a subduction zone fluid (or melt).

Figure 6 εNd(t) vs (⁸⁷Sr /⁸⁶Sr)_t plots for andesites in Sayaboury Province, Laos (basemap modified after Ref. [28]) (DM-Depleted mantle; LC-Lower crust; UC-Upper crust)

To summarize, the magmatic source of the middle triassic andesite magmas in Sayaburi area is an enriched mantle source modified by a subduction zone fluid.

5.2 Petrogenesis and tectonic setting

As mentioned above, the Middle Triassic andesite magmas in Sayaburi area were formed in volcanic island arc environment of orogenic belt, and the magmatic source is an enriched mantle source modified by a subduction zone fluid. In addition, as shown in Figure 7(a), the Ti vs Zr for all andesite samples are plotted in the volcanic arc area, and as shown in Figure 7(b), the Zr/Al₂O₃ vs TiO₂/Al₂O₃ data for most of the andesite samples are plotted in the Continental Arc + Post-collision Arc area. In Figures 7(c) and (d), Ce/P₂O₅ vs Zr/TiO₂ and La/Y vs Th data for the andesite samples are plotted in the ocean Continental Arc area. These characteristics indicate that the andesite formed in continental arc tectonic setting.

Owing to the lack of large-scale geological surveys and high-precision geochronology and geochemical data, the tectonic correlation and evolution of the northwestern Indochina Block are defined poorly. The Phanerozoic record of Southeast Asia preserves the history of opening and subsequent consumption of the Paleotethyan Ocean and the assembly of continental fragments into Asia as part of the broader-scale reconstruction of Pangea. However, uncertainty remains as to which of the many suture zones in Southeast Asia represents the relict of the main ocean, when final ocean closure occurred, and the assembly history of the Southeast Asia continental fragments. NW Thailand, NW Laos and SW Yunnan (SW China) are key areas for investigating and understanding Paleotethyan tectonic evolution due to the abundant preservations of the Late Paleozoic–Mesozoic igneous rocks, marine sediments and associated rocks.

Figure 7 Ba/Th vs (La/Sm)_N (a), Ba/Nb vs Ba/La (b), Zr/Yb vs Nb/Yb(c) and Th/Yb vs Ba/La (d) plots for andesites in Sayaboury Province, Laos

The latest information shows that Lancang Group is an early Paleozoic tectonic accretion complex, which contains Paleozoic island arc metamorphic volcanic rocks. It is considered to be the product of subduction of the original Tethyan ocean basin; the early Ordovician-Middle Triassic ophiolite developed in Tongchangjie, Manxin[31]; and the continental turbidite and oceanic sedimentary siliceous rocks are widely exposed in Wendong-Raub area. Rocks and serpentinized mafic-ultramafic intrusions, in which Paleozoic volcanic rocks in the western part sporadically develop paleo-Tethyan oceanic materials such as limestone and siliceous blocks without fixed forms [32]. The Late Carboniferous-Early Permian glacial- sea mixtures have not been found in the Hengduan Mountains-Indosinian orogenic system east of the Changning-Menglian-Chiangmai suture zone, and the Late Paleozoic and Mesozoic organisms have obvious South China-friendly characteristics [33]. Previous data show that the Changning-Menglian- Chiangmai Ocean inherited from the remnant ocean of the original Tethys and began to expand in the Early-Middle Devonian; in the Early Ordovician (471 Ma) or earlier, the Changning-Menglian- Chiangmai Ocean began to subduct eastward, forming the Sukhothai Terrane, the Jinghong-Nan- Uttaradit Suture Zone, the Simao Terrane, the Ailaoshan-Dian Bien Phu-Loei Suture Zone, the south Indochina Terrane, Song Ma Suture Zone, the Jinshajiang-Ailaoshan-Laocai Suture Zone, and South Indian branch from west to east [34]. A series of magmatic arc back-arc basins and continental debris [34] and continental debris (Mid-Triassic), the Changning-Menglian-Chiangmai Ocean closed, the Sibumasu-Simao/Indosinian block collided, the Triassic, Jurassic and Cretaceous marine- continental interfacies or continental strata unconformity above the underlying strata [35]. Subsequent collision events and post-collision events occurred at 237-230 Ma and 230-200 Ma, respectively. These evidences show that the Changning-Menglian-Chiangmai suture zone can be connected with the Longmuco- Shuanghu suture zone in central Tibet, representing the remains of the original-ancient Tethys ocean after its final extinction. This conclusion is also supported by palaeogeographic restoration data [5, 36-39].

The Mojiang-Luowenzhen volcanic arc belt develops a series of calc-alkaline volcanic rocks of island arc type from basic, neutral to acidic Late Carboniferous to Middle Triassic, and correspondingly develops intrusive rocks such as granite, granite diorite and diorite. PHAJUY et al [40] considered that the formation of island arc calc-alkaline volcanic rocks in the Late Carboniferous-Middle Triassic was controlled by the subduction of Changning-Menglian-Chiangmai, The Sayaburi andesite studied in this paper is located in the Mojiang-Luowenzhen volcanic arc belt and formed in the Middle Triassic (242 Ma), and the tectonic setting of Sayaburi andesite is continental island arc.

According to the results of this study, andesite was formed in the Middle Triassic (i.e. 242 Ma). In the Middle Triassic, the Jinghong-Nan-Uttaradit oceanic crust subducted to the Indosinian block. We believe that the geotectonic background environment of the andesite in the Sayaburi area is continental island arc, and this environment is related to the tectonic evolution of the Jinghong- Nan-Uttaradit back-arc basin.

6 Conclusions

1) The andesite in the Sayaburi Province of Laos formed in the Middle Triassic ((241.2±1.2) Ma).

2) The geotectonic background environment of the andesite in Sayaburi area is a continental island arc, and it is related to the tectonic evolution of the Jinghong-Nan-Uttaradit back-arc basin.

Acknowledgements

We are very grateful to Dr. CAO Hua-wen from Chengdu Center, China Geological Survey, and Dr. YU Maio from Central South University, China, for their great help in improving the manuscript.

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(Edited by FANG Jing-hua)

中文导读

老挝沙耶武里省中三叠世安山岩的成因与构造环境：地球化学、锆石U-Pb年代学和Sr-Nd同位素的约束

摘要：老挝沙耶武里省分布有大面积安山岩，该地区地质工作程度极低。本文拟在对该地区安山岩详细野外调查的基础上结合年代学、地球化学数据分析，探讨该套安山岩的地球化学特征、Sr-Nd同位素特征、源区特征、岩石成因及构造环境。老挝沙耶武里省安山岩锆石U-Pb年龄为(241.2±1.2) Ma。岩石属准铝质-弱过铝质钙碱性系列，LREE富集，分馏明显；而HREE相对亏损，分馏不明显，δEu平均值为0.96，具有弱的Eu异常或无Eu异常，相对富集Rb、K等大离子亲石元素，而Nb、Ta、P、Ti等高场强元素呈现低谷。安山岩的(⁸⁷Rb/⁸⁶Sr)t值介于0.702849～0.704687，ε_Nd(t)值介于3.533358～4.774175，t_DM(t)值介于633～835 Ma，t_DM2(t)值介于625～724 Ma。综合岩石学、地球化学和区域构造背景，研究区安山岩的岩浆源区为遭受了俯冲带流体改造的富集地幔源区，形成的大地构造背景环境为大陆岛弧环境，且与景洪-难河-程逸弧后洋盆构造演化相关。

关键词：老挝；安山岩；锆石U-Pb年代学；地球化学；构造环境

Foundation item: Projects(DD20160107, DD20150742) supported by the China Geological Survey; Project supported by the International Scientific Plan of the Qinghai-Xizang (Tibet) Plateau of Chengdu Center, China Geological Survey

Received date: 2019-01-29; Accepted date: 2019-04-03

Corresponding author: LIU Hong, Master, Engineer; Tel: +86-15397630283; E-mail: liuh@mail.cgs.gov.cn; ORCID: 0000-0002-2167- 9396