Introduction

Early Paleozoic igneous rocks, ranging in age from 525 to 460 Ma, exposed within the Himalayan Orogen Belt in NW India, Nepal, South Tibet and SW China (Bhanot et al. 1979; DeCelles et al. 1998, 2000, 2004; Godin et al. 2001; Xu et al. 2005; Liu et al. 2006; Gehrels et al. 2006; Cawood et al. 2007; Chen et al. 2007; Song et al. 2007; Zhang et al. 2008; Dong et al. 2009; Liu et al. 2009; Qi et al. 2010; Shi et al. 2010; Wang et al. 2011, 2012, 2013b; Li et al. 2012; Zhu et al. 2012a, b; Xing et al. 2015). In addition, Cambrian and Ordovician sequences are separated by an angular unconformity in NW India, Nepal and South Tibet (Brookfield 1993; Valdiya 1995; Le Fort et al. 1994; Hughes 2002; Liu et al. 2002; Zhou et al. 2004; Myrow et al. 2006a, b). In the early Paleozoic, these regions constituted part of the northern margin of Gondwana facing the proto-Tethys ocean. However, the detailed geological evolution of these regions during this time has poorly been established due to the hinder of late Paleozoic disruption and Cenozoic tectonothermal reworking associated with Tethys closure and Himalayan orogenic formation, respectively (e.g., Dewey et al. 1988; Metcalfe 1996, 2002, 2006, 2011, 2013; Yin and Harrison 2000; Yi et al. 2011; Pan et al. 2012; Cawood et al. 2013). Two end-member tectonic models have been proposed for the early Paleozoic orogenic event involving: (a) Pan-African orogeny associated with either the breakup of an earlier supercontinent or the final assembly of Gondwana (Murphy and Nance 1991; Miller et al. 2001; Xu et al. 2005; Yang et al. 2012); and (b) Andean-type orogeny following Gondwana assembly, caused by subduction of the proto-Tethyan Ocean beneath the Indian Craton and its adjacent micro-continental blocks (Cawood et al. 2007; Zhang et al. 2008, 2012b; Dong et al. 2010; Wang et al. 2011, 2012, 2013b; Zhu et al. 2012a).

The Sanjiang (also named Nujiang-Lancangjiang-Jinshajiang in Chinese literature) area in SW Yunnan (SW China) is an important area of the eastern Tethyan tectonic belt and preserved numerous geological relicts, which is associated with the final closure of the proto-Tethys ocean (e.g., Zhang et al. 1985; Cong et al. 1993; Zhong 1998; Wang et al. 2010, 2012; Liu et al. 2009). Early Paleozoic granitic rocks, gneiss and amphibolite with age around 499–462 Ma are recently identified within the Gongshan, Tengchong-Baoshan and Shan-Thai blocks in SW Yunnan (Chen et al. 2007; Song et al. 2007; Liu et al. 2009; Li et al. 2012; Yang et al. 2012; Wang et al. 2013b). These rocks are benefit for better understanding the tectonic setting of the area and tectonic affinity of the Gongshan, Tengchong-Baoshan and Shan-Thai blocks. For example, Fang et al. (1990) suggested that there is no affinity between the Tengchong-Baoshan Block and Gondwana. However, more and more data show the Tengchong-Baoshan and Shan-Thai Blocks are the micro-segments of the Gondwana (Metcalfe 1996; Zhong 1998; Liu et al. 2009; Wang et al. 2013b; Nie et al. 2014; Xing et al. 2015). The systematic works for the associated early Paleozoic igneous are required for probing the petrogenesis and tectonic setting of SW Yunnan. In this study, we reported Laser Ablation Induction Coupled Plasma Mass Spectroscopy (LA-ICP-MS), Sensitive High-Resolution Ion Microprobe (SHRIMP II) zircon U–Pb ages and geochemical data of the metaigneous rocks in the previously defined Lancang Group to constrain the source characteristics of the magma and discuss the tectonic implication on the accretionary orogenic history of the northern margin of Gondwana.

Geological background and petrography

Southwest Yunnan is one of the important branches of eastern Tethyan tectonic belt. The Tethyan–Alpine orogenic system has a change in direction from the Himalayan segment (WNW-trending) to the Southeast Asian segment (northerly trending) in SW Yunnan (Fig. 1a; Hutchison 1989; Metcalfe 1996, 2002, 2013; Zhang et al. 2008, 2012b; Wang et al. 2013b). The area includes Simao/Indochina, Baoshan/Shan-Thai and Tengchong blocks (Fig. 1a) which were separated by the Changning–Menglian and Longling–Ruili faults, respectively. The Simao/Indochina Block consists of a Proterozoic metamorphosed succession of pyroclastic rocks and carbonates (Zhong 1998), unconformably overlain by a Paleozoic package of carbonate and siliciclastic rocks with typical Cathaysia flora and fauna (Yunnan BGMR 1990; Zhong 1998; Feng 2002). The Baoshan, Tengchong and Shan-Thai blocks are components of the Sibumasu continental fragment and display stratigraphic and paleontological affinities to Gondwana continent (Fig. 1a, b; Fan and Zhang 1994; Metcalfe 1996, 2002; Zhong 1998; Feng 2002; Fontaine 2002; Wang et al. 2013b).

Fig. 1
figure 1

a Tectonic outline of Southeast Asian, b simplified geological map revised from 1:200,000 geological map of Cangyuan in SW Yunnan and c Stratigraphic column of the Lancang Group (revised from 1:200,000 geological map of Jinghong, Yunnan)

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The Lancang Group is a set of metamorphic volcano-sedimentary cycles, mainly exposed in Baoshan and Shan-Thai Blocks. The stratigraphic package includes pre-Mesozoic high-grade metamorphic rocks and Mesozoic–Cenozoic sedimentary and igneous rocks (Yunnan BGMR 1990; Zhong 1998; Wang et al. 2013b). Previous study considered this group consisting of four formations, which are in ascending order the Mengjingshan, Manlai, Huimin and Nankenghe Formations (Fig. 1c; Yunan BGMR 1979). The Mengjingshan Formation with thickness of over 300 m is mainly composed of light quartzite, granulite, sericite schist and sericite quartz schist in its upper part. The typical flysch sedimentary rocks indicate that the stratum is a production of high-speed accumulation with the crust subsidence intensely. The Manlai Formation consists of subdivide cycles from coarse to fine grains. The lower cycle is about 1400 m thick, and the upper cycle is nearly 1300 m thick; they predominantly consist of feldspathic quartz sandstone, gray sericite microcrystalline schist, sericite quartz schist, mica schist and metavolcanic rocks (Wei et al. 1984; Wu et al. 1984; Yunnan BGMR 1990; Shen et al. 2008). The Huimin Formation, 2500 m thick, comprises metamorphic basalt, andesite, andesitic tuff, rhyolitic tuff, and a few Fe-siliceous slate inbedding chlorite phyllite, sericite phyllite, chlorite schist, siderite layer and marble lens. The strata of Nankenghe Formation are mainly low metamorphic rocks, the structure and construction are less destroyed. The Nankenghe Formation is characterized by silica quartz sandstone, sericite quartz sandstone and microcrystalline schist in about 1000 m thick (Wei et al. 1984; Wu et al. 1984; Yang et al. 2012; Nie et al. 2014).

The Huimin Formation mainly exposed in Huimin and Manlai areas. Our study focuses on the Huimin Formation in both areas, and sampling locations are displayed in Fig. 1b, c. The represent samples (11ML-57B, -57F and -57H and DX-56) are andesites metamorphosed to lower-greenschist facies. They are dark-green in color with a schistose fabric and composed of feldspar, albite and chlorite along with the accessory mineral phases of titanite, apatite, zircon and Fe–Ti oxides. The major oxides and trace elemental data of the samples for the volcanic rocks in the Huimin Formation in the current study have been documented in Shen et al. (2008) and Nie et al. (2014).

Analytical methods

Zircon U–Pb LA-ICP-MS method

Zircons were separated using conventional heavy liquid and magnetic techniques and then purified by handpicking under a binocular microscope. After mounting in epoxy, polishing and coating of grains with carbon, the samples were photographed in transmitted and reflected light. The internal textures of zircons were examined using cathodoluminescence (CL) imaging prior to U–Pb isotopic analyses at the Institute of Geology and Geophysics (IGG), Chinese Academy of Sciences (CAS). U–Pb isotope analysis was conducted by Nu Plasma HR MC-ICPMS (Nu Instruments) with ArF-193 nm laser-ablation system (Resolution M-50) in the University of Hong Kong (11ML-57H). The instrumental settings and detailed analytical procedures have been described in Wu et al. (2006), Geng et al. (2014) and Gong et al. (2014). We used standards GJ-1 and 91500 to determine the elemental fractionation during sputter ionization. Off-line selection and integration of background and signals, time-drift correction and quantitative calibration were conducted by ICPMSDataCal (Liu et al. 2010). Zircon U–Pb age concordia and weighted average plots were made by Isoplot (Ludwig 2003). The analytical results are listed in Table 1.

Table 1 LA-ICP-MS and SHRIMP zircon U–Pb dating results of the previously defined Lancang Group metavolcanic rocks
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Zircon U–Pb SHRIMP method

The treatment processes of zircons before instrument tests are the same as we have introduced above. The zircons were analyzed for Pb–Th–U isotopes using the SHRIMP II ion microprobe at the Curtin University (DX-56), Australia. The instrumental conditions and data acquisition procedures are similar to those described by Williams and Claesson (1987) and Williams (1998). The SHRIMP runs in a primary ion beam of ca. 1.6–3.0 nA, 10 kV of O2−. Spots size range between 20 and 30 μm and each analysis site is rastered over 120 μm for 2 min to reduce any common Pb on the surface or contamination from the gold coating. The instrumental settings and detailed analytical procedures have been described in Williams (1998), Black et al. (2003) and Zi et al. (2012a, b). To maintain precision, one TEMORA analysis was performed after every three or four spots on the sample zircons during data collection. The common lead correction was applied using the measured 204Pb value (Compston et al. 1984). Errors for individual analyses are at 1σ level, unless otherwise stated. Uncertainties are quoted with 95 % confidence limits. Sites for dating were selected on the basis of CL and microscope images. Software SQUID 1.0 and Isoplot (Ludwig 2003) were used for data processing. The analytical results are listed in Table 1.

Geochemistry method

Samples were crushed and powdered to less than 200 mesh for whole-rock geochemical analysis at Langfang Integrity Geological Ltd., Hebei Province. Major oxides were determined by a XRF-100e spectrometry in the Guangzhou Institute of Geochemistry (GIG), CAS. Analytical error is usually <1 %, and detection limit is <0.05 % for most major elements. For trace elements, the rock powders were first digested by HF + HNO3 in Teflon bombs and then analyzed on IsoProbe MC-ICPMS in the GIG, CAS. Analytical precision is generally better than 5 % for most elements. Detailed sample-processing digesting procedure and analytical precision and accuracy for major elements and trace elements are described by Ma et al. (2014) and Liang et al. (2003), respectively. The analytical results for major and trace elements are shown in Table 2.

Table 2 Major oxides (wt%), trace element (ppm) and Sr–Nd isotopic analyses for the Ordovician metavolcanic rocks in SW Yunnan
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Sr and Nd isotopic ratios were measured by MCICP-MS at the GIG, CAS. The analytical procedures are the same as reported by Wei et al. (2002). The total procedure blanks are in the range of 200–500 pg for Sr and <50 pg for Nd. The mass fractionation corrections for Sr and Nd isotopic ratios were based on 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. The measured 87Sr/86Sr ratio of (NIST) SRM 987 standard and the 143Nd/144Nd ratio of the La Jolla standard are 0.710265 ± 12 (2σ) and 0.511862 ± 10 (2σ), respectively. 143Nd/144Nd and 147Sm/144Nd ratios of CHUR at the present time used for calculating εNd value are 0.512638 and 0.1967, respectively. 87Rb/86Sr and 147Sm/144Nd ratios were calculated using the Rb, Sr, Sm and Nd abundances measured by ICP-MS. The measured and age-corrected 87Sr/86Sr and εNd(t) are listed in Table 2.

Results

Zircon U–Pb geochronology

Zircon U–Pb dating was undertaken on samples 11ML-57H and DX-56 with the former analyzed by LA-ICP-MS and the latter by the SHRIMP method. The data are shown in Table 1 and Fig. 2.

Fig. 2
figure 2

Zircon U–Pb age concordia and weighted average plots for 11ML-57H (a, b) and DX-56 (c, d), respectively. The sampling locations are shown in Fig. 1b, c

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The zircon grains from 11ML-57H are mostly euhedral and short-columnar in morphology, transparent and colorless, and generally small in size (60–100 μm, 1:1-2:1 for length/width; Fig. 2a). Th/U ratios of twenty-four grains are in the range of 0.46–0.83 (Table 1). These analyses cluster around 460 Ma (Fig. 2a), yielding a weighted mean 206Pb/238U age of 462 ± 6 Ma with MSWD = 0.1 (n = 24; Fig. 2b). The CL images reveal oscillatory zonation with low to variable luminescence (Fig. 2a), together with high Th/U ratios, indicating an igneous origin (Hoskin and Black 2000; Wu and Zheng 2004). The age of 462 ± 6 Ma is interpreted as the eruption age of the sample.

Twenty-two spots on 22 grains were analyzed from sample DX-56. Zircons are mostly euhedral and short-columnar with well-developed oscillatory zoning, typical of a magmatic origin (Fig. 2c). Two major age peaks are present at around 456 and 794 Ma (Fig. 2c). Three spots give older ages around 2480 Ma. Only one spot has a Th/U ratio value of less than 0.1 (DX-56-21), whereas the other twenty-one spots have U and Th concentration of 25–703 ppm and 28–632 ppm, respectively, with the Th/U ratios ranging from 0.33 to 2.17 (Table 1). Thus, these ages with more than 790 Ma can be interpreted as inherited grains. Seven of twenty-two spots yield a weighted mean 206Pb/238U age of 454 ± 27 Ma with MSWD = 0.2 (n = 7; Fig. 2d), interpreted as the formation age of the sample.

Geochemical characteristics

Our geochemical data, together with the published data for the Huimin metavolcanic rocks (Shen et al. 2008; Nie et al. 2014), are used to constrain their petrogenesis and tectonic settings of Ordovician magmatic rocks along the proto-Tethyan margin of Gondwana continent. Loss on ignition (LOI) values for the samples range from 1.94 to 7.92 wt% (Table 2), suggesting that these rocks might have undergone some degree of low-temperature alteration (Rolland et al. 2002). In the plots of Zr and incompatible elements (Fig. 3a–f), these samples show linear correlation, suggestive of the insignificant mobile during the low-temperature alteration. Thus, only concentrations and ratios of the immobile elements (e.g., HFSEs and REEs) are used.

Fig. 3
figure 3

Zr (ppm) versus a TiO2 (wt%), b Y (ppm), c Hf (ppm), d Th (ppm), e Nb (ppm) and f La (ppm) for the Ordovician metavolcanic rocks in the previously defined Lancang Group (SW Yunnan)

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The metaigneous rocks contain 53.57–69.10 wt% SiO2 and 0.11–0.70 wt% P2O5 and are characterized by low TiO2 (0.64–1.00 wt%) and high Al2O3 contents (13.04–18.77 wt%). The samples plot in the fields of basaltic andesite and andesitic rocks on the Nb/Y–Zr/TiO2 × 10−4 plot (Fig. 4a; Winchester and Floyd 1977). Their MgO contents range from 3.21 to 9.12 % with Mg-numbers of 50–62, higher than those of normal arc volcanic rocks (e.g., NE Japan arc) at comparable SiO2 (Table 2; Fig. 4b). Thus these samples can be classified as high-Mg rocks according to the classification scheme of Kelemen (1995), as shown in Fig. 4b. They display geochemical characteristics similar to those of the Archean calc-alkaline sanukitoids of the Superior, Finland, Dharwar and Amazonian cratons (Fig. 4c; Shirey and Hanson 1984, 1986; Stern et al. 1989; Smithies and Champion 2000; Moyen et al. 2003; Halla 2005; Tatsumi 2008; Oliveira et al. 2009). These metaigneous samples have similar MgO and Na2O contents to those of the sanukitoid high-Mg andesite in the Japanese volcanic belt and low-silica adakite at comparable SiO2 (Fig. 4c; Tatsumi 2001, 2006, 2008; Moyen et al. 2003). On the Sr/Y–Y diagram (Fig. 4d; Kamei et al. 2004), they are generally plotted in the range of the sanukitoid high-magnesium andesite. Al2O3, FeOt, CaO and MgO generally show negative correlations with SiO2, but little correlations exist between SiO2 and TiO2 and P2O5 (Fig. 5a–f).

Fig. 4
figure 4

a Classification diagram of a Nb/Y versus Zr/TiO2 × 10−4, b SiO2 (wt%) versus Mg-number, c SiO2 (wt%) versus MgO (wt%) and d Y (ppm) versus Sr/Y for the Ordovician (454–460 Ma) volcanic rocks. The ranges for different rocks including normal arc lavas, Archean sanukitoids, Setouchi HMA are from Stern et al. (1989), Middlemost (1994), Kelemen (1995), Smithies and Champion (2000), Tatsumi (2001), Halla (2005), and Oliveira et al. (2009)

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Fig. 5
figure 5

SiO2 (wt%) versus a FeOt (wt%), b CaO (wt%), c Al2O3 (wt%), d MgO (wt%), e P2O5 (wt%), f TiO2 (wt%), g CaO/Al2O3 and h MnO (wt%) for the Ordovician metavolcanic rocks in the previously defined Lancang Group (SW Yunnan)

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The samples show similar chondrite-normalized REE patterns (Fig. 6a) and have moderately fractionated light rare-earth elements (LREEs) relative to heavy rare-earth elements (HREEs), with (La/Yb)N = 4.15–8.89 (Table 2). All the samples show negative Eu anomalies with Eu* = 0.20–0.33. On the primitive mantle-normalized incompatible element spider diagram (Fig. 6b), these samples are characterized by subparallel spiky patterns with enrichment in LILEs and depletion in HFSEs with significantly negative Nb–Ta anomalies [(Nb/La)N = 0.23–0.56] but insignificantly negative Zr–Hf anomalies [(Hf/Sm)N = 0.87–2.25]. Three samples from Manlai area show weakly negative Sr anomalies, contrary to typical adakite with pronounced positive Sr anomalies. Such patterns, irrespective of SiO2 content, are similar to those of the typical arc volcanic rocks.

Fig. 6
figure 6

a Chondrite-normalized REE patterns and b primitive mantle-normalized incompatible element for the Ordovician metavolcanic rocks. Normalized values for chondrite and primitive mantle are from Sun and McDonough (1989)

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Two samples (11ML-57B and 11ML-57H) were analyzed for Sr–Nd isotopic compositions. The measured 87Sr/86Sr ratios are 0.721356 and 0.722521, respectively. The 143Nd/144Nd values are 0.512054 and 0.512060, respectively. The initial Sr isotopic ratios back-calculated to 460 Ma are 0.7165 and 0.7171, and εNd(t) values are −7.63 and −7.62. The corresponding T DM values are 2.06 and 2.10 Ga for 11ML-57B and 11ML-57F (Table 2), respectively.

Discussion

Formation age of the Lancang metavolcanic sequence

The Lancang Group is composed of a metamorphic volcanic-sedimentary package, predominantly constituted by quartzite-schist, sandstone-volcanics, volcanics-schist and sandstone-schist, respectively. Previous estimations about the age of the package range from Mesoproterozoic to Paleozoic. Yunnan BGMR (1982) had assumed it to be the Mesoproterozoic sequence. These rocks were mapped as the Neoproterozoic sequence in the geological map of Jinghong and Menghai (Yunnan BGMR 1990). Zhong (1998) ascribed it as part of the Yangtze basement on the basis of a whole-rock Sm–Nd isochron age of 1982 ± 41 Ma from the Huimin Formation of the Lancang Group. Zhai et al. (1990) concluded that the Huimin volcanic rocks erupted during early Paleozoic based on a whole-rock Rb–Sr isochron age of 519 Ma. Shen et al. (2008) stated that the Huimin Formation was pre-Devonian sequence.

Our samples from the previously defined Lancang Group yielded weighted mean 206Pb/238U zircon ages of 462 ± 6 (n = 24) and 454 ± 27 Ma (n = 7), respectively. The CL structures of these grains have typical oscillatory zonation, and Th/U ratios range from 0.33 to 2.17 (except spot DX-56-21 with Th/U value of 0.04). Thus these ages can be interpreted as the eruption age of the metavolcanic rocks. Nie et al. (2014) recently obtained a U–Pb zircon age of 456 ± 7 Ma for the andesitic sample of the Huimin Formation. Thus, the previously defined Lancang Group, at least Huimin volcanic sequence, is most likely formed during the Ordovician period.

Origin of the Ordovician high-Mg andesitic rocks

All analyzed samples show little or no correlation between the LOI and Zr, La and Nb/La and Th/La, suggesting insignificant modification of elemental contents during low-temperature alteration. SiO2 correlates negatively with CaO and CaO/Al2O3 (Fig. 5b, g), indicative of fractionation of clinopyroxene and hornblende during magma evolution. This is also evidenced by the negative correlation between SiO2 and MgO, FeOt and Al2O3 (Fig. 5a, c, d). Apatite and Fe–Ti oxides fractionation should be insignificant since TiO2 and P2O5 contents are relatively constant irrespective of Zr (Fig. 3a) or SiO2 (Fig. 5e, f). Plagioclase fractionation is marked by negative correlations of CaO and Al2O3 with SiO2 (Fig. 5b, c) and significantly negative Eu anomalies (Fig. 6).

The samples have negative εNd(t) values (−7.63 and −7.62) and 147Sm/144Nd ratios of 0.132459–0.134479. TDM(Nd) ages of 2.06–2.10 Ga are much older than their formation age (~460 Ma). They exhibit similar initial Sr–Nd isotopic compositions to the Ordovician granitic rocks in SW Yunnan, the Greater Himalayan Crystalline Complex, and Lhasa metarhyolite, and also fall into the fields of the Greater Himalayan Crystalline Complex metasedimentary rocks and Ordovician S-type granites in the Lachlan Fold Belt (Fig. 7; Healy et al. 2004; Wang et al. 2007). The εNd(t) values are significantly lower than the synchronous Mandi and Shao La mafic rocks in Greater Himalayan Crystalline Complex and Lhasa metabasites (Miller et al. 2001; Chen et al. 2007; Liu et al. 2009; Wang et al. 2011, 2012; Zhu et al. 2011, 2012a, b; Wang et al. 2013b).

Fig. 7
figure 7

Initial Sr–Nd isotopic composition. The Sr–Nd isotopic data for Mandi and Shao La mafic rocks, along with the granite and metasedimentary rocks of the Greater Himalayan Complex (GHC), are from Parrish and Hodges (1996), Robinson et al. (2001), Imayama and Arita (2008), Miller et al. (2001), Visonà et al. (2010), Zhu et al. (2012a) and Wang et al. (2011, 2012)

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Three petrogenetic models have been proposed for the high-Mg andesitic rocks involving the partial melting of (1) eclogitic crust, (2) a young and hot slab, or (3) metasomatized source modified by the subduction-related components (Baker and Stolper 1994; Kelemen 1995; Yogodzinski et al. 1995; Hirose 1997; McCarron and Smellie 1998; Shimoda et al. 1998; Kawabata and Shuto 2005; Wang et al. 2006, 2009; Hoang et al. 2009; Zhang et al. 2012a; Honarmand et al. 2015). We firstly argue that cases (1) and (2) are not applicable to the Lancang high-Mg metavolcanic rocks based on the following observations. Our samples have relatively high Mg-numbers (50–62), Al2O3 (13.04–18.77 wt%) and low TiO2 (0.64–1.00 wt%) contents, enrichment in LILEs and depletion in HFSEs (Fig. 6b) with high LILE/HFSE ratios. The partial melting of a granulite or eclogite crust, containing minor rutile, generally produces magma with low Mg-number, high Al2O3 (>17 %) and TiO2 contents (Rapp et al. 1991, 1999; Sen and Dunn 1994). Thus, the Lancang metavolcanic rocks are unlikely to be derived from the melting of an eclogitic crust. Our high-Mg metavolcanic andesitic samples have weakly negative Eu, Ba and Sr anomalies, indicative of the absence of plagioclase in the source. In addition, the samples exhibit insignificant Ce anomalies, relatively high 87Sr/86Sr(t) ratios and negative εNd(t) values, against the possible model for derivation of a hot and young subducted slab for these rocks (Defant and Drummond 1990; Kelemen et al. 1993).

Melt extraction or slab- and sediment-derived metasomatism at exceptionally low pressures can form a high modal orthopyroxene source and lead to relatively high SiO2 contents in the rocks (Kushiro 1990; Gallagher and Hawkesworth 1992; Chalot-Prat and Boullier 1997). As a result, the arc-like elemental signatures (e.g., high LREE and LILE contents, high La/Nb, Ba/Th and Ba/La ratios and negative Nb–Ta–Ti anomalies) and the “crust-like” Sr–Nd isotopic compositions of the analyzed samples are likely attributed to a source modified by recycled sediment- or slab-derived components (Rapp et al. 1991, 1999; Evans and Hanson 1997; Sajona et al. 2000; Smithies and Champion 2000; Prouteau et al. 2001; Moyen et al. 2003; Smithies et al. 2003, 2007; Kamei 2004; Oliveira et al. 2009). High-Mg andesites that originate from a slab-metasomatized source (e.g., western Aleutians) generally exhibit high Sr contents and La/Yb ratios and depleted Sr–Nd isotopic system. However, our samples show insignificant Sr anomalies and EMII-like isotopic composition. Their Sr/La ratios gently increase with decreasing La/Yb ratios and the Th/Yb ratios increase with increasing Ba/La ratios (not shown). The Th/Yb ratios of Lancang high-Mg metavolcanic rocks range from 2.01 to 5.23, obviously higher than N-MORB (0.04; Sun and McDonough 1989) and E-MORB (0.25; Sun and McDonough 1989), having been ascribed to the addition of subducted sediments (Davidson 1987; Xu et al. 2014). As shown in the Fig. 8a, b, samples plot along the trend related to fluid-related/increasing hydrous metasomatism rather than melt-related enrichment. Such geochemical signatures appear to be consistent with the source involvement of subducted sediment rather than a slab-derived component.

Fig. 8
figure 8

a Nb/Y versus Ba/Y and b Th/Zr versus Nb/Zr for the Ordovician metavolcanic rocks showing an input of the subducted fluid-related metasomatism

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Tectonic implications

Available data show that the Sanjiang area in SW Yunnan is an important component of the orogenic zone associated with the final closure of the proto-Tethys ocean basin (Zhang et al. 1985; Cong et al. 1993; Zhong 1998). Evidence for early Paleozoic tectonothermal activity is widespread along the northern margin of East Gondwana from NW India to SW Yunnan involving an unconformity separating Ordovician from older strata, along with Cambrian–Ordovician magmatic activity, metamorphism and deformation (Cawood et al. 2007; Wang et al. 2013b). Widespread development of Cambrian and Ordovician volcanic rocks including acid and basic tuffs, basalts, andesites and felsic volcanic rocks in the western Tethyan Himalaya resembles to those from volcanic arc systems (Garzanti et al. 1986; Brookfield 1993; Valdiya 1995).

The Mandi mafic rocks in High Himalayan Crystalline of NW India formed at 496 ± 14 Ma and show geochemical affinity to a convergent margin setting (Miller et al. 2001). At Shenzha in the Central Lhasa Block, Ji et al. (2009) and Zhu et al. (2012a) identified the late Cambrian (~492 Ma) bimodal volcanic rocks related to the active continental margin, which are underlain by the lower Ordovician basal conglomerate (Ji et al. 2009; Li et al. 2010). SHRIMP zircon U–Pb ages for cumulate gabbro along the Longmucuo-Shuanghu suture zone range from 467 ± 4 to 432 ± 7 Ma (Li et al. 2008; Wang et al. 2008; Zhai et al. 2010). Yang et al. (2012) reported the late Cambrian (499.2 ± 2.1 Ma) mafic lavas with geochemical affinities to arc volcanic rocks in the Gongyanghe Group, which are unconformably overlain by lower Ordovician basal conglomerate in the Baoshan Block of SW Yunnan. Wang et al. (2013a) obtained zircon U–Pb age of 473.0 ± 3.8 and 444.0 ± 4.0 Ma for cumulate gabbro with geochemical affinity to back-arc basin setting at Nantinghe in the Baoshan Block of SW Yunnan. Meanwhile, Wang et al. (2013a) obtained LA-ICPMS zircon U–Pb age of 443.6 ± 4.0 and 439.0 ± 2.4 Ma for cumulate gabbro and gabbro in the Tongchangjie ophiolitic mélange along the Changning-Menglian tectonic belt, respectively. In addition, molybdenite samples from the dacite-hosted massive sulfide deposit at Dapingzhang of the southern Lancangjiang zone, which is interpreted as a product of back-arc volcanic setting, yield the zircon U–Pb age of 429 ± 2 Ma and Re–Os isotopic ages of 429 ± 6 and 442 ± 6 Ma (Yang et al. 2008; Li et al. 2010). Medium–basic to medium–acidic island-arc volcanic rocks were separated from the Precambrian Jitang Formation and re-defined as the Lower Paleozoic Youxi Formation in the central and northern Lancangjiang zones. Our metavolcanic samples from the previously defined Lancang Group in SW Yunnan formed in the Late Ordovician around 460 Ma and are classified as high-Mg andesitic rocks. They exhibit arc-like geochemical signatures (e.g., low TiO2, Ni, Cr and higher Al2O3 contents, and a marked enrichment in LILEs and LREEs and a depletion in HFSEs; Bailey 1981; Peate et al. 1997) and negative εNd(t) values, suggestive of subduction-related metasomatic enrichment, interpreted as the derivation of the metasomatic source with the input of subducted sediment. On tectonic discrimination diagrams (Fig. 9a–c), they plot in a suprasubduction setting. In addition, the peraluminous granites in the Tengchong, Baoshan and Shan-Thai Blocks in SW Yunnan, which were generated in the active continental margin setting related to accretionary orogenesis, have the zircon U–Pb ages of 498–460 Ma (Wang et al. 2013b and reference therein). All of these observations, along with the widespread development of early Paleozoic volcanic rocks in the Tethyan Himalaya and Sanjiang areas (e.g., Brookfield 1993; Garzanti et al. 1986; Valdiya 1995), suggest an evolution of the proto-Tethyan arc-basin system during early Paleozoic in South Tibet and Sanjiang areas.

Fig. 9
figure 9

a Th–Hf–Nb diagram (e.g., Wood 1980), b ATK diagram (e.g., Zhao 1989) and c TiO2 (wt%)-FeOt/MgO diagram (e.g., Glassley 1974; IAB: island-arc basalt; MORB: oceanic ridge basalt; OIB: oceanic island basalt) of the Ordovician metavolcanic rocks in SW Yunnan

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Ordovician basal conglomerate unconformably overlies the Cambrian-lower Ordovician Group in the Baoshan and Tengchong Blocks (Wang 2000), suggestive of a regional early Paleozoic orogenesis. Wang et al. (2013b) proposed the development of an early Paleozoic orogenesis from NW India to at least Namche Barwa. More importantly, the reconstruction of northeastern Gondwana at Late Ordovician indicates that the Tethyan Himalaya and associated East and Southeast Asian micro-continents (e.g., Baoshan, Tengchong, Shan-Thai, Pan-Cathaysia and Indochina) were located paleographically on the proto-Tethyan margin of East Gondwana (Fig. 10a, b; Cawood et al. 2007, 2013; Wang et al. 2010; Zhu et al. 2012a, b). Although the exact paleogeographic location of the Simao block along the East Gondwana margin is still unknown, it must be nearby or be part of the Pan-Cathaysia or Indochina blocks on basis of their paleobiological and paleoclimate affinity. Thus, these high-Mg rocks in the previously defined Lancang Group, together with synchronous mafic and granitic rocks, can be interpreted as the product of subduction of the proto-Tethyan ocean along the north margin of East Gondwana at the early Paleozoic (Chen et al. 2007; Liu et al. 2009; Zhang et al. 2008, 2012b; Wang et al. 2011, 2012, 2013b; Zhu et al. 2012a, b; Xing et al. 2015). The synthesis of our data and available regional observations suggest that these igneous rocks have been generated in a Late Ordovician island-arc setting related to subduction of the proto-Tethyan Ocean facing the Pan-Cathaysia continent. These Ordovician volcanic rocks in SW Yunnan might be the product of a regional accretionary orogen that extended along the northern margin of Gondwana during the Neoproterozoic to early Paleozoic.

Fig. 10
figure 10

a Tectonic reconstruction map of the India–Australia proto-Tethyan margin showing paleographical locations of the micro-continents during Ordovician (revised after Wang et al. 2010, 2013b) and b global paleogeographic map for the Ordovician (after Gehrels et al. 2011)

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Conclusions

  1. 1.

    The Huimin metavolcanic rocks in Manlai area of the previously defined Lancang Group of SW Yunnan give zircon U–Pb age of 462 and 454 Ma, indicative of Late Ordovician origin.

  2. 2.

    The volcanic rocks have Mg-numbers ranging from 62 to 50 with SiO2 of 53.57–69.10 wt%, compositionally corresponding to the high-Mg andesitic rocks. They show relatively high Al2O3 and low TiO2, with enrichments in LREE and LILEs and depletions in HFSEs, and have Eu and Nb–Ta negative anomalies. They have negative εNd(t) (−7.62 and −7.63) and Paleoproterozoic model ages (T DM of 2.10–2.06 Ga), similar to arc-like volcanic rocks.

  3. 3.

    These igneous rocks were derived from a mantle wedge source with involvement of subducted sediment in an Ordovician island-arc setting in response to subduction of the proto-Tethyan Ocean facing the Pan-Cathaysia.