Petrogenesis and Tectonic Setting of the Madeng Dacite, SW Sanjiang Indosinian Orogen: Evidence from Zircon U-Pb-Hf Isotopes, and Whole-Rock Geochemistry and Sr-Nd Isotopes

: The Sanjiang Indosinian orogen, located in the eastern part of the Paleo-Tethys tectonic domain, is a critical region to study the Paleo-Tethyan Ocean evolution. Middle Permian–Late Triassic magmatic rocks are widespread in the Deqin–Weixi–Madeng area of southwestern (SW) Sanjiang Indosinian orogen, yet their petrogenesis and tectonic setting remain disputed. In this study, LA-ICP-MS zircon U-Pb age and Hf isotopes, and whole-rock elemental and Sr-Nd isotope geochemistry of Madeng dacite were studied. The Madeng dacite was dated at ca. 241.7 and 243.4 Ma. The samples had high Al 2 O 3 (12.91 to 14.39 wt.%) but low MgO (0.62 to 1.76 wt.%) contents, and were alkali-rich (Na 2 O + K 2 O = 6.97 to 8.66 wt.%) with A/CNK > 1.1, strongly resembling peraluminous S-type granites. The rocks were enriched in Rb, K, Th, U and LREE, but depleted in Ba, Sr, Nb, Ta, P and Ti, and showed obvious negative Eu anomalies, suggesting fractionation of Ti-bearing minerals (e.g., rutile and ilmenite) and plagioclase. The dacite had an initial 87 Sr/ 86 Sr value of 0.705698 to 0.710277, and negative ε Nd (t) ( − 11.28 to − 10.64) and ε Hf (t) ( − 13.99 to − 8.60), indicating a continental meta-sedimentary source. Their average Nb/Ta (12.24) and Th/U (4.65) were also consistent with continental crust. According to the lithological assemblage and geochemical features, we propose that the Deqin–Weixi–Madeng area intermediate-felsic magmatism was generated in a subduction-related tectonic setting. of Late-Triassic of


Introduction
The Sanjiang Indosinian orogen is located in the southeastern margin of the Tibetan Plateau ( Figure 1A,B). The orogen comprises a series of NS-/NW-trending Paleozoic suture, island arc, and micro-continental slivers [1,2]. The major sutures include the Jinshajiang-Ailaoshan and Longmucuo-Shuanghu-Changning-Menglian. This orogen has undergone multi-stage tectono-magmat activity, and recorded the complete history of the opening and closure of the Paleo-Tethys Ocean [3][4][5][6][7]. Situated in the northern-central part of Sanjiang orogen, the Deqin-Weixi-Madeng area is the largest magmatic belt in the orogen, which is an ideal region for understanding and reconstructing the subduction and collision process following the closure of the Paleo-Tethys.
Recently, many geochronological studies on the Deqin-Weixi-Madeng magmatic rocks have indicated the presence of Early Permian-Late Triassic magmatism, which formed a series of basaltic to rhyolitic volcanics and coeval intermediate-felsic plutons. However, the petrogenesis and tectonic setting of these magmatic rocks remain controversial, which constrains understanding of the Paleo-Tethys evolution. Some workers suggested that these volcano-plutonic assemblages were generated by the west-dipping Jinshajiang Ocean

Geological Setting
The Sanjiang Indosinian orogen magmatic rocks can be divided into three segments with distinctive magmatism history along the strike [7], including a northern segment (north of Weixi), mainly encompassing the East Qiangtang and Zhongzan blocks; a middle segment (between Weixi and Changning), with magmatic rocks developed along the margin of Lanping basin, covered with sedimentary rocks; and a southern segment (south of Changning), with magmatic rocks developed along the margin of Simao basin. The geochronological data (see Supplementary Materials Table S1) [8,[10][11][12][13][14][15][16][17][18] of the middle segment have revealed a single major magmatism ( Figure 1C), showing a distinctive magmatic evolution in contrast with several episodes of magmatism in northern and southern segments [15]. The Deqin-Weixi-Madeng magmatic rocks are mainly exposed along the western and eastern margins of the Lanping basin in the middle segment. The magmatic rocks are distributed along a narrow N-S-trending belt (300 km long), intercalated with sandstone-siltstone and limestone. Magmatism in the middle segment likely started at 220-260 Ma (peaked at 248 Ma), and different magmatic phases are separated by periods of magmatic dormancy and sedimentation [7,14,15]. Some studies suggested that the Weixi-Madeng rocks were emplaced in one short magmatic phase (7 My) [15].
The Lanping basin is the largest Meso-Cenozoic basin in the Sanjiang Indosinian orogen and hosts abundant polymetallic deposits [19,20] (Figure 2). The main rock units exposed include Permian to Neogene sedimentary rocks and minor Permian-Middle Triassic igneous rocks on the basin margin. Thick (>1.5 km) volcanic sequence is exposed in the eastern Lanping basin (in the Weixi-Madeng-Misha), consisting mainly of rhyolite, dacite, and minor basalt [7] (Figure 2). The Madeng transection in this study (from Houdian in the west to Jiangweitang-Dapingzi in the east) can be divided into two parts separated by a thrust fault ( Figure 3A). The western part contains the Permian Shanlan Formation bioclastic limestone, sandstone, and schist, Upper Triassic Sanhedong Formation finegrained limestone, Eocene Jainchuan Formation andesitic-rhyolitic volcanic breccias, and the Miocene Sanyin Formation conglomerate sandstone with gypsum veins. The eastern part contains the Middle-Lower Triassic Pantiange Formation of dacite-rhyolite with minor basalt (from whence our samples came).

Sampling and Petrography
Representative, least-altered dacite samples were collected from the outcrops in Pantiange Formation at Madeng, with the sampling locations shown in Figure 3A,B.
Two representative dacite samples were chosen for zircon U-Pb-Hf isotope analyses. An additional seven samples were selected for major and trace element analysis and four for Sr-Nd isotope analysis. The coordinates and lithology of each sample are listed in Table 1.  [22]), the published data source: [12][13][14][15]23].    [14]). (B) Geological map of cross Section A-A' (modified after [15]

Sampling and Petrography
Representative, least-altered dacite samples were collected from the outcrops in Pantiange Formation at Madeng, with the sampling locations shown in Figure 3A,B.
The dacite is medium-to coarse-grained porphyritic with massive structure (Figure 4a,b). The phenocrysts include quartz, plagioclase, and feldspar, which are set in a cryptocrystalline groundmass of similar mineral content. Quartz (20%) is anhedral granular with undulose extinction. Plagioclase (25%) is subhedral-euhedral thick tabular (size: 0.5-3 mm) with polysynthetic twinning. K-feldspar (15%) is subhedral-anhedral granular with common Carlsbad twinning (Figure 4c,d).   Zircon grains were separated using conventional heavy liquid and magnetic separation techniques. Representative zircon grains were handpicked and then mounted in epoxy resin, polished, and carbon-coated. To examine the internal structure and choose the suitable analysis site, transmitted-/reflected-light micrographs and cathodoluminescence (CL) images were acquired at the Beijing Zhongke Mining Technology Co. Ltd. with a MIRA3 scanning electron microscope. Euhedral-subhedral zircon grains with clear os- Two representative dacite samples were chosen for zircon U-Pb-Hf isotope analyses. An additional seven samples were selected for major and trace element analysis and four for Sr-Nd isotope analysis. The coordinates and lithology of each sample are listed in Table 1. Zircon grains were separated using conventional heavy liquid and magnetic separation techniques. Representative zircon grains were handpicked and then mounted in epoxy resin, polished, and carbon-coated. To examine the internal structure and choose the suitable analysis site, transmitted-/reflected-light micrographs and cathodoluminescence (CL) images were acquired at the Beijing Zhongke Mining Technology Co. Ltd., Beijing, China, with a MIRA3 scanning electron microscope. Euhedral-subhedral zircon grains with clear oscillatory zoning, and without fractures or inclusions, were selected for dating, using an Agilent 7770e inductively coupled plasma mass spectrometry (ICP-MS) and a MicroLas COMPexPro102 (193 nm) laser ablation system (Agilent Technologies Inc., Palo Alto, CA, USA), following the method of [24]. The data were processed with ICPMSDataCal, which was calibrated with the zircon standard 91,500 and glass NIST-610. The zircon standard GJ-1, and Plešovice yield concordia age of 339.1 ± 1.8 Ma and 604.0 ± 1.9 Ma, were consistent with their recommended values [25]. Common Pb correction was subsequently carried out with the method of [26]. The age calculation and concordia plotting were made with Isoplot 3.0 software [27]. The age uncertainty is quoted at 2σ.

Whole-Rock Geochemistry and Sr-Nd Isotopes
Fresh and clean samples were milled to~200 mesh with an agate mill. Major and trace element analyses were carried out at the National Research Center for Geoanalysis, Chinese Academy Geological Science (Beijing, China). Major element oxides were analyzed on a PW4400 X-ray fluorescence spectrometer (XRF) with the analytical precision and accuracy being better than 5%. Trace element analysis was conducted at the same laboratory with solution ICP-MS (model PE300Q). About 50 mg of rock powder was weighted and then dissolved with HF-HNO 3 (1 mL: 1 mL) and digested in a Teflon bomb at 190 • C for over 24 h. The analytical precisions are better than 5% for elements at concentration >10 ppm, and less than 10% for elements with concentration <10 ppm.
Whole-rock Sr and Nd isotope analyses were performed with a Neptune Plus multicollector (MC)-ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd. (Hubei, China). About 50-200 mg of sample powder was dissolved in 1-3 mL HNO 3 and 1-3 mL HF in a closed Teflon bomb at 190 • C for over 24 h. Subsequently, the samples were digested in 1 mL HCl. The mass fractionation corrections for 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios were normalized to 86 Sr/ 88 Sr = 0.1194 and 146 Nd/ 144 Nd = 0.7219, respectively. For every seven samples, one international NBS987 and GSB standard was measured, which yielded 87 Sr/ 86 Sr = 0.710242 ± 0.000012 and 143 Nd/ 144 Nd = 0.512440 ± 0.000008, respectively, consistent (within error) with their recommended values [28,29]. The USGS BCR-2 basalt and RGM-2 rhyolite standards were chosen as the reference materials [30,31].

LA-ICP-MS Zircon Hf Isotope Analysis
This analysis was conducted on a Neptune Plus MC-ICP-MS, equipped with a Geo-Las HD laser-ablation system at Wuhan Sample Solution Analytical Technology Co. Ltd., Wuhan, China, Helium was used as the carrier gas, with the addition of a small amount of nitrogen to improve the signal sensitivity [32]. The analysis was performed with 44 µm spot size, 10 J/cm 2 energy density, and 10 Hz repetition rate. The analysis was made on the same U-Pb dated zircon grains. Instrumentational and data acquisition protocols were as described by [32]. During the analysis, the zircon standards of Plešovice, 91,500 and GJ-1 were analyzed, and obtained Hf isotope compositions (Plešovice = 0.282478 ± 0.000010, 91,500 = 0.282298 ± 0.000011 and GJ-1 = 0.282007 ± 0.000010) are consistent with the recommended value [33]. The initial 176 Hf/ 177 Hf and εHf(t) values were calculated with reference to the present-day chondritic reservoir 176 Hf/ 177 Hf = 0.282785 and 176 Lu/ 177 Hf = 0.0336 at the measured U-Pb ages [34]. Single-stage Hf model ages (T DM1 ) were calculated with reference to the depleted mantle at the present-day 176 Hf/ 177 Hf = 0.28325 and 176 Lu/ 177 Hf = 0.0384 [35]. Two-stage Hf model ages (T DM2 ) were calculated by assuming an average continental crust with 176 Lu/ 177 Hf = 0.015 [36].

Zircon U-Pb Age
Zircon grains from samples MD20-1-2 and MD20-5-2 have very similar characteristics. Most zircon grains are gray to black in CL images (Figure 5a,c). They are euhedral to subhedral and 100-300 µm in length (aspect ratios = 1:1 to 3:1). Most grains show a clear core-rim texture and well-developed oscillatory zoning in CL images (Figure 5a,c), suggesting an igneous origin [37].

Whole-Rock Major and Trace Elemental Geochemistry
The whole-rock major and trace element results are presented in Table S3 (Supplementary Materials). The loss on ignition (LOI) for all samples below 4 wt.% suggests insignificant post-magmatic alteration or weathering. All samples exhibit similar chemical compositions, with medium SiO2 (67.08-70.51 wt.%) and high Al2O3 (12.91-14.39 wt.%), total alkali (N2O + K2O = 6.97-8.66 wt.%) contents and an A/CNK value (molar Al2O3/(CaO + Na2O + K2O)) of 1.14-1.57. All samples fall into the dacite field in both the TAS and Nb/Y-SiO2 plots (Figure 6a,b) and are shoshonitic (Figure 6c). In the A/NK vs. A/CNK plot (Figure 6d), the samples are classified as strongly peraluminous. They also have high FeOT

Whole-Rock Major and Trace Elemental Geochemistry
The whole-rock major and trace element results are presented in Table S3 (

Zircon Lu-Hf Isotopic Compositions
The zircon Hf isotope data are presented in Table S4

Whole-Rock Sr-Nd Isotope Compositions
The Sr-Nd isotope data are presented in Table S5 (Supplementary Materials). Initial Sr isotope ratios and εNd(t) values were calculated with the obtained zircon U-Pb age of 242 Ma. The two samples have a much higher initial 87 Sr/ 86 Sr (0.738721 to 0.753399) than the present-day primitive mantle value of 0.7045 [45]. Both samples have high Rb (107-255 ppm) but relatively low Sr (39.4-66.7 ppm) contents. The εNd(t) value is negative (−11.28 to −10.64), with a TDM2 model age of 1881 to 1933 Ma.

Timing of Magmatism
We compiled 62 published geochronological data from Deqin-Weixi-Madeng area, which yielded an age from 220 to 260 Ma and represent the timing of regional magmatism (Table S1). Our new zircon ages of 243.4 and 241.7 Ma fall within this range, indicating that extensive volcanism occurred in Madeng during the Middle Triassic.

Whole-Rock Sr-Nd Isotope Compositions
The Sr-Nd isotope data are presented in Table S5 (Supplementary Materials). Initial Sr isotope ratios and ε Nd (t) values were calculated with the obtained zircon U-Pb age of 242 Ma. The two samples have a much higher initial 87 Sr/ 86 Sr (0.738721 to 0.753399) than the present-day primitive mantle value of 0.7045 [45]. Both samples have high Rb (107-255 ppm) but relatively low Sr (39.4-66.7 ppm) contents. The ε Nd (t) value is negative (−11.28 to −10.64), with a T DM2 model age of 1881 to 1933 Ma.

Genetic Type
The Madeng dacite is strongly peraluminous (A/CNK = 1.14-2.45), intermediate SiO 2 (67.08-70.51 wt.%) and high K 2 O (4.39-6.67 wt.%) with a relatively low differentiation index (DI) of 78.88-87.80 ( Figure 5), resembling weakly fractionated S-type granites rather than I-and A-types [48,49]. Apatite has a high solubility in strongly peraluminous magma, resulting in an increase in P 2 O 5 with increasing SiO 2 [50]. Therefore, the relationship between P 2 O 5 and SiO 2 can be used to distinguish the magmatic affinity. The positive correlation between P 2 O 5 and SiO 2 suggests that the Madeng dacite belongs to S-type granites (Figure 9a). In the 10,000 * Ga/Al diagram, all samples plot in the S-/I-type granite field with low Ga/Al ratios [51] (Figure 9b-d). In the SiO 2 vs. Zr, and ACF discrimination diagrams, all the Madeng dacite samples plot in the S-type granite field (Figure 9e,f), suggesting unfractionated S-type granite.

Magma Source
It has been shown that some incompatible element ratios, such as Nb/Ta and Th/U, can be a useful tracing index to constrain source regions [52]. Madeng dacite has Nb/Ta = 11.41-13.62 (average 12.24) and Th/U = 3.89 to 5.73 (average 4.65), close to the average crustal values (Nb/Ta = 11-12; Th/U = 3.8-6) but distinct from the average mantle values (Nb/Ta = 17.5; Th/U = 4) [38,53]. This indicates that the Madeng dacite was derived mainly from the partial melting of the crust. In addition, Madeng dacite has negative ε Hf (t) = −13.99 to −8.6 with a two-stage model age of 1821 to 1959 Ma, resembling rocks derived from an ancient crustal source (Figure 10a) [54]. This conclusion is also supported by the low whole-rock ε Nd (t) (−11.28 to −10.64) and initial 87 Sr/ 86 Sr (0.705698 to 0.710277) (Figure 10b). Minerals 2022, 12, x FOR PEER REVIEW 12 of 18

Magmatic Evolution
The linear correlations between MgO, Fe2O3T, Al2O3, TiO2, CaO, Na2O and SiO2 for the Deqin-Weixi-Madeng magmatic rocks indicate that fractionation occurred ( Figure  12). The decreasing MgO, Fe2O3T and Al2O3 with SiO2 suggest the fractionation of mafic minerals (e.g., amphibole), whilst the decreasing TiO2 with SiO2 indicates the fractionation of Ti oxides (e.g., rutile and ilmenite), as also supported by the distinct negative anomalies of Ti, Nb and Ta (Figure 7b). Moreover, the negative correlation between CaO and Na2O with SiO2, and the distinct negative Eu, Sr and Ba anomalies suggest the significant fractionation of plagioclase. In conclusion, the partial melting of ancient crust material and fractional crystallization plays an important role in the formation of Madeng dacite.
Previous studies proposed that strongly peraluminous S-type granites are mainly derived from the partial melting of meta-sediment (e.g., meta-pelite or meta-greywacke) [49,55]. The Madeng dacite is characterized by low molar CaO/(MgO + FeO T ) (0.05-0.27) and Al 2 O 3 /(MgO + FeO T ) (1.10 to 3.10) ratios, and plots within the metapelite field (Figure 11a). In addition, experimental data have suggested that the CaO/Na 2 O ratio of strongly peraluminous S-type granites is mainly controlled by their source composition [56]. Melt derived from metapelite has a lower CaO/Na 2 O ratio (<0.5) than that derived from meta-greywacke (mainly >0.5) [55]. The Madeng dacite has a low CaO/Na 2 O ratio (0.13-0.39), similar to melt derived from a metapelite source. In the CaO/Na 2 O vs. Al 2 O 3 /TiO 2 plot, all the samples plot in the metapelite field (Figure 11b). Furthermore, in the Rb/Sr vs. Rb/Ba discrimination plot (Figure 11c), the Madeng dacite has higher Rb/Sr (2.72-5.67) and Rb/Ba (0.19-0.83) ratios, again resembling metapelite-derived melts. Therefore, the partial melting of metapelite from the ancient crust was likely the dominant mechanism of the formation mechanism for the Madeng S-type dacite.

Magmatic Evolution
The linear correlations between MgO, Fe2O3T, Al2O3, TiO2, CaO, Na2O and SiO2 for the Deqin-Weixi-Madeng magmatic rocks indicate that fractionation occurred ( Figure  12). The decreasing MgO, Fe2O3T and Al2O3 with SiO2 suggest the fractionation of mafic minerals (e.g., amphibole), whilst the decreasing TiO2 with SiO2 indicates the fractionation of Ti oxides (e.g., rutile and ilmenite), as also supported by the distinct negative anomalies of Ti, Nb and Ta (Figure 7b). Moreover, the negative correlation between CaO and Na2O with SiO2, and the distinct negative Eu, Sr and Ba anomalies suggest the significant fractionation of plagioclase. In conclusion, the partial melting of ancient crust material and fractional crystallization plays an important role in the formation of Madeng dacite.

Magmatic Evolution
The linear correlations between MgO, Fe 2 O 3 T, Al 2 O 3 , TiO 2 , CaO, Na 2 O and SiO 2 for the Deqin-Weixi-Madeng magmatic rocks indicate that fractionation occurred ( Figure 12). The decreasing MgO, Fe 2 O 3 T and Al 2 O 3 with SiO 2 suggest the fractionation of mafic minerals (e.g., amphibole), whilst the decreasing TiO 2 with SiO 2 indicates the fractionation of Ti oxides (e.g., rutile and ilmenite), as also supported by the distinct negative anomalies of Ti, Nb and Ta (Figure 7b). Moreover, the negative correlation between CaO and Na 2 O with SiO 2 , and the distinct negative Eu, Sr and Ba anomalies suggest the significant fractionation of plagioclase. In conclusion, the partial melting of ancient crust material and fractional crystallization plays an important role in the formation of Madeng dacite. element (HFSE, e.g., Nb, Ta, P and Ti) depletions indicate a subduction setting [64]. The low Sr/Y ratio (<20) with low Y and Yb content and low YbN with low LaN/YbN are typical of normal arc rocks (Figure 13a,b), and the Deqin-Weixi-Madeng magmatic rocks plot largely in the volcanic arc granite field in the (Y + Nb) vs. Rb and Yb vs. Ta tectonic discrimination diagrams (Figure 13c,d). Consequently, we suggest that the Deqin-Weixi-Madeng intermediate-felsic magmatic rocks were formed in a subduction setting.
In brief, we suggest that the Paleo-Tethyan Longmucuo-Shuanghu Ocean may have continued to subduct during the Early Permian to Late Triassic. The long-lived dehydration of the Paleo-Tethys subducting slab and the partial melting of the mantle wedge may have generated (and accumulated) vast volumes of magma beneath the overriding plate. The crust of the overriding plate may have then been heated and partially melted to form intermediate-felsic magmas, which erupted in a short period of time (peak at 250 Ma).

Tectonic Implications
Our study demonstrated that the ca.243 Ma S-type felsic rocks in Madeng were formed by partial melting of meta-pelite rocks. Previous studies have shown that S-type granites could be formed in subduction and syn-/post-collision settings [55,59,60]. As mentioned above, the Deqin-Weixi-Madeng magmatic rocks were proposed to have formed under (1) a collision and post-collision extensional [8][9][10] or (2) a subduction tectonic setting [7,[11][12][13][14][15]. Some authors suggested that the Deqin-Weixi-Madeng magmatic rocks were generated during the closure of the remnant Jinshajiang Ocean, as supported by the occurrence of an ophiolite belt in the Zhongzan Block and foreland-basin molasse with bimodal volcanic rocks in the Jomda-Deqin-Weixi area. Some other authors, however, questioned the potential relationship between the ophiolite belt and remnant ocean and disputed the occurrence of the Jinshajiang suture between the Zhongzan and Changdu-Lanping-Simao Blocks. Furthermore, they proposed that regional magmatic arcs were formed by north-or east-dipping subduction of the Paleo-Tethyan Longmucuo-Shuanghu Ocean beneath the Yangtze block, based on the extensive occurrence of blueschist and eclogite in Longmucuo-Shuanghu-Changning-Menglian suture [61,62].

Author
In brief, we suggest that the Paleo-Tethyan Longmucuo-Shuanghu Ocean may have continued to subduct during the Early Permian to Late Triassic. The long-lived dehydration of the Paleo-Tethys subducting slab and the partial melting of the mantle wedge may have generated (and accumulated) vast volumes of magma beneath the overriding plate. The crust of the overriding plate may have then been heated and partially melted to form intermediate-felsic magmas, which erupted in a short period of time (peak at 250 Ma).

1.
LA-ICP-MS zircon U-Pb dating on the Madeng dacite yielded 241.7 and 243.4 Ma. Regional magmatic age correlation suggests that this Middle Triassic volcanism was likely formed in a subduction setting, indicating that the Paleo-Tethyan Longmucuo-Shuanghu Ocean subduction may have persisted in the Triassic.

2.
The Madeng dacite is characterized by being high-Al, alkali-rich, and low-Mg. The rocks are peraluminous S-type, and display clear LILE enrichments and HFSE depletions with markedly negative Eu anomalies. This suggests the fractionation of Ti-bearing minerals (e.g., rutile and ilmenite) and plagioclase. 3.