Geochronology and Geochemistry of the Zengudi and Tuobake Granite Porphyries in the Sanjiang Region, SW China: Petrogenesis and Tectonic Significance

The boundary between the Gondwana and Yangtze plate is still controversial. In southwest China, the Sanjiang region marks the collision zone which accreted several blocks coming from the northern Gondwana margin. In this region, subduction of the Paleo-Tethys Ocean and associated continental blocks during the Triassic Period led to the formation of an N–S trending complex involving intrusive and volcanic rocks. The intrusive rocks are important for constraining the evolution of the Paleo-Tethyan in southwestern China. This study presents new geochronological, geochemical, and Sr-Nd-Hf isotopic data of granite porphyries from northern Lancangjiang, in order to discuss the origin of these granites and their tectonic significance. Representative samples of the Zengudi and the Tuobake granite porphyries from the Yezhi area yielded weighted mean 206Pb/238U ages of 247–254 Ma and 246 Ma, respectively. The Zengudi granite porphyries display zircon Minerals2021, 11, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/minerals Article Geochronology and Geochemistry of the Zengudi and Tuobake Granite Porphyries in the Sanjiang Region, SW China: Petrogenesis and Tectonic Significance Mao-Jun Tian 1, Huan Li 1,*, Landry Soh Tamehe 1 and Zhen Xi 2 1 Key Labo atory of etall genic Prediction of Nonferrous Metals and Geological Environment Monitoring, Mi istry of Educ tion, School of Geosciences and Info-Physics, Central South University, Changsha 410083, China; tianmj@csu.edu.cn (M.-J.T.); tamehelandry@yahoo.com (L.S.T.) 2 School of Municipal and Geomatics Engineering, Hunan City University, Yiyang 413000, China; xizhen@hncu.edu.cn * Correspondence: lihuan@csu.edu.cn Abstract: The boundary between the Gondwana and Yangtze plate is still controversial. In southwest China, the Sanjiang region marks the collision zone which accreted several blocks coming from the northern Gondwana margin. In this region, subduction of the Paleo-Tethys Ocean and associated continental blocks during the Triassic Period led to the formation of an N–S trending complex involving intrusive and volcanic rocks. The intrusive rocks are important for constraining the evolution of the Paleo-Tethyan in southwestern China. This study presents new geochronological, geochemical, and Sr-Nd-Hf isotopic data of granite porphyries from northern Lancangjiang, in order to discuss the origin of these granites and their tectonic significance. Representative samples of the Z ngudi and the Tuobake granite porphyries from the Yezhi area yielded weighted mean 206Pb/238U ages of 247–254 Ma and 246 Ma, respectively. The Zengudi granite porphyries display zircon ԐHf(t) values of −12.94 to −2.63, ԐNd(t) values of −14.5 to −9.35, and initial 87Sr/86Sr ratios of 0.708 to 0.716. The Tuobake granite porphyries have zircon ԐHf(t) values of −14.06 to −6.55, ԐNd(t) values of −10.9 to −9.41, and initial 87Sr/86Sr ratios of 0.716 to 0.731. Both the Zengudi and Tuobake granite porphyries exhibit strongly peraluminous signatures with high A/CNK nAl2O3/(K2O + Na2O + K2O) ratios (1.07– 1.86 and 0.83–1.33, respectively). These granites are enriched in Rb and Th, and depleted in Ti, Nb, Ta, Sr, and P, with negative Eu anomalies (Eu/Eu* < 0.61). These geochemical and isotopic data indicate that the primary magma of the granite porphyries originated from partial melting of ancient continental crust as a result of basaltic magma underplating and underwent fractionation crystallization during their emplacement. We propose that the Triassic subduction of the Paleo-Tethys Ocean led to crust shortening and thickening in the Sanjiang region, while the northern Lancangjiang area was involved in the continental collision after the subduction of the Paleo-Tethys Ocean before 254 Ma. The boundary between the Gondwana and Yangtze plate is still controversial. In southwest China, the Sanjiang region marks the collision zone which accreted several blocks coming from the northern Gondwana margin. In this region, subduction of the Paleo-Tethys Ocean and associated continental blocks during the Triassic Period led to the formation of an N–S trending complex involving intrusive and volcanic rocks. The intrusive rocks are important for constraining the evolution of the Paleo-Tethyan in southwestern China. This study presents new geochronological, geochemical, and Sr-Nd-Hf isotopic data of granite porphyries from northern Lancangjiang, in order to discuss the origin of these granites and their tectonic significance. Representative samples of the Z ngudi and the Tuobake granite porphyries from the Yezhi area yielded weighted mean 206Pb/238U ages of 247–254 Ma and 246 Ma, respectively. The Zengudi granite porphyries display zircon ԐHf(t) values of −12.94 to −2.63, ԐNd(t) values of −14.5 to −9.35, and initial 87Sr/86Sr ratios of 0.708 to 0.716. The Tuobake granite porphyries have zircon ԐHf(t) values of −14.06 to −6.55, ԐNd(t) values of −10.9 to −9.41, and initial 87Sr/86Sr ratios of 0.716 to 0.731. Both the Zengudi and Tuobake granite porphyries exhibit strongly peraluminous signatures with high A/CNK nAl2O3/(K2O + Na2O + K2O) ratios (1.07– 1.86 and 0.83–1.33, respectively). These granites are enriched in Rb and Th, and depleted in Ti, Nb, Ta, Sr, and P, with negative Eu anomalies (Eu/Eu* < 0.61). These geochemical and isotopic data indicate that the primary magma of the granite porphyries originated from partial melting of ancient continental crust as a result of basaltic magma underplating and underwent fractionation crystallization during their emplacement. We propose that the Triassic subduction of the Paleo-Tethys Ocean led to crust shortening and thickening in the Sanjiang region, while the northern Lancangjiang area was involved in the continental collision after the subduction of the Paleo-Tethys Ocean before 254 Ma.


Introduction
The convergent subduction zone teraction between the crust and man the magmatic rocks and their tectonic bility of plate tectonic processes, mag the volcanic arc constrained the magm neity, and differences of igneous ro

Introduction
The convergent subduction zone is an important window for understanding the interaction between the crust and mantle as well as deciphering the relationship between the magmatic rocks and their tectonic environment [1][2][3]. In subduction zones, the variability of plate tectonic processes, magma sources, thermal states, and crust thicknesses in the volcanic arc constrained the magmatic activity and led to the segmentation, heterogeneity, and differences of igneous rocks [4][5][6][7]. Nowadays, exploring the plate tectonic

Introduction
The convergent subduction zone is an important window for understand teraction between the crust and mantle as well as deciphering the relationshi the magmatic rocks and their tectonic environment [1][2][3]. In subduction zones bility of plate tectonic processes, magma sources, thermal states, and crust thic the volcanic arc constrained the magmatic activity and led to the segmentation neity, and differences of igneous rocks [4][5][6][7]. Nowadays, exploring the pla

Introduction
The convergent subduction zone is an imp teraction between the crust and mantle as wel the magmatic rocks and their tectonic environm bility of plate tectonic processes, magma source the volcanic arc constrained the magmatic activ neity, and differences of igneous rocks [4][5][6][7].

Introduction
The convergent subduction zone is an important window for understanding the interaction between the crust and mantle as well as deciphering the relationship between the magmatic rocks and their tectonic environment [1][2][3]. In subduction zones, the variability of plate tectonic processes, magma sources, thermal states, and crust thicknesses in the volcanic arc constrained the magmatic activity and led to the segmentation, heterogeneity, and differences of igneous rocks [4][5][6][7]. Nowadays, exploring the plate tectonic processes by studying the origin and evolution of igneous rocks in orogenic belts is a hot topic for the tectonic history of continents [8][9][10][11].

Geological Setting
The Sanjiang region in southwest China has been generally considered a convergent margin subduction zone, which was involved during the tectonic evolution process of the Tethys (Figure 1a) [6]. This region can be further regarded as an excellent natural laboratory for studying the subduction-related tectonic processes. In the accretionary orogenic process caused by the collision of archipelagic arc-basin systems during Early Devonian to Middle Triassic [25], the Sanjiang region has experienced ocean-crust subduction and continent-continent collision [16,21], followed by complex thermal magmatism and metamorphism. Previous works on the tectonic history of this region have documented the formation of the Ganzi-Litang suture zone between the south China and Zhongza blocks [16,26,27], the Lancangjiang-Changning-Menglian suture zone between the Baoshan and Simao blocks [28,29], the Jinshajiang [12,[30][31][32] and the Ailaoshan-Songma suture zones between the Simao and Yangtze blocks [33][34][35] (Figure 1b,d). The Lancangjiang-Changning-Menglian and the Jinshajiang-Ailaoshan Oceans were opened during the Early-Middle Devonian and closed at Middle Triassic. While the Ganzi-Litang and Songma Oceans (southern extension of the Ailaoshan Ocean) were opened during the Early-Middle Devonian and closed at late Triassic. The remnant of the oceanic crust distributed in these suture zones has been systematically studied [6]. Three volcanic arc belts related to the subduction of the Paleo-Tethys Ocean were delineated in the Sanjiang region: the Yushu-Yidun belt formed during Late Triassic   [25], the Jiangda-Weixi belt formed during Late Permian to Triassic   [21,36], and the Yunxian-Lvchun-Ailaoshan belt formed during Permian to Early-Middle Triassic (285-265   [20,21]. Among these belts, the Jiangda-Weixi belt is the most complex volcanic arc characterized by fragmented blocks [6], which explains that its tectonic evolution is still unclear. The Zengudi and Tuobake granite porphyries are located in the Yezhi area in the north of the Sanjiang region. The Yezhi area is situated between the Jinshajiang suture zone and the Biluoxueshan-Chongshan shear zone and connects the Zhongzan and Lanping-Simao Blocks on the east and west, respectively (Figure 1a,b). The Yezhi area has undergone intense orogenic events in the East Tethyan tectonic zone as results of oceanic subduction and continental collision from Paleozoic to Mesozoic [16,19,21]. These orogenic activities were developed together with a remarkable volume of magmatic movements. Ophiolite and mafic rocks were crystallized in the Late Permian, whereas basic-acidic bimodal volcanic rocks and pyroclastic rocks were formed in the Early-Middle Triassic [16]. A series of ultrabasic, intermediate, and alkaline rocks are also exposed in the area [16,17]. Granitic rocks are mostly distributed on the west side of the study area. This region has alternatively experienced multiple periods of magmatism and sedimentation, which led to volcaniclastic successions [21,34]. Besides magmatic rocks, the exposed strata encompass Early to Middle Permian metamorphic sandstone, slate, and red sandstone (Figure 1c). The sedimentary succession is characterized by flysch and molasse [25].
The granite porphyries in the Yezhi area are distributed along the east side of the Jiangda-Weixi volcanic arc (Figure 1c). They form a long strip shape from south to north. The Zengudi granite porphyry intrudes into the Triassic Pantiange Formation volcanic rocks with an ellipsoid shape and is adjacent to the Triassic Shanglan formation metamorphicsedimentary rocks on the west (Figures 1c and 2a). The Tuobake granite porphyry displays an irregular long strip shape at the surface, intrusively connected to the Shanglan Formation (Figures 1c and 2b).

Sample Description
The crystals are mainly composed of quartz, plagioclase, K-feldspar, chlorite, and minor opaque minerals (Figure 2c). The quartz is xenomorphic granular and partly clastic with a particle size ranging between 0.3 to 2.0 mm (Figure 2d). Most plagioclase crystals occur as automorphic or subhedral laths (0.4-1.5 mm in size) showing polysynthetic and carlsbad-albite twins, with weakly sericitization and argillization. K-feldspar is mainly characterized by 0.2 to 1.5 mm automorphic or subhedral carlsbad twinning laths, and Minerals 2021, 11, 404 5 of 27 some crystals are xenomorphic granular. Chlorite has a lamellar shape with a size ranging between 0.2 and 2.0 mm.
The Tuobake granite porphyry samples also have a phanerocrystalline texture with a large amount of felsic minerals (approximately 80%) and less phenocrysts (approximately 10%) of the matrix. The crystals mostly comprise quartz, plagioclase, and K-feldspar. Most quartz crystals (0.4-2.5 mm in size) are rounded, resulting from excessive melting corrosion (Figure 2e). Plagioclase often occurs as automorphic or subhedral laths with a particle size ranging between 0.5 and 2.0 mm, displaying polysynthetic and carlsbad-albite twins (Figure 2f). K-feldspar crystals also appear as automorphic or subhedral carlsbad-albite twinning laths (0.4-1.5 mm in size), and some crystals are granular.

Analytical Procedures
We selected ten rock samples (five Zengudi and five Tuobake granite porphyries) for whole-rock geochemical analyses. Major element concentrations were determined using an X-ray fluorescence (XRF) instrument (Primus II, Rigaku, Japan) at the Wuhan Sample Solution Analytical Technology Co., Ltd. (Wuhan, China). The detailed sample digestion procedure was as follows: (1) the sample powder (200 mesh) was dried in an oven at 105 • C for 12 h; (2) approximately1 g of dried sample was accurately weighed and placed in the ceramic crucible and then heated in a muffle furnace at 1000 • C for 2 h. After cooling to 400 • C, this sample was placed in the drying vessel and weighed again to calculate the loss on ignition (LOI); (3) 0.6 g sample powder was mixed with 6 g co-solvent (Li 2 B 4 O 7 :LiBO 2 :LiF = 9:2:1) and 300 mg oxidant (NH 4 NO 3 ) in a Pt crucible, which was placed in the furnace at 1150 • C for 14 min. Then, the air quenching was applied to the sample for 1 min to produce flat discs on the fire brick for the XRF analyses.
Trace element compositions were also obtained at the Wuhan Sample Solution Analytical Technology Co., Ltd. using an Agilent 7700e ICP-MS instrument. The detailed sample digestion procedure was as follows: (1) the sample powder (200 mesh) was dried in an oven at 105 • C for 12 h; (2) 0.05 g sample powder was accurately weighed and placed in a Teflon bomb; (3) 1 mL HNO 3 and 1 mL HF were slowly added into the Teflon bomb; (4) the Teflon bomb was put in a stainless steel pressure jacket and heated at 190 • C in an oven for >24 h; (5) after cooling, the Teflon bomb was opened and placed on a hot plate at 140 • C to dry the sample. The 1 mL HNO 3 was added to the sample, which was dried again; (6) 1mL HNO 3 , 1 mL MQ water, and internal standard solution (1 ppm) were added to the sample and the Teflon bomb was resealed and placed in the oven at 190 • C for >12 h; (7) the final solution was transferred to a polyethylene bottle and diluted to 100 g by adding HNO 3 (2%).
Whole-rock Sr and Nd isotope measurements were carried out at the Isotope Laboratory of Wuhan Geological Survey Center, China Geological Survey. The Sr and Nd isotopic compositions were analyzed by thermoelectric ionization mass spectrometer using a Triton instrument. Sr and Nd single element solution from Alfa (Alfa Aesar, Karlsruhe, Germany) was used to optimize the instrument operating parameters. An aliquot of the international standard solution of 200 µg·L −1 NIST SRM 987 and 200 µg·L −1 JNdi −1 were regularly used for evaluating the reproducibility and accuracy of the instrument, respectively. Typically, the signal intensities of 88 Sr in NIST SRM 987 were > approximately 4.0 V, and the signal intensities of 144 Nd + in JNdi −1 were > approximately 2.5 V. The rare earth element (REE) solution from the Sr column was evaporated to incipient dryness and taken up with 0.18 M HCl. The converted REE solution was loaded into an ion-exchange column packed with LN resin. After the completion of draining the sample solution, the columns were rinsed with 0.18 M HCl to remove undesirable matrix elements. Then, the Nd fraction was eluted using 0.3 M HCl and gently evaporated to dryness prior to mass-spectrometric measurement. The Sr and Nd isotopic data were acquired in a static mode at low resolution. The routine data acquisition consisted of ten blocks of 10 cycles (4.194 s integration time per cycle). The mass fractionation corrections for Sr and Nd isotopic ratios were normalized to  [37,38].
Zircon grains were extracted from four rock samples (two Zengudi and two Tuobake granite porphyries), using electromagnetic selection and flotation at the Langfang Diyan Mineral Separation Company (Hebei, China). About 500 zircon grains were selected from each sample, then handpicked under a binocular microscope, mounted in epoxy resin, and finally polished to expose the grain core. Cathodoluminescence (CL) images of zircon grains were obtained at the Wuhan Sample Solution Analytical Technology Co., Ltd., using a scanning electron microscope coupled with an energy dispersive spectroscopy (EDS) system and a CL 3+ detector under operating conditions of 15 kV and 4 nA. These CL images were examined for selecting zircon grains without cracks and inclusions, which were further analyzed in the same laboratory by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). Zircon U-Pb dating and trace element analyses were performed using a 193 nm ArFexcimer (COMPexPro) with laser power of 8 J/cm 2 coupled with an Agilent 7700e ICPMS instrument. The spot size and frequency of the laser were set as 32 µm and 5 Hz, respectively. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction can be found in [39][40][41]. Zircon 91500 and GJ-1 were used as external standards for U-Pb dating and trace element calibration, respectively. Concordia diagrams and weighted mean calculations were performed with Isoplot/Ex_ver3 (Isoplot4 by Kenneth R. Ludwig of American).
Subsequently, in situ Hf isotope measurements were performed on the same zircon grain or spot that was previously analyzed for U-Pb dating at the Wuhan Sample Solution Analytical Technology Co., Ltd., Hubei, China. Zircon Lu-Hf isotope analyses were conducted using a Neptune Plus MC-ICP-MS in combination with a Geolas HD excimer laser ablation system. Detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument and analytical method are the same as described by Hu et al. 2012 [42]. All analyzed spots used a beam diameter of 44 µm and pulse frequency of 8 Hz with the GJ-1 zircon standard. The 179 Hf/ 177 Hf and 173 Yb/ 171 Yb ratios were used to calculate the mass bias of Hf (βHf) and Yb (βYb), which were normalized to 179 Hf/ 177 Hf = 0.7325 and 173 Yb/ 171 Yb = 1.132685 [43] using an exponential correction for mass bias. Interference of 176 Yb on 176 Hf was corrected by measuring the interference-free 173 Yb isotope and using 176 Yb/ 173 Yb = 0.79639 [43] to calculate 176 Yb/ 177 Hf. Similarly, the relatively minor interference of 176 Lu on 176 Hf was corrected by measuring the intensity of the interference-free 175 Lu isotope and using the recommended 176 Lu/ 175 Lu =0.02656 [44] to calculate 176 Lu/ 177 Hf.

Whole Rock Major and Trace Element Compositions
The whole rock major and trace element analytical results are shown in Table 1.  Figure 3b). In addition, these granitic rocks contain low contents of P 2 O 5 (0.043-0.16%), CaO (0.71-2.15%), TiO 2 (0.23-0.77%) and Fe 2 O 3 T (1.38-5.16%). The Zengudi granite porphyries are medium-to-high K and calc-alkaline rocks (Figure 3c,d). The CIPW standard mineral calculation shows that the contents of corundum range between 1.20 and 3.18, and high differentiation index (DI) from 78.74 to 90.99. The total rare earth element contents (∑REE) of the granite porphyry samples vary from 70.75 to 207.93 ppm, with high LREE/HREE ratios of 4.70-11.94. The La N /Yb N ratios range between 4.45 and 15.95. These granitic rocks show a negative Eu anomaly with Eu/Eu * ratios ranging from 0.35 to 0.61.  In both diagrams (Figure 3a,b), the Tuobake granites exhibit high silica and aluminum contents. According to the CIPW standard mineral calculation, the contents of corundum vary between 1.88 and 3.79. The ∑REE contents of these granite porphyry samples range from 159.62 to 241.79 ppm, with high LREE/HREE ratios of 6.79-7.37. These granitic rocks display La N /Yb N ratios ranging from 8.00 to 8.81, and negative Eu anomaly with Eu/Eu* ratios of 0.38-0.47.
The chondrite-normalized REE patterns of the Zengudi and Tuobake granite porphyries show LREE enrichment and obvious negative Eu anomalies (Figure 4a). The primitive mantle-normalized trace element spider diagram shows that these rocks are significantly depleted in Sr, Ba and high field strength elements (HFSE such as Ta, Nb, and Ti), but enriched in large ion lithophile elements (LILE such as Rb, U, K, and Th) ( Figure 4b).

Whole Rock Sr-Nd Isotopes
The whole rock Sr-Nd isotopic data of the studied granite porphyries is presented in Table 2. All the Zengudi granite porphyry samples have low contents of Rb (80-110 ppm) and Sr (68-92 ppm), but high initial 87  Abstract: The boundary between the Gondwana and Yangtze plate is still controversial. In southwest China, the Sanjiang region marks the collision zone which accreted several blocks coming from the northern Gondwana margin. In this region, subduction of the Paleo-Tethys Ocean and associated continental blocks during the Triassic Period led to the formation of an N-S trending complex involving intrusive and volcanic rocks. The intrusive rocks are important for constraining the evolution of the Paleo-Tethyan in southwestern China. This study presents new geochronological, geochemical, and Sr-Nd-Hf isotopic data of granite porphyries from northern Lancangjiang, in order to discuss the origin of these granites and their tectonic significance. Representative samples of the Zengudi and the Tuobake granite porphyries from the Yezhi area yielded weighted mean 206 Pb/ 238 U ages of 247-254 Ma and 246 Ma, respectively. The Zengudi granite porphyries display zircon ԐHf(t) values of −12.94 to −2.63, ԐNd(t) values of −14.5 to −9.35, and initial 87 Sr/ 86 Sr ratios of 0.708 to 0.716. The Tuobake granite porphyries have zircon ԐHf(t) values of −14.06 to −6.55, ԐNd(t) values of −10.9 to −9.41, and initial 87 Sr/ 86 Sr ratios of 0.716 to 0.731. Both the Zengudi and Tuobake granite porphyries exhibit strongly peraluminous signatures with high A/CNK nAl2O3/(K2O + Na2O + K2O) ratios (1.07-1.86 and 0.83-1.33, respectively). These granites are enriched in Rb and Th, and depleted in Ti, Nb, Ta, Sr, and P, with negative Eu anomalies (Eu/Eu* < 0.61). These geochemical and isotopic data indicate that the primary magma of the granite porphyries originated from partial melting of ancient continental crust as a result of basaltic magma underplating and underwent fractionation crystallization during their emplacement. We propose that the Triassic subduction of the Paleo-Tethys Ocean led to crust shortening and thickening in the Sanjiang region, while the northern Lancangjiang area was involved in the continental collision after the subduction of the Paleo-Tethys Ocean before 254 Ma.

Introduction
The convergent subduction zone is an important window for understanding the interaction between the crust and mantle as well as deciphering the relationship between the magmatic rocks and their tectonic environment [1][2][3]. In subduction zones, the varia-

Zircon U-Pb Ages
The zircon U-Pb dating results for the four granite porphyry samples are shown in Table 3. Zircons from the Zengudi granite porphyries (samples SC1801 and SC1802) have Th/U ratios = 0.12-0.52 (average = 0.35), which are comparable to those of typical magmatic zircons [55]. The 206 Pb/ 238 U ages of twenty-four zircon grains from sample SC1801 range between 243.9 Ma and 261. 7 Ma, giving a weighted mean age of 253.8 ± 2.2 Ma (MSWD = 4.4) (Figure 6a). One zircon from this sample has a 206 Pb/ 238 U age of 462.8 Ma (Table 1), which is regarded as the age of inherited zircon grain. Twenty-five zircon grains from sample SC1802 yield 206 Pb/ 238 U ages of 242.9 to 259.0 Ma, with a weighted mean age of 246.6 ± 0.54 Ma (MSWD = 5.3) (Figure 6b). Overall, the crystallization age of the Zengudi granite porphyries is estimated at 254-247 Ma. Zircons from the Tuobake granite porphyries (samples YZ1803 and YZ1804) have relatively high Th/U ratios = 0.15-0.69 (average = 0.48), which are similar to those of typical magmatic zircons [53]. The 206 Pb/ 238 U ages of twenty-five zircon grains from sample YZ1803 vary between 236.4 and 263.2 Ma, with a weighted mean age of 245.9 ± 2.6 Ma (MSWD = 3.3) (Figure 6c). One zircon from this sample yields a 206 Pb/ 238 U age of 430.8 Ma (Table 1), which is considered as the age of inherited zircon grain. For the sample YZ1804, twenty-three zircon grains yield a 206 Pb/ 238 U age varying from 241.1 to 257.8 Ma, with a weighted mean age of 245.8 ± 1.8 Ma (MSWD = 2.1) (Figure 6d). Consequently, the crystallization age of the Tuobake granite porphyries is estimated at 246 Ma, similar to that of the Zengudi granite porphyries.

Zircon Hf Isotopic Compositions
Zircon Lu-Hf isotopic data of the above four granite samples are shown in Table 4. A total of thirty-one zircons from the Zengudi granite porphyries and thirty-one zircons from the Tuobake granite porphyries were analyzed for 176 Hf/ 177 Hf isotopic ratios. The 176 Lu/ 177 Hf ratios of all the analyzed zircons are less than 0.002, indicating that there is no radiogenic Hf accumulation after zircon formation [56]. Thus, the results of 176 Hf/ 177 Hf ratios can represent the Hf isotopic composition of the magmatic system.
Zircons Abstract: The boundary between the Gondwana and Yangtze plate west China, the Sanjiang region marks the collision zone which accret the northern Gondwana margin. In this region, subduction of the P ated continental blocks during the Triassic Period led to the formatio involving intrusive and volcanic rocks. The intrusive rocks are impo lution of the Paleo-Tethyan in southwestern China. This study prese ochemical, and Sr-Nd-Hf isotopic data of granite porphyries from no to discuss the origin of these granites and their tectonic significance. Zengudi and the Tuobake granite porphyries from the Yezhi area yie ages of 247-254 Ma and 246 Ma, respectively. The Zengudi granite p values of −12.94 to −2.63, ԐNd(t) values of −14.5 to −9.35, and initial 87 The Tuobake granite porphyries have zircon ԐHf(t) values of −14.06 to −9.41, and initial 87 Sr/ 86 Sr ratios of 0.716 to 0.731. Both the Zengudi an exhibit strongly peraluminous signatures with high A/CNK nAl2O3/(K 1.86 and 0.83-1.33, respectively). These granites are enriched in Rb a Ta, Sr, and P, with negative Eu anomalies (Eu/Eu* < 0.61). These g indicate that the primary magma of the granite porphyries originat cient continental crust as a result of basaltic magma underplating and tallization during their emplacement. We propose that the Triassic s Ocean led to crust shortening and thickening in the Sanjiang Lancangjiang area was involved in the continental collision after the s Ocean before 254 Ma.

Introduction
The convergent subduction zone is an important window teraction between the crust and mantle as well as decipherin the magmatic rocks and their tectonic environment [1][2][3]. In s bility of plate tectonic processes, magma sources, thermal stat the volcanic arc constrained the magmatic activity and led to t neity, and differences of igneous rocks [4][5][6][7]. Nowadays, e

Introduction
The convergent subduction zone is an important window for unders teraction between the crust and mantle as well as deciphering the relatio the magmatic rocks and their tectonic environment [1][2][3]. In subduction zo bility of plate tectonic processes, magma sources, thermal states, and crus the volcanic arc constrained the magmatic activity and led to the segmenta neity, and differences of igneous rocks [4][5][6][7]. Nowadays, exploring the
Most peraluminous granites are formed by partial melting of crustal rocks [64][65][66][67] or metamorphosed sediments such as (1) metamorphosed mudstones and sandstones [66], (2) metamorphosed felsic igneous rocks (orthogneiss) [68], and (3) metamorphosed basaltic igneous rocks [69,70]. Previous studies have documented that the granitic magma originated from the orthogneiss or sandstone source exhibits high CaO/Na 2 O ratios and low Al 2 O 3 /TiO 2 ratios [69,71]. The studied Zengudi and Tuobake granite samples show high CaO/Na 2 O ratios (0.08-11.97) and low Al 2 O 3 /TiO 2 ratios (17.09-40.32). This indicates that partial melting of orthogneiss or sandstone in the source region may have produced the original magma for these granitic rocks. In the Na 2 O/CaO versus Al 2 O 3 /TiO 2 diagram (Figure 8a), most granite samples (except samples SC-H2 and SC-H3) plot in the field of psammitolite source. The w(CaO)/w(Na 2 O) ratios are usually used as a powerful index for distinguishing the source of granitic magma [58,65]. The granites originated from a clay-rich source are characterized by w(CaO)/w(Na 2 O) ratio < 0.3, whereas w(CaO)/w(Na 2 O) ratio > 0.3 reflects granites originated from a plagioclase-rich and clay-poor clastic source [72]. In this study, the w(CaO)/w(Na 2 O) ratios of the Zengudi and Tuobake granite porphyries are mostly > 0.3 (0.08-11.97 and 0.8-1.20, respectively), indicating that their granitic magmas were generated from a plagioclase-rich and clay-poor clastic source. In the Rb/Sr versus Rb/Ba diagram (Figure 8b), all samples plot in the field of clay-poor source and show a close affinity to pelite-derived melt. This further corroborates that the Zengudi and Tuobake granite porphyries were formed by the partial melting of psammitolite.
The Nd isotopic compositions of granitic rocks and their Hf isotopes of zircons can further reflect their magma sources [9,54,66,73]. The studied Zengudi and Tuobake granite porphyries show a narrow negative range of   86 and 0.83-1.33, respectively). These granites are enriched in Rb and Th, and depleted in Ti, Nb, Ta, Sr, and P, with negative Eu anomalies (Eu/Eu* < 0.61). These geochemical and isotopic data indicate that the primary magma of the granite porphyries originated from partial melting of ancient continental crust as a result of basaltic magma underplating and underwent fractionation crystallization during their emplacement. We propose that the Triassic subduction of the Paleo-Tethys Ocean led to crust shortening and thickening in the Sanjiang region, while the northern  (Simao Terrane). Thus, we infer that the Lancangjiang Paleo-Tethys suture zone is located at the west of this study area. The geochemical analyses of the studied granites show that these granite porphyries are strongly enriched in LILE (e.g., Rb, Th, K and U) but depleted in HFSE (e.g., Ta, Nb, and Ti) (Figure 4b)

Introduction
The convergent subduction zone is an important window teraction between the crust and mantle as well as deciphering the magmatic rocks and their tectonic environment [1][2][3]. In su bility of plate tectonic processes, magma sources, thermal state the volcanic arc constrained the magmatic activity and led to th neity, and differences of igneous rocks [4][5][6][7]. Nowadays, ex   Keywords: zircon U-Pb dating; granite porphyry; northern Lancangjiang; southern China; tectonic evolution; Paleo-Tethys

Introduction
The convergent subduction zone is an important window for understanding the interaction between the crust and mantle as well as deciphering the relationship between the magmatic rocks and their tectonic environment [1][2][3]. In subduction zones, the variability of plate tectonic processes, magma sources, thermal states, and crust thicknesses in the volcanic arc constrained the magmatic activity and led to the segmentation, heterogeneity, and differences of igneous rocks [4][5][6][7]. Nowadays, exploring the plate tectonic Keywords: zircon U-Pb dating; granite porphyry; northern Lancangjiang; southern China; tectonic evolution; Paleo-Tethys

Introduction
The convergent subduction zone is an important window for understanding the interaction between the crust and mantle as well as deciphering the relationship between the magmatic rocks and their tectonic environment [1][2][3]. In subduction zones, the variability of plate tectonic processes, magma sources, thermal states, and crust thicknesses in the volcanic arc constrained the magmatic activity and led to the segmentation, heterogeneity, and differences of igneous rocks [4][5][6][7]. Nowadays, exploring the plate tectonic  Keywords: zircon U-Pb dating; granite porphyry; northern Lancangjiang; southern China; tectonic evolution; Paleo-Tethys

Introduction
The convergent subduction zone is an important window for understanding the interaction between the crust and mantle as well as deciphering the relationship between the magmatic rocks and their tectonic environment [1][2][3]. In subduction zones, the variability of plate tectonic processes, magma sources, thermal states, and crust thicknesses in the volcanic arc constrained the magmatic activity and led to the segmentation, heterogeneity, and differences of igneous rocks [4][5][6][7]. Nowadays, exploring the plate tectonic  The initial isotopic ratios of the granite porphyries were corrected to 254 Ma and 246 Ma, respectively [16,17,22]. The Jinshajiang MORB is from [74]; Gangdese I-type granite is from [75]; I-type and S-type granites in Lancangjiang fault belt (LFB) is from [76]; and the Lancangjiang granite is from [15]. The Jinshajiang MORB represents mantle-derived magmas: .

Introduction
The convergent subduction zone is an important window for understanding the interaction between the crust and mantle as well as deciphering the relationship between the magmatic rocks and their tectonic environment [1][2][3]. In subduction zones, the variability of plate tectonic processes, magma sources, thermal states, and crust thicknesses in the volcanic arc constrained the magmatic activity and led to the segmentation, heterogeneity, and differences of igneous rocks [4][5][6][7]. Nowadays, exploring the plate tectonic Keywords: zircon U-Pb dating; granite porphyry; northern Lancangjiang; so evolution; Paleo-Tethys

Introduction
The convergent subduction zone is an important window for un teraction between the crust and mantle as well as deciphering the re the magmatic rocks and their tectonic environment [1][2][3]. In subducti bility of plate tectonic processes, magma sources, thermal states, and the volcanic arc constrained the magmatic activity and led to the segm neity, and differences of igneous rocks [4][5][6][7]. Nowadays, exploring

Introduction
The convergent subduction zone is an import teraction between the crust and mantle as well as the magmatic rocks and their tectonic environmen bility of plate tectonic processes, magma sources, t the volcanic arc constrained the magmatic activity neity, and differences of igneous rocks [4][5][6][7]. No  Keywords: zircon U-Pb dating; granite porphyry; northern Lancangjiang; southern China; tectonic evolution; Paleo-Tethys

Introduction
The convergent subduction zone is an important window for understanding the interaction between the crust and mantle as well as deciphering the relationship between the magmatic rocks and their tectonic environment [1][2][3]. In subduction zones, the variability of plate tectonic processes, magma sources, thermal states, and crust thicknesses in the volcanic arc constrained the magmatic activity and led to the segmentation, heterogeneity, and differences of igneous rocks [4][5][6][7]. Nowadays, exploring the plate tectonic   (Figure 10), most granite samples plot along the crustal evolution line. Furthermore, the Nb/Ta (11.44-13.48) and Zr/Hf (37.42-38.55) ratios of the studied granite porphyries are comparable to those of the continental crust [79,80]. Combined with their significant depletion in Ba and Sr (Figure 4b), all these characteristics suggest that the granitic magmas originated from the melting of Paleoproterozoic to Mesoproterozoic crustal materials.

Magma Evolutionary Processes of the Granite Porphyries
The Zengudi and Tuobake granite porphyries exhibit a wide range of SiO 2 concentrations (69.68-87.61%). Moreover, SiO 2 shows a clear negative correlation with major oxides (e.g., TiO 2 , P 2 O 5 , CaO, Al 2 O 3 , TFe 2 O 3 , MgO) and trace elements (e.g., Sc, Cr, Sr, Ni, and Zr ( Figure 11). This suggests a strong fractionation crystallization during magma evolution for the studied granites. On the La/Sm versus La diagram (Figure 12f), it is noticeable that the fractional crystallization is the main factor controlling the magma evolution. The conspicuous negative anomalies of Eu, low ∑HREE (8.65-30.7 ppm), and flat HREE patterns (Figure 4a) suggest that there were residual minerals (e.g., plagioclase and amphibole) in the magma source. The negative anomalies of Ba, Sr ( Figure 4b) and Eu (Figure 4a) imply the fractional crystallization of amphibole and plagioclase during magmatic evolution, respectively. On the other hand, the negative correlation between Sc, Cr, Sr, Ni, and Zr versus SiO 2 ( Figure 12) further confirms that the granitic magmas have undergone hornblende and plagioclase fractional crystallization during their magmatic evolutionary processes [82].  The Y/Yb and (Ho/Yb) N ratios are commonly applied to distinguish the residual phases in the magma source [83]. If Y/Yb > 10 and Ho N /Yb N > 1.2, garnet is the main residue mineral, whereas Y/Yb ≈ 10 and (Ho/Yb) N = 1 indicate that hornblende is the residue mineral [83]. In this study, the Y/Yb ratios range between 9.66 and 12.21 (average = 11.08), and the (Ho/Yb) N ratios vary from 0.97 to 1.21 (average = 1.12). Thus, we infer that the hornblende is the major residue mineral in the magma source for the Zengudi and Tuobake granite porphyries.
In summary, the Zengudi and Tuobake granite porphyries have likely undergone strong fractionation crystallization processes of plagioclase and amphibole (e.g., hornblende).
The Zengudi and Tuobake granite porphyries are located in the west margin of the Jiangda-Weixi arc volcanic belt (see Figure 1b), which represents an important part of the east Tethys. As mentioned above, the granite porphyries of the Zengudi and Tuobake are medium-to high-potassium, calc-alkaline, and S-type granite porphyries with intrusive ages of ca. 254-247 Ma and 246 Ma, respectively. We suggest that the Early Triassic granite porphyries of this study were probably formed in a volcanic arc setting for three reasons: (1) these rocks are characterized by high Al 2 O 3 contents and LILE/HFSE and La N /Yb N ratios, low Ni and Cr contents and (Nb/La) N ratios, and conspicuous negative Nb, Ta, Ti, and P anomalies. All these features indicate a typical arc magma affinity for the studied granites (Figure 13a,b). (2) The Zengudi and Tuobake granite porphyries have similar intrusive age (254-246 Ma) to that 251-244 Ma of the bimodal volcanic rocks from the Yezhi area and other parts of the Jiangda-Weixi volcanic arc which formed under a volcanic arc setting [14][15][16][17][18]. (3) Previous studies proposed that the Late Permian to late Early Triassic and Middle to Late Triassic rocks were separately formed within volcanic arc setting and syn-collision to post-collision tectonic settings [14,15,18,23]. The granite porphyries of this study were synchronously generated with the Lincang S-type arc granites from the southern Lancangjiang [18]. Moreover, trace elements can further provide useful information to constrain the tectonic setting of granitoids [87]. In the discrimination diagrams of Rb versus Y + Nb and Nb versus Y (Figure 14a,b), most of the Zengudi and Tuobake granite porphyry samples plot in the volcanic arc field. Comparable results have been obtained using the Rb-Hf-Ta tectonic discrimination diagrams (Figure 14c,d), which confirm that the studied granite porphyries were generated in a volcanic arc setting. Therefore, a volcanic arc setting is the most plausible explanation for the generation of Late Permian to Early Triassic magmatism in the Yezhi area in the northern Lancangjiang zone. Previous research has suggested that the breakup of the Paleo-Tethys Ocean in the Lancangjiang area started in the Late Devonian and the closure of this ocean occurred in the Late Triassic [17,30,31]. In the eastern margin of the Qamdo-Simao block (Figure 1b), the age of the Jiangda-Weixi volcanism ranges from 258 to 229 Ma [16,19,20,85]. The emplacement ages of the volcanic rocks from the western margin of the Simao Basin range from 238 to 229 Ma. These rocks were considered to be formed by southwestward to northeastward subduction of the Qamdo-Changning-Menglian Paleo-Tethys oceanic basin [34].
In this study, the Zengudi and Tuobake granite porphyries have emplacement ages of Late Permian (254 Ma) to Early Triassic (246 Ma), which are comparable to the SHRIMP zircon U-Pb age (249 Ma) obtained for the adjacent Baimaxueshan pluton [19,20] and bimodal volcanic rocks [16,17]. Hence, our new geochronological data confirm that the magma emplacement in this area corresponds to the early phase of the Indosinian Orogeny. Therefore, we suggest that the S-type magmatism was synchronous to the tectonic extrusion in the Jiangda-Weixi volcanic arc [16,85], and the shortened and thickened crust in the continental arc was formed by compression. During the Late Permian to Late Triassic, the Lancangjiang and Ailaoshan Oceans subducted beneath the Baoshan and Simao blocks, respectively, while the Songma Ocean experienced a southward subduction beneath the Indochina block.
Overall, the Zengudi and Tuobake granite porphyries (254-246 Ma), the Baimaxueshan granite (249 Ma) and the Lincang granite (250 Ma) from the northern Sanjang area were synchronously emplaced within the Jiangda-Weixi volcanic arc [17,39]. Subduction eastward of the Paleo-Tethys Ocean beneath the Indochina block began before 254 Ma (Figure 15a). From 254 to 246 Ma, the continuous subduction of the Paleo-Tethys Ocean led to a strong extrusion and slab break-off of oceanic lithosphere [90] (Figure 15b). During this late stage, partial melting of the thickened crust led to the formation of the large-scale S-type magmas, resulting in the synchronous emplacement of the Zengudi and Tuobake granite porphyries and the eruption of a large volume of volcanic rocks within the Jiangda-Weixi volcanic arc.

1.
The U-Pb dating of the Zengudi and Tuobake granite porphyries from the Sanjiang area reveal that these granites were emplaced between 254 and 246 Ma, indicating that their emplacement occurred at the closure of the Paleo-Tethys Ocean during Late Permian to Early Triassic.

2.
Whole-rock geochemistry and zircon Hf isotopic compositions suggest that the granite porphyries are typical volcanic arc rocks formed in the continental arc setting after the subduction of oceanic crust. These rocks were originated from plagioclase-rich and clay-poor clastic source magmas which have likely undergone intensive fractional crystallization.

3.
The crust of the Jiangda-Weixi volcanic arc belt was thickened by eastward subduction of the Baoshan block during the closure of the Paleo-Tethys, which resulted in partial melting of the thickened crust. Institutional Review Board Statement: Not Applicable.

Informed Consent Statement: Not Applicable.
Data Availability Statement: All data generated or analysed during this study are included in this published article.