Petrogenesis of late triassic high-silica granites in the North Qiangtang terrane, central Tibetan Plateau

1 Introduction

One of the distinguishing features that sets Earth apart from other planets in our Solar System and renders it able to sustain life is the felsic continental crust (Brown, 2013; Keller et al., 2015), and granitoids are essential for the formation of Earth’s stable continental crust (Campbell and Taylor, 1983). High-silica granite (SiO₂>70 wt%) is a unique rock type among the granitoids found on Earth, and can serve as a valuable indicator of the compositional maturity of continental crust (Glazner et al., 2008; Lee and Morton, 2015; Wu et al., 2017). High-silica granites are enriched in incompatible elements (such as Li, Be, Nb, Ta, Zr, and Hf) and are closely associated with rare-metal mineralization (Wu et al., 2020; Shuai et al., 2021; Deng et al., 2022). However, the origin of high-silica granitic magmatism is uncertain. Previous studies have proposed various genetic models, including melting of metasedimentary or granitic rocks (e.g., Glazner et al., 2008; Frost et al., 2016), fractional crystallization (Lee and Morton, 2015), melt-crystal segregation (e.g., Chen et al., 2021), and magmatic-fluid interaction (Ballouard et al., 2016). Zircon is an accessory mineral widely occurring in felsic igneous rocks, and could play a crucial role in precisely recording the geological age, magma temperature, and long-term evolution of the silicic magma system (e.g., Zhu et al., 2023; Miller et al., 2003).

The Tibetan Plateau has generally been recognized as a natural laboratory for study of the Tethyan tectonic–magmatic evolution, mineralization, and continental dynamics (Xu et al., 2015). The central Tibetan Plateau is characterized by the development of the Palaeo-Tethyan orogenic system, which is composed of multiple suture zones and terranes, high-pressure metamorphic belts, and island arc groups (Xu et al., 2015). With the tectonic evolution of the Palaeo-Tethyan Ocean, Triassic intermediate-acidic magmatic rocks were widespread in the central Tibetan Plateau. However, lively debate continues on the origin, petrogenesis, and geodynamic mechanisms of these magmatic rocks. For example, there is debate as to whether the Triassic granitic magmatism was derived from the melting of the Precambrian basement materials or related to the melting of the Hohxil-Bayan Har-Songpan-Garzê (HBSG) turbidites (Liu et al., 2021; Liu et al., 2023). There are still two competing models, a “collision-related” model and a “subduction-related” model, put forward to describe the Triassic tectonic evolution of the North Qiangtang terrane and the adjacent HBSG terrane (Yuan et al., 2010; Liu et al., 2021).

In this study, we present a comprehensive study of zircon U-Pb geochronology, zircon trace elements, whole-rock geochemistry, and Sr-Nd isotopes for two suites of newly identified high-silica granites in the central Tibetan Plateau. Those new results are used to constrain the petrogenesis of the Late Triassic high-silica granites, to reveal the Triassic crustal melting, and to further understand the geodynamic relationships of these granites with the Palaeo-Tethyan Ocean.

2 Geological background and sample description

There are four main Palaeo-Tethyan sutures in the central Tibetan Plateau, namely, from north to south, the A’nyemaqen-Kunlun suture, the Garzê-Litang suture, the Jinshajiang suture, and the Longmuco-Shuanghu suture. The HBSG terrane appears as an inverted triangular structure, from which the vast Triassic deep marine turbidites develop (Figure 1A; de Sigoyer et al., 2014). This voluminous series of turbidites are dominantly derived from the Dabie ultrahigh-pressure metamorphic rocks, the Qingling-Dabie orogen, the Kunlun arc, the South China terrane, or the Central Qiangtang ultrahigh-pressure metamorphic rocks (Weislogel et al., 2006; Zhang et al., 2012; Ding et al., 2013; Jian et al., 2019; Chen et al., 2023). Late Triassic to Early Jurassic magmatic rocks are widespread in the HBSG terrane, and mainly include granitoids, minor diorites, and volcanic rocks (Liu et al., 2021). The NQ terrane consists of Paleozoic to Mesozoic sedimentary rocks, carbonate rocks, and interbedded volcanic rocks, and is underlain by the Precambrian crystalline basement, with ages of 991–1,044 Ma (He et al., 2013). With the southward subduction of the Garzê-Litang Paleo-Tethyan Ocean, Triassic volcanic rocks, diorites, and granitoids are linearly distributed along the northern margin of the NQ terrane (Liu et al., 2016a; Liu et al., 2016b; Liu et al., 2021; Liu et al., 2022; Tan et al., 2020; Yang et al., 2012).

Figure 1

Figure 1. (A) Simplified tectonic map of the Tibetan Plateau. Continental terranes: HM, Himalaya; LS, Lhasa; SQT, South Qiangtang; NQT, North Qiangtang; HBSG, Hohxil-Bayan Har-Songpan-Garzê; QDM, Qaidam; AKS, A’nyemaqen-Kunlun suture; GLS, Garzê-Litang suture; JS, Jinshajiang suture; LSS, Longmuco-Shuanghu suture. (B) Simplified geological map of the study area in the central Tibetan Plateau, showing the main strata and Triassic igneous rocks. Age data are compiled from (1, 4, 7, 8, 14, and 19) Liu et al. (2021) (2, 7, 9, 10, and 16) Liu et al. (2019) (3, 6, and 12) Wang et al. (2008) (5, 11, 12, and 13) Tan et al. (2020) (15 and 17) Wang (2009) (18) Liu et al. (2016a) (20) Zhang et al. (2013) (21) Roger et al. (2003) (22) Zhao et al. (2014), Yang et al. (2012), Liu et al. (2019), and Liu et al. (2016b).

In this study, we investigated two different suites of typical Late Triassic granitic rocks developed in the northern margin of the NQ terrane, which are represented by the Yushu granite and the Longbao granitic porphyry (Figure 1). The Yushu granite intrudes into the Zhiduo-Yushu mélange, with an exposed area of ∼1.56 km². This pluton shows medium-to coarse-grained granular textures, and its granularity increases from the edge to the center (Figure 2A). The Yushu granite is composed of quartz (30%–38%), K-feldspar (30%–35%), plagioclase (15%–25%), muscovite (3%–5%), biotite (2%–4%), and minor accessory minerals including garnet, apatite, zircon, and opaque minerals. Most of the K-feldspar grains are generally coarse-grained perthites, and their inclusions consist of plagioclase, quartz, and muscovite (Figure 2D). Plagioclase occurs as subhedral crystals, and exhibits some alterations by secondary muscovite (Figure 2E). Flaky muscovite and biotite occur as subhedral to anhedral crystals, and they are partly replaced by a small amount of chlorite (Figures 2D–G). Garnet shows euhedral or subhedral grain with minor muscovite inclusions (Figure 2G).

Figure 2

Figure 2. Field photographs and microscope photographs for (A,D–G) the Yushu granite and (B,C and H,I) the Longbao granitic porphyry. The upper halves of the microscope photographs (e, f, g, and (I) show images obtained with plane-polarized light, and the bottom halves, images obtained with perpendicular polarized light. Mineral abbreviations: Qtz, quartz; Kfs, K-feldspar; Pl, plagioclase; Bt, biotite; Ms, muscovite; Grt, garnet.

The Longbao granitic porphyry intrudes as dykes into the host diorite pluton and shows obvious chilled margins (Figures 2B, C). This granitic porphyry consists of phenocrysts (35%–40%) and matrixes (60%–65%). The phenocrysts mainly contain subhedral to xenomorphic fine-grained quartz (∼30%), K-feldspar (∼35%), plagioclase (∼32%), and biotite (∼3%). Some quartz phenocrysts show the dissolution harbor (Figure 2H). K-feldspar phenocrysts mainly contain microcline and orthoclase (Figure 2H). Plagioclase phenocrysts display typical polysynthetic twinning, and contain some quartz and biotite inclusions (Figure 2I). The matrixes are composed of the feldspar-quartz microcrystals and some cryptocrystalline substance (Figure 2H).

3 Methods

3.1 Zircon U-Pb dating and trace element composition

The selection of typical samples HX06-2 (Longbao) and RN03-1 (Yushu) for U-Pb isotopic dating was based on petrographic observations. Zircon grains were separated by conventional heavy liquid and magnetic methods and then purified under a binocular microscope. Cathodoluminescence (CL) images illustrate the internal morphology of the analyzed zircon grains (Figure 3). Zircon U-Pb geochronology analyses were conducted synchronously by LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. The details of instrument operating conditions and data processing can be found in Liu et al. (2010). Uncertainties for individual analyses are quoted at the 1 sigma level, and errors for weighted mean ages are quoted at 95% confidence. A common Pb correction was applied using the method described by Andersen (2002). Weighted mean ²⁰⁶Pb/²³⁸U ages were calculated and plotted using the ISOPLOT software.

Figure 3

Figure 3. Representative cathodoluminescence and transmitted light images of zircons from the Longbao granitic porphyry (A) and the Yushu granite (B, B1, B2) samples. The corresponding ages, trace element concentrations, and elemental ratios are presented below the CL images. Yellow circles denote the positions of analysis of LA-ICP-MS for the U-Pb isotope, with spot size of 32 μm.

3.2 Whole-rock geochemistry and Sr-Nd isotopes

The whole-rock major element analyses were performed at the Analytical Institute of the Bureau of Geology and Mineral Resources, Hubei Province, China. The analytical precision of the spectrometer employed was better than 5%. Trace elements were measured using an Agilent 7500a ICP-MS instrument at the GPMR, following the methods of Liu et al. (2008). The samples were digested with a mixture of HF + HNO₃ for ICP-MS analyses.

Whole-rock Sr-Nd isotopic compositions were determined using a Thermo-type Neptune Plus MC-ICP-MS instrument at the GPMR. Mass fractionation corrections for the isotopic ratios were based on ⁸⁸Sr/⁸⁶Sr = 8.375209 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.721900. The JNdi-1 and NBS 987 standards were measured at a mean ⁸⁸Sr/⁸⁶Sr ratio of 0.710274 ± 0.000009 and a mean ¹⁴⁶Nd/¹⁴⁴Nd ratio of 0.512118 ± 0.000009, respectively.

4 Results

4.1 Zircon internal texture and U-Pb geochronology

A Longbao granitic porphyry sample (HX06-2) and a Yushu granite sample (RN03-2) were selected for zircon U-Pb isotopic dating; the results are listed in Table 1.

Table 1

Table 1. LA-ICP-MS zircon U-Pb data for the Longbao granitic porphyry (HX06-2) and the Yushu granite (RN03-2), central Tibetan Plateau.

Zircons from the sample HX06-2 were euhedral-subhedral prismatic crystals, and they had lengths of ∼50–130 μm, with an aspect ratio of 1:1 to 2:1. Most of them exhibited apparent oscillatory zoning, and some showed resorbed cores on CL images. Ten analyses of the zircon rims yielded younger ²⁰⁶Pb/²³⁸U ages of 212–222 Ma. The weighted mean ²⁰⁶Pb/²³⁸U age was 217 ± 3 Ma (MSWD = 0.51; Figures 4A–C), which represents the crystallization age of the Longbao granitic porphyry. The other ten spots yielded relatively old ²⁰⁶Pb/²³⁸U ages of 225–989 Ma, which can be interpreted as the ages of the inherited zircons.

Figure 4

Figure 4. Zircon U-Pb concordia diagrams (A,B) and (D,E) and weighted mean ages (C,F) for magmatic zircons of the Longbao granitic porphyry and the Yushu granite from the central Tibetan Plateau. Weighted mean ages for pooled ²⁰⁶Pb/²³⁸U analyses are quoted with the corresponding 95% confidence interval.

Zircons from the sample RN03-2 were euhedral to subhedral prismatic crystals with light yellow to brown colors. Their lengths ranged from 40 μm to 150 μm. Unlike the monotonous oscillatory zoning of zircon in the Longbao granitic porphyry, zircons from the Yushu granite showed complex internal structures, as revealed by CL and transmitted light images (Figure 3B). Zircon grains from the Yushu granite could be subdivided into two groups based on zircon internal structure and age. Group 1 zircons showed light-CL rims and oscillatory zonings on CL images, and they were transparent under transmitted light (Figure 3B), which makes them similar to the zircons from the Longbao granitic porphyry. Group 2 zircons exhibited distinct dark-CL rims on CL images, and these zircons were murky brown and generally translucent under transmitted light (Figure 3B). Nine spots on the zircon with light-CL rims yielded ²⁰⁶Pb/²³⁸U ages ranging from 207 Ma to 217 Ma, with a weighted mean ²⁰⁶Pb/²³⁸U age of 214 ± 2Ma (MSWD = 1.3). In the other group of zircons with dark-CL rims, eleven rim spots yielded ²⁰⁶Pb/²³⁸U ages of 196–204 Ma, with a weighted mean ²⁰⁶Pb/²³⁸U age of 200 ± 2 Ma (MSWD = 2.2; Figures 4D–F). In addition, fifteen analysis spots on old inherited cores yielded ²⁰⁶Pb/²³⁸U ages between 221 and 308 Ma.

4.2 Zircon trace element composition and Ti-in-zircon geothermometer

Trace element concentrations of zircon grains from the Longbao granitic porphyry sample (HX06-2) and the Yushu granite sample (RN03-2) were determined by LA-ICP-MS; the results are tabulated in Table 2. The results of magmatic zircon analysis were plotted to assess the conditions of zircon growth during the generation of the respective intrusions (Figure 5).

Table 2

Table 2. Trace elements of zircons for the Longbao granitic porphyry (HX06-2) and the Yushu granite (RN03-2) in the central Tibetan Plateau.

Figure 5

Figure 5. Zircon trace element (TE) results for the Longbao granitic porphyry (A) and the Yushu granite (B,C) samples. Graphs show CI chondrite-normalized (McDonough and Sun, 1995) rare earth element (REE) plots.

Zircons from the Longbao granitic porphyry exhibited steeply increasing chondrite-normalized REE patterns from La to Lu, and positive Ce and negative Eu anomalies (Figure 5A). The patterns for these zircons indicated low Hf, Y, Th, and U concentrations, with relatively high Ti concentrations and Th/U ratios (Figure 5B and Figure 6), which are similar to those of typical unaltered igneous zircons, as described by Hoskin and Schaltegger (2003).

Figure 6

Figure 6. Zircon trace element concentrations and ratios plotted against Hf concentration (left column) and age (right column) for the Longbao granitic porphyry and the Yushu granite samples.

The two groups of zircons from the Yushu granite exhibited markedly different trace element compositions. Zircons with light-CL rims had low Hf and Y concentrations, with moderate Th and U concentrations and Th/U ratios. Additionally, the chondrite-normalized REE patterns were steep, with positive Ce and negative Eu anomalies (Figure 5C). These could also be comparable to the characteristics of typical unaltered igneous zircons. However, zircons with dark-CL rims showed enrichment in LREE and MREE and high abundances of Hf, Y, Th, and U, with low Th/U ratios (Figure 6). These features are distinct from those of unaltered magmatic zircon, which could be interpreted as indicative of hydrothermal origin (Hoskin, 2005). Thus, the weighted mean ²⁰⁶Pb/²³⁸U age of 214 ± 2Ma can be interpreted as the crystallization age of the Yushu granite, while the younger weighted mean ²⁰⁶Pb/²³⁸U age of 200 ± 2 Ma for the dark rims may reflect the Pb loss caused by the late hydrothermal process. To estimate the crystallization temperature for minerals removed from their original petrologic context, we selected the Ti-in-zircon thermometry method of Ferry and Watson (2007). It is necessary to consider the activity factors of ${\mathrm{\alpha }}_{{\text{TiO}}_2}$ and ${\mathrm{\alpha }}_{{\text{SiO}}_2}$ before the application of Ti-in-zircon thermometry. Recent modeling results for different granite types have shown that SiO₂-rich and peraluminous granites have a narrow ${\mathrm{\alpha }}_{{\text{TiO}}_2}$ range of ∼0.5 and relatively consistent ${\mathrm{\alpha }}_{{\text{SiO}}_2}$ values of ∼1 (Schiller and Finger, 2019). Because all the samples were characterized by high SiO₂ contents and were slightly to strongly peraluminous, activity factors of ${\mathrm{\alpha }}_{{\text{SiO}}_2}$ = 1 and ${\mathrm{\alpha }}_{{\text{TiO}}_2}$ = 0.5 were used to estimate the Ti-in-zircon temperature. Calculations for zircons with light-CL rims from the Yushu granite showed a relatively high Ti-in-zircon temperature (771–913°C, average 842°C), and zircons from the Longbao granitic porphyry also exhibited a markedly high Ti-in-zircon temperature (873–972°C, average 904°C).

4.3 Whole-rock major and trace elements

The results of whole-rock geochemical analyses and the relevant parameters calculated are listed in Table 3. LOI values were relatively low (0.25–0.59 wt%), and neither the mobile nor the immobile elements exhibited any correlation with LOI (Supplementary Figure S1), suggesting insignificant alteration.

Table 3

Table 3. Bulk-rock major and trace elements data for the Yushu granite and the Longbao granitic porphyry, central Tibetan Plateau.

Samples of the Longbao granitic porphyry were characterized by high SiO₂ (75.98–76.78 wt%) and K₂O (4.40–5.43 wt%) content, but low CaO (0.99–1.15 wt%), TiO₂ (0.10–0.16 wt%), FeO^T (0.76–1.03 wt%), MgO (0.25–0.38 wt%), and P₂O₅ (0.02–0.04 wt%) content (Table 3). These were plotted in the field of high-K calc-alkaline and magnesian series in classification diagrams (Figures 7B, C). In addition, those samples were slightly peraluminous, with A/CNK ratios ranging from 1.02 to 1.06 (Figure 7D). Those samples had higher total rare earth element concentrations (∑REE = 103.8–125.2 ppm) than the Yushu granite samples. In chondrite-normalized REE diagrams, the samples were characterized by enrichment of LREE relative to HREE [(La/Yb)_N = 12.50–18.68], a flat HREE pattern [(Gd/Yb)_N = 0.92–1.32], and variable negative Eu anomalies (δEu = 0.43–0.76; Figure 8A). On a primitive mantle-normalized spider diagram, all the samples showed obvious depletion of Ba, Nb, Sr, P, and Ti, but enrichment of Th, U, and Rb (Figure 8B).

Figure 7

Figure 7. Classification diagrams for the granites studied. (A) (Na₂O+ K₂O) vs SiO₂ diagram (Middlemost, 1994); alkaline field (A) and sub-alkaline field (SA) after Irvine and Baragar (1971). (B) K₂O vs SiO₂ diagram (Peccerillo and Taylor, 1976). (C) FeO^T/(FeO^T + MgO) vs SiO₂ diagram (Frost et al., 2001). (D) A/NK vs A/CNK diagram (Maniar and Piccoli, 1989).

Figure 8

Figure 8. Chondrite-normalized REE patterns (A,C) and primitive mantle-normalized spider diagrams (B,D) for the granites studied. Normalizing values of chondrite and primitive mantle are from Taylor and McLennan (1985) and Sun and McDonough (1989), respectively.

Samples of the Yushu granite showed high SiO₂ (75.31–75.99 wt%) and K₂O (3.69–4.43 wt%) content, but low CaO (0.95–1.65 wt%), TiO₂ (0.03–0.08 wt%), FeO^T (0.69–1.05 wt%), MgO (0.15–0.30 wt%), and P₂O₅ (0.01–0.02 wt%) content (Table 3). They were plotted in the field of high-K calc-alkaline and magnesian series according to classification diagrams (Figures 7B, C). In addition, these samples were slightly to strongly peraluminous, with A/CNK ratios ranging from 1.05 to 1.13 (Figure 7D). The samples were characterized by moderate concentrations of total rare earth elements (∑REE = 93.2–98.6 ppm) and negative Eu anomalies (δEu = 0.25–0.55). On a chondrite-normalized diagram, all the samples exhibited flat REE patterns [(La/Yb)_N = 1.13–3.23 (Gd/Yb)_N = 0.77–1.13] (Figure 8C). On a primitive mantle-normalized spider diagram, all the samples were enriched in large-ion lithophile elements (LILEs), such as Rb, Th, and U, and exhibited depleted high field strength elements (HFSEs), such as Sr, Ba, P, and Ti (Figure 8D).

4.4 Whole-rock Sr-Nd isotopes

Whole-rock Sr-Nd isotopic compositions of the granites studied are listed in Table 4; initial Sr and Nd isotopic ratios (I_Sr) and ε_Nd(t) values were calculated based on the timing of 215 Ma. The sample from the Longbao granitic porphyry showed an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7120 and ε_Nd(t) value of −7.58, with the corresponding depleted-mantle model age (T_2DM) of 1.61 Ga. The samples from the Yushu granite were characterized by relatively high (⁸⁷Sr/⁸⁶Sr)_i ratios, in the range of 0.7121–0.7136, and low ε_Nd(t) values of −8.58 to −7.97, with a T_2DM age of 1.64–1.69 Ga.

Table 4

Table 4. Bulk-rock Sr-Nd isotopic compositions of the typical samples in the central Tibetan Plateau.

5 Discussion

5.1 Genetic types

In a seminal paper, Chappell and White (1974) classified the granites of the Lachlan Fold Belt (LFB) in south-eastern Australia into two end-members: I- and S-type granites. This classification has become well-established in most granites that have diverse sources and has developed into the current I-S-M-A classification of granitoids. M-type granites, which are distinguished by relatively low K₂O (<1.9 wt%) and Rb (<48 ppm) values, are derived directly from the melting of subducted oceanic crust or overlying mantle (Saito et al., 2004). A-type (alkaline, anhydrous, and anorogenic) granites exhibit notably high values of 10,000×Ga/Al (>2.6) and Zr+Nb+Ce+Y (>350 ppm) (Whalen et al., 1987), which are associated with ferroan granitic melts (Frost et al., 2001). The samples of Yushu granite and Longbao granitic porphyry were plotted in the field of high-K calc-alkalic and magnesian series (Figure 7) and exhibited low values of 10,000×Ga/Al; these characteristics do not correspond with those of the A- and M-type granites. The granites studied consisted primarily of plagioclase, alkali-feldspar, and quartz, with minor biotite and an absence of amphibole (Figure 2), tending to be close to haplogranite (near minimum-temperature melt; King et al., 1997). Additionally, it is very difficult to discriminate them as I-type or S-type, because both high-silica I- and S-type granites usually have analogous geochemical compositions and mineral assemblages (Wu et al., 2017).

5.2 Magma source and crustal melting

It is widely accepted that Sr-Nd isotopes can trace the magma sources of highly evolved granites, because it is difficult for magma differentiation to affect ε_Nd(t) values and Sm/Nd ratios (DePaolo, 1988). Samples from the Longbao granitic porphyry and the Yushu granite exhibited relatively high initial ⁸⁷Sr/⁸⁶Sr isotope ratios (0.7120–0.7136) and negative ε_Nd(t) values (−8.58 to −7.58), suggesting a large contribution from crustal components.

Two potential candidates involving the widespread Triassic turbidites and the Precambrian basement rocks have been proposed for the crustal sources of Triassic intermediate to acid rocks in the central Tibetan Plateau (Liu et al., 2021). Previous studies have demonstrated that the granites derived from melting of the Precambrian basement rocks had higher initial ⁸⁷Sr/⁸⁶Sr isotope ratios (0.7240–0.7390) and noticeably more negative ε_Nd(t) values (−13.26 to −11.13; Tao et al., 2014; Liu et al., 2023), which are distinct from those of the high-silica granites studied here. The Sr-Nd isotopic compositions of the two suites of high-silica granites overlapped with those of the Triassic turbidites in the HBSG terrane (Figure 9), further indicating that the widespread turbidites might be a good candidate for the crustal sources.

Figure 9

Figure 9. Plot of ε_Nd(t) vs (⁸⁷Sr/⁸⁶Sr)_i for the granites studied. Data sources: Triassic turbidites and Paleozoic metasediments in the HBSG terrane are from de Sigoyer et al. (2014), She et al. (2006), Zhang et al. (2012), and Zhao et al. (2022); Late Triassic S-type granitoids that were derived from the Paleoproterozoic basement rocks in the NQ terrane are from Liu et al. (2023) and Tao et al. (2014).

High-silica granites are usually considered to originate from pure crustal anatexis, and are usually linked with the process of crustal evolution (Clemens and Stevens, 2012). It is widely accepted that the continental crustal material is not water-enriched, and that the dehydration–melting reaction is the controlling factor in generating the crustal magma (Weinberg and Hasalová, 2015; Wu et al., 2020). Partial melting under vapor-absent and/or vapor-present conditions could produce different residual mineral phases and could cause various distributions of trace elements in the melts (Harris and Inger, 1992). Under vapor-absent conditions, the initial melt would have lower Ca, Sr, and Ba content and higher Rb/Sr ratios compared to vapor-present conditions, because more micas (enrichment of Rb than Sr) and less plagioclase (enrichment of Sr than Rb) are consumed (Harris and Inger, 1992; Gao et al., 2017). Although fractional crystallization of plagioclase could also result in low CaO and Sr content and a high Rb/Sr ratio, this was not the case for the high-silica granites examined. All the samples from the Longbao granitic porphyry had δEu values above 0.4, and this was also the case for most samples from the Yushu granite. These features indicate a relatively low to moderate degree of plagioclase fractionation (Gao et al., 2017; Zeng and Gao, 2017). Furthermore, fractional crystallization might be not the main factor controlling the generation of high-silica magma, because felsic melts usually have particularly high viscosity (Glazner, 2014). Thus, the low CaO and Sr content and the high Rb/Sr ratio might be more likely to have originated from the composition of the magma source, supporting the dehydration melting reaction (Figure 10). In addition, the proportions of An, Ab, and Or could represent the differences between vapor-absent and vapor-present conditions (Weinberg and Hasalová, 2015). On FeO-Na₂O/K₂O and An-Ab-Or diagrams based on experimental petrological data, the granites studied exhibited trends following the dehydration melting of Bt-Ms schists reported by Patiño Douce and Harris (1998) (Supplementary Figures S1A,B. Moreover, experimental studies have shown that melts derived from Ms-bearing and/or Bt-bearing sources exhibit higher K₂O/Na₂O ratios (>1.0) than melts derived from Amp-bearing sources (<1.0) (Gao et al., 2016). Samples from the Longbao granitic porphyry and the Yushu granite had relatively high ratios of K₂O/Na₂O (>1.0), suggesting that they might originate from Bt- and/or Ms-bearing sources. The variation in Rb/Sr, Sr, and Ba further supports the melting of a Ms-bearing crustal source (Figure 10).

Figure 10

Figure 10. (A) FeO vs Na₂O/K₂O diagram. (B) An-Ab-Or normative triangle diagram. The circle of dehydration melting of Bt-Ms schist is from Patiño Douce and Harris (1998). Gray arrows point in the direction of melt composition change from dehydration melting to water-present melting. (C) Rb/Sr vs Sr diagram. (D) Rb/Sr vs Ba diagram (Inger and Harris, 1993). VP, vapor-present; VA, vapor-absent; Bt, biotite; Ms, muscovite.

5.3 Geodynamic relationship with the paleo-tethyan ocean

The Triassic tectonic evolution of the NQ terrane and adjacent HBSG terrane remains hotly debated, and two conflicting models, the “collision-related model” and the “subduction-related model,” have been employed to interpret the generation of Triassic magmatism in the central Tibetan Plateau (Zhang et al., 2007; Yuan et al., 2010; Liu et al., 2021).

The “collision-related model” proposes that the Palaeo-Tethyan ocean closed during Middle Triassic times, and the subsequent crustal thickening and lithospheric delamination triggered the Middle to Late Triassic magmatism (Zhang et al., 2007; Yuan et al., 2010). However, it can be argued based on the identification of a Middle to Upper Triassic submarine fan in the HBSG terrane (Ding et al., 2013) that the ocean could not have closed until Late Triassic times. The zircon U-Pb ages of MORB-type mafic rocks from the ophiolites in the Garzê-Litang suture zone are 239–232 Ma, implying that the Palaeo-Tethyan ocean still existed during Middle Triassic times (Liu et al., 2016a). A series of adakites-high-Mg andesites-Nb-rich basalts with ages of ∼229 Ma have been identified in the Tuotuohe area, indicating that the oceanic subduction might have begun at ∼229 Ma (Wang et al., 2008). With the southward subduction of the Garzê-Litang ocean, arc volcanic rocks with ages of 230–209 Ma (Yang et al., 2012; Zhao et al., 2014; Liu et al., 2016b; Liu et al., 2020; Liu et al., 2022) developed along the northwest–southeast direction (Figure 1). In addition, new paleolatitude data indicate that the final closure of the Palaeo-Tethyan ocean might have occurred at 213–204 Ma (Song et al., 2015). The timing of the ductile deformation of the collision stage may be recorded in metamorphic minerals (e.g., biotite and muscovite), and ⁴⁰Ar-³⁹Ar analyses of these have yielded ages of 201–193 Ma (Yang et al., 2012; Zhang et al., 2013). Therefore, the high-silica granites (217–214 Ma) in this study were probably formed in a subduction environment.

Moreover, samples from the Longbao granitic porphyry and the Yushu granite exhibited high apparent Ti-in-zircon temperatures, indicating a significant thermal anomaly at the deep crustal levels. Wang et al. (2008) proposed that the oceanic lithosphere might have subducted to depths of ∼85 km at ∼229 Ma. With this deepening of the depths of subduction, intensive eclogite-facies metamorphism increased the density of the subducted oceanic lithosphere, subsequently leading to slab roll-back (Liu et al., 2021). Slab roll-back in turn would induce upwelling of the asthenosphere and provide sufficient heat for crustal melting. If this was the case, partial melting of a Ms-bearing crustal source would have induced the Late Triassic magmatism in the North Qiangtang terrane, including the Longbao granitic porphyry and the Yushu granite.

6 Conclusion

(1) Two suites of Late Triassic high-silica granites have been identified in the North Qiangtang terrane in the central Tibetan Plateau. The results of zircon U-Pb dating indicate that the crystallization ages of those high-silica granites are 217–214 Ma.

(2) The Late Triassic high-silica granites were mainly produced by dehydration melting of a Ms-bearing source, and the Triassic turbidites might be a good candidate for the magma source.

(3) The high-silica granites (217–214 Ma) examined in this study were probably formed in a subduction environment, and partial melting of Triassic turbidites induced by slab roll-back might be the key factor controlling the origin of Late Triassic magmatism in the North Qiangtang terrane.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author contributions

CJ: Writing–original draft. BL: Writing–review and editing. YC: Investigation, Writing–review and editing. ZW: Writing–review and editing. CC: Writing–review and editing.

Funding

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This work has obtained financial support from the National Natural Science Foundation of China (Grant 41602122 and 41502050), the China Geological Survey (Grants DD20230024 and DD20230315), the basic scientific research project of the Chinese Academy of Geological Sciences (Grant JKY202209).

Acknowledgments

We thank Zhao-Chu Hu, Lian Zhou, and Hai-Hong Chen for their help with the analytical work.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/feart.2024.1354185/full#supplementary-material

SUPPLEMENTARY FIGURE S1:Plots of FeOT, MgO, TiO2, P2O5, Na2O, K2O, Nb, Sr, Rb, Ba, La, and Th versus LOI for the studied granites studied.

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