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Article

Chronology, Geochemistry, and Tectonic Implications of Early Cretaceous Granitoids in the Ranwu Area, Eastern Gangdese Belt

by
Xinjie Yang
1,
Meiling Dong
1,*,
Yanyun Wang
1,
Chao Teng
1,
Dian Xiao
2,
Jun Cao
1,
Xiqing Chen
1 and
Jie Shao
1
1
Cores and Samples Center of Natural Resources, China Geological Survey, Langfang 065201, China
2
Sichuan-Tibet Technology Innovation Center (Chengdu) of National Railway Co., Ltd., Chengdu 610213, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(11), 1188; https://doi.org/10.3390/min15111188
Submission received: 30 September 2025 / Revised: 7 November 2025 / Accepted: 10 November 2025 / Published: 12 November 2025
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

The Gangdese Belt is sandwiched between the Yarlung–Zangbo Suture Zone (YZSZ) and the Bangong–Nujiang Suture Zone (BNSZ), and witnessed large-scale magmatic activity during the Early Cretaceous period. Currently, controversies remain regarding the petrogenetic mechanism and tectonic setting of the Early Cretaceous magmatism in the eastern Gangdese Belt. To clarify these controversies, this study conducted systematic petrogeochemical analysis on the Ranwu pluton in the eastern Gangdese Belt. The results show that the main rock types of the Ranwu pluton are monzogranite and granodiorite. The LA-ICP-MS zircon U-Pb ages of the monzogranite (sample CZTW1105) and granodiorite (sample CZTW2051) are 116.3 ± 0.5 Ma and 114.6 ± 0.6 Ma, respectively, collectively indicating that the Ranwu pluton formed during the Early Cretaceous period. The results of in situ zircon Hf isotope analysis show that the εHf(t) values range from −5.0 to 0.5, with corresponding Hf crustal model ages (TDMC) of 1139–1494 Ma. The Ranwu pluton belongs to the high-K calc-alkaline series and is classified as I-type granite. Combined with geochemical characteristics and tectonic setting discrimination diagrams, it is determined that the granites of this period have geochemical signatures of post-collisional granites and formed in a tectonic setting during the transition from a compressional to an extensional regime. The occurrence of Early Cretaceous post-collisional granites marks the end of the main orogenic stage in the Bangong–Nujiang Suture Zone, and the Gangdese Belt has since transitioned into a tectonic environment of the post-orogenic extensional stage.

1. Introduction

The Gangdese Belt, bounded by the Yarlung–Zangbo and Bangong–Nujiang suture zones, is one of the largest and most complex magmatic belts on the Tibetan Plateau, characterized by frequent magmatic episodes and diverse rock assemblages. Magmatic activity in this belt is widely regarded as closely related to the India–Eurasia plate collision, making it a long-standing focus of the international geological community [1,2,3]. The eastern segment of the Gangdese Belt witnessed large-scale magmatism during the Early Cretaceous period [4]. However, due to the lack of high-quality geochronological and geochemical data, the petrogenesis and geodynamic setting of this magmatism remain relatively poorly understood. Previous studies have proposed several mechanisms for the Early Cretaceous magmatism in the eastern Gangdese Belt, involving: (1) southward subduction of the Bangong–Nujiang oceanic crust beneath the Lhasa Terrane and slab break-off [5,6,7]; (2) remelting of the thickened continental crust during or after the collision between the Gangdese and Qiangtang Blocks following the closure of the Bangong–Nujiang Ocean [1,8]; (3) low-angle or flat-slab northward subduction of the Neo-Tethys (Yarlung–Zangbo) oceanic crust beneath the Lhasa Terrane [9,10]; (4) a bidirectional subduction system involving southward subduction of the Bangong–Nujiang oceanic crust and northward subduction of the Neo-Tethys oceanic crust beneath the Lhasa Terrane [11]; (5) a post-collisional extensional setting after the collisional compression between the Gangdese and Qiangtang Blocks [12,13].
The Ranwu pluton is located in the eastern segment of the Gangdese Belt. Previous studies have primarily focused on magma mixing processes within this pluton [14]. Based on detailed field investigations and systematic petrological studies, combined with petrogeochemical, zircon U-Pb geochronological, and zircon Hf isotopic analyses, this paper focuses on exploring the transition process from syn-collisional to post-collisional tectonic regimes in this area and determining its tectonic setting. By integrating additional datasets, this study provides key evidence for constraining the geodynamic background of the Early Cretaceous magmatism in the eastern Gangdese Belt.

2. Regional Geological Setting

Generally bounded by the Yarlung–Zangbo Suture Zone (YZSZ), the Bangong–Nujiang Suture Zone (BNSZ), and the Jinsha Suture Zone (JSSZ), the Tibetan Plateau is divided from south to north into the Himalayan Belt, the Gangdese Belt, the Qiangtang Block, and the Songpan–Ganzi Flysch Complex (Figure 1a). The Gangdese Belt is situated between the Yarlung–Zangbo Suture Zone (YZSZ) and Bangong–Nujiang Suture Zone (BNSZ), generally extending in a nearly east–west direction. It is approximately 2500 km long, 150–300 km wide from north to south, and covers an area of about 450,000 km2. Based on the distribution of the Shiquan River–Nam Tso Mélange Zone (SNMZ), Gar–Lunggar–Comai Fault (GLCF), and Luobadui–Milashan Fault (LMF), the Gangdese Belt can be further subdivided into four secondary units from south to north: South Gangdese (SG), Gangdese Retroarc Uplift Belt (GRUB), Central Gangdese (CG), and North Gangdese (NG) (Figure 1b).
South Gangdese is dominated by the Gangdese batholith and Linzizong volcanic rocks, with sporadic distributions of Jurassic–Cretaceous sedimentary strata. The Gangdise batholith is mainly composed of Late Cretaceous to Eocene granites. The Linzizong volcanic rocks are a set of Paleocene–Eocene volcanic rock strata in angular unconformity contact with the underlying sequences. The Gangdese Retroarc Uplift Belt (GRUB) is mainly composed of the Neoproterozoic Nyainqêntanglha Group, Carboniferous–Permian, and minor Triassic sedimentary strata. The eastern part of this secondary unit developed abundant Mesozoic intrusive rocks and is thrust southward over the South Gangdese along the Luobadui–Milashan Fault (LMF). Central Gangdese (CG) is primarily developed with Late Jurassic–Early Cretaceous volcano-sedimentary strata, which widely expose acidic volcanic rocks, volcaniclastic rocks, and their associated intrusive rocks. North Gangdese (NG) is mainly composed of Jurassic–Cretaceous volcano-sedimentary strata and their related intrusive rocks [4,14].
The study area is tectonically located in the eastern segment of the North Gangdese Belt (NG) and is administratively located in Ranwu Town, Baxoi County, Qamdo City, Tibet Autonomous Region. In the central–southern part of the area, the Early–Middle Proterozoic Demala Group, a set of amphibolite-facies metamorphic rock series, is extensively exposed and is considered part of a regionally extensive Precambrian metamorphic basement [15]. In the central part of the study area, the Devonian strata are poorly exposed, predominantly composed of limestone, whereas the Carboniferous–Permian sedimentary sequences are more widely outcropping and locally intercalated with volcanic rocks. Mesozoic strata in the northeastern part are mainly distributed along the Nujiang River or in intermontane basins. Almost all Early Mesozoic strata, as well as pre-Early Mesozoic strata and metamorphic rock series, have been intruded by Mesozoic–Cenozoic granites (Figure 1c).
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Figure 1. (a) Division of tectonic units of the Tibetan Plateau [7]; (b) Division of tectonic units of the Gangdese Belt [7]; (c) Simplified regional geological map of the study area [8,16,17].
Figure 1. (a) Division of tectonic units of the Tibetan Plateau [7]; (b) Division of tectonic units of the Gangdese Belt [7]; (c) Simplified regional geological map of the study area [8,16,17].
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3. Petrographic Characteristics

The Ranwu pluton trends NW (Figure 1c), with monzogranite and granodiorite constituting the dominant lithologies.
Monzogranite: Grayish-white in color, displaying a medium- to fine-grained granitic texture and massive structure. The principal mineral assemblage comprises plagioclase (30 vol%), K-feldspar (40 vol%), quartz (25 vol%), and biotite (5 vol%). Accessory minerals are predominantly zircon and apatite (Figure 2a,b).
Granodiorite: Grayish-white in color, exhibiting a fine-grained granitic texture and massive structure. The principal mineral assemblage consists of plagioclase (35%), K-feldspar (25 vol%), quartz (25 vol%), biotite (12 vol%), and hornblende (3 vol%). Accessory minerals include zircon, apatite, allanite, and titanite (Figure 2c,d).
Plagioclase grains throughout the rock are commonly altered by K-feldspar, with myrmekitic texture locally observable. Alteration affecting plagioclase manifests as minor kaolinization, sericitization, and local epidotization. The K-feldspar variety is perthitic and displays minor kaolinization. Biotite alteration is characterized by chloritization, epidotization, and minor limonitization. Hornblende occurs as subhedral prismatic to elongate prismatic crystals exhibiting green and brown coloration (Figure 2).

4. Analytical Methods and Results

Sample preparation for zircon dating was conducted at the Institute of Hebei Regional Geology and Mineral Survey (IHRGMS) in Langfang, Hebei Province. Zircon grains were separated using magnetic separation, heavy liquid separation, and gravity concentration techniques. Euhedral, transparent zircons devoid of visible fractures were hand-picked under a binocular microscope, mounted in epoxy resin on glass slides, and polished to expose the grain interiors. After preparation, cathodoluminescence (CL), reflected light, and transmitted light images of the zircons were acquired to characterize their external morphology, surface features, and internal structures, facilitating the selection of optimal analytical spots.
Zircon U-Pb dating and trace element analysis via laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) were performed simultaneously at Wuhan SampleSolution Analytical Technology Co., Ltd., Wuhan, China. The system comprised a GeoLasPro laser ablation unit (equipped with a COMPexPro 102 ArF 193 nm excimer laser and MicroLas optics, Göttingen, Germany) coupled to an Agilent 7900 ICP-MS, Santa Clara, CA, USA. Analyses employed a 32 μm laser spot diameter at a 5 Hz repetition rate. Trace element calibration standard sample: NIST 610; Isotope ratio calibration standard sample: 91500; Isotope ratio monitoring standard sample: GJ-1. The above samples are all international reference materials, and the recommended values are cited from GeoRem (http://georem.mpch-mainz.gwdg.de/). Detailed instrumental parameters and procedures follow Liu et al. (2008, 2010) [18,19]. Data reduction was performed using ICPMSDATACAL10.8 [19], with common Pb correction applied following Andersen (2002) [20]. Concordia diagrams and age calculations were generated using Isoplot (ver. 3.0; Ludwig, 2003) [21].
In situ zircon Lu-Hf isotope analysis was conducted at Wuhan SampleSolution Analytical Technology Co., Ltd. using a laser ablation multi-collector ICP-MS (LA-MC-ICP-MS) system. The setup consisted of a GeoLas HD laser ablation system (Coherent, Göttingen, Germany) coupled to a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Bremen, Germany). Analyses were performed with a laser energy density of ~10.0 J/cm2 in single-spot mode using a 44 μm spot size. Instrumental conditions and data acquisition protocols were described by Xu et al. (2004) [22]. The standard zircon 91500 was used in this correction to the 176Yb–176Hf interference, showing variable 176Yb/177Hf ratios of 0.0113–0.0122 with an average of 0.0118 for 91500. The 176Lu decay constant of λ = 1.867 × 10−11 yr−1 [23] was used to calculate the initial 176Hf/177Hf ratios. The chondritic values of 176Hf/177Hf = 0.0332 and 176Lu/177Hf = 0.282772 [24] were used in our calculation of the εHf(t) values. The depleted mantle evolution line is defined by (176Hf/177Hf)DM = 0.28325 and (176Lu/177Hf)DM = 0.0384 [25]. An 176Lu/177Hf ratio of 0.015 for the mean continental crust was used to calculate TDMC.
Rock slab preparation and major element analysis were completed at the Laboratory of the Cores and Samples Center of Natural Resources, China Geological Survey. Fresh samples were trimmed to remove weathered surfaces, coarsely crushed to 2–4 cm fragments, ultrasonically cleaned in 5% dilute HCl to remove surface contaminants, and pulverized to <200 mesh. Major elements were determined by wavelength-dispersive X-ray fluorescence spectrometry (XRF) with analytical precision better than ±5%. Trace element analysis was performed at the IHRGMS using an XSeries2 ICP-MS (Thermo Fisher Scientific, Bremen, Germany), maintaining both precision and accuracy within ±5%.

4.1. Zircon U–Pb Geochronology Results

Two samples from the Ranwu pluton, including monzogranite (CZTW1105) and granodiorite (CZTW2051), were selected for zircon LA-ICP-MS U-Pb isotopic dating. Dating results and zircon trace element data are presented in Table S1 and Table S2, respectively.
Cathodoluminescence (CL) images of zircons (Figure 3) reveal that grains from both samples are well-crystallized, predominantly prismatic or long-prismatic in shape, with lengths ranging from 160 to 440 μm and length-to-width ratios of 1.7:1 to 2.2:1. CL images display distinct oscillatory zoning. Zircon Th/U ratios range from 0.42 to 2.13 (all > 0.4), indicating a typical magmatic origin [26]. Chondrite-normalized REE patterns of zircons (Figure 4) exhibit significant negative Eu anomalies and positive Ce anomalies, characterized by LREE depletion and progressive HREE enrichment with left-inclined profiles, consistent with typical magmatic zircon patterns [26].
For monzogranite sample CZTW1105, 24 analytical spots yield a weighted mean 206Pb/238U age of 116.3 ± 0.5 Ma (MSWD = 1.5). All points are concordant, consistent with little Pb loss (Figure 5a). For the granodiorite sample CZTW2051, 25 analytical spots yield a weighted mean 206Pb/238U age of 114.6 ± 0.60 Ma (MSWD = 1.3) (Figure 5b). These two ages are consistent within analytical errors, thus constraining the crystallization age of the Ranwu pluton to 114–116 Ma, corresponding to the late Early Cretaceous period.

4.2. Zircon Hf Isotopic Characteristics

In situ zircon Hf isotopic analyses were conducted on zircons from the monzogranite (CZTW1105) and granodiorite (CZTW2051) samples of the Ranwu pluton, with results presented in Table S3. For 18 zircons from the monzogranite sample (CZTW1105), the 176Yb/177Hf and 176Hf/177Hf ratios range from 0.045895 to 0.081695 and 0.282631 to 0.282720, respectively, with 176Lu/177Hf ratios of 0.001640–0.002787. εHf(t) values range from −2.6 to 0.5, corresponding to the depleted mantle model ages (TDM) and crustal model ages (TDMC) of 770–903 Ma and 1139–1340 Ma, respectively. For 20 zircons from the granodiorite sample (CZTW2051), the 176Yb/177Hf and 176Hf/177Hf ratios range from 0.024087 to 0.078866 and 0.282560 to 0.282720, respectively, with 176Lu/177Hf ratios of 0.000832–0.002770. εHf(t) values range from −5.0 to 0.5, with the corresponding TDM and TDMC ages of 791–986 Ma and 1144–1494 Ma, respectively.

4.3. Whole-Rock Major- and Trace-Element Characteristics

Whole-rock geochemical analyses and related parameters for seven samples of the Ranwu pluton are presented in Table S4. The Ranwu pluton contains SiO2 ranging from 68.46% to 74.30%, Al2O3 from 13.28% to 15.74%, MgO from 0.27% to 1.00%, total FeO (FeO*) from 1.29% to 3.16%, with Mg# values of 42–51, K2O from 3.33% to 5.29%, Na2O from 2.97% to 3.54%, K2O/Na2O ratios of 0.97–1.78, K2O + Na2O contents of 6.75%–8.26%, alkalinity index (AKI) of 0.43–0.62, and a Rittmann index (σ) of 1.56–2.18. In the SiO2-K2O geochemical discrimination diagram, all samples plot in the high-K calc-alkaline series field (Figure 6a). The Ranwu pluton has an aluminum saturation index (A/CNK) ranging from 1.0 to 1.1, and all data points plot on the left side of the boundary between I-type and S-type granites (A/CNK = 1.1), belonging to the weakly peraluminous series and I-type granites (Figure 6b). All samples exhibit high differentiation indices (DI) of 75.75–91.05, with an average of 82.57.
Total rare earth elements (ΣREE) are relatively low, ranging from 79.70 ppm to 161.61 ppm. Chondrite-normalized REE patterns show a distinct right-inclined shape (Figure 7a), with enrichment in light rare earth elements (LREEs), LREE/HREE ratios of 3.94–14.5, and (La/Yb)N ratios of 3.2–18.7. Except for sample CZYQ2050 with δEu = 1.06, the δEu values of other samples range from 0.35 to 0.82, showing mostly weak negative Eu anomalies. In the primitive mantle-normalized trace element diagram (Figure 7b), large-ion lithophile elements (LILEs) such as Rb, Th, U, K, and Pb are relatively enriched, whereas high-field-strength elements (HFSEs) such as Nb, Ta, P, Zr, and Ti are significantly depleted.

5. Discussion

5.1. Petrogenetic Type

Currently, the MISA classification scheme (i.e., M-type, I-type, S-type, and A-type) is one of the most widely used schemes in granitoid genesis studies [30]. Among these, M-type granites typically originate from mantle sources, are often associated with ophiolites, and are characterized by low K2O content (<0.6%) and almost no K-feldspar [31]. No ophiolites are exposed around the Ranwu pluton, and its K2O content (3.33%–5.29%) is significantly higher than 0.6%, which is inconsistent with the typical characteristics of M-type granites.
The geochemical characteristics of the Ranwu pluton are significantly different from those of typical A-type granites, specifically: 10,000 × Ga/Al ratios (1.19–2.43), Zr (97–120 ppm), Nb (9.36–15.6 ppm), Ce (31.8–71.2 ppm), and Y (10.7–30.7 ppm) contents are all lower than the threshold values for typical A-type granites (10,000 × Ga/Al > 2.6, Zr > 250 ppm, Nb > 20 ppm, Ce > 100 ppm, Y > 80 ppm) [32]. In multiple petrological classification diagrams (Figure 8a–f), all sample points fall into the non-A-type granite field. Additionally, their zircon saturation temperatures (755–772 °C, average 765 °C) [33] are significantly lower than those of A-type granites (>800 °C) [34].
The Ranwu pluton lacks primary aluminous minerals (e.g., garnet, muscovite), a feature inconsistent with typical S-type granites [35]. In terms of trace element characteristics, S-type granites generally have low Th and Y contents, which either decrease or show little variation with increasing Rb; in contrast, I-type granites have relatively high Th and Y contents and often exhibit positive correlations with Rb [36]. The Ranwu pluton shows positive correlations in both Th-Rb and Y-Rb relationships (Figure 9a,b), indicating I-type granitic affinities.
Experimental studies have revealed that apatite solubility is low in metaluminous to weakly peraluminous magmas and decreases with increasing SiO2 content during magma differentiation; the opposite trend occurs in strongly peraluminous magmas [37,38]. Due to source differences, I-type granites typically exhibit lower A/CNK values than S-type granites [39], so their P2O5 often shows a negative correlation with SiO2. The Ranwu granites are weakly peraluminous (A/CNK < 1.1), and their P2O5 content decreases regularly with increasing SiO2 (Figure 9c), consistent with the evolutionary characteristics of I-type granites [40]. Meanwhile, in the ACF diagram for granites (Figure 9d) [41], sample points also fall into the I-type granite field.
In conclusion, the rock type of the Ranwu pluton belongs to I-type granites.
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Figure 9. I- and S-type granite discrimination diagrams for the Ranwu granites. (a) Rb vs. Y diagram [36]; (b) Rb vs. Th diagram [36]; (c) SiO2 vs. P2O5 diagram [39]; (d) ACF ternary diagram [41].
Figure 9. I- and S-type granite discrimination diagrams for the Ranwu granites. (a) Rb vs. Y diagram [36]; (b) Rb vs. Th diagram [36]; (c) SiO2 vs. P2O5 diagram [39]; (d) ACF ternary diagram [41].
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5.2. Magma Source

Zircon Hf isotopic composition is an important indicator for constraining the nature of magma sources [42,43,44,45]. Magmas derived from partial melting of ancient crust exhibit significantly lower zircon εHf(t) values than the chondritic uniform reservoir (CHUR). In contrast, magmas originating from juvenile crust or the mantle display εHf(t) values higher than CHUR. Crust-mantle mixed magmas typically have εHf(t) values close to CHUR. The genesis of I-type granites can involve two end-members: (1) mantle components mixed during crustal remelting [38,46,47]; (2) partial melting of intermediate-felsic igneous or metamorphic rocks within the crust [48]. The zircon εHf(t) values of the Ranwu pluton range from –5.0 to 0.5, indicating that the magma likely resulted from the mixing of magmas derived from both mantle and crustal sources. Given that most zircons exhibit negative εHf(t) values, we propose that the Ranwu pluton was likely formed through the mixing of a significant proportion of anatectic/remelted melts derived from ancient crustal materials with mantle-derived magmas. In the εHf(t) vs. U-Pb age diagram (Figure 10), the data points plot near the CHUR line, consistent with the characteristic features of typical crust-mantle mixed magmas in the northern Gangdese Belt [49,50]. Furthermore, in the ΣREE–Y/ΣREE diagram (Figure 11) [51], sample points plot within the crust-mantle mixing zone. The zircon Hf isotopic crustal model ages (TDMC) of 1139–1494 Ma are similar to the Precambrian basement ages of the Paleo-Mesoproterozoic Demala Group exposed in the Baxoi–Zayul area of the northern Lhasa Terrane [17]. This similarity provides further support for a crust-mantle mixing origin for the Ranwu pluton [7,52,53].
Mg# values of magmatic rocks also indicate source characteristics: rocks formed by lower crustal partial melting typically have Mg# < 40 [54]; products of basaltic partial melting have Mg# > 45 [55]; magmas mixed with more mafic materials than basalt have Mg# > 50 [40]; and magmas directly derived from partial melting of mantle wedge peridotite have Mg# > 60 [56]. The Mg# values of the Ranwu pluton (42–51) rule out the possibility of a purely crustal source or direct mantle melting, reflecting the addition of mantle materials. Previous regional geological surveys have shown that Early Cretaceous magmatic rocks in the Ranwu area are dominated by granitoids of the Boshulaling intrusive belt, with extremely limited exposure of andesites and basalts [57]. Therefore, large-scale intermediate-felsic magmatic rocks in this area could not have formed primarily through fractional crystallization of mafic magma [58,59,60,61]. A more reasonable genetic model involves upwelling mantle-derived magma, which provides heat to induce the partial melting of lower-crustal materials, thereby forming crust-mantle mixed magma.

5.3. Tectonic Setting of Magma Formation

The Early Cretaceous granites in the Bomi–Ranwu area, located southwest of the Bangong–Nujiang Suture Zone, stretch sub-parallel to the NW direction (Figure 1). Their magmatic activity was clearly controlled spatially and temporally by the southward subduction-collision processes of the Bangong–Nujiang Tethyan Ocean.
In the Al2O3 vs. SiO2 classification diagram (Figure 12a) [29], data points for these Early Cretaceous granites plot within the post-orogenic granite field. Post-orogenic granitoids are often used synonymously with post-collisional granitoids, marking the transition from crustal convergence-thickening to extension-thinning [62]. This is further supported by the Rb vs. Y + Rb tectonic setting discrimination diagram [63], where sample points fall into the post-collisional granite field (Figure 12b). The geochemical characteristics of enriching large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and relatively depleting high field strength elements (HFSEs) and heavy rare earth elements (HREEs) are also consistent with post-collisional granites [12]. Additionally, there is evidence of coeval A-type granites in the study area and adjacent regions, such as Early Cretaceous A-type granite discovered by the project team in northwestern Kangyu Township (Yang, X.J., unpublished data), A-type syenogranite reported by Xie et al. (2007) in Azagongla [57], and Burshulaling A-type granites reported by Xiao et al. (2024) [16], collectively indicating that the Bomi–Ranwu area entered a post-collisional setting during the Early Cretaceous period.
The Early Cretaceous Ranwu pluton is mainly composed of monzogranite and granodiorite, with a mineral assemblage of plagioclase, K-feldspar, quartz, biotite, and hornblende. This lithological and mineralogical assemblage is consistent with high-K calc-alkaline granitoids (KCG) formed during the post-collisional stage [12,64]. KCG primarily indicates tectonic regime transition (from compression to extension) rather than a specific geodynamic setting [64]. Combined with studies on post-collisional granites in the BNSZ [12], the discovery of high-K calc-alkaline post-collisional granites in the Ranwu area confirms that the North Gangdese Belt experienced a significant transition from a compressional to an extensional regime during the Early Cretaceous period. The weak development of primary structures in the Ranwu pluton indicates weak compressive stress during magma emplacement, supporting an extensional setting. In the lg (CaO/(Na2O + K2O)) vs. SiO2 diagram (Figure 12c) [65], sample points mainly fall in or near the compression-extension transition zone, further revealing that petrogenesis occurred during the transition from compression to extension. Regional geological evidence is consistent with this, viz., rift activity occurred in the Gangdese Belt from the Early Cretaceous Hauterivian Stage to the early Late Cretaceous Cenomanian Stage [13], and in the Early Cretaceous period, areas such as Coqen, Sangba, and Serong were also in a tectonic setting transitioning from compression to extension [12,66,67]. Collectively, these indicate that the Bomi–Ranwu area was in an extensional transition stage after compressional uplift during the Early Cretaceous period, marking the end of collisional orogeny along the BNSZ at the northern margin of the Gangdese Belt.
Samples of the Ranwu pluton are depleted in HFSEs (Nb, Ta, P, Zr, Ti) and LILEs (Ba, Sr), and enriched in LILEs (Rb, K, U, Th). This pattern is typical of island arc magmatic rocks, which are enriched in LILEs and depleted in HFSEs, and better matches the characteristics of continental margin arc magmatic rocks depleted in Nb, Ta, Ti, and Y [63,68,69]. The Th/Yb vs. Ta/Yb diagram by Gorton and Schandl (2000) (Figure 12d) [70] also indicates a formation in an active continental margin setting.
The Ranwu pluton shares similar crust-mantle mixed source characteristics with magmatic rocks in the northern Lhasa Terrane (Figure 10). Large-scale magmatism (110–120 Ma) in the northern Lhasa Terrane has been interpreted as the result of slab break-off of the southward-subducting Bangong–Nujiang Tethyan oceanic lithosphere [4,52,71]. After continental collision, shallow slab break-off below the suture zone can induce asthenospheric upwelling and underplating, leading to arc-like magmatism on both sides of the suture zone in a continental crustal extensional setting [72]. Numerical simulations [73,74] have confirmed that, under specific conditions, subducted slab break-off can promote the upwelling and influx of asthenospheric material into the crust via slab windows or slab rollback. This process triggers the contamination of magmas by mantle-derived materials and generates magmas with arc-like geochemical signatures in the overriding continental crust of the subduction zone.
In summary, the Ranwu pluton is a product of crust-mantle interaction, formed during the tectonic transition stage from the closure of the Bangong–Nujiang Tethys Ocean and Early Cretaceous collision between the Qiangtang and Lhasa blocks to post-collisional extension.
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Figure 12. Tectonic environment discrimination diagrams of the Ranwu Granites. (a) Al2O3 vs. SiO2 diagram [29]; (b) (Y + Nb) vs. Rb diagram [62]; (c) lg[CaO/(Na2O + K2O)] vs. SiO2 diagram [65]; (d) Th/Yb vs. Ta/Yb diagram [70]. Abbreviations: IAG = island arc granitoids, CAG = continental arc granitoids, CCG = continental collision granitoids, POG = post-orogenic granitoids, RRG = rift-related granitoids, CEUG = continental epeirogenic uplift granitoids, Syn-CLOG = syn-collision granites, Post-CLOG = post-collision granites, VAG = volcanic arc granites, ORG = ocean ridge granites, WPG = within-plate granites, ACM = active continental margins, WPVZ = within-plate volcanic zones, WPB = within-plate basalts, MORB = mid-ocean ridge basalts. Data are from the literature [8,12,16,17] and this study.
Figure 12. Tectonic environment discrimination diagrams of the Ranwu Granites. (a) Al2O3 vs. SiO2 diagram [29]; (b) (Y + Nb) vs. Rb diagram [62]; (c) lg[CaO/(Na2O + K2O)] vs. SiO2 diagram [65]; (d) Th/Yb vs. Ta/Yb diagram [70]. Abbreviations: IAG = island arc granitoids, CAG = continental arc granitoids, CCG = continental collision granitoids, POG = post-orogenic granitoids, RRG = rift-related granitoids, CEUG = continental epeirogenic uplift granitoids, Syn-CLOG = syn-collision granites, Post-CLOG = post-collision granites, VAG = volcanic arc granites, ORG = ocean ridge granites, WPG = within-plate granites, ACM = active continental margins, WPVZ = within-plate volcanic zones, WPB = within-plate basalts, MORB = mid-ocean ridge basalts. Data are from the literature [8,12,16,17] and this study.
Minerals 15 01188 g012

6. Conclusions

The study reaches the following conclusions:
(1) Zircon U-Pb dating constrains the emplacement age of the Rangwu pluton to 114–116 Ma. This age corresponds to a Late Early Cretaceous magmatic event associated with the formation of the Bangong–Nujiang Suture Zone (BNSZ).
(2) Samples of the Rangwu pluton are classified as high-K calc-alkaline I-type granites. Their geochemical signature is characterized by enrichment in large-ion lithophile elements and depletion in high-field-strength elements. Combined zircon εHf(t) values and calculated Mg# values indicate a hybrid crust-mantle source.
(3) Integrated research indicates that during the Late Early Cretaceous period, the Rangwu area, eastern Gangdese Belt, occupied a critical tectonic transition stage.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15111188/s1, Table S1: LA-ICP-MS zircon U-Pb data of the Ranwu pluton; Table S2: LA-ICP-MS zircon trace element data (ppm) of the Ranwu pluton; Table S3: LA-ICP-MS zircon Hf isotopic data of the Ranwu pluton; Table S4: Whole-rock major (wt.%), trace (ppm) element data of the Ranwu pluton.

Author Contributions

Conceptualization, X.Y. and M.D.; methodology, M.D.; formal analysis, X.Y. and M.D.; investigation, X.Y., M.D., Y.W., C.T., D.X., J.C., X.C. and J.S.; data curation, X.Y.; writing—original draft preparation, X.Y.; writing—review and editing, X.Y. and M.D.; visualization, X.Y.; supervision, M.D.; project administration, M.D.; funding acquisition, X.Y. and M.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Sichuan Science and Technology Program (No. 2024ZDZX0009), Geological Investigation Project from China Geological Survey (Nos. DD20221814 and DD20230138), and Science & Technology Fundamental Resources Investigation Program (No. 2022FY101800).

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Materials.

Acknowledgments

We are grateful to the editors and anonymous reviewers for their constructive reviews and comments that substantially improved this work.

Conflicts of Interest

Author Dian Xiao was employed by the company Sichuan-Tibet Technology Innovation Center (Chengdu) of National Railway Co., Ltd. The remaining 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.

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Minerals 15 01188 g002
Figure 2. Samples and photomicrographs of the Ranwu granite. (a,b) Monzogranite; (c,d) granodiorite. Abbreviations: Qtz = quartz, Kfs = K-feldspar, Pl = plagioclase, Bt = biotite, Hbl = hornblende.
Figure 2. Samples and photomicrographs of the Ranwu granite. (a,b) Monzogranite; (c,d) granodiorite. Abbreviations: Qtz = quartz, Kfs = K-feldspar, Pl = plagioclase, Bt = biotite, Hbl = hornblende.
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Figure 3. Cathodoluminescence images of zircon grains from the Ranwu pluton.
Figure 3. Cathodoluminescence images of zircon grains from the Ranwu pluton.
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Figure 4. Chondrite-normalized REE patterns of zircons from the Ranwu pluton (normalization values after Sun and McDonough, 1989) [27]. (a) Monzogranite sample CZTW1105; (b) Granodiorite sample CZTW2051.
Figure 4. Chondrite-normalized REE patterns of zircons from the Ranwu pluton (normalization values after Sun and McDonough, 1989) [27]. (a) Monzogranite sample CZTW1105; (b) Granodiorite sample CZTW2051.
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Figure 5. U-Pb concordia diagrams of the Ranwu pluton. (a) Monzogranite sample CZTW1105; (b) Monzogranite sample CZTW2051.
Figure 5. U-Pb concordia diagrams of the Ranwu pluton. (a) Monzogranite sample CZTW1105; (b) Monzogranite sample CZTW2051.
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Figure 6. SiO2 vs. K2O (a) and A/CNK vs. A/NK (b) diagrams of the Ranwu granites [28,29].
Figure 6. SiO2 vs. K2O (a) and A/CNK vs. A/NK (b) diagrams of the Ranwu granites [28,29].
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Figure 7. (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized trace element patterns for the Ranwu pluton (normalization values after Sun and McDonough, 1989) [27].
Figure 7. (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized trace element patterns for the Ranwu pluton (normalization values after Sun and McDonough, 1989) [27].
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Figure 8. Discrimination diagrams for genetic type of the Ranwu pluton. (a) Zr vs. 10,000 Ga/Al diagram; (b) Ce vs. 10,000 Ga/Al diagram; (c) Nb vs. 10,000 Ga/Al diagram; (d) Y vs. 10,000 Ga/Al diagram; (e) FeO*/MgO vs. (Zr + Nb + Ce + Y) diagram; (f) (K2O + Na2O)/CaO vs. (Zr + Nb + Ce + Y) diagram; FG: fractionated granites; OGT: unfractionated I- and S-granites [32].
Figure 8. Discrimination diagrams for genetic type of the Ranwu pluton. (a) Zr vs. 10,000 Ga/Al diagram; (b) Ce vs. 10,000 Ga/Al diagram; (c) Nb vs. 10,000 Ga/Al diagram; (d) Y vs. 10,000 Ga/Al diagram; (e) FeO*/MgO vs. (Zr + Nb + Ce + Y) diagram; (f) (K2O + Na2O)/CaO vs. (Zr + Nb + Ce + Y) diagram; FG: fractionated granites; OGT: unfractionated I- and S-granites [32].
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Figure 10. εHf(t) vs. crystallization age diagram (a), εHf(t) statistical histogram (b), and TDMC statistical histogram (c) of the Ranwu granites [49].
Figure 10. εHf(t) vs. crystallization age diagram (a), εHf(t) statistical histogram (b), and TDMC statistical histogram (c) of the Ranwu granites [49].
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Figure 11. ∑REE vs. Y/∑REE diagram of the Ranwu granites. A: crust-mantle mixed type granite; B: crustal type granite; C: mantle type granite [51].
Figure 11. ∑REE vs. Y/∑REE diagram of the Ranwu granites. A: crust-mantle mixed type granite; B: crustal type granite; C: mantle type granite [51].
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Yang, X.; Dong, M.; Wang, Y.; Teng, C.; Xiao, D.; Cao, J.; Chen, X.; Shao, J. Chronology, Geochemistry, and Tectonic Implications of Early Cretaceous Granitoids in the Ranwu Area, Eastern Gangdese Belt. Minerals 2025, 15, 1188. https://doi.org/10.3390/min15111188

AMA Style

Yang X, Dong M, Wang Y, Teng C, Xiao D, Cao J, Chen X, Shao J. Chronology, Geochemistry, and Tectonic Implications of Early Cretaceous Granitoids in the Ranwu Area, Eastern Gangdese Belt. Minerals. 2025; 15(11):1188. https://doi.org/10.3390/min15111188

Chicago/Turabian Style

Yang, Xinjie, Meiling Dong, Yanyun Wang, Chao Teng, Dian Xiao, Jun Cao, Xiqing Chen, and Jie Shao. 2025. "Chronology, Geochemistry, and Tectonic Implications of Early Cretaceous Granitoids in the Ranwu Area, Eastern Gangdese Belt" Minerals 15, no. 11: 1188. https://doi.org/10.3390/min15111188

APA Style

Yang, X., Dong, M., Wang, Y., Teng, C., Xiao, D., Cao, J., Chen, X., & Shao, J. (2025). Chronology, Geochemistry, and Tectonic Implications of Early Cretaceous Granitoids in the Ranwu Area, Eastern Gangdese Belt. Minerals, 15(11), 1188. https://doi.org/10.3390/min15111188

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