2025 Volume 120 Issue 1 Article ID: 240111
The northern Lhasa terrane in Tibet is widely developed with Cretaceous magmatic rocks, but their petrogenesis and tectonic setting are still controversial. This study focuses on the Badui pluton in the Luolong area of Tibet and conducts a series of analyses on these granitic rocks, including petrology, geochemistry, and zircon U-Pb geochronology. The Badui pluton mainly consists of biotite monzogranite and diorite porphyry veins with zircon U-Pb ages of 131.7 and 120.1 Ma, respectively, showing that they both formed in the Early Cretaceous. The biotite monzogranite has SiO2 and total alkali (Na2O+K2O) contents of 70.02-71.50 and 6.60-8.06%, respectively, and a Rittmann index (σ) of 1.53-2.40, indicating a metaluminous to weakly peraluminous I-type granite affinity. The diorite porphyry has SiO2 and total alkali (Na2O+K2O) contents of 58.78-60.85 and 5.49-6.57%, respectively. The rare earth and trace element compositions indicate that both the biotite monzogranite and diorite porphyry are enriched in light rare earth elements and large ion lithophile elements, however, depleted in heavy rare earth elements and high field strength elements such as Nb, Ta, and Ti. Together with the previous studies, formation of the Early Cretaceous Badui pluton is a response for the southward subduction of the Bangong-Nujiang Ocean. The diorite porphyry has high Sr, low Y characteristics and more depleted Hf isotopic composition, suggesting that subduction sediments were added during the partial melting of subducted oceanic crust, while the biotite monzogranite has opposite characteristics, suggesting that it was derived from partial melting of ancient lower crustal material.
The Lhasa terrane is one of the main tectonic units of the Tibetan Plateau, which experienced intense magmatic activity during the Cretaceous period, forming numerous volcanic and intrusive rocks (Fig. 1a) (Pan et al., 2012; Sui et al., 2013; Chen et al., 2014; Meng et al., 2022). These magmatic rocks record the evolution of the Neo-Tethys Ocean, which is crucial for understanding the formation and uplift of the Tibetan Plateau (Qu et al., 2012; Wu et al., 2014, 2015; Zhu et al., 2018). However, the magmatic activity of the Lhasa terrane there were significant differences between the northern and southern parts. The Cretaceous magmatic rocks of the southern Lhasa terrane have been widely studied, and they mainly consist of calc-alkaline volcanic rocks and granitic intrusions, reflecting a back-arc extensional environment caused by the northward subduction of the Neo-Tethys Ocean (Liu et al., 2017; Ma et al., 2017; Liu et al., 2018; Wang et al., 2020). In contrast, the Cretaceous magmatic rocks of the northern Lhasa terrane are mainly composed of alkaline volcanic rocks and gabbroic intrusions, indicating a forearc extensional environment resulting from the southward subduction of the Neo-Tethys Ocean. Moreover, there were also east-west variations in the Cretaceous magmatic activity of the northern Lhasa terrane, with weaker magmatism in the eastern region and intense magmatism in the central-western region. These variations may be related to the subduction direction, angle, speed, and tectonic nature of the Neo-Tethys Ocean plate and the Lhasa terrane itself (Wang et al., 2020). Therefore, more studies on the Cretaceous magmatic rocks developed in the eastern part of the northern Lhasa terrane will help to reveal the spatial distribution and formation mechanism of Cretaceous magmatic activity in this area and thus to better understand the evolutionary history of the Neo-Tethys Ocean and the uplift process of the Tibetan Plateau.
Cretaceous magmatism was widespread in the Lhasa terrane. The Cretaceous magmatic rocks in the southern Lhasa terrane have been extensively studied and are generally considered to be the magmatic record of the northward subduction of the Neo-Tethys Ocean. However, research on the Cretaceous magmatic rocks in the northern Lhasa terrane has mainly focused on the central and western regions, and studies on the coeval magmatic rocks in the eastern part of the northern Lhasa terrane are still very limited. The east-west difference of the Cretaceous magmatic rocks in the northern Lhasa terrane has not received enough attention. Zircon U-Pb geochronology studies show that the Cretaceous magmatic rocks in the northern Lhasa terrane mainly occurred between 120 and 100 Ma (Wang et al., 2020), but there are different understandings of their petrogenesis and tectonic setting: 1) remelting of thickened crust during or after the collision between the Lhasa and Qiangtang terranes (Chiu et al., 2009); 2) southward subduction of the Bangong Lake-Nujiang Ocean crust (Zhu et al., 2011b); and 3) northward low-angle or flat subduction of the Neo-Tethys Ocean crust (Leier et al., 2007). Theses different understandings seriously affect the understanding of the tectonic evolution of the Bangong Lake-Nujiang Ocean and the collision process between the Lhasa and Qiangtang terranes.
Therefore, we collected fresh samples from the Badui pluton in the Luolong area of the eastern part of the northern Lhasa terrane and conducted petrologic, geochemical and geochronological analyses, aiming at clarifying the petrogenesis and magmatic evolution of these rocks, as well as the dynamic background of the Early Cretaceous magmatism in the northern Lhasa terrane and its implications for regional tectonic evolution.
The Qinghai-Tibet Plateau, extending from north to south, consists of the Songpan-Ganzi terrane, Qiangtang terrane, Lhasa terrane, and the Himalayan belt. Among these, the Lhasa terrane, situated between the Bangong-Nujiang Suture Zone (BNSZ) and the Yarlung Zangbo Suture Zone (YZSZ), can be further divided into northern, central, and southern segments based on differences in basement characteristics and sedimentary cover. The northern segment is delineated by the Shiquan River-Nam Tso Mslange Zone (SNMZ), while the boundary between the central and southern segments is marked by the Luobadui-Milashan Fault (LMF).
The sedimentary cover in the north Lhasa terrane is mainly Jurassic-Cretaceous in age with minor Triassic deposits. Voluminous Mesozoic volcanic rocks (124-107 Ma), including andesite, dacite, rhyolite, and associated volcanic clastic rocks, are exposed within the Lower Cretaceous volcano-sedimentary sequence (Kapp et al., 2007; Zhu et al., 2009). A few Late Jurassic-Early Cretaceous igneous rock outcrops have been discovered in the northern Lhasa terrane. The Lhasa terrane has ancient basement rocks ranging in age from the Archean to Proterozoic (up to 2870 Ma) at its center with younger and juvenile crust (Phanerozoic) accreted towards both its northern and southern margins as revealed from four north-south traverses across the terrane (Zhu et al., 2011b). It has also been proved that central Lhasa subterrane was once a microcontinent that survived its long journey across the Paleo-Tethyan Ocean basins (Zhu et al., 2011b).
The study area is located in the eastern part of the Lhasa terrane. The magmatic rocks in the study area are mainly acidic intrusive rocks, including granodiorite and monzogranite, which have high silica, high potassium, and alkali-rich characteristics, reflecting a crustal partial melting origin (Zhu et al., 2011b). The acidic intrusive rocks in the study area mainly formed in the Early Cretaceous (120-100 Ma), related to the closure of the Bangong Lake-Nujiang Ocean and the collision of the Lhasa-Qiangtang terranes (Yin et al., 2023). The study area also developed Late Cretaceous (70-50 Ma) diorite porphyry veins, which have low silica, high magnesium, and iron-rich characteristics, reflecting the involvement of mantle material. The diorite porphyry veins in the study area widely intruded into the Early Cretaceous acidic intrusive rocks and the surrounding sedimentary strata, indicating a Late Cretaceous back-arc extensional environment. The intrusive rocks in the study area are distributed in a NW-SE direction. According to the feather of their spatial distribution and lithology, they can be divided into three units: Bamei pluton, Dongcuo pluton, and Badui pluton (Fig. 1b).
The Badui pluton is located near Badui village on the northwest side of the study area, with an exposed area of approximately 62.8 km2 which showing Figure 1b. The lithology is mainly monzonitic granite, with a small amount of biotite granodiorite in the center and edge of the intrusion, which is intrusive contact. The surrounding rocks in contact with the south side of the intrusion are the Middle Jurassic, which is mainly composed of purple-red fine sandstone, purple-red siltstone, gray-green siltstone, gray-green-gray fine sandstone, and gray-dark gray quartz sandstone as the main components and gray siltstone slate and lithic quartz sandstone as the minor components. The surrounding rocks in contact with the northeast side of the intrusion are the Lower Cretaceous, with a lithological combination of gray-black thin-layered slate and siltstone slate interbedded with thin-layered fine-grained metamorphic lithic sandstone. Diorite porphyry veins are widely intrusive in the study area. Diorite porphyry veins can also be seen intruding into the Badui pluton (Fig. 1b), but they are small in scale, mostly 0.1-2 meters wide veins extending from several meters to tens of meters and forming in the Late Cretaceous.
Sampling descriptionsThis study selected two types of rocks in this area, biotite monzogranite (N 30.45, E 96.15) and diorite porphyry (N 30.44N, E 96.16), and the detailed description is as follows:
Biotite monzogranite weathered surface is earthy yellow, and the fresh surface is light pink-red to pink-red, with a medium- to fine-grained monzonitic granite structure and blocky structure (Fig. 2a). The rock is composed of plagioclase (39%), potassium feldspar (21%), quartz (32%), biotite (6%), hornblende (1%), and metallic minerals (1%) (Fig. 2b). Potassium feldspar is mainly orthoclase, with a small amount of perthite, showing anhedral granular shape, containing quartz, plagioclase, biotite, hornblende as inclusion. Potassium feldspar grains are relatively large, reaching approximately 5-6 mm in length. They are slightly altered. Plagioclase is oligo-clase, showing subhedral plate-columnar shape, partially has zonal structure, with weak sericite and chlorite alteration; quartz shows an anhedral granular shape, with undulatory extinction. The grain size of quartz is 0.2-3 mm. Biotite shows brown flaky shape, with chloritoid and chlorite alteration. They typically measure approximately 0.2-0.5 mm in length. Hornblende is ferrohornblende, showing anhedral granular or subhedral long-columnar shape, with a small amount of chloritoid and chlorite alteration. Mineral grain size is 0.2-6 mm, of which 0.2-2 mm accounts for approximately 30%, 2-5 mm accounts for approximately 60%, 5-6 mm accounts for approximately 10%, slightly showing porphyritic texture.
Diorite porphyry is composed of phenocrysts of plagioclase (7%), hornblende (5%), and a matrix of plagioclase, hornblende, quartz, and metallic minerals, showing porphyritic texture, with a microcrystalline structure in the matrix (Figs. 2c and 2d). The plagioclase phenocrysts are andesine, showing subhedral plate-columnar shapes, with sericite, chlorite, clay, and calcitization, partially severely altered, and showing residual shapes; hornblende shows subhedral long-columnar or anhedral granular shapes, with chloritoid, chlorite, and calcite alteration, partially completely altered, only retaining hornblende crystal forms, and locally distributed; the phenocryst grain of hornblende size is 0.5-2.3 mm in the length. Plagioclase in matrix is andesine, showing a subhedral plate-columnar shape, with a small amount of zonal structure, with sericite, clay, chlorite, and calcite alteration, partially strongly altered, showing residual shape. It’s duplicated quartz shows an anhedral granular shape; mineral grain size is 0.02-0.5 mm, of which 0.02-0.2 mm accounts for approximately 90%, and 0.2-0.5 mm accounts for approximately 10%.
The whole-rock geochemical analysis was carried out at the Chengdu Institute of Comprehensive Utilization of Mineral Resources, Chinese Academy of Geological Sciences. The samples were crushed to 60-mesh-size using a corundum jaw crusher, and then ∼ 60 g of each crushed sample was powdered to <200 mesh in an agate ring mill. The samples were washed with distilled water containing 5% nitric acid, rinsed with pure distilled water and dried. Finally, the samples were ground to 200 mesh in an agate ring mill and stored for later use. The analysis was performed by an XRF spectrometer, with an analytical precision of 5%. Approximately 50 milligrams of rock powders were meticulously measured and placed into Teflon containers. Subsequently, a blend of concentrated HF and HNO3 acids, in a ratio of approximately 2:1 (volume/volume), was introduced. The containers were securely sealed and heated on a hotplate to approximately 120 degrees Celsius for a duration of 12 h, leading to the evaporation of the solutions until dry. Following this, another mixture of concentrated HF and HNO3 acids, this time in a 1:1 (volume/volume) ratio, was added to the containers. These were then heated in an oven at 195 degrees Celsius for a prolonged period of 48 h to ensure complete digestion. After this, the solutions were once again evaporated to dryness at about 120 degrees Celsius. Subsequently, 3 milliliters of concentrated HNO3 were added. The sealed containers were once more heated in an oven at 195 degrees Celsius for an additional 24 h. In the final step, the solutions were diluted to a total weight of 80 grams. One milliliter of a rhodium internal standard was incorporated, and the solutions were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS), with an analytical precision of 5%.
Zircon U-Pb dating and trace element analysesZircon U-Pb dating was performed by a laser ablation inductively coupled plasma-mass spectrometry at the Testing Center of Shandong Bureau of China Metallurgical Geology Bureau. Before the U-Pb dating and trace element analysis, a comprehensive examination of the morphology and internal structure of the target zircons was conducted using a binocular microscope and Cathodoluminescence (CL) imaging. This serves as a guide in identifying homogeneous and inclusion-free domains suitable for U-Pb dating and trace element analysis.
The instrument used was the GeoLasPro 193 nm ArF excimer laser ablation system. Helium served as the carrier gas, while was used as the supplementary gas. The analyses were conducted with a beam diameter of 32 µm. Each analysis takes ∼ 80 s with the first ∼ 15 s used to measure the gas blank (i.e., background), followed by 40 s of ablation and 25 s of washout time to allow the signal drop back to the gas blank values. A set of zircons 91500 and composition standards NIST 610 was inserted every 5-10 sample. The standard zircon 91500 was used for calibration. At the start of each analytical session, the primary zircon calibration standard GJ-1 was used for optimization of the ICP-MS and was analyzed using line scans instead of spot analyses. The trace element analysis of zircon was corrected using reference materials (NIST SRM 610) (Liu et al., 2010a), with 29Si serving as the normalized element. The isotopic ratios, elemen of the samples were calculated using the ICPMSDATACAL (Liu et al., 2010a, 2010b). U-Pb concordia diagrams were constracted using Isoplot 3.0 programs (Ludwig, 2003).
Zircon Lu-Hf isotope analysisIn situ zircon Hf isotope analyses were performed using a Geolas Pro laser-ablation system coupled to a Neptune plus Multiple-Collector ICP-MS at the Nanjing FocuMS Technology Co. Ltd. The analyses were conducted with a beam diameter of 44 µm. The ablated aerosol was carried by helium and then combined with argon and minor N2 (4 ml/min) before being introduced to the ICP-MS plasma. All Hf analyses were done on the same or near the spots of U-Pb dating. The accuracy of Lu-Hf isotope data was monitored by the analysis of reference material GJ-1, 91500, Plešovice and MUN-3 (Wiedenbeck et al., 1995). It is customary to retest these standard samples after every five test samples. These reference zircons have a very wide range of 176Yb/177Hf ratios (0.005-0.22) and thus provides the good indication of the veracity of 176Yb and 176Lu interference corrections on 176Hf. The 176Yb/177Hf radios we measured are within the range of these reference zircon. The average mean 176Hf/177Hf ratios of Plešovice, 91500, GJ-1 and SK10-2 was 0.281231 ± 0.000024 (2SD, n = 12), 0.282310 ± 0.000035 (2SD, n = 12), 0.282028 ± 0.000034 (2SD, n = 12), and 0.282752 ± 0.000053 (2SD, n = 12), respectively, which is in agreement within errors with reference values recommended by [Fisher et al., 2011 (MUN-3); Morel et al., 2008 (GJ-1); Sláma et al., 2008 (Plešovice); Wiedenbeck et al., 1995 (91500)]. We used correlations between measured 176Yb/177Hf and 176Hf/177Hf ratios to assess the effectiveness of correcting for homogeneous isotopic interference, and measured 178Hf/177Hf (1.46688-1.46746) and 180Hf/177Hf (1.88628-1.88704) ratios to evaluate the reliability of the experimental result (Spencer et al., 2020).
The whole-rock composition is listed in Supplementary Table S1 (Supplementary Tables S1-S5 are available online from https://doi.org/10.2465/jmps.240111).
Biotite monzogranite: SiO2 ranges from 70.02 to 71.50%, with an average of 70.56%, belonging to acidic intrusive rocks. The total alkali (K2O+Na2O) content ranges from 6.60 to 8.06%, with an average of 7.06%, and the Rittmann index (δ) is 1.53-2.40, belonging to the calc-alkaline series (Fig. 3a); K2O is 3.44-4.72 wt%, with an average of 3.34 wt%, belonging to high-K calc-alkaline rocks (Fig. 3b). In the A/CNK-A/NK diagram (Fig. 3c), all samples plot in the metaluminous to weakly peraluminous region and belong to I-type granite. The total rare earth element ΣREE ranges from 149.18 to 297.65 ug/g, (La/Yb)N = 7.79-27.15, showing LREE enrichment, HREE depletion (Fig. 4a). On the primitive mantle-normalized spider diagram (Fig. 4b), the large ion lithophile elements and HFSEs are strongly differentiated, with Rb, Th, U, and K enriched and Ba, Nb, Ta, Zr, and Ti depleted.
Diorite porphyry: the SiO2 content (58.78-60.85%) varies little and the total alkali Na2O+K2O content is 5.49-6.57%. In the TAS (total alkali silica) diagram, it is located in the diorite and monzonite region (Fig. 3a), and K2O is 0.83 wt%, with an average of 2.64 wt%, indicating high-K calc-alkaline series and island arc tholeiitic series (Fig. 3b). In the A/NK [molar ratio Al2O3/(Na2O + K2O)]-A/CNK [molar ratio Al2O3/(CaO + Na2O + K2O)] diagram, the samples fall between peraluminous and metaluminous (Fig. 3c). In terms of trace elements, the ΣREE value is 161.39-248.48 ug/g, and (La/Yb)N is 14.49-23.63, indicating that light and heavy rare earth elements have more obvious degree of fractionation than basic rocks (Fig. 4a). The trace element primitive mantle-normalized spider diagram shows that the intrusion is enriched in the large ion lithophile elements (LILE) such as Ba, Rb, and Th and depleted in the high field strength elements (HFSE) such as Ta, Ce, P, and Ti (Fig. 4b).
Zircon U-Pb dating and trace element compositionZircon crystals from sample show euhedral, elongated shape, with lengths ranging from 150 to 250 µm and widths ranging from 50 to 100 µm. Most length/width ratios vary from 2:1 to 3:1. In CL images, most zircon crystals display obviously euhedral concentric zoning (Fig. 5), implying their magmatic origin. At the same time, some zircons have inherited cores, indicating that the melt is saturated with zirconium. The two zircon samples selected for this study are from biotite monzogranite and diorite porphyry and LA-ICP-MS U-Pb isotopic data and trace element contents of zircon are listed in Supplementary Tables S2 and S3, respectively. Zircon Th content in the biotite monzogranite ranges from 342 to 2934 µg/g, the U content ranges from 460 to 3491 ppm, and the Th/U ratio is 0.30-1.10, indicating that they are magmatic zircons. The 206Pb/238U weighted average age of the sample is 131.7 ± 1.6 Ma (MSWD = 2.5, n = 29) (Fig. 5). The zircon Th content in the diorite porphyry sample ranges from 134 to 1186 µg/g, the U content ranges from 266 to 3047 µg/g, and the Th/U ratio is 0.24-0.77, greater than 0.1, also indicating magmatic origin. The weighted average 206Pb/238U age of the sample is 120.1 ± 1.6 Ma (MSWD = 2.4, n = 24).
The Hf isotope composition of the zircon grains from the diorite porphyry and biotite monzogranite samples are listed in Supplementary Table S4 and presented in Figure 6, and the εHf(t) values were calculated for the crystallization ages from the U-Pb dating. In detail, in diorite porphyry, all zircon samples have high 176Lu/177Hf ratios (0.00097-0.00434) and high εHf(t) values ranging from −6.47 to −0.03. The depleted mantle model ages (tDM2) for Hf isotopes of zircon are relatively younger than that of biotite monzogranite, ranging from 729 to 1039 Ma. Zircon Hf isotopes from biotite monzogranite are ranging from −7.80 to −4.73. The model ages (tDM2) of biotite monzogranite are older, ranging from 973 to 1095 Ma.
In this study, LA-ICP-MS zircon U-Pb dating was conducted on dioritic rocks from the Lhasa Bdui area. The biotite monzogranite and diorite porphyrite have crystallization ages of 131.7 ± 1.6 and 120.1 ± 1.6 Ma, respectively. As mentioned earlier, these moderately acidic rock types exhibit distinct geochemical characteristics and divergent rock crystallization ages with a difference of approximately 10 Ma, implying potential variations in their magmatic sources, evolutionary processes, and petrogenesis. Previous research has indicated that late Early Cretaceous magmatic activity occurred within the Tethys Himalayan belt while large-scale magmatic events spanning from 130 to 110 Ma took place in the middle Gangdise belt as well as the middle to western Gangdise belt in the North (Zhu et al., 2008, 2011a, 2011b; Zhang et al., 2012; Wang et al., 2020). The age data obtained in this study precisely document the extensive magmatic activity during this period, thereby providing hard evidence into such magmatic events in different tectonic settings of Cretaceous period in this area.
Peng et al. collated the ages of the Mesozoic igneous rocks exposed along the BNSZ, including the granites, volcanic rocks, and mafic intrusions. The data indicate that these igneous rocks were mainly generated at three stages: stage1 (190-150 Ma), stage2 (135-110 Ma), stage3 (110-70 Ma), related to the double side subduction-collision of Bangong-Nujiang Tethyan Ocean. In the eastern of the northern Lhasa terrane, there are many 120 Ma granitoids rocks, but there is no evidence of 130 Ma granitoids rocks (Fig. 1a). The age data obtained in this study just fill the gap, and also provide good evidence for that the closure of Bangong-Nujiang Tethyan Ocean is most likely diachronous (Peng et al., 2020).
Petrogenesis of biotite monzograniteFrom the perspective of rock-forming minerals, the Badui pluton rocks all contain iron-magnesium minerals such as hornblende and biotite, and accessory minerals contain apatite, without muscovite and kyanite; from the geochemical perspective, the rocks have high silica-alumina and low magnesium-titanium characteristics, and the aluminum saturation index is 0.95-1.05, belonging to calc-alkaline metaluminous to weakly peraluminous granites. In the aluminum saturation index diagram, all samples plot on the I-type side of the I-S-type granite boundary (Maniar and Piccoli, 1989). The above mineralogical and elemental geochemical characteristics all indicate that the biotite monzogranite in the Badui pluton is I-type granite. In the Sr/Y-Y diagram (Fig. 7a), the biotite monzogranite shows a non-adakitic origin. Therefore, it is unlikely to have formed by partial melting of a subducted oceanic crust. In addition, on the tectonic discrimination diagram (Rb versus Yb+Ta and Rb versus Y+Nb) (Fig. 8), the Badui biotite monzogranite plots in the field of volcanic arc granite (VAG), indicating a subduction setting. The oxygen fugacity calculated from zircon trace elements indicates that the biotite monzogranite formed in a low oxygen fugacity environment (Fig. 7d). According to the method provided by Miller et al., 2003, the zirconium saturation temperature is calculated to be around 650°, which is consistent to the result of zircon Ti thermometer (average 715 °C, Tables S1 and S3). According to Ballard et al. 2002, the ore-bearing porphyries were more oxidized than their barren counterparts. The mineralization possibility of diorite porphyry is obviously higher than that of biotite monzonitic granite.
Besides, the negative Hf isotopic composition of biotite monzogranite are consistent with the central Lhasa, which were derived from ancient lower crust (Fig. 7) (Zhu et al., 2009; Zhang et al., 2012). In summary, we consider that the biotite monzogranite in the Badui pluton formed in a low-oxygen fugacity, high-temperature environment under subduction conditions and that the source material was magma formed by partial melting of the lower crust.
Petrogenesis of diorite porphyryIn the Sr/Y-Y discrimination diagram, the diorite porphyry has high Sr/Y and low Y content, indicating that it is adakitic rock (Fig. 7a). Sr/Y and (La/Yb)N values are the two most important indicators for identifying adakites (Defant and Drummond, 1990; Martin et al., 2005). Theoretically, adakitic magmas produced by different degrees of partial melting of eclogite or garnet amphibolite should establish a positive correlation between Sr/Y and (La/Yb)N, as observed in adakites from lower continental crust (LCC) melting in the Dabie Mountains and the South Tan-Lu Fault (STLF) (Fig. 7c). Under this condition, in the residual magma containing garnet and lacking plagioclase, Sr and La are incompatible elements, while Y and Yb are compatible elements. Therefore, partial melting of subducted or thickened continental crust will produce adakites with coupled Sr/Y and La/Yb ratios (Liu et al., 2010a; Sun et al., 2012). In contrast, adakites from partial melting of subducted oceanic crust show ‘decoupling’ of Sr/Y and La/Yb ratios, with high and variable Sr/Y and low (La/Yb)N ratios (Fig. 7c). This is mainly due to the relatively low La content and La/Yb ratio (0.8) of the subducted oceanic crust, while the continental crust has a higher La/Yb ratio (5.3) (Sun and McDonough, 1989; Rudnick and Gao, 2014).
In addition, La is more mobile than Yb in the early stage of plate subduction (Kogiso et al., 1997), indicating that the forearc mantle-depth melting plate has a lower La/Yb ratio than MORB due to the loss of La during the early stage of plate subduction (Sun et al., 2012). Moreover, the increase in Sr content in MORB is a common process, but HREEs change little in the seawater alteration process (Alt et al., 1986; Nakamura et al., 2007), which will help to produce the high Sr/Y value of adakite. The diorite porphyry conforms to this characteristic of Sr/Y (49.26-140.54, av. 86.30), (La/Yb)N (18.32-41.74, av. 29.73), with no obvious correlation between Sr/Y and (La/Yb)N, similar to typical subduction zone adakites (Liu et al., 2010b; Sun et al., 2012). Therefore, we consider that the diorite porphyry adakite was formed by partial melting of subducted oceanic crust.
The depletion of K and the enrichment of Na, that is, the low K2O/Na2O ratio, can be regarded as another typical feature of the MORB component partial melting product, accompanied by residual garnet, clinopyroxene and hornblende (Defant and Drummond, 1990; Rapp and Watson, 1995; Martin et al., 2005). In contrast, partial melting of dry mafic lower crustal rocks (such as eclogite) without hornblende should produce K-rich melts. The adakitic rocks we studied have low K2O/Na2O ratios (generally less than 0.75, Liu et al., 2010c). In the K2O/Na2O-Al2O3 diagram (Fig. 7b), they are mostly distributed in the adakite region formed by oceanic plate melting, which is different from the lower crust. The Hf isotopes of our samples are generally higher than some from central Lhasa (Fig. 6). Combined with the adakitic melt has the positive Hf isotope and the diorite porphyry has more depleted Hf isotope evidence (−6.43 to −0.03), it shows that the adakitic melt from subduction of plate is influenced by other substances. Subduction input material (subduction plate) is mainly composed of igneous oceanic crust, underlying lithospheric mantle and seafloor sediments at the top of the plate. Subduction sediment, as the most dehydrating material in subduction process, plays a crucial role in the composition evolution of continental crust. According to Vervoort et al. 2011, subduction sediments have a much lower Hf isotope composition than oceanic crust basalt, and subduction sediments are highly likely to add melt during the process of subduction plate melting, which also explains the lower Hf isotope value of diorite porphyry.
In the Rb-Yb+Ta tectonic environment discrimination diagram (Fig. 8a), the samples are plotted in the volcanic arc granite range. In the Rb-Y+Nb tectonic environment discrimination diagram (Fig. 8b), the samples also mainly plot in the volcanic arc granite range, which is consistent with their rare earth and trace element characteristics showing arc volcanic rock properties (Pearce et al., 1984). Based on the above analysis, combined with the geochemical characteristics and zircon U-Pb age of the diorite porphyry veins in the Badui pluton, the source region of the Badui pluton should have been acidic magma formed by partial melting of subducted oceanic crust in an arc environment, and its emplacement time occurred at 120.1 ± 1.6 Ma, that is, in the Early Cretaceous. Therefore, the Badui pluton and the widely developed Early-Late Cretaceous granites in the northern Gangdese are considered products of the southward subduction of the Bangong Lake-Nujiang Ocean crust.
Implication of regional geological evolutionAccording to the formation and evolution of the Qinghai-Tibet Plateau combined with the regional geological characteristics, the study area can be divided into three stages: (1) late Paleozoic-Permian Paleo-Tethys Ocean subduction ocean-continent transition stage; (2) Mesozoic Neo-Tethys Ocean consumption stage; and (3) Cenozoic collision orogeny and plateau uplift stage. This paper mainly focuses on the second stage. The Neo-Tethys evolution stage refers to the geological history process from the Late Triassic to the Cretaceous (Li et al., 2020). This period can be roughly divided into two main tectonic evolution periods: the Jurassic-Early Cretaceous Bershulalin magmatic arc activity forming a forearc basin and the late Early Cretaceous Badui magmatic arc large-scale intrusive activity. During the Jurassic-Cretaceous period, the Neo-Tethys Ocean further subducted and was subducted southward, forming a multi-island arc basin system (Fan et al., 2015). No corresponding evidence of volcanic activity was found in the study area during this period, but evidence of Jurassic magmatic activity has been found in the Basu-Chayu area on the south side (Li et al., 2020). During this period, Jurassic fore-arc, interarc basin littoral-shallow marine clastic rocks, Cretaceous deltaic slate interbedded with thick-layered quartz sandstone lenses and platform carbonate rocks were widely developed in and around the area.
In the late Early Cretaceous (115-130 Ma), a large area of magmatic activity, such as granite, developed in the area. The petrological and geochemical characteristics of the granites, granodiorites, and their dark enclaves in the area show metaluminous to weakly peraluminous calc-alkaline I-type granitic rock characteristics (Chappell, 1999). The geochemical characteristics indicate that the Badui granite has obvious crust-mantle mixed source characteristics, showing arc granite features (Wilson, 1989). The tectonic discrimination diagram suggests that the study area may have been in a subduction tectonic setting in the late Early Cretaceous. The early biotite monzogranite was formed by partial melting of the lower crust induced by dehydration of the subducted oceanic crust, thus showing nonadakitic features. Approximately 10 Ma later, the subduction may process continued, leading to partial melting of the oceanic crust and forming adakitic magma, which intruded along the early tectonic weakness zone into the biotite monzogranite, thus forming diorite porphyry dyke (Fig. 9).
(1) The zircon U-Pb age of the biotite monzogranite samples from the Badui pluton is 131.7 Ma, and the zircon age of the diorite porphyry veins is 120.1 Ma, indicating that the intrusion formed in the Early Cretaceous.
(2) The diorite porphyry of the Badui pluton has high Sr and low Y characteristics, εHf(t) values ranging from −7.80 to −4.73, indicating that it may formed from partial melting of oceanic crust and mixing of subduction sediments, while the biotite monzogranite has the opposite characteristics, indicating that it was formed by partial melting of ancient lower crustal material.
(3) The Badui biotite monzogranite shows metaluminous to weakly peraluminous calc-alkaline I-type granitic rock characteristics, enriched in light REEs and large ion lithophile elements, and depleted in heavy REEs and HFSEs, εHf(t) values ranging from −6.47 to −0.03, reflecting that it formed in an active continental margin setting modified by subduction and should be a magmatic record of the southward subduction of the Bangong Lake-Nujiang Tethyan Ocean.
This study is supported by China Geological Survey Project (DD20220989).
Supplementary Tables S1-S5 are available online from https://doi.org/10.2465/jmps.240111.
