Chemistry of Zircon and Its Implication on the Petrogenesis of Cretaceous Volcanic Rocks from the Southeastern Coast of Zhejiang Province, South China

Abstract

The Cretaceous marks the peak of magmatic activity in southeastern (SE) China, which is attributed to the subduction of the paleo-Pacific plate beneath the South China Block. This region constitutes a significant igneous belt along the active continental margin of the western Pacific. Despite extensive research, the origin and evolution of Cretaceous felsic volcanic rocks are still debated. This study investigates the characteristics of zircon U-Pb-Hf isotopes and trace elements, and whole-rock geochemistry of Cretaceous volcanic rocks from the Wenzhou–Taizhou region in SE Zhejiang, and discusses their spatio-temporal patterns and petrogenesis. The results indicate that rhyolitic volcanic rocks formed during the period ca. 114 Ma and 95 Ma, representing two distinct magmatic episodes spanning the transition from the late Early to early Late Cretaceous. The late Early Cretaceous and early Late Cretaceous volcanic rocks are of a hybrid crust–mantle origin, as evidenced by their distinct Nb/Ta ratios, zircon ε_Hf(t) values, and variable trace element enrichments (Ti, Hf, U, Nb, and Yb). These compositional signatures suggest partial melting of late Paleoproterozoic to early Mesoproterozoic basement materials, with increasing mantle contributions over time. Both volcanic phases exhibit elevated Nb/Yb, Th/Nb, and U/Yb ratios, indicating a subduction-modified source akin to arc magmas. Together with calculated initial melt temperatures (<800 °C for Early Cretaceous, >800 °C for Late Cretaceous) and whole-rock rare-earth elements (REEs) distribution patterns (U-shaped with δEu = 0.37–0.65, seagull-shaped with δEu = 0.19–0.62, respectively), it is suggested that both phases of the volcanic magmas were generated through water-assisted (hydrous) melting, whereas the later phase formed at relatively higher temperatures and with a diminished water contribution via dehydration melting under extensional conditions. The generation of voluminous high-silica magmas in the SE China coastal region is probably linked to the rollback and retreat of the paleo-Pacific plate.

1. Introduction

The Late Mesozoic, particularly the Cretaceous period, was a crucial stage in the geological evolution of SE China, marked by intensive magmatism, widely linked to the subduction and consumption of the paleo-Pacific plate beneath the South China Block, shaping the tectonic framework of the whole region [1,2,3,4,5]. The products of this magmatic activity are predominantly distributed along the coastal areas of Zhejiang, Fujian, and Guangdong provinces (eastern Cathaysia Block), forming a giant volcanic-intrusive complex belt approximately 2000 km in length (Figure 1). This belt constitutes a significant component of the circum-Pacific continental margin magmatic zone [3]. Felsic volcanic products and their associated magmatic systems represent a critical part of continental crust. Consequently, felsic volcanic activity and its magmatic systems preserve essential records of the crustal differentiation and magmatic evolution [6,7]. Recent studies suggest that the large felsic magmatic systems are crustal level in scale, consisting of interconnected magma reservoirs at varying depths. These systems undergo complex processes, including magma generation, storage, migration, replenishment, and eruption [8,9,10,11].

However, the genetic mechanism of felsic volcanic rocks remains a contentious topic in igneous petrology. The two existing perspectives primarily suggest that felsic magmas are derived either from the partial melting of crustal rocks or from the fractional crystallization of mantle-derived magmas [8,15]. The source materials of Cretaceous felsic magmas in the SE coastal region are not well-constrained. The proposed models suggested their derivation from (1) young crustal sources with minimal mantle influence [16,17], (2) ancient crustal materials, such as metasedimentary or igneous rocks [18], or (3) mixing of crustal and mantle components in varying proportions [19]. Regardless of the magma source, the chemical and isotopic characteristics of Cretaceous A-type granites and volcanic rocks typically indicate their formation in extensional environments involving crust–mantle interactions [20]. The complex and variable geochemical and isotopic characteristics of these rocks underscore the need for more detailed geological and experimental investigations to refine the understanding of felsic magma sources.

Most research on Cretaceous volcanism in the SE China has focused on individual volcanic suites, providing valuable insights into localized magmatic processes. However, studies examining volcanism across different periods and tectonic environments are somewhat limited. In particular, systematic analyses of the material sources, mantle contributions, and crust–mantle interactions across diverse tectonic environments are scarce. To address this gap, this study investigates rhyolitic volcanic rocks from representative volcanic suites in the Wenzhou–Taizhou region of SE Zhejiang. By integrating laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data of zircon U-Pb-Hf isotopes, trace element composition of zircon, and whole-rock geochemical data with regional geological information and previous studies, we conducted a comparative study of Cretaceous volcanic rocks in the SE China coastal region. This study aims to provide new constraints on the Late Mesozoic tectonic dynamics of SE China, offering insights into broader magmatic and crustal evolutional processes associated with the paleo-Pacific plate geodynamics.

2. Geological Setting and Sampling

The South China Block comprises the Yangtze Craton and Cathaysia Block, amalgamated along the Jiangshan–Shaoxing Fault during the Neoproterozoic (Figure 1) [2]. The Mesozoic–Cenozoic volcanic belt along coastal region of SE China is a key segment of the circum-Pacific volcanic belt [21]. Mesozoic volcanism in this region was extensive and long-lived, spanning from the Late Triassic to the Late Cretaceous, with its peak during the Early Cretaceous. Over 90% of the volcanic rocks in this region are rhyolitic and are primarily distributed to the east of the Yuyao–Lishui–Zhenghe–Dapu–Lianhuashan fault zone. This belt features a series of well-preserved circular volcanic edifices extending from north to south, including Xiaoxiong, Kuocangshan, Nanyandangshan, Ningde, Daiyunshan, Pinghe, and Chaoan. These structures comprise calderas or volcanic domes of varying sizes [3,22].

Based on lithological and facies associations, contact relationships, lithostratigraphy, and isotopic ages, the previous studies have divided the Mesozoic volcanic rocks into lower and upper volcanic series [23]. In Zhejiang Province, the lower volcanic series containing two eruption cycles (ca. 145–130 Ma and ca. 122–110 Ma) includes the Moshishan Group in SE Zhejiang and the Jiande Group in NW Zhejiang, while the upper volcanic series spanning ages from ca. 110 to 87 Ma consists mainly of the Yongkang Group, Tiantai Group, and Xiaoxiong Formation in SE Zhejiang [3], which are distributed in faulted basins such as Xinchang, Tiantai, Xiaoxiong, Lishui, Jinqu, and Fanshan. The Wenzhou–Taizhou region of Zhejiang lies in the northeastern segment of the Late Mesozoic southeastern coastal arc-basin system, with the Wenzhou–Zhenhai Fault obliquely traversing the area. From north to south, this region hosts a series of volcanic edifices, including the Sanmen Xiaoxiong volcanic depression, Huangyan Wanghaigang volcanic dome, Wenling Dongliao volcanic dome, Yongjia Danuoyan caldera, Yueqing Yandangshan caldera, Wenzhou Daluoshan caldera, Pingyang Danan caldera, and Cangnan Fanshan volcanic depression (Figure 1). Among these, the Huangyan Wanghaigang volcanic dome, Yueqing Yandangshan caldera, and Wenling Dongliao volcanic dome are located in the northern Yandangshan area, while the Daluoshan caldera and Cangnan Fanshan volcanic depression are situated in the southern Yandangshan area. These volcanic edifices typically exhibit a ring-like distribution, with volcanic rocks predominantly occurring in the outer ring and subvolcanic intrusions concentrated in the inner ring (Figure 2). The lithologies primarily include granitic and syenitic porphyries. The intrusions form irregular stocks, with the marginal facies consisting of granite porphyry. The region is characterized by a suite of felsic volcanic rocks, mainly from the Xiaopingtian Formation of the Yongkang Group, Tangshang Formation of the Tiantai Group, and Xiaoxiong Formation volcanic rocks (Figure 2b). Rhyolite and rhyolitic tuff dominate the assemblage, with the underlying thin layers of basaltic trachyandesite, forming a bimodal volcanic sequence [24,25].

In this study, 10 fresh samples of rhyolitic tuffs were collected from several volcanic suites, including the samples of Huangyan Wanghaigang volcanic dome (CJ01, CJ02), the Yueqing Yandangshan caldera (ZA01, ZA02), and the Cangnan Fanshan volcanic depression (GQ01) of the Xiaopingtian Formation (Figure 3a); the Wenling Dongliao volcanic dome (WL01, CM01) of the Xiaoxiong Formation (Figure 3b), the WenzhouDaluoshan caldera (XY01, XY02, XY03) of the Tangshang Formation (Figure 3c). These samples consist predominantly of rhyolitic tuffs (Figure 3d–f). The rhyolitic tuffs from the Xiaopingtian and Xiaoxiong Formation exhibit a typical vitroclastic texture, consisting of quartz and feldspar crystal fragments, vitric ash fragments, and volcanic ash (Figure 3g,h). The crystal fragments display angular shapes with shattered edges, some of which exhibit partially melted and rounded tips. Their sizes range from <0.2 mm to 3–4 mm, primarily consisting of quartz, feldspar, and minor biotite. The vitric fragments are largely indiscernible in morphology, with partially devitrified varieties showing concentric arc-shaped patterns under cross-polarized light, indicative of semi-plastic vitric fragments. Volcanic ash forms densely packed, dust-like particles that exhibit complete extinction under cross-polarized light (Figure 3h). In contrast, the rhyolitic tuffs of the Tangshang Formation are characterized by a predominantly porphyritic texture, with phenocryst assemblages dominated by quartz, accompanied by minor plagioclase and biotite, all embedded in a micro-cryptocrystalline groundmass (Figure 3i).

3. Analytical Methods

3.1. Zircon U-Pb Isotope and Trace Element Analyses

The separation of zircon was conducted at Langfang Chengxin Geological Services Co., Ltd. (Langfang, China), using conventional crushing and electromagnetic separation techniques. Cathodoluminescence (CL) imaging, as well as transmitted and reflected light imaging of zircons, was performed at Wuhan Supu Analytical Technology Co., Ltd. (Wuhan, China). All analyses and testing were carried out at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan). Zircon U-Pb dating was conducted using an LA-ICP-MS system, with the laser ablation system being a GeoLas 2005 and the ICP-MS instruments consisting of an Agilent 7500a (Agilent Technologies, Santa Clara, CA, USA) and Neptune Plus (Thermo Fisher Scientific, Waltham, MA, USA). Analytical spots were selected from areas with well-defined oscillatory zoning, avoiding zircon cores. Detailed instrument operating conditions and data processing methods followed those outlined in [32,33]. Trace element concentrations in zircon were obtained simultaneously during LA-ICP-MS zircon U-Pb dating. The instrument was optimized using the U.S. National Institute of Standards and Technology (NIST) reference material SRM 610, which served as the external standard for trace element quantification. The standard zircon 91,500 (recommended age 1065 ± 4 Ma, 2σ) [34] was used as the external standard for dating, while the zircon reference material GJ-1 (recommended age 602.9 ± 4.1 Ma, 2σ) [33] was employed as a monitoring standard. Measured ages of 91,500 (1061.5 ± 3.2 Ma, 2σ) and GJ-1 (604 ± 6 Ma, 2σ) are well within 1% of the accepted age. Zircon trace elements were quantitatively calculated using a multi-external standard approach with USGS reference glasses (BCR2G, BIR1G) as the internal standard. After analysis, the obtained data were processed using the software ICPMSDataCal 10.7 [32]. Common lead correction was applied using the ²⁰⁴Pb method, with the isotopic composition modeled after Stacey and Kramers (1975) [35]. Age calculations and concordia diagram plotting were performed using Isoplot 3.0 [36].

3.2. Zircon Lu-Hf Isotope Analysis

Zircon Lu-Hf isotope analyses were conducted using a Neptune multi-collector MC-ICP-MS instrument (Thermo Fisher Scientific, Waltham, MA, USA). The ablation spot size, ranging from 44 to 50 μm, was adjusted according to the zircon grain size, and the analytical points were consistent with those used for U-Pb dating. Interference correction for Yb and Lu is of paramount importance for precise in situ measurements of Hf isotopes in zircon [37]. Exponential normalization mass bias corrections were applied to Yb and Hf isotopes using the ratios ¹⁷³Yb/¹⁷¹Yb = 1.13017 and ¹⁷⁹Hf/¹⁷⁷Hf = 0.7325, respectively [38]. The analogous physicochemical characteristics of Lu and Yb permitted the application of Yb isotope fractionation behavior as a proxy for Lu mass fractionation corrections. Using the calculated mass fractionation of Yb, the ¹⁷⁶Lu and ¹⁷⁶Yb interferences on ¹⁷⁶Hf are subtracted using the signals of ¹⁷⁵Lu and ¹⁷³Yb and values of 0.02656 for ¹⁷⁶Lu/¹⁷⁵Lu and 0.7876 for ¹⁷⁶Yb/¹⁷³Yb [33]. The standard zircon GJ-1 was used as a reference, with a reported ¹⁷⁶Hf/¹⁷⁷Hf value of 0.282013 ± 0.000019 [39]. Detailed instrument operating conditions and data acquisition procedures are the same as described in [39].

3.3. Whole-Rock Major and Trace Element Analyses

Whole-rock major element concentrations were determined using a Hitachi ZA3000 atomic absorption spectrophotometer (Hitachi High-Tech, Tokyo, Japan) and a TU-1901 ultraviolet–visible spectrophotometer (Beijing Pgeneral Instrument Co., Ltd., Beijing, China), with the relative output error less than 3%. The precision and accuracy of sample analyses met the requirements of GB/T 14506.31-2019 [40]. Trace element concentrations were analyzed using an Agilent 7500a ICP-MS (Agilent Technologies, Santa Clara, CA, USA), ensuring an analytical precision of over 5%. In order to monitor chemical pretreatment and mass spectrometry measurement, each batch of samples was analyzed with the same matrix (basalt BCR-2, basalt BHVO-2, andesite AGV-2, etc.). The margin of error of all trace element results for rock powder reference materials was guaranteed to be within 10%. Detailed sample preparation procedures, as well as information on analytical precision and accuracy, are provided in [32].

4. Results

4.1. LA-ICP-MS U-Pb Zircon Ages

LA-ICP-MS U-Pb age data for analyzed zircon grains from volcanic samples (ZA01, GQ01, CJ01, WL01, CM01, and XY01) in the coastal region of Zhejiang Province are provided in Supplementary Table S1. Cathodoluminescence (CL) images of the analyzed zircons, along with the analyzed spots and corresponding ²⁰⁶Pb/²³⁸U ages, are presented in Figure 4. The analyzed zircons are translucent, short-prismatic, and euhedral to subhedral, with lengths of 150–200 μm, widths of 75–100 μm, and aspect ratios of approximately 2:1. The zircon crystals exhibit straight prismatic faces and well-defined zoning patterns. Some zircons are segmented by brightly luminescent surfaces (Figure 4), yet all display high Th/U ratios (0.45–2.4) (Figure 5), indicative of a magmatic origin [41].

For the Zhen’an rhyolitic tuff sample (ZA01) from Xiaopingtian Formation, 18 zircon grains were analyzed. After excluding eight data points with concordance values below 90%, the remaining data points plot on or near the concordia curve (Figure 5a). Among these, four zircons yielded relatively older ²⁰⁶Pb/²³⁸U ages ranging from ca. 242 to 125 Ma, interpreted to be inherited grains. The remaining six analyses yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 113.6 ± 4.4 Ma (MSWD = 2.9, n = 6), representing the crystallization age of the tuff.

For the Ganqi rhyolitic tuff sample (GQ01) from Xiaopingtian Formation, 16 zircon spots were analyzed. Nine data points were excluded due to low concordance. The remaining seven data points plot on or near the concordia line (Figure 5b). These points yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 112.0 ± 2.4 Ma (MSWD = 2.4, n = 7), which represents the crystallization age of the tuff.

For the Cangji rhyolitic tuff sample (CJ01) from Xiaopingtian Formation, 16 zircon grains were analyzed. All data points plot on the concordia curve (Figure 5c). Two zircons with older ²⁰⁶Pb/²³⁸U ages of ca. 123 Ma and, 119 which may be of xenocryst origin. The remaining 14 analyses yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 106.1 ± 2.1 Ma (MSWD = 2.1, n = 14), considered as the crystallization age of the tuff.

A total of 16 zircon spots were analyzed from the Wenling rhyolitic tuff (WL01) from the Xiaoxiong Formation. All of which exhibited concordance values greater than 90% (Figure 5d). Among these, three zircons show relatively older ²⁰⁶Pb/²³⁸U ages from ca. 103 to 108 Ma. The remaining 13 grains yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 99.5 ± 1.6 Ma (MSWD = 0.3, n = 13), representing the crystallization age of the tuff.

For the Chumen rhyolitic tuff sample (CM01) from Xiaoxiong Formation, 18 zircon spots were analyzed. After excluding two data points with low concordance, the remaining data points plot on or near the concordia curve (Figure 5e). Among these, four zircons yielded relatively older ²⁰⁶Pb/²³⁸U ages ranging from ca. 146 to 123 Ma, which are interpreted to represent inherited zircons. The remaining 12 analysis points yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 100.2 ± 3.0 Ma (MSWD = 2.3, n = 12), representing the crystallization age of the tuff.

For the Xianyan rhyolitic tuff sample (XY01) from the Tangshang Formation, 18 zircon spots were analyzed. After excluding two data points with concordance values below 90%, the remaining 15 data points (excluding XY01-1.1, 4.1, and 18.1) plot on the concordia curve (Figure 5f). Among these, one zircon yielded a relatively older ²⁰⁶Pb/²³⁸U age of ca. 118.9 Ma. The remaining 15 analysis points yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 94.7 ± 2.0 Ma (MSWD = 0.5, n = 15), representing the crystallization age of the rhyolitic tuff.

In summary, the tuffs from the Zhen’an, Ganqi, and Cangji regions were formed in the late Early Cretaceous (ca. 114–106 Ma), while the volcanic rocks from Wenling, Chumen, and Xianyan crystallized in early Late Cretaceous (ca. 100–95 Ma). These two age groups represent magmatic pulses from the transition from the Early to Late Cretaceous period.

4.2. Zircon Trace Elements Composition and Lu-Hf Isotopes

The chemical compositions of 81 zircons with ages ranging from ca. 113 to 94 Ma (²⁰⁶Pb/²³⁸U ages) are presented in Supplementary Table S2. These zircons generally exhibit steeply rising REE patterns from La to Lu (Figure 6), along with pronounced positive Ce anomalies and negative Eu anomalies. These characteristics suggest that the zircon is likely derived from a common magmatic system.

In various samples, zircons exhibit a fundamentally overlapping trace element composition with a considerable range of variation. However, the concentrations of Ti, Hf, Th, and U in the sample CJ01 (with mean values of 4.18 ppm, 13,198 ppm, 245 ppm, 361 ppm) are relatively lower, all falling below the levels found in the five samples GQ01, ZA01, WL01, CM01, and XY01, where Ti (with an average over 20 ppm), Hf (with an average over 20,000 ppm), Th (with an average over 650 ppm), and U contents (with an average over 420 ppm) are observed, thus revealing characteristics of two distinct end-members. Furthermore, by applying zircon Ti thermometry [42], the crystallization temperatures of zircons were calculated. Zircon crystallization temperatures in late Early Cretaceous volcanic rocks are systematically lower than those in early Late Cretaceous counterparts. late Early Cretaceous zircons record temperatures below 800 °C, whereas early Late Cretaceous zircons consistently exceed 800 °C. Among the late Early Cretaceous samples, CJ01 yields the lowest temperatures (575–720 °C, average 663 °C), followed by WL01, CM01, and XY01. This is notably lower than the temperature values of other samples (Supplementary Table S2), suggesting that the volcanic rocks of CJ01 may have undergone a more complex magmatic process.

Most zircons exhibit significantly low Eu anomalies (δEu < 0.25). However, some analyzed grains, primarily from samples WL01, CM01, and XY01 formed in the early Late Cretaceous, display higher δEu values (>0.35), with some even reaching 0.6, indicating weaker negative Eu anomalies. Additionally, as δEu values decrease, the concentrations of Nb, U, and Hf show an increasing trend, while temperature, Ti content, and Th/U ratios exhibit a decreasing trend (Figure 6).

Figure 6. (a) Ti vs. δEu, (b) Hf vs. δEu, (c) U vs. δEu, (d) Th/U vs. Hf, (e) Ti vs. Hf, (f) Sm/Yb vs. Hf, and (g) Nb vs. δEu diagrams of trace element compositions in zircons; and (h) chondrite-normalized patterns of averaged REE content of zircons from representative volcanic samples. The normalizing values for REE are from [43]. (Pl = plagioclase; Fsp = feldspar; Zrn = zircon; Ap = apatite).

Figure 6. (a) Ti vs. δEu, (b) Hf vs. δEu, (c) U vs. δEu, (d) Th/U vs. Hf, (e) Ti vs. Hf, (f) Sm/Yb vs. Hf, and (g) Nb vs. δEu diagrams of trace element compositions in zircons; and (h) chondrite-normalized patterns of averaged REE content of zircons from representative volcanic samples. The normalizing values for REE are from [43]. (Pl = plagioclase; Fsp = feldspar; Zrn = zircon; Ap = apatite).

Based on the U-Pb age results, representative volcanic rocks (samples GQ01 and WL01) were selected for in situ microanalysis of Hf isotopes in zircons, and the results are presented in Supplementary Table S3. Overall, the Hf isotope compositions of zircons in the volcanic rock samples are relatively homogeneous, with ε_Hf(t) values ranging within 1–3 units (Figure 7).

In the rhyolictic tuff from Ganqi, the ¹⁷⁶Lu/¹⁷⁷Hf ratios of 11 zircons range from 0.000884 to 0.003439, with ¹⁷⁶Lu/¹⁷⁷Hf varying between 0.282440 and 0.282565. These zircons show negative ε_Hf(t) values ranging from −9.4 to −5.0. The two-stage Hf model ages (T_DM2) fall between 1.48 and 1.76 Ga (Figure 7).

In the rhyolitic volcanic rocks from Wenling, the ¹⁷⁶Lu/¹⁷⁷Hf ratios of 16 zircons range from 0.000873 to 0.002223, with ¹⁷⁶Lu/¹⁷⁷Hf varying between 0.282476 and 0.282560. The ε_Hf(t) values range from −8.2 to −5.4. The two-stage Hf model ages (T_DM2) fall between 1.68 and 1.50 Ga (Figure 7).

4.3. Whole-Rock Major and Trace Element Compositions

The major and trace element compositions of the representative volcanic samples are shown in Supplementary Table S4. In the TAS and K₂O-SiO₂ diagrams (Figure 8), the rhyolitic samples from the Early and Late Cretaceous transition are plotted in the rhyolite field (Figure 8a), exhibiting high and relatively stable SiO₂ contents (70.3%–76.9%), with Al₂O₃ contents ranging from 12.5% to 15.9%, K₂O ranging from 3.9% to 6.1% (Figure 6b), K₂O+Na₂O contents ranging from 7.5% to 10.6%, and an aluminum saturation index value (A/CNK) between 1.0 and 1.1, indicating weak peraluminous characteristics (Figure 8c).

In the late Early Cretaceous rhyolitic volcanic rocks, the Rittmann index (σ) ranges from 1.69 to 2.68, and the Rittmann alkalinity ratio (A.R) ranges from 3.3 to 4.4, showing sub-alkaline characteristics, belonging to the high potassium calc-alkaline series. In the early Late Cretaceous rhyolitic tuff, the σ ranges from 2.1 to 4.1, and the A.R ranges from 4.0 to 5.0, displaying calc-alkaline to alkaline characteristics, falling under the high potassium calc-alkaline to calcic-alkaline series (Figure 8).

The late Early Cretaceous rhyolitic volcanic rocks have an average ΣREE value of 182 ppm, with LREE/HREE ratios ranging from 9.5 to 13.2, showing significant light–heavy REE fractionation. The La_N/Yb_N ratios range from 8.9 to 17.3, and δEu values range from 0.38 to 0.65, indicating a moderate negative Eu anomalies. On the other hand, the early Late Cretaceous rhyolitic volcanic rocks have an average ΣREE value of 276 ppm, with an increase in total rare earth elements. The LREE/HREE ratios range from 9.8 to 16.2, showing enhanced light–heavy REE fractionation. The La_N/Yb_N ratios range from 8.6 to 20.9, and δEu values range from 0.2 to 0.6, displaying moderate to strong negative Eu anomalies. In the chondrite-normalized diagrams (Figure 9a), the REE distribution patterns of the rhyolitic volcanic rocks from the Early and Late Cretaceous transition show a slight rightward inclination.

In terms of trace element composition, the late Early Cretaceous rhyolitic volcanic rocks are characterized by high Sr/Y ratios (mean 6.2), low Rb/Sr ratios (mean 1.6), and Nb/Y ratios (mean 0.6). In contrast, the early Late Cretaceous volcanic rocks exhibit low Sr/Y ratios (mean 4.1), high Rb/Sr ratios (mean 3.1), and Nb/Y ratios (mean 0.7). On the spider diagrams (Figure 9b), both stages display enrichment in large-ion lithophile elements (LILEs) such as K, Rb, and U, and varying degrees of depletion in high-field-strength elements (HFSEs) such as Nb, Ta, Ti, and P. These features suggest that the separation and crystallization of alkali feldspar were more pronounced than those of plagioclase, and the rocks exhibit characteristics typical of arc-related melts. Additionally, the distribution patterns of both the late Early Cretaceous and early Late Cretaceous rhyolitic volcanic rocks resemble those of continental crust, implying a genetic link between these volcanic rocks and continental crustal sources.

5. Discussion

5.1. Spatio-Temporal Distribution of Magmatism in Zhejiang-Fujian Coastal Region

The southeastern coastal region of China is situated at the junction of the Pacific Plate and the Eurasian Plate, where Mesozoic volcanic rocks are widely distributed. Basaltic to andesitic volcanic rocks are sporadically exposed, while felsic volcanic rocks dominate, particularly in Zhejiang and Fujian provinces, where volcanic rocks are most prominent. Published geological data and our new results indicate two distinct periods of volcanic activity of the Early Cretaceous to Late Cretaceous in this region. Each period is characterized by bimodal volcanic rocks. During the Early Cretaceous period (ca. 145–118 Ma), volcanic activity attained its highest magma flux [53,54], forming hundreds of large volcanic suites. The lithological assemblages include post-collisional NE-trending regional extension high-potassium calc-alkaline rock series and extensive high-potassium calc-alkaline rhyolite-high-silica rhyolitic rocks (Figure 1). The volcanic rock succession is assigned to the Moshishan Group in Zhejiang Province. Subsequently, the large-scale volcanic activity essentially ceased, transitioning to a certain scale of post-orogenic A-type granite magmatic activity, primarily during the late Early Cretaceous to early Late Cretaceous (ca. 115–85 Ma; concentrated between ca. 101 Ma and 95 Ma). This period formed a massive volcanic belt extending approximately 2000 km in length and 400 km in width from the Zhao’an–Yunxiao area at the Fujian–Guangdong border to the Zhoushan Islands [2]. These volcanic rocks are predominantly distributed in the northeast-trending fault-control basins of Zhejiang and Fujian. Volcanic rocks in Zhejiang are divided into three lithostratigraphic groups, namely the Yongkang Group, the Tiantai Group, and the Xiaoxiong Formation, while it is known as the Shimaoshan Group in Fujian [55].

The crystallization ages of zircons in these felsic igneous rocks generally become younger from the inland towards the SE coastal regions [2] (Figure 1). The early stage volcanic rocks of the Early Cretaceous (ca. 145–118 Ma) period are primarily distributed in the west of the Yuyao–Lishui–Zhenghe–Dapu fault zone along the NE-SW trending, which is one of several parallel major faults in SE China. In contrast, the late-stage volcanic rocks (115–85 Ma) occur in the eastern coastal regions along this fault zone (Figure 2). Similarly, the Cretaceous granitic magmatism has progressively become younger from the inland areas towards the southeastern coastal region. The compiled U-Pb age data of igneous rocks from the coastal regions of Zhejiang and Fujian provinces (Supplementary File) reveal that the formation ages (ca. 113–90 Ma) of felsic intrusions in the coastal regions are similar to the late-stage volcanic rocks (Figure 10). Our new results indicate that the volcanic activity in the coastal regions of Zhejiang can be traced back to ca. 114–95 Ma. These ages roughly coincide with the peak period of volcanism in the SE coastal regions, forming a NNE-trending magmatic belt at the Early–Late Cretaceous transition in the coastal areas of eastern Zhejiang.

The volcanic sequences along the coast of Zhejiang Province are mainly include the Yongkang Group, Tiantai Group, and Xiaoxiong Formation. The Xiaopingtian Formation constitutes the uppermost stratigraphic level of the Yongkang Group of late Early Cretaceous age and consists predominantly of volcaniclastic rocks. The youngest crystallization age obtained from this formation defines the upper chronological limit of the Yongkang Group and marks the cessation of their respective volcanic cycle. The Xiaoxiong Formation and Tiantai Group, which succeed the Yongkang Group, constitute a Late Cretaceous volcanic-sedimentary and sedimentary sequence intercalated with volcanic rocks. They overlie the older strata with an angular unconformity. The crystallization ages obtained from the base of the Xiaoxiong Formation and Tiantai Group mark the initiation of their respective volcanic cycles. According to isotopic ages (Supplementary File), the ages of the Xiaopingtian Formation in the Yongkang Group range from ca. 115 to 105 Ma [28,29]. This was succeeded by the Xiaoxiong Formation and the contemporaneous Tiantai Group, which yield crystallization ages spanning from ca. 99 to 88 Ma. Specifically, volcanic rocks at the base of the Tangshang Formation (Tiantai Group) in the Lishui Basin are dated at ca. 95 Ma [30]. The basal ages of the Xiaoxiong Formation are about ca. 98 Ma in the Xiaoxiong area of Sanmen, ca. 96 Ma in the Shunxi area of Pingyang, and ca. 99 Ma in the Fanshan area of Cangnan [31]. Consequently, the interval from ca. 105 Ma to 99 Ma represents a volcanic hiatus between the late Early Cretaceous and the Late Cretaceous.

5.2. Petrogenetic Insights from Integrated Whole-Rock and Zircon Chemistry

5.2.1. Fractional Crystallization

It is widely acknowledged that fractional crystallization is a fundamental process in magmatic evolution, influencing compositional changes in magma, as evidenced by published research [22,56,57]. The whole-rock geochemistry indicates mineral crystallization differentiation. Studied volcanic rocks from two distinct periods exhibit high SiO₂ content (70.3%–76.3%), and there is a noticeable negative correlation between MgO, TFeO, CaO, TiO₂, P₂O₅, Al₂O₃, and SiO₂ in these volcanic rocks (Figure 11), indicating that the magma differentiation process is primarily controlled by the fractional crystallization of minerals such as pyroxene, Fe-Ti oxides, apatite, and feldspar. Additionally, their trace elements also display good linear relationships (Figure 12). In the REE patterns (Figure 9a), all volcanic rock samples exhibit distinct negative Eu anomalies (δEu of 0.2–0.7). On the spider diagrams of trace elements, these volcanic rocks show pronounced Rb positive anomalies and depletions in Ba, Sr, P, and Ti (Figure 9b), with the CM01 volcanic rock displaying a prominent Rb positive anomaly and a particularly significant Ba negative anomaly, reflecting the greater influence of alkaline feldspar over plagioclase in the magma evolution process [58].

Zircon is one of the most common accessory minerals in felsic magmatic rocks. The trace element composition of zircon can reliably reflect geochemical characteristics of its parental magma. For example, the Zr/Hf ratio, δEu, and Sm/Yb ratio in zircon can be used to trace the co-crystallization of minerals such as feldspar, ilmenite, monazite, apatite, and zircon in the parent magma [56]. Specifically, more evolved melts tend to precipitate zircons with higher Hf content and lower Zr/Hf and δEu values. Zircons in these volcanic rocks exhibit dark cathodoluminescence (CL) oscillatory zoning and bright sector zoning, along with negative correlations between Hf, U, Nb, and δEu, as well as between Th/U, Ti, Sm/Yb, and Hf (Figure 6). These features suggest that fractional crystallization of minerals such as monazite/apatite/ilmenite, plagioclase, and zircon was involved in the magma differentiation [57]. Additionally, zircons from the CJ01 volcanic rocks show significantly lower concentrations of Ti, Hf, U, and Nb compared to other samples, indicating a less evolved nature [59] (Figure 6). The geochemical composition of igneous rocks can be influenced by various factors, including magma source, fractional crystallization, and magma mixing [22]. The distinct Nb/Ta ratios and zircon εHf(t) values support a hybrid crust–mantle origin for these rocks, implying significant contributions from mantle-derived components during magma generation, likely in deep-crustal hot zones. However, evidence for late-stage, shallow-level magma mixing is limited. In the magma mixing model (Figure 11a), the samples form coherent trends rather than defining a binary mixing array. For instance, the CJ01 sample data do not show significant deviations compared to other samples, plotting towards the less-evolved end of the fractional crystallization trend. Moreover, it plots toward the less-evolved end within Stage 1, consistent with its lower Ti-in-zircon (Supplementary Table S2) and lower Hf–U in zircon (Figure 6) relative to more evolved counterparts. These results suggest that the two phases of felsic volcanism were dominated by fractional crystallization processes, likely originating from high-silica melts that underwent varying degrees of fractional crystallization and differentiation of minerals such as apatite, zircon, ilmenite, and feldspar (Figure 12).

5.2.2. Magma Source

Felsic magmas can form through various processes, such as crustal remelting [60], differentiation of mafic magmas [61], as well as mixing, assimilation, storage, and homogenization of different magmas [62]. The Late Mesozoic magmatic activity in the entire southeastern China is dominated by felsic magmatic processes, with relatively fewer intermediate and mafic igneous rocks. Although mafic xenocrysts have been discovered in some volcanic rocks like Yandang Mountain, Furong Mountain, and Yunshan [46,57], their exposure is limited, indicating that the majority of the felsic magmas are unlikely to have directly differentiated from mantle-derived mafic magmas [2]. Moreover, most samples contain inherited zircon grains (e.g., samples ZA01, CM01, and XY01), suggesting that crustal assimilation may have played a role in the formation of these volcanic rocks, but it is challenging to explain such a significant volume of felsic magmatic activity solely through assimilation processes. The current concepts suggest that felsic magmas are primarily formed through deep crustal melting [63]. The enriched zircon Hf isotope compositions (ε_Hf(t) ranging from −9.4 to −5.0) in this study also suggest significant involvement of crustal materials in the formation of these volcanic rocks.

The remelting of crustal basement is primarily regulated by its composition. The basement of the Cathaysia Block, dominantly formed during the Paleoproterozoic (Figure 7), constitutes a fundamental control [45]. Moreover, Xu et al. [45] reported that the Middle to Late Jurassic volcanic sequences in Zhejiang exhibit T_DM2 ages between 2.10 and 1.85 Ga, similar to the Nd model ages (2.2–1.8 Ga) of the metamorphic basement of the Cathaysia Block [64], indicating a significant correlation between the magma sources and the basement metamorphic rocks. Therefore, the two-stage Hf model ages (T_DM2) of the zircons range from 1.48 to 1.76 Ga (Figure 7), suggesting the involvement of a late Paleoproterozoic to early Mesoproterozoic crust as a probable source.

Similarly, the zircon Hf isotope compositions of Late Mesozoic, especially Cretaceous, volcanic rocks in the coastal regions of SE China exhibit a characteristic trend from enrichment to depletion over time [4,5,22,55], a phenomenon also evident in contemporaneous intrusive rocks [65] (Figure 7), In the early Late Cretaceous period in the SE coastal region of Zhejiang, the ε_Hf(t) values of intrusions at Yueqing Yandangshan, Maoliling, and Wanghaigang range from −10 to −2.5, −12.2 to −2.9, and −12.2 to −4.4, respectively [46,66,67]. In the early Late Cretaceous period, representative intrusions at Xiaoxiong in Sanmen and Taohuadao exhibit ε_Hf(t) values ranging from −6.7 to −4.2 and −5.5 to −4.4, respectively [31], displaying even more depleted characteristics, implying the addition of juvenile material. These results can be interpreted as the Late Mesozoic magmatic activity in SE China being a result of crust–mantle interactions, where the mantle-derived magmas not only act as a heat source, inducing partial melting of overlying crustal materials, but also directly participate as end members in magma mixing [55]. Therefore, when discussing magma sources using whole-rock Nd and zircon Hf isotopes, the potential contribution of juvenile mafic crust with similar isotopic compositions to the mantle should not be overlooked. On the other hand partial melting of mafic crust can generate felsic rocks with isotopic compositions resembling those of the mantle [16,68,69]. In this scenario, major and trace element analyses of whole rocks may help distinguish contributions from the crust and mantle [70].

The two stages of felsic volcanic rocks in this study are both high-potassium calc-alkaline rocks, rich in potassium and silica. They represent a unique rock composition with dual geochemical characteristics of both the crust and mantle, indicating the involvement of crustal materials in the petrogenesis [71]. This also suggests that felsic magmas cannot evolve solely from mafic magmas. The genesis of high-silica magmas can occur through deep melting of continental crust and the separation of mafic andesite melt products. The typical products of deep melting of continental crust are large-volume high-silica igneous rocks [72,73], Additionally, the A/CNK ratio (more than 1) in the felsic volcanic rocks in this region, reflecting weakly peraluminous characteristics, together with zircon Hf isotopes (Figure 7), is consistent with a substantial crustal contribution within a mixed crust–mantle system [74]. Furthermore, the depletion of trace elements such as Ba, Ti, P, Nb, Zr, and Ta (Figure 9b) is consistent with continental crust rock compositions [52]. In conjunction with the corresponding REE patterns, these geochemical features collectively indicate derivation from a crustal source. In the late Early Cretaceous, the volcanic rocks exhibit Nb/Ta ratios of 11.8–13.3, while in the early Late Cretaceous, the Nb/Ta ratios range from 14.3 to 17.3 (Nb/Ta for chondrites being 17.5, and for the crust being 11–12 [75]), approaching the values of depleted mantle (depleted mantle Nb/Ta being 15.5 [76]); the Lu/Yb ratios overall range from 0.14 to 0.17 (mantle Lu/Yb of 0.14–0.15 [43]), close to mantle standard values. This indicates that the material provided by oceanic crust is limited. Hence, the materials of these volcanic rocks are mainly derived from ancient crustal sources, with some contribution from the mantle, increasing gradually as the rocks become younger over time.

5.2.3. Partial Melting Conditions of the Crust

The analyses above have indicated that in the volcanic rocks of different regions along the coast of Zhejiang, magmas enriched in isotopic end-members primarily originate from the partial melting of ancient crustal basement material. Such extensive crustal melting implies widespread and intense geothermal anomalies, which can be caused by major heat sources such as radioactive element decay heat, heating from deep-seated high-temperature mantle-derived magmas, heating from the emplacement of hot mantle-derived magmas, and heat released during the crystallization process [77]. Among these mechanisms, the process of underplating is considered to be closely related to the genesis of felsic rocks [78,79], and evidence of Late Mesozoic basalt underplating in SE China comes from geophysical, petrogenetic, and metamorphic studies [2,65,80]. However, thermodynamic simulations do not support the idea of large-scale melting of crustal rocks. For instance, the previous studies have suggested through thermodynamic simulation research that the heat flow from basaltic magma underplating is unlikely to cause extensive crustal melting, especially in cases where the crust is relatively thin [81]. Even if the lower crust is composed of gabbro, basaltic underplating alone may not be the primary mechanism for generating large-scale partial melting of the lower continental crust [82,83]. However, if the generation of the felsic melts involves water-assisted melting (which could be a significant mechanism driving regional granitic magmatic activity [70,84,85]), this issue can be addressed.

In a magmatic system, the relationship between U and Nb is influenced by factors such as variations in mantle source composition (including depleted and undepleted mantle sources), melting depth, dehydration of oceanic crust [86], and crustal assimilation [87] processes. Analyzing the bivariate plots of zircon U, Th, Hf, Yb, Nb, and Y, most data points are situated within the Arc-related domain, exhibiting elevated Th/Nb ratios (mostly exceeding 10) and U/Yb ratios (mostly surpassing 0.5) [87]. Zircons formed in subduction zones tend to display higher U/Nb values, attributed to relative Nb depletion and U enrichment (Figure 13a–c). All age groups demonstrate the influence of at least some degree of fluid-assisted melting. Furthermore, Zircon Th/U ratios serve as a diagnostic indicator of tectonic stress regimes, with systematically lower values (<1.0) characteristic of compressional domains and elevated ratios diagnostic of extensional environments [27,88]. The Th/U values of zircons exhibit an increasing trend over time (Figure 13d), reflecting the continuous strengthening of lithospheric extension. Therefore, the supply of H₂O and heat may provide an additional mechanism for the high heat flow [89], lithospheric thinning, and asthenospheric upwelling [90], providing thermal energy and a source of juvenile material for large-scale crustal melting.

Notably, zircon saturation temperatures [91] based on whole-rock compositions reveal that the late Early Cretaceous volcanic rocks have lower average initial melt temperatures (769 °C) compared to the early Late Cretaceous volcanic rocks (808 °C) (Figure 14). Similarly, titanium-in-zircon thermometry [42] yields consistent results, indicating that the late Early Cretaceous volcanic rocks (ZA01 having mean value of 764 °C; GQ01 of 782 °C; CJ01 of 652 °C) exhibit lower temperatures than the early Late Cretaceous volcanic rocks (WL01: 823 °C; CM01: 819 °C; XY01: 820 °C). Particularly, the zircon saturation temperatures for CJ01 (566–709 °C) are comparable to those observed in water-assisted crustal melting scenarios in the Cordilleran batholiths [84]. Furthermore, these volcanic phases display distinct whole-rock REE patterns. The late Early Cretaceous volcanic rocks exhibit U-shaped REE patterns with slight negative Eu anomalies, while the early Late Cretaceous volcanic rocks show seagull-shaped REE patterns with pronounced negative Eu anomalies. The contrasting REE patterns robustly constrain the mineral fractionation regimes governing each volcanic phase. The U-shaped patterns of the late Early Cretaceous rocks signify MREE depletion, a hallmark of amphibole fractionation within a hydrous magmatic system. Conversely, the seagull-shaped patterns of the early Late Cretaceous rocks, with their profound negative Eu anomalies and steep HREE slopes, reflect a regime dominated by extensive feldspar alongside accessory apatite and zircon fractionation [92,93]. This continuous compositional trend between cold-wet and hot-dry magma types [92] observed in these two volcanic episodes (Figure 9) can be interpreted as a response to the rollback and subduction of the paleo-Pacific plate [4,65]. This phenomenon is similar to that observed in Cretaceous felsic rocks from the coastal region of Fujian Province [16]. Early Cretaceous I-type granites in Fujian typically contain 5%–10% hydrous minerals such as biotite and amphibole, whereas late Cretaceous granites show a reduced abundance of hydrous minerals, ranging from 3% to 5%, suggesting a decrease in water content in the younger magmas [16]. Therefore, both episodes of the volcanic magmas in this study were generated through water-assisted (hydrous) melting, whereas the later phase formed at relatively higher temperatures and with a diminished water contribution via dehydration melting [85].

5.3. Origin and Evolution Model of the SE China Cretaceous Volcanic Rocks

Large-scale felsic volcanic eruptions are typically associated with the formation of collapse calderas [94]. Following caldera formation, replenishment processes can lead to the activation of residual magma or crystallized material, resulting in the formation of rhyolite domes or shallow intrusions, recording the complex history of the crustal magmatic system [94]. Along the Zhejiang–Fujian coast, numerous such caldera structures have developed, such as Yandangshan, Yunshan, Xiaoxiong, Kuangcangshan, Wanghaigang in Huangyan, Xianyan, and Daluoshan. They often constitute volcanic–plutonic complexes, with their origins explained by the recently proposed model of crustal-magmatic systems [31,46,47,57], suggesting that the magma systems of large felsic volcanoes operate at a crustal-penetrating scale, interconnected by deep and shallow magma chambers. Mantle-derived magmas accumulate, store, and evolve in the lower crust over the long term and are connected to the middle-upper crustal magma chambers, emphasizing the role of magma replenishment and crustal material activation in driving magma differentiation and influencing the evolution, growth, and reworking of continental crust [8,9,10,11,95]. Numerous studies indicate that crystal-poor rhyolites separate from crystal-rich granite magma reservoirs within the optimum crystallinity window (40%–70%) [92]. New data on volcanic rocks from these typical calderas, combined with previous research, indicate that both volcanic phases underwent significant crystal fractionation and magma replenishment processes, along with certain crustal assimilation, contributing to the geochemical diversity of volcanic rocks along the southeastern coast of Zhejiang.

Due to the stability of plagioclase at relatively shallow depths and its instability under high-pressure conditions [96], residual plagioclase in the source region or plagioclase fractionated during the crystallization process can lower the δEu (Eu_N/(Sm_N × Gd_N)^0.5) ratio of the melt, thus affecting the δEu ratio of subsequently crystallizing zircon. δEu is also influenced by the oxygen fugacity of the melt [41,97]. The zircons in both volcanic phases exhibit a plagioclase separation trend (Figure 6 and Figure 12), implying magma differentiation at relatively shallow crustal depths (Figure 12). Garnet is commonly used to demonstrate high-pressure environments formed at significant depths, although there are exceptions. Garnet has high partition coefficients for HREEs and Y, and its crystallization leads to an increase in Dy/Yb in the melt. Dy/Yb > 0.4 in zircon is typically associated with garnet, although fractionation/presence of amphibole in residual minerals before zircon crystallization may result in lower Dy/Yb values even when associated with garnet. Zircons from these two phases show low Dy/Yb and high Yb characteristics, suggesting the presence of shallow crustal magma chambers (Figure 15).

Furthermore, the previous studies have consistently indicated that the origin of late Mesozoic felsic volcanic rocks in SE China is associated with the westward subduction and retreat of the paleo-Pacific plate [90,98,99]. However, the mechanisms by which the lower crust became water-rich and fusible, as well as the triggers for its partial melting, remain unclear. The late Mesozoic magmatism in SE China has been widely recognized as forming in a tectonic setting analogous to an Andean-type active continental margin, which underwent a transition from compression to extension due to slab rollback [1,2,85,100]. In such a back-arc extensional setting, the partial melting of crustal rocks can be driven by both heat flux (dehydration melting) and fluid flux (hydration melting). The fluids are ultimately sourced from the subducted slab at the adjacent convergent margin [85]. Previous research has demonstrated that the large volume of felsic magma reflects extensive crustal melting, which is controlled by the composition of the lower crust, particularly its water content [101]. Previous research also highlighted that a water-rich and fertile lower crust, typical of long-lived paleo-continental margins and active continental margins, is a critical feature for generating such voluminous felsic magmas [101]. Our data show that these two volcanic episodes are primarily associated with arc/subduction environments (Figure 13), consistent with their formation in this rollback-induced extensional regime within an active continental margin [1,2,100].

Fluids derived from dehydration of minerals like amphibole and biotite in the slab induce melting in the mantle wedge, providing a heat source (with a temperature difference of over 500 °C with the lower crust) for basaltic underplating and intrusions into the crust, contributing to the formation of a “MASH zone” [102], leading to partial melting of significant amounts of granitic magma in the middle-lower crust [72]. Between ca. 115 and 100 Ma, the arc back-arc extension triggered extensive lower crustal hydrous partial melting due to the retreat of the paleo-Pacific plate, resulting in the differentiation of mafic magmas in the lower crust and the mixing of crustal material (Figure 16). This process generally does not directly lead to high-potassium rhyolitic magmas but generates abundant andesitic or dacitic magmas [103], and further differentiation occurs in the shallow crust [6,92,104], leading to the formation of high-silica volcanic rocks (Figure 8; SiO₂ greater than 70%). Subsequent enhanced plate retreat, possibly transitioning to a rift tectonic setting [22,70], allowed for a sustained supply of “heat” and “juvenile material” from the mantle. Consequently, transcrustal magma chambers likely had relatively long lifespans [27], and over a period of ca. 100–90 Ma, fractional crystallization and weak crustal contamination produced magmatic rocks at various levels, including shallow high-silica magmatic rocks [105]. Simultaneously, the high-angle retreat of the slab caused the migration of magmatic activity toward the coastal zone (Figure 1), and the involvement of these mafic magmas in the formation of felsic magmatic rocks led to increased magma temperatures (Figure 14) and relatively juvenile isotopic compositions (Figure 7). Additionally, the release of heat from high-temperature basaltic magmas further consumed the fertile materials in the lower crust, resulting in the formation of late episode volcanic rocks (Figure 9a and Figure 16).

6. Conclusions

(1) The volcanic rocks from the coastal region of SE Zhejiang crystallized in the range between ca. 114 Ma and 95 Ma. Along with intrusive rocks emplaced between ca. 113 and 90 Ma, they define a NNE-trending volcanic-intrusive belt, spanning the Early and Late Cretaceous.

(2) Integrating whole-rock major and trace element compositions with zircon trace elements and Hf isotopes, this study subdivides Cretaceous volcanism into two phases formed by mixing/differentiation of underplated basaltic magmas with melts derived from a crustal source dominated by late Paleo- to early Mesoproterozoic components.

(3) Both phases of the volcanic magmas were generated through water-assisted (hydrous) melting, whereas the later phase formed at relatively higher temperatures and with a diminished water contribution via dehydration melting of the lower crust, driven by intense lithospheric extension in a back-arc setting induced by the retreat of the paleo-Pacific plate.