Formation of the Liaotianshan Volcano in Southeastern China: Implications for the Evolution and Recharge of Crustal Magma System

Abstract

Large-volume volcanic activity may offer significant insights into the evolution of silicic magma systems. The Liaotianshan volcano represents one of the earliest and best-preserved examples in SE China, comprising two stages of silicic volcanic rocks followed by extrusive rhyolite porphyries within the conduit. In this study, we present petrological and geochemical analyses, along with zircon dating, of the Liaotianshan volcano. LA–ICP–MS zircon U–Pb dating results revealed that two-stage eruptions occurred between approximately 160 and 157 Ma, and the latest batch of magma was extruded from the conduit around 153 Ma. Volcanic rocks from both stages exhibit similar geochemical compositions, characterized by pronounced depletion in high-field-strength elements and enrichment in large-ion lithophile elements, with the majority of zircon ε_Hf(t) values falling within a narrow range of –9.8 to –5.4. In contrast, the extrusive rhyolite porphyries display distinct geochemical characteristics, demonstrating enrichment in heavy rare earth elements relative to light rare earth elements [(La/Yb)_N = 0.14–0.61], obvious negative Eu anomalies (Eu/Eu* = 0.01–0.03), and positive Ce anomalies, alongside markedly depleted zircon Hf isotopic compositions. We propose that the volcanic rocks from the two stages were formed by the reworking of the Paleoproterozoic crustal basement with occasional recharge of parental magma, while the extrusive rhyolite porphyries resulted from the mixing of crustal-derived magma and significantly depleted mantle-derived materials. The Liaotianshan volcano was formed in contradiction to the model of melt extraction and crystal accumulation within a magma chamber, instead reflecting the evolutionary history and replenishment dynamics of the crustal magma system.

1. Introduction

The emplacement and eruption mechanisms of large-volume silica-rich magmas are of crucial significance for comprehending the differentiation and evolution of the continental crust [1,2,3]. However, the origin of the volcanic rocks and their association with plutonic rocks remain highly controversial. Some scholars propose a close relationship between volcanic and plutonic rocks, suggesting that rhyolitic melts could accumulate at the top of a magma reservoir and subsequently erupt, leaving the plutonic rocks as remnant cumulates [4,5]. In contrast, others contend that rapid eruptions of silicic magmas occur at high magma flux, leaving behind only minor plutonic residues [6], while significant plutonic assembly is generated at low magma flux [7,8]. Nevertheless, it is widely agreed that shallow plutons in large silicic calderas have a close genetic connection with the coexisting volcanic rocks [4,7,9]. The eruptions commonly partially evacuate the magma chamber, leaving behind its remnants as associated shallow plutons and sometimes extrusions in the volcanic conduit [4]. Therefore, the origin of large-volume volcanism is of paramount importance for the comprehension of the nature and evolution of the mid- to upper crustal magma system, as it records the complex history of magma accumulation, fractionation, mixing, and replenishment [4,7,8].

Late Mesozoic silicic magmatism is intensive along the coastal area of SE China, forming a large-scale volcanic–plutonic complex belt [10,11] (Figure 1). Numerous studies on the Cretaceous volcanic–plutonic complexes have been conducted, suggesting that the volcanic rocks and subvolcanic plutons within a volcano were co-magmatic [9,12,13]. However, detailed evaluation of the relationship between Jurassic volcanic and shallow plutonic rocks has received only minimal attention. Here, we present a case study on the Liaotianshan volcano, one of the earliest and best-preserved Late Mesozoic volcanos in SE China [14], to constrain the crystallization ages, the nature of the magma sources, and magmatic differentiation, which would assist in assessing the genetic relationship between the volcanic and plutonic rocks.

2. Geological Background

The South China Block is composed of two major Precambrian continental blocks: the Yangtze Block in the northwest and the Cathaysia Block in the southeast. These blocks were merged during the Neoproterozoic along the Jiangshan–Shaoxing fault [15] (Figure 1). In the eastern Cathaysia Block, Paleoproterozoic to Neoproterozoic granites and high-grade metamorphic rocks are extensively distributed, and they are intruded by Early Paleozoic and Late Mesozoic granites or overlain by Paleozoic sediments and Late Mesozoic silicic volcanic rocks [11,16]. The Late Mesozoic volcanic rocks mainly comprise rhyolitic pyroclastic rocks and rhyolite lavas. They outcrop in several large volcanic fields that contain calderas or volcano complexes closely associated with shallow intra-caldera plutons, covering a total outcrop area of about 90,000 km² [9,10,11,17] (Figure 1). Traditionally, these volcanic rocks were classified into lower and upper series, which were emplaced during ca. 160–120 Ma and 110–85 Ma, respectively [11].

The Liaotianshan volcano, located in southwestern Fujian Province, shows an irregular semi-elliptical shape with an outcrop area of about 23 km², unconformably overlying the lower Carboniferous and lower–middle Permian strata, as well as early Jurassic biotite syenogranite (Figure 2a). The center of the volcano is situated within the steep and elevated Liaotianshan–Nanshanding area. The complete volcanic eruption process can be broadly divided into two stages based on the lithology and lithofacies association. The products of the first eruption stage primarily consist of tuffaceous sandstones and mudstones associated with eruption–sedimentary facies, along with rhyolitic/dacitic crystal-lithic tuffs in air-fall facies; these are sporadically distributed around the periphery of the volcanic basin. The second eruption stage was marked by more intense volcanic activity that generated rhyodacites and rhyolitic crystal-lithic tuffs, breccia-bearing rhyolitic tuffs, and ignimbrites interbedded with tuffaceous sandstones and mudstones. The products of this second eruption stage constitute the main body of the volcano. The volcanic conduit is positioned at Liaotianshan and is filled with rhyolite porphyries exhibiting extrusion facies. Various lithologies and facies are arranged in semi-concentric patterns, forming an overall inward-dipping ring structure with dip angles ranging from 11° to 35° (Figure 2b).

3. Samples and Methods

The samples analyzed in this study were collected from the northeastern flank of the Liaotianshan volcano. The collection includes four rhyolitic crystal-lithic welded tuffs from the first eruption stage, sixteen volcanic rocks from the second eruption stage, and five rhyolite porphyries from the volcanic conduit (Figure 2; Table S1). The rhyolite porphyries are flesh-red in color and possess a porphyritic texture. Phenocrysts constitute 10%–15% of the rock, with grain sizes ranging from 0.5 to 4.5 mm. These phenocrysts are primarily composed of quartz, plagioclase, and alkali feldspar, while the matrix displays a microgranular texture consisting of fine quartz (Figure 3a,b). The second-stage breccia-bearing rhyolitic welded tuffs, situated near the conduit, are characterized by gray or purple hues and comprise fine lithic fragments alongside crystal fragments and other components. These rocks contain 3%–5% lithics, with sizes reaching up to centimeters. These breccias are irregularly shaped and randomly distributed, comprising materials such as rhyolite and tuff (Figure 3c,d). In contrast, the first-stage crystal-rich welded tuffs, located further away from the conduit, show little to no presence of lithics; they predominantly consist of crystal fragments including alkali feldspar, plagioclase, and quartz, typically not exceeding a content level of 20%. Notably, at the base of these first-stage-welded tuffs is an increased concentration of ductile pumice fragments (Figure 3e,f).

All analyses were carried out at the Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration, Guilin University of Technology, China.

Zircon cathodoluminescence (CL) images were obtained to characterize the internal structures. Zircon U–Pb dating was performed using an Agilent 7500 inductively coupled plasma–mass spectrometry (ICP–MS) instrument equipped with a GeoLas HD laser sampler. Zircon trace element analyses were performed concurrently with the U–Pb isotopic dating analyses. All analyses employed a beam diameter of 32 μm, a repetition rate of 6 Hz, and a laser energy of 10 J/cm². The specific instrumental settings and analytical procedures adhered to those outlined by [18]. The Lu–Hf isotopic composition measurements involved analysis within similar domains of the zircon crystal used for U–Pb dating as determined from CL images. In situ zircon Hf isotope analyses were conducted using a GeoLas HD laser ablation system in conjunction with a Neptune Plus multi-collector–ICP–MS instrument. The specific instrumental settings and analytical procedures are the same as [14].

Whole-rock major element analyses were conducted using X-ray fluorescence techniques. Loss-on-ignition (LOI) values for each sample were determined after heating to 1000 °C. Trace element analyses were performed with an Agilent 7900CX ICP–MS instrument following the acid dissolution of the samples. The analytical precision was better than ±5% for major elements and ranged from ±2% to ±5% for most trace elements.

4. Results

4.1. Zircon U–Pb Geochronology

The zircon crystals from different samples display consistent features, shown as euhedral to subhedral elongated prisms with lengths ranging from 100 to 200 μm. The CL images of zircons separated from the first- and second-stage volcanic rocks show well-developed oscillatory zoning, with certain zircons exhibiting narrow dark bands that indicate fluctuations in the zircon crystallization rate and magma replenishment processes [19] (Figure 4). In contrast, the CL images of zircon from extrusive rhyolite porphyry are significantly darker, with only a limited number of grains displaying faint oscillatory zoning. Additionally, some grains show internal structural damage associated with elevated concentrations of radioactive elements thorium and uranium within the zircon (Figure 4).

The results of LA–ICP–MS zircon U–Pb dating indicate that most analyses of the first- and second-stage volcanic rocks are concordant or nearly concordant (Figure 5; Table S2). One sample from the first-stage volcanic products yields a weighted mean ²⁰⁶Pb/²³⁸U age of 159.9 ± 0.9 Ma, while six samples from different layers of the second-stage volcanic successions yield similar ages ranging from 157.2 ± 1.0 to 158.8 ± 1.0 Ma. However, the dating results of the extrusive rhyolite porphyry yield few concordant ages (Figure 5a); even the limited number of concordant ages are significantly younger than previously reported values (153.2 ± 0.7 Ma [14]). Additionally, the first-stage volcanic rocks contain several inherited zircons from the Neoproterozoic (1046–945 Ma), Paleozoic (402 Ma), and Middle Triassic (246–243 Ma) periods, whereas the second-stage volcanic rocks include a limited number of captured zircons from the Paleozoic (420–369 Ma).

4.2. Zircon Trace Elements

The trace elements of zircons in the analyzed first- and second-stage volcanic rock samples largely overlap with each other (Figure 6; Table S3). These zircons exhibit significantly depleted concentrations of light rare earth elements (REEs) but enriched concentrations of heavy REEs, accompanied by pronounced negative Eu anomalies (Eu/Eu* ≤ 0.62, and 84% of them ≤0.40) and positive Ce anomalies (Ce/Ce* ≥ 1.48). With the exception of a few grains exhibiting slightly enriched La contents (> 1 ppm), potentially due to micro-inclusions and therefore excluded from subsequent statistical analysis [20], the remaining zircons display consistent Th (45.0–1901 ppm), U (76.5–2673 ppm), and Ti (1.32–11.3 ppm) concentrations, while demonstrating enriched Hf (7883–13740 ppm) and Y (424–3549 ppm) contents. The trace elements of zircons in the extrusive rhyolite porphyry, on the contrary, exhibit distinct features characterized by conspicuous enrichment of light REEs. Meanwhile, the concentrations of heavy REEs exhibit an order of magnitude higher than those observed in the zircons from both the first- and second-stage volcanic rocks.

4.3. Zircon Hf Isotopes

The zircon Lu–Hf isotopic composition of four samples from the second-stage volcanic rocks was analyzed, which exhibit a generally homogeneous distribution, displaying a symmetrical bell-shaped pattern. Apart from four analyses exhibiting ε_Hf(t) values ranging from –13.6 to –10.3, the remaining grains display ε_Hf(t) values within a narrow range of –9.8 to –5.4 (Figure 7; Table S4). Excluding the outliers, the weighted mean ε_Hf(t) values for the analyzed second-stage volcanic rock samples are –8.3 ± 0.4, –7.8 ± 0.4, –7.7 ± 0.4, and –8.2 ± 0.4, respectively. These values align with those of the first-stage volcanic rocks (–8.0 ± 0.3) but exhibit a lower magnitude compared to that of the extrusive rhyolite porphyry (–3.9 ± 0.4) [14]. Moreover, the zircon ε_Hf(t) values correspond to two-stage model ages (T_DM2) of 1.80–1.52 Ga for the majority and 2.04–1.83 Ga for the outliers.

4.4. Whole-Rock Major and Trace Elements

The first- and second-stage volcanic rocks exhibit similar major elemental compositions, with combined SiO₂ contents varying widely from 71.69 wt% to 82.42 wt% and total alkali (K₂O + Na₂O) contents of 2.99–9.46 wt%, thus classifying them as rhyolites (Figure 8; Table S5). Conversely, the extrusive rhyolite porphyries possess a narrow major elemental composition range, with SiO₂ contents spanning from 75.31wt% to 76.16 wt%. Furthermore, all the analyzed samples demonstrate enriched Al₂O₃ contents (>10.37%), while presenting depleted TiO₂ (<0.34 wt%), total Fe₂O₃ (Fe₂O₃^T, <2.30 wt%), MnO (<0.07 wt%), MgO (<0.44 wt%), and P₂O₅ (<0.04 wt%) contents. Overall, the above major-element oxides decrease linearly as SiO₂ increases (Figure 8).

The first- and second-stage volcanic rocks demonstrate similar trace elemental compositions as well, featuring pronounced depletion in high-field-strength elements (HFSEs), such as Nb, Ta, Ti, and P, and enrichment in large-ion lithophile elements (LILEs), such as Rb and K (Figure 9). The volcanic rocks also present fractionation characterized by enrichment in light REEs [(La/Yb)_N = 2.21–33.90], with moderately negative Eu anomalies (Eu/Eu* = 0.07–0.50) and slightly negative Ce anomalies (Ce/Ce* = 0.60–0.96 for most of the samples). In contrast, the extrusive rhyolite porphyries display distinct trace elemental characteristics, showing enrichment in heavy REEs [(La/Yb)_N = 0.14–0.61], with obvious negative Eu anomalies (Eu/Eu* = 0.01–0.03) and positive Ce anomalies (Ce/Ce* = 1.00–3.85).

5. Discussion

5.1. Affinities of the Zircons and Their Constraints on the Timescale of the Volcanic Activity

Mounting evidence suggests that many large intrusions do not stem from a single magma emplacement followed by crystallization; instead, they originate from the cumulative emplacement of pulsating magmas over timescales ranging from thousands to millions of years [25,26]. Likewise, large-volume volcanic eruptions can also endure for the same or even a longer time scale [9]. A potential ≤100 ky overestimation of zircon-based eruption ages compared to ⁴⁰Ar/³⁹Ar-based deposition ages for volcanic tephra has been proposed [27]. This pre-eruption time of zircon crystals is negligible in comparison with the analytical uncertainties. Therefore, the zircon U–Pb dating results are regarded as the eruption and deposition ages of the studied Liaotianshan volcanic successions.

As noted previously, zircons in the first- and second-stage volcanic rocks display comparable morphology, internal structure, and trace elemental compositions (Figure 6). Moreover, these zircons show a linearly decreasing trend in Ti contents and Th/U and Eu/Eu* values as Hf contents increase (Figure 10), demonstrating them to be magmatic zircons, which crystallized within a stable magmatic evolution system [28]. On the contrary, zircons in the extrusive rhyolite porphyry display entirely different internal structure and trace elemental characteristics (Figure 6). Additionally, they possess distinct Th and Ti contents, as well as (Sm/La)_N and Eu/Eu* values, which are different from those of the grains in the first- and second-stage volcanic rocks (Figure 10). Their low (Sm/La)_N and Ce anomalies further suggest a late-stage hydrothermal origin [28]. From this perspective, the latest-stage extrusive rhyolite porphyries in the Liaotianshan volcano crystallized in a fundamentally different magma from the previous volcanic rocks.

The differences in the zircon trace elemental patterns lead to another consequence that the dating analyses for the extrusive rhyolite porphyry fail to yield reasonable results (Figure 5). Fortunately, Liu et al. [14] presented a qualified age of 153.2 ± 0.7 Ma for the rhyolite porphyry, which is approximately 4 Ma younger than the adjoining second-stage volcanic rocks. The time hiatus is sufficiently long for significant changes to occur in the magmatic system. On the other hand, the first- and second-stage volcanic rocks have indistinguishable ages (159.9 ± 0.9 to 157.2 ± 1.0 Ma), hence suggesting that they were derived from a successive magmatic cycle. In conclusion, the large-volume eruption primarily took place between approximately 160 and 157 Ma in the study area, possibly occurring in several rounds due to interbedded sediments. Subsequently, the most recent batch of magma was extruded from the conduit around 153 Ma; however, its composition has undergone notable changes.

5.2. Evolution and Replenishment of the Magma

Apart from the indistinguishable eruption and deposition ages, the first- and second-stage volcanic rocks have highly similar zircon Lu–Hf isotopic compositions (Figure 7). Consequently, both stages of volcanic rocks could have originated from a common magma source. It is widely acknowledged that the continental crust constitutes a significant contributor to the voluminous silicic magmas [29]. The enriched zircon Lu–Hf isotopic compositions indicate that the Liaotianshan volcanic rocks originated predominantly from the partial melting of the Paleoproterozoic crustal basement. Moreover, the narrow range of zircon ε_Hf(t) values, in conjunction with low magma temperatures (<829 °C) determined using a Ti-in-zircon thermometer, implies the scarce involvement of juvenile materials. With that being said, crustal contamination might play a limited role in the volcanic activity, owing to the small number of inherited zircons present in the volcanic products.

As shown in Harker diagrams (Figure 8), the first- and second-stage volcanic rocks exhibit a linear tendency of selected major-element oxides that correlate well with SiO₂ contents. A favorable linear correlation among trace elements was also observed among these volcanic rocks (Figure 11). All these volcanic samples present negative Ba, Eu, Sr, and Ti anomalies, as well as distinct positive Rb anomalies, in their normalized trace element patterns (Figure 9), indicating the removal of alkali feldspar and plagioclase during magma differentiation [30].

It has been shown that, in some cases, the earliest volcanic products within a silica-rich volcano have the lowest SiO₂ content, demonstrating the least-evolved features [9]. The subsequent volcanic units are likely the result of crystal fractionation from a parental magma close to that of the earliest volcanic products, such as in the Cretaceous Yandangshan caldera, SE China [9]. However, the first-stage volcanic rocks in the Liaotianshan volcano yield similar ranges of content for SiO₂ and other major-element oxides (Figure 8), as well as (La/Yb)_N and Eu/Eu* values (Figure 9), when compared to the second-stage volcanic rocks. In addition, the SiO₂ contents of the volcanic rocks from the early to late parts of the volcano fluctuate on multiple occasions (Figure 12). This may indicate that corresponding magma recharge events took place during the prolonged volcanic activity [31]. The injection of parental magma led to the magma reservoir being chemically less evolved. The recharge of the magma chamber by the arrival of a new magma batch is almost ubiquitous at an active volcano [32,33].

5.3. Petrogenesis of the Extrusive Rhyolite Porphyries

The latest product within a volcano is typically composed of shallow plutons or subvolcanic porphyries associated with the previously formed volcanic rocks [4,7,9,17]. In this case, the latest volcanic product likely formed by crystal accumulation in the magma chamber, resulting from the extraction and eruption of rhyolitic melts [30,34,35]. Thus, these rocks usually display geochemical characteristics that plot in a direction opposite to the fractional crystallization vectors defined by the previously formed volcanic rocks [5,7,9]. However, extrusive rhyolite porphyries in the Liaotianshan volcano do not exhibit the least-evolved geochemical characteristics. Their contents of SiO₂ and other major-element oxides are positioned in the middle of the ranges observed for first- and second-stage volcanic rocks (Figure 8). Moreover, the modeling of trace elements reveals a well-evolved trend that aligns with the associated volcanic rocks, which is inconsistent with the melt extraction and residual cumulates model (Figure 11). Therefore, the extrusive rhyolite porphyries within the Liaotianshan volcano are inferred to have originated from a magma distinct from that which produced the first- and second-stage volcanic rocks, as also indicated by the zircon trace element patterns.

The extrusive rhyolite porphyries display significantly depleted zircon Hf isotopic compositions in comparison to the first- and second-stage volcanic rocks (Figure 7), implying an alternative petrogenesis involving partial melting of different resources or mixing between ancient- and mantle-derived magmas. In fact, Late Mesozoic volcanic–plutonic rocks in SE China exhibit a universal trend of increasing radiogenetic zircon Hf isotopic compositions over time [13,14,36,37]. This phenomenon has been extensively discussed and widely accepted as resulting from an increasing contribution of mantle-derived materials, plausibly driven by enhanced lithospheric extension associated with paleo-Pacific subduction [13]. We advocate for the magma mixing model to explain the generation of the extrusive rhyolite porphyries, not only because it accounts for their less-evolved characteristics, such as depletion in incompatible elements, but also due to its alignment with the storage configuration of the magma reservoir within the crust (see below).

Recent models regarding the formation, storage, and chemical differentiation of magma within the Earth’s crust have been emphasized to encompass the entire crust, i.e., the trans-crustal magmatic systems [3,38]. A deep crustal hot zone could be generated by mantle-derived basaltic magma injecting into the lower crust, which generates and promotes the partial melting of pre-existing crustal rocks [39]. The crust-derived melts ascend from the hot zone to form a deep acidic magma reservoir in the mid- to upper crust, feeding and recharging a shallow upper crustal magma chamber, which is necessary for the generation of high-silica rhyolitic rocks [40]. The prolonged volcanic activity of the Liaotianshan volcano, along with occasional injections of parental magma, further corroborates the existence of both a shallow magma chamber and a deep magma reservoir. Developed in contradiction to the melt extraction and crystal accumulation model, the volcanic and extrusive rocks within the Liaotianshan volcano exemplify a scenario that reflects the evolutionary history and replenishment dynamics of the crustal magma system.

6. Conclusions

The first- and second-stage volcanic rocks within the Liaotianshan volcano were formed continuously during 160–157 Ma, and the latest rhyolite porphyries extruded around 153 Ma. These volcanic rocks originated from partial melting of Paleoproterozoic crustal basement, with occasional injections of parental magma resulting in a less chemically evolved magma reservoir during its evolution. In contrast, the geochemical differences observed in the extrusive rhyolite porphyries are primarily attributed to contributions of mantle-derived materials into the ancient crust-derived magmas. The Liaotianshan volcano developed in contradiction to the model of melt extraction and crystal accumulation within a magma chamber, instead reflecting the evolutionary history and replenishment dynamics of the crustal magma system.