Zircon Record of Supercontinent Amalgamation in Back-arc Volcanic Rocks

Episodic supercontinental amalgamation has profoundly inuenced the evolution of the geosphere, hydrosphere, atmosphere and biosphere. However, the timing of supercontinent formation has mainly been constrained by the global age spectra of detrital zircon. Here, we show that the zircons in back-arc volcanic rocks not only reect the evolution of local magmatism but also contain a record of global continental amalgamation events. We found that the young (<100 ka) zircons in volcanic rocks from the Okinawa Trough have old (108 Ma to 2.7 Ga) inherited zircon, which were captured as the magma ascended through the rifting continental crust. Moreover, the ages of the inherited zircons correspond to ve supercontinent amalgamation events. Specically, the Archaean inherited zircons, which have positive (cid:0) Hf (t) and low δ 18 O values, correspond to the formation of juvenile global continental crust. In contrast, the negative (cid:0) Hf (t) and high δ 18 O values of post-Archaean inherited zircons indicate that their parental magma contained recycled, old crust due to the enhanced crustal thickening and crust-mantle interactions during supercontinent assembly. Therefore, inherited zircons in back-arc volcanic rocks not only reect the evolution of local magmatism but also contain a record of global supercontinental amalgamation events.


Introduction
The history of episodic supercontinent amalgamation and dispersal has profoundly in uenced Earth processes, surface environments and biogeochemical cycles [1][2][3] . Since zircons are physicochemically robust, accumulate as common accessory phases in sedimentary detritus 4 , and can be isotopically dated through the U and Th decay systems, thereby resolving timescales from gigayears (Ga) to kiloyears (ka) [5][6] , the timing of supercontinent formation was mainly constrained by the global age spectra of detrital zircon from the world's rivers 1,2 . Based on the distinct peaks of global U-Th-Pb ages in detrital zircon, the timing of assembly of the Kenorland, Nuna, Rodinia, Gondwana and Pangaea supercontinents has been revealed 1,2,7 . At present, however, no studies have reported records of these ve global supercontinent amalgamation events in zircons from local back-arc volcanic eruptions. In this study, we used magmatic zircons, captured from the deep crust during mantle-derived magma ascent through the rifting upper crust in the Okinawa Trough (OT) to analyse the era of magmatic activity in this region and provide information about global supercontinental amalgamation events.
The OT, which is located on the western Paci c active continental margin (Fig. S1), is a back-arc basin formed by the northwestward subduction of the Philippine Sea Plate beneath the Eurasian Plate initiating during the middle to late Miocene (i.e., from >15 Ma to ~6 Ma) 8 . The crustal thickness decreases from >25 km in the northern OT to ~10 km on the axis of the SOT graben 9 . The southernmost part of the OT (SPOT), which is considered an embryonic crustal rifting zone where the crust (25-30 km) has not experienced signi cant thinning 10 , is characterized by a cluster of active volcanoes dominated by dacites and rhyolites 10,11 . These <0.2 Ma silicic magmas evolved via mixing of a mantle-derived basaltic magma and a crustal felsic magma, followed by extensive fractional crystallization 10,11 .
Zircons were separated from the three submarine volcanic rock samples at the three stations (C1, T9 and R10-H2; Figs. S1 and S2), which are calc-alkalic rhyolites and dacite (Fig. S3) and have the most isotopically enriched compositions of all the volcanic rocks in the OT (Fig. S4). We used secondary ionization mass spectrometry (SIMS), laser ablation multicollector inductively coupled mass spectrometry (LA-MC-ICP-MS), and cathodoluminescence (CL) imaging to investigate the U-Th-Pb-O-Hf isotope and trace element compositions of Quaternary zircons and old zircon cores captured from the deep crust during mantle-derived magma ascent through the rifting continental crust in the southern Okinawa Trough (SOT). We used these data to investigate the era of magmatic activity in this region and to provide information on global supercontinent amalgamation events.

U-Th-Pb Ages
We used CL imaging to identify light-CL zircons (LCLZs) with dark-CL zircon cores (DCLZs) in the samples (Fig. S5). The LCLZs are in U-Th radioactive disequilibrium (Fig. S6), and the U-Th ages (<100 ka) of the LCLZs from the rhyolites and the dacite are similar and coeval (Fig. 1A). The cumulative probability density function (PDF) curve peaks at 30 ka for the LCLZs in the rhyolites, whereas the PDF curve peaks at 19 and 45 ka for the LCLZs in the dacite (Fig. 1B). In contrast to the LCLZs, the DCLZs in both the dacite and the rhyolites are older (U-Pb ages of 108 Ma to 2.7 Ga) (Figs. 2 and S5; Table S2), and the age distribution of the DCLZs (Table S2) is consistent with the ages of the ve supercontinent amalgamation events (Fig. 2).

Discussion
Long-lived silicic magma reservoirs beneath the OT The LCLZs are euhedral with oscillatory zoning (Fig. 1A) and have high Th/U ratios (>0.4; Fig. S8), exhibiting steep chondrite-normalized REE patterns with variable HREE enrichments, prominent positive Ce anomalies, and strong negative Eu anomalies (Fig. S9), which are characteristic of magmatic zircons 12 , indicating a magmatic origin rather than a detrital or metamorphic origin [12][13] . Moreover, the LCLZs exhibit homogeneous δ 18 O and εHf (t) values (Figs. 2 and S7), suggesting that all the LCLZs crystallized from their parent magma 14 . The crystallization temperatures decrease systematically with increasing Hf concentration (Fig. S10), which is a fractionation proxy 15 , suggesting that the decrease in temperature was accompanied by protracted zircon crystallization. Multiple age spots on individual LCLZs with continuous and uninterrupted oscillatory zoning (Fig. 1A) revealed age differences of up to 85 ka between the core and rim domains, providing further evidence of protracted zircon crystallization. Overall, the LCLZs crystallized in a long-lived upper crustal magma reservoir for ~100 ka, which is consistent with the results of other studies conducted on the longevity of silicic magma systems in continental arc settings 15 . The outermost crystal rims represent the youngest phase of zircon crystallization (1.8 +3.6/-3.5 ka) ( Fig. 1; Table S1). We interpret this to be the approximate eruption age.
Chen et al. 16 and Huang et al. 17 obtained U-series ages of 88.7-12.7 ka for the silicic rocks from the middle and northern OT. These ages suggest that volcanic activity has occurred throughout the OT since the Late Pleistocene 16,17 . This volcanic activity was likely sustained by long-lived silicic magma reservoirs beneath the submarine volcanoes 15 .
Provenance of the zircon xenocryst cores in the silicic magmas A prominent feature observed in this study is that some of the LCLZs in the volcanic rocks had DCLZs with U-Pb ages of ~108 Ma to 2.7 Ga ( Fig. S5; Table S2). Based on the Hf-O isotope compositions of the LCLZs, the Quaternary volcanic rocks were likely produced by the mixing of a mantle-derived ma c magma with ~10-20% crust-derived silicic magma (Figs. S7C -7D), which is supported by the enriched radiogenic Sr-Nd isotope compositions of the whole-rock samples 11 (Fig. S4). The involvement of a high percentage of a crustal component is consistent with the presence of inherited ancient zircons (the DCLZs) with rim overgrowths (the LCLZs) 18 (Fig. S5). The presence of the DCLZs in the subductionrelated volcanic rocks is either due to the direct incorporation of the DCLZs into the mantle source via subducted sediment 19,20 or the capture of fragments of ancient continental crust (containing the DCLZs) during magma ascent [21][22][23] . Experimental results have demonstrated that the retention times of O isotopes in 20-120 μm zircons at a temperature of 900°C are between 160 and 5700 years 24 . For hotter mantle magmas, the time required for the δ 18 O to reach diffusive equilibrium with the homogeneous mantle value is much shorter than 5.7 ka 25 . The high saturation level of ma c melts causes the fast dissolution of pre-existing zircons under normal conditions 26 . Thus, it would not be possible for continent-derived zircon crystals that have been recycled back into the mantle by subduction to preserve highly variable O isotope signatures at mantle temperatures 25 . Therefore, the DCLZs with highly variable δ 18 O values (Fig.  2) must be remnants of a deeply buried basement that underwent extensive partial melting by and mixing with the ascending magma. In other words, these DCLZs are unmelted remnants of the basement.

Remnants of the Cathaysian Block underlying the embryonic crustal rifting zone
During this study, Archean zircon cores were found in both the dacite and the rhyolites. The Archaean DCLZs have the following characteristics. (1) They exhibit clear, bright, and broad oscillatory zoning in the CL images and euhedral to subhedral rather than oval shapes (Fig. 3A), indicating that these zircons are magmatic rather than detrital. (2) They have high Th/U ratios of 0.59 (Table S3), suggesting a magmatic rather than a metamorphic origin (Th/U<0.1) 13 . (3) They lack signi cant Eu anomalies, exhibit relatively at HREE patterns (Fig. 3B), and have lower contents of incompatible elements, such as Y, Hf, and U, than the younger zircon rims (Table S3), suggesting that they crystallized from a less evolved magma 12 . (4) They have oxygen isotope compositions (5.74-5.85‰; Fig. 2B; Table S2) similar to those of mantle zircons (5.3 ± 0.3‰) 27 and positive εHf (t) values (4.66 to 4.88; Table S2) that plot between the evolutionary trends of the depleted mantle 28 and chondrites 29 (Fig. 3C), suggesting that the parent rocks of the DCLZs were mainly derived from the depleted mantle. (5) They have εHf (t) values within the lower crustal range 30 (Fig. 3C). Collectively, these lines of evidence suggest that the Archaean DCLZs were derived from a ma c source that was extracted from the basaltic lower crust of the SPOT, which formed through melting of the depleted mantle ~2.9 Ga (Fig. 3C).
In addition, drilling results show that the 174 Ma granitoids in the adjacent East China Sea Basin (Fig. 4A) have crustal Hf model ages of 2.9-2.5 Ga, implying that their parent magmas were derived from reworking of the Archaean lower crust 31 . Similarly, the Archaean zircons (2.7−2.5 Ga) in the studied volcanic rocks suggest the presence of unexposed Archaean lower crust beneath the SPOT (Fig. 4B). The ages and Hf isotope compositions of these zircons are also similar to the zircons found in the lower crustal xenoliths in the western Cathaysia Block, southern China (Fig. 4A) 32 . Moreover, one DCLZ with a Neoproterozoic age (741.7 Ma) had a very low εHf (t) value (−15.7) and an Hf model age (T DM2 ) of 2.6 Ga (Table S2) Table S2), suggesting that their parent magmas were derived from the reworking of Proterozoic crustal components or from mixing of crust-derived magmas and juvenile material 28 . Thus, the older basement has experienced reworking and complex modi cation related to the addition of juvenile material since the Neoarchean. In addition, their highly variable δ 18 O values suggest that they may have originated from the metasedimentary basement in the upper and middle crust overlying the Archean crystalline basement of the lower crust (Fig. 4B).
Implications for the timing of continent amalgamation events Notably, the age distribution of the DCLZs coincides with ve supercontinent amalgamation events (Fig.  2). We propose that the Archaean DCLZs (2.5−2.7 Ga), with positive εHf(t) values and low oxygen isotope values (5.74−5.85‰) ( Fig. 2; Table S2), are unique and related to the formation of juvenile continental crust, i.e., crust that segregated rapidly from the mantle without signi cant involvement of older crustal materials 33 . This event occurred during the amalgamation of Kenorland and is the largest global event that affected volcanism on all continents 34 (Fig. 2) 1,2,35 . The negative εHf (t) values of these DCLZs indicate that their parent magmas contained recycled older crust 28 . This interpretation may be consistent with the fact that the crustal thickening caused by supercontinent assembly results in more crust-mantle interaction 36 . As such, the DCLZs from the OT act as a record of global continental amalgamation events. DCLZs have also been found in other subduction-related volcanic rocks [21][22][23] , making these records an essential tool for improving our understanding of the supercontinent cycle and the amalgamation process.

Conclusions
In summary, zircon U-Th-Pb ages and trace elements as well as Hf-O isotopes results support the following model for the two silicic magma eruptions from the southwest OT: (1) long-lived magma reservoirs beneath the submarine volcanos with a protracted time scale at least 100 ky; (2) Zircon xenocrysts acquired during the magma ascent through the upper crust rather than recycled from the subducted sediments; (3) The occurrence of these Archean zircon suggests the presence of unexposed Archean materials in the southern OT crust, which experienced complex modi cation related to the addition of juvenile material and reworking from Neoarchean time onward; (4) The Archean zircon xenocryst cores correspond to the global formation event of "juvenile" continental crust. Post-Archean zircon xenocryst cores indicate their parent magma contains recycled older crustal material. This may consistent with the fact that crustal thickening result in crust-mantle interaction during supercontinent assembly. Therefore, zircons in back-arc volcanic rocks not only re ect the local evolution of continents, but also record the global continental amalgamation. In the future, the study of zircon from other backarc volcanic rocks in the western Paci c is expected to nd zircons that record the supercontinent cycle and amalgamation.

Methods
Zircons were separated from the three volcanic rock samples (Fig. S2). Representative zircon grains were mounted in epoxy resin and polished to expose grain centers. For all epoxy mounts of zircon grains, cathodoluminescence (CL) images were obtained prior to analysis, and used to guide the analysis locations. First, secondary ion mass spectrometry (SIMS) analyses were performed for zircon U-Th-Pb-O isotopes and trace elements. The same grains of zircon were then analyzed by LA-MC-ICPMS for a determination of Lu-Hf isotopes.
Zircon 238 U-230 Th disequilibrium ages and trace element compositions were obtained by SIMS using a CAMECA IMS 1270 at the University of California Los Angeles (UCLA) following the analytical protocols described by Schmitt et al. 37 and Bell and Harrison 38 . Zircon U-Pb and O analyses were performed using a CAMECA IMS 1280 at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), following the analytical procedure described by Li et al. 39 and Tang et al. 40     Plots of (a) εHf(t) and (b) δ18O vs. time for the LCLZs and DCLZs. The light grey dots represent global zircon εHf(t) (Roberts and Spencer,41) and δ18O data (Spencer et al.7). The vertical bars represent supercontinent amalgamation events1,2,7.