Introduction

Magmatic arcs witness the interplay between endogenous and exogenous processes, including magmatism, crustal thickening, uplift, erosion, sedimentation and burial of detritus1,2,3,4. Magmatism produces new crust, which later interacts with the hydrosphere and atmosphere through erosion and weathering. On the other hand, crustal materials from the surface are recycled to Earth’s interior. This chain of processes in magmatic arcs play important roles in driving much of the mass exchange between Earth’s interior and surface. The inward transport of surface materials, including volatiles, has profound influence on the cycling of carbon, oxygen, sulfur, etc. on Earth’s surface and may alter the chemical and physical properties of the deep crust and even mantle.

Nearly every Phanerozoic arc in the world exhibit crustal signatures in geochemistry, suggesting pervasive crustal recycling in the formation of arc crust. Conventional views link crustal recycling processes to slab subduction, including sediment subduction and subduction erosion (± relamination) have been widely invoked to explain the crustal signatures seen in most arc magmas5,6. Yet the recent work on continental arcs hints at thrust faults as potential recycling channels7,8,9,10.

Here, we examined a suite of migmatites from a Neoproterozoic magmatic arc in western China. We used combined petrologic, geochronologic and geochemical studies of these samples to understand the nature of the recycled materials and evaluate how thrust faults may contribute to rock recycling in compressional arc settings.

Results

Geological setting and samples

The Yangtze Block in Eastern Asia consists of Archean–Paleoproterozoic crystalline basement surrounded by Neoproterozoic fold belts. It is bounded by the Tibetan Plateau to the west, the North China Block to the north and the Cathaysia Block to the southeast. It was placed in a marginal position in Rodinia supercontinent and has underwent a long-term evolution and complex tectonic-magmatic processes in a continental margin setting during Neoproterozoic11,12,13. The western margin of the Yangtze Block became tectonically active since the early Neoproterozoic; it started with intra-oceanic arc magmatism before 971 ± 16 Ma (ref. 14) and then transitioned to Andean-type magmatism at ca. 870 Ma (ref. 15). This ancient subduction relic was lately imaged by deep seismic reflection profile16. The prolonged magmatic history gave rise to linearly distributed Neoproterozoic arc magmatic rocks spanning over 800 km (Supplementary Fig. 1a). The Longmenshan Thrust Belt to the northwestern margin of the Yangtze Block exposed abundant Neoproterozoic plutonic complexes due to the major Miocene extrusion and thrust process17, of which the largest one is known as the Pengguan Complex, comprising voluminous 860–750 Ma plutonic rocks (Supplementary Fig. 1b, c). The Huangshuihe Group in the core region of the Pengguan Complex exists as a huge roof pendant of the plutonic rocks and consists of metamorphic rocks of schist, metapelite, quartzite and meta-pyroclastic rock. Ductile deformation, faults, mylonite with S-C fabric, and migmatitic lineation are extensive in the sequences.

Two main types of migmatites were identified in the field (Supplementary Fig. 2): the inhomogeneous migmatites (or diatexite) contain abundant blocks of melanosome and associated aplite vein; the stromatic migmatites preserve a regular layered structure and are characterized by centimeter-thick, foliation-parallel leucosome, melanosome and mesosome. Patch-shaped neosomes are abundant in stromatic migmatites and formed during incipient partial melting. Large leucosomes (~50 cm in width) occur occasionally and are usually fed by a few small leucosome veins. The stromatic migmatites have a NNW-dipping foliation (S1) (~355°/48°) defined by oriented biotite or feldspar augen. The S1 foliation is parallel to bedding planes defined by the metapelite (Supplementary Fig. 2c) and is folded locally by syn-anatectic deformation on varying scale (Supplementary Fig. 2e, f). The fold axial planes (S2) generally display E–W striking, S-dipping orientation. Besides, the study area was superimposed by massive high-angle, S-verging thrust faults (Supplementary Fig. 2g), which should be linked with post-Mesozoic structural tectonics17.

Six stromatic migmatite and one leucosome samples in the Huangshuihe Group were collected in this study (Supplementary Fig. 1d). The main minerals in migmatite are plagioclase, biotite, K-feldspar, quartz and muscovite (Supplementary Fig. 3). Anatexis of primary mineral assemblage led to prevalent zircon overgrowth and muscovite-rimmed biotite in the migmatite (Supplementary Figs. 3 and 5–7). Entrainment of peritectic phase, which consists of small spessartine-rich garnet grains, biotite, muscovite, quartz, plagioclase, K-feldspar and Fe-oxides, was found in 16YX-1-1 (Supplementary Fig. 3 and Supplementary Data 4). The reaction of “biotite + MnO, Al2O3, SiO2 (from melt) = garnet + muscovite”18 may control garnet paragenesis. These observations are indicative of near-solidus partial melting with local melt segregation.

Zircon U–Pb–Hf–O isotopes

Most zircon grains in the Pengguan migmatites have core-rim structures. The zircon core domains, presumably derived from arc magmatic detritus, show limited variation in their ages, concentrating at ~830–870 Ma, with few at ~930 Ma (Fig. 1a), and have mantle-like to slightly elevated δ18O values (5.3 to 7.4‰) (Fig. 1b). Their εHf(t) values vary from –3 to +13, with most being positive, indicative of heterogeneous but generally juvenile sources. Zircon overgrowth rims are slightly younger than the maximum depositional age for each sample, with U − Pb dates generally ranging from ~815 Ma to ~860 Ma (Supplementary Data 3). The overgrowth rims have significantly higher δ18O values (9.3 to 13.3‰) compared with those of core domains, despite their similar εHf(t) range (–3 to +8 except one analysis of –9) as core domains (Supplementary Data 2). Zircon grains from the leucosome sample show homogeneous δ18O values (11.1 to 13.4‰) with a large range of εHf(t) values (–6.9 to +8.4) (Supplementary Data 2). All εHf(t) values were calculated to t = 850 Ma in order to facilitate comparison.

Fig. 1: Zircon U−Pb age distributions and zircon δ18O-εHf(t) values of the Pengguan migmatites.
figure 1

a Core and rim age distribution of zircon grains extracted from Pengguan migmatites. Unimodal core and rim zircon U-Pb dates distribution is shown in the diagram. Anatexis and maximum depositional ages (MD Age, see discussion for the calculation) of each sample are marked with light red and blue, respectively. Zircon U−Pb dates with concordant U −Th−Pb isotopes were used here. Error bars are shown at one standard error. The concordant criteria were adopted from Spencer et al.40. b δ18O-εHf(t = 850 Ma) diagram for zircon core and rim domains of three migmatite samples and zircon grains from leucosome. Error bars are shown at one standard error for δ18O value and two standard errors for εHf(t) value. C = core domain, R = rim domain. Mantle zircon δ18O = 5.3 ± 0.6‰ (ref. 41).

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Anatexis P-T conditions

We reconstructed the metamorphic P-T conditions for the Pengguan migmatites using Perple_X 6.9.0 (http://www.perplex.ethz.ch). The bulk rock composition of sample 16YX-1-1 was chosen for calculation because this sample clearly documents: (1) mineral-melt interaction; (2) coexistence of minerals (Mn-rich garnet + biotite + muscovite + quartz + plagioclase + K-feldspar + Fe-oxides) and; (3) minor partial melting with no evident melt migration. In the calculated P-T pseudosection, the mineral assemblage of the Pengguan migmatite falls in a narrow domain (domain 1 in Fig. 2a) near the solidus. Using Si pfu in muscovite from 16YX-1-1 (3.08 to 3.14, in moles per formula unit; Supplementary Data 4), which is sensitive to pressure in the K-feldspar + phlogopite + quartz system19, we further constrained the anatexis P–T conditions to ~670 °C and 5.9−8.1 kbar (Fig. 2). The low anatexis temperature is also consistent with the extremely low Th/U ratios of the zircon overgrowth rims (Fig. 1a). At near-solidus temperatures, Th concentration in the melt is largely buffered by Th-rich accessory minerals (such as monazite and allanite)20.

Fig. 2: Reconstructed anatexis P-T conditions for the Pengguan migmatites.
figure 2

a P–T pseudosection calculated for the Pengguan migmatite (16YX-1-1) in the MnNKCFMASH system (Quartz and H2O in excess). Internally consistent thermodynamic dataset of Holland and Powell42 was used. Mineral assemblage in the thin green belt along the solidus is consistent with the mineral composition and field observations of sample 16YX-1-1. The predicted Si isopleths content in muscovite (3.06–3.18, in molar per formula unit) are shown by the white dotted lines. Gt–garnet, Bi–biotite, Mus–muscovite, Pl–plagioclase, Fsp–K-feldspar, Ilm–ilmenite, sil–sillimanite, ky–kyanite, Sp–spinel, Crd–cordierite. b P-T conditions of the Pengguan migmatite formation (green line) projected on geothermal gradients in various geologic settings (modified after Rothstein and Manning25, Hopkins et al.43 and Peacock23).

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Discussion

The Pengguan migmatites are peraluminous with aluminum saturation indices (ASI) of 1.10–1.44 (Supplementary Data 1). Muscovite and peritectic garnets are observed in all samples, indicating peraluminous composition of the protoliths. These migmatites also show geochemical signatures similar to those arc magmatic rocks and forearc sediments from Peninsular Ranges batholith, but distinct from those of MORB and intraplate volcanics (Fig. 3), suggesting that the protoliths of these migmatites are dominated by arc-related magmatic detritus. This view is also consistent with the observation that the magmatic cores of zircon in the migmatites have very similar age distributions to that of the arc-related magmatic rocks in the study area (Fig. 1a). The absence of pre-Neoproterozoic zircon xenocrysts hints that forearc magmatic detritus may have served as the protoliths of the Pengguan migmatite. The consistent and juvenile Hf isotopes of the zircon cores and overgrowth rims lends further support for the arc origin of the migmatite precursor materials. We thus suggest the Pengguan migmatites documented a process that recycled the arc crust itself, and we refer to this process as endogenic recycling to distinguish it from recycling of oceanic sediments or oceanic crust into subduction zones.

Fig. 3: Geochemical affinity of the Pengguan migmatites compared with those of 860–820 Ma plutonic rocks nearby, the plutonic rocks from Peninsular Ranges batholith (PRB)44 and forearc sediments derived from PRB45.
figure 3

a The Nb/Yb-Th/Yb discrimination diagram is after Pearce46. b The A–CN–K–CIA (Chemical index of alteration) diagram is after Jiang and Lee45. OIB – Ocean-island basalt; E- and N-MORB – enriched and normal mid-ocean-ridge basalt. Data sources are provided in Supplementary Data 1.

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We note that the zircon overgrowth rims have systematically higher δ18O values than the magmatic cores (Fig. 1b), which is indicative of equilibrium with high-δ18O anatectic melts during crystallization. High δ18O is a diagnostic signature of low-temperature water-rock interaction at Earth’s surface. Thereby the protoliths of the Pengguan migmatites must have undergone some extent of chemical weathering and O isotope exchange at low temperatures before being buried and remelted. Downward infiltration of meteoric water may be another important mechanism to introduce oxygen isotopic heterogeneity to the deep crust. But this mechanism would likely cause water-rock interaction at high temperatures and impart low δ18O signature to the rocks, as has been clearly seen in the lower oceanic crust21. In addition, the maximum penetration depth of meteoric water ranges from 5 to 18 km (ref. 22), which is less than the depth of anatexis (~18 to 24 km) calculated for our migmatites. We thus exclude interaction with downward infiltrated meteoric water as a likely mechanism to explain the high δ18O recorded by the zircon rims of this study.

An important question pertains to how the magmatic detritus that had been initially deposited at the surface was transported to the hot deep crust. In magmatic arc settings, recycling of surface rocks has generally been associated with slab subduction. Subducting slabs can directly bring trench sediments to the deep crust or even mantle5,6. Subduction erosion has also been recognized as an important mechanism for downward transport of shallow crustal materials5. Slab tops are cold (dT/dP = <34 °C/kbar) (Fig. 2b; estimated from Peacock23) and melting of the sediments deposited at the slab surface is generally considered difficult at crustal depths24. Phase equilibrium modeling shows that the Pengguan migmatites formed at ~670 °C and 5.9−8.1 kbar. These P–T conditions translate into a hot geothermal gradient of 83−114 °C /kbar or 25 to 34 °C /km, considerably hotter than slab top geothermal gradients but consistent with those seen in arc crust with continuous magmatic inflation25 (Fig. 2b). This would imply that the Pengguan forearc detrital sediments, shortly after their deposition, were rapidly transported to the deep crust beneath the active arc volcanic front. We suggest the most likely recycling mechanism is via deep thrust faults in the upper continental plate rather than by slab subduction (Fig. 4). Downward flow of wall rocks during magma ascent26 could be another mechanism in transporting surface materials to the deep crust, but we think it less likely occurred because: (1) wall rock xenoliths were not seen in the plutonic rocks, and (2) vertical flow foliation or lineation is absent in the wall rocks. In compressional magmatic arcs, including mature island arcs and continental arcs, fold and thrust belts may extensively develop in the forearc and inboard side of the arc and serve as important crustal recycling channels. Typical examples include the thrust fault systems in Lachlan orogen27, Japan arc28 and the Cordilleran continental arc system7.

Fig. 4: Cartoon showing rapid endogenic recycling of arc magmatic rocks through thrust channels in continental arcs (not to scale).
figure 4

a Thrust-driven rock recycling in this study: from arc magmatism, to erosion and weathering at the surface, to forearc sedimentation, then to burial and remelting. b Sketch diagram illustrating the self-recycling process in arc system.

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The nearly identical age distributions of the zircon cores and overgrowth rims (Fig. 1a) hint at a fast rock cycle, from arc magmatism to water-rock interaction, then to burial and remelting. To estimate the timescale and rate of burial for the Pengguan forearc magmatic detritus, we took the weighted average value of the 50% of youngest U − Pb dates with concordant U − Th−Pb isotopes from zircon core domains as the maximum depositional age, and the weighted average age of zircon rims as the remelting (anatexis) age (Supplementary Data 2 and Fig. 1a). In doing so, we see that the maximum depositional ages are less than 1 to 14 Myr older than the remelting ages for each sample. With the errors of zircon dating taken into account, the magmatic protoliths of the Pengguan forearc sediments must have been exhumed, deposited in a sedimentary basin, and then buried to the depth of crustal anatexis on a ~10 Myr timescale. This would imply an efficient burial process with minimum burial rate of 2–3 mm/y.

Our findings point to endogenic recycling as an important mechanism driving internal redistribution of arc crustal materials. The role of such endogenic recycling in the formation of arc crust has been largely overlooked in the past. Radiogenic isotopes are widely employed to constrain crustal recycling processes, but given the short residence time (e.g., 10 Myr), radiogenic isotopes can be completely blind to endogenic recycling. We speculate that extensive endogenic recycling may also generate significant decoupling between radiogenic and stable isotope compositions in recycled materials and their derivative melts, which may further complicate the use of isotope-based proxies in tracing crustal recycling in arc settings.

Rapid endogenic recycling may be facilitated by thrust fault networks. Deep thrust faults may serve as critical transport channels connecting the surface and deep arc crust with ongoing magmatism. And by transporting hydrated surface crustal materials to the deep crust, endogenic recycling enhances the overall differentiation of arc crust. Because large-scale thrust faults necessarily form in compressional settings, efficient endogenic recycling may partly explain why thick arc crust formed in compressional settings (e.g., continental arcs) tend to be more differentiated than thin arc crust formed in extensional settings (e.g., immature island arcs)29.

Methods

In situ Zircon isotopes

Zircon grains were separated using conventional density and magnetic techniques, mounted in epoxy resin disk, and polished to expose their internal texture. In situ U–Th–Pb–Hf–O isotope analyses were carried out guided by cathodoluminescence (CL) images and transmitted and reflected photographs. The CL images were taken with a Carl Zeiss Supra 55 field-emission scanning electron microscope (FE-SEM) coupled to a GATAN MonoCL4 detector at the State Key Laboratory for Mineral Deposits Research in Nanjing University (MiDeR-NJU) under following conditions: an accelerating voltage of 3 kV, working distance of 11.5 mm, and aperture size of 30 μm.

Zircon U–Pb isotopes were analyzed using the Cameca IMS-1280HR second ion mass spectrometry (SIMS) at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (SKLabBIG GIG CAS) under the following operating conditions: 7 scan cycle, ~8 nA primary O2– beam, 20 × 30 µm spot size, and ~5400 mass resolving power. Calibration of Pb/U ratios is relative to the primary standard zircon Plešovice30 and is based on an observed linear relationship between ln (206Pb/238U) and ln (238U16O2/238U)31. A long-term uncertainty of 1.5% (1 RSD) for 206Pb/238U measurements of the standard zircon was propagated to the unknowns, despite that the measured 206Pb/238U error in a specific session is generally around 1% (1 RSD) or less. U and Th concentrations of unknowns were also calibrated relative to the standard zircon Plešovice, with Th and U concentrations of 78 and 755 ppm, respectively30. Measured compositions were corrected for common Pb using non-radiogenic 204Pb. A secondary standard zircon Qinghu32 was analyzed as unknown to monitor the reliability of the whole procedure. Data reduction was carried out using the Isoplot/Ex 3 software33. Eleven measurements of the Qinghu zircon standard during the course of the study yielded a weighted mean 238U/206Pb age of 159 ± 2 Ma (MSWD = 0.72), consistent with its recommended value of 159 ± 0.2 Ma (ref. 32).

After U–Pb dating, the sample mount was re-ground for ~5 µm to ensure any oxygen implanted in zircon surface from the O2– beam used for U–Pb analysis is completely removed. Zircon oxygen isotope analyses were also conducted using SIMS at SKLabBIG GIG CAS. The 133Cs+ primary ion beam was accelerated at 10 kV, with an intensity of ~2 nA and focused to an area of ɸ 10 μm on the sample surface and the size of analytical spots is about 20 μm in diameter (10 μm beam diameter +10 μm raster). Oxygen isotopes were measured in multi-collector mode using two off-axis Faraday cups. The measured oxygen isotopic data were corrected for instrumental mass fractionation (IMF) using the Penglai zircon standard34 (δ18OVSMOW = 5.31 ± 0.1‰), which was analyzed once every four unknowns, using sample-standard bracketing method. The internal precision of a single analysis generally was better than 0.1‰ (1σ) for the 18O/16O ratio. As discussed by Kita et al.35 and Valley and Kita36, internal precision for a single spot (commonly < 0.1‰, 1σ) is not a good index of analytical quality for stable isotope ratios measured by SIMS. Therefore, the external precision, measured by the spot-to-spot reproducibility of repeated analyses of the Penglai standard, 0.30‰ (2σ, n = 24) is adopted for data evaluation. The Qinghu zircon was used as secondary zircon standard, and seventeen measurements of the standard yielded a weighted mean value of δ18O = 5.39 ± 0.08‰ (2σ; MSWD = 1.3), consistent with the reported value of 5.4 ± 0.2‰ (2σ)32.

Zircon Lu–Hf isotopic analyses were conducted using a GeoLas 193 nm laser‐ablation system attached to a Neptune (Plus) MC‐ICP‐MS at MiDeR-NJU. Beam diameter of ~44 μm was preferentially adopted to zircon domain with large size, while ~32 μm beam diameters was adopted to zircon domain with its size < 44 μm. Each diameter-change operation will be followed by analysis of zircon standard to ensure the stability of the experiment. Ablation pulse rate and energy density are 10 Hz and 10.5 J/cm2 respectively. The ablation times were 60 s. Helium carrier gas transported the ablated sample from the laser-ablation cell via a mixing chamber to the ICPMS torch. Masses 172Yb, 173Yb, 175Lu, 176Hf + Yb+Lu, 177Hf, 178Hf, 179Hf, and 180Hf were measured in Faraday cups; all analyses were carried out in static-collection mode. Hf reference solution JMC475 was analyzed during analytical session to allow normalization of the fundamental mass spectrometer performance. Interference of 176Yb on 176Hf has been corrected by measuring the 172Yb isotope and using 176Yb/172Yb to calculate 176Yb/177Hf. The appropriate value of 176Yb/172Yb was determined by spiking the JMC475 Hf standard with Yb and a 176Yb/172Yb = 0.588596 was used for this correction. Interference of 176Lu on 176Hf is corrected by measuring the 175Lu isotope and using 176Lu/175Lu = 0.02658 to calculate 176Lu/177Hf. The interference corrected 176Hf/177Hf was normalized assuming 179Hf/177Hf = 0.7325 for mass bias correction. Reference zircon Mudtank and 91500 were used to monitor accuracy and precision of Hf isotope ratios and instrumental drift with respect to the Lu/Hf ratios. The obtained 176Hf/177Hf ratios were 0.282295 ± 0.000009 (n = 15; MSWD = 2.9) for 91500, and 0.282487 ± 0.00008 (n = 14; MSWD = 3.6) for Mudtank, and were consistent with the recommended values37,38. The obtained 178Hf/177Hf and 180Hf/177Hf ratios were 1.467207 ± 0.000017 (n = 13; MSWD = 1.6) and1.886870 ± 0.000049 (n = 14; MSWD = 2.8) for zircon 91500, 1.467216 ± 0.000023 (n = 13; MSWD = 4) and 1.886871 ± 0.000038 (n = 13; MSWD = 2.1) for zircon Mudtank. The stable 178Hf/177Hf and 180Hf/177Hf ratios overlap at 2σ with recommended values reported by Thirlwall and Anczkiewicz39.

Whole-rock geochemistry

Major elements were analyzed using a Thermo ARL9900XP X–ray fluorescence spectrometer (XRF) at the MiDeR-NJU. The analytical precision is generally better than 2% for all elements. Whole-rock rare earth and other trace elements were analyzed using an ICP-MS (Finnigan MAT–Element II) instrument at MiDeR-NJU. Each sample was precisely weighted 30 mg and then was put into a 15 ml Savillex digestion vessel. After being dissolved by HNO3 and the injection of 1 ml 500 ng/ml internal standard Rh solutions, the samples are ready for analyzing. Analytical precision for most elements by ICP- MS is better than 5%. Major and trace element composition data of the migmatite and leucosome samples are provided in Supplementary Data 1.

Mineral composition

The mineral major element compositions were determined using a JEOL 53 JXA-8100 electron probe microanalysis (EPMA) at the MiDeR-NJU. The instrument was operated in wavelength-dispersion mode with a beam diameter of 1–2 µm, a 15 kV accelerating voltage, and a 20 nA beam current. Element peaks and backgrounds were measured for all elements with counting times of 10 and 5. Natural and synthetic standards were used. Detection limits were better than 0.02 wt % for the oxides of most elements. All EPMA data were automatically reduced using the ZAF correction program. Mineral major content results are provided in Supplementary Data 4.