Magmatic arcs witness the interplay between endogenous and exogenous processes, including magmatism, crustal thickening, uplift, erosion, sedimentation and burial of detritus1–4. 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. 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.
Geological setting and samples
Western margin of the Yangtze Block became tectonically active since the early Neoproterozoic. Intra-oceanic arc magmatism started before 971±16 Ma10 and then transitioned to Andean-type magmatism at ca. 870 Ma11. This prolonged magmatic history gave rise to linearly distributed arc magmatic rocks spanning over 800 km at present coordinate (Fig. S1a). Post-Triassic orogenic process along the Longmenshan Thrust Belt12 exposed numerous Neoproterozoic plutonic complexes in western Yangtze Block, of which the largest one is known as the Pengguan Complex, comprising voluminous 860–750 Ma plutonic rocks (Fig. S1b). The Huangshuihe Group in the core region of the Pengguan Complex serves as a huge roof pendant and consists of metamorphic rocks of schist, quartzite and pyroclastic rock. Ductile deformation, faults, mylonite with S-C fabric, and migmatitic lineation are extensive in the sequences (Fig. S2).
Six migmatite and one leucosome samples in the Huangshuihe Group were collected in this study. The migmatites contain compositional layering, local carbonate interlayer and was intruded by mafic dikes (Fig. S2). Patch-shaped neosomes are abundant in the migmatites (Fig. S2) and formed during incipient partial melting. Large leucosomes (around 50 cm width) occur occasionally and are usually fed by a few small leucosome veins.
The main minerals in migmatite are plagioclase, biotite, K-feldspar, quartz and muscovite (Fig. S3). Anatexis of primary mineral assemblage lead to prevalent zircon overgrowth and muscovite-rimmed biotite in the migmatite (Fig. S3&5). 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(Fig. S3; Table S4). The reaction of “biotite + MnO, Al2O3, SiO2 (from melt) = garnet + muscovite”13 may control garnet paragenesis. These observations are indicative of near-solidus partial melting with local melt segregation.