S- to I-type magmatic transition captures continental arc evolution in Central and South Pamir

Le résumé fourni par la source

Continental arcs are pivotal to crustal evolution, serving as primary sites for magma generation, crustal differentiation, and lithospheric materials recycling. However, the mechanisms driving magmatic type transitions within continental arcs remain debated. Here we investigate the geological evolution of the Pamir continental arc through zircon U-Pb dating and Hf-O isotopes and whole-rock elements and Sr-Nd isotopes for two-mica granites, biotite granites−granodiorites−diorites, and enclaves from Central and South Pamir. Our results demonstrate two distinct granitic suites (S- and I-type) with mafic enclaves recording two magmatic episodes, characterized by distinct spatial-temporal patterns. The S-type two-mica granites (122−108 Ma) in the Northern belt of Pamir represent initial Early Cretaceous arc magmatism. These two-mica granites show high SiO2 (69.7−79.5 wt%) contents and elevated (La/Yb)N ratios, with enriched isotopic compositions [87Sr/86Sri = 0.7078−0.7189, εNd(t) = −10.6 to −8.9] and variable zircon εHf(t) values (−11.3 to +4.9) and δ18O values (6.0‰−10.7‰). They formed through muscovite dehydration melting of the Central Pamir metasedimentary basement. The mafic enclaves in the two-mica granites display arc-type trace-element patterns and variable isotopic compositions [87Sr/86Sri = 0.7050−0.7091, εNd(t) = −7.2 to −0.9, and εHf(t) = −11.9 to +6.0]. They were derived from metasomatized subcontinental lithospheric mantle and mixed with the Central Pamir basement during magma ascent. The subsequent widespread I-type biotite granites−granodiorites−diorites across South Pamir mark both magmatic flare-up and maturation of the arc system. These biotite granites−granodiorites−diorites have variable SiO2 (59.9−71.2 wt%) contents and isotopes [87Sr/86Sri = 0.7063−0.7129, εNd(t) = −11.2 to −4.8, zircon εHf(t) = −16.8 to +7.8, and δ18O = 5.9‰−13.2‰]. These rocks were mainly produced by partial melting of the South Pamir metaigneous basement, followed by fractional crystallization of plagioclase, hornblende, and biotite, and mixing with minor mantle-derived depleted melts. Their mafic enclaves have similar Sr-Nd isotopic [87Sr/86Sri = 0.7086−0.7113, εNd(t) = −10.6 to −10.2] but distinct zircon Hf-O isotopic [εHf(t) = 4.4−9.9, δ18O = 6.2‰−7.3‰] compositions relative to host biotite granites−granodiorites, indicating a mixture of asthenospheric mantle and South Pamir basement. We propose that the basement differences between Central (metasedimentary) and South Pamir (metaigneous) resulted from the Neo-Tethys flat-slab subduction−induced sediment backarc thrusting. The tectonic transition from flat-slab subduction to slab foundering triggered asthenospheric upwelling coupled with metasediment depletion, driving magma source shifts from the Central Pamir metasedimentary rocks to South Pamir metaigneous rocks. The S- to I-type magmatic transition in the Pamir continental arc was controlled by synergistic effects of evolving crustal structure and tectonic regime reorganization and holds broader implications for understanding the evolution of global continental arcs.

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