6533b871fe1ef96bd12d0e44

RESEARCH PRODUCT

Zircon ages of metamorphic and magmatic rocks within peridotite-bearing mélanges: Crucial time constraints on early Carboniferous extensional tectonics in the Chinese Tianshan

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Abstract We dated and geochemically characterized peridotite-bearing melanges in the Chinese South Tianshan and within the Main Tianshan Shear Zone. The Yushugou-Tonghuashan melange in the Chinese South Tianshan exposes a tectonic juxtaposition of a diapirically emplaced metaperidotite (predominantly lherzolite) massif with a high-grade metamorphic terrane (ca. 10 km long; protolith age ≥ ca. 445–466 Ma). Metamorphic zircons of a mafic granulite (eNd(t) = 5.0) yielded a weighted mean 206Pb/238U age of 341 ± 8 Ma that we interpret as the time of granulite-facies metamorphism. The youngest zircon rims of an intermediate granulite (eNd(t) = − 4.3) have a mean age of 332 ± 13 Ma that records a retrogressive metamorphic event. These ages determine the timing (ca. 341–332 Ma) of mantle diapirism through continental crust. A dolerite dike (eNd(t) = 2.3) emplaced into metaperidote has a crystallization age of 335 ± 5 Ma, that, on the basis of geochemistry, we interpret as representing E-MORB-OIB magmatism that accompanied mantle diapirism. An undeformed pink granite (eNd(t) = − 3.6) intrudes the melange matrix and has an emplacement age of 324 ± 5 Ma, thus providing an upper time limit for a tectonic movement that led to melange formation. The older time limit of deformation (ca. 362–352 Ma) is constrained by the youngest ages of thermo-tectonically modified zircons in a mylonitized metagabbro and a foliated meta-andesite. Magmatic zircons in the meta-andesite (eNd(t) = − 3.3) and a dacite (eNd(t) = − 5.6) have eruption ages of 433 ± 4 Ma and 435 ± 3 Ma that date formation of the melange matrix. A microgabbro dike (eNd(t) = 10.0) cutting metasediments (i.e. melange matrix) has an emplacement age of 279 ± 3 Ma and contains abundant zircon xenocrysts ranging in age from Archean to late Paleozoic. Similarly, a dolerite dike, a meta-andesite, an intermediate granulite, and a metagabbro all contain inherited Precambrian to Paleozoic zircons. We accordingly conclude that the melange was formed by mantle diapirism in the early Carboniferous and was subsequently affected by an early Permian volumetrically minor mafic episode. Two foliated garnet granites (eNd(t) = − 7.0 and − 6.9), in direct contact with metaperidotite within the Main Tianshan Shear Zone have emplacement ages of 341 ± 2 Ma and 329 ± 4 Ma that reflect an early Carboniferous episode of crustal anatexis. They also contain Neoproterozoic zircon xenocrysts. Also within the Main Tianshan Shear Zone, a gabbro (eNd(t) = 8.7) has an emplacement age of 297 ± 3 Ma and contains abundant Neoproterozoic to late Paleozoic zircon xenocrysts. Based on the regional geological relationships, we associate early Carboniferous mantle diapirism and crustal anatexis with lower crustal delamination-induced asthenospheric upwelling. The numerous Precambrian zircon xenocrysts support a Chinese Tianshan–Tarim connection. The early Permian mafic episode coincided in time with the development of an early Permian unconformity in the Chinese North Tianshan.