The meta-gabbroic complex of Fushui in north Qinling orogen: A case of syn-subduction mafic magmatism

摘要

SIMS zircon geochronology and oxygen isotopes, as well as LA-ICPMS zircon Lu-Hf isotopic analyses, were carried out on a suite of rocks from the Fushui complex to probe its origin and tectonic significance for the evolution of the Qinling Group in the north Qinling orogenic belt. The Fushui complex comprises several phases of meta-gabbros: the dominant rock type is light-colored meta-gabbro, followed by mica-bearing and dark-colored meta-gabbros, with abundant dark gabbroic enclaves with different sizes and shapes and occasional occurrence of mica-bearing gabbroic pegmatite dikes. Age determination shows that the magmatic intrusion began at about 500 Ma and surged at 490-480 Ma with the latest zircon crystallized at about 476 Ma. This age spectrum is remarkably coeval with the peak metamorphism (500-480 Ma) of the surrounding Qinling Group metamorphic rocks dated by zircons from eclogites, retrograded eclogites, HP mafic granulites and UHP felsic gneisses, representing syn-subduction mafic magmatism. These late Cambrian to early Ordovician zircons display a slightly enriched Hf isotopic feature with a limited range in epsilon Hf(t) (-5 to -2) and yield uniform T-DM age (1.15-1.38 Ga). They also show a relatively small variation in delta O-18 (6.69 +/- 0.44 parts per thousand (2 sigma)-8.75 +/- 0.34 parts per thousand (2 sigma)), much higher than the normal mantle value. The isotopic feature suggests that the Fushui complex was originally derived from enriched lithospheric mantle, which was metasomatized by melts from ancient continental sediments and altered MORB basalts. The Carboniferous zircons, dominantly occurring as rims of early Ordovician cores, display very similar Hf T-DM ages (1.19-1.27 Ga) and oxygen isotopes as their counterparts, corresponding to their reoystallization at 335 Ma during amphibolite facies metamorphism when exhumed. Our study on the Fushui complex, together with the recently reported zircon U-Pb age data on the surrounding HP-UHP metamorphic rocks demonstrates that the Qinling Group preserves a complete cycle of tectonic evolution in an orogenic belt from an oceanic basin spreading, and mini-continent formation to deep subduction of mini-continent and multiple stage exhumations. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.