Abstract:

 A combined study of UPb zircon dating, wholerock chemical and Sr-Nd isotopic composition was carried out for the Meiwu pluton in the middle part of the west Qinling. U-Pb zircon dating shows that the Meiwu pluton intruded at 245242 Ma. The Meiwu pluton, a complex batholith, is composed of quartzdiorite, granodiorite and biotite granite, with abundant mafic magmatic enclaves (MMEs) in the granodirite. The different rock types in the Meiwu pluton are characterized by distinct chemical evolution trend. Geochemical and SrNd isotopic composition indicate the magma of the Meiwu granodiorite and biotite granite was dominantly derived from crustal source. However, the granodiorite show relatively high Mg# (5058), Cr (217×10-6, (4693)×10-6) and Ni ((1250)×10-6), suggesting a minor contribution from the mantle. The quartzdiorite and mafic magmatic enclaves display high Mg# (63 and 62), Cr (217×10-6 and 318×10-6) and Ni (61×10-6 and 110×10-6), respectively, indicating that their magma were dominant mantlederived. Geochemical and U-Pb zircon ages, combined with regional setting, suggest the Meiwu Pluton in the middle part of the west Qinling resulted from breakoff of the subducted Animaque oeanic slab at early stage of postcollisional setting. The slab breakoff model can well explain the linear distribution of the Early Indosinian plutons and rapid crustal uplift during the Middle Triassic in the west Qinling.

Key words:  west Qinling, complex batholith, zircon U-Pb dating, magma source, slab breakoff