Petrogenesis of an early Eocene gabbro-granite complex in Kachang (SW Yunnan) and its implications for Eocene magmatism in the Tengchong terrane of SW China

摘要

Major and trace element, zircon U-Pb and Hf-isotope data are reported for mafic intrusions and host granite from the Kachang pluton (North Yingjiang of SW Yunnan) in the Tengchong Terrane, in order to investigate their sources, petrogenesis and tectonic implications. The zircon U-Pb age of the mafic rocks (55Ma) is identical to that of the host granite (56.7 +/- 0.6Ma). The mafic rocks have high MgO concentrations (up to 13.43wt.%) at low SiO2 contents (low to 42.73wt.%) and slight negative to positive epsilon Hf(t) values (-2.26 to +0.59). They are enriched in LILE and LREEs and depleted in HFSEs, which can be explained as melts derived from a enriched mantle, with some crustal contamination. The host granite have high SiO2 contents (69.18-72.65wt.%), highly negative epsilon Hf(t) values (-9.08 to -5.14), suggesting mainly derived from an ancient crustal source. Field observations, geochronology, geochemistry and zircon Hf isotopic compositions point to a complex petrogenesis, where enriched mantle- and crust-derived magma mixing was coupled with crystal fractionation, thus explaining the genetic link between mafic and felsic magmas, result of mafic magma upwelling triggered by the subduction rollback of the Neotethyan slab. Our new data, along with the data reported (especially zircon U-Pb dating and Lu-Hf isotope data) in the Tengchong Terrane, indicate that the spatial and temporal variations and changing magmatic compositions over time in the Tengchong Terrane closely resemble those of the Lhasa Terrane in southern and central Tibet.