| The Chengba Complex at the southern margin of the Gangdese batholith, southern Tibet, consists mainly of granodiorite with subordinated dioritic enclaves and leucogabbro. Zircon U-Pb analyses results demonstrate that the granodiorites formed at 29.40±0.18 Ma to 29.42±0.25 Ma, whereas the fine-grained dioritic enclaves formed at 30.02±0.15 Ma.,and the leucogabbro formed at 29.68±0.21 Ma. Bulk-rock major and trace element, isotope(Sr-Nd-Hf) analyses indicate that there kinds of rock are enriched in LREE and LILE, depleted in HREE and HFSE and have similar relatively high zircon εHf(t) values(+1.1~+6.2) but negative whole-rock εNd(t) values(-2.9~-5.9).Combined with literature data, out new results suggest that the Chengba granodiorite,leucogabbro and dioritic enclaves have been formed within a time period of ~ 1 myr. Similar isotopic characteristics suggest that they were derived from a similar source. the dioritic enclaves are relatively high in Mg O,Mn O,Na2 O,and Fe O; the granodiorites are characterized by elatively high Si O2(65.2%~66.2%),K2O(3.2%~4.0%) and Sr, but low Fe O Na2 O and Mg O; with the highest Sr/Y ratio,the leucogabbros are very high in Na2 O,Ca O,but very low in Fe O,Mg O,K2 O,Rb,Ba. If the dioritic enclaves is the most similar to the original magma, the fractional crystallization of hornblende can explain the characteristics of granodiorite. The fractional crystallization of biotite can explain the characteristics of leucogabbro.In conclusion, the chengba granodiorite, leucogabbro, dioritic enclaves have similar formation time, isotopic characteristics, geochemical characteristics. The high Sr/Y Chengba granodiorite and leucogabbro in contrast to low Sr/Y ratios in the mafic enclaves, possibly do not represent primary magma composition. Instead, the high Sr/Y and high Ba characteristics of the Chengba granodiorite could be resulted from fractional crystallization of parent magmas with low Sr/Y ratios and low Ba contents like those mafic enclaves. |
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