Permian high Ti/Y basalts from the eastern part of the Emeishan Large Igneous Province, southwestern China: Petrogenesis and tectonic implications

摘要

The source characteristics of the widespread Permian high Ti/Y basalts in the Emeishan Large Igneous Province (LIP) constitute important themes to evaluate the possible connection with a mantle plume beneath the Yangtze Block in southwestern China. Here we investigate the geochemical and isotopic signature of basalts from the Guangxi and Guizhou regions in the eastern margin of the Emeishan LIP and report the occurrence of high Ti/Y basalts in Guangxi. The zircons separated from Guangxi basalts yield a U-Pb concordia age of 257 Ma, which is consistent with the age of the plume-related eruption of the Emeishan LIP. Both the Guangxi and Guizhou basalts studied here display evolved Sr-Nd isotopic composition and DUPAL Pb isotopic composition, with (~(87)Sr/~(86)Sr)I = 0.705231 to 0.706147, positive to slightly negative εNd® values of -0.13 to +0.68, and A7 =5-11, A8 = 70-84. The Guangxi basalts possess low SiO_2 (44.82-49.71 wt.%), and TiO_2 (2.29-3.54 wt.%), but are enriched in LREE and LILE with (La/Sm)_N values of 2.2-2.6, high Ce/Yb ratios (19.6-30.0) and slightly negative Nb and Ta anomalies. The Guizhou basalts display higher SiO2 (47.49-50.27 wt.%), TiO_2 (3.93-4.92 wt.%), Zr, Nb and La contents. They also show higher Ce/Yb (30.2-37.8) and Sm/Yb (3.56-4.19) ratios as compared to those of the Guangxi basalts, and do not possess negative Nb and Ta anomalies. The distinct differences between the Guangxi and Guizhou basalts may be caused by different degrees of partial melting of the garnet peridotite in their source region. We propose that the Guizhou basalts were derived from partial melting of metasomatized veins in the Yangtze continental lithosphere which were heated by the upwelling Emeishan plume, and the Guangxi basalts represent higher degrees of partial melting of the sub-continental lithospheric mantle and undergone minor crustal contamination during their ascent.