Distinct mantle sources of low-Ti and high-Ti basalts from the western Emeishan large igneous province, SW China: implications for plume-lithosphere interaction

摘要

The Late Permian Emeishan large igneous province (ELIP), with an areal extent of over 500,000 km(2), at the western margin of Yangtze craton, is increasingly regarded as the result of the impingement of a mantle plume onto the lithosphere. However, petrogenesis of the continental flood basalt remains controversial. The best-exposed lava succession in the western ELIP is studied in order to further constrain their petrogenesis and plume-lithosphere interaction. The basaltic lava flows of the ELIP are geochemically classified into low-Ti (LT) and high-Ti (HT) types. The LT type lavas exhibit low Ti/Y (<500) and epsilonNd(t) (-0.34similar to-3.76) but comparatively high Mg# (44-67) and (Sr-87/Sr-86)(i) (0.705-0.708). whereas the HT type lavas have high Ti/Y (>500), epsilonNd(t) (-1.17similar to0.43) but lower Mg# (31-53) and (Sr-87/Sr-86)(i) (0.705-0706). The LT basalts can be 9 further subdivided into: LT1 and LT2. LT1 lavas exhibit relatively higher Mg# (51-67) and (Sr-87/Sr-86)(i) ratio (0.706-0.707). lower Nb/La ratio (<0.9) and initial epsilonNd(t) (-6.74similar to-0.34) than the LT2 type lavas (Nb/La>1.1; epsilonNd(t)=-1.17similar to0.43). Detailed stratigraphic work indicates that there was a temporal progression from LT1 to LT2 to HT-type magmas. This compositional shift cannot be explained simply in terms of a declining extent of crustal contamination of a mantle-derived melt with time. Instead, it seems that the LT and HT type magmas originated from distinct mantle sources and parental magmas. Geochemical features of die early stage LT1 lavas are indicative of a significant contribution from the enriched continental lithosphere mantle, whereas the compositional shift from LT1 to LT2 reveals a trend from predominantly shallower lithospheric mantle to deeper mantle with time. Late stage HT magma formed from a deeper mantle source that may possibly be a mantle plume. (C) 2004 Elsevier B.V. All rights reserved.