Geochemistry of three layered mafic-ultramafic intrusions,in the Panxi area and constraints on their sources

A series of layered mafic and ultramafic intrusions aligned along a N-S trending fault zone are exposed in the Panxi area of the Emeishan continental flood basalt province, SW China. They are temporally and spatially associated with flood basalts and therefore considered to represent the magma storage system. Three representative intrusions, i.e. Panzhihua (V-Ti ores), Limahe (Cu-Ni ores) and Xinjie (V-Ti and Cu-Ni-PGE), were chosen for major element, trace element and Sr-Nd isotope studies. The Xinjie intrusion is geochemically intermediate between the Panzhihua and Limahe intrusions. In terms of the features of major elements, the Panzhihua intrusion has relatively high Fe, Ti, Ca and Al contents, whereas the Limahe intrusion has relatively high Mg contents. All the three intrusions have similar chronrite-normalized REE patterns and primitive mantle-normalized trace element patterns, which resemble those of the Lijiang picrite that is characterized by slight enrichment of LREE and LILE. However, the Limahe intrusion displays negative Nb, Ta and Ti anomalies, suggesting modest contamination with crustal materials or the lithospheric mantle in the source area. They display an approximately linear trend on the diagram of log (Th/Yb)(PM) vs. log (Ta/Th)(PM), in which the Panzhihua intrusion displays higher (Ta/Th)(PM), and lower (Th/Yb)(PM), whereas the Limahe intrusion have reverse ratios as. compared with the former, possibly implying contamination with upper crustal materials, and the Xinjie intrusion falls between them, showing contamination with lower crust. Each intrusion has a quite distinct isotopic composition. The Xinjie intrusion has homogeneous (Sr-87/Sr-86), ratios (0.70610 - 0.70636) and epsilon(Nd) (t) values 1.01 to 1.75). The Limahe intrusion is characterized by high (Sr-87/Sr-86), ratios (0.70631 - 0.70930) and low epsilon(Nd) (t) values (-0.85 to -4.13), whereas Panzhihua has the similar isotopic composition to that of the Emeishan picrite: 0.70434 - 0.70510 for (Sr-87/Sr-86)(t) and 1.1 similar to 3.2 for epsilon(Nd) (t). However, the Sr-Nd isotopic compositions of all the three intrusions ( except for some samples from the Limahe intrusion) fall in the field of the Emeishan basalt province and OIB. Concludingly, we propose that all the three intrusions were derived from a mantle plume, but the plume-derived magmas were contaminated by different materials: the lithospheric mantle for Panzhihua, lower crust for Xinjie and upper crust for Limahe. The sources of the three intrusions probably reflect the evolutionary process of the plume-derived melts.