Deep magma evolution in the extensional Youjiang Basin in late Yanshanian period: Evidence from geochemical characteristics of Baiceng ultramafic rock, Guizhou Province

The ultramafic rocks in Baiceng area are located on the southwest margin of Yangtze block. There exist multi-stage magmatic activities and quite a few ultramafic rocks in this area. Lots of research work on ultramafic rocks has been done in this area, but some important questions remain unsolved. In this paper, the authors analyzed major elements, minor elements (including REE) and PGE for 11 samples of ultramafic rocks from Baiceng area. The results show that their major elements vary in a narrow range, implying the characteristics of high potassium and low sodium. These rocks are also enriched in LILE (Ba, Sr, Rb), HFSE (Nb) and rare earth elements. The enrichment of LREE forms a right inclined curve, which indicates definite fractionation of REE. The anomalies of Eu and Ce are negative. The total content of platinum group element is low, and the differentiation is not significant. Geochemical characteristics of these ultramafic rocks indicate that the rocks were formed under a tectonic setting of large-scale lithospheric extension in Youjiang Basin. The magma was derived from the mantle with a low degree of partial melting, with garnet and sulfide being left over during the melting. Obvious crustal contamination never happened during the process of magma rising. The crystallization and differentiation level of magma was not high. The fractionation of clinopyroxene and olivine occurred during magma intruding while there was no fractionation of plagioclase.