Chapter 5: Petrogenesis of the basalts from Lanjiafan Formation and Evaluation of Tectonic setting

Most trends on variation diagrams are the result of mixing. Below consider some of the more important mixing process.

5.1 Alteration

It is observed from the field evidence as well as petrography that the studied basalts are experienced varying degrees of alteration, which were observed by the presence of the secondary minerals such as calcite and a larger LOI varying from 3.87wt%-13wt%. Elements which are mobile during alteration include SiO₂, CaO, Na₂O, K₂O, indicating that these major elements easily migrate with alteration occurs and low strength elements (Humphris et al., 1978). However, the high field strength elements (HFSE, such as Nb, Ta, Zr, Hf, Th, U, and Y) and the rare earth elements (REE) are relatively immobile in all even the most severe hydrothermal alteration (Pearce, 1975). In contrast, large LILE (LILEs, such as Cs, Rb, Sr, Rb, Pb and Ba) show no linear relation with zirconium. For this reason only high field strength elements Ti, Zr, Y, Nb, Ta, Hf, Th and REE have been used in the discussion of the source properties and petrogenesis of these mafic volcanic rocks.

5.2 Fractional crystallization

Fractional crystallization is a major process in the evolution of many igneous rocks, and is frequently the cause of trends seen on variation diagrams for igneous rocks, especially basalts rocks. However, the basalts from the Lanjiafan Formation have Mg# varying from 46.2 to 89.3, slightly higher than those of the primitive mantle derived magmas (about 70-72; Irving and Green 1976; Frey et al., 1987). Besides, they are relatively low in Cr (0.68-37.8) and Ni (14-123), indicating a fractional crystallization to a certain degree in the magmatic chambers or during the ascending processes of the parental magma most likely of plagioclase (Xu et al., (2015). Accordingly, the petro graphical observations show the presence of plagioclase as the dominant mineral. For the alkaline basalts, the weak Eu and Sr anomalies imply a minor fraction crystallization of plagioclase, (Frey et al., 1978). For sub alkaline basalts, the negative Eu (0.58Niu et al, 2009