Crystallisation of Ca-bearing nepheline in basanites from Kajishiyama, Tsuyama Basin, Southwest Japan

Abstract Ca-bearing nepheline found in the Kajishiyama basanite, Tsuyama Basin, southwest Japan, was investigated to clarify its genesis in silica-undersaturated magmas. The basanite contains olivine and augite as phenocrysts and microphenocrysts, with Ca-bearing nepheline, olivine, augite, ulvöspinel, plagioclase, alkali feldspar, apatite and zeolites in the groundmass. Zeolites are more abundant in coarser-grained samples. The whole-rock composition of the basanite is characterised by low SiO2 and P2O5 contents and high total Fe, MgO, Na2O, K2O, Ba and Sr contents. The Ca-bearing nepheline, ~20 μm in size, occurs in the mesostasis of the Kajishiyama basanite and contains up to 2.31 wt.% CaO and 16.75 wt.% Na2O, in contrast to nepheline from the Hamada nephelinite, southwest Japan. The approximate compositional formula of the Kajishiyama nepheline with the highest Ca content is (Ca0.467Ba0.013Na5.286K0.919□Total1.385)Σ8.070(Si0.912Al6.980Cr3+0.003Fe3+0.067 Mg0.017)Σ7.979Si8.000O32; i.e. Ne65.50Ks11.39Qxs11.22CaNe11.89. Basanites are defined as being nepheline-normative, however they are high in normative plagioclase, the amount of which increases with fractionation of the magma. Nepheline crystallised after plagioclase, at the last stage of magmatic solidification is enriched in Ca. Such Ca-rich nepheline only forms from a magma which is high in normative plagioclase, as is the case in the Kajishiyama basanite. In contrast, Ca-poor nepheline is precipitated from nephelinitic magmas that crystallise melilite instead of plagioclase, even when Ca contents are high.