A new pyroxene fractionation trend from a layered basic intrusion

Abstract

Pyroxene fractionation trends in igneous rocks depend on the magma composition and the conditions of crystallization. Four main types of pyroxene crystallization trend have been recognized in terrestrial igneous rocks (Fig. 1). These are the clinopyroxene trend of mildly alkaline basic magmas^1,2; the clinopyroxene trend of strongly alkaline basic magmas³; the two-pyroxene trend typical of tholeiitic magmas^4–6; and the two-pyroxene trend typical of calc-alkaline intrusions^7–9. The calc-alkaline pyroxene trends show restricted iron-enrichment as high $P_{{\text{H}}_{\text{2}}\text{O}}$ causes the crystallization of calcic amphibole (and biotite) in place of pyroxenes at a fairly early stage of fractionation. Ca-rich and Ca-poor pyroxenes from the Fongen–Hyllingen complex which coexist with hydrous mafic phases and initially have compositions very similar to those from calc-alkaline complexes, show extreme iron-enrichment in the late differentiates. This is the first recorded example of such extreme pyroxene fractionation in elevated $P_{{\text{H}}_{\text{2}}\text{O}}$ conditions and this fractionation trend, accompanied by other distinct mineralogical features, warrants designation as a new type: the Fongen–Hyllingen trend.