Geochemical constraints on the differentiation processes that were active in the Sept Iles complex

Abstract

Major- and trace-element abundances in the major units (gabbro, anorthosite, monzonite, syenite, and granite) of the unmetamorphosed Sept Iles complex have been evaluated to determine if these rocks can be related by simple magmatic processes or if it is necessary to invoke separately derived magmas. Major-element mass-balance and trace-element distribution calculations indicate that the diorite and quartz syenite were produced by fractional crystallization of plagioclase and augite, together with minor hypersthene and ilmenite, from a parental gabbroic magma. The Sr depletion of the granite, as compared with the quartz syenite, cannot be developed readily by partial melting and is better explained by fractional crystallization models. Major-element mass-balance solutions indicate that the granite was formed by removal of alkali feldspar, plagioclase, amphibole, and ilmenite from a quartz syenitic magma. Depletion of REE in the granite was probably the result of amphibole or REE-rich accessory mineral fractionation. It is unlikely that an unrelated, independently generated granitic magma could have a composition so related to the remainder of the complex. Therefore, fractional crystallization of a parental gabbroic magma is the dominant process that controlled the diversity of magma in the complex.