High-temperature alteration during cooling of mafic intrusions: Insights from the Saint-Jean-du-Doigt intrusive complex (Armorican Massif, France)

Abstract

We characterize the alteration of the Saint-Jean-du-Doigt mafic-dominated, bimodal intrusive complex in the Armorican Massif by means of petrographic investigation and geochemical analysis. The pluton crops out along the seashore, providing a well-exposed, continuous E-W cross-section. The 3D geological modeling confirms that the western part corresponds to the lower part of the intrusion, whereas the eastern part represents the intrusion roof. The petrographic observations highlight an alteration gradient from the lower part to the intrusion roof. Whereas calcic plagioclase is well preserved in the lower part, it is partially saussuritized in the central part and completely altered to albite, epidote and sericite in the intrusion roof. Clinopyroxene is common and partially replaced by actinolite in the lower part, and absent in the intrusion roof. Amphibole is extremely common throughout the pluton. Mineralogical and chemical analyses of amphibole show a crystallization sequence from primary magmatic (brown) magnesio-hastingsite over transitional magnesio-hornblende to secondary actinolite, with a range of estimated crystallization temperatures between ∼900 and ∼450 °C. Ilmenite crystals are fluid inclusion-rich in the central part and the intrusion roof, but fluid inclusion-free in the lower part. Qualitative analyses of the fluid inclusion-bearing ilmenite show that it is enriched in Mo, Sn, Sb, Pb and W, strongly suggesting that these elements are concentrated in the fluid phase. Fluid-inclusion bearing ilmenite of the upper part of the intrusion is then partially replaced by titanite and rutile at temperatures around 600 °C. The abundance of hydrous mineral phases and pegmatites, common evidence for fluid-assisted mineral replacement, and volatiles trapped in ilmenite strongly suggest that the Saint-Jean-du-Doigt intrusive complex had a high volatile content. We therefore propose that the observed progressive alteration from the lower to the upper part of the intrusion mainly results from the interaction of the gabbroic rocks with exsolved magmatic fluids of the intrusive complex. We finally suggest that the petrological and geochemical features observed in the mafic rocks of the Saint-Jean-du-Doigt intrusive complex track the magmatic-hydrothermal transition of the pluton.