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Olivier Namur, Université de Liège (Belgium)
Bernard Charlier, Université de Liège (Belgium)
Michael D Higgins, Université de Chicoutimi (Canada)
Jacqueline Vander Auwera, Université de Liège (Belgium)
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The differentiation of ferrobasalts in plutonic environments is still a matter of debate which has been mainly centered on the Skaergaard intrusion. Here, we present preliminary results of a detailed petrological study of the Sept Iles Layered Intrusion, for which a ferrobasaltic parent magma has been recognized. The aim of this study is to better constrain the evolution of Fe and Si during ferrobasaltic differentiation. The Sept Iles Layered Intrusion belongs to the 564±4 Ma [1] old Sept Iles Intrusive Suite which supposedly results from the activity of a mantle plume related to the opening of the St-Laurent rift system [2]. The Sept Iles Layered Intrusion is made of a 6 km-thick unmetamorphosed series of troctolite and gabbro (Layered Series) overlain by the anorthosites of the Upper Border Series probably developed by floatation of plagioclase and the mainly granitic rocks of the Upper Series. Examination of 220 thin sections collected in the nearly continuous drill cores from the 6000m-thick Layered Series gives the following sequence of crystallization: plagioclase and olivine, +magnetite and + ilmenite, +Ca-rich pyroxene - olivine, +apatite and +olivine. The most primitive cumulus assemblage is An68 and Fo70 and occurs after a 120m-thick basal reversal (An64 to An68; Fo68 to Fo70). The most evolved compositions are An37 and Fo40.
Based on modal and cryptic layering, three megacyclic units (MCU) have been recognized. MCU I is 2000m-thick and is characterized by a smooth differentiation trend of cumulus plagioclase (An67-An42), olivine (Fo70-Fo55), Ca-rich pyroxene (Mg#), magnetite (Cr) and ilmenite. The top of MCU I is characterized by an 200m-thick level of apatite-rich gabbros. In MCU II (3000m thick), plagioclase composition is nearly constant (An60-An55) during the first 2100m with however small reversals frequently marked by the disappearance of Ca-rich pyroxene and locally Fe-Ti oxides. The last 900m of this unit displays sharp decrease of An (plagioclase) and Fo (olivine) and is marked by the appearance of cumulus apatite, forming 250m-thick horizons of oxide-apatite-rich gabbros called the Critical Zone. MCU III starts on top of the Critical Zone and corresponds to a new compositional reversal (An37 to An68). It is only 800m-thick but its upper part is hidden in the Saint-Laurent river.
References [1] Higgins, M.D. and van Breemen, O. (1998) J. Geol. 106, 421-431. [2] Higgins, M.D. (2005) Lithos. 83, 199-213.
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