Differentiation of Tholeiitic Basalt to A-Type Granite in the Sept Iles Layered Intrusion, Canada

Abstract

The undeformed 564 Ma Sept Iles layered intrusion (Quebec, Canada) is a large igneous body of c. 20 000 km3. From the base to the top, it consists of a Layered Series dominated by troctolite and gabbro, an anorthositic Upper Border Series and a dominantly granitic Upper Series. The parent magma of the Layered Series is inferred to be an iron-rich tholeiitic basalt (48 wt % SiO2; 15 wt % FeOt). Whole-rock compositions from the chilled margin, dykes cross-cutting the Layered Series and silicic rocks from the Upper Series display continuous major and trace element geochemical trends ranging from basalts to ferroan metaluminous A-type granites (77 wt % SiO2). Initial 143Nd/144Nd (0·51201-0·51207) and 87Sr/86Sr (0·70353-0·70548) indicate a juvenile-mantle source and minimal contamination by old crust (1-2%) during crystallization. Geochemical modeling, using the MELTS thermodynamic calculator combined with equations predicting mineral-melt equilibria from experiments on tholeiitic basalts, indicate that basaltic to monzonitic melt compositions are in equilibrium with the troctolites and gabbros of the Layered Series. Fe-Ti oxides saturate early in the Layered Series, after 14% fractionation of plagioclase-olivine cumulates. Further fractionation of Fe-Ti oxide-bearing gabbros drives the residual liquids toward silica enrichment and iron depletion. Major and trace element modeling indicates that the A-type granites from the Upper Series were produced by protracted fractional crystallization of an iron-rich basaltic parent magma, at a fraction of residual liquid of only 8%. The observed relative volumes of mafic cumulates and silicic rocks in the intrusion are in agreement with the calculations. Most of the intermediate compositions correspond to magmatic mafic enclave-bearing granitoids and display geochemical evidence of hybridization. Intermediate compositions produced by fractional crystallization are scarce and a Daly gap occurs from 57 to 67 wt % SiO2. This gap could result either from the fractional crystallization process or from silicate-liquid immiscibility during that compositional interval. © The Author 2011. Published by Oxford University Press. All rights reserved.