Staff directory
Jean-Paul Liégeois
Earth Sciences
Geodynamics and mineral resources
Geodynamics and mineral resources
Publication details
Namur, O., Charlier, B., Toplis, M.J., Higgins, M.D., Hounsell, V., Liégeois, J.P. & Vander Auwera, J. 2011. ‘Differentiation of tholeiitic basalt to A-type granite in the Sept Iles layered intrusion, Canada’. Journal of Petrology 52: 487-539. DOI: 10.1093/petrology/egq088. I.F. 4.289.
Article in a scientific Journal / Article in a Journal
The undeformed 564 Ma Sept Iles layered intrusion (Quebec,
Canada) is a large igneous body of c. 20 000km3. 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; 15wt
% 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.