Petrogenetic and tectonic implications of the geochemistry of lavas and intrusions in the Roy Group, Chibougamau area
Jean H Bédard (GSC-Q), François Leclerc, Lyal Harris (INRS-ETE), Patrice Roy, Patrick Houle (MRNF), Normand Goulet (UQAM)

Our new subdivision of the Gilman Formation includes at its base, tholeiitic (La/Yb pm = 1.0-2.5) basalts and andesitic basalts of the David Member. Gabbroic to pyroxenitic sills are abundant in the David. These intrusions, with rare exceptions, have a tholeiitic affinity and inverse modelling suggests they may be comagmatic with the David Member.

The overlying Allard Member comprises andesitic basalt flows and andesitic tuffs (La/Yb pm = 3.8-6.2), overlain by calc-alkaline rhyodacitic flows and tuffs (La/Yb pm = 4.1-65.6). The geochemistry of Allard rocks suggests an important crustal contribution, which becomes dominant for the more felsic members. The overlying Bruneau Member is composed of tholeiitic (La/Yb pm = 0.6-1.6) basalts and andesitic basalts. The latter are more primitive than those of the David Member and do not represent a structural repetition, but rather the arrival of new primitive mantle-derived magmas.

The overlying Blondeau Formation is composed of andesites, spherulitic rhyolites and turbidites. Swarms of sills are abundant along this transition zone (Cummings Complex). The Cummings consists of three thick sills, variably dissected by reverse faults. Chilled margins and metasedimentary septa prove their igneous origin. Inverse modelling indicates that two major magma suites were involved, one tholeiitic and one calc-alkaline, each giving rise to a complete lithological series (peridotite, pyroxenite, and gabbro). Rocks in the calc-alkaline suite occasionally contain igneous hornblende.

Field evidence clearly indicates the stratigraphy is largely intact near Chibougamau. The abundance of feeder sills implies a proximal magmatic source. Two mafic-felsic cycles are preserved, recording a changeover from mantle-derived magmas with little crustal interaction (David and Bruneau) to mixtures between mantle-derived magmas and magmas derived from the continental crust (Allard and ?Blondeau). The simplest model to explain these features would be that of a mafic lava plain emplaced in shallow water in an ensialic setting. During mafic eruption, huge volumes rapidly travel through the continental crust with little interaction. Part of the basaltic magma remains trapped at the base of the crust, where heat exchanges and crustal melting linked with basalt fractionation take place, to generate the Allard Member. The crustal magmatic system then dwindles, until it is reactivated by the arrival of new mantle-derived basaltic magma.