The Petrology and Structural Relations of Metagabbros from the Western Grenville Province, Canada

Deformation in the Grenville province culminated at 1.15-1.0 Ga. and possibly extended back to 1.3 Ga., making it the youngest structural unit within the Canadian shield. Recent work in the Central Gneiss Belt (CGB) of SW Grenville province has outlined a number of domains of different structural, litholgical and metamorphic character. These discrete crustal blocks are bounded by broad, ductile shear zones which commonly include tectonic fragments of gabbroic and anorthositic material in various stages of strain and degradation of the primary fabric. The shear zones are the lower crustal reflection of ductile thrusting during the culmination of the Grenvillian orogeny and contain kinematic indicators that consistently imply northwest-directed transport. The metagabbros have a continental basaltic signature and were probably emplaced in an extensional setting at around 1.2 Ga. They then underwent a complex and multi-stage textural evolution controlled by reaction kinetics. Early reactions were diffusion-controlled and involved the subsolidus formation of garnet-bearing coronas that separated plagioclase from olivine and opaque oxides. This probably occurred during cooling from magmatic temperatures at depths of around 35 km in the crust where calcic plagioclase was unstable and became more sodic, thus liberating Ca for corona growth and exsolving excess Al as spinel. Olivine coronas have been modelled in terms of steady state diffusion in order to estimate relative component mobilities during corona growth. The following results were obtained: restricted mobility of aluminium and to a lesser extent Si, was rate-controlling during corona growth; uphill Ca-diffusion may have been important; Mg and Fe have similar mobility with Mg at least as mobile as Ca and probably more so. Metastable under the changing regional metamorphic conditions, the delicate coronas were susceptible to further P-T-aH20 constrained reactions which eventually led to the formation of two pyroxene granulites or garnet amphibolites. The host rocks equilibrated under granulite and upper amphibolite facies conditions at 1.16 to 1.03 Ga. and the P.T estimates range from 8-10 Kbars and 700-750 0 C, with no significant differences between the various lithotectonic domains. This implies that provided the shear zones were sites for large crustal displacements, the cation distributions employed in the geothermobarometers must have been reset by continuous reactions after thrusting, and further that the fluid activity controlled whether granulite or upper amphibolite facies assemblages became stable on both local and regional scales. Relict high pressure assemblages are present in garnet peridotites and sapphirine-bearing mafic boudins collected from the ductile shear zones and as in the coronite metagabbros, unfavourable kinetics were again responsible for the preservation of the discontinuous reaction textures. In the sapphirine-bearing meta-eclogite the primary mineralogy involving high-Al Clinopyroxene, pyrope-rich garnet and kyanite±plagioclase, records pressures of around 14-16 Kbars whereas subsequent decompression products including sapphirine, corundum, spinel, plagioclase, quartz and orthopyroxene, equilibrated at around 11-12 Kbars. As it is unlikely that the host rocks went through such a high pressure event, it is proposed that the shear zones were active and incorporated material fron depths of at least 50-55 Km in the thickened Grenvillian crust during the culmination of the Grenvillian orogeny.