Mineral chemistry of ultramafic and mafic plutonic rocks of the Appalachian ophiolites, Québec, Canada

Abstract

This paper presents the first detailed mineralogical record of ophiolitic plutonic rocks in the Thetford Mines-Asbestos area, Québec. The plutonic rocks form the lower 5.5-km section of the 8-km-thick Thetford Mines Ophiolite. The ophiolitic cumulate sequence is in tectonic contact with underlying thick, plastically deformed, harzburgitic peridotite now largely serpentinized. The mineralogy of this peridotite is a typical example of a very depleted ultramafic rock since the minerals are poor in hygromagmaphile elements and enriched in the high-partition-coefficient elements (Ni, Cr) and Mg. The cumulates form a rock suite varying from the bottom to the top of the sequence as follows: dunite, chromitite, wehrlite, clinopyroxenite, hornblende gabbro. Olivine from the very base of the sequence is highly magnesian (Fo 91–94) and Ni-rich (NiO: 0.30-0.41 wt.%). Chromite grains are characterized by high Cr# (0.60-0.87) and varying Mg# (0.30-0.62) related to host-rock composition and higher Fe 3+# than harzburgite spinels. The TiO 2 content is usually very low (< 0.22 wt.%) but increases in gabbros (up to 1.1 wt.%). The compositional variation of clinopyroxene is very restricted. Diopside grains are depleted in TiO 2, Na 2O and Al 2O 3 and have variable Cr 2O 3 content (0-0.8 wt.%) modified during partial replacement of the grains into actinolitic hornblendes. Primary amphiboles are low-TiO 2 (< 1 wt.%) magnesian hornblendes. Calculations using mineral chemistry data suggest that the primitive liquid which was in equilibrium with early fractionated minerals, was highly magnesian, Ni- and Cr-rich and conversely TiO 2-and alkali-poor. This composition is comparable to that of the boninitic lavas of the lower volcanic group overlying the gabbroic rocks. By taking into account subsolidus re-equilibration it is possible to recalculate an average primitive magma composition. The genesis of this liquid results from a 20–30% partial melting of a source depleted in hygromagmaphile elements. It is concluded that both magmatic processes and source composition are responsible for the genesis of low-TiO 2 magnesian liquids and that these conditions may not be restricted to a single geodynamic environment. Paleogeographic reconstructions of Appalachian terranes in Cambrian-Ordovician times, the occurrence of boninitic lavas and low-Ti cumulate sequence constrain the formation of the Thetford Mines ophiolite in a marginal basin or the frontal arc region of an island-arc system. It is stressed that geodynamic significance of such liquids must be considered in relation to a geological context in order to ascertain their real implication.