Formation and evolution of granite magmas during crustal reworking: the significance of diatexites

A continuous section through reworked Archaean crust records the generation of granitic magma and its subsequent development in the Opatica subprovince in the Canadian shield. There, the transition from palaeosome to granite was a closed-system process through intermediate stages of patch migmatite and diatexite. The average degree of partial melting was less than 30%, but the melt fraction was redistributed within individual diatexite layers during deformation. Regions that lost melt became residual diatexites enriched in TiO2, FeO7, MgO, CaO, Sc, Cr, Co, Sr, rare earth elements (REE) and high field strength elements (HSFE). Melt accumulated to create diatexite magmas enriched in large ion lithophile elements (LILE); but contaminated with residuum material. Such diatexite magmas are parental to granites found at higher crustal levels in the terrane. Flow of the diatexite magma in response to deformation separated some of its residuum into schlieren. Parautochthonous plutons were created where ascending granitic magma locally ponded below impermeable layers and structures. Magma left the anatectic region in dykes and lost its remaining residuum as it crystallized. Consequently, the allochthonous granite magmas that rose though 20 km of crust to feed the highest level plutons in this region are highly fractionated and essentially free of residuum.