Synkinematic granite emplacement in a shear zone: The Pleasant Hills pluton, Canadian Appalachians

Abstract

The 361–356 Ma (Late Devonian) Pleasant Hills pluton comprises a series of sheets of mafic, felsic, and hybrid intermediate plutonic rocks that were emplaced along a positive flower structure associated with the crustal-scale Cobequid fault zone. This dextral strike-slip fault zone forms the boundary of the Meguma and Avalon terranes. The sequence of emplacement is inferred from crosscutting structures. Radiometric dating shows that the pluton was crystallized within a few million years. Gabbroic magma was probably derived from partial melting of the mantle beneath the extensional Magdalen basin, and successive batches of mafic magma were progressively richer in H 2 O, Ti, Zr, and Nb. Felsic magmas were probably derived by partial melting of the lower crust. Small batches of felsic magma show differing trace-element evolution that reflects changes in magma temperature and type of volatiles, younger magmas being generally richer in high field strength elements and fluorine. Rapakivi-textured granite is common. Heat-transfer and mass-transfer along the fault zone resulted in thermal softening and lubrication of all branches of the flower structure, so that there was repeated magma flow through each branch. The presence of magma controlled the localization of strain, which is expressed as synmagmatic deformational structures, and deformation continued under solid-state conditions to produce mylonitic fabrics. Each branch of the flower structure acted as a pathway for many magma batches, and only small volumes of magma reached the highest structural levels. The pluton was emplaced by progressive widening of, and magma emplacement along, branches of the flower structure as a result of country rock translation. This translation involved both lateral and upward movement of blocks within the Cobequid fault zone. Dextral slip is recognized on the southern margin of the escaping block and reverse oblique sinistral slip on its northern margin. The Pleasant Hills pluton shows progressive creation of space for magma along branches of the flower structure and gradual expansion of the pluton from a series of dike-like bodies to a larger composite pluton through the emplacement of numerous small, geochemically distinct, magma batches.