SYNKINEMATIC GRANITE EMPLACEMENT IN A SHEAR ZONE - THE PLEASANT HILLSPLUTON, CANADIAN APPALACHIANS

The 361-356 Ma (Late Devonian) Pleasant Hills pluton comprises a serie s of sheets of mafic, felsic, and hybrid intermediate plutonic rocks t hat were emplaced along a positive flower structure associated with th e crustal-scale Cobequid fault zone. This dextral strike-slip fault zo ne forms the boundary of the Meguma and Avalon terranes, The sequence of emplacement is inferred from crosscutting structures. Radiometric d ating shows that the pluton was crystallized within a few million year s. Gabbroic magma was probably derived from partial melting of the man tle beneath the extensional Magdalen basin, and successive batches of mafic magma were progressively richer ire H2O, Ti, Zr, and Nb, Felsic magmas mere probably derived by partial melting of the lower crust, Sm all batches of felsic magma show differing trace-element evolution tha t reflects changes in magma temperature and type of volatiles, younger magmas being generally richer in high held strength elements and fluo rine, Rapakivi-textured granite is common. Heat-transfer and mass-tran sfer along the fault zone resulted in thermal softening and lubricatio n of all branches of the flower structure, so that there was repeated magma flow through each branch, The presence of magma controlled the l ocalization 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 r eached the highest structural levels. The pluton was emplaced by progr essive widening of and magma emplacement along, branches of the flower structure as a result of country rock translation, This translation i nvolved 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 marg in, 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 plu ton through the emplacement of numerous small, geochemically distinct, magma batches.