The emplacement and deformation of granitic rocks in a transpressional shear zone: the ox mountains igneous complex

Abstract

The structural evolution of the Ox Mountains Granodiorite {478±12Ma) during and after its emplacement is described. This pluton has been emplaced within and synchronously with, a major transpressional shear zone which is expressed as a 11km wide belt of strongly deformed NE-SW striking, steeply dipping metasediments. The steep shear zone cleavage is intensified to a mylonitic fabric in a braided system of high strain zones formed through out the Ox Mountains Inlier. Pervasive ductile, sinistral deformation in the shear zone is interupted by the emplacement of the pluton and by the synchronous development in the country rocks of series of brittle thrust structures, which produced a displacement upwards and towards the centre of the shear zone. These thrusts are intimately associated with the emplacement of moderately inclined granodiorite sheets belonging to the main intrusive phase indicating a component of vertical extension in the country rocks at this time. The CMC is a heterogeneous, four component pluton internally composed of a series of large sheets or dykes. Minor muscovite granite sheets emplaced along the northern contacts, preceeded the main intrusive sheets of Group 1 and Group 2 granodiorite with associated diorites. Sheets of tonalites and minor components completed the emplacement history. A prolonged history of sinistral transpressional shearing has deformed the pluton. Discrete sinistral shears indicate an early localization event is overprinted by a main ductile penetrative fabric which cross-cuts ail internal contacts. This fouation is deformed by extensive sinistral S-C fabrics. Later deformation becomes increasingly partitioned into late sinistral and dextral shear zones which are locally mylonitic. Microstructural evidence suggests that the main foliation was formed under lower amphibolite facies and deformed by a steady state flow process. The S-C fabric and late shear zone formation, best developed in the granodiorites and granites, may have been initiated by a switch in the predominant alkalifeldspar deformation mechanism from crystal plastic to a diifusive mass transfer process. This may be a retrogessive effect, and the product is a grain size reduction which may lead to ultramylonite production. The emplacement model for the Ox Mountains Granodiorite is constrained by the original geometry of the dykes or sheets. These data rule out emplacement of the OMG in a releasing bend or pull-apart structure. Strain data does not allow a forceful mechanism and a permissive emplacement model is preferred, in which vertical extension during the intrusive episode created an area of dilation in which dyking occurred. This was caused by oblique movements on two upwardly converging high strain zones outside the pluton. Two satellite plutons, the Lough Talt Adamelhte and the Easkey Lough Adamellite were emplaced in extensional cavities created by reactivated sinistral movements on one of the high strain zones at a much later date. (c400Ma).Transpressional shear zones may initiate or enhance melting in the lower crust and mantle lithosphere where thermal perturbation has occurred. The shear zones may provide conduits for the melts and emplacement sites, especially where high transpressional strains are accommodated by vertical uplift. [brace not closed]