Quartz deformation adjacent to a mid-crustal shear zone, the Renvyle-Bofin Slide, Connemara, western Ireland

A. Rahimi-Chakdel, A.P. Boyle & D.J. Prior

Mid-crustal shear zones commonly juxtapose rocks of differing geological history. In north Connemara, the Renvyle-Bofin Slide (RBS) is one such mid-crustal shear zone involving predominantly sinistral strike-slip shearing. To the north is a monotonous sequence of psammites, semi pelites and rare pelites (locally called the Kylemore Formation) that was intruded by a series of ultrabasic and basic igneous bodies, the Dawros-Currywongaun-Doughruagh Complex (DCDC). To the south is the classic Argyll Group shallow water sequence comprising quartzites, marbles, psammites and pelites. In addition to the lithological contrast, there is also a metamorphic and structural difference. Around the DCDC, rocks north of the RBS are sillimanite bearing and record metamorphic temperature up to 880° C. These rocks are characterised by a pronounced north plunging stretching lineation, which seems to be related to the transpressional bend in the RBS to the west of the DCDC. South of the RBS, pelites contain staurolite and record temperature up to 625° C. Rocks south of the RBS are characterised by a strike parallel NW-SE, W-E stretching lineation.

One way of gaining a better understanding of the deformation across this structure is to look at quartz microstructure, in particular quartz crystal preferred orientations (CPO). In the study area (see map below), samples were collected from two localities where quartz-rich lithologies crop out against each other across the RBS. Microstructural analysis was performed using a CamScan CrystalProbe for automatic electron backscatter diffraction (EBSD). Complete crystallographic orientations were determined for at least several thousand individual quartz grains in each sample to provide a dataset for exploring the active deformation mechanism(s).

The hotter rocks N of the RBS, with a N-plunging stretching lineation, show a simple "single crystal" type pattern characteristic of dislocation slip at high temperature. The cooler rocks S of the RBS, with a NW-SE to W-E stretching lineation, by contrast, show a more complex pattern which reflects not just the different stretching direction S of the RBS, but perhaps also superimposition of strains recording more than one quartz CPO component. This latter possibility will be discussed further.