MINERALOGICAL COMPARISON OF METAMORPHOSED ROCKS FROM THE BALLANTRAE OPHIOLITE AND THE SLISHWOOD GAP SERPENTINITE, UK AND IRELAND

In the area of Scotland and Ireland we visited two sites with the intention, through the research of the minerals and textures found in each, to determine potential origins and whether those locations could share a common genesis prior to their metamorphism. In Ireland, the Slishwood Gap Serpentinite (SGS) is estimated at an age of 900 million years and is likely to be ultramafic in origin. The Ballantrae Ophiolite (BO), found in Scotland, is interpreted to have been accreted from an island arc and estimated to be 470-490 million years in age. During the spring 2012, samples were collected at both sites. Multiple samples were prepared for thin section analyses and X-Ray Diffraction. To prepare the samples for XRD, they were crushed and sieved to a size <63µm before analyzing with a Rigaku Miniflex ranging in 2Ɵ from 5° to 70°. Analyses of thin sections from the SGS revealed antigorite as the most prevalent serpentine mineral in these samples, making up more than half of the mineral base (approx. 70%). A smaller but also noticeable presence of opaque minerals, mostly magnetite (5-10%), was observed. Other minerals frequently occurring included lizardite and crysotile, additional polymorphs of serpentine, as well as dolomite. There were a few larger pyroxene crystals found in some of the samples. Interpretation of diffraction patterns revealed the presence of serpentine polymorphs, antigorite, lizardite, and crysotile as well as dolomite. In contrast, thin sections of the BO revealed minerals including quartz, plagioclase feldspar, and K-feldspar. The textures were congruent with metasedimentary rocks as well as siltstone and greywacke textures. The preservation of sedimentary structures is consistent with low grade metamorphism along with some evidence of foliation. XRD shows the presence of quartz, albite, calcite, and clinochlore. Another sample taken from the BO is a conglomerate. It contains mostly well rounded pebbles of different lithologies. Rocks from the BO likely originated from a marine, sedimentary environment associated with an accretionary terrain. Based on the minerals found in each it can be said with reasonable certainty that they originated from very distinctly different parent rocks. The metamorphism that altered these rocks, while low grade in each, resulted in very different minerals and textures.