The petrology of metalimestones from the uppermost Argyll Group and lowermost Southern Highland Group (Dalradian) of Northern Ireland

Thin sections of seven samples of metacarbonate rocks from the Tayvallich Subgroup (Argyll Group) within the Dalradian of Northern Ireland, have been studied by optical petrography and cathodoluminescence (CL) imaging. Two of the metacarbonate samples are metadolostones and the remaining five are calcitic metalimestones. Mineral assemblages are dominated by calcite or dolomite, but subordinate quartz, feldspar and white mica are ubiquitous. Grain boundaries are only moderately well equilibrated texturally and the CL imaging reveals variable chemical heterogeneity on the grain-scale. In the dolomite-bearing samples, CL images reveal relict, diffuse calcitic domains; calcite-dolomite boundaries do not match optically-visible grain boundaries. Inclusion trails reveal dolomite growth zones. Locally, these show growth of euhedral crystal surfaces into voids, some of which are now lined with clay. This indicates that dolomitisation postdates metamorphism and deformation and is thought to have occurred at a much later date. Calcitic metalimestone samples from Torr Head have some thin, poorly-crystalline, calc-silicate bearing layers that contain epidote, green chlorite, very fine-grained ?plagioclase and ?grossular or ?vesuvianite. These are thought to represent volcanic or volcaniclastic debris. The petrography of thirteen thin sections of samples described in a previous report (Thomas, 2000) is also included.
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