Characteristics of Metamorphic Textures of the Loch Eil Group of the Moine Highlands

Abstract

Recent studies on the Moine supergroup suggests that ages over a large extent have been poorly constrained, and lack of research conducted is a result of structural complexity and mineralogical monotony. The purpose of this research is to fill in said gaps in knowledge and data, with the following research question: how many metamorphic events are being recorded in the rocks of the southern Moine highlands. It is hypothesized that there are 2 events at 800 Ma, caused by different phases of the Knoydart event, 1 event at 450 Ma, caused by the Caledonia event, and possible contact metamorphism at 425 Ma, caused by the Strontian granitic intrusions. The methodology being implemented consists of microscope analysis to broadly identify mineral assemblages and metamorphic textures in the samples, followed by major element analysis on the microprobe, which is then proceeded by x-ray mapping of garnet grains present in the samples, U-Th-Pb analysis of monazite grains to help establish a range of ages, and geothermobarometric modelling to establish temperature and pressure conditions of the samples. For the purpose of this analysis, garnet geothermobarometers such as GASP, GMBP, and garnet-biotite-plagioclase-quartz will be used, and the data being inputted into the models will be extract from major element analysis. The mineral assemblages present in the four samples being used in the analysis show varying percentages of quartz, feldspars, and micas, with garnet and monazite being abundant in one sample. Primary metamorphic textures present in the samples are symplectite, undulatory extinction, and schistosity. Current limitations are due to overprinting, which affects age precision, distinguishing between older and younger fabrics, and deciphering between competing P-T-t paths, as well as lack of aluminosilicates in the samples as another limitation. Through geothermobarometric modelling, it has been determined that the temperatures being recorded are ranging from 550-660°C with some outliers plotting around 750°C, and the pressure conditions are ranging from 2-8 kbar, with most of those pressure conditions ranging from 3-5 kbar. Keywords: Moine, garnet, monazite, geothermometry, geobarometry