From 1960 to August, 1966, the activity of Oldoinyo Lengai took the form of quiet extrusion of carbonatite lava. In August, 1966, the style of activity changed abruptly and violent ash eruptions took place. The activity varied from minor emissions of ash to major Plinian and Vulcanian type eruptions. A new ash-cone built up within the crater and ash was widely distributed on the slopes of the volcano and over the surrounding countryside.The ash consists of sodium carbonate mixed with crystals of nepheline, pyroxene, wollastonite, apatite, melanite and pyrite. Also blocks of ijolite and melteigite were ejected during the activity.
The Grampian and Appin groups of the southwestern Monadhliath Mountains form the earliest known syn-rift sequences of the Scottish central Highlands. They were likely to have formed in an intracontinental setting and represent deposition of mixed clastic and carbonate shallow and deep marine strata. The Grampian Group of the southern Monadhliath Mountains was deposited during a period of initial basin rifting (NW–SE extension) followed by a phase of thermal subsidence. Syn-rift sediments comprise a 2.5–6 km thick turbidite system. Thermal subsidence brought about the basinward progradation of shallow marine shelf sediments resulting in the infilling of pre-existing basin topography. The overlying Appin Group commenced with deposition of a shallow marine sequence alternating between nearshore tidal sand and offshore mud deposition. This formed in response to renewed rifting and concomitant subsidence. Accelerated rifting resulted in localized footwall uplift and erosion while sedimentation continued in the hanging-wall areas. Resultant subsidence, perhaps partly thermally driven, caused gradual basin widening and produced an onlapping marine sequence. There followed a period of progressive clastic deprivation when carbonates were precipitated, and at the onset of anoxic conditions, deposition of organic muds. The fundamental structural elements responsible for the formation of the Grampian and Appin group basins were also influential in the orogenic evolution of the basin-fill. Half-graben fills were deformed to produce regionally extensive folds such as the Stob Ban–Craig a’ Chail Synform.
Metasedimentary rocks of the Neoproterozoic Grampian and Appin groups exposed in the Glen Roy district (Monadhliath Mountains, Scotland) were strongly folded during an initial progressive tectonothermal event of the Caledonian orogeny. The attitude and style of the first folds varies from open, upright NE-trending structures in the west (e.g. the Appin Synform) to recumbent NW-facing nappes in the east (e.g. the Treig Synform). These early nappes occur only south of the Corrieyairack Complex and their amplitude decreases towards the NW. Ductility contrasts induced sliding such as that locally recorded along the Grampian-Appin group lithostratigraphical boundary. Subsequently the early structures were deformed by tight, upright ‘D2’ folds. NW-thrusting along the Eilrig Shear Zone, during ‘D2’, is interpreted as occurring in response to a transfer of bulk transport to deeper structural levels relative to ‘D1’. Prograde metamorphism accompanied deformation, with the thermal peak in the Glen Roy district (T = 500–600 °C; P = 7.0–8.0 kbar) coinciding with ‘D2’ folding. It is suggested that the regional variation in the style of deformation was controlled, at least in part, by pre-existing major structures, most notably fundamental, transverse basement lineaments. Temporal changes in the style of deformation occurred in response to changing crustal architecture during progressive deformation. The study demonstrates the diachronous nature of deformation and also illustrates how its is possible to preserve areas with simple deformation histories in the centre of a major orogenic belt.
Synopsis Detailed mapping of the area between Loch Lochy and Loch Ness (Scotland), SE of the Great Glen, has identified a 1 to 1.2 km-wide belt of quartz-feldspar-mylonites, garnetiferous phyllonites and relatively undeformed arkosic psammites underlying a sequence of metasediments correlated with the Neoproterozoic Grampian Group. The mylonites and garnet-phyllonites define a previously undescribed major ductile shear zone, here referred to as the Eilrig Shear Zone, which has an approximate strike length of at least 20 km. The shear zone is underlain by a sequence of pebbly arkosic psammites of unknown stratigraphical affinity. Kinematic indicators present within the various mylonitic rocks of the Eilrig Shear Zone yield a consistent NW-directed sense of overthrusting. Initial P–T estimates obtained for the garnet-bearing phyllonites indicate that ductile deformation resulting in mylonitization occurred during prograde amphibolite facies metamorphism. It is concluded that the Eilrig Shear Zone represents a major, possibly regional, ductile structure which resulted in the overthrusting of the Grampian Group towards the NW early in the structural history of the area.
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