A Morarian age for the ‘younger Moines’ of central and western Scotland
MUCH of our recent knowledge of the Moine metasediments1,2, which extend from the Moine Thrust across the Great Glen Fault to the boundary of the Dalradian Supergroup3,4 (Fig. 1), stems from the western Moine region. Here, late-Precambrian, Morarian ages (740–780 Myr) have been established from pegmatites in the Morar Division5, and a Grenvillian age (1,030 ±50 Myr) from the Granitic Gneiss of Ardgour in the Glenfinnan Division6. The ‘Central Highland Granulites’, generally regarded as ‘younger Moine’2,6, are little known geologically. Here we briefly outline the results of field (by M.A.J.P.) and radiometric investigations (by O.v.B.) in the Central Highland Granulites and the western Moines, which have fundamental implications for the evolution of these extensive metamorphic rocks, and for the nature of the Grenvillian–Caledonian interval in this sector of the North Atlantic Crust.
The stratigraphy of late Precambrian metasediments between Glen Roy and Lismore
Synopsis The Moinian and Dalradian stratigraphy of the Glen Roy-Lismore area is systematically described and its terminology and definition is revised. The highest formation of the succession, the Lismore Limestone Formation, is of Lower Dalradian age and stratigraphically underlies the Portaskaig Boulder Bed. Succession, Glen Roy-Lismore. Thickness Islay Middle Dalradian Portaskaig B. B. LOWER DALRADIAN Ballachullish Group Lismore Limestone Fmn. ~1200 m Islay Limestone Fmn. (lower part correlated with Lismore Limestone Fmn.) Cuil Bay Slate Fmn. 300 m Appin Phyllite Fmn. 250 m Appin Limestone Fmn. 150 m Appin Quartzite Fmn. 300 m Ballachulish Slate Fmn. 400 m Ballachulish Limestone Fmn. 250 m Lochaber Group Leven Schist Fmn. ~2000 m Glen Coe Quartzite Fmn. 400 m Binnein Schist Fmn. 400 m Binnein Quartzite Fmn. 300 m Eilde Schist Fmn. 400 m Eilde Quartzite Fmn. 600 m Moinian Eilde Flags Fmn. >1000 m At Loch Leven the Lochaber Group is 4 km thick and the entire Lower Dalradian succession 7 km thick. To the north and west the quartzite units wedge out and the succession thins to about 5 km. This regional disparity is attributed to major facies changes and not structural modification as previously maintained (Bailey, 1934). There is a sedimentary contact between the Moinian and Dalradian Assemblages across most of the area. A detailed geological map of Lismore Island is presented together with a stratigraphic table of the Lismore Limestone Formation, which comprises a thick, partly cyclical sequence of alternating limestones and slates. The Eilde Flags Formation, the Lochaber Group and the Ballachulish Group constitute three lithological associations, each diagnostic of particular depositional conditions within a shallow shelf sea environment. The chemical compositions of the limestones and slates are tabulated and briefly discussed.
Igneous Rock Associations 23. The Bushveld Complex, South Africa: New Insights and Paradigms
SUMMARYThe Bushveld Complex has continued to serve as the basis for study into the fundamental nature of petrological processes for layered intrusion formation and for oxide and sulphide hosted Platinum Group Element (PGE)–Cu–Ni ore deposits. These studies have included discoveries in terms of the physical extent of Bushveld magmatism, both laterally and internally. Lateral variations in the mafic to ultramafic Rustenburg Layered Suite of the Northern Lobe of the complex have also revealed petrologically distinctive Upper Critical Zone equivalent rocks (the so-called Flatreef) with enhanced contamination and mineralization traits that reflect a transition between Eastern and Western Lobe equivalent stratigraphy and Platreef-style complexity. Traditional magma mixing models have been re-examined in light of radiogenic isotopic evidence for crustal involvement early in the chromite precipitation or formation process, combined with evidence for associated heterogeneous fluid contents, cryptic layering profiles, and textural evidence. A wide variety of alternative ore-genesis models have been proposed as a consequence. The fundamental mechanics of magma chamber processes and the existence of the magma chamber as an entity have been called into question through various lines of evidence which have promoted the concept of progressive emplacement of the complex as a stack of not-necessarily-quite-sequentially intruded sills (with or without significant quantities of transported phenocrysts), emplaced into variably crystallized and compacted crystal-liquid mush mixtures, modified by compaction-driven late magmatic fluid (silicate and aqueous) activity. Alternatively, petrological and geochemical observations have been used to discount these interpretations in favour of more conventional cooling and gravity-driven accumulation of silicate and ore minerals in a large, liquid-dominated system.RÉSUMÉLe complexe de Bushveld a demeuré à la base d’études sur la nature fondamentale des processus pétrologiques de formation d’intrusions litées et des gîtes des éléments du groupe platine (ÉGP)-Cu-Ni hébergés dans les oxydes et les sulfures. Ces études ont comporté des découvertes sur l’étendue physique, à la fois latérale et interne, du magmatisme de Bushveld. Les variations latérales de la suite stratifiée et mafique à ultramafique Rustenburg du lobe nord du complexe ont également révélé des roches équivalentes pétrologiquement distinctes de la zone critique supérieure (le communément désigné Flatreef) avec des traits de contamination et de minéralisation accrus qui reflètent une transition entre la stratigraphie équivalente des lobes est et ouest et la complexité de type Platreef. Les modèles traditionnels de mélanges magmatiques ont été réexaminés à la lumière de preuves isotopiques radiogéniques indiquant une implication de la croûte au début du processus de précipitation ou de formation de la chromite, combinées à des preuves de contenu fluide hétérogène associé, de profils de litage cryptique et de preuves texturales. Ainsi, une grande variété de modèles alternatifs de genèse de minerai a été proposée. La mécanique fondamentale des processus de la chambre magmatique et l'existence de la chambre magmatique en tant qu'entité ont été remises en question au moyen de divers éléments de preuve qui ont mis en avant le concept de mise en place progressive du complexe sous forme d'un empilement non-nécessairement séquentiel de sills injectés (avec ou sans quantités significatives de phénocristaux transportés) mis en place dans des mélanges de bouillie cristaux/liquide à cristallisation et compaction variable, modifiés par une activité tardive de fluide magmatique (silicaté et aqueux) induite par la compaction. Alternativement, des observations pétrologiques et géochimiques ont été utilisées pour écarter ces interprétations en faveur d'un processus plus conventionnel de refroidissement et d’accumulation de minérais silicatés et minéralisés induite par la gravité dans un vaste système à dominance liquide.