R. M. Forster

Bio: R. M. Forster is an academic researcher. The author has contributed to research in topics: Basalt & Mafic. The author has an hindex of 1, co-authored 1 publications receiving 8 citations.

A geochemical and petrological study of the Tertiary minor intrusions of Rhum, northwest Scotland

Abstract: The Rhum minor intrusion suite is petrographically and compositionally variable, the majority of the basic rocks are aphyric dolerites but olivine phyric (picritic) and plagioclase phyric (big feldspar) dolerites are also present. In addition, several medium to coarse grained gabbro and peridotite plugs occur. Intermediate rocks are not abundant and acid dykes are rare. The basic and intermediate intrusions occur in a variety of structural environments all over the island, the gabbros and peridotites are generally emplaced as plugs in the northern and southern Torridonian tracts, while the few acid dykes are usually closely associated with the plutonic acid rocks. Twelve classes of dolerite (Groups 1-12) and five classes of gabbro have been distinguished on geochemical and petrographic grounds. The suite as a whole is characterised by high MgO levels when compared with rocks of analogous differentiation in other Tertiary suites of North-West Britain, Several of the group 1 picrites are believed to have existed as mafic liquids with 15-20 wt% MgO at high crustal levels. In addition the basic members (dolerite groups 2 to 4) and several of the gabbros are characterised by higher Ni and Cr and lower Al(_2)O(_3) than comparable British Tertiary rocks. Discriminant Function analysis of the compositional variation within the basic members of the Rhum minor intrusion suite has shown that the majority of these rocks are completely transitional between alkali olivine basalt and tholeiite basalt and contrast with the mildly alkaline Skye Main Lava Series and the mildly tholeiitic Ardnamurchan cone sheets. In contrast, the dolerite groups 5 and 6 are very similar to some of the Skye magma types and consequently are believed to be outlying members of the Skye regional dyke swarm. Of the intermediate rocks, the dolerite groups 8 and 9 and the gabbroic differentiates are tholeiitic in affinity and the dolerite groups 10 to 12 are alkalic in affinity. Detailed field and geochemical investigation has shown there is a progression from tholeiitic basic magmas to high MgO more alkalic composition with time. Despite the known presence of a large upper crustal magma chamber on Rhum, the geochemical variation of the majority of the basic rocks is cottatible with processes operating below 15 km within the lower crust, or in the upper mantle at depths up to 80 km. In contrast the big feldspar (group 7) dolerites and the magmas which gave rise to the peridotite plugs are believed to have been tapped from a high level crustal magma chamber. In addition the divergence towards more extreme alkalic and tholeiitic compositions in the intermediate rocks coupled with their general porphyritic nature and their major oxide and trace element variation suggests that they are the product of low pressure fractionation processes.

The Rum Layered Suite

Abstract: Palaeocene igneous activity in the Rum Central Complex culminated in the formation of the ultrabasic and gabbroic rocks of the Layered Suite. Its three components, the Eastern Layered Series, the Western Layered Series and the Central Series, represent a continuum in time during which replenishments of picritic (MgO 15-20 wt.%) and basaltic magmas ponded in thin sill-like bodies at the Lewisian gneiss - Torridonian sandstone unconformity, each contributing incrementally to a layered cumulate sequence. The magmas were probably guided during ascent by the long-lived Long Loch Fault. Peridotite (olivine - chrome-spinel) cumulates formed from picritic magma. The residual (basaltic) magma mixed with resident residual magma from earlier batches, and with small amounts of siliceous rheomorphic melts from country rocks, forming (isotopically contaminated) allivalitic (= troctolitic), plagioclase - olivine cumulates or, less commonly, gabbroic (plagioclase - olivine - clinopyroxene) cumulates. Residual basaltic magma was probably also intruded as gabbroic sheets and plugs, and extruded as lavas. Widespread slump and shear structures indicate mechanical instability of unconsolidated cumulate mushes, especially in the allivalites. Ultrabasic breccias are common in the Central Series, and are attributed to (i) disruption of earlier cumulates as new batches of magma rose along an elongate, north-south feeder zone and (ii) collapse of cumulates into this zone during episodic magma withdrawal. Equilibrated textures, lack of compositional zoning in olivine and pyroxene, offsets between compositional and modal variation at unit and other lithological boundaries, the occurrence of finger structures and other replacement features, and the compositional modification of ultrabasic rocks adjoining late-stage gabbroic veins, all attest to the pervasive influence of migrating intercumulus liquids during crystallization and consolidation of the cumulates.

The Rhum Layered Complex, Inner Hebrides, Scotland

Abstract: The lower Eocene Rhum Central complex has a core of ultra-basic rocks and gabbros which formed at a late stage in the centre’s evolution. Three major components are recognised: the Eastern Layered Series of alternating layers (up to 80m thick) of feldspathic perioditite and allivalite, totalling c. 600m in thickness; the Western Layered Series of layered gabbros conformably overlain by layered feldspathic peridotites; and a later intrusive Central Series consisting of feldspathic peridotites and subordinate gabbros, dunites and layered allivalites. The Central Series is noteworthy for its abundant development of ultrabasic breccias. Emplacement of the ultrabasic and gabbroic rocks was initially controlled by the Main Ring Fault but the Long Loch Fault zone exercised control during intrusion of the Central Series. The Eastern and Western Layered Series are considered to be parts of a once-continuous steep-sided intrusion with a domed roof that extended only slightly above the level of the Rhum mountains (c.800m altitude).

A review of silicic pyroclastic rocks of the British Tertiary Volcanic Province

Abstract: Summary Silicic airfall and ashflow tuffs constitute a significant component of the extrusive products of the British Tertiary Volcanic Province. These deposits typically formed early in the evolution of individual volcanic centres as proximal facies accumulations intercalated with polylithic pyroclastic breccias, and occur throughout the evolution of the Province. Rhyolites and, less commonly, trachytes occur as lavas in close spatial and temporal association and, together with the silicic pyroclastic rocks, are interpreted as the extrusive equivalents of the felsic and felsic-mafic (mixed-magma) subvolcanic intrusions which are important members of the central intrusive complexes of the Province. The field relations of the silicic tuffs suggest that the associated pyroclastic breccias are also often extrusive and together they form crater infills.

Petrology and evolution of the Northern part of the Rhum Ultrabasic Complex

Abstract: The central region of the Rhum ul-trabasic complex comprises three major sequences of olivine, and plagioclase-olivine cirulates: (i) the westerly continuation of the Eastern Layered Series; (ii) the northernmost extension of a redefined Western Layered Series, and (iii) a newly-defined Central Series. The structural relationship of the Eastern and Western Layered Series is unresolved, but they are transgressed by the large dyke-like mass of the Central Series. The Long Iodh Fault played a significant role in the development of the essentially in situ Central Series.Striking differences between the three series reflect fundamental differences in the way they accumulated and solidified. Field, textural and geochemical evidence favour a multiple intrusion origin for the macro-layering. Gravitational deposition from flawing magma currents, and in situ crystallization, produced the finescale layering of the plagioclase-olivine and olivine cumulates respectively. A complex pattern of cryptic variation in both the cumulus and intercumulus phases supports the magrra replenishment model'. Field, • petrographic, geochemical and experimental studies of the transitional basalt picrite dyke suite have derrnstrated the existence, on Rhum, of high-magnesian liquids with up to 20.5% MgO. The suite as a whole is characterized by 1CM incoirpatible-elenent contents and slightly L&EE-enriched chondrite-normalized REE patterns. The high-MgO liquids are believed to be the result of small degrees of melting (15-20%) of garnet Therzolitè at a depth of Ca. 65 kin, leaving a lherzolite residuum. Similar magnes Ian liquids were probably parental to the ultrabasic complex. The nature of the rrera entering the rum chamber varied greatly. Magma supplying the chamrber during the development of the Eastern and Western Layered Series carried only minor amiits of liquidus olivine (and spinel) ca. 10%. In contrast that supplying the Central Series chanber was highly olivine-phyric (ca. 50% crystals). Intrusive offshoots from the Central Series porphyritic (picritic) magma are represented by the numerous non-layered peridotitic minor intrusions, and lax-temperature (basaltic) magma by the gabbroic minor intrusions. Available evidence suggests that mantle fusion in the Rhum area pre-dated that of Skye. Close chemical resenbiarices between the plcritic basaltic rragmas of Rhum and the magnesian basalts from known Tertiary spreading centres suggest there were genuine, but abortive, attempts to produce oceanic crust beneath Rhum.