Peridotite Sills and Metasomatic Gabbros in the Eastern Layered Series of the Rhum Complex
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Journal of the Geological Sociev, London, Vol. 145, 1988, pp. 207-224, 19 figs, 1 table. Printed in Northern Ireland Peridotite sills and metasomatic gabbros in the Eastern Layered Series of the Rhum complex J. H. BEDARD,’ R. S. J. SPARKS,R. RENNER, M. J. CHEADLE & M. A.HALLWORTH Department of Earth Sciences, Downing Street, Cambridge University, Cambridge CB2 3EQ, UK Abstract: Mapping of the Eastern LayeredSeries (ELS) of the Rhumultrabasic complex on the northern flank of Hallival shows that peridotite and allivalite (troctolite or gabbro) layers are laterally discontinuous and vary both in thickness and lithology. Peridotite generally has sharp upper and lower contacts against the allivalites, which sometimes cut across the layering in the allivalite. Reaction, dissolution and hybridization effects betweenperidotite and allivalite are developed locally. Some troctolite layers terminate as isolated, fingered blocks in peridotite. There are many small peridotite bodies which are clearly intrusive into allivalite and have previously beenidentified as distinct peridotite sheetsand plugs. They are petrographically almost identical to the major stratiform peridotites and in some cases are apophyses from them. We propose that many of the peridotite layers in the ELS formed as thick sills of picritic magma emplaced into a partly solidified, layered troctolite complex. The stratiform gabbros of the ELS are heterogeneous, layered rocks that commonly contain relicts of troctolite and anorthosite. Wavy (metre-scale) contacts between gabbro and troctolite cut across pre-existing grain-size, modal and rhythmic layering with little disruption. These metasomatic gabbros mimic the textures, grain-size and rhythmic layering of their troctolitic protoliths. We propose that many of the ELS gabbros formed as a result of interaction between porous troctolites and a low-temperature basaltic melt. Residual basaltic melt segregated from solidifying peridotite may have caused this metasomatism. The Eastern Layered Series (ELS) of the Rhum ultrabasic systematic cryptic variations of mineral composition through complex is composed of alternations of thick layers (tens of individual cumulate cycles, and the feldspathiccumulates metres) of peridotite and feldspar-rich allivalite. Allivalite is should be in isotopic equilibrium with the underlying olivine a local termfor conformable feldspathic rocks (troctolite cumulate peridotites. and gabbro). Harker (1908) argued that this macro-layering In the modified replenishment model proposed by resulted from alternating injections of two magma types that Huppert & Sparks (1980) andTait (1985) (Fig. la), the had differentiated at depth. Modern experimental petrology dense replenishing picritic magma ponds under the resident and phase equilibria indicate that these rocks are not liquid basalt, interrupting the deposition of troctolite or gabbro. compositions but crystal cumulates. As consequence,a The ponded picrite may assimilate its floor while Harker’s sill-complex modelhas been neglected since the simultaneously crystallizing olivine and minor chrome study by Brown (1956). spinel. Turbulent convection in the ponded picrite layer Brown (1956) defined fifteen major peridotite-allivalite inhibits crystal settling andkeeps olivine phenocrysts in alternations in the ELS. The absence of systematic suspension. When convection wanes, the suspended olivine differentiation with stratigraphic height, in contrast to the phenocrysts settle out en mane to form the peridotite layer. Skaergaardintrusion, led him to propose that these This can accountfor a lack of systematic cryptic layering in a repetitions of peridotite and allivalite layers were the result peridotite layer. Protracted fractionation of olivine from the of periodicreplenishment by primitive basalt. ,The replenishing picrite results in a density inversion, allowing replenishment model has been accepted by everyone since, the fractionatedresidua to mixwith the resident basaltic although opinion has swung to the view that the replenishing magma above. In most cases, the mixture is no longer magmas were picritic rather than basaltic (Donaldson 1975; saturated only in olivine, and so peridotitedeposition is Gibb 1976; Huppert & Sparks 1980; Tait 1985). succeeded by deposition of troctolite or gabbro. Since the In Brown’s (1956) basaltic replenishment model a feldspathic cumulates precipitate from hybrid magmas, they peridotite-allivalite cyclic unit reflects closed-system frac- need not be in isotopic equilibrium with the olivine tionation.Peridotites (olivine cumulates) accumulate from cumulate peridotites beneath. mafic parentalbasalt. The magma evolves until feldspar An axiom held by many students of layered intrusions is joins olivine onthe liquidus andtroctolite cumulates are that layers accumulate sequentially on the floor. Thus, the then deposited. Further fractionation leads to co-saturation law of superposition holds and layers young upwards. This is in clinopyroxene and the accumulation of gabbros. Periodic clearly the case in Skaergaard(Wager & Brown 1968) or replenishments of the chamber by magnesian basalt Kiglapait (Morse 1969), where there is no other wayof rejuvenate the system and cause repetitions of the cumulate explaining the systematic variations of mineral composition cycle. In this model there shouldbe progressive and andmodes with stratigraphicheight. On Rhum, however, this assumption has not been adequately justified, and we Presentaddress: Geological Survey of Canada, 601 Booth question its validity. Street, Ottawa, Ontario, Canada K1A OE8. Another interpretation of the peridotite-allivalite 207 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/2/207/4889416/gsjgs.145.2.0207.pdf by guest on 25 September 2021 208 J. H. BEDARD ET AL. Geology of the Eastern Layered Series This section is principally based on detailed mapping and traverses completed on the north flank of Hallival on Units 7through 10 (Fig. 2b); complemented by a review of published and unpublished work by other researchers. We begin by describing the dominant ELS lithologies (perido- tite, troctolite and gabbro). We then consider the different types of peridotite-allivalite contacts and their relationships tothe structures and layering that occur within the allivalites. Subsequently, we discuss the large-scale ge- ometrical relationships of allivalite and peridotite. Finally, we document vertical and lateral lithological variations within the allivalites, and describe the gabbro-troctolite contact relationships. General background:the Rhum ultrabasic complex Fig. 1. (a) Replenishment model of Huppert & Sparks (1980). The TheRhum complex of the British Tertiary Volcanic dense, replenishing picritic magma ponds under thebasalt and Province was emplaced into Lewisian gneisses and interrupts the deposition of 'basaltic' cumulates. After a period of Torridoniansandstones during the Palaeocene (58 Ma, peridotite deposition, the two layers mix and gabbroic or troctolitic Musset 1984; Emeleus 1987). The basic and ultrabasic rocks cumulate are then deposited.If the picrite/basalt interface can be divided into (Fig. 2a)aCentral Series (CS), a intersects the sloping cumulate floor, then time-correlative WesternLayered Series (WLS) and anEastern Layered cumulates may display lateral facies changes. Because the Series (ELS)(Emeleus 1987). The WLS, as redefined by replenishing picrite is hot and undersaturated in plagioclase and McClurg (1982: HarrisBay, Transitionand Ard Mheall pyroxene, it could melt the basaltic cumulate floor and cause Series) is principally composed of alternations of dunitic and channelling. Two-stage intrusive model proposed in this paper, (b) harrisitic peridotite. The CS is mostly composed of same symbols as in Fig. la. On the right is a schematic representation of the cumulates derived from a magma chamber feldspathic peridotiteand peridotitic breccias (Emeleus 1987). In places there are rhythmically-repeated, modally- replenished by two-phase basalts (01 + plag). This leads to the development of a troctolitic complex. Subsequently, picritic magma gradedlayers (fig. 12 in Emeleus 1987), which commonly rises through the earlier troctolitic cumulates, forming sills on its contain xenoliths of peridotiteand allivalite that closely way to the chamber/cumulate interface. Assimilation of troctolite resemble typical ELS lithologies (figs 9 & 10 in Emeleus allows some of the picritic sills to coalesce. Residual three-phase 1987). The CS peridotites and breccias transgress the (plag + cpx + 01) magma is eventually expelled from partially layered facies andextend beyond the Main Ring Fault solidified picritic sills and reacts with the host troctolites to form (MW on Fig. 2a). Their emplacement appears to have been metasomatic gabbros. controlled by the N-S Long Loch Fault (LLF on Fig. 2a). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/2/207/4889416/gsjgs.145.2.0207.pdf by guest on 25 September 2021 PERIDOTITEGABBROSAND OFRHUMCOMPLEX THE 209 ISLE OF RHUM INNER HEBRIDES intrusion.Discordant bodies of intrusive gabbro are locally abundant. The ELS peridotites The ELS peridotites range from massive equigranular rocks, to layeredvarieties, torare chaotic intrusive breccias. Breccias may containfragments of troctolite,pyroxenite, peridotite or chromitite (McClurg 1982; Volker 1983). Where internal layering occurs it is usually sub-parallel to the peridotite-allivalite contacts and is defined by rhythmic variation in the grain size,