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The Original Lithologies and Their Metamorphism

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The Original Lithologies and Their Metamorphism BASIC PLUTONIC INTRUSIONS OF THE RISÖR­ SÖNDELED AREA, SOUTH NORWAY: THE ORIGINAL LITHOLOGIES AND THEIR METAMORPHISM IAN C. STARMER Department of Geology, University of Nottingham, England Present address: Dept. of Geo/ogy, Queen Mary College, London, E. l STARMER, I. C.: Basic plutonic intrusions of the Risör-Sändeled area South Norway: The original Jithologies and their metamorphism Norsk Geologisk Tidsskri/1, Vol. 49, pp. 403-431. Oslo 1969. The emplacement of Pre-Cambrian basic intrusions is shown to have been a polyphase process and to have occurred Jargely between two major periods of metamorphism. The original igneous lithologies are discussed and a regional differentiation series is demonstrated, with changes in rock-type accompanied by some cryptic variation in the component minerals. Metamorphism after consolidation converted the intrusives to coronites and eaused partial amphibolitisation. Corona growths are described and attributed to essentially isochemical recrystal­ lisations in the solid-state, promoted by the diffusion of ions in inter­ granular fluids. The amphibolitisation of the bodies is briefly outlined, and it is concluded that the formation of amphibolite involved the intro­ duction of considerable amounts of extraneous water. The criterion for demoostrating the onset of amphibolitisation is thought to be the intemal replacement of pyroxene by homblende, contrasting with the replacement of plagioclase by various amphiboles during corona growth. Both amphibolites and coronites have been scapolitised, and this may have been an extended process which certainly continued after amphi­ bolitisation. Regional implications are considered and it is suggested that a !arge mass of differentiating magma may have underlain the whole region in Pre-Cambrian times. INTRODUCTION The 'hyperites' of the Kragerö region, to the north, were intensively studied by Brögger (1934) and were re-investigated from a genetical viewpoint by Reynolds & Predrickson (1962). More recently, Frodesen has carried out a detailed geochemical study of a coronite body immediately to the north of the preseht area (Frodesen 1968 a, b). In the Risör-Söndeled district the genesis of the hyperites can be divided into a number of phases and processes. After differentiation and intrusion some of the rocks underwent late-magmatic or deuteric alteration. Clouding and marginal alteration of plagioclase then occurred in conjunction with corona growths around ferromagnesian minerals and intense metamorphism converted the rocks to amphibolite. Chlorine metasornatism locally eaused the scapolitisation of both coronite and amphibolite. The period of intrusion can be placed in a geologkal time-sequence and the original igneous lithologies are seen to follow a differentiation trend in- 404 IAN C. STARMER Fig. J. The generallocation LOCATION OF of the hyperite bodies. HYPERITIC volving a troctolite-troctolitic norite series, a troctolite-olivine gabbro series, and a number of late-stage, olivine-free gabbros. The differentiation is on a regional scale and only small parts of the series are shown by individual masses at the present level of exposure. In the north of the area, around Söndeled, amphibolite bands are often concordantly interlayered with metasediments and the whole complex has been affected by a series of interneting fold phases. Some bands show affini­ ties with the large, discordant coronite bodies, but others represent intrusives, lavas or tuffs in the sedimentary. sequence (Starmer 1967). The present work is concemed only with the larger, discordant masses which exhibit good corona growths and igneous differentiation trends. FIELD RELATIONS The general distribution of the hyperites is shown on Fig. l and a more detailed indication of their field relations and rock-type is given on Figs. 2-4. The exposed surfaces of some bodies consist largely of amphibolite or metagabbro but this is often a thin veneer with relatively unaltered coronite projecting through sporadically. For this reason the maps (Figs. 2-4) show onlythe general distribution of coronite and amphibolitised derivatives since small patches of the former occur in amphibolite and vice versa. This type of alteration is the result of differential permeation of water during meta­ morphism. Considered in three-dimensions, most bodies have a dominantly coronite core which passes outwards into an amphibolitised margin. The masses are often stock-like or lensoid in shape and form distinct topographic features. The larger bodies are markedly discordant to the earlier-formed lithological banding and regional foliation, but smaller masses mayshow some concordance at the present level of exposure. BASIC PLUTONIC INTRUSIONS 405 IN THE AREA EAST OF S0NDELED 05km. Legend Re } �·"' Faults _..r'Vert. �l Fo!iation _- Minor Folds -<"5_ 7 85• � EJ �2 -<'5_ 6 75• nke and Amphibolite - 55-85" lp � -< Metagabbro lliiiiil3 ........ <55" ICS 1967 0 Pegmatite �4 Fig. 2. The area East of Söndeled. The coronites are: -C) clinopyroxene-rich gabbro,­ l) noritic troctolite,- 2) troctolitic gabbro,- 3) olivine gabbro,- 4) gabbro. In the north of the area, around Söndeled, (Fig. 2), the basic rocks intruded a complex of quartzites, amphibolites, and biotite-rich schists and gneisses, which had previously undergone Upper Amphibolite facies metamorphism and accompanying folding. Along the Risör peninsula (Fig. 3) they invaded a zone of migmatites consisting of the above lithologies intimately mixed with 'granitic' gneisses (sensu lato). In both of the above areas, the solidified intrusions were subjected to a later metamorphism of Upper Amphibolite grade which eaused extensive amphibolitisation. Deformations bent the sur­ rounding rocks around these resistant masses, which often controlied the wavelengths of major structures (e.g. at Avreid, Fig. 4). Marginal shearing also occurred towards the end of this period of metamorphism and deforma­ tion. In the South of the area, around Laget (Fig. 4), basic rocks intruded Granulite facies lithologies which had retrogressed (under Upper Amphibolite conditions) and had undergone regional granitisation (Starmer 1967). The largest body consists of troctolitic and olivine gabbro coronite with a small mass of somewhat sheared olivine-free gabbro on its western flank. Am­ phibolitised margins are extremely thin and are in no way comparable with those of bodies to the north, but some amphibolite has formed adjacent to pegmatites and in a series of north-south striking shears and fissures. Around 5 406 IAN C. STARMER �·' Faults � Minor Folds _..,.. Vertical ..-<" 75·85" ....r 65·75" _.<' 55·65" .--r <55" t lkm . RIS0R PENINSULA AND �3 Fig. 3. The Risör peninsula and Barmen. The coronites are:- l) troctolite, clinopyroxene and bronzite troctolite,- 2) noritic troctolite, troctolitic norite,- 3) troctolitic gabbro,- 4) olivine gabbro,- 5) gabbro. this main body, 'granitic' augen gneisses have been transformed to an adinole of plagioclase-quartz gneiss containing abundant rutile, sphene and occasional diopside. Exposures of noritic troctolite coronite to the southeast (Fig. 4) are heavily amphibolitised and seem to represent discordant, dike-like bodies within the augen gneiss. The coronites retain their original igneous texture; the amphibolites may be massive, gneissic or even schistose (when rich in biotite), but the term 'metagabbro' is reserved for rocks with a remnant coarse igneous-textured fabric of granoblastic andesine aggregates pseudomorphing original labradorite laths (reaching 5 cm length) and horobiende aggregates replacing the origi­ nal ferromagnesian minerals. Retrogression of both amphibolite and metagab­ bro has occasionally produced biotite-rich schists. Many coronites have a megascopic purple coloration imparted by the plagioclase laths but others may be black or brownish grey in colour. Several lithologies are related to the 'hyperites'. Anthophyllitejgedrite­ rich rocks which have formed by magnesia metasomatism, show a selective concentration around the intrusives, and when they occur at some distance from the exposed margins ma y be related to sub-surface extensions. BASIC PLUTONIC INTRUSIONS 407 - t � ..,. "' . ..,. ICS 1967 Fig. 4. The Avreid-Laget area. The coronites are: - l) troctolite, clinopyroxene and bronzite troctolite, - 2) noritic troctolite, - 3) troctolitic gabbro, - 4) olivine gabbro, - 5) gabbro. Apatite dikes have developed (particularly at Hasdalen) and rutile-hearing veins occur at Laget and Stamsöy. Plagioclase-rich segregations (consisting of oligoclase or andesine, usually with associated quartz) have formed layers, lenses, ptygmatic-folds, and veins in amphibolite and metagabbro. The adjacent rock is frequently enriched in homblende andfor biotite and both of these minerals may occur within the felsic segregations. Larger borlies of plagioclase-rich pegmatite (consisting of albite, oligoclase or sodic andesine with varying amounts of quartz, homblende or biotite) have intrusive relationships and appear to have been derived from extemal sources. Large 'granitic' pegmatites are often associated with the hyperite masses which are thought to have provided zones of low pressure to which the pegma­ titic fluids could migrate. Where pegmatites cut coronite they cause local amphibolitisation and where they have intruded amphibolite, the latter is often enriched in biotite. 08 IAN C. STARMER SPINEL - ""d �� x' m� z .. m .:C 3 Fig. 5. The recalculated modes of the original intrusions, showing some cryptic variation of component minerals. THE
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