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Geology and Geochemistry of a Metamorphosed Picrite-Ankaramite Dyke Suite from the Seiland Province, Northern Norway

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Geology and Geochemistry of a Metamorphosed Picrite-Ankaramite Dyke Suite from the Seiland Province, Northern Norway Geology and geochemistry of a metamorphosed picrite-ankaramite dyke suite from the Seiland province, northern Norway BRIAN ROBINS & MAHER A. TAKLA Robins, B. & Takla, M. A.: Geology and geochemistry of a metamorphosed picrite-ankaramite dyke suite from the Seiland province, northern Norway. Norsk Geologisk Tidsskrift, Vol. 59, pp. 67-95. Oslo 1979. ISSN 0029-196X. Dilational maficdykes intruded during and after the second phase of early Caledonian deformation in the Seiland province were emplaced into complex fracture systems. Despite subsequent metamorphism, some dykes retain chilled margins, internal boundaries reflecting several magmatic pulses, central concentrations of phenocrysts, ocelli, and ultramafic nodules. Olivine, aluminous clinopyroxene, labradoritic plagioclase, nickeliferous pyrrhotite, magnetite-hercynite ss., ferri-ilmenite, the absence of Ca-poor pyroxene, and groundmass olivine in the freshest dykes indicate alkali olivine basalt affinities. The dykes have a mean major-element composition in the alkali picrite range, and the chemical variation is explained by accumulation of olivine and clinopyroxene. Na and K variations appear to be inherited from a pre-accumulation history. Mean Rb/Sr, K/Rb, K/Sr, and Y/Nb ratios correspond with common basalts, Ca/Sr is high and K, Nb, Zr, Rb, and the REE are positively correlated. Normalized REE patterns vary in absolute enrichment, but are light-REE enhanced. The primary magmas were generated by varying degrees of partial metting of an inhomogeneous mantle. B. Rabins, Geologisk institutt, Avd. A, Universitetet i Bergen, Norway. M. A. Takla, Geology Department, Cairo University, Giza, A. R. Egypt. The Caledonian magmatic activity of the north­ yr-1 decay constant for Rb87 (Pringle & Sturt em and eastern parts of the Seiland province of 1969). northern Norway is believed to have evolved Tholeiitic basalt magmas were followed early from low-potassium tholeiites emplaced early in in the prolonged second phase of deformation the tectono-thermal cycle to more alkaline (D2) by the emplacement of suites of high-potas­ magmas intruded during the later stages of the sium calc-alkaline plutonics ranging from low-si­ plutonic development. This is reflected in the lica diorites to monzogranites. These were sub­ variations in composition and character of the sequently re-intruded by widespread gabbros major and minor intrusives of the region (Robins associated with pertbositic syenite derivatives & Gardner 1975). (e.g., Fig. l (3)), possibly representing the The earliest major intrusions are tholeiitic eruption of basalt magmas of transitional chem­ layered gabbros which were emplaced either istry (Robins & Gardner 1975). during the course of the first phase of deforma­ The involvement of magmas of alkali olivine tion (D1) of the late Precambrian to Cambrian basalt affinity during the late-D2 and post-D2 shelf-type sediments, or during the peak of the evolution of the Seiland province is suggested Barrovian almandine-amphibolite facies meta­ by: morphism which followed (Ramsay & Sturt 1963, Roberts 1968, Sturt 1969, Sturt & Taylor The mineralogy and chemistry of the late-D2 1972). An example of this dass of pluton, the layered 'clinopyroxene' gabbros, as exempli­ subject of a detailed petrological investigation, is field by the Rognsund intrusion (Fig. l (4)). the Hasvik gabbro (Fig. l (1)), which preserves a In particular, the aluminous and under­ typically tholeiitic chilled margin and cryptic saturated composition of their single pyroxene layering (Robins & Gardner 1974). Anatexis has been interpreted in terms of a nepheline­ within the contact metamorphic aureo1e of this normative basaltic parent (Robins & Gardner intrusion has been dated at 530 ± 35 m.y. by 1974); Rb/Sr whole-rock isochron using the 1. 39 X w-u The presence of tectonite-type lherzolite 68 B. Rabins & M. A. Takla NORSK GEOLOGISK TIDSSKRIFT l (1979) S b R Fig. I. Generalized geological map of the northem and eastern parts of the Seiland province (after Robins & Gardner 1974), PERlOOTITE showing the locations of the CUNOPYROXENE GABBRO$ sampled dyke complexes. The numbered intrusions referred to SYENOGABBROS in the text are as follows: l - THOLEIITIC GABBRO$ the Hasvik gabbro; 2- the DIORITES Lille Kufjord gabbro; 3 -the 10 METASEDIMENTS Seiland syenogabbro; 4-the Rognsund gabbro. nodules in occasional bojitic or picritic dykes, Robins 1972, 1974), suggests alkali olivine some of which were emplaced into the basalt magmatism. Rognsund gabbro before the development of the evolved alkaline rocks. The major-ele­ Definitive evidence of the intrusion of alkali ment geochemistry of three of the dykes also olivine basalt magmas during the later stages of supports alkali basalt-type activity (Robins the Caledonian plutonic evolution of the Seiland 1975); province is restricted to the three major-element The general contemporaneity of some basic analyses of the nodule-bearing dyke rocks men­ dykes with the late-orogenic alkaline rocks tioned above, and five major-element analyses of belonging to the alkaline pyroxenite-syenite the syn-D2 basic dykes emplaced into the Hasvik and nepheline syenite-litchfieldite suites, and gabbro (Fig. l) (Robins 1975, Robins & Gardner also with carbonatites (Sturt & Ramsay 1%5, 1975). NORSK GEOLOGISK TIDSSKRIFT l (1979) Picrite-ankaramite dyke suite from Sei/and 69 Fig. 2. Mafic dykes with dis tinet chilled margins truncating the deform ed rhythmic layering of the Rognsund gabbro and cut by a sheared amphibole syenite pegmatite, illustrating the type of criteria by which the emplacement of the dyke complexes rnay be dated. The petrological analysis of widespread late­ in the slightly earlier syenogabbros, the post- or D2 and post-D2 dyke swarms present in the late-D2 tholeiitic Lille Kufjord gabbro (Fig. l, Seiland area reported here, was undertaken with (2)), or the post-D2 nepheline syenite intrusion the specific aims of defining the affinities of the on Stjernøy. basic and ultrabasic magmas emplaced during In each case, the known relative age of the the later plutonic development of the Seiland host provides an upper age limit for the dykes province and to relate them, if possible, to the emplaced into it. Thus the dykes emplaced into synchronous major layered intrusions. An inter­ the syenogabbros or the perthositic syenite of pretation of dyke chemistry also necessitates the Stjernodden (BD 76-89) can be expected to span consideration of possible chemical changes in­ a wider age spectrum than those cutting the later duced by subsequent deformation and metamor­ nepheline syenite. All of the dykes studied are phism in the almandine-amphibolite and green­ believed to have been emplaced after the major schist facies. episode of folding beginning the second phase of regional Caledonian deformation. Only those dykes sampled from Oldervik (Hakkstabben) General description of the dykes (BD 90-95, 108-117) are from a metagabbro host and dyke complexes of syn-D1 age; many of these are relatively undeformed and cut irregular coarse-grained Timing of emplacement picritic bodies emplaced into the metagabbro during the second deformation phase. Dykes BD A variety of dyke complexes exposed along the 24-36 were, however, collected from a gabbro of coastlines of Seiland and Stjernøy were sampled unknown relative age (Robins 1971). A lower (Fig. l), the majority being situated within syn­ limit for the length of time during which the D2 clinopyroxene gabbros, with some emplaced sampled dyke complexes could have been em- 70 B. Rabins & M. A. Takla NORSK GEOLOGISK TIDSSKRIFT l (1979) o _j; � J> - - o · gi � " V>Q.>-. LOCAL OEFORMATION I BO P374, BO BO BO 90-95 376 76-89 52-59 1-23 108-117 96-100 67-75 BH 20, LB 32, 59-67 � �eridotites 67 SECONO .____,�=======: P7, 11 Clinopyroxene H9 PHASE OF gobbros OEFORMATION s Syenogobbros Colc-alkal ine intrusions Regional Metamorphic Tholeiitie gabbros Fig. 3. Summary diagram for the plutonic Peak evolution of the northern and eastern parts of the Seiland province, the maxi­ FIRST mum intervals of time available for the PHASE OF Tholeiitic gabbros emplacement of the sampled dykes, and OEFORMATION l the hosts to the various dyke complexes. N placed is determinable in those cases where dis­ tinctive later intrusions, such as members of the alkali ne suites, cut the earlier complexes. By this means, it can be established that the dykes BD 1-23 and 67-75 were intruded between the em­ placement of the Rognsund gabbro host and amphibole syenite pegmatites which cross-cut all the basic intrusions (Fig. 2). The majority of the • • dykes within the nepheline syenite were prob­ .. • ably emplaced during a much narrower time­ span, being post-dated by carbonatitic activity. X The maximum intervals of relative time available X for the development of the sampled dyke com­ � X 3. X plexes are illustrated graphically in Fig. X )( X XX x,(' X X X X External fe atures X X Basic dykes and dyke complexes, in varying states of preservation and of different ages, are • Pole to foliation ubiquitous features in both metasediments and s major intrusions alike in many parts of the " Pole to dyke Seiland province. At some localities, dyke com­ Fig. 4. Stereographic projection of poles to 50 dykes in the plexes reach an extraordinary density, though Grandnes dyke complex demonstrating their random orienta­ tion relative to that of the rhythmic layering and tectonic linear swarms are not developed on more than a fabric of their host. The absence of tlat-lying intrusions local scale. The orientation of dykes appears to may be a consequence of the essentially two-dimensional be a result of local stress fields combined with exposures. NORSK GEOLOGISK TIDSSKRIFT l (1979) Picrite-ankaramite dyke suite from Sei/and 71 Fig. 5. A general view of a typical dyke complex at Grandnes, the locality at which samples BD 1-23 were selected. Note the several generations of mafic dykes, their intricate form, and the deformed syenite pegmatites which cut all the intrusions. fracture patterns existing at the time of magma emplaced in a mesotype gabbro varying in colour ingress.
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