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The Egersund dike swarm of SW Norway is made up of 11 basaltic dikes trending ESE-WNW. Two groups are defined: porphyritic dikes with bytownite phenocrysts of tholeiitic affinity and aphyric dikes of tholeiitic to alkaline affinity. Baddeleyite in the most alkaline dike gives a U-Pb intrusive age of 616 Ϯ 3 Ma. The swarm is parallel to the Neoproterozoic southwestern (present-day orientation) Tornquist margin of Baltica. It is coeval with the Long Range swarm of Labrador which is parallel to the southeastern proto-Appalachian margin of Laurentia. Both swarms are related to rifting that resulted in the opening of Iapetus ocean. Chronocorrelation of Neoproterozoic rift-related magmatic suites in western Baltica and eastern Laurentia suggests diachronic opening of Iapetan oceanic basins at ca. 610 Ma along the northwestern Baltoscandian margin of Baltica and at ca. 550 Ma along the proto-Appalachian margin of Laurentia and Tornquist margin of Baltica.

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compositions (Nd 750 = +5•46 to −0•87; I Sr 750 = 0•7021-The Seychelles islands consist of undeformed and unmetamorphosed, 0•7061) that can be modelled as basaltic magmas derived from metaluminous monzogranites and granodiorites of Neoproterozoic depleted mantle, variably contaminated (0-15%) by Archaean age (>750 Ma). Subsolvus, and lesser hypersolvus granitoids are silicic crust. All petrologic, petrographic, geochemical, isotopic and crosscut by coeval dolerite dykes, dominantly of olivine tholeiite chronologic data for Neoproterozoic magmatic rocks of the Seychelles, composition. Field relations suggest that mixing between granitoid coupled with palaeomagnetic data indicating its position at the and doleritic magmas generated a variety of minor intermediate margin of the Rodinia supercontinent at >750 Ma, are at least rocks that occur as irregular masses and enclaves; their compositions consistent with, if not suggestive of, a continental or Andean-type plot as linear arrays between those of dolerites and granitoids. Two arc setting. We argue, therefore, that the conventionally accepted groups of granitoids can be distinguished based on colour, chemistry notion of an extensional (i.e. rift or plume) setting for Seychelles and isotopic signature. Mahé Group granitoids are grey, with magmatism is vulnerable. relatively low incompatible element concentrations and primitive isotopic signatures that cluster at Nd 750 = +2•85 ± 0•17 and I Sr 750 = 0•7031 ± 0•0008 (some samples with impossibly low I Sr <0•700 were affected by open-system processes). Praslin Group granitoids are characteristically red to pink, with Rb >180 ppm, KEY WORDS: Seychelles; Neoproterozoic; granitoid; Andean-type arc; U >4•2 ppm, Th >20 ppm, Pb >30 ppm, and correspondingly petrogenesis evolved and variable isotopic signatures (Nd, T = +0•80 to −3•71 and I Sr, T = 0•7032-0•7263). Both groups of granitoids appear to have been derived from a mixed source dominated by a juvenile, mantle-derived component, with variable amounts of an ancient, possibly Archaean, silicic source constituent or contaminant. INTRODUCTION Potential source materials are unexposed in the Seychelles, but the The Seychelles (Figs 1 and 2), correctly interpreted by juvenile component may resemble >750 Ma intermediate to mafic both Wegener (1924) and Du Toit (1937) as a continental metavolcanic rocks of NW Madagascar or NE India; candidates fragment left behind after Gondwana break-up, have for the ancient silicic component, whose signature is slightly enhanced long been considered as a geological curiosity, repin granitoids of the Praslin Group, may be represented by 2•5-3•3 resenting small, but conspicuous granitic islands in the Ga tonalitic gneisses of the Banded Gneiss Complex in Rajasthan Indian Ocean, at least 1000 km from the prominent (NW India), or by similar materials present in central-northern continental land masses of Madagascar or Africa. About 25 of the >100 islands in the Seychelles archipelago Madagascar. Seychelles dolerites show variable Nd and Sr isotopic

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Igneous rocks of the Devonian Kola Alkaline Carbonatite Province (KACP) in NW Russia and eastern Finland can be classified into four groups: (a) primitive mantle-derived silicaundersaturated silicate magmas; (b) evolved alkaline and nepheline syenites; (c) cumulate rocks; (d) carbonatites and phoscorites, some of which may also be cumulates. There is no obvious age difference between these various groups, so all of the magma-types were formed at the same time in a relatively restricted area and must therefore be petrogenetically related. Both sodic and potassic varieties of primitive silicate magmas are present. On major element variation diagrams, the cumulate rocks plot as simple mixtures of their constituent minerals (olivine, clinopyroxene, calcite etc). There are complete compositional trends between carbonatites, phoscorites and silicate cumulates, which suggests that many carbonatites and phoscorites are also cumulates. CaO/Al 2 O 3 ratios for ultramafic and mafic silicate rocks in dykes and pipes range up to 5, indicating a very small degree of melting of a carbonated mantle at depth. Damkjernites appear to be transitional to carbonatites. Trace element modelling indicates that all the mafic silicate magmas are related to small degrees of melting of a metasomatised garnet peridotite source. Similarities of the REE patterns and initial Sr and Nd isotope compositions for ultramafic alkaline silicate rocks and carbonatites indicate that there is a strong relationship between the two magma-types. There is also a strong petrogenetic link between carbonatites, kimberlites and alkaline ultramafic lamprophyres. Fractional crystallisation of olivine, diopside, melilite and nepheline gave rise to the evolved nepheline syenites, and formed the ultramafic cumulates. All magmas in the KACP appear to have originated in a single event, possibly triggered by the arrival of hot material (mantle plume?) beneath the Archaean/Proterozoic lithosphere of the northern Baltic Shield that had been recently metasomatised. Melting of the carbonated garnet peridotite mantle formed a spectrum of magmas including carbonatite, damkjernite, melilitite, melanephelinite and ultramafic lamprophyre. Pockets of phlogopite metasomatised lithospheric mantle also melted to form potassic magmas including kimberlite. Depth of melting, degree of melting and presence of metasomatic phases are probably the major factors controlling the precise composition of the primary melts formed.

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