Carbonate – amphibole – clinopyroxene rocks and carbonatites from the Afrikanda complex, in the Kola Peninsula, Russia, contain a number of oxysalt minerals, including major calcite (15–95 vol.%), subordinate hydroxylapatite, ancylite-(Ce), calcio ancylite-(Ce), and minor burbankite, khanneshite, nyerereite, shortite, bradleyite, strontianite, britholite-(Ce) and barite. T hree mineral parageneses differing in the mode of occurrence of calcite are distinguished: (1) calcite – magnesiohastingsite – diops ide rock, (2) segregations of perovskite and titanite, and (3) calcite carbonatite. Cathodoluminescence studies document a complex evolutionary history of primary Sr-enriched calcite (0.6–1.4 wt.% SrO) involving late-stage resorption and replacement by a low Sr variety (<0.5 wt.% SrO). The presence of nyerereite, shortite, bradleyite, burbankite and khanneshite as solid inclusions in the early-crystallized minerals (primarily oxides and hydroxylapatite) indicates initially high activities of Na in the system. The transition from nyerereite (inclusions in magnetite) to shortite (in perovskite) signifies evolution of the carbonatite system toward Ca-enriched compositions. Crystallization of ancylite-(Ce) and calcio-ancylite-(Ce) is related to low-temperature hydrothermal processes, whereas burbankite and khanneshite probably represent primary magmatic phases. Low-temperature (200–250 °C) hydrothermal alteration accompanied by isotope-exchange processes produced variations in the oxygen isotopic composition of the Afrikanda rocks ( 18 O in the range 9.3 to 12.1‰ SMOW). Subtle variations in the isotopic composition of carbon ( 13 C in the range –2.5 to –1.7‰ PDB) suggest interaction with a meteoric-hydrothermal fluid with a low CO 2:H2O ratio. The observed high 13 C values of the calcite are consistent with heterogeneity of the mantle beneath the Kola Craton; the heterogeneity probably was due to a subduction-related source of contamination.

Calcite – amphibole – clinopyroxene rock from the afrikanda complex, kola peninsula, russia: mineralogy and a possible link to carbonatites. ii. oxysalt minerals

Nd, Pb and Sr isotopic data from nephelinite lavas from the Tertiary nephelinite-carbonatite complex of Napak, eastern Uganda, show large isotopic variations that can only be attributed to open-system behaviour. Possible explanations of the data include mixing between nephelinitic melts derived from an isotopically heterogeneous mantle, or interaction between a HIMU melt and mafic granulites. In both models crystal fractionation, involving olivine and clinopyroxene, played an important role. Major element chemistry, textural evidence and isotopic data from clinopyroxene phenocrysts from the olivine-bearing nephelinites, suggest that the pyroxenes did not crystallize from their host liquids. The isotopic data from the clinopyroxene phenocrysts support an interpretation of crystal fractionation in an open magma system that was undergoing continuous isotopic change. This study emphasises the importance of using combined isotopic data from both whole rock and mineral phases to interpret the evolutionary history of a single eruptive centre.

Nd, Pb and Sr isotopic data from the Napak carbonatite-nephelinite centre, eastern Uganda: an example of open-system crystal fractionation

Samples of olivine nephelinite from Napakvolcano,eastern Uganda, containtwo populations of clinopyroxenephenocrysts, one chromian and the othertitanium-bearing aluminian diopside. The chromian diopsidecrystalsareamongthe most magnesian to occur in nephelinite. The diopside phenocrysts show normal, reverse and oscillatory zoning, and some have an embayed texture suggestiveof resorption. Disequilibrium in Nd, Pb,andSr isotopicratios forboth populations ofclinopyroxeneand their host rocks is interpreted in terms of crystallization in an open system. Populations of chromian and Ti-bearing aluminian diopside from the same lavas have distinct isotopic ratios, indicativeof derivation and crystallization from different melts. We attribute the zoning, tcxtural features, and bimodality of the Napak diopsides to the mixing of at least two nephclinitic magmas at a shallow crust.il level.

Diopside phenocrysts from nephelinite lavas, Napak Volcano, eastern Uganda; evidence for magma mixing

Insights into the evolution of an alkaline magmatic system: An in situ trace element study of clinopyroxenes from the Ditrău Alkaline Massif, Romania

The Alnö igneous complex intruded local migmatites and granite country-rocks of Svecofennian age at *580 Ma and produced a suite of early alkaline silicate intrusives (e.g., ijolites, nepheline syenites) and dykes (e.g., trachytes, phonolites) and later a suite of carbonatite sheet intrusions for which Alnö is widely known. The Alnö igneous complex displays an older and deformed northern ring complex and the main “southern” ring complex. An explosive carbonatite vent with melilitite lapilli is apparently a late event, as are kimberlitic alnöite dykes. The latter seem not to be diamantiferous. Carbonatites are rare carbonate-rich magmatic rocks that are usually associated with alkaline silica-undersaturated intrusions. Although they make up a very small portion of the crust’s volume, they occur in all continents and from Archaean to the present and are of great relevance for our understanding of crustal and mantle processes. The Alnö complex is one of only several tens of known alkaline and carbonatite ring-intrusions in the world (see Appendix 1) and is the type-locality for the rock type alnöite (“porphyritic trap” of Hisinger 1808; “melilite-basalt” of Törnebohm 1883; and “alnöite” Rosenbusch 1887). The Alnö intrusion was emplaced at 584 ± 7 Ma (Meert et al. 2007) into Palaeoproterozoic country-rock and comprises plutonic and alkaline dyke rocks, carbonatites, and ultramafic lamprophyres. In addition, a carbonatite vent-breccia with accretionary lapilli provides evidence for explosive volcanic activity (von Eckermann 1960b; Kresten 1990). Alnö’s deeper plumbing system, and that of most other carbonatite ring-complexes, remains poorly constrained, however, leaving us with considerable uncertainties as to how exactly the plutonic and the volcanic environments in carbonatite centres are connected (see e.g. Andersson et al. 2013). © Springer International Publishing AG, part of Springer Nature 2018 P. Kresten and V. R. Troll, The Alnö Carbonatite Complex, Central Sweden, GeoGuide, https://doi.org/10.1007/978-3-319-90224-1_1 1 Discussions on the genesis, origin and emplacement of carbonatite have been ongoing for many decades (e.g. von Eckermann 1948, 1960a, 1960b; Le Bas 1977; Kresten 1980, 1990; Treiman and Essene 1985; Gittins 1988; Phipps 1988; Wallace and Green 1988; Keller and Hoefs 1995; Vuorinen and Skelton 2004; Mitchell 2005; Fischer et al. 2009; Rukhlov and Bell 2010). Carbonatites frequently occur in ring-complexes, and have been related to stable cratonic regions, orogenies, rifting and extension within continental margins, and to mantle plumes (Le Bas 1977; Gittins 1988; Phipps 1988; Wallace and Green 1988; Meert et al. 2007; Fischer et al. 2009; Rukhlov and Bell 2010). They have been found in all continents with the Kaiserstuhl volcanic complex in the Rhine graben (Le Bas 1977; Keller 1989) and the Oldoinyo Lengai volcano in the East African Rift Valley (Fischer et al. 2009; Mattsson et al. 2014) being the youngest examples. The latter is the only known active carbonatite volcano, albeit of unique composition (Dawson 1962; Rukhlov and Bell 2010). As most carbonatites are known from plutonic complexes and only a few volcanic examples are available, there is a significant gap in our understanding of the plutonic-eruptive association for this unusual group of rocks as plutonic and eruptive episodes are rarely preserved side by side (Gernon et al. 2012). The plutonic and volcanic occurrence of carbonatites at Alnö (von Eckermann 1948; Kresten 1990) thus provides an opportunity to further our understanding of magma storage and transport in carbonatite ring centres. Indeed, the discoveries of carbonatite occurrences around the world have gone from 56 to around 530 between 1987 and today, and 49 of them are considered extrusive. They extend back to the Archean and the oldest carbonatites that have been found to date are more than 2000 million years old and are located in South Africa and Finland. About 40 of the extrusive carbonatite occurrences are dominated by calcite, 7 by dolomite and the remaining two are known to be of alkaline natrocarbonatite composition (Mitchell 2005; Jones et al. 2013). Moreover, the number of occurrences of carbonatites has increased with decreasing age, which indicates that carbonate formation has probably become more common with time and that the conditions for carbonatite formation become more widespread (Jones et al. 2013). However, critics of this scheme argue that preservation of old carbonatites may not be too likely, thus implying a possibly continuous amount of carbonatite occurrences through geological time. The mantle plume model is a popular model but recent discoveries of strong repeated lithospheric controls over the course of carbonatite ages have been made. Newer discoveries of carbonatite-silicate volcanism from the Neogene to the Quaternary, comprising occurrences in France, Spain and Italy allow researchers to further our knowledge about extrusive carbonatite volcanism and its mantle origins 2 1 An Introduction to Carbonatites and Carbonatite Complexes

The Alnö Carbonatite Complex, Central Sweden

Compositional data are presented for pyroxenes from the undersaturated rocks of the Fen alkaline complex. Four groups of pyroxenes are present: (l) Al-Na diopsides rich in CaAlrSiOu, forming the cores of phenocrysts in damkjernite; (2) Ti-Al salites rich in CaTiAlrO., occuring as rims to phenocrysts and as strongly zoned discrete crystals in damkjernite and vibetoite; (3) an acmitic-hedenbergite series found in urtites, ijolites, and silicocarbonatites; (4) an acmitic series characteristic only of the later stages of urtite crystallization. Damkjernites are considered to have evolved at high temperatures over a very wide pressure range and the closely related vibetoites at relatively lower pressures and temperatures under HrO-rich conditions. Other rocks in the complex formed at low pressures under COr-rich conditions, possibly over a range of temperatures. Pyroxene and mica compositions indicate that urtite is not simply related to ijolites by fractional crystallization and that slvites are unlikely to be the end product of extensive crystallization of the ijolite-silicocarbonatite series. The Fen complex is considered to represent the root ofan undersaturated volcano and the rocks present are the result of the crystallization under different pressures, temperatures, and volatile conditions of several batches of the same undersaturated (nephelinite or alkali basalt) magma. Thus none of the rocks currently exposed at Fen are related by in-situ ftactional crystallization of a single batch of magma.

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Papers

Pyroxenes of the Fen alkaline complex, Norway