Petrogenetic processes in the ultramafic, alkaline and carbonatitic magmatism in the Kola Alkaline Province: a review

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 silica-undersaturated 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/Al2O3 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

Petrogenesis of Archean PGM-bearing chromitites and associated ultramafic-mafic-anorthositic rocks from the Guelb el Azib layered complex (West African craton, Mauritania)

The Archean Guelb el Azib layered complex (GAC) in the West African craton of Mauritania is composed of an association of serpentinites, chromitites, amphibolites and anorthosites with few fine-grained amphibolite dykes. The complex forms tectonic slices in 2.9-3.5. Ga TTG gneiss terrains in close association with supracrustal rocks (BIFs, impure marbles, amphibolites). It was affected by a main granulite-facies grade metamorphism (up to 900. °C at 5-6. kbar) with subsequent retrogression in amphibolite and greenschist facies conditions. The preserved igneous macrostructures, the mineral compositions and the nature of relic magmatic assemblages have been used to constrain the composition of the parental melts and the conditions of crystallization. According to petrological observations and to comparison with experimental data, the formation of the complex can be explained by fractionation of a slightly hydrous high-alumina basaltic melt at low pressure. The early fractionation of olivine and the absence of massive clinopyroxene fractionation before plagioclase saturation led to crystallization of highly calcic plagioclase with Fe-, Al-rich but Cr-poor chromite from a hydrous tholeiitic parental magma, similar to worldwide Archean tholeiites. The complex shares many similarities with Archean anorthosite layered complexes, possibly formed in a supra-subduction zone environment according to results obtained on similar 2.9-3.0. Ga complexes from Greenland and India (namely Fiskenaesset and Sittampundi). Three phases of PGE mineralization affected the GAC chromitites: (i) igneous crystallization of laurite; (ii) formation of late magmatic IPGE sulpho-arsenides (irarsite-hollingworthite) and (iii) hydrothermal Pt-Pd mineralization represented by sperrylite and rustenburgite. © 2012 Elsevier B.V

A Sr-, Nd- and Pb-isotopic investigation of the transition between two megacyclic units of the Bjerkreim-Sokndal layered intrusion, south Norway

The Bjerkreim-Sokndal layered intrusion forms part of the Proterozoic South Rogaland Igneous Complex of southwest Norway. The intrusion has been divided into two parts; an anorthositic to gabbro-noritic Layered Series, and a generally more massive mangeritic, quartz-mangeritic to charnockitic upper part. On the basis of repeated lithological sequences, the Layered Series has been divided into a number of megacyclic units which are interpreted as reflecting magma chamber replenishment. Initial (940 Ma) Sr, Nd and Pb isotope ratios for plagioclases separated from samples crossing the transition between megacyclic unit III and megacyclic unit IV range from 87Sr/86Sr(i) = 0.7048, 143Nd/144Nd(i) = 0.51156 (ε(Nd) = +2.6) and 206Pb/204Pb(i) = 17.50; to 87Sr/86Sr(i) = 0.7061, 143Nd/144Nd(i) = 0.51143 (ε(Nd) = +0.1) and 206Pb/204Pb(i) = 17.73. The isotopic compositions vary systematically with stratigraphic height, and the interplay between the three isotopic systems is used to divide the stratigraphic profile into two parts: replenishment and post-replenishment. Across the replenishment interval isotopic variations are consistent with mixing between a resident and a replenishing magma. The replenishing magma was characterized by 87Sr/86Sr(i) = 0.7048, 143Nd/144Nd(i) = 0.51156 (ε(Nd) = +2.6) and 206Pb/204Pb(i) = 17.50. The positive ε(Nd) of this magma indicates that its source region was either in the mantle or was itself recently derived from the mantle. In contrast the resident magma had 87Sr/86Sr(i) = 0.7061; 143Nd/144Nd(i) = 0.51143 (ε(Nd) = +0.1) and 206Pb/204Pb(i) = 17.73 indicating that it had assimilated crustal material. In addition, the isotopic correlations across the replenishment interval indicate that prior to mixing the concentrations of Nd and Pb in the resident magma had been strongly reduced, probably by fractionation of apatite. Above the replenishment interval, Nd and Pb isotopic variations are apparently decoupled from Sr isotopic variations. Two mechanisms are proposed for achieving this. The first is heterogeneous mixing of up to 20% of a third magmatic component into an already isotopically stratified magma chamber. The second is the presence of 10%-20% of post-cumulus plagioclase that crystallized from a migrating intercumulus melt. (C) 2000 Elsevier Science B.V. All rights reserved