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In many alkaline complexes, large amounts of ultramafic rocks occur together with carbonatites, melilitolites and other alkaline silicate rocks. There is an ongoing debate if and how these contrasting lithologies were formed by differentiation of a common, mantle-derived silicate magma or rather by metasomatic processes between carbonatite and country rocks. In order to find petrological evidence for one or the other, two key examples, the Gardiner (E Greenland) and Kovdor (Russia) complexes are compared in this study. Despite their similar tectonic setting and succession of rock types, they show significant differences in the texture and mineral composition of ultramafic rocks. Ultramafic rocks from Kovdor include calcite- and biotite-rich dunites and pyroxenites without typical cumulate textures. They consist of Ni-poor forsterite, Cr-poor diopside and Ni-Cr-poor spinel and are possibly metasomatic reaction products between mantle-derived carbonatite melts and silicic host rocks. Similar ultramafic rocks are associated with carbonatites e. g. at Palabora (South Africa), Afrikanda (Russia), and Salitre (Brazil). In contrast, the ultramafic rocks from Gardiner show well-preserved cumulate textures and consist of Ni-rich forsterite, Cr-rich diopside as well as Cr-Ni-Ti-rich spinel and also contain F-Cl-rich apatite. They record an increase in aSiO2 from dunite to pyroxenite at similar f O2 (ΔFMQ ~ +1.2, with FMQ = fayalite-magnetite-quartz buffer), indicating that these rocks represent cumulates of an evolving, moderately oxidized mafic melt derived from a Ti-rich mantle source, similar to other rocks of the North Atlantic igneous province. In contrast to systems like Kovdor where carbonatite metasomatism is likely dominant, Ti-rich parental silicate magmas can abundantly crystallize Ti phases, as recorded by massive perovskite cumulates in Gardiner melilitolites. This can effectively scavenge HFSE from the magmatic system early in its evolution and likely explains HFSE-barren carbonatites at Gardiner, while those from Kovdor are highly HFSE-enriched. In summary, the results of our study provide strong textural and mineral chemical evidence that ultramafic rocks in alkaline complexes can be of both cumulate and metasomatic origin; the specific type has an important bearing on their HFSE enrichment and on the types of ores present in such complexes. [ABSTRACT FROM AUTHOR]

The origin of magmas that formed the Bushveld Complex remains highly debated in spite of many decades of intense research. Previous geochemical–petrological studies have shown a strong mantle derivation resulting ultimately in highly economic ore bodies of platinum group elements and chromium. However, geochemistry also points to the contribution of a significant crustal component, which may have been derived singly or in combination from a number of different sources. These include subcontinental lithospheric mantle that was enriched prior to Bushveld magma formation, possibly by subduction, assimilation of lower and upper crust during magma ascent, and contamination during magma chamber accretion within sedimentary rocks of the enclosing Transvaal Supergroup. In this study, the contributions of these various reservoirs will be evaluated by employing Hf isotopic data of well-characterized zircon grains in mafic, felsic and metasedimentary rocks, together with Zr–Hf bulk-rock compositions. The results reveal that magmatic zircon grains in mafic cumulate rocks from the floor to the roof of the c. 9 km thick Rustenburg Layered Suite (RLS) show essentially the same variations in εHf2·055 Ga from −7·5 to −10·2 as those of metamorphic zircon grains and overgrowths in the immediate surrounding quartzite and metapelitic rocks, as well as in granitic melt batches, granophyres, and the upper Rooiberg volcanics. The same values are also obtained by estimating the average Hf isotopic compositions of detrital zircon grains in many quartzite and metapelitic rocks from the surrounding Magaliesberg (εHf2·055 Ga = −6·2 to −10·8, six samples, maximum deposition age at 2080 Ma) and Houtenbeck formations (εHf2·055 Ga = −7·1 to −8·9, three samples, maximum deposition age at 2070 Ma), and by a six-point isochron of a garnet-schist from the Silverton Formation (εHft = −6·6 ± 0·7; age = 2059·4 ± 2·7 Ma). Zircon morphologies, zoning patterns, Hf isotopic data and petrological constraints furthermore reveal that metamorphic zircon was precipitated from aqueous fluids and/or felsic melts at temperatures between 550 and 900 °C, and that the Hf isotopic composition became homogenized during fluid transport in the contact aureole. However, results of numerical modelling indicate that fluid infiltration had only a minor effect on the Zr–Hf budget and Hf isotopic composition of the RLS, and that these parameters were mainly controlled by the mixing of melts derived from three major sources: (1) the asthenospheric mantle (>20 %); (2) enriched subcontinental lithospheric mantle (<80 %); (3) assimilation of significant amounts of crust (up to 40 %). The modelling furthermore suggests that assimilation of lower Kaapvaal Craton crust was minor (<15 %) during B1 (high-Mg andesite) magma formation, but up to 40 % during B3 (tholeiite) magma formation. The minor variation in εHft of zircon throughout the entire stratigraphy of the RLS resulted from the interplay of three dominant contributing factors: (1) intrusion of hot (>1200 °C) mantle-derived magmas with relatively low Zr–Hf concentrations having a similar εHf2·055 Ga of −8·5 ± 1·9 to that of upper crust rocks surrounding the RLS; (2) significant assimilation of volcanic and metasedimentary rocks with high Zr–Hf concentration; (3) mingling, mixing and/or diffusive exchange of Zr and Hf between crust and mantle-derived melts and aqueous fluids prior to late-magmatic crystallization of zircon at temperatures between 700 and 900 °C. This study shows that the combination of Zr–Hf bulk-rock data with Hf isotopic data of well-characterized zircon grains provides a powerful tool to quantify various mantle and crustal reservoirs of mafic layered intrusions, and allows new insights into magma chamber and related contact metamorphic processes. [ABSTRACT FROM AUTHOR]

The orogenic peridotites and pyroxenites of the Ulten Zone (north Italy) record multistage metasomatism by crust-derived melts and fluids within a Late-Variscan mantle wedge. We acquired new major and trace element data as well as garnet and whole-rock iron speciation for a representative suite of samples, with the aim to further constrain element cycling and the redox state attending the development of the major mineralogical and textural rock types that occur within the Ulten Zone. Initially, spinel peridotites were refertilized by mafic melts in the hot and shallow mantle wedge, followed by garnet formation as the peridotites were carried towards a cool, subducting slab of continental crust by corner flow. Upon exhumation, ingress of aqueous, crust-derived fluids provoked amphibole-forming reactions, which caused gradual consumption of garnet and clinopyroxene and transformation from coarse- to fine-grained assemblages. Since Si, Al, Na, Ti, Ca and HREE formerly stored in reactants were not fully accommodated in newly-formed phases, these elements were partially removed from the bulk-rock, generating more depleted compositions resembling residues after partial melting. Unexpectedly, the remaining garnet retains low Fe3+/ΣFe (<0·046) even after the bulk-rocks became strongly enriched in Fe3+ during metasomatism and retrogression (Fe3+/ΣFe = 0·11–0·23), which was mostly stored in coexistent amphibole and interstitial serpentine. Low Fe3+/ΣFe in garnet is consistent with Δlog f O2 = FMQ-1·7 to FMQ-0·3 at 2 GPa. Combined with previous studies, this is evidence for garnet growth within a heterogeneously oxidized mantle wedge, reflecting a variable extent of percolation by oxidizing aqueous fluids. During metasomatism, concomitant variable enrichment in LILE, LREE and some HFSE, and significant compositional differences between sampling localities, reflect both variable fluid/rock ratios at small spatial scales but also indicate chromatographic effects that likely relate to different positions relative to the subducting crust releasing fluids into the mantle wedge. Hydration by dilute fluids during retrogression did not result in additional enrichment in fluid-mobile elements, but caused further replacement of garnet and clinopyroxene. This study highlights the control that changing mineralogies, developed in response to interaction with various crustal melts and fluids under variable pressure-temperature and redox conditions in a continental subduction zone, exert on the retention or release of major and trace elements in peridotite. In particular, formation and/or persistence of amphibole and dolomite, as documented in the present study, suggest that the subduction-modified mantle wedge is an efficient trap for volatiles and fluid-mobile elements. [ABSTRACT FROM AUTHOR]