Geochemically, the large family of alkaline plutonic rocks (both Qtz-undersaturated and -oversaturated compositions) can be subdivided into metaluminous [(Na₂O + K₂O) < Al₂O₃] and peralkaline [(Na₂O + K₂O) > Al₂O₃] types. In this paper, we discuss two important aspects of the mineralogical evolution of such rocks. With respect to their Fe–Mg phases, a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite (± ilmenite). The relative stability of these phases is controlled by oxygen fugacity and Na activity in the crystallizing melts. If Na activity in the melt is high enough, arfvedsonite + aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions. Major mineralogical differences within this rock group exist with respect to their high field strength element (HFSE)-rich minerals: most syenitic rocks, known as miaskites, contain zircon, titanite or ilmenite as HFSE-rich minerals, whereas in agpaites complex Na–K–Ca–(Ti, Zr) silicates incorporate the HFSE. Similarly, only a small group of peralkaline granites are found to lack zircon, titanite or ilmenite but instead contain Na–K–Ca–(Ti, Zr) silicates. Here, we present a detailed phase petrological analysis of the chemical parameters (µNa₂O, µCaO, µK₂O) that influence the transition from miaskitic to agpaitic rocks. Based on the occurrence of Ti and Zr minerals, several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished: a high-Ca trend, which is exemplified by the alkaline rocks of the Kola Province, Russia, and a Ca-depletion trend, which is displayed by the alkaline rocks of the Gardar Province, South Greenland. Both trends show significant Na-enrichment during magmatic evolution. High-Ca agpaites evolve from nephelinitic parental melts that did not crystallize large amounts of plagioclase. In contrast, agpaites showing Ca-depletion originate by extensive fractionation of plagioclase from basaltic parental melts. In some peralkaline granites evolutionary trends are observed that culminate in agpaite-like HFSE-mineral associations in the most evolved rocks.

The formation of peralkaline melts is generally recognized to be related to enrichment of the mantle in sodium or potassium during a metasomatic event that predated magma generation. The wide range in mineral assemblages, mineral compositions, and fluid compositions in peralkaline melts are, to a large extent, a function of the sodium/potassium content of these mantle-metasomatizing fluids, which also governs redox conditions. We propose that sodic fluids will reduce the mantle assemblage by depleting ferric iron from garnet to form the aegirine component in pyroxene. In contrast, potassic fluids would oxidize the mantle assemblage by extracting Al₂O₃ from the garnet to make phlogopite. We suggest that during differentiation of peralkaline melts, once Fe–Ti oxides have been depleted from the assemblage, simple crystallization reactions of common solid phases such as aegirine or arfvedsonite control the oxygen fugacity by equilibria such as Hence, the more persodic a melt is, the more aegirine will be crystallized and the more reduced this melt will be. This is because without a mineral donor for ferric iron, the crystal chemical forcing of the coupled crystallization of Na with Fe³⁺ drives oxidation of the dominant Fe²⁺ in the melt and thereby reduction of the C–H–S–O-bearing fluid and/or melt phase. This is in convincing agreement with observations from a variety of silica-undersaturated peralkaline complexes such as Ilimaussaq in Greenland, Mt. St. Hilaire in Canada, Lovozero and Khibina in Russia, and Tamazeght in Morocco, and explains the occurrence of magmatic methane in the most sodic of these complexes. In silica-oversaturated peralkaline melts such as comendites and pantellerites, however, this equilibrium appears to be less influential and these melts are accordingly less reduced. In perpotassic melts, in contrast, no K–Fe³⁺ pyroxene is stable and K can only be incorporated into dominantly Fe²⁺-bearing phases such as biotite or K-amphibole, if no K-feldspar is stable. Here, redox conditions in the coexisting fluid and/or melt are either unaffected or even driven to more oxidized values by the crystallization reactions, which is in accordance with observations on the ultrapotassic rocks of the Roman Province (Italy) or the kalsilite-bearing Murun complex in Siberia. We conclude that peralkalinity in magmatic rocks is tightly connected to oxygen fugacity, that both the generation and the evolution and crystallization of peralkaline melts are governed by redox equilibria, and that these depend strongly on the Na/K ratio as well as on the (Na + K)/Al ratio of the melt. The specific features of peralkaline rocks, such as the difference between agpaitic and miaskitic rocks, the unusual enrichment of high field strength elements and the occurrence of magmatic hydrocarbons in some of them, can all be explained by the oxygen fugacity-controlling model reactions discussed in this study.

The Eocene Tamazeght complex, High Atlas Mountains, Morocco is a multiphase alkaline to peralkaline intrusive complex. A large variety of rock types (including pyroxenites, glimmerites, gabbroic to monzonitic rocks, feldspathoidal syenites, carbonatites and various dyke rocks) documents a progression from ultramafic to felsic magmatism. This study focuses on the silicate plutonic members and the genetic relationships between the various lithologies. Based on detailed petrographic and mineral chemical data we show that the various units crystallized under markedly different oxygen fugacity and silica activity conditions and demonstrate how these parameters influence both the phase assemblage and the detailed chemical evolution of the fractionating phases. Nepheline, olivine–clinopyroxene and hornblende–plagioclase thermometry indicate equilibration temperatures ≥800°C for all major rock types. Highly oxidized conditions (close to the hematite–magnetite buffer) are characteristic of the garnet-rich pyroxenites, ultrapotassic glimmerites and associated olivine-shonkinites. The parental magmas to these rocks evolved from low initial a_SiO2 values of 0·1 to values of 0·5–0·8 during nepheline and alkali feldspar saturation. In contrast, the monzonitic rocks evolved from initially high a_SiO2 values (up to 0·75) down to about 0·1 at intermediate values of oxygen fugacity (ΔFMQ = +2–5 to −1, where FMQ is the fayalite–magnetite–quartz buffer). For nepheline syenites and malignites, more reduced conditions (ΔFMQ = −2) and intermediate a_SiO2 values (between 0·25 and 0·5) dominate. We conclude that fractional crystallization is not a likely mechanism to explain the large variety of lithologies present in the Tamazeght complex. It is more probable that successive melting of a compositionally heterogeneous mantle source region gave rise to several melt batches with distinct chemical and physico-chemical characteristics. Low-degree melts from a K-phase-bearing mantle domain resulted in the formation of ultrapotassic glimmerites, whereas garnet-rich pyroxenites and olivine-shonkinites may have originated from hybrid melts and partly from a pyroxene-dominated source. Less alkaline lithologies such as monzonites potentially reflect larger degrees of melting and the increased importance of a basaltic component, whereas nepheline syenites and malignites may be explained by lower degrees of melting and a more alkaline character for the parental melt of these rocks.