A global review on agpaitic rocks
2017-07-08

Peralkaline igneous rocks are defined by a molar (Na + K)/Al ratio > 1 and are subdivided into miaskitic and agpaitic varieties depending on their mineralogy. In the more common miaskitic types, rare earth elements (REEs) and high field strength elements (HFSEs) are largely stored in zircon and titanite, while agpaitic varieties contain a wealth of mostly halogen-bearing Na-Ca-HFSE minerals instead. Among those, minerals of the eudialyte, rinkite, and wöhlerite groups are the most common ones. The present review on the geological and mineralogical information available on agpaitic rocks provides a summary of the fluid inclusion record of miaskitic and agpaitic rocks as fluids play a key role in the evolution of peralkaline rocks. Magmas that crystallize peralkaline rocks are generally believed to originate from low-degree partial melting of geochemically enriched mantle lithologies, combined with prolonged differentiation processes at shallow crustal levels. Agpaitic and hyperagpaitic rocks (the latter containing appreciable amounts of water-soluble minerals) represent the most evolved stages of peralkaline systems. They form either parts of plutonic to subvolcanic composite magmatic complexes, which consist of several agpaitic and/or miaskitic intrusive units, or they occur as sills, laccoliths, domes, dykes, or even as lavas. However, as agpaitic rocks are notably rare compared to miaskitic rocks (about 100 vs. several thousand occurrences worldwide), their formation requires special conditions that are not generally met during the evolution of peralkaline rocks. Despite their rarity, agpaitic and hyperagpaitic rocks form important deposits of critical metals such as REE, Zr, Nb, and U and are interesting targets for otherwise rare elements such as F, Be, Sn, Zn, and Ga. The relative timing when magmas reach their agpaitic stage is highly variable. Magmatic–agpaitic assemblages can form only if early-magmatic crystallization conditions were reduced enough (low fO2) to enable subsequent Fe enrichment, an increase in peralkalinity, retention of halogens, and extreme enrichment of HFSEs in the evolving magmas, as only these contribute to the direct crystallization of agpaitic minerals. Late-magmatic interstitial agpaitic assemblages indicate that the required enrichment levels of the above-mentioned constituents were reached only during the final differentiation stages of magmas. Hydrothermal agpaitic assemblages precipitate from highly saline brines released from peralkaline magmas and are capable of transporting HFSEs. All three varieties of agpaitic assemblages occur in plutonic, subvolcanic, and volcanic rocks. Although many modern and detailed studies have dealt with plutonic–subvolcanic agpaitic rocks, most volcanic occurrences are insufficiently studied, mainly because of difficult outcrop or logistic situations. However, especially the volcanic examples raise questions on the details of why and how degassing of halogens at such shallow emplacement levels is sufficiently prevented to precipitate halogen-bearing agpaitic assemblages. Thus, a thorough geochemical and petrological investigation of such localities is desired. The presently valid definitions of agpaitic and miaskitic rocks are not appropriate, and therefore, alternative definitions are suggested. Similarly, the frequently used classification scheme for nepheline syenites must be abandoned as it is inconsistent and excludes very similar mineral assemblages observed in other partly even quartz-bearing rock types. Variable processes may produce sequences of mineral assemblages that belong to different groups of this classification. Therefore, we suggest that rather than dividing agpaitic rocks into specific subgroups, careful textural studies that distinguish early-magmatic, late-magmatic, and hydrothermal phase assemblages are warranted. This is the only way to understand the effect of various physicochemical parameters (such as P, T, fO2, aSiO2, aH2O, peralkalinity, and the activity of other compounds including halogens) during different evolutionary stages of these mineralogically and texturally diverse rocks.