Hyperagpaitic rocks are highly peralkaline nepheline syenites in which minerals such as ussingite [Na2AlSi2O8(OH)] and naujakasite (Na6FeAl4Si8O26) crystallize instead of, or in addition to, feldspars and feldspathoids; eudialyte is succeeded by steenstrupine-(Ce) and members of the lovozerite and lomonosovite groups; highly water-soluble minerals such as villiaumite (NaF) and natrosilite (Na2Si2O5) form part of magmatic mineral assemblages. Hyperagpaitic magmatic rocks in the Ilímaussaq alkaline complex in South Greenland include intrusive bodies of villiaumite- and naujakasite- bearing lujavrite (i.e. melanocratic, silica-undersaturated syenite), and late veins and pegmatites. The transition from agpaitic to hyperagpaitic magmatic conditions is controlled by increasing peralkalinity (or aNa2Si2O5) and changes in activities of water, halogens and phosphorus. The relative importance of these parameters can be evaluated by chemographic analysis based on observed mineral assemblages and mineral compositions. Highly peralkaline melts can crystallize agpaitic albite+microcline+nepheline+arfvedsonite+eudialyte±sodalite magmatic assemblages over a wide range of alkali, water and halogen activities. In most cases, a net increase in HF activity is needed to induce villiaumite crystallization, suggesting hyperagpaitic conditions, but because of the orientation of the villiaumite-saturation surface in log aNa2Si2O5-log aH2O-log aHF space, loss of an aqueous fluid may stabilize villiaumite without increasing peralkalinity. Naujakasite indicates elevated aNa2Si2O5, ussingite elevated aH2O, and steenstrupine-(Ce) and vuonnemite high aNa2Si2O5 and aP2O5. Closed-system fractionation of agpaitic magma may lead to hyperagpaitic residual liquids if aegirine, eudialyte or sodalite do not crystallize early, which is not applicable to the main line of magma evolution in Ilímaussaq, with abundantly eudialyte-bearing kakortokite and sodalite-bearing naujaite cumulates. Hyperagpaitic residual liquids can have developed if fractionation of these minerals were suppressed, or by open-system processes such as intraplutonic assimilation of sodalite-bearing cumulates in lujavritic magma at low aHCl.
Bibliographical noteArticle ends on p. 1363.
- agpaitic rocks
- hyperagpaitic rocks
- Ilímaussaq complex