The magmatic and fluid evolution of the motzfeldt intrusion in South Greenland: Insights into the formation of agpaitic and miaskitic rocks

Abstract

The 1.275 Ga Motzfeldt intrusive complex in the Gardar failed-rift Province in South Greenland formed from six successively intruding melt batches (SM1-SM6) interpreted to be derived from a common magma source at depth. Five units (SM1-SM5) crystallized an alkaline to peralkaline, miaskitic mineral assemblage of amphibole, clinopyroxene, feldspar, nepheline, Fe-Ti oxides, zircon, apatite, fluorite and rarely olivine. The last magmatic batch (SM6) is characterized by an agpaitic mineral assemblage of aegirine, nepheline, alkali-feldspar, eudialyte and rare fluorite or sodalite. Coexisting mafic minerals constrain the crystallization conditions of the miaskitic rocks to about 850-600°C, whereas solidus temperatures below 500°C are indicated by coexisting alkali feldspars in the agpaitic rocks. Oxygen fugacities during the orthomagmatic stage are below the FMQ (fayalite-magnetite-quartz) buffer (ΔFMQ -0.5 to -2.0) whereas late hematite provides evidence of a higher relative oxygen fugacity during late-stage alteration. The Nd and oxygen isotope compositions of amphiboles and pyroxenes are homogeneous throughout the complex and suggest a common, mantle-derived magma source for all six units which is comparable with other Gardar intrusions. The hydrogen isotopic composition of amphiboles (δD-99 to -132‰) indicates low-temperature fluid-rock interaction with low fluid-rock ratios. Fluid inclusion studies indicate that H2O-NaCl fluids present during the magmatic stages in the miaskitic units had salinities of <10 wt %NaCl eq. Calcite crystals in fluid inclusions within these rocks suggest that CO2 or HCO3- was an important component of the original fluid phase. In contrast, the agpaitic unit is characterized by a CH4-H2O-NaCl fluid. The C-O-H isotope compositions of the fluid inclusions in all units are consistent with mixing between a small volume of magmatic fluid and a large volume of meteoric water. The chemical evolution of the Motzfeldt complex is a type example of the connection of the transition from miaskitic to agpaitic mineral assemblages with redox-dependent fluid-solid equilibria. The transition from a relatively oxidized to a relatively reduced fluid is correlated with a change from a more reduced, Fe2+-bearing miaskitic (taramite-arfvedsonite, zircon, SM1-SM5) to a more oxidized, Fe3+-bearing agpaitic assemblage (aegirine, eudialyte, SM6). We suggest that coupled fluid-solid redox equilibria involving Fe2+, Fe3+, CO2 and CH4 in this case were simply driven by temperature decrease and an overall increase in Na (+ K) in the melt. This observation sheds light on the heavily debated miaskite-agpaite transition. The combined temperature and compositional effect stabilized Fe3+, CH4 and enhanced the solubility of Zr (possibly as Na-Zr-Si-O complexes) in the latest stage melt (SM6) resulting in the crystallization of an agpaitic mineral assemblage. In terms of redox conditions during crystallization, the Motzfeldt rocks represent an intermediate case in the Gardar Province between more oxidized, CO2-dominated intrusions such as the syenite-carbonatite complex of Grønnedal-Ika and CH4-dominated, more reduced complexes such as the peralkaline granitic Puklen and the agpaitic Ilímaussaq complex. © The Author 2008. Published by Oxford University Press. All rights reserved.