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 850^6008C, whereas solidus temperatures below 5008C are indicated by coexisting alkali feldspars in the agpaitic rocks. Oxygen fugacities during the orthomagmatic stage are below the FMQ (fayalite^ magnetite^quartz) buffer (FMQ ^0 � 5t o ^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 (dD^99 to ^132%) indicates lowtemperature 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 of510 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^ NaClfluid.The C^O^H isotope compositionsofthefluid 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.Thetransitionfromarelatively oxidized to a relatively reduced fluid is correlated with a change from a more reduced, Fe 2þ -bearing miaskitic (taramite^arfvedsonite, zircon, SM1^SM5) to a more oxidized, Fe 3þ -bearing agpaitic assemblage (aegirine, eudialyte, SM6).We suggest that coupled fluid^solid redox equilibria involving Fe 2þ ,F e 3þ ,C O2 and CH4 in this case were simply driven by temperature decrease and an overall increase in Na (þ K) in the melt.This observation sheds light on the heavily debated miaskite^ agpaite transition.The combined temperature and compositional effect stabilized Fe 3þ ,C H 4 and enhanced the solubility of Zr (possibly as Na^Zr^Si^O complexes) in the latest stage melt (SM6) resulting in the crystallization ofan agpaitic mineral assemblage. In terms of redox conditions during crystallization, the Motzfeldt rocks represent an intermediate case in the Gardar Province between more oxidized, CO2dominated intrusions such as the syenite^carbonatite complex of