Quantification of Magmatic and Hydrothermal Processes in a Peralkaline Syenite-Alkali Granite Complex Based on Textures, Phase Equilibria, and Stable and Radiogenic Isotopes

Abstract

The Puklen complex of the Mid-Proterozoic Gardar Province, South Greenland, consists of various silica-saturated to quartz-bearing syenites, which are intruded by a peralkaline granite. The primary mafic minerals in the syenites are augite ± olivine + Fe–Ti oxide + amphibole. Ternary feldspar thermometry and phase equilibria among mafic silicates yield T = 950–750°C, a SiO2 = 0·7–1 and an f O2 of 1–3 log units below the fayalite–magnetite–quartz (FMQ) buffer at 1 kbar. In the granites, the primary mafic minerals are ilmenite and Li-bearing arfvedsonite, which crystallized at temperatures below 750°C and at f O2 values around the FMQ buffer. In both rock types, a secondary post-magmatic assemblage overprints the primary magmatic phases. In syenites, primary Ca-bearing minerals are replaced by Na-rich minerals such as aegirine–augite and albite, resulting in the release of Ca. Accordingly, secondary minerals include ferro-actinolite, (calcite–siderite) ss , titanite and andradite in equilibrium with the Na-rich minerals. Phase equilibria indicate that formation of these minerals took place over a long temperature interval from near-magmatic temperatures down to ∼300°C. In the course of this cooling, oxygen fugacity rose in most samples. For example, late-stage aegirine in granites formed at the expense of arfvedsonite at temperatures below 300°C and at an oxygen fugacity above the haematite–magnetite (HM) buffer. The calculated δ 18 O melt value for the syenites (+5·9 to +6·3‰) implies a mantle origin, whereas the inferred δ 18 O melt value of <+5·1‰ for the granitic melts is significantly lower. Thus, the granites require an additional low-δ 18 O contaminant, which was not involved in the genesis of the syenites. Rb/Sr data for minerals of both rock types indicate open-system behaviour for Rb and Sr during post-magmatic metasomatism. Neodymium isotope compositions (eNd 1170 Ma = −3·8 to −6·4) of primary minerals in syenites are highly variable, and suggest that assimilation of crustal rocks occurred to variable extents. Homogeneous e Nd values of −5·9 and −6·0 for magmatic amphibole in the granites lie within the range of the syenites. Because of the very similar neodymium isotopic compositions of magmatic and late- to post-magmatic minerals from the same syenite samples a principally closed-system behaviour during cooling is implied. In contrast, for the granites an externally derived fluid phase is required to explain the extremely low e Nd values of about −10 and low δ 18 O between +2·0 and +0·5‰ for late-stage aegirine, indicating an open system in the late-stage history. In this study we show that the combination of phase equilibria constraints with stable and radiogenic isotope data on mineral separates can provide much better constraints on magma evolution during emplacement and crystallization than conventional whole-rock studies.