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

• Published in: Journal of Petrology
• Main Authors: MARKS, MICHAEL, VENNEMANN, TORSTEN, SIEBEL, WOLFGANG, MARKL, GREGOR
• Format: Text
• Language: English
• Published: Oxford University Press 2003
• Subjects: ARTICLES; Ferro; Greenland
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/44/7/1247; https://doi.org/10.1093/petrology/44.7.1247

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 SiO 2 = 0·7–1 and an f O 2 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 O 2 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 δ18O melt value for the syenites (+5·9 to +6·3‰) implies a mantle origin, whereas the inferred δ18O melt value of <+5·1‰ for the granitic melts is significantly lower. Thus, the granites require an additional low-δ18O 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 (εNd 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 ...