Quantification of magmatic and hydrothermal processes in a peralkaline syenite-alkali granite complex based on textures, phase equilibria, and stable and radiogenic isotopes


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Quantification of magmatic and hydrothermal processes in a peralkaline syenite-alkali granite complex based on textures, phase equilibria, and stable and radiogenic isotopes
Journal of Petrology
Marks M., Vennemann T.W., Siebel W., Markl G.
Statut éditorial
Date de publication
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 = 950-750degreesC, 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 750degreesC 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 similar to300degreesC. 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 300degreesC and at an oxygen fugacity above the
haematite-magnetite (HM) buffer. The calculated delta(18)O(melt) value
for the syenites (+5.9 to +6.3parts per thousand) implies a mantle
origin, whereas the inferred delta(18)O(melt) value of <+5.1parts per
thousand for the granitic melts is significantly lower. Thus, the
granites require an additional low-delta(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
(epsilonNd(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 epsilon(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 epsilon(Nd) values of about -10 and low
delta(18)O between +2.0 and +0.5parts per thousand 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.
Open Access
Création de la notice
29/09/2012 17:23
Dernière modification de la notice
25/09/2019 7:11
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