Prehnite--Epidote Phase Relations in the Nordre Aputitëq and Kruuse Fjord Layered Gabbros, East Greenland
Layered gabbros at Nordre Aputit�q and Kruuse Fjord were emplaced during extensional tectonism that led to the formation of the North Atlantic basin in the Early Tertiary. Sub-solidus reactions between the gabbros and hydrothermal fluids formed superimposed alteration assemblages in fractures, cavit...
| Published in: | Journal of Petrology |
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| Main Authors: | , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1987
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| Subjects: | |
| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/28/6/1193 https://doi.org/10.1093/petrology/28.6.1193 |
| Summary: | Layered gabbros at Nordre Aputit�q and Kruuse Fjord were emplaced during extensional tectonism that led to the formation of the North Atlantic basin in the Early Tertiary. Sub-solidus reactions between the gabbros and hydrothermal fluids formed superimposed alteration assemblages in fractures, cavities, and the adjacent gabbros. The earliest secondary minerals are Ca-Al amphibole + clinopyroxene + biotite � plagioclase that form thin veins or porous pegmatitic masses. These minerals are crosscut, overgrown or partially replaced by one or more generations of prehnite and epidote bearing assemblages associated with filling of the fractures and cavities, and with extensive wall rock albitization. Wide variations in the partitioning of Fe3+ and Al between coexisting prehnite and epidote solid solutions occur in these altered gabbros. The partitioning data define distinct clusters in terms of associated mineralogy and paragenetic relations. This, together with prehnite and epidote compositions from active geothermal systems, are used to evaluate the thermodynamic properties of the intercrystalline exchange reaction:When compared to the compositions of prehnite and epidote in the Nordre Aputit�q and Kruuse Fjord intrusions, it is concluded that the latest and lowest temperature generations of prehnite and epidote display disequilibrium partitioning of Fe3+ and Al, manifested by the occurrence of prehnite that is relatively enriched in Fe3+ Thermodynamic analysis of phase relations in the system Na 2 O-CaO-Al 2 O 3 -Fe 2 O 3 -FeO-SiO 2 -H 2 O-HCl is used to determine local equilibrium constraints on Fe3+-Al substitution in prehnite and epidote. It appears that paragenetic and compositional relations of prehnite and epidote are sensitive indicators of local fluctuations in fluid composition and temperature. The complex magmatic and structural history of the gabbros at Nordre Aputit�q relative to Kruuse Fjord is considered to be responsible for the differences in mineral paragenesis and compositions of prehnite and ... |
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