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Record: oai:ARNO:505887

AuthorH.J. Zwart
TitleMetamorphic history of the Central Pyrenees part 1; Arize, Trois Seigneurs and Saint-Barthelemy Massifs (sheet 3).
JournalLeidse Geologische Mededelingen
AbstractThe relation between orogenic movements and metamorphism is discussed. Schistosity and especially lineations are characteristic for metamorphites of the synkinematic phase. Lineations show a regular pattern. Late-kinematic metamorphism accompanied by different kinds of movement result in irregular rock flowage and rheomorphism. The structures of synkinematic mica-schists, gneisses, amphibolites and marbles are discussed.
In the three satellite massifs a series of gneisses and granitic rocks exists which are the products of silica and sodium metasomatism of originally pelitic rocks. The time and duration of the metamorphism determines the final state of the rocks. Synkinematic metamorphism alone gave rise to the garnet-augen-gneisses which, being rather dry, can be classified in the granulite facies. The lower part of these augen-gneisses are converted into schistose (not linear) gneisses and granites by post-kinematic feldspathisation. At the same time many anhydrous minerals are replaced by hydrous ones. Late-kinematic feldspathisation without a preceding synkinematic feldspar phase, leads to the formation of migmatites (sillimanitegneisses), and by continuing metamorphism to quartz-diorites. The transitional rocks between the garnet-augen-gneisses and the migmatites are the granitic biotite-muscovite-gneisses. Rheomorphism and mobilization of tho quartz-diorites is an important feature and probably leads finally to the intrusive biotite-granodiorites.
The muscovite-granites and gneisses which in part are also synkinematic, show a strong late phase of microclinization, due to potash metasomatism, originating from the underlying migmatites. In the mica-schists also, a syn- and post-kinematic phase of metamorphism can be detected.
The biotite-granodiorites show a different texture compared with the quartz-diorites of the migmatite-series. Their age is younger than the last phase of metamorphism, since the biotite-granodiorites did not participate in a late stage of muscovitisation, which is characteristic for most of the metamorphic rocks. These granodiorites are considered as intrusive magmatic bodies, originating from deeper levels, where continuing rheomorphism has lead to complete liquefaction.
Chemical analyses showed that the migmatites and the basal gneisses are enriched in silica, sodium and some calcium. Aluminium, iron, some magnesium, and titanium are removed. The quartz-diorites lost part of their potash. The muscovite-granites and gneisses show a strong enrichment in silica, sodium and potash. Aluminium, iron and magnesium are expelled.
Characteristic for the synkinematic phase is abundance of anhydrous minerals which suggest metasomatism in a dry state. Post-kinematic metasomatism goes together with introduction of water. The behaviour of water is considered to be responsible for the structural difference between syn- and post-kinematic rocks.
Finally the repartition of the various gneisses in the three satellite massifs is discussed.
Document typearticle
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