Power, Gregory Michael (1974) The geology of the Precambrian rocks of La Hague, Manche, France. Doctoral thesis, Keele University.

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Abstract

The Rrecambrian rocks of La Hague have been divided into three areas of older gneisses, an orthogneiss complex intrusive into these gneisses and a series of post-tectonic granitic rocks.
The gneisses of the Nez de Jobourg, Omonville and Greville areas are shown to have had a similar evolutionary history. Evidence is presented for an early series of semi-pelitic sediments together with basic rocks, possibly lavas, into which granitic rocks were intruded. Deformation under low pressure amphibolite facies conditions resulted in the formation of a gneissose banding. The 2620m.y. age recorded by Leutwein et al., (1973) is tentatively assigned to this event although it may date the production of the main foliation under amphibolite facies metamorphism which followed. The Nez de Voidries quartz dioritic gneiss and the Nez de Jobourg granodioritic gneiss were emplaced at a late stage during this second deformation. A third deformation formed small scale asymmetric folds of the main foliation. This deformation may have been preceded and was certainly followed by emplacement of basic dykes.
The Thiebot complex, intruded next, is shown to consist of the Moulinet and Jardeheu quartz dioritic gneiss, the Thiebot granodioritic gneiss and the Red granitic gneiss. Chemically and petrographically these rocks form a calc-alkaline series and could have originated by equilibrium fusion of a lower crustal source and fractional crystallization at a higher level. The K-feldspar of the Red granitic gneiss is dominantly only intermediate microcline whilst the Thiebot granodioritic gneiss visually contains both monoclinic K-feldspar and intermediate microcline. This is interpreted as a primary variation and unlikely to be the result of reheating. It is suggested that, the foliation in the Thiebot complex was formed close to the time of intrusion.
The Omonville streaky gneiss cuts the Jardeheu quartz dioritio gneiss. It is normally uniform in chemical and mineralogical composition but locally exhibits strong fractionation.
Brioverian schists at Landemer and Bale de la Querviere are younger than the gneisses of the Greville area but their age relations to the Thiebot complex cannot be established. It is shown that they have suffered an early and a main phase of deformation under greenschist facies conditions end also contain later minor structures. The gneisses of the Greville area are sheared and this shearing appears most intense near the schists. It is argued that this was principally produced during the main deformation of the schists although some shearing may be of post-Cambrian origin.
The St. Martin mcnzonite pre-dates the St. Germain granite and may be older than the other post-tectonic granites. It is characterised by K-feldspar megacrysts and fine grained green inclusions. As suggested by Jeremine (1924), the inclusions probably represent an earlier phase produced from the same magma.
The Northern granites are demonstrated to form a related series of intrusive phases emplaced in the order, meladiorite, leucodiorite, Houffet granodiorite, La Becchue quartz diorite, St. Germain granite then the Cap de la Hague granodiorite and Ecuty granite, although the order of precedence of these last two is not known. Each phase is shown to have a distinctive mineralogical and chemical composition. Together, although not in strict sequence of intrusive age, they form a calc-alkaline series with the Ecuty granite showing strong fractionation of major and trace elements. The sequence of structural states of the K-feldspars in the later phases is consistent with that found in many series of granitic rocks and conditions were particularly suit-able for the production of maximum microcline in the Ecuty granite. There is evidence for sodium metasomatism in the St. Germain granite, possibly related to the emplacement of the Cap de la Hague granodiorite. Local zones of cataclastic deformation occur in the granites and may result from post-Cambrian movements.
The Cambrian sediments suggest post-tectonic molasse type sedimentation. New descriptions emphasise the importance of thrusting of the Cambrian over the underlying rocks and nowhere is there unequivocal evidence of an original undisturbed sedimentary surface between Precambrian and Cambrian.
The foregoing descriptions enable the strong similarities in geological history between La Hague and the nearby Channel Islands to be recognised and discussed.

Item Type: Thesis (Doctoral)
Subjects: Q Science > QE Geology
Divisions: Faculty of Natural Sciences > School of Geography, Geology and the Environment
Depositing User: Lisa Bailey
Date Deposited: 03 Apr 2019 08:50
Last Modified: 03 Apr 2019 08:50
URI: http://eprints.keele.ac.uk/id/eprint/6149

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