Mineralogical and Δ18o-Δd Isotopic Study of Kaolinized Micaschists At
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Catena 133 (2015) 97–106 Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena Mineralogical and δ18O–δD isotopic study of kaolinized micaschists at Penestin, Armorican Massif, France: New constraint in the kaolinization process A. Gaudin a,V.Ansana,⁎,T.Rigaudierb a Laboratoire de Planétologie et Géodynamique de Nantes (LPGN), Université de Nantes, UMR 6112 CNRS, 2 rue de la Houssinière, BP92208, 44322 Nantes, France b Laboratoire CRPG, UMR 7358 CNRS-UL, 15 rue Notre Dame des Pauvres, BP20, 54501 Vandoeuvre les Nancy, France article info abstract Article history: Many kaolinite occurrences developed from Paleozoic rocks forming the basement of the Armorican Massif in west- Received 23 February 2015 ern France. Their origin, hydrothermal alteration and/or weathering, is still discussed. In order to provide new con- Received in revised form 30 April 2015 straints to this debate, we studied the kaolinite occurrence at Penestin in the southern part of the Armorican Massif Accepted 8 May 2015 using several approaches, including field and petrological observations, mineralogical analyses (XRD) and stable ox- Available online xxxx ygen–hydrogen isotope analyses. An alteration profile was developed from faulted micaschists showing, from the fi fi Keywords: bottom to the top of the pro le, a reddish saprolite zone with pure white kaolinite ssures cutting across the schis- – Weathering tosity and a white kaolinite zone, 5 10 m thick. The detailed study by XRD of the layer silicates enabled us to char- Paleoenvironment acterize a mineralogical sequence with increasing alteration intensity, which is coherent with a weathering Kaolinite evolution: mica, chlorite → illite/vermiculite (chlorite) mixed-layer mineral → vermiculite → vermiculite/kaolinite Micaschist mixed-layer mineral → kaolinite. Moreover, an intense iron leaching process occurs within the kaolinite zone, which Stable isotope is characterized by a bleaching of this zone combined with iron oxyhydroxide precipitation along/within fractures XRD and cracks. The isotopic analyses of kaolinite from the zone and fissures gave δ18O values ranging from 19.64 to 21.21‰, and δD values ranging from −69.4 to −65.8‰. This confirms that kaolinite was formed by low- temperature water–rock interactions in contact with meteoric fluids during weathering processes. Isotopic and stratigraphic data suggest that this weathering occurred before the Messinian (~7 Ma) and probably dates back to the Eocene, during which the climate was sub-tropical at this location. © 2015 Elsevier B.V. All rights reserved. 1. Introduction sometimes comprise ferricrete or silcrete, but are totally devoid of bauxite. Kaolinite clay outcrops, up to 10 m thick, are abundant in the Armor- Moreover, kaolinite occurrences have also been reported for ican Massif (western France; Fig. 1) and developed both from igneous, Hercynian leucogranites and granodiorites (e.g. Ploemeur, Lorient, metamorphic and sedimentary rocks. Their genesis and their absolute Bérrien and other quarries north of Rennes (Fig. 1)). These are spatially ages are still discussed today: Are they due to weathering in a tropical associated with quartz veins and tourmaline resulting from medium- climate and/or hydrothermalism related to magmatic events? Are temperature fluid circulation; thus some authors have attributed their they formed by the same process whatever the protolith? Are they formation to hydrothermal processes related to magmatic events Paleozoic or Tertiary era? (Charoy, 1975; Chauris, 1984; Nicolas, 1956). However, a supergene or- More generally, kaolinite may be produced in different contexts, in- igin is now favored from a later mineralogical study (Bellion, 1979), a cluding weathering in tropical or sub-tropical wet climatic conditions, SEM textural and morphological study of kaolinite particles (Estéoule- diagenetic processes or hydrothermal alteration. Many kaolinite occur- Choux, 1981), and especially from a recent δ18OandδD isotopic study rences in the Armorican Massif developed from Paleozoic sedimentary (Boulvais et al., 2000). The weathering process is assumed to have and metamorphic rocks (Fig. 1). Some authors have suggested that occurred during the Eocene, which would be consistent with sub- their formation results from an intense weathering process yielding tropical climatic conditions during this period (Estéoule-Choux, 1967). thick (40 to 50 m, even 90 m) kaolinite-rich weathering profiles The southern area of the Armorican Massif at Penestin shows thick (Bellion, 1979; Boulvais et al., 2000; Estéoule-Choux, 1967, 1983; kaolinitic outcrops developed from faulted Hercynian micaschists with Thiry et al., 2006; Thomas, 1999; Wyns, 1991, 1996). These profiles veins of quartz and granite close to syn-tectonic leucogranites (Fig. 1). We focused our study on a coastal cliff showing successively from the ⁎ Corresponding author. bottom to the top: micaschist, a saprolite and a kaolinite zone. These al- E-mail address: [email protected] (V. Ansan). tered outcrops have never been studied in terms of mineralogical http://dx.doi.org/10.1016/j.catena.2015.05.006 0341-8162/© 2015 Elsevier B.V. All rights reserved. 98 A. Gaudin et al. / Catena 133 (2015) 97–106 Fig. 1. Schematic map of the Armorican Massif located in the northwestern part of France, in which major crustal scale shear zones (NASZ: North Armorican ShearZoneandSASZ:South Armorican Shear Zone) and their associated syn-kinematic Carboniferous leucogranites are mapped. Main outcrops containing alteration products, including kaolinite minerals, are plot- ted according to their protolith lithology (locations after Boulvais et al., 2000). The studied site is Penestin, south of the SASZ and 80 km northwest of Nantes. assemblage and process. They enable two hypotheses to be addressed: The studied area is located to the south of the SASZ, near the estuary is the mineralogical assemblage arranged as (1) a weathering profile of the Vilaine river (Fig. 2). It consists of a relatively high-grade meta- or (2) a hydrothermal deposit? This latter hypothesis is based on abun- morphic belt with amphibolite facies (Triboulet and Audren, 1988), dant faults inside micaschists (protolith) that are injected by numerous containing metapelites, micaschists, and more or less migmatized veins of quartz and granite, contemporaneous with carboniferous gneisses, called the Vilaine formation (Audren et al., 1975). Micaschists, leucogranites (e.g. along the Southern Armorican Shear Zone (SASZ) which are present in our site, show numerous spatial mineralogical het- and in the Guérande area, in Fig. 1). erogeneities with a main association containing muscovite, chlorite, al- In order to discriminate the formation process, we studied in detail bite and garnet, with biotite, staurotide and sillimanite locally (Audren the altered products formed from the micaschist. Boulvais' study et al., 1975). In a constant way, micaschists are very rich in exuded (2000) was essentially based on the isotopic compositions of alteration quartz, preferentially located in the hinges of folds. Gneiss, located at products, assuming that kaolinite remains stable. However, his infer- the outlet of the Vilaine river between the Pointe de Cofrenau and the ence is debatable because, for example in the Manaus region in Brazil, Pointe du Halguen (Fig. 2), is a granitic mylonite whose schistosity is Lucas et al. (1996) suggested that kaolinites are newly precipitated at underlined by muscovite, chlorite and flattened quartz surrounding each level of the profile and maintained at dynamic equilibrium with broken K-feldspars (Audren et al., 1975). To the north, migmatized local physical–chemical conditions by precipitation–dissolution reac- gneisses (Fig. 2) show varied petrology including silico-aluminous tions. Therefore, in this study, we combined several approaches includ- gneiss and leptynic gneiss with amphibolitic veins (Audren et al., ing field data, petrological observations, XRD mineralogical analyses 1975). Numerous syn-kinematic leucogranites were emplaced along and stable oxygen–hydrogen isotope analyses. the SASZ, between 340 and 300 Ma (Bernard-Griffiths et al., 1985). These are peraluminous granites with an igneous mineralogy consisting of quartz, albite, orthoclase, biotite and muscovite (Strong and Hanmer, 2. Geological setting 1981). The granites were formed by partial melting of mixed sedimen- tary sources (Peucat et al., 1988). The northern contact between The Armorican Massif comprises an Upper Proterozoic to Paleozoic micaschists and migmatized gneisses (Fig. 2) would correspond to a basement cropping out in western France (Fig. 1), separated by two late regional detachment zone associated with a network of extensional main shear zones. The northern part belongs to the Cadomian orogenic faults in micaschists that occurred during the emplacement of granites belt of Late Proterozoic age (Chantraine et al., 2001), whereas the cen- (e.g. Guérande; Fig. 1), ca. 310 Ma ago (Gapais et al., 1993). In the tral and southern parts correspond to the southern branch of the Penestin area, numerous extensional faults crosscut the micaschist European Variscan belt, which developed during the Upper Paleozoic and are often associated with veins of quartz and granite. (Le Corre et al., 1991). The bedrock consists of sedimentary, metamor- After the Hercynian orogeny, the Armorican Massif remained in con- phic and plutonic rocks that were deformed during the Cadomian and tinental environmental