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Ubiquitous Fibrous Antigorite Veins from the Lanzo Ultramafic Massif, Internal Western Alps (Italy): Characterisation and Genetic Conditions

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Ubiquitous Fibrous Antigorite Veins from the Lanzo Ultramafic Massif, Internal Western Alps (Italy): Characterisation and Genetic Conditions Per. Mineral. (2007), 76, 169-181 doi:10.2451/2007PM0014 SPECIAL ISSUE: From Petrogenesis to Orogenesis http://go.to/permin An International Journal of l PERIODICO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage Ubiquitous fibrous antigorite veins from the Lanzo Ultramafic Massif, Internal Western Alps (Italy): characterisation and genetic conditions CHIARA GROPPO* and ROBERTO COMpagNONI Centro interdipartimentale “G. Scansetti” per lo studio sull’asbesto ed altri particolati tossici, Universita’ degli Studi di Torino, 7 via Pietro Giuria, 10125, Torino, Italy Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, 35 via Valperga Caluso, 10125, Torino, Italy Accepted, April 2007 ABSTRACT. — Discontinuous veins, 1 to 20 cm 1 a ca. 20 cm e lunghe da alcuni cm a svariati dm. thick and from few centimetres to several decimetres Le vene antigoritiche consistono di fasci di fibre long, of fibrous antigorite are widespread in the rigide e fragili di tipo “cross” che mostrano una serpentinites of the Lanzo Ultramafic Massif, Western caratteristica struttura a bande parallele alle salbande Alps. The cross fibre antigorite veins consist of rigid della vena. L’antigorite fibrosa è stata caratterizzata and brittle bundles of fibres and typically show a mediante microscopia ottica in luce polarizzata, banded structure parallel to the vein selvages. The microscopia elettronica (SEM-EDS, EMPA e TEM) fibrous antigorite has been characterised by optical e spettroscopia vibrazionale (FTIR e µ-Raman). In microscopy, electron microscopy (SEM-EDS, EMPA base al valore sperimentale di m = 17 (m = numero and TEM), and vibrational spectroscopy (FTIR and di tetraedri lungo un’intera lunghezza d’onda della µ-Raman). The determined value of m = 17 (m = number of tetrahedra along an entire wavelength) and struttura dell’antigorite) e ai dati termobarometrici thermobarometric data on the associated mafic rocks delle metabasiti e rodingiti associate è possibile and rodingites, suggest that fibrous antigorite veins ipotizzare che le vene ad antigorite fibrosa si siano formed during exhumation at T = 350-400 °C and P = sviluppate durante l’esumazione in condizioni di 0.3-0.6 GPa, i.e. under greenschist facies conditions. facies Scisti Verdi a T = 350-400 °C e P = 0,3-0,6 GPa. The fibrous habit of antigorite may be explained by L’abito fibroso dell’antigorite può essere spiegato two different mechanisms, i.e. the crack-seal process mediante due diversi meccanismi: riempimento di and the dissolution-precipitation creep mechanism. fratture (crack-seal) oppure scorrimento viscoso Both these processes are compatible with the banded con dissoluzione e precipitazione (dissolution- structure of the studied veins and the estimated P-T precipitation creep). Entrambi i processi sono in grado conditions of formation. di spiegare la struttura a bande delle vene studiate e sono compatibili con le condizioni termobariche RIASSUNTO. — Nelle serpentiniti del Massiccio stimate per la loro genesi. Ultrabasico di Lanzo (Alpi Occidentali) sono comuni vene di antigorite fibrosa potenti da ca. KEY-WORDS: fibrous antigorite, optical and electron microscopy, spectroscopy, P-T estimates, vein * Corresponding author, E-mail: [email protected] growth 170 C. GROPPO and R. COMpagNONI INTRODUCTION Bodinier, 1988) (Fig. 1). According to Pognante et al. (1985), Rampone and Piccardo (2000), Since the discovery of the close association Müntener et al. (2004) and Piccardo et al. (2004), between the exposure to chrysotile and the the Lanzo Massif is a portion of sub-continental development of several diseases (asbestosis, lung lithosphere emplaced at shallow levels during the cancer and pleural mesothelioma) (Nicholson et al., opening of the Mesozoic Piemonte-Liguria basin 1978; Mossman, 1993), a great stimulus has been and subsequently involved in a subduction event. given to the study of serpentinites and asbestos- The polyphase Alpine metamorphic evolution, bearing veins. Though only chrysotile and tremolite well recorded in the serpentinised portion of the are officially considered as asbestos by most Massif, consists of a first oceanic stage followed European Countries [the other four fibrous minerals by an eclogite-facies peak (Compagnoni et al., – actinolite, anthophyllite, riebeckite (crocidolite), 1980; Bodinier, 1988; Pognante, 1991). During and grunerite (amosite) – considered as asbestos the post-climax exhumation, the Lanzo Massif and used industrially - do not occur in Italy], other experienced a clockwise P-T path from eclogite- to minerals with fibrous habit may occur in serpentinite greenschist-facies conditions (Compagnoni et al., veins, such as polygonal serpentine, diopside, 1980; Pognante, 1991). carlosturanite, balangeroite, and antigorite (for a review, see: Belluso et al., 1995; O’Hanley, 1996; Alpine metamorphic veins in the Lanzo Massif Alberico et al., 1997). In the geologic literature, fibrous antigorite is reported as “asbestiform Four generations of Alpine metamorphic veins antigorite”, “picrolite” or “splintery antigorite” have been recognised in the serpentinised portion (Hess et al., 1952; Riordon, 1955; Udovkina et of the Lanzo Massif (Compagnoni et al., 1980; al., 1987; Belluso et al., 1995; Viti, 1995; Viti and Groppo and Compagnoni, 2003, 2005). The first vein generation consists of balangeroite, chrysotile, Mellini, 1996). FeNi-alloys (mainly awaruite and taenite) and In the Lanzo Ultramafic Massif (Western Alps), magnetite: it probably developed during an early different generations of metamorphic veins have stage of the peridotite serpentinisation under been recognised (Compagnoni et al., 1980; Groppo prograde high-pressure metamorphic conditions and Compagnoni, 2003, 2005), most of them (Compagnoni et al., 1983; Groppo et al., 2004). containing fibrous minerals. A careful examination The second vein generation, related to the high- of about one hundred vein specimens from several pressure metamorphic peak at about T = 500- localities of the Massif (Fig. 1) by means of 600 °C and P = 1.2 – 2.0 GPa (Sandrone and conventional (optical microscopy, X-ray diffraction, Compagnoni, 1983; Pognante, 1991; Pelletier and electron microscopy, and IR spectroscopy) and non- Müntener, 2006), consists of lamellar antigorite, conventional (µ-Raman spectroscopy: cf. Groppo et olivine, Ti-clinohumite, diopside and magnetite. al., 2006) techniques has shown that the commonest The third vein generation consists of antigorite type is the antigorite ± diopside-bearing vein (in the ± diopside crystallised under greenschist facies following referred to as antigorite vein), which is conditions (Castelli et al., 1995). The fourth vein the subject of this paper. generation, filled with short fibre chrysotile, forms local stockwork-type mineralisations, such as that formerly exploited in the Balangero asbestos mine GEOLOGICAL SETTING (Compagnoni et al., 1980). The Lanzo Ultramafic Massif is located in the innermost part of the Piemonte Zone of Calcschists Samples with meta-ophiolites, Western Alps. It is a large The studied samples were collected from 10 body of fresh tectonitic spinel-plagioclase lherzolite localities from the serpentinised rim of the Lanzo with subordinate harzburgite and rare dunite, Ultramafic Massif (Fig. 1). The discontinuous converted to antigorite-serpentinite at its rim and antigorite-bearing veins, typically 1 to 20 cm thick along shear zones (Nicolas, 1966; Boudier, 1976; and from few centimetres to several decimetres Compagnoni et al., 1980; Pognante et al., 1985; long, cut across massive serpentinite consisting Ubiquitous fibrous antigorite veins from the Lanzo Ultramafic Massif, Internal Western Alps (Italy)... 171 of an antigorite matrix with interlocking or massive appearance in the fresh specimen, on interpenetrating microstructure (Wicks and weathered surfaces or after grinding exhibits a Whittaker, 1977) and rare mineralogical relicts fibrous habit. (mainly clinopyroxene) of the peridotitic protolith. The veins consist of rigid and brittle bundles of fibres with massive appearance in the hand RESULTS: FIBROUS ANTIGORITE CHARACTERISATION specimen, light greyish-whitish to pale green in colour and with a splintery fracture (Fig. 2). Optical microscopy The contact with the host rock is sharp and the fibres, up to 15-20 cm in length, are generally More than one hundred specimens of antigorite oriented perpendicularly to the vein selvages. It veins have been observed under the polarizing is noteworthy that the antigorite, in spite of its microscope. In thin section, the fibrous antigorite- Fig. 1 – Simplified tectonic map of the Lanzo Ultramafic massif (modified from Nicolas, 1966): sample localities are shown as circled numbers. The inset shows location of the Lanzo Massif within a simplified tectonic sketch-map of the Western Alps. Mont Blanc-Aiguilles Rouges (MB) of the Helvetic-Dauphinois Domain; SB: Grand St. Bernard Zone of the external Penninic Domain; Internal Crystalline Massifs of the Penninic Domain: Monte Rosa (MR), Gran Paradiso (GP), Dora-Maira (DM); DB: Dent Blanche nappe of the Austroalpine Domain. SA: Southern Alps. TO: Town of Torino. 172 C. GROPPO and R. COMpagNONI Fig. 2 – Fibrous antigorite vein consisting of rigid and brittle bundles of fibres with massive appearance. bearing veins show a peculiar banded structure et al. (2004) and Renard et al. (2005). Small parallel
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