The Zermatt-Saas Zone mainly consists of serpentinite, metagabbros, and metabasalts with minor calcsilicates and quartzites (Fig. 1) (ERNST & DAL PIAZ, 1978 ...
Rend. Online Soc. Geol. It., Vol. 21 (2012), pp. 313-314. © Società Geologica Italiana, Roma 2012
On the metamorphic ophiolites of the western Alps: 40 years after the Penrose Conference GISELLA REBAY (*), MARIA IOLE SPALLA (**), SILVANA MARTIN (***) & ROGER POWELL (°)
RIASSUNTO
GEOLOGICAL SETTING
Quaranta anni dopo la Penrose Conference: considerazioni sulle ofilioti metamorfiche delle Alpi occidental. Con una serie di esempi si intende mostrare qui come sia progredita la maniera di interpretare le ofioliti di alta pressione delle Alpi Occidentali negli ultimi quaranta anni. Il ricorso alla modellizzazione termodinamica delle fasi all’equilibrio combinato con l’analsi srutturale a differenti scale e con una accurata analisi petrografica permette di individuare unità tettonometamorfiche con una evoluzione metamorfica Alpina uniforme. Questo può consentire la ricostruzione dell’evoluzione completa della litosfera oceanica della Tetide nel dominio alpino.
Key words: HP ophiolites, HP serpentinites, metagabbro, tectono-metamorphic evolution, Western Alps.
INTRODUCTION Ophiolites outcrops offer a unique opportunity to study directly the oceanic lithosphere, as they represent one of the few exposures of otherwise unreachable rock bodies. In the last 40 years relevant progress has been made on the direct observation and study of ocean floor geology, but there still remain many problems to be solved. Ophiolites are mostly found in sutures in collisional chains, and therefore the original relationships amongst rocks are reworked by tectonics and metamorphism. Nonetheless, our ability in unravelling deformation and metamorphic processes has been significantly improved allowing new insights not only in the transformations to which the oceanic rocks have been subjected in subduction zones, but also on their structure.
The suture zone of the Western European Alps, deformed under HP-conditions during the Alpine subduction and collision (e.g. DAL PIAZ et alii, 2001 and refs therein) is made of metaophiolites and metasediments. This ophiolitic suture, constituted by the Zermatt-Saas and Combin Zones, is tectonically sandwiched between the continental internal massifs and the external nappes respectively belonging to the Penninic and Austroalpine and Helvetic domains (e.g. MARTHALER & STAMPFLI, 1989; DAL PIAZ et alii, 2001). The Zermatt-Saas Zone mainly consists of serpentinite, metagabbros, and metabasalts with minor calcsilicates and quartzites (Fig. 1) (ERNST & DAL PIAZ, 1978; BECCALUVA et alii, 1984; PFEIFFER et alii, 1989, MARTIN et alii, 1994). These rocks preserve HP or UHP mineral assemblages, developed during the Alpine convergence, and overprinted by exhumation-related metamorphism (e.g. ERNST & DAL PIAZ, 1978; REINECKE, 1991; LI et alii, 2004; Rebay et al., 2012). The age of the oceanic protoliths spans from 164 to 153 Ma (RUBATTO et alii, 1998) and structural and mineral relics show that these protoliths were affected by oceanic metamorphism before the Alpine subduction (e.g. DAL PIAZ et alii, 1980, LI et alii, 2004). Relict oceanic textures are well documented in the Zermatt-Saas Zone (e.g. BARNICOAT & FRY, 1986). The Combin unit is mainly made of metasediments (the Schistes Lustrés Unit) and metabasalts with blocks of ophiolites metre up to kilometre-sized, scattered into these calcschist-rich series (TRICART & SCHWARTZ, 2006). In the Piedmont Zone the pelagic sediments record a metamorphic evolution under highP/low-T conditions in the blueschists facies (e.g. POGNANTE, 1991), that, lateron, during exhumation, was re-equilibrated under greenschist facies conditions.
_________________________ (*) Dipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, via Ferrata, 1, 27100 Pavia, Italy. (**) Dipartimento di Scienze della Terra “A. Desio”, Università di Milano and CNR-IDPA, via Mangiagalli, 34, 20133 Milano, Italy. (***) Dipartimento di Geoscienza, Università di Padova, via G. Gradenigo, 6, 35131 Padova, Italy. (°)School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia.
EXAMPLES Some significant examples of the improvement of our knowledge on the metamorphic ophiolites from the western Alps are here portrayed. For instance, it has been shown that eclogitised serpentinites from the Valtournanche valley (REBAY et alii, in press) have undergone an HP metamorphic evolution while preserving relationships and parageneses that are indicative of an oceanic
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metamorphic evolution. Here structures and relict parageneses indicate that these mantle rocks underwent a complex oceanic metamorphism involving their almost complete hydration before the subduction. Evidences of such a pre-Alpine evolution, that can be found in serpentinites and enclosed metagabbro, deserve at least a short discussion. Similar relations and parageneses can be observed in the Voltri Group (see REBAY et alii, 2004, and refs therein). In both examples this early oceanic evolution has important bearing on the subsequent alpine HP evolution where fluids played again an important role (SCAMBELLURI & RAMPONE, 1999). Metagabbros from the Monviso ophiolite show similar evidences (MESSIGA et alii, 1999) of widespread hydration during oceanic evolution. Moreover, it is relevant the oceanic palaeo-hydrothermal system preserved in the St Marcel Valley (MARTIN et alii, 2008; REBAY & POWELL, in press). In this case, different rocktypes development of peculiar HP mineral assemblages took place, depending on the bulk rock composition, which varies from mafic to ultramafic and comprises eclogites, glaucophanites, talcschists and chloriteschists. Again, the presence of Cu-sulphide mineralisations and the nearby MnPraborna mine testify ocean floor hydrothermal activity (TUMIATI et alii, 2010, and refs therin). In general, a detailed analysis of rocks such as the above mentioned ones, allows, not only the reconstruction of their Alpine metamorphic evolution, but also the detection of the nature and evolution of their oceanic protoliths. CONCLUSIONS A multi-scale structural study combined with petrological analysis and thermodynamic modelling in selected areas of the western Alps brought to the conclusion that a large portion of the western Alps ophiolites underwent a widespread hydration due to oceanic metamorphism before being involved in the subduction zone and subjected to metamorphism from HP to UHP type. The reconstruction of the complex Alpine metamorphic evolution allows to reconstruct their oceanic evolution. The determination of the different tectonometamorphic units in the ophiolites is essential both to understand the nature of the processes taking place in subduction zones, and to decipher the structure and relationships in the oceanic lithosphere before the subduction. REFERENCES BARNICOAT A.C. & FRY N. (1986) – High-pressure metamorphism of the Zermatt-Sass ophiolite, Switzerland. J. Geol. Soc. London, 143, 607-618. DAL PIAZ G.V., CORTIANA G., DEL MORO A., MARTIN S., PENNACCHIONI G. & TARTAROTTI P. (2001) - Tertiary age and paleostructural infereces of the eclogitic imprint in the Austroalpine outliers and Zermatt-Saas Zone ophiolite, western Alps. Int. J. Earth Sci., 90, 668-684.
ERNST G.W. & DAL PIAZ G.V. (1978) - Mineral parageneses of eclogitic rocks and related mafic schists of the Piemonte ophiolite nappe, Breuil-St Jacques area, Italian Western Alps. Am. Miner., 63, 621-640. MARTHALER, M. & STAMPFLI, G.M. (1989) - Les Schistes lustrés à ophiolites de la nappe du Tsaté: un ancien prisme d'accrétion issu de la marge active apulienne. Schweiz. Miner. Petrogr. Mitt., 69, 211-216. MARTIN, S., REBAY, G., KIENAST, J.R., MEVEL, C. (2008) – An ophiolitic palaeo-hydrothermal field metamorphosed in the eclogite-facies from the italian western Alps, Saint Marcel valley. Ofioliti, 33, 49-63. MARTIN S., TARTAROTTI P. & DAL PIAZ G.V. (1994) - The Mesozoic ophiolites of the Alps: a review. Boll. Geof. Teorica Applicata, 36, 141-144, 175-220, Trieste. MESSIGA B., KIENAST J.R., REBAY G., RICCARDI M.P., TRIBUZIO R., (1999) ) - Cr-rich magnesiochloritoid eclogites from the Monviso ophiolites (Western Alps, Italy). J. Metamorph. Geol., 17, 287-299. LI X.P., RAHN M. & BUCHER K. (2004) - Serpentinites of the Zermatt-Saas ophiolite complex and their texture evolution. J. Met. Geol., 22, 159-177. POGNANTE U., (1991) - Petrological constrains on the eclogiteand blueschist- facies metamorphism and P-T-t path in western Alps. J. Metamorph. Geol., 9, 5-17. REBAY, G., TALLONE, S., D’ANTONIO, D., SCAMBELLURI, M., BISTACCHI, A., MESSIGA, B. & GOSSO, G. (2004). Metamorphosed Ophiolitic Units in the Ligurian Alps. In: (eds.) Pasquaré, G., Venturini, C. and Groppelli, G., Mapping Geology in Italy, Apat, 259-264, Selca, Firenze. REBAY, G., SPALLA, M.I. & ZANONI, D., (2012) - Interaction of deformation and metamorphism during subduction and exhumation of hydrated oceanic mantle: Insights from the Western Alps. J. Metamorph. Geol., in press. REBAY, G. & POWELL, R., (2012) - Eclogite-facies sea-floor hydrothermally-altered rocks: calculated phase equilibria for an example from the Western Alps at Servette. Ofioliti, in press. REINECKE T. (1991) - Very high pressure metamorphism and uplift of coesite-bearing metasediments from the ZermattSaas Zone, Western Alps., Eur. J. Mineral. 10, 7-17. RUBATTO D., GEBAUER D. & FANNING M., (1998) - Jurassic formation and Eocene subduction of the Zermatt-Saas-Fee ophiolites: implications for the geodynamic evolution of the Central and Western Alps. Contrib. Miner. Petrol., 132, 269-287. SCAMBELLURI, M. & RAMPONE, E., (1999). Mg-metasomatism of oceanic gabbros and its control on Ticlinohumite formation during eclogitization. Contrib. Miner. Petrol., 135, 1-17. TRICART P. & SCHWARTZ S. (2006) - A north-south section across the Queyras Schistes lustrés (Piedmont zone,Western Alps): Syn-collision refolding of a subduction wedge. Eclogae geol. Helv., 99, 429–442. TUMIATI, S., MARTIN, S., GODARD, G. (2010). Hydrothermal origin of manganese in the high-pressure ophiolite metasediments of Praborna ore deposit (Aosta Valley, Western Alps). Europ. J. Min., 22, 577-594.
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