A Jurassic oceanic core complex in the high-pressure Monviso ophiolite (western Alps, NW Italy) Andrea Festa1, Gianni Balestro1,*, Yildirim Dilek2, and Paola Tartarotti3 1DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI TORINO, VIA VALPERGA CALUSO, 35, 10125 TORINO, ITALY 2DEPARTMENT OF GEOLOGY AND ENVIRONMENTAL EARTH SCIENCE, MIAMI UNIVERSITY, OXFORD, OHIO 45056, USA 3DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI MILANO, VIA MANGIAGALLI, 34, 20133 MILANO, ITALY ABSTRACT The eclogite-facies Monviso ophiolite in the western Alps displays a complex record of Jurassic rift-drift, subduction zone, and Cenozoic collision tectonics in its evolutionary history. Serpentinized lherzolites intruded by 163 ± 2 Ma gabbros are exposed in the footwall of a thick shear zone (Baracun shear zone) and are overlain by basaltic lava flows and synextensional sedimentary rocks in the hanging wall. Mylonitic serpentinites with sheared ophicarbonate veins and talc-and-chlorite schist rocks within the Baracun shear zone represent a rock assemblage that formed from seawater-derived hydrothermal fluids percolating through it during intra-oceanic extensional exhumation. A Lower Cretaceous calc-schist, marble, and quartz-schist metasedimentary assemblage unconformably overlies the footwall and hanging- wall units, representing a postextensional sequence. The Monviso ophiolite, Baracun shear zone, and the associated structures and mineral phases represent core complex formation in an embryonic ocean (i.e., the Ligurian-Piedmont Ocean). The heterogeneous lithostratigraphy and the structural architecture of the Monviso ophiolite documented here are the products of rift-drift processes that were subsequently overprinted by subduction zone tectonics, and they may also be recognized in other (ultra)high-pressure belts worldwide. LITHOSPHERE; v. 7; no. 6; p. 646–652 | Published online 24 August 2015 doi:10.1130/L458.1 INTRODUCTION subsequent continental collision–related exhu- REGIONAL GEOLOGY OF THE WESTERN mation. Yet, slivers of mafic-ultramafic rock as- ALPS AND THE MONVISO OPHIOLITE Submersible surveys, geophysical studies, semblages in collision zones have been widely and deep ocean drilling projects during the last used in numerous studies to document the oc- The western Alps (Fig. 1A) evolved between two decades have provided new insights into the currence of remnants of oceanic basins and to the colliding Adria microplate and the European mode and nature of magmatic, tectonic, and hy- reconstruct their paleogeographies in the geo- plate during the late Eocene–early Oligocene. drothermal processes that occur along slow- to logical past (Decandia and Elter, 1972; Lagabri- Eastward subduction of the Ligurian-Piedmont ultraslow-spreading ridges (e.g., Cannat, 1993; elle, 1994; Dilek and Thy, 1998; Manatschal et oceanic lithosphere during the Early Creta- Tucholke et al., 1998; Dilek, 2002; Karson et al., 2011; Balestro et al., 2014; Dilek and Furnes, ceous–middle Eocene resulted in ophiolite em- al., 2006). These studies have revealed the oc- 2014). However, results of such reconstructions placement (Rosenbaum and Lister, 2005, and currence on the seafloor of oceanic detachment may lead to misleading interpretations for the reference therein), underthrusting of the Euro- faults and associated shear zones with deformed tectonic settings of the investigated ophiolites pean continental margin beneath Adria (Platt mafic-ultramafic rocks. Detachment faults along and for the extent of the inferred ocean basins et al., 1989), and tectonic imbrication along nonvolcanic rifted margins and in young oceanic in which they formed, if the primary seafloor WNW-vergent thrust faults (Ricou and Siddans, lithosphere accommodate high-magnitude exten- structures of these ophiolites go undetected. 1986). In the central part of the belt, eclogite- sion, causing the exhumation of lower-crustal In this paper, we document the internal facies ophiolite units (e.g., Zermatt-Saas zone gabbros and upper-mantle peridotites on the sea- structure of the eclogite-facies Monviso ophio- Auctorum) and blueschist-facies metasedimen- floor, forming oceanic core complexes (Cannat et lite in the western Alps (Fig. 1), and we show tary units (Combin zone and “Schistes Lustrés” al., 2006; Karson et al., 2006; Smith et al., 2014). that despite the overprint of high-pressure sub- Auctorum) are tectonically sandwiched between These rocks display mineral assemblages and duction-zone metamorphism and deformation European and Adriatic continental margin units structural fabrics developed during the interplay of its lithological units, this ophiolite displays (Fig. 1A; Dal Piaz et al., 2003). of ductile and brittle deformation episodes, fluid- a well-preserved record of the intra-oceanic The Monviso ophiolite is exposed in the rock interactions, and metasomatism associated extensional tectonics that affected it during the southern part of the western Alps (Fig. 1), with their exhumation (Boschi et al., 2006). opening of the Ligurian-Piedmont Ocean. We where it rests tectonically on the Dora Maira Recognition of detachment faults and core further discuss the crustal architecture and the unit, which was part of the European continen- complex structures in fragments of ancient oce- occurrence of a talc-and-chlorite schist shear tal margin (Dal Piaz et al., 2003), and below the anic lithosphere is often difficult because of the zone (i.e., Baracun shear zone of Balestro et al., Queyras Schistes Lustrés unit, which consists multistage, intense deformation and metamor- 2015) in Monviso, which represent evidence of of carbonaceous metasedimentary rocks with phism they experienced during subduction and Jurassic oceanic core complex development, meta-ophiolite bodies (Lombardo et al., 1978; documented for the first time in the eclogitized Tricart and Lemoine, 1991). The Monviso ophi- *Corresponding author: [email protected]. ophiolite units in the western Alps. olite includes lherzolitic mantle rocks intruded 646 © 2015 Geological Society of Americawww.gsapubs.org | For permission | toVolume copy, contact7 | Number [email protected] 6 | LITHOSPHERE Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/7/6/646/3040038/646.pdf by guest on 11 April 2021 A Jurassic oceanic core complex in the high-pressure Monviso ophiolite (western Alps) | SHORT RESEARCH (Figs. 1A and 2), where it is marked by up to Alps Dinarides N 7°E N tens-of-meters-thick talc-and-chlorite schist Apennin A Adria paleomargin units d r ia rocks, separating metaperidotites and 163 tic Se Adria a ± 2 Ma metagabbros (Rubatto and Hermann, es crust Ty rrhenian lt European paleomargin 2003) in the footwall from metabasalt and Sea e Ionian b units (DM: Dora Maira) calc-schist with ophiolite-derived detrital inter- Sea e in calations in the hanging wall (Figs. 2, 3A, and 250 km p B l Ophiolite Units 3B). Both the hanging-wall and footwall assem- A n Blueschist-facies units blages and the Baracun shear zone (Figs. 2 and N 45° r e e (Q: Queyras Schistes 3A–G) are unconformably overlain by a Lower t Fig.2 Lustrés) Cretaceous (Lagabrielle, 1994) metasedimen- s s e Neogen Eclogite-facies units tary sequence (i.e., postextensional succession), W basin (MO: Monviso ophiolite which is metamorphosed along with the rest of P the assemblage (i.e., high-pressure eclogitic- Po valley 1: Upper tectonic units H Q 2: Lower tectonic units) facies metamorphism). 1 2 DM Insubric fault Footwall and Hanging-Wall Units of the Baracun Shear Zone MO Penninic thrust European 20 km Trace of geological Lithological units in the footwall of the Bar- crust cross section acun shear zone include massive serpentinite A with poorly preserved relics of the original min- W E eral phases and textures. Metagabbroic intru- Monviso ophiolite sions are meters to tens of meters thick and are Upper tectonic units (480° C, 2.2 GPa) Lower tectonic units (550° C, 2.6 GPa) composed mainly of Mg-Al metagabbro charac- (i.e. Forciolline and Vallanta) (i.e. Viso Mozzo, Lago Superiore, Basal Serpentinite) terized by the occurrence of Cr-omphacite. Less metagabbro common Fe-Ti metagabbro intrusions occur as Queyras Fig. 4C meters-thick dikes and contain eclogitic assem- Schistes blages of garnet-omphacite-rutile. At the con- Lustrés Dora tacts with their host metaperidotites, all metagab- Maira bro intrusions are extensively rodingitized. The massive serpentinite immediately beneath the metasediments metabasalt Mg-Al and Fe-Ti metagabbros Baracun shear zone includes 50-cm- to 1-m- mylonitic serpentinite and talc-schist massive serpentinite tectonic contact thick mylonitic serpentinite with sheared ophi- C 500 m carbonate veins (Fig. 3H). These carbonate-rich veins, which also crosscut the talc-and-chlorite Figure 1. (A) Simplified tectonic map of the western Alps. HP—high pressure. (B) Location of map in A. (C) Representative structural cross section of the Monviso (modified from Angiboust schist of the Baracun shear zone (Figs. 3H–3I), et al., 2011; Balestro et al., 2013). are overprinted by S1 foliation and F2 folds, con- straining the timing of hydrothermal activities to a pre–eclogitic-facies metamorphic stage. by Middle–Upper Jurassic metagabbros (163 zone, during which S1 foliation was developed; The hanging-wall units above the Baracun ± 2 Ma; Rubatto and Hermann, 2003). Both the (2) late Eocene–early Oligocene continental shear zone consist mainly of calc-schist and metaperidotites and metagabbros are overlain collision stage, which caused W-vergent