Discussion
Serpentinized peridotites with minor interbedded mafic-ultramafic rocks, namely, cm-scale cpx-rich layers, amp-bearing chloriteschists, dunite, amphibole-rich layers, and chloriteschists constitute the sole for the volcanic and cover sequence of the Antrona ophiolites (see also Tartarotti et al., 2011). A multi-scalar structural study here peformed on selected samples of the Antrona ultramafic rocks reveals the occurrence of relict microstructures, textures and mineralogy attributable to a pre-Alpine oceanic origin, prior to the dominant Alpine tectono-metamorphic imprint affecting the whole ophiolite sequence and well-documented in the literature (Colombi and Pfeifer, 1986; Colombi, 1989; Pfeifer et al., 1989; Turco and Tartarotti, 2006).
The ultramafic portion of the Antrona ophiolite mostly consists of foliated serpentinites which preserve pre-Alpine mantle porphyroclastic textures (see Figs. 3), although crystal-plastic recrystallization with neoblasts formation related to the Alpine orogenic history has likely occurred. Olivine represents the best preserved mineral phase and is here interpreted as mantle relic, as suggested by its microstructural and textural features. Intracrystalline deformation may also be inferred by the occurrence of subgrains (see Figs. 3c, 3d) and deformation lamellae inside olivine porphyroblasts, commonly found in mantle tectonitic peridotites (Den Tex, 1969; Mercier and Nicolas, 1975). These and other microstructures typical of mantle peridotites, such as exholved clinopyroxene porphyroclasts, and porphyroclastic holly-leaf-shaped spinel (e.g. Gueguen and Nicolas, 1980; Tartarotti et al., 2002; Dick et al., 2010) were observed not only in serpentinized peridotites but also in dunite samples and in coarse-grained ol-cpx-spl-rich rocks (see Fig. 3). Dunite layers in the Antrona serpentinites can thus be interpreted as being residual rocks hosted in the mantle harzburgite/lherzolite main body. Tartarotti et al. (2011) have shown that olivine and clinopyroxene compositions in the Antrona serpentinites are comparable with those of abyssal peridotites from modern oceanic lithosphere (e.g., Dick 1989; Tartarotti et al. 2002; Seyler et al., 2003; Dick et al. 2010; Warren and Shimizu, 2010), inferring that the slightly higher Fe content in olivine and Ca content in clinopyroxene could be related to rock recrystallization during the Alpine orogenic evolution.
Finally, further possible evidence of relict mantle textures are: rectangular pseudomorphic aggregates of serpentine (after probable pyroxene), and the ol+chl+amp pseudomorphs replacing earlier (ortho?) pyroxene in coarse-grained ol-cpx-spl- rich rocks (see also Tartarotti et al., 2011). As far as spinel crystals found in the studied ultramafic rocks, it is noteworthy the occurrence of a well preserved holly-leaf habit (recalling the typical texture of mantle peridotites), in spite of the internal reworked texture revealing a Ferritchromite composition. Similar Ferritchromite-rich spinel crystals characterized by a porous texture and a Cr-chlorite rim (like that found in our samples; see Figs. 3, 4, 5) has been described in a few localities, such as the Kalkan ophiolite of the southern Urals, and interpreted as being due to hydration and oxidation reactions consuming Al-rich chromite during prograde metamorphism (Merlini et al., 2009). Other Authors (e.g. Mellini et al., 2005) suggest that Ferritchromite is of hydrothermal origin formed after the oceanic serpentinization of Al-rich spinel. Ferritchromite porphyroclasts rimmed by Cr-chlorite have been recently found in the blueschist facies ophiolites of the Southern Apennine ophiolites (Sansone et al., in press) and interpreted as being related to oceanic alteration. Although an oceanic origin for the studied Ferritchromite samples cannot be ruled out, further investigations, such as TEM analyses are needed in order to precisely characterize the internal structure of Ferritchromite.
In order to better constrain the origin of relict olivine in the studied samples, a Quantitative Texture Analysis (QTA) using neutron diffraction was carried out in rock samples containing olivine crystals. QTA analysis reports a well defined LPO of olivine in the three samples though ANT64 displays the highest F2 value (3.43). Such a difference in calculated texture strength may be also referred to differences in grain-sizes (Table 1) that may influence the F2 values. Similarly the higher values of Rw, Rb and GoF observed for ANT64 do not necessarily correspond to a lower quality refinement, since the comparison of experimental and computed patterns and PFs points to a well reproduced LPO. The discrepancies may be due to several factors as grain-size variations and presence of other mineral phases in the analysed rocks as well the shape of the analysed sample with respect to the D19 setup. All these factors may imply an increase of the refinement factors (Chateigner, 2005; Toby, 2006).
The relationship between the olivine LPO and the dominant slip system may be inferred if meso- or microscopic shear planes are visible and may be used as reference to compare natural LPO with experimental or theoretical LPO (Jung and Karato, 2001; Karato, 2008). Olivine LPO in sample ANT64 displays orientations similar to those described by several authors as characteristic of type D and E distribution (Drury, 2005; Karato et al., 2008 and reference therein; Tartarotti et al., 2011; Tommasi et al., 2000). Similar patterns, comparable with type E distribution, are also present in samples ANT138 and ANT120 olivine, albeit the absence of a clearly defined mesoscopic foliation reduces the reliability of the comparison.
This comparison suggests slip systems [100](0kl) and [100](001) as active slip systems in the studied samples (Karato et al., 2008), which require T>800°C for P=1.5 for the activation (Carter and Ave'lallemant, 1970) and a relatively low content (< 800 ppm H/Si) of water in olivine (Jung and Karato, 2001; Karato, 2008).
Since the relationship between olivine fabric and the dominant slip systems suggests [100](0kl) and [100](001) as active slip systems (Karato et al., 2008) and the activation of these slip systems is referred as possible at T>800°C for P=1.5 GPa (Carter and Ave'lallemant, 1970), although it is sensitive to a pressure increase (e.g., Raterron et al., 2009), we confirm a mantle origin for the olivine fabric, as inferred by Tartarotti et al. (2011), inherited from the pristine mantle structure. An origin of the olivine LPO related to the Alpine evolution is discarded since temperature values as high as 800 °C have never been estimated for the Antrona ophiolites.
Moreover, the development of D-type olivine LPO is considered to be a sign of the presence of a moderate amount of water in mantle olivine crystals (<800H/Si ppm after Paterson, 1982). Such an amount of water is considered to be relevant for style in tectonics, having implications for the mechanics of subduction processes, by switching on highly localized weak faulting instead of a broad, slow creeping flow (Regenauer-Lieb, 2003), the transition occurring above a water concentration of 200 ppm H/Si.
This interaction and incorporation of water within olivine crystal defects may have occurred before oceanic alteration, where further water is interacting with olivine and other rock-forming minerals as suggested by chemical reactions and X-rays chemical maps in spinel domains (see Figs. 4, 5).
Other serpentinite bodies in the Alpine metaophiolites, namely, the Mount Avic serpentinites (Tartarotti and Martin, 1991; Diella et al., 1994; Fontana et al., 2008), the Zermatt-Saas serpentinites (Li et al., 2004), and the southern Lanzo ophiolitic massif (e.g., Piccardo et al., 2005) have been interpreted as being of oceanic origin, although not supported by quantitative fabric analyses. During the Alpine evolution, mylonitised serpentinites developed due to strain gradients and most of the earlier fabrics have been erased, leading to structural transposition of early S1 foliation consisting of porphyroblastic olivine, clinopyroxene, spinel, and serpentine and formation of S2 foliation. Conversely, within low strain volumes, serpentinites and host dunite and other ultramafic rocks could partly preserve their original structure.