Geological Setting
The Southern Apennine chain is a fold-and-thrust belt (Fig. 1) formed between the upper Oligocene and Quaternary (Scandone, 1967; Ogniben, 1969; D’Argenio et al., 1973, 1975; Pescatore and Slaczka, 1984; Mostardini and Merlini, 1986; Casero et al., 1988; Roure et al., 1991; Hippolyte et al., 1994; Cello and Mazzoli, 1999; Cello et al., 2000; Mazzoli et al., 2001; Patacca and Scandone, 2007), resulting from the convergence between the African and European plates and the simultaneous rollback of the SE-directed Ionian subduction, which caused the opening of the Tyrrhenian back-arc basin (Gueguen et al., 1998; Cello and Mazzoli, 1998; Doglioni et al., 1999). Relicts of the late Creaceous-Oligocene accretionary wedge, resulting from subduction towards NW of the western Tethys Ocean, occur at the highest structural levels of the Apennine chain. The accretionary wedge includes the Liguride Complex, consisting of HP/LT metamorphic sequences (the Frido Unit; Vezzani, 1969, 1970), as well as sequences devoid of a metamorphic overprint (the North-Calabrian Unit; Bonardi et al., 1988). Ophiolitic slices are present both in the Frido Unit and in the North-Calabrian Unit (Bonardi et al., 1988). Ophiolites of the Frido Unit are characterized by a HP/LT metamorphic overprint (Lanzafame et al., 1979; Spadea, 1982; Beccaluva et al., 1982), whereas those of the North-Calabrian Unit do not exhibit any subduction-related metamorphism (Bonardi et al., 1988).
The ophiolitic rocks studied in this paper belong to the Frido Unit (Fig. 2) consisting of serpentinites, derived from mantle lherzolite and subordinate harzburgite (Sansone et al., 2012), metagabbros, metabasalts, and their respective sedimentary cover, showing a very low-grade metamorphic overprint (Vezzani, 1970; Lanzafame et al. 1979; Spadea, 1982, 1994). Metabasalts either occur as metadolerite dikes intruded in the serpentinites or as volcanic rocks with a relict pillow structure (Lanzafame et al. 1979; Spadea, 1982, 1994). Dikes and serpentinites have been affected by ductile to brittle deformation connected to oceanic lithosphere stretching and to emplacement of the ophiolites in the orogenic wedge. For this reason, dikes are mostly preserved as 1 to 10 m-sized blocks enclosed in a strongly deformed serpentinite matrix (Fig. 3). Ophiolites are associated with tectonic slices essentially made up of medium-to high grade metamorphic rocks (such as amphibolite, gneiss and granofels).
Figure 2. Simplified geological map of the study area (see text), modified after Monaco et al. (1995).
Figure 3. Examples of outcrops of metadolerite dikes in the study area.
a) Metadolerite dike protruding from serpentinites covered by grass at Timpa della Guardia. b) Dike surrounded by cataclastic serpentinites at the Timpa Castello quarry.
Metadolerite dikes hosted in serpentinized peridotites display different kinds of texture and degrees of re-crystallization (Sansone, 2010). Texture ranges from intersertal to grano-xenoblastic and mylonitic with increasing deformation. Metadolerites recorded two main metamorphic events: an ocean-floor metamorphism and an orogenic overprint (Sansone et al., 2011). Ocean-floor metamorphism was accompanied by rodingitization and spilitization. In the study area rodingites are exposed as greyish-white, cm- to dm-thick dikes cutting through serpentinites (Spadea, 1982; Sansone et al., 2011; Sansone and Rizzo, 2012). Garnet found in rodingites replaces plagioclase and retains high hydrogrossularite–grossularite contents (86 - 95 mol% Grs; 0.02 - 8 mol % Prp; 4 - 6 mol % Alm.; 0.2 - 0.4 mol % Sps; Sansone et al., 2011).
The serpentinites of the Frido Unit derive from serpentinization of mantle peridotites with porphyroclastic texture (Sansone et al., 2012). Serpentinization produced a pseudomorphic mesh texture defined by serpentine+magnetite that statically replace olivine crystals, and by yellow-brown bastite replacing orthopyroxene. Pseudomorphic texture is crosscut by various sets of submillimetric veins mostly filled with serpentine fibers. These serpentine veins are responsible for the foliation visible in the field.