Peridotite bodies in the Alpujarride complex

The Ronda peridotite bodies (including in this term the ultramafic bodies on both sides of the Gibraltar arc) are systematically situated between the upper two tectonic units of the Alpujarride complex Jubrique Unit (top) and Blanca Unit (bottom). These Alpujarride units have similar lithologies and metamorphic records from the western end of the Alboran domain to the south of Granada (Fig.1, Azañón and Alonso-Chaves, 1995, Azañón et al., 1998).

Locally, Malaguide Complex rocks directly overlie thin peridotite slices in the areas subjected to late Miocene extension, such as that evidenced northeast of the Sierra Bermeja (Fig. 3). However, when primary top boundaries are well preserved, as in the Alpujata, Bermeja and Carratraca massifs (Figs. 2 and 3), garnet lherzolites of the peridotite top underlie garnet gneiss in the footwall of the Jubrique Unit. In this case, the contact between the mantle and crustal rocks is included in the so-called Bermeja-Jubrique shear zone (Balanyá et al., 1997), which can be considered to have been a tectonic paleo-Moho at some time in the pre-Miocene times (Balanyá et al., 1997; Reuber et al., 1982). The Jubrique Unit contains a 5-km-thick metamorphic succession of, from bottom to top (Figs. 3 and 4), garnet gneiss, migmatite gneiss, staurolite-bearing schist, chloritoid-bearing schist, phyllites, quartzites, calc-silicates and carbonate rocks. It has upward-decreasing metamorphic zoning, with isogrades subparallel to the lithologic contact and to the main foliation (Sp). The complete lithologic sequence of the Jubrique Unit roughly corresponds to a segment of condensed middle-upper crust (Balanyá et al., 1997), from metapelitic granulites at the bottom to scarcely recrystallized Triassic carbonates at the top.

Figure 3. Western Alboran Domain in the Betic chain

Western Alboran Domain in the Betic chain

Geological map of the western Alboran Domain in the Betic chain. Note the thin slices of peridotites (mainly serpentinites) that connect the main massifs, generally associated with large omissions of the units above. F: Flysch Trough Units; S: Subbetic; TS: tertiary sediments; Q: Quaternary. Numbers on the Jubrique unit represent metamorphic zones as in Figure 4.

Where the bottom of the Ronda peridotite crops out, the Blanca unit underlies the ultramafic bodies. The Blanca Unit group comprises a set of rocks showing high-temperature, medium-pressure metamorphism, ranging from granites to high-grade schist and marbles. Under the Bermeja body (Fig. 3) and Ceuta slice (Fig. 1), directly below the peridotites, there is a peculiar rock that consists of cordierite granitoid rich in heterogeneous fragments of lithologies characteristic of Alpujarride units, including peridotites and serpentinites. This fragment-bearing granitoid has features of both tectonic (Sánchez-Gómez et al., 1995) and “magmatic” (Lundeen, 1978) breccia. The Alpujarride fragments vary considerable in size: although most of them are centimetric, some reach a few hundred meters long. These fragments reveal the main Alpujarride penetrative foliation (Sp) and a superposed crenulation cleavage (Sánchez-Gómez et al., 1995). Late leucocratic dykes intrude fragment-bearing granitoids, peridotites and lower levels of the Jubrique unit.

In the Alpujata massif (Fig. 3), the peridotites overlie an inverse, condensed, but complete Alpujarride sequence, from garnet gneiss to marble (Fig. 4). The main difference with the Jubrique and other Alpujarride units is that sillimanite is ubiquitous in all the metapelite levels and marbles have high-temperature minerals such as diopside and forsterite (Westerhof, 1977).

Figure 4. Lithologic columns and PT trajectories

Lithologic columns and PT trajectories

Lithologic columns and PT trajectories of different rock levels within the units overlying (Jubrique Unit) and underlying (Blanca Group Units) the two main peridotite massifs in the western Betics. Granitoid bodies, over 22 my in age, mostly intruding the lower units. D1 to D4, tectonometamorphic events (see text for further explanation). Z1 to Z6 metamorphic mineral zones: Z1= pyrophyllite+ carpholite; Z2= kyanite+ chloritoid; Z3= staurolite+ garnet; Z4= Sillimanite; Z5= Sillimanite+ K feldspar; Z6= K feldspar+ kyanite+ garnet+ cordierite. Granitoid generation and granitoid-related paths are restricted to the D4 event. Aluminosilicate triple junction according to Bohlen et al. (1991). Together with stability phase fields, temperature determinations were principally based on garnet-biotite (metapelites) and garnet-clinopyroxene geotermometers (pyroxenite lenses). GASP barometer, silica content of phengites, and MgCrd=Py+Si+Qz, and Qz+MCtd=Chl+ky+W equilibria were mainly employed for pressure determinations. PT-paths from Balanyá et al. (1997), Tubía y Gil Ibarguchi (1991) and Sánchez-Gómez (1997).

Beyond the main peridotite massifs, thin, discontinuous layers of serpentinite and small, elongated, serpentinized peridotite bodies frequently mark the boundary between the Blanca-group units and the units above it (the Jubrique and, in some instances, a Malaguide unit)(Fig. 3). The nearly uninterrupted rosaries of serpentinite slices suggest an initial continuity of the main peridotite bodies (Navarro-Vilá and Tubía, 1983; Sánchez-Gómez, 1997).

The connection between the distinct massifs can be seen in the geologic map of Figure 3. Here, the succession of serpentinite bodies implies that roughly NE-SW extension separated the components of a continuous peridotite slab, as illustrated by the lateral thinning toward the NE and SW of the Alpujata body. On the other hand, the separation of the main bodies of Bermeja and Alpujata point to a different, E-W extension system. Thus, the north metamorphic branch of the Gibraltar arc outlines a roughly orthogonal pattern of total extension.

Similar superimposed extension has been reported in other areas of the Betic chain (Crespo-Blanc et al., 1995, García-Dueñas et al., 1992). The general boudin-like shape of most Alpujarride units, among them the Ronda peridotite bodies, can be explained as the effect of Miocene extension related to a rifting process that culminated in the formation of the Alboran basin.