Mantle in the mountains Day 2

Nice view which they were admiring when we found them

This was a very long day as we had to go right back down to the coast and along to Estepona, where Jan and I lost the rest of the convoy but with the help of Emily Satnav found them again on the road up the hill towards Stop 1 in the parking area at Puerto de Penas Blancas: this is by the junction near Km 14  at the start of the track up to Pico de los Realajes de Sierra Bermeja.
At the point where we found them we had a good view to Gibraltar and the Rif mountains, the extension of the arcuate mountain belt and, in between, the Alboran basin which has subsided 7 km.  

"How do you date a dyke?" Location 1

I'd come across the term "bermeja" before and now found that it meant "ginger coloured" - very appropriate since this is the rusty orange weathered colour of the iron rich peridotite, of which these hills are made. The peridotite sheet forms a massif in the Betic Internal Zone.

It's a complicated body with cm scale heterogeneity. There are 4 main zones but as well as the ultramafic harzburgite and lherzolite there is a suite of other rocks. The zones aren't inverted, the zone order is a function of preservation of stages in its history.
Rh-Os dating gives an age of 1.36 Ga, this is when the rock emerged from the mantle and was incorporated in continental crust.
In Jurassic the plate thinned, rifted, mantle melting with uprise, decompression. This allowed mafic veins to form.

Location 1

Close up of the leucocratic dyke material

At the back of the parking area we found the peridotite is cut by a leucocratic dyke, with brittle fracturing, sloping down to left at 30°. Presence of plagioclase indicates shallow mantle. Further up hìll (near the cyclist mirador) is annealed granular spinel peridotite. Higher still is spinel peridotite with tectonic foliation. Odd that higher pressure facies at top and lower pressure plagioclase lower down but these represent different time steps in the story. Right near top boundary are spinel peridotites with garnet.

There's serpentinisation related to the granitic dyke along a serpentinised zone. The dyke probably originated below the peridotite and a partial melt was injected upwards. Composition is quartz and feldspar and tiny black specks probably tourmaline. Also blue green diopside (chromite).

Location 2

We drove on down the hill, turning left and going about two miles on a forestry track which was level but rough. 

Leaving the granular spinel peridotite we found stripes (foliation) in rocks which were picked out by weathering. We were passing into the garnet peridotite zone where thin mafic layers contain garnets.

Peridotite with thin mafic layers dotted with garnets

This is near the contact with the overlying crust: mafic layers were formed in ultramafic mantle by partial melting, then deformed and stretched. In some places garnets are little swells in thin layers; these rocks were described by Tom as  "marginal mylonites". We also saw lineation formed by alignment of elongate cpx.

Differential weathering of layered peridotite

Back to junction where some "hippy sort" was wandering around doing some type of shaman ritual drum beating! Get all sorts on the costas!

Los Reales mirador

Location 3

A steep, winding road took us up to the refugio where we parked at Plazoleta Salvador Guerrero and walked 250 m or so to the viewpoint which was spectacular: views all way around the arcuate orogeny through the Betics, Gibraltar and round to Rif. 

Ibex

Discussed Alboran Basin, volcanics, seismics and, as we returned to the cars, saw an ibex.

Location 4

Continuing back down towards Estepona we stopped by an arroyo. Not on peridotite, because no pine trees! We are learning!
The bedrock here is migmatite breccia as seen Day 1, loose in road cut. Boulders conrain chrome diopside from higher up, sometimes in layers.

Hand specimen of peridotite from Arroyo de la Cala with green chrome diopside