Newsletter 2 – March 2016

CREEPers on the outcrops: Cap de Creus field trip

CREEP is not only indoor!

The second short course held in Montpellier was followed by a field trip in the close locality Cap de Creus, Spain, where the easternmost part of the Pyrenees crop out with outstanding examples of crustal deformation. There we could see by ourselves the geometries and other features of highly deformed rocks, which are the product of the mechanisms explained during the previous days in Montpellier.

Cropping-out rocks belong to the Variscan (~300Ma) basement, which is essentially composed by sedimentary sequences, pre-Variscan and Variscan intrusives (mainly granodiorites and pegmatites) that underwent high-temperature low-pressure metamorphism, even up to migmatitisation. The field trip programme focused on the two main shear zones that wrap to the north and to the south the antiform fold of the basement, whose hinge follows a NW-SE trend.

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Fig. 1   The stunning landscape of Cala Serena. The narrow dell on the left follows the geometry of a mylonitic shear zone.

The first day we visited the outcrops near C. Serena (fig. 1), C. Prona and P. Farallons. This domain belongs to the northern NE-dipping mylonitic belt hosted in high-grade syllimanite-bearing metasedimentary sequences and minor intrusions. The easy walk follows a path in the Mediterranean scrub with stunning examples of strain localisation every few footsteps.  Here the partitioning of deformation is straightforwardly shown in high-strain mylonitic domains and folded low strain domains (fig. 2).  Locally, pegmatite dikes and quartz veins are dragged into these shear zones producing nice drag folds and pinch and swell structures (fig. 3). Other notable features are the extreme isoclinal folding and the development of spectacular sheath folds in the high strain domains, which are locally unveiled by the erosion. At Punta dels Farallons crops out a migmatite complex intruded by thick pegmatite dikes and other felsic intrusives including a peculiar garnet-rich granodiorite. Although deformation mechanisms are less efficient in pegmatites, these dikes show nice internal partitioning of strain locally highlighted by boudinaged schorl tourmaline crystals and domino-like sheared feldspar crystals.

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Fig. 2 (left) Mylonitic shear zone at Cala Prona showing nice strain localisation and drag folding.

Fig. 3 (right) Pegmatite dike dragged inside a mylonitic shear zone and showing ductile pinch and swell structure.

The second day we investigated the outcrops located in the southern SW-dipping shear zone near the town of Roses, just on the coast in beneath the lighthouse. Here the dominant lithology is a deformed granodiorite intruded by several leucocratic dikes (pegmatites to aplites), which emplaced with diachronous syn- or post- deformation ages. Here are exposed highly localised shear zones of at least two generations, which host a green-schists mineral association.  The sharpness of these mylonites and the offset of unambiguous markers (for example the displaced dike in fig. 4) allow a good estimation of the finite strain. Notable the schlieren texture of mafic enclaves and local magma mingling.

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Fig. 4 Leucocratic dike (on the right) showing a clear sinistral displacement of the shear zone.

It is worthless to say that this field trip was warmly welcomed by us CREEPers. Indeed, these two days have been a good way to see the rheology of rocks from the point of view of a field geologist. For some ESRs from other background, it was also an excellent way to combine the knowledge from short course to the real field, which can be a better understanding of the geologic world.

Moreover we enjoyed the really nice weather of the upcoming Mediterranean spring, the nights in Cadaques, really delicious food and, most of all, the time spent together. We are for sure looking forward to the next CREEP trip in the open air!

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Fig. 5 Andréa Tommasi (with orange shoes on the right) and the CREEPers.

Giacomo, Linfeng and Wen

 

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