The structure of the NE Cap de Creus peninsula.
Relationships with metamorphism and magmatism.

ABSTRACT

This work concentrates in the tectonics of the deepest-seated domains exposed in the northern Cap de Creus peninsula, and in the associated magmatism and metamorphism. The Cap de Creus peninsula forms the most easterly outcrop of Paleozoic basement along the Axial Zone of the Pyrenees, in which penetrative foliations, metamorphism and magmatism are Hercynian in age. Rocks outcropping in the study area correspond to a metasedimentary sequence with minor interlayered meta-igneous rocks.
The presence of a metamorphic gradient, where grade increases towards the north, reflects a prograde low pressure regional metamorphism, initiated during the early deformational events, and ranges from the chlorite - muscovite zone in the south to the sillimanite - K-feldspar zone in the north. Locally, in medium and high grade domains, small migmatite areas are present, together with a sequence of calc-alkaline granitoids, emplaced sequentially from the more basic magmas to the more acid ones. A swarm of pegmatite dykes extends over the anatectic and perianatectic domains up to the cordierite-andalusite zone. A retrograde metamorphism is heterogeneously distributed along late structures, especially along mylonitic bands, and appears superimposed on the prograde metamorphic pattern.
Polyphasic structures are interpreted as part of a progressive deformational history, with an evidenced continuity of events from prograde to retrograde metamorphic conditions. However, three main deformational events have been distinguished. The early event (D1) developed a penetrative, originally shallow-dipping, schistosity (S1). Afterwards, a sequence of regional deformation events caused steep structures. Each event was inhomogeneous in space and characterized by progressive non-coaxial deformation, leading to a complex structural pattern. The D2 event produced NE-SW trending folds affecting the S1 schistosity in prograde metamorphic conditions. Fold-related strain gradients across the area define a structural zonation, with domains of low and high strain and a broad shear zone-like geometry. High strain deformation took place preferentially in the north, and was associated to migmatization and granitoid emplacement. A transitional stage between D2 and D3 events (named D2-3) is characterized by the development of sub-vertical E-W trending folds. This deformational stage took place around the time of peak metamorphism grading to high temperature retrogression in the north (where the pegmatite dyke swarm emplaced syntectonically), but under clear retrograde conditions in central and southern domains. Progressive deformation at retrograde conditions produced NW-SE-trending D3 structures and gave rise to a second structural zonation, with a fold belt covering the southern part of the studied area, and a mylonite belt tracking the higher grade metamorphic zones.
The studied relationships between high temperature structures (D2 and D2-3) in the metasediments and the deformational features in the granitoids and pegmatites support the synchronicity of deformational processes and magmatism. In addition, strain and kinematic analysis of these structures reveals that, dispite the existence of complex relationships between geometry and kinematics, deformation took place in a broadly transpressive regime, involving subvertical extension and NNW-SSE subhorizontal bulk shortening with a dextral component.