P-T-t-d evolution of orogenic middle crust of the Roc de Frausa Massif (Eastern Pyrenees): a result of horizontal crustal flow and Carboniferous doming?

摘要

Structural, petrological and textural studies are combined with phase equilibria modelling of metapelites from different structural levels of the Roc de Frausa Massif in the Eastern Pyrenees. The pre-Variscan lithological succession is divided into the Upper, Intermediate and Lower series by two orthogneiss sheets and intruded by Variscan igneous rocks. Structural analysis reveals two phases of Variscan deformation. D1 is marked by tight to isoclinal small-scale folds and an associated flat-lying foliation (S1) that affects the whole crustal section. D2 structures are characterized by tight upright folds facing to the NW with steep NE-SW axial planes. D2 heterogeneously reworks the D1 fabrics, leading to an almost complete transposition into a sub-vertical foliation (S2) in the high-grade metamorphic domain. All structures are affected by late open to tight, steeply inclined south-verging NW-SE folds (F3) compatible with steep greenschist facies dextral shear zones of probable Alpine age. In the micaschists of the Upper series, andalusite and sillimanite grew during the formation of the S1 foliation indicating heating from 580 to 640 degrees C associated with an increase in pressure. Subsequent static growth of cordierite points to post-D1 decompression. In the Intermediate series, a sillimanite-biotite-muscovite-bearing assemblage that is parallel to the S1 fabric is statically overgrown by cordierite and K-feldspar. This sequence points to similar to 1kbar of post-D1 decompression at 630-650 degrees C. The Intermediate series is intruded by a gabbro-diorite stock that has an aureole marked by widespread migmatization. In the aureole, the migmatitic S1 foliation is defined by the assemblage biotite-sillimanite-K-feldspar-garnet. The microstructural relationships and garnet zoning are compatible with the D1 pressure peak at similar to 7.5kbar and similar to 750 degrees C. Late- to post-S2 cordierite growth implies that F2 folds and the associated S2 axial planar leucosomes developed during nearly isothermal decompression to <5kbar. The Lower series migmatites form a composite S1-S2 fabric; the garnet-bearing assemblage suggests peak P-T conditions of >5kbar at suprasolidus conditions. Almost complete consumption of garnet and late cordierite growth points to post-D2 equilibration at <4kbar and <750 degrees C. The early metamorphic history associated with the S1 fabric is interpreted as a result of horizontal middle crustal flow associated with progressive heating and possible burial. The upright F2 folding and S2 foliation are associated with a pressure decrease coeval with the intrusion of mafic magma at mid-crustal levels. The D2 tectono-metamorphic evolution may be explained by a crustal-scale doming associated with emplacement of mafic magmas into the core of the dome.