P-T­-t-d constraints on the Late Variscan evolution of the Eastern Pyrenees

Abstract

Structural, petrological, microstructural and geochronological studies were combined with pseudosection modeling in the Roc de Frausa Massif (Eastern Pyrenees). The massif is constituted by Upper Proterozoic–Early Cambrian rocks and represents a mid crustal section intruded by igneous rocks. The aim is to compare the thermal evolution of different crustal levels in a single orogenic event. The rocks constituting the massif were part of the northern margin of Gondwana and were involved in the Variscan orogeny when Gondwana collided with Laurentia-Baltica.

Two main Variscan deformation events are distinguished. D1 is marked by tight to isoclinal small-scale folds and a sub-horizontal foliation. D2 structures are tight upright folds facing to the NW with steep NE–SW axial planes. In the high-grade metamorphic domains it transposes S1 foliation by a sub-vertical S2 foliation. D3 structures are characterized by NW–SE folds compatible with steep dextral shear zones that retrograde the pre-Variscan rocks and the Variscan intrusives to greenschist facies. N and S of the studied area F3 folds involve Mesozoic rocks, and therefore it can be attributed to the Alpine orogeny.

In the micaschists of the Upper crustal levels, andalusite porphyroblasts with S1 inclusion trails and sillimanite in S1 pressure shadows indicate heating from 580 °C to 640 °C. Cordierite includes the former minerals and does not exhibit pressure shadows pointing to isothermal decompression from 3.4 to 2.6 kbar. A calc-alkaline granitoid intruded on top of this level interkinematically between D1 and D2 (314–311 Ma). Intermediate crustal levels are dominated by schists with sillimanite−biotite−muscovite in the S1 fabric overgrown by cordierite and K-feldspar with no pressure shadows. These assemblages point to decompression from 5 to 3 kbar at 640−660 °C. A gabbro-diorite stock intruded in this level coeval with the D2 in two magmatic pulses (312 and 307 Ma). In the inner aureole four types of migmatites have been characterized, with different textures, mineral assemblages, mineral chemistry and whole-rock compositions. The biotite−sillimanite−K-feldspar−garnet assemblage together with garnet zoning is compatible with heating within the S1 fabric at peak-pressure of 7 kbar and 730 °C. F2 folds with subvertical melt-filled S2 foliation were overgrown by cordierite indicating mainly syn-D2 decompression to 4.5 kbar. The different whole-rock and mineral compositions and the preservation of the mineral assemblages of migmatitic rocks can be explained by different episodes of melt-loss and fluid infiltration in the metasediments at 790 °C and 5.5 kbar and incorporation of the fluids into the melt. The fluids would be released by the crystallizing gabbro-diorite. Lower crustal levels are dominated by fold-structured migmatites occurred in the interval 320–315 Ma. They present a biotite−cordierite composite S1-S2 biotite-bearing fabric with relic garnet embedded in plagioclase, thus precluding deciphering the early metamorphic evolution. Cordierite overgrowing both S1 and S2 fabric points to late equilibration at 3 kbar and 700 °C.

The early metamorphic history associated with the S1 fabric is interpreted as a result of lower crustal horizontal flow and could be related to moderate crustal thickening. D2 event is characterized by decompression associated to highly heterogeneous exhumation of the metamorphic complex during the last stages of the Variscan evolution.

Se ha realizado un estudio estructural, petrológico, geoquímico, geocronológico y de modelización térmica en rocas pelíticas e ígneas del macizo de Roc de Frausa (Pirineo oriental) con la finalidad de comparar la evolución térmica de diferentes niveles de la corteza terrestre en un mismo evento orogénico. Las rocas que conforman el macizo formaban parte del margen norte de Gondwana y quedaron involucradas en la orogenia varisca al colisionar Gondwana con Laurentia-Baltica. Las rocas constituyen una serie Proterozoica superior a Paleozoica inferior afectadas por tres episodios deformativos, los dos primeros de edad varisca y el tercero alpina.

En los micaesquistos con andalucita–sillimanita del nivel cortical superior, la andalucita y la sillimanita indican un incremento de temperatura de 580 °C a 640 °C coetáneo a la fábrica S1. La cordierita indica una descompresión isotérmica de 3.4 kbar a 2.6 kbar. Por encima de este nivel se emplazó un granitoide calco-alcalino intercinemáticamente entre D1 y D2 (314–311 Ma). Los niveles corticales medios están formados por esquistos con sillimanita-biotita-moscovita sin-S1 sobrecrecidos por cordierita y feldespato potásico post-S1. Estas asociaciones señalan una descompresión desde 5 a 3 kbar a 640–660 °C. Un stock gabro-diorítico intruyó sincrónico a D2 en dos pulsos magmáticos (312 y 307 Ma). En la aureola de contacto se distinguen cuatro tipos de migmatitas con diferentes texturas, asociaciones minerales y composiciones de roca y mineral que se explican mediante un modelo de perdida de fundido alternando con episodios de infiltración de fluido. Las asociaciones minerales indican condiciones PT de 7 kbar y 730 °C sin-D1 y una descompresión sin-D2 a 4.5 kbar. Los niveles corticales inferiores están formados por migmatitas cristalizadas entre 320–315 Ma. La cordierita sobrecrece las fábricas S1 y S2 ricas en biotita y con granate relicto incluido en plagioclasa e indica un equilibrio tardío a 3 kbar y 700 °C.

D1 se interpreta como el resultado de un flujo horizontal de la corteza inferior, que podría relacionarse con un engrosamiento cortical moderado. D2 se caracteriza por una descompresión isotérmica que puede explicarse por una exhumación varisca tardía de los complejos metamórficos.