Petrología de la aureola metamórfica de la granodiorita de Barcelona en la Sierra de Collcerola (Tibidabo)

Abstract

The Paleozoic materials outcropping in the Sierra de Collcerola área (Tibidabo) range from Upper Ordovician up to Carboníferous. These materials approximately define a syncline structure wich minor folds facing to SE or SSE and show the development of a marked slatycleavage. The structures of WNW-ESE direction are cut near Barcelona by a granodioritic intrusion (granodiorite of Barcelona). This intrusion does not show any evidence of regional deformation and presents an oblique contact to the Hercynian structures of the host rocks, affecting, therefore, a great variety of lithologies, the main ones being: 1) metapelites and metapsamites; 2) calcareous and calc-silicate rocks, and 3) metabasites (likely derived from gneous protoliths). The materials affected by the contact metamorphism belong to the Silurian and Upper Ordovician. Previously to the intrusion the Paleozoic materials were affected by a regional metamorphism, syn-kinematic with the development of the main schistosity, which did not surpass conditions of the greenschists facies. As a result of the intrusion of the granodiorite, the host materials underwent a considerable recrystallization which affected rocks situated at even more than two km from the contact. The calcareous and calc-silicate rocks are the first to show the effects of contact metamorphism; metabasites go next, and finally metapelites and metapsamites show those effects too. Calcareous and calc-silicate rocks are the more reactive too during the metamorphic event giving rise to a great variety of minerals through the contact aureole: chlorite, biotite, actinolite, epidote, homblende, clinopyroxene, idocrase, grossular/andradite and scapolite in this order; in the basic rocks a generalized development of homblende and locally of pyroxene took place, while in the pelitic and semipelitic rocks cordierite and andalusite generally formed (the latter only in those levels corresponding to ancient black shales of the Silurian), and only locally corundum, spinel, sillimanite and scapolite were developed. An important feature of most of the studied rocks is the existence in them of local equilibria; this must be Iikely the result of minor variations in the composition of the vapor phase and/or bulk rock system. The changes in the chemical composition of the tluids could be due to the existence of numerous millimetric to metric altemances of contrasting lithologies (e. g. pelites-earbonates or pelites-basic rocks), although they could also greatly be due to the intlux of tluids proceeding from the emplacement of the granodiorite. In effect, most rocks of the aureole present mineral associations of the homblendehomfels facies, nevertheless, locally, in places situated at 200 m of the contact, associations of the pyroxene-hornfels facies developed (corundum-spinel-sillimanite, etc.), which suggests that besides normal heating by thermal conduction, there must exist and additional heat supply, Iikely related to convecting tluids, induced by the intrusion of the granodiorite. Although under such circurnstances estimation of P-T conditions of the contact metamorphism is very difficult, on the basis of the chemical composition of analyzed minerals, and taking into account stratigraphic considerations for the situation previous to the emplacement of the granodiorite, it may be suggested that the maximum P-T conditions reached during the contact metamorphism were around 1.5 kbar and 700°C.