Evidence for different substitution relationships between aluminosilicates in two domains east of the O110 de Sapo gneiss antiform in Galicia (NW Spain) suggests a high dependence of metamorphic evolution on the local tectonic and granitoid intrusion history. The Vivero pelite belt constitutes a pinched synformal domain located between the Ollo de Sapo antiform to the west and the Lugo dome to the east. Within this synform, black, A1rich Silurian pelites contain kyanite that, along with muscovite, formed pseudomorphically after early pre-D2 andalusite. Syn-D2 inversion of andalusite to kyanite indicates an evolution from early, low-P to medium-P regimes. Locally, near the Vivero transtensional fault, which constitutes the E boundary of the synform new, late post-D2 pleochroic andalusite grew from staurolite and biotite indicating reinstallment of a new low-P regime during and after the D3 deformation phase, which is related to movement along the fault zone. The Ol10 de Sapo domain contains only a high T-low P series of metamorphic assemblages. The peak assemblage within this domain was cordierite-biotite-K-feldsparandalusite/fibrolite related to emplacement of a pre-to syn-D2 swarm of sheet-like peraluminous granitoids located mainly in the core of the antiform. Early pre-to syn-D2 staurolite-biotite + garnet, and late sillimanite-staurolite-biotite assemblages present on the west side of the Lugo dome testify to an early stage of nearly isothermal decompression followed by decompression with local heating. Stabilization of late andalusite in the Vivero pelite belt and of sillimanite west of the Lugo dome appear almost synchronous and clearly related to a syn-D3 thermal peak induced by Upper Carboniferous granitoid intrusion along the Vivero transtensional fault separating both domains.
Kyanite replaces andalusite in a belt of Ordovician and Silurian pelitic rocks that form a narrow synform pinched between high-grade antiforms in NW Variscan Iberia. Kyanite occurs across the belt in Al-rich, black pelites in assemblages I: kyanite-chloritoid-chlorite-muscovite and II: kyanite-staurolitechlorite-muscovite. In I, kyanite occurs in the matrix and in kyanite-muscovite aggregates that pseudomorph earlier andalusite porphyroblasts. The aggregates are found across the belt and can still be recognized in assemblage II and even in III: andalusite-staurolite-biotite-muscovite, this latter being a hornfelsic Silurian schist where kyanite is relic and staurolite occurs in the matrix, and is resorbed inside new massive pleochroic andalusite. KFMASH and MnKFMASH pseudosections have been constructed using Thermocalc for Al-rich and Al-poorer compositions from the belt. Chloritoid zoning in Al-rich rocks containing assemblage I, plus chloritoid-chlorite thermometry complemented with garnet-chlorite thermometry in Al-poorer lithologies, mean that the path is one of increasing pressure and temperature. Conditions prior to assemblage I, with earlier andalusite stable, are those of the andalusite-chloritoidchlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims. The passage from assemblage I to II implies a prograde path within the kyanite field. Assemblage III represents peak conditions, indicating a prograde staurolite-consuming reaction across a KFMASH field, leading eventually to a locally found andalusite-biotite-muscovite hornfels. The lowest pressure stages are recorded by cordierite-biotite in Al-poor pelites. Garnet-bearing MnKFMASH assemblages in Al-poorer pelites record conditions similar to assemblages II and III. The replacement of andalusite by kyanite in assemblage I is attributed to downdragging of andalusite-bearing rocks into a synform as testified by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage. Prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the Vivero fault.
Metapelitic rocks in the low pressure contact metamorphic aureole around the Susqueda igneous complex, Spain show a number of features that make them an ideal testing ground for the modelling of silica-undersaturated melting. Rocks in the aureole experienced localized depletion in silica by the segregation of quartz veins during a pre-anatectic, regional cordierite-andalusite grade metamorphic event. These rocks were then intruded by gabbroic to dioritic rocks of the Susqueda igneous complex that formed a migmatitic contact metamorphic aureole in the country rocks. This migmatisation event caused quartz-saturated hornfels and restite formation in rocks that had experienced no quartz vein segregation in the previous regional metamorphic event, but silica-undersaturated melting in those rocks that were previously depleted in silica. Silica-undersaturated melting is investigated using a new petrogenetic P-T projection and equilibrium pseudosections calculated in the KFMASH and NCKFMASH systems, respectively. The grid considers quartz absent equilibria and a range of phases that form typically in silica-undersaturated bulk compositions, for example corundum. It is shown that the quartz-rich precursors in the Susqueda contact aureole produced about 10% melt during contact metamorphism. However, most of this melt was extracted leaving behind rocks with restitic bulk compositions and minor leucosome segregation. It is suggested that the melt mixed with the host igneous rocks causing an apparent magmatic zoning from diorite in the centre of the complex to tonalite at the margins. In contrast, the quartz-poor precursors (from which the quartz veins segregated) melted in the silica-undersaturated field producing a range of assemblages including peritectic corundum and spinel. Melting of the silica-undersaturated rocks produced only negligible melt and no subsequent melt loss.
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