Abstract:The Peña Negra complex, in central Spain, provides a superb opportunity for the study of Variscan partial melting processes. The heat source for the anatexis has been controversial. The presence of mafic rocks in the vicinity has given rise to the suggestion that mafic magmas of mantle origin provided the requisite heat for the generation of the anatectic granites. Experimental petrology can be used to support this concept, but the volumes of mafic magma necessary for the anatexis appear improbably large. Geophysical models for Central Iberia do not support the presence of such large basic complexes at depth. Furthermore radiometric dating of the scarce mafic outcrops indicates that they are too old to have been involved in the granitic magma genesis. The alternative interpretation is that melting of the protolith was promoted by the radioactive decay of K, U and Th. Anatexis was enhanced by the existence of a fertile protolith and by the presence of shear structures that permitted volatile fluxing. The situation is believed to have been analogous to that in other European Variscan terrains.
A large anatectic complex of Hercynian age, the Peña Negra complex, crops out in the central-north area of the Sierra de Gredos, Avila batholith, central Spain. Migmatites and anatectic granitic rocks were generated during the Upper Carboniferous from a Neoproterozoic -Lower Cambrian schist-greywacke complex with minor interlayered orthogneisses. Granitic rocks appear as subhorizontal sheets of megacrystic granodiorite and as lenticular bodies of leucogranite. These formed by in situ anatexis and correspond to melt-restite segregations under two different conditions, namely high melt-fraction in the case of the granodiorite, and low melt-fraction in the case of the leucogranites. The leucogranites display large enclaves of sillimanite-rich material, representing the restitic fraction. Granodiorite was generated as consequence of crustal thickening under conditions of T ≈ 750 to 800°C and P ≈ 4 kbar, coinciding with the second phase of the Hercynian orogeny. After this event, an extensional episode involved smaller degrees of decompression-induced melting, and generated small batches of cordierite leucogranites from an already depleted source. Anatexis was enhanced, in both cases, by shear structures and by the existence of a fertile protolith.Keywords: Hercynian orogeny, Peña Negra Complex, Schist-Greywacke Complex, migmatites, anatexis, cordierite, Spain. SOMMAIREUn massif anatectique important d'âge hercynien, que nous appelons le complexe de Peña Negra, affleure dans la partie centre-nord de la Sierra de Gredos, batholithe d'Avila, en Espagne centrale. Les migmatites et les roches granitiques anatectiques ont été formées au cours d'un événement carbonifère supérieur aux dépens d'un socle de schistes et grauwackes avec intercalations mineures d'orthogneiss, d'âge néoprotérozoïque ou cambrien inférieur. Les roches granitiques semblent se présenter en feuilles subhorizontales de granodiorite mégacristique et en lentilles de leucogranite. Ces roches ont été formées par anatexie in situ et illustreraient une ségrégation du liquide silicaté de la fraction résiduelle sous deux régimes différents, le premier impliquant une fraction de liquide élevée (cas de la granodiorite) et l'autre, une fraction de liquide réduite (cas du leucogranite). Les lentilles de leucogranite contiennent de grosses enclaves enrichies en sillimanite, qui représentent la fraction résiduelle. La formation de la granodiorite résulte de l'épaississement de la croûte et a eu lieu entre environ 750 et 800°C à une pression de 4 kbar, lors de la seconde phase de l'orogenèse hercynienne. Suite à cet événement, une période d'extension a provoqué une fusion plus limitée accompagnant la décompression, et donc a produit de plus petits volumes de leucogranite à cordiérite à partir d'une source déjà rendue relativement stérile. L'anatexie a été favorisée, dans les deux cas, par la présence de structures de cisaillement et d'un protolithe fertile.(Traduit par la Rédaction)
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