Summary
A metadolerite of the ophiolites of the Northern Apennines (Vignola, Panaro Valley, near Modena, Italy) behaves as ocean-floor tholeiites when Ti, P, Zr, Cr, Y and Nb are considered. However, it deviates from typical oceanic tholeiites in that(1) it contains primary alkali amphiboles, (2) its clinopyroxenes plot in the diagram TiO2−MnO−Na2O not in the fields of the ocean-floor basalts, and (3) the calculated uranium content of the liquid in equilibrium with its magmatic minerals is similar to the one reported for alkali basalts.
These results put constraints on the generalized significance attributed to the ophiolites of the Northern Apennines, i. e. not all of them were generated at spreading ridges similar to the “normal ridge segments” of the present-day ocean-floors, as has been generally accepted by now.
The metamorphic parageneses indicate that the rock underwent low-pressure transformations, probably in oceanic environment, followed by recrystallizations at higher pressure, reasonably in concomitance with an orogenesis.
Zusammenfassung
Ein Metadolerit aus den Ophiolithen des Nördlichen Apennins (Vignola im Panaro-Tal bei Modena, Italien) entspricht in den Gehalten an Ti, P, Zr, Cr, Y und Nb den Tholeiiten der Ozeanböden. Er weicht hingegen von typischen ozeanischen Tholeiiten dadurch ab, daß er 1. primäre Alkaliamphibole enthält, daß 2. seine Klinopyroxene im Diagramm TiO2−MnO−Na2O nicht in die Felder der Tholeiite der Ozeanböden fallen und daß 3, der berechnete U-Gehalt für die mit seinen magmatischen Mineralen im Gleichgewicht stehenden liquiden Phase ähnlich jener ist, die für Alkalibasalte angegeben wird.
Diese Ergebnisse schränken die generelle Bedeutung, die den Ophiolithen des Nördlichen Apennins bisher allgemein zugeschrieben wurde, ein: Nicht alle entstanden an sich ausdehnenden Rücken ähnlich jenen der heutigen Ozeanböden.
Die metamorphen Parageneses zeigen, daß das Gestein bei niedrigem Druck, wahrscheinlich in ozeanischer Umgebung, Umwandlungen erlitt; dann folgten Rekristallisationen unter höherem Druck, die wohl zeitlich mit einer Orogenese zusammenfielen.
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Capedri, S., De Negri, G. & Venturelli, G. Clinopyroxenes and amphiboles in a metadolerite from the northern apennines. Implications on the palaeogeographic role of ophiolites. TMPM Tschermaks Petr. Mitt. 26, 21–37 (1979). https://doi.org/10.1007/BF01081288
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DOI: https://doi.org/10.1007/BF01081288