Summary
The Miocene volcanic suite of Sierra de Gata (Betic Cordillera) is composed of dacites, rhyodacites and rhyolites, besides basaltic andesites and andesites. The latter are very fresh and are the least fractionated rock types; thus they have been investigated in order to put some constraints on the genetic interpretation of the volcanic sequence. These rocks are composed of plagioclase, orthopyroxene, clinopyroxene and magnetite both as phenocrysts and in the groundmass. Olivine and ilmenite occur occasionally. The andesites exhibit “orogenic” chemical features. They are characterized by very low Ni, Co and Cr contents and show negative Eu anomalies, which support fractionation of mafic phases and plagioclase at depth. The petrogenesis of the andesites, however, cannot be related simply to fractionation processes of a single mafic magma since the lack of good interelemental correlations, the scattering of Sr-isotope ratios and the isotope disequilibrium between the phases conflict with such a simple mechanism. A model which can better explain the petrogenesis of the analyzed andesites considers the intervention of different mafic melts, generated in a heterogeneous mantle and characterized by different chemical and isotopic compositions, which fractionated at depth and concomitantly interacted significantly with the continental crust.
Zusammenfassung
Die miozäne Vulkanitabfolge der Sierra de Gata (Betische Kordillere) wird von Daciten, Rhyodaciten, Rhyoliten sowie basaltischen Andesiten and Andesiten aufgebaut, letztere sind in einem guten Erhaltungszustand and zeigen die geringste Fraktionierung. Sie wurden daher genauer untersucht, um Aussagen über die Genese dieser Vulkanite machen zu können.
Phenokrysten und Grundmasse bestehen aus Plagioklas, Orthopyroxen, Klinopyroxen und Magnetit. Manchmal kommen Olivin und Ilmenit hinzu. Die Geochemie der Andesite folgt typisch “orogenen” Trends. Sie sind durch sehr niedrige Ni-, Co- und Cr-Gehalte und negative Eu-Anomalien charakterisiert; dies belegt eine Fraktionierung der mafischen Gemengteile und des Plagioklases in der Tiefe. Die petrogenetische Entwicklung der Andesite kann allerdings nicht einfach mit Fraktionierungsprozessen eines einzigen mafischen Magmas erklärt werden. Das Fehlen von Element-Korrelationen, das starke Streuen der Sr-Isotopenverhältnisse und das Isotopenungleichgewicht zwischen den einzelnen Mineralphasen stehen in Widerspruch zu einem derartigen einfachen Modell. Ein Modell, das die Petrogenese der untersuchten Andesite besser erklärt, zieht die Beteiligung verschiedener mafischen Schmelzen-sie bildeten sich in einem heterogenen Mantel und sind durch Unterschiede im Chemismus und ihrer Isotopenzusammensetzung charakterisiert—in Betracht. Dabei kam es zu Fraktionierung dieser Magmen in der Tiefe und zu beträchtlichen Reaktionen mit kontinentalem Krustenmaterial.
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Toscani, L., Venturelli, G., Barbieri, M. et al. Geochemistry and petrogenesis of two-Pyroxene andesites from Sierra de Gata (SE Spain). Mineralogy and Petrology 41, 199–213 (1990). https://doi.org/10.1007/BF01168495
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DOI: https://doi.org/10.1007/BF01168495