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Garnet-spinel-pyroxenite xenoliths from hyblean plateau (South-eastern Sicily, Italy)

Granat-Spinell-Pyroxenit-Xenolithe aus dem Iblei-Plateau (Südost-Sizilien, Italien)

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Summary

Rare garnet-spinel pyroxenite xenoliths occur in some basaltic tuff-breccia levels of Miocene age from the Valle Guffari (Hyblean Plateau, Sicily), together with a number of spinel-bearing mantle xenoliths. The garnet-bearing pyroxenites may be divided into two groups (“a” and “b”) on textural and mineralogical bases. Garnet-bearing spinel websterites with a fully recrystallized texture represent the first group (a). Here the garnet (Py54.5 A1m32 Gr13.5), with a diffuse kelyphitic alteration, forms a reaction corona between coarse spinel grains and the in contact pyroxenes. The transition from the spinel-pyroxenite to the garnet-pyroxenite field may depend on isobaric cooling from higher (magmatic?) temperatures. Garnet-pyroxene geothermometry indicates that the last equilibration most probably occurred at P = 1.0 GPa (ca.), T = 750 °C (ca).

The second lithotype (b) is an orthopyroxene-bearing garnet-spinel clinopyroxenite, exhibiting a complex texture. It consists of zones of coarse clinopyroxene grains enclosing euhedral spinel passing to zones where tiny rounded crystals of the same pyroxene and spinel are enclosed in relatively large patches of extensively kelyphitisized garnet (Py64.8 Alm25.6 Gr9.6). Garnet also occurs as inclusion-free grains up to 4 mm in diameter. P-T calculations give significantly higher values than for the former case (a). The origin of the b-type garnet may also depend on subsolidus reaction of spinel and pyroxenes after an isobaric cooling from still higher temperatures, but a primary magrnatic origin might also be possible, especially for the “granular” garnets.

P-T estimates for both the pyroxenite types closely match a steady geotherm for 100 mW/m2 surface heat flow. Such a relatively intense heat flow may suggest the occurrence of huge masses of hot magma intruding the Hyblean lithospheric mantle and lower crust at different levels.

Zusammenfassung

Seltene Granat-Spinell-Pyroxenit-Xenolithe kommen in einigen basaltischen Tuff-Breckzien Horizonten miozänen Alters aus dem Valle Guffari (lblei-Plateau, Sizilien) zusammen mit einer Anzahl von Spinell-führenden Mantel-Xenolithen vor. Aufgrund textureller und mineralogischer Kriterien können die Granat-führenden Pyroxenite in zwei Gruppen („a” und „b”) unterteilt werden. Granat-führende Spinell-Websterite mit vollkommen rekristallisierter Textur repräsentieren die erste Gruppe („a”). Hier bildet Granat (Py54.5 Alm32 Gr13.5) mit einer diffusen kelyphitischen Umwandlung, einen Reaktionssaum zwischen grobkörnigem Spinell und Pyroxenen, mit denen er in Kontakt ist. Der übergang vom Spinell-Pyroxenit- zum Granat-Pyroxenit-Feld kann auf isobarische Abkühlung von höheren (magmatischen ?) Temperaturen zurückgehen. Granat-Pyroxen-Geothermometrie zeigt, dass die letzte Equilibrierung sehr wahrscheinlich bei P = 1.0 GPa (ca.), T = 750°C (ca.) erfolgte.

Der zweite Typ von Granat-führenden Pyroxeniten ist ein („b”) Orthopyroxenführender Granat-Spinell-Klinopyroxenit, der komplexe Texturen zeigt. Er besteht aus Zonen von grobkörnigem Klinopyroxen mit Einschlüssen von idiomorphem Spinell, der in Zonen übergeht, wo kleine gerundete Kristalle des gleichen Pyroxens und Spinells in relativ große Bereiche von extensiv kelyphitisiertem Granat (Py64,8 Alm25,6 Gr9,6) eingeschlossen sind. Granat kommt auch als einschlußfreie Körner mit bis zu 4 mm Durchmesser vor. P-T Berechnungen geben wesentlich höhere Werte als für die Gesteine des Types („a”). Die Entstehung der b-Typ-Granaten kann auch durch Subsolidus-Reaktion von Spinell und Pyroxen nach isobarischer Abkühlung von noch höheren Temperaturen beeinflußt sein; ein primärer magmatischer Ursprung könnte auch möglich sein, besonders für die „körnigen” Granate.

P-T Abschätzungen für beide Pyroxenit-Typen sind gut einer Geotherme für 100 mW/m2 Wärmefluß an der Oberfläche zuzuordnen. Ein solcher, relativ intensiver Wärmefluß könnte auf das Vorkommen von großen heißen Magmenkörpern hinweisen, die den lithosphärischen Mantel unter dem Iblei-Plateau und die untere Kruste in verschiedenen Niveaus intrudierten.

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Authors and Affiliations

  1. Dipartimento di Scienze Geologiche, Sezione Scienze della Terra, Università di Catania, Corso Italia 55, 1-95129, Catania, Italy

    P. Atzori, P. Mazzoleni, R. Punturo & V. Scribano

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  1. P. Atzori
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  2. P. Mazzoleni
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  3. R. Punturo
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  4. V. Scribano
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Atzori, P., Mazzoleni, P., Punturo, R. et al. Garnet-spinel-pyroxenite xenoliths from hyblean plateau (South-eastern Sicily, Italy). Mineralogy and Petrology 66, 215–226 (1999). https://doi.org/10.1007/BF01164494

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