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 spinelwebsterites with a fully recrystallized texture represent the first group (a). Here the garnet (PY54.5 Alm32 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 Alto25.6 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 magmatic origin might also be possible, especially for the "granular" garnets. P-T estimates for both the pyroxenite types closely match a steady geotherm for 100mW/m 2 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.

Garnet-spinel-pyroxenite xenoliths from Hyblean Plateau (South-eastern Sicily, Italy)

MAZZOLENI, Paolo;PUNTURO, Rosalda;SCRIBANO, Vittorio
1999

Abstract

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 spinelwebsterites with a fully recrystallized texture represent the first group (a). Here the garnet (PY54.5 Alm32 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 Alto25.6 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 magmatic origin might also be possible, especially for the "granular" garnets. P-T estimates for both the pyroxenite types closely match a steady geotherm for 100mW/m 2 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.
Seltene Granat-Spinell-Pyroxenit-Xenolithe kommen in einigen basaltischen Tuff-Breckzien Horizonten mioz~nen Alters aus dem Valle Guffari (Iblei-Plateau, Sizilien)zusammen mit einer Anzahl yon Spinell-ffihrenden Mantel-Xenolithen vor. Aufgrundtextureller und mineralogischer Kriterien k6nnen die Granat-ffihrenden Pyroxenite inzwei Gruppen (,,a" und ,,b") unterteilt werden. Granat-ftihrende Spinell-Websterite mitvollkommen rekristallisierter Textur reprfisentieren die erste Gruppe (,,a"). Hier bildetGranat (PY54.5 Alm32 Gr13.5) mit einer diffusen kelyphitischen Umwandlung, einenReaktionssaum zwischen grobk6rnigem Spinell und Pyroxenen, mit denen er inKontakt ist. Der U-bergang vom Spinell-Pyroxenit- zum Granat-Pyroxenit-Feld kann aufisobarische Abkfihlung von h6heren (magmatischen ?) Temperaturen zur~ckgehen.Granat-Pyroxen-Geothermometrie zeigt, dass die letzte Equilibfierung sehr wahrscheinlichbei P = 1.0 GPa (ca.), T = 750 °C (ca.) effolgte.Der zweite Typ von Granat-ffihrenden Pyroxeniten ist ein (,,b") OrthopyroxenffihrenderGranat-Spinell-Klinopyroxenit, der komplexe Texturen zeigt. Er besteht ausZonen von grobk6rnigem Klinopyroxen mit Einschltissen von idiomorphem Spinell, derin Zonen tibergeht, wo kleine gerundete Kristalle des gleichen Pyroxens und Spinells inrelativ groge Bereiche von extensiv kelyphitisiertem Granat (PY64,8 Alm25,6 Grg,6)eingeschlossen sin& Granat kommt auch als einschlugfreie K6rner mit bis zu 4 mmDurchmesser vor. P-T Berechnungen geben wesentlich h6here Werte als ftir dieGesteine des Types (,,a"). Die Entstehung der b-Typ-Granaten kann auch durchSubsolidus-Reaktion von Spinell und Pyroxen nach isobarischer Abktihlung yon nochh6heren Temperaturen beeinflul3t sein; ein prim~irer magmatischer Ursprung k6nnteauch m6glich sein, besonders ftir die ,,k6rnigen" Granate.P-T Absch~itzungen ftir beide Pyroxenit-Typen sind gut einer Geotherme ftir100 mW/m; W/irmeflug an der Oberflfiche zuzuordnen. Ein solcher, relativ intensiverWfirmeflug k6nnte auf das Vorkommen von grogen heigen Magmenk6rpern hinweisen,die den lithospNirischen Mantel unte
Xenoliths; Pyroxenite; Garnet; Hyblean area; Sicily; Petrology
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/20.500.11769/2615
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