As it is known, deep-seated xenoliths provide us direct information about the inaccessible portions of the lithosphere. The present work focuses on a suite of xenoliths collected within Miocene tuff-breccias at Valle Guffari (Hyblean Plateau, south-eastern Sicily). They mainly consist of: a) ultramafics, consisting of spinel facies peridotites and pyroxenites (some garnet bearing) with rare phlogopite occurrence; and b) feldspar-bearing suite, represented by metabasites and anorthosites. The size of collected specimens ranges from about 2 to 30cm (most abundant 6-8cm); the shape is rounded and most of them are relatively fresh and suitable for our investigation. Spinel facies peridotites (harzburgite and lherzolite) mostly show protogranular texture, with kinkbanded olivine and orthopyroxene, with smaller clinopyroxene and spinel. Infrequent porphyroclastic texture, and very rare equigranular mosaic texture also occur. Finally, intergranular glass and veinlets of metasomatic glass are ubiquitous. Figure 1. Vp, Vs and Vp-related seismic anisotropy of the investigated Hyblean xenoliths, together with photomicrographs of common protogranular texture. CONFERENZA A. RITTMANN Nicolosi (Catania) 29 | 31 ottobre 2014 102 Spinel-bearing pyroxenites consist of Cr-diopside websterites, Al-augite websterites, spinel and garnet clinopyroxenites, with ubiquitous intergranular glass. Cr-diopside websterites, which often occur as veins within peridotite, exhibit xenoblastic granular or polygonal texture; garnet may be also present. Al-diopside websterites exhibit polygonal texture, with exoluted pyroxene. Al-augite pyroxenite, which are the ultracoarse specimens (>20mm) are not very abundant; they exhibit igneous cumulitic texture with pyroxene megacrysts and neo-blastic sub grains. Finally, feldspar bearing metagabbroes and anorthosite xenoliths are considered to represent the inaccessible sub-Hyblean crustal portion [Scribano et al., 2006]. Previous studies [Atzori et al., 1999] highlighted as equilibration conditions of pyroxenites are about 0.98GPa at 740°C (spinel pyroxenite) and 1.32 GPa at 1040°C (garnet pyroxenite). Unfortunately, no reliable PT values could be obtained for metabasites because of evident disequilibration features; nevertheless, Nimis and Ulmer [1998] estimates provided P values of 0.5 GPa for clinopyroxene within metagabbros. The seismic properties of selected deep-seated xenoliths (spinel harzburgite, pyroxenite and metabasite; Punturo et al., 2000] considered to be representative of the sub-Hyblean main lithologies, were experimentally determined with a multi-anvil apparatus at confining pressure up to 600MPa (room temperature) and up to 600°C (at 600MPa; Fig.1). Laboratory measurements of 3D distribution of Vp and Vs permitted the intrinsic P-wave related seismic anisotropy and shear wave splitting to be also determined, together with density changes as a function of pressure and temperature. Finally, in order to understand the contribution of Crystallographic Preferred Orientation of olivine and pyroxenes to seismic anisotropy of deep lithosphere, we also carried out measurements on thin sections with a microscope equipped with Universal Stage. Results provide direct petrographic, compositional and petrophysical data which may contribute to define the nature of the sub-Hyblean lithosphere and permit available geophysical data to be constrained and interpreted at a better extent. References Atzori P. Mazzoleni P., Punturo R., Scribano V., (1999). Garnet-bearing pyroxenite xenoliths from Hyblean Plateau (South-Eastern Sicily, Italy). Mineralogy and Petrology, 27, 1-17. Nimis P., Ulmer P., (1998). Clinopyroxene geobarometry of magmatic rocks. Part 1: an expanded structural geobarometer for anhydrous and hydrous, basic and ultrabasis systems. Contributions to Mineralogy and Petrology, 133, 122-135. Punturo R., Kern H., Scribano V., Atzori P., (2000). Petrophysical and petrological characteristics of deepseated xenoliths from Hyblean Plateau, south-eastern Sicily, Italy: suggestions for a lithospheric model. Mineral Petrogr Acta 43, 1-20. Scribano V., Sapienza G., Braga R., Morten L., (2006). Gabbroic xenoliths in tuffbreccia pipes from the Hyblean Plateau: insights into the nature and composition of the lower crust underneath Southeastern Sicily, Italy. Mineralogy and Petrology, 86, 63–88. DOI 10.1007/s00710-005-0092-6.

Petrophysical Properties of the Sub-Hyblean Lithosphere: insight from Deep-Seated Xenoliths

PUNTURO, Rosalda;Occhipinti R.
2014-01-01

Abstract

As it is known, deep-seated xenoliths provide us direct information about the inaccessible portions of the lithosphere. The present work focuses on a suite of xenoliths collected within Miocene tuff-breccias at Valle Guffari (Hyblean Plateau, south-eastern Sicily). They mainly consist of: a) ultramafics, consisting of spinel facies peridotites and pyroxenites (some garnet bearing) with rare phlogopite occurrence; and b) feldspar-bearing suite, represented by metabasites and anorthosites. The size of collected specimens ranges from about 2 to 30cm (most abundant 6-8cm); the shape is rounded and most of them are relatively fresh and suitable for our investigation. Spinel facies peridotites (harzburgite and lherzolite) mostly show protogranular texture, with kinkbanded olivine and orthopyroxene, with smaller clinopyroxene and spinel. Infrequent porphyroclastic texture, and very rare equigranular mosaic texture also occur. Finally, intergranular glass and veinlets of metasomatic glass are ubiquitous. Figure 1. Vp, Vs and Vp-related seismic anisotropy of the investigated Hyblean xenoliths, together with photomicrographs of common protogranular texture. CONFERENZA A. RITTMANN Nicolosi (Catania) 29 | 31 ottobre 2014 102 Spinel-bearing pyroxenites consist of Cr-diopside websterites, Al-augite websterites, spinel and garnet clinopyroxenites, with ubiquitous intergranular glass. Cr-diopside websterites, which often occur as veins within peridotite, exhibit xenoblastic granular or polygonal texture; garnet may be also present. Al-diopside websterites exhibit polygonal texture, with exoluted pyroxene. Al-augite pyroxenite, which are the ultracoarse specimens (>20mm) are not very abundant; they exhibit igneous cumulitic texture with pyroxene megacrysts and neo-blastic sub grains. Finally, feldspar bearing metagabbroes and anorthosite xenoliths are considered to represent the inaccessible sub-Hyblean crustal portion [Scribano et al., 2006]. Previous studies [Atzori et al., 1999] highlighted as equilibration conditions of pyroxenites are about 0.98GPa at 740°C (spinel pyroxenite) and 1.32 GPa at 1040°C (garnet pyroxenite). Unfortunately, no reliable PT values could be obtained for metabasites because of evident disequilibration features; nevertheless, Nimis and Ulmer [1998] estimates provided P values of 0.5 GPa for clinopyroxene within metagabbros. The seismic properties of selected deep-seated xenoliths (spinel harzburgite, pyroxenite and metabasite; Punturo et al., 2000] considered to be representative of the sub-Hyblean main lithologies, were experimentally determined with a multi-anvil apparatus at confining pressure up to 600MPa (room temperature) and up to 600°C (at 600MPa; Fig.1). Laboratory measurements of 3D distribution of Vp and Vs permitted the intrinsic P-wave related seismic anisotropy and shear wave splitting to be also determined, together with density changes as a function of pressure and temperature. Finally, in order to understand the contribution of Crystallographic Preferred Orientation of olivine and pyroxenes to seismic anisotropy of deep lithosphere, we also carried out measurements on thin sections with a microscope equipped with Universal Stage. Results provide direct petrographic, compositional and petrophysical data which may contribute to define the nature of the sub-Hyblean lithosphere and permit available geophysical data to be constrained and interpreted at a better extent. References Atzori P. Mazzoleni P., Punturo R., Scribano V., (1999). Garnet-bearing pyroxenite xenoliths from Hyblean Plateau (South-Eastern Sicily, Italy). Mineralogy and Petrology, 27, 1-17. Nimis P., Ulmer P., (1998). Clinopyroxene geobarometry of magmatic rocks. Part 1: an expanded structural geobarometer for anhydrous and hydrous, basic and ultrabasis systems. Contributions to Mineralogy and Petrology, 133, 122-135. Punturo R., Kern H., Scribano V., Atzori P., (2000). Petrophysical and petrological characteristics of deepseated xenoliths from Hyblean Plateau, south-eastern Sicily, Italy: suggestions for a lithospheric model. Mineral Petrogr Acta 43, 1-20. Scribano V., Sapienza G., Braga R., Morten L., (2006). Gabbroic xenoliths in tuffbreccia pipes from the Hyblean Plateau: insights into the nature and composition of the lower crust underneath Southeastern Sicily, Italy. Mineralogy and Petrology, 86, 63–88. DOI 10.1007/s00710-005-0092-6.
2014
Hyblean xenoliths; petrophysics; litosphere
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.11769/74241
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