The geology of the Betic area has been characterized by several tectono-magmatic episodes related to multiple orogenic cycles and extensional phases (Puga et al., 2011; Bianchini et al., 2011 and references therein) ultimately leading to widespread subduction-related and anorogenic volcanism. The last magmatic phase (Pliocene) is represented by Na-alkaline basalts at Tallante that entrained and exhumed abundant deep-seated xenoliths of both mantle and crustal provenance. These xenoliths attracted an intense petrological interest testified by an impressive number of scientific papers. Unfortunately, most of these studies focused on the ultramafic xenoliths ignoring the crustal lithologies that were investigated only by the paper of Vielzeuf (1983). In this contribution we present new data on felsic (i.e. crustal) xenoliths from Tallante that integrate the petrological information provided by the ultramafic parageneses, highlighting the deep lithosphere stratigraphy of the area as well as constraints for the interpretation of geophysical data and models. The felsic xenolith is characterized by a metamorphic peak assemblage composed of quartz + spinel + sillimanite. The occurrence of cordierite rims between spinel and quartz is related to post-peak metamorphism. To obtain P-T information from this texture, we calculated a P-T pseudosection for the specific mineral reaction texture spinel + quartz = cordierite. The microbulk composition was modeled in the SiO2-Al2O3-FeO-Fe2O3-MgO system. Water has not been taken into account in order to simulate the likely anhydrous conditions occurring during the peak metamorphic stage. The observed paragenesis Spinel-Quartz-Sillimanite indicates temperature around 1000 °C for pressure of 7 kbar, increasing to 1125 °C at 9 kbar. The formation of cordierite rims between quartz and spinel requires, in the chosen system, decompression down to < 6 kbar (T < 950 °C). This evolution was accompanied by partial melting testified by glasses films and blebs. Therefore, felsic xenoliths from Tallante attained P-T condition overlapping those recorded by the ultramafic xenoliths of mantle provenance that are represented by spinel-plagioclase peridotite. This suggests that an intimate association of crust and mantle lithologies with pervasive interlayering and interfingering at metric to hectometric scale, plausibly not detectable by geophysical approaches, characterizes the Moho discontinuity in this area. Melting preferentially occurred in the crustal domains (characterized by lower solidus conditions) generating silica-rich magmas that veined the surrounding peridotite domains, also inducing orthopyroxene-rich metasomatic aureole. The proposed petrological scenario is discussed taking also into account the investigations on the neighboring massifs of Ronda and Beni Bousera where the fossil crust-mantle boundary is exposed, providing fresh insights for the configuration and evolution of the Moho discontinuity in collisioned plate boundaries, that are widespread throughout the peri-Mediterranean realm. The hypotized eteropic crust-mantle boundary would represent a suitable source region for exotic magma types such as lamproites that are common within the Mediterranean area (Tommasini et al. 2011). References - Bianchini et al., 2011. Lithos 124, 308-318. - Puga et al., 2011. Lithos 124, 255-272. - Tommasini et al. Earth Planet. Sci. Lett. 301, 469-478. - Vielzeuf, 1983. Contrib. Mineral. Petrol. 82, 301-311
Crustal xenoliths from Tallante (Betic Cordillera, Spain): insights for an interlayered crust-mantle boundary?
BIANCHINI, Gianluca;
2011
Abstract
The geology of the Betic area has been characterized by several tectono-magmatic episodes related to multiple orogenic cycles and extensional phases (Puga et al., 2011; Bianchini et al., 2011 and references therein) ultimately leading to widespread subduction-related and anorogenic volcanism. The last magmatic phase (Pliocene) is represented by Na-alkaline basalts at Tallante that entrained and exhumed abundant deep-seated xenoliths of both mantle and crustal provenance. These xenoliths attracted an intense petrological interest testified by an impressive number of scientific papers. Unfortunately, most of these studies focused on the ultramafic xenoliths ignoring the crustal lithologies that were investigated only by the paper of Vielzeuf (1983). In this contribution we present new data on felsic (i.e. crustal) xenoliths from Tallante that integrate the petrological information provided by the ultramafic parageneses, highlighting the deep lithosphere stratigraphy of the area as well as constraints for the interpretation of geophysical data and models. The felsic xenolith is characterized by a metamorphic peak assemblage composed of quartz + spinel + sillimanite. The occurrence of cordierite rims between spinel and quartz is related to post-peak metamorphism. To obtain P-T information from this texture, we calculated a P-T pseudosection for the specific mineral reaction texture spinel + quartz = cordierite. The microbulk composition was modeled in the SiO2-Al2O3-FeO-Fe2O3-MgO system. Water has not been taken into account in order to simulate the likely anhydrous conditions occurring during the peak metamorphic stage. The observed paragenesis Spinel-Quartz-Sillimanite indicates temperature around 1000 °C for pressure of 7 kbar, increasing to 1125 °C at 9 kbar. The formation of cordierite rims between quartz and spinel requires, in the chosen system, decompression down to < 6 kbar (T < 950 °C). This evolution was accompanied by partial melting testified by glasses films and blebs. Therefore, felsic xenoliths from Tallante attained P-T condition overlapping those recorded by the ultramafic xenoliths of mantle provenance that are represented by spinel-plagioclase peridotite. This suggests that an intimate association of crust and mantle lithologies with pervasive interlayering and interfingering at metric to hectometric scale, plausibly not detectable by geophysical approaches, characterizes the Moho discontinuity in this area. Melting preferentially occurred in the crustal domains (characterized by lower solidus conditions) generating silica-rich magmas that veined the surrounding peridotite domains, also inducing orthopyroxene-rich metasomatic aureole. The proposed petrological scenario is discussed taking also into account the investigations on the neighboring massifs of Ronda and Beni Bousera where the fossil crust-mantle boundary is exposed, providing fresh insights for the configuration and evolution of the Moho discontinuity in collisioned plate boundaries, that are widespread throughout the peri-Mediterranean realm. The hypotized eteropic crust-mantle boundary would represent a suitable source region for exotic magma types such as lamproites that are common within the Mediterranean area (Tommasini et al. 2011). References - Bianchini et al., 2011. Lithos 124, 308-318. - Puga et al., 2011. Lithos 124, 255-272. - Tommasini et al. Earth Planet. Sci. Lett. 301, 469-478. - Vielzeuf, 1983. Contrib. Mineral. Petrol. 82, 301-311I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
