Neogene to Quaternary volcanic rocks from the Hyblean area (south-eastern Sicily) mostly consist of basalts with both alkaline and tholeiite affinity, erupted in submarine settings in most cases. Minor basanites and nephelinites also occur. Nephelinites mark the resuming of eruptive activity in this area during Late Miocene, after a non-magmatic period of 50 Ma (i.e., the Eocene hiatus), either in form of diatremes or lava flows. Hence, nephelinite lavas were the final volcanic products on Early Pleistocene. The Hyblean nephelinites show both aphyric (especially in diatremic facies) to strongly porphyritic textures with high color index. The porphyritic types consist of olivine, strongly zoned Ti-rich augite phenocrysts immersed in a groundmass of Ti-augite, nepheline, abundant apatite and Fe-Ti oxide micrograins. Sodalite-group micro-phenocrysts sometimes occur in the Hyblean nephelinites. Scarce amounts of interstitial glass also occur, displaying either a nepheline-like, hauyne-like or feldspar-like compositions. In the latter case, acicular plagioclases, a few micrometers in size, are immersed in the glass. The glassy groundmass is often enriched in S, Cl and P. This would imply high S and Cl fugacity in the eruptive system during late stages of its cooling history at variable fO2 conditions, generally well above the QFM buffer as deduced by their mineral assemblages. In some cases, the melanocratic character of these volcanic rocks is strengthened by the occurrence of Mg-rich olivine and variously resorbed orthopyroxene xenocrysts, being fragments of mantle peridotites. Whole rock analyses of Hyblean nephelinites display conflicting geochemical aspects: on the one hand, some features conform to world wide nephelinites (e.g., SiO2 content as low as about 38 wt.%; rather high contents of Ba, Sr, Th, Ta, Nb, LREE, Pb and P; Chondrite-normalised REE patterns being roughly linear hence displaying a strong REE fractionation). On the other hand, Hyblean nephelinites share some geochemical characteristics with the coexisting basalts (e.g., the Sr-Nd isotopic compositions indicate a time-integrated depleted source; some geochemical proxies widely used for fingerprinting magma type closely fit those of Ocean Island Basalts; K2O, TiO2, Zr, Hf, Y contents are similar to those of coexisting alkaline basalts). In addition, important variations are present among the published nephelinite analyses with respect to some major and trace elements (e.g., Na2O content varies from about 2 wt.% to about 4 wt.%; MgO from about 11 wt.% to about 16 wt.%; CaO from 11 wt.% to 15 wt.%). More interestingly, detailed field sampling indicates strong chemical variations even within a distinct lava flow [unpublished data of the Authors]. The origin of Hyblean nephelinites has been previously related to the influence of metasomatic agents (i.e., carbonatite fluid) in the mantle source. Conversely, we put forward the hypothesis, plenty conforming the above mentioned data, that these nephelinites originated from an ascending basaltic magmas that assimilated different amounts of deep-seated evaporite-like salts (Ca-Mg sulfates, halite, carbonates, minor phosphates), providing an adequate magma: salt volume ratio. In this respect, it is opportune to recall that recent works suggest the serpentinite nature of a large part of the unexposed section of the Hyblean lithospheric section. Since the serpentinization process requires pure H2O, the involved seawater-derived serpentinizing fluid will undergo total out-salting. In particular, about 10.5 kg of salts can be deposited per m3 of serpentinized peridotite. Salts are likely accumulating in pores, fractures and major tectonic discontinuities of the serpentinite wall rocks, giving rise to the accumulation of huge salt deposits through time. In addition, hot aqueous fluids, due to the local dehydration of the serpentinite at the contact with the ascending basaltic magma, can form highly mobile saline brines. The latter, in their supercritical state, let the salt assimilation by slightly superheated basalt magma a thermodynamically viable process. In case of fluid oversaturated magmatic systems, diatreme eruptions have occurred; otherwise, lava flows have been emplaced.

En-Route Formation of highly Silica-Undersaturated Melts through Interaction between Ascending Basalt and Serpentinite-Related Saline Brines: Inference from Hyblean Cenozoic Nephelinites, Sicily

SCRIBANO, Vittorio;
2014

Abstract

Neogene to Quaternary volcanic rocks from the Hyblean area (south-eastern Sicily) mostly consist of basalts with both alkaline and tholeiite affinity, erupted in submarine settings in most cases. Minor basanites and nephelinites also occur. Nephelinites mark the resuming of eruptive activity in this area during Late Miocene, after a non-magmatic period of 50 Ma (i.e., the Eocene hiatus), either in form of diatremes or lava flows. Hence, nephelinite lavas were the final volcanic products on Early Pleistocene. The Hyblean nephelinites show both aphyric (especially in diatremic facies) to strongly porphyritic textures with high color index. The porphyritic types consist of olivine, strongly zoned Ti-rich augite phenocrysts immersed in a groundmass of Ti-augite, nepheline, abundant apatite and Fe-Ti oxide micrograins. Sodalite-group micro-phenocrysts sometimes occur in the Hyblean nephelinites. Scarce amounts of interstitial glass also occur, displaying either a nepheline-like, hauyne-like or feldspar-like compositions. In the latter case, acicular plagioclases, a few micrometers in size, are immersed in the glass. The glassy groundmass is often enriched in S, Cl and P. This would imply high S and Cl fugacity in the eruptive system during late stages of its cooling history at variable fO2 conditions, generally well above the QFM buffer as deduced by their mineral assemblages. In some cases, the melanocratic character of these volcanic rocks is strengthened by the occurrence of Mg-rich olivine and variously resorbed orthopyroxene xenocrysts, being fragments of mantle peridotites. Whole rock analyses of Hyblean nephelinites display conflicting geochemical aspects: on the one hand, some features conform to world wide nephelinites (e.g., SiO2 content as low as about 38 wt.%; rather high contents of Ba, Sr, Th, Ta, Nb, LREE, Pb and P; Chondrite-normalised REE patterns being roughly linear hence displaying a strong REE fractionation). On the other hand, Hyblean nephelinites share some geochemical characteristics with the coexisting basalts (e.g., the Sr-Nd isotopic compositions indicate a time-integrated depleted source; some geochemical proxies widely used for fingerprinting magma type closely fit those of Ocean Island Basalts; K2O, TiO2, Zr, Hf, Y contents are similar to those of coexisting alkaline basalts). In addition, important variations are present among the published nephelinite analyses with respect to some major and trace elements (e.g., Na2O content varies from about 2 wt.% to about 4 wt.%; MgO from about 11 wt.% to about 16 wt.%; CaO from 11 wt.% to 15 wt.%). More interestingly, detailed field sampling indicates strong chemical variations even within a distinct lava flow [unpublished data of the Authors]. The origin of Hyblean nephelinites has been previously related to the influence of metasomatic agents (i.e., carbonatite fluid) in the mantle source. Conversely, we put forward the hypothesis, plenty conforming the above mentioned data, that these nephelinites originated from an ascending basaltic magmas that assimilated different amounts of deep-seated evaporite-like salts (Ca-Mg sulfates, halite, carbonates, minor phosphates), providing an adequate magma: salt volume ratio. In this respect, it is opportune to recall that recent works suggest the serpentinite nature of a large part of the unexposed section of the Hyblean lithospheric section. Since the serpentinization process requires pure H2O, the involved seawater-derived serpentinizing fluid will undergo total out-salting. In particular, about 10.5 kg of salts can be deposited per m3 of serpentinized peridotite. Salts are likely accumulating in pores, fractures and major tectonic discontinuities of the serpentinite wall rocks, giving rise to the accumulation of huge salt deposits through time. In addition, hot aqueous fluids, due to the local dehydration of the serpentinite at the contact with the ascending basaltic magma, can form highly mobile saline brines. The latter, in their supercritical state, let the salt assimilation by slightly superheated basalt magma a thermodynamically viable process. In case of fluid oversaturated magmatic systems, diatreme eruptions have occurred; otherwise, lava flows have been emplaced.
2039-6651
Volcanic rocks; Hyblean Plateau; Nephelinite
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.11769/108074
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