We present petrographic, geochemical, and isotopic (Sr, Nd, and Pb) data of a representative suite of spl-peridotite xenoliths (mg>88) hosted in alkali basalts from numerous outcrops in the Tertiary Veneto Volcanic Province (VVP; SE Alps, Italy), compared to various world-wide mafic inclusions (French Massif, Australia, China, Philippines, Russia, Kerguelen). The VVP spl-harzburgites and -lherzolites carry textures ranging from protogranular, porphyroclastic, granuloblastic to pyrometamorfic. These samples are characterized by a continuous depletion trend from the cpx-rich lherzolites to harzburgites, with CaO, Al2O3, TiO2, and Na2O contents decreasing with mg increasing (Morten, 1987; Beccaluva et al., 2001). Then, the VVP xenoliths spinels show a strong Cr/(Cr+Al) ratio increase at a slight Mg/(Mg+Fe2+) ratio decrease, thus reflecting a variably depleted mantle source. The VVP xenoliths display a large range of enrichment in LREE, K, Rb, Sr and P, suggesting post depletion metasomatic episodes (Morten et al., 2002). Whereas most of the VVP xenoliths’ multi-element spectra, incompatible element and isotope ratios are similar to the VVP host basalts, thus with a strong HIMU signature (Macera et al. submitted), some depleted samples show geochemical features typical of crust derived material. These characteristics cannot be related to significant interaction with the local lower continental crust, as represented by several analyzed gabbroic xenoliths. Nevertheless negative Nb and Ta anomalies in analogous peridotitic samples have been previously ascribed to metasomatism inferred by plume rising material in the upper mantle (Bedini et al., 1997). Comparing the VVP peridotites with several mafic xenoliths from various geodynamical environments, we suggest that this crust affinity could be alternatively explained by the presence of a not perfectly homogenized upper crustal component in the source region, probably induced by subduction related episode(s). In this contest, the isotopic composition of the VVP mafic xenoliths is a crucial tool to understand the geochemical history of the Alpine subcontinental mantle.
Mantle xenoliths hosted in alkali basalts in subduction environment.
MACERA, PATRIZIA;
2003-01-01
Abstract
We present petrographic, geochemical, and isotopic (Sr, Nd, and Pb) data of a representative suite of spl-peridotite xenoliths (mg>88) hosted in alkali basalts from numerous outcrops in the Tertiary Veneto Volcanic Province (VVP; SE Alps, Italy), compared to various world-wide mafic inclusions (French Massif, Australia, China, Philippines, Russia, Kerguelen). The VVP spl-harzburgites and -lherzolites carry textures ranging from protogranular, porphyroclastic, granuloblastic to pyrometamorfic. These samples are characterized by a continuous depletion trend from the cpx-rich lherzolites to harzburgites, with CaO, Al2O3, TiO2, and Na2O contents decreasing with mg increasing (Morten, 1987; Beccaluva et al., 2001). Then, the VVP xenoliths spinels show a strong Cr/(Cr+Al) ratio increase at a slight Mg/(Mg+Fe2+) ratio decrease, thus reflecting a variably depleted mantle source. The VVP xenoliths display a large range of enrichment in LREE, K, Rb, Sr and P, suggesting post depletion metasomatic episodes (Morten et al., 2002). Whereas most of the VVP xenoliths’ multi-element spectra, incompatible element and isotope ratios are similar to the VVP host basalts, thus with a strong HIMU signature (Macera et al. submitted), some depleted samples show geochemical features typical of crust derived material. These characteristics cannot be related to significant interaction with the local lower continental crust, as represented by several analyzed gabbroic xenoliths. Nevertheless negative Nb and Ta anomalies in analogous peridotitic samples have been previously ascribed to metasomatism inferred by plume rising material in the upper mantle (Bedini et al., 1997). Comparing the VVP peridotites with several mafic xenoliths from various geodynamical environments, we suggest that this crust affinity could be alternatively explained by the presence of a not perfectly homogenized upper crustal component in the source region, probably induced by subduction related episode(s). In this contest, the isotopic composition of the VVP mafic xenoliths is a crucial tool to understand the geochemical history of the Alpine subcontinental mantle.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
