Preliminary geochemical and petrological data on the Tertiary ultramafic xenoliths from the Veneto Volcanic Province (VVP; SE Alps, Italy) suggest the presence of a plum-cake mantle region at P ≅ 1.5-2.5 Gpa and T ≅ 1250 ¡K (spl-lherzolite field). Major elements geochemistry of the spl-harzburgite and -lherzolite xenoliths reflects a depleted mantle source. Nevertheless, the VVP xenoliths are also characterized by cryptic metasomatism, with varied enrichment in LREE, K, Rb, Sr, and P, indicating a refertilized mantle source after the depletion episode(s). Their PM-normalized spiderdiagrams are comparable to those observed for the VVP alkaline basalts having a plume-related origin (HIMU-DM; Macera et al., 2002). Disregarding their mineralogy, the VVP xenoliths do not show uniform Nb and Ta anomalies, suggesting variable interactions with crustal material at depth in the mantle. Finally, the VVP mafic xenoliths display a large spread in incompatible trace elements ratios, testifying to the heterogeneity of their mantle source. These features may be explained in terms of a plume-induced metasomatism (HIMU-type mantle upwelling) on a variably depleted subcontinental lithosphere (DM) beneath SE Alps, where a not perfectly homogenized crustal component was also present. We suggest that plume material has fed the VVP upper mantle starting at least 65-60 Ma ago. Different textures shown by the VVP xenoliths, ranging in general from protogranular in the Adige Valley outcrops to porphyroclastic in the Lessini area, may be a useful tool to estimate width and intensity variation of the shallow expression of mantle diapirism beneath SE Alps at Tertiary time.
"Plum-cake" subcontinental mantle beneath SE Alps as resulting from the geochemistry of mantle xenoliths
MACERA, PATRIZIA;
2002-01-01
Abstract
Preliminary geochemical and petrological data on the Tertiary ultramafic xenoliths from the Veneto Volcanic Province (VVP; SE Alps, Italy) suggest the presence of a plum-cake mantle region at P ≅ 1.5-2.5 Gpa and T ≅ 1250 ¡K (spl-lherzolite field). Major elements geochemistry of the spl-harzburgite and -lherzolite xenoliths reflects a depleted mantle source. Nevertheless, the VVP xenoliths are also characterized by cryptic metasomatism, with varied enrichment in LREE, K, Rb, Sr, and P, indicating a refertilized mantle source after the depletion episode(s). Their PM-normalized spiderdiagrams are comparable to those observed for the VVP alkaline basalts having a plume-related origin (HIMU-DM; Macera et al., 2002). Disregarding their mineralogy, the VVP xenoliths do not show uniform Nb and Ta anomalies, suggesting variable interactions with crustal material at depth in the mantle. Finally, the VVP mafic xenoliths display a large spread in incompatible trace elements ratios, testifying to the heterogeneity of their mantle source. These features may be explained in terms of a plume-induced metasomatism (HIMU-type mantle upwelling) on a variably depleted subcontinental lithosphere (DM) beneath SE Alps, where a not perfectly homogenized crustal component was also present. We suggest that plume material has fed the VVP upper mantle starting at least 65-60 Ma ago. Different textures shown by the VVP xenoliths, ranging in general from protogranular in the Adige Valley outcrops to porphyroclastic in the Lessini area, may be a useful tool to estimate width and intensity variation of the shallow expression of mantle diapirism beneath SE Alps at Tertiary time.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
