The Quaternary volcanism of the Garrotxa area (NE Spain) developed during the rift-type extensional tectonics affecting the eastern margin of Iberia since the late Oligocene and related to the opening of the Valencia trough. Location of the eruptive centres is controlled by the main NW-SE faults that limit the different elevated or depressed blocks configuring the tectonic structure of the area. The Garrotxa volcanic activity is characterised by small strombolian cones and lava flows formed by very short monogenic eruptions, occasionally phreatomagmatic. As result of this activity, more than 40 volcanic cones and about 30 main lava flows can be recognised in this region. The volcanic rocks consist of basanites, alkali basalts, and trachybasalts that are aphanitic, vesicular, hipohialine and porphyritic in texture, in which olivine and clinopyroxene (± plagioclase and oxides) are the main mineral phases. They show relatively homogeneous geochemical features, with FeOT/MgO and Na2O/K2O weight ratios ≥ 1. For most of the samples, major and trace element correlations exclude significant stagnation and magmatic evolution in crustal reservoirs. Primitive mantle-normalized multi-element diagrams are characterized by a typical bell-shaped, OIB-like trend, with progressive enrichment of the most incompatible elements, except for Th, Pb, K and Rb, and significant peaks at Ta and Nb, sharing their geochemical patterns with the whole Cenozoic anorogenic magmatism of the European-Mediterranean area. Significant variability of 143Nd/144Nd and 206Pb/204Pb ratios in the Garrotxa volcanics testify for a common heterogeneous mantle source, and are likely related to mixing processes occurred between the local lithospheric mantle and a sublithospheric, geochemical enriched component. Sr-, Nd-, and Pb-isotope compositions of the Garrotxa magmas confirm their similarity to the EAR (Enriched Asthenospheric Reservoir)-derived basalts of the Mediterranean region, showing a HIMU (High μ = high 238U/204Pb)- DMM (Depleted MORB Mantle) signature. Noticeable, the Garrotxa basalts display geochemical features that are very close to those of Cape Verde, a key “end-member” in the petrogenesis of the European magmatism since the Cretaceous (Piromallo et al., 2008). In our model, the rift-related extensional tectonics favors upwelling of sub-lithospheric, plume-like mantle into the local lithosphere, originating EAR-type basalts via adiabatic decompression melting. In conclusion, the Garrotxa volcanic activity must be included within the members of the within-plate Tertiary-Quaternary magmatism of the European-Mediterranean area.
The Garrotxa Volcanic Field (NE Spain): new geochemical constraints
MACERA, PATRIZIA;
2011-01-01
Abstract
The Quaternary volcanism of the Garrotxa area (NE Spain) developed during the rift-type extensional tectonics affecting the eastern margin of Iberia since the late Oligocene and related to the opening of the Valencia trough. Location of the eruptive centres is controlled by the main NW-SE faults that limit the different elevated or depressed blocks configuring the tectonic structure of the area. The Garrotxa volcanic activity is characterised by small strombolian cones and lava flows formed by very short monogenic eruptions, occasionally phreatomagmatic. As result of this activity, more than 40 volcanic cones and about 30 main lava flows can be recognised in this region. The volcanic rocks consist of basanites, alkali basalts, and trachybasalts that are aphanitic, vesicular, hipohialine and porphyritic in texture, in which olivine and clinopyroxene (± plagioclase and oxides) are the main mineral phases. They show relatively homogeneous geochemical features, with FeOT/MgO and Na2O/K2O weight ratios ≥ 1. For most of the samples, major and trace element correlations exclude significant stagnation and magmatic evolution in crustal reservoirs. Primitive mantle-normalized multi-element diagrams are characterized by a typical bell-shaped, OIB-like trend, with progressive enrichment of the most incompatible elements, except for Th, Pb, K and Rb, and significant peaks at Ta and Nb, sharing their geochemical patterns with the whole Cenozoic anorogenic magmatism of the European-Mediterranean area. Significant variability of 143Nd/144Nd and 206Pb/204Pb ratios in the Garrotxa volcanics testify for a common heterogeneous mantle source, and are likely related to mixing processes occurred between the local lithospheric mantle and a sublithospheric, geochemical enriched component. Sr-, Nd-, and Pb-isotope compositions of the Garrotxa magmas confirm their similarity to the EAR (Enriched Asthenospheric Reservoir)-derived basalts of the Mediterranean region, showing a HIMU (High μ = high 238U/204Pb)- DMM (Depleted MORB Mantle) signature. Noticeable, the Garrotxa basalts display geochemical features that are very close to those of Cape Verde, a key “end-member” in the petrogenesis of the European magmatism since the Cretaceous (Piromallo et al., 2008). In our model, the rift-related extensional tectonics favors upwelling of sub-lithospheric, plume-like mantle into the local lithosphere, originating EAR-type basalts via adiabatic decompression melting. In conclusion, the Garrotxa volcanic activity must be included within the members of the within-plate Tertiary-Quaternary magmatism of the European-Mediterranean area.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
