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The Tertiary volcanism of SW Sardinia is characterized by close field association of lower sequences characterized by calc-alkaline basaltic andesites, andesites and minor basalts (28.5 ± 1 to 17 ± 0.7 Ma) and by an upper sequence (up to 500 m thick) (mainly composed of ignimbrites) ranging from dacites to calc-alkaline rhyolites and comendites in the upper part (17.6 ± 0.8 to 13.8 ± 0.7 Ma). In this paper we focused on petrological aspects of the upper sequence in an attempt to understand the origin of peralkaline rocks generally characterizing intraplate volcanic settings (e.g., main Ethiopian rift) associated with rock series of transitional character. In Sardinia, comendites are associated with calc-alkaline rocks related with subduction of oceanic lithosphere. In this context, ignimbrites from the Northern sector have been interpreted as products of partial melting of continental crust. Five major groups were distinguished on the basis of stratigraphy, mineral assemblages, whole-rock and glass chemistry: 1. (a) G 1 dacites (Pl An68–42 + Opx + Cpx ± altered Ol and locally Pl An58–43 + Bt + Amph ± Opx ± Cpx); 2. (b) G 2 rhyodacites (Pl An46–29 + San Or66 + Opx + Cpx ± Ol ± Bt); 3. (c) G 3 rhyolites “ transitional ” to peralkaline rhyolites (Pl An40–26 + San Or55–42 ± Anor ± Opx ± Cpx); 4. (d) G 4 comendites (Na-San Or46–39 ± Anor + Qz + Na-Px + Na-Amph); 5. (e) G 5 post-comenditic rhyolites “ transitional ” to peralkaline rhyolites (Pl An26–24 + Na-San Or42–40 ± Anor). Ti-Mt + Ilm ± Ap appear to be the common accessory phases in all groups; however, Zir + Mt were also found in the G 3 and G 4 groups. Petrographic data and variation of chemical compositions of the main mineral phases (i.e. % An in plagioclases and Mg# in mafic minerals) suggest that the whole sequence is genetically related. Geothermometric and geobarometric calculations gave liquidus temperatures decreasing from G 1 to G 4 from 1100° to ∼950°C and pressures of ∼5–7 kbar. The latter should correspond to the depth of magma evolution for the investigated rocks. The agpaitic index increases with SiO 2 and shifts from subalkaline to peralkaline values at the transition from G 3 to G 4 . The chemical evolution observed in the upper sequence could be governed by dominant crystal/liquid fractionation processes of the observed mineral assemblage, leading to residual peralkaline liquids. Particularly, trace elements show a general enrichment of incompatible elements and a strong enrichment for Zr and Nb, accompanied by depletion of compatible elements (Sr and Ba). Furthermore, the chondrite-normalized rare-earth element (REE) diagram shows a comparable pattern characterized by a progressive REE enrichment from G 1 to G 4 with an increasing negative Eu anomaly, which confirms that feldspar is the dominant fractionating phase. Mass-balance calculations are consistent with a magmatic system dominated by crystal/liquid fractionation processes. This model confirms that the comendite rocks can be related with the subalkaline upper sequence from SW Sardinia. ( 87 Sr 86 Sr ) o at 16.5 and 15 Ma for subalkaline and peralkaline units in the upper sequence are 0.70750 and 0.70640, respectively, and are well comparable with the values for the andesites. This confirms that the whole sequence from the Sulcis area is genetically related and crystal/liquid fractionation was the dominant process in the evolution of the upper sequence. |
