Geology and petrography of the volcanic sequence exposed along the Moscarello escarpment, lower eastern slope of Mount Etna: evidence for a migration of the eruptive centres. In the course of investigations on the earliest subaerial alkalic products of the Etnean activity, an unconformity within the "Timpe di Moscarello" volcanic succession was recognised. This succession was formerly related (Romano, 1982) to the activity of eruptive centres of the "Trifoglietto Unit" (Tr, 80-35 ka BP, Gillot et alii, 1994); the unconformity however allows a lower lithostratigraphic unit (defined as Timpe di Moscarello lavas), here referred to the "Ancient Alkaline Centres" (AAC; 225-120 ka BP, Gillot et alii, 1994), to be distinguished from an upper one (Praino lavas), referred to Tr on the grounds of field data and petrologic characters (Busa et alii, 1999). The succession is extensively covered by products of the "Mongibello Unit". The Timpe are escarpments actually related to important faults, trending NNW-SSE, that belong to the northerly extension of Iblean-Maltese system, and are cut by deep gulleys connected to a minor fault system, trending WNW-ESE (fig. 2). In detail, the outcrops of the AAC volcanic strata, continuously exposed at the base of the "Timpe", show dips varying from place to place, in a way showing that several eruptive centres existed aligned parallel to the fault escarpment (fig. 2). In this part of the sequence lavas are chiefly hawaiites, and "high P" and "low P" varieties were distinguished among them, by their petrographic and geochemical features, in accordance to earlier references (Cristofolini et alii, 1991a; Finoc-Chiaro, 1995). The "high P" and "low P" levels are interlayered, suggesting that at least two distinct parent magmas, originated by different melting degrees of mantle sources, might have been alternatively involved in feeding the eruptive activity (Busa et alii. 1999). In the upper part of the sequence the lava flows range from sub-horizontal to slightly dipping to the east, and appear as coming from eruptive centres, located west of the escarpment. As these lavas are dominantly Ti-depleted mugearites to benmoreites, as are the typical Tr products, they are here referred to this unit. They are associated with rare hawaiites very similar to the "high P" members of the AAC succession, and chemical data (fig. 7) show that the Tr benmoreites might well have originated by differentiation from a magma with this composition (Busa et alii, 1999, 2001). The westerly displacement of the eruptive activity apparently produced then a shift toward an uprise and differentiation of melts from only the "high P" magma source.

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