Alkali metasomatism as a process for trondhjemite genesis: evidence from Aspromonte Unit, north-eastern Peloritani, Sicily

Rocks of trondhjemitic composition are widespread in the North-Eastern PeloritaniBelt within the Aspromonte Unit, a Hercynian medium- to high-grade metamorphiccomplex intruded by late-Hercynian peraluminous grani-tes and later affected byMP-LT Alpine metamorphism. Among these trondhjemitic bodies, the Pizzo Bottinotrondhjemites form one of the largest, outcropping over about 6 km2 and up to 400mthick. These rocks display concordant to discordant relationships with associated metamorphic rocks and are often cut by late-Hercynian leucogranitic dykes. The field, petrographic and geochemical features of these trondhjemites are consistent with an igneous origin. Petrographic and geochemical evidences suggest that the trondhjemitic characterof the Pizzo Bottino rocks is due to an alkali metasomatism process involving cationic exchange of Na and Ca for K and consequent replacement of K-feldspar by oligoclase in the original granitoids. The major and trace element contents of the Pizzo Bottino trondhjemites are in fact comparable to those of the peraluminous late-Hercynian granitoids from the southern Calabrian-Peloritani Arc (CPA), when the elementsdirectly involved in the alkali metasomatism process (Na, Ca, K, Sr, Ba, Rb) arenot considered. The behaviour of REE elements, plus Th and U, also seems to be partially controlled by metasomatic processes, because their abundances vary with the K/Na ratio. Metasomatism seems to be the only viable mechanism involved in the genesis of the Pizzo Bottino trondhjemi-tes. Other trondhjemite generation processes such as fractionation from basaltic parents and partial melting of metabasaltic or metasedi-mentary sources are ruled out on geological, petrographic and isotopic (Sr, Nd) grounds. Lastly, regional considerations place the metasomatic event during the late Hercynian, after the emplacement of the original granitoids and preceding the intrusion of the leucograni-tic dykes, which are not affected by metasomatism.