Ageing of the Thetyan crust documented by xenoliths from Hyblaean diatremes (Sicily): Implication for crustal assimilation during magma emplacement

Previous study on xenoliths from Miocenic diatremes from Hyblaean area (Sicily) suggested that the local Meso- Cenozoic sedimentary and volcanic sequence rests upon a fossil oceanic core-complex belonging to the Ionian Thetys lithospheric domain. Hyblaean xenoliths have also suggested that the postulated core-complex hosted long living hydrothermal systems which have produced diverse geochemical and mineralogical modification to the host ultramafic and gabbroic rocks (e.g. hydration, alkalization, sulfidization, carbonation) through time, as well the deposition of hydrothermal brines at different crustal levels. The Upper Miocene volcanic rocks cropping out in the Central-Eastern section of the area were erupted after a 50 Ma non-magmatic period (the Eocene hiatus). The interaction between ascending magmas and the fossil hydrothermal system produced dehydration of the serpentinite wall rock with formation of fluidized eruptive systems (i.e. diatremes). The basalt magma assimilated some hydrous peralkaline anatectit melts thereby attaining a silica-undersaturated, sodic (nephelinitic) character. Magmas that fed the Pleistocene volcanism in the northern margin of the Plateau intersected a section of the fossil hydrothermal system, which was particularly rich in hydrothermal evaporites as testified by the high contents in S, Cl, F, H2O, Ca, Na, Sr, Ba, relatively high Zr/Hf and occurrence of sodalite series phenocrysts. More in general, we put forward the idea that selective assimilation of hydrothermally altered wall rocks may explain the geochemical paradox of the Hyblaean basaltic lavas, which display Nd-Sr isotope ratios distinctive of MORB-type magmas and distribution of some trace elements more typical of alkaline-series magmas.