MINERALOGY AND PETROLOGY OF MAFIC INCLUSION-BEARING TRACHYTES FROM MT ARCI VOLCANIC MASSIF (WESTERN SARDINIA)

Abundant felsic-intermediate rocks (subalkaline to mildly alkalic) with minor basaltic lavas characterize the activity of the Mt. Arci volcanic complex during Pliocene. Mildly alkaline rocks include trachytes, trachyandesites and evolved basalts. Trachytes occur as porphyritic lava flows (TL) and small volume of pyroclastic rocks (TP). Both trachyte types are oversaturated, silica-rich and metaluminous. TL exhibit a complex feldspar assemblage (plagioclase with "antirapakivi" texture, Na-sanidine, ternary feldspars) along with augite, orthopyroxene, Fe-Ti oxides and rare amphibole phenocrysts. They host mafic to intermediate porphyritic and cumulitic inclusions (SiO2 = 50.4 - 58.6 wt %) containing olivine, augite, plagioclase, alkalifeldspar, and showing obvious evidence of entrainment in a partially molten state. TP are slightly porphyritic and characterized by coexistence of plagioclase (K - oligoclase) + sanidine; biotite is the dominant mafic phase. Geochemistry of TL differs from PT by distinctly lower Ba, Sr, Th, Pb and higher Na, K and Zr contents. Two-feldspar equilibrium assemblage indicates near water-saturated conditions of crystallization, in contrast with a strong water undersaturation of the TL, which allowed extensive plagioclase resorption. Genetic relationships between TL and TP can be excluded on the basis of both chemical and mineralogical considerations. Simple fractional crystallization models satisfactorily account for the origin of the small volumes of trachyandesitic lavas (TA) and trachytic pyroclastics starting from a mildly alkaline basaltic parent, but fail to reproduce trachytic lavas, although the involvement of mildly alkaline basalts in their origin is supported by the composition of some magmatic inclusions. Evolution in a magmatic reservoir compositionally zoned from mildly alkaline basalt to trachyte, with recurrent internal mixing, best explains the features of TL, even including their marked Zr enrichment. The proposed origin for the porphyritic inclusions involves (a) hybridization along the interface between differentiated and underlying mafic magma and (b) entrainment, dispersion and quenching of small amounts of these hybrids within the trachyte.