THE ROLE OF FRACTIONAL CRYSTALLIZATION, CRUSTAL MELTING AND MAGMA MIXING IN THE PETROGENESIS OF RHYOLITES AND MAFIC INCLUSION-BEARING DACITES FROM THE MONTE-ARCI VOLCANIC COMPLEX (SARDINIA, ITALY)

Rhyolites are metaluminous to slightly peraluminous rocks with a wide range of SiO2 contents (67-75%). They fall into two groups: (1) Mafic inclusion-bearing rhyolites (MIBR). These have initial 87 Sr/86Sr ratios between 0.70526 and 0.70897 and commonly show relatively high values of Ti, Mg, Fe, Ca, Sr and compatible trace elements (Ni, Cr, V); cordierite-bearing, highly restitic, crustal xenoliths are rare. (2) Inclusion-free rhyolites (IFR). They include both porphyritic lavas (IFR1) and obsidians (IFR2, IFR3), which share low values of Ti, Fe, Mg, Ca, Sr and are strongly depleted in compatible trace elements. IFR 1-2 have a marked LREE/HREE fractionation and a narrow range of 87Sr/86Srratios (0.70885-0.70972), whereas IFR3 have larger Sr isotopic ratios (0.71529-0.71553), low Th, Zr, Hf. LREE contents and low LREE/HREE. Nd isotopic composition is quite uniform for all rhyolite types, with relatively low 143Nd/144Ndratios (0.51216-0.51228, epsilon(Nd) ranging between -9.3 and -6.9). MIBR appear to be the product of fractionation of subalkaline (tholeiitic) magmas + assimilation of a crustal component moderately enriched in Sr-87 and/or with low Sr content. IFR 1-2 may be derived from partial melting of moderately 87Sr-rich crustal lithologies with retention of garnet and plagioclase (+/-K-feldspar) in the restite, although AFC processes (followed by effective separation from the basalts) cannot be excluded. Obsidians IFR3 need a crustal source with higher 87Sr-/86Sr; their peculiar geochemical features most probably reflect the complex behaviour during partial melting of accessory minerals carrying REE, Th, Zr, Hf. Among the eruptive products of Phase 2, mafic inclusion-bearing dacites (MIBD) provide evidence of basalt-rhyolite mixing: (a) coexistence of glasses with SiO2 content ranging from 64 to 72 wt.%; (b) a phenocryst assemblage including phases of basaltic and rhyolitic provenance; and (c) abundant crystal-rich inclusions related to a basaltic intrusion. The extrusion of these lavas has been preceeded by an explosive eruption of rhyolitic tuffs (87Sr/86Sr initial ratio = 0.71097). The most satisfactory petrogenetic model for MIBD includes: (1) differentiation of subalkaline (tholeiitic) magma to a silica-poor and Ti-Fe-P rich liquid; (2) partial melting of wall-rocks yielding a rhyolitic magma segregated at the top of the basaltic reservoir; and (3) explosive eruption of the uppermost portion of the magma chamber followed by mixing of rhyolite with the underlying part of the system to form a hybrid dacite.