Unravelling the complex interaction between mantle and crustal magmas encoded in the lavas of San Vincenzo (Tuscany, Italy). Part I: Petrography and Thermobarometry

Abstract

The San Vincenzo Volcanic Complex was emplaced ~4.4Ma. ago and consists of cordierite-bearing lavas which are the result of a complex interaction between mantle-derived and crustal anatectic magmas. The lavas are mostly characterized by porphyritic, glassy peraluminous rhyolites hosting variable contents of magmatic enclaves (clinopyroxene-bearing latites and amphibole-bearing clinopyroxene crystal mushes), sialic and ultramafic cognates (syenogranites, anorthosites, cordierite–biotite and pyroxenite inclusions), and crustal rocks (sillimanite–cordierite xenoliths, cordierite and biotite xenocrysts) of centimetric-to-millimetric size.Mineral chemistry shows large variations as well. Plagioclase and sanidine are represented respectively by An21–79Or1–13 and An≤1Or57–77. Cordierite has a Mg# of 51–78%, while garnet shows almandine compositions with low CaO (≤2wt.%) and variable MnO contents (1–5wt.%). Clinopyroxene indicates large ranges of Mg# (68–92%) and Al2O3 (0.5–6.3wt.%), and relatively high CaO contents (up to 24wt.%); orthopyroxene shows both ferroan enstatite (Mg#=60–78%) and magnesian ferrosilite (Mg#=39–44%) compositions; whereas amphibole shows only Mg-rich calcic compositions. On the basis of textural characteristics, as well as Ti and XMg variations, we have identified six different types of biotite associated with oxide minerals such as ilmenite and spinels of both aluminium (Al>1 in Y site) and iron (Fe>1 in Y site) subgroups.Compositional/textural relationships indicate crystallization at both equilibrium and disequilibrium conditions. Minerals with euhedral habits and homogeneous compositions usually occur in the same thin sections of partly-equilibrated crustal xenoliths (and xenocrysts) and zones of “active” mixing between mantle-derived and crustal magmas characterized by “needle-like” and skeletal microlites, and subhedral microphenocrysts of amphibole and biotite. These hybrid-mixed features, as well as the occurrence of calcic amphibole, garnet and magnetite, are exclusive to lavas cropping out in the northern sector of the volcanic complex (i.e. Acqua Calda Quarry). By contrast, the rhyolites in the southern area (i.e. Rozze Valley) mostly show mingled textures characterized by “coherent” globular blobs of mantle-derived magma (up to 15cm in size) that did not efficiently homogenize with the rhyolite.To determine the crystallization depths and physico-chemical conditions of the San Vincenzo magmatic feeding system, we applied some thermobarometers to single phases (amphibole and melt inclusions) or petrographic domains (sillimanite–cordierite xenoliths, syenogranites and anorthosites, piroxenites) showing crystallization at equilibrium conditions. In the framework of the estimated conditions, we used H2O–CO2 solubility, viscometric and densimetric models to obtain additional constraints on sub-volcanic processes.The overall results suggest that the thermal interaction of a hot mantle-derived magma (~1100°C) with the Tuscan crust (≤700°C) occurred at the MOHO transition (~24km). This interaction determined (i) partial melting of the lower-crust siliciclastic rocks, (ii) migration of the anatectic magma to shallower levels and (iii) formation of a deep storage region (16–24km) filled with mantle-derived magma. In the deep reservoir, the mantle-derived magma evolved at conditions of 900–1010°C, H2Omelt 5.2–8.6wt.% and +2≤ΔNNO≤+4 through assimilation of carbonate–evaporite metasediments and fractionation of clinopyroxene (±plagioclase±ferroan enstatite±biotite) to form ultramafic and mafic crystal mushes and cumulates. The overlying siliciclastic rocks underwent partial melting at 710–850°C (H2Omelt≤4wt.%) leading to the genesis of a second reservoir filled with anatectic magma at depths of 8–17km. Locally, the T–H2Omelt conditions in the shallower reservoir were as high as 920°C and ~5wt.%, when the mantle-derived magma was injected into the rhyolite magma to form the mingled lavas which were afterwards erupted in the Rozze Valley area. The hybridization of the rhyolites occurred at 870–910°C (H2Omelt=5–7wt.%), suggesting that the Acqua Calda Quarry lavas were the last to be erupted.