Crustal- vs. mantle-derived magmas in the San Vincenzo rhyolites (Tuscan Magmatic Province, Italy) as constrained by texture and thermobarometry

This work focuses on the physical-chemical conditions of genesis and evolution of the San Vincenzo rhyolites, one of the most significant examples of mixing between anatectic and mantle-derived magmas [1 and reference therein]. Rhyolite lavas often show quenched andesite/latite enclaves with clinopyroxene and plagioclase phenocrysts and a large variety of petrographic domains (e.g. quartz syenite autholiths, sillimanite-cordierite-quartz-spinel-ilmenite xenoliths, clinopyroxene-rich cumulates), and restitic and melt inclusion-bearing magmatic phases (plagioclase, sanidine, quartz, biotite, cordierite). Melt inclusion-free, mafic phenocryts (amphibole, garnet, clino- and orthopyroxenes), often showing reaction/overgrowth rims of biotite, are relatively widespread. In particular, the evidence of amphibole nucleation, growth and quenching is found in mixed domains next to the basic enclaves, whereas clinopyroxene-rich cumulates show inclusion and interstitial amphiboles with homogeneous compositions. Various thermobarometric methods [2, 3, 4] were applied to the EMP data from the different domains showing equilibrium textures and compositions. In addition, we calibrated new cordierite-saturation thermobarometric formulations for peraluminous H2O-undersaturated melts. These latest formulations are particularly suitable for constraining the P-T conditions of San Vincenzo peraluminous melts (ASI 1.05-1.32, H2O 0.2-3.5 wt%) as only a few plagioclase inclusions are inferred to approach water-saturation (H2O 3.8-4.1 wt%). In contrast, single-crystal amphibole thermobarometry indicates higher H2O contents (3.7-6.8 wt%) and shows a continuous P-T crystallization pattern from 991°C and 1085 MPa (amphibole crystals from clinopyroxene-rich cumulates) to 869°C and 207 MPa (micro-phenocryst cores) crossing the MOHO [22-23 km, ca 600 MPa, 5] at 900-950°C. These results are consistent with a population of plagioclase-melt inclusion pairs indicating temperatures of 885-950°C. Clinopyroxene-liquid thermobarometry constrains the crystallization of basic enclave phenocrysts to higher-T (1060-1120°C) and mantle depths (800-1250 MPa). Sanidine-melt, sanidine-plagioclase and most of the plagioclase-melt pairs indicate temperatures of 837-720°C, closely matching the entrapment conditions of H2O-undersaturated melt inclusions of cordierite phenocrysts (790-850°C; 275-370 MPa). Temperatures of micro-quartz syenite autoliths (662-713°C) and those of “flame”-like restitic biotites (640-735°C) are similar. Finally, the application of THERMOCALC program (average P–T calculation mode) to a sillimanite-cordierite-quartz-spinel-ilmenite xenolith suggests equilibrium conditions of 700-750°C and 320-350 MPa. This textural and thermobarometric picture is consistent with the formation and evolution of the anatectic magma of San Vincenzo by “gas sparging” [i.e. rising of temperature and melting via fluid percolation, 6], magma mixing, assimilation and fractional crystallization processes at upper-mantle/crustal depths. References. [1] Poli, G. & Perugini, D. (2003): Periodico di Mineralogia, 72, 141-155; [2] Ridolfi, F., Renzulli, A., Puerini, M. (2010): Contributions to Mineralogy and Petrology, 160, 45-66; [3] Putirka, K.D. (2008): Reviews in Mineralogy and Geochemistry, 69, 61-120; [4] Henry, D.J., Guidotti, C.V., Thomson, J.A. (2005) American Mineralogist,, 90, 316–328; [5] Accaino, F., Tinivella, U., Rossi, G., Nicolich, R. (2005): Journal of Volcanology and Geothermal Research, 148, 46-59; [6] Bachmann, O., Bergantz, G.W. (2006): Journal of Volcanology and Geothermal Research, 149, 85-102.