REGIONAL PETROLOGY OF THE SAN-FRANCISCO VOLCANIC FIELD, ARIZONA, USA

A wide variety of rock types has been erupted during the past 5 Ma in the San Francisco Volcanic Field (SFVF) of north-central Arizona. Neph eline-normative to olivine hypersthene- and quartz-normative basalts f orm the mafic end of a spectrum that includes various lineages of rela tively high-Na and high-K, intermediate and silicic volcanics. These e volved magmas have been erupted in individual centres that range in si ze from small craters to the San Francisco Peaks themselves, with a vo lume of similar to 50 km(3). Isotopic variability (Sr-87/Sr-86 similar to 0.7030-0.7046) and a spread of La/Yb in the basalts indicate a var iety of upper-mantle sources A number of petrographically distinct bas alt types are related by varying degrees of olivine and/or clinopyroxe ne accumulation. The fractional crystallization of olivine, clinopyrox ene, plagioclase and oxides (+/-amphibole), from alkali olivine basalt parents, to produce a relatively Na-rich lineage of hawaiite, benmore ite/high-Na andesite and trachyte is supported by least-squares fracti onation and mixing calculations. Mixing with K-rich, crustally derived (radiogenic and unradiogenic Sr-87/Sr-86 and unradiogenic Pb-206/Pb-2 04) rhyolite has Produced other trends towards alkali-rich andesite, d acite and rhyolite. Within some individual silicic centres, distinctiv e major and trace element geochemistry, petrograpihic features and iso topic data are interpreted to result from a combination of fractional crystallization of basalt in deep crustal magma chambers (cumulate blo cks are widespread in basaltic rocks), crustal anatexis, and direct mi xing of evolved basaltic and rhyolitic melts In other centres, a combi nation of assimilation-fractional crystallization involving crustal co mponents developed Partially digested oligoclase-andesine and quartz x enocrysts, identical in composition to phases present in granulite xen olith assemblages, also occur widely in the basalt-rhyolite compositio nal spectrum. Reverse zoning of plagioclase and ferromagnesian phenocr ysts and disequilibrium assemblages are common. The majority of the vo lcanic rocks are alkalic with respect to major element (LUGS) classifi cations, but mineralogical characteristics include the presence of phe nacrystic and groundmass orthopyroxene in many of the intermediate roc ks. Some meta- and peraluminous rhyolites have low La/Yb, massively ne gative Eu anomalies, < 20 p.p.m Ba and Sr, and extremely manganiferous fayalite (similar to 10 wt% MnO). Rare peralkaline high-SiO2 rhyolite s contain similar to 900 p.p.m. Zr.