Contrib Mineral Petrol (1986) 93 :297-311 Contributions to Mineralogy and Petrology 9 Springer-Verlag 1986 Petrology and evolution of transitional alkaline- sub alkaline lavas from Patmos, Dodecanesos, Greece: evidence for fractional crystallization, magma mixing and assimilation G. Paul Wyers and Michael Barton Department of Geology and Mineralogy, The Ohio State University, Columbus, OH 43210, USA Abstract. Petrographic, mineral chemical and whole-rock major oxide data are presented for the lavas of the Main Volcanic Series of Patmos, Dodecanesos, Greece. These la- vas were erupted about 7 m.y. ago and range in composi- tion from ne-trachybasalts through hy-trachybasalts and trachyandesites to Q-trachytes. To some extent, the ne-tra- chybasalts are intermediate in composition to the alkaline lavas found on oceanic islands and the calc-alkaline lavas of destructive plate margins. Major oxide variation is large- ly explicable in terms of fractional crystallization involving removal of the observed phenocryst and microphenocryst phases viz. olivine, plagioclase, clinopyroxene and Ti-mag- netite in the mafic lavas, plagioclase, clinopyroxene, mica and Ti-magnetite in the evolved lavas. Apatite, which oc- curs as an inclusion in other phenocrysts or as micropheno- crysts must also have been removed. However, mass bal- ance calculations indicate that the chemistry of the hy-tra- chybasalts is inconsistent with an origin via fractional crys- tallization alone and the complex zoning patterns and re- sorbtion phenomena shown by phenocrysts in these lavas show that they are hybrids formed by the mixing of 80-77% ne-trachybasalt with 20-23 % trachyandesite. It is estimated that the mixing event preceded eruption by a period of 12 h-2 weeks suggesting that mixing triggered eruption. Combined fractionation and mixing cannot explain the rel- atively low MgO contents of the hy-trachybasalts and it is concluded that assimilation also occurred. Assimilation, and especially addition of volatiles to the magmas, may be responsible for the evolutionary trend from ne-normative to hy-normative magmas and was probably facilitated by intensified convection resulting from mixing. A model is presented whereby primitive magma undergoes fractiona- tion in an intracrustal magma chamber to yield more evolved liquids. Influx of hot primitive magma into the base of the chamber facilitates assimilation, but eventually mixing yields the hy-trachybasalts and finally the ne-trachy- basalts are erupted. Introduction In recent years there has been increasing recognition of the complexity of the processes involved in magma evolu- tion. It is now widely realized that the compositions of Offprint requests to ." G.P. Wyers many magmas erupted through the continental crust are influenced by magma mixing and assimilation in addition to fractional crystallization. As emphasized by O'Hara (1977) and O'Hara and Matthews (1981), it is essential that the operation of such processes are recognized, and the effects quantified, before attempts are made to reconstruct upper mantle source region characteristics. The Tertiary to recent lavas from the southern Aegean Sea, Greece, have evolved via fractionation, mixing and assimilation (Barton etal. 1983; Huijsmans etal. 1983, 1986). Most of the volcanic complexes thus far described in detail occur along the Hellenic arc which is associated with the overriding of the African Plate by the Aegean microplate (Huijsmans et al. 1986). In this paper, we de- scribe the petrology of alkaline-sub-alkaline lavas from Pat- mos, which is located some 100 km to the north of the Hellenic arc (Fig. 1). Specifically, we use mineral chemical and whole-rock major oxide data to evaluate the processes operative during evolution of the Main Lava Series (see Wyers et al. in prep.). Our major objective is to demonstrate that significant conclusions may be drawn on the basis of major element data alone. In companion papers (Wyers and Barton in prep. ; Barton and Wyers in prep.) we use trace element and isotopic data to further constrain the ideas developed in the present communication and we esti- mate the conditions of crystallization. Outline of geology Patmos is an almost entirely volcanic island with an area of approx- imately 38 km 2. The geology has been described in some detail by Robert (1973) who recognized the following major volcanic units: high-K, high-A1basalts; intermediate potassic lavas (latite, quartz-latite, potassic trachyte); quartz-potassic alkaline lavas (quartz-trachytes, phonolites) and pyroclastics; and sodic alkaline lavas (phonolites, trachytes). We have adopted a classification scheme based entirely upon whole-rock chemistry. This scheme, which was previously used by Johnson et al. (1976) for alkaline volcanics on islands off the coast of Papua (New Guinea) is illus- trated in Fig. 2 and is based upon the amount of normative nephe- line or quartz (which includes SiO2 from normative hypersthene) and the Thornton-Tuttle 0960) differentiation index. We deliber- ately chose the scheme to avoid the necessity of using names nor- mally associated with talc-alkaline or typically alkaline lava series and it has been used successfullyfor lavas on some oceanic islands (e.g., Tristan da Cunha; Baker et al. 1964). The distribution of the major volcanic units is shown in Fig. 3, which is modified from Robert (1973). The predominant rock-types