Geochemistry of a transitional ne-trachybasalt ? Q-trachyte lava series from Patmos (Dodecanesos), Greece: further evidence for fractionation, mixing and assimilation

Abstract

Trace-element and preliminary Sr- and O-isotopic data are reported for a transitional alkaline-sub-alkaline lava series (MVS) from Patmos, Greece. The lava types belonging to this series are ne-trachybasalt, hy-trachybasalt, hy-trachyandesite and Q-trachyte. Rb, Sr and Ba contents, as well as K/Rb ratios, of the ne-trachybasalts differ from those of alkali basalts of oceanic islands and those of K-rich alkaline lavas of continental regions and are consistent with the occurrence of these volcanics in a destructive plate margin environment. Qualitatively, the variations shown by many trace elements throughout the MVS are explicable in terms of magma evolution via fractional crystallization involving removal of the observed phenocryst phases. Cross-cutting REE patterns can be explained by removal of small amounts of apatite. However, certain features of the data cannot be reconciled with the operation of fractional crystallization alone. These are: a) the compatible behavior of Ba throughout the MVS; b) the moderately (as opposed to highly) incompatible behavior of Zr, Rb and Nb relative to Th; and c) the significant decrease of K/Th, Rb/Th, Zr/Th, Zr/Nb, Nb/Th, Yb/Th, Ta/Th, U/Th and Zr/Ta ratios especially (but not exclusively) in the mafic part of the series. Quantitative modeling indicates that the hy-trachybasalts are anomalously enriched in both highly incompatible and highly compatible elements and these lavas are shown to be hybrids formed by mixing of ne-trachybasalt and hy-trachyandesite. Mixing proportions of the end members calculated from incompatible element abundances (∼19% ne-trachybasalt) differ from those calculated from compatible element abundances (∼62% ne-trachybasalt) and are inconsistent with proportions calculated from published mineral chemical data. In addition, mixing cannot account for the observed variations in incompatible element ratios and this is taken as evidence for the simultaneous operation of assimilation. Isotopic variations (87Sr/ 86Sr from 0.7049 to 0.7076 and 18O/16O from 4.7 to 8.6‰) and the positive correlation of isotope ratios with SiO2 and Th contents provide conclusive proof that assimilation occurred. Calculations show that the isotopic characteristics and the concentrations of many trace elements in the Q-trachytes can be explained by fractional crystallization of ne-trachybasalt combined with assimilation of average continental crust (87Sr/86Sr-0.710), and that large amounts of assimilation are not necessary (Ma/Mc=0.55). REE data are not well explained by this model and suggest a crustal end-member enriched in LREE relative to the average crust. Zr and Hf data are also not well explained and indicate that the assimilant was depleted in HFSE relative to average crust or that HFSE are held back in relatively insoluble phases such as zircon in the restite during assimilation. Nevertheless, the results of the modeling demonstrate that Ba concentrations may decrease during AFC processes and that high Sr contents (∼1500 ppm in the MVS ne-trachybasalts) do not render mafic, parental magmas immune to the effects of assimilation in terms of their 87Sr/86Sr ratios. The results of this study confirm conclusions based upon major-oxide and mineral chemical data for the MVS lavas but, more importantly, show that careful analysis of trace element data allows the various processes involved in magma evolution to be identified and quantified, even in the absence of major oxide and isotopic data. Finally, it is reiterated that magma mixing and assimilation may be coupled processes in the magma chambers beneath many volcanic centers, and recognition of this fact has profound implications for studies of magmas erupted at continental margins and through continental crust.