Volcano-stratigraphy and geochemistry of collision-related volcanism on the Erzurum-Kars Plateau, northeastern Turkey

The Eastern Anatolia Region exhibits one of the world's best exposed and mo st complete transects across a volcanic province related to a continental c ollision zone. Within this region, the Erzurum-Kars Plateau is of special i mportance since it contains the full record of collision-related volcanism from Middle Miocene to Pliocene. This paper presents a detailed study of th e volcanic stratigraphy of the plateau, together with new K-Ar ages and sev eral hundred new major- and trace-element analyses in order to evaluate the magmatic evolution of the plateau and its links to collision-related tecto nic processes. The data show that the volcanic units of the Erzurum-Kars Pl ateau cover a broad compositional range from basalts to rhyolites. Correlat ions between six logged, volcano-stratigraphic sections suggest that the vo lcanic activity may be divided into three consecutive Stages, and that acti vity begins slightly earlier in the west of the plateau than in the east. T he Early Stage (mostly from 11 to 6 Ma) is characterised by bimodal volcani sm, made up of mafic-intermediate lavas and acid pyroclastic rocks. Their p etrography and high-Y fractionation trend suggest that they result from cry stallization of anhydrous assemblages at relatively shallow crustal levels. Their stratigraphy and geochemistry suggest that the basic rocks erupted f rom small transient chambers while the acid rocks erupted from large, zoned magma chambers. The Middle Stage (mostly from 6-5 Ma) is characterised by unimodal volcanism made up predominantly of andesitic-dacitic lavas. Their petrography and low-Y fractionation trend indicate that they resulted from crystallization of hydrous (amphibole-bearing) assemblages in deeper magma chambers. The Late Stage (mostly 5-2.7 Ma) is again characterised by bimoda l volcanism, made up mainly of plateau basalts and basaltic andesite lavas and felsic domes. Their petrography and high-Y fractionation trend indicate that they resulted from crystallization of anhydrous assemblages at relati vely shallow crustal levels. AFC modelling shows that crustal assimilation was most important in the deeper magma chambers of the Middle Stage. The ge ochemical data indicate that the parental magma changed little throughout t he evolution of the plateau. This parental magma exhibits a distinctive sub duction signature represented by selective enrichment in LILE and LREE thou ght to have been inherited from a lithosphere modified by pre-collision sub duction events. The relationships between magmatism and tectonics support m odels in which delamination of thickened subcontinental lithosphere cause u plift accompanied by melting of this enriched lithosphere. Magma ascent, an d possibly magma generation, is then strongly controlled by strike-slip fau lting and associated pull-apart extensional tectonics. (C) 1998 Elsevier Sc ience B.V. All rights reserved.