ALBERT

Description: High-Mg ultrapotassic volcanic rock occurrences of lamproitic affinity are exposed in southwestern Anatolia, mostly within the Menderes Massif. From north to south the lamproitic volcanism shows increasingly younger ages ranging from 20 to 4 Ma. Volcanism is contemporaneous with more voluminous shoshonitic, high-K calc-alkaline, and ultrapotassic magmatic activity in the Simav–Selendi, Uşak, Kırka, Köroğlu, Afyon and Isparta–Gölcük areas. The southward decrease in the age of the volcanism correlates with changes in geochemical composition, particularly a decrease in 87Sr/86Sr, 207Pb/204Pb, Zr/Nb and Th/Nb, and an increase in 143Nd/144Nd, 176Hf/177Hf, 206Pb/204Pb, 208Pb/204Pb and Ce/Pb, thus delineating a systematic change from orogenic (crust-like) to anorogenic (convecting mantle-like) signatures. Rare earth element compositions of clinopyroxene phenocrysts demonstrate an increasing role for residual garnet for locations in the central parts of the Menderes Massif, indicating a lithosphere thickness greater than 80 km. In contrast, K2O abundances remain nearly constant at around 7%, indicating buffering by phlogopite in the mantle source. Magma genesis in southwestern Anatolia is controlled by post-collisional extensional events initiated after major lithospheric thickening. Geochemical constraints suggest that the mantle source experienced two main geodynamic stages. The first stage caused ultradepletion of the mantle and subsequent metasomatic enrichment, which allowed coupling of the geochemical signatures of ultradepleted harzburgite with those of crust-derived sediments. This happened during the final closure stages of the southern Neotethys Ocean and the accretion of forearc oceanic lithosphere (island-arc type), as shallowly subducted material to the Anatolian lithosphere. The second stage is post-collisional, and is related to the collapse of the orogenic belt and the development of extension-related horst and graben structures. This stage is concurrent with the initiation of a thermal anomaly originating from a gap, identified by seismic tomography, in the subducted slab under western Anatolia. We propose that the lithospheric mantle underwent intense ‘asthenospherization’ owing to lithosphere–asthenosphere interaction, caused by the southward expansion of this gap during slab roll-back. The geochemical resemblance of the lamproites to more voluminous, contemporaneous shoshonitic magmas implies their derivation from a heterogeneous mantle source that had been affected by similar processes. These mantle processes may be closely associated with the major episode of uplift in the Menderes Massif.

Description: The Demir Kapija ophiolitic complex in southern Macedonia–FYROM (Former Yugoslav Republic of Macedonia) represents the southernmost exposure of the Tethyan Eastern Vardar ophiolitic unit in the Eastern Mediterranean. It consists of a mafic volcanic sequence (pillow basalts, sheeted dyke diabases and gabbros) that was subsequently intruded by island arc magmas with and without adakitic affinity. The mafic volcanic sequence is characterized by slightly increased ratios of large ion lithophile elements to high field strength elements (LILE/HFSE), flat rare earth element (REE) patterns, radiogenic 143Nd/144Nd (up to 0·51272) and high TiO2 contents (which reflect Pl + Ol + Cpx fractionation). The relationship between TiO2 and MgO contents indicates that Ti saturation was eventually reached and that Ti-magnetite fractionated. The mafic volcanic sequence of the ophiolite complex formed around 166·4 Ma in a short-lived intra-oceanic back-arc basin by slab roll-back of the Western Vardar Ocean. The rocks with and without adakitic affinity are spatially and temporally closely related. Their crystallization age is around 164 Ma. Our data suggest that two subgroups of arc lavas evolved as discrete volcanic lineages that are not related by fractional crystallization of a common parental magma, and that two different parental magmas are required. The arc lavas with adakitic affinity show some of the typical features of adakites; that is, low heavy REE, elevated Sr/Y, high LILE and high light REE. Major and trace element compositions of clinopyroxene phenocrysts resemble those of typical adakite-derived clinopyroxene. The very high Th/La, Th/Yb and Ba/Yb ratios and the reduced 143Nd/144Nd values (around 0·51245) of the rocks with adakitic affinity are considered to reflect contributions of sedimentary material to their mantle source. By analogy with adakites, these rocks are interpreted as the product of slab + sediment melting in an unusually hot subduction zone (subduction of young oceanic crust). In contrast, the arc lavas without adakitic affinity are related to a different parental melt, similar to common arc magmas. The Demir Kapija ophiolite formed in a short-lived intra-oceanic back-arc basin during subduction initiation within a back-arc. The arc intrusions are related to the change from an extensional to a compressional regime.