Petrogenesis of Lamprophyres from Central Sakarya (NW Anatolia, Turkey): Intraplate Magmatism at the Onset of Slab Rollback

Mid-Eocene lamprophyre and camptonite (alkaline lamprophyre) dikes from Central Sakarya, NW Anatolia, document local NE-SW crustal extension immediately following closure of the western Neotethys. The rocks are highly phyric with clinopyroxene megacrysts up to 10 cm in length; smaller phenocrysts (<1 cm) of calcic amphibole, plagioclase feldspar and Fe-Ti oxides are also abundant in all samples. Camptonites (~6.9 wt.% MgO) contain normative olivine and minor nepheline, and phenocrysts of amphibole (Mg# 0.8-0.9), clinopyroxene (Mg# 0.8) and plagioclase feldspar (An₈₄). Pargasitic amphibole began crystallizing in these rocks at 25-30 km, presumably along the base of the crust at the top of the subducted slab, with later overgrowths and phenocrysts forming at mid-crustal depths of 6-12 km. Mafic megacrysts commonly contain an optically continuous patchwork of pyroxene and amphibole that reflects fluctuating silica saturation levels during crystal growth and/or fluid-driven modification. Sr-Pb isotopic data indicate melts were derived from a mixture of ambient MORB mantle and heterogeneous subcontinental lithospheric mantle. We also infer a significant role for sediments in the bulk rock geochemistry. Pb anomalies are positive relative to primitive mantle but absent relative to GLOSS; Th/La and Hf/Nd are negatively correlated, and Pb isotopic values of the samples overlap Miocene-Recent Central Anatolian mafic lavas inferred to reflect bulk incorporation of sediments and/or melange. We interpret the lamprophyres to reflect melting at the earliest stage of slab rollback and southwestward migration of the subduction zone now located in the westernmost Aegean Sea. Isotopic and trace element similarity between the lamprophyres and Central Anatolian lavas speaks to a shared petrogenesis related to regional delamination of the Neotethyan slab during post-collisional relaxation. In contrast, younger mafic lavas from NW Anatolia record a greater contribution from asthenospheric materials as the sinking slab opens new pathways for ascent of sublithospheric mantle.