The petrology of Paleogene volcanism in the Central Sakarya, Nallıhan Region: Implications for the initiation and evolution of post-collisional, slab break-off-related magmatic activity

Abstract

Zircon ages, mineral chemistry, whole-rock major and trace element compositions, as well as Sr–Nd isotopic ratios of basaltic (basanite, basalt, and hawaiite with MgO=3.90–10.06 and SiO2=43.18–48.16) to andesitic (SiO2=50.86–61.27) and rhyolitic (SiO2=71.11–71.13) volcanic rocks (E-W emplaced Nallıhan volcanics) in the Lower Eocene terrestrial sedimentary units in the Central Sakarya Zone were studied and compared with those of the northerly located E-W-trending Eocene volcanic rocks (the Kızderbent Volcanics with 52.7–38.1Ma radiometric ages) that are thought to be related to slab break-off process following the continental collision in the NW Anatolia. Zircon U–Pb ages of the Nallıhan volcanics vary from 51.7±4.7 to 47.8±2.4Ma.Clinopyroxene from the basaltic and andesitic rocks record crystallization conditions from ~7–8kbars (~23km) and ~1210°C, to 4.5–1.5 kbars (~14–1.5km) and 1110–1010°C crystallization conditions, respectively. The olivine-bearing, high-MgO (up to 10wt%) basaltic rocks of the Nallıhan volcanics have nepheline-normative and Na-alkaline compositions, while the andesitic to rhyolitic rocks show calc-alkaline affinity with mainly sodic character. This is the first time this type of volcanic rock has been described in this region. The initial Sr isotopic ratios of both basaltic and andesitic–rhyolitic samples from the Nallıhan volcanics are similar (~0.7040–0.7045), indicating that fractional crystallization processes were not accompanied by crustal contamination and that the magma chambers were likely stored within ophiolitic units. Trace element ratios suggest that the Nallıhan volcanics were derived from E-MORB- or OIB-like enriched mantle sources, while the Kızderbent volcanics had N-MORB-like depleted mantle sources. Both volcanic units were produced by partial melting of spinel-bearing (shallow) mantle sources that had undergone subduction-related enrichment processes, with the degree of enrichment having been greater for the Kızderbent volcanics.The geochemical features of both the Nallıhan and Kızderbent volcanics are best explained as the result of slab break-off, in which the Nallıhan volcanics (located closer to the original subduction front) were produced mainly by the melting of upwelling asthenospheric mantle. Further back from the subduction front, the upwelling interacted with more highly metasomatized sub-arc mantle that underwent partial melting to produce the Kızderbent volcanics. This geodynamic scenario can be used for understanding other post-collisional slab break-off-related magmatic activities.