Zircon ages from the Beypazarı granitoid pluton (north central Turkey): tectonic implications

Abstract

The Beypazarı granitoid is emplaced in a Late Cretaceous volcanic arc in north central Turkey and provides evidence for processes that occurred during the closure of the Neo-Tethys Ocean. Zircons from its northern granodiorite and quartz monzonite exposure are dated in rock thin section and display characteristic igneous zoning in cathodoluminescence (CL). Its oldest Late Cretaceous ages (95.4 ± 4.2–91.3 ± 6.5 Ma, 238U/206Pb, ±1σ) time early crystallization from rising melts, but inspection of the youngest grains in composite CL–secondary electron images shows they are located along grain boundaries surrounded by alteration textures and were possibly affected by fluid-driven metamorphism. Ages obtained from zircon mid-rims (82.7 ± 6.0–70.5 ± 3.4 Ma) record continuous crystallization of the pluton. Here, we model the Late Cretaceous development of the Beypazarı granitoid wherein subduction of Neo-Tethys Oceanic plate, multiple injections and mixing of magma, and crustal contribution during ascent are the cause of geochemical heterogeneities within the pluton and account for protracted zircon crystallization. The model speculates that the pluton is in the overriding tectonic plate of a series of stacked, north-dipping subduction zones that record the closure of Neo-Tethys ocean basins during the geologic assembly of Turkey.