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, 238 U/ 206 Pb, ±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-Tethyan 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-Tethyan ocean basins during the geologic assembly of Turkey.

Neotethyan closure history of western Anatolia: a geodynamic discussion

“…Lithologies represented in chaotic formations intercalated between these two units suggest that only the oceanic domain of the Cycladic basin, evident in the Aegean, did not significantly extend into western Anatolia and prolonged eastwards likely as a deep-sea basin floored by thinned continental crust. Besides, we express doubts about the possibility for an Afyon Ocean (Shin et al 2012;Speciale et al 2014) to have once separated the Tavşanlı and the Afyon platforms, given missing evidence for distinct ophiolitic belts (alleged by these authors) and a tectonic-magmatic scenario contradicting metamorphic data. On the other hand, the timing of sedimentation cease in the various Lycian nappes suggests that the upper Köyceğiz Nappe and the intermediate Teke Dere Nappe were accreted at distinct times to the trench, and should be restored to two distinct palaeogeographic domains on either side of the Cycladic basin (Fig.…”

“…1). Near the junction of the Izmir-Ankara, Inner-Tauride and Ankara-Erzincan sutures, Triassic schists of the Sakarya Zone of Pontides were intruded by the Beypazarı magmatic complex, for which an arc-type geochemical signature (Öztürk et al 2012) and Upper Cretaceous crystallisation ages (>95-70 Ma; Speciale et al 2014) have been documented. Internally undeformed calc-alkaline plutons of mainly Lower Eocene age (54-45 Ma; U-Pb on zircon and Ar-Ar on white mica and hornblende) (Harris et al 1994;Sherlock et al 1999;Altunkaynak et al 2012;Shin et al 2013) are intrusive in the Tavşanlı Zone and the overlying ophiolite (Fig.…”

“…5c) (Dilek and Altunkaynak 2009;Ersoy and Palmer 2013); and (3) coeval subduction zones with Palaeocene slab break-off (Fig. 5d) (Shin et al 2012;Speciale et al 2014). Hereafter, the tectonic models for western Anatolia, distributed accordingly into three categories, are presented with emphasis on their foundations, and their shared and distinctive aspects.…”

“…5d) The model proposed first by Shin et al (2012) and then refined by Speciale et al (2014) is essentially based on new zircon U-Pb in situ geochronologic data from intrusions in the region of Sivrihisar and west of Ankara (Fig. 1).…”

“…In the models of Shin et al (2012) and Speciale et al (2014), the Tavşanlı and Afyon zones are considered as two microcontinents separated by a Neotethyan oceanic domain ('Afyon Ocean' ; Fig. 2).…”