U-Pb SIMS dating of synkinematic granites: timing of core-complex formation in the northern Anatolide belt of western Turkey

Journal of the Geological Society, Mar 2005 by Ring, Uwe, Collins, Alan S

Abstract:

Secondary ion mass spectrometry (SIMS) U-Th-Pb dating of magmatic zircon from the synkinematic Egrigöz and Koyunoba granites and a leucogranite dyke dates core-complex formation in the northern Anatolide belt of western Turkey at 24-19 Ma. The granites intrude into the footwall of the Simav detachment and are strongly elongated in the NNE direction parallel to tectonic transport on the detachment. Although large parts of the granites are undeformed, localized mylonitic to ultramylonitic deformation occurs directly beneath the Simav detachment and preserves evidence of progressive deformation from ductile to brittle conditions. Oscillatory zoned rims of long-prismatic zircon from the Egrigöz and Koyunoba granites yield identical and well-constrained intrusion ages of 20.7 ± 0.6 Ma and 21.0 ± 0.2 Ma, whereas inherited grains range from Palaeoproterozoic (2972 ± 13 Ma) to Neoproterozoic (653 ± 6 Ma to 500 ± 5 Ma) in age. A leucogranite dyke yields an intrusion age of 24.4 ± 0.3 Ma, with inherited Neoproterozoic (640 ± 7 Ma to 511 ± 6 Ma) grains. Our data, in conjunction with published ^sup 40^Ar/^sup 39^Ar biotite ages, indicate very rapid cooling (greater than c. 200 °C Ma^sup -1^) for the granites during and after synkinematic emplacement.

Keywords: U-Pb, Anatolide belt, detachment faults, metamorphic core complexes, granites.

Synkinematic granites are important for dating erogenic processes and thus for understanding the evolution of mountain belts. A close relationship between granite emplacement and movement along major extensional detachments has been documented (Coney 1980; Gans et al. 1989; Hill et al. 1992; Lee & Lister 1992), hence such a relationship helps to constrain the age of the detachments. The Anatolide belt of western Turkey exposes remarkable late-orogenic extensional detachments (Bozkurt 2000; Gessner et al. 2001a; Isik & Tekeli 2001; Seyitoglu et al. 2002; Isik et al. 2004). A bivergent system of Pliocene to Recent detachments bounds the Central Menderes metamorphic core complex (Gessner et al. 2001a) (Fig. 1). Towards the margins of the Anatolide belt, detachment faulting is older (Seyitoglu et al. 1992; Isik & Tekeli 2001; Ring et al. 2003a; Isik et al. 2004) but the exact timing of core-complex formation is not well constrained.

We report Late Oligocene to Early Miocene secondary ion mass spectrometry (SIMS) U-Th-Pb zircon ages of 20.7 ± 0.6 Ma from the Egrigöz granite, 21.0 ± 0.2 Ma for the Koyunoba granite and 24.4 ± 0.3 Ma (all 2σ errors) for a leucogranite dyke, and demonstrate the coeval relationship between movement of the Simav detachment and these synkinematic intrusions. In conjunction with one recently reported ^sup 40^Ar/ ^sup 39^Ar white mica age of 22.9 ± 0.5 Ma for mylonitization of a pre-shear-zone rock (Isik et al. 2004) this constrains the timing of core-complex formation at the northern margin of the Anatolide belt of western Turkey.

Setting

Previous research revealed that the architecture of the Anatolide belt in western Turkey comprises three major tectonometamorphic units, which are characterized by different Iithologies, protolith ages, pre-collisional palaeogeography and erogenic history (Ring et al. 1999a). This heterogeneous tectonic pile was assembled during Eocene collision-related crustal shortening (Gessner et al. 200Ic). The Lycian Nappes and the Vardar-Izmir-Ankara suture zone represent the upper tectonometamorphic unit, both of which were high-pressure metamorphosed and deformed in the Late Cretaceous (before c. 75 Ma) (Collins & Robertson 1997, 1998; Sherlock et al. 1999; Oberhansli et al. 2001; Ring & Layer 2003). The middle tectonometamorphic unit has been correlated with the Cycladic blueschist unit in the Aegean (Candan et al. 1997; Ring et al. 1999a). The Menderes nappes make up the lower tectonometamorphic unit and, unlike the other two, do not show Late Cretaceous to Eocene highpressure metamorphism. The high-pressure rocks of the middle and upper tectonometamorphic units are separated from the Menderes nappes by a huge out-of-sequence thrust, the Eocene Cyclades-Menderes thrust (Gessner et al. 200Ic) (Fig. 1).

After Late Cretaceous to Eocene crustal shortening, the Anatolide belt underwent at least two periods of extensional faulting. In the northern part of the Anatolide belt, Isik & Tekeli (2001) and Isik et al. (2004) identified the top-to-the-NNE Simav detachment (Fig. 2). The Simav detachment reactivated the Eocene Cyclades-Menderes thrust so that the rocks of the Cycladic blueschist unit and the Vardar-izmir-Ankara zone make up the hanging wall of the detachment. The footwall consists of Precambrian gneiss of the Menderes nappes. Locally this gneiss is caught up in the detachment zone and occasionally occurs in a hanging-wall position. The Egrigöz and Koyunoba granites, and also the Alaçam granite further west, the Demirçi granite further south (Fig. 1 ) and a number of smaller granites and granitic dykes, intrude into footwall gneiss. The timing of movement on the Simav detachment is not well constrained, but is thought to be older than a 16-15 Ma volcano-sedimentary sequence that unconformably overlies the detachment (Seyitoglu et al. 1992; Isik & Tekeli 2001). Apatite fission-track ages of 20-19Ma (Gessner et al. 2001a; Ring et al. 2003a) and one single ^sup 40^Ar/^sup 19^Ar white mica age of 22.9 ± 0.5 Ma from a mylonite derived from Precambrian gneiss (Isik et al. 2004) suggest an age off. 23-19 Ma for the Simav detachment.