40Ar-39Ar and Rb-Sr geochronology of high-pressure metamorphism and exhumation history of the Tavsanli Zone, NW Turkey

Abstract

Geochronological investigations in high-and ultra-high-pressure metamorphic rocks are problematic since ®rstly the low temperatures lead to ®ne grain size and disequilibrium assemblages, and secondly the problem of``excess argon'' aects 40 Ar-39 Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically signi®cant. This study presents new data from the Tavsanli Zone; 40 Ar-39 Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, con®rming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger 40 Ar-39 Ar ages in the cores of coarser white micas, which in conjunction with 40 Ar-39 Ar ages from the ®nest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma A1 and exhumation to the surface via thrusting. The 40 Ar-39 Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks.

Quartz and calcite microfabric transitions in a pressure and temperature gradient, Sivrihisar, Turkey

“…The 88-82 Ma 40 Ar/ 39 Ar results for phengite in the northern domain are similar to other Late Cretaceous ages obtained in other studies (Harris et al, 1994;Okay & Kelley, 1994;Okay et al, 1998;Sherlock et al, 1999). These ages are interpreted as indicating cooling during exhumation of HP rocks from the subduction zone.…”

“…Zircon U-Pb ages range from 78.4 ± 8.5 Ma to 42.4 ± 2.3 Ma (Shin, Catlos, Jacob, & Black, in press). The hornblende 40 Ar/ 39 Ar age for the granitoid is 53 ± 3 Ma (Sherlock, Kelley, Inger, Harris, & Okay, 1999) (ca. 54 Ma when recalculated using the Renne et al (2010) age of GA1550 biotite).…”

“…Because most of the 40 Ar/ 39 Ar muscovite ages at 57-64 Ma are older than the hornblende 40 Ar/ 39 Ar age of the Sivrihisar pluton (~53 Ma (Sherlock et al, 1999)), muscovite likely grew or recrystallized during regional metamorphism prior to pluton emplacement. However, samples most proximally located to the pluton likely experienced some to complete resetting of the argon clock.…”

Kinematic reconstructions and magmatic evolution illuminating crustal and mantle dynamics of the eastern Mediterranean region since the late Cretaceous

“…24 ± 3 kbar, 430 ± 30 °C) were reached between 88 and 78 Ma (figures 5 and 7) [Okay, 1986[Okay, , 2002Sherlock et al, 1999;Seaton et al, 2009;Plunder et al, A C C E P T E D M A N U S C R I P T…”

“…The late Cretaceous HP-LT Tavşanlı and Afyon-Ören units were then exhumed along a cold retrograde path reaching the surface in the early Eocene [Özcan et al, 1988;Sherlock et al, 1999;Rimmelé et al, 2003a;Ring and Layer, 2003;Pourteau et al, 2010, A C C E P T E D M A N U S C R I P T…”