³⁹Ar/⁴⁰Ar Ages from the Yozgat Batholith: Preliminary Data on the Timing of Late Cretaceous Extension in the Central Anatolian Crystalline Complex, Turkey

Abstract

Isotopic dating of sheared and unsheared rocks can be important in understanding deformational processes in orogenic belts. This study examines 40 Ar= 39 Ar dating of granitoids and mylonitic rocks to constrain intrusive and deformational events within the northern part of the central Anatolian crystalline complex (CACC). The Kerkenez granitoid within this complex, comprising primarily quartz monzonite and hornblende granite, contains discrete ductile shear zones. These zones are characterized by protomylonite and mylonite formations with metamorphism conditions that reach lower amphibolite facies, mylonitic foliations and lineations, and asymmetric kinematic indicators (e.g., asymmetric porphyroclasts, composite shear bands) with top-to-the-northwest shear senses. Considering the high closure temperatures (∼500°C for hornblende and ∼350°C for K-feldspar), both hornblende quartz monzonite and hornblende granite in the Kerkenez granitoid may have cooled rapidly, suggesting that hornblende quartz monzonite may have been emplaced at around 81:2 ± 0:5 Ma and that it is older than hornblende granite, which has a well-defined plateau age (72:6 ± 0:2 Ma). On the basis of intrusive relations and our 40 Ar= 39 Ar age data, we can constrain the upper age limit (∼81 Ma) on the regional metamorphism in the northern part of the CACC. The 40 Ar= 39 Ar dating of hornblendes in two mylonite samples from a ductile shear zone yields plateau ages of 71:6 ± 0:3 and 71:7 ± 0:2 Ma, respectively. K-feldspars in the same samples yield plateau ages of 71:6 ± 0:2 and 81:3 ± 0:2 Ma. Therefore, we adopt 71:6 ± 0:3 and 71:7 ± 0:2 Ma as the cooling ages of hornblende and K-feldspar, respectively, in the ductile shear zone. On the other hand, an age of 81:3 ± 0:2 Ma for deformed K-feldspar appears to reflect not the age of ductile deformation but rather the age of undeformed hornblende quartz monzonite. These age data suggest that the shear zones formed soon after the emplacement and cooling of hornblende granite. The cooling event of the shear zones is interpreted to be associated with the beginning of extension in the region. Furthermore, these data imply that metamorphism, emplacement, and cooling of the intrusives and ductile shearing of the intrusions were coeval in the region and occurred in the Late Cretaceous.

Crustal homogenization revealed by U–Pb zircon ages and Hf isotope evidence from the Late Cretaceous granitoids of the Agaçören intrusive suite (Central Anatolia/Turkey)

“…Age data from hornblende-bearing granitoids in other parts of the CACC (see Fig. 1) were presented for the Behrekdag batholith as a 79.5 ± 1.7 Ma K-Ar hornblende age (Ilbeyli et al 2004), for the Kerkenez granitoid as a 81.2 ± 0.5 Ma 40 Ar/ 39 Ar hornblende age (Isik et al 2008), and for the Akçakoyunlu granitoid as 79.3-77.6 Ma K-Ar hornblende ages (Boztug et al 2007c), all to be interpreted as cooling ages. Therefore, it is plausible to suggest that the Late Cretaceous magma generation within the CACC Table 3 in Whitney et al 2003), on the other hand, show a mean 206 Pb/ 238 U age of 83.9 ± 2.4 Ma (at 95% CI; excluding the point with 66.5 ± 0.5 Ma), which is consistent with our data.…”

“…1), which yielded a 73.5 ± 1.0 Ma Rb-Sr whole-rock age (Ataman 1972) and a 70.0 ± 1.0 Ma 40 Ar/ 39 Ar amphibole and biotite age by Kadioglu et al (2006); the Baranadag granitoid (Fig. 1) with a 74.0 ± 2.8 Ma U-Pb titanite age by Köksal et al (2004), a 76.4 ± 1.3 Ma K-Ar hornblende age by Ilbeyli et al (2004), and a 74.1 ± 4.9 Ma 207 Pb/ 206 Pb zircon evaporation age by Boztug et al (2007b); the Kerkenez granitoid with a 72.6 ± 0.2 Ma 40 Ar/ 39 Ar hornblende age by Isik et al (2008), and the Adatepe and the Yassiagil granitoids ( Fig. 1) with 79.8-68.0 Ma K-Ar cooling ages by Boztug et al (2007c).…”

The discovery of a low-angle normal fault in the Taurus Mountains: the İvriz detachment and implications concerning the Cenozoic geology of southern Turkey*

“…The cooling period of the Baranadağ quartzmonzonite is 69-72 Ma and apatite fission track ages of 57-60 Ma have been provided (Boztuğ and Jonckheere, 2007;Boztuğ et al, 2009). 40 Ar/ 39 Ar dating of 72.11 ± 1.46 Ma andesite has been reported in the basin fill of the AyhanBüyükkışla basin, which is related to the exhumation of the CACC's Hırkadağ massif (Advokaat et al, 2014;Lefebvre et al, 2015) (location 5 in Figure 1). Recent studies using paleomagnetic reconstructions suggest that the CACC experienced nearly E-W extensional exhumation above an eastward-dipping subduction during late Cretaceous times (Lefebvre, 2011;Lefebvre et al, 2013Lefebvre et al, , 2015Nairn et al, 2013;van Hinsbergen et al, 2016).…”

“…To the north of the Niğde massif, on the northern side of the CACC, the Kerkenez granitoid of the Yozgat batholith has extensional mylonitic shear zones dated 71.6 ± 0.3 Ma and 71.7 ± 0.2 Ma (Ar/Ar, hornblende), showing a top-to-the-NW shear sense (Işık et al, 2008) (location 2 in Figure 1). [1] Top-to-the-NE sense of shear on a detachment on the Niğde massif (Gautier et al, 2002(Gautier et al, , 2008, [2] top-to-the-NW sense of ductile shear from the Yozgat batholith (Işık et al, 2008), [3] top-to-the-SW sense of ductile shear on the Emizözü shear zone (Işık, 2009), [4] Kaman detachment and top-to-the-NW sense of shear (Lefebvre, 2011;Lefebvre et al, 2011), [5] top-to-the-NE sense of shear of the Hırkadağ detachment (Lefebvre, 2011;Advokaat et al, 2014;Lefebvre et al, 2015), [6] top-to-the-N-NE sense of shear on the İvriz detachment (this paper), [7] location of the hypothetical Aydos main breakaway fault (this paper), [8] NE stretching lineations in the SE of Altınekin (Eren, 2000). NM: Niğde massif, AD: Akdağ massif, KM: Kırşehir massif, YB: Yozgat batholith, AG: Ağaçören granitoid, BA: Baranadağ quartz-monzonite, HM: Hırkadağ massif.…”

Paleomagnetic constraints on the timing and distribution of Cenozoic rotations in Central and Eastern Anatolia

“…This was followed by the latest Cretaceous to Eocene accretion of the Tauride fold-andthrust belt that largely consists of carbonate nappes (Demirtasli et al, 1984;Gutnic et al, 1979;Monod, 1977;Özgül, 1984). The Taurides mostly escaped metamorphism and accreted, while the Kırşehir Block and Afyon Zone were exhumed by extension in the Late Cretaceous to early Eocene time (Gautier et al, 2002(Gautier et al, , 2008Isik, 2009;Isik et al, 2008;Lefebvre et al, 2011Lefebvre et al, , 2015.…”