Ar-39/Ar-40 ages from the Yozgat batholith: Preliminary data on the timing of Late Cretaceous extension in the central Anatolian crystalline complex, Turkey

摘要

Isotopic dating of sheared and unsheared rocks can be important in understanding deformational processes in orogenic belts. This study examines Ar-40/Ar-39 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 (similar to 500 degrees C for hornblende and similar to 350 degrees 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 Ar-40/Ar-39 age data, we can constrain the upper age limit (similar to 81 Ma) on the regional metamorphism in the northern part of the CACC. The Ar-40/Ar-39 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.