Unusually slow cooling of metamorphic rocks in an orogenic belt: A case from the Bolu Massif (NW Turkey)

In this study, we deal with the metamorphic evolution of the amphibolite-facies tectonometamorphic unit of the Bolu Massif, which forms part of the late Neoproterozoic basement of the Istanbul Zone within the Tethyan realm (NW Turkey). The amphibolite-facies tectonometamorphic unit occurs tectonically beneath a tectonometamorphic unit consisting of greenschist-facies metavolcanic rocks intruded by a late Neoproterozoic granitoid. It is ca. 50 km long and 7-8 km wide, and consists of migmatitic amphibolite (~79% of outcrop area), dikes/sill and stocks of trondhjemite (~20%) and minor serpentinite and metapyroxenite (~1%). Comparable amphibolite-facies rocks crop out in several isolated exposures in a narrow belt (~300 km long and up to 20 km wide) parallel to the Intra-Pontide suture which separates the Istanbul Zone from the Sakarya Zone. Amphibolite contains mineral assemblages involving hornblende, plagioclase, ±biotite, ±epidote and ±quartz. Rutile, titanite, apatite and zircon are common accessories. Zr-in-titanite thermometry after Hayden et al. (2008) yields temperatures of 727 ± 17 °C at assumed pressures of 0.7 GPa, while the Zr-in-rutile thermometer after Tomkins et al. (2007) gave significantly lower temperatures (643 ± 7 °C at 0.7 GPa). Because of the high-variance mineral assemblage, metamorphic pressures are difficult to constrain. The presence of igneous epidote in trondhjemite suggests pressures ≥ 0.7 GPa. The equilibration conditions (640-730 °C, 0.7-0.8 GPa) are close to the wet solidus of basalts, suggesting that the migmatization probably occurred by water-present melting. U-Pb dating on zircons from amphibolite and trondhjemitic veins yielded consistent age values of 260-255 Ma. On the other hand, titanite and rutile from the amphibolite yielded lower age values of 231 ± 6 and 181 ± 6 Ma (2σ), respectively. K-Ar hornblende and Rb-Sr biotite whole data from the literature are 222-205 ± 8 and 161-155 ± 2 Ma (2σ), respectively. The coeval nature of the U-Pb zircon dates from amphibolites and trondhjemitic leucosomes suggest that the peak of metamorphism occurred at 260-255 Ma. Given the commonly accepted closure temperatures of minerals for respective isotopic systems, these highly scattered dates suggest that the cooling from the metamorphic peak to 300 °C occurred over an extended period of time (ca. 100 Ma), corresponding to a cooling rate of 4-5 °C/Ma. The absence of marine sedimentary rocks of Late Triassic and latest Early Cretaceous age suggests that the Istanbul Zone was above sea level between Late Triassic and latest Early Cretaceous time (ca. 237 to 100 Ma), and there was no evidence for significant crustal-scale extension. Exhumation at the earth’s surface occurred during the Late Cretaceous, as deduced from unconformably overlying Campanian limestones and the development of Campanian to Ypressian marine sedimentary successions. We suggest that the late Permian metamorphism occurred in a geodynamic setting associated with magmatism in an extensional setting, and static relaxation of the perturbed geotherm between Late Triassic and Early Cretaceous, and exhumation occurred in an extensional setting during the development of sedimentary basin at Late Cretaceous. This study is supported by two grants (TUBITAK #122R002 and ITUBAP #42913).