INFLUENCE OF TRANSPRESSION ON THE DURATION OF METAMORPHISM AND PARTIAL MELTING IN THE MID-CRUST

The influence of partial melting on the evolution of an orogen depends in part on the magnitude/duration of partial melting and the tectonic regime during melting. Different regions of the Central Anatolian Crystalline Complex (CACC) experienced different metamorphic histories with respect to duration of high-T metamorphism, extent of partial melting, and timing/mode of exhumation, although they contain similar rock types, represent similar mid-crustal levels, and were deformed during Late Cretaceous contraction.

U-Pb SHRIMP analyses of thin discontinuous rims on zircon from the peraluminous Uckapili granitoid and a sillimanite schist in the Nigde massif (southern CACC) give essentially concordant ages between 88-78 Ma. The continuous range of ages indicates at least 10 m.y. of crustal melting and high-T metamorphism, or variable Pb loss owing to a post-78 Ma high-T event. In the absence of evidence for the latter, we interpret the spread of ages to reflect protracted high-T metamorphism and partial melting. SHRIMP ages for metamorphic monazite from a sillimanite schist are 84.7 ± 0.7 Ma.

In contrast, the Kirsehir massif (northern CACC) does not contain evidence for extensive crustal melting/migmatization. High-T metamorphism occurred over a shorter duration, with a tight cluster of metamorphic monazite ages at 84.1 ± 0.8 Ma (U-Pb SHRIMP). The remarkable agreement between the monazite ages from the Nigde and Kirsehir massifs suggests that both regions experienced peak metamorphism at ~84-85 Ma (ca. 750-700 °C).

Despite similarities, the two massifs record dramatically different histories with respect to extent of partial melting and timing/mechanism of exhumation. We propose that the prolonged heating/melting of the Nigde massif, and possibly its tectonic exhumation as a core complex, were driven by oblique deformation in a wrench zone, whereas the more limited high-T metamorphism with only minor crustal melting in the Kirsehir massif occurred in a head-on collision zone.