Interpretation of tectonic versus magmatic processes for resetting apatite fission track ages in the Niğde Massif, Turkey

Abstract

Apatite fission track (AFT) ages may record the timing of exhumation-related cooling of formerly deeper rocks or the timing or simple cooling of rocks already at or near the Earth's surface. The interpretation of AFT ages is therefore challenging in regions that experienced coeval or sequential bedrock exhumation and magmatism. The Niğde Massif, central Turkey, is an example of a metamorphic complex for which the tectonic significance of the AFT ages has been questioned owing to the location of the massif in a volcanic province. The metamorphic rocks of the Niğde Massif have Miocene AFT ages of 17–9 Ma, ∼ 30–50 million years younger than the timing of initial unroofing of the rocks to the Earth's surface at ∼ 60–45 Ma. According to one model, fission tracks in apatite may have been completely reset when the massif was partly to entirely reburied in the Tertiary. However, the massif is located in a region of major Neogene volcanism during which ignimbrite was deposited on exhumed metamorphic basement, and there is disagreement as to whether the Miocene AFT ages represent cooling related to final exhumation via denudation or cooling following heating by volcanism or other magmatic activity. To evaluate different cooling scenarios, we examine AFT age and track length characteristics in the context of model temperature–time paths. Integrated thermal modeling, modeling of apatite fission track age and track length data, and geological observations suggest that the AFT ages represent final exhumation and cooling in the Miocene and few to no thermal effects from volcanic deposits or other Neogene magmatic activity.