Cooling history and exhumation of the Nepheline Syenites, NW Iran: Constraints from Apatite fission track

Abstract

Thermal history and apatite fission-track ages were determined for the Kaleybar, Razgah and Bozqush alkaline intrusions which display Eocene-Oligocene stratigraphic age. These subduction-related intrusions are located in the Alborz-Azerbaijan magmatic belt which is characterized by a Paleogene magmatic flare-up associated with extensional/transtensional tectonism. The mean of Uranium content and apparent age for apatites of the Bozqush, Kaleybar, and Razgah were obtained 21.8 (±3.8), 9.5 (±5.7), and 24.5 (±11.3) ppm and 29 (±1.8), 36.6 (±3.0), and 40.7 (±1.3) Ma (σ), respectively, which represented the time that the rocks of intrusions were last at temperatures of 60 ˚C to 110 ˚C. The results indicate that the apatite apparent ages are in concord with the stratigraphic ages. The apatite fission track ages and track lengths distribution were combined to construct time-temperature history by inverse modeling, which represented the all samples resided in the partial annealing zone (PAZ) for a significant period of time. The apatite fission track analysis indicated relatively complex cooling history for the host rocks because of the magmatic activity, as it was occurred during Cenozoic in the Alborz-Azerbaijan magmatic belt. The time-temperature curves of the studied intrusions begun with a relatively rapid initial cooling and followed by long residence at the PAZ temperature (heating stage). The time-temperature paths indicate that the start of rapid recent cooling to the surface temperatures was occurred at 5 Myrs.