Natural long‐term annealing of the zircon fission track system around a granitic pluton

Abstract

We measured 24 ages and 14 length distributions of fission tracks in zircons separated from basement sandstones around a granitic pluton to investigate natural long‐term annealing behaviors of the zircon fission track (ZFT) system. Samples were collected from three Cretaceous coherent units of the Shimanto accretionary complex, Shikoku, Japan, along an eastward traverse at distances of between 1.6 and 10.6 km from the intrusion contact of 15 Ma Takatsukiyama Granite exposing about 5 km in diameter. Sampling was carried out around the expected position of the transition zone of track retention, i.e., the fossil partial annealing zone (FPAZ), based on previous regional ZFT age distributions. Approaching the contact, original ZFT ages of ∼100 Ma show a continuous reduction to ∼15 Ma at ∼3 km distance. Their single‐grain age distributions accordingly change from widespread multimodal patterns reflecting provenance ages to unimodal ones characterizing the secondary heating by the granite intrusion. Mean and distributions of ZFT lengths also show notable variations within the FPAZ, as a result of mixing both preexistent tracks that were shortened by the Miocene heating and unannealed ones having original, long lengths that have been formed and accumulated after the cooling of the pluton. These facts, along with observed track segmentation and annealing anisotropy, suggest that the shortening patterns of tracks in nature (a heating duration of 105–106 years, estimated from thermal modeling) are indistinguishable from those previously demonstrated at laboratory timescales. This basically validates the conventional approach of extrapolating laboratory‐based kinetic parameters to geologic timescales. The observed FPAZ and thermal modeling results, in conjunction with laboratory heating data previously reported and recent constraints from an ultradeep borehole, favor a relatively narrow temperature range for the partial annealing zone, i.e., ∼230° to ∼330°C for a heating duration of ∼106 years.