Fission track ages on apatite from Miocene igneous rocks in the Kii Peninsula, Japan

Abstract

This paper reports fission track FT ages on apatite from Miocene igneous rocks of the Kii Peninsula. A total of twelve ages was determined, of which nine data passed a statistical test. Ages of around Ma were obtained for the Muro Pyroclastic Flow Deposit, the Sekibutsu Tuff, and the granite porphyry of the Kumano Acidic Rocks, and were indistinguishable from the zircon FT ages we reported previously Iwano et al., , Jour. Geol. Soc. Japan, ,-, indicating instantaneous cooling. On the other hand, two plutonic bodies of the Omine Granitic Rocks yielded apatite FT ages of about Ma that were significantly younger than the zircon FT ages approximately Ma , suggesting relatively slow cooling.

Instantaneous paleomagnetic record from the Miocene Kozagawa Dike of the Kumano Acidic Rocks, Kii Peninsula, Southwest Japan: cautionary note on tectonic interpretation

“…The clustering of 10 site-mean directions for the Kozagawa granite porphyry dike, as represented by a small a 95 and large k, suggests that the directions are a geologically instantaneous record of the paleomagnetic field. This is consistent with the following geological observations: (i) the Kozagawa granite porphyry forms a single, continuous, narrow intrusion within the composite Kozagawa Dike and is <100 m wide (Aramaki & Hada 1965); and (ii) the Kozagawa granite porphyry has a finegrained groundmass that implies rapid cooling as does FT dating of zircon and apatite (Hasebe et al 1993;Iwano et al 2007Iwano et al , 2009). The most plausible interpretation is that the remanent magnetization in this granite porphyry dike, which is characterized by a southwest declination and steep negative inclination, is a TRM that records a geologically instantaneous feature (or snapshot) of the paleomagnetic field.…”

“…The Kozagawa granite porphyry yields a K-Ar biotite age of 14.3 Ϯ 0.2 Ma (Sumii et al 1998). Similar ages of 14-15 Ma have been determined by fission track (FT) dating of zircon and apatite, which are nearly identical and indicative of rapid cooling (Hasebe et al 1993;Iwano et al 2007Iwano et al , 2009). These radiometric ages are consistent with biostratigraphic age constraints from the host marine sediments of the Kumano Group.…”

“…; Iwano et al . , ). The most plausible interpretation is that the remanent magnetization in this granite porphyry dike, which is characterized by a southwest declination and steep negative inclination, is a TRM that records a geologically instantaneous feature (or snapshot) of the paleomagnetic field.…”

“…; Iwano et al . , ). These radiometric ages are consistent with biostratigraphic age constraints from the host marine sediments of the Kumano Group.…”

Miocene clockwise rotation of Southwest Japan

“…5.2.1 (GMT; Wessel and Smith, 1998), with the ETOPO1 global relief model (Amante and Eakins, 2009 . The diatom biochronology is after Yanagisawa and Akiba (1998), and the planktonic foraminiferal biochronology is after Anthonissen and Torii, 198315 Ma , 2000, Otofuji et al 1985a Torii 1983 Uto et al, 1997;, , 2009.6°, I 66.1°, α 95 3.7°, n 21; Torii, 1983Torii, 52.9° 400 m , 1998, 2012, 2003, 2004, 1978, Torii, 1983Hoshi et al, 2000Hoshi et al, , 2003, 2004Fig. 4 Hoshi et al, 2013Otofuji et al 1985a15.2 Ma 15 14 Ma , Hasebe et al, 1993, , 2009, , 1982, Fig.…”