LA-ICP-MS zircon U-Pb age and Hf isotope data from the granitic rocks in the Iwakuni area, Southwest Japan: re-evaluation of emplacement order and the source magma
Journal
Geosciences Journal
Journal Volume
23
Journal Issue
6
Pages
917-931
Date Issued
2019
Author(s)
Mateen T.
Okamoto K.
Wang K.-L.
Lee H.-Y.
Abe S.
Mita Y.
Rehman H.U.
Terabayashi M.
Yamamoto, Hiroshi
Abstract
U-Pb-Hf isotopes of zircons from the granitic intrusive rocks in the Iwakuni area of Southwest Japan are determined by LA-ICP-MS analysis. The Gamano Granodiorite belonging to the Older Ryoke Granites of the Ryoke Belt yielded U-Pb mean age of 92.3 ¡Ó 2.6 Ma (MSWD = 1.8). The Namera Granite belonging to the Younger Ryoke Granites of the Ryoke Belt yielded U-Pb mean age of 106.3 ¡Ó 1.6 Ma (MSWD = 2.3). The Shimokuhara Granite belonging to the Hiroshima Granites of the San-yo Belt yielded U-Pb mean age of 103.8 ¡Ó 1.5 Ma (MSWD = 0.32). A granite porphyry sample taken from about 15 m thick dike discordantly intruded into pelitic schist of the Ryoke metamorphic rocks yielded U-Pb mean age of 92.5 ¡Ó 1.6 Ma (MSWD = 0.63). Hf isotope data, obtained from the same dated zircon grains, show initial 176Hf/177Hf isotopic ratios referred to as ¡§£`Hf(t)¡¨ generally ranging between¡V4.8 and +1.1 except an anomalously low value (¡V15.0) from an inherited core. The £`Hf(t) values, calculated with average crustal 176Lu/177Hf ratio of 0.015, correspond to Hf isotope crustal model ages between 1474 to 1087 Ma (Mesoproterozoic). The Gamano Graodiorite has long been regarded to be a syn-tectonic intrusion, but the zircon ages and £`Hf(t) values are similar to those of the post-tectonic granite porphyry dike. The results of this study together with available U-Pb zircon ages of granitic rocks in the Iwakuni area reported elsewhere reveal that the order of emplacement indicated by the U-Pb zircon ages are much discrepant with the traditional classification, the Older Ryoke, the Younger Ryoke. It is better to refrain from the use of ¡§Older¡¨ and ¡§Younger¡¨ for the collective names of the granitic intrusive bodies. ? 2019, The Association of Korean Geoscience Societies and Springer.
Subjects
granitic rocks
Hf isotope
Ryoke Belt
San-yo Belt
U-Pb age
SDGs
Type
journal article