Article https://doi.org/10.1007/s12303-019-0003-1 pISSN 1226-4806 eISSN 1598-7477 Geosciences Journal GJ 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 Tayyaba Mateen 1 , Kazuaki Okamoto 2 , Sun-Lin Chung 3,4 , Kuo-Lung Wang 3,4 , Hao-Yang Lee 3 , Shuhei Abe 2 , Yuji Mita 2 , Hafiz U. Rehman 1 , Masaru Terabayashi 5 , and Hiroshi Yamamoto 1 * 1 Graduate School of Science and Engineering, Kagoshima University, Kagoshima 890-0065, Japan 2 Department of Earth Sciences, Faculty of Education, Saitama University, Saitama 338-8570, Japan 3 Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan 4 Department of Geosciences, National Taiwan University, Taipei 106, Taiwan 5 Faculty of Engineering and Design, Kagawa University, Takamatsu 761-0396, Japan 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 ± 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 ± 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 ± 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 ± 1.6 Ma (MSWD = 0.63). Hf isotope data, obtained from the same dated zircon grains, show initial 176 Hf/ 177 Hf isotopic ratios referred to as “ Hf (t)” generally ranging between –4.8 and +1.1 except an anomalously low value (–15.0) from an inherited core. The  Hf (t) values, calculated with average crustal 176 Lu/ 177 Hf 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 intru- sion, 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 indi- cated 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. Key words: Ryoke Belt, San-yo Belt, granitic rocks, U-Pb age, Hf isotope Manuscript received July 30, 2018; Manuscript accepted January 11, 2019 1. INTRODUCTION Southwest Japan has been divided into the Inner Zone (continental side) and the Outer Zone (oceanic side) along the Median Tectonic Line. Cretaceous to Paleogene granitoids are extensively distributed in the Inner Zone and these granitoids are situated in a part of Mesozoic to Early Cenozoic active margins of circum-Pacific regions (e.g., Jahn et al., 1990; Jahn, 2010). The Ryoke Belt is the predominant tectonic unit of the Inner Zone. In the Chugoku district, the San-yo Belt is juxtaposed along the northern side of the Ryoke Belt (Fig. 1a). The Ryoke Belt is composed mainly of voluminous granitic intrusives known as the Ryoke granitoids and a variety of metamorphic rocks, a typical example of low-pressure/temperature type (e.g., Okudaira et al., 1993; Yamamoto, 1994; Brown, 1998). The San-yo Belt is dominated by granitic intrusives and felsic volcanic rocks. Ishihara (1971, 1978) distinguished the W-Sn-ore rich San-yo Belt from the ore-barren Ryoke Belt. Calc-alkaline magmatic activity in island arcs or continental margins is regarded to contribute to the growth of continental crust (e.g., Jakes and White, 1971; Taylor, 1977; Green, 1980). Magmatic growth of the crust is most evident on the active margins of circum-Pacific regions (Jahn et al., 1990). Therefore, *Corresponding author: Hiroshi Yamamoto Graduate School of Science and Engineering, Kagoshima University, Korimoto 1-21-24, Kagoshima 890-0065, Japan Tel: +81-992858130, Fax: +81-992594720, E-mail: hyam@sci.kagoshima-u.ac.jp The Association of Korean Geoscience Societies and Springer 2019