Behavior of rare elements in Late Cretaceous pegmatites from the Setouchi Province, Inner Zone of Southwest Japan

Abstract

This paper reports the paragenesis and reaction textures of rare earth minerals such as monazite, zircon, allanite, thorite, xenotime, and apatite in pegmatites within the Setouchi Province, Inner Zone of Southwest Japan. One of the zircon grains described shows parallel growth with thorite, suggesting their limited solid solution. Xenotime grains are sometimes disseminated by thorite and overgrown by zircon. Monazite in most pegmatites is corroded and replaced with an aggregate of allanite, apatite, thorite, and occasional xenotime and zircon. Coremantle domains in zircon grains in one of the pegmatites give high Th/U ratios (0.8 -0.4) with lower U and Th contents, and rim domains give low Th/U ratios (0.5 -0.2 or less) with higher U and Th contents. The decrease of the Th/U ratio toward the rim results from the change of co -precipitating phase from monazite to thorite. The indistinguishable monazite age (81.7 ± 6.4 Ma) and zircon age for the rim domains (82.9 ± 2.1 Ma) suggest that the paragenetic change occurred during a single evolution of pegmatite. The composition of the metasomatic fluid changed from Ca -poor to Ca -rich, causing monazite corrosion and the consequent precipitation of thorite and allanite. This, in turn, regulated the Th and U contents and the Th/U ratio in associated zircon, as well as the fluid composition.

Submarine slope–fan sedimentation in an ancient forearc related to contemporaneous magmatism: The Upper Cretaceous Izumi Group, southwestern Japan

“…Abbreviations for stratigraphy: Coniac., Coniacian; Sant., Santonian. References for radiometric age data: [1] Aoki et al (), [2] Imaoka, Nakajima, and Itaya (), [3] Takagi et al (), [4] Takagi et al (), [5] Takagi, Soda, and Yoshimura (), [6] Kishi et al (), [7] Imaoka et al (), [8] Imaoka and Ikawa (), [9] Imaoka et al (), [10] Yuge, Imaoka, and Iizumi (), [11] Ikawa et al (), [12] Suzuki, Adachi, and Nureki (), [13] Suzuki and Adachi (), [14] Suzuki, Adachi, and Kajizuka (), [15] Umeda et al (), [16] Tazaki et al (), [17] Kagami et al (), [18] T. Sato et al (), [19] Aoki et al (), [20] Nakahata, Isozaki, and Tsutsumi (), [21] Sonehara and Yagi (), [22] Matsumoto, Sawada, and Kagami (), [23] K. Sato et al (), [24] A. Noda et al (), [25] A. Noda, Danhara, Iwano, and Hirata (2017a), [26] Nakajima et al (), [27] Iida et al (), [28] D. Sato, Matsuura, and Yamamoto (2016a), [29] D. Sato, Yamamoto, and Takagi (2016b), [30] D. Sato (), [31] Terakado and Nohda (), [32] D. Sato (), [33] H. Ito et al (), [34] Miyata (), [35] Shibata and Uchiumi (), [36] Seike et al (), [37] Morioka, Tainosho, and Kagami (), [38] Watanabe et al (), [39] Kawakami and Suzuki (). Detailed version of this figure is presented in supporting information…”

Geochemical variation of the Late Cretaceous-Paleogene granitoids across the Ehime-Hiroshima-Shimane transect, Japan

“…Oxygen isotopic ratios, δ 18 O values of quartz of the granitoids, decrease north from 13 ‰ at the Ryoke metamorphic belt, to 9 ‰ around the Kabe area (Honma and Sakai, 1976). Small pegmatites often occur in the Seto Inland Sea region, which can contain rare metal minerals (Sato et al, 2014), and productive amounts of Be-bearing minerals, such as phenacite hosted in the skarn body and danalite in the intruded alkaline granites at Mihara mine (Aoki and Hida, 1974).…”

“…The southernmost part of the studied granitoids are those in the Takanawa Peninsula, which are considered to belong mostly to the Ryoke metamorphic and plutonic rocks (Hirokawa, 1965;Okamura, 1967), except for biotite granites around the northern tip where occurring together metasomatic syenites (Murakami, 1959) and rare metal-bearing pegmatites (Minagawa et al, 1978(Minagawa et al, , 2001Sato et al, 2014), both of which are characteristic of the Sanyo belt granitoids (e.g., Aoki and Hida, 1974). The Ryoke granitoids were once classified as "older" and "younger" (Miyahisa and Hiraoka, 1970) without any radiometric age data.…”