Age gap between the intrusion of gneissose granitoids and regional high‐temperature metamorphism in the Ryoke belt (Mikawa area), central Japan

Abstract

The relationships between the intrusion of gneissose granitoids and the attainment of regional high‐T conditions recorded in metamorphic rocks from the Ryoke belt of the Mikawa area, central Japan, are explored. Seven gneissose granitoid samples (tonalite, granodiorite, granite) were collected from three distinct plutonic bodies that are mapped as the so‐called “Older Ryoke granitoids.” Based on bulk‐rock compositions and U–Pb zircon ages obtained by laser ablation inductively coupled plasma mass spectrometry, the analyzed granitoids can be separated into two groups. Gneissose granitoids from the northern part of the area give weighted mean 206Pb/238U ages of 99 ±1 Ma (two samples) and 95 ±1 Ma (one sample), whereas those from the southern part yield 81 ±1 Ma (two samples) and 78–77 ±1 Ma (two samples). Regional comparisons allow correlation of the northern granitoids (99–95 Ma) with the Kiyosaki granodiorite, and mostly with the Kamihara tonalite found to the east. The southern granitoids are tentatively renamed as “78–75 Ma (Hbl)−Bt granite” and “81–75 Ma Hbl−Bt tonalite” (Hbl, hornblende; Bt, biotite). and seem to be broadly coeval members of the same magmatic suite. With respect to available age data, no gneissose granitoid from the Mikawa area shows a U–Pb zircon age which matches that of high‐T metamorphism (ca 87 Ma). The southern gneissose granitoids (81–75 Ma), although they occur in the highest‐grade metamorphic zone, do not seem to represent the heat source which produced the metamorphic field gradient with a low dP/dT slope.

Multistage zircon growth recording polyphase metamorphic evolution caused by pulsed granitoid intrusions into a low‐P/T type metamorphic belt: P–T–D–t evolution of migmatites in the Ryoke belt, southwest Japan

“…The Ryoke belt was formed as a result of Cretaceous plutono‐metamorphic activity at the continental margin of East Asia (Brown, 1998; Kawakami et al, 2013, 2019; Kinoshita, 1995; Nakajima, 1994; Skrzypek et al, 2016, 2018; Suzuki & Adachi, 1998; Takatsuka et al, 2018a, 2018b) and is composed of abundant granitoids and low‐ P / T type metamorphic rocks equilibrated at up to granulite facies conditions (Brown, 1998; Ikeda, 1998a, 1998b, 2002; Kawakami, 2001; Miyashiro, 1961; Miyazaki, 2010; Okudaira et al, 1993). Partial melting of pelitic metamorphic rocks is common in the highest‐grade metamorphic zones (Brown, 1998; Kawakami, 2001, 2004; Kawakami & Ikeda, 2003; Kawakami & Kobayashi, 2006; Miyazaki et al, 2008).…”

“…TTL, tanakura tectonic line; ISTL, Itoigawa-Shizuoka tectonic line, MTL, median tectonic line. (b) Geological map of the Mikawa area showing sample localities of this study (white squares with sample numbers), published U-Pb zircon ages of granitoids and pegmatite (white circles with black numbers;Takatsuka et al, 2018aTakatsuka et al, , 2018b, and metamorphic rocks (white circles with blue numbers;Takatsuka et al, 2018b). Because pegmatite GY47A(Takatsuka et al, 2018b) is sampled from almost the same locality with GY101B, only a white square for GY101B is shown in the map.…”

“…Because pegmatite GY47A(Takatsuka et al, 2018b) is sampled from almost the same locality with GY101B, only a white square for GY101B is shown in the map. Geological map is compiled and modified afterMakimoto et al (2004),Miyazaki et al (2008),Nakashima et al (2008),Miyazaki (2010),Yamasaki and Ozaki (2012),Endo and Yamasaki (2013), the Geological Survey of Japan (2015),Miyake et al (2014Miyake et al ( , 2016, andTakatsuka et al (2018a). The geological map to the south of the MTL is not shown for simplicity.…”

GEOFCM: a new method for statistical classification of geochemical data using spatial contextual information

“…Th-U-total Pb isochron dating of monazite shows that the Older and Younger Ryoke granitoids were emplaced during 100-85 and 85-75 Ma, respectively (Suzuki et al, 1994;Kutsukake, 2002). Recently, Takatsuka et al (2018) measured the U-Pb zircon ages of granitoids collected from the Kamihara tonalite body and Tenryukyo granite in the southern part of the studied area and showed that the southwestern part of the Tenryukyo granite is younger than others (~81-75 Ma) and has a different origin, suggesting that age data is a powerful tool to classify the plutons in this area, although the dataset used here does not contain the 'younger part' of the Tenryukyo body. There are a total of 11 recognized granitoid bodies in the study area ( Fig.…”

Zircon U–Pb ages of the Ryoke granitoids from the Takanawa Peninsula, northwest Shikoku, southwest Japan

“…Because of the high closure temperature of U-Pb system in zircon, zircon U-Pb dating is considered to be an effective method for determining magmatic ages of granitoids and has been successfully applied to various granitoid bodies throughout the Ryoke Belt (e.g., Herzig et al, 1998;Watanabe et al, 2000;Nakajima et al, 2004;Iida et al, 2015;Skrzypek et al, 2016;Takatsuka et al, Takanawa Peninsula (modified after Ochi , 1982). Numbers in italics refer to zircon U-Pb ages.…”