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.

三河地域における片麻状花崗岩類の貫入と領家広域変成作用の高温達成時期の関係を調べた. いわゆる「古期領家花崗岩類」3岩体から7試料を採取し. 全岩化学組成とLA-ICP-MSによるU–Pbジルコン年代を測定した. 北部の片麻状花崗岩類は99 ±1 Ma(2試料)と95 ±1 Ma (1試料)を示し. 神原トーナル岩と清崎花崗岩に対比できる. 一方. 南部の片麻状花崗岩類は81 ±1 Ma(2試料)と78 ±1−77 ±1 Ma (2試料)を示すことから. これらはほぼ同時期に活動した同源のマグマと推定される. 三河地域の片麻状花崗岩類からは. 本研究も含め. これまで高温変成作用の年代(約87 Ma)に合致するU–Pbジルコン年代は得られていない. 南部の片麻状花崗岩類は. 最高変成度地域に産するにもかかわらず. 高度変成岩類に比べて有意に若い年代(81−75 Ma)を示す. このことは南部の片麻状花崗岩類が領家広域変成作用の熱源ではないことを示唆する.