Sensitive high‐resolution ion microprobe U‐Pb ages of the Latest Oligocene‐Early Miocene rift‐related Hidaka high‐temperature metamorphism in Hokkaido, northern Japan

Abstract

Abstract In order to define the timing of granulite facies metamorphism, sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb analyses were performed on zircons of three pelitic granulites from the lower metamorphic sequence of the Hidaka Metamorphic Belt, southern central Hokkaido, Japan. Both rounded and prismatic zircons were found in the granulite samples. The rounded zircons had thin (10–20 µm) concentric overgrowth rims on detrital cores, while the prismatic zircons did not have detrital cores. Both the overgrowth rims on the rounded zircons and the entire prismatic zircons were formed under granulite facies metamorphism and consistently yield Latest Oligocene–Early Miocene ages (23.7 ± 0.4 Ma to 17.2 ± 0.5 Ma; 206Pb/ 238U ages (n = 31) with low Th/U ratios, mostly <0.1). The internal structure of zircons and their SHRIMP U‐Pb ages provide strong evidence in support of the granulite facies event occurring during the Latest Oligocene‐Early Miocene. The detrital cores of rounded zircons show a huge variety of ages; Mesoarchean to Paleoproterozoic, Paleozoic to Mesozoic and Paleogene. The interior and marginal portions of the Eurasian continent including cratonic areas are suggested for their source provenances. These wide variations in age suggest that the protolith of the granulites of the lower metamorphic sequence were deposited near the trench of the Eurasian continental margin during Paleogene. The protolith of the lower metamorphic sequence of the Hidaka metamorphic belt was thrust under the upper metamorphic sequence, which had already been metamorphosed in early Paleogene. The Latest Oligocene‐Early Miocene Hidaka high‐temperature metamorphic event is presumed to have been caused by asthenospheric upwelling during back‐arc rifting of the Kuril and Japan basins.