Fission track and U–Pb zircon ages of psammitic rocks from the Harushinai unit, Kamuikotan metamorphic rocks, central Hokkaido, Japan: constraints on metamorphic histories

Abstract

Accurate pressure-temperature-time (P-T-t) paths of rocks from sedimentation through maximum burial to exhumation are needed to determine the processes and mechanisms that form high-pressure and low-temperature type metamorphic rocks. Here, we present a new method combining laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb with fission track (FT) dates for detrital zircons from two psammitic rock samples collected from the Harushinai unit of the Kamuikotan metamorphic rocks. The concordant zircon U-Pb ages for these samples vary markedly, from 1980 to 95 Ma, with the youngest age clusters in both samples yielding Albian-Cenomanian weighted mean ages of 100.8 ± 1.1 and 99.3 ± 1.0 Ma (2σ uncertainties). The zircon U-Pb ages were not reset by high-P/T type metamorphism, because there is no indication of overgrowth within the zircons with igneous oscillatory zoning. Therefore, these weighted mean ages are indicative of the maximum age of deposition of protolithic material. By comparison, the zircon FT data yield a pooled age of ca. 90 Ma, which is almost the same as the weighted mean age of the youngest U-Pb age cluster. This indicates that the zircon FT ages were reset at ca. 90 Ma while still at their source, but have not been reset since. This conclusion is supported by recorded temperature conditions of less than about 300°C (the closure temperature of zircon FTs), as estimated from microstructures in the deformed detrital quartz grains in psammitic rocks, and no shortening of fission track lengths in the zircon. Combining these new data with previously reported white mica K-Ar ages indicates that the Harushinai unit was deposited after ca. 100 Ma, and underwent burial to its maximum depth before being subjected to a localized thermal overprint during exhumation at ca. 58 Ma.

The basement geology of Japan from A to Z

“…Metabasites of the Iwashimizu Complex show oceanic island geochemical characteristics (G. Kimura, Sakakibara, & Okamura, 1994;Nakano & Komatsu, 1979;Sakakibara et al, 1999). Similarities in sedimentary ages and basalt geochemistry suggest links between the Iwashimizu Complex and the accretionary units in the Idonnappu terrane (Iwasaki et al, 1995;A. Okamoto et al, 2015;Watanabe et al, 1994).…”

“…Metabasites of the Iwashimizu Complex show oceanic island geochemical characteristics (G. Kimura, Sakakibara, & Okamura, ; Nakano & Komatsu, ; Sakakibara et al, ). Similarities in sedimentary ages and basalt geochemistry suggest links between the Iwashimizu Complex and the accretionary units in the Idonnappu terrane (Iwasaki et al, ; A. Okamoto et al, ; Watanabe et al, ). The third group is found in the Pankehoronai Complex and is characterized by strongly foliated and folded pelitic schist with lesser amounts of similarly deformed siliceous, psammitic, and mafic schists. This group is characterized by pumpellyite‐actinolite facies metamorphism.…”

Three-dimensional S-wave attenuation structure in and around source area of the 2018 Hokkaido Eastern Iburi Earthquake, Japan

“…The low-Qs zone depth-extending in parallel to the high-Vp values (the Region A) is possibly consistent with the development of high-attenuation properties at the frontal areas of the collision zones. Along the HMR, metamorphic materials, such as the serpentine, are often exposed (e.g., Okamoto et al 2015). Large amplitude decays which would be due to the serpentine were discussed from seismic observations (e.g., Nakamura et al 2006;Nakajima et al 2013).…”

Zircon fission‐track and U–Pb double dating using femtosecond laser ablation–inductively coupled plasma–mass spectrometry: A technical note

“…U–Th–Pb dating using LA–ICP–MS analyses of zircon or other U‐bearing minerals has been well developed (Chew, Sylvester, & Tubrett, 2011; Hirata & Nesbit, 1995; Košler, Tubrett, & Sylvester, 2001; Yokoyama et al, 2018). Recently, the zircon fission‐track (FT) dating method has been extended by combining FT analyses with LA–ICP–MS analyses to simultaneously determine both FT and U–Pb ages (Hasebe, Tamura, & Arai, 2013; Košler & Sylvester, 2003; Okamoto et al, 2015; Shen et al, 2012). This new method is not only useful for dating zircon, but also for analyzing the thermal history and provenance of samples by using the different closure temperatures of each chronometer (Carter & Bristow, 2000; Carter & Moss, 1999).…”