Mega-porphyritic rhyolite, ca. 2 km long and ca.150 m wide, occurs in the Nakatsugawa Complex of a Jurassic accretionary complex in the southwestern North Kitakami Belt, Kitakami Massif. The rhyolite has been considered as one of the Early Cretaceous dikes occurring ubiquitously in the massif. However, it is proved to be an early Permian body based on ca. 280 Ma obtained by zircon U–Pb dating. Considering that the rhyolite including abundant potassium feldspar is unlikely to a component of an accretionary complex and that many tectonic blocks such as the Ordovician ultramafic and plutonic rocks or the Paleozoic high-P/T schists occur in the Nedamo Belt located south of the North Kitakami Belt, the rhyolite is likewise thought to be a tectonic block emplaced into the accretionary complexes by a post-Jurassic tectonic movement. This mega-porphyritic rhyolite is possible to be a fragment of the late Paleozoic island-arc igneous bodies, which is almost missing in the present Japanese islands.
Advances in inductively coupled plasma mass spectrometry (ICP-MS) have facilitated multielemental analyses of various geological samples. However, time-efficient measurement of high-matrix samples is challenging. In this study, we demonstrate the experimental setup of a new ICP-MS, Agilent 7900, equipped with a collision/reaction cell (CRC), an ultra-high matrix introduction (UHMI) system, and a third-generation integrated sample introduction system (ISIS3). The CRC effectively reduces molecular interference. To obtain a sufficiently low background equivalent concentration and blank/standard ratio, the He gas flow rate was set to 4.5 mL/min. The UHMI system is an aerosol dilution system that minimizes the matrix effect, reducing the interference from doubly charged ions, element oxide ions, plasma-based polyatomic ions, acid-based polyatomic ions, and matrix-based polyatomic ions. The ten-fold dilution mode (HMI-8) was very effective in minimizing the formation rate of oxide ions and matrix-based polyatomic ions. For the quantitative multi-element analysis of geochemical samples, the matrix effect caused by the decrease in ionization efficacy owing to coexisting major elements is a severe problem. However, the matrix effect for trace elements has not been successfully corrected using the internal standard method for natural sample measurements. Therefore, we used a matrix-matched standard solution prepared from JB-1a by adding Li, Be, Ni, Cu, Zn, As, Mo, Ag, Cd, Sn, Sb, Cs, Tl, Pb, and Bi. The ISIS3 reduced the measurement time by approximately 80 min when measuring 42 elements in 50 samples compared with the previous sample introduction system, in which the measurement solution was diluted with water. With the optimized setting parameters, we confirmed that the concentrations of 42 elements in the eight geochemical reference materials were comparable to their referenced values.
Mesozoic strata are distributed within three zones separated by NNW–SSE trending major faults in the Ofunato area, southeastern Kitakami Massif, Northeast Japan. The representative stratigraphic sections from the Triassic to the unconformably overlying Early Cretaceous strata are seen in the western zone. We reviewed the stratigraphic divisions and examined the zircon U–Pb ages for three strata: the Triassic Myojinmae and the Lower Cretaceous Kobosoura formations in the western zone, and the undivided Early Cretaceous strata in the central zone. The Triassic Myojinmae Formation is characterized by terrestrial conglomerate and reddish purple sandstone indicating 219.2 ± 4.1 Ma (Norian) as a weighted mean U–Pb age for the youngest cluster of detrital zircons. Considering the occurrence of Monotis fossils, the formation is regarded as the Norian deposits. Early Cretaceous strata (Ofunato Group) in the western zone consist of terrestrial conglomerate (Hakoneyama Formation), marine deposits (Funagawara and Hijochi formations) and marine to terrestrial volcaniclastics (Kobosoura Formation) in ascending order. A weighted mean U–Pb age of zircons for the welded tuff in the Kobosoura Formation indicates 124.7 ± 0.6 Ma (early Aptian). Early Cretaceous strata of the eastern zone are composed of thick volcanics (Ryori Formation) and marine to terrestrial overlayers (Attari Formation). In the central zone, the undivided Early Cretaceous strata consisting mainly of volcaniclastics are distributed and a weighted mean U–Pb age of the zircons for its felsic tuff indicates 121.9 ± 0.6 Ma (early Aptian). Integrating previous studies on fossil ages, whole sequences of the Early Cretaceous strata in the Ofunato area were formed by a transgression with andesite-dominated volcanism in the Hauterivian to Barremian, and then by a subsequent regression with felsic volcanism in the Barremian to early Aptian. In the early Aptian, terrigenous clastics including ejecta might have partly coevally deposited on the land surfaces while intrusions of felsic-andesitic magma caused the Cretaceous granitoids and dyke rocks in the Kitakami Massif.