Publication

In situ eruptions and accreted blocks of Mesozoic basalts exhumed in present-day forearc settings, around the Pacific Rim record paleovolcanic events in intraplate oceanic settings since the closure of the Panthalassa Ocean. Basaltic and andesitic volcanic and intrusive rocks occur in the present-day forearc area of the Late Cretaceous Nemuro Belt in northeastern Japan. Most basalts in this area are K-rich (i.e. shoshonites), similar to rocks erupted in oceanic back arc or petit-spot settings. In this study, we investigated the occurrence, geochemistry, and age of these shoshonitic rocks (ca. 70 Ma), along with associated adakitic andesites (ca. 62 Ma). The petrogenesis of these igneous rocks and petrography of the associated Nemuro sandstones were used to constrain the nature of the Nemuro paleo-arc during the Late Cretaceous to early Paleogene. The shoshonitic, adakitic, and basaltic rocks in the Nemuro sandstones formed in an intraoceanic juvenile arc setting. Previous paleomagnetic studies have suggested that the Nemuro and Lesser Kuril paleo-arcs were approximately ENE–WSW-trending, located in the central paleo-Pacific, and extensions of the Olyutorsky (eastern Kamchatka) and Terpenia (eastern Sakhalin) paleo-arcs near the mid-ocean ridges of the Kula/Izanagi and Pacific plates in the central Pacific. Our findings are consistent with these previous results, although the shoshonites in the lower part of the Nemuro Group indicate that the Nemuro paleo-arc drifted rapidly northward or northwestward after ca. 65 Ma.