Phengite ⁴⁰Ar/³⁹Ar age of garnet-bearing pelitic schist pebble obtained from conglomerate in the Nedamo Terrane, Northeast Japan

Abstract

Conglomerate containing garnet-bearing pelitic schist pebble was obtained from the Nedamo Terrane, an Early Carboniferous accretionary prism of northeast Japan. Phengite in the schist pebble is dated to be 347-317 Ma based on the ⁴⁰Ar/³⁹Ar spot-heating method. The schist in the pebble can be correlated with the Renge Metamorphic Rocks of southwest Japan, and the Yamagami Metamorphic Rocks in the Kitakami-Abukuma massif. The age data suggest that high-P/T metamorphism which produced the garnet-bearing pelitic schist may have been in progress within Early Carboniferous (359-318 Ma) and the schists were rapidly exhumed to provide pebbles, or that the conglomerate containing the pebbles may belong to a post-Early Carboniferous accretionary unit which is not identified yet. In case that the age has been all rejuvenated by secondary effects, the schist pebble may have been derived from pre-Late Devonian high-P/T metamorphics as the Matsugadaira Metamorphic Rocks, probably 380 Ma.