Early Cretaceous Sr‐rich silicic magmatism by slab melting in the Kitakami Mountains, northeast Japan

Abstract

Early Cretaceous plutonic rocks in the Kitakami Mountains, northeast Japan, related to the westward subduction along the eastern margin of the East Asian continent, intruded the Jurassic accretionary complex and a microcontinental block. These plutonic rocks are classified into types A and B. The former is further subdivided into high‐Sr, low‐Sr, and intermediate series. High‐Sr series rocks occur as the central parts of three zoned plutons in the North Kitakami belt; named the Hashigami, Tanohata, and Miyako plutons. The high‐Sr series rocks show distinct chemical characteristics compared with those of the low‐Sr series. In particular, the high‐Sr series is characterized by SiO2 greater than 61 wt %, higher Na2O and lower K2O concentrations, and slightly higher Al2O3 and lower total FeO and MgO concentrations. In trace element chemistry, the high‐Sr series rocks are richer in Sr and Ga, and poorer in Rb, Pb, and Y. These characteristics resemble those of the high‐Al2O3 trondhjemite, tonalite, and dacite (high‐Al TTD). Mass balance calculation indicates that the high‐Sr series magma can be derived by partial melting of hypothetical oceanic crust (mid‐oceanic ridge basalt (MORB), plus 5% altered MORB, plus 5% sediments) leaving garnet, clinopyroxene, quartz, rutile, and apatite as restite. On the basis of the phase relations for tholeiitic basalt and the estimated geothermal gradient of the subducted slab, slab melting is considered to have occurred at a depth of 2.0 to 2.2 GPa (about 70 to 80 km), and the age of subducted oceanic crust may be younger than 10 to 25 Ma. The coexistence of the magma derived from the subducted slab with that derived from the mantle wedge is a particularly important characteristic of the Early Cretaceous magmatism in the Kitakami Mountains. This suggests that the mantle beneath the Cretaceous forearc was warmer than for normal forearcs. Subduction of the Farallon‐Izanagi ridge may have caused the warm mantle.