Three types of greenstone from the Hidaka belt, Hokkaido, Japan: Insights into geodynamic setting of northeastern margin of the Eurasian plate in the Paleogene

Abstract

The Hidaka belt in Central Hokkaido, Japan, consists of an early Paleogene subduction complex, referred to as the Hidaka Supergroup, dominated by clastic rocks. The southern area of the Hidaka Supergroup is referred to as the Nakanogawa Group, which gradually leads to the high-temperature Hidaka metamorphic belt in the western part. We collected 17 samples of greenstone from the entire Hidaka belt and examined their wholerock major and trace element geochemistry. Including those described in previous reports, three distinct types of greenstone exist in the Hidaka belt. Type 1 greenstone is an ocean island basalt-type greenstone. The multielement and rare earth element (REE) patterns for this type of greenstone show a steep slope up to the left, with Ti/V > 62 and Zr/Nb < 15. Type 2 greenstone is a mid-ocean ridge basalt (MORB)-type greenstone that shows relatively flat chondrite-normalized REE patterns and a gentle slope up to the left on the normal-MORBnormalized multi-element patterns with Ti/V = 26-53 and Zr/Nb = 21-117. Type 3 greenstone shows multielement and REE patterns similar to those of Type 1, but with a clear relative depletion of Nb, Ta, and Ti. In addition, its Ti-V relations are similar to those of Type 2 greenstone. Type 1 and Type 3 greenstones occur only in the Nakanogawa Group. Type 2 greenstone is mostly distributed in the northern the Hidaka Supergroup. Type 1 and Type 3 greenstones were generated by igneous activity on the Izanagi Plate, which was being subducted during the formation of the subduction complex of the Hidaka belt. Type 2 greenstone is interpreted as a product of a spreading axis that was active during the formation of the same subduction complex. Whereas Type 2 greenstone has been regarded as having a typical MORB-like geochemical signature, our results show slightly different, Indian Ocean MORB-type trace element patterns. These geochemical signatures are different than those of the amphibolites in the Hidaka metamorphic belt. The protolith of the amphibolite is not equivalent to the Type 2 greenstones and is probably an accreted fragment of an older oceanic plate. Type 2 greenstone was presumably generated from upper mantle with an Indian mantle-like geochemical signature during the Izanagi-Pacific ridge subduction on the western margins of the Pacific Ocean around 48 Ma.

The emplacement ofin situgreenstones in the northern Hidaka belt: The tectonic relationship between subduction of the Izanagi–Pacific ridge and Hidaka magmatic activity

“…It is noted that minor elements (Nb, P, Zr, Ti, and Y) that were an important basis for identifying N-MORB-like geochemical features in previous studies (Miyashita et al, 1997;Miyashita & Yoshida, 1994) cannot distinguish between enriched MORB and N-MORB. Thus, the data reported by Yamasaki and Nanayama (2020) are essentially consistent with the data previously reported in terms of minor element composition. Yamasaki and Nanayama (2020) mentioned that MORB generation from upper mantle with an Indian mantle composition during the IPR subduction in the western margins of the Pacific Ocean is probably a fundamental cause of the enriched MORB-like multi-trace element composition of this greenstone.…”

“…Thus, the data reported by Yamasaki and Nanayama (2020) are essentially consistent with the data previously reported in terms of minor element composition. Yamasaki and Nanayama (2020) mentioned that MORB generation from upper mantle with an Indian mantle composition during the IPR subduction in the western margins of the Pacific Ocean is probably a fundamental cause of the enriched MORB-like multi-trace element composition of this greenstone.…”

“…Miyashita et al (1997) proposed a multi-ridge collision model for the occurrences of in situ greenstones in central Hokkaido, including the Hidaka belt and Idonnappu zone. However, their model has not been validated since, except by Yamasaki and Nanayama (2020).…”

Origin and evolution of the Paleo‐Kuril arc inferred from detrital zircon U–Pb chronology in eastern Hokkaido, NE Asia

“…This claim provides an important basis for our hypothesis that the subduction of the PKA was initiated during 85–80 Ma (see Section 7.4). In contrast, the adjacent Tachiiwa greenstone body (Figure 3) was reported to be an ocean‐island basalt‐like camptonite resulting from hotspot‐like igneous activity on the Izanagi Plate (Yamasaki & Nanayama, 2020). Nonetheless, the amphibole K–Ar age for this body is ca.…”

Supplemental Material: Subduction of the Izanagi-Pacific Ridge–transform intersection at the northeastern end of the Eurasian plate

“…For panel F, Sample O-1b (in situ greenstone in the HMZ) from Yamasaki and Nanayama (2020) was used for the composition of the greenstone (apart from the imprecise Lu content). The Partition coefficients and modal melting proportions from Sen and Dunn (1994;equation 6 at 1.5 GPa) were used in the calculations, and the restite unmixing model of White and Chappell (1977) were adopted.…”