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 whole–rock 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 multi–element 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–MORB–normalized multi–element patterns with Ti/V = 26–53 and Zr/Nb = 21–117. Type 3 greenstone shows multi–element 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.