REE abundances of the effusive and intrusive rocks from the Shimokawa mining area, Hokkaido, Japan

The Shimokawa mining area is an example of collision orogenesis formed by accretion of Shimokawa diabase complex and Nisama igneous complex. The former, which consists of pillow lava, diabase sheet and diabase dike, is believed to be formed with turbidite strata in a fore-arc basin of Cretaceous age. Whereas, the latter is a gabbroic intrusion mainly consisting of olivine gabbro, norite and diorite. Major chemical element compositions and REE abundances of main rock types of above two complexes are discussed. It is inferred that Shimokawa diabases are of tholeiitic rock series. The differentiation index (D. I.) shows that pillow lava can not be distinguished from the diabase sheet whereas diabase dike has significantly high value. REE abundances of above three rock types decrease with the increase of D. I. value, especially towards lighter element. Three rock types of the Nisama igneous complex are wholly of calc-alkaline rock series. Both REE abundances and D. I. values increase in the order norite-olivine gabbro-diorite. Chondrite-normalized REE patterns and (La)EF/(Sm)EF in terms of ∑ FeO/MgO led to the conclusion that the Shimokawa diabases belong to solid type and the Nisama igneous rocks to liquid type, respectively. The Shimokawa diabase complex, which has been believed to belong to MORB by many investigators, might be formed in an island arc environment.