日本の花崗岩類におけるジルコニウム含有量の特徴

抄録

Zr contents of granitic rocks of the major intrusive bodies in Japan were evaluated on the Harker diagrams. The contents show three patterns as (i) increasing type, (ii) decreasing type, and (iii) decreasing after increasing type. The first type is best shown in the M-type magnetite-series of Tanzawa tonalites, which was generated in Zr-poor mafic igneous source rocks at depth, then never saturated with Zr in magmas; thus the contents increase with the crystallization. The second type is common in the ilmenite series originated in crust materials, which was saturated with Zr at the beginning of crystallization; thus the content decreases as the silica increases. In the third-type magma, Zr increases with the increasing silica content, then around 66% SiO₂ starting to decease after saturation of Zr. This is most commonly seen in the magnetite-series granitoids. Thus, variation of the Zr contents in granitoids is essentially controlled by the original Zr contents of the granitic magmas in Japan.