Crystal chemistry of zircon from granitic rocks, Japan:genetic implications of HREE, U and Th enrichment

摘要

Zircon from granitic rocks, Japan is classified into two types, based on minor element content as determined by elec-tron microprobe; a heavy rare earth element (HREE)-U-Th-poor (Type 1) and a HREE-U-Th-rich type (Type 2). Type-1 zircons characteristically occurring in common granites contain up to 2.72 wt% HREE_2O_3 including Y and Sc; up to 0.44 wt% ThO_2; up to 2.10 wt% UO_2, while type-2 zircons characteristically occurring in granitic pegmatites contain up to 19 wt% HREE_2O_3including Y and Sc; up to 11 wt% UO_2; and up to 6.6 wt% ThO_2. The crystal structure refinement of zircon from the Takenouchi granitic pegmatite has for the first time confirmed that type-2 zircons exist as a single-crystal phase. High amounts of HREE, Y, and Sc are preferentially incorporated by xenotime substitution HREE~(3+) + P~(5+) → Zr~(4+)+Si~(4+). Furthermore, incorporation of U and Th into zircon is primarily governed by coupling the thorite substitution with the coffinite substitution U~(4+)+Th~(4+)→ 2Zr~(4+). Zircon crystals from granites and granite pegmatites, extraordinarily enriched in HREE, U, and Th, are generally anhydrous, which strongly suggests that they were formed at the magmatic stage of granitic rock formation. Enrichments of HREE, U and Th are generally observed in both the zircons from granitic pegmatites and the rims of zircons from granites, and originated in fractionation processes in the last stage of crystallization of granitic magma. The discovery of type-1 and type-2 zircons in granitic rocks from the magmatic arc in Japan means that the former were formed from the granitic magmas at high temperature while the latter are intimately related to the formation of granitic pegmatites below 500 ℃.