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MINOR-ELEMENT SYSTEMATICS OF FLUORAPATITE AND ZIRCON INCLUSIONS IN ALLANITE-(Ce) OF FELSIC VOLCANIC ROCKS FROM THREE OROGENIC BELTS: IMPLICATIONS FOR THE ORIGIN OF THEIR HOST MAGMAS

Mihoko Hoshino^1,, Mitsuyoshi Kimata¹, Masahiro Shimizu¹ and Norimasa Nishida²

¹ Earth Evolution Sciences, Life and Environmental Sciences, University of Tsukuba, Tennodai 1–1–1, Tsukuba, Ibaraki, 305-8572, Japan
² Chemical Analysis Division, Research Facility Center for Science and Technology, University of Tsukuba, Tennodai 1–1–1, Tsukuba, Ibaraki, 305-8577, Japan

E-mail address: hossy716{at}geol.tsukuba.ac.jp

We have investigated allanite phenocrysts and the apatite and zircon inclusions they contain from three localities: (1) SK100 ash beds, Niigata, Japan, (2) the youngest Toba Tuff (YTT), Sumatra, Indonesia, and (3) the Bishop Tuff (BST) in eastern California, to gather geochemical inferences about the magmas from which these phases crystallized using an electron microprobe. Back-scattered electron (BSE) images reveal the presence of euhedral crystals of apatite (5–30 µm) and zircon (5–20 µm) incorporated into euhedral phenocrysts of allanite. Textural evidence suggests that apatite, zircon and allanite crystallized in that order. Furthermore, the chondrite-normalized REE pattern of apatite inclusions in these allanite phenocrysts resembles that of apatite in syenites and unfractionated carbonatites. This characteristic suggests that apatite inclusions crystallized from relatively alkaline magmas rich in REE, CO₃^2– and PO₄^3–. Such a magmatic signature could perhaps represent that of a boundary-layer melt. In addition, the F/Cl and Mn/Fe values of the apatite inclusions imply that the relative SiO₂ contents of such magmas increase in order SK100 YTT BST. Variations in Y and Hf contents and Zr/Hf value among the zircon inclusions also show that they crystallized from mafic (SK100), intermediate (YTT), and felsic (BST) magmas. The host rocks are similar to magnetiteseries granites from Japan, according to two characteristics of the allanite: (1) appreciable amounts of Mg (0.078–0.165 apfu) and Ti (0.057–0.137 apfu) and (2) chondrite-normalized REE pattern. The petrogenetic processes envisaged for crystallization of allanite phenocrysts and their apatite and zircon inclusions can explain differences in (1) the REE pattern between allanite and apatite, (2) variations in minor elements among allanite, apatite and zircon, and (3) occurrences of minerals associated with the allanite phenocrysts.

Keywords: allanite phenocrysts, apatite inclusion, zircon inclusion, host magma, rhyolite, Niigata, Japan, Toba tuff, Indonesia, Bishop tuff, California.