Origin of zircon in jadeitite from the Nishisonogi metamorphic rocks, Kyushu, Japan

Abstract

On the basis of internal structures, laser ablation U-Pb ages and trace element compositions, the origin of zircon in jadeitite in the Nishisonogi metamorphic rocks was examined. The zircon comprises euhedral zoned cores overgrown by euhedral rims. The cores contain inclusions of muscovite, quartz, albite and possibly K-feldspar, yield 238 U-206 Pb ages of 126 ± 6 Ma (±2 SD, n = 45, MSWD = 1.0), and have Th ⁄ U ratios of 0.48-1.64. The rims contain inclusions of jadeite, yield 238 U-206 Pb ages of 84 ± 6 Ma (±2 SD, n = 14, MSWD = 1.1), and have Th ⁄ U ratios of <0.06. The cores are richer in Y, Th, Ti and rare earth elements (REEs), but the rims are richer in Hf and U. Chondrite-normalized REE patterns of the cores indicate higher Sm N ⁄ La N ratios, lower Yb N ⁄ Gd N ratios and larger positive Ce anomalies compared with those of the rims. Thus, the cores and rims have different 238 U-206 Pb ages and trace element compositions, suggesting two stages of zircon growth. Although the 238 U-206 Pb ages of the rims are consistent with the reported 40 Ar ⁄ 39 Ar spot-fusion ages of matrix muscovite in the jadeitite, the 238 U-206 Pb ages of the cores are older. The mineral inclusions and high Th ⁄ U ratios in the cores are best explained by crystallization from felsic magma. Therefore, the cores are considered relicts from igneous precursor rocks. The rims surrounding the inherited cores possibly precipitated from aqueous fluids during jadeitite formation. The elevated U concentrations in the rims suggest that infiltration of external fluids was responsible for the precipitation. This study provides an example of jadeitite formation by metasomatic replacement of a protolith.

Fluid-metapelite interaction in an ultramafic mélange: implications for mass transfer along the slab-mantle interface in subduction zones

“…The ultramafic mélanges contain tectonic blocks of metagabbro, mafic schist, pelitic schist, and albitite in a matrix of chlorite-actinolite schist, talc schist, and schistose serpentinite (Nishiyama 1989(Nishiyama , 1990. Small blocks (<5 m in diameter) of jadeitite, omphacitite, rodingite, and zoisitite are found locally (Nishiyama 1978;Shigeno et al 2005Shigeno et al , 2012aMori et al 2011).…”

Jadeitites and associated metasomatic rocks from serpentinite mélanges in the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan: a review

“…This indicates the origin of the jadeitite as metasomatic replacement (R-type as defined by Tsujimori and Harlow, 2012), whereas both R-type and P-type (fluid-precipitates) are recognized in jadeitites from the Renge belt (Kunugiza and Goto, 2010;Tsujimori and Harlow, 2012). Thus the Nishisonogi jadeitite has a unique feature among Japanese jadeitites, which has been well documented by Nishiyama (1978), Shigeno and Mori (2003), Shigeno et al (2005Shigeno et al ( , 2012a, and Mori et al (2008Mori et al ( , 2010Mori et al ( , 2011. This paper reviews the nature of the Nishisonogi jadeitite in terms of occurrence, petrography, P-T conditions, and geochronology to coincide with the occasion of the selection of jadeitite as the 'national rock' of Japan.…”

“…In many cases, the cores show healed microfractures in bright CL images. The zircon grains contain mineral inclusions of muscovite, quartz, albite, and K-feldspar in the cores (Mori et al, 2011;Yui et al, 2012). Jadeite and omphacite inclusions occur only in the rims; note that K-feldspar is absent from the zircon rims and jadeitite matrix.…”

“…The Tone jadeitite contains many zircon grains as a matrix mineral and as inclusions in jadeite (Mori et al, 2011;Yui et al, 2012). The zircon is colorless, with doubly terminated, euhedral crystals of 50-800 µm in length.…”

“…The zircon rims are considered to be the products of overgrowth during the jadeitite formation. Mori et al (2008Mori et al ( , 2010Mori et al ( , 2011 reported U-Pb ages of Th/U ratios of zircon in the Tone jadeitite, by using a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The unfractured cores yielded 238 U-206 Pb ages of 126 ± 6 Ma (±2 SD, n = 45, MSWD = 1.0) and Th/U ratios of 0.48-1.64 (Fig.…”