Behavior of zircon in the upper-amphibolite to granulite facies schist/migmatite transition, Ryoke metamorphic belt, SW Japan: constraints from the melt inclusions in zircon

Abstract

23Behavior of zircon at the schist/migmatite transition is investigated. Syn-metamorphic 24 overgrowth is rare in zircon in schists, whereas zircon in migmatites has rims with low Th/U that 25 give 90.3 ± 2.2 Ma U-Pb concordia age. Between inherited core and the metamorphic rim, a thin, 26 dark-CL annulus containing melt inclusion is commonly developed, suggesting that it formed 27 contemporaneous with the rim in the presence of melt. In diatexites, the annulus is further truncated 28 by the brighter-CL overgrowth, suggesting the resorption and regrowth of the zircon after near-peak 29 metamorphism. Part of the zircon rim crystallized during the solidification of the melt in migmatites. 30Preservation of angular-shaped inherited core of 5-10 μm in zircon included in garnet suggests 31 that zircon of this size did not experience resorption but developed overgrowths during near-peak 32 metamorphism. The Ostwald ripening process consuming zircon less than 5-10 μm is required to 33 form new overgrowths. Curved crystal size distribution pattern for fine-grained zircons in a diatexite 34 sample may indicate the contribution of this process. Zircon less than 20 μm is confirmed to be an 35 important sink of Zr in metatexites, and ca. 35 μm zircon without detrital core are common in diatexites, supporting new nucleation of zircon in migmatites. 37In the Ryoke metamorphic belt at the Aoyama area, monazite from migmatites records the 38 prograde growth age of 96.5 ± 1.9 Ma. Using the difference of growth timing of monazite and zircon, 39 the duration of metamorphism higher than the amphibolite facies grade is estimated to be ca. 6 Myr. 40 41 Keywords: zircon, migmatite, melt inclusion, glass, crystal size distribution, duration of 42 metamorphism. 43 Vavra et al. 1999; Bowman et al. 2011). In the polymetamorphic orthogneiss from northern Labrador, 56 Canada, almost no zircon grows in the amphibolite facies gneisses, and it starts to grow near the 57 amphibolite-granulite facies transition (Schiøtte et al. 1989). Vavra et al. (1999) described the zoning 58 pattern of zircon from the amphibolite-granulite facies transition of the Ivrea Zone (Southern Alps) 59in detail. In the Ivrea Zone, this grade of metasediments accompanies partial melting, and all the 60 zircon overgrowth was supposed to have formed entirely in an anatectic environment. They observed 61 an angular shape of inherited core of zircon in metasediments and interpreted that it is not affected 62 by the partial dissolution process. Since dust-like tiny zircons are abundant in the metasediments, 63 they assumed the Ostwald ripening as a possible growth mechanism of zircon overgrowth, and 64 considered that such a process took place during the prograde metamorphism. They recognized three 65 patterns of zircon overgrowth based on morphology and internal structure as follows; (i) prismatic 66 (prism-blocked) with low Th/U ratio and dark-cathodoluminescence (dark-CL), (ii) stubby with 67 medium Th/U ratio, and (iii) isometric with high Th/U ratio and bright-CL. The f...

Age gap between the intrusion of gneissose granitoids and regional high‐temperature metamorphism in the Ryoke belt (Mikawa area), central Japan

“…Nakajima et al () considered these zircon rims as overgrowths formed during anatexis and interpreted the age as that of regional metamorphism. In a similar migmatite from the Grt−Crd zone of the Aoyama area (Figure ), Kawakami et al () report melt inclusions in zircon rims and proposed that such rims formed at suprasolidus conditions and record the age of near‐peak to early retrograde metamorphism. Therefore, we regard the ca 87 Ma age (Nakajima et al, ) as a reliable estimate of near‐peak to early retrograde, regional high‐ T metamorphic conditions in the Mikawa area.…”

A high‐Tmetamorphic complex derived from the high‐PSuo metamorphic complex in the Omuta district, northern Kyushu, southwest Japan

“…The rims show very low Th/U values (< 0.08), and because the gneisses are migmatitic pelitic gneisses, they underwent partial melting under conditions of 720 °C and 6.2 kbar (Ikeda et al, ), falling in the sillimanite‐stable part of the amphibolite facies. Partial melting and subsequent solidification under the upper amphibolite facies conditions would have led to the dissolution of detrital zircon and precipitation of metamorphic zircon (Kawakami et al, ). The zircon rims and their very low Th/U ratios characterize metamorphic zircon rims in migmatitic gneisses (Kawakami et al, ).…”

Possible polymetamorphism and brine infiltration recorded in the garnet–sillimanite gneiss, Skallevikshalsen, Lützow–Holm Complex, East Antarctica

“…Recent advance in understanding the growth/recrystallization timing of zircon and monazite during metamorphic cycle, especially the roles of partial melting and fluid infiltration events in formation/dissolution/recrystallization of these minerals enable to reconstruct the lower crustal evolution in more detail (e.g., Rubatto et al 2001;Rubatto, 2002;Harley and Kelly, 2007;Rubatto et al, 2013;Kawakami et al, 2013;Korhonen et al, 2013;Harley and Nandakumar, 2014).…”