Tourmaline and boron as indicators of the presence, segregation and extraction of melt in pelitic migmatites: examples from the Ryoke metamorphic belt, SW Japan

Yamaguchi

Miyake

et al. 2012

Contrib Mineral Petrol

Self Cite

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...

Section: Introduction 45 46mentioning

confidence: 65%

Section: Introduction 45 46mentioning

confidence: 99%

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

Yamaguchi

Miyake

et al. 2012

Contrib Mineral Petrol

Self Cite

“…The bold dashed line is the same reaction line as given in (a). Tur-out (Ryoke) is empirically derived high-temperature stability limit of tourmaline in Ryoke metapelitic rocks (Kawakami 2004). Breakdown reaction line of alkali-free dravite (þboron-rich fluid) is from Werding & Schreyer (1984), and that of dravite is from Schreyer & Werding (1997).…”

Section: Source Of Boron In a Mafic -Ultramafic Environmentmentioning

confidence: 99%

Kornerupine sensu stricto associated with mafic and ultramafic rocks in the Lützow-Holm Complex at Akarui Point, East Antarctica: what is the source of boron?

Motoyoshi

Shearer

et al. 2008

Kornerupine, (A, Mg, Fe)(Al, Mg, Fe) 9 (Si, Al, B) 5 O 21 (OH, F), is known from only five mafic or ultramafic settings worldwide (of the .70 localities overall). We report a sixth occurrence from Akarui Point in the Lützow-Holm Complex, East Antarctica, where two ruby corundum (0.22-0.34 wt% Cr 2 O 3 )-plagioclase lenses are found at the same structural level as boudinaged ultrabasic rocks in hornblende gneiss and amphibolite. Ion microprobe analyses of kornerupine give 13-59 ppm Be, 181-302 ppm Li, and 5466-6812 ppm B, corresponding to 0.38-0.47 B per 21.5 O; associated sapphirine also contains B (588-889 ppm). Peak metamorphic conditions are estimated to be 770-790 8C and 7.7-9.8 kbar. Kornerupine encloses tourmaline and plagioclase, which suggests the prograde reaction tourmaline (1) þ plagioclase (.An34) þ sapphirine + spinel ! kornerupine þ corundum (ruby) þ plagioclase (,An82) + (fluid or melt). Alternatively, kornerupine and tourmaline could have formed sequentially under nearly constant P-T conditions during the infiltration of fluid that was originally B-bearing, but then progressively lost Na (or gained Ca) and B through reaction with mafic rocks. Kornerupine later reacted with H 2 O-CO 2 fluid in cracks at P-T conditions in the andalusite stability field: kornerupine þ plagioclase þ (Na, K, + Si in fluid) ! tourmaline þ biotite þ corundum (sapphire) + magnesite + andalusite þ (Ca in fluid). Secondary tourmaline differs from the included tourmaline in containing less Ti and having a higher Na/(Na þ Ca þ K) ratio. There are two possible scenarios for introducing B into the lenses: (1) infiltration of boron-bearing aqueous fluids released by prograde breakdown of muscovite in associated metasedimentary rocks; (2) hydrothermal alteration of mafic and ultramafic rocks by seawater prior to peak metamorphism. The latter scenario is consistent with an earlier suggestion that Akarui Point could be part of an ophiolite complex developed between the Yamato-Belgica and Rayner complexes.

“…1b), there are two large areas where Ryoke metamorphic rocks are widely exposed. Low-to high-grade metamorphic rocks are widely distributed in the Wazuka area (Wang et al 1986;Takeuchi & Wang 1999;Ozaki et al 2000), and high-grade metamorphic rocks are widely distributed in the Aoyama area (Hayama et al 1982;Takahashi & Nishioka 1994;Kawakami 2001aKawakami , b, 2002Kawakami , 2004Kawakami & Kobayashi 2006). The Older Ryoke granites with gneissose texture dominate to the south of the Ryoke metamorphic rocks (Yoshizawa et al 1966;Hayama et al 1982).…”

Section: Introductionmentioning

confidence: 99%

CHIME monazite dating as a tool to detect polymetamorphism in high‐temperature metamorphic terrane: Example from the Aoyama area, Ryoke metamorphic belt, Southwest Japan

Suzuki

2011

Island Arc

Self Cite

Chemical Th-U-total Pb isochron method (CHIME) monazite dating was carried out for pelitic-psammitic migmatites and the Ao granite (one of the Younger Ryoke granites) from the Aoyama area, Ryoke metamorphic belt, Southwest Japan. The Ao granite gives an unequivocal age of 79.8 Ϯ 3.9 Ma. The monazite grains in migmatites yield an age of 96.5 Ϯ 1.9 Ma with rims and patchy domains of 83.5 Ϯ 2.4 Ma. The 83.5 Ϯ 2.4-Ma overprinting on migmatites over the garnet-cordierite zone suggests a wide and combined effect of thermal input and fluid activity on the monazite grains caused by the contact metamorphism by the Younger Ryoke granites including the Ao granite. This contact metamorphism has not been detected from the major metamorphic mineral assemblage previously, possibly because the migmatites already possessed the high-temperature mineral assemblage before the granite intrusions and were immune from contact metamorphism in terms of major metamorphic minerals. However, monazite records contact metamorphism clearly. Therefore, the field mapping of the CHIME monazite age is a powerful tool for recognition of polymetamorphism in high-temperature metamorphic terrains where later thermal effects can not be easily detected by the growth of new major metamorphic minerals.