Magmatic zoisite and epidote in tonalite of the Ryoke belt, central Japan

Abstract

Magmatic epidote and zoisite commonly occur in Cretaceous tonalite of the Hazu area in the Ryoke belt, central Japan. The tonalite is mainly composed of amphibole, biotite, plagioclase, quartz, and epidote/zoisite with minor ilmenite, magnetite, pyrite, zircon, and apatite. Small amounts of K-feldspar occur as an interstitial phase between other felsic phases or perthitic patches in plagioclase. Epidote occurs as inclusions in plagioclase, as interstitial phase in the matrix, and as secondary phase in chlorite pseudomorphs after biotite, and in saussuritized plagioclase. The X Fe [¼ Fe 3þ /(Al þ Fe 3þ)] value of the secondary epidote ranges from 0.27 to 0.39. Epidote inclusions in plagioclase and interstitial grains contain less Fe 3þ (X Fe ¼ 0.08-0.29), Fe 3þ-poor epidote with X Fe , 0.18 occurs only as inclusion. Zoisite with X Fe value of 0.01-0.07 occurs only as inclusions in plagioclase, and usually has thin lamella-like layers of Fe 3þ-poor epidote with X Fe ¼ 0.09-0.14. The Fe 3þ-poor epidote with X Fe , 0.20 and zoisite included in plagioclase occasionally form aggregates with K-feldspar and quartz. A thin sodic plagioclase zone develops at the boundary between the Fe 3þ-poor epidote and zoisite inclusions and their host plagioclase. Such a reaction texture is not observed at the boundary between Fe 3þ-richer epidote inclusions with X Fe. 0.20 and their host plagioclase. Epidote grains with X Fe. 0.20 in plagioclase and the matrix are a magmatic phase that crystallized directly from the tonalite magma. The Fe 3þ-poor epidote (X Fe , 0.20) and zoisite were probably formed by a local reaction between the trapped melt and its host plagioclase, and these are considered not to have been in equilibrium with the tonalite magma. Compositions of amphibole-plagioclase assemblages allowed for temperature estimates in the range of 730-770 C and minimum pressures of 0.47-0.57 GPa for the epidote/zoisite-bearing tonalites of the Hazu area. Epidote/ zoisite-free tonalites occur in other areas of the Ryoke belt. There may be several tonalite bodies that record different intrusion processes and solidification depths in the Ryoke belt.

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

“…This could be due to the difference between the samples dated in this study and that analyzed by Suzuki & Adachi (). Our samples except GY48D were collected from 30 m to 3000 m away from pelitic gneiss; they contain allanite but no monazite is found, which is in agreement with the description of the main granitoid facies in southwest Mikawa (Masumoto et al, ). Conversely, the sample analyzed by Suzuki and Adachi is a leucocratic biotite granite which occurs at the margin of the pluton; it is in contact with pelitic gneiss and contains monazite.…”

“…Concordant 206 Pb/ 238 U ages obtained from FIGURE 4 Selected Harker diagrams for gneissose granitoids from the Mikawa and Toyone areas. The composition of the studied samples is plotted together with published data from Ishihara and Chappell (2007), Ishihara and Terashima (1977), Kutsukake (1993), Masumoto et al (2014), Morishita and Suzuki (1993), Nakai (1976), Asahara (2009), andYamasaki (2013). See text for detailed discussion 37 analyses range from 82 Ma to 74 Ma, and their weighted mean is 77.1 AE0.6 Ma (37 spots, MSWD = 1.2; Figure 6e).…”

“…The main mineral assemblage is Pl+Qtz+Bt+Hbl with minor Kfs (Pl, plagioclase; Qtz, quartz; Bt, biotite; Hbl, hornblende; Kfs, K‐feldspar) and cummingtonite (Kutsukake, ). Magmatic epidote is present in Kamihara tonalite samples from southern Mikawa while it is absent in those from northern Mikawa and Toyone, suggesting that the former record a deeper intrusion level than the latter (Masumoto, Enami, Tsuboi, & Hong, ). A tonalite sample from Toyone yields a CHIME monazite age of ca 95 Ma (Nakai & Suzuki, ).…”

“…Selected Harker diagrams for gneissose granitoids from the Mikawa and Toyone areas. The composition of the studied samples is plotted together with published data from Ishihara and Chappell (), Ishihara and Terashima (), Kutsukake (), Masumoto et al (), Morishita and Suzuki (), Nakai (), Tsuboi and Asahara (), and Yamasaki (). See text for detailed discussion…”

“…Bulk-rock analyses were performed for all samples except for sample GY98A and weathered sample GY90A. The results are summarized in Table 1 and plotted in Figure 4, together with data from previous studies of the Kamihara tonalite and Tenryukyo granite (Ishihara & Chappell, 2007;Ishihara & Terashima, 1977;Kutsukake, 1993;Masumoto et al, 2014;Morishita & Suzuki, 1993;Nakai, 1976;Tsuboi & Asahara, 2009;Yamasaki, 2013).…”