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Zoisite-Clinozoisite Relations in Low- to Medium-Grade High-Pressure Metamorphic Rocks and Their Implications

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Zoisite-Clinozoisite Relations in Low- to Medium-Grade High-Pressure Metamorphic Rocks and Their Implications MINERALOGICAL MAGAZINE, DECEMBER I980, VOL. 43, PP. IOO5-I3 Zoisite-clinozoisite relations in low- to medium-grade high-pressure metamorphic rocks and their implications MASAKI ENAMI Department of Earth Sciences, Nagoya University, Nagoya 464, Japan AND SHOHEI BANNO Department of Earth Sciences, Kanazawa University, Kanazawa 920, Japan SUMMARY. Coexisting zoisite and clinozoisite in seven- electron-probe microanalysis of coexisting zoisite teen specimens from six localities in Japan have been and clinozoisite are described below, with our view studied with the electron-probe microanalyser. Zoisite on the temperature-dependence of the gap in the and clinozoisite are commonly zoned, but compositional temperature range of low- to medium-grade meta- gaps between them are systematic. Referring to the metamorphic grade of the host rocks, a temporary and morphism of high-pressure intermediate type. schematic phase-diagram for the system Ca2AIaSi3OI2- (OH)-Ca2AI2Fea+Si3012(OH) is presented. With in- Mode of occurrence of coexisting zoisite and creasing temperature, in the range of low- to medium- clinozoisite grade metamorphism, the compositional gap between the two epidote-group minerals shifts towards higher Fe 3+ Fig. I shows specimens localities. Brief accounts compositions. of the geology and petrology of these areas and the mode of occurrence of coexisting zoisite and EPIDOTE-GROUP minerals with the general for- clinozoisite are described below. mula Ca2(A1,Fea+)aSi3012(OH) have two series Iratsu and Tonaru epidote-amphibolite masses. of solid solutions, zoisite and clinozoisite-pistacite. The Iratsu and Tonaru masses are metamorphosed The chemical compositions of coexisting zoisite and layered gabbros that occur in the epidote clinozoisite have been reported by many authors amphibolite-facies area in central Shikoku (Banno (Banno, I964; Myer, 1966; Ackermand and Raase, et al., 1976; also for general petrology, cf. Ernst I973; Hietanen, I974; Raith, i976), but their et al., I97o and Banno et al., 1978 ). This is the stability relations are insufficiently understood. highest metamorphic grade areas in the Sanbagawa There are two opposite views on which of zoisite metamorphic terrain. Main rock-types are epidote and clinozoisite is the higher-temperature phase. amphibolite, garnet-epidote amphibolite, horn- Banno 0964), Holdaway (I97z), and Raith (1976) blende eclogite, quartz eclogite, bimineralic favoured the view that zoisite is the higher- eclogite, and zoisite rock. The coexistence of zoisite temperature phase and hence the compositional and clinozoisite is common in these masses, and gap between zoisite and clinozoisite shifts towards has been found in three rock-types; leucocratic the Fe 3 +-rich region with increasing temperature, epidote amphibolite, hornblende eclogite, and whereas Ackermand and Raase (1973) considered zoisite rock. In the following these epidote amphi- the opposite to be true. Because of the difficulty of bolites are sometimes referred to as metagabbros, the direct determination of the phase relations by but the samples we have studied do not exhibit the synthetic experiments, petrographic data have to textures and mineralogies of metamorphism other be fully used to solve this problem. We have studied than the Sanbagawa event. the compositional gap between zoisite and clino- In the leucocratic epidote amphibolite the occur- zoisite in specimens collected at six localities in rences of zoisite and clinozoisite are classified into Japan (Iratsu, Tonaru, Fujiwara, Nishisonogi, three types by texture. In the first type they occur Yoshimi, and Omi). The results mainly based on as subhedral grains measuring 5o-5oo pm. This is Copyright the Mineralogical Society 1006 M. ENAMI AND S. BANNO Bessi area " ~ .~ Go ::::::::::::::::::::::::::::: I R : Iratsu mass TO : Tonaru mass FU : Fujiwara mass YOSHIMI HA : Higashi-akaishi mass P BESSI ( IRATSU, TONARU, FUdlWARA ) NISHISONOGI FIG. I. Localities of the samples investigated in this paper. A metamorphic zone map of the Bessi district is shown in the upper-left of the figure. the commonest among the zoisite+clinozoisite- boundary of two layers, where the zoisite+ bearing specimens we have dealt with. The second clinozoisite + hornblende + paragonite + muscovite type consists of an aggregate of fine-grained (about +quartz assemblage occurs. Paragonite and 2o-50 #m), subhedral zoisite and clinozoisite (speci- muscovite are subordinate in amount. In this men ME75o425oi). The third type is the inter- rock-type, zoisite and clinozoisite are fine-grained, growth of zoisite and clinozoisite in a single grain measuring about 4o-20o /~m. The zoisite and (specimen ST7o55a ). clinozoisite in this rock-type are not in mutual In hornblende eclogite, clinozoisite is fine- contact but their chemical relation is the same as grained (50-lOO pm) and subhedral, and occurs in those found in mutual contact in other specimens. the interstices of large subhedral grains (o.5- Fujiwara albite-epidote amphibolite mass. The I.O mm) of zoisite. Fujiwara mass is a complex of ultrabasic and basic Zoisite rock is metamorphosed anorthosite, rocks (Onuki et al., 1978; Enami, in press). It occurs originally forming layers in gabbro, and consists in the garnet zone of Higashino et al. (1977) and of zoisite-rich leucocratic bands intercalated with Banno et al. (I978) of the Sanbagawa terrain in hornblende-rich ones. Zoisite is rare in the central Shikoku, and is situated about 8 km east hornblende-rich band, whereas clinozoisite is rare of the Iratsu mass, probably belonging to the lower in the zoisite-rich one, but they coexist at the albite-epidote amphibolite facies. Yokoyama ZOISITE-CLINOZOISITE RELATIONS too7 (1976) estimated the equilibration temperature of Metagabbro of the Omi area. The Omi area is the mass to be about 35o-45o ~ on the basis of located in the southern part of the Niigata Prefec- the antigorite + brucite and epidote + actinolite ture and is a glaucophanitic metamorphic terrain assemblages. (Banno, I958). The coexisting zoisite and clino- The coexistence of zoisite and clinozoisite was zoisite are found in a metagabbro which occurs found in a thin section containing hornblende + as a tectonic block in the albite-epidote amphi- dinopyroxene + chlorite + muscovite + zoisite + bolite facies area (Maruyama, pers. comm. I978 ). clinozoisite + albite. Clinopyroxene is a relic from The specimen studied has hornblende + zoisite + pre-metamorphic assemblage. The clinozoisite is clinozoisite + albite. Prehnite occurs in veins. Sub- subhedral, is surrounded by aggregates of prismatic hedral clinozoisite is in contact with zoisite, forming zoisite, and is optically homogeneous. Most of the an aggregate of fine (2o-3o #m) subhedral grains. zoisites show sector zoning (Enami, 1977), but some optically homogeneous grains do occur and were Chemistry of coexisting zoisite and clinozoisite used in the analyses. Metasomatized metagabbro in the Nishisonogi Zoisite and clinozoisite were analysed by an area. The Nishisonogi area is a high-pressure dectron-probe microanalyser, Hitachi Model metamorphic terrain located in the western part XMA 5A, with accelerating voltage of 15 kV, of the Nagasaki Prefecture, Kyushu. Metagabbros specimen current 0.o2 pA, and beam diameter of this area mainly consist of actinolite, chlorite, 5/~m. ornphacite, zoisite, clinozoisite, muscovite, para- Chemical analyses were performed on 8 speci- gonite, and albite. Jadeite (but not with quartz) mens from the Iratsu mass, 3 from the Tonaru mass, and Na-amphiboles also occur. The associated I from the Fujiwara mass, 2 from the Nishisonogi rocks are inferred to belong to the garnet zone of area, 2 from the Yoshimi area, and I from the Omi the Sanbagawa belt, on the basis of common area. Table I lists the analyses. presence of garnet in pelitic schists (Nishiyama, The coexistence of zoisite and clinozoisite was I978; Nishiyama, pers. comm. i978 ). confirmed on four specimens by X-ray powder The coexisting zoisite and clinozoisite were patterns of magnetic and less-magnetic fractions found in banded metagabbros consisting of of epidote concentrates following the criteria of amphibole-rich and zoisite-rich bands. Their Seki (1959) as well as on three samples by a mineral parageneses are tremolite + omphacite + single-crystal method using an automated four- chlorite + muscovite + clinozoisite and zoisite + circle X-ray diffractometer. In all these specimens clinozoisite + chlorite + muscovite + albite, respec- clinozoisite is distinctly richer in Fe 3 § than zoisite, tively. The zoisite and clinozoisite in the zoisite-rich and they may be identified by the difference of unit are in mutual contact. Grain sizes are variable interference colour that clinozoisite is yellow to ranging from lOO/~m to 2 mm. lavendar-blue whereas zoisite is grey or bluish grey. Metagabbro of the Yoshimi area. This area is Zonal structure. Many of the clinozoisites located to the east of the Kanto mountains, where examined are optically heterogeneous, but the a small isolated faulted block of metamorphosed zoisites appear to be homogeneous optically, basic and ultrabasic complexes is exposed in an although compositional variation was found by area of Tertiary sediments. Main rock-types are microanalysis. Three types of zoning patterns were garnet amphibolite, schistose amphibolite, horn- recognized in the clinozoisites. In the first type blendite, pyroxenite, two-mica gneiss, and serpen- observed in a specimen from the Tonaru mass tinite (Murai,
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