Serpentinization of olivine-rich troctolite from core 227, Integrated Ocean Drilling Program (IODP) Hole U1309D ranges from 10 to 90. Two episodes of serpentinization are recognized. The first, dominant in weakly serpentinized samples, is an approximately isochemical (except for water) replacement of olivine (Fo(8485)) by a mixture of serpentine (antigorite, Mg-number 92) and brucite (amakinite-rich; Mg-number 65). The compositions of the minerals in type 1 veins are a reflection of FeMg exchange between olivine and the brucite serpentine formed during early serpentinization. The early serpentinite veins (type 1) are thin ( 005 mm), irregular, and exploit pre-existing cracks in olivine. The presence of antigorite suggests that early serpentinization occurred at T 300C. Type 1 veins reflect rock-dominated serpentinization, became isolated early in their history, and persist as relicts in all but the most altered samples. The main episode of serpentinization is manifested by through-going lizardite (average Mg-number 96)magnetite veins (type 2). Type 2 veins define an anastomosing foliation, may be several millimeters in width and appear to exploit pre-existing, favorably oriented type 1 veins. Type 2 veins reflect open-system serpentinization. Magnetite in these veins formed by oxidation of the Fe in brucite and serpentine, whereas addition of silica to the system converted the Mg-component of the brucite to serpentine. Magnetite forms one or more distinct bands in the interior of the vein and is never in direct contact with relict olivine. A bruciteserpentine mixture, similar to that found in type 1 veins, but with lizardite instead of antigorite, is commonly present at the margins of type 2 veins (i.e. where they are in reaction contact with relict olivine). We interpret type 2 veins as a steady-state system where brucite continually forms at the olivinevein contact and then reacts out in the interior of the vein. This continual formation and destruction of brucite imposes an exceptionally low a(SiO2) on the system. Magnetite and olivine are never in contact in type 2 veins (or anywhere else) because the olivine-out reaction yields ferroan brucite and not magnetite. The desilication of serpentine in the type 2 veins is a reflection of the inherent instability of Fe-rich serpentine with respect to magnetite at low silica activity. Thus, the composition of serpentine in equilibrium with magnetite in serpentinites is a function of serpentinemagnetite and not serpentineolivine equilbria.

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Onset and Progression of Serpentinization and Magnetite Formation in Olivine-rich Troctolite from IODP Hole U1309D

The oligoclase-biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (XCa= 0.10–0.28)-bearing assemblages in pelitic schists, and represents the highest-grade zone of the Sanbagawa metamorphic terrain. Mineral assemblages in pelitic schists of this zone, all with quartz, sodic plagioclase, muscovite and clinozoisite (or zoisite), are garnet + biotite + chlorite + paragonite, garnet + biotite + hornblende + chlorite, and partial assemblages of these two types. Correlations between mineral compositions, mineral assemblages and mineral stability data assuming PH2O = Psolid suggests that metamorphic conditions of this zone are about 610 ± 25°C and 10 ± 1 kbar.



Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase-biotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high-pressure type of metamorphic belt and with those in a generalized‘upper staurolite zone’as an example of a medium-pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as:

Petrology of pelitic schists in the oligoclase‐biotite zone of the Sanbagawa metamorphic terrain, Japan: phase equilibria in the highest grade zone of a high‐pressure intermediate type of metamorphic belt

A Laser Raman microspectrometry method was applied to metamorphic quartz in quartz-eclogite-, epidote-amphibolite-, and amphibolite-facies rocks to assess the quantitative correlation between the Raman frequency shift and metamorphic pressure. Quartz crystals sealed in garnet and other phases have a higher frequency shift than those in the matrix. Furthermore, the quartz inclusions show a frequency shift speciÞ c to the individual host crystals in eclogites (garnet ≈ kyanite > omphacite ≈ epidote). These observations imply that the residual pressures retained by quartz inclusions depend on elastic parameters of the host crystals, as discussed by previous researchers. The Raman frequency shift of quartz inclusions in garnet systematically increases with increasing peak metamorphic pressures from the amphibolite facies (0.30–0.55 GPa/470–570 °C), through the epidote-amphibolite facies (0.8–1.1 GPa/470–635 °C) to the quartz-eclogite facies (2.1–2.5 GPa/660–710 °C). Calibrations based on experimental work suggest that the measured Raman frequency shifts signify residual pressures of 0.1–0.2, 0.4–0.6, and 0.8–1.0 GPa for these three groups of metamorphic rocks, respectively. Normal stresses (internal pressures) of quartz inclusions in garnet, numerically simulated with an elastic model, and inferred pressure-temperature conditions at peak metamorphic stage are compatible with the residual pressures estimated from the frequency shifts. Laser Raman microspectroscopic analysis of quartz is a simple and effective method for (1) comparison of pressure conditions in metamorphic rocks formed under various pressure-temperature conditions, and (2) detection of a higher-pressure signature in metamorphic rocks extensively recrystallized during the subsequent exhumation and hydration stage.

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Laser Raman microspectrometry of metamorphic quartz : A simple method for comparison of metamorphic pressures

A detailed metamorphic zonal mapping is being in progress on the Sanbagawa metamorphic belt in central Shikoku. The mapping is based upon the distribution of index minerals, garnet and biotite in pelitic schists, and on the sliding equilibrium among silicate and oxide minerals. The distribution of mineral zones has revealed a peculiar thermal structure of the metamorphic complex that the highest-grade rocks occur in the middle of apparent stratigraphy. A large scale recumbent fold, with south vergency and extending for more than 20km, is postulated as a possible structural interpretation.It is concluded, as the most probable model we could imagine at the moment, that before the maximum temperature of metamorphism was reached, the Sanbagawa schists had been metamorphosed in more or less normal thermal regime that the temperature had increased downwards. Then a large scale recumbent fold took place, separating the higher-grade rocks from the heat source and bringing them in between the lower-grade ones. This recumbent fold was accompanied by the start of the uplift of the whole metamorphic complex, while continuing metamorphic reactions with decreasing temperature and pressure.The fact that the Sanbagawa belt is overturned suggests that a very distinctive crustal shortening took place in the present day Sanbagawa terrain in the Mesozoic time, and that the present day distribution of pre-Tertiary geologic units in the outer zone of the south-western Japan can hardly be in situ.

Thermal structure of the sanbagawa metamorphic belt in central shikoku

The Sanbagawa (Sambagawa) metamorphic belt in south-west Japan suffered a high-pressure intermediate metamorphism of Cretaceous age. The regionally highest grade rocks (epidote-amphibolite facies) of the belt occur in the Besshi district in central Shikoku, and locally contain higher grade masses of coarse-grained metabasic and ultrabasic lithologies, such as the Higashiakaishi peridotite, Iratsu epidote-amphibolite, Nikubuchi peridotite and Sebadani metagabbro masses (Kunugiza et al. 1986) (Fig. 1). These masses have been regarded as large-scale tectonic blocks (maximum of 7 × 3 km2) in a mélange zone which formed at the initial stage of the uplifting of the Sanbagawa belt (Takasu 1984, Kunugiza et al. 1986). In this paper, quite a diverse origin and metamorphic history of these blocks will be revealed, and the tectonics associated with the development of the tectonic blocks will be discussed. Metamorphic history of the tectonic blocks Sebadani (SB) metagabbro mass The Sebadani (SB) metagabbro mass, 0.3 × 0.2 km2, is located in the albite-biotite zone, and consists of garnet-epidote amphibolites and relict eclogite which survived epidote amphibolization. The banding of melanocratic hornblende-rich and leucocratic epidote-rich layers records the layering of previous pyroxene and plagioclase layers of cumulate gabbro. Takasu (1984) recognized two stages of eclogite equilibrium in the SB mass; a core-core pair of 720–750°C and 12–20 kbar, and a rim-rim pair of 610–630°C and 10–17 kbar. The basic Sanbagawa schists, up to 20 m from the SB mass, underwent contact metamorphism at high pressure that produced garnet and omphacite (maximum Jd 36%) porphyroblasts from

P-T histories of peridotite and amphibolite tectonic blocks in the Sanbagawa metamorphic belt, Japan

Genesis of epidote amphibolite masses in the Sanbagawa metamorphic belt of central Shikoku

The Iratsu epidote amphibolite mass is a metamorphosed layered gabbro, which has mineralogy of the albite-epidote amphibolite fades in the facies series of the Sanbagawa metamorphism (Banno et al., 1976). Lime stone occurs frequently in the mass, especi ally on the northern slope of the Mt. Gongen-Mt. Jiyoshi-Mt. Futatsudake ridge and also sporadically in other areas. Banno (1964) described that limestone of the Tonaru mass, a similar metamorphosed gabbro near the Bessi mine, has diposide+ grossular assemblage, but details of mine ralogy have not been studied. The author found a limestone having similar skarn mine ralogy from the Oki valley, eastern part of the mass. Petrography of the rock with comments on its genesis will be given below.

Notes on petrography and rock-forming mineralogy (1)

Petrography of the peridotite-metagabbro complex in the vicinity of Mt. Higashi-akaishi, Central Shikoku. Part I. Megascopic textures of the Iratsu and Tonaru epidote amphibolite massas

Notes on petrography and rock-forming mineralogy (6) Glaucophane in the Iratsu amphibolite in the Sanbagawa belt in central Shikoku.:GLAUCOPHANE IN THE IRATSU AMPHIBOLITE IN THE SANBAGAWA BELT IN CENTRAL SHIKOKU

https://www.jstage.jst.go.jp/article/ganko1941/74/9/74_9_332/_pdf

Osamu Iwata

Papers

Genesis of epidote amphibolite masses in the Sanbagawa metamorphic belt of central Shikoku

Notes on petrography and rock-forming mineralogy (1)

Petrography of the peridotite-metagabbro complex in the vicinity of Mt. Higashi-akaishi, Central Shikoku. Part I. Megascopic textures of the Iratsu and Tonaru epidote amphibolite massas

Notes on petrography and rock-forming mineralogy (6) Glaucophane in the Iratsu amphibolite in the Sanbagawa belt in central Shikoku.:GLAUCOPHANE IN THE IRATSU AMPHIBOLITE IN THE SANBAGAWA BELT IN CENTRAL SHIKOKU

Notes on petrography and rock-forming mineralogy (6)