Prograde Metamorphism of the Horokanai Ophiolite in the Kamuikotan Zone, Hokkaido, Japan

The Horokanai ophiolite is a segment of metamorphosed oceaniccrust and upper mantle, tectonically replaced into the KamuikotanZone of Hokkaido, Japan. Metamorphic grade, ranging from thezeolite faciles (Zone A) through the greenschist facile (ZoneB) and the greenschist-amphibolite transitional facile (ZoneC), to the amphibolitic and granulizes facile (Zone D) increasesprogressively downwards with zone boundaries subparallel tothe ophiolite pseudostraitigraphy. The granulite facile rocksinclude both metagabbros and their underlying ultranafic rocks.Coexisting minerals from several tens of samples covering allthe minerals zones were analysed by means of an electronprodemicroanalyser; the results are presented, along with brief considerationof their compositional variation with metamorphic grade. Thefaciles series of metamorphism of the Horokanai ophiolite correspondsto the low-pressure type with a temperature range of 100-750?C, which is broadly comparable to that inferred for ocean-floormetamorphism. The major difference is the presence of the granulitefacile rocks in the Horokanai ophiolite and its absence in ocean-floormetamorphism.     

Lawsonite-Pumpellyite-Epidote Stabilities in glaucophane schists in the Horokanai-Kamietanbetsu area of Kamuikotan zone,Hokkaido, Japan

Summary The Horokanai-Kamietanbetsu area is underlain by a coherent sequence of metabasic rocks and metasediments, which underwent high-pressure glaucophane schist facies metamorphism during the late Mesozoic. On the basis of the assemblages of Ca-Al hydrous silicates in the metabasites, the Horokanai-Kamietanbetsu area is divided into three mineral zones.Phase relations of pumpellyite, lawsonite, epidote and associated minerals in glaucophane schists of the study area are analysed in terms of the system A₂O₃-Fe₂O₃-FeO-MgO-CaO with excess of quartz, albite and H₂O-predominant fluid. The Al-Fe³⁺ substitution of Ca-Al hydrous silicates in buffered assemblages changes systematically with metamorphic grade. An analysis of a five component-seven phase system by means of Schreinemakers' method explains the paragenetic relations of the study area.The composition-paragenetis relations of pumpellyite are constructed for the three mineral zones in the Horokanai-Kamietanbetsu area by defining the compositional invariant point for lawsonite-pumpellyite-Na-amphibole-Na-Ca pyroxene-chlorite and the univariant line for pumpellyite-epidote-Na-amphibole-chlorite on an X_Fe ²⁺-X_Fe ³⁺ diagram. The changes of the position of the invariant points and the univariant lines indicate the direction of increase of grade.

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Die Stabilität von Lawsonit-Pumpellyit-Epidot in Glaukophanschiefern des Gebietes von Horokanai-Kamietanbetsu, Kamuikotan Zone. Hokkaido, Japan

Zusammenfassung Die Horokanai-Kamietanbetsu Zone besteht aus einer kohärenten Abfolge von metabasischen Gesteinen und Metasedimenten, die während des spdten Mesozoikums von einer Hochdruck-Glaucophanschiefer-Metamorphose betroffen wurde. Aufgrund der Ca-Al Silikatassoziationen in den Metabasiten kann das Gebiet von Horokanai-Kamietanbetsu in drei Zonen unterteilt werden.Die Phasenbeziehungen von Pumpellyit, Lawsonit, Epidot und den assoziierten Mineralen in Glaucophanschiefern des Untersuchungsgebietes werden auf der Basis des Systems Al₂O₃-Fe₂O₃-FeO-MgO-CaO mit ÜberschuLß von Quarz, Albit und einer H₂O-dominierten fluiden Phase analysiert. Die Al-Fe³⁺ Substitution von Ca-Al Hydrosilikaten in gepufferten Assoziationen ändert sich systematisch mit dem Grad der Metamorphose. Die Analyse eines fünf Komponenten—sieben Phasensystems nach Schreinemakers ermöglicht eine Interpretation der paragenetischen Beziehungen im Untersuchungsgebiet.Die Beziehungen von Zusammensetzung und Paragenese von Pumpellyit in den drei Mineralzonen des Untersuchungsgebietes werden dadurch ermittelt, daß man den invarianten Punkt der Zusammensetzung für Lawsonit-Pumpellyit-Na-Amphibol-Na-Ca-Pyroxen-Chlorit und die univariante Linie für Pumpellyit-Epidot-Na-Amphibol-Chlorit auf einem X_Fe ²⁺-X_Fe ³⁺-Diagram definiert. Die Änderungen der Position der invarianten Punkte und Linien lassen die Richtung der Zunahme des Grades der Metamorphose erkenne.

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With 6 Figures     

The Kamuikotan zone in Hokkaido, Japan: tectonic mixing of high-pressure and low-pressure metamorphic rocks

Abstract. In the Kamuikotan zone, central Hokkaido, Japan, two distinct types of metamorphic rocks are tectonically mixed up, along with a great quantity of ultramafic rocks; one type consists of high-pressure metamorphic rocks, and the other of low-pressure ones. The high-pressure metamorphic rocks are divided into two categories. (1) Prograde greenschist to glaucophaneschist facies rocks derived from mudstone, sandstone, limestone, a variety of basic rocks such as pillow and massive lavas, hyaloclastite and tuff, and radiolarian (Valanginian to Hauterivian) chert, among which the basic rocks and the chert, and occasionally the sandstone, occur as incoherent blocks (or inclusions) enveloped by mudstone. (2) Retrograde amphibolites with minor metachert and glaucophane-calcite rock, which are tectonic (or exotic) blocks enclosed within prograde mudstone or serpentinite, or separated from these prograde rocks by faults. The K-Ar ages of the prograde metamorphic rocks (72, 107 and 116 Ma on phengitic muscovites) are younger than those of the retrograde rocks (109, 132, 135 and 145 Ma on muscovites, and 120 Ma on hornblende). The low-pressure metamorphic rocks consist of the mafic members of an ophiolite sequence with a capping of radiolarian (Tithonian) chert with the metamorphic grade ranging from the zeolite facies, through the greenschist (partly, actinolite-calcic plagioclase) facies to the amphibolite (partly, hornblende-granulite) facies. The low-pressure metamorphism has a number of similarities with that described for'ocean-floor'metamorphism. The tectonic evolution of such a mixed-up zone is discussed in relation to Mesozoic plate motion.     

Platinum–Group Minerals in Podiform Chromitite from the Kamuikotan Zone, Hokkaido, Northern Japan

Abstract: Ru–Os–Ir alloys have been found in two podiform chromitites located at the Chiroro and Bankei mines in the Sarugawa peridotite complex in the Kamuikotan zone, Hokkaido, Japan. This is the first report on the occurrence of PGM (= platinum-group minerals) from chromitites in Japan. The Ru–Os–Ir alloys most typically form polyhedra associated with other minerals (Ni–Fe alloys and heazlewoodite) in chromian spinel. The PGM are possibly pseudomorphs after some primary PGM such as laurite and are chemically highly inhomogeneous, indicating a low-temperature alteration origin. This is consistent with intense alteration (formation of serpentine, uvarovite and kämmererite) imposed on the Kamuikotan chromitites. High-temperature primary PGE (platinum–group elements)–bearing sulfides were possibly recrystallized at low temperatures into a new assemblage of PGM, Ni-Fe alloys and sulfides. Placer PGM around the peridotite complexes are chemically different from the PGM in dunite and chromitite possibly due to the, as yet, incomplete search for the rock-hosted PGM. The PGE content in chromitites is distinctly higher in those in the Kamuikotan zone than in those in the Sangun zone of Southwest Japan, consistent with the more refractory nature (Cr# of spinel, up to 0.8) of the former than the latter (Cr# of spinel, 0.5).     

Phase relations in garnet-bearing metabasites of prehnite-pumpellyite facies from the Darbut-Sartuohai ophiolite, Western Junggar of Xinjiang, China

Summary Previous petrogenetic grids for very low grade metamorphism do not apply to garnet-bearing prehnite-pumpellyite facies rocks. Garnet-bearing metabasites with Act – absent of prehnite-pumpellyite facies have been found in the Darbut-Sartuohai ophiolite, Xinjiang, China. Results of thermodynamic calculation in the CMASH system using the internally consistent mineral thermodynamic database of Berman (1988) show that equilibria reactions among the end-member species in mineral paragenesis of Pmp-Prh-Grs-Zo-Chl-Qtz intersect at a pressure of 4.75 kbar and temperature of 350 °C. The association of grossular-rich garnet and pumpellyite occurs at pressures and temperatures similar to the Pmp-Act facies. Based on the petrogenetic grid derived in this study, peak metamorphic conditions for metabasites from the Darbut-Sartuohai ophiolite are 310 to 330 °C, and 3.0 to 4.0 kbar, which are affected by the substitution of Fe³⁺Al₋₁. This study shows that grossular-rich garnet can appear in metabasites of the prehnite-pumpellyite facies, but it depends on the substitution between Fe³⁺ and Al in the CMASH+Fe³⁺ system.

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Zusammenfassung Phasenbeziehungen in Grandit-führenden Metabasiten (Prehnit-Pumpelleyit Fazies) vom Darbut-Sartuohai Ophiolit, westliches Junggar (Xinjiang, China) Bereits existierende petrogenetische Netze für niedrig-gradige Metamorphose k?nnen nicht auf Grandit-führende Metabasite angewendet werden. Solche Metabasite der Prehnit-Pumpellyit Fazies wurden in Darbut-Sartuohai Ophiolit gefunden. Thermodynamische Berechnungen mit dem Datensatz von Berman (1988) zeigen, dass die Endglieder der Mineralparagenese Pmp-Prh-Grs-Zo-Chl-Qtz einen invarianten Punkt bei 4.75 kbar und 350 °C bilden. Daher kommen Grossular-reicher Grandit und Pumpellyit unter ?hnlichen PT-Bedingungen der Pmp-Act Fazies vor. Anhand des petrogenetischen Netzes, das in der hier vorliegenden Arbeit berechet worden ist, konnten die Peak-metamorphen Bedingungen für den Darbut-Sartuohai Ophiolit mit 310–330 °C und 3.0–4.0 kbar ermittelt werden. Diese Bedingungen sind vom Fe³⁺Al₋₁ Austausch abh?ngig. Diese Arbeit zeigt, dass Grossular-reiche Granate im CAMSH-System in der Prehnit-Pumpellyit Fazies selbst bei Anwesenheit von Chlorit stabil sind.

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Received September 25, 1999; revised version accepted July 3, 2000     

Mode of occurrence and significance of jadeite in the Kamuikotan metamorphic rocks, Hokkaido, Japan

Abstract In the Kamuikotan zone, jadeite occurs in pelitic rocks, in metaplagiogranites, in veins in amphibolites and mafic sedimentary rocks, and in jadeite-albite rocks. In the first and second types, jadeite is associated with quartz, and is often in direct contact with it. However, such rock-types never occur as part of the coherent metamorphic sequence, but are found only as exotic blocks enclosed in serpentinite. Thus, jadeite + quartz-bearing assemblages are not regarded as representative of the Kamuikotan metamorphism. Lawsonite and aragonite, however, commonly do occur in the Kamuikotan metamorphic rocks, and this metamorphism belongs to a subfacies of the lawsonite-albite facies, in which aragonite is stable. The serpentinite matrix which carried jadeite + quartz-bearing pelites and metaplagiogranites into the metamorphic sequence is interpreted as a tectonic rather than a sedimentary melange.     

Phase relations of pumpellyite-actinolite facies metabasites in the Sanbagawa metamorphic belt in central Shikoku, Japan

Sanbagawa metabasites metamorphosed at conditions near the upper limit of the pumpellytic-actinolite facies were examined in terms of phase equilibria in the five component system Al₂O₃---Fe₂O₃---CaO---MgO---FeO. The Fe³⁺ content of epidote measured as X^Ep_Fe (=Fe/(Fe + Al) of epidote) in the assemblage epidote-chlorite-actinolite-pumpellyite decreases gradually towards the higher-grade, pumpellyite-free areas. The progressive change in X_Fe^Ep can be detected within one metabasite bed 200 meters thick near the upper limit of the pumpellyite-actinolite facies. The Mg---Fe²⁺ substitution, as expressed by variation of Fe/(Fe + Mg) in chlorite (0.40–0.55) has little effect on the Fe³⁺ + Al) ratios of epidote and pumpellyite in the above-mentioned assemblage. The lowet X_Fe^Ep in the pumpellite-bearing assemblage is 0.15 and hence the upper limit of the pumpellyite-actinolite facies is defined by the appearance of an epidote-chlorite-actinolite assemblage with X^Ep_Fc = C.15     

Sub-greenschist facies assemblages of metabasites from south-eastern Peloritani range (NE-Sicily)

Summary Pre-Hercynian magmatic rocks are widespread in the Palaeozoic basement of the Peloritani range. The metabasites of the Mongiuffi-Melia and Gallodoro areas represent the largest and the most important lower-Ordovician magmatic products of this range. These rocks were metamorphosed during the Hercynian event and preserve relict igneous minerals and textures. A detailed study of the metamorphic assemblages has allowed the identification of two stages of metamorphic crystallisation; they are mainly distinguished by the differentXCO₂ of the fluid phase. The first metamorphic event produced three calcite-free sub-greenschist facies assemblages that contain ubiquitous quartz+albite+titanite+chlorite+epidote along with pumpellyite or prehnite or actinolite. The second metamorphic episode produced a Calcite+Chlorite assemblage, non-diagnostic to evaluate P-T conditions of metamorphism. The first stage assemblages are only preserved in small domains of the rock. As even very low amounts of CO₂ in the fluid phase drastically inhibit the formation of diagnostic sub-greenschist facies calc-silicate assemblages, it appears that a more CO₂-rich fluid must have been introduced during the second event. We suggest that this introduction of more CO₂-rich fluid occurred during the development of S₂ crenulation cleavage.

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Sub-grünschieferfazielle Mineralvergesellschaftungen in Metabasiten der südöstlichen Peloritani Range (NE Sizilien)

Zusammenfassung Prähercynische magmatische Gesteine sind weitverbreitet im paläozoischen Basement der Peloritani Range. Die Metabasite aus dem Mongiuffi-Melia und dem Gallodoro Gebiet sind die größten und wichtigsten magmatischen Produkte des unteren Ordoviziums in diesem Gebiet. Sie wurden während der hercynischen Orogenese metamorphisiert und sie bewahren reliktische magmatische Minerale und Strukturen. Eine Detailstudie der metamorphen Mineralvergesellschaftungen erlaubt die Identifikation von zwei metamorphen Kristallisationsstadien, die sich hauptsächlich im XCO₂ der fluiden Phase unterscheiden. Das erste Ereignis resultierte in der Bildung von drei Calcit-freien sub-grünschieferfaziellen Mineralvergesellschaftungen, die verbreitet Quarz+Albit+Titanit+Chlorit+Epidot mit Pumpellyit oder Prehnit oder Aktinolith führen. Die zweite metamorphe Episode führte zur Bildung der Vergesellschaftung Calcit+Chlorit; diese ist für die Abschätzung der P-T Bedingungen der Metamorphose ungeeignet. Die erste Vergesellschaftung ist nur in kleinen Domänen in den Gesteinen erhalten geblieben. Da bereits sehr geringe Mengen an CO₂ in der fluiden Phase die Bildung der diagnostischen sub-grünschieferfaziellen kalk-silikatischen Mineralassoziation verhindern, scheint es, daß das CO₂-reiche Fluid während des zweiten Stadiums zugeführt wurde. Wir meinen, daß die Zufuhr dieser CO₂-reichen Fluide während der Entwicklung der S2 Krenulationsschieferung erfolgt sein muß.

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Chemical variation in metabasites from a proterozoic amphibolite-granulite transition zone,South Norway

The metabasites were originally minor intrusions which are now characterised by wholly metamorphic textures and mineral assemblages diagnostic of an amphibolite-granulite facies transition. Two forms of chemical variation can be distinguished: (1) an igneous-type differentiation, involving tholeiitic iron enrichment accompanied by increases in some incompatible elements, and decreases in the transition trace elements; (2) metamorphic fractionation effects resulting in deficiencies in K, Rb, Sr, Ba, Zr, and enhancement in Na in the highest grade rocks. These distributions closely parallel those in the host acid-intermediate charnockitic gneisses. The data preclude the possibility that the deficiencies were caused by removal of melts from the granulites. The metamorphism closely followed intrusion, and it is unlikely that the fractionations were caused by secondary dehydration of once hydrous assemblages. The preferred model involves intrusion and crystallisation directly under high-grade conditions, possibly with CO₂-rich fluids playing an important role in suppressing the formation of hydrous minerals.     

Double seismic zone beneath the middle of Hokkaido, Japan, in the southwestern side of the Kurile Arc

The vertical section of microearthquakes, determined accurately by using the Hokkaido University network, shows two dipping zones (the double seismic zone) 25–30 km apart in the depth range of 80–150 km beneath the middle of Hokkaido in the southwestern side of the Kurile arc. Hypocentral distribution of large earthquakes (m_b > 4) based on the ISC (International Seismological Centre) bulletin also shows the double seismic zone beneath the same region. The hypocentral distribution indicates that the frequency of events occurring in the lower zone is four times greater than that in the upper zone. The difference in seismic activity between the two zones beneath Hokkaido is in contrast with the region beneath northeastern Honshu in the northeastern Japan arc.Composite focal mechanisms of microearthquakes and individual mechanisms of large events mainly characterize the down-dip extension for the lower zone as is observed beneath northeastern Honshu. For the upper zone, however, the stress field is rather complex and not necessarily similar to that beneath northeastern Honshu. This may be considered to indicate the influence of slab contortion or transformation in the Hokkaido corner between the Kurile and the northeastern Japan arcs.     

Parageneses and compositions of amphiboles from Franciscan jadeite–glaucophane type facies series metabasites at Cazadero, California

Abstract Sodic amphiboles are common in Franciscan type II and type III metabasites from Cazadero, California. They occur as (1) vein-fillings, (2) overgrowths on relict augites, (3) discrete tiny crystals in the groundmass, and (4) composite crystals with metamorphic Ca–Na pyroxenes in low-grade rocks. They become coarse-grained and show strong preferred orientation in schistose high-grade rocks. In the lowest grade, only riebeckite to crossite appears; with increasing grade, sodic amphibole becomes, first, enriched in glaucophane component, later coexists with actinolite, and finally, at even higher grade, becomes winchite. Actinolite first appears in foliated blueschists of the upper pumpellyite zone. It occurs (1) interlayered on a millimetre scale with glaucophane prisms and (2) as segments of composite amphibole crystals. Actinolite is considered to be in equilibrium with other high-pressure phases on the basis of its restricted occurrence in higher grade rocks, textural and compositional characteristics, and Fe/Mg distribution coefficient between actinolite and chlorite. Detailed analyses delineate a compositional gap for coexisting sodic and calcic amphiboles. At the highest grade, winchite appears at the expense of the actinolite–glaucophane pair. Compositional characteristics of Franciscan amphiboles from Ward Creek are compared with those of other high P/T facies series. The amphibole trend in terms of major components is very sensitive to the metamorphic field gradient. Na-amphibole appears at lower grade than actinolite along the higher P/T facies series (e.g. Franciscan and New Caledonia), whereas reverse relations occur in the lower P/T facies series (e.g. Sanbagawa and New Zealand). Available data also indicate that at low-temperature conditions, such as those of the blueschist and pumpellyite–actinolite facies, large compositional gaps exist between Ca- and Na-amphiboles, and between actinolite and hornblende, whereas at higher temperatures such as in the epidote–amphibolite, greenschist and eclogite facies, the gaps become very restricted. Common occurrence of both sodic and calcic amphiboles and Ca–Na pyroxene together with albite + quartz in the Ward Creek metabasites and their compositional trends are characteristic of the jadeite–glaucophane type facies series. In New Caledonia blueschists, Ca–Na pyroxenes are also common; Na-amphiboles do not appear alone at low grade in metabasites, instead, Na-amphiboles coexist with Ca-amphiboles throughout the progressive sequence. However, for metabasites of the intermediate pressure facies series, such as those of the Sanbagawa belt, Japan and South Island, New Zealand, Ca–Na pyroxene and glaucophane are not common; sodic amphiboles are restricted to crossite and riebeckite in composition and clinopyroxenes to acmite and sodic augite, and occur only in Fe₂O₃-rich metabasites. The glaucophane component of Na-amphibole systematically decreases from Ward Creek, New Caledonia, through Sanbagawa to New Zealand. This relation is consistent with estimated pressure decrease employing the geobarometer of Maruyama et al. (1986). Similarly, the decrease in tschermakite content and increase in NaM₄ of Ca-amphiboles from New Zealand, through Sanbagawa to New Caledonia is consistent with the geobarometry of Brown (1977b). Therefore, the difference in compositional trends of amphiboles can be used as a guide for P–T detail within the metamorphic facies series.     

雅鲁藏布缝合带巴尔蛇绿岩边部石英菱镁岩成因

石英(滑石)菱镁岩是基性岩、超基性岩与富CO2流体反应形成的一套硅化-碳酸盐化蚀变岩,常伴生金、汞、菱镁矿、碱金属矿化而在国外备受关注,但国内相关研究较薄弱。巴尔蛇绿岩位于西藏阿里地区,石英菱镁岩主要出露在蛇绿岩体东北边界,出露厚度约20 m,近北西西向延伸数千米。本文系统研究了雅鲁藏布缝合带西段巴尔蛇绿岩边部石英菱镁岩矿物学及地球化学特征。根据尖晶石镜下特征,划分为两个阶段:一阶段石英菱镁岩中尖晶石稳定存在,二阶段石英菱镁岩中尖晶石几乎全部蚀变(代表更强程度蚀变作用)。石英菱镁岩主要地球化学组成为Si O2、Mg O及CO2,8件样品主量元素含量变化较大,Si O2/Mg O比值波动较大,反映石英菱镁岩形成为非等化学过程。蛇纹石化橄榄岩与石英菱镁岩有着相似微量元素分布规律,表明同源性,但两者仍有较明显差异:两阶段石英菱镁岩LREE都较地幔橄榄岩略富集,从地幔橄榄岩到两阶段石英菱镁岩,HREE先亏损再略富集。微量元素标准化图解中,Sr在橄榄岩与石英菱镁岩中都为正异常,其含量随蚀变强度增强而增加。此外,两阶段石英菱镁岩均无Au、Hg矿化。     

日本北海道音调津的球状石墨

本文利用光学显微镜、X射线衍射仪、透射电镜等手段对日本北海道音调津球状石墨的成因、结构等进行研究,认为日本北海道音调津球状石墨中除了石墨外,还存在镍黄铁矿、黄铜矿、磁黄铁矿等金属硫化物矿物.通过透射电子显微镜观察发现音调津石墨中大部分是椭圆状石墨颗粒,高分辨晶格条纹主要是非晶态结构,但是同时也发现了结晶良好的石墨.高结晶度石墨的存在可能是在硫化物Fe-Ni-Cu的触媒作用下形成的.天然球状石墨的微结构和性状研究对天然石墨的形成和开发应用有重要意义,对新型碳材料的制备技术有重要科学意义.     

Observations on the boundary between zeolite facies and prehnite-pumpellyite facies

Using graphical analysis of the system CaO-Al₂O₃-SiO₂-H₂O-CO₂, this paper derives a topology relating the minerals calcite, laumontite, wairakite, prehnite, quartz, and zoisite. Simple thermodynamic reasoning allows this system to be applied to natural rocks and indicates that the first appearance of the assemblage epidote-chlorite-quartz (±albite) should mark the upper boundary of zeolite facies. This assemblage forms at the expense of laumontite+prehnite, laumontite+calcite, or laumontite+pumpellyite, with wairakite likely to replace laumontite as the stable zeolite at low pressures. In natural systems this proposed facies boundary is multivariant and, hence, it is likely to be strongly sensitive to compositional variables. For example, Na-bearing wairakite will be more stable than pure Ca-wairakite and increasing abundance of Fe³⁺ will tend to stabilize epidote+quartz at the expense of the zeolites. Because of this, monitoring the composition of minerals such as epidote, prehnite, or wairakite from lowvariance assemblages may provide a more-sensitive indicator of metamorphic grade than the presence or absence of any particular mineral assemblage.     

Piemontite from the manganiferous hematite ore deposits in the Tokoro Belt,Hokkaido, Japan

Summary Piemontites occur in manganiferous hematite ore deposits and radiolarian chest in the Nikoro Group, Tokoro Belt, eastern Hokkaido, Japan. The piemontite-bearing chest and ore bodies have suffered low-grade metamorphism of high pressure intermediate type. In ore bodies, piemontite forms veinlets with quartz and/or pumpellyite-(Mn²⁺) containing Mn³⁺ in Y site. In chest, piemontite occurs not only in veinlets but also in radiolarian tests with pumpellyite-(Mn²⁺). The mineral assemblages characterized by piemontite, pumpellyite-(Mn²⁺), okhotskite, hematite and bixbyite indicate that chest and ore deposits were metamorphosed under extremely highfO₂ condition. Some piemontites in ores contain as much as 1.12 Mn³⁺, and the sum of Mn³⁺ and Fe³⁺ attains 1.46 per formula unit, whereas piemontites in chest contain less (Mn³⁺ + Fe³⁺). This difference in compositions may essentially be ascribed to the difference in the host rock compositions. On the other hand, Mn³⁺ and Fe³⁺ contents of piemontites in ores vary considerably by Al (Mn³⁺, Fe³⁺) and Mn³⁺ Fe³⁺ substitutions. This phenomenon may be interpreted in terms of the local availability of Mn³⁺ and Fe³⁺ in the host rocks.The low-temperature stability limit of piemontite is evaluated from the relations between piemontite and pumpellyite and from the estimated P-T conditions of piemontite crystallization in chert and ore deposits.

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Piemontit aus den manganreichen Hematit-Lagerstätten des Tokoro-Gürtels, Hokkaido, Japan

Zusammenfassung Piemontite treten in manganführenden Hämatitlagerstätten und Radiolariten in der Nikoro-Gruppe des Tokoro-Gürtels, Ost-Hokkaido, Japan; auf. Die Piemontit-füh-renden Radiolarite und Erzkörper zeigen eine niedrig temperierte (Low-grade Bereich), Hochdruck (intermediate-type)-Metamorphose. In den Erzkörpern bildet Piemontit Gänge zusammen mit Quarz und/oder Mn³⁺ (in der Y-Position)-führendem Pumpellyit-(Mn²⁺). In den Radiolariten tritt Piemontit nicht nur in Gängen, sondern auch zusammen mit Pumpellyit-(Mn²⁺) in Radiolarien auf. Die Mineralparagenese Piemontit, Pumpellyit-(Mn²⁺), Okhotskit, Hämatit und Bixbyit deutet darauf hin, daß die Radiolarite und Erzlagerstätten unter hohenfO₂-Bedingungen metamorphisiert worden sind.In den Erzkörpern enthalten einige Piemontite bis zu 1.12 Mn³⁺ und die Summe von Mn³⁺ und Fe³⁺ erreicht 1.46 pro Formeleinheit. Die Piemontite in den Radiolariten zeigen geringere Mn³⁺ + Fe³⁺ Gehalte. Diese Unterschiede in der Zusammensetzung sind auf die unterschiedlichen Trägergesteine zurückzuführen. Außerdem variieren die Mn³⁺ und Fe³⁺-Gehalte der Piemontite in den Erzkörpern deutlich auf Grund der Substitution von Al (Mn³⁺, Fe³⁺) und Mn³⁺ Fe³⁺. Dieses Phänomen kann durch die lokale Verfügbarkeit von Mn³⁺ und Fe³⁺ im Trägergestein interpretiert werden.Die niedrige Temperatur-Stabilität von Piemontit kann durch die Assoziation Piemontit-Pumpellyit und durch die bestimmten P-T-Bedingungen der Piemontit-Kristallisation in den Radiolariten und Erzlagerstätten abgeschätzt werden.

     

Contact zone interaction of metabasites with metapelites: Amphibolite facies mineral assemblages, chemical profiles and their origin, the Puolankajärvi Formation, Finland

The amphibolite facies, iron-rich metapelites (garnet-staurolite mica schists) of the early Proterozoic Puolankajärvi Formation are intercalated with metamorphosed basic rocks which vary from iron-rich amphibolites to magnesium-rich layered sills and serpentinites. Garnet-cummingtonite-hornblende assemblages are abundant at the contacts of the metapelites with amphibolites, while garnet-chlorite, garnet-gedrite±chlorite, garnet-gedrite-cummingtonite±chlorite and chlorite-cummingtonite-hornblende assemblages occur at their contacts with the layered magnesium-rich sill.

The bulk composition profiles of these contacts, normalised with respect to chromium and/or aluminium, show conspicuous enrichment in Fe, S, Ga and Co and depletion in Si, K, Ba and Rb. Some elements (e.g. Mg, Mn, Ni, P, Zr and Hf) are depleted or enriched in only small amounts, while others (e.g. Ca, Na, Sr and the rare earth elements) may show both enrichment and depletion in the course of a profile.

Although early infiltration during the hydration of metabasites and tectonic reworking may have produced some interaction of the basites with the pelites, the major metasomatic event is considered to have bile elements. The composition of the metapelites, for example, approached that of magnesium-rich ultramafites through potassium depletion, while the ultramafites became markedly enriched in iron. To conserve the volume, silica was depleted from the contact which underwent iron enrichment.     

Relationship between mid-Cretaceous (upper Albian–Cenomanian) ammonoid facies and lithofacies in the Yezo forearc basin, Hokkaido, Japan

An examination of inner shelf, outer shelf, and slope deposits in the Yezo forearc basin, northern Japan, provides new insights into the relationship between mid-Cretaceous ammonoid facies and lithofacies. Although undergoing post-mortem transport to some degree, the ammonoids were not moved to areas outside of their original habitat. This assumption is based on the condition of the outer shell surface, general absence of fragmentation, and sedimentary structures. Desmoceras predominates in the upper Albian–Cenomanian succession regardless of lithofacies, the family Gaudryceratidae is the second-most dominant group in each lithofacies, the abundance of Zelandites decreases offshore, and other groups, including Acanthoceratidae, are uncommon but occur in both inshore and offshore facies. External shell ornamentation does not necessarily vary according to lithofacies differences, while the shape of the whorl section does vary with lithofacies as a reflection of ambient environments. The smooth, slender Zelandites and the compressed morph of the smooth Desmoceras predominate in high-energy regimes represented by frequent hummocky cross-stratification and current ripple marks of an inner shelf. In contrast, the depressed morph of Desmoceras predominates in low-energy, offshore, muddy sea-floor regimes.     

Pseudotachylite from the Main Zone of the Hidaka metamorphic belt, Hokkaido, northern Japan

Pseudotachylite veins have been found in the mylonite zone of the Hidaka metamorphic belt, Hokkaido, northern Japan. They are associated with faults with WNW-ESE to ENE-WSW or NE-SW trends which make a conjugate set, cutting foliations of the host mylonitic rocks with high obliquity. The mylonitic rocks comprise greenschist facies to prehnite-pumpellyite facies mineral assemblages. The mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near-surface environments. An analysis of the deformational and metamorphic history of the Hidaka Main Zone suggests that the ambient rock temperature was 200–300° C immediately before the formation of the Hidaka pseudotachylite. Three textural types of veins are distinguished: cryptocrystalline, microcrystalline and glassy. The cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline-type veins. The microcrystalline type is largely made up of quench microlites of orthopyroxene, clinopyroxene, biotite, plagioclase and opaque minerals with small amounts of amphibole microlites. The interstices of these microlites are occupied by glassy and/or cryptocrystalline materials. The presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km. The bulk chemical composition of the pseudotachylite veins is characterized by low SiO₂ and a high water content and is very close to that of the host mylonitic rocks. This indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases. Local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts. Pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high Al contents. The presence of pigeonite and high-Al pyroxene microlites, of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins. The melt temperatures were estimated to be up to 1100° C using a two-pyroxene geothermometer. Using published data relating water solubilities in high-temperature andesitic magmas to pressure, a depth estimate of about 4 km is inferred for the Hidaka pseudotachylites. Evidence derived from pseudotachylites in the Hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals.     

First record of a Cretaceous cheilostome bryozoan from Hokkaido, Japan

A small collection of recrystallised, encrusting colonies of a single species from the Mikasa Formation (lower Middle Cenomanian), represents the first record of cheilostome (malacostegan or anascan) bryozoans from Hokkaido, Japan.