Garnet Types from the Cazadero Area, California

This is the third in a series of papers on glaucophane schistsfrom the Franciscan Formation near Cazadero, California. Previouspapers describe three distinct types of glaucophane-bearingFranciscan metamorphic rocks near Cazadero. The purpose of this study is to investigate the garnets presentin metamorphic types III (bedrock schists) and IV (tectonicblocks) as defined by Coleman & Lee (1963). Twenty-fourgarnet analyses are presented. Sixteen of these are from (aragonite-bearing)type III glaucophane schists, and eight are from type IV glaucophaneschists. Type IV rocks include California eclogites. Type III rocks include metabasalt, metachert, metashale, meta-ironstone,and metacarbonate that were formed under high pressure and relativelylow temperature. These rocks contain garnets that display awide range of composition, but the dominant molecules representedare consistently almandine, spessartine, and grossular. Type IV rocks are mainly metabasalts that were probably formedunder higher temperatures and pressures than type III rocks.There is a distinct difference between garnets from type IIIrocks and those from type IV (including eclogites); the lattercontainless spessartine and more pyrope, and the dominant moleculesare almandine and grossular. The four analyses of garnets fromCalifornia eclogites have an average pyrope content of aboutten molecular per cent, and they extend the range of compositionreported for eclogite garnets. Quantitative spectrographic determinations of minor elementsare listed for each of the garnets described. The values determinedfor some of the minor elements have a wide range and a capriciousdistribution over a few feet of outcrop area. As a group, both the garnets from type III rocks and those fromtype IV are pyralspites with large contents (as much as 35 molecularper cent) of ugrandite. This unusual admixture of the pyralspiteand ugrandite garnet series may have resulted in part from theconditions (high pressures and relatively low temperatures)under which the enclosing rocks were recrystallized.     

Glaucophane-bearing Metamorphic Rock Types of the Cazadero Area, California

A detailed field and laboratory study has been made of a well-exposedglaucophane schist sequence within the Jurassic and CretaceousFranciscan Formation of northern California. Three types ofglaucophane-bearing metamorphic rocks have been distinguishedin and around the area of the detailed study. Each is characterizedby distinctive textures and mineral assemblages that are interpretedto represent different grades of metamorphism within the glaucophaneschist facies. From the combination of small- and large-scale mapping in thearea described it is clear that coarsely schistose blocks, tensof feet in diameter, commonly rest directly upon and withinless intensely metamorphosed terrain. In the Cazadero area theseisolated blocks of coarsely crystalline rocks are concentratedin a band that is roughly concordant with some of the majorfaulting, and their metamorphic fabric shows no consistent relationto local or regional structures. It is tentatively suggestedthat these blocks have been transported upward tectonicallyand that they are not stratigraphically equivalent to the othertypes of glaucophane-bearing metamorphic rocks in the area. Chemical and petrographic evidence indicates that basalt andsediments have been converted to glaucophane-bearing rocks underconditions of metamorphism that were essentially isochemical,except for fugitive components and some minor elements.     

Lawsonite-bearing eclogitic metabasites in the Cazadero area, northern California

Summary In the Cazadero area, northern California, Lawsonite-bearing eclogitic metabasites occur in association with glaucophane schists. Lawsonite-bearing eclogitic metabasites are coarse-grained, and characteristically lack albite. Representative mineral assemblages are; (1) garnet + omphacite + Lawsonite + epidote + glaucophane + chlorite + quartz, (2) garnet -F- omphacite + Lawsonite + pumpellyite + glaucophane + actinolite + quartz, (3) garnet + omphacite + Lawsonite + pumpellyite + epidote + glaucophane + quartz. They can be represented on an A1₂O₃-Fe₂O₃-MgO-Na₂O diagram in which all minerals are projected from quartz, Lawsonite, almandine garnet, and H₂O-predominant fluid. On the basis of the garnet-clinopyroxene geothermometry and phase relations, the metamorphic conditions for the formation of Lawsonite-bearing eclogitic metabasites are estimated at 360-445 °C and more than 9 ± 1 kbar. Lawsonite-bearing eclogitic metabasites formed near the univariant curve albite = jadeite +quartz. A petrogenetic grid constructed by Schreinemakers' method shows that the Lawsonite-bearing eclogitic metabasites in the Cazadero area formed under transitional P-T conditions between those of the garnet-bearing glaucophane schists in New Caledonia and lawsonitebearing eclogitic metabasites in Corsica.Zusammenfassung Im Gebiet um Cazadero, Nordkalifornien, kommen Lawsonit-führende eklogitische Metabasite in Assoziation mit Glaukophanschiefern vor. Den grobkörnigen Lawsonitführenden Metabasiten fehlt charakteristischerweise Albit. Repräsentative Mineralparagenesen sind: (1) Granat + Omphacit + Lawsonit + Epidot + Glaukophan + Chlorit + Quarz, (2) Granat + Omphacit + Lawsonit + Pumpellyit + Glaukophan + Aktinolith + Quarz, (3) Granat + Omphacit +Lawsonit + Pumpellyit + Epidot + Glaukophan + Quarz. Sie lassen sich in einem A1₂O₃-Fe₂O₃-MgO-Na₂O Diagramm, in dem alle Minerale von Quarz, Lawsonit, Almandin-reichem Granat und einem H₂O-dominierten Fluid projiziert werden, darstellen. Die Metamorphosebedingungen der Lawsonitführenden eklogitischen Metabasite werden auf Grund von Granat-Klinopyroxenthermometrie und der Phasenbeziehungen mit 360-445°C und mehr als 9 ± 1 kbar abgeschätzt. Die Lawsonit-führenden eklogitischen Metabasite bildeten sich nahe der univarianten Reaktion Albit = Jadeit +Quarz. Ein petrogenetisches Netz, konstruiert nach der Schreinemakers Methode, zeigt, daß die P-T Bedingungen der Lawsonitführenden eklogitischen Metabasite im Gebiet von Cazadero im übergangsbereich zwischen jenen von Granat-führenden Glaukophanschiefern in Neukaledonien und Lawsonit-führenden eklogitischen Metabasiten in Korsika liegen.

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Lawsonit-führende eklogitische Metabasite im Gebiet um Cazadero, Nordkalifornien

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With 9 Figures     

Petrology of Franciscan Metabasites Along the Jadeite-Glaucophane Type Facies Series, Cazadero, California

The Cazadero blueschist allochthon lies within the Central MelangeBelt of the Franciscan assemblage in the northern Coast Rangeof California. Mineral compositions and assemblages of morethan 200 blueschists from Ward Creek were investigated. Theresults delineate lawsonite-, pumpellyite-, and epidote-zones.The lawsonite and pumpellyite zones are equivalent to the TypeII metabasites of Coleman & Lee (1963) and are characterizedby well-preserved igneous textures, relict augite, and pillowstructures, whereas epidote zone rocks are equivalent to theType III strongly deformed and schistose metabasites. Chlorite,phengite, aragonite, sphene, and minor quartz and albite areubiquitous. The lawsonite zone metabasites contain lawsonite ( < 3 wt.per cent Fe₂O₃), riebeckite-crossite, chlorite, and Ca-Na-pyroxene;some rocks have two distinct clinopyroxenes separated by a compositionalgap. The clinopyroxene of the lowest grade metabasites containsvery low X_jd. In pumpellyite zone metabasites, the most commonassemblages contain Pm + Cpx + Gl + Chl and some samples withhigher Al₂O₃ and/or Fe₂O₃ have Pm + Lw + Cpx + Chl, Actinolitejoins the above assemblage in the upper pumpellyite zone wherethe actinolite-glaucophane compositional gap is well defined.The epidote zone metabasites are characterized by the assemblagesEp + Cpx + two amphiboles + Chl, Lw + Pm + Act + Chl, and Ep+ Pm + two amphiboles + Chl depending on the Fe₂O₃ content ofthe rock. In the upper epidote zone, winchite appears, Fe-freelawsonite is stable, pumpellyite disappears and omphacite containsvery low Ac component. Therefore, the common assemblages areEp + winchite + Lw, and Lw + Omp + winchite. With further increasein metamorphic grade, epidote becomes Al-rich and lawsoniteis no longer stable. Hence Ep + winchite + omphacite ? garnetis characteristic. Mineral assemblages and paragenetic sequences delineate threediscontinuous reactions: (1) pumpellyite-in; (2) actinolite-in;and (3) epidote-in reactions. Using the temperatures estimatedby Taylor & Coleman (1968) and phase equilibria for Ca-Na-pyroxenes,the P–T positions of these reactions and the metamorphicgradient are located. All three metabasite zones occur withinthe aragonite stability field and are bounded by the maximumpressure curve of Ab = Jd + Qz and the maximum stabilities ofpumpellyite and lawsonite. The lawsonite zone appears to bestable at T below 200?C with a pressure range of 4–6?5kb; the pumpellyite zone between 200 and 290?C and the epidotezone above 290?C with pressure variation between 6?5 and 9 kb.The metamorphic field gradient appears to have a convex naturetowards higher pressure. A speculative model of underplatingseamounts is used to explain such feature.     

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.     

Coexisting Amphiboles from Blueschist Facies Metamorphic Rocks

Four pairs of associated calcic and sodic amphiboles from blueschistfacies metamorphic rocks were analyzed with the electron microprobeand studied by single-crystal X-ray diffraction techniques.Except for ranges in the ratios Mg/(Mg+Fe) and Fe³⁺/(Fe³⁺+Al+Ti),the sodic amphiboles are similar in chemical composition. Theamount of calcium in the M(4)-site ranges only from 0·18to 0·21 ion per formula unit. The calcic amphiboles,in addition to a range in Mg/(Mg+Fe), vary in Na/(Na+Ca) ratio(0·29–0·48). Three of the calcic amphibolescontain less than 1·5 calcium ions per formula unit,indicating a significant solid solution of sodic amphibole componentsin the calcic amphibole phase. The a and b unit-cell parametersof the calcic amphiboles decrease with increased content ofthe sodic component.     

Phase Petrology and Mineral Chemistry of Coexisting Amphiboles from Telemark, Norway

Electron microprobe analyses of coexisting amphiboles, garnet,and cordierite in specimens collected from a single outcropprovide the basis for phase diagrams thought to be representativeof fixed external conditions in the sillimanite zone. A projectionthrough the compositions of quartz and plagioclase is employedto characterize the mineral facies and to facilitate comparisonswith other metamorphic terrains. Limiting recalculations whichbracket the possible range of Fe⁺³ contents in calcic amphiboleanalyses by electron probe permit an additional projection throughthe composition of magnetite, a ubiquitous phase. From thesedata the apparent non-ideality exhibited by various combinationsof coexisting cumming-tonite, anthophyllite, gedrite, and calcicamphibole is explained by intracrystalline cation ordering.     

Amphiboles of the Khibiny alkaline pluton, Kola Peninsula, Russia

The rocks of the Khibiny pluton contain 25 amphibole varieties, including edenite, fluoredenite, kaersutite, pargasite, ferropargasite, hastingsite, magnesiohastingsite, katophorite, ferrikatophorite, magnesiokatophorite, magnesioferrikatophorite, magnesioferrifluorkatophorite, ferrimagnesiotaramite, ferrorichterite, potassium ferrorichterite, richterite, potassium richterite, potassium fluorrichterite, arfvedsonite, potassium arfvedsonite, magnesioarfvedsonite, magnesioriebeckite, ferriferronyboite, ferrinyboite, and ferroeckermannite. The composition of rock-forming amphiboles changes symmetrically relative to the Central Ring of the pluton; i.e., amphiboles enriched in K, Ca, Mg, and Si are typical of foyaite near and within the Central Ring. The Fe and Mn contents in amphiboles increase in the direction from marginal part of the pluton to its center. Foyaite of the marginal zone contains ferroeckermannite, richterite, arfvedsonite, and ferrorichterite; edenite is typical of foyaite and hornfels of the Minor Arc. Between the Minor Arc and the Central Ring, foyaite contains ferroeckermannite, arfvedsonite, and richterite; amphiboles in rischorrite, foidolite and hornfels of the Central Ring are (potassium) arfvedsonite, (potassium) richterite, magnesiokatophorite, magnesioarfvedsonite, ferroeckermannite, and ferriferronyboite; amphiboles in foyaite within the Central Ring, in the central part of the pluton, are arfvedsonite, magnesioarfvedsonite, ferriferronyboite, katophorite, and richterite. It is suggested that such zoning formed due to the alteration of foyaite by a foidolite melt intruded into the Main (Central) Ring Fault.     

新疆阿克苏前寒武纪蓝片岩中多硅白云母的研究

新疆阿克苏蓝片岩是目前世界范围内保存最好的前寒武纪蓝片岩之一.一般认为蓝闪石片岩是在低温高压条件下形成的,且与板块俯冲有密切联系.本文对阿克苏蓝片岩地体的泥质片岩和砂质片岩中的白云母进行了电子探针分析,通过对得到的Si含量数据进行统计,探讨了蓝片岩地体不同区域所受压力的变化,进而用矿物组合、矿物形变、多硅白云母压力计等分析方法讨论了变质温压区间和温压关系.研究结果表明阿克苏蓝片岩带南侧岩石承受的变质压力明显大于北侧,这可能指示了古洋壳板块从北向南的俯冲方向.     

Co-existing Blue and Blue-green Amphiboles from Ile de Groix (Morbihan, France)

Seventeen co-existing blue and blue-green amphibole pairs fromamphibolites of zones I, II and III of Ile de Groix have beenanalysed by electron microprobe. The size of the miscibilitygap, related to variations of compositions of blue and associatedblue-green amphiboles in well defined P and T conditions, asa function of whole rock compositions, is shown. Blue amphibolesare glaucophanes of relatively homogeneous composition (0.018< Ca/(Ca + Na) < 0.102); associated blue-green amphibolesshow larger substitutions, from actinolitic hornblendes to pargasitichornblendes (0.43 < Ca/(Ca + Na) < 0.81). A comparisonwith pairs crystallized in other physical conditions shows variationsin blue amphiboles with P, in blue-green amphiboles with T,inside glaucophane schist facies. The miscibility gap seemsto be the narrowest for glaucophane schist facies conditions,which are near the greenschist facies boundary: P 8 kb, T 450 °C.     

Amphiboles of a Metagabbro--Amphibolite Sequence, Hidaka Metamorphic Belt, Hokkaido

The western part of the Hidaka Metamorphic Belt, Hokkaido, consistsof primary pyroxene gabbro and lesser amounts of olivine gabbrothat have been dynamically metamorphosed to metagabbro—gabbroicamphibolite-amphibolite-epidote amphibolite during uplift andshearing about 23 m.y. ago. Textures and the presence of relic and recrystallized amphiboleand plagioclase in the same rock indicate incomplete reactionand non attainment of equilibrium during recrystallization. EPMA and bulk analyses of 165 amphiboles indicate a continuousoverall compositional range from actinolite to dark green hornblende(with 100 mg/(Mg+Fe²⁺+Fe³⁺Mn) ratios varying from 89.5 to 32.0)marked by increasing Al, Fe, Ti, and Na. A compositional gapis usually present between relic and recrystallized amphibolesin any one rock which becomes more prominent with increasingshearing. In addition to host rock chemical control, amphibole compositionis largely dependent on the An content of coexisting plagioclase.Locally epidote and sphene exert a strong influence on bothamphibole and plagioclase compositions. Amphibole Ti and Mncontents decrease with shearing and Fe enrichment of the hostrocks largely as a result of the incoming of rutile, sphene,and Fe-Ti oxides. Analysis of host rock oxidation ratio andamphibole compositions indicates that the rocks essentiallybehaved as closed systems to oxygen during metamorphism. Al^1V-Al^IV, Al^IV-Fe³⁺, and Al^IV-(Na, K)^A are the main substitutionsin the amphiboles. Within any one rock the recrystallized amphibolesare enriched in Al, Fe, Ti, and Na relative to the relice amphiboles.Increasing metamorphism results in a progressive change of amphiboles(recrystallized) to more Fe and Si (rather than Al) rich compositionsreflecting the trend towards greenschist where Fe-actinolite(+Mg chlorite) would be stable. Differentiation of the amphiboles is within the limits of SiAlreplacement and the compositional limits of the early stagereaction rim and replacement amphiboles in the host olivineand pyroxene metagabbros.     

豫南中温榴辉岩中角闪石的变质演化

在该区中温榴辉岩的各个演化阶段中，出现了不同成分的角闪石。石榴石环带及其核部的闪石等矿物包囊体记录了前榴辉阶段及其进变质演化的特征。在榴岩阶段晚期，蓝闪石稳定出现，其成分环带反映了压力降低的连续过程；角闪石－斜长石后成合晶为石榴石和绿辉石的退变质产物；退变质后期，钙质闪石大量出现。角闪石的矿物组合及其成分变化，反映了中温榴辉岩的顺时针变质演化过程。     

Eclogites and Blueschists from Northeastern Oman: Petrology and P-T Evolution

Eclogites and blueschists occur in the basement and continentalshelf deposits in the Saih Hatat tectonic window of northeasternOman, where they structurally underlie the Semail ophiolite.These eclogites and blueschists constitute part of a coherenthigh-P/T metamorphic terrain comprised of interlayered metabasites,metapelites, and quartzofeldspathic and calcareous mica schists.The metabasites record a continuous regional trend of increasingP and T from west to east, where crossite-epidote schists incentral and western Saih Hatat grade into blueschists and eclogitesat As-Sifah, allowing for the subdivision of this terrain intothree metamorphic zones. P-T conditions range from 4?5 to 5?5kb atT <340?C for zone A to >10?2 kb at 500–580?Cfor zone C. Zoning patterns of garnets, pyroxenes, and phengite, and thecompositions of pyroxene and amphibole inclusions in garnet,indicate that the eclogites and enclosing high-grade blueschistsof zone C followed a ‘clockwise’ P-T rpath of increasingP and T followed by increasing T and decreasing P and a finalstage of retrogression. Garnets and pyroxenes crystallized atdifferent stages of P-T evolution of the terrain in the variouseclogite pods and boudins. This conclusion and the contrastingmineralogies of the eclogites and the enclosing blueschistsare attributed to differences in bulk-rock chemistry, fluidavailability, or fluid compositions within the terrain. The blueschists and eclogites of this terrain formed as a resultof A-type subduction and crustal thickening of the Oman continentalmargin along an east-northeast-dipping thrust or ‘subductionzone’ which was initiated by the change in plate motionbetween Africa and Eurasia at 131 Ma. High P/T metamorphismcontinued until the final stages of ophiolite emplacement inthe late Cretaceous, as indicated by the metamorphism of thethrust sheets overlying the coherent terrain.     

中国夕卡岩矿床中的角闪石

文章综合分析研究了中国21个夕卡岩矿床中的130个角闪石的成分分析数据。根据夕卡岩类型及其伴生金属矿化的不同，把角闪石分为4大类：钙夕卡岩中的角闪石多属钙角闪石，包括绿钙闪石、铁角闪石、镁绿钙闪石、铁浅闪石、阳起石、铁阳起石、铁镁钙闪石和铁韭闪石等；镁夕卡岩中的角闪石以透闪石为主，局部有浅闪石或韭闪石；锰质夕卡岩中的角闪石有锰质阳起石、锰质透闪石、锰直闪石和锰镁闪石；碱质夕卡岩中的角闪石属钠-钙角闪石或钠角闪石类，包括钠透闪石、镁亚铁钠闪石、亚铁钠闪石、镁铝钠闪石和镁钠闪石。碳酸盐围岩和有关侵入岩的成分对角闪石的类型、成分及其伴生金属矿化起重要的作用。     

山东五莲碱性三元长石的微结构研究

笔者对产于山东五莲县七宝山辉石二长岩中的碱性长石进行了电子探针,Ｘ射线衍射,透射电子显微镜以及＾２９ＳｉＭＡＳＮＭＲ谱研究,确定五莲碱性三元长石（Ｏｒ５０．３,Ａｂ４４．３Ａｎ５．４）为钾长石和钠长石共格出溶连晶,其基体钾长石为单斜对称,在透射电子显微镜下未见其呈现任何微结构,而在钠长石中见到宽度为１０．４ｎｍ（钠长石的８ｂ０）的片晶相互形成周期钠长石律双晶,钠长石叶片的宽度平均为３９ｎｍ（ｎ＝４     

福建东山坑北复式岩体中蓝绿色角闪石矿物学特征及其形成环境探讨

角闪石作为福建东山坑北复式岩体的主要变质特征矿物之一,其详细的成因矿物学研究对解释该套岩体的变质变形特征和条件具有一定的指导意义。本文应用电子探针分析和斜长石-角闪石温压计、黑云母-石榴子石温压计对该区花岗岩中蓝绿色角闪石进行了成分分析和温压条件的研究。研究表明该区变质变形花岗岩中蓝绿色角闪石为铁浅闪石,大部分为岩浆结晶和次生交代的角闪石。通过斜长石-角闪石温压计计算得出其压力为0.40-0.72 GPa,变质温度大约为543-557℃。通过石榴石-黑云母温度计计算其变质温度为546-590℃。角闪石的成因矿物学研究表明坑北岩体的变质程度为低角闪岩相-高绿片岩相。变质作用相当于燕山造山运动的第Ⅲ幕,是与深大断裂有关的低压高温型区域。     

Iranshahr Blueschists as Results of Subduction of the Neotethys Inner Makran Oceanic Crust, SE Iran

<正>The Makran accretionary prism is one of the most extensive subduction accretions on Earth.It is characterized by intense folding,thrust faulting and dislocation of the Cenozoic units consisted of sedimentary,igneous and metamorphic rocks.It is located in the southeast of Iran and southwest of Pakistan.Rock     

Regional flow perturbation folding within an exhumation channel: A case study from the Cycladic Blueschists

Kilometre-scale cylindrical folds and associated parasitic folds that trend at small angles to the transport lineation are analysed along a 100-km-long transport-normal segment of the Cycladic Blueschists in an attempt to reconstruct the 3D structural architecture within an exhumation channel. Reversals in the polarity of both fold vergence and the hinge/lineation obliquity occur in a flow-normal direction, defining transport-parallel culmination and depression surfaces that root downwards onto an underlying detachment. Fold patterns generated around culmination and depression surfaces support models of flow-perturbation folding where folds initiate at small angles or sub-parallel to transport in response to wrench-dominated differential shearing. Successive culmination and depression surfaces are separated from one another by along strike distances of ∼20 km, although atypical fold geometries developed in the flanks of major culmination and depressions follow their own patterns, revealing that smaller perturbations occur within the larger scheme. Major culminations are interpreted to reflect regions of surging flow marked by increased velocity during exhumation, whilst the opposite is true for depressions. This behaviour implies that on a regional scale, differential shear varies laterally in an irregular-sinusoidal manner defining areas of relative high and relative low displacement within the exhumation channel.     

中国东北黑龙江杂岩体蓝闪石片岩岩石学研究

黑龙江杂岩主要由蓝闪石片岩构成,蓝闪石片岩主要包括绿帘石、绿泥石、钠长石、钠质角闪石、多硅白云母和石英以及为数不多的榍石、钛铁矿和黑硬绿泥石。其中,钠质角闪石包括蓝闪石和镁质钠闪石。变质条件的压力（P）温度（T）评价条件为400－425℃和0.69－0.86 MPa,对应岩相为绿帘蓝闪石片岩相。黑龙江杂岩蓝闪石片岩的地球化学研究表明,其原岩是可与洋岛型（OIB）玄武岩和一些洋中脊型（E-MORB）玄武岩相对比的大洋玄武岩,这说明黑龙江杂岩蓝闪石片岩的玄武岩原岩是在海底山或者在大洋隆起条件下由富集源形成的;具有洋岛型玄武岩地球化学特征的变质玄武岩以及富锰的变质燧石、大理石、变质硬砂岩和蛇绿岩鳞片的加入证明黑龙江杂岩是消减—增生杂岩,它含有变形洋壳的碎块和在被改造的前震旦纪佳木斯岩体西边缘上形成于侏罗纪的增生楔岩石;黑龙江杂岩在原岩成分上可与活动大陆边缘许多增生的杂岩相当。     

Amphiboles from Dalradian metasedimentary rocks of NE Scotland: environmental inferences and distinction from amphiboles of meta-igneous amphibolites

Summary Several formations within the lower part of the Dalradian Appin Group are characterised by calcareous and calc-silicate lithologies. Amphibole occurs in these metasedimentary units in a variety of habits, from fine-grained disseminated crystals, through radiating aggregates permeating the whole rock, to dark green layers consisting of over 90% amphibole. Electron microprobe analyses of these amphiboles exhibit simple compositional trends from tremolite, through tremolitic hornblende to magnesio-hornblende. The only significant substitutions are from tremolite towards alumino-tschermakite [MgSi Al^viAl^iv] and from tremolite towards ferri-tschermakite [MgSi Fe^viAl^iv].This is in marked contrast to amphiboles from nearby suites of metavolcanic rocks and intrusive metadolerites which exhibit four coupled substitutions, leading from tremolite towards the ferro-actinolite, edenite, alumino-tschermakite and ferritschermakite end-members. These distinctive compositional trends may aid the distinction between para- and ortho-amphibolites elsewhere.Tremolitic amphibole commonly constitutes a high proportion of the calc-silicate rocks, the only other phases present being quartz ± phlogopite. Carbonate minerals are notably absent. This suggests that the sedimentary protolith was high in Mg, such as a dolomitic marl, possibly containing some magnesite. Sedimentological considerations suggest deposition in tropical latitudes in restricted coastal lagoons forming part of an extensive stable shelf bordering a mature landscape.

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Amphibole in Metasedimenten der Dalradians, NE Schottland: Rückschlüsse auf prämetamorphe Ablagerungsbedingungen und Unterscheidung von Amphibolen aus orthomagmatischen Amphiboliten

Zusammenfassung Mehrere Formationen im unteren Teil der Appin Gruppe der Dalradians sind durch karbonatische und kalk-silikatische Lithologien gekennzeichnet. Amphibole treten in diesen metasedimentären Einheiten in verschiedener Ausbildung auf, als feinkörnige, disseminierte Kristalle, als radialstrahlige, die Gesteine durchwachsende Aggregate und in dunkelgrünen, zu mehr als 90% aus Amphibolen bestehenden Lagen. Elektronenstrahl-Mikrosondenuntersuchungen belegen einfache mineralchemische Zusammensetzungen, die Tremolite, tremolitische Hornblende und Magnesio-Hornblende umfassen. Die einzige bedeutende Substitution ist die zwischen Tremolit und Alumino-Tschermakit [MgSi Al^viAl^iv] und zwischen Tremolit und Ferri-Tschermakit [MgSi = Fe^viAl^iv].Im Gegensatz dazu zeigen Amphibole aus den nahegelegenen Metavulkaniten und intrusiven Metadoleriten vier Substitutionstrends von Tremolit zu Ferro-Aktinolith, Edenit, Alumino-Tschermakit und Ferri-Tschermakit. Diese unterschiedlichen Trends könnten auch anderswo bei der Unterscheidung zwischen Para- und Orthoamphiboliten behilflich sein. Tremolitische Amphibole machen normalerweise einen Großteil der kalk-silikatischen Gesteine aus, in denen ansonsten nur Quarz ± Phlogopit auftritt. Karbonate fehlen bemerkenswerterweise. Dies weist auf Mg-reiche sedimentäre Protolithe, wie dolomitische Mergel mit möglicherweise etwas Magnesit, hin. Sedimentologische Überlegungen lassen die Ablagerung in abgeschnürten Küstenlagunen tropischer Breiten, die Teil eines weiträumigen stabilen Schelfes waren, vermuten.

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The Managing Editor regrets that the complete text of this paper has already been published in Mineral Petrol 49: 45–62 (1993)