A Petrogenetic Grid for Reactions Producing Biotite and other Al-Fe-Mg Silicates in the Greenschist Facies

This report presents an analysis of phase relations among biotite,muscovite, chlorite, stilpnomelane, actinolite and K-feldsparin a ten component system within the greenschist facies. Itis based on study of the chemical composition of these minerals,on calculations to balance chemical equations-among them, andon their field distribution. A petrogenetic grid resulting fromthis treatment consists of a single invariant point and multipleunivariant lines, the number depending on what part of the assemblageset is held constant. The reactions which involve biotite aresimilar to previously proposed reactions for the biotite isograd.At high pressure, biotite is produced from muscovite+stilpnomelane+actinolite?K-feldspar.At low pressure, chlorite+K-feldspar?stilpnomelane?actinolitereact to form biotite. A biotite-free reaction, not previouslyidentified, divides the chlorite zone into high pressure andlow pressure fields, characterized by the assemblages muscovite+stilpnomelane+actinoliteand chlorite+K-feldspar, respectively. In the blueschist facies,muscovite plus stilpnomelane and/or actinolite are stable insteadof biotite.     

Phase relations of biotite and stilpnomelane in the greenschist facies

Phase relations of biotite and stilpnomelane and associated silicate minerals have been studied in rocks of the greenschist facies, chiefly from Otago, New Zealand and western Vermont, but also from Scotland, Minnesota-Michigan iron range, and northwest Washington. That stilpnomelane in the greenschicht facies crystallizes initially with nearly all iron in the ferrous state is indicated by chemical analyses, high p-T experiments, and phase relationships. Alteration of stilpnomelane after metamorphism not only oxidizes iron but leaches potassium; corrections for both effects must be made in using analyses of brown stilpnomelane in studies of phase relations. Two discontinuous reactions which produce biotite at the biotite isograd have been identified:

1. muscovite+stilpnomelane+actinolite→ biotite+chlorite+epidote
2. chlorite+microcline→ biotite+muscovite. Biotite produced by the first of these reactions has a limited range of variation in Fe/Mg. As grade advances within the biotite zone more magnesian and ferruginous biotites become stable in consequence of the two continuous reactions:
3. muscovite+actinolite+chlorite→ biotite (Mg-rich)+epidote
4. muscovite+stilpnomelane→ biotite (Fe-rich)+chlorite.

Stilpnomelane is stable in muscovite-free rocks throughout the biotite zone, and even up to the grade at which hornblende becomes stable. Phengitic muscovite is stable throughout the biotite zone in New Zealand and thus apparently does not contribute to the formation of biotite until a higher grade is reached.     

Reactions leading to the formation and breakdown of stilpnomelane in the Otago Schist, New Zealand

Semi‐pelitic rocks ranging in grade from the prehnite–pumpellyite to the greenschist facies from south‐eastern Otago, New Zealand, have been investigated in order to evaluate the reactions leading to formation and breakdown of stilpnomelane. Detrital grains of mica and chlorite along with fine‐grained authigenic illite and chlorite occur in lower‐grade rocks with compactional fabric parallel to bedding. At higher grades, detrital grains have undergone dissolution, and metamorphic phyllosilicates have crystallized with preferred orientation (sub)parallel to bedding, leading to slaty cleavage. Stilpnomelane is found in metapelites of the pumpellyite–actinolite facies and the chlorite zone of the greenschist facies, but only rarely in the biotite zone of the greenschist facies. Illite or phengite is ubiquitous, whereas chlorite occurs only rarely with stilpnomelane upgrade of the pumpellyite‐out isograd. Chemical and textural relationships suggest that stilpnomelane formed from chlorite, phengite, quartz, K‐feldspar and iron oxides. Stilpnomelane was produced by grain‐boundary replacement of chlorite and by precipitation from solution, overprinting earlier textures. Some relict 14 Å chlorite layers are observed by TEM to be in the process of transforming to 12 Å stilpnomelane layers. The AEM analyses show that Fe is strongly partitioned over Mg into stilpnomelane relative to chlorite (K_D≈2.5) and into chlorite relative to phengite (K_D≈1.9). Modified A′FM diagrams, projected from the measured phengite composition rather than from ideal KAl₃Si₃O₁₀(OH)₂, are used to elucidate reactions among chlorite, stilpnomelane, phengite and biotite. In addition to pressure, temperature and bulk rock composition, the stilpnomelane‐in isograd is controlled by variations in K, Fe³⁺/Fe²⁺, O/OH and H₂O contents, and the locus of the isograd is expected to vary in rocks of different oxidation states and permeabilities. Biotite, quartz and less phengitic muscovite form from stilpnomelane, chlorite and phengite in the biotite zone. Projection of bulk rock compositions from phengite, NaAlO₂, SiO₂ and H₂O reveals that they lie close to the polyhedra defined by the A′FM minerals and albite. Other extended A′FM diagrams, such as one projected from phengite, NaAlO₂, CaAl₂O₄, SiO₂ and H₂O, may prove useful in the evaluation of other low‐grade assemblages.     

黑龙江依兰地区黑硬绿泥石片岩岩石学与P-T条件研究

依兰地区黑龙江杂岩中黑硬绿泥石片岩矿物组合为黑硬绿泥+绿泥石+多硅白云母+钠长石+透闪石+钾长石±黑云母.岩石矿物学研究表明黑硬绿泥石片岩形成于250～400℃,6～9kbar的变质条件下.黄褐色黑硬绿泥石呈束状、放射状集合体.黑硬绿泥石片岩形成于佳木斯地块向西与松嫩地块俯冲拼贴过程中蓝片岩变质作用的后期,压力稍有降低而温度略有升高的变质变形环境,是蓝片岩向绿片岩转变的蓝闪绿片岩相变质条件下的产物.     

Low pressure cordierite-bearing migmatites from Kelly's Mountain,Nova Scotia

The Kelly's Mountain gneiss complex of Cape Breton Island, Nova Scotia, is a migmatitic paragneiss dominated by biotite- and cordierite-bearing assemblages. Metamorphic grade throughout the complex is in the upper amphibolite facies, with garnet absent and only retrograde muscovite present. In the high grade core of the complex the reaction biotite+andalusite+quartz=cordierite+K-feldspar+sillimanite+ilmenite+H₂O is preserved. The pelitic migmatites contain cordierite- and K-feldspar-rich leucosomes and biotite-rich melanosomes. Minor clinopyroxene-bearing amphibolite in the complex does not show migmatitic textures. The migmatites are interpreted as in situ peraluminous partial melts on the basis of phase relations and textural criteria. Retrograde metamorphism under conditions of high fluid pressure locally produced muscovite after K-feldspar and muscovite+green biotite+chlorite after cordierite in paragneiss, and sphene after ilmenite in amphibolite. Peak metamorphic conditions of 1–3.5 kb and 580–700° C are estimated. The high geothermal gradient inferred from these conditions was probably caused by the intrusion of diorites associated with the gneiss complex. The Kelly's Mountain complex represents a rare example of migmatites formed in the low-pressure facies series, and illustrates some of the reactions involving melting in high grade pelitic rocks.     

Contrasting P-T Paths in the Eastern Himalaya, Nepal: Inverted Isograds in a Paired Metamorphic Mountain Belt

Petrology and phase equilibria of rocks from two profiles inEastern Nepal from the Lesser Himalayan Sequences, across theMain Central Thrust Zone and into the Greater Himalayan Sequencesreveal a Paired Metamorphic Mountain Belt (PMMB) composed oftwo thrust-bound metamorphic terranes of contrasting metamorphicstyle. At the higher structural level, the Greater HimalayanSequences experienced high-T/moderate-P metamorphism, with ananticlockwise P–T path. Low-P inclusion assemblages ofquartz + hercynitic spinel + sillimanite have been overgrownby peak metamorphic garnet + cordierite + sillimanite assemblagesthat equilibrated at 837 ± 59°C and 6·7 ±1·0 kbar. Matrix minerals are overprinted by numerousmetamorphic reaction textures that document isobaric coolingand re-equilibrated samples preserve evidence of cooling to600 ± 45°C at 5·7 ±1·1 kbar.Below the Main Central Thrust, the Lesser Himalayan Sequencesare a continuous (though inverted) Barrovian sequence of high-P/moderate-Tmetamorphic rocks. Metamorphic zones upwards from the loweststructural levels in the south are: Zone A: albite + chlorite + muscovite ± biotite; Zone B: albite + chlorite + muscovite + biotite + garnet; Zone C: albite + muscovite + biotite + garnet ± chlorite; Zone D: oligoclase + muscovite + biotite + garnet ± kyanite; Zone E: oligoclase + muscovite + biotite + garnet + staurolite+ kyanite; Zone F: bytownite + biotite + garnet + K-feldspar + kyanite± muscovite; Zone G: bytownite + biotite + garnet + K-feldspar + sillimanite+ melt ± kyanite. The Lesser Himalayan Sequences show evidence for a clockwiseP–T path. Peak-P conditions from mineral cores average10·0 ± 1·2 kbar and 557 ± 39°C,and peak-metamorphic conditions from rims average 8·8± 1·1 kbar and 609 ± 42°C in ZonesD–F. Matrix assemblages are overprinted by decompressionreaction textures, and in Zones F and G progress into the sillimanitefield. The two terranes were brought into juxtaposition duringformation of sillimanite–biotite ± gedrite foliationseams (S₃) formed at conditions of 674 ± 33°C and5·7 ± 1·1 kbar. The contrasting averagegeothermal gradients and P–T paths of these two metamorphicterranes suggest they make up a PMMB. The upper-plate positionof the Greater Himalayan Sequences produced an anticlockwiseP–T path, with the high average geothermal gradient beingpossibly due to high radiogenic element content in this terrane.In contrast, the lower-plate Lesser Himalayan Sequences weredeeply buried, metamorphosed in a clockwise P–T path anddisplay inverted isograds as a result of progressive ductileoverthrusting of the hot Greater Himalayan Sequences duringprograde metamorphism. KEY WORDS: thermobarometry; P–T paths; Himalaya; metamorphism; inverted isograds; paired metamorphic belts     

Reactions to define the biotite isograd in the Ryoke metamorphic belt,Kii Peninsula,Japan

Two types of biotite isograd are defined in the low-grade metamorphism of the Wazuka area, a Ryoke metamorphic terrain in the Kii Peninsula, Japan. The first, BI₁, is defined by the reaction of chlorite+K-feldspar= biotite+muscovite+quartz+H₂O that took place in psammitic rocks, and the second, BI₂, by the continuous reaction between muscovite, chlorite, biotite and quartz in pelitic rocks. The Fe/Mg ratios of the host rocks do not significantly affect the reactions. From the paragenesis of pelitic and psammitic metamorphic rocks, the following mineral zones were established for this low-pressure regional metamorphic terrain: chlorite, transitional, chlorite-biotite, biotite, and sillimanite. The celadonite content of muscovite solid solution in pelitic rocks decreases systematically with the grade of metamorphism from 38% in the chlorite zone to 11% in the biotite zone. Low pressure does not prohibit muscovite from showing the progressive change of composition, if only rocks with appropriate paragenesis are chosen. A qualitative phase diagram of the AKF system relevant to biotite formation suggests that the higher the pressure of metamorphism, the higher the celadonite content of muscovite at BI₁, which is confirmed by comparing the muscovites from the Barrovian and Ryoke metamorphism.     

Progressive sequence of reactions of the Ryoke metamorphism in the Yanai district, southwest Japan: the formation of cordierite

The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low- to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K-feldspar have a higher K component in an A'KF diagram than those in rocks lacking K-feldspar. This fact reflects an increase in the K₂O content in muscovite, but in biotite it reflects an increase of not only the K₂O content but also of the octahedral vacancy.
At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low-pressure type metamorphism. Some textures in one of the high-grade areas, the K-feldspar-cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K-feldspar+water
and   andalusite=sillimanite.quartz=cordierite+K-feldspar+water
This implies that this zone probably has a P–T  path involving isobaric heating.     

Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central Argentina

The Tres Arboles ductile fault zone in the Eastern Sierras Pampeanas, central Argentina, experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes. Much of the fault zone is characterized by a (D2) ultramylonite, composed of fine‐grained biotite + plagioclase, that lacks a well‐defined preferred orientation. The D2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep. These ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase, sillimanite and garnet inherited from the gneissic and migmatitic protolith (D1) of the hangingwall. The assemblage of garnet + sillimanite + biotite suggests that D1‐related fabrics developed under upper amphibolite facies conditions, and the persistence of biotite + garnet + sillimanite + plagioclase suggests that the ultramylonite of D2 developed under middle amphibolite facies conditions. Greenschist facies, mylonitic shear bands (D3) locally overprint D2 ultramylonites. Fine‐grained folia of muscovite + chlorite ± biotite truncate earlier biotite + plagioclase textures, and coarser‐grained muscovite partially replaces relic sillimanite grains. Anorthite content of shear band (D3) plagioclase is c. An30, distinct from D1 and D2 plagioclase (c. An35). The anorthite content of D3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite. Biotite is partially replaced by muscovite and/or chlorite, particularly in areas of inferred high strain. Quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric. All D3 reactions require the introduction of H⁺ and/or H₂O, indicating an open system, and typically result in a volume decrease. Syntectonic D3 muscovite + quartz + chlorite preferentially grew in an orientation favourable for strain localization, which produced a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of aqueous fluids, on a scale of hundreds of micrometre. Local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids. However, the predominant greenschist facies deformation (D3) along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system.     

The origin of biotite in archaean meta-sediments near Yellowknife,N. W. T., Canada

Data on mineral compositions, modal proportions and textural relations are used to deduce the biotite-forming reaction in Archaean meta-greywackes and meta-pelites of a low-pressure facies series. Biotite originated by reaction of chlorite, muscovite and ilmenite, producing rutile, K-feldspar and quartz as subordinate reaction products. Chlorite composition did not change as the reaction progressed but muscovite became depleted in K and (Mg+Fe+Mn) while gaining a little Ti. The composition of biotite produced in the meta-pelites is to some extent dependent on how much biotite was formed.     

A petrogenetic grid for pelitic schists in the system SiO2-Al2O3-FeO-MgO-K2O-H2O

A quantitative petrogenetic grid for pelitic schists in the system KFMASH that includes the phases garnet, chlorite, biotite, chloritoid, cordierite, staurolite, talc, kyanite, andalusite, sillimanite, and pyrophyllite (with quartz, H₂O and muscovite or K-feldspar in excess) is presented. The grid is based on thermodynamic data of Berman et al. (1985) and Berman (1988) for endmember KFASH and KMASH equilibria and natural Fe-Mg partitioning for the KFMASH system. Calculation of P-T slopes and the change in Fe/(Fe+Mg) along reactions in the KFMASH system were made using the Gibbs method. In addition, the effect on the grid of MnO and CaO is evaluated quantitatively. The resulting grid is consistent with typical Buchan and Barrovian parageneses at medium to high grades. At low grades, the grid predicts an extensive stability field for the paragenesis chloritoid+biotite which arises because of the unusual facing of the reaction chloritoid+biotite + quartz+H₂O = garnet+chlorite+muscovite, which proceeds to the right with increasing T in the KFMASH system. However, the reaction proceeds to the left with increasing T in the MnKFASH system so the assemblage chloritoid + biotite is restricted to bulk compositions with high Fe/(Fe+Mg+Mn). Typical metapelites will therefore contain garnet+chlorite at low grades rather than chloritoid + biotite.     

Tschermak Substitution in Low- and Middle-grade Pelitic Schists

In low- and middle-grade pelitic metamorphic rocks, the extentsof Tschermak substitution in muscovite, chlorite and biotitechange regularly with bulk-rock composition and external conditions.This paper gives a theoretical analysis of the changes. From equipotential lines for Al₂O₃ plotted on AFM diagrams,we have derived a series of Thompson-type muscovite compositiondiagrams, which show how the celadonite content of muscovitevaries with the associated ferromagnesian minerals or with thebulk-rock composition under constant external conditions. Thedistribution coefficient of the exchange reaction for Tschermaksubstitution between muscovite and chlorite varies greatly notonly with temperature but also with the extent of this substitutionin the two minerals because of their strong deviation from ideality.Muscovites with a high celadonite content (phengites) occurin low-temperature rocks in any of the high-, medium- and low-pressuretypes of metamorphism, probably because the exchange equilibriumfor Tschermak substitution between muscovite and chlorite isnot sensitive to pressure. When both Mg-Fe and Tschermak substitutionoccur in muscovite and some other silicates, a metapelite containingthree AFM phases together with muscovite and quartz has at leasttwo independent net-transfer reactions that take place sideby side with changing external conditions. The mass balancerequirement in the rock is imposed on a linear combination ofthe two reaction equations, leading to a constraint on the stoichiometricequations among phase components and the progressive compositionalchanges of muscovite, chlorite and biotite. From such a viewpoint,we examine reactions and progressive mineralogical changes inmetapelites, beginning with K-feldspar-bearing low-grade metapeliticrocks, in which biotite appears by reaction of K-feldspar withchlorite at a temperature lower than that of the biotite isograddefined for K-feldspar-free pelitic rocks. When both Mg-Fe and Tschermak substitution occur in muscoviteand some other silicates, a metapelite containing three AFMphases together with muscovite and quartz has at least two independentnet-transfer reactions that take place side by side with changingexternal conditions. The mass balance requirement in the rockis imposed on a linear combination of the two reaction equations,leading to a constraint on the stoichiometric equations amongphase components and the progressive compositional changes ofmuscovite, chlorite and biotite. From such a viewpoint, we examinereactions and progressive mineralogical changes in metapelites,beginning with K-feldspar-bearing low-grade metapelitic rocks,in which biotite appears by reaction of K-feldspar with chloriteat a temperature lower than that of the biotite isoerad definedfor K-feldsoar-free oelitic rocks. The equations for the reactions that produce biotite or biotite? garnet in K-feldspar-free metapelites have been derived. Combinedwith the composition relations of coexisting muscovite, chlorite,biotite and garnet, they lead to the inference that progressof these reactions with rising temperature causes a decreaseof the celadonite content of muscovite, the antigorite contentof chlorite and the phlogopite content of biotite in the metapelitesof the chlorite, biotite and lower garnet zones, and that thistrend may not continue into the staurolite zone. This inferencehas been verified by examination of the analytical data of mineralsin four progressive metamorphic terranes covering the high-,medium- and low-pressure types. The composition of chlorite is buffered by the garnet-producingreaction so that chlorites in garnet-bearing metamorphic rocksdevelop (Mg ? Fe)/Al₂ ratios close to that of almandine at elevatedtemperatures. The maximum possible celadonite content in muscoviteunder given external conditions is realized in K-feldspar-bearingrocks, and decreases with rising temperature.     

Geothermobarometry of a stilpnomelane–garnet-bearing metapegmatite: P–T constraints on the Eo-Alpine metamorphic overprint of the Austroalpine nappes north of the Tauern Window

The Ordovician Kellerjochgneiss (Schwaz Augengneiss) is a polymetamorphic orthogneiss-bearing unit and is part of the Austroalpine basement nappes north of the Tauern Window. Within the Kellerjochgneiss a small, strongly deformed metapegmatite dike occurs. The pegmatite crosscuts the gneiss discordantly and contains the mineral assemblage muscovite 1,2+plagioclase+K-feldspar+chlorite+quartz+garnet 1 (Alm_67–76Andr_0.9–2Sps_17–28Prp_0.4–5)+garnet 2 (Grs_36–46Alm_24–32Andr_8–21Sps_15–17Prp_0–1)±stilpnomelane±biotite±clinozoisite. The magmatic protolith assemblage is comprised of relict K-feldspar, quartz and garnet 1. Textural observations indicate that biotite and muscovite cores (muscovite 1) are either part of the magmatic- or an earlier (Variscan?) metamorphic assemblage. Geothermobarometry of the metapegmatite was done on the latest-stage (Eo-Alpine) mineral assemblage garnet 2+muscovite 2+chlorite+stilpnomelane+plagioclase+quartz. Calculations of H₂O-absent intersections in the system [KCNFMAS] with the multi-equilibrium program THERMOCALC v.3.1 yielded P–T estimates of 4.4 to 6.7 kbar and 321°C to 376°C. Calculations of the P–T conditions by using the assemblage muscovite 2+chlorite+stilpnomelane+quartz yielded slightly higher pressures of 6.4 to 7.2 kbar at temperatures of 310–325°C. Correlating these P–T data with geochronological data from the neighbouring lithologies (Kellerjochgneiss, Innsbruck Quartzphyllite, Wildschönau Schists) and with structural investigations from these units indicate that the P–T estimates obtained in this investigation represent the Eo-Alpine metamorphic overprint. Hence, these unusual rocks provide important information on the Eo-Alpine P–T conditions since most samples studied from the investigated Austroalpine basement nappes north of the Tauern Window rarely contain mineral assemblages suitable for geothermobarometry.     

Petrology of an Andalusite-Type Regional Metamorphic Terrane, Panamint Mountains, California

The terrane in the Panamint Mountains, California, was regionallymetamorphosed under low-pressure conditions and subsequentlyunderwent retrograde metamorphism. Prograde metamorphic isogradsthat mark the stability of tremolite + calcite, diopside, andsillimanite indicate a westward increase in grade. The studywas undertaken to determine the effects of the addition of Caon the types of assemblages that may occur in pelitic schists,to contribute to the understanding of the stability limits inP – T – aH₂O – X_Fe of the pelitic assemblagechlorite + muscovite + quartz, and to estimate the change inenvironment from prograde to retrograde metamorphism. Peliticassemblages are characterized by andalusite + biotite + stauroliteand andalusite + biotite + cordierite. Within a small changein grade, chlorite breaks down over nearly the entire rangein Mg/(Mg + Fe) to biotite + aluminous mineral. Chlorite withMg/(Mg + Fe) = 0.55 is stable to the highest grade, and thegeneralized terminal reaction is chlorite + muscovite + quartz= andalusite + biotite + cordierite + H₂O. Calcic schists arecharacterized by the assemblage epidote + muscovite + quartz+ chlorite + actinolite + biotite + calcite + plagioclase atlow grades and by epidote + muscovite + quartz + garnet + hornblende+ biotite + calcite + plagioclase at high grades. Epidote doesnot coexist with any AFM phase that is more aluminous than garnetor chlorite. Lithostatic pressure ranged from 2.3 kb to 3.0kb. During prograde-metamorphism temperatures ranged from lessthan 400° to nearly 700°C, and X_H2O (assuming P_H2O +P_CO3 = P_total) is estimated to be 0.25 in siliceous dolomite,0.8 in pelitic schist, and 1.0 in calcic schist. Temperatureduring retrograde metamorphism was 450° ± 50°C,and all fluid were H₂O-rich. A flux of H₂O-rich fluid duringfolding is believed to have caused retrograde metamorphism.The petrogenetic grid of Albee (1965b) is modified to positionthe (A, Cd) invariant point relative to the aluminosilicatetriple point, which allows the comparison of facies series thatinvolve different chloritoid-reactions.     

点苍山变质杂岩新生代变质-变形演化及其区域构造内涵

点苍山变质杂岩体是哀牢山-红河韧性剪切带四个变质杂岩体之一,遭受了多期多阶段变质-变形作用改造。本文重点针对点苍山杂岩的新生代变质-变形作用,尤其是以富铝质高级变质岩即夕线石榴黑云片麻岩和侵位于其中的糜棱岩化细晶花岗质岩石开展了深入研究。对夕线石榴黑云片麻岩的显微构造分析与矿物共生组合研究,确定了高角闪岩相和低角闪岩相变质矿物共生组合,分别为:石榴石(Grt)+夕线石(Sil)+钾长石(Kfs)+黑云母(Bi)+斜长石(Pl)±石英(Q)和夕线石(Sil)+白云母(Ms)+黑云母(Bi)+石英(Q)。对其中的变质锆石进行SHRIMP U-Pb测试,获得了新生代三个阶段的变质作用年龄,即54.2±1.7Ma、31.5±1.5Ma和27.5±1.2Ma.本文还深入研究了侵位于高级变质岩中的一个花岗岩质糜棱岩的宏观与显微构造特点,其LA-ICP-MS年龄为24.4±0.89Ma,代表着同剪切就位花岗质岩浆侵位和结晶年龄。夕线石榴黑云片麻岩中变质锆石从2150～27Ma多期多阶段表观年龄的发育,表明点苍山变质杂岩体具有复杂的构造演化史。点苍山杂岩的多阶段新生代构造-热演化归咎于印度-欧亚板块会聚与碰撞作用(约54Ma)、造山后伸展作用(大约40～30Ma)和沿着哀牢山-红河剪切带大规模左行走滑变形作用(约27～21Ma)。     

Intersecting Isograds in the Whetstone Lake Area, Ontario

Pelitic and calcareous rocks in the Whetstone Lake area havean unusually wide range of chemical composition. Metamorphicreactions have been deduced that represent the observed ‘discontinuities’in compatible mineral assemblages, and by plotting the reactantand the product assemblage of each reaction on a map, metamorphicisograds have been delincated ‘from both sides’.For the pelitic rocks, successively higher-grade isograds arebased on the following reactions: (1)chlorite+muscovite+garnetstaurolite+biotite+quartz+water; (2) chlorite+muscovite+staurolite+quartz kyanite+biotite+water; (3) kyanitesillimanite; (4)staurolite+museovite+quartzsillimanite+garnet+biotite+water. A fifth isograd, based on the reaction (5) biotite+calcite+quartzCa-amphibole+K-feldspar+carbon dioxide+water intersects the isograds based on reactions (2), (3), and (4)in such a manner as to indicate that the H₂O/CO₂ fugacity ratiowas significantly higher in the vicinity of a granite plutonthan in the metasedimentary rocks remote from the pluton. Chemicalanalyses of the coexisting minerals in reaction (5) indicatethat the real reaction may involve plagioclase, epidote, sphene,and Fe-Ti oxides as well.     

Glaucophane-bearing assemblage overprinted by greenschist-facies metamorphism in the Variscan Kaczawa complex, Sudetes, Poland

An Early Palaeozoic (Ordovician ?) metamudstone sequence near Wojcieszow, Kaczawa Mts, Western Sudetes, Poland, contains numerous metabasite sills, up to 50 m thick. These subvolcanic rocks are of within-plate alkali basalt type. Primary igneous phases in the metabasites, clinopyroxene (salite) and kaersutite, are veined and partly replaced by complex metamorphic mineral assemblages. Particularly, the kaersutite is corroded and rimmed by zoned sodic, sodic–calcic and calcic amphiboles. The matrix is composed of actinolite, pycnochlorite, albite (An ≤ 0.5%), epidote (Ps 27–33), titanite, calcite, opaques and, occasionally, biotite, phengite and stilpnomelane. The sodic amphiboles are glaucophane to crossite in composition with Na^B from 1.9 to 1.6. They are rimmed successively by sodic–calcic and calcic amphiboles with compositions ranging from magnesioferri-winchite to actinolite. No compositions between Na^B= 0.92 and Na^B= 1.56 have been ascertained. The textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist (or blueschist–greenschist transitional) assemblage. The presence of glaucophane and no traces of a jadeitic pyroxene + quartz association indicate pressures between 6 and 12 kbar during the high-pressure episode. Temperature is difficult to assess in this metamorphic event. The replacement of glaucophane by actinolite + chlorite + albite, with associated epidote, allows restriction of the upper pressure limit of the greenschist recrystallization to <8 kbar, between 350 and 450°C. The mineral assemblage representing the greenschist episode suggests the P–T conditions of the high-pressure part of the chlorite or lower biotite zone. The latest metamorphic recrystallization, under the greenschist facies, may have taken place in the Viséan.     

Stilpnomelan als alpinmetamorphes Produkt im Mont-Blanc-Granit

Stilpnomelane is known to be index-mineral of lowest metamorphic facies in the northern zone of the Swiss Alps. In Montblanc-region the first stilpnomelane due to alpine metamorphism will be found in the Montblanc-granite, whereas until now the adjacent metamorphic rocks in the north of granite have not yielded any stilpnomelane. This could be explained by chemical control in the metamorphic rocks or by instability of stilpnomelane because a new formed biotite has been found in the shearing-zones in the northern part of granite and stilpnomelane is disappearing.Metamorphic reactions in the very coarse granite are bound to the limits between constituents or to the interior of larger crystals. Stilpnomelane very often has been found in microcline situated near to biotite or chlorite of primary origin. Towards the southern part of granite stilpnomelane will be found in larger quantities and occasionally a new formed chlorite will appear. This evolution is accompanied by the appearance of larger quantities of patch-perthite and chessboard-perthite in the microcline-megacrysts.Probably a green biotite coexists with stilpnomelane, but majority of alpine biotite has been formed in a later phase and is accompanied by a strong change of quartz-fabric and disappearance of stilpnomelane.One might suppose, that the change of lowest greenschist-facies (stilpnomelane) to a higher degree (biotite) even over very short distances depends more on the influence of changing chemical composition in the coexisting aqueous-gaseous phases than on a real increase of pressure and temperature.

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Ich danke Herrn Prof. Dr. E. Niggli, Bern, für die Möglichkeit des ständigen Erfahrungsaustausches und die Bereitstellung der Nonius-Kamera und Herrn Prof. Dr. H. G. F. Winkler, Göttingen, für wertvolle Hinweise und kritische Stellungnahme. Endlich sei Herrn Prof. Dr. P. Bearth, Basel, herzlich gedankt für die Diskussion und die Mitteilung weiterer Fundpunkte von Stilpnomelan.Wertvolle Unterstützung erhielt diese Arbeit durch einen Kredit des Schweizerischen Nationalfonds.     

Metamorphism of the Basement of the Qilian Fold Belt in the Minhe-Ledu Area, Qinghai Province, NW China

The basement of the central Qilian fold belt exposed along the Minhe-Ledu highway consists of psammitic schists, metabasitic rocks, and crystalline limestone. Migmatitic rocks occur sporadically among psammitic schist and metabasitic rocks. The mineral assemblage of psammitic schist is muscovite + biotite + feldspar + quartz ± tourmaline ± titanite ± sillimanite and that of metabasitic rocks is amphibole + plagioclase + biotite ± apatite ± magnetite ± pyroxene ± garnet ± quartz. The migmatitic rock consists of leucosome and restite of various volume proportions; the former consists of muscovite + alkaline feldspar + quartz ± garnet ± plagioclase while the latter is either fragments of psammitic schist or those of metabasitic rock. The crystalline limestone consists of calcite that has been partly replaced by olivine. The olivine was subsequently altered to serpentine. Weak deformations as indicated by cleavages and fractures were imposed prominently on the psammitic schists, occasionally on me     

The greenschist facies in part of eastern Otago,New Zealand

Rocks of the greenschist facies in eastern Otago, New Zealand, have been investigated in an area some thirteen to sixteen kilometers wide and sixty-five kilometers long extending northeastwards approximately normal to the boundary of the schist with lower grade rocks. Quartzo-feldspathic schists predominate but greenschists and metacherts occur sporadically throughout the area. At the southwestern edge of the area schists are in the chlorite zone, slightly above the high-grade limit of pumpellyite. Metamorphic grade increases toward the northeast into the biotite zone which occupies about half the terrane studied and is believed to be everywhere little advanced in metamorphic grade past that of the biotite isograd. Some 130 mineral specimens have been partially analysed with the electron probe. Results derived from these data as well as other mineralogical investigation are as follows: Albite contains a maximum of 1% anorthite plus orthoclase in epidote-bearing rocks from all parts of the area.Compositions of epidotes range from 12% to 32% Ca₂Fe₃(SiO₄)₃(OH), but most lie between 15% and 20%, a compositional field thought by Strens (1965) and Holdaway (1965) to occupy a miscibility gap in the epidote series. Zoning in some epidotes suggests a history of early growth of small, sparse iron-rich epidotes, and later growth of relatively large amounts of iron-poor epidote probably caused by breakdown of prehnite and/or pumpellyite. Muscovites vary widely in celadonite content; but the composition shows little if any dependence on metamorphic grade within the area studied. Most tend to be celadonite-rich, and in this respect are similar in composition to muscovites from rocks of the glaucophane-schist facies.Chlorites range widely in Mg/Fe; but Al/Mg+Fe is relatively uniform. Chlorites associated with actinolite tend to have higher Mg/Fe than those associated with stilpnomelane. Following the classification of Foster (1962) most chlorites are brunsvigite and some are ripidolite. Textural and chemical relations between biotite and coexisting minerals demonstrate that, contrary to some previous suggestions, biotite is not a relict mineral. An alteration product of chlorite bears strong resemblance to biotite, and previous misidentification of this mineral as biotite has caused much confusion regarding the distribution and metamorphic significance of biotite in Otago schists.An attempt to determine the reaction producing biotite is not successful. Possibly biotitebearing rocks have slightly higher biotite component than rocks of the chlorite zone. All newly formed amphibole found in eastern Otago is pale green, Al- and Na-poor actinolite. One of the chemical conditions necessary for the formation of actinolite in schists of eastern Otago is a relatively high Mg/Fe+Al ratio.Stilpnomelane is an integral part of assemblages in which it occurs, being developed under conditions of relatively low and in rocks with a high Fe/Mg + Al ratio. The present highly oxidized state of all stilpnomelane observed in this study is probably not a primary feature of the mineral but developed after metamorphism.Porphyroblastic garnets are accessory constituents in about half the quartzo-feldspathic schists collected from the biotite zone but are extremely rare in specimens of the same lithology from the chlorite zone. Either a garnet-producing reaction began in quartzo-feldspathic schists at about the biotite isograd, or rocks of biotite zone tend to have slightly higher garnet component than those of the chlorite zone. Composition of the garnets ranges widely, extremes being: 77% spess., 18% gross., 5% alm.; 25% spess., 50% gross., 25% alm.; 15% spess., 30% gross., 55% alm. Most of the variation in composition is controlled by host rock composition, but garnets at higher grade tend to have lower spessartine content. The garnets are zoned; generally Mn decreases and Fe increases from core to rim.For the most part chemical equilibrium among different grains and minerals was closely approached over distances of at least a few millimeters. However, profound disequilibrium exists within some individual grains, such as a zoned garnet which over a distance of only 15 microns ranges in spessartine content from 77% in the core to 35% on the rim.This report is a condensed version of part of the author's Ph.D. thesis (Brown, 1966), University of California, Berkeley.