Chemical Study of Minerals from the Moine Schists of the Ardnamurchan Area, Argyllshire, Scotland

The mineralogy of the Moine schists of the Ardnamurchan areahas been studied, using twenty-two analyses of muscovites, eighteenof biotites, seven of garnets (including five partial analyses),and one analysis of an epidote. The muscovites range in composition from phengite-ferrimuscovitein the biotite grade of metamorphism to nearly pure muscovite(sensu stricto) in the staurolite grade. The amounts of aluminiumin octahedral and tetrahedral co-ordination in the muscovitesincrease as the grade of metamorphism increases. In the associatedbiotites the amount of octahedral aluminium shows only slightvariations which are not related to the grade of metamorphism,but the amount of tetrahedral aluminium increases with increasein the grade of metamorphism. Muscovites and biotites from epidote-bearingrocks contain smaller amounts of tetrahedral aluminium thanmicas from rocks without epidote at the same grade of metamorphism.The amounts of tetrahedral aluminium in micas from rocks ofsimilar composition can probably be used as an index of thegrade of metamorphism. Magnesium and ferrous iron show a consistent distribution relationbetween coexisting muscovites and biotites. The value of thedistribution coefficient for these cations between the two micasis about 2.5 to 3.0, with magnesium preferentially concentratedin the muscovite. The value of the distribution coefficientdoes not vary appreciably within the medium grades of regionalmetamorphism. There is also a consistent pattern of distributionof magnesium, ferrous, and ferric iron between the two micas,but ferric iron is relatively concentrated in muscovite in rocksthat contain epidote without garnet; it is possible that thebiotites in these rocks attain a maximum possible content ofFe3+ at about 10 per cent of the total Mg+Fe²⁺+Fe³⁺ content.The consistent distribution of magnesium and iron between coexistingmicas suggests that it can be used as an index of the attainmentof chemical equilibrium with respect to these two elements inmetamorphic rocks. All but one of the analysed micas have deficiencies in the occupancyof the large cation site; there appears to be a consistent relationshipin the extent of the large cation deficiency between the twocoexisting micas, wjth the biotites having a greater deficiencythan the muscovites. Sodium is relatively concentrated in themuscovites but does not show a consistent distribution relationshipbetween the coexisting micas. The analysed garnets are almandine-spessartines with appreciablesolid solution towards grossularite. The garnets that containthe largest amounts of grossularite are those from the epidote-bearingrocks. In the rocks without epidote the garnets show a poorcorrelation of decreasing calcium content with increasing gradeof metamorphism.     

Chemistry and genesis of garnets in metamorphic rocks

250 analyses of garnets in metamorphic pelitic, arkosic, and pyroclastic rocks from the Stavanger area have been made by means of a microprobe (ARL-EMX). The relations between chemistry and metamorphic grade are discussed. The dependence of the Mn content upon temperature, pressure, chemistry of the parent rocks and oxygen fugacity is a very sophisticated one.It is shown that the Mn content of the garnets depends on the oxygen fugacity of the system and that the oxygen fugacity is strongly linked to the behaviour of graphite during increasing metamorphism.Examples described by Miyashiro (1953) and Engel and Engel (1960) are in agreement with the model presented here.Some observations show an influence of primary sedimentary enrichments of manganese upon the first occurrence of garnets in the course of metamorphism.     

Multi-stage Garnet in the Internal Brianconnais Basement (Ambin Massif, Savoy): New Petrological Constraints on the Blueschist-facies Metamorphism in the Western Alps and Tectonic Implications

Three types of garnet have been distinguished in pelitic schistsfrom an epidote–blueschist-facies unit of the Ambin andSouth Vanoise Briançonnais massifs on the basis of texture,chemical zoning and mineral inclusion characterization. Type-1garnet cores with high Mn/Ca ratios are interpreted as pre-Alpinerelicts, whereas Type-1 garnet rims, Type-2 inclusion-rich porphyroblastsand smaller Type-3 garnets are Alpine. The latter are all characterizedby low Mn/Ca ratios and a coexisting mineral assemblage of blueamphibole, high-Si phengite, epidote and quartz. Prograde growthconditions during Alpine D₁ high-pressure (HP) metamorphismare recorded by a decrease in Mn and increase in Fe (±Ca)in the Type-2 garnets, culminating in peak P–T conditionsof 14–16 kbar and 500°C in the deepest parts of theAmbin dome. The multistage growth history of Type-1 garnetsindicates a polymetamorphic history for the Ambin and SouthVanoise massifs; unfortunately, no age constraints are available.The new metamorphic constraints on the Alpine event in the massifsdefine a metamorphic T ‘gap’ between them and theirsurrounding cover (Briançonnais and upper Schistes Lustrésunits), which experienced metamorphism only in the stabilityfield of carpholite–lawsonite (T < 400°C). Thesedata and supporting structural studies confirm that the Ambinand South Vanoise massifs are slices of ‘eclogitized’continental crust tectonically extruded within the SchistesLustrés units and Briançonnais covers. The correspondingtectonic contacts with top-to-east movement are responsiblefor the juxtaposition of lower-grade metamorphic units on theAmbin and South Vanoise massifs. KEY WORDS: Alpine HP metamorphism; Ambin and South Vanoise Briançonnais basements; metamorphic gaps; multistage garnets; Western Alps     

Chromite in Komatiites, II. Modification during Greenschist to Mid-Amphibolite Facies Metamorphism

Chromite compositions in komatiites are influenced by metamorphicprocesses, particularly above 500°C. Metamorphosed chromiteis substantially more iron rich than igneous precursors, asa result of Mg–Fe exchange with silicates and carbonates.Chromite metamorphosed to amphibolite facies is enriched inZn and Fe, and depleted in Ni, relative to lower metamorphicgrades. Relative proportions of the trivalent ions Cr³⁺, Al³⁺and Fe³⁺ are not greatly modified by metamorphism up to loweramphibolite facies, although minor Fe³⁺ depletion occurs duringtalc–carbonate alteration at low temperature. SignificantAl is lost from chromite cores above 550°C, as a resultof equilibration with fluids in equilibrium with chlorite. ElevatedZn content in chromite is restricted to rocks with low (metamorphic)Mg/Fe ratios, and is the result of introduction of Zn duringlow-temperature alteration, with further concentration and homogenizationduring prograde metamorphism. Cobalt and Mn also behave similarly,except where carbonate minerals are predominant in the metamorphicassemblage. Chromite at amphibolite facies is typically extensivelyreplaced by magnetite. This is the result of incomplete metamorphicreaction between chromite and chlorite-bearing silicate assemblages.Magnetite compositions at the inner chromite–magnetiteboundary are indicators of metamorphic grade. KEY WORDS: chromite; komatiite; spinel; metamorphism; Zn     

Progressive Metamorphism of Metabasic Rocks from the Haast Schist Group of Southern New Zealand

Progressive mineralogical changes are described for metabasicrocks through a Barrovian-type metamorphic series ranging fromgreenschist to amphibolite facies in the Southern Alps of NewZealand. Wet chemical and electronmicroprobe analyses of coexisting phasesillustrate (a) that chlorite composition becomes progressivelymore Mg-rich towards higher grades. Chlorite and biotite zonechlorites have Mg/Fe <1.00, while in the oligoclase zonethe chlorite Mg/Fe >2.00. (b) Biotite and epidote compositionsshow no systematic variation with metamorphic grade and arecontrolled by bulk rock composition. For epidote, compositionis directly related to oxidation states during metamorphism.(c) Zoning profiles from atoll and normal porphyroblastic almandine-richgarnets are illustrated, and their relationship to compositionalchanges with metamorphic grade discussed, (d) Coexisting compositionsof albite and oligoclase through the garnet zone outline theform of the peristerite solvus. Myrmekitic textures in oligoclaseare ascribed to release of silica during progressive eliminationof albite. Element distribution between coexisting minerals is graphicallyinvestigated. Isogradic samples show very similar element distributions,suggesting general equilibration. Distribution of any elementbetween two phases for the facies series as a whole, however,is clearly influenced by changing concentrations of other ionsin one or both minerals. In particular for pairs containingcalciferous amphibole, the distribution coefficient is dependentupon tetrahedrally co-ordinated Al⁺³ of the amphibole.     

Mineralogy of New Caledonian metamorphic rocks

A Cretaceous to low-Tertiary sequence of interbedded pelites, cherts, basic and acidic volcanics and calcareous lenses has been metamorphosed by an Oligocene event. A complete intergradational metamorphic sequence is exposed in the Ouégoa destrict. The following metamorphic zones have been recognised: — (1) lowest-grade rocks consisting of quartz-sericite phyllites and pumpellyite metabasalts (2) lawsonite zone, characterized by the association of lawsonite and albite (3) epidote zone, characterised by epidote-omphacite-sodic hornblendealmandine bearing metabasalts and epidote-albite-almandine-glaucophane bearing metasediments; calcareous metasediments may also carry omphacite. The epidote and lawsonite zones are separated by a narrow belt of transitional rocks. Garnets occur in metasediments throughout the lawsonite zone as rare tiny crystals (<0.03 mm diam.). Garnets first appear in metabasalts in lawsonite-epidote transitional rocks. Garnets are widespread and abundant in epidote-zone metasediments and metabasalts. 45 garnets from rocks representative of all lithologies and metamorphic grades have been analysed with an electron-probe microanalyser. The garnets were consistently zoned. Garnets in lawsonite and low-grade epidote zones show a “bell-type” zoning with cores enriched in Mn relative to Fe and rims enriched in Fe, Mg and frequently Ca. Garnets from high-grade epidote-zone metapelites and metabasalts show, in addition, a shallow oscillatory zoning with complimentary variations in Mn and Fe equivalent to 5 mole- % spessartine and almandine. The Fe-for-Mn substitutional zoning, believed to be caused by a diffusion/saturation effect similar to that of the Rayleigh fractional model (Hollister, 1966), appears to have had superimposed on it the effects of parent-rock chemistry and metamorphic grade which control in a complex manner the composition of the cores and the rims of garnets. Garnets from different rock types and metamorphic grade are compositionally distinct. Garnets from lawsonite-zone rocks, irrespective of parent-rock chemistry appear to be spessartine. Garnets from epidote-zone metaigneous rocks and most metasediments are almandine. Garnets from epidote-zone metasediments with bulk-rock compositions which are manganiferous, or have high oxidation ratios, or both, may be spessartine-rich. Garnets from metabasalts are consistently more pyropic in both core and rim compositions than garnets from pelitic metasediments; the pyrope content of cores and rims of garnets from equivalent rock types and mineral assemblages increases with increasing metamorphic grade. Cores of garnets from epidote-zone pelites are richer in grossular than garnets from lower-grade pelites. The reaction which brings almandine garnet into Ouégoa district blueschist assemblages simultaneously with the replacement of lawsonite by epidote involves components of chlorites and sodic amphiboles and can be represented by the following simplified equation: ferroglaucophane+Fe-rich chlorite+lawsonite → glaucophane+Mg-rich chlorite+epidote+almandine.     

An Exotic Eclogite/Blueschist Slice in a Barrovian Style Metamorphic Terrain, Alanya Nappes, Southern Turkey

A 600 m thick, more than 40 km long slice of eclogite faciesrocks, called the Sug?z? Nappe, occurs in the Alanya area, southernTurkey, sandwiched between two other crystalline nappes whichdo not show HP/LT metamorphism. All three nappcs were affectedby a later Barrovian-type metamorphism and penetrative deformationwhich welded the nappes into a single tectonic unit. The SugOzuNappe consists predominantly of garnet-mica schists with lensesof eclogite and blueschist metabasites. The mineral assemblagein the eclogites, garnet+omphacite+glaucophane+paragonite+quartz+phengite+rutile, is estimated to have formed at 13?5?1?5 kb and510?25?C. The eclogite assemblage is variably overprinted bya later Barrovian metamorphism with the development of barroisite,chlorite, and albite. The extent of the Barrovian overprintwas controlled by the supply of fluid to the rocks. The Barrovian metamorphism increases in grade downwards in thestructural sequence; biotite and garnet isograds are mappedin the lowermost Mahmutlar Nappe, made up mostly of metapelitesand metapsammites. The metapelites in the garnet zone consistof garnet+biotite+chlorite+muscovite+albite+oligoclase+quartz+ilmenite;metamorphic conditions are estimated as 6?5?1?0 kb and 469?13?C. The HP/LT rocks of the Sug?z? Nappe underwent a cooling of about100?C during a dry uplift from a depth of about 48 km to 21km where they were intercalated with the other nappes, and affectedby a Barrovian metamorphism caused by the incoming fluids. Thecase of the Alanya Nappcs illustrates that the Barrovian overprintobserved in many eclogites and blueschists may not be due toincreasing temperature during uplift, but simply due to theintroduction of a fluid phase during part of the uplift P-Tpath.     

Manganese haloes surrounding metamorphic stratabound base metal deposits

Primary manganese haloes "survive" medium to high-grade regional metamorphism: stratabound base metal deposits in Namaqualand, N. W. Cape Province (Proterozoic) and in the Eastern Alps (Paleozoic and Mesozoic) reveal an increase in Mn as the ore horizon (and associated BIF) is approached from the hangingwall or the footwall. Electron microprobe analyses show Mn to be preferentially concentrated in garnets; compositions vary considerably and include pure spessartine (40 % MnO). Compositional inhomogeneities (zoning) appear to be confined to garnets in lower grade metamorphic terranes. Iron contents of associated sphalerite vary within meters of the stratigraphic sequence and even within one polished section; sphalerite geobarometry does not appear promising in these environments.     

Zones of Progressive Regional Burial Metamorphism in Part of the Tasman Geosyncline, Eastern Australia

A succession of metamorphic zones can be recognized along theMolong Geanticline, an ancient rise within the Tasman Geosynclineof Eastern Australia. In order of increasing grade, the zonesin the volcanogenic rocks are: (1) an albite-quartz zone, containingneither prehnite, pumpellyite, actinolite, nor biotite; (2)a prehnite zone; (3) a prehnite-pumpellyite zone; (4) an actinolitezone; and (5) a biotite zone. Zeolite associations are not developed.Zones 4 and 5 lie within the greenschist facies. The rocks alongthis geanticline are generally only slightly deformed, lackingcleavage. The grade of this metamorphism within the Molong Geanticlineincreases partly in accordance with increasing stratigraphicdepth and partly increases eastwards towards the adjacent HillEnd Trough. Within the Trough, rocks show metamorphic assemblagesakin to those of Zones 4 and 5 as defined within the MolongGeanticline, but deformation is more extreme and a regionalcleavage is characteristic. In the region about the Hill EndTrough and the adjacent geanticlines there is an apparent variationfrom essentially static burial metamorphism of the geanticlinesthrough transitional types to tectonic dynamothermal metamorphismof the trough. Both metamorphisms are envisaged as having arisenduring the same general period of metamorphism, but deformationof the geanticlines was much less than deformation in the interveningHill End Trough.     

Garnet Compositions at Broken Hill, New South Wales, as Indicators of Metamorphic Processes

Electron microprobe analyses of garnets in the finely bedded‘banded iron formations’ (BIF) of the Willyama Complexat Broken Hill reveal marked compositional changes from onegarnet to the next on a scale of 1–2 mm. Further, systematicanalytical traverses across bedding and along bedding show thecompositions of the garnets to change markedly from one finebed to the next, but to remain extremely uniform within individualbeds. In view of the minuteness of the domains involved it appearsevident that compositional variation cannot be attributed tovariations in metamorphic pressures, temperatures or oxygenfugacities. Neither can they be attributed to variation in garnet-matrixpartition functions, as most of the garnets occur in one simplematrix—quartz. It is concluded that in spite of the high (sillimanite) gradeof the relevant metamorphism, any equilibration of garnet compositions,and hence any associated inter-grain metamorphic diffusion,has been restricted to a scale of less than 1 mm; that garnetcompositions here reflect original rock composition on an ultra-finescale, and have no connotations concerning metamorphic grade;that, hence, the garnets must derive from a single precursormaterial, earlier suggested to be a manganiferous chamositicseptechlorite; and that the between-bed variation: within-beduniformity of garnet composition reflects an original patternof chemical sedimentation—a pattern preserved with theutmost delicacy through a period of 1800 x 10⁶ years and a metamorphicepisode of sillimanite grade.     

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)。     

Sm–Nd garnet dating of polyphase metamorphism: northern Coast Mountains, south-eastern Alaska, USA

Sm–Nd ages of garnet from the northern Coast Mountains of south-eastern Alaska, USA, constrain the timing of thermal events in polyphase metamorphic rocks of the western metamorphic belt and provide new data on the spatial extent of Cretaceous regional metamorphism. Bulk garnet–whole-rock Sm–Nd ages for a sillimanite-zone amphibolite (Taku Inlet) and a biotite-zone metapelite (Tracy Arm) are 77±17 Ma and 59±12 Ma, respectively. Garnet core–whole-rock (80±9 Ma), core–matrix (84±9 Ma), rim–whole-rock (59±4 Ma) and rim–matrix (62±4 Ma) ages were obtained from a sample collected 200  m west of a Palaeocene Coast plutonic–metamorphic complex sill-like pluton that separates medium-grade metamorphic rocks from high-grade metamorphic rocks and voluminous Tertiary plutons in the core of the orogen. The garnet core ages of c. 80 Ma indicate that the regional metamorphic grade reached garnet zone prior to the intrusion of the plutons and high-grade metamorphism of rocks to the east. Similar ages for the younger plutons, the youngest garnets and the rim of a multistage garnet ( c. 59 Ma) indicate a later episode of contact metamorphic garnet growth. Documentation of pre-71 Ma garnet-zone metamorphism along the western edge of the Coast plutonic–metamorphic complex confirms that Albian to Late Cretaceous metamorphism associated with crustal thickening affected this part of the orogen. The similarity of garnet Sm–Nd ages to independent age estimates for metamorphic events confirms that this technique provides useful estimates for the timing of Late Cretaceous to Tertiary thermal events. The c. 20  Myr difference between garnet core and rim ages suggests that the Sm–Nd isotope systematics of a single garnet grain can be used for distinguishing between multiple metamorphic events.     

喜马拉雅结晶岩系中的石榴子石

高喜马拉雅地区广泛分布着一套结晶岩系,它是由前寒武系铁铝榴石-闪岩相的巴罗型区域变质岩系、混合岩和第三纪花岗岩组成。在我国境内聂拉木一带出露宽度达50公里。应思淮(1973)把这套岩系命名为珠穆朗玛群,张旗(1979)把它叫做聂拉木群。对于其中区域变质岩的变质带划分,存在不同的看法。     

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.     

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.     

Petrology of eclogites from north of Shahrekord, Sanandaj-Sirjan Zone, Iran

Summary Metabasic rocks were recently found within a ductile shear zone in the north of Shahrekord, being a part of the structural zone of Sanandaj-Sirjan, SW Iran. The rocks give evidence of a so far unrecognized eclogite facies metamorphic event and testify to high pressure metamorphism in the Sanandaj-Sirjan Zone, near the Main Zagros Reverse Fault, which is the assumed suture zone between the Arabian plate and the Iranian block. The eclogites occur as lenses or blocks within ortho- and paragneisses. The petrographic features and reaction textures display at least two main metamorphic stages: (1) a peak eclogite facies stage, and (2) a subsequent amphibolite facies stage. The eclogite facies metamorphism is indicated by omphacite + garnet + sodic-calcic amphiboles (barroisite, magnesiokatophorite and magnesiotaramite) + phengite + rutile + (clino-)zoisite + quartz ± dolomite. The garnets are mainly almandine-rich, which fits with the C-type eclogite classification. Calcic amphiboles (hornblende, tschermakite and pargasite) + plagioclase are secondary phases formed during the retrograde amphibolite-facies metamorphism. P-T estimates for the eclogite facies give pressures of 21–24 kbar and temperatures of 590–630 °C (geothermometry) and 470–520 °C (THERMOCALC), respectively. Geothermobarometry for the amphibolite-facies metamorphism yields 10–11 kbar and 650–700 °C. Author’s address: Ali Reza Davoudian, Department of Natural Resources, Shahrekord University, Shahrekord, Iran     

Metabasic Dyke Swarms in a High-Grade Metamorphic Terrane——A Case Study in the Taipingzhai-Jinchangyu Area,Eastern Hebei Province

Geological and petrological studies indicate that three phases of metabasic dykes are present in theTaipingzhai-Jinchangyu area within the high-grade metamorphic terrane of eastern Hebei. Garnet andhornblende in metabasic dykes of the second and third phases occur separately, forming two mineral aggregateareas gl+cpx+pl±hy and hb+cpx+pl-hy. P_(H_2O) in the rocks appears to be the main factor controlling theformation of the two aggregate areas. Both were formed simultaneously at the same metamorphic temperature. The second-phase basic dykes underwent metamorphism of pyroxene-granulite facies at a temperature ofsome 825C: later the dykes, together with the third-phase basic dykes experienced metamorphism ofamphibole-granulite facies at a temperature of about 750C under pressure of 0.9GPa.     

Garnet Lu–Hf dating of retrograde fluid activity during ultrahigh-pressure metamorphic eclogites exhumation

Previous studies on the atoll-shaped garnets in ultrahigh-pressure (UHP) metamorphic eclogites from the Dabie orogen, east-central China, suggest a fluid-enhanced overgrowth origin at the onset of exhumation. The atoll-garnets bearing eclogite place better constraints on the timing of the retrograde fluid activity and are a straightforward target to gain insight into the isotopic equilibrium and/or disequilibrium during exhumation. Comprehensive textural, chemical and Lu–Hf geochronological analyses on the atoll garnet-bearing eclogite show that the retrograde fluid activity event likely occurred at ca. 221 Ma. The Lu–Hf age of 221.0?±?2.3 Ma marks the last garnet overgrowth episode during exhumation rather than prograde metamorphism. This somewhat restricted study suggests that dating the prograde-zoning-preserved garnets may bias results towards a particular metamorphic event rather than the prograde timing, as previously thought. The general assumption that larger garnet crystals in metamorphic rocks are older should be made with caution, and it is likely invalid in atoll garnet-bearing metamorphic eclogites because the preliminary garnet cores have been largely consumed. These observations highlight that linking textural and chemical analyses is crucial for interpreting geochronological data.     

苏鲁榴辉岩在俯冲峰期-折返早期的递进塑性变形过程及对超高压变质岩石折返动力学的启示

榴辉岩的变形特征严重影响俯冲板片的流变学行为,而石榴石的变形机制又直接制约着榴辉岩的变形属性。本文在结合前人研究的基础上,通过对中国大陆科学钻探（CCSD）主孔榴辉岩中石榴子石的典型变形特征的详细对比和分析,进一步证明了榴辉岩在俯冲峰期条件下可以发生强烈的塑性变形。且俯冲峰期-折返早期经历了从以恢复作用主导的位错蠕变,到动态重结晶、扩散蠕变,再到静态重结晶和矿物生长的连续递进变形过程。其中,从动态重结晶到高温静态重结晶的转换,结合短时增温、局部熔融等证据,共同证明了在苏鲁超高压变质岩石折返过程中存在一个短暂的低应力“热”区间,在此前后,折返机制和驱动力发生了转变,由浮力作用下的快速折返转为以挤压透入性剪应力作用下的缓慢抬升阶段。     

Distinct metamorphic evolution of alternating silica-saturated and silica-deficient microdomains within garnet in ultrahigh-temperature granulites: An example from Sri Lanka

Here we report the occurrence of garnet porphyroblasts that have overgrown alternating silica-saturated and silica deficient microdomains via different mineral reactions. The samples were collected from ultrahigh-temperature (UHT) metapelites in the Highland Complex, Sri Lanka. In some of the metapelites, garnet crystals have cores formed via a dehydration reaction, which had taken place at silica-saturated microdomains and mantle to rim areas formed via a dehydration reaction at silica-deficient microdomains. In contrast, some other garnets in the same rock cores had formed via a dehydration reaction which occurred at silica-deficient microdomains while mantle to rim areas formed via a dehydration reaction at silica-saturated microdomains. Based on the textural observations, we conclude that the studied garnets have grown across different effective bulk compositional microdomains during the prograde evolution. These microdomains could represent heterogeneous compositional layers (paleobedding/laminations) in the precursor sediments or differentiated crenulation cleavages that existed during prograde metamorphism. UHT metamorphism associated with strong ductile deformation, metamorphic differentiation and crystallization of locally produced melt may have obliterated the evidence for such microdomains in the matrix. The lack of significant compositional zoning in garnet probably due to self-diffusion during UHT metamorphism had left mineral inclusions as the sole evidence for earlier microdomains with contrasting chemistry.