Thermobarometry, Geochronology and the Interpretation of P-T-t Data in the Britt Domain, Ontario Grenville Orogen, Canada

Textural evidence, thermobarometry, and geochronology were usedto constrain the pressure-temperature-time (P—T—t)history of the southern portion of the Britt domain in the CentralGneiss Belt, Ontario Grenville Province. Typical metapeliticassemblages are quartz+plagioclase+ biotite + garnet + kyanite alkali feldspar sillimanite rutile ilmenite staurolite gahnite muscovite. Metatonalitic assemblages have quartz+ plagioclase + garnet biotite + hornblende + rutile + ilmenite.Metagabbroic rocks contain plagioclase + garnet + clinopyroxene+ biotite + ilmenite hornblende rutile quartz. Notabletextural features include overgrowths of sillimanite on kyaniteand of spinel on staurolite. The spinel overgrowths can be modeledby the breakdown of staurolite via the reaction Fe-staurolite= hercynite +kyanite + quartz + H₂O. The decomposition of stauroliteto her-cynite has a steep dP/dT slope and constrains the lateprograde path of a staurolite metapelite. Garnet—Al₂SiO₅—plagioclase—quartz(GASP) barometry applied to metapelitic garnets that preservecalcium zoning reveals a pressure decrease from 11 to 6 kbat an assumed temperature of 700 C. Garnet—plagioclase—ilmenite—rutile—quartzand garnet—clinopyroxene—plagioclase—quartzbarometry is in good agreement with pressures obtained withthe GASP barometer. Geochronologic data from garnet, allanite,and monazite in metapelitic rocks give ages that fall into twogroups, 1–4 Ga and 1.1 Ga, suggesting the presence ofat least two metamorphic events in the area. It is most reasonableto assign the 1.4 Ga age to the high-pressure data and the 1.1Ga age to the lower-pressure data. Collectively the P—T—tdata indicate a complex and protracted history rather than asingle cycle of burial and uplift for this part of the GrenvilleProvince.     

Petrology of Biotite-Cordierite-Garnet Gneiss of the McCullough Range, Nevada I. Evidence for Proterozoic Low-Pressure Fluid-Absent Granulite Grade Metamorphism in the Southern Cordillera

Proterozoic migmatitic paragneisses exposed in the McCulloughRange, southern Nevada, consist of cordierite+almanditic garnet+biotite+sillimanite+plagioclase+K-feldspar+quartz+ilmenite+hercynite.This assemblage is indicative of a low-pressure fades seriesat hornblende-granulite grade. Textures record a single metamorphicevent involving crystallization of cordierite at the expenseof biotite and sillimanite. Thermobarometry utilizing cation exchange between garnet, biotite,cordierite, hercynite, and plagioclase yields a preferred temperaturerange of 590–750?C and a pressure range of 3–4 kb.Equilibrium among biotite, sillimanite, quartz, garnet, andK-feldspar records a_H2O between 0?03 and 0?26. The low a_H2Otogetherwith low f_O2 (QFM) and optical properties of cordierite indicatemetamorphism under fluid-absent conditions. Preserved mineralcompositions are not consistent with equilibrium with a meltphase. Earlier limited partial melting was apparently extensiveenough to cause desiccation of the pelitic assemblage. The relatively low pressures attending high-grade metamorphismof the McCullough Range paragneisses allies this terrane withbiotite-cordierite-garnet granulites in other orogenic belts.aosure pressures and temperatures require a transient apparentthermal gradient ofat least 50?C/km during part of this Proterozoicevent in the southern Cordillera. ^*Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024-1567     

The Formation of Chrysoberyl in Metamorphosed Pegmatites

Mineral assemblages in pegmatite samples from Kolsva, Swedenand Marikov, Czechoslovakia show that chrysoberyl is alwaysaccompanied by quartz, and is a breakdown product of primarypegmatitic beryl. Textures and the mineral-forming process forthe Kolsva pegmatite are explained by the reactions beryl +K-feldspar + H+ = chrysoberyl + quartz + SiO_{2, aq} + K⁺ + H₂Oor alternatively beryl —K—feldspar + H₂O = chrysoberyl+ quartz + melt. Mineral assemblages from mica-rich parts ofthe pegmatite include sillimanite—K—feldspar, muscovite—K—feldspar—sillimanite,and annite—magnetite—spinel—sillimanite—garnet.Details about the composition and the textural relationshipsof these minerals are given; they indicate a post-pegmatiticmetamorphic event at P—T conditions near to the anatecticregime. The samples from Marikov show textures, which are explainedby the reactions beryl + albite + H⁺ = chrysoberyl + quartz+ Na⁺ + H₂O or alternatively beryl + albite + H₂O = chrysoberyl+ quartz + melt. Breakdown of muscovite produces sillimaniteaccording to the reactions beryl + albite + muscovite + H⁺ =chrysoberyl + quartz + sillimanite + Na⁺ + K⁺ + H₂O or alternativelyberyl + albite + muscovite + H₂O = chrysoberyl + quartz + sillimanite+ melt. Similar reaction textures and mineral assemblages were foundin other chrysoberyl-bearing pegmatites (Maroankora, Madagascar;Helsinki, Finland; Haddam, Greenfield, Greenwood, U.S.A.). Hydrothermal experiments located the reaction beryl + alkalifeldspar + H₂O = chrysoberyl + phenakite + melt at P—Tconditions between the K—feldspar—quartz—H₂Osolidus and the K—feldspar—albite—quartz-H₂Osolidus. It is concluded that the formation of Al-rich minerals likechrysoberyl and sillimanite in pegmatites is due to a post-pegmatiticevent at high P—T conditions. The question as to whichof the alternative set of reactions is more likely, the ionicequilibria or the anatectic chrysoberyl formation, must be leftopen. The previous hypothesis of a desilification of a pegmatitewhich intruded into SiO₂-poor country rocks, or of the assimilationof Al₂O₃-rich country rocks, cannot explain the mineral assemblagesof the two pegmatites.     

大陆碰撞过程中的巴罗式变质作用及原地深熔作用:以南阿尔金为例

南阿尔金吐拉地区所出露的变质泥质岩和变质基性岩普遍经历了中压麻粒岩相变质作用,其中变泥质岩以出现石榴子石+夕线石+长石+黑云母+石英为特征,而基性麻粒岩则以石榴子石+单斜辉石+紫苏辉石+斜长石+石英为特征,具有典型中压相系的麻粒岩相变质作用矿物组合,即显示"巴罗式"变质作用特征。野外宏观特征显示这套变泥质岩普遍经历了原地深熔作用,并局部发生混合岩化作用。岩相学观察结果显示泥质片麻岩保留了关键的深熔作用显微结构证据:(1)石榴子石内部发育有钾长石、石英和斜长石组成的矿物集合体,可能代表了早期熔体的假象;(2)黑云母颗粒边界发育尖锐的、不规则的微斜长石,而且黑云母边界溶蚀明显,形成锯齿状不规则的边界,指示深熔作用可能与黑云母的分解密切相关,即黑云母可能为深熔作用的主要反应相;(3)石英、斜长石或石榴子石颗粒边界发育圆珠状不规则的钾长石,而且颗粒边界或三联点中尖锐状钾长石与周围矿物的形成较小的二面角,有些甚至相互连通呈网络状,这也与它们继承了熔体结构特征一致;(4)不规则钾长石(或微斜长石)分布在石榴子石和夕线石附近,指示石榴子石和夕线石可能为深熔作用的残留相。锆石U-Pb定年结果显示麻粒岩相变质作用和相关深熔作用时代基本一致,主要发生在~450Ma。因此,吐拉地区的中压麻粒岩相变质作用和深熔作用明显要晚于南阿尔金地区榴辉岩和高压麻粒岩的峰期变质时代40~50Myr,而是与榴辉岩折返过程中麻粒岩相叠加变质作用的时代较为接近。但南阿尔金~450Ma的变质作用、深熔作用和岩浆作用是否为独立的构造热事件抑或深俯冲板片折返阶段的产物,这还需要今后进一步的工作验证。     

Petrology of Granulites from Anakapalle--Evidence for Proterozoic Decompression in the Eastern Ghats, India

The granulite complex at Anakapalle, which was metamorphosedat 1000 Ma, comprises orthopyroxene granulites, leptynite, khondalite,mafic granulites, calc-silicate rock, spinel granulites, andtwo types of sapphirine granulites—one quartz-bearingand migmatitic and the other devoid of quartz and massive. Reactiontextures in conjunction with mineral-chemical data suggest severalcontinuous and discontinuous equilibria in these rocks. In orthopyroxenegranulites, dehydration-melting of biotite in the presence ofquartz occurred according to the reaction biotite+quartz= garnet (Py₃₇)+K-feldspar+orthopyroxene + liquid. Later, this garnet broke down by the reaction garnet (Py₃₇)+quartz= orthopyroxene + plagioclase. Subsequently, coronal garnet (Py₃₀) and quartz were producedby the same reaction but proceeding in the opposite direction.In spinel granulites, garnet (Py₄₂) and sillimanite were producedby the breakdown of spinel in the presence of quartz. In thetwo types of sapphirine granulites, garnet with variable pyropecontent broke down according to the reaction garnet = sapphirine + sillimanite + orthopyroxene. The highest pyrope content (59 mol %) was noted in garnets fromquartz-free sapphirine granulites compared with the quartz-bearingone (53 mol % pyrope). The calculated positions of the mineralreactions and diserete P-T points obtained by thermobarometrydefine a retrograde P-T trajectory during which a steep decompressionof 1.5 kbar from P-T_max of 8 kbar and 900C was followed bynear-isobaric cooling of 300C. During this decompression, garnetwith variable pyrope contents in different rocks broke downon intersection with various divariant equilibria. Near-isobariccooling resulted in the formation of coronal garnet around second-generationorthopyroxene and plagioclase replacing earlier porphyroblasticgarnet in orthopyroxene granulites. It has been argued thatthe deduced P-T trajectory originated in an extensional regimeinvolving either a crust of near-normal thickness of a slightlyoverthickened crust owing to magmatic underaccretion.     

Regression Modelling of Metamorphic Reactions in Metapelites, Snow Peak, Northern Idaho

The second of two periods of regional metamorphism that affectedpelitic rocks near Snow Peak caused complete re-equilibrationof mineral assemblages and resulted in a consistent set of metamorphicisograds. Metamorphic chlorite and biotite occur in the lowestgrade rocks. With increasing grade, garnet, staurolite, andkyanite join the assemblage, resulting in a transition zonecontaining all the above phases. At higher grade, chlorite,and finally staurolite disappear. Mass balance relations at isograds and among minerals of low-varianceassemblages have been modelled by a non-linear least-squaresregression technique. The progressive sequence can be describedin terms of schematic T-X_H2O relations among chlorite, biotite,garnet, staurolite, and kyanite at P_total above the KFMASH invariantpoint involving those phases. The first appearance of garnetwas the result of an Fe-Mg-Mn continuous reaction. As temperaturerose, the garnet zone assemblage encountered the stauroliteisograd reaction, approximated by the model reaction: 3?0 chlorite + 1?5 garnet + 3?3 muscovite + 05 ilmenite = 1?0staurolite + 3?1 biotite + 1?5 plagioclase + 3?3 quartz + 10?3H₂O. The staurolite zone corresponds to buffering along this reactionto the intersection where chlorite, biotite, garnet, staurolite,and kyanite coexist. The transition zone assemblage formed byreaction at this T–X _H2O intersection which migrates towardmore H2O-rich fluid composition with progressive reaction. Thenet reaction at the intersection is approximated by the transitionzone reaction: 1?0 chlorite +1?1 muscovite + 0?2 ilmenite = 2?7 kyanite + 1?0biotite + 0?4 albite + 4?2 H₂O. Chlorite was commonly the first phase to have been exhaustedand the remaining assemblage was buffered along a staurolite-outreaction, represented by the model reaction: 1?0 staurolite + 3?4 quartz + 0?4 anorthite + 1?4 garnet + 0?1ilmenite + 7?9 kyanite + 2?0 H₂O. Consumption of staurolite by this reaction resulted in the highestgrade assemblage, which contains kyanite, garnet, biotite, muscovite,quartz, plagioclase, ilmenite, and graphite.     

The role of synmetamorphic igneous rocks in the metamorphism and partial melting of metasediments,Northwest Adirondacks

Field and petrologic studies along the Adirondack Lowlands — Highlands boundary near Harrisville, NY, indicate that heat from the synmetamorphic intrusion of the Diana syenite complex (intrusion temperature of 1,050° C) played a major role in the local metamorphic thermal regime and was responsible for extensive partial melting of adjacent metasedimentary units (Major Paragneiss of Engel and Engel). Metamorphic temperatures inferred from two — feldspar and spinel — quartz assemblages decrease from 850–950° C along the Diana — metasediment contact to 650–700° C, 2–3 km away from the contact. Metamorphic pressures are 7±0.5 kb as determined from coexisting plagioclase — garnet — sillimanite — quartz, kyanite — sillimanite, and garnet — rutile — ilmenite — sillimanite — quartz (GRAIL). In the paragneiss, migmatites consisting of quartz — microcline perthite — sodic plagioclase leucosomes are generally concordant with the melanosome consisting of biotite — sillimanite — garnet — spinel — plagioclase ±corundum±cordierite. Qualitatively the amount of partial melt and occurrences of corundum-bearing assemblages decrease away from the Diana contact. Activity of H₂O inferred from coexisting biotite — sillimanite — quartz — garnet — K-feldspar ranges from 0.01 to 0.17 and is five to ten times lower in corundum-bearing rocks.Melting proceeded via vapor-absent reactions involving biotite in response to localized heating by synmetamorphic intrusion of magma. This unusually preserved, synmetamorphic contact aureole in a regional granulite terrane supports the concept that granulites owe their origin to magma intrusion and/or the ponding of magmas at the base of the crust.     

Genetic Mechanism of Mineral Inclusions in Zircons from the Khondalite Series, Southeastern Inner Mongolia

The early Precambrian khondalite series is widely distributed in the Jining-Zhuozi-Fengzhen-Liangcheng area, southeastern Inner Mongolia. The khondalite series mainly consists of sillimanite garnet potash feldspar (or two-feldspar) gneiss and garnet biotite plagioclase gneiss. These gneissic rocks have commonly experienced granulite-facies metamorphism. In zircons separated from sillimanite garnet potash feldspar gneisses, many mineral inclusions, including Sil, Grt, Ky, Kfs, Qtz and Ap, have been identified by the Laser Raman spectroscopy. Generally, prograde metamorphic mineral inclusion assemblages such as Ky + Kfs + Qtz + Ap and Ky + Grt + Kfs + Qtz are preserved in the core of zircon, while peak granulite-facies metamorphic minerals including Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap are identified in the mantle and rim of the same zircon. However, in some zircons are only preserved the peak metamorphic minerals such as Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap from core to ri     

Oxygen Fugacity and Water Activity during Thermal Peak and Retrogression of Granulites around the Sarvapuram Area,Karimnagar Granulite Terrane,Andhra Pradesh,India

The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity(XH2 O) conditions obtained with the win TWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log f O2 values ranging from-11.38 to-14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks.     

浙西南遂昌—大柘地区八都岩群印支期变质变形序列

浙西南遂昌-大柘地区八都岩群在印支期变质事件影响下发生变质变形,通过详细野外调查和岩相学研究,可将其划分为3期变质变形序列：S₁变形期,NW向片麻理记录的残留紧闭褶皱,共生矿物组合为石榴子石变斑晶及其内部定向分布的包裹体矿物,石榴子石+黑云母+石英（泥质）和石榴子石+角闪石+斜长石+石英（长英质）;S₂变形期,区域性宽缓褶皱及NE向缓倾透入性片麻理,共生矿物组合为石榴子石变斑晶及定向分布的基质矿物,矽线石+石榴子石+黑云母+石英+斜长石±钾长石（泥质）和石榴子石+钾长石+斜长石+黑云母+石英（长英质）;S₃变形期,NE向陡倾透入性片麻理及韧脆性断裂大部分被花岗斑岩脉填充,共生矿物组合为石榴子石变斑晶及其周围退变矿物,石榴子石+矽线石+堇青石+斜长石+黑云母+石英±钾长石（泥质）和角闪石+斜长石+黑云母+钛铁矿（长英质）。结合前人研究成果,八都岩群印支期变质事件峰期变质程度达到麻粒岩相,显示顺时针近等温降压（ITD）型的p-T演化轨迹,S₁-S₃变质变形反映出从俯冲碰撞到快速折返冷却的演化过程,伴随S₃同期侵位的花岗斑岩锆石U-Pb定年结果,将该演化过程完成时间约束在229.7 Ma,可能是浙西南地区对印支期古特提斯洋域内印支-华南-华北板块之间俯冲-碰撞过程的响应。     

The Garnet+Hornblende Isograd in Calcic Schists from an Andalusite-Type Regional Metamorphic Terrain, Panamint Mountains, California

Calcic schists in the andalusite-type regional metamorphic terrainin the Panamint Mountains, California, contain the low-varianceassemblage quartz+epidote+muscovite+biotite+calcic amphibole+chlorite+plagioclase+spheneat low grade. Near the sillimanite isograd, chlorite in thisassemblage is replaced by garnet. The low variance in many calcicschists allows the determination of the nature of the reactionthat resulted in the coexistence of garnet+hornblende. A graphicalanalysis of the mineral assemblages shows that the reactioncan not be of the form biotite+epidote+chlorite+plagioclase+quartz=garnet+hornblende+muscovite+sphene+H₂Obecause garnet+chlorite never coexisted during metamorphismand the chlorite-bearing and garnet-bearing phase volumes donot overlap. The compositions of the minerals show that withincreasing grade amphibole changed from actinolite to pargasitichornblende with no apparent miscibility gap, the partitioningof Fe and Mg between chlorite and hornblende changed from K_D(Mg/Fe, chl&amp) < 1 to K_D > 1, the partitioning betweenbiotite and hornblende changed from K_D (Mg/Fe, bio/amp) <1 in chlorite-zone samples to K_D > 1 in garnet + hornblende-zonesamples, and the transition to the garnet-bearing assemblageoccurred when the composition of plagioclase was between An₅₅and An₈₀. Both the graphical analysis and an analytical analysisof the compositions of the minerals using simplified componentsderived from the natural mineral compositions indicate thatat the garnet+hornblende isograd the composition of hornblendewas colinear with that of plagioclase and biotite, as projectedfrom quartz, epidote, muscovite, and H₂O. During progressivemetamorphism, chlorite+biotite+epidote+quartz continuously brokedown to form hornblende+muscovite+sphene until the degeneracywas reached. At that point, tie lines from hornblende couldextend to garnet without allowing garnet to coexist with chlorite.Thus, the garnet+hornblende isograd was established throughcontinuous reactions within the chlorite-grade assemblage ratherthan through a discontinuous reaction. In this type of isograd,the low-grade diagnostic assemblage occurs only in Mg-rich rocks;whereas the high-grade assemblage occurs only in Fe-rich rocks.This relation accounts for the restricted occurrence of garnet+hornblendeassemblage in low-pressure terrains. In Barrovian terrains,garnet+chlorite commonly occurs, and the first appearana ofgarnet+hornblende can simply result from the continuous shiftof the garnet+chlorite tie line to Mg-rich compositions.     

Origin and evolution of a peraluminous silicic ignimbrite suite: The Violet Town Volcanics

The Violet Town Volcanics are a 373 Ma old, comagmatic, S-type volcanic sequence mainly comprising crystal-rich intracaldera ignimbrites. Rock types vary from rhyolites to rhyodacites, all containing magmatic cordierite and garnet phenocrysts. Variation in the suite is primarily due to fractionation of early-crystallized quartz, plagioclase and biotite (plus minor accessory phases) in a high-level magma chamber prior to eruption. Early magmatic crystallization occurred at around 4 kb and 850° C with melt water contents between 2.8 and 4 wt.%. This high-temperature, markedly water-undersaturated, restite-poor, granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain, leaving a granulite facies residue.Table of Less Common Abbreviations Used Pkb pressure in kilobars - T° C temperature in degrees Celsius - mole fraction of water in the fluid - aH₂O activity of water - Bi biotite - Cd cordierite - Gt garnet - Py pyrope - Gr grossular - Alm almandine - Sp spessartine - He hercynite - Ilm ilmenite - Kfs potassium feldspar - Opx orthopyroxene - Pl plagioclase - An anorthite - Q quartz - Sill sillimanite - Ap apatite     

Metamorphism and Structure of Penfold Creek Area, near Quesnel Lake, British Columbia

The Quesnel Lake area lies within the Omineca Crystalline Belt,and is underlain by the northern extremity of the Shuswap MetamorphicComplex. Closely spaced and steeply dipping isograds mark themargins of the metamorphic belt. In the Penfold Creek area onlyone and one-half miles separate the biotite and sillimaniteisograds. Related to this sharp increase in metamorphic gradethere is a marked change in the fold style. In the chloritezone are similar folds, showing a strong axial-plane cleavage,and tight refolded isoclines dominate in the sillimanite zone.Three periods of deformation and two periods of prograde metamorphismhave been recognized, with the first metamorphic period beingassociated with Phase 2 deformation and the second being post-Phase2. Regression analysis of possible mineral reactions in these rocksshows that they formed under conditions of a gradient in temperatureand possibly H₂O activity, and that most of the assemblages,although containing many phases, have at least two degrees offreedom. Thermodynamic analysis of mineral equilibria indicatesthat equilibrium was closely approached between solid solutionsinvolving garnet, plagioclase, biotite, white mica, and staurolitein the presence of Al₂SiO₅, quartz, and H₂O. The conditionsof metamorphism of rocks containing Al₂SiO₅ are estimated tobe 7000 ± 1500 bars, 680 ± 30 °C, activityof H₂O = 0.80 (approx.). Partial melting did not occur becauseof the reduced activity of H₂O. ^*Present address: Overseas Division, Institute of Geological Sciences, 5 Princes Gate, London SW7, England.     

Melting of Biotite + Plagioclase + Quartz Gneisses: the Role of H2O in the Stability of Amphibole

Biotite + plagioclase + quartz (BPQ) is a common assemblagein gneisses, metasediments and metamorphosed granitic to granodioriticintrusions. Melting experiments on an assemblage consistingof 24 vol. % quartz, 25 vol. % biotite (XMg = 0·38–0·40),42 vol. % plagioclase (An_26–29), 9 vol. % alkali feldsparand minor apatite, titanite and epidote were conducted at 10,15 and 20 kbar between 800 and 900°C under fluid-absentconditions and with small amounts (2 and 4 wt %) of water addedto the system. At 10 kbar when 4 wt % of water was added tothe system the biotite melting reaction occurred below 800°Cand produced garnet + amphibole + melt. At 15 kbar the meltingreaction produced garnet + amphibole + melt with 2 wt % addedwater. At 20 kbar the amphibole occurred only at high temperature(900°C) and with 4 wt % added water. In this last case themelting reaction produced amphibole + clinopyroxene ±garnet + melt. Under fluid-absent conditions the melting reactionproduced garnet + plagioclase II + melt and left behind a plagioclaseI ± quartz residuum, with an increase in the modal amountof garnet with increasing pressure. The results show that itis not possible to generate hornblende in such compositionswithout the addition of at least 2–4 wt % H₂O. This reflectsthe fact that conditions of low aH₂O may prevent hornblendefrom being produced with peraluminous granitic liquids fromthe melting of biotite gneiss. Thus growth of hornblende inanatectic BPQ gneisses is an indication of addition of externalH₂O-rich fluids during the partial melting event. KEY WORDS: biotite; dehydration; gneisses; hornblende; melt     

Reaction Textures in a Suite of Spinel Granulites from the Eastern Ghats Belt, India: Evidence for Polymetamorphism, a Partial Petrogenetic Grid in the System KFMASH and the Roles of ZnO and Fe2O3

Spinel granulites, with or without sapphirine, occur as lensesin garnetiferous quartzofeldspathic gneisses (leptynites) nearGokavaram in the Eastern Ghats Belt, India. Spinel granulitesare mineralogically heterogeneous and six mineral associationsoccur in closely spaced domains. These are (I) spinel–quartz–cordierite,(II) spinel–quartz–cordierite–garnet–orthopyroxene–sillimanite,(III) spinel–cordierite–orthopyroxene–sillimanite,(IV) spinel–quartz–sapphirine–sillimanite–garnet,(V) spinel–quartz-sapphirine–garnet and (IV) rhombohedral(Fe–Ti) oxide–cordierite–orthopyroxene–sillimanite.Common to all the associations are a porphyroblastic garnet(containing an internal schistosify defined by biotite, sillimaniteand quartz), perthite and plagioclase. Spinel contains variableamounts of exsolved magnetite and is distinctly Zn rich in thesapphirine-absent associations. X_Mg in the coexisting phasesdecreases in the order cordierite–biotite–sapphirine–orthopyroxene–spinel–garnet–(Fe–Ti)oxides. Textural criteria and compositional characteristicsof the phases document several retrograde mineral reactionswhich occurred subsequent to prograde dehydration melting reactionsinvolving biotite, sillimanite, quartz, plagioclase and spinel.The following retrograde mineral reactions are deduced: (1)spinel + quartz cordierite, (2) spinel + quartz garnet + sillimanite,(3) garnet + quartz cordierite + orthopyroxene, (4) garnet+ quartz + sillimanite cordierite, (5) spinel + cordierite orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz sapphirine, (7) spinel + sapphirine + quartz garnet + sillimanite,and (8) spinel + quartz sapphirine + garnet. A partial petrogeneticgrid for the system FeO–MgO–Al₂O₃–SiO₂–K₂O–H₂Oat high fo₂, has been constructed and the effects of ZnO andFe₂O₃ on this grid have been explored Combining available experimentaland natural occurrence data, the high fo₂ invariant points inthe partial grid have been located in P–T space. Geothermobarometricdata and consideration of the deduced mineral reactions in thepetrogenetic grid show that the spinel granulites evolved throughan anticlockwise P–T trajectory reaching peak metamorphicconditions >9 kbar and 950C, followed by near-isobaric cooling(dT/dP = 150C/kbar). This was superimposed by an event of near-isothermaldecompression (dT/dP = 15C/kbar). The studied spinel granulites,therefore, preserve relic prograde mineral associations andreaction textures despite being metamorphosed at very high temperatures,and bear evidence of polymetamorphism. KEY WORDS: spinel granulite; Eastern Ghats; India; polymetamorphism; geothermometry; geobarometry Corresponding author     

The interpretation of reaction textures in Fe‐rich metapelitic granulites of the Musgrave Block,central Australia: constraints from mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3

Fe‐rich metapelitic granulites of the Musgrave Block, central Australia, contain several symplectic and coronal reaction textures that post‐date a peak S2 metamorphic assemblage involving garnet, sillimanite, spinel, ilmenite, K‐feldspar and quartz. The earliest reaction textures involve spinel‐ and quartz‐bearing symplectites that enclose garnet and to a lesser extent sillimanite. The symplectic spinel and quartz are in places separated by later garnet and/or sillimanite coronas. The metamorphic effects of a later, D3, event are restricted to zones of moderate to high strain where a metamorphic assemblage of garnet, sillimanite, K‐feldspar, magnetite, ilmenite, quartz and biotite is preserved. Quantitative mineral equilibria calculations in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (KFMASHTO) using Thermocalc 3.0 and the accompanying internally consistent dataset provide important constraints on the influence of TiO₂ and Fe₂O₃ on biotite‐bearing and spinel‐bearing equilibria, respectively. Biotite‐bearing equilibria are shifted to higher temperatures and spinel‐bearing equilibria to higher pressures and lower temperatures in comparison to the equivalent equilibria in K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH). The sequence of reaction textures involving spinel is consistent with a D2 P–T path that involved a small amount of decompression followed predominantly by cooling within a single mineral assemblage stability field. Thus, the reaction textures reflect changes in modal proportions within an equilibrium assemblage rather than the crossing of a univariant reaction. The D3 metamorphic assemblage is consistent with lower temperatures than those inferred for D2.     

Granulite-Facies Metamorphism at Molodezhnaya Station, East Antarctica

Granulite-facies quartzofeldpathic gneisses metamorphosed 1000m.y. ago are exposed around Molodezhnaya Station (67°40'S,46°E) in East Antarctica. In addition to quartz, K-feldspar,and plagioclase, the fourteen samples studied in detail consistof the assemblages biotite-orthopyroxene-magnetite, biotite-garnet-orthopyroxene-ilmenite±magnetite, biotite-garnet ± ilmenite ± magnetite,biotite-garnet-sillimanite-ilmenite ± rutile, and biotite-garnet-cordierite-ilmenite-(sillimanite-rutile).Garnets are pyrope-almandine (13 to 34 mol per cent pyrope).Biotite (X_Fe = 0.33 to 0.57) is rich in TiO₂ (4 to 6.3 wt percent) and its Al₂O₃ content depends on the mineral assemblage.Orthopyroxene (X_Fe = 0.45 to 0.60) contains 1.5 to 3.0 weightper cent Al₂O₃. By and large, the minerals are chemically homogeneousand compositional variations are systematic, which indicatecrystallization under equilibrium conditions. On the basis ofthe compositions of coexisting garnet-biotite, garnet-cordierite,garnet-plagioclase (with sillimanite), and garnet-plagioclase-orthopyroxene,temperatures and pressures during the granulite-facies metamorphismare estimated to be 700°C ± 30°C and 5.5 ±1 kb. Water pressure apparently was significantly less thantotal pressure. Alteration during events following the granulite-facies metamorphismhas resulted in chemical zoning in garnet, in which grain edgesare more iron-rich than cores, heterogeneous biotite compositions,and anomalous trends involving MnO. Temperatures based on biotiteand garnet-edge compositions range from 410 to 580°C. Differences in the chemical potential (µ) of water andoxygen in the fluid phase can explain compositional variationsamong the three sillimanite-bearing samples and the relativelyiron-rich compositions of garnet and biotite associated withcordierite. Apparently, the water released by the formationof cordierite remained in the rock, forcing µH₂O to increaseas cordierite formed. Buffering of fluid phase composition bythe mineral assemblage suggests that water was not removed fromthe Molodezhnaya rocks by flushing with CO₂-rich fluids duringmetamorphism, a hypothesis evoked to explain ‘dry’mineral assemblages in other granulite-facies terrains.     

Biotite and garnet compositional variation and mineral equilibria in Grenville gneisses of the Otter Lake area, Quebec

Garnet-biotite gneisses, some of which contain sillimanite or hornblende, are widespread within the Otter Lake terrain, a portion of the Grenville Province of the Canadian Shield. The metamorphic grade is upper amphibolite to, locally, lower granulite facies. The atomic ratio Fe²⁺/(Fe²⁺+ Fe³⁺) in biotite ranges from 0.79 to 0.89 (ferrous iron determinations in 10 highly pure separates), with a mean of 0.86. Mg and Fe²⁺ atoms occupy 67–78% of the octahedral sites, the remainder are occupied by Fe³⁺, Ti, and Al, and some are vacant. Mg/(Mg + Fe²⁺), denoted X, in the analysed samples ranges from 0.32 to 0.65. Garnet contains 1–24% grossular, 1–12% spessartine and X ranges from 0.07 to 0.34. Compositional variation in biotite and garnet is examined in relation to three mineral equilibria: (I) biotite + sillimanite + quartz = garnet + K-feldspar + H₂O; (II) pyrope + annite = almandine + phlogopite; (III) anorthite = grossular + sillimanite + quartz. Measurements of X (biotite) and X (garnet) are used to construct an illustrative model for equilibrium (I) which relates the observed variation in X to a temperature range of 70°C or a range in H₂O activity of 0.6; the latter interpretation is preferred. In sillimanite-free gneisses, the distribution of Mg and Fe²⁺ between garnet (low in Ca and Mn) and biotite is adequately described by a distribution coefficient (KD) of 4.1 (equilibrium II). The observed increase in the distribution coefficient with increasing Ca in garnet is ln K_D= 1.3 + 2.5 × 10^?2 [Ca] where [Ca] = 100 Ca/(Mg + Fe²⁺+ Mn + Ca). The distribution coefficient is apparently unaffected by the presence of up to 12% spessartine in garnet. In several specimens of garnet-sillimanite-plagioclase gneiss, the Ca contents of garnet and of plagioclase increase in unison, as required by equilibrium (III). The mean pressure calculated from these data (n= 17) is 5.9 kbar, and the 95% confidence limits are ±0.5 kbar.     

Type-locality granulites: high-pressure rocks formed at eclogite-facies conditions

Summary The type-locality granulites from the Granulitgebirge of Saxony, Germany, are rocks of broadly granitic composition containing minor garnet and kyanite within a commonly mylonitised matrix of feldspars and quartz. Petrographic evidence indicates a primary assemblage of ternary feldspar + quartz + garnet + kyanite + rutile, most likely resulting from partial melting of a granitic protolith, for which equilibrium temperature and pressure conditions of >1000 °C and >1.5 GPa have been deduced. These extreme (for crustal rocks) conditions, and the inferred peak assemblage, are supported by the newly-developed Zr-in-rutile geothermometer and experimental studies on the same bulk composition, respectively. As these conditions lie above those required for plagioclase stability in quartz tholeiites, they are thus in the eclogite facies. Widespread modification of the peak assemblage, for example mesoperthite formation after ternary feldspar, deformation-induced recrystallisation of perthites to two-feldspar + quartz aggregates, biotite replacing garnet, Ca-loss at garnet rims, sillimanite replacing kyanite or secondary garnet growth, makes reliable interpretation of equilibrium assemblages and compositions very difficult and explains the spread of published pressure-temperature values and consequent confusion about formation depths and the validity of tectonometamorphic models. Such extreme metamorphic conditions in rock compositions typical for the upper continental crust, reflecting a hot subduction environment, has important consequences for understanding some collisional orogens.     

Pressure-temperature Evolution of the Metapelites in the Motuo Area,the Eastern Himalayan Syntaxis

The Motuo area is located in the east of the Eastern Himalayan Syntaxis. There outcrops a sequence of high-grade metamorphic rocks, such as metapelites. Petrology and mineralogy data suggest that these rocks have experienced three stages of metamorphism. The prograde metamorphic mineral assemblages(M1) are mineral inclusions(biotite + plagioclase + quartz ± sillimanite ± Fe-Ti oxides) preserved in garnet porphyroblasts, and the peak metamorphic assemblages(M2) are represented by garnet with the lowest XSps values and the lowest XFe# ratios and the matrix minerals(plagioclase + quartz ± Kfeldspar + biotite + muscovite + kyanite ± sillimanite), whereas the retrograde assemblages(M3) are composed of biotite + plagioclase + quartz symplectites rimming the garnet porphyroblasts. Thermobarometric computation shows that the metamorphic conditions are 562–714°C at 7.3–7.4 kbar for the M1 stage, 661–800°C at 9.4–11.6 kbar for the M2 stage, and 579–713°C at 5.5–6.6 kbar for the M3 stage. These rocks are deciphered to have undergone metamorphism characterized by clockwise P-T paths involving nearly isothermal decompression(ITD) segments, which is inferred to be related to the collision of the India and Eurasia plates.