Garnet-bearing granite from the Třebíč pluton, Bohemian Massif (Czech Republic)

Summary Garnet occurs as a significant mineral constituent of felsic garnet-biotite granite in the southern edge of the Třebíč pluton. Two textural groups of garnet were recognized on the basis of their shape and relationship to biotite. Group I garnets are 1.5–2.5 mm, euhedral grains which have no reaction relationship with biotite. They are zoned having high X_Mn at the rims and are considered as magmatic. Group II garnets form grain aggregates up to 2.5 cm in size, with anhedral shape of individual grains. The individual garnet II grains are usually rimmed by biotite and have no compositional zoning. The core of group I garnets and group II garnets contains 67–80 mol% of almandine, 5–19 mol% of pyrope, 7–17 mol% of spessartine and 2–4 mol% of grossular. Biotite occurs in two generations; both are magnesian siderophyllites with Fe/(Fe + Mg) = 0.50–0.69. The matrix biotite in granites (biotite I) has high Ti content (0.09–0.31 apfu) and Fe/(Fe + Mg) ratio between 0.50 and 0.59. Biotite II forms reaction rims around garnet, is poor in Ti (0.00–0.06 apfu) and has a Fe/(Fe + Mg) ratio between 0.61 and 0.69. The textural relationship between biotite and garnet indicates that garnet reacted with granitic melt to form Ti-poor biotite and a new granitic melt, depleted in Ti and Mg and enriched in Fe and Al. In contrast to the host durbachites (hornblende-biotite melagranites), which originated by mixing of crustal melts and upper mantle melts, the origin of garnet-bearing granites is related to partial melting of the aluminium-rich metamorphic series of the Moldanubian Zone.     

Coexisting garnets and biotites from Precambrian gneisses of the south coast of Western Australia

Garnet-biotite (-cordierite) phase relations in high-grade gneisses of the south coast of Western Australia reflect at least two metamorphic episodes. Chemical uniformity of the interiors of garnet and cordierite grains suggest thorough equilibration during a major phase of metamorphism. Narrow Mg-depleted rims on garnet grain boundaries in contact with biotite or cordierite, and complementary Mg-enriched rims on contiguous cordierites are the result of subsequent retrograde re-equilibration. The absence of reaction zoning in biotites suggests more complete retrograde modification of this mineral.Comparison between granulite and amphibolite facies garnet-biotite pairs shows that Mn contents of both minerals are higher, and Ti contents of the biotites are lower, in the lower-grade rocks. These differences, although not entirely unrelated to grade, are more directly controlled by variations in host rock chemistry and modal amounts of garnet and biotite.Partitioning of Mg, Fe²⁺ and Mn between garnet and biotite is fairly uniform, with no clear differences between granulite and amphibolite facies pairs. Application of the Mg-Fe²⁺ distributions to the geothermometers devised by Perchuk, Thompson, and Goldman & Albee yields variable T estimates of 600–680°C, 580–780°C, and 475–715°C respectively, for the main metamorphism. These estimates are low compared with the T indicated for the granulite facies rocks by other evidence (i.e. > 750°C at 5 kb P_T). The Mg-Fe²⁺ distributions between contiguous garnet-biotite rims suggest that retrograde re-equilibration occurred at least 20–140°C below the T of the main metamorphism.     

Symplectite breakdown of Ca-rich almandines in upper amphibolite-facies Skagit Gneiss,North Cascades,Washington

In upper amphibolite-facies Skagit Gneiss, certain rocks, usually carrying hornblende, display post-kinematic breakdown of almandine-rich garnets to symplectite consisting of plagioclase plus biotite and/or hornblende. Other almandine-rich garnets, widespread in hornblende-free schists and gneisses, remained stable to the end of the metamorphic cycle. Analysed garnets with symplectite coronas have 14 to 30 mol.% grandite. A set of garnets free of such coronas have 6 to 9% grandite and contain relatively more pyrope.In all symplectite-bearing rocks selected for mineral analyses, symplectite plagioclase is more calcic than the earlier main-fabric plagioclase. Fe and Mn are higher, and Mg and Ti lower, in symplectite than in main-fabric biotite and hornblende, except where late re-equilibration has been locally accomplished. Main-fabric biotite, hornblende, and cummingtonite partly preserve a record of original element partitioning between these phases and garnet, indicating that equilibrium was approached during the essentially syn-kinematic main stage of the metamorphism. Between the main-stage phases and their post-kinematic symplectite counterparts, equilibrium has rarely been attained, despite high T and presence of H₂O. Instead, there are sequences of arrested chemical exchanges and of highly incomplete attainments of successive equilibria. The principles revealed by this study are thought to be more broadly applicable to questions of equilibrium vs. disequilibrium in regional-metamorphic sequences.Combined with petrographic-petrologic data, the mineral analyses permit one to calculate model equations that quantitatively describe symplectite-forming reactions, including the amounts of materials added to and removed from garnets transformed into symplectites. Na, K, minor Ba, H₂O, and part of the Ti present were added; major portions of the garnets' Fe and of their subordinate Mn were removed. Transfers of Mg, Al, and Si during symplectite formation were minor, if any. Source and disposal of added and removed substances are discussed. The mass balance of symplectite formation ranges from significant losses in all hornblende-bearing reactions examined to minor gains in some only-biotite-bearing reactions. V is positive for all reactions examined and ranges from a few per cent for subordinate only-hornblende-bearing symplectites to somewhat over 50% for certain only-biotite-bearing symplectites; intermediate values are obtained for symplectites carrying both hornblende and biotite.     

高温变泥质岩石中石榴石-黑云母地质温度计的应用——以胶北荆山群富铝岩石为例

在对胶北荆山群麻粒岩相富铝岩石中石榴石、黑云母的成分环带进行深入研究基础上,选取不同粒径、与不同矿物相邻的石榴石、黑云母各微区点成分,利用石榴石-黑云母温度计分别进行了温度估算。确定在黑云母含量较高的岩石(V_(Grt)/V_(Bt)≤1)中,利用大颗粒石榴石(d≥1500μm)晶体核部(或靠近长英质矿物一侧的晶体幔部)成分与基质中远离石榴石等镁铁矿物处于长英质矿物之间的黑云母核部成分配合。通过石榴石-黑云母温度计可以获得相当可信的变质峰期温度。但是对于黑云母含量极低的岩石(V_(Grt)/V_(Bt)≥6),由于黑云母的成分普遍遭到了强烈改造。使得温度估算结果异常偏低,因此不适合采用石榴石-黑云母温度计估算峰期温度。同一岩石中,采用不同的相邻石榴石-黑云母矿物对晶体边缘成分获得的温度值差异较大,反映它们在峰期后发生Fe-Mg交换反应并达到封闭温度平衡状态的程度不同,因此利用石榴石-黑云母温度计难以获得准确的封闭温度。通过热力学计算,建立了一个新的石榴石-黑云母温度计公式。确定胶北荆山群所经历的变质峰期温度为720～770℃,峰期后最低相对封闭温度为480～500℃。     

Eo-Alpine Metamorphism in the Uppermost Unit of the Cretan Nappe System — Petrology and Geochronology

The uppermost unit of the Cretan nappe system contains a variegated series of high-grade metamorphic rocks. In the Léndas area, amphibolites are present characterized by the assemblage

$$\text{brown}\;\text{hornblende}\;+\;\text{diopside}\;+\;\text{plagioclase}\;\text{(An 50)}$$

while associated metapelitic gneisses consist of

$$\text{garnet}\;+\;\text{cordierite}\;+\;\text{biotite}\;+\;\text{sillimanite (andalusite)}\;\pm\;\text{K-feldspar}\;+\;\text{plagioclase (An 40-50)}\;+\;\text{quartz}.$$

Judging from relevant experimental data for the gneiss assemblage including the Fe/Mg distribution on coexisting garnet and cordierite, the P-T conditions of metamorphism are estimated at about 700° C and 5 kb water vapour pressure.K/Ar determinations on hornblendes from three amphibolites yielded cooling ages of 71.3, 71.2, and 71.1 (±1.7) m.y. respectively; biotites from three paragneisses gave 70.2 ± 1.4, 69.7 ± 1.2, and 67.9 ± 1.4 m.y. respectively. Assuming a sealing temperature against argon diffusion of 300° C, for biotite, and 500° C, for hornblende, a cooling rate of 100–200° C/m.y. is calculated. Thus a late Cretaceous (eo-Alpine) metamorphic event is established in the post-Cretaceous nappes of Crete.     

Garnet-amphibolites at Yokokawa in the Abukuma metamorphic belt,Japan

The mineralogy and petrochemistry of the garnet-amphibolites from the highgrade part of the Abukuma metamorphic belt have been studied, using five analyses of rocks, five of hornblendes, three of garnets and one analysis of cummingtonite, Garnetiferous amphibolites are rich in Fe, whereas non-garnetiferous ones are rich in Mg, especially in cummingtonite-amphibolite. The chemical composition of hornblendes associated with garnet is pargasitic and rich in FeO and poor in CaO, but that of non-garnetiferous rocks is rich in MgO. The garnets are rich in almandine molecule. Mg/Mg + Fe²⁺ ratios of both hornblendes and garnets correspond with those of the host rocks. The development of garnet in the Adirondack metabasites belonging to the upper almandine-amphibolite and granulite facies is observed in Mg-rich rocks as well as in Fe-rich rocks, in which both garnet and hornblende are rich in Mg respectively. However, under the conditions of the andalusite-sillimanite type metamorphism as shown in the Abukuma Plateau, Fe-rich garnet occurs in Fe-rich basic rocks, but cummingtonite occurs in Mg-rich ones instead of Mg-rich garnet. Finally, the problem of polymetamorphism is discussed. The cummingtonite-amphibolite may be the product of polymetamorphism, and Mg-rich garnet which had been present previously was decomposed to cummingtonite and plagioclase by the subsequent regional metamorphism of andalusite-sillimanite type.     

Modeling of retrograde diffusion zoning in garnet: evidence for slow cooling of granulites from the Highland Complex of Sri Lanka

Summary ?Diffusion modeling of zoning profiles in garnet rims from mafic granulites is used to estimate cooling rates in the Proterozoic basement of Sri Lanka, which represents a small, but important fragment of the Gondwana super-continent. Metamorphic peak temperatures and pressures, estimated with two-pyroxene thermometry and garnet–clinopyroxene–plagioclase–quartz (GADS) barometry, yield 875±20 °C and 9.0±0.1 kbar. These peak metamorphic conditions are slightly higher than results obtained by garnet-biotite Fe–Mg exchange thermometry of 820±20 °C. Reset flat zoning profiles were observed in most garnets. Only narrow garnet rims touching biotite exhibit retrograde zoning in terms of Fe and Mg exchange. The garnet zoning observed requires a slow cooling history. Equilibrium was achieved along grain boundaries during or close to peak metamorphism. During subsequent cooling to lower temperatures, only local exchange between garnet and biotite occurred. A cooling rate of 1–5 °C/Ma is estimated. The estimated temperature-time history from garnet profiles is in good agreement with the cooling history inferred from mineral radiogenic ages in the literature. Received December 11, 2001; revised version accepted August 28, 2002     

The genesis of garnet and cordierite in acid volcanic rocks: Evidence from the Cerberean Cauldron,Central Victoria,Australia

In the acid volcanic rocks of the Cerberean Cauldron of Central Victoria, four almandine garnet types can be distinguished. Types 1 and 2 are the most important. Type 1 garnets, about 1 mm across, euhedral and free of inclusions, occur mainly in the Rubicon Rhyolite. Type 2 garnets, up to 1 cm across, often irregular and always with inclusions, are most abundant in the Lake Mountain Rhyodacite. Type 1 garnets are enriched in Fe and Mn and depleted in Mg and Ca with respect to Type 2 garnets. Zoning patterns in Type 1 garnets show enrichment of Fe and Mn in the cores. Conversely Type 2 garnets have Fe and Mn enriched rims, although the zoning is frequently less regular. Fe rich cordierite occurs, mainly in the Rubicon Rhyolite, as nearly euhedral sixlings, without inclusions. They are frequently altered but where fresh show zoning patterns like those of the Type 1 garnets.These characteristics are best explained if the Type 2 garnets and cordierite are residuals of high grade metamorphic assemblages in granitic liquids produced by partial melting of pelitic rocks. Type 1 garnets and biotite subsequently crystallized from the liquid and the zoning patterns of Type 2 garnets and cordierite were modified near their rims by reaction.     

Trace element and Sm–Nd 'age' zoning in garnets from peridotites of the Caledonian and Variscan Mountains and tectonic implications

ABSTRACT Ion probe traverses across garnets from peridotites of the Caledonides of Norway and the Variscides of Poland show zoning patterns for Y, V, Zr, Cr, Ti and the REE. The complexly zoned patterns of garnets from the Bystrzyca Górna peridotite, Poland, are interpreted in terms of a changing P–T history (isobaric cooling followed by decompression and cooling). Weak rimward gradients in REE concentrations in garnets from the Almklovdalen and Sandvika peridotites, Norway, may be relicts of the original growth history of the garnets, but the nearly flat Y, V, Zr, Cr and Ti profiles from the same garnets imply a later period of near-homogenization at uniform P–T. Crushed garnet separates from each body were separated into three or more fractions on the assumption that density and magnetic susceptibility vary with Fe/Mg ratio, and Fe/Mg ratios change from garnet core to rim. Sm-Nd garnet–clinopyroxene ‘ages’ were determined for each fraction to determine whether they are also zoned. Four garnet fractions from the Góry Sowie peridotite give nearly the same ages (397–412 Ma) that are believed to span the interval of garnet growth. Garnet fractions from the Norwegian peridotites define scattered ages (816–1350 Ma) that are suspect, but hint at a Sveconorwegian equilibration event. The data indicate the Variscan and Norwegian peridotites had different histories, despite superficial mineralogical and tectonic similarities. Norwegian garnet peridotites had a long pre-Caledonian history and were extracted from a relatively cold mantle whereas the Variscan garnet peridotites had a comparatively short pre- or Eo-Variscan history and were extracted from a hot mantle.     

Zoned minerals in garnet peridotite nodules from the Colorado Plateau: implications for mantle metasomatism and kinetics

Some garnet peridotite nodules from The Thumb, a minette neck on the Colorado Plateau in the southwestern United States, contain zoned minerals. Zoning does not exceed 1.5 wt.% for any oxide, but some relative changes are large: in one garnet TiO₂ and Cr₂O₃ ranges are 0.05–0.65 and 3.5–5.0 wt.%, respectively. In two porphyroclastic nodules, garnet rims are depleted in Mg and enriched in Fe, Ti, and Na compared to cores, and one garnet is irregularly zoned in Ti and Cr. Olivine crystals in these rocks are unzoned, and pyroxene zoning is slight, yet matrix olivine and pyroxene contain more Fe and Ti and less Mg and Cr than inclusions of these phases in garnet. In three coarse nodules, garnet rims are Ti-rich compared to cores, and Ca, Fe, Mg, and Cr zoning patterns are complex. Several nodules appear to have partially equilibrated near 1200° C and 35 kb, and under these conditions cation mobility in pyroxene was greater than in garnet. The zoning partly reflects Fe and Ti metasomatism in the mantle. Calculations indicate that Fe-Mg gradients in garnet could have persisted for only a short time in the mantle, perhaps thousands of years or less, so the metasomatism occurred shortly before eruption. The minette host, a likely source of the Fe and Ti, is rich in light rare earth elements: since the nodules are much poorer in these elements, little or no infiltrated minette was trapped in them. Diffusion is a possible mechanism for nodule metasomatism. Some fertile peridotite nodules from kimberlites may have been affected by similar events. Compositional differences between inclusions in garnet and matrix phases are intriguingly similar to some of the differences between most peridotite inclusions in diamonds and common lherzolite phases.     

Mineral chemistry of banded migmatites from Hafafit and Feiran areas, Egypt

The migmatitic rocks exposed in Hafafit and Feiran areas exhibit some migmatitic structures as the banded, agmatic, boudinage and schlieren structures. The dominant type of these structures is the stromatic migmatites. Electron microprobe analyses of plagioclases, biotites and amphiboles from Hafafit and Feiran areas, in the Eastern Desert and Sinai, Egypt, are carried out and the metamorphic conditions are discussed. The present study revealed marked differences in the composition of plagioclases, biotites and amphiboles from Hafafit and Feiran localities. The obtained data indicated that plagioclases of the Feiran migmatites are of andesine and oligoclase composition, and display anorthite content from An₂₀ to An₃₈; whereas the Hafafit migmatites show a wider range of plagioclases from An₁₀ to An₆₀, and therefore plagioclases have labradorite, andesine and oligoclase composition. This may be due to the slow rate of the crystallisation processes. The analyses indicated that biotites of the studied areas are of metamorphic origin showing significant variation in Fe–Mg. It is worth mentioning that biotites from Hafafit migmatites have Mg–biotite composition while that of Feiram migmatites have Fe–biotite composition. High Mg and low Fe contents in biotite suggest higher crystallisation temperature. The composition of amphiboles in Hafafit migmatites is ferro-tschermakitic hornblende, while amphiboles from Feiram migmatites are magnesio-hornblende. High Ti content in the hornblende of Feiran migmatites suggests that they were formed at slightly higher temperatures and lower pressure than the Hafafit migmatites (i.e. Feiram migmatites and Hafafit migmatites were formed at granulite and amphibolite facies, respectively). Discrimination diagrams show that the muscovite is of secondary origin. Moreover, the present study confirmed that these migmatites are mainly formed by metamorphic differentiation via partial melting.     

Petrology and geothermometry of retrograded Hercynian charnockites and host gneisses, Agly Massif, French Pyrenees

Abstract A Hercynian charnockite occurs within high-grade gneisses in the Agly Massif, French Pyrenees. Its thermal history has been evaluated using the Fe-Mg distribution coefticient ( K _D) between garnet and biotite. These minerals have different origins but similar compositions in the charnockites and host gneisses. In the charnockite, the Bi–Ga pairs are the retrograde products of Opx alteration. This Opx reaction with feldspar can be written. Opx + PI + Fluid 1(H₂O + Al + K + Fe + Ti) = Bi + Ga + Q + Fluid 2(H₂O + Na). The garnets are relatively Ca poor (4–2.5% grossular); they are automorphic and zoned in the gneisses and poikiloblastic in the charnockites. Both types show a retrograde rim (of few hundred microns'width) across which Fe and Mn increase as Mg decreases. The biotites show a good correlation between the octahedral cations (Ti⁴⁺+ Fe²⁺) and (Mg²⁺+ Al³⁺_VI); Ti and Fe both increase, whereas Mg and Al_VI decrease. There is an inverse linear correlation between Fe²⁺ and Mg²⁺ and the Fe/Mg ratio increases as Ti increases. The relation between Ti and K ^Ga-Bi_DFe-Mg is less clear: it seems that K _D slightly decreases as Ti increases. The equilibration temperatures of Ga–Bi pairs are discussed: the charnockite Ga-Bi pairs have equilibrated between 550°C and 600°C; whereas those of the gneisses have equilibrated between 550°C and 650°C. Two main thermal steps appear: one in the gneisses between 600-650°C and a second one in both the gneisses and the charnockites between 550°C and 600°C.     

阜平杂岩中低品位磁铁矿的形成与深熔作用的关系

在中-高级变质的阜平杂岩中可以形成低品位磁铁矿.除碎屑岩中继承的磁铁矿外,新生变质磁铁矿多呈斑晶,可出现于多种岩石类型,如变基性岩、中性岩、酸性岩和变沉积岩中,表明新生磁铁矿的形成不受层位控制.磁铁矿可由物理重结晶和化学反应2种形式形成,重结晶过程主要为矿物颗粒的加粗,但没有明显的脱水反应.变质化学反应形成的磁铁矿与各单元所经受的后期变质事件改造有关,这类磁铁矿的出现与岩石中TFe的含量没有必然的联系,关键在于变质反应中是否有适量的铁组分的迁移和富集.变质反应过程中,初期黑云母变质转化形成角闪石,即变质反应不全是脱水或吸水过程,表明阜平杂岩主要的变质过程发生在含水体系中.在进一步的变质改造中,黑云母、角闪石可深熔转化形成磁铁矿.在片麻岩的含水熔融过程中,Mg、Ca优先迁移,而Fe(Ti,Al)迁移微弱,造成Fe(Ti,Al)与Mg组分的分离,残留的相对富铁组分形成磁铁矿、钛铁矿.磁铁矿结晶时没有明显的挤压或剪切,张应力可能占主导地位,相应的深熔作用主要发生在构造静应力期或体系略微抬升的过程中.     

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.     

Complex chemical zoning in eclogite facies garnet reaction rims: the role of grain boundary diffusion

In metapelites of the Saualpe complex (Eastern Alps) continuous 10 µm to 20 µm wide garnet reaction rims formed along biotite-plagioclase and biotite-perthite interfaces. The pre-existing mineral assemblages are remnants of low pressure high temperature metamorphism of Permian age. The garnet reaction rims grew during the Cretaceous eclogite facies overprint. Reaction rim growth involved transfer of Fe and Mg components from the garnet-biotite interface to the garnet-feldspar interface and transfer of the Ca component in the opposite direction. The garnets show complex, asymmetrical chemical zoning, which reflects the relative contributions of short circuit diffusion along grain boundaries within the polycrystalline garnet reaction rims and volume diffusion through the grain interiors on bulk mass transfer. It is demonstrated by numerical modelling that the spacing of the grain boundaries, i.e. the grain size of the garnet in the reaction rim is a first order control on its internal chemical zoning.     

Distribution and significance of cordierite in paragneisses of the Hudson Highlands,southeastern New York

Cordierite occurs locally and sporadically in biotite-quartz-two feldspar paragneisses of the Precambrian Highlands complex in southeastern New York. Cordieritic and associated non-cordieritic gneisses were compared to determine the significance of cordierite for the metamorphic history of the complex.Microprobe analyses of the ferromagnesian phases show the following ranges in Fe/Mg (mol.): cordierite 0.19–0.43; biotite 0.33–0.73; garnet 1.98–3.56. Feldspar compositional ranges are: plagioclase An_25–53; K-feldspar in microperthite Or_62–87Ab_12–37 An_0–1. Garnet and plagioclase associated with cordierite are depleted in Ca relative to those in cordierite-free assemblages.Textural evidence, phase rule considerations and consistent distribution coefficients for FeO and MgO in coexisting garnet, cordierite and biotite from each locality examined suggest that all phases formed in at least local equilibrium during the hornblende-granulite subfacies metamorphism. The assemblages studied limit the conditions of metamorphism to between 700 and 750° C and 3.0 to 5.5. Kb. total pressure, with P _T greater than . Differences in mineral compositions and partitioning coefficients among the sampled areas suggest slight local differences within these ranges.Mineral and modal analyses of cordierite-bearing and cordierite-free gneisses show the latter to be enriched in calcium and potassium and depleted in alumina relative to the former. We conclude that the rarity of cordierite in the Highlands paragneisses reflects a scarcity of parent rocks of suitable composition rather than unfavorable physical conditions.     

A Report on a Biotite-Calcic Hornblende Geothermometer

This paper presents a biotite-calcic hornblende geothermometer which was empirically calibrated based on the gamet-biotite geothermometer and the gamet-plagioclase-hornblende-quartz geobarometer, in the ranges of 560-800℃ (T) and 0.26-1.4 GPa (P) using the data of metadolerite, amphibolite, metagabbro, and metapelite collected from the literature. Biotite was treated as symmetric Fe-Mg-AlVI-Ti quaternary solid solution, and calcic hornblende was simplified as symmetric Fe-Mg binary solid solution. The resulting thermometer may rebuild the input garnet-biotite temperatures well within an uncertainty of ±50℃. Errors of ±0.2 GPa for input pressure, along with analytical errors of ?% for the relevant mineral compositions, may lead to a random error of ±16℃ for this thermometer, so that the thermometer is almost independent of pressure estimates. The thermometer may clearly discriminate different rocks of lower amphibolite, upper amphibolite and granulite facies on a high confidence level. It is assume     

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.     

Regional vs contact metamorphism of garnet metapelites in the vicinity of Late Variscan granites (Central Armorican Domain, Brittany, France)

Late Variscan granites intruded Brioverian (Upper Proterozoic) and Lower Paleozoic pelitic sequences to the north of the South Armorican shear zone. In the vicinity of the granites, Brioverian garnet micaschists contain pre/syn-S₂ assemblages with garnet + staurolite and post-S₂ assemblages with staurolite ± andalusite. Andalusite appeared pre/syn- and post-S₂ in garnet-free micaschists. The garnets in the Brioverian micaschists are zoned with increasing Mg and decreasing Mn and Ca from core to inner rim. Only poor garnet zonations occur in Paleozoic hornfelses of enclaves in the Rostrenen granite. The results of a microstructurally controlled application of garnet–biotite geothermometers and garnet–plagioclase geobarometers are similar to P–T trends obtained by the Gibbs method of garnet zonation modelling in the system NCFMnMASH. The P–T paths of a pre/syn-S₂ regional metamorphism are clockwise between 500–550°C/8 kbar and 700°C/5 kbar, followed by cooling decompression. They contrast with isobaric contact metamorphism between 500 and 700°C at 2.5–3 kbar in Paleozoic hornfelses. This points to a two-stage Variscan metamorphism with a pre-granitic pressure-dominated event in the Brioverian micaschists, followed by Late Variscan contact metamorphism, and suggests the existence of a pre-granitic tectonic boundary between the micaschists and overlying low-grade sequences.     

Prograde and retrograde history of eclogites from the Eastern Blue Ridge, North Carolina, USA

Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H₂O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.