Large Garnet Research Articles

Perpotassic granulites from southern Bohemia

Garnetiferous, perpotassic granulites associated with voluminous, felsic calc-alkaline garnet-kyanite granulites in the Blanský les massif are described as a new rock type. A K2O content near 13 wt%, Zr content ranging from 17 to 5877 ppm, primary Ca-Mg-Fe garnet (pyrope up to 33 mol %), and the absence of primary plagioclase and quartz characterize the new type of granulite. Calcium is incorporated only in garnet and apatite. Owing to the content of garnet the bulk syenitic compositions are metaaluminous to peraluminous. The perpotassic granulites are accompanied by subordinated melanocratic perpotassic granulite and alkali-feldspar melanocratic granulite containing up to 20 vol % quartz. These rock types also contain highly variable amounts of zircon. High-pressure, noneutectic partial melting of crustal rocks and crystallization under upper mantle conditions is suggested as the main process in generation of perpotassic granulites. A polyphase, ductile to brittle deformation and recrystallization in pyroxene granulite and later in amphibolite facies accompanied obduction of the granulite massif which carries numerous enclaves of pyrope peridotite. Euhedral equant zircon crystals up to 4 mm long enclosed in large garnet crystals and in the alkali feldspar matrix of perpotassic granulite represent a uniform, high-temperature population with a nearly concordant U-Pb system (Aftalion et al. 1989). The Hercynian age of this zircon indicates the role of the Hercynian anatexis of crustal rocks under upper mantle conditions in generation of granulites in the Moldanubian zone of the Bohemian massif.

Metamorphic segregations associated with garnet and orthopyroxene porphyroblast growth: two examples from the Larsemann Hills, East Antarctica

In the Larsemann Hills, East Antarctica, there are two important types of zoned segregations that formed during a Proterozoic, granulite facies metamorphic event: (1) segregations with a large garnet porphyroblast in the core, potassium feldspar in the mantle, and biotite, cordierite and plagioclase in the matrix; and (2) segregations with a large orthopyroxene porphyroblast in the core, potassium feldspar in the mantle, and biotite and quartz in the matrix. The main difference between these and zoned metamorphic segregations with leucocratic mantles described by previous authors is that the mantle phases are not a depleted subset of the matrix assemblage, but part of the product assemblage. The concentrical arrangement in the garnet segregations can be explained by the sequence in which a number of garnet- and potassium feldspar-producing reactions were met during temperature increase, with internal buffering of the activity of H2O. These reactions are modelled in the system, K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH). The main reaction responsible for the garnet porphyroblast-bearing segregations is a KFMASH divariant equilibrium, quartz+biotite+cordierite = garnet+potassium feldspar+melt, taking place in a quartz+biotite+ cordierite+garnet assemblage. Potassium feldspar and melt produced by this reaction are associated spatially with garnet as a consequence of the sparse nucleation of garnet, and the chemical potential gradients generated prior to the interception of this equilibrium. In order to account for the mineral assemblage of the mantles, and the preservation of the porphyroblasts during retrogression, some melt loss from the segregation is necessary. The orthopyroxene segregations can be explained in an analogous manner. Within this model, the PT conditions in which the two types of segregations formed are consistent with the M1 pressures and temperatures suggested by previous work: about 4.5 kbar and 750° C.

Magmatic and metasomatic differentiation in the petrogenesis of Archaean metabasites from Tigharry, North Uist

Synopsis The polymetamorphic supracrustal sequence at Tigharry, North Uist is associated with amphibolites, garnet–amphibolites, biotite–amphibolites, cummingtonite–garnet amphibolites, actinolitic hornblende–chlorite rocks and dolomite–actinolite–anthophyllite–chlorite rocks. These are a cogenetic group of tholeiitic basic and ultrabasic rocks belonging to the Archaean “banded basic” suite of the Lewisian complex in the Outer Hebrides. Diverse metabasite assemblages and cryptic variations of mineral composition are controlled by strong, predominantly magmatic chemical differentiation and the geochemistry is very similar to that of layered ultramafic/gabbro bodies of the mainland Lewisian complex. Volumetrically less important anthophyllite/gedrite–andesine–kyanite–phlogopite–quartz rocks and hornblende–anthophyllite–garnet–labradorite–staurolite–quartz rocks which have been described previously as metasediments are texturally and compositionally members of the basic/ultrabasic suite. The chemistry of these orthoamphibole-bearing rocks is not purely magmatic and reflects metasomatism (essentially Ca-depletion) of basic material either before or during metamorphism. Chlorite and biotite-rich assemblages including some with large garnets may also be of partially metasomatic derivation. The prograde assemblages of the basic rocks equilibrated during Proterozoic (Laxfordian) metamorphism under P–T conditions of 600–650°C and 6–9 kb. The associated metapelitic rocks contain kyanite and compositionally heterogeneous garnets whose complex zoning and inclusion fabrics may reflect growth during both the Laxfordian and Scourian metamorphic cycles.

Magmatic and metamorphic controls on chemical variations within the Eiksunddal eclogite complex, Sunnmøre, western Norway

The Eiksunddal eclogite complex is one of the larger group B (“country-rock”) eclogites within the Western Gneiss Region (WGR) of Norway. Whole-rock chemical data suggest a magmatic origin for the rocks within the eclogite complex. The chemical variations among the eclogitic rocks were controlled by accumulation of olivine (Fo 80) and plagioclase (An 70) and probably magnetite. The composition of the interstitial (“trapped”) liquid is derived by linear regression methods and is a high-Al orthopyroxene normative basalt. The most likely conditions of crystallization are: T ≈ 1150°C, P < 10 kbar. The Al-rich nature of the interstitial liquid may have implications for the formation of anorthosites and leucogabbros associated with similar mafic rocks elsewhere in the WGR. Trace-element characteristics are compatible with a continental origin of these rocks. During the subsequent metamorphic history, the marginal parts of the complex have suffered variable degrees of contamination. Abundant amphibole inclusions in large garnets in garnet websterites suggest local hydration at pre-eclogitic stages. This may be associated with introduction of radiogenic Sr as previously reported by W.L. Griffin and H.K. Brueckner. During prograde metamorphism, alkali exchange occurred during growth of garnet, clinopyroxene and biotite at the expense of pargasitic amphibole and orthopyroxene. The most extensive metasomatism however, was probably associated with the introduction of fluid during amphibolitization (retrogression) of the eclogitic rocks. Three model reactions — eqs. 5–7 — qualitatively explain the relation between degree of retrogression and the observed metasomatic effects. Whether Si and Ca are gained or lost during retrogression seems to be controlled by the extent of amphibole growth in diopside + plagioclase symplectites initially formed by breakdown of omphacite. Alumina has behaved in a more or less immobile way during metasomatism. Low mobility of Al is also reflected by the formation of subsilic (Al-rich) amphiboles and Al-rich plagioclase during garnet breakdown. This demonstrates the importance of kinetic factors in controlling mineral chemistry.

Thermo-tectonic evolution of a metamorphic internal zone documented by axial projections and petrological P-T paths, Wopmay orogen, northwest Canada

Axial projections of the transversely folded 1.9 Ga Wopmay orogen document stratigraphic and structural relations in a metamorphic internal zone having more than 30 km of structural relief. The greater part of the metamorphic belt is separated from underlying autochthonous Archean basement and thin Early Proterozoic cover by a sole thrust coextensive with that beneath the external thrust-fold belt Site-specific metamorphic pressure-temperature ( P-T ) paths are derived from core-to-rim microprobe analyses of large zoned poikiloblastic garnets that contain abundant inclusions of biotite, plagioclase, quartz, and, in some cases, aluminosilicate. For garnet-absent units, qualitative P-T paths are deduced from overprinting mineral assemblages. The structural and metamorphic data combined with the results of geochronological and contiguous-external-belt studies show that regional metamorphism in the internal zone is the result of three overlapping, thermally additive, thermotectonic regimes: (1) onset of metamorphism during crustal thinning, (2) plutonic heat advection during crustal shortening, and (3) thermal relaxation, concurrent with uplift, as a consequence of crustal thickening.

Parting in giant garnets as an indicator of Late Brittle Deformation at Gore Mountain, Warren County, N.Y.

The Barton garnet deposit contains large garnet crystals (2.5 ‐ 25 cm) disseminated through a gabbro‐derived garnet amphibolite. These crystals are characterized by two sets of parting planes which intersect at about 80° and have a constant attitude throughout the amphibolite. The results of X‐ray diffraction studies on the parting planes of the garnet crystals by the back‐reflection Laue method and by X‐ray diffractometry show that the parting‐plane sets are not crystallographically controlled by the lattice of the garnet and that the garnets are not oriented in the amphibolite. Each parting set in the giant garnets and its corresponding parallel joint set in the amphibolite matrix are interpreted as brittle fractures which developed in response to a discrete tectonic episode.

Reaction of zoning of garnet

Compositional zoning of garnet in metamorphic or igneous rocks preserves evidence of the equilibration history of the sample and can be interpreted in terms of a growth-fractionation, diffusion-exchange, or diffusion-reaction model. Diffusion zoning is usually assumed to result from exchange reactions between garnet and other phases as the partitioning coefficient varies in response to changing environmental conditions, primarily temperature. However, in many natural environments where garnet grew originally in divariant equilibrium with other phases, changing conditions can promote continuous or “divariant” reactions and consequent compositional shifts of phases that can be much greater in some systems showing these reactions than those related to the small changes of partitioning. Diffusional zoning related to overstepping of these continuous reactions must be related to incongruent reaction and necessitates formulation of a kinetic diffusion-reaction model involving moving phase boundaries as well as solid-state diffusion. Three samples containing zoned garnets from the metamorphic aureole around the Ronda ultramafic intrusion in southern Spain are used to illustrate two possible models of diffusion-reaction processes. The examples are particularly informative because the reactions are demonstrably irreversible and evidence of the reaction system is preserved. Partitioning data indicates that compositions of product phases are not in equilibrium with the original garnet and do not vary with extent of reaction; therefore, exchange reactions with garnet were not possible and garnet changed composition only by incongruent reaction. After a small amount of reaction, Mg/Fe of the rim composition approaches a value apparently in equilibrium with product phases, but the garnets are zoned inward to the original garnet composition preserved in the interior. Grossularite content is approximately constant and spessartite content variable but small, thus, the rim composition of pyrope or almandine is assumed to be fixed by the external reaction process and is taken as a boundary condition in the following models. The zoning profile of pyrope or almandine component between the fixed rim and core compositions (assumed to extend to ∞) is described in semiinfinite, half-space models appropriate for large garnets with narrow rims. The first model corresponds to a reaction system in which all garnet compositions are metastable (case 1) and zoning depends on the independent variables of the diffusion constant, velocity of the interface between garnet and matrix, and time. The second model, corresponding to systems in which the initial garnet composition is metastable but an equilibrium composition is stable (case 2), depends on the independent variables diffusion constant, time, and a function of reaction compositions. In case 1 the consumption velocity is assumed constant and a steady state zoning profile is reached at large time, whereas, in case 2, the velocity decreases with the concentration gradient and steady state is not possible. The models were tested using a reaction time estimated from cooling models of the aureole, mass of garnet consumed, determined petrographically, and phase compositions. The two cases are somewhat independent in that different parameters are independent variables. The estimate of the diffusion constant of 10−18±2 cm2/sec (assumed to be a mutual or binary coefficient for almandine and pyrope) is considered reasonable for the temperature range of reaction (probably 600–900° C), and the two models are consistent considering the probable error and possible real temperature differences. It is obvious that details of the metamorphic reaction system must be known to successfully apply diffusion models. Kinetic models, involving consumption or growth of the phase as well as diffusion are probably necessary when dealing with natural rocks. Several possible and interesting complications, such as cross coupling between components, can be investigated if more data were available. Experimental determination of diffusion constants allow natural reaction rates to be estimated by this method. Diffusion zoning is an important consideration that could increase the efficiency of experimentation with chemically recalcitrant phases.

Narrow Linewidth and High Infrared Transmission in Hydrothermally Grown Yttrium Iron Garnet

The properties of yttrium iron garnet (YIG) grown from high-temperature—high-pressure aqueous (hydrothermal) solvents have been studied. YIG with a room-temperature ferromagnetic linewidth as low as 0.3 Oe and with an infrared extinction coefficient at 1.14 μ as low as 0.5 has been prepared. Such YIG should be useful for lasers, laser optical devices, and microwave devices, and the hydrothermal technique is shown to have advantages over conventional flux growth for the reproducible production of large garnets. Linewidth is shown to correlate with proton content which is affected by the concentration of Ca2+ ions in the lattice. Absorbtion at 1.14 μ is uniformly low in hydrothermal YIG, and this fact is discussed. The role of impurities, dopants, and conditions of growth are discussed.

Some aspects of a horizontal schistosity

AbstractA Pre‐Cambrian horizontal schistosity was formed in regionally metamorphosed rocks around Lannilis, Finistére, during repeated fold movements. Only horizontal movement occurred since the superimposed load due to deep burial was great. Biotite crystallisation along horizontal planes resulted in a schistosity except in the principal thrust zones where decussate biotite textures and large garnets developed. During the early movements, ‘ac’ girdles of quartz axes were formed but these girdles were rotated during the later movements along the schistosity. The degree of rotation and the form of the associated structures cannot be interpreted easily either by ‘Sow’ on planes perpendicular to a compression axis or by ‘slip’ along the schistosity.

A Note on the Development of Large Garnets in Sutherland Migmatites

AbstractIn the outer zone of the Sutherland Migmatite Complex, four-inch garnets are developed in “veins” in garnetiferous rocks which, adjacent to the “veins”, are devoid of garnet. The redistribution of material has been brought about by activity along certain privileged paths. Comparison is made with the gigantic 3-ft. garnets of the Adirondacks.

GENESIS OF SOME ADIRONDACK GARNET DEPOSITS

New mineralogic and petrographic data, derived from study of a number of garnet-rich deposits in the southeastern Adirondacks, are presented, with particular emphasis on the Barton deposit at Gore Mountain. This deposit, long the world9s largest producer of abrasive garnet and widely known for its giant hornblende-sheathed garnets, has been ascribed to various primary and secondary modes of genesis. The new data indicate that the Barton ore is an extreme metagabbro facies whose large garnets were derived by progressive recrystallization growth from microscopic corona garnet during the peripheral metamorphism of the gabbro by intrusive syenite magma. The progressive changes from corona garnet (C type) to very small poikiloblasts (P type), to megascopic non-poikilitic porphyroblasts (X type), and finally to very large hornblende-sheathed crystals (XH type) are traced petrographically and compositionally. These basic garnet types, representing different degrees of recrystallization growth of the original, probably deuteric, corona garnet, are recognized in many gabbroic and anorthositic facies as well as in hybrid facies of syenite-gabbro and syenite-anorthosite derivation. Because of this genetic relationship, garnet composition and textural type facilitate the petrogenetic interpretation of a number of garnetiferous facies and deposits, and throw new light on certain old petrologic problems of the Adirondack province. Chemical analyses, specific gravities, optical and x-ray diffraction data, micrometric modes, micrographs, maps, and graphs are presented.

Studies of garnet from Suisyô-yama in the Hida mountains

A large garnet vein, associated chiefly with quartz and sparingly with hematite and epidote, occurs in dioritic rock forming Suisvô-yama in the Hida mountains. There are two kinds of garnet distinguishable in colour, greenish grey and wine-red, the former forming the main part of the mass and the latter appearing in a thin lining form in the cavities filled up by quartz which can be taken as the product formed by pneuma-tolytic action after the formation of the garnet mass. Hence the wine-red garnet is considered as a thermally altered variety of the greenish grey garnet. Main physical properties, namely, the index of refraction, specific gravity and cell-dimention, of the wine-red garnet were carefully deter-mined. For the refractive indices determined by the minimum deviation method for a crystal, three different values were obtained such as nD=1.872, 1.881 and 1.890, owing to the zonal growth. The specific gravity and the cell-dimention are given as 3.831 at 4°C and 11.996 A respectively From these physical properties and the presence of MnO which was chemi-cally determined, the chemical composition of the garnet, calculated by. Kôzu's formula, can be given as And=87.34, Gr=4.75, Alm=3.54 and Sp=4.37%. The present studies were carried out under the guidance of Prof. S. Kôzu, to whom the author's hearty thanks are due.