Corona textures between kyanite, garnet and gedrite in gneisses from Errabiddy, Western Australia   总被引：2，自引：0，他引：2  

JUDY BAKER  ROGER POWELL  MICHAEL SANDIFORD  JAN MUHLING 《Journal of Metamorphic Geology》1987,5(3):357-370

Abstract Corona textures, which developed in alternating layers in rocks in a supracrustal belt at Errabiddy, Western Australia, involved:
(a) The production of staurolite, cordierite and quartz or sapphirine between Kyanite and/or sillimanite and gedrite; and
(b) The production of cordierite between garnet and gedrite.
These textures are inconsistent with development along the same pressure–temperature path in the system FeO–MgO–Al₂O₃–SiO₂–H₂O, but can be accounted for if CaO, mainly in garnet, is taken into account. The sapphirine-bearing kyanite–gedrite textures are explained by lower a (SiO₂) during their development. The assemblages indicate a consistent pressure–temperature ( P–T ) trajectory involving substantial uplift with only a slight decrease in temperature. The history of these rocks includes reheating of originally high-grade rocks that had cooled to a stable conductive geotherm, followed by substantial, essentially isothermal uplift. The tectonic environment for this was presumably the one responsible for emplacement of the high-grade terrain in the upper crust.  相似文献   

An internally consistent thermodynamic data set for phases of petrological interest   总被引：102，自引：8，他引：94  

T. J. B. HOLL  & R. POWELL 《Journal of Metamorphic Geology》1998,16(3):309-343

The thermodynamic properties of 154 mineral end-members, 13 silicate liquid end-members and 22 aqueous fluid species are presented in a revised and updated data set. The use of a temperature-dependent thermal expansion and bulk modulus, and the use of high-pressure equations of state for solids and fluids, allows calculation of mineral–fluid equilibria to 100  kbar pressure or higher. A pressure-dependent Landau model for order–disorder permits extension of disordering transitions to high pressures, and, in particular, allows the alpha–beta quartz transition to be handled more satisfactorily. Several melt end-members have been included to enable calculation of simple phase equilibria and as a first stage in developing melt mixing models in NCKFMASH. The simple aqueous species density model has been extended to enable speciation calculations and mineral solubility determination involving minerals and aqueous species at high temperatures and pressures. The data set has also been improved by incorporation of many new phase equilibrium constraints, calorimetric studies and new measurements of molar volume, thermal expansion and compressibility. This has led to a significant improvement in the level of agreement with the available experimental phase equilibria, and to greater flexibility in calculation of complex mineral equilibria. It is also shown that there is very good agreement between the data set and the most recent available calorimetric data.  相似文献   

燕山造山带后石湖山碱性环状杂岩体的成因与形成过程     

文霞  马昌前  桑隆康  Roger Mason  佘振兵  熊富浩 《地球科学》2013,38(4):689-714

后石湖山杂岩体是与垮塌破火山口有关的碱性环状杂岩体, 主要由呈环形分布的碱性火山岩、环状岩墙(斑状石英正长岩)、嵌套的中心复式岩株(晶洞碱长花岗岩和斑状碱长花岗岩)和锥状岩席(石英正长斑岩和花岗斑岩)组成.LA-ICPMS锆石U-Pb年代学分析表明, 斑状石英正长岩环状岩墙、石英正长斑岩和花岗斑岩锥状岩席的侵位年龄分别为119±3Ma、121±2Ma和121±2Ma.该环状杂岩体火山岩与侵入岩的形成年龄相近, 体现了它作为火山-侵入杂岩体的特征.斑状石英正长岩富碱(Na₂O+K₂O=10.0%~10.5%), K₂O含量较高(5.21%~5.42%), 具正的Eu异常(Eu/Eu*=1.05~1.40).碱长花岗岩和斑岩均具有富碱、高FeOtot/MgO、Ga/Al、Zr、Nb和REE值(Eu除外), 以及低Al₂O₃、CaO、MgO、Ba、Sr和Eu含量的特征, 都属于A型花岗岩质岩石.其中斑岩为铝质A型花岗岩, 具有高的初始岩浆温度(880~901℃).所有A型花岗质岩石均具有较富集的Nd同位素组成, ε_Nd(t)值变化于-13.9~-12.2之间.斑状石英正长岩是下地壳中-基性麻粒岩和片麻岩部分熔融产生的熔体与幔源玄武质岩浆混合, 后又发生单斜辉石分离结晶的产物; 碱长花岗岩源于上地壳长英质岩石部分熔融产生的熔体与幔源玄武质岩浆混合, 随后经历长石的分离结晶作用而成; 斑岩是受幔源岩浆底侵加热的上地壳长英质岩石的部分熔融产生的熔体, 并经历了长石的分离结晶作用而产生.该环状杂岩体的形成过程可以概括为: (1)火山爆炸性喷发形成大量的碱性火山熔岩和火山碎屑岩; (2)地下岩浆房空虚导致压力下降, 其顶板围岩失稳而沿火山口周围近直立的环状断裂垮塌, 形成塌陷的破火山口.与此同时, 下覆岩浆房的岩浆被动挤入环状断裂而形成斑状石英正长岩环状岩墙; (3)浅部地壳的长英质岩浆房过压, 促使其高温过碱质A型花岗质岩浆上升侵位形成了中心的斑状碱长花岗岩岩株, 这些岩浆的上涌导致上覆围岩产生倾角中-陡的、内倾的锥状裂隙, 为石英正长斑岩锥状岩席侵位提供了空间; (4)浅部岩浆房复活, 高温过碱质A型花岗质岩浆再度上升侵位形成被嵌套的晶洞碱长花岗岩岩株.同样, 这种岩浆的再度上侵导致上覆围岩产生了倾角较陡而内倾的锥状裂隙, 为花岗斑岩锥状岩席提供了侵位空间.后石湖山碱性环状杂岩体的形成是华北东部早白垩世与克拉通破坏相关的伸展构造体制下的产物, 这种构造体制可能与古太平洋板块的俯冲作用有关.   相似文献   

An improved and extended internally consistent thermodynamic dataset for phases of petrological interest,involving a new equation of state for solids   总被引：15，自引：0，他引：15  

T. J. B. HOLLAND  R. POWELL 《Journal of Metamorphic Geology》2011,29(3):333-383

The thermodynamic properties of 254 end‐members, including 210 mineral end‐members, 18 silicate liquid end‐members and 26 aqueous fluid species are presented in a revised and updated internally consistent thermodynamic data set. The PVT properties of the data set phases are now based on a modified Tait equation of state (EOS) for the solids and the Pitzer & Sterner (1995) equation for gaseous components. Thermal expansion and compressibility are linked within the modified Tait EOS (TEOS) by a thermal pressure formulation using an Einstein temperature to model the temperature dependence of both the thermal expansion and bulk modulus in a consistent way. The new EOS has led to improved fitting of the phase equilibrium experiments. Many new end‐members have been added, including several deep mantle phases and, for the first time, sulphur‐bearing minerals. Silicate liquid end‐members are in good agreement with both phase equilibrium experiments and measured heat of melting. The new dataset considerably enhances the capabilities for thermodynamic calculation on rocks, melts and aqueous fluids under crustal to deep mantle conditions. Implementations are already available in thermocalc to take advantage of the new data set and its methodologies, as illustrated by example calculations on sapphirine‐bearing equilibria, sulphur‐bearing equilibria and calculations to 300 kbar and 2000 °C to extend to lower mantle conditions.  相似文献   

Influence of ferric iron on the stability of mineral assemblages     

J. F. A. DIENER  R. POWELL 《Journal of Metamorphic Geology》2010,28(6):599-613

Ferric iron is present in all metamorphic rocks and has the ability to significantly affect their phase relations. However, the influence of ferric iron has commonly been ignored, or at least not been considered quantitatively, mainly because its abundance in rocks and minerals is not determined by routine analytical techniques. Mineral equilibria calculations that explicitly account for ferric iron can be used to examine its effect on the phase relations in rocks and, in principle, allow the estimation of the oxidation state of rocks. This is illustrated with calculated pseudosections in NCKFMASHTO for mafic and pelitic rock compositions. In addition, it is shown that ferric iron has the capacity to significantly increase the stability of the corundum + quartz assemblage, making it possible for this assemblage to exist at crustal P–T conditions in oxidized rocks of appropriate composition.  相似文献   

Retrograde melt–residue interaction and the formation of near‐anhydrous leucosomes in migmatites     

R. W. WHITE  R. POWELL 《Journal of Metamorphic Geology》2010,28(6):579-597

Considering physical segregation of melt from its residue, the chemical potentials of the components (oxides) are the same in both when segregation occurs. Then, as P–T conditions change, gradients in chemical potential are established between the melt‐rich domains and residue permitting diffusional interaction to occur. In particular, on cooling, the chemical potential of H₂O becomes higher in the melt segregation than in the residue, particularly when biotite becomes stable in the residue assemblage. Diffusion of water from the melt to the residue promotes crystallization of anhydrous products from the melt and hydrous products in the residue. This diffusive process, when coupled with melt loss from the rocks subsequent to some degree of crystallization, can result in a significant degree of anhydrous leucosome being preserved in a migmatite with only minor retrogression of the residue. If H₂O can diffuse between the melt segregation and all of the residue, then no apparent selvedge between the two will be observed. Alternatively, if H₂O can diffuse between the melt segregation and only part of the residue, then a distinct selvedge may be produced. Diffusion of H₂O into the residue may be in part responsible for the commonly anhydrous nature of leucosomes, especially in granulite facies migmatites. Diffusion of other relatively mobile species such as Na₂O and K₂O has a lesser effect on overall melt crystallization but can change the proportion of quartz, plagioclase and K‐feldspar in the resultant leucosome. The diffusion of H₂O out of the melt results in the enhanced crystallization of the melt in the segregation and increases the amount of resulting anhydrous leucosome relative to the amount produced if melt crystallized in chemical isolation from the residue. For high residue:melt ratios, the proportion of resulting near‐anhydrous leucosome can approach that of the proportion of melt present at the onset of cooling with only minor loss of melt from a given segregation required. Crystallization of melt segregations via the diffusion of H₂O out of them into the host may also play a major role in driving melt‐rich segregations across key rheological transitions that would allow the expulsion of remaining melt from the system.  相似文献   

Mineral activity–composition relations and petrological calculations involving cation equipartition in multisite minerals: a logical inconsistency     

T. J. B. HOLLAND  R. POWELL 《Journal of Metamorphic Geology》2006,24(9):851-861

Equipartition is an assumption that preserves the same relative fraction of the cations on each site, originally used just to distribute cations such as Fe and Mg over two or more sites during mineral recalculation. This approximation has been used in almost all thermometry and barometry applications in petrology involving pyroxene, amphibole, chlorite and biotite mica. It has also become the default approach used in deriving activity–composition relations in multisite minerals where details of element partitioning among the sites is unknown and therefore assumed to be random. It is shown that, where a third element, such as Al, resides on just one of the sites, equipartition introduces a serious logical and numerical inconsistency between the enthalpy and entropy of mixing. In particular, application of equipartition is demonstrated to be equivalent to treating the solution as one in which all cations reside on one type of site. For example, in the case of aluminous orthopyroxene the equipartition assumption implies that Fe, Mg and Al are distributed across two identical sites, and that end‐members such as Mg‐tschermak's pyroxene have enthalpies characterized by a totally disordered distribution of Mg and Al over the M2 and M1 sites. Clearly this is unsupportable, and solid solutions should be treated with appropriate thermodynamics for order–disorder. For phases where element partitioning data are unavailable, we offer a simple alternative strategy, using the ordering of Fe and Mg on M1 and M2 sites in biotite as an example.  相似文献   

How well known are the thermodynamics of Fe–Mg–Ca garnet? Evidence from experimentally determined exchange equilibria     

M. GUIRAUD  R. POWELL 《Journal of Metamorphic Geology》1996,14(1):75-84

ABSTRACT Many thermodynamic calculations relating to metamorphic rocks hinge on the thermodynamic parameters of garnet. Though some models are widely used, it is not clear whether their underlying premise is correct: that a single set of equations can be written for the activities of the end-members of garnet covering the whole compositional range. A voluminous body of data can be used to constrain the thermodynamics of garnet, namely Fe–Mg exchange experimental data involving garnet and another mineral, particularly clinopyroxene, orthopyroxene and olivine. However, examination of these data reveals inconsistencies, apparently stemming from differences between the thermodynamics of low-Ca and high-Ca garnets, with a boundary of about X^g_Ca= 0.15. In the two regions, for the high P–T of the experimental data, the thermodynamics follow the regular model, with values for the interaction parameters in the low Ca region of about w^g_FeMg= 50R and w_aFe–w^g_MgCa=– 1300R, in which R is the gas constant, and in the high Ca region of about w^g_FeMg= 1100R and w^g_CaFe–w^g_MgCa=– 2200R. Using the subregular, rather than the regular, model does not remove the discrepancy. The cause of the discrepancy needs to be identified if reliable calculations on rocks are to be made.  相似文献   

A comparison of mineral magnetic, geochemical and mineralogical techniques for compositional studies of glacial diamictons     

JOHN WALDEN  JOHN P. SMITH  ROGER V. DACKOMBE 《Boreas: An International Journal of Quaternary Research》1996,25(2):115-130

Data from heavy mineral analysis, X-ray fluorescence spectrometry (XRF) and X-ray diffractrometry (XRD) are compared with those obtained from mineral magnetic analysis for a range of glacial diamicton samples taken from the Quaternary sequence in the Isle of Man. These data show that the mineral magnetic approach offers a useful means of characterizing glacial sediments for purposes of lithostratigraphic correlation and provenance indication and that the magnetic data are of a comparable quality to those obtained from standard applications of the more traditional techniques. The heavy mineral and XRD data indicate that little post-depositional chemical alteration of the detrital composition of the sediments has taken place. The mineral magnetic, heavy mineral and XRF analysis suggests both similar lithostratigraphic relationships between the various diamicton units and consistent indications of their likely provenance. These are consistent with published, field-based, stratigraphic relationships of the same sediments and ice-flow directions in the Irish Sea Basin during the Devensian.  相似文献   

Calculated phase equilibria involving chemical potentials to investigate the textural evolution of metamorphic rocks   总被引：3，自引：2，他引：1  

R. W. WHITE  R. POWELL  J. A. BALDWIN 《Journal of Metamorphic Geology》2008,26(2):181-198

Evolving pressure–temperature conditions during metamorphism drive changes in the stable mineral assemblage, mineral proportions and mineral compositions in rocks. These changes are achieved via the diffusion of components between minerals, fluid and melt, the driving force for diffusion being the gradients in chemical potential of the components developed spatially within the rock. This study utilises recent developments in the software thermocalc to investigate quantitatively chemical potential relationships in rocks, with the phases involved being (solid) solutions. Phase diagrams with chemical potentials as axes are used to understand better the spatial rearrangement of components during the metamorphic evolution of rocks and the metamorphic textures that result. In contrast to qualitative chemical potential diagrams, quantitative diagrams can be contoured for mineral composition, allowing consideration of chemical zoning in minerals. Furthermore, the amount of material required to diffuse to equalise chemical potentials can be calculated. We start by demonstrating the approach via an example of retrograde corona development in an ultra-high-temperature granulite. Whereas the use of chemical potentials to consider the retrograde development of corona textures is well known, they are also significant in considering the prograde history. The role of chemical potentials in prograde metamorphic textural evolution is highlighted in consideration of the consumption and growth of aluminosilicates during the kyanite-to-sillimanite reaction, and the growth of garnet porphyroblasts.  相似文献