DOCSLIB.ORG

On the Mechanism of Prograde Metamorphic Reactions in Quartz-Bearing Pelitic Rocks

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
On the Mechanism of Prograde Metamorphic Reactions in Quartz-Bearing Pelitic Rocks ContI'. Mineral. and Petrol. 20, 244-267 (1969) On the Mechanism of Prograde Metamorphic Reactions in Quartz-Bearing Pelitic Rocks DUGALD M. CAR1\UCHAEL Dept. of Geological Sciences, .l\..lcGill University, Montreal, Canada Received September 2, 1968 Abstract. An empirical method is described whereby the sequence of textural changes in pelitic rocks from one zone to the next may be reconciled with the balanced metamorphic reaction inferred to have been in progress. It consists in deducing from the textures of a single thin section a set of metasomatic cation-exchange reactions, which proceed in different microscopic domains of the rock, but which add up on the scale of the whole thin section to give the balanced metamorphic reaction. Each metasomatic subsystem is closed to aiuminulll, but open to the more mobile cations, which are free to diffuse from one subsystem to another. subject to the requirement of short.range electrostatic neutrality, and to the assumption that the system is closed on the scale of the whole thin section. Evidence in support of the central postulate that aluminum is relatively immobile is found in 1. the preservation of bedding laminations, on a finer scale than the staurolite porphyroblasts which transect them without disrupting them. 2. The fact that quartz, the only abundant aluminum-free mineral in pelitic rocks, is by far the most common mineral in veins and" pressure shadows". 3. The fact that the reactiolls so deduced provide reasonably precise descriptions of such common textures as the sillimanite needles in biotite and quartz, and the abundant quartz "inclusions" in staurolite. Introduction Prograde regional metamorpltic reactions generally do not reverse themselves during cooling and erosion, so that with few exceptions, the mineral assemblage in a metamorphic rock apparently records the most intense metamorphic con­ ditions to which that rock was subjected. Several lines of evidence indicate tJIat metamorphic assemblages commonly represent a close approach to thermodynamic equilibrium under these conditions, but little is known of the path by wltich the rocks approach this state. Does the pattern of metamorphic zones that may be mapped in space correspond to the time sequence of changes at anyone point, wltile the metamorphic conditions were slowly intensifying 1 If so, then each mineral assemblage will form directly from the assemblage that precedes it in the spatial sequence of zones, by means of the chemical reaction that "relates" the two zones, and the isograds will migrate slowly through the rock body, keeping pace with rising temperature and pressure. A sequence of rock specimens collected across an isograd should record each stage of the reaction by which the mineral assemblage of the lower-grade zone was transformed into that of the higher­ grade zone. It has long been a problem that the textures of such a sequence of specimens, as seen in thin section, generally do not accord with the reaction that may be deduced by comparing the mineral assemblages on either side of the isograd (cf. :Mechanism of Metamorphic Reactions in Pelitic Rocks 245 WILLIAMSON, 1953; TURNER and VEltHOOGEN, 1960, p.480 and 486; CRINNER, 1961). There is a gradual reduction and ultimate disappearance of the reactant minerals. compensated by the appearance and growth of chemically equivalent product minerals, but generally the products do not grow directly in contact with the reactants. Instead, the products and reactants are in different domains of the same thin section, separated from each other by minerals that are stable on both sides of the isograd. This prohlem has led some petrologists to douht that the metamorphic reactions deduced from ohserved changes in mineral compatibilities actually happen in metamorphic rocks (cf. RAST, 1965; CKAKRABORTY and SEN, 1967; GANGULY, 1968). ATHERTON (1965) has invoked the ambiguous textures of regional meta· morphic rocks in support of YODER'S (1955) view that rocks attain their meta­ morphosed sta.te directly, without passing through each of the progressive zones in turn. This inference. if correct, is a serious challenge to the hypothesis that metamorphic assemblages generalJy represent a close approach to equilibrium. If ea~h mineral assemblage forms directly from the well·crystallized assemblage tha't precedes it in the spatial sequence of zones, then the possibility of a given assemblage forming outside its own stability field would be remote, because the free energy change between the initial state and the stable state would be too small to bracket any conceivable metastable states. On the other hand, if it were possible for a deeply-buried shale to remain in an unmetamorphosed condition throughout the necessarily long period of time required to heat it to the tempera­ ture at which a high-grade assemblage would be stable, then each of the lower­ grade assemblages would be an accessible metastable state, having a free energy between that of the shale and that of the stable high-grade assemblage. Depending on kinetic factors, the shale might come to rest in any of these metastable states, or conceivably it might remain unmetamol'phosed during the subsequent cooling and erosion, and reappear at the surface of the earth bearing no imprint of the high temperature and pressure to which it had been subjected. Clearly, any attempt to correlate the physical conditions of metamorphism with natural meta­ morphic assemblages is futile if the natu.ra.1 metamorphic reactions are too sluggish to keep pace with rising temperature. The a-im of this paper is to show that certain common isogradic textures, ob· served in a typical regional metamorphic terrane, can be reconciled with the meta.­ morpltic reactions inferred to have been in progress, so that there is no reason to doubt that the regional metamorphism was progressive. and no need to invoke post-metamorphic recrystallization. Constant·Aluminum Replacement A crystalline phase may dissolve congruent,ly, whereby all of its constituent ions go into solution, or it may dissolve incongruently, whereby some of its ions go into solut'ion, and the others rearrange themselves to form a different solid phase. However, if the solution contains ions of a kind not present in the crystalline phase, a third possihility arises. The crystaBine phase may "dissolve" by ion exchange. some of its ions going into solution, and the others combining with ions from the solut.ion to form a different solid pha-se. 246 D. M. CAR.J,UCHAEL: This third possibility is already qualitatively familiar to the geologist, as replace­ ment. LINDGREN (1918) has discussed constant-volume replacement, and BARTH (1948) has discussed constant-oxygen replacement. These are not the only pos­ sibilities; in fact, depending on how the boundaries of a system are defined, any one of the extensive variables may be held constant, as a reference against which to measure changes in the other extensive variables (GIBUS, 1928; THOMPSON, 1959). Students of metasomatism and bydrothermal alteration have found that the extens.ive variable most nearly held constant in many natural hydrothermal systems is the aluminum content (MEYER, 1950). That is to say, aluminum appears to be the least mobile of the major components of metasomatic systems. Inasmuch as metamorphic reactions are metasomatic if considered on a small enough scale, the question arises whether the growth of the products of meta­ morphic reactions in aluminous rocks might approximate a process of constant­ aluminum replacement. The concept of the limit of migration of different chemical species during metamorphism (HARK}JR, 1893) is useful in this rega,rd. If aluminum is significantly less mobile than any other major component, then its limit of migration will be significantly smaller than that of any other component, and a "local system" (THOMPSON, 1959) may be envisaged which would be large enough to encompass the limit of migration for aluminum, but small enough that the other components could dilfuse into or out of the system in response to externally controlled gradients in their activities. A chemical reaction taking place in such a system would be a process of constant-aluminum replacement. Evidence f,hat the Limit 01 ~ligration 01 Aluminum is Small There is reason to surmise that aluminum may be relatively immobile during metamorphism, both because its diffusion coefficient is likely to be small (FYFE et at., 1958, p. 62; TURNER and VERHOOGEN, 1960, p. 479), and because its equi­ librium concentration in the intergranular "dispersed phase" (cf. HEIER, 1965) is likely to be small. The higher the concentration of an ion in the dispersed phase, the steeper the chemical potential gradient that can be established for the ion, and hence the faster its possible rate of diffusion (FIeFE et al., 1958, p. 79; MUELl4ER, 1967). MOREY (1957) has shown that the solubility of alumina in superheated steam is three orders of magnitude smaller than that of silica. Moreover, the solubility of aluminosilicate minerals in a quartz-bearing assemblage is likely to be restricted by the common-ion effect; the solubility of albite in a quartz-saturated solution is much smaller than the solubility of albite itself (W. S. FYFE, personal communi­ cation, 1966). In the absence of convincing experimental evidence that aluminum is relatively immobile, some indirect evidence recorded in the textures of pelitic rocks may be cited. The fol1owing examples are from the vVhetstone Lake area, southeastern Ontario, mapped by the writer for the Geological Survey of Canada during the summers of 1965 and 1966. Small·Scale Constancy of Aluminum, Concentration A convenient numerical measure of the volumetric concentration of aluminum in a minera.I may be derived by dividing the number of aluminum ions in a miner- Mechanism of Metamorphic Reactions in Pelitic Rocks 247 aI's formula by tbe molar volume of that mineral, and multiplying by 100. Some common minerals of pelitic schists are listed below, in order of increasing con­ centration of aluminum, based on chemical data of ALBEE (1962), HOUNSLOW and MOORE (1967), and DEER et al.
Load more

Recommended publications
  • The Metamorphosis of Metamorphic Petrology
    Downloaded from specialpapers.gsapubs.org on May 16, 2016 The Geological Society of America Special Paper 523 The metamorphosis of metamorphic petrology Frank S. Spear Department of Earth and Environmental Sciences, JRSC 1W19, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180-3590, USA David R.M. Pattison Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada John T. Cheney Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA ABSTRACT The past half-century has seen an explosion in the breadth and depth of studies of metamorphic terranes and of the processes that shaped them. These developments have come from a number of different disciplines and have culminated in an unprece- dented understanding of the phase equilibria of natural systems, the mechanisms and rates of metamorphic processes, the relationship between lithospheric tecton- ics and metamorphism, and the evolution of Earth’s crust and lithospheric mantle. Experimental petrologists have experienced a golden age of systematic investigations of metamorphic mineral stabilities and reactions. This work has provided the basis for the quantifi cation of the pressure-temperature (P-T) conditions associated with various metamorphic facies and eventually led to the development of internally con- sistent databases of thermodynamic data on nearly all important crustal minerals. In parallel, the development of the thermodynamic theory of multicomponent, multi- phase complex systems underpinned development of the major methods of quantita- tive phase equilibrium analysis and P-T estimation used today: geothermobarometry, petrogenetic grids, and, most recently, isochemical phase diagrams. New analytical capabilities, in particular, the development of the electron micro- probe, played an enabling role by providing the means of analyzing small volumes of materials in different textural settings in intact rock samples.
    [Show full text]
  • Geology and Petrology of the Hirao Limestone and the Tagawa Metamorphic Rocks−With Special Reference to the Contact Metamorphism by Cretaceous Granodiorite
    GeologyJournal and of Mineralogicalpetrology of the and Hirao Petrological Limestone Sciences, and the Volume Tagawa 99 ,metamorphic page 25─41, 2004 rocks 25 Geology and petrology of the Hirao Limestone and the Tagawa metamorphic rocks−with special reference to the contact metamorphism by Cretaceous granodiorite * ** *** Mayuko FUKUYAMA , Kensaku URATA and Tadao NISHIYAMA *Graduate School of Science and Technology, Kumamoto University, Kurokami 2－39－1, Kumamoto 860－8555 **Department of Geography, Tokyo Metropolitan University, Minami－Osawa 1－1, Hachioji, Tokyo 192－0397 ***Department of Earth Sciences, Kumamoto University, Kurokami, 2－39－1, Kumamoto 860－8555 The Hirao Limestone located in northeast Kyushu is bordered on the north by unmetamorphosed Paleozoic stra- ta (the Kagumeyoshi Formation), and on the south by the Tagawa metamorphic rocks, a member of the Sangun Metamorphic Rocks. They are thermally metamorphosed due to the Cretaceous Hirao granodiorite intrusion. As a result, neither the fossil record nor the record of the regional (Sangun) metamorphism has been preserved. We investigated the geologic relationships among these units. The tectonic collage model associated with ac- cretionary processes proposed by Kanmera and Nishi (1983) seems the most reasonable. The Hirao Limestone and the uppermost part of the Tagawa metamorphic rocks may be a large olistolith formed during the accretion of the Palaeozoic formation. Petrological studies show that the area was thermally metamorphosed as revealed by widespread occur- rences of biotite in pelitic rocks both in the Tagawa metamorphic rocks and in the Kagumeyoshi Formation. Garnet occurrences in the pelitic schists close to the Hirao granodiorite suggest about 700°C for the peak meta- morphic temperature of contact metamorphism, based on garnet－biotite geothermometers.
    [Show full text]
  • The Dalradian Rocks of the North-East Grampian Highlands of Scotland
    Revised Manuscript 8/7/12 Click here to view linked References 1 2 3 4 5 The Dalradian rocks of the north-east Grampian 6 7 Highlands of Scotland 8 9 D. Stephenson, J.R. Mendum, D.J. Fettes, C.G. Smith, D. Gould, 10 11 P.W.G. Tanner and R.A. Smith 12 13 * David Stephenson British Geological Survey, Murchison House, 14 West Mains Road, Edinburgh EH9 3LA. 15 [email protected] 16 0131 650 0323 17 John R. Mendum British Geological Survey, Murchison House, West 18 Mains Road, Edinburgh EH9 3LA. 19 Douglas J. Fettes British Geological Survey, Murchison House, West 20 Mains Road, Edinburgh EH9 3LA. 21 C. Graham Smith Border Geo-Science, 1 Caplaw Way, Penicuik, 22 Midlothian EH26 9JE; formerly British Geological Survey, Edinburgh. 23 David Gould formerly British Geological Survey, Edinburgh. 24 P.W. Geoff Tanner Department of Geographical and Earth Sciences, 25 University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow 26 27 G12 8QQ. 28 Richard A. Smith formerly British Geological Survey, Edinburgh. 29 30 * Corresponding author 31 32 Keywords: 33 Geological Conservation Review 34 North-east Grampian Highlands 35 Dalradian Supergroup 36 Lithostratigraphy 37 Structural geology 38 Metamorphism 39 40 41 ABSTRACT 42 43 The North-east Grampian Highlands, as described here, are bounded 44 to the north-west by the Grampian Group outcrop of the Northern 45 Grampian Highlands and to the south by the Southern Highland Group 46 outcrop in the Highland Border region. The Dalradian succession 47 therefore encompasses the whole of the Appin and Argyll groups, but 48 also includes an extensive outlier of Southern Highland Group 49 strata in the north of the region.
    [Show full text]
  • How Do Metamorphic Fluids Move Through Rocks? an Investigation of Timescales, Infiltration Mechanisms and Mineralogical Controls
    How do metamorphic fluids move through rocks? An investigation of timescales, infiltration mechanisms and mineralogical controls Barbara I. Kleine ©Barbara I. Kleine, Stockholm University 2015 ISBN 978-91-7649-120-1 Cover picture: A relict of glaucophane from Fabrika Beach. Printed in Sweden by Holmbergs, Malmö 2015 Distributor: Publit Abstract This thesis aims to provide a better understanding of the role of mountain building in the carbon cycle. The amount of CO2 released into the atmosphere due to metamorphic processes is largely unknown. To constrain the quantity of CO2 released, fluid-driven reactions in metamorphic rocks can be studied by tracking fluid-rock interactions along ancient fluid flow pathways. The thesis is divided into two parts: 1) modeling of fluid flow rates and durations within shear zones and fractures during greenschist- and blueschist-facies metamorphism and 2) the assessment of possible mechanisms of fluid infiltration into rocks during greenschist- to epidote-amphibolite-facies metamorphism and controlling chemical and mineralogical factors of reaction front propagation. On the island Syros, Greece, fluid-rock interaction was examined along a shear zone and within brittle fractures to calculate fluid flux rates, flow velocities and durations. Petrological, geochemical and thermodynamic evidence show that the flux of CO2-bearing fluids along the shear zone was 100-2000 times larger than the fluid flux in the surrounding rocks. The time- averaged fluid flow velocity and flow duration along brittle fractures was calculated by using a governing equation for one-dimensional transport (advection and diffusion) and field-based parameterization. This study shows that fluid flow along fractures on Syros was rapid and short lived.
    [Show full text]
  • Joint Meeting
    Joint Meeting 19. Jahrestagung der Deutschen Gesellschaft für Kristallographie 89. Jahrestagung der Deutschen Mineralogischen Gesellschaft Jahrestagung der Österreichischen Mineralogischen Gesellschaft (MinPet 2011) 20.-24. September 2011 Salzburg Referate Oldenbourg Verlag – München Inhaltsverzeichnis Plenarvorträge ............................................................................................................................................................ 1 Goldschmidt Lecture .................................................................................................................................................. 3 Vorträge MS 1: Crystallography at High Pressure/Temperature ................................................................................................. 4 MS 2: Functional Materials I ........................................................................................................................................ 7 MS 3: Metamorphic and Magmatic Processes I ......................................................................................................... 11 MS 4: Computational Crystallography ....................................................................................................................... 14 MS 5: Synchrotron- and Neutron Diffraction ............................................................................................................. 17 MS 6: Functional Materials II and Ionic Conductors ................................................................................................
    [Show full text]
  • Petrogenesis of Metamorphic Rocks, 8Th Edition
    Petrogenesis of Metamorphic Rocks . Kurt Bucher l Rodney Grapes Petrogenesis of Metamorphic Rocks Prof.Dr. Kurt Bucher Prof. Rodney Grapes University Freiburg Department of Earth and Environmental Mineralogy Geochemistry Sciences Albertstr. 23 B Korea University 79104 Freiburg Seoul Germany Korea [email protected] [email protected] ISBN 978-3-540-74168-8 e-ISBN 978-3-540-74169-5 DOI 10.1007/978-3-540-74169-5 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2011930841 # Springer-Verlag Berlin Heidelberg 2011 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: deblik Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Preface This new edition of “Petrogenesis of Metamorphic Rocks” has several completely revised chapters and all chapters have updated references and redrawn figures.
    [Show full text]
  • Petrology and Geochemistry of Selected Talc-Bearing Ultramafic Rocks and Adjacent Country Rocks in North-Central Vermont
    Petrology and Geochemistry of Selected Talc-bearing Ultramafic Rocks and Adjacent Country Rocks in North-Central Vermont GEOLOGICAL SURVEY PROFESSIONAL PAPER 345 Petrology and Geochemistry of Selected Talc-bearing Ultramafic Rocks and Adjacent Country Rocks in North-Central Vermont By ALFRED H. CHIDESTER GEOLOGICAL SURVEY PROFESSIONAL PAPER 345 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1962 UNITED STATES DEPARTMENT OF THE INTERIOR STEW ART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The U.S. Geological Survey Library has cataloged this publication as follows : Chidester, Alfred Herman, 1914- Petrology and geochemistry of selected talc bearing- ultra- mafic rocks and adjacent country rocks in north-central Vermont. Washington, U.S. Govt. Print. Off., 1961. vii, 207 p. illus., maps (7 fold. col. in pocket) cliagrs.. tables. 29 cm. (U.S. Geological Survey. Professional paper 345) Bibliography: p. 205-207. 1. Petrology Vermont. 2. Talc Vermont. 3. Mines and mineral resources Vermont. 4. Geochemistry Vermont. 5. Rocks, Igne­ ous Vermont. I. Title. (Series) For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, B.C. CONTENTS Page Geology of the Barnes Hill, Waterbury mine, and Mad Abstract. __________________________________________ 1 River localities Continued Introduction. ______________________________________ 3 Petrography Continued Location and history. ___________________________ 3 Schist Continued Barnes Hill locality _________________________ 3 Mineralogy, etc. Continued Page Waterbury mine locality_____--_---__________ 4 . Ilmenite, rutile, and sphene_ _________ 52 Mad River locality_________---_----__-______ 4 Garnet.___________________________ 52 Previous investigations __________________________ 5 Apatite__. _________________________ 53 Fieldwork and acknowledgments. -___-_---________ 5 Epidote and allanite _______________ 53 Geologic setting. ___________________________________ 6 Other minerals _____________________ 53 Regional setting.
    [Show full text]
  • Effect of Metamorphic Reactions on Thermal Evolution in Collisional Orogens
    J. metamorphic Geol., 2009, 27, 579–600 doi:10.1111/j.1525-1314.2009.00847.x Effect of metamorphic reactions on thermal evolution in collisional orogens T. LYUBETSKAYA AND J. J. AGUE Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109, USA ([email protected]) ABSTRACT The effects of metamorphic reactions on the thermal structure of a collisional overthrust setting are examined via forward numerical modelling. The 2D model is used to explore feedbacks between the thermal structure and exhumation history of a collisional terrane and the metamorphic reaction progress. The results for average values of crustal and mantle heat production in a model with metapelitic crust composition predict a 25–40 °C decrease in metamorphic peak temperatures due to dehydration reactions; the maximum difference between the P–T–t paths of reacting and non-reacting rocks is 35–45 °C. The timing of the thermal peak is delayed by 2–4 Myr, whereas pressure at peak temperature conditions is decreased by more than 0.2 GPa. The changes in temperature and pressure caused by reaction may lead to considerable differences in prograde reaction pathways; the consumption of heat during dehydration may produce greenschist facies mineral assemblages in rocks that would have otherwise attained amphibolite facies conditions in the absence of reaction enthalpy. The above effects, although significant, are produced by relatively limited metamorphic reaction which liberates only half of the water available for dehydration over the lifetime of the prograde metamorphism. The limited reaction is due to the lack of heat in a model with the average thermal structure and relatively fast erosion, a common outcome in the numerical modelling of Barrovian metamorphism.
    [Show full text]
  • The Study of Metamorphic Rocks, Zonation and Isogrades in Garnet Rocks in the Hamadan Area
    Current World Environment Vol. 7(1), 55-59 (2012) The Study of Metamorphic Rocks, Zonation and Isogrades in Garnet Rocks in the Hamadan Area ZAHRA HOSSEIN MIRZAEI BENI and ZOHREH HOSSEIN MIRZAEI BENI Young Research Club, Khorasgan (Isfahan) Branch, Islamic Azad University, Isfahan (Iran). (Received: March 03, 2012; Accepted: April 15, 2012) ABSTRACT The study area is a part of the Sanandaj- Sirjan metamorphic belt. We can divide Hamadan metamorphic rocks in three groups: regional metamorphic rocks, contact metamorphic rocks and migmatites. In this area we can’t completely divide zonation of contact and regional metamorphic. In some places that contact metamorphic has influenced to low degree regional metamorphic rocks, contact metamorphic zonations are clearly appear, but when contact and regional metamorphic have a same degree or regional metamorphic has high degree than contact metamorphic, we can’t distinguish them easily. In Hamadan area regional metamorphic zones are Chlorite± Biotite zone (we haven’t garnet rocks in this zone), Biotite± Garnet zone (divided in two sub zone, Biotite and Garnet zone), Andalusite zone, Staurolite zone, Staurolite± Andalusite zone, Sillimanite- Muscovite zone and Sillimanite- Potassium feldspar± Cordierite zone, also contact metamorphic zones are Cordierite zone and Cordierite- Potassium feldspar zone. Key words: Contact metamorphic; Garnet; Isogrades; Metamorphic zonation; Migmatites; Regional metamorphic. INTRODUCTION minerals chemistry in the study area are rich in almandine. Garnet crystallizes in cubic system and mostly in dodecahedron (rhomb-dodecahedron) Geological Setting and trapezohedron (tetragon-trioctahedron) crystal The study area is a part of the Sanandaj- forms. General chemical formula of this mineral is: Sirjan metamorphic belt. The Alvand plutonic 2+ 2+ R3R’2(SiO4)3, which bivaliant cations (i.e.
    [Show full text]
  • Metamorphic Reactions and Protoliths of Metamorphic Rocks Development of Metamorphic Rocks
    Lecture FOUR Metamorphic Reactions and Protoliths of Metamorphic Rocks Development of Metamorphic Rocks The yielded metamorphic rocks, with specific mineral composition and textures is a function of: Protolith nature i.e: whole rock chemistry (pelitic (Argillaceous), semipelitic, calcareous (limestone and dolomite), mafic-ultramafic, basic igneous, granitic, Mn-rich sediments, ironstone, laterites... etc.) P-T-X conditions (the intensity of temperature and the intensity and type of pressure (simple compressed or twisted and broken) influence and the presence or absence of fluids and their chemistry during metamorphism) Time (how long the rock subjected to HT and HP?. By which the grain size was evolved, and the reactions were proceed) Metamorphic Rocks components and development A metamorphic rock consists of individual grains of several solid minerals and a network of grain boundaries, which at the time of metamorphism may have held an aqueous fluid, providing pathways for transport through the rock. garnet Omphacite garnet Development of Metamorphic minerals For a new mineral to appear by a chemical reaction, a number of processes have to operate in concern: Nucleation: nuclei (embryo crystals) of the new mineral appear Interface reactions - dissolution: reactant minerals break down, their chemical constituents going into solution Interface reactions - growth: material is added onto the nuclei to build larger crystals Mass transfer: material is transported through the rock from sites of breakdown to sites of growth Nucleation, Mineral growth and Grain size -Completed reaction produces an amount of product (mineral phases). The microstructure, will depend on the relative rates of nucleation and growth of minerals - Grain size in a metamorphic rocks is a function in: - Intensity of P-T conditions, - nuclation rate, and -Time interval of metamorphism -Coarse-grained rocks are the product of long sustained metamorphic conditions (possibly over millions of years) at HT and HP (e.g.
    [Show full text]
  • Abstract Volume
    ABSTRACT VOLUME THE 9th IGNEOUS AND METAMORPHIC STUDIES GROUP MEETING 15-18 January 2017 Glenburn Lodge, Muldersdrift Organised by: Marlina Elburg, Jérémie Lehmann, Trishya Owen-Smith & Sebastian Tappe Department of Geology, University of Johannesburg Sponsored by: UNUSUAL FEATURES WITHIN THE STRATIGRAPHY OF THE CRITICAL ZONE SOUTH OF MOKOPANE Acheampong, K.O1., Kinnaird, J.A1., Nex, P.A.M.1 1 School of Geosciences, University of the Witwatersrand, Wits 2050, South Africa; [email protected] The mafic rocks of the Bushveld Complex differ to the north and south of the Ysterberg-Planknek Fault, part of the array that is the Thabazimbi-Murchison Lineament. To the north, there is sporadic Lower Zone, succeeded by a PGE-Cu-Ni-bearing Platreef, Main Zone and Upper Zone successions. To the south is Lower Zone, the Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member and Main Zone. This investigation on farm of Grasvally aims to provide a better understanding of the mafic rocks south of Mokopane, in order to provide an improved understanding of the relationship between the geology and mineralisation of this region. The farm Grasvally is situated approximately 20 km south of Mokopane. The geology of the Grasvally area comprises mainly mafic to ultramafic Bushveld rocks of Lower Zone to Main Zone affinity which were emplaced above sediments of the Pretoria Group. The GNPA member was originally divided into two major sub-units by Hulbert (1983) but more recently it has been divided into Lower Mafic Unit (LMF), the Lower Gabbronorite Unit (LGN) and the Mottled Anorthosite Unit (MANO) (de Klerk, 2005). The GNPA member may be the equivalent of the Platreef to the north of the Ysterberg-Planknek Fault.
    [Show full text]
  • EGU2010-9229, 2010 EGU General Assembly 2010 © Author(S) 2010
    Geophysical Research Abstracts Vol. 12, EGU2010-9229, 2010 EGU General Assembly 2010 © Author(s) 2010 Interpreting metamorphic and metasomatic reactions from mineral textures Håkon Austrheim (1) and Andrew Putnis (2) (1) Physics of Geological Processes, University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway ([email protected]) , (2) University of Münster, Institut für Mineralogie, Münster, Germany ([email protected]) Metamorphism and metasomatism both involve the reequilibration of mineral assemblages due to changes in pres- sure, temperature and/or chemical environment. The distinction between metasomatism (which is a metamorphic reaction involving a significant change in chemical composition) and metamorphism (essentially isochemical) may appear to be well defined, but since no reaction can occur without transport these definitions require that we specify an arbitrary length scale to distinguish the two processes. It is generally accepted that material transport, even on a local scale, involves a fluid phase, and that dissolution and reprecipitation reactions are involved in converting one assemblage to another (Carmichael, 1969; Philpotts and Ague, 2009). Given that the fluid phase is not just pure water, a key question is the thermodynamic as well as the kinetic role of the fluid phase in the reaction. In a metamorphic event P-T paths are inferred from sequences of discontinuous reactions assumed to take place as a rock crosses univariant reaction lines in P-T space and continuous reactions involving chemical reequilibration in coexisting minerals. Any fluid is assumed to merely play a catalytic role and not affect the thermodynamics of the reaction. The criteria for a ‘metamorphic event’ are primarily based on mineral textures in a rock, but this is subject to misinterpretation, as pointed out by Vernon et al.
    [Show full text]