DOCSLIB.ORG

Charnockitic Magmatism in Southern India

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Charnockitic Magmatism in Southern India Charnockitic magmatism in southern India H M Rajesh1∗ and M Santosh2 1Department of Geographical Sciences and Planning, University of Queensland, St Lucia, 4072 Queensland, Australia. 2Department of Natural Environmental Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan. ∗e-mail: [email protected] Large charnockite massifs cover a substantial portion of the southern Indian granulite terrain. The older (late Archaean to early Proterozoic) charnockites occur in the northern part and the younger (late Proterozoic) charnockites occur in the southern part of this high-grade terrain. Among these, the older Biligirirangan hill, Shevroy hill and Nilgiri hill massifs are intermediate charnockites, with Pallavaram massif consisting dominantly of felsic charnockites. The charnockite massifs from northern Kerala and Cardamom hill show spatial association of intermediate and felsic charnock- ites, with the youngest Nagercoil massif consisting of felsic charnockites. Their igneous parentage is evident from a combination of features including field relations, mineralogy, petrography, ther- mobarometry, as well as distinct chemical features. The southern Indian charnockite massifs show similarity with high-Ba–Sr granitoids, with the tonalitic intermediate charnockites showing simi- larity with high-Ba–Sr granitoids with low K2O/Na2O ratios, and the felsic charnockites showing similarity with high-Ba–Sr granitoids with high K2O/Na2O ratios. A two-stage model is suggested for the formation of these charnockites. During the first stage there was a period of basalt under- plating, with the ponding of alkaline mafic magmas. Partial melting of this mafic lower crust formed the charnockitic magmas. Here emplacement of basalt with low water content would lead to dehy- dration melting of the lower crust forming intermediate charnockites. Conversely, emplacement of hydrous basalt would result in melting at higher fH2O favoring production of more siliceous felsic charnockites. This model is correlated with two crustal thickening phases in southern India, one related to the accretion of the older crustal blocks on to the Archaean craton to the north and the other probably related to the collision between crustal fragments of East and West Gondwana in a supercontinent framework. 1. Introduction example, in Limpopo Belt, South Africa, Bohlen- der et al (1992) proposed in situ biotite dehydra- Charnockites characterized by orthopyroxene- tion to form orthopyroxene in the presence of a bearing granitic mineral assemblages are a common fluid phase with locally different activity of CO2 constituent of granulite-facies metamorphic ter- for the metamorphic charnockite and crystalliza- rains. However, the relative importance of igneous tion from calc-alkaline magmas derived by partial versus metamorphic processes involved in their ori- melting of lower crustal rocks for the spatially asso- gin is debated. They are either granitic rocks meta- ciated igneous charnockite. morphosed to the granulite facies (metamorphic Charnockitic rocks usually show spatial asso- charnockites; e.g., Newton et al 1980) or rocks ciation of rocks (charnockite–charnoenderbite– whose pyroxene crystallized directly from magma enderbite) differing in modal abundance of (igneous charnockites; e.g., Wendlandt 1981). For the dominant feldspar species. There is a Keywords. Charnockitic magmatism; southern India; intermediate charnockite; felsic charnockite; petrogenetic model. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 113, No. 4, December 2004, pp. 565–585 © Printed in India. 565 566 H M Rajesh and M Santosh ∗ ∗ Figure 1. Fe /(Fe + MgO) vs. SiO2 and Na2O+K2O − CaO vs. SiO2 plots illustrating the representative trends of I-type granitoids, A-type granitoids, intermediate charnockites, and felsic charnockites. great deal of scientific literature covering the rocks interpreted by Dobmeier and Raith (2000) charnockite (opx-bearing granite)–charnoenderbite as enderbite are, in reality, charnockites or char- (opx-bearing granodiorite)–enderbite (opx-bearing noenderbites. Hence, Bhattacharya and Sen (2002) tonalite) (Holland 1900; Tilley 1936; Le Maitre proposed a common origin for charnockite and 2002) suite of rocks although no consensus exists enderbite at Chilka Lake. about the origin of at least some charnockite Charnockites, like granitoids, have a common occurrences including classic localities. A case compositional range and can be broadly divided of ongoing uncertainty is the origin of arrested into intermediate charnockites and felsic charnock- charnockite in the Chilka Lake area, India. Dob- ites, with most reported occurrences showing vari- meier and Raith (2000) postulated that these rocks ation from intermediate to felsic compositions formed as a result of localized synkinematic fluid (Rajesh 2004b). Intermediate charnockites are migration and were genetically linked to the host dominantly calc-alkalic (in terms of modified alkali- leptynite (garnet-biotite gneiss) in which they are lime index) and ferroan to magnesian (in terms enclosed. These authors also proposed that the of Fe-number), while felsic charnockites are dom- charnockites are completely unrelated to ender- inantly alkali-calcic and ferroan (figure 1). The bite layers in the same outcrops. In a comment, relatively iron-enriched nature of felsic charnock- Bhattacharya and Sen (2002) claimed that many ites imparts to them a predominantly tholeiitic Charnockitic magmatism in southern India 567 Figure 2. Shaded relief image of southern India showing the distribution of major charnockite massifs (cf. Rajesh 2004b, c). Not all charnockite massifs are considered in this study. The different crustal blocks are also shown. TB – Trivandrum Block; NB – Northern Block. affinity, in contrast to the predominantly calc- metamorphic charnockites showing relict composi- alkaline affinity of the intermediate charnockites tional banding and obliteration of foliation with the (Rajesh 2004b). adjacent gneiss, to patches and veins of charnock- The formation of charnockites may provide ite (incipient charnockites), representing in situ important constraints in trying to unravel the stages of granulite formation driven by the influx mechanisms involved in granulite formation, of CO2-rich fluids (carbonic metamorphism; New- because their granitic and often undepleted chem- ton et al 1980). The aim of this paper is to focus istry, which is more akin to the melt fraction on the characteristics of the magmatic charnock- rather than the restite, precludes their formation ites (charnockite hereafter, unless otherwise men- by removal of a hydrous partial melt. As dis- tioned) that constitute the major granulite masses cussed by Kilpatrick and Ellis (1992), the high in southern India. In the process we compile geo- temperature origin of charnockites compared to I- chemical data of these massifs to discuss and type granitoids, together with their more enriched develop an understanding of how geochemical char- incompatible element abundances, clearly excludes acteristics relate to their source rocks, petrogenetic the possibility that they represent the remelting of processes and tectonic environments. granulitic residues formed after the extraction of I- type granitoids, a model proposed for the origin of A-type granitoids by Collins et al (1982). Genera- 2. Geologic setting tion of charnockite magmas by high-temperature, water-deficient melting of mafic to intermediate, The southern granulite terrain (south of Dhar- high-K sources compositionally similar to alkaline war craton) includes several regional en echelon basalts or calc-alkaline basaltic andesites is one of Neoproterozoic shear zones which dissect the ter- the alternatives (e.g., Rajesh 2004a). rain into different Late Archaean and Proterozoic In the granulite-facies terrain of southern India, crustal blocks such as the Madras, Northern, Nil- charnockites include intrusions of gabbroic and giri, Madurai and Trivandrum blocks (Harris et al tonalitic-enderbitic composition occurring over 1994) (figure 2). The northern part of the southern large areas (massif igneous charnockites), massive granulite terrain consists primarily of charnockite 568 H M Rajesh and M Santosh massifs that form highland areas interspersed with of Palghat-Cauvery shear zone contains ample evi- lowlands consisting of felsic rocks generally in dence for Pan-African activity (metamorphism and amphibolite facies (the Madras, Northern, and magmatism). Compared to the older charnockite Nilgiri blocks). The nature of contacts between massifs from Madras-Northern-Nilgiri blocks, the highland and lowland rocks is highly controver- Madurai and Trivandrum blocks expose early- to sial, with faults suggested, but not proved, at late-Proterozoic charnockite massifs (Bartlett et al many of the boundaries of the massifs. With the 1995; Jayananda et al 1995; Miller et al 1996; exception of granitoids, carbonatites, and ultra- Mohan and Jayananda 1999). Recent studies from mafic rocks, Pan-African ages are absent from the Madurai block have suggested a polymetamorphic northern part of this granulite terrain. Granulite- and multistage P-T (700–1000◦C and 5–12 kbar; facies metamorphism affected the area at ca. 2.5 Ga Harris et al 1982; Mohan and Windley 1993; Raith and 2.8–3.0 Ga. The Nilgiri block was metamor- et al 1997; Satish Kumar et al 2002) evolution phosed under medium-high pressure (6–10 kbar; for the terrane. Nd-model ages from the Madurai Harris et al 1982; Janardhan et al 1982; Raith block range from 2.1–3.1 Ga (Harris et
Load more

Recommended publications
  • ANTIPERTHITE and MANTLED FELDSPAR TEXTURES in CHARNOCKITE (ENDERBITE) from S.W. NIGERIA Fnnp. H. Hunrenn, Departmenl of Geology
    THE AMERICAN MINERALOGIST, VOL 50, \OVEMBER-DECEMBER, 1965 ANTIPERTHITE AND MANTLED FELDSPAR TEXTURES IN CHARNOCKITE (ENDERBITE) FROM S.W. NIGERIA Fnnp. H. Hunrenn, Departmenl of Geology,Fourah Bay College, Freelown,Siena Leone. ABSTRACT The strongly antiperthitic plagioclases of a hypersthene-quattz-dioritic (enderbite) member of a charnockite series occurring in S.W. Nigeria v''ere found to be commonly mantled by alkali feldspar (orthoclase microperthite). The potassium-rich phase of the orthoclase microperthite is in optical continuity with the potassium-rich phase of the anti- perthites. The petrography and mineralogy of these feldspars are described, including qualitative data obtained by microprobe analysis, and the probability of a common origin for the antiperthite and mantle alkali feldspar by exsolution processes discussed. Modal analyses of the proportions of alkali feldspar to plagioclase for trvelve individual crystals from the same rock specimen gave sufficientiy constant values to help substantiate the proposal of exsolution origin. INrnolucrroN Antiperthitic plagioclaseis a very common feature of the charnockitic rocks, to such an extent that it is often quoted as a characteristicof such rocks (e.g.,Heinrich, 1956).Because of the practical difficultiesof experi- mental and synthetic studies inherent in the sluggish transformation kinetics of the plagioclases,antiperthite texture has not receivedas great attention as the perthite textures from researchworkers. Development of antiperthite both by exsolutionand replacementprocesses is accepted (Deer et el., t963). An exsolution origin for the antiperthites in granulite faciesrocks has beenconsidered by Sen (1959).This worker investigated the potassium content of a seriesof naturai plagioclasefeldspars from amphibolite facies,granulite faciesand volcanic rocks by meansof par- tial chemical analyses.He found a definite increasein potassium content utith increase in temperature of formation and sought to explain this trend by lattice structural consideratiorls,e.g.
    [Show full text]
  • The Charnockite-Anorthosite Suite of Rocks Exposed in Central Dronning
    The charnockite-anorthosite suite of rocks exposed in central Dronning Maud Land, East Antarctica: a study on fluid-rock interactions, and post-entrapment change of metamorphic fluid inclusions Die charnockitischen und anorthositischen Gesteinsserien im zentralen Dronning Maud Land: Fluid-Gesteins-Wechselwirkungen und die Veränderung metamorpher Fluid-Einschlüsse nach ihrer Bildung Bärbel Kleinefeld Geologie der Polargebiete Fachbereich Geowissenschaften Universität Bremen Postfach 330440 28334 Bremen Die vorliegende Arbeit ist die inhaltlich unveränderte Fassung der Dissertation, die im Juli 2002 dem Fachbereich Geowissenschaften der Universität Bremen unter gleichem Titel vorgelegt wurde. Tableofcontents Table of contents Abstract.......................................................................................................................1 Zusammenfassung.................................................................................................... 3 1. Introduction ........................................................................................................5 1.1. Fluid-rockinteractionsindeep-seatedcrustalrocks...............................5 1.2. Previousstudiesandscopeofthethesis..................................................7 2. The charnockite – anorthosite suite of rocks..................................................9 2.1. Classifyingrocksofthecharnockiteseries ..............................................9 2.2. Massif-typeanorthosites.........................................................................
    [Show full text]
  • On Charnockites ⁎ B
    Available online at www.sciencedirect.com Gondwana Research 13 (2008) 30–44 www.elsevier.com/locate/gr GR Focus On charnockites ⁎ B. Ronald Frost , Carol D. Frost Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, USA Received 4 June 2007; received in revised form 20 July 2007; accepted 24 July 2007 Available online 7 August 2007 Abstract Charnockitic rocks form extensive orthogneiss plutons in many granulite terranes and are less commonly found in unmetamorphosed plutons, which have formed in various tectonic regimes. Geochemically, clearly igneous charnockites cover nearly the whole range of granite chemistry, from magnesian to ferroan and from calcic to alkalic. Pyroxenes from unmetamorphosed charnockitic rocks have compositions ranging from magnesian to very iron-rich and record temperatures as high as 1000 °C. Oxygen fugacities for these plutons range from below FMQ to Δ log FMQN+2, values that cover nearly the whole range found in other granitic rocks. This range in bulk chemistry and intensive parameters is a reflection of the many mechanisms that produce charnockites. They may form in rifting environments, where they are ferroan, alkali-calcic to alkalic and metaluminous. Many of these ferroan charnockites are isotopically primitive, suggesting that they have been derived largely or entirely from differentiation or melting of tholeiitic melts. Charnockites are also found in deeply eroded arcs, where they are magnesian, calcic to calc-alkalic and metaluminous. Some charnockitic magmas may form by crustal melting or have incorporated a large component of crustal melt; these plutons tend to be weakly to moderately peraluminous and to have intermediate values of FeO/(FeO+MgO).
    [Show full text]
  • Metamorphism
    Title page INTRODUCING METAMORPHISM Ian Sanders DUNEDIN EDINBURGH LONDON Contents Contents v Preface ix Acknowledgements x 1 Introduction 1 1.1 What is metamorphism? 1 1.1.1 Protoliths 1 1.1.2 Changes to the minerals 1 1.1.3 Changes to the texture 3 1.1.4 Naming metamorphic rocks 3 1.2 Metamorphic rocks – made under mountains 3 1.2.1 Mountain building 3 1.2.2 Directed stress, pressure and temperature in a mountain’s roots 4 1.2.3 Exhumation of a mountain’s roots 6 1.3 Metamorphism in local settings 6 1.3.1 Contact metamorphism 7 1.3.2 Hydrothermal metamorphism 7 1.3.3 Dynamic metamorphism 9 1.3.4 Shock metamorphism 9 2 The petrography of metamorphic rocks 11 2.1 Quartzite and metapsammite 11 2.1.1 Quartzite 11 2.1.2 Metapsammite 13 2.2 Metapelite 13 2.2.1 Slate 14 2.2.2 Phyllite and low-grade schist 16 2.2.3 Minerals and textures of medium-grade schist 17 2.2.4 The regional distribution of minerals in low- and medium-grade schist 20 2.2.5 Pelitic gneiss and migmatite 22 2.2.6 Metapelite in a contact aureole 23 2.2.7 The significance of Al2SiO5 for inferring metamorphic conditions 23 2.3 Marble 24 2.3.1 Pure calcite marble 24 2.3.2 Impure marble 26 2.3.3 Metasediments with mixed compositions 29 CONTENTS 2.4 Metabasite 30 2.4.1 Six kinds of metabasite from regional metamorphic belts 31 2.4.2 The ACF triangle for minerals in metabasites 36 2.4.3 P–T stability of metabasites, and metamorphic facies 38 vi 2.4.4 A metabasite made by contact metamorphism 40 2.5 Metagranite 41 2.5.1 Granitic gneiss and orthogneiss 41 2.5.2 Dynamic metamorphism
    [Show full text]
  • PRELIMINARY EVALUATION of BEDROCK POTENTIAL for NATURALLY OCCURRING ASBESTOS in ALASKA by Diana N
    Alaska Division of Geological & Geophysical Surveys MISCELLANEOUS PUBLICATION 157 PRELIMINARY EVALUATION OF BEDROCK POTENTIAL FOR NATURALLY OCCURRING ASBESTOS IN ALASKA by Diana N. Solie and Jennifer E. Athey Tremolite (UAMES 34960) displaying the soft, friable fibers of asbestiform minerals. Sample collected from the Cosmos Hills area, Kobuk District, Alaska, by Eskil Anderson. Image courtesy of the University of Alaska Museum Earth Sciences Department. June 2015 Released by STATE OF ALASKA DEPARTMENT OF NATURAL RESOURCES Division of Geological & Geophysical Surveys 3354 College Road, Fairbanks, Alaska 99709-3707 907-451-5020 dggs.alaska.gov [email protected] $2.00 (text only) $13.00 (per map sheet) TABLE OF CONTENTS Abstract ................................................................................................................................................................................................................................. 1 Introduction ........................................................................................................................................................................................................................ 1 General geology of asbestos ......................................................................................................................................................................................... 2 Naturally occurring asbestos potential in Alaska ..............................................................................................................................................
    [Show full text]
  • Streaming of Saline Fluids Through Archean Crust
    Lithos 346–347 (2019) 105157 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Streaming of saline fluids through Archean crust: Another view of charnockite-granite relations in southern India Robert C. Newton a,⁎, Leonid Ya. Aranovich b, Jacques L.R. Touret c a Dept. of Earth, Planetary and Spaces, University of California at Los Angeles, Los Angeles, CA 90095, USA b Inst. of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Science, Moscow RU-119017, Russia c 121 rue de la Réunion, F-75020 Paris, France article info abstract Article history: The complementary roles of granites and rocks of the granulite facies have long been a key issue in models of the Received 27 June 2019 evolution of the continental crust. “Dehydration melting”,orfluid-absent melting of a lower crust containing H2O Received in revised form 25 July 2019 only in the small amounts present in biotite and amphibole, has raised problems of excessively high tempera- Accepted 26 July 2019 tures and restricted amounts of granite production, factors seemingly incapable of explaining voluminous bodies Available online 29 July 2019 of granite like the Archean Closepet Granite of South India. The existence of incipient granulite-facies metamor- phism (charnockite formation) and closely associated migmatization (melting) in 2.5 Ga-old gneisses in a quarry Keywords: fl Charnockite exposure in southern India and elsewhere, with structural, chemical and mineral-inclusion evidence of uid ac- Granite tion, has encouraged a wetter approach, in consideration of aqueous fluids for rock melting which maintain suf- Saline fluids ficiently low H2O activity for granulite-facies metamorphism.
    [Show full text]
  • Petrographical Characters of Some Important Host Rocks in Vizianagarm Manganese Ores Belt (A.P.), INDIA
    UNIVERSITY OF MAURITIUS RESEARCH JOURNAL – Volume 16 – 2010 University of Mauritius, Réduit, Mauritius Petrographical Characters of Some important host rocks in Vizianagarm Manganese Ores Belt (A.P.), INDIA. F. N Siddiquie Department of Geology,, Aligarh Muslim University India Email: [email protected] Paper Accepted on 17 February 2010 Abstract Petrographical study of various rocks types has been carried out in the present study area with the discovery of a new rock type- Crystalline algal Limestone. An attempt has been made to describe the petrographical characters of the following rock types encountered in the present study area. 1. Calc-granulite 2. Garnet-sillimanite-feldspathic gneiss 3. Garnetiferous gneiss 4. Feldspathic quartzite 5. Crystalline algal limestone 6. Shale 7. Hypersthene gneiss. 8. Granitic gneiss. The older Khondalite Group mainly consists of calc-granulite, garnet sillimanite gneiss, feldspathic quartzite and garnetiferous gneiss. The occurrence of crystalline algal limestone and red and green shales have been F.N Siddiquie reported for the first time. The younger Charnockite Group mainly consists of hypersthene gneiss (porphyrroblastic and non-porphyroblastic) and granitic gneisses. Granite and pegmatite occur as intrusive bodies in this group of rocks. The Khondalites and Charnockites show a complex structural pattern resulting from repeated periods of deformation. The general strike of rocks is NW-SE with local swings at places. These rocks at places show relict banding. Stratigraphically these are included within a thick succession of Pre- Cambrian rocks belonging to the Khondalite and Charnockite Groups of Dharwar Supergroup, that form a part of Eastern Ghat Complex of India. The manganiferous rocks that have been encountered in the Vizianagarm Manganese Ore Belt (A.P.) India are known as Kodurites.
    [Show full text]
  • A Systematic Nomenclature for Metamorphic Rocks
    A systematic nomenclature for metamorphic rocks: 1. HOW TO NAME A METAMORPHIC ROCK Recommendations by the IUGS Subcommission on the Systematics of Metamorphic Rocks: Web version 1/4/04. Rolf Schmid1, Douglas Fettes2, Ben Harte3, Eleutheria Davis4, Jacqueline Desmons5, Hans- Joachim Meyer-Marsilius† and Jaakko Siivola6 1 Institut für Mineralogie und Petrographie, ETH-Centre, CH-8092, Zürich, Switzerland, [email protected] 2 British Geological Survey, Murchison House, West Mains Road, Edinburgh, United Kingdom, [email protected] 3 Grant Institute of Geology, Edinburgh, United Kingdom, [email protected] 4 Patission 339A, 11144 Athens, Greece 5 3, rue de Houdemont 54500, Vandoeuvre-lès-Nancy, France, [email protected] 6 Tasakalliontie 12c, 02760 Espoo, Finland ABSTRACT The usage of some common terms in metamorphic petrology has developed differently in different countries and a range of specialised rock names have been applied locally. The Subcommission on the Systematics of Metamorphic Rocks (SCMR) aims to provide systematic schemes for terminology and rock definitions that are widely acceptable and suitable for international use. This first paper explains the basic classification scheme for common metamorphic rocks proposed by the SCMR, and lays out the general principles which were used by the SCMR when defining terms for metamorphic rocks, their features, conditions of formation and processes. Subsequent papers discuss and present more detailed terminology for particular metamorphic rock groups and processes. The SCMR recognises the very wide usage of some rock names (for example, amphibolite, marble, hornfels) and the existence of many name sets related to specific types of metamorphism (for example, high P/T rocks, migmatites, impactites).
    [Show full text]
  • Basic Intrusion, Charnockite.Rapakivi Granite Plutonism Aod Crusta! Depletion, S
    RENolCONTI DELL,\ SOCIETA lTAUANA 01 MINERALOG IA E PETROLOGIA, 1988, \bI. 4}.2, pp. '4).,,4 Basic intrusion, charnockite.rapakivi granite plutonism aod crusta! depletion, S. W. Sweden FREDERIK H. HUBBARD Depanment of Geolosy, Dundee University, Dundee, Scotland AUSTRACT. - The southern segment of the South-West South-West Swedish Province by a series of Swedish Province (SWSP) of the Precambrian of the major thrust zones, it has characteristics Baltic Shield has features which distinguish it from the northern sesment of the same province and the peculiar to this region pf the Swedish Svecofennian sequences to the east. Repeated cycles of Precambrian. These include an abundance of syn and late·orogenic b..sic magma injection maimained meta-basic intrusions, the occurrence of high crusta! temperatures and led to progressive granulite fades assemblages within the general depletion of the rode! at, and adjacent to, the conduits. This is shown by the development of loc:::aI ~u1ile facies upper amphibolite facies terrain and zones in the seneral amphibolite facies terrain and the distinctive plutonic associations of alkaline occurrence of plutonic charnockite Jrulsses. granite and charnockite (e.g. QUENSEL P., The syn-orogenic: basic magmas caused depletion largely 1951; CALDENruS et al., 1966, MOHREN E., by the generation of hydrous solutions derived from dehydration of mafic minerals in the rountry rocks LARSSON W., 1968, HUBBARD F.H., 1975, ronsequent on the heat influx associated with the 1978). A Rb-Srisochron age of 1420Ma was intrusion. These hydrothermlll solutions metasomatised obtained by WELIN and GORBATSCHEV (1978) and migmatised the basic rocks, formed ap!ite from a composite charnock.ite and granite segregatioO$, or were exhausted to higher level.
    [Show full text]
  • Earth's Oldest Rocks
    Chapter 12 The Oldest Terrestrial Mineral Record: Thirty Years of Research on Hadean Zircon From Jack Hills, Western Australia Aaron J. Cavosie1, John W. Valley2 and Simon A. Wilde3 1Curtin University, Perth, WA, Australia; 2University of WisconsineMadison, Madison, WI, United States; 3Curtin University of Technology, Perth, WA, Australia Chapter Outline 1. Introduction 255 3.4.1 Lithium 266 2. Jack Hills Metasedimentary Rocks 256 3.4.2 Ti Thermometry 266 2.1 Age of Deposition 257 3.4.3 Rare Earth Elements, Yttrium and Phosphorous 266 2.2 Metamorphism 258 3.4.4 Other Trace Elements (Al, Sc, Sm/Nd, Xe) 267 2.3 Geology of Adjacent Rocks 258 3.5 Hafnium Isotopic Compositions 267 3. Studies of Jack Hills Zircon 259 3.6 Mineral Inclusion Studies 268 3.1 Images of Jack Hills Zircon 259 4. Early Earth Processes Recorded by Jack Hills Zircon 270 3.2 Age of the Hadean Zircon Population 259 4.1 Derivation of Jack Hills Zircon From Early Mafic 3.2.1 The UePb Story 259 Crust (εHf) 270 3.2.2 U Abundance, Radiation Damage, and Pb Loss 261 4.2 Existence of >4300 Ma Granitoid 270 3.2.3 The Oldest Grains in the Jack Hills 262 4.3 Significance of Surface Alteration on the Early Earth 271 3.2.4 Distribution of Hadean Grains 264 4.4 Impact Events Recorded in Jack Hills Zircon? 271 3.3 Oxygen Isotopes 264 Acknowledgments 273 3.4 Trace Elements 266 References 273 1. INTRODUCTION Little is known of the early Earth because of the absence of a rock record for the first 500 million years after accretion.
    [Show full text]
  • Metamorphic Rocks
    Metamorphic Rocks Adapted by Lyndsay R. Hauber, Michael B. Cuggy, & Joyce M. McBeth (2018) University of Saskatchewan from Deline B, Harris R & Tefend K. (2015) "Laboratory Manual for Introductory Geology". First Edition. Chapter 11 "Metamorphic Rocks" by Karen Tefend, CC BY-SA 4.0. 5 5.1 INTRODUCTION Metamorphic rocks form by the physical, and sometimes chemical, alteration of a pre-existing rock, whether it is igneous or sedimentary. In some cases, even metamorphic rocks can be altered into a completely different metamorphic rock. With igneous rocks forming from the melt produced by any rock type and a sedimentary rock forming from the weathered product of any rock type, the alteration of any rock to produce a metamorphic one completes the components of what is known as the rock cycle. Basically, the rocks we encounter today that we classify as either igneous, metamorphic, or sedimentary, could have belonged to a different rock classification in the past, as rocks are recycled throughout geologic time, driven by the motion of the tectonic plates. It is easy to see that increasing the temperature of a rock can produce magma, and that rocks on the surface of the earth can break up into sediment that can ultimately lithify into a sedimentary rock. But how can we alter a solid rock into a new rock, without melting it or making it become sediment? All rocks are formed at certain temperatures and pressures on or more commonly, beneath the earth’s surface, and these rocks are the most stable at the conditions under which they form.
    [Show full text]
  • 5. Myrmekite Formed by Exsolution?
    1 ISSN 1526-5757 5. MYRMEKITE FORMED BY EXSOLUTION? Lorence G. Collins email: [email protected] February 3, 1997 In order to remain open to other possibilities for the origin of myrmekite, I report the following two sites in this presentation in which bulbous myrmekite on the border of perthitic orthoclase is suggested by other investigators to form by exsolution. Weinsberg (Moldanubian) Granite, Austria At the first site, the Weinsberg (Moldanubian) granite and an older quartz- monzonite facies of the South Bohemian pluton in Austria both contain orthopyroxene and clinopyroxene and must have crystallized from magmas at high temperatures. The granite contains orthoclase megacrysts (as much as 10 to 20 cm long), which (1) are chemically zoned with Ba-rich cores (1 to 2 % mol Cs; and locally as much as 6 to 8 % Cs) and (2) are mineralogically zoned with aligned inclusions of plagioclase whose subhedral to euhedral faces are parallel to potential faces of the orthoclase (from illustrations at poster session, III Hutton Symposium, Koller and Kloetzli, 1995; personal communication, Koller, 1996). The megacrysts are perthitic and locally are bordered by small bulbous myrmekite with tiny quartz vermicules. Plagioclase lamellae are absent in the orthoclase adjacent to the myrmekite. The myrmekite is also associated with symplectic biotite which Koller and Kloetzli (1995) suggest is formed by the breakdown reaction orthopyroxene + K-feldspar + water = biotite + quartz. Discussion Koller (personal communication at the Hutton Symposium, 1995) suggested that the myrmekite bordering the orthoclase megacrysts was formed by exsolution. If that is true, it should have contrasting characteristics with myrmekite formed by K-metasomatism.
    [Show full text]