DOCSLIB.ORG

Pressure-Temperature Paths from Garnet-Zoning: Evidence for Multiple Episodes of Thrust Burial in the Hinterland of the Sevier Orogenic Belt

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Pressure-Temperature Paths from Garnet-Zoning: Evidence for Multiple Episodes of Thrust Burial in the Hinterland of the Sevier Orogenic Belt American Mineralogist, Volume 87, pages 115–131, 2002 Pressure-temperature paths from garnet-zoning: Evidence for multiple episodes of thrust burial in the hinterland of the Sevier orogenic belt THOMAS D. HOISCH,1,* MICHAEL L. WELLS,2 AND LORI M. HANSON1,† 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, U.S.A. 2Department of Geosciences, University of Nevada, Las Vegas, Nevada 89154, U.S.A. ABSTRACT Garnets are present in two horizons of the schist of Stevens Spring from the Basin Creek area of the Grouse Creek Mountains in northwest Utah. The two horizons possess different bulk composi- tions, which resulted in garnet growth by different reactions along the same pressure-temperature (P-T) path. Garnet in the upper horizon grew from the breakdown of chlorite at upper greenschist- facies conditions and garnet in the lower horizon grew from the breakdown of staurolite at upper amphibolite-facies conditions. From the upper horizon, five garnets from three samples were ana- lyzed. All display growth zoning, ragged morphologies associated with secondary rim consumption, and yield garnet-biotite geothermometry temperatures of ~460–490 °C. From the lower horizon, one garnet from each of two samples was analyzed. These also display growth zoning, but differ from garnet in the upper horizon in that they are dominantly idioblastic and yield garnet-biotite geothermometry temperatures of ~635 °C. Garnet-biotite geothermometry calculated for every point along detailed compositional traverses across the garnets revealed localized reequilibration along rims, cracks and inclusions in both generations of garnet. Garnets from the upper horizon display prograde reequilibration and the garnets from the lower horizon display retrograde reequilibration. Numerical simulations of garnet growth using the Gibbs method with Duhem’s theorem were car- ried out to determine P-T paths. The P-T path defined by the modeling of five garnets from the upper horizon is an isothermal pressure increase of ~1.7 kbar. The P-T path defined by the modeling of two garnets from the lower horizon has a steep P-T trajectory (dP/dT = 32 bars/°C) and a total pressure change of ~0.9 kbar. Both paths are indicative of thrust burial; however, the two paths cannot be reconciled as products of a single thrust episode. These data are interpreted to indicate two different episodes of thrust burial during the Sevier Orogeny, separated by ~150 °C of heating and partial exhumation. INTRODUCTION 1983; Selverstone and Spear, 1985; Spear et al. 1990). Addi- Since the early efforts in thermal modeling of tectonic pro- tionally, arrays of P-T determinations have been interpreted as cesses (e.g., Oxburgh and Turcotte 1974; England and Thomp- recording P-T paths (e.g., Hodges and Royden 1984; McGrew son 1984), construction of pressure-temperature (P-T) paths et al. 2000). from metamorphic rocks has proven to be a powerful tool in The samples analyzed in this study are schist of Stevens studies of ancient orogenic belts. Events including thrust burial, Spring from the Basin Creek area in the northern Grouse Creek intrusive heating, and extensional exhumation have now been Mountains, Utah (Figs. 1 and 2). Garnet growth was simulated widely interpreted from the study of P-T paths (e.g., Florence using the Gibbs method with Duhem’s theorem (Spear 1989, and Spear 1993; Lang 1996; Selverstone and Spear 1985; Spear 1993) to determine P-T paths. Garnets grew in two horizons of et al. 1990; Whitney et al. 1999). differing bulk composition; each records growth at different The general approaches used in the construction of P-T paths temperatures and along different segments of the P-T path, are varied and include geothermobarometry of mineral inclu- making it possible to determine the P-T path more completely sion suites in porphyroblasts that may record progressive equi- than would have been possible with either one alone. Garnet in librium conditions during porphyroblast growth (St-Onge the upper horizon grew during upper greenschist-facies meta- 1987), inferences made from reaction textures that show the morphism along the prograde path, and garnet in the lower direction of reactions in P-T space (e.g., Thost et al. 1991), and horizon grew later during upper amphibolite-facies metamor- studies of chemical zoning in minerals, specifically garnet (e.g., phism (discussed in detail below). Florence and Spear, 1993; Lang 1996; Spear and Selverstone The two sampled horizons of the schist of Stevens Spring are the uppermost and lowermost horizons, each about 5–10 meters thick. The distance orthogonal to foliation separating * E-mail: [email protected] the two sampled horizons is about 300 m (Fig. 2). Between the † Present address: Department of Natural Resources/Air Qual- two horizons is a package of interlayered amphibolite, quartz- ity Bureau, 7900 Hickman Road, Suite 1, Urbandale, IA 50322. biotite gneiss, and garnet-free pelitic schist. There are no high- 0003-004X/02/0001–115$05.00 115 116 HOISCH ET AL.: P-T PATHS FROM GARNETS, SEVIER OROGENIC BELT FIGURE 1. Simplified geologic and location map for parts of the Raft River, Grouse Creek, and Albion Mountains. Abbreviations are as follows: MM, Middle Mountain; CR, City of Rocks; VM, Vipont Mountain. Modified from Armstrong (unpublished mapping), Compton (1972, 1975), Miller (1980), Todd (1980), Wells (1996), and new mapping by M.L. Wells (this study). Top inset, tectonic map of the western U.S., shows location of metamorphic core complexes (black fill), leading edge of the Sevier fold and thrust belt (thick line with sawteeth), and outline of the Cenozoic Basin and Range Province. Bottom inset is a generalized map illustrating location of northern Great Basin metamorphic core complexes (diagonal wavy pattern) and study area in the hinterland of the Sevier orogenic belt. Thrusts of the foreland fold and thrust belt are shown by barbed lines; hatchured lines are major normal faults of the Wasatch fault system. angle faults between the sample sites, and there is no evidence (Holdaway 2000). Pressure constraints were obtained in rocks for foliation-parallel faulting within the schist of Stevens Spring, from the upper horizon using MBPG (muscovite-biotite-pla- and thus the two sampled horizons are considered to have un- gioclase-garnet) geobarometry. Although rocks from the lower dergone the same P-T history. Sample stations UH1 and UH2 horizon possess a mineral assemblage appropriate for the ap- are from the upper horizon on the south side of Basin Creek, plication of MBPG geobarometry and geobarometry based on and LH1 is from the lower horizon on the north side of Basin the anorthite breakdown reaction (GASP geobarometry), they Creek (Fig. 2). were not used because the low grossular content of the garnets Throughout the schist of Stevens Spring, a schistosity con- and the low anorthite content of the plagioclases create prob- sisting of granoblastic quartz and feldspar and planar aligned lems with the propagation of uncertainties (Todd 1998). Crude micas is overprinted by a greenschist-facies ductile fabric that pressure constraints for rocks in the lower horizon were ob- is expressed as shear bands, strain shadows around tained through considerations of staurolite and sillimanite sta- porphyroblasts, and dynamically recrystallized quartz. Samples bilities. were examined petrographically to determine which were best suited for detailed microprobe analysis. Samples showing the GEOLOGIC SETTING least visible alteration, the least overprint of the younger fab- The metamorphic rocks studied in the Grouse Creek Moun- ric, and a mineral assemblage appropriate for thermobarometry tains are part of a discontinuous belt of isolated occurrences of and for determining P-T paths based on garnet growth zoning amphibolite-facies Barrovian metamorphic rocks in the meta- (at least quartz + muscovite + biotite + plagioclase + garnet) morphic core complexes of the western Cordillera (Fig. 1). The were judged to be suitable. The temperatures of garnet growth regional metamorphism resulted from crustal thickening within were determined using garnet-biotite geothermometry the hinterland of the Sevier orogenic belt, mostly in Cretaceous HOISCH ET AL.: P-T PATHS FROM GARNETS, SEVIER OROGENIC BELT 117 FIGURE 2. Geologic map (a), geologic cross section (b), and tectonostratigraphic column (c) of the Basin Creek area, northern Grouse Creek Mountains. Sample localities for studied pelitic schist are shown with labeled circles. Locations of kyanite- bearing metamorphic rocks are indicated with triangles on a, in close proximity to studied samples. All map units are keyed to the tectonostratigraphic column c, with the exception of Oligocene granite (black fill). time, with local enhancement of deformation fabrics and con- graphically elevated hinterland crust experienced gravitation- tact-regional metamorphic effects around Cretaceous plutons ally driven horizontal extension synchronous with protracted (e.g., Snake Range, Miller and Gans 1989). The principal ar- shortening within the foreland fold and thrust belt. guments for the thrust-burial genesis for the metamorphism Throughout the Raft River, Grouse Creek, and Albion Moun- are metamorphic pressures locally recording burial of two to tains, a remarkably consistent yet greatly attenuated sequence three times that of stratigraphic burial (e.g., Hodges et al. 1992) of Proterozoic to Permian greenschist- to amphibolite-facies and systematic lateral metamorphic pressure gradients within metasedimentary
Load more

Recommended publications
  • Bayesian Probabilistic Reconstruction of Metamorphic P–T Paths Using Inclusion Geothermobarometry
    Journal of Mineralogical and Petrological Sciences, Volume 113, page 82–95, 2018 Bayesian probabilistic reconstruction of metamorphic P–T paths using inclusion geothermobarometry † † Tatsu KUWATANI*, , Kenji NAGATA**, , Kenta YOSHIDA*, Masato OKADA*** and Mitsuhiro TORIUMI* *Japan Agency for Marine–Earth Science and Technology (JAMSTEC), Yokosuka 237–0061, Japan **Artificial Intelligence Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tokyo 135–0064, Japan ***Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa 277–8561, Japan †PRESTO, Japan Science and Technology Agency (JST), Kawaguchi 332–0012, Japan Geothermometry and geobarometry are used to study the equilibration of mineral inclusions and their zoned host minerals, which provide information on the P–T conditions of inclusions at the time of their entrapment. However, reconstructing detailed P–T paths remains difficult, owing to the sparsity of inclusions suitable for geothermometry and geobarometry. We developed a stochastic inversion method for reconstructing precise P–T paths from chemically zoned structures and inclusions using the Markov random field (MRF) model, a type of Bayesian stochastic method often used in image restoration. As baseline information for P–T path inversion, we introduce the concepts of pressure and temperature continuity during mineral growth into the MRF model. To evaluate the proposed model, it was applied to a P–T inversion problem using the garnet–biotite geothermom- eter and the garnet–Al2SiO5–plagioclase–quartz geobarometer for mineral compositions from published datasets of host garnets and mineral inclusions in pelitic schist. Our method successfully reconstructed the P–T path, even after removing a large part of the inclusion dataset.
    [Show full text]
  • Valid Garnet–Biotite (GB) Geothermometry and Garnet–Aluminum Silicate–Plagioclase–Quartz (GASP) Geobarometry in Metapelitic Rocksb
    Lithos 89 (2006) 1–23 www.elsevier.com/locate/lithos Valid garnet–biotite (GB) geothermometry and garnet–aluminum silicate–plagioclase–quartz (GASP) geobarometry in metapelitic rocksB Chun-Ming Wu a,*, Ben-He Cheng b a Laboratory of Computational Geodynamics, The Graduate School, Chinese Academy of Sciences, P.O. Box 4588, Beijing 100049, China b Sinopec International Petroleum Exploration and Production Corporation, Beijing 100083, China Received 5 September 2004; accepted 2 September 2005 Available online 28 November 2005 Abstract At present there are many calibrations of both the garnet–biotite (GB) thermometer and the garnet–aluminum silicate– plagioclase–quartz (GASP) barometer that may confuse geologists in choosing a reliable thermometer and/or barometer. To test the accuracy of the GB thermometers we have applied the various GB thermometers to reproduce the experimental data and data from natural metapelitic rocks of various prograde sequences, inverted metamorphic zones and thermal contact aureoles. We have concluded that the four GB thermometers (Perchuk, L.L., Lavrent’eva, I.V., 1983. Experimental investigation of exchange equilibria in the system cordierite–garnet–biotite. In: Saxena, S.K. (ed.) Kinetics and equilibrium in mineral reactions. Springer- Verlag New York, Berlin, Heidelberg. pp. 199–239.; Kleemann, U., Reinhardt, J., 1994. Garnet–biotite thermometry revised: the effect of AlVI and Ti in biotite. European Journal of Mineralogy 6, 925–941.; Holdaway, M.J., 2000. Application of new experimental and garnet Margules data to the garnet–biotite geothermometer. American Mineralogist 85, 881–892., Model 6AV; Kaneko, Y., Miyano, T., 2004. Recalibration of mutually consistent garnet–biotite and garnet–cordierite geothermometers. Lithos 73, 255–269.
    [Show full text]
  • Crustal Evolution and Hydrothermal Gold Mineralization in the Katuma Block of the Paleoproterozoic Ubendian Belt, Tanzania
    Crustal Evolution and Hydrothermal Gold Mineralization in the Katuma Block of the Paleoproterozoic Ubendian Belt, Tanzania Dissertation zur Erlangung des Doktorgrades an der Mathematisch-Naturwissenschftiliches Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Emmanuel Owden Kazimoto Kiel 2014 Crustal Evolution and Hydrothermal Gold Mineralization in the Katuma Block of the Paleoproterozoic Ubendian Belt, Tanzania Dissertation zur Erlangung des Doktorgrades an der Mathematisch-Naturwissenschftiliches Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Emmanuel Owden Kazimoto Kiel 2014 Gedruckt mit der Unterstützung des Deutschen Akademischen Austauschdienstes Referent: Prof. Dr. Volker Schenk Korrereferentin: Prof. Dr. Astrid Holzheid Tad der mündlichen Prüfung: 1-7-2014 Zum Druck genehmigt Der Dekan Vorwort Die vorliegende Arbeit wurde als monographische Dissertation verfasst, jedoch ist in den drei Kapiteln jeweils eine eigenständige Einleitung und Diskussion vorhanden. Für die einzelnen Kapitel wurde bewusst ein unabhängiger Aufbau gewählt, da diese losgelöst voneinander in internationalen Fachzeitschriften publiziert werden sollen. Daher finden sich in jedem Kapitel eine Einleitung, Diskussion und Literaturverzeichnis wieder, auch die Länge und etwaige Formatierungen sind in Hinblick auf die jeweiligen Vorgaben der Fachzeitschriften bewusst gewählt. Der Leser sei darauf hingewiesen, dass es durch den gewählten Aufbau zu Wiederholungen kommen kann und möge diesen Sachverhalt bei der Lektüre berücksichtigen. Kiel, June 2014 Emmanuel Owden Kazimoto Acknowledgements I would like to thank the German Academic Exchange Programme (DAAD) and The Ministry of Education and Vocational Training of Tanzania (MOEVT) through the Tanzania Commission for Universities (TCU) for providing funds that facilitated my stay in Germany and enabled me to attain my PhD degree. I am also grateful to the University of Dar es Salaam for the financial support of my fieldworks through the Sida Earth Science Project.
    [Show full text]
  • Petrology and Geothermobarometry of Grt-Cpx and Mg-Al-Rich
    Petrology and Geothermobarometry of Grt-Cpx and Mg-Al- rich Rocks from the Gondwana Suture in Southern India: Implications for High-pressure and Ultrahigh-temperature Metamorphism Hisako Shimizu*, Toshiaki Tsunogae Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan, [email protected] and M. Santosh Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan Introduction The Palghat-Cauvery Shear/Suture Zone (PCSZ) in southern India represents a system of dominantly E-W trending shear zones that separate the Archean Dharwar Craton to the north and Neoproterozoic granulite blocks to the south. Available geochronological data on high- grade metamorphic rocks from this region have confirmed the widespread effect of a ca. 550- 530 Ma thermal event related to the collisional amalgamation of the Gondwana supercontinent (e.g., Collins et al., 2007a, Santosh et al., 2009). Recent petrological investigations of high- grade metamorphic rocks of the PCSZ around Namakkal district identified prograde high- pressure (HP, P >12 kbar) metamorphism and peak ultrahigh-temperature (UHT) metamorphic history of this region (e.g., Shimpo et al., 2006; Nishimiya et al., 2010), which has been correlated to deep subduction prior to the collision and exhumation of the orogen during Neoproterozoic to Cambrian (Santosh et al., 2009). The PCSZ is therefore regarded as the trace of the Gondwana suture zone that continues westwards to the Betsimisaraka suture in Madagascar (Collins and Windley, 2002), and eastwards into Sri Lanka and probably into Antarctica. However, P-T paths related to tectonic settings of this region are still under debate as both clockwise (e.g., Shimpo et al., 2006) and counterclockwise (e.g., Sajeev et al., 2009) P- T paths have been reported from this region.
    [Show full text]
  • Actually Consists of 2 Cleavages
    Types of foliations • Crenulation Cleavage- – Actually consists of 2 cleavages – The first may be a slaty cleavage or schistosity that becomes microfolded – Fold axial planes typically form at high angle to the σ1 of the second compressional phase 1 Progressive development (a → c) of a crenulation cleavage for both asymmetric (top) and symmetric (bottom) situations. From Spry (1969) Metamorphic Textures. Pergamon. Oxford. 2 Figure 23.24a. Symmetrical crenulation cleavages in amphibole-quartz-rich schist. Note concentration of quartz in hinge areas. From Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag. 3 Figure 23.24b. Asymmetric crenulation cleavages in mica-quartz-rich schist. Note horizontal compositional layering (relict bedding) and preferential dissolution of quartz from one limb of the folds. From Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag. 4 Figure 23.25. Stages in the development of crenulation cleavage as a function of temperature and intensity of the second deformation. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Development of S2 micas depends upon T and the intensity of the second deformation 5 Types of lineations a. Preferred orientation of elongated mineral aggregates b. Preferred orientation of elongate minerals c. Lineation defined by platy minerals d. Fold axes (especially of crenulations) e. Intersecting planar elements. Figure 23.26. Types of fabric elements that define a lineation. From Turner and Weiss (1963) Structural 6 Analysis of Metamorphic Tectonites. McGraw Hill. Analysis of Deformed Rocks • If two or more geometric elements are present, we can add a numeric subscript to denote the chronological sequence in which they were developed and superimposed- • Deformational events: D1 D2 D3 … • Metamorphic events: M1 M2 M3 … • Foliations: So S1 S2 S3 … • Lineations: Lo L1 L2 L3 … • Plot on a metamorphism-deformation-time plot showing the crystallization of each mineral 7 Deformation vs.
    [Show full text]
  • Megaliths and Geology: a Journey Through Monuments, Landscapes and Peoples
    Megaliths and Geology Megálitos e Geologia MEGA-TALKS 2 19-20 November 2015 (Redondo, Portugal) Edited by Rui Boaventura, Rui Mataloto and André Pereira Access Archaeology aeopr ch es r s A A y c g c e o l s o s e A a r c Ah Archaeopress Publishing Ltd Summertown Pavilion 18-24 Middle Way Summertown Oxford OX2 7LG www.archaeopress.com ISBN 978-1-78969-641-7 ISBN 978-1-78969-642-4 (e-Pdf) © the individual authors and Archaeopress 2020 Financial support for the meeting Mega-Talks 2 has been provided by the project Moving megaliths in the Neolithic (PTDC/EPH-ARQ/3971/2012), funded by FCT (Fundação para a Ciência e a Tecnologia, Portugal) and the Municipality of Redondo (Portugal). All rights reserved. No part of this book may be reproduced, stored in retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying or otherwise, without the prior written permission of the copyright owners. This book is available direct from Archaeopress or from our website www.archaeopress.com Contents Introduction: Megaliths and Geology: a journey through monuments, landscapes and peoples ........................... iii Moving megaliths in the Neolithic - a multi analytical case study of dolmens in Freixo-Redondo (Alentejo, Portugal). Rui Boaventura, Patrícia Moita, Jorge Pedro, Rui Mataloto, Luis Almeida, Pedro Nogueira, Jaime Máximo, André Pereira, José Francisco Santos & Sara Ribeiro ............................................................... 1 Funerary megalithism in the south of Beira Interior: architectures, spoils and cultural sequences. João Luís Cardoso .................................................................................................................................................................. 25 A look at Proença-a-Nova’s Megalithism (Beira Baixa Intermunicipal Community, UNESCO Global Geopark Naturtejo, Portugal).
    [Show full text]
  • Mineral Chemistry and Geothermobarometry of Gabbroic Rocks from the Gysel Area, Alborz Mountains, North Iran Química Mineral Y
    ISSN-E 1995-9516 Universidad Nacional de Ingeniería COPYRIGHT © (UNI). TODOS LOS DERECHOS RESERVADOS http://revistas.uni.edu.ni/index.php/Nexo https://doi.org/10.5377/nexo.v33i02.10779 Vol. 33, No. 02, pp. 392-408/Diciembre 2020 Mineral chemistry and geothermobarometry of gabbroic rocks from the Gysel area, Alborz mountains, north Iran Química mineral y geotermobarometría de rocas gabroicas del área de Gysel, montañas de Alborz, Irán del norte Farzaneh Farahi, Saeed Taki*, Mojgan Salavati Department of Geology, Faculty of Basic Sciences, Lahijan Branch, Islamic Azad University, Lahijan, Iran. Corresponding autor email: [email protected] (recibido/received: 02-June-2020; aceptado/accepted: 04-August-2020) ABSTRACT The gabbroic rocks in the Gysel area of the Central Alborz Mountains in north Iran are intruded into the Eocene Volcano-sedimentary units. The main gabbroic rocks varieties include gabbro porphyry, olivine gabbro, olivine dolerite and olivine monzo-gabbro. The main minerals phases in the rocks are plagioclase and pyroxene and the chief textures are sub-hedral granular, trachytoidic, porphyritic, intergranular and poikilitic. Electron microprobe analyses on minerals in the rock samples shows that plagioclase composition ranges from labradorite to bytonite, with oscillatory and normal chemical zonings. Clinopyroxene is augite and orthopyroxene is hypersthene to ferro-hypersthene. Thermometry calculations indicate temperatures of 650˚C to 750˚C for plagioclase crystallization and 950˚C to 1130˚C for pyroxene crystallization. Clinopyroxene chemistry reveals sub-alkaline and calc-alkaline nature for the parental magma emplaced in a volcanic arc setting. Keywords: Gabbro, mineral chemistry, thermobarometry, plagioclase, pyroxene, Alborz, Iran RESUMEN Las rocas gabroicas en el área de Gysel de las montañas Alborz Central en el norte de Irán se introducen en las unidades sedimentarias del volcán Eoceno.
    [Show full text]
  • Geothermobarometry in Pelitic Schists
    American Mineralogist.Volume 78, pages68 I-693, 1993 Geothermobarometryin pelitic schists:A rapidly eyolyingfieldx M. J. Hor,o,q.wav, Brsw.l;rr MuxrroploHyAy Department of Geological Sciences,Southern Methodist University, Dallas, Texas 75275, U.S.A. Ansrnrcr The study of pelitic rocks for the purpose of deciphering pressure-temperature(P-Z) information is an important area for collaboration betweenmineralogists and petrologists. Accurate geothermobarometryis especiallyimportant for studiesof pressure-temperature- time paths of terranesduring tectonism and for studies of the movement of metamorphic fluids. During the past decade, new developments have included thermodynamic data bases,sophisticated crystal structure determinations and associatedsite assignments,and analysesfor Fe3+ and light elements (especially H and Li) for many minerals within a petrogeneticcontext. It is now necessaryto apply thesenew findings to geothermobarome- try in several important ways: (l) The stoichiometric basis for hydrous minerals should be revised in light of highly variable H and variable Fe3*, which can now be estimated with considerableaccuracy as a function of grade or mineral assemblage.(2) Mole fraction (activity) models should be based on the best crystal chemical considerations.For some minerals, H may be omitted from the model if all the substitutionsinvolving H are coupled substitutions. (3) Thermodynamic data should be basedon careful analysisof all available experiments and secondarycomparison with natural assemblages.(4) The possibility of nonideal solid solution should be considered,as ideality is merely a specialcase of nonide- ality. It is better to estimate binary interaction parameters than to assumethat they are zero. However, it is difficult to determine ternary interaction parameters.In such cases, little error is likely to result from assumingthat strictly ternary interaction parametersare zero, while evaluating all the binary terms.
    [Show full text]
  • Metamorphism and the Origin of Granitic Rocks Northgate District Colorado
    Metamorphism and the Origin of Granitic Rocks Northgate District Colorado GEOLOGICAL SURVEY PROFESSIONAL PAPER 274-M Metamorphism and the Origin of Granitic Rocks Northgate District Colorado By T. A. STEVEN SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 274-M A discussion of the progressive metamorphism, granitixation, and local rheomorphism of a layered sequence of rocks, and of the later emplacement and deuteric alteration of an unrelated granitic stock UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957 UNITED STATES DEPARTMENT OF THE INTERIOR FRED A. SEATON, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. CONTENTS Page Page Abstract_________________________________ 335 Pre-Cambrian geology—Continued Introduction-_______________________________________ 335 Dacite porphyry—____ ——— __ —— _____________ 364 Acknowledgments__ ___--_____-____-_____-______-_ 336 Intrusive quartz monzonite_-____--_-__-_--_-_-_. 365 Geologic setting._______ — _________________________ 336 Petrography ________—— —— _______________ 365 Pre-Cambrian geology—___________________________ 337 Main body of the stock____________— 366 Hornblende gneiss___-_________-_-_____-________ 338 Marginal dikes_________-____-__-__——— 366 Quartz monzonite gneiss_________________________ 342 Satellitic dikes___-___.__________ 367 Biotite-garnet gneiss___________________________ 345 Wall-rock alteration_________ _ __——_ 368 Pegmatite_________________________________
    [Show full text]
  • Download the Scanned
    THn AMERIceN M INERALoGIST JOURNAL OF THE MINERALOGICAL SOCIETY OF AMERICA Vol.22 DECEMBER, 1937 No. 12,Part 1 DEVELOPMENT OF PLAGIOCLASE PORPHYROBLASTSI G. E. Gootsrooo, U niver si.tyoJ W ashington, S eattle, W ashington. Assrnacr Porphyroblastic textures are common to hornfels in the Cornucopia area oJ the Wal- lowa Mountains of northeastern Oregon. Recent petrographic studies indicate that they are also common to some of the rocks of the Cascades of Washington. Plagioclase porphyro- blasts range from initial allotrioblastic forms to fully developed idioblastic crystals. They exhibit many characteristic features which may include poikioblastic structures, helizitic inclusions and complex aggregates. The arrangement within the crystal of inclusions of either identifiable minerals or turbid material, is apparently directly related to the stage of development of the porphyroblast. This included matter differs chiefly in its arrange- ment from the products of endogenetic or subsequent alteration. Recognition of these metamorphic textures and structures is one of the points essential to the interpretation of recrystallization-replacement as applied to igneous-appearing rocks which are believed to have been formed in situ by processesof additive hydrothermal metamorphism. In a previous paper the writer outlined the development of qaartz porphyroblasts in a siliceoushornfels.2 The quartz porphyroblasts in the siliceoushornfels were too small (0.5 mm.) to be conspicuousin the hand specimens,but were readily seenunder crossednicols, although in plane light they merged almost completely with the groundmass of the horn- fels, and contained the sameabundance of small inclusions as the ground- mass. The siliceoushornfels is not the most abundant; biotitic and horn- blendic varieties are far more common.
    [Show full text]
  • Nucleation and Growth History in the Garnet Zone
    Spear and Daniel Geological Materials Research v.1, n.1, p.1 Three-dimensional imaging of garnet porphyroblast sizes and chemical zoning: Nucleation and growth history in the garnet zone Frank S. Spear and Christopher G. Daniel Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York 12180 <[email protected]> (Received July 1, 1998; Published October 30, 1998) Abstract The three-dimensional (3-D) growth history of two garnet zone samples (Grt + Chl + Bt + Ms + Pl + Qtz + Ilm) from southwestern Maine was examined by serial sectioning and 3-D reconstructions of compositional zoning from backscatter images and X-ray maps. Mn, Fe, Mg, and Ca zoning is broadly concentric. The concentration of Mn in garnet cores generally correlates with size (d = 50 to 750 microns), indicating progressive nucleation. In detail, all elements show irregular, patchy zoning in the cores. Assuming constancy of Mn on the rims of all garnet crystals in a rock volume plus no subsequent diffusional modification, Mn concentration can be used as a Òtime lineÓ for garnet growth. Examination of the evolution of individual garnet crystals reveals that multiple nuclei formed simultaneously in the core regions and that nuclei expanded by growth in amoeba-shape forms along preexisting mineral grain boundaries (primarily quartz and plagioclase), dissolving the interior grains until the grains were either gone or encapsulated, at which time dissolution ceased. Amoeba-shaped garnet crystals coalesced as they grew and, simultaneously, new nuclei appeared in the nearby matrix. The net result was a single garnet porphyroblast that formed by the growth and coalescence of multiple nuclei.
    [Show full text]
  • Petrologic Significance of Fe-Rich Staurolite in Pelitic Schists of the Silgará Formation, Santander Massif
    EARTH SCIENCES RESEARCH JOURNAL Earth Sci. Res. J. Vol. 20, No. 1 (March, 2016): C1 - C7 PETROLOGY Petrologic significance of Fe-rich staurolite in pelitic schists of the Silgará Formation, Santander Massif Carlos Alberto Ríos R.1*, Oscar Mauricio Castellanos A.2 1*. Escuela de Geología, Universidad Industrial de Santander, Bucaramanga, Colombia, e-mail: [email protected] 2. Programa de Geología, Universidad de Pamplona, Colombia ABSTRACT Keywords: Staurolite, pelitic schists, Silgará Medium grade metapelites of the Silgará Formation at the Santander Massif (Colombian Andes) have been Formation, metamorphism, Santander Massif. affected by a medium-pressure/high-temperature Barrovian type of metamorphism, developing a sequence of metamorphic zones (biotite, garnet, staurolite and sillimanite). These rocks record a complex tectono- metamorphic evolution and reaction history. Metapelitic rocks from the staurolite zone are typically foliated, medium- to coarse-grained, pelitic to semipelitic schists that contain the mineral assemblage biotite + garnet + staurolite ± kyanite; all contain muscovite + quartz + plagioclase with minor K-feldspar, tourmaline, apatite, zircon, epidote, calcite, and Fe–Ti oxides. Field and microscopic evidences reveal that Fe-rich staurolite in pelitic schists is involved in several chemical reactions, which explains its formation and transformation to other minerals, which are very important to elucidate the reaction history of the Silgará Formation metapelites. Significado Petrológico de Estaurolita Rica en Fe en Esquistos Pelíticos de la Formación Silgará, Macizo de Santander RESUMEN Palabras clave: Estaurolita, esquistos pelíticos, Medium grade metapelites of the Silgará Formation en el Macizo de Santander (Andes Colombianos) han sido Formación Silgará, metamorfismo, Macizo de afectadas por un metamorfismo de tipo Barroviense, el cual se ha producido en condiciones de media presión y Santander.
    [Show full text]