CONTACT AUREOLE RHEOLOGY of the WHITE HORSE PLUTON By
Total Page：16
File Type：pdf, Size：1020Kb
CONTACT AUREOLE RHEOLOGY OF THE WHITE HORSE PLUTON by WAYNE THEODORE MARKO, B.S. A THESIS IN GEOSCIENCE Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Approved •—.—• «^ Chairperson of the Committee Accepted Dean of the Graduate School August, 2004 ACKNOWLDGEMENTS This work would was made possible with funding provided by the NSF grant for contact aureole research awarded to Dr. Aaron Yoshinobu at Texas Tech University as well as by teaching assistantships provided by the Texas Tech Geosciences Department. 1 would like extend thanks to my committee members, Dr. Aaron Yoshinobu, Dr. Calvin Barnes and Dr. George Asquith for their insightful observations and guidance with various aspects of this work. Lastly, I thank my family and friends for their support and camaraderie during the course of the project. n ABSTRACT The 160 Ma White Horse pluton intruded a thick sequence of miogeoclinal Paleozoic carbonate rocks in the northeastern Great Basin Region, Nevada. The dominantly quartz monzonite pluton (-16 km^ of exposure) lacks internal fabric, concentric zoning, and stoped blocks, but hosts several smaller granite and granodiorite bodies as well as numerous microdiorite mafic enclaves. The structural aureole extends 7 km along the eastern side of the elliptical intrusive body. Continuous and discontinuous spaced axial planner foliations and harmonic to disharmonic, tight to isoclinal folds wrap around the western margin of the pluton. Folds verge toward and away from the pluton and a rim anticline is preserved along the pluton margin. In several locations fold axes are cut by the pluton host rock contact. The aureole was shortened approximately 54% during emplacement. However, a maximum of 52 % of the exposed pluton area is unaccounted for after the ductile aureole strain is restored. The pluton contact geometry is highly variable and parallel to subparallel with host rock anisotropy. Along the southern aureole the contact dips 14° to the north (towards the intrusion) and in the southeast the contact turns over to dip 45° to the southeast (away from the intrusion). In the northeastern aureole the contact dips 31° to the northeast while at structurally higher elevations the contact dips 45° to the southwest. Several dikes cut host rock structure and turn to propagate towards one another, preserving the late stage stoping process. Aureole tectonites consist of carbonate mylonites and coarse mylonites interlayered with annealed marble. Intracrystalline slip, grain boundary migration, subgrain rotation and grain size ni sensitive diffusion creep accommodated ductile deformation. Significant syntectonic grain growth occurred during carbonate grain recrystallization. Minimum sfress estimates from piimed calcite tectonites range from 30 to 34 MPa. Minimum strain rate estimates range from 10""s'' at 600° C to 10''^s"' at 400° C. Temperatures correspond with the peak thermal gradient predicted for the aureole. Magma chamber construction was accomplished by contemporaneous brittle and ductile translation of host rocks. Stoping accounts for missing host rock area, truncated bedding, and the consistent subparallel orientation of the pluton contact with host rock structure. Ductile deflection of the host rocks was accomplished by lateral expansion of the magma chamber after magma ascent had ceased. This process produced pluton- vergent and divergent folds and a rim anticline. Calculated aureole strain rates preclude the possibility of simple end-member emplacement diking and diapiric models. The large viscosity contrast between the host rocks and the magma as well as the lack of fabric within the intrusion suggests aureole deformation (chamber construction) was driven by buoyancy and/or overpressure and was completed before the magma had achieved a yield strength. IV TABLE OF CONTENTS ACWKNOWLDGMENTS ii ABSTRACT lii LIST OF TABLES vii LIST OF FIGURES viii LIST OF ABBREVIATIONS xi CHAPTER I. INTRODUCTION 1 Tectonic setting 3 Regional geology in the Basin and Range province 6 II. GEOLOGY OF THE WHITE HORSE PLUTON AND CONTACT ARUEOLE 13 Introduction 13 White Horse pluton 14 Contact aureole of the White Horse pluton 15 Cross sections 19 Pluton host-rock contact 21 Thermal aureole and metamorphic grade 22 III. HOST ROCK RHEOLOGY 40 Introduction 40 Microstructures 40 Summary of microstructures in primary phases 41 Aureole petrofabrics 43 Distribution of petrofabrics within the aureole 45 Grain size measurements 45 Micro-boudinage 47 C-axis orientation in calcite 47 IV. DISCUSSION 61 Timing of deformation 61 Sfrain 63 Mechanism of folding 63 Interpretation of microstructures and petrofabrics 66 Interpretation of deformation mechanisms 68 Piezometry and strain rate 70 Effective viscosity 74 Structural restoration of ductile strain 74 Role of stoping 76 Interpretation of magmatic structures 76 Alternative emplacement models 77 Emplacement model 79 V. CONCLUSIONS 92 REFERENCES 94 APPENDIX: ROCK DESCRIPTIONS 103 vi LIST OF TABLES 1.1 Jurassic age structure in the vicinity of the White Horse pluton (modified from EUson, 1995). 9 2.1 Summary of fold geometry. 25 3.1 Summary of grain size measurements. 50 4.1 Summary of deformation in the Goshute Mountains (from Silberling and Nichols, 2002). 84 4.2 Summaryof differential stress measurements. 85 4.3 Contact aureole strain rates and viscosities. 86 Vll LIST OF FIGURES 1.1 Simplified map of Jurassic intrusions and regional structure in the 10 Basin and Range Province (modified fronrEUson et al. 1990 and Miller and AUmendinger, 1991). 1.2 Geologic map of the White Horse Mountain Region (simplified 11 from Silberling and Nichols, 2002). 1.3 Litho-tectonic section for the Goshute Range locally near the White 12 Horse intrusion. Jg = Jurassic granite and associated dikes(?), Tg = Tertiary dike(?). 2.1 Geologic map and geologic cross sections of the White Horse pluton in pocket and contact aureole. 2.2 Mafic enclave in granitic host with xenocrysts(?) of plagioclase. 26 2.3 Southern hemisphere projection of poles to magmatic foliation planes. 27 2.4 Fossils preserved outside of the structural aureole. 28 2.5 Aureole foliations. 29 2.6 Poles to bedding both inside and outside of the contact aureole. 30 2.7 Stretching lineation. 31 2.8 Simplified geologic map of lineation, small fold, and major fold distributions in the contact aureole. 32 2.9 Examples of minor parasitic folds in the aureole. 33 2.10 Fold from the northern aureole near cross section B-B'. 34 2.11 View of NE pluton/host-rock contact. 3 5 2.12 Photograph of folds in cross section G-G'. 36 vni 2.13 Schematic diagram of the pluton host rock contact and selected cross sections. 37 2.14 SimpUfied geologic map showing distribution of aureole petrofabrics, porphyroblasts and folds. 38 2.15 Temperature-mole fraction (C02) phase stability diagram for selected carbonate reactions at 2 Kbar. 39 3.1 Photomicrographs of undeformed limestone and dolostone from 51 outside of the aureole. 3.2 Close-up of dolomite microstructures (white line denotes bulk rock SPO). 52 3.3 Subhedral dolomite crystal embayed by calcite grains. 53 3.4 Calcite microstructures (white lines mark macroscopic SPO). 54 3.5 Quartzite grain boundaries (crossed polars). 55 3.6 Quartzite samples from within the structural aureole. 56 3.7 Aureole petrofabrics shown with increasing distance from the pluton contact from top to bottom and measured distance from the contact. 57 3.8 Grain boundary maps and 3-D volumetric percentage (V(R)) and frequency percentage (h(r)) histograms of grain diameters. 58 3.9 Stain measurements on boudinaged tremolite grain in sample WH-89-02. 59 3.10 Southern hemisphere projection of calculated calcite c-axis orientations and calculated extension and compression directions for e-twin lamellae formation. 60 4.1 Folding models (modified from Twiss and Moores, 1992). 87 4.2 Restored cross sections A-A' and C-C depicting the current thickness of the aureole and the restored length of quartzite marker beds (red lines). 88 IX 4.3 A 2-D plan view strain restoration assuming -54% strain everywhere within the contact aureole. 89 4.4 Block diagram model of the White Horse pluton magma chamber and host rock evolution. 90 4.5 Final structural geometry. 91 LIST OF ABREVIATIONS degrees °C temperature in degrees Celsius 2-D two dimensions 3-D three dimensions A/(NCK) weight percent of [aluminuni/(i antig antigorite cc calcite CO2 carbon dioxide Dl first deformation event dol dolomite di diopside epsilonnd epsilon neodymium EW east-west fo forsterite (olivine) GBMR grain boimdary migration recr}- H2O water K temperature in degrees Kelvin K-Ar potassium-argon Kbar kilobars Km kilometers XI LPO lattice-preferred orientation m meters Ma million years l^ni micrometers MPa megapascals N north NE northeast Pa»s pascal seconds S seconds S south SE southeast SSE south to southeast SPO shape-preferred orientation Sr strontium T0,T1,T2,T3,T4 time one, time two, time three, time four tc talc tr tremolite h(r)%) two-dimension radii frequency percentage V(R)%) three-dimension volumetric frequency percentage xn CHAPTER I INTRODUCTION Magma emplacement studies often suffer from a lack of constraints on the structural evolution of contact aureoles. In fact, a recent and provocative model for evolution of sub-arc magma chambers is based primarily on the internal structures and/or geochronological data from various phases of a composite pluton (e.g., Glazner, Bartley, Coleman, Gray, and Taylor, 2004). While many insightfiil end-member models have been developed to characterize such processes as diapirism (Marsh, 1982; Weinberger and PodUchakov, 1994) and ballooning (Furlong and Meyers, 1985; Paterson and Vernon, 1995), few studies of natural plutons have been able to quantify the nature and magnitude of deformation attending magma emplacement into the crust. A few studies have attempted to estimate strain rates in aureoles (e.g., Karlstrom, Miller, Kingsbury, and Wooden, 1993). Nyman, Law, and Morgan (1995) used microstructures and isotopic constraints to evaluate temperature and strain rate profiles in the aureole of the Papoose flat pluton.