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Tectonometamorphic Evolution of an Allocthonous Terrane, Gory Sowie Block, Northeastern Bohemian Massif (Poland)

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Tectonometamorphic Evolution of an Allocthonous Terrane, Gory Sowie Block, Northeastern Bohemian Massif (Poland) TECTONOMETAMORPHIC EVOLUTION OF AN ALLOCTHONOUS TERRANE, GORY SOWIE BLOCK, NORTHEASTERN BOHEMIAN MASSIF (POLAND) A thesis presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Master of Science Stephen J. Zahniser November 2004 This thesis entitled TECTONOMETAMORPHIC EVOLUTION OF AN ALLOCTHONOUS TERRANE, GORY SOWIE BLOCK, NORTHEASTERN BOHEMIAN MASSIF (POLAND) by Stephen J. Zahniser has been approved for the Department of Geological Sciences and the College of Arts and Sciences by David A. Schneider Assistant Professor of Geological Sciences Leslie A. Flemming Dean, College of Arts and Sciences Zahniser, Stephen J. M.S. November 2004. Geological Sciences Tectonometamorphic evolution of an allocthonous terrane , Gory Sowie Block, northeastern Bohemian massif (Poland) (76pp.) Director of Thesis: David Schneider The Gory Sowie Block (GSB), of SW Poland’s Sudete Mountains, contains numerous ultra-high temperature granulites (UHT) and small relict ultra-high pressure (UHP) eclogites enveloped within amphibolite-facies gneisses and amphibolites. The GSB is bounded by ductile fault zones, bisected by the Sudetic Boundary fault and partially overlies ophiolitic sequences. The GSB experienced a polyphase metamorphic history spanning Caledonian and Variscan accretionary events (440 - 330 Ma) with peak metamorphic conditions of 1000°C and >20 kbar occurring ca. 400 Ma, indicated by U- Pb zircon ages on garnet peridotites and felsic granulites. This project involved 40Ar/39Ar hornblende and mica plateau ages from primarily GSB host gneisses. The results indicate that the western GSB experienced a diachronous cooling event with the northern region cooling from upper amphibolite-facies (sillimanite/kyanite zone) ca. 385 Ma and southern region from middle to lower amphibolite-facies (garnet/biotite zone) ca. 375 Ma with cooling rates ranging from 40 - 25°C m.y.-1. U-Th total-Pb monazite geochronometric results obtained during this study reveal both amphibolite-facies metamorphism at 385 Ma, concordant with the 40Ar/39Ar cooling ages, and homophazation of the gneisses and late-fluid mobilization along tectonic boundaries associated with large scale metamorphism of the surrounding Sudetic terranes at ca. 360 Ma. Additionally, concordant 40Ar/39Ar hornblende and biotite plateau ages obtained along the far eastern margin indicate a regional heating event at 336.8 ± 0.8 Ma., most likely associated with Niemcza shearing, Approved: David Schneider Assistant Professor of Geological Sciences Acknowledgments My deepest thanks to my friend and advisor Dave Schneider for helping me see this project to completion and giving me the opportunity to broaden my intellectual and scientific horizons through his thoughts and insights. Additional thanks to Maciej Manecki, Damian Nance, Gregory Springer, Matt Heizler, Robert Tracy and Bartek Budzyn for their insights and contributions in the field, lab and review processes. Also, much thanks to Jake Glascock for being a friend, providing significant contributions in the field, and all around keeping me sane. Thanks to Zack Wessel, Scott Dodson and Brent Barley for teaching me the fundamentals of Geology through many late nights of pool. Tricia Piercey, Stacia Gordon and Shelly Rose for their friendship, insights and editing skills. The Douglas’s for being supportive neighbors and helping put the Deere out to pasture. My sincere gratitude to my family for their continued support in every aspect of my life and helping me through the hard times. I would like to extend my prayers to Carol Popovich and the Popovich family. Financial support for this project was provided through the National Research Council. v Table of Contents Page Acknowledgments……………………………………………………………….… iii Abstract……………………………………………………………………………. iv List of Figures………………………………………………………………………vii List of Tables………………………………………………………………………. viii 1. Introduction………………………………………………………………………9 2. Geologic Setting………………………………………………………………….12 2.1. Variscides……………………………………………………………12 2.2. Bohemian Massif…………………………………………………… 13 2.3. Sudetes……………………………………………………………… 14 2.4. Gory Sowie Block…………………………………………………...15 3. Previous Tectonometamorphic Chronometry……………………………………19 4. Petrology & Petrography………………………………………………………... 22 4.1. Methods & Approach………………………………………………..22 4.2. Gneisses & Migmatites……………………………………………...22 4.3. Granulites……………………………………………………………25 4.4. Eclogites, Metabasites & Serpentites………………………………..26 5. 40Ar/39Ar Thermochronology…………………………………………………….29 5.1. Analytical Procedures………………………………………………. 30 5.2. 40Ar/39Ar Results…………………………………………………….32 6. U-Th-total-Pb Geochronology…………………………………………………...44 6.1. Analytical Procedures………………………………………………. 44 vi 6.2. Total-Pb Results……………………………………………………..46 7. Discussion………………………………………………………………………..55 7.1. Geochronology……………………………………………………....55 7.2. Tectonic Implications………………………………………………..57 8. Conclusions………………………………………………………………………63 References…………………………………………………………………………..65 Appendix A…………………………………………………………………………72 vii List of Figures Figure Page 1 Lithostratigraphic units of the Bohemian Massif...…………………………13 2 Geologic map of the Sudete Mountains...…………………………………..16 3 Geologic map of the Gory Sowie Block..…………………………………..18 4 Simplified map of Gory Sowie Block with sample locations..……………..23 5 Field shot and photomicrograph of a typical gneiss outcrop………………. 24 6 Field shot and photomicrograph of a granulite outcrop…………………….25 7 Field shot and photomicrograph of a eclogite (retrogressed?) outcrop……. 27 8 Metamorphic isograd map of the Gory Sowie……………………………...28 9a 40Ar/39Ar age spectra………………………………………………………..33 9b 40Ar/39Ar age spectra..………………………………………………………34 10 Cumulative probability graphs of in situ monazite total-Pb analysis……… 49 11 Y-chemical maps and monazite chrontour diagrams of U-Th total-Pb analysis……………………………………………………………………...50 12 Thermal history of the Gory Sowie Block………………………………….56 13 P-T-t path for the Gory Sowie Block……………………………………….59 viii List of Tables Table Page 1 Analytical 40Ar/39Ar isotopic data…………………………………………..35 2 Analytical U-Th total-Pb elemental data…………………………………... 51 9 1. Introduction Ultrahigh grade metamorphic terranes exposed at the surface represent crustal material that once resided at deep-lithospheric depths (>100 km) due to plate subduction or collision-induced crustal thickening. Conditions at these depths often exceed 25 kbar and 800°C and result in the formation of mineral assemblages that include coesite ± cordierite ± garnet ± clinopyroxene ± kyanite ± sillimanite (O’Brien, 1997a). The presence of eclogite and granulite facies rocks exposed at the surface provides an opportunity to study orogenic roots; moreover, preservation of ultrahigh pressure (UHP) and ultrahigh temperature (UHT) mineral assemblages suggests rapid exhumation, often associated with either return flow along a subduction zone or orogenic collapse. Unraveling the frequently complex thermal and unroofing histories of UHP and UHT rocks through the use of thermochronological and geochronological analyses, coupled with thermobarometric estimates, is instrumental for the construction of quantitative unroofing models of orogenic systems. An understanding of crustal deformation, metamorphism and the roots of orogenic systems has largely been gained through the study of exhumed rocks. Exhumation generally occurs through tectonic (normal faulting, ductile thinning) and erosional denudation. These processes are an important component of the Earth system due to their influence on topography, sedimentation and ultimately mantle dynamics. Various models have been applied to the exhumation of deep seated rocks, but most researchers agree that a density driven buoyancy gradient is the primary driving force behind the emplacement of eclogites and possibly granulites into the lower and middle crust during the initial unroofing processes. Serpentization occurring at ultrahigh 10 pressures during or after collision typically reduces local mantle rock densities from 3.3 to 2.7 g cm-3, thus amplifying the buoyancy force (Ring et al., 1999). However, eclogite and granulite sequences typically occur as blocks or lenses within an amphibolite-facies matrix, suggestive of subsequent forces other than simple buoyancy in controlling final emplacement and unroofing. Exhumation of ultrahigh grade terranes is thus considered a multistage process (e.g., Hacker et al., 1995). Within the northeastern Bohemian massif, the Gory Sowie Block (GSB) of the Sudetes Mountains (southwestern Poland) contains known UHT and UHP exposures. The GSB is proposed to have attained peak metamorphic conditions during a Late Caledonian UHP event followed by localized UHT granulite-facies metamorphism and later regional (retrograde?) amphibolite-facies metamorphism, during the Variscan orogeny (Brueckner et al., 1996, O’Brien, 1997b). Peak metamorphism is preserved in exposures of garnet peridotites, granulites locally containing kyanite with sillimanite overgrowths, and migmatitic mafic and ultramafic zones contained within the local host gneisses (O’Brien, 1997a; Kryza & Pin, 2002). Sillimanite-kyanite intergrowth is indicative of a distinctive high-temperature metamorphic event characterized by near isothermal decompression, likely during rapid exhumation (O’Brien, 1997a; Platt & Whitehouse, 1999; Kryza & Pin, 2002). Unlike adjacent portions of the Sudetes, the GSB contains the only documented ages implicating a Caledonian component (Zelazniewicz, 1987;
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