EXHUMATION OF THE ORLICA-SNIEZNIK DOME, NORTHEASTERN BOHEMIAN MASSIF (POLAND AND CZECH REPUBLIC) 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 Masters of Science Jacob M. Glascock November 2004 This thesis entitled EXHUMATION OF THE ORLICA-SNIEZNIK DOME, NORTHEASTERN BOHEMIAN MASSIF (POLAND AND CZECH REPUBLIC) BY Jacob M. Glascock has been approved for the Department of Geological Sciences and the College of Arts and Sciences by David Schneider Assistant Professor of Geological Sciences Leslie A. Flemming Dean, College of Arts and Sciences Glascock, Jacob M. M.S. November 2004. Geological Sciences Exhumation History of the Orlica Snieznik Dome, Northeastern Bohemian Massif (Poland and Czech Republic) (80 p.) Director of Thesis: David Schneider The Orlica-Snieznik Dome (OSD), located in the northeastern Bohemian massif (Czech Republic and Poland), represents a Variscan massif consisting of widespread amphibolite-facies gneisses and migmatites enclosing eclogite and granulite crustal-scale lenses. 40Ar/39Ar thermochronology yielded cooling ages for white mica and biotite between 341 ± 1 Ma to 337 ± 0.6 Ma and 342 ± 1 Ma to 334 ± 0.6 Ma from the Snieznik mountains. One amphibolite-derived hornblende yielded an integrated Ar-Ar age of ca. 400 Ma. The Orlica mountains yielded cooling ages between 338 ± 0.9 Ma to 335 ± 0.5 Ma. U-Th-total Pb monazite geochronology confirms two thermal events, likely commencing at ca. 400 Ma with granulite facies metamorphism. The cooling ages of the gneisses and schists are consistent across the dome and represent rapid wholesale cooling of the OSD, on an order of 50 oC/m.y. indicative of exhumation-related, amphibolite- facies metamorphism directly following UHP conditions. Approved: David Schneider Assistant Professor of Geological Sciences ACKNOWLEDGMENTS I would like to thank my advisor, David Schneider, for his guidance, support, and friendship, and I am honored to have worked alongside him throughout these last two years. This experience has led to a whole new understanding of my capabilities as a professional and has allowed me to achieve a new level of confidence. I have gained valuable skills and the ability to apply those skills in a variety of settings, but most importantly I have gained a friend. The last two years were an incredible experience of which I had the privilege of sharing with fellow student and friend, Stephen Zahniser. His camaraderie has helped me through some tough times and will always be greatly appreciated. Along with Dave, the three of us see no evil, hear no evil, and speak no evil. My thanks again to the both of you. I would also like to thank my committee members, Greg Nadon and Doug Green, as well as Maciej Manecki for their guidance and support throughout my college education. Their insight, intellect and genuine interest in my graduate work are greatly appreciated. My gratitude also extends out to Dr. Matt Heizler (NMT) and Dr Robert Tracey (VT). This project was funded by the National Research Council/National Science Foundation. I would not have succeeded this far without my family and friends, so I would like to thank my better half; Stephanie Miller, my parents and relatives, my sister and brothers, Bartek Budzyn, David Patterson, Joey Smith, Brett Laverty, Molly Hart, the Kahns, the Lyles, the Kristofcos, and Paul Huffer, and anyone else who has helped me along during this adventure. 5 TABLE OF CONTENTS Page Abstract…………………………………………………………………………………3 Acknowledgments………………………………………………………………………4 List of Figures…………………………………………………………………………..6 List of Tables………………………………………………………………………….. 7 1. Introduction…………………………………………………………………………..8 2. Tectonic Setting: Variscides………………………………………………………… 10 3. Geologic Setting: Sudetes……………………………………………………………13 4. Previous Geochronology……………………………………………………………..18 5. Petrology and Petrography…………………………………………………………...20 6. 40Ar/39Ar Thermochronology………………………………………………………... 25 6.1. Analytical Procedure……………………………………………………….27 6.2. 40Ar/39Ar Analytical Results………………………………………………. 29 7. U-Th-total Pb Geochronology………………………………………………………. 51 7.1. Analytical Procedure……………………………………………………….52 7.2. U-Th-total Pb Analytical Results…………………………………………..53 8. Discussion……………………………………………………………………………58 9. Conclusions..………………………………………………………………………... 66 10. References…………………………………………………………………………..67 Appendix A: Sample Petrology and Petrography………………………………………73 6 LIST OF FIGURES Figure .Page 1. Modified terrane map of the Bohemian massif and adjacent zones………………… 11 2. Geologic map of the Sudetes, Poland and Czech Republic………………………….14 3. Simplified geologic map of the Orlica-Snieznik Dome and adjacent regions……….15 4. Sample map of collection sites from the Orlica-Snieznik Dome…………………….21 5. Field and petrographic photos of representative rock units………………………….23 6. 40Ar/39Ar age spectra………………………………………………………………… 33 7. Elemental images of monazite grains from szx-10A………………………………...54 8. Distribution of weighted mean monazite total-Pb ages and probability curves of monazite total-Pb age results for szx-10A…………..………………………………….56 9. Distribution of weighted mean monazite total-Pb ages and probability curves of monazite total-Pb age results for szx-10A…………..………………………………….57 10. P-T-t evolution of the Orlica-Snieznik Dome………………………………………63 7 LIST OF TABLES Table Page 1. Analytical 40Ar/39Ar isotopic data…………………………………………………36-50 2. Analytical EMPA monazite U-Th-total Pb elemental data…………………………..55 8 1. Introduction Ultrahigh pressure (UHP) metamorphic terranes represent crust that has undergone a complex interplay of deep lithospheric and thermal processes, and their subsequent rapid exhumation results in the preservation of the deep-earth petrology. These mineral assemblages, which can lie within the coesite stability field, are typically indicative of conditions exceeding 28 kbar and 700 ºC suggesting deep (~100 km) subduction of continental and/or oceanic crust at major plate margins. The descent phase of these blocks is fairly well understood; however two outstanding enigmatic features of UHP terranes are the mechanism and rate of the ascent phase of deformation. Notably, though, the simple preservation of the UHP assemblages in orogenic belts implies the rocks were rapidly unroofed and exhumed. Previous models of deep exhumation describe two mechanisms in which preserved continental UHP terranes were rapidly brought to the surface. One such process is the removal of the overburden through surface erosion and normal faulting in which the subsequent extensional collapse of the orogenic belt played the role of the driving mechanism (e.g., Vanderhaeghe et al., 1999). Another widely accepted model is the rapid ascent of UHP eclogites and granulites through the lower and middle lithosphere by means of a density-regulated buoyant crust (e.g., Hacker et al., 1995). However most UHP rocks are enveloped by an amphibolite-facies metamorphic matrix, which implies multiple stages of exhumation, and likely combines both mechanisms since buoyancy cannot be the only driving force for exhumation and other processes must 9 be acting to allow further ascent of the UHP blocks (Hacker et al., 1995; Qingchen and Bolin, 2000; Ernst and Liou, 2000). Detailed exhumation histories of UHP terranes can be revealed through thermochronological and geochronological analyses of the minerals found within the UHP rocks and surrounding metamorphic matrix, which will allow a quantitative evaluation of the heating and cooling rates associated with ascent. Specifically, this study focuses on the Orlica-Snieznik Dome (OSD) located in the northeastern Bohemian massif of the Czech Republic and Poland. The OSD, a well-known UHP terrane, represents an ideal location for a geochronometric investigation because it can enable a more specific placement of metamorphic and cooling ages into an already existing geologic framework. The equivocal structure of the dome varies with interpretation but has typically been described as a Neoproterozoic-Early Paleozoic basement-cover sequence consisting of granitic orthogneisses and various paragneisses; a characteristic signature of the widespread Variscan tectonics of Europe (Floyd et al., 1996). Previous thermochronology from the northeastern Bohemian massif are limited, but suggest denudation between ca. 340-310 Ma that is believed to reflect cooling from major Variscan Barrovian metamorphism (Maluski et al., 1995). This study provides new results, which are primarily 40Ar/39Ar thermochronometric and complimentary U-Th-total Pb monazite geochronometric data from enveloping amphibolite-facies metamorphic rocks in order to place timing constraints on the ascent phase (exhumation) of the enclosed eclogites and granulites of the OSD. 10 2. Tectonic Setting: Variscides The Variscan orogeny has been considered to represent a diachronous obduction- collision zone with a deformational history spanning over 100 m.y. resulting in an oblique dextral collisional regime. The Variscan belt is a collage of arc terranes and microplates that amalgamated within this collisional regime between 480 and 250 Ma (Matte, 2001). The system, as wide as 1000 km, extended from the present day Caucasus Mountains of Eurasia through the Mauritanian Belt of Africa to the Alleghenian belt of the Appalachians and the Ouachitas of North America at the end of the Carboniferous. Collision involved the Armorican Terrane Assemblage (ATM) situated between Laurentia-Baltica to the northwest