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Toward an Improved Understanding of Intraplate Uplift and Volcanism: Geochronology and Geochemistry of Intraplate Volcanic Rocks and Lower-Crustal Xenoliths

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Toward an Improved Understanding of Intraplate Uplift and Volcanism: Geochronology and Geochemistry of Intraplate Volcanic Rocks and Lower-Crustal Xenoliths Lehigh University Lehigh Preserve Theses and Dissertations 2017 Toward an Improved Understanding of Intraplate Uplift nda Volcanism: Geochronology and Geochemistry of Intraplate Volcanic Rocks and Lower-Crustal Xenoliths Leonard Daniel Ancuta Lehigh University Follow this and additional works at: http://preserve.lehigh.edu/etd Part of the Environmental Sciences Commons Recommended Citation Ancuta, Leonard Daniel, "Toward an Improved Understanding of Intraplate Uplift nda Volcanism: Geochronology and Geochemistry of Intraplate Volcanic Rocks and Lower-Crustal Xenoliths" (2017). Theses and Dissertations. 2486. http://preserve.lehigh.edu/etd/2486 This Dissertation is brought to you for free and open access by Lehigh Preserve. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of Lehigh Preserve. For more information, please contact [email protected]. Toward an Improved Understanding of Intraplate Uplift and Volcanism: Geochronology and Geochemistry of Intraplate Volcanic Rocks and Lower-Crustal Xenoliths by Leonard D. Ancuta A Dissertation Presented to the Graduate and Research Committee of Lehigh University in Candidacy for the Degree of Doctor of Philosophy in Department of Earth and Environmental Sciences Lehigh University January 2017 © 2016 Copyright Leonard D. Ancuta ii Approved and recommended for acceptance as a dissertation in partial fulfillment of the requirements for the degree of Doctor of Philosophy Leonard D. Ancuta Toward an Improved Understanding of Intraplate Uplift and Volcanism: Geochronology and Geochemistry of Intraplate Volcanic Rocks and Lower-Crustal Xenoliths 4 November 2016 Defense Date Peter K. Zeitler Dissertation Director (Must Sign with Blue Ink) Approved Date Committee Members Anne S. Meltzer Bruce D. Idleman Richard W. Carlson (external) iii ACKNOWLEDGMENTS I would like to thank many people for their help and support throughout my time at Lehigh. For their support, feedback and contributions throughout the dissertation processes, I would like to thank my committee, Dr. Peter Zeitler, Dr. Anne Meltzer, Dr. Bruce Idleman, and Dr. Richard Carlson. My advisor, Dr. Peter Zeitler has been constant source of support and encouragement through the entire processes of this dissertation. I would like to thank Richard Carlson for his significant contributions and for allowing me the opportunity to collaborate with him and other scientists at the Department of Terrestrial Magnetism at Carnegie Institution for Science. Bruce Idleman has been very supportive with both comments and feedback as well as endless help in the lab at Lehigh. At Carnegie Institution for Science I would like to thank Mary Horan, Timothy Mock and Emma Bullock for their significant help in the lab. I would like to acknowledge the National Science Foundation’s Continental Dynamics Program and Petrology and Geochemistry Program for funding support, without which this work would not be possible. iv TABLE OF CONTENTS List of Figures vi List of Tables viii Abstract 1 Preface 3 Chapter 1: Whole-Rock 40Ar/39Ar Geochronology, Geochemistry, and Stratigraphy of Intraplate Cenozoic Volcanic Rocks, Central Mongolia 9 Chapter 2: Geochemical Evidence for a Sub-Lithospheric Source for the Intraplate Cenozoic Volcanic Rocks in Central Mongolia 52 Chapter 3: Geochronology and Geochemistry of Lower-Crustal Xenoliths: Exploring the Formation of the Lower Crust Beneath Central Mongolia 98 Conclusion 147 Supplemental data tables 151 Vita 195 v LIST OF FIGURES Figure 1.1 - Location map 12 40 39 Figure 1.2 - Ar/ Ar release spectra 29 Figure 1.3 - Age distribution of Cenozoic volcanic rocks 31 Figure 1.4 - Okrhon volcanic stratigraphy 34 Figure 1.5 - Okrhon field photo 35 Figure 1.6 - Incision rates vs. time interval 35 Figure 1.7 - MgO vs. elevation 37 Figure 1.8 - La/Yb vs. La and La/Yb vs. Yb 39 Figure 2.1 - Location map 57 Figure 2.2 - TAS plot of Cenozoic basalts 70 Figure 2.3 - PM normalized trace-element diagrams 72 Figure 2.4 - Basalt Os concentrations vs. 187Os/188Os 73 Figure 2.5 - Basalt isotope variation vs. age 75 Figure 2.6 - Basalt 206Pb/204Pb vs 207Pb/204Pb 77 Figure 2.7 - Basalt Sr vs Os isotopic composition 83 Figure 2.8 - Basalt Pb vs Os isotopic composition 84 Figure 2.9 - Basalt Sr vs Nd isotopic composition 86 Figure 3.1 - Location map 103 Figure 3.2 - Crustal xenolith photomicrographs 115 Figure 3.3 - Crustal xenolith P-T conditions 118 Figure 3.4 - Zircon LA-ICP-MS U-Pb ages 120 vi LIST OF FIGURES CONT. Figure 3.5 - Cathodoluminescence maps of zircon 121 Figure 3.6 - PM normalized trace-element diagram 123 Figure 3.7 - Arc normalized trace-element diagram 123 Figure 3.8 - Pb diffusion in lower crustal zircons 132 Figure 3.9 - Isotope variation vs. silica and temperature 136 Figure 3.10 - Sr and Nd isotopic composition vs silica 139 vii LIST OF TABLES Table 2.1 - Basalt isotopic compositions 78 Table 2.2 - Mixing model parameters 81 Table 3.1 - CPX and OPX mineral compositions for Thermometry 117 Table 3.2 - Crustal xenolith isotopic compositions 125 Table S1 - Basalt geochronology sample descriptions 152 Table S2 - Basalt geochronology data 171 Table S3 - Previously published basalt geochronology data 176 Table S4 - Basalt major and trace element data 180 Table S5 - U-Pb age data 187 Table S6 - Crustal xenolith major and trace element data 193 viii ABSTRACT The origin of diffuse intraplate volcanism and 4000 m high topography of the Hangay Mountains in central Mongolia is enigmatic, as it is not explained by or predicted by traditional plate tectonic models. Chapter One presents new whole-rock 40Ar/39Ar ages that range from Holocene to ~30 Ma for the Cenozoic volcanic rocks in central Mongolia. The total volume of the Cenozoic volcanic rocks is approximately 1540 km3, but prior to erosion, the volume may have been as high as 2900 km3. Volcanism began to increase gradually in the early Miocene and peaked in the middle Miocene, with a gradual decrease in volume through the Holocene. The low total volumes and the lack of an age progressive hot-spot track rule out the presence of a mantle plume. The long-term gradual increase and subsequent decrease in volcanic output may also rule out delamination. Chapter Two presents the results of a geochemical study to understand the source of the volcanic rocks. Isotopic compositions of the volcanic rocks fall between prevalent mantle (PREMA) and enriched mantle (EM1) and are similar to other Cenozoic basalts from east Asia, though markedly different from the depleted MORB mantle (DMM) isotopic signature of the lithospheric mantle in Mongolia, suggesting the volcanic rocks must be derived from a sub-lithospheric source. A mantle upwelling beneath the region is likely, though the specific cause can not be identified by geochemistry and geochronology alone. In Chapter Three, two-pyroxene granulite lower crustal xenoliths brought to the surface by the Cenozoic volcanism were studied using U- Pb geochronology and geochemistry. U-Pb geochronology indicates the lower crust in central Mongolia began to form by at latest the Permo-Triassic, likely during the formation of the Central Asian Orogenic Belt (CAOB), which is consistent with arc-like 1 geochemical signatures. Recent seismology studies indicate that the high elevations of the Hangay region could be isostatically supported by a thick crust. My new data suggests the lower crust, and by inference the high topography of the Hangay, formed in the Late Paleozoic to Early Mesozoic during the formation of the CAOB, which is significantly older than any other previous estimates. 2 PREFACE Following the widespread acceptance of plate tectonic theory 50 years ago, significant advances have been made to improve our understanding of the geodynamic process that shaped Earth. Despite this great leap forward in understanding earth, many important questions remain that must be addressed to move toward a complete understanding of the tectonic processes that shape the lithosphere. In this dissertation, I seek to answer some fundamental questions about the origin of intraplate volcanism, development of high topography in the middle of continents, and the formation of the lower crust, which remain unresolved in our understanding of tectonics and in the evolution of the lithosphere. The lower crust is the foundation of the crust; it can influence the surface in many ways; thick roots provide isostatic support for high topography, delamination of dense lower crust can give rise to intraplate volcanism and dynamically supported topography (Kay and Mahlburg Kay, 1993), or at the other extreme the lower crust along with the mantle lithosphere add significant stability to cratons over billion-year timescales (Jordan, 1978). In some locations, sometimes referred to as hot-spots, basaltic volcanism punches through the crust and dramatically changes the surface, sometimes bringing with it volcanic gases that have also been shown to change climate (Robock, 2000). Additionally, hot-spot like volcanism may also influence the landscape through the development of dynamically supported topography (He et al., 2003; Saunders et al., 2007). Other than the geodynamic implications, understanding the evolution of the crust and features that develop on it like high-topography and volcanic systems is vital because of the clear impact on life. The outer shell of Earth supports and is home to all life. The 3 formation and destruction of topography has a direct and long-term impact on the evolution of ecosystems and biodiversity (Bentley et al., 2014; Oliveros et al., 2016). It has also been shown that the development of topography on the continents can alter climate patterns (Han et al., 1995). Volcanic eruptions can destroy civilizations, and in more extreme cases they have contributed to mass extinction events by changing climate (Kamo et al., 2003). As with most unresolved questions, the geodynamic processes that give rise to volcanism in intraplate settings are contentious. The unifying theory of plate tectonics explains and predicts the existence of volcanism and high topography at plate boundaries.
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