3.26 Ga Mendon Formation, Barberton, Greenstone Belt, South Africa
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Louisiana State University LSU Digital Commons LSU Master's Theses Graduate School 2013 High Resolution Tephra and U/Pb Chronology of the 3.33 – 3.26 Ga Mendon Formation, Barberton, Greenstone Belt, South Africa Nicholas Burns Decker Louisiana State University and Agricultural and Mechanical College, [email protected] Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_theses Part of the Earth Sciences Commons Recommended Citation Decker, Nicholas Burns, "High Resolution Tephra and U/Pb Chronology of the 3.33 – 3.26 Ga Mendon Formation, Barberton, Greenstone Belt, South Africa" (2013). LSU Master's Theses. 2176. https://digitalcommons.lsu.edu/gradschool_theses/2176 This Thesis is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Master's Theses by an authorized graduate school editor of LSU Digital Commons. For more information, please contact [email protected]. HIGH RESOLUTION TEPHRA AND U/PB CHRONOLOGY OF THE 3.33 – 3.26 GA MENDON FORMATION, BARBERTON GREENSTONE BELT, SOUTH AFRICA A Thesis Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College In partial fulfillment of the requirements for the degree of Master of Science in The Department of Geology and Geophysics by Nicholas Burns Decker B.S., Louisiana State University, 2010 May 2013 Acknowledgements The author is especially grateful to Dr. Gary Byerly, his advisor, for his ever insightful guidance, an excellent field season, and a worthwhile project. He would also like to thank Dr. Byerly, Dr. Don Lowe and his student Lizzy Stefurak, and Dr. Melanie Thompson-Stiegler for their contributions to his research for which they will be coauthors on when this manuscript is submitted for publication to Precambrian Research. The author would like to acknowledge Dr. Samuel Bentley for allowing him use of his handheld XRF system and to Dr. Bentley’s students, Ashley Howell, Jill Bambrick Banks, and Kathryn Denommee for their help operating it. The author is also eternally grateful to Rick Young for helping process samples and repair lab equipment. Finally, the author would like to thank the staff of the WSU GeoAnalytical Lab and the Stanford SUMAC facility for their help acquiring and processing geochemical data. ii Table of Contents Acknowledgements ......................................................................................................................... ii Abstract ........................................................................................................................................... v 1. Introduction ................................................................................................................................. 1 2. Geologic Setting.......................................................................................................................... 4 2.1. Barberton Greenstone Belt ....................................................................................... 4 2.2. Mendon Formation................................................................................................... 4 2.3. Komatiites and Pyroclastic Komatiitic Volcanism ................................................ 10 3. Methods..................................................................................................................................... 12 3.1. Bulk Rock X-Ray Fluorescence............................................................................. 12 3.2. Handheld X-Ray Fluorescence .............................................................................. 12 3.3. Geochronology ....................................................................................................... 15 4. Mendon Stratigraphy ................................................................................................................ 16 4.1. Umbaumba Gorge Section ..................................................................................... 17 4.2. The 620 Section ..................................................................................................... 19 5. Geochemistry ............................................................................................................................ 21 5.1. Bulk Rock X-Ray Fluorescence............................................................................. 21 5.2. Handheld X-Ray Fluorescence .............................................................................. 25 5.3. Geochronology ....................................................................................................... 29 6. Discussion ................................................................................................................................. 39 6.1. Chemostratigraphy ................................................................................................. 39 6.2. Relationship to the Weltevreden Formation .......................................................... 42 6.3. Volcanism and Basin Relationships ....................................................................... 43 6.4. Tectonism ............................................................................................................... 45 7. Conclusions ............................................................................................................................... 49 References ..................................................................................................................................... 51 iii Appendix: Handheld XRF Test Analyses ..................................................................................... 56 Vita ................................................................................................................................................ 66 iv Abstract The Mendon Formation in the Barberton Greenstone Belt of South Africa marks the boundary between the Onverwacht and Fig Tree Groups. These groups are characterized by mafic to ultramafic volcanism and felsic volcanism with related epiclastic sedimentation, respectively. This transition marks the end of komatiitic volcanism in the Barberton Greenstone Belt and is accompanied by numerous impact-related spherule layers. This study characterizes the upper Mendon Formation texturally and geochemically over a wide areal extent and across structure and facies change in an attempt to better understand the evolution of tectonic processes at this boundary. A suite of whole rock and handheld X-Ray Fluorescence analyses are presented in conjunction with textural information, stratigraphic relationships, and U/Pb ages to create a temporal and chemostratigraphic framework for the Mendon Formation. Local and regional stratigraphic variations, including absence of distinctive layers and variation in layer thickness, seen across the Mendon preclude ascription of a single stratigraphy that accurately describes the >1.2 km of section present in this formation. These variations indicate diachronous deposition of the Mendon over a wide areal extent and into multiple basins or sub-basins by more than one magmatic source. 204Pb-corrected 206Pb/238U and 207Pb/235U concordia model ages of 3279 ± 9.1 Ma and 3287.3 ± 2.9 Ma for two samples from upper portions of the Mendon provide temporal context for deposition during the Mendon Formation. Two samples from the basal 10 m of the Fig Tree Group, above the S2 spherule bed that marks the boundary between the Onverwacht and Fig Tree Groups, give model ages of 3267.8 ± 6.9 Ma and 3261 ± 18 Ma. These ages provide added constraints for the Onverwacht-Fig Tree boundary and confirm that the Weltevreden Formation is roughly age-correlative with the uppermost Mendon Formation. While these formations are in part age-correlative and have similar lithologies, they do not appear to be v genetically related. The dominance of ultramafic volcanic rocks and the paucity of felsic volcanic and terrigenous sedimentary rocks within the Mendon and Weltevreden Formations indicate that the primary mode of crustal formation was likely plume-related magmatic accretion and not subduction. The relatively sharp transition within the BGB from ultramafic volcanic sequences to more felsic volcanic and epiclastic sedimentary sequences is everywhere marked by impact-related spherule layers, which suggest that major impacts may have played a role in the evolution of early Earth tectonics to more modern, subduction-related styles. vi 1. Introduction Greenstone belts are somewhat enigmatic Archean crustal sequences that represent a large portion of preserved early crust and are typically composed of a variety of volcanic and sedimentary rocks that can be considered relatively unique to this early stage of Earth’s evolution. There is still considerable debate over the tectonic processes and surface environments that gave rise to these terranes (de Wit et al., 2011; Furnes et al., 2013; Grove and Parman, 2004; Lowe and Byerly, 2007; Shirey and Richardson, 2011; Van Kranendonk, 2011a). One of the best preserved of these crustal sequences is the Barberton Greenstone Belt (BGB) of South Africa. Within this relatively well-studied supracrustal sequence, three stratigraphic groups appear to represent profoundly different tectonic settings: the Onverwacht Group, predominantly mafic to ultramafic lavas deposited on shallow marine platforms and likely the products of mantle plumes; the conformably overlying Fig Tree Group, predominantly felsic volcaniclastic