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Structural and Geochemical Analyses of Fragment-Rich Pseudotachylite Bodies and the Vredefort Granophyre of the Vredefort Impact Structure, South Africa

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Structural and Geochemical Analyses of Fragment-Rich Pseudotachylite Bodies and the Vredefort Granophyre of the Vredefort Impact Structure, South Africa Structural and Geochemical Analyses of fragment-rich pseudotachylite bodies and the Vredefort Granophyre of the Vredefort Impact Structure, South Africa Thesis submitted to the Department of Earth Sciences, Free University of Berlin, Germany, in partial fulfilment of the requirements for the degree of Doctor rerum Naturum. Department of geoscience (FB Geowissenschaften) at the Freie Universität Berlin supported by the Museum für Naturkunde, Humboldt-Universität Daniel Lieger Berlin 2011 Structural and Geochemical Analyses of fragment-rich pseudotachylite bodies and the Vredefort Granophyre of the Vredefort Impact Structure, South Africa Daniel Lieger 1. Gutachter: Prof. Dr. Harry Becker 2. Gutachter: Prof. Dr. Ulrich Riller Disputation am: 16.05.2011 1 ZUSAMMENFASSUNG Meteoriteneinschläge (Impakte) werden heute als eine der wichtigsten Prozesse in der planetaren Geologie angesehen. Obwohl detaillierte strukturelle, petrografische und geochemische Studien von terrestrischen Impaktstrukturen existieren, sind deren Daten und Zusammenhänge kaum verstanden. Unsicherheiten existieren vor allem in der geometrischen und kinematischen Bedeutung von Dislokationszonen, der zeitlichen und mechanischen Bedeutung zwischen kontinuierlicher und diskontinuierlicher Deformation während der Zentralbergbildung sowie in der Entstehung und Platznahme der pseudotachylitischen Schmelze von großen fragmentreichen Körpern. Die vorliegende Arbeit untersucht die Entstehung des Zentralberges und der fragmentreichen pseudotachylitischen Zonen der komplexen, 2.023±4 Ga alten Vredefort Impaktstruktur in Südafrika. Der zentrale Bereich der Vredefort Impaktstruktur, der sogenannten Vredefort Dome, repräsentiert den erodierten Rest der angehobenen Gesteinspakete des Zentralberges. Der Vredefort Dome umfasst einen 40 Kilometer breiten archaischen Kern (Vredefort Core) bestehend aus migmatischen Gneisen mit einer Tonalit-Trondhjemit-Granodiorit- Vergesellschaftung sowie granitoide Plutone und Gesteine des Grünsteingürtels. Dieser innere Bereich des Vredefort Domes ist umgeben von dem 15-20 Kilometer breiten „Collar“, der aus steil gestellten bis überkippten, 3.07 bis 2.1 Ga alten Metasedimentgesteinen sowie intrudierten Alkaligraniten und Dioriten besteht. Das Vorkommen der Pseudotachylite ist die Gemeinsamkeit aller Gesteine des gesamten Vredefort Domes. Die Pseudotachylite kommen als unregelmäßig verlaufende, Millimeter bis Zentimeter breite Gänge sowie Dezimeter bis einige zehner Meter mächtige Körper mit meist scharfem Kontakt zum Nebengestein vor. Die Präsenz von stoßwellenmetamorphen Mineralen in einigen kleinen Gängen zeigt, dass diese Gänge das Produkt von Wechselwirkung der Stoßwelle mit dem Zielgestein sind. Zusätzlich entsteht bei der Wechselwirkung Reibungswärme, die ebenfalls verantwortlich für lokal gebildete Schmelzen ist. Reibungsschmelzen in Impaktstrukturen können auch ohne Einwirkung der Stoßwelle entstehen, indem Kraterrandschollen an übersteilten Kraterrändern an listischen Störungen abgleiten und Pseudotachylite bilden. Jedoch können weder stoßwelleninduzierte Schmelzen noch 2 Reibungsschmelzen die Vorkommen von Meter mächtigen Pseudotachyliten in den Impaktstrukturen erklären. Die umfangreiche strukturelle Analyse dieser Arbeit von dem prä-impakt Mineralgefüge und fragmentreichen Pseudotachyliten des Vredefort Domes hat gezeigt, dass die Gefüge und Pseudotachylite radial und konzentrisch hinsichtlich des Kraterzentrums angeordnet sind. Dabei ergab die Gefügeanalyse eine nordwest-südost streichende Spiegelebene. Die Gefügesymmetrie ist konsistent mit einer vertikalen Dehnung und Anhebung des inneren Kernbereichs sowie einer auswärts gerichteten Rotation und Dilatation des äußeren Kernbereichs, was durch numerische Modellierungen von Impaktereignissen berechnet wurde. Die symmetrische Anordnung, Geometrie und Brekzienintensität von pseudotachylitischen Körper stimmen mit der Verformung überein, die ebenfalls die numerischen Modelle wiedergaben. Mit Hilfe petrografischer und geochemischer Analysen sollte der Impaktschmelzsee als Quelle für die großen fragmentreichen Pseudotachylite bestätigt werden. Der Vergleich von geochemischen Daten der fragmentreichen Pseudotachylite mit denen der unmittelbaren Nebengesteinstypen zeigte, dass diese signifikant voneinander abweichen und dass die Assimilation des Nebengesteins die Schmelzzusammensetzung der pseudotachylitischen Schmelze modifizierte. Die chemischen Trends der kompositionellen Abweichung von Meter breiten fragmentreichen Pseudotachyliten vom direkten Nebengestein sind konsistent mit der Präsenz einer allochthonen granitoiden Schmelze, deren Zusammensetzung dem Vredefort Granophyr ähnelt. Der Vredefort Granophyr gilt als das Produkt der Differentiation des originalen Impaktschmelzsees. Die Untersuchung des Schmelztransportes anhand Zentimeter zu Meter breiten pseudotachylitischen Körpern ergab einen gerichteten Schmelztransport von großen in kleine pseudotachylitische Körper und Apophysen. Mit Hilfe der Mineralogie von Sulfiden und Silikaten wurde die Temperatur dieser Schmelze zwischen 1200-1700 °C bestimmt. Die Analysen der strukturellen und geochemischen Daten zeigten, dass in der Spätphase der Kraterbildung eine allochthone Schmelze aus dem überhitzten Impaktschmelzsee in Extensionsbrüche im Kraterboden intrudiert sein muss, um fragmentreiche Pseudotachylite zu bilden. Es ist wahrscheinlich, dass während des Kollapses des Zentralberges und der auswärts gerichteten Gesteinsbewegung die Schmelzbewegung dem Druckgradienten folgte und in fragment-reiche Dilatationszonen intrudierte. 3 Zusätzlich zur geochemischen Analyse der pseudotachylitischen Körper wurden geochemische Daten vom Vredefort Granophyr ermittelt, um den Einfluss der Impaktschmelze auf fragmentreichen Pseudotachylite zu bestimmen. Strukturell ähneln sich die Vorkommen der fragmentreichen Pseudotachylite und Vredefort Granophyre. Die Vredefort Granophyre sind ebenfalls radial und konzentrisch hinsichtlich des Kraterzentrums angeordnet. Jedoch durchschlagen sie, im Gegensatz zu den fragmentreichen Pseudotachyliten, den Kontakt zwischen dem Archaischen „Core“ und dem proterozoischen „Collar“. Geochemische Untersuchungen von Vredefort Granophyrgängen ergab eine chemische Heterogenität zwischen Gängen im Vredefort „Core“ und denen, die im „Core“ und „Collar“ verlaufen sowie innerhalb einzelner Gänge. Innerhalb der Gänge wurden zusätzlich neben den chemischen auch texturelle Unterschiede sowie unterschiedliche Fragmentverteilungen festgestellt, die sich in zwei Zonen trennen lassen. Diese beiden Zonen innerhalb der Vredefort Granophyre geben dabei zwei Intrusionsphasen an. Die strukturelle Betrachtung dieser Gänge deutet darauf hin, dass die Vredefort Granophyr-Impaktschmelze in ein Bruchsystem intrudierte, welches erst während der isostatischen Ausgleichsbewegung der darunterliegenden Kruste zehntausende Jahre später angelegt wurde. 4 ABSTRACT Despite the fact that large meteorite impacts belong to one of the most processes in planetary geology, detailed structural, petrographic and geochemical studies of terrestrial impact structures, let alone of large complex ones, are little understood. Major uncertainties centre around the geometric and kinetematic significance of prominent dislocations, the temporal and mechanical relationships between continuous and discontinuous deformation during formation of the central uplift as well as the formation and emplacement of pseudotachylitic melt in large breccia bodies. This study investigated the formation of the central uplift structure and pseudotachylitic breccia zones of the large, complex 2023±4 Ma old Vredefort Impact Structure. The central portion of the impact structure, the so-called Vredefort Dome, is the eroded relic of structurally uplifted rocks, generally referred to as the central uplift. The Vredefort Dome consists of a core, ca. 40 km in diameter, of a predominantly polydeformed Archean migmatitic gneisses, with subsidiary tonalite-trondhjemite-granodiorite assemblage, syn-tectonic granitoids and remnants of greenstone units. The core is surrounded by a 15-20 km wide ‘collar’ of subvertical to overturned, 3.07 and 2.1 Ga supracrustal strata that were deposited unconformably on the Archean crystalline basement rocks. The Vredefort dome is well known for its prominent pseudotachylite zones. The geometry of these zones ranges from mm- to cm-wide veins, dm- to m-scale dike-like bodies to tens of metres wide, irregular but overall planar zones. The presence of shock-metamorphic minerals in some very thin bodies indicates that these bodies were formed as a result of shock heating, whereby shear heating by friction may well have contributed to localized melting. However, neither in situ shock-induced nor frictional melting can explain the local occurrence of hundreds of meter wide pseudotachylite bodies. Comprehensive structural analysis of pre-impact mineral fabrics and properties of fragment-rich pseudotachylite in the Vredefort Dome have shown that these fabrics and pseudotachylite zones have commonly radial or concentric trends. The high spatial coverage of the data revealed that fabric strike is symmetric about a NW-SE striking vertical mirror plane passing through the Dome centre. The fabric symmetry is consistent with vertical stretching and uplift of the inner core and outward rotation and dilation in the outer core zone as is predicted for rocks at the current erosion level by numerical modelling of the impact event. More specifically, the well- 5 developed centro-symmetric
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