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The Origin of Intraplate Volcanism on the New Zealand Micro- Continent Zealandia

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The Origin of Intraplate Volcanism on the New Zealand Micro- Continent Zealandia The Origin of Intraplate Volcanism on the New Zealand micro- continent Zealandia Dissertation zur Erlangung des Doktorgrades an der Mathematischen-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Christian Timm Kiel 2008 Hiermit erkläre ich, dass ich die vorliegende Doktorarbeit selbständig und ohne unerlaubte Hilfen erstellt habe. Ferner habe ich weder diese noch eine ähnliche Arbeit an einer anderen Abteilung oder Hochschule im Rahmen eines Prüfungsverfahrens vorgelegt, veröffentlicht oder zur Veröffentlichung vorgelegt. …………………………….. Christian Timm Referent/in…………………………………………………Prof. Dr. Kaj Hoernle, Ph.D. Koreferent/in………………………………………………Prof. Dr. Colin Devey Tag der mündlichen Prüfung………………………………04.07.2008 Zum Druck genehmigt: Kiel, ………………………………………………………… Prof. Dr. J. Grotemeyer (Dekan) Kurzfassung Der Ursprung von Intraplatten-Vulkanismus wird hauptsächlich mittels einer stationären thermalen Anomalie unterhalb der Lithosphäre (Mantel Plume), oder durch kontinentales „Rifting“ erklärt. Diese beiden Prozesse sind oftmals mit voluminösem Vulkanismus assoziiert. Der langlebige und diffus auftretende Vulkanismus auf dem Neuseeländischen Mikro-Kontinent, Zealandia, hingegen kann nicht durch diese Theorien erklärt werden und wird in dieser Dissertation genauer untersucht. Bis in die mittlere Kreidezeit (~ 100 Mio. Jahre) stellte Zealandia einen Teil im NW des ehemaligen Superkontinents Gondwana dar und lag über einer aktiven Subduktionszone. Subduktionbezogener Vulkanismus endete allerdings zwischen 100 – 90 Mio Jahren mit der beginnenden Separation Zealandias von Gondwana. Das beginnende Auseinanderdriften der beiden Landmassen ging einher mir voluminösem Intraplatten-Vulkanismus. Seitdem ist Zealandia ca. 6000 km im Bezug auf den darunter liegenden Mantel nach NW zur heutigen Position gedriftet. Das gesamte Känozoikum hindurch trat diffus über das Plateau Zealandias verteilt Intraplatten-Vulkanismus in Form von monogenetischen vulkanischen Feldern und Schildvulkanen auf. Die mafische Känozoischen Vulkangesteine (MgO > 5 Gew %) ähneln Ozean Insel Basalten und liegen auf Isotopendiagrammen zwischen den Mantelendgliedern MORB, EMII, und HIMU, was auf fundamental unterschiedliche Bildungsgeschichten dieser vulkanischen Gesteine hindeutet. Anhand der seismischen Tomographie ist unterhalb des östlichen Teils von Zealandia ein Bereich mit niedrigeren seismischen Wellengeschwindigkeiten (höchstwahrscheinlich heiß) zwischen ~600 und 1450 km entdeckt worden, die sich bis in die Antarktis verfolgen lässt. Die Isotopensignaturen der Känozoischen Vulkangesteine lassen sich durch eine Mischung der HIMU-artigen kreidezeitlichen vulkanischen Gesteine mit der verarmten (MORB-ähnlichen) oberen Asthenosphäre erklären. Daher liegt die Vermutung nahe, dass der unterhalb von 600 km auftretende Plume, der zur Separation Zealandias von Gondwana beitrug, den oberen Mantel mit primitiven -I- „Plume-Domänen“ versehen hat und die Känozoischen Vulkangesteine demnach höchstwahrscheinlich Mischschmelzen aus diesen beiden Endgliedern darstellen. Die EMII-typ Isotopensignaturen hingegen deuten auf die Interaktion der tief gebildeten asthenosphärischen Schmelzen mit der darüber liegenden kontinentalen Lithosphäre (Mantel, metasomatiert durch Subduktion im Mesozoikum und Kruste) hin. Ein möglicher Prozess zur eigentlichen Schmelzbildung unterhalb Zealandias ist jedoch lithosphärische Delamination durch Rayleigh-Taylor Instabilitäten. Die Bildung von Rayleigh-Taylor Instabilitäten entlang des Übergangs von dichterer unterer Lithosphäre und weniger dichter oberer Asthenosphäre führt zum Aufströmen von warmen asthenosphärischem Mantel und dadurch zur Bildung dekompressiver Schmelzen. -II- Abstract The origin of intraplate volcanism has been predominately attributed to a stationary thermal anomaly beneath the lithosphere (mantle plume) or to continental rifting. Both mechanisms generally result in voluminous volcanic activity. However, diffusely and continuously occurring low-volume intraplate volcanism on the New Zealand micro-continent, Zealandia is inconsistent with these processes. Until the mid-Cretaceous Zealandia was part of the former super-continent Gondwana, situated at its NW margin above an active subduction zone. Subduction ended at ~ 100 Ma and volcanism changed from subduction- to rift-related as Zealandia started to separate from Gondwana, which was associated with voluminous eruptions until ~ 90 Ma. Since the final separation of Zealandia from Gondwana, the micro- continent drifted ~ 6000 km to the NW with respect to the upper asthenosphere to its recent position. During the Cenozoic diffuse intraplate volcanism occurred randomly distributed on the continental plateau, forming two volcanic endmembers: 1) monogenetic volcanic fields and 2) composite shield volcanoes. The mafic Cenozoic volcanic rocks of these volcanic centers (MgO > 5wt %) are strongly akin to ocean island basalts and plot between three isotopic endmembers: MORB, HIMU and EMII, suggesting fundamental differences in the formation of these volcanic rocks. Seismic tomography reveals a low velocity zone (most likely hot) between ~600 and 1450 km beneath Zealandia, which can be traced to Antarctica. Since mixing of the HIMU-type Cretaceous volcanic rocks with depleted MORB can explain most of the isotopic compositions of the Cenozoic lavas, the deep-seated plume (which possibly contributed to the separation Zealandias from Gondwana) polluted the depleted upper asthenospheric mantle (MORB) with fertile (HIMU-type) domains. The EMII-type isotopic signature in some of the volcanic reflects interaction (partly extensively) of the asthenospheric melts with continental lithospheric (mantle, metasomatized by subduction during the Mesozoic and crust). A possible mechanism to trigger partial melts, however is lithospheric removal by Rayleigh- -III- Taylor instabilities. The downwelling of a dense lower lithosphere allowed the warm and less dense upper asthenosphere to stream into the resulting cavities and form partial melts by decompression. -IV- Vorwort Diese Arbeit stellt in Ihrer Gesamtheit eine monographische Dissertation dar, die aus vier unabhängigen Kapiteln besteht (Chapter I-IV). Diese Arbeit entstand auf Anregung durch Prof. Dr. Kaj Hoernle, welche im Rahmen der durch die Deutsche Forschungsgemeinschaft geförderten Projekte HO1833/12-1 und HO1833/12-2 durchgeführt wurden. Ihm gilt mein besonderer Dank für die Vergabe dieser Arbeit, seine Unterstützung, progressive Diskussionen und wichtige Denkanstösse und vor allem, den roten Faden nicht zu verlieren! Prof. Dr. Colin Devey möchte ich für die Übernahme des Koreferats danken. Ein Weiterer besonderer Dank gilt Dr.F. Hauff, Dr. J. Fietzke, Dr. P vd Bogaard und Dipl.-Ing. J. Sticklus, S. Hauff für Ihre Assistenz und unschätzbarer Hilfe bei der Isotopenanalytik, ohne deren Unterstützung von technischer Seite, wie auch bei der Datenauswertung, die Durchführung der Dissertation nicht möglich gewesen wäre. Dagmar Rau danke ich für die RFA Analytik. Außerdem bin ich dem IFM-GEOMAR für die Kostenübernahme der “In-House” Analytik (Hauptelement- und Sr-Nd-Pb-Hf Isotopenanalytik) zu großem Dank verpflichtet, denn ohne die Übernahme dieser Kosten wäre diese Arbeit nicht möglich gewesen. Dr. J. Geldmachen, Dr. S. Duggen und Dr. M. Portnyagin danke ich für Ihre Diskussionsbereitschaft und hilfreichen und stimulierenden Kommentare. Desweiteren danke ich Dieter Garbe-Schönberg und seinem Team, insbesondere Ulrike Westernströer, Petra Fiedler, Heidi Blaschek und Inge Dold für Ihre Unterstützung bei der Probenaufbereitung für die ICP-MS Analytik. -V- Spezieller Dank geht an das PGP Team und Prof. Yuri Podlachikov in Oslo, welches mir zu einem tollen Aufenthalt in Oslo (März/April 2007) ermöglicht hat und hier insbesondere and die Doktoranden WG (Karthik, Christoph, Marcin und Karen), genauso wie Lars Rüpke und Timm John. Ebenso bin ich dankbar für den tollen Neuseelandaufenthalt (Februar-April 2005) und hier insbesondere Andrea Todt für die nette Beherbergung in Dunedin und Dr. James White von der University of Otago für die Reparatur des defekten Getriebes... Für Ihre Hilfe als Hiwis möchte ich mich bei Christoph Marshall, David Rose, Jochen Kollofrath und Anna Erichsen bedanken. Ken Heydolph, Nikolaus Bigalke, Jens Schneider, Rieka Harders, Mathias Marquardt, Sebastian Münn, Brian Haley und Gerd Lube (und natürlich allen anderen die dabei waren auch) gilt mein Dank für die vielen netten Stunden in der Mensa, die guten und unterhaltsamen Gespräche, Kaffeepausen etc.. Hier sei insbesondere der SFB Kaffee hervorgehoben... Mein größter Dank allerdings geht an meine Eltern und insbesondere an meine werdende eigene Familie und hier natürlich an meine Jule, die mir unendlich viel Kraft gegeben und meine Launen zum Ende der Arbeit hin geduldig ertragen hat. Vielen Dank! -VI- Content Kurzfassung………………………………………………………………………………I Abstract………………………………………………………………………………….III Vorwort…………………………………………………………………………………..V List of Figures…………………………………………………………………………...XI List of Tables…………………………………………………………………………...XII Chapter I 1.1 Geological frame..…………………………………………………………………….1 1.2. Brief overview of the mantle structure and composition…………………………….3 1.3 Outline of the thesis and future perspectives….……………………………………...4 Chapter II Geochemical Evolution of Intraplate Volcanism at Banks Peninsula, New Zealand: Mantle Melts versus Crustal Contamination Abstract…..……………………………………………………………………………...12 2.1 Introduction..………………………………………………………………………...13
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