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Friedrich-Alexander Universität Erlangen-Nürnberg Petrologische und Geochemische Untersuchungen an ultramafischen und mafischen Gesteinen der Shackleton Range, Ost-Antarktis Zeugen des Zusammenschlusses Gondwanas und letzte Relikte eines einstigen Ozeans? Der Naturwissenschaftlichen Fakultät/ Dem Fachbereich Geographie und Geowissenschaften der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades Dr. rer. Nat. vorgelegt von Tanja Romer aus Illertissen i Als Dissertation genehmigt von der Naturwissenschaftlichen Fakultät/ vom Fachbereich Geographie und Geowissenschaften der Friedrich-Alexander-Universität Erlangen-Nürnberg Tag der mündlichen Prüfung: 01.06.2017 Vorsitzende/r des Promotionsorgans: Prof. Dr. Georg Kreimer Gutachter/in: Prof. Dr. Esther Schmädicke Prof. Dr. Reiner Klemd ii Zusammenfassung Im östlichen Teil der Antarktis liegt die Shackleton Range. Es handelt sich hierbei um ein Kollisionsorogen, das nach heutigen Erkenntnissen der panafrikanischen Orogenese zugeordnet wird. Hinweise darauf finden sich im nördlichen Bereich (z.B. Haskard Highlands) der Shackleton Range. Hier treten granatführende, ultramafische Gesteine als Linsen, eingeschlossen in hochgradig metamorphen Gneisen auf. Die Linsen setzen sich hauptsächlich aus granat- und/oder spinell-führenden Pyroxeniten und untergeordnet auch Peridotiten zusammen. Die nähere Umgebung der Linsen wird vor allem durch Amphibolite dominiert. Die Pyroxenite enthalten teilweise eine Verwachsung von Granat und Olivin und sind damit ein eindeutiger Indikator für eine eklogitfazielle Metamorphose in diesem Bereich. Weiterhin zeugen sie als ultramafische Gesteine von einer möglichen Suturzone. In dieser Forschungsarbeit konnte mittels Mikrosondenanalytik an Granat, Ortho- und Klinopyroxen, Spinell, Olivin und Amphibol für die ultramafischen Gesteine ein Teil des im Uhrzeigersinn verlaufenden P-T-Pfads rekonstruiert werden. Thermobarometrische Berechnungen ergaben maximale Metamorphosetemperaturen von 800 bis 850 °C. Die maximal erreichten Drücke dürften zwischen 20 bis 23 kbar gelegen haben. Die darauf folgende Druckentlastung konnte durch einen kelyphitischen Saum um Granat, bestehend aus Spinell, Ortho- und Klinopyroxen auf etwa 2 kbar eingegrenzt werden. Diese Berechnung beruht auf dem geringeren Cr-Gehalt des kelyphitischen Spinell im Vergleich zu dem granularen Spinell der Matrix. Die Ergebnisse lassen sich sehr gut mit einer Metamorphose im eklogitfaziellen Bereich korrelieren. Weiterhin deutet der metamorphe Gradient von 11 bis 12 °C/km auf eine ehemalige Subduktionszone hin. Einige der Proben konnten datiert werden. Hierbei ergab die Sm-Nd-Datierung der pyroxenitischen und amphibolitischen Proben ein Eduktalter von 918 ± 460 Ma, basierend auf den Nd-Isotopenhäufigkeiten und den Gesamtgesteinsdaten. Das Sm- Nd-Metamorphosealter der untersuchten Granate liegt zwischen 510 bis 530 Ma. Die Datierung mit der Ar-Ar-Methode an Amphibolen aus Pyroxeniten sowie Amphiboliten ergab dazu vergleichbare Metamorphosealter von etwa 500 bis 520 Ma. Diese Ereignisse können somit eindeutig der panafrikanischen Orogenese zugeordnet werden. Die bestimmten Protolithalter lassen darauf schließen, dass die Gesteine während des Neoproterozoikums oder späten Mesoproterozoikums entstanden sind. Diese Daten stimmen mit Altern von Pyroxeniten und mafischen Gneisen aus dem nördlichen Teil der Shackleton Range, z.B. Northern Haskard Highlands, Bernhardi Heights und Lord Nunatak überein (Talarico et al., 1999b; Will et al., 2010). Ferner sind in der Lanterman Range, im Transantarktischen Gebirge, Gesteine mit einem vergleichbaren Eduktalter (750 - 700 Ma) vorhanden (Di iii Vincenzo et al., 1997). Auf Grundlage der geochemischen Analysen dieser Forschungsarbeit werden die Proben in vier Gruppen, Pyroxenite, Amphibolite mit MgO-Gehalt ≥ 14 Gew.%, Amphibolite mit MgO-Gehalt ≤ 12 Gew.% und Harzburgite, unterteilt. Bei den Mg-reichen Amphiboliten handelt es sich mutmaßlich um amphibolitfaziell rekristallisierte Pyroxenite. Die Herkunft der Harzburgite lässt sich nicht eindeutig klären. Sie entstammen entweder dem Mantel oder stellen sehr frühe Kumulate da, höchstwahrscheinlich ist beides vorhanden. So zeigt eine der Proben Signaturen von Mantelderivaten (z.B. Verteilung der Seltenen Erden), während die restlichen Proben typische Merkmale von Olivin-Kumulaten besitzen. Generell lässt die Elementverteilung von Haupt- und Spurenelementen (z.B. Zr- und Nb-Gehalte), sowie die verschiedenen Verhältnisse der Elemente zueinander (z.B. Th/Nb-, La/Sm- und Zr/Nb-Verhältnis) darauf schließen, dass die Magmenquelle fast aller Proben im Bereich einer ozeanischen Insel lag. Die Abreicherung von mobilen Elementen wie z.B. Sr und Ba legt nahe, dass die primäre Magmenquelle der untersuchten Gesteine durch sekundäre Prozesse verändert wurde. Im Weiteren sprechen z.B. Harker-Diagramme, Ce-Yb-Verhältnisse und normierte Spurenelementverteilungsmuster für eine starke Differenzierung der Magmensuite. In einigen Fällen deuten die Th-, La- und Nb-Gehalte auf eine Interaktion mit krustalem Material hin. Insgesamt zeigen die Ergebnisse dieser Forschungsarbeit, dass die hier untersuchten Proben vor etwa 918 ± 460 Ma im Bereich einer ozeanischen Insel entstanden sind und im Rahmen der panafrikanischen Orogenese eine Hochdruck-Metamorphose erfuhren. Die ultramafischen Gesteine sind somit höchst wahrscheinlich erste Zeugen einer eklogitfaziellen Metamorphose im Raum der östlichen Antarktis, welche die Kollision von Ost- und Westgondwana widerspiegeln. Die Subduktion mit anschließender Kontinent-Kontinent-Kollision lässt sich auf einen Zeitraum von 530 bis etwa 510 Ma fixieren. Diese Forschungsarbeit und schon vorangegangene Untersuchungen aus dieser Region lassen mutmaßen, dass sich die daraus entstandene Suturzone über die Herbert Mountains, wo neoproterozoische Ophiolithe gefunden wurden, weiter zu den Lord Nunataks zieht. Es ist anzunehmen, dass es sich bei dieser Zone um die Kuunga Sutur handelt. Diese Sutur markiert die letzte Kontinent- Kontinent-Kollision im Bereich der östlichen Antarktis und dürfte somit vom kompletten Zusammenschluss Gondwanas zeugen. iv Abstract The Shackleton Range is located in the eastern part of Antarctica. It is, according to the recent studies, a collisional orogen, which can be most likely associated with Pan-African orogeny. Indications are found in the northern area (Haskard Highlands) of the Shackleton Range. Here come garnet leading ultramafic rocks as lenses enclosed in highly metamorphic gneisses. The lenses consist mainly of garnet- and/or spinel-leading pyroxenites and subordinate peridotites. The surrounding area of the lenses is dominated by amphibolites. The pyroxenites contain partially an intergrowth of garnet and olivine and are therefore a clear indicator of an eclogite facies metamorphism in this area. Furthermore the ultramafic rocks are tracers of an existing suture zone. A part of the clockwise P-T–path of the ultramafic rocks could be reconstructed in this research project by using microprobe analysis of garnet, ortho- and clinopyroxene, spinel, olivine and amphibole. Thermobarometric calculations showed maximum metamorphic temperatures of 800 to 850 °C. The maximum achieved pressure is, after the calculations, between 20 to 23 kbar. The following decompression could be limited, by a kelyphitic rim of spinel, ortho- and clinopyroxen around garnet, to 2 kbar. This calculation is based on the lower concentration of chrom in the kelyphitic spinel in comparison with the spinel of the matrix. The results match well with an eclogite facies metamorphism. Furthermore, the metamorphic gradient of 11 - 12 °C/km indicates a former zone of subduction. In addition, some samples were dated. Here, the Sm -Nd dating of pyroxenitic and amphibolitic samples showed a Protolithage of 918 ± 460 Ma, based on the Nd isotopic abundances and the whole rock data. The Sm -Nd metamorphic age of the examined garnet is between 510 - 530 Ma. The dating of amphiboles from pyroxenites and amphibolites with the Ar-Ar method yielded comparable metamorphic ages of 500 - 520 Ma. Thus these events can be attributed to the Pan-African orogeny. The Protolithages suggests that the minerals formed during the Neoproterozoic or late Mesoproterozoic. These data are consistent with ages of mafic gneisses and pyroxenites from the northern part of the Shackleton Range, for example, Northern Haskard Highlands, Bernhardi Heights and Lord Nunatak (Talarico et al., 1999b; Will et al., 2010). Furthermore, rocks of similar age (750 - 700 Ma) are present in the Lanterman Range, in the Transantarctic Mountains (Di Vincenzo et al., 1997). According to the geochemical analyzes of this research study, the samples are divided into four groups, pyroxenites, amphibolites with a concentration of MgO ≥ 14 Gew.%, amphibolites with a MgO concentration of ≤ 12 Gew.% and harzburgites. The former amphibolites are most likely amphibolite facies recrystallized pyroxenites. The origin of the harzburgites can not clearly be reconstructed. They originate either from the mantle or represents very early cumulates, v possibly it exists both. Generally, the element distribution of major and trace elements (e.g. Zr- and Nb-contents) as well as the various ratios of the elements to each other (e.g. Th/Nb- , La/Sm- and Zr/Nb-ratio) suggests that the magma source of most samples is in the range of an oceanic island. The depletion of mobile elements such as Sr and Ba suggests that the primary magma source of the examined rock has been altered by secondary processes. Furthermore,
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