Petrogenesis of low Ti and high Ti magmatic rocks of the Jurassic ferrar large igneous province, Antarctica [Elektronische Ressource] / von Ricarda Hanemann

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• Publié par: friedrich-schiller-universitat_jena
• Publié le: 01 janvier 2009
• Nombre de lectures: 8
• Langue: English
• Poids de l'ouvrage: 33 Mo

Extrait

Petrogenesis of low-Ti and high-Ti
magmatic rocks of the Jurassic
Ferrar Large Igneous Province, Antarctica
Dissertation
zur Erlangung des akademischen Grades doctor rerum naturalium
(Dr. rer. nat.)
vorgelegt dem Rat der Chemisch-Geowissenschaftlichen Fakultät
der
Friedrich-Schiller-Universität Jena
von Dipl.-Geol. Ricarda Hanemann
geboren am 25.08.1975 in JenaGutachter:
1. Prof. Dr. L. Viereck-Götte
2. Prof. Dr. F. Holtz
Tag der öffentlichen Verteidigung: 28.11.2007ABSTRACT
The Mid-Jurassic Ferrar Large Igneous Province is one of the Mesozoic continental plateau
basalt provinces associated with the break-up of the Gondwana supercontinent. The Ferrar
magmatic rocks, exposed in a linear belt along the Palaeo-Pacific margin of Gondwana,
comprise numerous sills, dikes and lava flow sequences as well as the Dufek layered mafic
intrusion. Uniform crust-like trace element and isotope signatures indicate a single magma
source for the > 3000-km long Ferrar magmatic province. A subcontinental lithospheric mantle
source has been proposed characterised by crustal enrichment due to Palaeozoic subduction
along the cratonic margin of Gondwana. However, the thermal source for the voluminous melt
generation is still under debate as the crust-like isotope data obscure the detection of a possible
mantle plume involvement. Furthermore, the genesis of the two magma series present within
the Ferrar province, a low-Ti and a high-Ti series, is an unsolved issue up to now.
Against this background, the present thesis investigates Ferrar magmatic rocks from northern
Victorialand and from George V Land aiming a better understanding of the petrogenesis of the
Ferrar magmatic rocks. The main subject is directed towards the understanding of the evolution
of the two compositionally distinct Ferrar magma series including the differentiation history, the
melt generation and some magma source constraints. Beside major and trace elements (incl.
rare earth elements – REE), platinum-group elements (PGE) were analysed in selected Ferrar
rocks to further describe the evolutionary processes and additionally, to decipher mantle source
characteristics, since the thermal source for the generation of the large volumes of Ferrar
magmas is still a matter of debate.
The analysed basaltic andesites and andesites exhibit the common phase assemblage of
tholeiitic differentiation sequences composed of varying amounts of pyroxenes, plagioclase,
oxides and mesostasis. All samples have been categorised as low-Ti (LTS: TiO ~ 0.4 – 1.2 2
wt%) and high-Ti series (HTS: TiO ~ 1.7 – 2.1 wt%). In contrast to the LTS, rocks of the HTS 2
generally lack orthopyroxene and contain clinopyroxene and plagioclase that are richer in FeO
and Na O, respectively. They exhibit higher concentrations of incompatible elements (e.g. FeO, 2
TiO , Y, Zr, REE) at only slightly enriched SiO contents relative to the LTS. 2 2
The low-Ti rocks are characterised by wide petrographical as well as mineral- and whole-rock
chemical variations. The mineral chemistry and specific element concentrations of low-Ti rocks
exhibit good correlations with the degree of differentiation of the distinctly evolved samples.
These variations are interpreted to result from fractional crystallisation during low-pressure in
situ differentiation. From the chilled margins of low-Ti sills (Mg# ~ 62), which are suggested to
represent the pre-intrusive composition of the low-Ti magmas, cumulates and differentiates
(Mg# ~ 74 – 33) developed after magma intrusion in the upper crust. The mineral and bulk-rock
chemical data further indicate that the magmas of the chemically more evolved but uniform HTS
(Mg# ~ 31 – 24) are not generated by continued low-pressure fractionation of the LTS magmas.
The uniform trace element signatures (and published isotope data) of the two Ferrar magma
series indicate that commonly proposed models for the generation of distinct magma series in
other LIPs, such as derivation from different magma sources or differences during melt
generation, can be excluded. Assuming an identical primary magma for both Ferrar magma
series, different conditions during their pre-emplacement differentiation are suggested to explain
the two distinct compositions.
In consideration of previously published experimental results, the compositional differences
observed for the two Ferrar magma series are used to develop a differentiation model
suggesting distinctly different water activities, oxygen fugacities and pressure conditions for
both magma series. The influence of these intensive parameters on the Ferrar magma evolution has been examined by equilibrium crystallisation experiments performed at 1100 °C and 2 kbar
in internally heated pressure vessels using a chilled margin sample from a low-Ti sill as starting
material. The obtained phase relations and compositions exhibit systematic variations with
changing run conditions. The comparison of the experimental results with the natural phase
assemblage of the analysed low-Ti and high-Ti rocks indicates that the high-Ti magma series
experienced pre-emplacement differentiation under lower pressure, lower water activity and
lower oxygen fugacity than the low-Ti magma series.
From these results, it is suggested that the low-Ti magmas experienced the pre-emplacement
differentiation mainly at the base of the continental crust, while the high-Ti magmas stagnated
and differentiated at shallower crustal level. The complex interaction of the different intensive
variables during magma differentiation in the proposed independent temporary magma
reservoirs largely influenced the fractionating mineral phases and hence can well explain the
distinct evolution of the two Ferrar magma series.
This simplified polybaric differentiation model including the transfer of the increasingly evolving
low-Ti and high-Ti magmas to the upper crust is assumed to be essentially controlled by sudden
major tectonic changes related to rifting processes associated with the initial break-up of the
Gondwana supercontinent. Magma emplacement along pre-existing lithospheric discontinuities
would explain the unusual distribution of the Ferrar magmatic rocks.
However, the present geochemical data do not give any evidence of a mantle plume
contribution during generation of the Ferrar magmas. The PGE abundances, thought to shed
some light on this problem, exhibit distinct correlations with the differentiation index of the
samples indicating that the primary signatures are also largely influenced by the pronounced in
situ differentiation. The mainly sulphur-undersaturated conditions during differentiation of the
Ferrar magmas inferred from the PGE systematics confirm the refractory nature of their mantle
source that has also been deduced from specific major and trace element features of the
studied rocks. These signatures indicate a derivation of the primary Ferrar magmas by partial
melting of a secondarily hydrated source within the subcontinental lithospheric mantle, depleted
by previous events of melt extraction. Nevertheless, the controlling mechanism of melt
generation producing the voluminous Ferrar magmas can not be evaluated conclusively by
means of the present geochemical data and remains an outstanding issue for an
interdisciplinary discussion.KURZFASSUNG
Die Jurassische Ferrar Large Igneous Province (FLIP) repräsentiert eine der mit dem
Gondwana-Zerfall assoziierten Mesozoischen Plateaubasaltprovinzen. Die magmatischen
Gesteine der Ferrar Provinz treten in einem schmalen Gürtel entlang des Paläopazifischen
Randes Gondwanas auf und umfassen zahlreiche Lagergänge, Gänge und Lavastromabfolgen
sowie die geschichtete mafische Dufek Intrusion. Einheitliche krustenähnliche Spurenelement-
und Isotopencharakteristika veranlassen zur Annahme einer identischen Magmenquelle für die
sich über mehr als 3000 km erstreckende Ferrar Provinz. Eine subkontinentale lithosphärische
Mantelquelle wird vorgeschlagen, welche im Zuge Paläozoischer Subduktionsprozesse entlang
des Gondwana-Kratons an Krustenkomponenten angereichert wurde. Jedoch wird die Wärme-
quelle für die großvoluminöse Schmelzbildung noch immer kontrovers diskutiert, da die
krustalen Isotopensignaturen die Identifizierung der Beteiligung eines Mantelplumes verhindern.
Des Weiteren ungeklärt ist die Genese der auftretenden low-Ti und high-Ti Ferrar
Magmaserien.
Vor diesem Hintergrund wurden in der vorliegenden Arbeit magmatische Ferrar Gesteine aus
dem nördlichen Victorialand und dem George V Land mit dem Ziel untersucht, einen
wesentlichen Beitrag zum besseren Verständnis der Petrogenese der Ferrar Gesteine zu
liefern. Zentrale Fragestellung ist die Entwicklung der beiden geochemisch verschiedenen
Magmaserien, welche die Beschreibung ihrer Differentiationsgeschichte, ihrer Schmelzbildung
sowie die Charakterisierung ihres Magmenursprungs beinhaltet. Zusätzlich zu Haupt- und
Spurenelementen (einschließlich der Seltenen Erdelemente – REE) wurden an ausgewählten
Proben die Konzentrationen der Platingruppenelemente (PGE) analysiert, um sowohl die
magmengenetischen Prozesse als auch Mantelquellensignaturen detaillierter beschreiben zu
können, da die thermische Quelle, erforderlich für die