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Geochemistry and petrogenesis of high-Mg tholeiites and lamprophyres in the Vestfold Hills, Antarctica

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Seitz, H-M (1991) Geochemistry and petrogenesis of high-Mg tholeiites and lamprophyres in the Vestfold Hills, Antarctica. PhD thesis, University of Tasmania.

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Abstract

The Vestfold Hills, Antarctica, an ice-free region of ea. 400 km2, is a well
exposed terrane of deep Archaean and Proterozoic crust which has been strongly
interfolded and transposed. Between 2350 and 1100 Ma a dense network of mafic
dykes intruded at mid- to upper crustal depths. Crosscutting relationships distinguish
9 different dyke sets intruded in three magmatic episodes. These three episodes
exhibit distinct directional trends suggesting rotation of the regional crustal stress
field over the period 2350 to 1150 Ma. During the first magmatic episode high-Mg
tholeiites were emplaced at about 2350 Ma together with Fe-rich tholeiites. A second
magmatic period has been inferred at ~1800 Ma in which a further set of Fe-rich
tholeiites was emplaced. Radiogenic dating of the 1800 Ma dykes is not well
constrained and field observations from this study did not clarify the relationship of
these dykes to the older (2350 Ma) Fe-rich tholeiites. A third magmatic episode,
which occurred between 1350 and 1150 Ma, emplaced two sets of lamprophyric
dykes and Fe-rich tholeiites, in at least four separate events.
Previous studies have concluded that the 2350 Ma old high-Mg tholeiites were
emplaced at deep crustal levels (7 - 8 kbar). New pressure estimates based on
aluminium exchange between clinopyroxene and plagioclase and on pressure
dependent trends of magmatic evolution reveal that these dykes were more likely
emplaced at considerably lower levels ( < 5 kb), possibly as low as 1-2 kbar. This requires much faster uplift of the Vestfold Hills block, during the late Archaean and early Proterozoic, than previously thought
The 2350 Ma high-Mg tholeiites can be divided into 3 geochemically distinct
groups which are not related by simple crystal fractionation processes. The high
Si02 and MgO, and low Ti02 magmas and are enriched in platinum group elements
and gold: all characteristic of siliceous high magnesian basalts (SHMB). A
fundamental question arises as to whether these geochemical characteristics are
primary features inherited from the mantle or whether the magmas have been
modified through assimilation of continental crust. This has been assessed by
evaluating the extent to which differentiation processes are able to account for
geochemical variations within one of the largest dykes in the Vestfold Hills has been
studied in detail. This dyke is found to be of virtual homogeneous composition,
apart from some minor variations in major- and trace elements which are consistent
with limited in situ differentiation. Major and minor element geochemistry shows no
evidence exists for crustal contamination in this dyke.
A large noritic ring complex (ea. 7 - 8 km in diameter) is associated with the
intrusion of the 2350 Ma high-Mg tholeiite suite. It consists of several partly isolated
bodies which vary in width from 70 to 400 m. This complex comprises three
lithological units: (1) Homogeneous Norite, the most uniform and voluminous unit,
containing cumulus orthopyroxene with clinopyroxene and plagioclase as
intercumulus phases, (2) the Mottled Norite, a unit characterized by large 1 - 2 cm
plagioclase - alkali-feldspar aggregates, and (3) the Rubbly Norite, a less common
variant, distinguished by patches of bronzitic orthopyroxene, cognate and inherited
xenoliths, and globules of sulphide. The xenoliths are mostly orthopyroxenites but
also include websterites, sapphirine-bearing fine-grained quartzite, and feldspathic
gneiss. The sulphide-rich zones range in size from ~ 4 to 30 m in width, with
extensions up to 400 m in length in the southern part of the complex. Evidence for
multiple intrusion suggests the Rubbly Norite was subsequently emptaced into the
the Homogeneous and Mottled Norite units.

Item Type: Thesis (PhD)
Keywords: Basalt, Lamprophyres
Copyright Holders: The Author
Copyright Information:

Copyright 1991 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Includes bibliographical references (p. 286-303). Thesis (Ph.D.)--University of Tasmania, 1992. 3 folded maps in pocket at back of vol

Date Deposited: 03 Feb 2015 03:20
Last Modified: 09 Jan 2017 23:09
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