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Mafic dykes of the East Antarctic shield : experimental, geochemical and petrological studies focusing on the proterozoic evolution of the crust and mantle

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Kuehner, SM (1986) Mafic dykes of the East Antarctic shield : experimental, geochemical and petrological studies focusing on the proterozoic evolution of the crust and mantle. PhD thesis, University of Tasmania.

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Abstract

The Vestfold Hills block of the East Antarctic Shield is an Archean
granulite facies complex crossed by hundreds of mafic dykes. The dykes
compose five chemically distinct suites emplaced in three separate magmatic
episodes during the Early-Middle Proterozoic. Sampling of Chilled margins
coupled with detailed documentation of cross-cutting relationships form the
basis for studying the chemical and temporal evolution of each dyke suite
as well as providing insight into the chemical evolution of the
subcontinental mantle during the Proterozoic. Furthermore, the emplacement
of the swarms is bracketed by tectonothermal metamorphic events at
ca.2500 Ma and ca.1100 Ma. Previous studies of the metamorphic assemblages
which developed during these events have provided constraints on the
P,T, time path of the Shield. A principal purpose of the present study has
been to determine emplacement depths of the dyke suites in order to further
restrict the possible uplift path of the shield during the -1400 Ma
interval separating, the final two major deformations. High pressure experimental techniques were used to determine the load
pressure at the emplacement depth of two samples representing dyke suites
intruded at ca.2400 Ma and ca.1360 Ma. This was accomplished by
experimentally reproducing the chilled margin phenocryst assemblage and the
phase compositions after first rigorously evaluating crystal/liquid KD's to
ensure the studied material represented liquid compositions: The results
of the experimental studies were also applied to compositionally similar
dykes from the Napier Complex of Enderby Land. The studies indicate the
Napier Complex and the Vestfold Hills are characterized by -2000 Ma of
crustal stability following the peak metamorphic event at ca .3100 Ma. Both
terrains followed identical isobaric cooling paths, to ca.2500 Ma, After
which the Napier Complex remained at deep crustal levels ( -28 km) until
ca.1000 Ma, while the Vestfold Hills was exhumed at a rate of -1cm/1000 a
until ca. 1100 Ma. This long period of crustal stability experienced by
both terrains was terminated by a Himalayan-style tectonic event which
resulted in isothermal decompression at a minimum net rate of 11cm/1000 a
in the Napier Complex, while crustal loading, depressed the Vestfold Hills
from depths of -16 km to -22 km resulting in the formation of garnet in the
mafic dykes. In the absence of more recent deformational events, it is
assumed that these terrains experienced slow, erosion controlled uplift
since ca.1000 Ma. The oldest undeformed mafic dyke swarm in the Vestfold Hills was
emplaced at ca.2400 Ma and is composed of two contemporaneous suites, a
Si02 -rich, high-Mg tholeiite suite and a Fe-rich tholeiite suite, which
compare favourably to basaltic komatiite -Fe tholeiite associations of
Archean greenstone belts. Normalized plots of incompatible trace elements,
phase compositions, and major element ratios divide the high-Mg suite into
three distinct subgroups. The trace element characteristics and isotopic
studies prohibit these subgroups from being related by crystal
fractionation and previous isotopic studies preclude crustal contamination
as a source of chemical variations. Major and trace element evaluation
also indicates that two of the chemically distinct subgroups were derived
from a primitive liquid extracted from "chondritic" mantle sources leaving
an olivine + orthopyroxene residue. Superimposed upon the chondritic
characteristics is evidence for selective trace element contamination
through a "wall-rock" reaction-type process, possibly with a plagioclasebearing
lherzolite. Comparison of an estimated parental liquid composition
with experimental melting studies indicates magma extraction took place at
pressures of -10 kbar (35 km), consistent with the geochemical signature
indicating partial melting with a plagioclase-bearing mantle. The more
differentiated Fe-rich tholeiites are depleted in LIL elements compared to
the high-Mg tholeiites which excludes any simple relationship between the
suites by a fractional crystallization process. Three petrographically and chemically distinct groups can be
identified within the Fe-rich suite. Two groups can be related in both
major and trace element variations by crystal fractionation, but field
relationships preclude this simple interpretation. All the groups have REE
patterns indicating derivation from an unfractionated mantle, which
precludes a relationship with the high-Mg suite by polybaric melt
extraction of a single source. The two closely related groups display
enrichment in P and Zr in excess of that due to crystal fractionation.
This may represent a relatively deep level wall-rock reaction process as
the third group displays elemental enrichments identical to that of the
high-14g tholeiites (formed at -10 kbar).
The youngest and volumetrically largest dyke swarm was emplaced in
the Vestfold Hills at ca.1360 Ma and is dominated by Fe-rich tholeiites
with subsidiary, though widespread alkaline dykes. The major and trace
element diversity of the Fe-rich suite can be accounted for by a crystal
fractionation dominated process. However, the geochemical trends are
defined by clusters of analyses which in themselves define differentiation
trends that cannot be related to fractionation of the phenocryst phases. Furthermore, the ordering of the groups does not relate to a simple,
monotonic emplacement sequence. The nature of the small-scale geochemical
trends and the complex emplacement style may be the result of "open system"
magma chamber processes. Rare earth element profiles, as well as the
incompatible element plots indicate the parental liquids to this suite were
derived from a primitive source which had not experienced previous melting
events.

Item Type: Thesis (PhD)
Keywords: Dikes (Geology)
Copyright Holders: The Author
Copyright Information:

Copyright 1986 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:

Library has additional copy on microfiche. Thesis (Ph.D.)--University of Tasmania, 1987. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:31
Last Modified: 05 Apr 2016 22:49
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