Culshaw et al., 1998. The Post Hill amphibolite and its basal mylonite occupy a thrust klippe that
largely escaped the effects of the later deformation Marten, 1977.
3. Petrochemistry of the Post Hill Amphibolite
Field relations and limited geochemical data two samples with major element analyses from
White, 1976 suggested to Gower et al. 1982 that volcanic activity represented by the Post Hill am-
phibolite may have been related to continental rifting, although Wilton 1996 refuted this sug-
gestion based on geochemical data from a single additional sample. Samples from the structurally
higher Metasedimentary formation have bulk rock chemistry indicating a minimal component
of Archaean detritus Gower et al., 1982, consis- tent with o
Nd
of + 2 for anatectic granite within this formation Kerr, 1989.
To better constrain the tectonic setting of mafic volcanism, 14 samples were collected from the
Post Hill amphibolite in the Post Hill area, from which eight typical samples a – h, Table 1 were
selected for major and trace element analyses. In addition, two samples collected from the thinner
extensions of the amphibolite to the southwest and northeast were also analysed i and j, respec-
tively, Table 1. Sample locations are shown in Fig. 2. The samples consist mainly of pleochroic
green or green-blue amphibole 50 – 80, with less abundant, mainly untwinned plagioclase and
minor quartz, epidote, and opaque minerals. Bi- otite is present rarely. Most samples are fine-
grained with a moderate to strong foliation, and preserve no relict igneous textures.
The analysed samples show a narrow range in SiO
2
content from 48.5 – 51.4 Table 1. They are tholeiitic, with FeO
t
MgO ranging from 1.5 to 3.0 Fig. 3a. The sample suite does not have the
high TiO
2
and FeO
t
of the two samples from White 1976 that led Gower et al. 1982 to
conclude that the rocks formed in a continental rift. The data are similar to the analysis presented
by Wilton 1996 for a sample from the northeast- ern extension of the amphibolite on Kaipokok
Bay D. Wilton, written communication 1998. The latter sample is included for comparison in
Fig. 3 and Fig. 4. The samples of Post Hill amphibolite have a
range in composition similar to that shown by Mesozoic mafic volcanic and intrusive rocks of
eastern North America, a classic rifted continental margin assemblage e.g. Wang and Glover, 1992.
Wang and Glover 1992 showed that such mafic rocks have a range in compositions due to diverse
factors such as variations in source rocks and degree of crustal contamination. Like the Meso-
zoic suite, the Post Hill amphibolite ranges from compositions typical of island arc tholeiites
through to typical within-plate basalt Fig. 3b, c. However, some features, such as low Y Fig. 3d,
Fig. 4 are more consistent with island arc suites than with continental rifting suites. In general, the
analyses span the range between the average low- potassium tholeiite island arc tholeiite and the
average within-plate tholeiite Fig. 4.
Given the demonstrably wide chemical range possible in continental rifting suites such as the
Mesozoic rocks of eastern North America, and a close relationship between the Post Hill amphibo-
lite and underlying and overlying sedimentary se- quences that both contain Archaean detritus see
below, we suggest that geochemical data for the Post Hill amphibolite indicate a rifted continental
margin setting rather than an island arc setting.
4. U – Pb analytical techniques
