Geoscience Reference
In-Depth Information
Worked Example 7.3 Dyke rooting and overlapping spreading centres in the
Oman Ophiolite
The Oman Ophiolite is a 350 km long, well-
preserved tract of Cretaceous ocean fl oor, thrust
over the northeast coast of Arabia. Studies in the
1980s (Rothery 1983) and early 1990s (Macleod
and Rothery 1992) made the crucial fi eld
observation of places where dolerite dykes from
the sheeted dyke complex could be traced
downward into 'high-level gabbro' of identical
composition but coarser grain size (e.g. Figure
7.15b). This gabbro was interpreted as the upper
part of a magma chamber from which the dykes
were fed, and indeed it was possible to fi nd
chilled margins of gabbro against gabbro,
representing the roots of individual dykes.
Integration of mineral texture and fabric
observations with these broader fi eld
observations proved its worth when it was found
that, next to and below dyke rooting zones, the
'high-level gabbro' often has a weakly lineated
fabric. This is expressed by orientation of
pyroxene crystals consistent with generally
upward fl ow towards the overlying dykes,
although sometimes this fabric has been
destroyed by recrystallization during
hydrothermal alteration (when pyroxenes were
largely replaced by amphibole).
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Equally important, elsewhere (within a few tens
of kilometres) the dykes from the sheeted dyke
complex could be traced into cumulate gabbros,
where they cut across the cumulate layering.
Dykes could not have been fed from these places,
so it was argued that they must have been
injected along strike from a nearby magma
chamber. About half a dozen of these
'penetration zones' were identifi ed, and it was
suggested that they marked segmentation of the
spreading axis at which the Oman Ophiolite was
created. Such segmentation was consistent with
overlapping spreading centres like those that
began to be identifi ed on the East Pacifi c Rise in
the late 1980s (and subsequently in many other
places, including marginal basins) rather than
segmentation defi ned by transform faults.
Although mid-ocean ridge magma chambers have
subsequently come to be regarded as small and/
or ephemeral, the important distinction between
dyke rooting and dyke penetration remains valid.
another the enclosed crystal must have grown before the
enclosing crystal, and well-formed crystals probably grew
before badly-shaped crystals that appear to be fi lling in spaces.
In the fi eld, such evidence can be assessed only for coarse-
grained rocks, and even for those it is better to study thin-
sections under the microscope.
Pegmatites
You may fi nd exceptionally coarse intergrown crystals in parts
of a pluton, or coarse patches within an otherwise medium-
grained sill. This is probably pegmatite (Table 7.2, p. 146), in
which the size of the crystals is nothing to do with cooling
rate, but was instead controlled by the availability of
nucleation sites and/or the abundance of volatiles in the last
residue of the melt. Pegmatites usually occur in veins or lenses.
Things to note in particular are the shape of the pegmatite body
and the clarity of its edges. Does it appear that the main
igneous body was solid and brittle, or mushy and ductile,
when the pegmatite was active?
