Geoscience Reference
In-Depth Information
Worked Example 7.2 Chilling statistics in Cyprus
W
section A
E
5 _
_ 17
21 _ 29
8
1
1
23 4
9
1
1
12
13
18 19
20
3
3
7
W
section B
E
35 _ 38
40 _ 46
48 _ 52
54 _ 58
61 _ 63
70 _
_ 79
7
73
7
32
3
3
3
4
5
5
6
6
6
6
6
72
7
7
7
7
8
50
Chilled margin
Hydrothermal breccia veins
Epidote veins
0
metres
1
Figure 7.16 Schematic diagram of an east-west traverse across sheeted dykes in Oman (similar to
those in Figure 7.15). The tick on the chilled margins shows the direction of chilling. Numbers identify
dykes, but not their order of intrusion. The east end of section A and the west end of section B are
separated by a 20 m fault zone of poor and confused exposure. There is another fault zone between dykes
9 and 10. (From Lippard et al. 1986.)
In the early 1970s, the sea-fl oor spreading origin
of ophiolites was gaining support, but was not
yet fully accepted. Rupert Kidd and Joe Cann
(1974) studied chilled margins in the sheeted
dyke complex of the Troodos Ophiolite (Cyprus),
in places that consisted of nothing but dykes,
striking approximately N-S. They argued that
the absence of non-dyke 'host rock' means that
this unit must have been formed by 100%
spreading, and they cited evidence from chilled
margins for additional support. They found that
usually the chilled margin of one dyke abutted
against the chilled margin of its neighbour (so
that their relative ages could not be told), and
reasonably explained this as a younger dyke
being injected along the edge of a previous dyke.
However, they observed that about one dyke in
ten had been split by a younger dyke, which
must have been injected into the interior of the
older dyke.
They documented the direction of chilling in
these split dykes, and found that there was a
statistically signifi cant preponderance of chilled
margins in which chilling was towards the east.
From this, they argued that the eastern halves of
split dykes were more common in the ophiolite
than western halves of split dykes, and hence that
the Troodos Ophiolite as a whole represents ocean
fl oor that formed on the east of the spreading axis.
Conversely, in the Oman Ophiolite, where the
sheeted dykes also strike roughly N-S, there is a
preponderance of chills towards the west (Figure
7.16), suggesting derivation from the west side of
a spreading axis.
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