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characterized by the presence of phenocrystal phlogopite and augite in a ground-
mass of sanidine, analcite (pseudomorphous after leucite). The cocitic rocks have
high K
2
O(4
5 wt%), low K
2
O/Na
2
O(
1.0) and K
2
O/Al
2
O
3
ratios (0.4
0.6).
-
*
-
4.2 Ultrapotassic Rocks of Australia
4.2.1 West Kimberley
A host of potassium-rich intrusive rocks were emplaced (Prider 1939, 1982; Wade
and Prider 1940; Alsopp et al. 1985; Nelson et al. 1986) in the Fitzroy area of the
Kimberley region of Western Australia (Fig.
4.6
). As many of the intrusions contain
diamond (Atkinson et al. 1982, 1984), these rocks have been subjected to numerous
petrological, geochemical, and geochronological studies. The lamproites are con-
west at the south western margin
of the Kimberly block (King Leopold Mobile Belt, KLMB). The belt is then
extended across the Leonard shelf to the southern margin of the Fitzroy Trough.
fined to a broad irregular zone, which trend east
-
124
0
128
0
TIMOR SEA
Kununurra
KIMBERLEY
BLOCK
16
0
King
Sound
Argyle
1
Derby
4
E
C
3
2
Halls Creek
N
CANNING BASIN
Proterozoic Mobile Belt
Anticline
Syncline
Fault
Concealed fault
Kimberlite
Olivine lamproite
Leucite lamporite
Diamond occurrence
0
200km
0
20
Fig. 4.6 Distribution of lamproites and diamond occurrences in relation to the tectonic feature of
the Kimberley region of Western Australia (after Jaques et al. 1984) (1 Mount North, 2 Old
Leopold Hill, 3 Mount Rose, 4 Seltrust Pipe 2; Approximate positions only)
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