Geology Reference
In-Depth Information
According to Smith and Lorenz (1986) there are more than 100 Miocene lam-
proites in West Kimberley, Australia (Jaques et al. 1984), covering 75,000 km 2
from Ellendale through Noonkanbah to Fitzroy crossing. There are a few dykes,
which intruded into lower Proterozoic granites and metamorphic rocks of the King
Leopold mobile zone; most of the lamproitic bodies intrude Phanerozoic sediments
of the Leonard shelf, Fitzroy trough at the north-eastern margin of the Canning
basin. According to them there are 48 lamproites in the Ellendale Field on the
Lennard Shelf (Atkinson et al. 1984). Forty-three lamproites were emplaced in an
elongated belt (40 km long and 10 km across, trending 305
) aligned with Oscar
Fault, which is one of the major step faults bounding the Fitzroy Trough. This
trough started forming during the Ordovician. Major graben development took
place during the Devonian. There was sedimentation till the Triassic and around this
time, the early structures were reactivated with right lateral and vertical movements
on the step of the graben.
The lamproites are of two categories:
°
(1) Leucite lamproites comprising variable proportion of phlogopite, diopside,
richterite with less than 5 % modal olivine, and
(2) Olivine lamproite without leucite but containing phlogopite, diopside, rich-
terite and 30 vol% modal olivine.
The ranges of SiO 2 contents of leucite lamproites and olivine lamproites are
42 wt%, respectively. The H 2 O + concentration of leucite lamproites
and olivine lamproites are around 2 and 7 wt%, respectively. The CO 2 content is
<0.25 wt%. Smith and Lorenz (1986) found that, of the 46 Ellendale lamproites, 14
may be classi
50
55 and 35
-
-
ed as olivine lamproites; 28 belong to the category of leucite lam-
proites and four are transitional rock types with 28 % modal olivine and appreciable
amount of leucite.
The Ellendale diatremes intrude sandstone of Permian formation and are usually
less then 100 m in diameters and sometimes more than 1 km across. There are many
diatremes which are elongated in plan view with their long axis trending west-
northwest. Usually a diatreme is of the shape of champagne-glass with a narrow
feeder vent, overlain by a broad shallow crater. It is often observed that during the
final stage of volcanism the magma ascended through the conduit into the centre of
the craters, spreading out in the form of a lava lake or dome. According to Smith
and Lorenz, the near surface
overlie the crater sediments and some-
times may overlap into the country rock. They found that the near surface
lamproites
lam-
proites
are highly vesicular and brecciated often incorporating clastics from
underlying sediments below the crater.
Smith and Lorenz concluded that volcanic activity related to nearly all Ellendale
pipes started with phreatomagmatic phase. During this period the lamproitic magma
interacted explosively with copious amount of groundwater from sedimentary rocks
of the Permian granite formation. This was followed by a maar formation, which
then collapsed due to ejection of juvenile, country rock clasts with the formation of
diatrame underneath. Because of the phreatomagmatic activity at the surface, many
thin pyroclastic beds of base surge origin, formed on the rim of the crater or within
 
Search WWH ::




Custom Search