Geology Reference
In-Depth Information
8.2 Continental Mafic-Ultramafic Intrusions
These bodies are large, discordant, funnel- or saucer-shaped intrusions,
generally with inward dipping contacts, causing them to be thicker in the
centre than at the perimeter, that is they are
lopoliths
(cf. Chapter 6). Most of
them contain medium- to coarse-grained ultramafic, gabbroic, leucogabbro and
even anorthositic lithologies as shown in Figure 8.3; together these rock-types
form a series of repetitive, or rhythmic layers (with igneous lamination) which
crystallised within a magma chamber fed by mafic/ultramafic magma. The
terminology for layered rocks is based on classic studies which formulated the
concept that the layers represent concentrations of minerals accumulated by min-
eral settling; the layered rocks have been termed
cumulates
. However, it is now
realised that some layered rocks cannot be a result of such crystal settling, and
that layering might form by a number of processes. Hence the term 'cumulate'
may be applied to such rocks in a non-genetic sense, in which crystal settling is
a possible but non-essential process in the origin of cumulate rocks. Examples
of a mafic-ultramafic layered intrusion include the Bushveld intrusion (see
Section 10.2); Skaergaard intrusion, Greenland, The Dufek and Dais intrusions
in Antarctica; Stillwater, USA; and the Rum Complex, Scotland, among others.
Figure 8.3
Alternating layers of olivine rich (peridotite) and plagioclase rich
(gabbro/leucogabbro) in the central complex of Rum, Scotland.
