Geoscience Reference
In-Depth Information
Chapter 15
The hard rock cafe
There are three kinds of rocks:
Ingenious, Metaphoric, and Sedentary,
named after the three kinds of
geologists.
(3) Primitive mantle that can yield basalts by par-
tial melting.
(4) Cumulates
or
residues
that
have
been
intruded by basalt.
(5) High-pressure or high-temperature melts.
Anon.
Peridotites contain more than 40% olivine. They
are divided into fertile or infertile (or barren).
Fertile peridotites can be viewed as having an
appreciable basaltic component. The terms
'enriched' and 'depleted' are often used inter-
changeably with 'fertile' and 'infertile' but have
trace-element and isotopic connotations that are
often inconsistent with the major-element chem-
istry. Table 15.1 gives compositions for represen-
tative ultramafic rocks.
Garnet lherzolites are composed mainly of
olivine and orthopyroxene (Table 15.2). Olivine
is generally in the range of 60--70 vol. % and
orthopyroxene 30--50%. The average clinopyrox-
ene and garnet proportions are about 5% and 2%,
respectively.
The major oxides in peridotites and lherzo-
lites generally correlate well (Figure 15.2 and
Table 15.3). The lherzolite trend can be explained
by variable amounts of clinopyroxene and gar-
net. Olivine- and orthopyroxene-rich rocks, pre-
sumably from the mantle, are found in foldbelts,
ophiolite sections, oceanic fracture zones and, as
xenoliths, in kimberlites and alkali-rich magmas.
They are rare in less viscous magmas such as
tholeiites. Olivine and orthopyroxene in varying
proportion are the most abundant minerals in
peridotites. These are dense refractory minerals,
and peridotites are therefore generally thought
Ultramaf ic rocks
Ultramafic rocks (UMR) are composed chiefly of
ferromagnesian minerals and have a low silicon
content compared with the crust, mafic rocks
and basalts. The term is often used interchange-
ably with
ultrabasic
; pyroxene-rich rocks are ultra-
mafic but not ultrabasic because of their high
SiO
2
content. Peridotites, lherzolite, dunite and
harzburgite are specific names applied to ultra-
mafic rocks that are chiefly composed of olivine,
orthopyroxene, clinopyroxene and an aluminous
phase such as plagioclase, spinel or garnet. Ultra-
mafic rocks are dense and mainly composed of
refractory minerals with high seismic velocities.
Basic rocks, such as basalts, become dense at
high pressure (for example, eclogite) and can
have properties comparable to the more refrac-
tory peridotites. Some eclogites overlap basalts in
their bulk chemistry. The relationships between
these rocks are shown in Figure 15.1.
Peridotites can represent
(1) The refractory residue left after basalt extrac-
tion.
(2) Cumulates formed by the crystallization of a
magma.
