Geology Reference
In-Depth Information
1. The
abundant
granite-granodiorite
(tonalite-diorite-gabbro
series
(cf.
Figure 3.10), often known as the
calc-alkaline
association.
2. The less abundant alkali granite-alkali syenite-syenite-foid syenite (usually
nepheline syenite) series, known as the
alkaline
association.
These associations have strict geochemical definitions based on chemical
ratios, such as CaO/(Na
2
O
K
2
O) (see Gill, 2010), but, as will be clear
from (Figure 3.11 and appendix Figure A.1), the geochemical differences are
reflected in the nature of the feldspars and, less obviously, in the mafic mineral
content. Rocks of the alkaline series have relatively low abundances of mafic
minerals (for example, alkali syenite and syenite in Figure 3.13) and, instead,
their low silica (basic) end-members extend closer to the feldspathoid apex in
Figure 3.11 - nepheline is a typically abundant mineral. In contrast to the dark-
browns and black colours of pyroxenes and amphiboles in diorites and gabbros,
the smaller amounts of mafic minerals of alkali rocks are often distinctive, con-
sisting of spectacular green or dark-blue alkali pyroxenes and amphiboles (for
example, aegirine and riebeckite - compare formulae with common pyroxenes
and amphiboles in Table 3.4). Thus a secondary characteristic distinguishing
alkali granite from calc-alkaline granite, for example is often the presence of
dark-blue amphibole needles in the former and of black prismatic amphiboles
and/or micas in the latter.
In the low silica, ultrabasic rocks, there are two important groups to consider
(cf. Figure 3.13):
+
1. A leucocratic group, the
anorthosites
.
2. A melanocratic group, the
peridotites
and
pyroxenites
.
Anorthosites are pale-brown or white monomineralic rocks dominated by
plagioclase feldspar; varieties with more than circa 15% of mafic minerals are
termed anorthositic gabbros. They are sometimes found in association with
alkaline rocks such as syenites and nepheline syenites and sometimes with
layered mafic-ultramafic rocks. In contrast with anorthosites, rocks that are
virtually devoid of feldspar but have abundant mafic minerals (
>
90%), and
range from dark-green or brown to black in colour, are known collectively
as
ultramafic
. These are subdivided further, according to their particular com-
binations of mafic minerals, into pyroxenites (
>
60% pyroxene), peridotites
(
40% olivine) and several other minor categories that are not easily recog-
nised in the field (Figure 3.14).
Dunite
, an olive-green monomineralic rock with
90 - 100% olivine, is perhaps the most distinctive, but recognition of the other
types depends on the separate identification of orthopyroxene and clinopyroxene
which is sometimes difficult in hand specimens. Orthopyroxenes are generally
of paler brown or green colours than clinopyroxenes and sometimes weather to a
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