Geology Reference
In-Depth Information
(a)
Figure 1.6.
(a) Thin section showing medium grained equigranular granite texture; scale in mm.
quartz, together with feldspar and mica. Both alkali (K and Na rich, orthoclase, microcline) and pla-
gioclase (Na and Ca rich, albite, oligoclase, andesine, etc.) feldspars are present in most granites.
Relative frequencies of alkali and plagioclase feldspars, plus grain size, form the basis of subdivi-
sion and classification of granitic rocks (
Fig. 1.4).
Many of the names given to granitic rocks are self explanatory, so that a granodiorite has miner-
alogical characteristics intermediate between a granite and a diorite; but others, still occasionally
used and of historical interest, are of local derivation. Thus, another name for granodiorite is tonalite,
after the Tonale Alps in northern Italy. The nearby Adamello Alps have given their name to adamel-
lite. Syenites take their name from Syene, now known as Aswan, in Egypt, and monzonite, well
known from its occurrence in the Yosemite region of the Sierra Nevada, California, from Monzoni,
in the Tyrol.
Granites (and other crystalline rocks) in which the constituent crystals are completely bounded by
faces are said to be euhedral. Where such faces are absent the rock is described as anhedral, and
where the rock is partly euhedral, partly anhedral, it is said to be subhedral. The texture of rocks
with anhedral crystals, with shapes dictated by adjacent grains, is described as allotriomorphic
or xenomorphic, in contrast with euhedral or subhedral forms with a hypidiomorphic (or sub-
idiomorphic) texture. Granites in which the constituent crystals are all of approximately the same size
are described as equigranular (Fig. 1.6a). They may, however, be fine grained (e.g. aplite) or coarse
(e.g. pegmatite). Some crystalline rocks display a markedly bimodal grain size with very large crys-
tals or phenocrysts set in a finer groundmass and are described as porphyritic (Fig. 1.6b). Granites are
light coloured, or leucocratic (as opposed to dark-coloured or melanocratic), commonly pink or grey
in overall colour, and typically massive. Granodiorite is by far the most common of the granitic rocks,
ocurrences being equal in extent to all the other types together. Granitic rocks grade into a group of
crystalline rocks in which free quartz is accessory and not essential, but which are again classified on
the basis of contained feldspars. These rocks are known generally as granitoids. Clearly, granitic
rocks vary in composition and texture, not only between, but also within, batholithic or other plutons.
For this reason alone, different parts of the same pluton may vary in their responses to weathering
agencies, and hence give rise to different landform assemblages.
Crystal orientation in granitoids is generally random, with only weak and local alignment of
crystal axes, as for instance in the immediate vicinity of some fractures, presumably those along
