SHALLOW-LEVEL INTRUSIONS - The Field Description of Igneous Rocks - page 121

mafic mineral composition may be termed a basalt , for example basaltic
dyke , although the terms dolerite and diabase are also used. Diabase implies
a particular intergrown textural relationship between plagioclase and pyroxene
(often observable only in thin section and termed ophitic texture) and is
synonymous in US usage; however, in the UK the term diabase is rarely used,
but may be sometimes used for a dolerite in which the feldspars and mafic
minerals are highly altered. Intermediate and acid dykes are termed finegrained
diorite, granodiorite and granite, and, if they are aphanitic, may be termed
andesite, dacite and rhyolite as described for lava flows. Glassy aphanitic minor
intrusions may be termed pitchstone or obsidian as defined in Section 4.2.
Although most minor intrusions may be matched in composition with common
rock types described earlier, a rather uncommon group, the lamprophyres (see
also Chapter 3) which are distinct in mineral composition and texture from
normal plutonic and volcanic rocks, also form minor intrusions.
Sills (for example, Figure 6.3), range from a few centimetres to hundreds of
metres in thickness, although the commonest sills are between 1 and 20 m in
thickness. Commonly sills occur as multiple intrusions forming 'nested' sill com-
plexes (for example, Figure 6.1). They may show attenuated extremities which
separate into elongate 'fingers' (Figure 6.2). Sills may be easily confused with
basaltic lava flows (especially where both occur in the same sequence). However,
close inspection will reveal chilled margins on the top and the bottom of sills and
they will lack the surface characteristics of lavas (as described in Chapter 4).
Dykes (for example, Figure 6.4) like sills, usually vary from a centimetre to
many metres, or even hundreds of metres, in width but large elongated discordant
Figure 6.3 Thick sills (dark brown) in Karoo sediments, Dry Valleys, Antarctica.