Geology Reference
In-Depth Information
Figure 6.11
Intrusion breccia of more mafic (angular blocks) into a more fel-
sic host, formed by magma mingling in shallow sheet intrusions Ardnamurcan,
Scotland.
segregations and irregular diffuse streaks termed
schlieren
.Theyvaryfrom
fine-grained aplites through medium-grained (including granophyre) to coarse-
grained texture and may have large crystals (up to 10 - 20 cm long), particularly
of pyroxene, amphibole and Fe-Ti oxide. They are then termed
pegmatites/
pegmatitic schlieren
. The composition of segregations in sills generally reflects
the composition of the crystallising magma. Felsic segregations in quartz-bearing
basalt/dolerite sills are fine-grained diorite, granodiorite and granophyre whereas
those in quartz-free, olivine basalt/dolerite sills are often quartz-free gabbro or
diorite, containing zeolites and/or foid minerals. Such pegmatites can be very
important to help age date such intrusions.
6.3 Volcanic Plugs and Diatremes
Erosion of composite volcanoes may reveal subcircular exposures of fine-grained
rocks that may be interpreted as
volcanic plugs
. Such plugs are also termed
volcanic pipes, and are generally a few hundred metres in diameter, but may
extend up to a kilometre. The three-dimensional form of volcanic plugs is gen-
erally cylindrical. Most plugs are composed of shallow intrusions/lavas, but a
