Geology Reference
In-Depth Information
c
d
L
d
f
L
c
aplite/pegmatite dykes
strike and dip
of jointing
strike of flow
layers
pitch of flow
lines
Figure 7.17
Block diagram showing the relationship between flow structures
and joint systems in an intrusive mass. c, Cross joints; d, diagonal joints; f,
flat-lying joints; l, longitudinal joints.
apart by their grain size - aplites are fine grained (sometimes with a sugary
or saccahroidal granular texture) while pegmatites are much coarser grained
rocks with individual crystals growing to several centimetres or even metres in
size in extreme cases (see Section 2.5.2). Pegmatites can also be mineralised
due to late stage enrichment in trace metals (Chapter 10). They often form
contemporaneously with primary cooling joints through a process of auto
intrusion (hydrofracturing) of cooling magma. Both can occur in isolation but
are more commonly found as cognate sets with sharp margins. They provide
excellent time markers and their dips and strikes useful structural information
on the orientation of late stage thermal stresses set up during cooling. They
can often be seen cutting surrounding country rock where they can be traced
back to their host pluton.
