Geology Reference
In-Depth Information
These interactions are likely to be geologically instantaneous, occurring over a
period of hours to weeks. If interpreted correctly, the nature of the resulting
interaction between coeval but contrasting magma types can reveal much about
the timing and mode of pluton construction during cooling and crystallisation.
Mixing and mingling is also preserved in erupted volcanic products as streaky
pumice.
During magma emplacement, a process that requires displacement of pre-
existing country rock, incorporation of surrounding material called
xenoliths
(Greek for 'stranger rock') is almost inevitable. This class of material picked
up from rock hosting an intrusion can itself be igneous where different magma
types are coalescing into a batholith, but is more commonly (meta) sedimentary
in origin. It is vital not to confuse country rock (metasedimentary) xenoliths
with magmatic enclaves in the field.
Xenocrysts
can be crystals of entrapped
igneous country rock or crystals delved into the host by invading magma.
As a final point it is worth considering the intrusion geometry in which
magma mingling takes place. In the
plutonic or subvolcanic zone
, this is most
likely to be predominantly horizontal where mafic magma is injected near to the
chamber floor, although where mafic magma is intruded as synplutonic dykes
and sills the geometry of mingling may vary widely and apparently chaotically.
In contrast, mingling and mixing during volcanic eruptions will take place in
sub vertical conduits.
