Geology Reference
In-Depth Information
9
MAGMA MIXING AND MINGLING
Igneous petrologists have long recognised that plutonic rocks in the field, and
granitic rocks in particular, contain within them a variety of lithologies that are
either not of igneous origin, or are igneous but appear compositionally distinctive
from their host. The generic name given to this extraneous material is enclave
and five board categories are now recognised: (i) xenoliths, fragments of older
country rock caught up during magma ascent and emplacement, (ii) magmatic
enclaves, originally liquid or semi-liquid blobs derived from comagmatic intru-
sions, (iii) restite, solid material derived from the magma source region (iv),
autoliths, cognate material derived from the host but differing slightly in com-
position, (v) xenocrysts or anitcrysts, individual crystals or accretions of crystals
or pheoncrtsys transferred into the host magma during mingling or mixing, or
from a different part of the intrusion (for example, crystal mush on the cham-
ber floor or walls). The structural and textural relationship between enclave
type and host, if decoded and read properly, can reveal a wealth of informa-
tion about the intrusive history of the magmatic system under investigation. For
example, depending upon the rheology and relative volumes of host and invad-
ing magma, the nature of the preserved interactions range from near complete
mixing and blending of two compositionally distinct fluids with similar vis-
cosities to late stage dyking of fluid mafic magma into a rigid, near-crystallised
plutonic host. This open system behaviour, where mafic and silicic magmas have
the opportunity to interact more-or less continuously at any level from the base
of the continental crust right through to eruption at the surface, is now regarded
as commonplace. However, confusion can arise when, for example magmatic
enclaves are misidentified as metasedimentary xenoliths, and vice versa, and
where the terms magma mixing and magma mingling are used interchangeably
to describe the same thing (they are in fact quite different). Thus when faced
with unravelling the complexities of co-genetic magmatic systems in the field,
caution and careful field observation are required.
9.1 Magma Rheology
Magmas are complex, multiphase fluids that contain solids (crystals, bits of
pre-existing rocks), dissolved gas and bubbles, carried in a continuous liquid
(melt) phase. The physical properties of magma strongly control their ability to
