Geology Reference
In-Depth Information
final stages of magma
emplacement
. Their presence and abundance can indicate
proximity to pluton margins. For example, xenoliths may occur as 'trains' of
blocky material that can be traced over a short distance into surrounding the
country rock.
In some cases xenoliths may show evidence of high temperature metamor-
phism and even internal melting. When encountered in the field, efforts should be
made to identify their lithology with particular reference to surrounding country
rock, the degree of metamorphism and preserved orientation, remembering that
depending upon time of incorporation during magma emplacement the xenolith
may have been transported some distance from its point of origin, or remain
effectively in-situ.
9.6.1 Xenocrysts
The presence of corroded felsic and mafic minerals (xenocrysts) are often found
in magmatic enclaves and generally interpreted as the result of mechanical trans-
fer during the mafic-felsic magma interaction. A common type of xenocryst
found in mafic to intermediate enclaves that have intruded into granite magma
are
oscelli
(Italian for eye), quartz xenocrysts rimmed by biotite or amphibole.
Other textural features indicative of magma mingling and mixing are rapakivi
feldspars where sodic or potassium feldspar is rimmed by plagioclase (e.g.
Chapter 7, Figure 7.23), and minerals of any kind which show evidence of
reabsorption triggered by thermal or chemical reactions as a given crystal at
home in its host melt suddenly finds itself displaced into magma of a different
composition and or temperature under hybrid (enclave) magma conditions (e.g.
Figure 9.13). If present together these features point to strong non-equilibrium
conditions suggestive of an origin through magma mingling or mixing.
9.7 A Word of Warning
Telling apart xenoliths from magmatic enclaves in the field can at times be very
difficult. Both have genetic implications for the origin and source region of
granite magmas that warrant caution if you get them wrong. If in doubt the best
thing is to record what you can (shape, size, orientation, if possible mineralogy)
and take a sample for follow-up petrographical identification. Record the mate-
rial in your notebook as generic enclave. Also, for magmatic enclaves, do not go
by colour alone as a guide to composition. This is because magmatic enclaves
range in composition from mafic to felsic. Felsic composition enclaves can and
do occur in granitic rocks but look dark. In this case the dark colour is due to
the fine grain size not composition and it is dangerous therefore to assume that
dark coloured enclaves are always remnants of mafic or intermediate magma.
Because of this, many geologists use the non-genetic term microgranular enclave
to describe magmatic enclaves in the field.
