Geology Reference
In-Depth Information
as slumps, flame structures and graded bedding are testament to these examples
(see Chapter 8). Other types of layering are formed by localised fractionation
in situ
rather than by large scale settling, and also layering formed by the
wholesale replacement of material due to percolating fluids through a crystal
mush can be responsible for a variety of complicated igneous laminations.
Examples of igneous layering and lamination will be dealt with in more detail in
Chapters 7 and 8.
2.4.3 Inclusions in host rocks
During the ascent of magma bodies to their level of emplacement or extrusion,
fragments of solid rock material from the surrounding region or from the walls
and roof of the magma chamber may become incorporated and remain as foreign
material in the solidified igneous rock. These foreign bodies are known as xeno-
liths and they comprise two main types: (i) those with relict meta-sedimentary
textures and (ii) those with igneous textures.
1. Xenoliths in the first category originated as fragments of country rock that
were either engulfed by the upward-moving magma or fell into a magma
chamber before crystallisation became complete. Undisturbed sedimentary
rock fabrics, such as stratification, may be preserved within these typically
angular inclusions but, more often, these xenoliths become severely meta-
morphosed by the heat of the magma, producing finely-banded hornfelsic
textures. Xenoliths in lava flows erupted onto continental crust are particu-
larly useful for studies of crust and upper mantle structure, as they may have
sampled most of the layers through which they have passed (for example,
upper mantle peridotite, high-grade metamorphic rocks of the lower crust,
shales and limestones of the upper crust). Country rock xenoliths engulfed
by intrusive magmas often show evidence of reaction with the magma. At
its most advanced stage, this results in partial assimilation of the xenoliths,
leaving rounded dark-coloured inclusions, rich in recrystallised biotite and
amphibole, with apparently igneous textures.
2. Xenoliths of igneous origin: these range from the olivine and peridotite
xenoliths, often called 'nodules', that are common in volcanic rocks to frag-
ments of pre-existing igneous bodies or of early-crystallised igneous material
that became incorporated into the residual magma from the walls of a magma
chamber (for example, Figure 2.12). All igneous-textured xenoliths derived
from a source in common with their host magma may be referred to as
cognate xenoliths; the term autolith is appropriate for igneous xenoliths
which represent earlier-crystallised igneous material from the same magma.
Generally, however, these terms have genetic implications and should be
used with caution until a full petrological and petrogenetic analysis of the
igneous body has been completed.
