Page 30 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

unusually high temperature. Apparent porphyritic magma(s), and gives a useful, though rough guide to the
texture can also occur in non-welded pumice-rich source composition. However, the original total
deposits in which alteration and diagenetic compaction abundances and relative proportions of each phase are
mask the outlines of porphyritic pumice clasts (45.5-6; usually significantly modified during fragmentation and
Part 5). transportation, and are not easily inferred from the
abundances and relative proportions of crystal
Crystals and crystal fragments (1) components in the final volcaniclastic deposit. That
limitation aside, data on the assemblage, total
Crystals and crystal fragments are found in a wide abundance, relative abundance, size and shape of crystal
variety of volcaniclastic deposits (1.3, 25.3-4, 32.4-5). fragments can provide a very effective and reliable basis
They are ultimately derived from porphyritic magmas for distinguishing and mapping volcaniclastic units in
and from crystalline or porphyritic country rock. Both the field. These parameters are approximately constant,
primary volcanic and surface sedimentary processes of or else vary systematically within single emplacement
fragmentation can effectively separate crystals from units in primary pyroclastic deposits and also,
their host, and concentrate them in crystal-rich commonly, within mass-flow resedimented
volcaniclastic deposits (Cas, 1983). volcaniclastic deposits. Systematic variations in the
crystal fragment population may reflect compositional
Whole crystals and crystal fragments are liberated zonation in the source porphyritic magma and/or sorting
during explosive eruptions of porphyritic magma. A of crystal fragments according to size or density during
small proportion of crystal fragments in pyroclastic transport and deposition (especially common in fallout
rocks may be derived from the disintegration of igneous deposits).
and metamorphic wall rocks. In pyroclastic deposits,
angular fragments of euhedral crystals are typically Vesicles (2)
more abundant than complete euhedral crystals, and
show a relatively wide grain size range, the upper limit Volatiles exsolved from lavas, shallow intrusions and
of which is determined by the maximum phenocryst size densely welded tuffs accumulate in bubbles called
in the porphyritic source magma. Crystal fragments, vesicles, that may be permanently entrapped on
especially euhedra, may have a thin partial selvedge of solidification and preserved. Vesicles are also formed
glassy pumice or scoria. Some crystals within pumice or by steam bubbles enclosed in some fine-grained, moist
scoria clasts are fractured in situ, and the fragments ash deposits generated by explosive eruptions (Lorenz,
show jigsaw-fit texture. 1974; Rosi, 1992). Amygdales are former vesicles that
have been partially or completely infilled with
Quench fragmentation of porphyritic magma is another secondary minerals.
means of generating free crystals and crystal fragments,
and these can be significant in the coarse sand- and Vesicles are common in silicic, intermediate and mafic
granule-size components of hyaloclastite, especially lava flows, in both subaerial and subaqueous settings
resedimented hyaloclastite. In situ quench fragmentation (2.1-4, 6.4, 17.4, 20). Variations in their size, shape and
of porphyritic lavas commonly affects phenocrysts, abundance in lavas reflect the interplay of several
producing jigsaw-fit or near jigsaw-fit, monominerallic controls, including original magma volatile content and
crystal fragment clusters. Subsequent alteration and viscosity, rates of decompression and diffusion,
deformation of quench-fragmented phenocrysts and coalescence and interference of adjacent vesicles, and
glassy groundmass may result in an apparent pyroclastic deformation during flowage. Some subaerial basaltic
texture. flows consist of an upper and sometimes lower vesicle-
rich zone, separated by a poorly vesicular interior. The
Crystals in volcanogenic sedimentary deposits may be upper zone is broader, more vesicular and contains
derived by reworking and resedimentation of non- larger bubbles than the lower zone, probably as a result
welded, crystal-bearing pyroclastic or autoclastic of coalescence of rising bubbles during solidification
deposits, and by surface weathering and erosion of (Sahagian et al., 1989). Dimroth et al. (1978) noted an
crystal-bearing volcanic rocks, such as porphyritic lava equivalent increase in vesicularity towards the tops of
or crystal-rich welded ignimbrite. Crystal fragments of subaqueous basaltic sheet flows in the Archean of
either origin become increasingly rounded by surface Quebec, Canada. A different pattern occurs in "spongy"
processes, and evidence of the original clast-forming pahoehoe (Walker, 1989b): vesicles are spherical and
mechanisms may be destroyed. Note that some primary increase in size and abundance symmetrically inward
phenocrysts are rounded prior to eruption, due to from the margins to the centers of lava flow units (19.6).
magmatic resorption. This distribution is interpreted to result from vesicle
growth and coalescence in static lava that has
Crystal fragments are typically confined to, and may appreciable yield strength, and is principally developed
dominate the sand or coarse ash grain size of in medial to distal parts of subaerial basaltic flows.
volcaniclastic deposits. The mineral assemblage
represented by crystals and crystal fragments in Pipe vesicles are slender cylindrical cavities up to
volcaniclastic deposits, especially pyroclastic and mass- several millimeters across and tens of centimeters in
flow resedimented syn-eruptive volcaniclastic deposits, length (16.1, 17.2). They are commonly found near the
strongly reflects that present in the porphyritic source bases of subaerial pahoehoe lava flows (Waters, 1960;

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