Page 182 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

tectonic cleavage and lineation. The alignment of these banded and flow-folded lavas and intrusions resemble
dark phyllosilicate lenses closely resembles the flattened thinly bedded and tectonically folded thinly bedded
pumice clast fabric of welded pyroclastic deposits volcaniclastic rocks respectively. Flow banding can,
(eutaxitic texture) (42.4, 43.1). Furthermore, in strongly however, be distinguished from bedding due to its
foliated and lineated rocks, phenocrysts and perlitic characteristic wavy, coarsely to finely bulbous,
groundmass have been stretched and partially cauliflower-shaped outline (44.3-4, 44.7). Flow folds
dismembered into fragments, which further contributes can be distinguished from tectonic folds by their
to clastic appearance (Allen, 1988). In altered and irregular distribution and shape, and local areas where
deformed volcanic sequences, the relationship of tectonic cleavage is not axial planar to the folds.
tectonic structures to apparent clast shape and texture
must always be evaluated, in order to determine the Spherulitic or microlitic cores of lavas and
origin of apparent clastic textures and of foliations that shallow intrusions
resemble eutaxitic texture.
The cores of lavas and shallow intrusions have
Mixed glassy and spherulitically devitrified zones groundmasses with abundant, small, closely packed
spherulites or microlites, and are characterized by
Mixed glassy and spherulitically devitrified zones massive granular textures, similar to well sorted
originally comprised massive coherent volcanic glass sandstone (in outcrop, hand specimen and thin-section).
with scattered spherulites and lithophysae, or bands of Diagenetic and hydrothermal alteration of these rocks
glass alternating with bands of spherulites and generally commenced along cooling joints and
lithophysae, or a combination of both (44.3). Diagenetic hydraulic fractures, producing comparatively simple
and hydrothermal alteration of the mixed glassy and fracture-controlled vein networks. Moderate to intense
devitrified zones in lavas and intrusions was similar to alteration resulted in additional patchy (rarely
alteration of the totally glassy margins described above, homogeneous) replacement of chemically unstable
but was also strongly influenced by the compositional components between fractures, typically die feldspar
domains formed by partial devitrification (compare microlite-rich groundmass, followed by feldspar
44.3, 44.4, 44.7). Bands, nodules and patches of phenocrysts. This patchy alteration and resulting
quartzofeldspathic devitrification structures (spherulites, compositional contrasts produced false clastic and
lithophysae) were recrystallized, silicified and/or polymict textures in originally coherent lavas and
partially replaced by feldspar during alteration but, in shallow intrusions in some areas. Furthermore, the
general, maintained a quartzofeldspathic composition widespread recrystallization of spherulites, during
(44.4). In contrast, glassy areas between the devitrified alteration and metamorphism, has enhanced the granular
domains were replaced mainly by more sericite- or texture of these rocks, making the distinction from true
chlorite-rich assemblages (44.4-5, 44.7). Consequently, volcaniclastic textures particularly difficult.
the glassy domains and devitrified domains now differ
in colour and composition, and could be misinterpreted General trends and implications
to have been of different original composition.
Furthermore, phenocrysts are more prominent in the In conclusion, diagenetic and hydrothermal alteration
dark, phyllosilicate-altered, originally glassy domains caused the textures in both the glassy autoclastic and
than in the pale, quartzofeldspathic-altered, spherulitic glassy coherent facies of the lavas and shallow
domains. This results in apparent variation in crystal intrusions to evolve toward matrix-supported, monomict
content between the two domains, and hence polymict and polymict clastic textures typical of volcaniclastic
clastic appearance. mass-flow deposits including pyroclastic flow deposits.
With increasing alteration intensity and deformation,
The original outline or morphology of devitrification these false clastic textures converged in appearance to
bands and nodules has been preserved, except in cases resemble the textures of welded pyroclastic flow
of intense alteration. However, both diagenetic and deposits (42.4, 43.1). Spherulite-defined flow banding is
hydrothermal alteration and subsequent regional well preserved and even enhanced in weakly altered
metamorphism have recrystallized the original internal rocks. However, moderate to strong alteration caused
fibrous devitrification structure to mosaics of anhedral originally flow-banded and nodular devitrified lavas and
quartz and feldspar. Intense alteration caused intrusions to resemble, respectively, thinly bedded and
redistribution of the pre-existing compositional domains pebbly granular volcaniclastic deposits. The more
into massive or weakly banded, irregularly mottled crystalline cores of the lavas and shallow intrusions are
alteration textures (cf. Allen, 1988). less altered, except near the centre of hydrothermal
systems. They are characterized by massive sandy or
Consequently, diagenetic and hydrothermal alteration granular texture, with local areas of fracture-controlled
have generally imparted a clastic appearance to pseudobreccia texture, which comprises granular
originally mixed glassy and spherulitically devitrified pseudoclasts in a more altered pseudomatrix.
zones in lavas and intrusions. Spherulites and
lithophysae resemble rounded to subrounded volcanic Consequently, apparent pyroclastic and other
clasts, and strongly spherulitic or lithophysae-rich rocks volcaniclastic textures must be examined critically, with
closely resemble sandy to pebbly, granular all likely post-depositional processes in mind, in order
volcaniclastic deposits (44.4, 44.6). Altered planar flow- to correctly interpret their origin. Furthermore, there
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