Page 181 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

in both the silicic and mafic lavas and shallow alteration phase (42.3-8, 43.2-4). The farther that the
intrusions include variable amounts of hyaloclastite and prominent fracture- and matrix-controlled alteration
intrusive hyaloclastite (peperite) breccia, whereas the phase progressed out from fractures and matrix, the
cores are largely coherent. more matrix-rich and matrix-supported was the resultant
alteration texture. This two-phase style of alteration was
Glassy margins extremely common and has produced pseudo-clastic
textures in many units. The pseudoclastic textures can
The originally glassy, permeable margins show more occur at a range of scales in one rock unit,
pervasive, intense and complex textural changes corresponding to the range in types and scales of
resulting from diagenetic and hydrothermal alteration original fracture networks (compare 42.1, 42.3, 42.6). In
than do the crystalline, relatively impermeable cores. outcrop, these altered units resemble coarse-grained,
These differences are attributed to the instability of clast- to matrix-supported pseudo-breccias. In hand
glass and the influence of permeability on alteration. specimens and thin-sections, original perlitic textures in
Most of the originally glassy lavas and shallow the coherent glass have been transformed by alteration
intrusions, especially around the ore deposits, display into aggregates of splintery and arcuate particles that
two or more superimposed alteration phases. Each can closely resemble pyroclastic glass shards (Allen,
alteration phase consists of an area of rock or a set of 1988).
domains that can be distinguished by a particular
alteration mineral assemblage or by greatly different Subsequent alteration was (1) mainly restricted to
proportions of minerals in similar mineral assemblages, fractures and the matrix of breccias and strongly
and also, generally, by different colour and texture. controlled by fracture and matrix permeability; or (2)
Different alteration phases in the same rock result from overprinted on previously formed alteration domains of
different alteration stages, even though the time between similar composition. For example, second-generation
stages may be very short. Preservation of overprinting phyllosilicate alteration preferentially occurred within
relationships, different stages of textural change and previously phyllosilicate-altered domains, and
different intensities of alteration, allow reconstruction of silicification preferentially occurred in existing domains
the sequence of alteration. of quartz- or feldspar-rich composition.

The early alteration resulted in extensive and The general evolution of alteration style that occurs in
preferential replacement of glassy parts by a many areas, from pervasive to more vein-like, can
combination of fracture- and/or matrix-controlled probably be attributed to the increasing stability of the
alteration, and pervasive alteration. In detail, alteration mineral assemblage and decreasing permeability as
commenced along permeable fractures (quench, perlitic alteration progressed. Early pervasive alteration phases
and hydraulic fractures), and in the matrix of breccias now mainly comprise pale feldspar-rich or darker
(42, 43). As alteration progressed, alteration fronts phyllosilicate-rich assemblages. Subsequent
moved out from the fractures or matrix toward the overprinting alteration phases are mainly pale quartz-
centers of unfractured domains. Two situations arose: rich or darker phyllosilicate-rich assemblages.
(1) In some areas, alteration fronts of the first major
alteration stage extended through all glassy parts, In addition to abundant false clastic textures, this
including right to the centre of the unfractured glassy polyphase and domain-controlled alteration style has
domains, and produced one pervasive alteration phase. also produced apparent polymict appearance because:
In these areas, any second alteration stage also generally (1) pseudomatrix and pseudoclast domains appear to
commenced along fractures and in the matrix of have different original composition, due to their
breccias, and extended out from these into the rest of the different colour and alteration mineralogy (42.4, 43.2,
rock. This alteration was rarely completely pervasive, 43.4);
possibly because the rocks were no longer glassy, and (2) each phase of alteration commonly varies in
isolated relics of the first pervasive alteration phase intensity, and consequently colour, within a single
remained (42.6). outcrop; originally monomict lava breccia or coherent
(2) In other areas, the first alteration stage ceased lava with a uniform texture can assume a polymict
before complete replacement of the rock, leaving appearance, as a result of patchy variation in alteration
isolated kernels of glass (43.3), that were subsequently intensity (42.6, 43.1);
altered during a second alteration stage and formed a (3) at moderate intensities of alteration, phenocrysts
different alteration phase. (especially feldspar) are more prominent in dark
phyllosilicate domains than in pale siliceous or
In both situations, incipient alteration, restricted to feldspathic domains (43.1, 14.3), resulting in apparent
fractures and matrix, enhanced primary clastic textures variation in crystal content between different alteration
by creating a contrast in composition and color between domains and greatly contributing to clastic, polymict
the fractures and matrix and the areas they enclosed. appearance.

More advanced alteration produced pseudoclastic In areas of strong tectonic deformation, the
textures, comprising isolated domains or pseudoclasts of phyllosilicate-rich, mechanically weak alteration
one alteration phase, enclosed within a continuous patches (pseudoclasts) have been flattened and stretched
interconnected pseudo-matrix domain of a different into lenticular shapes (pseudo-fiamme) parallel to
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