Page 138 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

B. The matrix of the vapour-phase crystallised, non-
welded ignimbrite shown in 23.1A consists of
uncompacted, un-deformed, cuspate, rod and spike
shaped, partly devitrified, bubble-wall shards. Pale
brown, very fine grained, vapour-phase
crystallisation minerals infill the original pore spaces
in the matrix and pumice clasts. Plane polarised light.

Toconao Ignimbrite, 4 Ma; specimen Ta, Toconao,
northern Chile.
2. Slightly welded, glassy rhyolitic ignimbrite
A. This slab of ignimbrite shows the extremely poor
sorting, and matrix-supported fabric, typical of most
pyroclastic flow deposits. Pale, coarsely porphyritic
pumice lapilli (P) and a wide variety of angular
volcanic lithic lapilli (L) are supported in a grey ash
matrix consisting of crystals, crystal fragments and
glass shards. Most pumice lapilli are only slightly
flattened and the ignimbrite is not strongly welded.
The coherence is partly due to vapour-phase
crystallisation of fine-grained silica (tridymite/
cristobalite) and feldspar in the pore spaces soon after
emplacement.
Ongatiti Ignimbrite, 750 ka; Hinuera Stone Quarry,
Hinuera, New Zealand.
B. In thin-section, shards with cuspate (C) and platy
(P) shapes are evident in the matrix of the ignimbrite
in 23.2A. Although many shards retain original
bubble-wall shapes, those adjacent to crystal
components show the effects of slight welding and
are partly deformed and compacted. Crystal
components, feldspar (Fl) and pyroxene (Px), are
moderately abundant. Pale brown, very fine ash and
fine-grained vapour-phase minerals occur between
the shards. Plane polarised light.

Ongatiti Ignimbrite, 750 ka; specimen CV4B,
Hinuera Stone Quarry, Hinuera, New Zealand.
3. Slightly welded ignimbrite
A. The conspicuous volcaniclastic texture of this
ignimbrite comprises abundant crystal fragments,
scattered black and grey lithic clasts, pale brown
relict pumice wisps and dark brown, finer matrix.
The framework components (crystals, lithic clasts,
pumice wisps) are poorly sorted and matrix-
supported. The relict pumice wisps define a weak
foliation (parallel to the arrow).

Sapinero Mesa Tuff, 28.5 Ma; specimen SMTe-1, San
Juan caldera, San Juan Mountains, Colorado, USA

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