Page 142 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Plate 25 — Lithophysae and spherulites in welded ignimbrite
1. Lithophysae in densely welded ignimbrite

These lithophysae (L) have central, lens to star
shaped vugs which are lined by radially arranged
crystal fibres. Lithophysae result from glass
devitrification and begin to form early in the cooling
history of primary pyroclastic deposits and lavas. In
ignimbrites, they are a reliable indicator of dense
welding because they only form in coherent glass,
that is, in deposits that have undergone thorough
welding compaction.

Carpenter Ridge Tuff, 27.5 Ma; Bachelor-caldera,
San Juan Mountains, Colorado, USA.

2. Spherulites in devitrified welded ignimbrite
Dark grey spherulites (arrow) have nucleated on
crystal fragments in the matrix of welded, dacitic
ignimbrite. The rest of the matrix has a mottled grey
and blue-grey appearance due to patchy variation in
the style and grain size of devitrification. A genetic
interpretation can be inferred from textural and
lithofacies evidence in outcrop: broken crystals and
lithic fragments indicate the rock is volcaniclastic;
the presence of spherulites indicates hot emplacement
and primary welding; the poorly sorted, unstratified
lithofacies are consistent with welded ignimbrite
(rather than welded pyroclastic fall deposits).
Bulgonunna Volcanic Group, Late Carboniferous;
Glendon Creek, northern Queensland.
3. Crystal- and lithic-rich welded ignimbrite
A. Dominant components of this ignimbrite are
feldspar, hornblende and biotite crystal fragments,
glassy lenses (flattened pumice lapilli), volcanic
lithic lapilli and much finer matrix. Aligned flattened
pumice lapilli define a eutaxitic foliation (parallel to
arrow) that is characteristic of welded ignimbrite.

Nelson Mountain Tuff, 27 Ma; specimen NMT4, San
Luis caldera, San Juan Mountains, Colorado, USA.

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