Page 136 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

3. Lithic clast concentration zone in ignimbrite
Irregular pods rich in angular, dense lithic clasts
sometimes occur in ignimbrites as a result of sorting
processes operating during flowage. These examples
occur near the base of the crystal-rich ignimbrite host
and are almost conformable with the steeply dipping
pumice foliation (S 0, arrow). The matrix of the
ignimbrite has well preserved, welded shards
indicating hot emplacement from primary pyroclastic
flows.

Bulgonunna Volcanic Group, Late Carboniferous;
Burdekin River, northern Queensland.

4. Lithic- and crystal-rich, welded ignimbrite
This ignimbrite consists of angular, quartz-phyric
volcanic lithic clasts (L), quartz- and feldspar-phyric,
formerly glassy (now chlorite) fiamme (F) and shard-
and crystal-rich matrix. Welding textures are well
preserved in thin-section (27.1B) providing evidence
that the unit was emplaced hot and is primary.

Tyndall Group, Mount Read Volcanics, Cambrian;
Comstock Valley, western Tasmania.
5. Lithic segregation pipes in ignimbrite
Lithic clast-rich segregation pipes in pyroclastic flow
deposits result from streaming of hot gas that
preferentially removes much of the fine ash. Such
structures provide clear evidence of hot
emplacement. The illustrated examples occur in
ignimbrite that is extremely poorly sorted and
comprises rhyolitic pumice lapilli (P), a variety of
volcanic lithic clasts (L) and abundant shard- and
crystal-rich matrix.

Carpenter Ridge Tuff, 27.5 Ma; outflow sheet from
Bachelor caldera, San Juan Mountains, Colorado,
USA.

127