Page 158 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

1. Normally graded, volcaniclastic lithic breccia-
sandstone

A, B. Two main lithofacies are evident in this
outcrop: (1) a very thick bed graded from
volcaniclastic lithic breccia (Bx) at the base to
diffusely stratified sandstone (T) at the top; (2) planar
bedded, crystal-rich sandstone (St). Pink and green
colour banding in the sandstone is defined by
alternating pink albite-rich and green chlorite-rich
alteration. Both lithofacies are dominated by juvenile
volcanic components (angular quartz and feldspar
crystals, angular volcanic lithic fragments) that may
have been produced by explosive eruptions, but the
lithofacies organisation is typical of deposits from
successive, subaqueous, high-concentration,
volcaniclastic, granular mass flows. The lithofacies
are markedly enriched in relatively dense
components (crystals, lithic fragments), and depleted
in vitric components (pumice, shards) compared with
primary pyroclastic flow deposits.

Mount Read Volcanics, Cambrian; Anthony Road,
western Tasmania.
2. Polymict, volcaniclastic lithic breccia
A. Lithic breccia at the bases of graded units shown
in 32.1 is polymict. The most abundant lithic
fragments are dark blue-grey, angular, quartz- and
feldspar-bearing, welded ignim-brite and porphyritic
lava. They are supported in quartz and feldspar
crystal-rich matrix, and surrounded by pinkish haloes
of albite alteration.

Mount Read Volcanics, Cambrian; Anthony Road,
western Tasmania.

B. A photomicrograph of the crystal-rich matrix of
the breccia in 32.2A shows angular quartz (Q) and
feldspar (Fl) crystal fragments, and welded
ignimbrite lithic (L) fragments separated by fine
grained, patchy albite and chlorite (alteration
products). Plane polarised light.

Mount Read Volcanics, Cambrian; specimen ARA
4B, Anthony Road, western Tasmania.

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