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Welded Breccias in Andesite Lavas

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Welded Breccias in Andesite Lavas Journal of the Geological Society, London, Vol. 150, 1993, pp. 897-902, 7 figs. Printed in Northern Ireland Welded breccias in andesite lavas R. S. J. SPARKS l, M. V. STASIUK l, M. GARDEWEG 2 & D. A. SWANSON 3 1Department of Geology, University of Bristol, Bristol BS8 1RJ, UK eServicio Nacional de Geologia y Mineria Chile, Avda. Santa Maria 0104, Casilla 1347, Santiago, Chile 3US Geological Survey, Department of Geological Sciences, University of Washington, Seattle, Washington, USA Abstract: Flow breccias, formed at the margins of blocky andesite lavas, can be reheated, welded and sometimes deformed to form rocks reminiscent of welded pyroclastic rocks. Reheating occurs due to advection of heat from the flow to the basal breccias. Surface breccias also infill extensional crevasses which are later closed and compressed. Strongly deformed welded breccia bands form within the flow levees. The angular breccia clasts can be deformed into fiamme. The formation of welded clastic rocks is usually associated lava is nearly 80 km long with a volume of at least 2 km 3 and with pyroclastic processes, in which the deposits are a thickness up to 60 m. It contains 30-35% phenocrysts emplaced at sufficiently high temperature that the particles mainly of plagioclase with subordinate amounts of clinopyro- sinter together at point contacts (i.e. weld). The clasts xene, hypersthene and magnetite in a fresh glassy matrix. sometimes deform plastically with accompanying decrease in The lava flowed down the broad ancestral Tieton River porosity. The most common occurrence is in ignimbrites valley, to near Rimrock Lake, where it entered a narrow where pumice clasts set in an ash matrix are welded and canyon cut into Tertiary Cascade rocks and Columbia River deformed into fiamme and eutaxitic foliation is developed Basalt lavas, and finally spilled into the Naches valley where (Smith 1960). Welded textures can also develop in proximal it ponded and stopped (Fig. 1). The Tieton River recut its pumice fall deposits (Sparks & Wright 1979), in agglutinates canyon completely through the andesite, leaving numerous of fall-out origin (Fisher & Schmincke 1986) and in remnants on the canyon walls. In the canyon at the flow pumice-fall deposits overlain by thick lava (Christiansen & base, the lava is underlain by river gravels, and at its Lipman 1966; Schmincke 1967). Welded textures can be margins it lies against talus and sub-vertical walls of older mimicked in compaction of pumice deposits altered by rocks. We describe marginal exposures which illustrate the low-temperature diagenesis (Branney & Sparks 1990). main facies variations in the breccias. In the context of the Combined effects of devitrification, perlitic fracture, following discussion we define matrix as fine autoclastic hydrothermal alteration and tectonic deformation on particles of ash grade (<2 mm diameter) apparently formed originally glassy lavas can produce pseudo-pyroclastic by flow-fragmentation of andesite. textures (Allen 1988). At a locality 50 km from the vent near the channel axis In this paper we describe welded textures developed in (Fig. 1, locality 1) the lava is 50 m thick and shows a 5 m autoclastic breccias within blocky andesite lava flows. The thick basal zone below columnar jointed lava resting on welded facies occur at the bases and margins of flows and as consolidated river gravels (Fig. 2a). From the bottom, the bands within lava levees. The rocks range from incipiently basal zone grades from a loose matrix-rich breccia up into a welded breccias to those with strong deformation textures coarse clast-supported breccia, with point welding at the and formation of fiamme from originally angular blocks. We contacts of blocks, and then into densely welded breccia in interpret the development of welded breccias in terms of which angular, equant lava clasts are set in a dense glassy heat transfer and deformation within active andesite flows. black matrix (Fig. 2b). The angular blocks are typically a Pichler (1981) also described welding textures within lava few centimetres in diameter. The densely welded matrix- flows and attributed them to internal brecciation and supported breccia passes up into columnar lava where deformation in the flow. These observations further autoclastic textures are no longer apparent. The columnar highlight the caution needed in interpretation of rocks with jointing is continuous into the basal breccia zone but welded fragmental textures. Although andesite lava is a becomes more poorly defined and eventually dissappears common rock type encountered in the geological record downwards. At a more proximal location, 20 km from the there are few descriptions of their internal structures and vent (Fig. 1, locality 2) the 60 m thick lava is underlain by textures. In contrast there are many descriptions of basalt 1 m of tuffaceous soil containing angular flattened pumice. lavas. The features described in this paper are widespread The top of the soil shows a 10-20 cm thick layer of banded, and might easily be misinterpreted as pyroclastic in origin. dense glassy welded material which contains plant-debris imprints. The basal blocky breccia (1-2m in thickness) Observations varies laterally over a few metres from an open matrix-free block breccia to a matrix-rich breccia with individual clasts The Tieton andesite, Washington, (USA) up to 1 m diameter. The matrix-free breccia is welded at The Tieton Andesite lava (c. 60% SiO2) erupted from the point contacts while the matrix-rich breccia is strongly Goat Rocks Volcano in the Cascades of Washington at 1 Ma welded and dense. Some blocks are slightly flattened. age (Warren 1941; Swanson 1978; Swanson et al. 1989). The An excellent exposure of a lateral margin is exposed 897 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/5/897/4892374/gsjgs.150.5.0897.pdf by guest on 29 September 2021 898 R.S.J. SPARKS ET AL. layered lava and were clearly derived by break up of the I o Tieton Andesite ~ 0 20 km layered lava that makes up the inner wall and levre crest. (~) l_x~calitynumber I I I I I The layered levre rocks can be broadly divided into 10lake /l/qO4e,~~ layers of massive steel-grey glassy lava with fine millimetric to centimetric banding and layers of deformed and strongly welded flow breccia. The rock types alternate in layers in the range of 10 cm to 200 cm width (Figs 4 and 6a). Tight flow folds can be seen in the flow-banded rock. The welded breccia layers consist of deformed steel grey lava clasts set in a fine rusty red welded matrix of ash grade size (<2 mm). The breccia rocks vary from clast supported to matrix supported (Fig. 6). Deformation in the breccias varies from moderate (Fig. 6b) where the original angular shape of the i'~"~~a ......................... 12@1"46W30' N ~l clasts is readily apparent to strongly deformed examples which merge into flow-banded rock (Fig. 6c). Most of the clasts have been deformed into lenticular glassy lenses of Fig. 1. Map of the Tieton Andesite showing locations described in fiamme (Fig. 6) enclosed in a strongly welded autoclastic ash text. Size of most of the intra-canyon andesite remnants grade matrix. Isolated samples of the welded breccia exaggerated for clarity. Localities 1 and 3 are visible from highway resemble welded ignimbrites (Fig. 6a). along Tieton River. Locality 2 is at northwest base of flow remnant Thin-sections of the welded breccias confirm that they overlooking Rimrock Lake. Inset map shows location of field area are very poorly sorted with deformed clasts ranging from a in State of Washington. few hundred microns to clasts the size of the section. The fine autoclastic welded ash matrix is unresolveable, but the typical vitroclastic texture of deformed glass shards of 45 km from the vent (Fig. 1, locality 3) where the lava banks ignimbrites are not apparent. against a 35 ° talus slope made of Columbia River basalt clasts. The basal matrix-rich breccia (thickness about 0.5 m) Other examples. Since recognizing welded textures in the is friable and only point contact welded in contrast to the flow breccias described above we have observed that these locality near the channel axis where the breccia is rocks are widespread in lavas of intermediate compostiion. predominantly densely welded. Many of the andesites of Lascar Volcano show similar Thin-sections of the welded breccias of the Tieton welded and deformed facies. On a recent visit by two of us andesite show both angular and more plastic irregularly (M.V.S. and R.S.J.S.) to Sollipulli Volcano in southern shaped clasts set in a fine crystal-rich matrix. The individual Chile (38°S) we observed both undeformed and highly de- clasts typically have a glassy to cryptocrystalline groundmass formed welded basal and lev6e breccias in most of the whereas the autoclastic matrix is devitritified with little andesite lavas. We have also observed similar facies in residual glass and abundant broken crystal fragments. phonolite lavas on Tenerife. Andesite lava of Lascar Volcano, Chile Interpretations A young andesite lava (63-64% SIO2) was examined on the The blocks in the welded breccias of the Tieton and Lascar south-western flanks of Lascar Volcano 23°S in northern andesites are observed to be angular, even in cases where Chile (Fig. 3). The lava has not been dated but it is fresh they have been strongly plastically deformed. Blocky and very well-preserved. The lava is a fine-grained glassy andesite flows develop a carapace of angular blocks and andesite with sparse (15%) microphenocrysts. The lava is autoclastic fines. These fragmental surfaces originate in the about 3.3km in length, 700m wide and approximately cooled outer parts of the flows where a brittle solid crust 30-40 m thick.
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