sign in sign up
<<
Home , Breccia, Sollipulli

Journal of the Geological Society, London, Vol. 150, 1993, pp. 897-902, 7 figs. Printed in Northern Ireland

Welded in

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 , 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 which are later closed and compressed. Strongly deformed welded bands form within the flow levees. The angular breccia clasts can be deformed into fiamme.

The formation of welded clastic rocks is usually associated 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 with subordinate amounts of clinopyro- sinter together at point contacts (i.e. weld). The clasts xene, hypersthene and magnetite in a fresh glassy . sometimes deform plastically with accompanying decrease in The lava flowed down the broad ancestral Tieton River porosity. The most common occurrence is in valley, to near Rimrock Lake, where it entered a narrow where clasts set in an ash matrix are welded and canyon cut into Cascade rocks and Columbia River deformed into fiamme and eutaxitic foliation is developed 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 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 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 of 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 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 30-40 m thick. The lava flowed down the c. 25 ° slope of the forms which is continuously disrupted by flow. The summit stratocone onto gentler terrain (slope c. 12°) of fragmental material in the welded breccias derives from this deposits. The flow margins are composed of cooled surface material. In order to weld and be plastically prominent levres up to 30m high and 50-70m wide deformed the breccias must have been through a period of bounding a central channel. The crest of the levres re-heating. We now discuss how they were re-heated in the represents the maximum thickness attained by the lava surge context of the thermal and deformational evolution of a and the inner walls have been exposed to a depth of up to lava. 10 m as the advancing lava drained the channel. The structure of the levre in the Lascar andesite is A dvective heating shown in Fig. 4 and is illustrated in Figs 5 and 6. The inner As lava moves away from the source heat is lost to the wall is composed of flow-banded layered lava with steep dips ground and atmosphere principally by thermal conduction, into the channel at 60-70 ° (Figs 5a and 6b). The strike of resulting in cooling of the marginal regions of the flow. the layering is oblique to the channel levee boundary by However continued flow of hot lava from the vent carries between 10 ° and 20 °. The outer margins of the lev6e consists heat downstream and eventually the margins of the flow can of a very coarse chaotic blocky breccia (Fig. 6a). Coherent be reheated locally if the flow is sustained over a sufficiently blocky outcrops of layered flow-banded lava with the same long period. strike and dip as the inner walls protrude through the We now consider the temperature variations across a chaotic breccia especially towards the top of the levre. Some cooling flow. In a longtitudinal section down the central axis of the blocks in the breccia consist of flow-banded and of the channel to the flow front the thickness of a cooled

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/5/897/4892374/gsjgs.150.5.0897.pdf by guest on 29 September 2021 WELDED BRECCIAS IN ANDESITES 899

(b)

(a)

region at the lava surface increases downstream by conduction. A layering develops in which a brecciated blocky layer at the flow surface overlies a viscous layer of cooled lava. At the flow front caterpillar-track motion and flow front avalanching results in the flow overturning the surface layers of breccia and viscous cooled lava. An inverted layer of the breccia and cooled lava forms between the ground and the high temperature interior of the lava. Initially this basal region can cool further as heat is lost to the underlying ground. At any fixed distance from the vent the basal region of breccia and cooled lava derived from the flow surface will eventually heat up due to two effects. First, the base of a flow is the region of high stress so that the cool viscous layer will be progressively thinned due to shear strain. Thinning will be resisted by the increased viscosity in the basal cooled viscous layer, but will nevertheless result in a decrease in thickness with time as the flow advances downstream. Second, the advection of heat from new interior hot lava moving behind the flow front will result in conduction of heat into the basal region which will lower viscosity and enhance shear thinning. The temperature history at the base or margins of a lava is determined by a competition between conductive heat loss to the surroundings and advection of heat by the flow of lava in the channel. The general physical principles that govern the thermal evolution of magmatic flows have been presented by Huppert & Sparks (1989). In the initial stages of flow the margins will be chilled, but with prolonged flow they will be reheated. The increases in temperature at the flow margins is predicted by theory (Huppert & Sparks 1989) and demonstrated in analogue laboratory experiments (Stasiuk et al. 1993). In a lava advection results in thermal thinning of the cooled viscous layer and heating of the basal breccias. Ultimately the ground can melt to cause thermal Fig. 2. Field photographs showing (a) an outcrop of Tieton lava erosion if it has a melting temperature lower than the lava with columnar jointing overlying a basal breccia and (b) strongly temperature (Peterson & Swanson 1974; Hulme 1982; welded flow breccia from base of flow. Locality 1 in Fig. 1. Huppert 1989).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/5/897/4892374/gsjgs.150.5.0897.pdf by guest on 29 September 2021 900 R.S.J. SPARKS ET AL.

viscous region will increase with distance from vent, because it is derived by overturning of the cooled upper surface which increases in thickness with distance. The basal region will therefore take longer to reheat and shear thin as its 'thickness increases. Second the length of time of advective heating will decrease with distance from vent, due to the advance of the flow. Points closer to the vents will have been heated by advection for longer periods. Lascar Summit Heat advection and shearing will also effect the margins Crater of the channel. The lateral margins have lower velocities and the margins will heat more slowly than the flow base at the same distance. This explains why the marginal basal breccias in the Tieton andesite are less welded than the basal breccias in the channel axis at the same distance. Lava deformation history The Lascar andesite provides a good example of deformation and morphological evolution which is common in many lavas. Andesite lavas develop extensional parabolic fractures or crevasses which open up on the surface. Their ~ ,~ shape is a consequence of channel flow which rotates the fracture due to the increasing shear rate towards the channel margins. The crevasses are extensional fractures which can be seen to penetrate several metres into the lava interior from the surface. In longtitudinal-section the fractures dip upstream and in cross-section they dip into the channel interior. We propose that the Lascar welded breccia bands ~-Andesit-e originate in extensional crevasses which fill with surface Lava breccia as they open up (Fig. 7). The crevasses become closed up by compression for two reasons. First they are rotated at the margins of the channel and in the high shear region at the flow margins. The parabolic flow profile results in the being rotated and extended along its length with a corresponding shortening normal to the axis of the crevasse. The clasts are subjected to shear strain which is Fig. 3. Aerial photograph of Lascar andesite on south-west flanks more intense towards the flow margins and increases with of the volcano showing lava flow described in this paper. The lava time. The highly strained flow margins ultimately can is outlined for clarity. Scale I • 12 500. become incorporated into the lava levee as a central channel develops, abandoning stagnant marginal lava to form the lev6e (Naranjo et al. 1992). Second the lava front becomes The occurrences of welded breccias in the Tieton cooler and more viscous as the flow moves onto a lower andesite are attributed to heat advection and shear thinning. slope and decelerates (as was the case in the Lascar lava). The more proximal locality shows the most pronounced The lava piles up behind the front and compressive stresses welding with deformation of clasts and welding of the top close the crevasses. few centimetres of the underlying soil. This is as expected Breccia that falls into a crevasse will reheat and be for two reasons. First the thickness of the basal cooled deformed under compression as the crevasse closes. We propose that the welded bands in the lev6es of the Lascar andesites formed by this mechanism as illustrated in Fig. 7, f ~ Weldeddeformed which explains the geometry of the steeply dipping inwardly I~ ~ "~/ ~,,~brecci~ inclined alternations of strongly deformed breccia and 5m ~ -bandedlava massive lava that characterize the lev6es of Lascar. This model requires that the material composing the lev6es was once participating in the marginal shear flow and then becomes abandoned as the boundary between stagnant levee and active channel moves inwards. Inward migration of the boundary between channel and lev6e and a decrease in channel depth have been documented in the eruption of "'/¢//, the andesite (Naranjo et al. 1992) and is characteristic of an eruption with declining flow rate through time. The Lascar lev6es thus preserve the deformed highly Fig. 4. Schematic cross-section through margins of the Lascar lava, sheared margins of flow which formed in the early stages of showing alteration of inwardly dipping layers of deformed welded high flow rate and which were then abandoned as flow rate breccia and flow-banded to massive lava. decreased.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/5/897/4892374/gsjgs.150.5.0897.pdf by guest on 29 September 2021 WELDED BRECCIAS IN ANDESITES 901

(a) (b) Fig. 5. Photographs of Lascar lava lev6e showing (a) outer margins of lev6e with blocky fragmental surface and (b) inner wall with alternation of massive glassy lava with bands of deformed and strongly welded breccia.

surface and at the flow front in the cooled outer surface of Conclusions the flow overriden by the flow. In long-lived flows the Although andesite lavas lose heat as they move, the bases reheating can cause the breccias to weld and breccia clasts to and margins are reheated by advection of heat from new hot be deformed into 'fiamme-like' lenticles. Ultimately the lava and are deformed because these are the regions of breccia may be reconstituted back into lava destroying the highest shear stress. Brecciated material formed on the flow original clastic textures. Cooled surface breccia can also be

(a) (b)

Fig. 6. -normalized REE plots and MORB-normalized multi-element plots for Well 209/3-1 . Solid lines, core 1 samples; dashed line, sample 2-1. REE plots are normalized using the values given by Wakita et al. (1971), and the multi-element plots normalized to MORB (using the values of Saunders & Tarney 1984).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/5/897/4892374/gsjgs.150.5.0897.pdf by guest on 29 September 2021 902 R.S.J. SPARKS ET AL.

i o i \ co (c) 1 ~~.:~.-.~~ \ -~ t Fig. 6.--(Continued)

incorporated into extensional crevasses. The breccia material will then be reheated, welded and deformed from elastic bands within the lava. Matrix-rich deformed breccias / \ I can resemble in hand sample. The preservation / \ [ of these bands in the lava levres of andesite flows show that / \ I their lava levres were part of the actively shearing flow margins, in proximal regions basal welded deformed rocks / / \ I / 4 \ of lava flow origin may be hard to distinguish from welded \ fall-out deposits and agglutinates, especially if the later deposits have also been reheated and sheared by the overlying lava.

The field studies in Chile wcrc supported by a Royal Society Fig. 7. Schematic model of how deformed welded breccias can form Overseas Field Work Grant to R.S.J.S.M.V.S. acknowledges a by opening of extensional crevasse fractures which are closed and Commonwealth Scholarship for graduate studies at Bristol. P. deformed by the high shear rates at the flow margins. Open Hancock made some valuable comments on the deformational symbols represent undeformed breccia fill and closed symbols regimes in the Lascar lavas. Reviews by N. Harris and two referees represent deformed and welded breccia. No scale is involved and helped to improve the manuscript. the width of the crevasses is exaggerated for clarity. The diagram shows the deformation of a single crevasse as it is progressively deformed downstream. References ALLEN, R.L. 1988. False pyroclastic textures in altered silicic lavas, with implications for w~lcanic-associated mineralization. Economic Geology, 83, 1424-1446. BRANNEY, M.J. & SPARKS, R.S.J. 199{). Fiammc formed by diagencsis and burial-compaction in soils and subaqucous . Journal of the Stromboli, Tyrrhenisches Meer. Sammlung Geologoscher Furer, 69, Geological Society. London, 147, 919-922. Borntraeger, Berlin. CIIRISTIANSEn, R.L &. LIt'MAn, P.W. 1966. Emplacement and thermal SOIMmCKE, H.U. 1967. Fused and peperites in southern-central history of a lava flow near Forty Mile Canyon, Southern Nevada. Washington. Geological Society of America Bulletin, 78, 319-330. Geological Society of America Bulletin, 80, 1-8. SMrrfI, R.L. 1960. Zones and zonal variations in welded ash-flows. US FlsJlEr, R.V. & SCHMINCKE, tt.U. 1986. Pyroclastic rocks. Springcr Vcrlag, Geological Survey Professional Paper, 354F, 149-159. Heidelberg, Germany. SPARKS, R.S.J. & WrIGm', J.V. 1979. Welded air-fall tufts. In: CtIAPIN, C.E. HULME, G. 1982. A review of lava flow processes in relation to the formation & ELS'rON, W.E. (eds) Ash Flow Tufts. Geological Society of America of lunar sinuous rilics. Geophysical Surveys, 5, 245-279. Special Paper, 180, 150-166. HUPPER'r, H.E. 1989. Phase changes following the initiation of a hot turbulent S'rASIUK, M.V., JAUPART, C. & SPARKS, R.S.J. 1993. The influence of cooling flow over a cold solid surface. Journal of Fluid Mechanics, 198, 293-319. on lava flow. Geology 21, 335-338. -- & SPARKS, R.S.J. 1989. Chilled margins in igneous rocks. Earth and SWANSON, D.A. 1978. Geologic map of the Tieton River area, Yakima Planetary Science Letters, 92, 397-405. County, south-central Washington, scale 1 : 48 000. US Geological Survey, NARANJO, J.A., SPARKS, R.S.J., STASIUK, M.V., MORENO, H. & ABLAY, G.J. Miscellaneous field studies map MF-968. 1992. Morphological, structural and textural variations in the 1988-1990 --, CAMERON, K.A., EWARTS, R.C., PRINGLE, P.T. & VANCE, J.A. 1989. andesite lava of Lonquimay Volcano, Chile. Geological Magazine, 129, volcanism in the Cascade Range and Columbia Plateau, 657-678. southern Washington and northernmost Oregon. New Mexico Bureau of PE'IERSON, O.W. & SWANSON, D.A., 1974. Observed formation of lava tubes. Mines and Resources Memoir, 47, 1-50. Speleology, 2, 209-224. WARREN, W.C. 1941. Relation of the Yakima basalt to the Keechelus PICHLER, H. 1981. ltalienische Vulkan-Gebeite III: Lipari. Vulcano, Andesite Series. Journal of Geology, 49, 795-814.

Received 26 November 1992; revised typescript accepted 2 April 1993.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/5/897/4892374/gsjgs.150.5.0897.pdf by guest on 29 September 2021