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Structures within large volume rhyolite lava flows of the Devonian Comerong Volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome, New Zealand
Abstract
Many rhyolitic units within the Late Devonian Comerong Volcanics erupted as lava flows and domes, some up to 18 km long and 350 m thick. The textural and structural characteristics that distinguish the flows and domes as lava rather than rheomorphic ignimbrite include unbroken phenocrysts, zones of autobrecciation, and finely developed flow layering with individual layers continuous for several metres. The flow layering is typically contorted into isoclinal folds with forms suggesting fluidal deformation and is interlayered with and gradational into restricted zones of pumice-rich lapilli tuff and zones of lenticulite breccia. The lenticulite breccia comprises discontinuous, lenticular rhyolite fragments, the long axes of which define a foliation parallel to flow layering. Lenticles in the breccia vary from elongate layers up to 1 m long and several millimetres thick to short fragments less than 10 cm long and several centimetres wide. Similar zones of lenticulite breccia consisting of glassy lenticular clasts in a devitrified, spherulitic “matrix” of cristobalite and albite, exist within the Late Pleistocene Ngongotaha dome near Rotorua, New Zealand. The lenticulite breccia is considered to form by aqueous diffusion and selective devitrification of the rhyolite along anastomosing fluid paths and to be modified by mechanical fracturing of the lava in a zone of high shear stress.Geochemically the rhyolites are high-Si and A-type, with high Zr and Y contents indicating that they formed from high-temperature, relatively anhydrous, F-rich melts. A-type granitoids crop out intermittently along the length of, and adjacent to the volcanic complex and are comagmatic with the rhyolite. The high temperature, low bubble and phenocryst content, and a high eruptive rate of the rhyolite, likely resulted in a low effective viscosity and extensive flow units.
... Silicic lava flows were initially described with blocky and rubbly crusts with metric clast sizes lending them the label "block lava flows" (Finch 1933;Harris et al. 2017). The base of these flows contains basal breccia which is thought to be formed either by brittle fragmentation during flow and deformation of basal shear zones (Dadd 1992;Latutrie et al. 2017;Smith 1996) or by the lava flow overriding its own upper and frontal parts during a "caterpillar track" motion . Classical models for silicic lava flow formation, including lava flows associated with Snake River-type volcanism, involve the presence of basal breccia as a diagnostic feature for effusive eruptions (cf. ...
... These second-and third-order fractures are analogous to Riedel shear fractures described for lava flows by, for example, Richard et al. (1995) and Zheng et al. (2021) which swap their sense of shear across different sets of first-order fractures and reflect the complex stress accommodation in a brittle-ductile material (cf. Dadd 1992;Gonnermann and Manga 2005). The greater the displacement during simple shear, then the higher the probability of shear lens formation (Fig. 9). ...
... Breccia formation in silicic lava flows can be caused by the lava flow overriding its own frontal breccia or it can develop in situ along basal shear zones (Dadd 1992;Latutrie et al. 2017;Manley and Fink 1987;Smith 1996). Shear accommodation in basal shear zones of silicic lava flows can generate brecciation by the propagation of faults during the ductile-brittle transition (Smith 1996). ...
Large-volume silicic eruptions in large igneous provinces are unique in the geological record because there are no observed analogues. Their facies architecture do not strictly follow the diagnostic features proposed in the literature for rheomorphic ignimbrites and lava flows. The models proposed for their emplacement depend on conditions such as high temperature (> 950 °C) and low viscosity (< 106 Pa s) coupled with unusually high effusion rates. Low-Ti dacitic lavas from the Paraná-Etendeka Large Igneous Province (LIP) have a wide variety of morphologies and lithofacies on both sides of the Atlantic Ocean. A common facies association framework for the Caxias do Sul/Grootberg dacites (outcropping in Namibia and southern Brazil) include the presence of platy and thinly jointed facies with a columnar jointed massive core and an amygdaloidal upper facies, with an absence of basal breccia. In this work, we describe and interpret features observed in the basal portion of these silicic units, focusing on the thinly jointed facies. This basal facies includes what has been termed as “zebra-like banding.” Zebra-like banding is an apparent structure that originates from a fracture network which involves horizontal and long first-order fractures, oblique and short second-order fractures, oblique short to long third-order fractures, and intersection zones of fourth-order structures. First-order fractures are related to pure shear, the second- and third-order fractures are Riedel shears, and the fourth-order structures represent angular contacts between different sets containing first-, second-, and third-order fractures. The combination of first-, second-, and third-order fractures evolves to shear lenses. The zebra-banding” is caused by oxidation/reduction halos developing in the host rock during post-emplacement fluid circulation along the fractures, culminating in precipitation of silica polymorphs and zeolite. The fracture system, including all orders of fracture, formed during late to post emplacement stages, and accommodated pure and simple shear in a ductile–brittle basal zone during oscillations in effusion rate. Fracturing of the basal zone, rather than the formation of basal breccia, demonstrates stress accumulation in this part of the flow and helps to explain why the presence of basal breccia is not a diagnostic feature in distinguishing large-volume silicic lava flows from rheomorphic ignimbrites.
... Previous work has found an array of structural and textural characteristics in silicic lava flows (e.g., Fink, 1983;Manley and Fink, 1987;Anderson and Fink, 1992;Dadd, 1992;Smith and Houston, 1994;Smith, 1996;Castro and Cashman, 1999;Castro et al., 2002;Magnall et al., 2017). Manley and Fink (1987) defined five textural facies that have since been found at most rhyolitic lava flows: coarsely vesicular pumice, finely vesicular pumice, obsidian, crystalline rhyolite, and breccia. ...
... We thus see evidence of the basal shear zone breaking up into blocks in the same manner observed by Dadd (1992) and Smith (1996) at the base of a silicic lava flow. Dadd (1992) observed that the basal breccia could be found as pockets in the lowermost portion of the massive lava and concluded that this was consistent with mechanical fracturing in a zone of high shear stress. ...
... We thus see evidence of the basal shear zone breaking up into blocks in the same manner observed by Dadd (1992) and Smith (1996) at the base of a silicic lava flow. Dadd (1992) observed that the basal breccia could be found as pockets in the lowermost portion of the massive lava and concluded that this was consistent with mechanical fracturing in a zone of high shear stress. Smith (1996) also argued that in basal zones subject to high shear stress, brittle failure involves fracturing along horizontal shear planes as well as vertically. ...
Emplacement dynamics of highly viscous, silicic lava flows remain poorly constrained due to a lack of consideration of crystal-rich cases. Emplacement models mostly apply to glassy or microlitic, vesiculated rhyolitic flows. However, crystalline, vesicle-free silicic lava can flow differently. We studied the Grande Cascade unit, which is a vesicle-free, phenocryst-rich, trachytic flow in the Monts Dore massif, France. Field work was carried out to define internal structures, and oriented samples were collected for chemical, petrological, and anisotropy of magnetic susceptibility analyses, allowing us to estimate emplacement temperature and viscosity.
These data allow us to define a new silicic lava flow subtype that is low in temperature (800−900 °C), high in silica content (up to 66.8 wt%), high in viscosity (109−1011 Pa s), rich in phenocrysts (∼35%), and lacks vesicles. Brittle deformation of the lava occurs upon extrusion, generating a cataclasite basal layer and thin (3-m-thick) shear zone that accommodates all of the stress, allowing most of the flow’s volume to slide over its base as a 40-m-thick plug in which there is no deformation. Blocks are rare, of a single size (10 ± 1 cm), and result from localized break-up of the basal shear zone.
Emplacement dynamics are different from those of glassy, pumiceous lava flows. They are closer to glacier dynamics, where most of the volume slides over a thin basal shear zone and till is generated there by abrasion and milling of the underlying layer. For the Grande Cascade lava flow, abrasion means that the flow lacks its classical blocky crust and instead the flow base is marked by a layer rich in fine-grained material. The structures and emplacement dynamics of this crystal-rich flow are consistent with ideal, gravity-driven shear flow. We thus argue for a global reassessment of silicic-rich lava emplacement based on crystal content and using a multidisciplinary approach focused on well-exposed examples in the rock record.
... Silicic lava flows were initially described with blocky and rubbly crusts with metric clast sizes lending them the label "block lava flows" (Finch 1933;Harris et al. 2017). The base of these flows contains basal breccia which is thought to be formed either by brittle fragmentation during flow and deformation of basal shear zones (Dadd 1992;Latutrie et al. 2017;Smith 1996) or by the lava flow overriding its own upper and frontal parts during a "caterpillar track" motion . Classical models for silicic lava flow formation, including lava flows associated with Snake River-type volcanism, involve the presence of basal breccia as a diagnostic feature for effusive eruptions (cf. ...
... These second-and third-order fractures are analogous to Riedel shear fractures described for lava flows by, for example, Richard et al. (1995) and Zheng et al. (2021) which swap their sense of shear across different sets of first-order fractures and reflect the complex stress accommodation in a brittle-ductile material (cf. Dadd 1992;Gonnermann and Manga 2005). The greater the displacement during simple shear, then the higher the probability of shear lens formation (Fig. 9). ...
... Breccia formation in silicic lava flows can be caused by the lava flow overriding its own frontal breccia or it can develop in situ along basal shear zones (Dadd 1992;Latutrie et al. 2017;Manley and Fink 1987;Smith 1996). Shear accommodation in basal shear zones of silicic lava flows can generate brecciation by the propagation of faults during the ductile-brittle transition (Smith 1996). ...
... Previous studies of silicic lavas have inter- preted their emplacement in terms of the slow spreading of crystal-poor domes, e.g., for Holo- cene rhyolite lava flows in the western USA (Fink, 1980a(Fink, , 1980b(Fink, , 1993DeGroat-Nelson et al., 2001) and older rhyolites in Australia (Dadd, 1992;Smith and Houston, 1995) and New Zealand (Dadd, 1992;Stevenson et al., 1994aStevenson et al., , 1994b. As a lava flow advances, cooled surface fragments cascade down the lava flow front and are overridden by the core in a tank- tread-style motion typical of block and 'a'? lava flows (Fink, 1983;Harris et al., 2004). ...
... Previous studies of silicic lavas have inter- preted their emplacement in terms of the slow spreading of crystal-poor domes, e.g., for Holo- cene rhyolite lava flows in the western USA (Fink, 1980a(Fink, , 1980b(Fink, , 1993DeGroat-Nelson et al., 2001) and older rhyolites in Australia (Dadd, 1992;Smith and Houston, 1995) and New Zealand (Dadd, 1992;Stevenson et al., 1994aStevenson et al., , 1994b. As a lava flow advances, cooled surface fragments cascade down the lava flow front and are overridden by the core in a tank- tread-style motion typical of block and 'a'? lava flows (Fink, 1983;Harris et al., 2004). ...
Understanding lava flow processes is important for interpreting existing lavas and for hazard assessments. Although substantial progress has been made for basaltic lavas our understanding of silicic lava flows has seen limited recent advance. In particular, the formation of lava flow breakouts, which represent a characteristic process in cooling-limited basaltic lavas, but has not been described in established models of rhyolite emplacement. Using data from the 2011−2012 rhyolite eruption of Puyehue-Cordón Caulle, Chile, we develop the first conceptual framework to classify breakout types in silicic lavas, and to describe the processes involved in their progressive growth, inflation, and morphological change. By integrating multi-scale satellite, field, and textural data from Cordón Caulle, we interpret breakout formation to be driven by a combination of pressure increase (from local vesiculation in the lava flow core, as well as from continued supply via extended thermally preferential pathways) and a weakening of the surface crust through lateral spreading and fracturing. Small breakouts, potentially resulting more from local vesiculation than from continued magma supply, show a domed morphology, developing into petaloid as inflation increasingly fractures the surface crust. Continued growth and fracturing results in a rubbly morphology, with the most inflated breakouts developing into a cleft-split morphology, reminiscent of tumulus inflation structures seen in basalts. These distinct morphological classes result from the evolving relative contributions of continued breakout advance and inflation. The extended nature of some breakouts highlights the role of lava supply under a stationary crust, a process ubiquitous in inflating basalt lava flows that reflects the presence of thermally preferential pathways. Textural analyses of the Cordón Caulle breakouts also emphasize the importance of late-stage volatile exsolution and vesiculation within the lava flow. Although breakouts occur across the compositional spectrum of lava flows, the greater magma viscosity is likely to make late-stage vesiculation much more important for breakout development in silicic lavas than in basalts. Such late-stage vesiculation has direct implications for hazards previously recognized from silicic lava flows, enhancing the likelihood of flow front collapse, and explosive decompression of the lava core.
... Observation of these processes in active silicic flows is, however, problematic because access to the flow interior is impossible, and erosion still has not had time to provide outcrop exposure. In addition, some features of silicic lava flows can easily be confused with ignimbrite facies and thus must be described and interpreted with some care (e.g., Dadd 1992;Sparks et al. 1993;Manley 1996b). ...
... The basal breccia can also be found as pockets in the lowermost portion of the massive lava. Dadd (1992) observed such structures in rhyolite flows concluding that these structures were consistent with mechanical fracturing of the lava in a zone of high shear stress. Such features are thus consistent with a zone subject to extremely high shear stress undergoing brittle deformation to cause the rock to fragment (Smith 1996). ...
A 70-m-thick, 2200-m-long (51 × 10⁶ m³) trachytic lava flow unit underlies the Puy de Cliergue (Mt. Dore, France). Excellent exposure along a 400-m-long and 60- to 85-m-high section allows the flow interior to be accessed on two sides of a glacial valley that cuts through the unit. We completed an integrated morphological, structural, textural, and chemical analysis of the unit to gain insights into eruption and flow processes during emplacement of this thick silicic lava flow, so as to elucidate the chamber and flow dynamic processed that operate during the emplacement of such systems. The unit is characterized by an inverse chemical stratification, where there is primitive lava beneath the evolved lava. The interior is plug dominated with a thin basal shear zone overlying a thick basal breccia, with ramping affecting the entire flow thickness. To understand these characteristics, we propose an eruption model that first involves processes operating in the magma chamber whereby a primitive melt is injected into an evolved magma to create a mixed zone at the chamber base. The eruption triggered by this event first emplaced a trachytic dome, into which banded lava from the chamber base was injected. Subsequent endogenous dome growth led to flow down the shallow slope to the east on which the highly viscous (10¹² Pa s) coulée was emplaced. The flow likely moved extremely slowly, being emplaced over a period of 4–10 years in a glacial manner, where a thick (>60-m) plug slid over a thin (5-m-thick) basal shear zone. Excellent exposure means that the Puy de Cliergue complex can be viewed as a case type location for understanding and defining the eruption and emplacement of thick, high-viscosity, silicic lava flow systems.
... Rotorua Caldera, located in the northern part of the TVZ (Fig. 1B), is a large caldera that has produced one of the most widespread ignimbrites in the TVZ; the Mamaku ignimbrite, which covers >3200 km 2 (Wilson et al., 1984;Milner et al., 2002Milner et al., , 2003Gravley et al., 2007;Cole and Spinks, 2009). Although the ignimbrite itself is well documented, the post caldera collapse domes are only mentioned briefly in scientific literature (Dadd, 1992;Wood, 1992;Milner et al., 2002;Cole and Spinks, 2009). At Rotorua caldera, a criss-cross pattern of basement and rift structures defines the margin of the caldera and individual blocks (see Fig. 4) that Milner et al. (2002) suggested to have formed during eruption of the Mamaku ignimbrite. ...
... Published gravity data from Rogan (1982) and Hunt (1992), along with sparse surface faults and features from Hunt (1992), Milner (2001) and Cole et al. (2010), provided evidence for an inferred fault map of Rotorua Caldera using alignment of domes and internal structures at Ngongotaha Dome (this paper). Previous work on Ngongotaha Dome includes theses by Shepherd (1991), Richnow (1999) and Dravitzki (1999) and published work of Dadd (1992). ...
... Intercalated with the mafic flows are felsic lavas, rare felsic pyroclastic rocks and sedimentary rocks. Felsic lavas in the Comerong Volcanics are typically flow foliated and are up to 350 m thick and 18 km long (Dadd 1992b). Sedimentary rocks include a basal, largely monomict breccia with clasts derived from the basement Ordovician metasedimentary rocks, and minor volcaniclastic mudstone, sandstone and conglomerate. ...
... A period of rhyolitic volcanism followed the eruption of the BAM. Cohesive flows characterised the early rhyolite volcanism in most of the study area (Dadd 1992b); however, small pyroclastic flows were emplaced in the south at the base of the volcanic pile. The second mafic magma type to erupt was the DRBM. ...
The Comerong Volcanics are a Middle to Late Devonian bimodal sequence located in the southeastern Lachlan Orogen of NSW, Australia. Magmatism related to subduction was ongoing and located to the east of the continent during this period placing the Comerong Volcanics in a continental back arc setting. Mafic rocks in the Comerong Volcanics occur in three stratigraphically distinct units and range in composition from tholeiitic andesite to basalt. An inverse relationship between volume of erupted lava and degree of fractionation of the magma is evident from the base of the volcanic complex to its top. The lowermost mafic unit is the least voluminous and most fractionated and consists of flows with low Mg#, Ni and Cr and high incompatible trace element abundances. The overlying unit comprises both moderately and extremely fractionated basalt and is characterised by flows with a relatively high TiO2 content. Lavas in this unit can be divided into higher and lower-Ti types. The uppermost unit is the most voluminous and the least fractionated with relatively high MgO, Ni and Cr and low incompatible trace element abundances. Compositions are similar to the low-Ti lavas in the unit below. Trace element characteristics of all lavas suggest they were derived from a heterogeneously enriched source in the subcontinental lithospheric mantle. Enrichment of the source in LILE and depletion in Ti and Nb likely reflect earlier subduction in the orogen. The mafic rocks were erupted in a continental within-plate setting; a setting consistent with the field relationships.
... Similar textures have been observed in the Pleistocene Ngongotaha Rhyolite, New Zealand, in which "lenticulite breccia" is interlayered with variably devitrified, continuously flow-banded rhyolite (Dadd, 1992b). The elongate lenticular lenses consist of obsidian and "range from massive with sparse spherulites and perlitic cracking to clasts 'veined' with devitrified rhyolite and having a wispy texture" (Dadd, 1992b, p.44). ...
... The lenses are strongly aligned and resemble fiamme (Fig. 4). Dadd (1992b) concluded that the fiamme textures were defined by patchy spherulitic and/or devitrified domains versus obsidian domains in otherwise coherent lava. We have observed that the phenocrysts are locally broken and not continuous past clast margins. ...
Fiamme are aligned, “flame-like” lenses found in welded ignimbrite. Fiamme also occur in welded pyroclastic fall deposits, secondary welded pumice-rich facies, diagenetically altered and compacted non-welded, pumiceous, volcaniclastic facies and lavas. Fiamme can be formed in a variety of ways. The common genetic use of the term fiamme for pumice clasts that have undergone welding compaction is too narrow. “Fiamme“ is best used as a descriptive term for elongate lenses or domains of the same mineralogy, texture and composition, which define a pre-tectonic foliation, and are separated by domains of different mineralogy, texture or composition. This descriptive term can be used regardless of the origin of the texture, and remains appropriate for flattened pumice clasts in welded ignimbrite.
... A porphyritic rhyolite with approximately 20% plagioclase and minor altered ferromagnesian minerals is folded intimately with the phenocryst-poor rhyolite in the central portion of the Flow foliation in outcrop is de¢ned by wispy parting planes and a texture developed by the fragmentation of the £ow foliation to produce false ¢amme (e.g. Dadd, 1992). The foliation is folded into complex commonly isoclinal folds. ...
... Units D3¡1 and D4¡1 are likely to have been moderately viscous as they form thick units of limited lateral extent and have silica contents of approximately 76% and 72% respectively (Table 1). Felsic magmas such as these tend to be viscous, unless they are unusually hot, peralkaline (Dadd, 1992) or under high con¢ning pressure. ...
Silurian mafic lava flows and rhyolite sills in the Passamaquoddy Bay area, southeastern Canada, interacted with wet sediment to produce peperitic breccia. The pahoehoe-type mafic flows flowed over and bulldozed into wet, unconsolidated silt and sand to produce both blocky and fluidal peperite at their lower contacts. Soft-sediment deformation structures in the siltstone adjacent to the flows, irregularly shaped vesicles in the siltstone, and siltstone-filled vesicles in the mafic lava all indicate the sediment was wet at the time of flow emplacement. Sills of rhyolite emplaced into the same sedimentary sequence also interacted with the wet sediment and produced peperite at both the upper and lower contacts. Evidence for the rhyolitic magma interacting with wet sediment includes sediment-filled vesicles in rhyolite, quartz-filled vesicles in siltstone and angular, non-vesicular shards of rhyolite in clastic dykes. Peperite formed by this process is exclusively blocky. The rhyolitic magma in the Passamaquoddy Bay sequence and other examples of high-viscosity magma producing only blocky peperite suggest that in contrast to low-viscosity magmas, which produce a combination of blocky and fluidal peperite, only blocky peperite will form associated with highly viscous rhyolite magma. The sediment that interacted with both the mafic and rhyolitic lavas is of similar texture indicating that, in this case, sediment texture did not control peperite type. The change from fluidal to blocky peperite texture may also be controlled in part by the magma temperature and/or confining pressure, and the generation and maintenance of a steam film at the magma/wet sediment interface.
... The massive to brecciated felsic unit displays the salient attributes of a coherent¯ow in which autobrecciation processes were prevalent and produced breccia during¯ow advance (Bonnichsen and Kauffman, 1987). The jostled angular clasts displaying a jig-saw puzzle ®t that overlie the massive portion of the¯ow, are indicative of an in-situ¯owtop breccia (Dadd, 1992), which commonly develops laterally and vertically from massive parts of lobatē ows or domes (Bonnichsen and Kauffman, 1987). The abundance of quartz and feldspar phenocrysts is typical of a high viscosity¯ow with extrusion below liquidus, as suggested by the limited 500 m lateral extent and breccia abundance. ...
... Viscous felsic lava commonly forms short, thick, stubby¯ows or domes and¯ow length rarely exceeds 1±2 km (Williams and McBirney, 1979). The¯ow unit compares favorably to subaerial¯ow features (Bonnichsen and Kauffman, 1987;Dadd, 1992). The thin, massive to brecciated felsic unit may be the distal segment of a¯ow from a small dome (e.g. ...
The 200-m thick, volcano-sedimentary Raquette Lake Formation, located in the south-central Archean Slave Province, represents a remnant arc segment floored by continental crust. The formation overlies the gneissic Sleepy Dragon Complex unconformably, is laterally interstratified with subaqueous mafic basalts of the Cameron River volcanic belt, and is considered the proximal equivalent of the turbidite-dominated Burwash Formation. A continuum of events associated with volcanism and sedimentation, and controlled by extensional tectonics, is advocated. A complex stratigraphy with three volcanic and three sedimentary lithofacies constitute the volcano-sedimentary succession. The volcanic lithofacies include: (1) a mafic volcanic lithofacies composed of subaqueous pillow–pillow breccia, and subaerial massive to blocky flows, (2) a felsic volcanic lithofacies representing felsic flows that were deposited in a subaerial environment, and (3) a felsic volcanic sandstone lithofacies interpreted as shallow-water, wave- and storm-reworked pyroclastic debris derived from explosive eruptions. The sedimentary lithofacies are represented by: (1) a conglomerate–sandstone lithofacies consistent with unconfined debris flow, hyperconcentrated flood flow and talus scree deposits, as well as minor high-energy stream flow conglomerates that formed coalescing, steep-sloped, coarse-clastic fan deltas, (2) a sandstone lithofacies, interpreted as hyperconcentrated flood flow deposits that accumulated at the subaerial–subaqueous interface, and (3) a mudstone lithofacies consistent with suspension sedimentation in a small restricted lagoon-type setting. The Raquette Lake Formation is interpreted as a fringing continental arc that displays both high-energy clastic sedimentation and contemporaneous effusive and explosive mafic and felsic volcanism. Modern analogues that develop along active plate margins in which continental crust plays a significant role include Japan and the Baja California peninsula.
... Phase 2c massive to blocky dacite flows and thick massive to lobate rhyolitic flows are typical viscous flow products (Kano et al., 1991;Dadd, 1992;Manley, 1992). Fragmental debris associated with the rhyolitic lobes is generated by autoclastic and thermal granulation processes, and is considered hyaloclastite and carapace breccias (Cas and Wright, 1987;McPhie et al., 1993), or lobe-hyaloclastite flow deposits (Gibson et al., 1997). ...
... The NVC is similar to shield volcanoes in terms of dominant effusive volcanism, whereas the andesitic base, a large volume of felsic rocks and a large central cauldron zone are consistent with composite volcanoes. High temperature of emission, elevated heat retention capability, high magma discharge rates and low magmatic water content as well as deep-water setting could explain the dominant effusive style of the NVC felsic rocks (Cas and Wright, 1987;Dadd, 1992;Manley, 1992Manley, , 1996. Despite the fact that Mt. ...
The 4 km-thick Archean Normétal volcanic complex (NVC), composed of basaltic andesite, dacite, and rhyolite, is represented by five distinct volcanic phases and one sedimentary phase. Initial volcanic construction features effusive mafic volcanism characterized by massive, pillowed and pillow breccia flows and local massive dacite (phases 1 and 2a). Prominent felsic volcanism of phase 2 commences locally with tuffs, lapilli tuffs and lapilli tuff breccias derived either from hydroclastic or autoclastic fragmentation processes (phase 2b). The principal constructive phase of the NVC (phase 2c) is composed of pillowed andesite, massive dacite, and dominant massive, flow banded and lobate rhyolite flows. Autoclastic or hydroclastic brecciation of the former have produced rhyolitic tuff, lapilli tuff and lapilli tuff breccia. Rhyolitic volcanism continued with eruption of lava flows (phase 3) and the intrusion of dykes and felsic endogenous domes (phases 3 and 4). A subsequent 20–70 m-thick sedimentary unit, composed of volcaniclastic turbidites and pelagic background sediments, constitutes a marker horizon indicating volcanic quiescence. Renewed volcanism of phase 5 is characterized by mafic to felsic turbiditic lapilli tuffs and tuffs, and mafic to felsic flows or intrusions. The felsic lapilli tuffs, tuffs and flows host the Normétal VMS deposit. The geometry and volcanic stratigraphy of the NVC suggests emission of viscous, phenocryst-rich felsic flows from three principal centers, including a parasitic western vent, the major central 6 km-wide cauldron structure and an eastern vent. Voluminous viscous felsic lava over a large area supports the inference of numerous vents whereby individual centers coalesced to produce a composite or complex stratovolcano. Proximal to distal facies changes, variable rhyolitic unit and lobe closures argue for multiple conduits. The VMS deposits are located at the western edge of the central cauldron. Geochemical analyses show two complete compositional spectrums (phases 1, 2, 4 and 5) from basaltic andesite to rhyolite. The Zr/Y and LaN/YbN ratios of phases 1, 2, 4 and 5 show a transitional affinity whereas phase 3 is tholeiitic to slightly transitional. Multi-element diagrams suggest that all phases are consistent with subduction-related processes. The mafic-felsic NVC, a composite volcano that formed upon a shield type volcano, displays subaqueous effusive dominant volcanic construction at depth below storm wave base, as indicated by pillowed flows, turbiditic and pelagic sedimentary rocks, and massive sulphide deposits. Geochemistry and physical volcanology of the NVC are consistent with construction of an immature arc volcano. The submerged Izu-Bonin arc volcanoes may be modern analogues.
... In formerly contiguous Australia, many silicic Paleozoic plutonic and volcanic units contemporaneous with New Zealand rocks are recognised, representing upper crustal exposures. Late Devonian-Carboniferous volcanic units in Australia span a wide compositional rangefrom basalts to rhyolites of I-, A-and S-type affinityand a wide geographical spread from the New England Fold Belt in the north to Tasmania in the south (Bryan et al. 2004;Dadd 1992). The central Victorian magmatic province in southeast Australia is characterised by the extensive development of Late Devonian-Early Carboniferous silicic volcanics and intrusives that are considered correlatives of New Zealand's Karamea Suite (Clemens and Wall 1984;Tulloch et al. 2009;Turnbull et al., 2016). ...
Two felsic volcanic clasts from conglomerates in Fiordland and northwest Nelson provide U–Pb zircon ages of 359 and c. 362 Ma respectively; ages that occur within the interval between voluminous Western Province plutonic S-type magmatism of the Karamea and Ridge suites. Both volcanic clasts have inherited zircon populations, and trace element signatures suggesting derivation from crustal melts. The dacitic clast from the mid-Cretaceous Seek Cove Formation in southwest Fiordland has whole-rock chemistry and an O–Hf–zircon isotopic signature consistent with derivation from the Takaka Terrane. A rhyolite clast from the Pupu Conglomerate in northwest Nelson also has chemistry consistent with derivation from a Takaka Terrane source. We therefore correlate both volcanic clasts to the S-type Ridge Suite. No comparable S-type plutonic source of the same age has been recognised in the northwest Nelson section of the Takaka Terrane. These volcanic clasts represent the oldest recognised episode of S-type Ridge Suite magmatism in the Takaka Terrane.
... That is, vertical fractures develop through the horizontal shear planes, to cut the layer into blocks. Basal breccias can be tens of meters thick (Manley and Fink 1987) and welded (Sparks et al. 1993) and comprise angular, closely packed, matrix-supported clasts of lava and alien (loose bedrock) material rolled up into the lava-derived breccia (Dadd 1992). Grinding and crushing of the basal breccia by the weight and sliding action of the overlying lava can contribute to a finegrained component of the breccia mixture. ...
Lava flows occur worldwide, and throughout history, various cultures (and geologists) have described flows based on their surface textures. As a result, surface morphology-based nomenclature schemes have been proposed in most languages to aid in the classification and distinction of lava surface types. One of the first to be published was likely the nine-class, Italian-language description-based classification proposed by Mario Gemmellaro in 1858. By far, the most commonly used terms to describe lava surfaces today are not descriptive but, instead, are merely words, specifically the Hawaiian words ‘a‘ā (rough brecciated basalt lava) and pāhoehoe (smooth glassy basalt lava), plus block lava (thick brecciated lavas that are typically more silicic than basalt). ‘A‘ā and pāhoehoe were introduced into the Western geological vocabulary by American geologists working in Hawai‘i during the 1800s. They and other nineteenth century geologists proposed formal lava-type classification schemes for scientific use, and most of them used the Hawaiian words. In 1933, Ruy Finch added the third lava type, block lava, to the classification scheme, with the tripartite system being formalized in 1953 by Gordon Macdonald. More recently, particularly since the 1980s and based largely on studies of lava flow interiors, a number of sub-types and transitional forms of all three major lava types have been defined. This paper reviews the early history of the development of the pāhoehoe, ‘a‘ā, and block lava-naming system and presents a new descriptive classification so as to break out the three parental lava types into their many morphological sub-types.
... When this substrate is soft, unconsolidated sediment, the effects of loading and heat can significantly modify the local stratigraphic architecture, either by local compaction and induration (Christiansen and Lipman 1966;Henry et al. 1990) or sediment fluidisation and peperite formation (Kokelaar 1982). Exposure of the contact between rhyolite lava flows and underlying sediment is rare, notably because rhyolite lava domes tend to form as opposed to lava flows (Branney et al. 2008) and can be poorly exposed either due to insufficient erosion or talus cover (Dadd 1992;Manley 1996). A road-cut exposure on Route 95 (N 43°24′ 26.1″, W 116°51′ 54.4″) through the Jump Creek Rhyolite (JCR), a Editorial responsibility: P-S Ross SR-type rhyolite lava flow, in the Owyhee Mountains, SW Idaho, provides a valuable opportunity to study the contact between a voluminous rhyolite lava flow and the underlying sediments, allowing for the construction of a detailed 2-D cross-section through the lava pile and the volcaniclastic sediment substrate, the Sucker Creek Formation (SCF) (Bonnichsen et al. 2004;Bonnichsen and Godchaux 2006). ...
In the Northern Owyhee Mountains (SW Idaho), a >200-m-thick flow of the Miocene Jump Creek Rhyolite was erupted on to a sequence of tuffs, lapilli tuffs, breccias and lacustrine siltstones of the Sucker Creek Formation. The rhyolite lava flowed over steep palaeotopography, resulting in the forceful emplacement of lava into poorly consolidated sediments. The lava invaded this sequence, liquefying and mobilising the sediment, propagating sediment subvertically in large metre-scale fluidal diapirs and sediment injectites. The heat and the overlying pressure of the thick Jump Creek Rhyolite extensively liquefied and mobilised the sediment resulting in the homogenization of the Sucker Creek Formation units, and the formation of metre-scale loading structures (simple and pendulous load casts, detached pseudonodules). Density contrasts between the semi-molten rhyolite and liquefied sediment produced highly fluidal Rayleigh-Taylor structures. Local fluidisation formed peperite at the margins of the lava and elutriation structures in the disrupted sediment. The result is a 30–40-m zone beneath the rhyolite lava of extremely deformed stratigraphy. Brittle failure and folding is recorded in more consolidated sediments, indicating a differential response to loading due to the consolidation state of the sediments. The lava-sediment interaction is interpreted as being a function of (1) the poorly consolidated nature of the sediments, (2) the thickness and heat retention of the rhyolite lava, (3) the density contrast between the lava and the sediment and (4) the forceful emplacement of the lava. This study demonstrates how large lava bodies have the potential to extensively disrupt sediments and form significant lateral and vertical discontinuities that complicate volcanic facies architecture.
... Lo cally, there is a no tice able flow fold ing. More over, due to shear ing (Dadd, 1992), stretch ing and brecciation of flow-banded vol ca nic rocks are spo rad i cally ob served. The vol ca nic rocks are not al tered. ...
The Penguin Island volcano is located on the southern shelf of King George Island (South Shetland Islands, West Antarctica). Its activity is regarded as connected with the opening of the Bransfield Strait. Penguin Island is dominated by a 180 m high basaltic stratocone (Deacon Peak) with a 350 mwide crater containing a small basaltic plug inside and radial dykes, and it has asecond principal vent-the Petrel Crater maar - that was formed during a phreatomagmatic eruption about 100 years ago. A low-potassium, calc-alkaline sequence of basaltic lava flows with intercalations of beach deposits (Marr Point Formation) forms the basement of the stratocone. The Marr Point Formation lava flows have never been dated before. Combined whole rock 40Ar- 39Ar isotopic dating and magneto stratigraphy were applied for this purpose. We obtained an isotopic 40Ar- 39Ar plateau age of 2.7 ±0.2 Ma, and together with the palaeomagnetic data, middle Pliocene age (Piacenzian) is implied for the basaltic plateau of Penguin Island.
... que se movían como una unidad. Este mecanismo sería homólogo al responsable de la formación de lentículas de brecha producidas en lavas riolíticas en los sectores foliados del flujo, producidos por cizalla (Dadd 1992). Estimación de temperatura y viscosidad: La deformación de los cristaloclastos en el flujo reomórfico se produce a una temperatura por encima de la temperatura de transición del vidrio (T g ) en condiciones de baja presión (presión de carga dada por la columna de material suprayacente). ...
The petrographic analysis of the vitrophyre that makes up the basal layer of the Las Lajas rhyolite ignimbrite lead to the identification of textures that indicate a flow of the set of phenocrysts-matrix as a response to the load pressure and to a ground slope (~20°). The majority of the phenocrysts, feldspar, quartz, and biotite are subhedral and anhedral crystals which have fragmented prematurely during the magma vesiculation during eruption. After the "sedimentation" of the phenocrysts and the molten material that formed the matrix, the majority of the phenocrysts fractured once again, possibly due to the thermal contraction and expansion and, in some cases, self-collision. The pending laminar flow below the set of molten crystals in the base of the unit generates a shear strain that displaces the fragments of the brittle broken crystals. At the same time, the biotite breaks along the cleavage planes and has a ductile deformation due to shocks with other solid crystals and by adapting to the matrix's deformation flow direction. The deformation due to the vitrophyre was produced at a temperature above 730 °C (glass transition temperature for calc-alkaline rhyolitic melt) and with a viscosity for the phenocrysts-matrix below the 8.64 E+12 PS s. This fragmentation mechanism due to flow would be the equivalent to the one produced in rhyolitic lava and lava domes and is similar to that produced in the submagmatic flow, although with a higher strain rate that involves cataclasis of crystals.
... There are many studies of lava dome growth (e.g., Williams, 1932;Christiansen and Lipman, 1966;Cole, 1970;Huppert 1982;Murase et al., 1985;Fink and Manley, 1987;Swanson et al., 1987;Duffi eld and Dalrymple, 1990;Swanson and Holcomb, 1990;Anderson and Fink, 1990;Dadd, 1992;Miller, 1994;Nakada et al., 1995;Fink and Bridges, 1995). They mostly concern the geometrical and/or mechanical aspects, as well as the rheological properties, of erupted magmas (Huppert, 1982;Blake 1990). ...
Simple experiments were carried out to study the fracturing of the outer crust of lava domes during emplacement. Analog magma was injected vertically from a reservoir into a feeder conduit and flows on a rigid planar base. A cohesive mixture of sand and flour poured on the dome simulated the lava dome crust generated by cooling. Results showed that two opposite end members have to be considered: symmetrical versus asymmetrical deformation for thin and thick brittle shells, respectively. Thin crusts produce gently dipping slopes with mainly radial fractures. In contrast, thick crusts generate steeply dipping flank slopes on which deformation is restricted. Between the end members, there is a general change from one style to the other. For natural domes, the experiments indicate that a certain thickness of crust is necessary to produce explosive activity: thick crust will cause more violent events.
... It varies from moderate , with microlites orientation mainly around the phenocrysts corners, to well-developed, forming flow layering . Flow layering (Dadd 1992) occurs locally in a rhyolite dome in the northern portion of the area (seeFig. 5), and consists of a structure with planar to stretched and folded layers (Fig. 2c ) and predominantly subvertical orientation. ...
The Iricoumé Group correspond to the most expressive Paleoproterozoic volcanism in the Guyana Shield, Amazonian craton. The volcanics are coeval with Mapuera granitoids, and belong to the Uatumã magmatism. They have U-Pb ages around 1880 Ma, and geochemical signatures of A-type magmas. Iricoumé volcanics consist of porphyritic trachyte to rhyolite, associated to crystal-rich ignimbrites and co-ignimbritic fall tuffs and surges. The amount and morphology of phenocrysts can be useful to distinguish lava (flow and dome) from hypabyssal units. The morphology of ignimbrite crystals allows the distinction between effusive units and ignimbrite, when pyroclasts are obliterated. Co-ignimbritic tuffs are massive, and some show stratifications that suggest deposition by current traction flow. Zircon and apatite saturation temperatures vary from 799 • C to 980 • C, are in agreement with most temperatures of A-type melts and can be interpreted as minimum liquidus temperature. The viscosities estimation for rhyolitic and trachytic compositions yield values close to experimentally determined melts, and show a typical exponential decay with water addition. The emplacement of Iricoumé volcanics and part of Mapuera granitoids was controlled by ring-faults in an intracratonic environment. A genesis related to the caldera complex setting can be assumed for the Iricoumé-Mapuera volcano-plutonic association in the Pitinga Mining District.
... que se movían como una unidad. Este mecanismo sería homólogo al responsable de la formación de lentículas de brecha producidas en lavas riolíticas en los sectores foliados del flujo, producidos por cizalla (Dadd 1992). Estimación de temperatura y viscosidad: La deformación de los cristaloclastos en el flujo reomórfico se produce a una temperatura por encima de la temperatura de transición del vidrio (T g ) en condiciones de baja presión (presión de carga dada por la columna de material suprayacente). ...
El análisis petrográfico del vitrófiro basal de la ignimbrita riolítica Las Lajas, permitió detectar texturas que indican flujo del conjunto cristaloclastos-matriz como respuesta a la presión de carga e inclinación del substrato. La mayoría de los cristaloclastos (feldespato, cuarzo y biotita), son trozos subhedrales a anhedrales de cristales fragmentados prematuramente durante la vesiculación del magma en el momento de la erupción. Con posterioridad, la mayoría de los cristaloclastos se fractura nuevamente, posiblemente por contracción y expansión térmica diferencial. El flujo laminar pendiente abajo del conjunto cristales-fundido en la base de la unidad, produjo deformación por cizalla que generó planos de flujo en la matriz, desplazó los fragmentos de los cristales fracturados frágilmente y en algunos casos produjo rotura de cristales por choques mutuos. En el caso de la biotita, además de romperse por los planos de clivaje, se deformó de manera dúctil por choques con otros cristales rígidos o adaptándose a los planos de flujo de la matriz. La deformación por flujo del vitrófiro se habría producido a una temperatura por encima de 730°C (temperatura de transición del vidrio para fundidos riolíticos calco-alcalinos) y con una viscosidad para el conjunto cristaloclastos-matriz por debajo de 8,64 E+12 Pa s. Este mecanismo de fragmentación por flujo sería homólogo al que se produce en lavas riolíticas y similar al producido en el flujo submagmático aunque con una tasa de deformación más alta que involucra cataclasis de cristales.
... It varies from moderate, with microlites orientation mainly around the phenocrysts corners, to well-developed, forming flow layering. Flow layering (Dadd 1992) occurs locally in a rhyolite dome in the northern portion of the area (see Fig. 5), and consists of a structure with planar to stretched and folded layers (Fig. 2c) and predominantly subvertical orientation. This fabric is characterized by the interbedding of thin layers with different crystallization degrees, since primary vitric to fine phaneritic (Fig. 2d). ...
The Iricoumé Group correspond to the most expressive Paleoproterozoic volcanism in the Guyana Shield, Amazonian craton. The volcanics are coeval with Mapuera granitoids, and belong to the Uatumã magmatism. They have U-Pb ages around 1880 Ma, and geochemical signatures of A-type magmas. Iricoumé volcanics consist of porphyritic
trachyte to rhyolite, associated to crystal-rich ignimbrites and co-ignimbritic fall tuffs and surges. The amount and morphology of phenocrysts can be useful to distinguish lava (flow and dome) from hypabyssal units. The morphology of ignimbrite crystals allows the distinction between effusive units and ignimbrite, when pyroclasts are obliterated.
Co-ignimbritic tuffs are massive, and some show stratifications that suggest deposition by current traction flow. Zircon and apatite saturation temperatures vary from 799◦C to 980◦C, are in agreement with most temperatures of A-type melts and can be interpreted as minimum liquidus temperature. The viscosities estimation for rhyolitic and trachytic compositions yield values close to experimentally determined melts, and show a typical exponential decay with water addition. The emplacement of Iricoumé volcanics and part of Mapuera granitoids was controlled by ring-faults in an intracratonic environment. A genesis related to the caldera complex setting can be assumed for the Iricoumé-Mapuera volcano-plutonic association in the Pitinga Mining District.
... Stimac et al. 1996; Yurtmen and Rowbotham 1999) and is closely associated with K-feldspar within spherulites in devitrified obsidian domes and rhyolitic lavas (e.g. Swanson et al. 1989; Dadd 1992). The cristobalite stability field is between 1,470 and 1,713 °C at <1 GPa (Deer et al. 1996); hence, it exists as a metastable phase in dome-forming volcanic systems, where temperatures are typically ≤850 °C. ...
Cristobalite is commonly found in the dome lava of silicic volcanoes but is not a primary magmatic phase; its presence indicates that the composition and micro-structure of dome lavas evolve during, and after, emplacement. Nine temporally and mineralogically diverse dome samples from the Soufrière Hills volcano (SHV), Montserrat, are analysed to provide the first detailed assessment of the nature and mode of cristobalite formation in a volcanic dome. The dome rocks contain up to 11 wt.% cristobalite, as defined by X-ray diffraction. Prismatic and platy forms of cristobalite, identified by scanning electron microscopy (SEM), are commonly found in pores and fractures, suggesting that they have precipitated from a vapour phase. Feathery crystallites and micro-crystals of cristobalite and quartz associated with volcanic glass, identified using SEM-Raman, are interpreted to have formed by varying amounts of devitrification. We discuss mechanisms of silica transport and cristobalite formation, and their implications for petrological interpretations and dome stability. We conclude: (1) that silica may be transported in the vapour phase locally, or from one part of the magmatic system to another; (2) that the potential for transport of silica into the dome should not be neglected in petrological and geochemical studies because the addition of non-magmatic phases may affect whole rock composition; and (3) that the extent of cristobalite mineralisation in the dome at SHV is sufficient to reduce porosity—hence, permeability—and may impact on the mechanical strength of the dome rock, thereby potentially affecting dome stability.
... Studies of dome growth are abundant (e.g. Williams 1932;Christiansen and Lipman 1966;Cole 1970;Huppert et al. 1982;Murase et al. 1985;Fink and Manley 1987;Swanson et al. 1987;Anderson and Fink 1990;Duffield and Dalrymple 1990;Swanson and Holcomb 1990;Dadd 1992;Miller 1994;Fink and Bridges 1995;Nakada et al. 1995). These mostly deal with the mechanical aspects of lava dome emplacement. ...
Simple experiments have been conducted to study the strain evolution in lava dome cross sections. A viscous fluid is injected vertically from a reservoir into a feeding conduit. Silicone putty is used as analogue magma. Two-dimensional experiments allow the assessment of the internal strain within the dome. Particle paths are symmetrical on either side of a central line passing through the feeding conduit and display parabolic trajectories. The highest strain zone is located above the extrusion zone. In cross sections, stretch trajectories show a remarkable concentric pattern, wrapping around the extrusion zone of the analogue magma. To the lateral margins, a triple junction of stretch trajectories defines an isotropic point in the strain field. In the main central part of the dome, an intermediate zone of reversed sense of shearing is caused by a change in the sign of the velocity gradient with respect to that in the upper and lower zones. Knowledge of this evolving strain pattern can provide a better understanding of the evolution of natural domes. Also, it can help to unravel the kinematic history of ancient domes partly removed by erosion.
... Some flows are large, with one 350 m thick and possibly 18 km long. The large volume of individual flows is attributed to their high temperature and high F/H,0 which resulted in relatively low viscosity (Dadd, 1992). ...
The Middle to Late Devonian Yalwal Volcanics, Comerong Volcanics, Boyd Volcanic Complex and associated gabbroic and A-type granitic plutons form part of a continental volcano-tectonic belt, the Eden-Comerong-Yalwal Volcanic Zone (EVZ), located parallel to the coast of southeastern Australia. The EVZ is characterised by an elongate outcrop pattern, bimodal basalt-rhyolite volcanism, and a paucity of sedimentary rocks. Volcanic centres were located along the length of the volcanic zone at positions indicated by subvolcanic plutons, dykes, rhyolite lavas and other proximal vent indicators including surge bedforms in tuff rings, and hydrothermal alteration. Previous interpretations that suggested the volcanic zone was a fault bounded rift are rejected in favour of a volcano-tectonic belt. The Yellowstone-Snake River Plain region (Y-SRP) in the USA is an appropriate analogue. Both regions have basalt lavas which range in composition from olivine tholeiite to ferrobasalt, alkalic rhyolitic rocks enriched in Y, Zr and Th, large rhyolite lava flows, plains-type basalt lava flows, and a paucity of sedimentary rocks. The Y-SRP is inferred to have developed by migration of the American plate over a fixed hot spot leading to a northeast temporal progression of the focus of volcanic activity. Application of a similar hot spot model to the EVZ (using a length of 300 km and a time range for volcanic activity of 5–10 Ma), suggests that during the Middle to Late Devonian the Australian plate was moving at a rate of between 3 and 6 cm/yr relative to the hot spot and that the northern extent of the volcanic zone at any time was a topographically high region with rhyolitic activity, similar to present day Yellowstone. As the focus of activity moved northward, the high region subsided and the depression was flooded by basalt. The EVZ was much wider (up to 70 km) and much longer than the belt defined by present-day outcrop and was of comparable scale to the Y-SRP. The main difference between the two volcanic belts is the lack of large pyroclastic flows and identifiable caldera complexes in the EVZ.
... The conditions of temperature, pressure and strain rate under which such a transition occurs has been studied experimentally in sedimentary and metamorphic rocks (e.g., Paterson, 1958; Heard, 1960 ). Field evidence gests a similar transition occurs during simultaneous cooling and flow of volcanic rocks; for example, Dadd (1992) suggested that lenticular breccias in the basal zone of rhyolite lavas of the Comerong Volcanics in southeastern Australia and the Ngongotaha lava dome, New Zealand may have formed from banded lavas by stretching and mechanical brecciation within the basal zone due to high shear strains. A transition from ductile to brittle processes is indicated by structures such as folds truncated on one or both limbs as have been described in rhyolite flows (Christiansen and Lipman, 1966). ...
Structural and textural evidence indicates that the basal breccia of the Tertiary Minyon Falls Rhyolite of Tweed Volcano, eastern Australia, formed mainly by fragmentation within the basal shear zone of the lava flow. Cooling at the flow base led to lava behaviour passing through the ductile-brittle transition while subjected to progressive deformation. The intact lavas of the basal zone record intense ductile shearing in multiple folds, rotated phenocrysts, strong alignment of crystallites and micro-folding of these crystal alignments. Minor brittle structures, including faults on fold limbs and microfaults, occur in the intact lava immediately above the basal breccia. These structures represent incipient brecciation and complete brecciation has occurred by intense faulting of previously intact lava. Brittle deformation migrated upward into overlying intact lava as cooling occurred. This process is distinctly different from overriding and incorporation of surface breccias which is commonly invoked as the primary process of basal breccia formation.
... It varies from moderate, with microlites orientation mainly around the phenocrysts corners, to well-developed, forming flow layering. Flow layering (Dadd 1992) occurs locally in a rhyolite dome in the northern portion of the area (see Fig. 5), and consists of a structure with planar to stretched and folded layers (Fig. 2c) and predominantly subvertical orientation. This fabric is characterized by the interbedding of thin layers with different crystallization degrees, since primary vitric to fine phaneritic (Fig. 2d). ...
The Iricoumé Group correspond to the most expressive Paleoproterozoic volcanism in the Guyana Shield, Amazonian craton. The volcanics are coeval with Mapuera granitoids, and belong to the Uatumã magmatism. They have U-Pb ages around 1880 Ma, and geochemical signatures of α-type magmas. Iricoumé volcanics consist of porphyritic trachyte to rhyolite, associated to crystal-rich ignimbrites and co-ignimbritic fall tuffs and surges. The amount and morphology of phenocrysts can be useful to distinguish lava (flow and dome) from hypabyssal units. The morphology of ignimbrite crystals allows the distinction between effusive units and ignimbrite, when pyroclasts are obliterated. Co-ignimbritic tuffs are massive, and some show stratifications that suggest deposition by current traction flow. Zircon and apatite saturation temperatures vary from 799°C to 980°C, are in agreement with most temperatures of α-type melts and can be interpreted as minimum liquidus temperature. The viscosities estimation for rhyolitic and trachytic compositions yield values close to experimentally determined melts, and show a typical exponential decay with water addition. The emplacement of Iricoumé volcanics and part of Mapuera granitoids was controlled by ring-faults in an intracratonic environment. A genesis related to the caldera complex setting can be assumed for the Iricoumé-Mapuera volcano-plutonic association in the Pitinga Mining District.
Libro recomendado para proyectos de investigación en los campos de Estratigrafía, Sedimentología y Minería. Presentado por el Dr. Leandro Echavarria (Colorado School of Mines) 2008
Two sparsely plagioclase-phyric rhyolite domes, 50–100 m thick and 125–250 m long as observed in 2-dimensional, vertically dipping exposures, are partly mantled by a rhyolite volcaniclastic complex that, on the south side of the domes, also forms a half-cone-like mound at least 60 m thick and 500 m long. The domes and mantling volcaniclastic complex are overlain by three aphyric to sparsely phyric rhyolite flows that are 70–120 m thick and 550–>1200 m long. Domes and flows have a lower columnar-jointed, highly fractured, locally brecciated subfacies that grades upward into 40–95% crackled and disaggregated breccia. Crackled breccia is a highly fractured subfacies in which clast-like areas are bounded by closely spaced joints and minor matrix, but there is only limited rotation of clasts. Crackled breccia was produced by quench fracturing combined with hydraulic action of water converted to steam or supercritical fluid within the fractures. Overlying disaggregated breccia comprises rotated particles and 25–50% matrix. It is most abundant near flow margins and is a crumble breccia produced by flow advance or expansion. The volcaniclastic mound comprises lenticular, interlayered facies of (1) resedimented, phreatomagmatically generated, heterolithic, rhyolitic tuff and lapilli-tuff, (2) isolated to close-packed rhyolite lobes that are 0.5–5 m thick and 0.5–>50 m long; the lobes are, at least in part, pillows, and (3) units comprising small (
A Miocene submarine lava dome at Tate-iwa, Atsumi, Yamagata, Japan, displays well-preserved primary morphological features. The dome currently rises above a wave-cut coastal platform and is 55 m high and 90–180 m across. It comprises a massive core and a lava lobe–hyaloclastite rim, both of which are composed of compositionally uniform, feldspar–phyric dacite (SiO2=64 wt.%). The boundary between these two zones is distinct but gradational. The massive core consists of homogeneous, coherent dacite and is characterized by flow banding along the margin and by columnar joints radiating from the centre to the margin. The lava lobe–hyaloclastite rim encircles the massive core and consists of a complex of 80–90% dacitic lava lobes and 10–20% hyaloclastite. The lava lobes are 1–6 m thick, 3–12 m wide and more than 5 m long. Each lobe consists of a radially columnar-jointed core and a glassy rim 10–30 cm thick. The hyaloclastite comprises polyhedral dacite clasts 5–100 cm across in a matrix of dacite fragments up to 5 mm across. The lava lobes and hyaloclastite have gradational contacts in places. Paleobathymetric studies based on foraminifera in sediment beneath the dome, and the K–Ar age of the dome, suggest that it was extruded in a middle-bathyal environment (1500–2000 m below sea level) at 12.9±0.6 Ma. The internal structures of the dome suggest that it formed by a combination of exogenous growth involving extrusions of small dacite lobes, and endogenous growth involving a continuous magma supply and simple expansion from the interior. The presence of small lava lobes along the rim suggests that the magma had relatively low viscosity at the time of extrusion, in spite of its high silica content. Magma temperatures calculated by two-pyroxene and Fe–Ti oxide geothermometers were 999–1042 and 957–1005 °C, respectively. The inferred low viscosity may be attributed to the high temperature of the magma and/or a high confining pressure resulting from the deep-sea environment.
The 52-m-thick rhyolitic sill at Onedin, in northwestern Australia, intruded wet, unconsolidated sediment of the Koongie Park Formation during the Palaeoproterozoic. The sill is composed of five main zones. A thin (1–2 m thick) basal peperitic contact zone with sparse large (
The Somers Ignimbrite Formation, part of an Upper Cretaceous calc-alkaline volcanic association dated at 89 ± 2 Ma, forms the summit area of Mt. Somers, mid-Canterbury, New Zealand. It is the eroded remnant of a succession of 12 high-grade to extremely high-grade ignimbrite sheets each of which are designated as members. Each member comprises numerous 'sub-units' (< 100 mm-12 m thick) which exhibit variable degrees of welding and may represent discrete flow-units. On a finer scale, compositional layering (1-30 mm) occurs with alternations of darker and lighter coloured poikilomosaic and felsitic bands which are commonly folded due to rheomorphism. Broken phenocrysts are common, particularly in the felsitic bands. Clastic dikes and clasts of tuff are common in Member 1 and at the base of Member 2, and fine grained tuff layers, interpreted as co-ignimbrite deposits, occur at the top of some sub-units within these two members. Welding occurred during (rather than following) initial flow emplacement while rheomorphism continued during late stages of emplacement and autobrecciation into the post-depositional phase, largely in response to local variation in underlying topography. Vertical chemical zonation within the Somers Ignimbrite Formation suggests progressive extraction from a compositionally zoned magma chamber.
Three widespread felsic volcanic units, the Eucarro Rhyolite, Pondanna Dacite Member and Moonaree Dacite Member, have been distinguished in the Mesoproterozoic Gawler Range Volcanics. These three units are the largest in the Gawler Range Volcanics, each in excess of 500 km3. Each unit is ∼300 m thick and includes a black, formerly glassy base, a granophyric columnar-jointed interior, and an amygdaloidal outer margin. The units are very gently dipping and locally separated by thin (<20 m) lenses of either ignimbrite (Mt Double Ignimbrite), tuffaceous sandstone or faults. The youngest unit, the Moonaree Dacite Member, covers a central area with a diameter greater than 80 km. The southern two units have east–west extents in the order of 180 km, but are much less extensive from south to north (5–60 km). All three units are dominated by euhedral phenocrysts and are relatively crystal rich. Both the Eucarro Rhyolite and Moonaree Dacite Member contain clasts of basement granitoid and other lithologies and are locally heterogeneous in texture and composition. Some granitoid clasts have disintegrated, liberating feldspar and quartz crystals into the surrounding host. These liberated crystals cause textural variations, but can be identified on the basis of shape (amoeboid or skeletal) and/or size (megacrysts). Textural and lithofacies characteristics are consistent with the interpretation that these units are lavas; the strongly elongate distribution and wide extent of the Eucarro Rhyolite and Pondanna Dacite Member could indicate that vents were aligned along an extensive east–west-trending fissure system. Stratigraphic nomenclature has been revised to better reflect the presence of the three emplacement units. The oldest unit, the Eucarro Rhyolite, is dominated by plagioclase-phyric rhyolite that locally includes granitoid clasts and megacrysts. Along the northern margin, the rhyolite is amygdaloidal and has mingled with a quartz-rich rhyolite (Paney Rhyolite Member). The Eucarro Rhyolite and Paney Rhyolite Member replace the formerly defined ‘Eucarro Dacite’, ‘Nonning Rhyodacite’, ‘Yannabie Rhyodacite’ and ‘Paney Rhyolite’. The two younger units, Pondanna Dacite Member and Moonaree Dacite Member, are compositionally and spatially distinct, newly defined members of the Yardea Dacite.
Very thick units of massive pumice and lithic clast-rich breccia in the Early Permian Berserker beds at Mount Chalmers, Queensland, are deposits from cold, water-supported, volcaniclastic mass flows emplaced in a below-wave base submarine setting. Adjacent to syn-volcanic andesitic and rhyolitic sills and dykes, the pumice-lithic breccia shows a well-developed eutaxitic texture. The eutaxitic foliation is parallel to intrusive contacts and extends as far as a few metres away from the contact. At these sites, pumice clasts are strongly flattened and tube vesicles within the pumice clasts are compacted and aligned parallel to the direction of flattening. Some lenticular pumice clasts contain small (2 mm), round, quartz-filled amygdales and spherulites. Further away from the sills and dykes, the pumice clasts have randomly oriented, delicate tube vesicle structure and are blocky or lensoid in shape. Round amygdales were generated by re-vesiculation of the glass and the spherulites indicate devitrification of the glass at relatively high temperatures. The eutaxitic texture is therefore attributed to re-heating and welding compaction of glassy pumice-lithic breccia close to contacts with intrusions. In cases involving sills, secondary welding along the contacts formed extensive, conformable, eutaxitic zones in the pumice-lithic breccia that could be mistaken for primary welding compaction in a hot, primary pyroclastic deposit.
Momo-iwa, Rebun Island, Hokkaido, Japan, is a dacite cryptodome 200–300 m across and 190 m high. The dome is inferred to have intruded wet, poorly consolidated sediment in a shallow marine environment. The internal structure of the dome is concentric, with a massive core, banded rim, and narrow brecciated border, all of which are composed of compositionally uniform feldspar-phyric dacite. Boundaries between each of the zones are distinct but gradational. The massive core consists of homogeneous coherent (unfractured) dacite and is characterized by radial columnar joints 60–200 cm across. The banded rim encircles the massive core and is 40 m wide. It is characterized by large-scale flow banding parallel to the dome surface. The flow banding comprises alternating partly crystalline and more glassy bands 80–150 cm thick. The outermost brecciated border is up to 80 cm thick, and consists of in situ breccia and blocky peperite. The in situ breccia comprises polyhedral dacite clasts 5–20 cm across and a cogenetic granular matrix. The blocky peperite consists of polyhedral dacite clasts 0.5–2 cm across separated by the host sediment (mudstone). The internal structures of the dome suggest endogenous growth involving a continuous magma supply during a single intrusive phase and simple expansion from the interior. Although much larger, the internal structures of Momo-iwa closely resemble those of lobes in subaqueous felsic lobe-hyaloclastite lavas.
Ignimbrite ‘TL’ on Gran Canaria is a complex, compositionally zoned rheomorphic tuff, that locally exhibits features previously considered to be diagnostic of lavas. It is made up of two locally overlapping lobes of ignimbrite that were emplaced during a single eruptive episode. The eastern lobe is high-grade, with rheomorphic zones and localised patches that are lava-like. The western lobe is extremely high-grade, more extensively lava-like, and welded to its top surface. Both parts are zoned, with a basal comendite-rich zone grading up, through a mixed zone, into an upper trachyte-rich zone. Lithic contents, and the relative proportions of comendite and trachyte pyroclasts vary with height. Each comendite-rich zone is vitroclastic, whereas each trachyte-rich zone is partly lava-like with local gradations into vitroclastic ignimbrite. Mixed zones are intermediate in character, and locally show compositional banding. Gradational zoning in massive ignimbrite, best seen in lower strain zones, and welding fabrics that are pervasively lineated and oblique to bedding, suggest that deposition was sustained, agglutination was rapid, and rheomorphic deformation began during the sustained deposition. The viscosity and porosity of the agglutinate varied with height because successively deposited pyroclast populations varied in grainsize, composition and temperature. The hot agglutinate continued to compact and shear downslope after the density currents had dissipated, causing further rheomorphic folding, thrusting, attenuation and autobrecciation. The western lobe locally overlies the partly welded top of the eastern lobe, in part because it advanced rheomorphically across it for at least 300 m. Hot-state loading and auto-intrusion occurred due to unstable density layering in the chemically zoned agglutinate. Deformation behaviour changed during cooling and degassing, and because of heat transfer between juxtaposed agglutinates, and localised retention of dissolved volatiles where there was an overlying impermeable cap.
Although rhyolitic lavas and lava domes are characterised by evenly porphyritic textures, not all the phenocrysts are whole euhedra. We undertook image analysis of 46 rhyolitic lava and lava dome samples to determine the abundance and shape of quartz and feldspar phenocryst fragments. Phenocryst fragments were identified in nearly all samples. On average, fragments amount to ∼5% of the total phenocryst population, or ∼0.5 modal%. The abundance of fragments in lavas and lava domes is not related to the groundmass texture (whether vesicular, flow banded, massive, glassy or crystalline), nor to distance from source. Fragments are, however, more abundant in samples with higher phenocryst contents. The phenocryst fragments in rhyolitic lavas and lava domes are mainly medium to large (0.5–3.5 mm), almost euhedral crystals with only a small portion removed, or chunky, equant, subhedral fragments, and occur in near-jigsaw-fit or clast-rotated pairs or groups. The fragments probably formed in response to decompression of large melt inclusions. Shear during laminar flow then dismembered the phenocrysts; continued laminar shear separated and rotated the fragments. Fractures probably formed preferentially along weaknesses in the phenocrysts, such as zones of melt inclusions, cleavage planes and twin composition planes. Rare splintery fragments are also present, especially within devitrified domains. Splinters are attributed to comminution of solid lava adjacent to fractures that were later healed. For comparison, we measured crystal abundance in a further 12 rhyolite samples that include block and ash flow deposits and ignimbrite. Phenocryst fragments within clasts in the block and ash flow samples showed similar shapes and abundances to those fragments within the lava and lava domes. Crystal fragments are much more abundant in ignimbrite (exceeding 67% of the crystal population) however, and dominated by small, equant, anhedral chunks or splinters. The larger crystals in the ignimbrite are subrounded. The phenocrysts within ignimbrite pumice lapilli are also more intensely fractured than those in lavas and lava domes. Thus, in deformed and altered volcanic successions, data on crystal fragment abundance and shape can help discriminate lavas from pyroclastic facies.
A partly eroded, subaerial, dacite cryptodome at Showa-Shinzan, Usu volcano, Hokkaido, Japan, displays its internal structures, and provides an excellent opportunity to study contact relationship between cryptodome and overlying sediment. The margin of the cryptodome comprises two facies: inner coherent dacite and outer dacite breccia. The coherent dacite facies is ~5 m in the exposed section, and consists of homogeneous or weakly flow-banded, feldspar-phyric dacite. The dacite breccia facies envelope the coherent dacite facies, and is 4-5 m thick. The breccia is monomictic, non-stratified and consists of angular dacite clasts up to 15 cm across in a cogenetic matrix. The overlying sediment directly covers the dacite breccia facies, and comprises reddish-brown, fluvial deposit. The dacite breccia formed by fracturing of coherent dacite in response to cooling contraction, and shearing of the fractured dacite due to movement of the growing cryptodome.
Several silicic units of the Trans-Pecos volcanic field have outcrop and thin-section scale features of lava flows but areal extents and aspect ratios of ignimbrites. These voluminous rocks (up to hundreds of cubic kilometres per unit) are quartz trachytes to low-silica rhyolites (68% to 72%SiO2). Lava flow features include flow banding and folding, elongated vesicles, and autobreccias and vitrophyres at the base and top of units. Pyroclastic flow features include sheetlike geometry, lateral extents up to 70 km, aspect ratios as low as 1:700, and areal extents up to 3000 km2. A few of these units are clearly rheomorphic ignimbrites, but others show no unambiguous evidence of a primary pyroclastic origin. Although no adequate explanation currently exists for the origin of the latter, we evaluate two end-member hypotheses: (1) they are ignimbrites in which extreme rheomorphism has obliterated primary internal features, and (2) they are highly viscous lavas with unusually high heat retention or effusion rates that allowed them to spread over great areas. Either origin requires a rock type and eruptive mechanism not commonly recognized.
Describes and interprets the textural units and their distribution using an implacement model for rhyolitic obsidian flows. Maps and photographs of the surface structure are explained by a deformational model that takes account of the mechanical properties of the different textural units. Finally, the structure of domes is used to interpret local and tectonic stress patterns at the time of extrusion.-from Author
In the Lachlan Fold Belt of southeastern Australia, Upper Devonian A-type granite suites were emplaced after the Lower Devonian
I-type granites of the Bega Batholith. Individual plutons of two A-type suites are homogeneous and the granites are characterized
by late interstitial annite. Chemically they are distinguished from I-type granites with similar SiO2 contents of the Bega Batholith, by higher abundances of large highly charged cations such as Nb, Ga, Y, and the REE and lower
Al, Mg and Ca: high Ga/Al is diagnostic. These A-type suites are metaluminous, but peralkaline and peraluminous A-type granites
also occur in Australia and elsewhere.
Partial melting of felsic granulite is the preferred genetic model. This source rock is the residue remaining in the lower
crust after production of a previous granite. High temperature, vapour-absent melting of the granulitic source generates a
low viscosity, relatively anhydrous melt containing F and possibly Cl. The framework structure of this melt is considerably
distorted by the presence of these dissolved halides allowing the large highly charged cations to form stable high co-ordination
structures. The high concentration of Zr and probably other elements such as the REE in peralkaline or near peralkaline A-type
melts is a result of the counter ion effect where excess alkali cations stabilize structures in the melt such as alkali-zircono-silicates.
The melt structure determines the trace element composition of the granite.
Separation of a fluid phase from an A-type magma results in destabilization of co-ordination complexes and in the formation
of rare-metal deposits commonly associated with fluorite. At this stage the role of Cl in metal transport is considered more
important than F.
New analyses of 131 samples of A-type (alkaline or anorogenic) granites substantiate previously recognized chemical features, namely high SiO2, Na2O+K2O, Fe/Mg, Ga/Al, Zr, Nb, Ga, Y and Ce, and low CaO and Sr. Good discrimination can be obtained between A-type granites and most orogenic granites (M-, I and S-types) on plots employing Ga/Al, various major element ratios and Y, Ce, Nb and Zr. These discrimination diagrams are thought to be relatively insensitive to moderate degrees of alteration. A-type granites generally do not exhibit evidence of being strongly differentiated, and within individual suites can show a transition from strongly alkaline varieties toward subalkaline compositions. Highly fractionated, felsic I- and S-type granites can have Ga/Al ratios and some major and trace element values which overlap those of typical A-type granites.A-type granites probably result mainly from partial melting of F and/or Cl enriched dry, granulitic residue remaining in the lower crust after extraction of an orogenic granite. Such melts are only moderately and locally modified by metasomatism or crystal fractionation. A-type melts occurred world-wide throughout geological time in a variety of tectonic settings and do not necessarily indicate an anorogenic or rifting environment.
Peralkaline welded tuffs from the islands of Gran Canaria, Canary Islands, and Pantelleria, Italy, show abundant evidence for post-depositional flow. It is demonstrated that rheomorphism, or secondary mass flowage, can occur in welded tuffs of ignimbrite and air-fall origin. The presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process. Deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding. Internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets. It is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes, provided that the apparent viscosity of the magma is sufficiently low. Compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type.
Petrographic and whole-rock analytical data suggest that obsidian flows (now represented by perlitic pitchstone), rhyolitic breccias, air-fall tuffs and flow-banded rhyolites which crop out in the vicinity of the Nxwala Perlite Mine were derived from a single felsic magma body. Reconstruction of the geological history of these rocks indicates that the Nxwala pyroclastics and lavas represent products of a plinian eruption associated with a period of rhyolitic dome building and dome collapse.-Authors
The Providencia rhyodacite lava flow of southern Baja California is an unusually extensive salic extrusion. Remnants of the flow overlie lower to middle Miocene volcanic rocks and occur in a 27-km-long belt near the city of La Paz. Isopachs of the flow show a maximum thickness of 120 m and indicate a minimum volume of 8.6 km3. Persistent flow bands are closely spaced and parallel the base of the flow. These flow bands are thin, planar lithophysal cavities that give the rock a distinct parting. In the upper part of the flow the banding is strongly deformed into isoclinal to open folds. Flow directions, developed from fold axial information, together with the isopach data, suggest that the rhyodacite flowed at least 23 km north-northwest from its source south of La Paz. The Providencia rhyodacite (68-72.5% Si02,3.8% Na20, and 4.5% K20) contains about 5% phenocrysts (plag > opx > Fe-Ti oxides) set in a devitrified groundmass of fine-grained alkali feldspar and tridymite ?). Lithophysal planar cavities are lined with large (as long as 3 mm) vapor-phase crystals whose paragenetic relationships define a crystallization order from oldest to youngest: (1) fayalite + thick laths of brown hornblende, (2) a-quartz, (3) hematite, and (4) tridymite + apatite + rare biotite + rare fibrous green hornblende. Field evidence suggests, but does not prove, that the Providencia rhyodacite is a primary lava flow rather than a remobilized pyroclastic flow. A high volatile content together with a high eruption temperature acted in concert to maintain a low viscosity, a fact that probably facilitated flow of the lava to great distances.
Surface mapping and microscopic observation of textures in glassy and pumiceous rocks from several groups of silicic lava flows and domes, along with drill cores of the Inyo Scientific Drilling Project, indicate that development of the textural stratigraphy of the flows is controlled by a combination of cooling, microfracturing, and migration of gases released by crystallization. The Inyo cores have provided near-vent and distal views of the interior of the 550-yr-old Obsidian Dome rhyolite flow, as well as a profile through the unerupted portion of its feeder dike. The flow stratigraphy revealed in the drill core and in the fronts of several other Holocene-age silicic flows consists of a finely vesicular pumice carapace underlain successively by obsidian, coarsely vesicular pumice, obsidian with lithophysae, crystalline rhyolite, more obsidian with lithophysae, and basal breccia.
The obsidian layers form where rapid cooling inhibits diffusion of ions and prevents crystallization. The transition from surface pumice to obsidian is controlled by the depth at which overburden pressure suppresses vesiculation. The thickness of the rigid crust is determined by the rapid decrease in the lava’s temperature-dependent yield strength with depth. The coarsely vesicular pumice layer forms as gases released by crystallization rise through microcracks and are trapped beneath the rigid pumiceous surface layer. Thickness and buoyancy of the coarsely vesicular pumice layer increase with flow length, eventually giving rise to diapirs that rise to the surface of the largest flows. Increasing gas content of the coarsely vesicular pumice layer in active flows can also lead to such volcanic hazards as explosive craters on distal flow surfaces or pyroclastic flows triggered by collapse of flow fronts.
The Wall Mountain Tuff was deposited about 36 m.y. ago in paleovalleys extending from west of Salida to beyond Castle Rock, Colorado, a distance of at least 140 km. The Gribbles Run paleovalley, 16 km northeast of Salida, is exceptionally well exposed by modern dissection and reveals a complex interplay of primary and secondary structures formed during deposition of the Wall Mountain Tuff. Deposition occurred in laminar boundary layers between the bottom and sides of the channel and ash flows passing above. The overlapping sequence of events during deposition was (1) agglutination and incipient collapse of glassy particles, (2) laminar shearing of the compacting and welding mass to form a primary foliation analogous to flow banding in lavas, (3) expulsion of gases from the collapsing spongy mass and concentration of these gases along shear planes, (4) formation of gas pockets in places where the volume of gases expelled exceeded that which could be accommodated on shear planes, (5) elongation of gas pockets and pumice to form a primary lineation in the plane of the foliation, (6) statistical alignment of the long axes of solid particles parallel to the direction of flow and imbrication of the long axes so that they dip sourceward relative to the foliation planes, (7) development of primary flow folds with axes perpendicular to the lineation, and (8) end of forward motion. By the time forward motion ceased within a given layer, the tuff had the rheological properties of a rhyolite lava. The high viscosity of the welded tuff preserved open cavities and prevented differential compaction over lithic fragments or primary folds.
More rapid deposition along the sides of the paleovalley than along its axis caused inward accretion of welded tuff with steep primary flow foliation to form a U-shaped cross-channel profile. Secondary folds, whose axes parallel the lineation, and concurrent growth faults formed locally by creep toward the valley axis. Spectacular internal unconformities developed where undeformed tuff was deposited over primary or secondary folds, yet all the tuff welded together to form a remarkably uniform simple cooling unit. Episodic downstream movements of a few metres, as the Wall Mountain Tuff adjusted to its bed and to its rising center of gravity, opened swarms of tension cracks along certain horizons. These tension cracks dip steeply in the downstream direction and provide a useful indicator of flow direction. Other structures useful in determining flow direction are imbricated crystals, streamlined ridges and grooves in lineated gas cavities, and upstream dip of axial planes on asymmetric primary folds.
Emplacement temperature is the dominant factor in determining whether a tuff undergoes primary or secondary welding. If the temperature is well above the softening point, the glassy particles will agglutinate and collapse during deposition in the laminar boundary layer, and the tuff will show megascopic laminar flow structures (primary welding). If the temperature is below the softening point, deposition occurs as loose ash, and welding is a postemplacement process (secondary welding). Most tuffs undergo secondary welding and only the preferred orientation of solid particles and the grain-size distribution remain as evidence of laminar flow during deposition. The type of welding may vary both laterally and vertically within some ash-flow sheets and impose a significant facies variation.
The metaluminous Watergums granite, from a bimodal association of A-type granite/rhyolite with basalt in SE Australia, has been experimentally studied so that constraints can be placed on its origin. Petrological, geochemical and experimental data support an origin by direct, high-T partial melting of a melt-depleted I-type source rock in the lower crust.-J.A.Z.
The effect of fluorine on melt viscosities of five compositions in the system Na2O-Al2O3-
SiO2h as been investigateda t one atmospherea nd 1000-1600'Cb y concentric-cylinder
viscometry. The compositions chosen were albite, jadeite and nepheline on the join
NaAlOlSiO2 and two others of the join at 75 mole percent SiO2, one peralkaline and one
peraluminous. All melt viscosities were independent of shear rate over two orders of
magnitude, indicating Newtonian behavior. All viscosity-temperature relationships were
Arrhenian within error. Fluorine reduces the viscosities and activation energies of all melts
investigated. The viscosity-reducing power of fluorine increases with the SiO2 content of
melts on the join NaAlO2-SiO2 and is a maximum at Na/Al (molar) = I for melts containing
75 mole percent SiO2. Fluorine and water have similar effects on aluminosilicate melt
viscosities, probably due to depolymerization of these melts by replacement of Si-O-(Si,
Al) bridges with Si-OH and Si-F bonds, respectively. Evidence from slag systems shows
that fluorine also reduces the viscosity of depolymerized silicate melts. The viscous flow of
phonolites, trachytes and rhyolites will be strongly afected by fluorine. It appears that
fluorine contents of igneous rocks may be combined with water in calculation schemes for
determining the viscosity of natural melts.
Large, Miocene-age rhyolite lava flows occur in the Bruneau-Jarbidge area of the central Snake River Plain and in the adjoining Jacks Creek area. The flows typically are 100-150 m thick and have volumes ranging from 10 to 200 km3 . The flow interiors consist of thick central zones of massive devitrifled rhyolite overlying zones of basal vitrophyre. These massive central zones are capped by structurally complex upper zones with both glassy and devitrifled rhyolite. The upper zones contain gas cavities of varying dimensions and abundance, including swarms of cavities, each a meter or more across. Sheeting joints, in some places accompanied by pencil and dimple joints, are abundant in the upper zones, in the top part of the central zones, and in the marginal parts of the flows. Flow margins consist of bulbous lobes of massive rhyolite separated by steeply to chaotically jointed zones. Few flow margins are less than 25 m thick. The basal and upper zones and the marginal parts of the flows contain abundant breccia formed by en masse flowage and explosive steam release. All of the rhyolite flows are believed to have erupted from fissures. Most flowed onto preexisting soils and other sedimentary materials. Small amounts of air-fall ash occur beneath a few flows near their eruptive fissures, but these deposits are thinner and less widespread than the fallout ash blankets that are beneath many of the southwestern Idaho welded-tuff units. A combination of high effusion rates, high temperatures, and large volumes probably imparted sufficiently low bulk viscosities to the lavas to allow them to flow away from their eruptive fissures to form sheets instead of steep-sided domes. Several large-volume, high-temperature, densely welded, ash-flow-tuff sheets occur in southwestern Idaho. These pyroclastic flows may have coalesced into pools of silicate liquid capable of en masse flowage after emplacement. They are similar in appearance to the rhyolite lava flows in that region. However, a combination of physical characteristics can be used to distinguish the two types of flows. Good indicators that a given rhyolite sheet may be a lava flow-rather than a unit emplaced as ash hot enough to form a liquid pool and flow-are the presence of blunt flow margins, abundant basal and marginal flow breccias, pervasive flow layering, laterally persistent zones of mismatched vertical shrinkage joints, complex contacts between basal vitrophyres and overlying zones of devitrifled rhyolite, and abundant zones with pencil jointing, combined with the absence of lithic fragments, pumice fragments, bubble-wall shards, extensive phenocryst breakage, internal subhorizontal ash-emplacement layering, and subparallel flow marks.
Most phases of silicic lava dome growth have some associated explosive activity. Tephra produced during this activity have depositional characteristics, grain sizes, and grain shapes that reflect different mechanisms of dome growth and destruction. It is therefore possible to interpret the explosive history of a dome through study of adjacent tephra deposits even though the dome may no longer be present. Five stages of dome growth and their associated tephra deposits are considered here. (1) Crater formation before extrusion of a dome, including phreatic, phreatomag- matic (ph-m), and Plinian pumice eruptions, produces a tephra sequence at the base of a dome consisting of deposits rich in accidental lithic clasts from crater walls, overlain by beds of fine-grained tephra and coarse-grained pumice. (2) Magma pulses during dome growth (ph-m, in part) produce tephra consisting of mixhues of juvenile pumice and clasts derived from the partly solidified dome. (3) Ph-m interaction between new magma and a water-saturated dome produces uniform tephra consisting of angular clasts of dome lava. (4) Explosive eruptions that fonow collapse of a gravitationaMy unstable dome produce tephra that consists of angular, partly pumiceous clasts of dome lava which fragment due to expansion of metastable water ah release of contining pressure. (5) Posteruptive destruction of the dome by phreatic eruptions results in pyroclasts consisting of he-grained, hydrothermally altered clasts derived from dome lavas. Major kinetic processes before explosive dome eruptions are the relatively slow diffusion of magmatic volatiles from magma to hcture planes and foliations within the dome, and the relatively fast diffusion of meteoric water into magma by mechanical mixing. These basic processes control most explosive activity at domes in cases of either expulsion of new magma or collapse of an unstable dome.
Snowflake texture consists of more or less randomly oriented laths of alkali feldspar poikilitically enclosed by small patches of optically continuous silica, commonly alpha-quartz. The texture results from devitrification of glassy, densely welded tuff but not, apparently, from devitrification of glassy lava-flow rocks. During devitrification, alkali feldspar crystallizes first, followed by silica; iron oxide minerals become concentrated around the peripheries of devitrified regions.
At least $100 km.^{3}$ of trachytic and peralkaline soda rhyolitic ash flows were erupted during late Miocene to early Pliocene time from a source near the center of Gran Canaria, Canary Islands. Their deposits cover more than $350 km.^{2}$ and are typical of ash-flow deposits elsewhere in that they show vertical changes in color, degree of crystallization, and denseness of welding. They underwent unusually intense vapor-phase crystallization, chiefly to aegirine-augite, Na-amphibole(?), and alkali feldspar. Several structures unusual for ash-flow deposits typify those of Gran Canaria. The structures are: (1) stretched pumice fragments; (2) broken and rotated pumice fragments; (3) tension cracks in the matrix; (4) hollows around rotated inclusions; (5) folds; (6) imbricated pumice fragments; (7) ramp structures. The structures are found throughout the $350-km.^{2}$ area, and those with directional significance show a uniform trend away from the source of the ash flows. The structures indicate that the ash...
Using values of viscosity and its temperature dependence, density, heat capacity and thermal conductivity appropriate to rhyolitic liquids an analysis of the macroscopic heat balances of these liquids flowing under constant stress has been carried out. The analysis shows that for shear couples and shear stresses expected in the flow of silicic lava on the earth's surface thermal feedback may occur after time periods ranging from a few seconds to several days and may cause rapid increases in temperature. For silicic magmas flowing in a dike or circular conduit under a constant pressure gradient these thermal instabilities may occur after a few days to several weeks for dike half thicknesses or conduit radii ranging from a few centimeters to a few meters.For silicic lava flows this phenomenon may be important in producing commonly observed flow structures such as pumiceous banding and color banding. Local temperature increases would have the effect of reducing local water solubility, increasing diffusion rates, and increasing nucleation and growth rates of vapor bubbles, thus causing highly vesicular bands that parallel the shear planes. Temperature increases could also cause increases in fO2 within a fluid layer resulting in oxidation and red color banding.For silicic lavas flowing in dikes or conduits, the possible thermal instabilities are accompanied by instabilities in volume rate of flow which may contribute to rapid vesiculation of upward-moving magma and result in explosive volcanism.
Altered silicic volcanic rocks with the appearance of welded and nonwelded pyroclastic flow deposits, volcaniclastic debris flows, and massive- to thin-bedded tuffaceous rocks form a major component of a Silurian succession that hosts Zn-Cu-Pb massive sulfide deposits at Benambra, southeastern Australia. However, critical evaluation of rock textures indicates that these silicic volcanic rocks are mainly lavas and associated autoclastic facies with remarkably deceptive false pyroclastic and volcaniclastic textures. The false textures were produced mainly in originally glassy lava by the combined effects of devitrification, perlitic fracture, and pervasive hydrothermal alteration. These processes are related to the emplacement and cooling history of individual lavas and to a regional, essentially synvolcanic, hydrothermal system. Tectonic foliation of the more altered and mechanically weakened rocks has led to partial dismembering of phenocrysts and groundmass fabrics, thereby further enhancing clastic appearance. Additional study results are discussed.
The foliation in welded tuffs is defined by planar alignment of glass shards, platy crystals, and flattened pumice fragments. The textural features and interrelationships of these elements are clearly the result of the deformation of ash-flow material. The strain involved in developing the alignment can be determined by measuring deformed objects of known original shape. The orientation and shape of the two-dimensional finite strain ellipse, computed from measured bubbles and Y-shaped shards, demonstrates that the long axis parallels the foliation and that there is a close correspondence between the ratio of the principal strains and the bulk density. The measured strain is inhomogeneous on at least two scales. Single cooling units show a regular and continuous vertical variation in deformation: the upper and lower portions are nearly undeformed, whereas the middle portion of the sheet is strongly deformed. On a small scale, the strain varies systematically around rigid lithic fragments and crystals and reaches high ratios at the tops and bottoms of these objects, while pressure shadow zones develop at the sides, which may have complex strain histories. All the lines of evidence point to compaction as the only mechanism involved in the production of the observed characteristic features of the Bishop Tuff, a Pleistocene ash-flow sheet in eastern California. Deformation in the tuff is defined by (1 + e 1) = 1.0. During flow and the earliest stages of compaction, pumice lapilli behave as rigid bodies. At about 50 percent porosity, the pumice collapses with the matrix, but at a more rapid rate, and the final forms are flattened in the plane of the foliation. Final shape ratios may reach 25 by simple volume loss; further flattening by essentially volume constant deformation may account for the relatively high mean ratios in fully compacted tuff, and for large ratios reported by others. A comparison of these results with selected specimens of other tuff units and published data strongly suggests that compaction is the dominant mechanism in producing the strong parallel alignment of textural components in all these welded tuffs. In cases where late or postcompactional deformation has also taken place, its effects are superimposed on the earlier compactional features; even so, the results appear to be more closely related to processes in the compactional stage. Extensive flow at or near final tuff densities does not explain most of these features.
Experimental devitrification of natural rhyolitic glass was undertaken in an attempt to produce, under known conditions, analogs of naturally devitrified rocks. The results should facilitate the recognition and interpretation of devitrification textures. Previous studies have shown that alkali-rich aqueous solutions increase devitrification rates sufficiently to produce solid pieces of devitrified obsidian in the laboratory, but none of these has provided a systematic study of experimentally produced devitrification textures. Cylinders of natural volcanic glass, 4.6 mm in diameter and 10 to 20 mm long, were sealed in 5-mm-diameter gold capsules, along with either pure water or alkali-rich solutions. Runs were made in externally heated pressure vessels in the temperature range 240° to 700°C and in the pressure range 0.5 to 4 kb. Textures recognized in the products of the runs closely resembled hydration and devitrification texture s in some rhyolitic rocks. Although hydration fronts and strain birefringence were developed during some experimental runs, no perlitic fractures were observed. Devitrified products contained spherulites, micropoikilitic quartz, orb texture, axiolites, and miarolitic cavities. Two stages of devitrification are distinguished. The glassy stage is characterized by glassy or felsitic textures with isolated spherulites, and the spherulitic stage by spherulitic textures and micropoikilitic quartz. A hypothetical third stage in this succession, not represented in products from this study, most likely has a granophyric or granitic texture, with no evidence of glassy precursors.
Analytical data are presented for the following elements: Cs, Rb, Ba, K, Sr, Ca, Na, Fe, Mg, Cu, Co, Ni, Li, Sc, V, Cr, Ga, Al, Si, La, Y, and Zr. Eight samples were analysed by the spark source method for rare earths, Tl, Pb, Hf, Sn, Nb, Mo, Bi, and In. In addition to data on rhyolitic volcanics, a small number of intermediate volcanics and eugeosynclinal sediments were analysed for comparative purposes.The following features are shown by the trace element data:(a)
The rhyolitic rocks have consistently lower concentrations of most trace and minor elements when compared with recent estimates of average concentrations in granites. None of the criteria for strong fractionation (e.g. low K/Rb, Ba/Rb and K/Cs ratios) are present.
(b)
The data do not indicate any systematic differences between the rhyolitic lavas and ignimbrites, although the very young rhyolitic pumices are consistently more basic in their element concentrations compared to the other rhyolitic analyses.
(c)
The residual glasses (and devitrified matrices) are depleted, relative to the total rock compositions, in Fe, Mg, Ca, Sr, V, Sc, and Al, and enriched in Cs, Rb, K, Ba, and Si. Zr is depleted in the residual glasses separated from rhyolites, but not in the andesitic residual matrices.
(d)
The rare earth fractionation patterns of the rhyolitic and andesitic extrusives are very similar, being intermediate between chondritic and sedimentary patterns i.e., there is no evidence of strong fractionation. The rhyolitic patterns also indicate a slight Eu depletion.
(e)
Comparable trace and minor element behaviour (with the possible exception of Zr) seems to exist through the rhyolite-andesite compositional range. This is supported by the whole rock-residual liquid trends for the various elements studied, which broadly coincide with the observed whole rock trends, both through the rhyolitic-andesitic compositonal range, and within the rhyolitic compositional range.
The data are finally discussed in the light of the possible origin of the rhyolitic magmas. It is believed that the analytical data presented are qualitatively consistent with the recently proposed idea that the magmas are derived by partial fusion of the associated Triassic-Jurassic eugeosynclinal greywacke-argillite sedimentary sequence.
The late Pleistocene caldera complex of the Sierra La Primavera, Jalisco, México, contains well-exposed lava flows and domes, ash-flow tuff, air-fall pumice, and caldera-lake sediments. All eruptive units are high-silica rhyolites, but systematic chemical differences correlate with age and eruptive mode. The caldera-producing unit, the 45-km3 Tala Tuff, is zoned from a mildly peralkaline first-erupted portion enriched in Na, Rb, Cs, Cl, F, Zn, Y, Zr, Hf, Ta, Nb, Sb, HREE, Pb, Th, and U to a metaluminous last-erupted part enriched in K, LREE, Sc, and Ti; Al, Ca, Mg, Mn, Fe, and Eu are constant within analytical errors. The earliest post-caldera lava, the south-central dome, is nearly identical to the last-erupted portion of the Tala Tuff, whereas the slightly younger north-central dome is chemically transitional from the south-central dome to later, moremafic, ring domes. This sequence of ash-flow tuff and domes represents the tapping of progressively deeper levels of a zoned magma chamber 95,000 ± 5,000 years ago. Since that time, the lavas that erupted 75,000, 60,000, and 30,000 years ago have become decreasingly peralkaline and progressively enriched only in Si, Rb, Cs, and possibly U. They represent successive eruption of the uppermost magma in the post-95,000-year magma chamber.
Eruptive units of La Primavera are either aphyric or contain up to 15% phenocrysts of sodic sanidine ≧quartz >ferrohedenbergite >fayalite>ilmenite±titanomagnetite. Whereas major-element compositions of sanidine, clinopyroxene, and fayalite phenocrysts changed only slightly between eruptive groups, concentrations of many trace elements changed by factors of 5 to 10, resulting in crystal/glass partition coefficients that differ by factors of up to 20 between successively erupted units. The extreme variations in partitioning behavior are attributed to small changes in bulk composition of the melt because major-element compositions of the phenocrysts and temperature, pressure, and oxygen fugacity of the magma all remained essentially constant.
Crystal settling and incremental partial melting by themselves appear incapable of producing either the chemical gradients within the Tala Tuff magma chamber or the trends with time in the post-caldera lavas. Transport of trace metals as volatile complexes within the thermal and gravitational gradient in volatilerich but water-undersaturated magma is considered the dominant process responsible for compositional zonation in the Tala Tuff. The evolution of the post-caldera lavas with time is thought to involve the diffusive emigration of trace elements from a relatively dry magma as a decreasing proportion of network modifiers and/or a decreasing concentration of complexing ligands progressively reduced trace-metal-site availability in the silicate melt.
In this contribution, shear zones are treated as zones of inhomogeneous deformation in which strain softening has occurred. The mylonites which form in ductile shear zones are the softened medium. The development of mylonite microstructures and fabrics are discussed from this point of view. Seven possible softening processes are discussed. They are the advent of superplasticity, geometrical softening, continual recrystallization, reaction softening, chemical softening, pore fluid effects and shear heating. Attention is also given to the brittle deformation of hard minerals in a soft ductile matrix. It is concluded that these fracture because of localized stress concentrations and that their microstructures give no information on deformation in the matrix. An oblique shear band foliation may develop in mylonites as shear strains continue to increase and may destroy the earlier-formed mylonite foliation. It is thought that the foliation may indicate hardening of the soft mylonite.
The Middle to Late Devonian Yalwal Volcanics, Comerong Volcanics, Boyd Volcanic Complex and associated gabbroic and A-type granitic plutons form part of a continental volcano-tectonic belt, the Eden-Comerong-Yalwal Volcanic Zone (EVZ), located parallel to the coast of southeastern Australia. The EVZ is characterised by an elongate outcrop pattern, bimodal basalt-rhyolite volcanism, and a paucity of sedimentary rocks. Volcanic centres were located along the length of the volcanic zone at positions indicated by subvolcanic plutons, dykes, rhyolite lavas and other proximal vent indicators including surge bedforms in tuff rings, and hydrothermal alteration. Previous interpretations that suggested the volcanic zone was a fault bounded rift are rejected in favour of a volcano-tectonic belt. The Yellowstone-Snake River Plain region (Y-SRP) in the USA is an appropriate analogue. Both regions have basalt lavas which range in composition from olivine tholeiite to ferrobasalt, alkalic rhyolitic rocks enriched in Y, Zr and Th, large rhyolite lava flows, plains-type basalt lava flows, and a paucity of sedimentary rocks. The Y-SRP is inferred to have developed by migration of the American plate over a fixed hot spot leading to a northeast temporal progression of the focus of volcanic activity. Application of a similar hot spot model to the EVZ (using a length of 300 km and a time range for volcanic activity of 5–10 Ma), suggests that during the Middle to Late Devonian the Australian plate was moving at a rate of between 3 and 6 cm/yr relative to the hot spot and that the northern extent of the volcanic zone at any time was a topographically high region with rhyolitic activity, similar to present day Yellowstone. As the focus of activity moved northward, the high region subsided and the depression was flooded by basalt. The EVZ was much wider (up to 70 km) and much longer than the belt defined by present-day outcrop and was of comparable scale to the Y-SRP. The main difference between the two volcanic belts is the lack of large pyroclastic flows and identifiable caldera complexes in the EVZ.
Recent studies of igneous rocks have taken a refreshing new direction, mainly as the result of greater awareness of the important role played by physical properties of magmas in determining the eruptive behavior and compositional variations of volcanic rocks. This review summarizes the present state of knowledge, along with some of the recent rheological studies having a direct bearing on interpretations of volcanic phenomena and processes of crystallization and differentiation of shallow magmatic intrusions.
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The stratigraphy, volcanic evolution and tectonic setting of the Comerong Volcanics, Australia
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A look at some rhyolite lavas of the Comerong Volcanics
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An extensive, hot, vapour-charged rhyodacite flow
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Geology of the N.S.W. south coast
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A look at some rhyolite lavas of the Comerong Volcanics, southeastern N.S.W., Australia
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Origin of pumiceous and glassy textures in rhyolite flows and domes
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