Silicic Lava Flows Research Articles

Emplacement dynamics of the plumbing system and lava pile of the Paraná Magmatic Province in Morro da Igreja, Santa Catarina, Brazil

The mechanisms of magma ascent, transport and emplacement of the volcanic pile from LIPs are key issues regarding the understanding of the complex construction of volcanic systems and magma transport through the crust. The integrated approach of morphology of the volcanic and subvolcanic bodies, whole rock geochemistry and ASM data provides a robust tool to unravel the flow dynamics of the volcanic bodies and the sequence of the magmatic events. Such approach allows building up a model of construction of the lava pile considering the role played by the plumbing system. This paper investigates the emplacement mechanism of sill and lava flows of the Serra Geral Group (SGG), in the southern part of Mesozoic Paraná-Etendeka Magmatic Province (PEMP) in Brazil. Geologic and geochemical analysis were integrated with anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and rock magnetism experiments on 14 sampling sites in the Morro da Igreja region, top of the lava pile in Santa Catarina. The area is composed of low-Ti basaltic (with predominance of rubbly pahoehoe) and silicic lava flows, both intruded by a tabular sill that reaches a few hundred meters wide. The mafic lava flows are composed of Gramado, Urubici and Esmeralda magma-type basalts and andesite basalts. The silicic lava flows are massive or foliated, classified as Palmas type dacites. The sill has regular columnar joints and is classified as Esmeralda type. Magnetic mineralogy data suggests magnetite or Ti-poor magnetite as the main magnetic mineral for all the igneous rocks, and AARM results show anomalous fabric for the lava flows and normal or intermediate fabric for the sill. Besides the anomalous fabric sites, AMS data provide reliable directional data to infer flow direction for the sill, with initial propagation towards NE, followed by a preferential SE direction of magma flow. For the silicic rocks, the presence of vertical and inclined magnetic foliation suggest a lava dome geometry. The dynamics of the magmatic flow of the sill and lava flows associated with the compositional characteristics of the rocks allows stablishing the stratigraphy of the magmatic events in the area as well as the proposition of an emplacement model for the sequence.

Emplacement dynamics of a crystal-rich, highly viscous trachytic flow of the Sancy stratovolcano, France

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.

An andesitic sill complex in the Southern Permian Basin: volcanogenetic model and stratigraphic implications

Subvolcanic intrusions are highly variable in shape and structure, and occur in nearly all parts of the upper crust, as a result of extensive volcanic activity. Processes of subvolcanics interacting with the host rock are insufficiently understood, as they are rarely exposed. In the southernmost part of the Flechtingen-Altmark Subprovince, (sub)volcanic rocks of the Flechtingen Volcanic Complex (FVC) are exposed in several quarries. It is built up of silicic tuffs, ignimbrites and lava flows, but also of intermediate lavas and extended sill sheets. Additionally, major granitic intrusions exposed by drillings are associated with the FVC. In the Mammendorf quarry, a sill intruded in between lithified turbiditic series of early Carboniferous (Visean–Serphukovian) age at the base, and widely consolidated volcanoclastic deposits of late Carboniferous (late Pennsylvanian) age at the top. Various magma-host rock interactions were found indicating brittle and ductile deformation patterns occurring at the basal contact, and secondary fluidal mixing predominantly occurring at the top contact, most probably caused by fluids accompanying the intruding magma. We present an extended volcanogenetic model for the FVC. Volcanic activity initiated at 302 ± 3 Ma with fallout deposits represented by mostly re-deposited silicic ashfall deposits of the Flechtingen Formation, and cumulated in depositing major ignimbrite series, most likely forming a caldera. Later, the sills intruded at the rheological boundary of the lithified Mississippian turbiditic series and the partly consolidated volcanoclastic series. Finally, major granitic intrusions emplaced in the basement rocks at around 298 ± 4 Ma. The study contributes to clarify stratigraphic constraints of late Carboniferous to early Permian continental deposits and sheds light on stratigraphy of significant late Paleozoic volcanic deposits of the Flechtingen-Altmark Subprovince in the Southern Permian Basin.

Characterization of volcanic structures associated to the silicic magmatism of the Paraná-Etendeka Province, in the Aparados da Serra region, southern Brazil.

The Paraná-Etendeka Magmatic Province is associated with the distensive tectonics that caused the rupture of the Gondwana continent during the Lower Cretaceous and generated an intense volcanism that covers South America and the NW portion of Namibia in Africa. In Brazil, this volcanic sequence is named Serra Geral Group and predominantly consists of basalts and subordinated silicic rocks. The goal of this study is to characterize the geomorphological features observed in the Aparados da Serra region, southern Brazil, and to evaluate the relationship between these structures and the primary silicic volcanic structures. The geomorphological features were first identified using remote sensing and then correlated with flow structures observed in the field, as well as petrographic and geochemical data. AMS data were used to determine magnetic patterns and the direction of magmatic flow of the rocks. Despite the low degree of anisotropy, clear patterns of lineation and foliation were identified in the studied rocks. Our data shows that Units I and II correspond to silicic lava flows linked to effusive fissure eruptions, presenting a dome morphology caused by differential erosion. Unit III rocks may correspond to true volcanic domes, whereas the Unit IV corresponds to the effusive feeder structures.

Bimodal volcanics from Isbil stratovolcano, Dhamar–Radáa volcanic field, Yemen Republic: contribution to stratigraphy, geochemistry, and eruption evolution

The present paper addresses the geochemical characteristics and stratigraphy of the bimodal volcanics, including silicic (felsic) volcanics and basaltic lava flows constituting Isbil stratovolcano. It is located in the eastern part of Dhamar–Rada volcanic field (DRVF), SE, Yemen. The study area has been subjected to tensional tectonic regime involving extensive effusive-extrusive volcanism (in major) that unconformably overlies Paleozoic and Mesozoic strata of structural elements. These volcanic centers are restricted along strike-slip faults and near to a detachment fault that were feeders for cinder cones and silicic caldera. NNW, NW, and E-W are the three major trends of faults recognized and these are related to the progressive rifting of Red Sea and Aden Gulf. Four volcanic sequences have been well identified and recognized, namely, (1) the lower mafic sequence and (2) the middle silicic sequence. This is followed by (3) upper mafic sequence and (4) upper silicic volcanics sequence crosscutting the surrounding rocks. The stratigraphic and structural characteristics of the substrata are responsible for this eruptive complex in Isbil volcano, forming a shift in depositional environment from subaqueous to subaerial regime. On the basis of the geochemical data, the present silicic lava (rhyolite and dacite) has high A/CNK ratios, Ga/Al ratios, and concentrations of high field strength elements (HFSEs) such as Zr and Nb, typical of peraluminous domain giving rise to within plate magmatism. The mineralogical and chemical characteristics of the silicic rocks are best interpreted as hybrid products of crystal fractionation, coupled with crustal assimilation and/or magma mixing derived from heterogeneous source. On the other hand, the basaltic lava flows exhibit alkaline nature, characterized by enrichment in LILE, LREE, and HFS elements and depletion in HREE. The very low Sm/Nd and Rb/Sr ratios may point to lithospheric mantle origin. The thermal and mass exchange between mantle-derived basaltic magma and crustal material above a mantle plume may be controlled the compositional evolution of the Isbil volcano.

The origin and evolution of breakouts in a cooling-limited rhyolite lava flow

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-Cordon 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 multiscale satellite, field, and textural data from Cordon 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 Cordon 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.

Morphology and stratigraphy of Serra Geral silicic lava flows in the northern segment of the Torres Trough, Paraná Igneous Province

The impetus for this research was the enigma regarding the origin of the extensive silicic volcanic units in the Parana-Etendeka Igneous Province. Are they rheoignimbrites, lava flows, or a combination of both? The Sao Joaquim silicic eruptive sequence is comprised of Palmas-Type aphyric dacites and is located in the northern segment of Torres Trough, a dissected mountainous region in southern Brazil. These dacite outcrops form plateaus and remnant hills that are scattered above a basaltic andesitic landscape. The original morphology of the silicic volcanic flows was constrained based on the internal architecture, geometry, and cyclicity of the lithofacies, in conjunction with petrographic and geochemical information. The data suggests the presence of ancient large tabular and lobate silicic lava flows (~100 m thick and ~10–40 km extents). No pyroclastic features were found. Eight interdigitating units were mapped. These large aspect ratios are similar to ones for basaltic flows and those of Snake River-type rhyolites, and they suggest high effusion rates and high temperatures as well as the presence of well-insulated cooled crusts. Lavas would have overflowed from long fissure vents that had created an extensive volcanic silicic surface body composed of conjugated and interdigitated flows above the north segment of the Torres Trough region.

Structures and lithofacies of inferred silicic conduits in the Paraná-Etendeka LIP, southernmost Brazil

Extensive silicic units in the Paraná-Etendeka LIP have been long interpreted as pyroclastic density currents (rheomorphic ignimbrites) derived from the Messum Complex in Namibia. In recent literature, however, they have been characterized as effusive lava flows and domes. In this paper we describe structures and lithofacies related to postulated silicic lava feeder conduits at Mato Perso, São Marcos and Jaquirana-Cambará do Sul areas in southern Brazil. Inferred conduits are at least 15–25m in width and the lithofacies include variably vesicular monomictic welded and non-welded breccias in the margins to poorly vesicular, banded, spherulitic and microfractured vitrophyres in the central parts. Flat-lying coherent vitrophyres and massive obsidian are considered to be the subaerial equivalents of the conduits. Large-scale, regional tectonic structures in southern Brazil include the NE-SW aligned Porto Alegre Suture, Leão and Açotea faults besides the Antas Lineament, a curved tectonic feature accompanying the bed of Antas river. South of the Antas Lineament smaller-scale, NW-SE lineaments limit the exposure areas of the inferred conduits. NE-SW and subordinate NW-SE structures within this smaller-scale lineaments are represented by the main postulated conduit outcrops and are parallel to the dominant sub-vertical banding in the widespread banded vitrophyre lithofacies. Upper lava flows display flat-lying foliation, pipe-like and spherical vesicles and have better developed microlites. Petrographic characteristics of the silicic vitrophyres indicate that crystal-poor magmas underwent distinct cooling paths for each inferred conduit area. The vitrophyre chemical composition is defined by the evolution of trachydacitic/dacitic vitrophyres with 62–65wt% SiO2 to rhyodacite and rhyolite with 66–68wt% SiO2. The more evolved rocks are assigned to the latest intrusive grey vitrophyre outcropping in the center of the conduits. Degassing pathways formed during fragmentation and fracturing episodes within the conduits may have helped to inhibit the explosivity of the eruptions. Based on the documented lithofacies architecture, we attribute the source of the silicic lava flows in the studied localities to tectonic-controlled, local conduits, rather than pyroclastic density currents from distant vent areas.

Basement control of alkalic flood rhyolite magmatism of the Davis Mountains volcanic field, Trans-Pecos Texas, U.S.A.

Voluminous silicic lava flows, erupted ~37.4Ma from widespread centers within the Davis Mountains Volcanic Field (DMVF), covered approximately 10,000km2 with an initial volume as great as 1000km3. Lava flows form three major stratigraphic units: the Star Mountain Rhyolite (minimum 220km3) of the eastern Davis Mountains and adjacent Barilla Mountains, the Crossen Formation (~75km3) of the southern Davis Mountains, and the Bracks Rhyolite (~75km3) of the Rim Rock region west of the Davis Mountains proper. Similar extensive rhyolite lava also occurs in slightly younger units (Adobe Canyon Rhyolite, ~125km3, 37.1Ma), Sheep Pasture Formation (~125km3, ~36Ma) and, less voluminously, in the Paisano central volcano (~36.9Ma) and younger units in the Davis Mountains. Individual lava flows from these units formed fields as extensive as 55km and 300-m-thick.Flood rhyolite lavas of the Davis Mountains are marginally peralkaline quartz trachyte to low-silica rhyolite. Phenocrysts include alkali feldspar, clinopyroxene, FeTi oxides, and apatite, and, rarely, fayalite, as well as zircon in less peralkaline units. Many Star Mountain flows may be assigned to one of four geochemical groupings. Temperatures were moderately high, ranging from ~911 to 860°C in quartz trachyte and low silica rhyolite. We suggest that flood rhyolite magma evolved from trachyte magma by filter pressing processes, and trachyte from mafic magma in deeper seated plutons.The Davis Mountains segment of Trans-Pecos Texas overlies Grenville basement and is separated from the older Southern Granite and Rhyolite Province to the north by the Grenville Front, and from the younger Coahuila terrane to the south by the Ouachita Front. We suggest that basement structure strongly influenced the timing and nature of Trans-Pecos magmatism, probably in varying degrees of impeding the ascent of mantle-derived mafic magmas, which were produced by upwelling of asthenospheric mantle above the foundered Farallon slab. Basalt was able to penetrate Coahuila crust in the Big Bend region. Thicker Grenville crust under the Davis Mountains retarded ascent of mafic magmas, allowing mafic plutons to differentiate into silicic magma that was eventually erupted as flood lava. North of the Grenville Front, magmatism was further delayed and thicker, older crust there may have helped concentrate magmatism under the Davis Mountain region. Only after the onset of Basin and Range faulting was true basalt erupted over much of the Trans Pecos.

Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)

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) coulee 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.

Cenozoic Eruptive Stratigraphy and Structure in Taiz area of Yemen

The present study focuses on the field description of a bimodal volcanic rock centre and fault types present in Taiz area located in the southern part of Yemen. Taiz area serves as one of the key areas to understand the Afro-Arabian bimodal volcanism and the emplacement of Afar plume and the relationship between extension tectonics and magmatism. Taiz area comprises bimodal volcanic rocks encompassing mafic and silicic lava flows and pyroclastic rocks. The lava flows were erupted in five phases – three major basic and two acid phases. Lower basalt sequence (Tb1) is the oldest and main phase of flood basalt volcanism, formed as part of a wide spread volcanism within the Afro-Arabian region. This is followed by lower silicic sequence phase (Tr1) consisting of varied assortment of lithologies such as rhyolite flows with subordinate ignimbrites, welded ash, pyroclastic breccia, and random pumice and obsidian. The middle basalt sequence phase (Tb2) has been formed from the flows fed by fissure-dyke systems and separated from Tr1 by red colored weathering band of saprolitic bole (paleosol). The latest silicic sequence phase (Tr2) with limited exposures in the region, comprises rhyolitic plug domes, rhyolitic lava flows and locally welded and unwelded volcaniclastic deposits. In places, the silicic volcanic rocks have been diagenetically altered to bentonitic clay minerals and zeolites. The youngest phase of volcanic rocks represented by the basaltic rocks in the region (Tb3) is exposed in few outcrops along the southeastern slope of the mapped area and conformably capping the Tr2. It includes the basaltic flows intercalated with mafic conglomerate and tuff layers. The study area has been subjected to tensional tectonic regime throughout much of the Tertiary and extensions led to volcanism, granitic rock intrusions and formation of structural elements such as normal faults and deep joints. NW, NE, and E-W are the three major trends of faults recognized and these are related to the progressive rifting of Red Sea and Gulf of Aden.

Exceptional mobility of an advancing rhyolitic obsidian flow at Cordón Caulle volcano in Chile

The emplacement mechanisms of rhyolitic lava flows are enigmatic and, despite high lava viscosities and low inferred effusion rates, can result in remarkably, laterally extensive (>30 km) flow fields. Here we present the first observations of an active, extensive rhyolitic lava flow field from the 2011-2012 eruption at Cordón Caulle, Chile. We combine high-resolution four-dimensional flow front models, created using automated photo reconstruction techniques, with sequential satellite imagery. Late-stage evolution greatly extended the compound lava flow field, with localized extrusion from stalled, ~35 m-thick flow margins creating >80 breakout lobes. In January 2013, flow front advance continued ~3.6 km from the vent, despite detectable lava supply ceasing 6-8 months earlier. This illustrates how efficient thermal insulation by the lava carapace promotes prolonged within-flow horizontal lava transport, boosting the extent of the flow. The unexpected similarities with compound basaltic lava flow fields point towards a unifying model of lava emplacement.

Shift from lamproite-like to leucititic rocks: Sr–Nd–Pb isotope data from the Monte Cimino volcanic complex vs. the Vico stratovolcano, Central Italy

In the Italian peninsula leucite-free and -bearing ultrapotassic rocks occur intimately associated but well separated in time. Silica-saturated ultrapotassic and associated shoshonitic magmas erupted during the Pliocene to Lower Pleistocene. Silica under-saturated leucite-bearing ultrapotassic rocks, mainly leucitites, were emplaced sometime later, during the Middle–Upper Pleistocene. The transition from leucite-free to -bearing rocks is diachronous and in some cases is complicated by the occurrence of crustal-derived magmas coeval with early leucite-free magma.The Monte Cimino–Vico area is a key locality for studying the transition from leucite-free to -bearing ultrapotassic rocks. There a volcanic complex is overlain by some 500ka younger leucite-bearing rocks of Vico stratovolcano. Most of the potassic and ultrapotassic rocks of the Monte Cimino volcanic complex are characterised by high Mg-# (66.4–77.8), and compatible element abundances in spite of their high silica contents (52.9–58.0wt.%). Ultrapotassic leucite-free rocks transitional to lamproites are olivine-bearing orthopyroxene- and plagioclase-free latites and K-rich basaltic trachyandesite and they are confined in the final mafic activity of the Monte Cimino volcanic complex. Two-pyroxene parageneses are observed in the early Monte Cimino volcanic complex giving equilibration temperature as high as 1050°C and suggesting a sub-crustal derivation of their parental magmas. A two-pyroxene high-K calc-alkaline magma is then inferred for the genesis of the early Monte Cimino activity (e.g. dome complex and silicic lava flows and ignimbrites), which was modelled through crystal-fractionation from a high-K basaltic andesitic magma plus mixing with crustal anatectic rhyolitic magma.Final mafic lavas fill the isotopic and chemical gap between shoshonite-like and lamproite-like rocks, and each lava flow could be considered as a mantle-derived term from a heterogeneous sub-continental lithospheric mantle. Final mafic lavas are high-silica and -MgO ultrapotassic rocks due to crystallisation of primitive magmas generated by partial melting of a vein metasomatised network within a lithospheric upper mantle source, at low pressure and high PH2O.The shift to the younger Vico volcano, in which leucite-bearing magmas prevail, is thought to be due to the arrival in the mantle source of newly formed metasomatic agent from the undergoing slab characterised by the increasing of a sedimentary carbonate recycled component. This new component appears in the Middle Pleistocene and it is responsible for the composition of the magmatism of the Roman region. The recycling of carbonate-bearing pelitic sediments within the mantle wedge produces a carbonate-rich metasomatic agent. The latter is responsible for stabilisation of phlogopite-bearing wehrlitic veins within the mantle wedge. Partial melting of this newly formed vein network under high XCO2 generated silica under-saturated, and then leucite-bearing, ultrapotassic primary melts. A further shift is observed among the Vico post-caldera magmas, in which the appearance of a deep asthenospheric component is argued to be channelled late through slab-tears.

Miocene basaltic magmatism in the Goldfield-Superstition volcanic province, central Arizona: Geochemistry, mineralogy, and petrology

Basaltic lavas form part of the Miocene (∼20.5 to 18 Ma) Goldfield-Superstition silicic large igneous province in central Arizona. Most of the basalt erupted early in the development of this southern Basin and Range volcanic province, and only small amounts of basalt co-erupted with the silicic volcanism. We examined 50 samples of basalt from lower, middle, and upper stratigraphic positions of the province to establish basalt petrogeneses, the characteristics of basalt sources, relationships among compositionally different lavas, and the igneous processes that relate various basalt types. We base our study largely on major and trace elements, mineral compositions, and a small data set for Sr, Nd, and Hf isotopes. Lower-section basalts include the Weekes Wash basalts, which are transitional between alkalic and tholeiitic (SiO2 49–51 wt. %) and with MgO 8.1–10.2 wt. %. They are associated with lavas that are andesitic (SiO2 ∼59–62 wt. %; MgO 8–2.5 wt. %) or are seemingly andesitic due to alteration and felsic xenocrysts. Weekes Wash basalts have incompatible-element abundances that correlate positively with MgO. Their87Sr/86Sr ratios are ∼0.705. Olivines, rarely fresh, have Fo86–89 cores, and clinopyroxenes have Mg#s 86–89. Overlying the Weekes Wash are the Cottonwood Spring basalts, which are alkalic (SiO2 ∼45.5–47.5 wt. %), and can be categorized into subgroups defined by lower and higher incompatible-element abundances, or low Ti-P and high Ti-P (e.g., Ba ∼1100 versus 1800 ppm; La ∼75 versus 110 ppm). The Cottonwood Spring basalts are the closest to primary lavas we observed (MgO 10.1–11.8 wt. %), and their87Sr/86Sr ratios are ∼0.705–0.706. Their olivines are Fo86–89, and their clinopyroxenes have Mg#s 86–90. Overlying the Cottonwood Spring basalts is the Apache Gap Fe-Ti-enriched basalt (TiO2 ∼2.6 wt. %), which has the lowest MgO (∼5.6–7.5 wt. %) and incompatible-element abundances observed for any basalts in the province (e.g., Ba ∼400 ppm; La ∼25 ppm). All lower-section basalts have primitive-mantle normalizations showing Nb-Ta negative anomalies. Middle-section basalts erupted among silicic lava and pyroclastic flows from ∼19 to 18.5 Ma, and they compositionally resemble Weekes Wash basalts. Upper-section (post-18.5 Ma) lavas are the Willow Springs hawaiite (∼9.5 wt. % MgO;87Sr/86Sr ∼0.705) and Black Mesa basanite (SiO2 ∼44 wt. %; MgO ∼8 wt. %; CaO 14.7 wt. %;87Sr/86Sr ∼0.706). A Nb-Ta anomaly is clear in the hawaiite but weak in the basanite, as the basanite has the highest Nb and Ta observed (∼60 and ∼3 ppm; lowest Zr/Nb, ∼4 versus all others >7). Relevant interpretations are the following. Absence of ultramafic mantle xenoliths in Goldfield-Superstition basalts suggests that magmas occupied crustal reservoirs. The two subgroups of the Cottonwood Spring basalts attest to small-scale trace element and isotopic heterogeneities in lithospheric mantle sources that, based on Nb-Ta and Ce versus Ce/Yb modeling, had subduction zone characteristics and were garnet bearing. For Weekes Wash basalts, the decreasing incompatible-element abundances with decreasing MgO is consistent with a hybrid origin by the low-Ti-P subgroup of Cottonwood Spring basalts having assimilated lower crust. Trace-element modeling based on assimilation fractional crystallization (AFC) demonstrates that mixing Cottonwood Spring basalt with ∼50 percent (partial) melts of lower-crust pyroxenite in proportions from 80:20 to 50:50 yields Weekes Wash basalt compositions. Apache Gap lava represents AFC processes of high-Ti-P Cottonwood Spring basalts in a crustal reservoir, enabling Fe-Ti enrichment after ∼60 percent crystallization, as estimated by mass balancing, of mainly clinopyroxene and plagioclase. Upper-section hawaiite models as a differentiate after ∼50 percent crystallization of mainly clinopyroxene and plagioclase from a composition resembling low-Ti-P Cottonwood Spring basalt. Black Mesa basanite originated in a garnet-bearing lithospheric mantle source generally similar to those for Cottonwood Spring basalts, but additionally carbonitized (e.g., basanite has SiO2-undersaturation; high CaO) to yield magma relatively enriched in Nb and Ta. Basanite undersaturation is consistent with a smaller percent of source melting at the close of Goldfield-Superstition basaltic magmatism compared to initial source melting. This study of the Goldfield-Superstition volcanic province demonstrates that magmas produced from Miocene lithospheric mantle spanning ∼2.5 million years took various paths, from erupting as nearly primitive lavas from their lithospheric sources, to interacting with lower crust where they assimilated, differentiated, and provided heat to create a silicic large igneous province. Finally, the basalts reflect some changes in source and melting characteristics over the time of their emplacements, but they sustained lithospheric-source and ancient subduction characteristics throughout.

Atmospheric control of the cooling rate of impact melts and cryolavas on Titan’s surface

As on Earth, Titan’s atmosphere plays a major role in the cooling of heated surfaces. We have assessed the mechanisms by which Titan’s atmosphere, dominantly N 2 at a surface pressure of 1.5 × 10 5 Pa, cools a warm or heated surface. These heated areas can be caused by impacts generating melt sheets and (possibly) by endogenic processes emplacing cryolavas (a low-temperature liquid that freezes on the surface). We find that for a cooling cryolava flow, lava lake, or impact melt body, heat loss is mainly driven by atmospheric convection. Radiative heat loss, a dominant heat loss mechanism with terrestrial silicate lava flows, plays only a minor role on Titan. Long-term cooling and solidification are dependent on melt sheet or flow thickness, and also local climate, because persistent winds will speed cooling. Relatively rapid cooling caused by winds reduces the detectability of these thermal events by instruments measuring surface thermal emission. Because surface temperature drops by ≈50% within ≈1 day of emplacement, fresh flows or impact melt may be difficult to detect via thermal emission unless an active eruption is directly observed. Cooling of flow or impact melt surfaces are orders of magnitude faster on Titan than on airless moons (e.g., Enceladus or Europa). Although upper surfaces cool fast, the internal cooling and solidification process is relatively slow. Cryolava flow lengths are, therefore, more likely to be volume (effusion) limited, rather than cooling-limited. More detailed modeling awaits constraints on the thermophysical properties of the likely cryomagmas and surface materials.

プロト伊豆‐マリアナ島弧の衝突付加テクトニクス―レビュー―

This paper reviews the geology, petrology, and tectonics of the Proto-Izu-Mariana arc against the Honshu arc, Japan, since 15 Ma, to formulate current topics related to the Pacific-type orogeny. Since the first pioneering work by Sugimura (1972), which placed the plate boundary on-land Japan north of the Izu peninsula collision-accretion tectonics of the Proto-Izu-Mariana arc against the Honshu arc started to be extensively investigated through multi-disciplinary methods. Aoike (1999) proposed a comprehensive scenario of five successive accretions of Proto-Izu to the present, with all bounded by top- and bottom-faults since 15 Ma. Oroclinal bending of the Honshu arc began simultaneously with the collision and indentation of the Proto-Izu arc, as is well documented by the Neogene paleomagnetic declination of sedimentary rocks in the Kanto district (Niitsuma et al., see summary, 1989). The entire Izu-Mariana arc is subducting without any accretion now under Honshu, as is well documented by seismic tomography imaging by Hasegawa et al. (see Hasegawa et al., 2010). Tanzawa Tonalite–Trondhjemite–Granodiolite (TTG) pluton intruded into the already-accreted Tanzawa Group at 4 Ma (Tani et al., 2010), resulting in a contact metamorphic aureole ranging from amphibolite through greenschist to zeolite facies at a depth of ca. 10 km, and tectonically denudated along the Kan-nawa fault to be exposed on the surface by 1.0 Ma. The Proto-Izu-Mariana arc was built on the Pre-Paleogene, presumably Cretaceous MORB crust on which boninitic and fractionated silicic lava flows accumulated since 48 Ma, which is older than the bending of the Hawaii–Emperor seamount chain at 43 Ma. Hence, we need another plate off Proto-Ogasawara arc, called North New Guinea plate (Seno, 1985), and ridge subduction underneath Ogasawara arc to initiate subduction magmatism. Both calk-alkali rock series (CA) and coeval blueschist–eclogite rocks are exposed along the inner wall of the Izu–Mariana trench. This strongly suggests that extensive tectonic erosion occurred, resulting in the coexistence of two rock units that were formed 100-150 km from each other horizontally, i.e., rocks on the volcanic front and rocks on the descending oceanic slab. Petrogenesis of juvenile crust and TTG rocks are being debate with two models: basaltic arc magma fractionation (Kawate and Arima, 1998) and delaminated mafic lower crust (Nakajima and Arima, 1998). In the latter model, a difficulty is derived from density at the Moho depth and small size of delaminated residue. Instead, extensive tectonic erosion that transports not only the upper crust but also the lower mafic crust together with parts of the descending slab could be more plausible than these two models in order to present a thinner mafic lower crust.

The pre-eruptive volatile contents of recent basaltic and pantelleritic magmas at Pantelleria (Italy)

Pantelleria Island, located in the Sicily Channel Rift Zone (Italy), is the type locality for the peralkaline rhyolitic rocks called pantellerites. In the last 50 ka, after the large Green Tuff caldera-forming eruption, volcanic activity at Pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows, lava domes and poorly dispersed pantelleritic pumice fall deposits. Basaltic cinder cones and lava flows are only present outside the caldera in the NW sector of the island. The most recent basaltic (Cuddie Rosse, ∼ 20 ka) and pantelleritic (Cuddia Randazzo and Cuddia del Gallo, ∼ 6 ka) pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions. The melt inclusions in pyroxene and olivine phenocrysts of Cuddie Rosse scoriae have an alkali basalt composition. The dissolved volatiles comprise 0.9–1.6 wt.% H 2O, several hundred ppm of CO 2, 1600–2000 ppm of sulphur and 500–900 ppm of chlorine. The water–carbon dioxide couple gives a confining pressure ∼ 2 kbar prior to the eruption. This result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption. The melt inclusions in feldspar, fayalite and aenigmatite phenocrysts of Cuddia del Gallo and Cuddia Randazzo pumice have a pantelleritic composition (Agpaitic Indices 1.3–2.1), up to 4.4 wt.% H 2O, 8700 ppm Cl, 6000 ppm F, and CO 2 below the detection limit. Sulphur averaging 420 ppm has been measured in Cuddia Randazzo melt inclusions. These data indicate relatively high volatile contents for these low-energy Strombolian-type eruptions. Melt inclusions in Cuddia del Gallo pumice show the most evolved composition (Agpaitic Indices 2–2.1) and the highest volatile content, in agreement with fluid saturation conditions in the magma chamber prior to the eruption. This implies a confining pressure of ∼ 1 kbar for the top of the pantelleritic reservoir. The composition of melt inclusions and mineralogical assemblage of Cuddia Randazzo pumice indicate that it has a lower evolutionary degree (Agpaitic Indices 1.3–1.8) and lower pre-eruptive Cl and H 2O contents than Cuddia del Gallo pumice. An increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the Cuddia del Gallo explosive eruption. Evidence of widespread pre-eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the Cuddia Randazzo event.

Geologic mapping of the Zal region of Io

We produced a regional geologic map of the Zal region of Io's antijovian hemisphere using Galileo mission data. We discuss the geologic features, summarize the map units and structures that are present, discuss the nature of volcanic activity, and present an analysis of the volcanic, tectonic, and gradational processes that affect the region. The Zal region consists of five primary types of geologic materials: plains, mountains, paterae floors, flows, and diffuse deposits. The flows and patera floors are similar, but are subdivided based on uncertainties regarding emplacement environments and mechanisms. The Zal region includes two hotspots detected by Galileo : one along the western scarp of the Zal Patera volcano and one at the Rustam Patera volcano (name submitted to IAU). A third hotspot at the nearby At'am Patera volcano (name submitted to IAU) is the source of diffuse and pyroclastic materials that blanket north Zal Mons. The western bounding scarp of Zal Patera is the location of a fissure vent that is the source of multiple silicate lava flows. The floor of Zal Patera has been partially resurfaced by dark lava flows, although portions of the patera floor appear bright and unchanged during the Galileo mission. This suggests that the floor did not undergo complete resurfacing as a flooding lava lake but does contain a compound flow field. Mountain materials exhibit stages of degradation; lineated material degrades into mottled material. We have explored the possibility that north and south Zal Mons were originally one structure. We propose that strike-slip faulting and subsequent rifting separated the mountain units, opened a fissure which serves as a vent for lava flow, and created a depression which, by further extension during the rifting event, became Zal Patera. With comparison to other regional maps of Io, this work provides insight into the general geologic evolution of Io.

Automated identification of lava flow structures using local Fourier spectrum of digital elevation data

The rheological parameters of a lava flow can be inferred by studying structural features on the solidified lava surface. We have developed a technique using the S‐transform (a localized Fourier transform) to analyze and interpret digital elevation data. We examined structural features of a silicic lava flow, the Medicine Lake dacite flow in California. The S‐transform accurately identified and located both repeating and single‐instance structures. Several classes of structures were identified in the data, including the following: localized short wavelength features (1.3–3.6 m in wavelength) identified as large blocks and repeating medium wavelength features (5–8, 10–16, 18–26, 30–36, and 56–67 m in wavelength) identified as multiple generations of crustal folding. Fold wavelengths determined by the S‐transform and a model of compressional folding associated with near surface viscosity gradients were used to evaluate the rheology of the Medicine Lake dacite flow. The similarity of the amount of strain during folding events and the wavelength ratio of successive generations of folding suggest that the flow rheology remained relatively constant during folding of the flow surface. Repeating long wavelength features (67–70‐ and 105–140‐m wavelengths) were identified as the spacing of crease structures. Crease structures were recognized by the absence of short wavelength features at the location of their smooth flanks and by a central spike corresponding to the central trough. The limitations and applicability of this approach lie in the accuracy and resolution of the digital elevation data used.

Interpretation of very long period tremors at Ambrym volcano, Vanuatu, as quasi‐static displacement field related to two distinct magmatic sources

A 3‐component broadband seismometer (STS‐2) has been deployed on the SW slope of Ambrym, Vanuatu, a large‐caldera volcano with two persistent lava lakes from July to November 2000. Well‐polarized Very Long Period Tremors (18–22s; VLPTs) occurred quite continuously, whereas other seismic signal with periods of 9s, 6–7s, 5s, 2.3–2.7s, 1.95s and 1s were intermittent. VLPTs signals show two clearly distinct directions pointing towards the caldera. Polarization orientations suggest two sources: the first one, and the most active, fits with the zone from which silicic lava flows erupted in 1986, in the central‐to‐eastern part of the caldera. The second one corresponds to the active zone formed by the complex Marum volcanic center and the Benbow cone, in the western part of the depression structure. The depths are estimated between 2.7–2.9 km with respect to sea level. VLPTs are interpreted as the quasi static‐displacement field of two nondestructive sources. They indicate the persistent continuing magmatic activity beneath the eastern floor of the caldera, although no volcanic activity is observed in this area, and the more obvious, but less active, corresponding to the lava lakes. These data also corroborate the complex internal structure of the Ambrym caldera, suggested by geological data and bimodal magmatism (basaltic, ∼50% SiO2, and silicic, ∼60% SiO2).