Silicic Centers Research Articles

Effusive and explosive eruptive history of the Ilopango caldera complex, El Salvador

The Ilopango caldera in central El Salvador is the source of at least 13 voluminous rhyolitic eruptions over the last two million years, including the 431 CE Tierra Blanca Joven (TBJ) which blanketed areas inhabited by the Mayan civilization, and the 1.707 ± 0.022 Ma Olocuilta ignimbrite, one of the largest Quaternary eruptions in Central America. These rhyolitic deposits have been the subject of numerous detailed studies. In contrast, the abundant effusive and monogenetic eruptions at Ilopango have not been studied. These small-volume eruptions range from basalt to rhyolite (49.7–72.8 wt% SiO2) and occur both within the caldera margin and on the south flank of the Antiguo Ilopango stratovolcano. Here we present 28 new 40Ar/39Ar ages and geochemical data from domes, lavas, dikes, and ignimbrites from the Ilopango caldera and the Antiguo Ilopango stratovolcano as well as from adjacent volcanic systems. Collectively, these new data are used to construct a revised eruptive sequence for the Ilopango caldera that spans almost three million years. The new chronostratigraphy for Ilopango is highlighted by two volcanic cycles. Each cycle starts with rhyolite dome effusions, and then a prolonged period of explosive rhyolitic volcanism followed by a ~120–175 kyr period of less evolved eruptions. The spatio- and chemo-temporal patterns in the Ilopango eruptive sequence are similar in many ways to those observed in other well-studied Quaternary silicic centers.

Post-Caldera Eruptions at Chalupas Caldera, Ecuador: Determining the Timing of Lava Dome Collapse, Hummock Emplacement and Dome Rejuvenation

Determining the lithology, extent, origin, and age of hummocks can be challenging, especially if these are covered by successive deposits and lush vegetation. At Chalupas caldera, a late-Pleistocene silicic center that lies astride the Eastern Cordillera of northern Ecuador, we have tried to overcome these difficulties by combining geological observations and sampling, laboratory analysis (geochemistry, scanning electron microscope analysis and radiometric dating) and remote sensing techniques. Chalupas is the second largest caldera in the Northern Volcanic Zone of South America and its VEI 7 eruption, which occurred ∼0.21 Ma, has garnered the attention of the volcanological community. Our research highlights new observations of the post-caldera activity at Chalupas, beginning with the growth of Quilindaña stratovolcano (∼0.170 Ma), followed by the formation of Buenavista dome that is located 5 km eastward of Quilindaña’s summit. At the eastern foot of Buenavista dome we identify hummocky terrain covering an area of ∼20 km2. Collectively, the suite of techniques that we used helped to highlight geological features that shed light on the provenance of the hummocks and demonstrate that this topography may have originated from gravitational breccia flows from Buenavista lava dome. Numerical simulations were also performed to represent breccia flow transit and emplacement over the present caldera landscape and to view the potential hazard footprints of a future Buenavista dome collapse. For modeling we employed volumes of 20–120 Mm3 to visualize the consecutive traces of mass flow deposition and how the traces correspond to the hummocky landscape. Following the partial collapse of Buenavista lava dome, its rejuvenation is represented by tephra layers of several small eruptions that are dated at about 40 ky BP. These tephras represent some of the youngest eruptive activity recognized at Chalupas caldera. Our results contribute to the overall knowledge about Chalupas and demonstrate that eruptions at this important caldera are more recent than was previously reported.

Magmatic evolution of the Boku Volcanic Complex, Main Ethiopian Rift

The Boku volcanic complex is a Quaternary center situated on the axial segment of the Main Ethiopian Rift (MER), located 92 km South-east from Addis Ababa. The main objective of this study is to understand the magmatic evolution of the volcanic complex and to develop a model to answer some outstanding questions related to bimodal products of rift related volcanism, using geological mapping, petrographic and geochemical approaches. The Boku complex is characterized by two main phases of activity: pre-caldera/caldera forming eruptive activity and post-caldera eruptive activity. The volcanic stratigraphy consists from bottom to top of a sequence of rhyolitic lava flows, pumice flows, welded ignimbrite, pumice fall, rhyolitic lava dome, obsidian flow, lower basaltic lava flow, ash flow, basaltic scoria and upper basaltic lava flows. The lithologic varieties together with the geochemical results indicate that the Boku eruptive products are bimodal in composition; no intermediate compositions are found. The mafic rocks are transitional to weakly subalkaline basalts while the silicic rocks are predominantly peralkaline rhyolites. These two groups of rocks are co-genetic and related to each other by fractional crystallization processes starting from mantle-derived basaltic magma with a small component of crustal contamination. The available geophysical, geochemical and field data suggest that the evolution of the evolved silicic center which hosts a bimodal rock distribution can be explained as a result of prolonged stagnation of transitional basaltic melt (sourced from the mantle) at relatively high pressures, where these evolve to intermediate compositions. The transitional basaltic melts can occasionally erupt to the surface along weakness lines such as faults that infrequently cut the lower part of the shallow reservoir, generating basalts from the intra-caldera and lateral eruptive centers. We consider the intermediate magma is mechanically trapped at mid-crustal depths because of its higher crystal load (ca. 50%). Silicic magma is formed at shallow depth by prolonged fractional crystallization of the intermediate magma and minor assimilation of crustal material. These silicic magmas generate both effusive and explosive eruption products in the overall stratigraphy of the volcanic complex.

Geological evolution of the Boset-Bericha Volcanic Complex, Main Ethiopian Rift: 40Ar/39Ar evidence for episodic Pleistocene to Holocene volcanism

The Boset-Bericha Volcanic Complex (BBVC) is one of the largest stratovolcanoes of the northern Main Ethiopian Rift (MER). However, very little is known about its eruptive history, despite the fact that approximately 4 million people live within 100km of the complex. Here, we combine field observations, morphometric analysis using high-resolution LiDAR data, geochemistry and 40Ar/39Ar geochronology to report the first detailed account of the geological evolution of the BBVC, with a focus on extensive young lava flows covering the two edifices, Gudda and Bericha. These lavas exhibit a bimodal composition ranging dominantly from basaltic rift floor lavas and scoria cones, to pantelleritic trachytes and rhyolite flows at Gudda, and comenditic rhyolites at Bericha. Further, several intermediate compositions are associated with fissure vents along the Boset-Kone segment that also appear to link the silicic centres. We divide the BBVC broadly into four main eruptive stages, comprising: (1) early rift floor emplacement, (2) formation of Gudda Volcano within two main cycles, separated by caldera formation, (3) formation of the Bericha Volcano, and (4) sporadic fissure eruptions. Our new 40Ar/39Ar geochronology, targeting a representative array of these flows, provides evidence for episodic activity at the BBVC from ~120ka to the present-day. We find that low-volume mafic episodes are more frequent (~10ka cyclicity) than felsic episodes (~100ka cyclicity), but the latter are more voluminous. Over the last ~30ka, mafic to intermediate fissure activity might have reinvigorated felsic activity (over the last ~16ka), manifested as peralkaline lava flows and pyroclastic deposits at Gudda and Bericha. Felsic episodes have on average a higher eruption rate (2–5/1000years) and productivity at Gudda compared to Bericha (1–2/1000years). The young age of lavas and current fumarolic activity along the fault system, suggest that the BBVC is still potentially active. Coincident episodic activity within the BBVC and at several rift segments in the MER is observed, and facilitates continental rifting.

Origin and Evolution of Silicic Magmas in Oceanic Arcs; an in situ Study from St Lucia, Lesser Antilles

Processes linked with the genesis, evolution and emplacement of silicic complexes in arcs are still poorly constrained. Of particular interest are the depth of magma production, the relative contribution of crystal fractionation versus crustal partial melting and the timescales involved. The Soufriere Volcanic Complex (SVC) on St Lucia is one of the largest silicic centres in the Lesser Antilles arc. Here we present the results of a detailed mineralogical study, including in situ Sr isotopes in plagioclase and in situ δ18O in dated zircons, of both SVC and Pre-SVC volcanic rocks to place constraints on the processes intrinsic to the development and evolution of the silicic complex. These data suggest that the production of silicic magma in the SVC occurs in two stages. The first stage involves differentiation of mafic magma by crustal assimilation and mineral fractionation in the middle–lower crust of the arc to produce magmas with intermediate compositions. These intermediate magmas are water-rich (∼7 wt %) and have high 87Sr/86Sr, Ba, Sr and La/Sm (∼5) compared with Pre-SVC lavas. Near-constant trace element and isotopic compositions throughout the SVC lifespan indicate that the same process was persistent over the last 600 kyr. In the second stage, the intermediate magmas are transferred to a shallower and more differentiated chamber (∼6 km depth). During ascent, any crystals or xenocrysts residual from stage one in the deeper chamber become fully resorbed and the magma crystallizes calcic amphibole microphenocrysts, followed by anorthite-rich plagioclase close to or at the water saturation depth. During mixing upon recharge within the shallow chamber, anorthite-rich plagioclase from the recharging magma is partially resorbed; so are the crystals in equilibrium with the resident differentiated magma. The recharge event probably causes chamber-wide convection. Mixing is thought to trigger eruption of the silicic complex magmas.

Initiation of large-volume silicic centers in the Yellowstone hotspot track: insights from H2O- and F-rich quartz-hosted rhyolitic melt inclusions in the Arbon Valley Tuff of the Snake River Plain

During the onset of caldera cluster volcanism at a new location in the Snake River Plain (SRP), there is an increase in basalt fluxing into the crust and diverse silicic volcanic products are generated. The SRP contains abundant and compositionally diverse hot, dry, and often low-δ18O silicic volcanic rocks produced through time during the formation of individual caldera clusters, but more H2O-rich eruptive products are rare. We report analyses of quartz-hosted melt inclusions from pumice clasts from the upper and lower Arbon Valley Tuff (AVT) to gain insight into the initiation of caldera cluster volcanism. The AVT, a voluminous, caldera-forming rhyolite, represents the commencement of volcanism (10.44 Ma) at the Picabo volcanic field of the Yellowstone hotspot track. This is a normal δ18O rhyolite consisting of early and late erupted members (lower and upper AVT, respectively) with extremely radiogenic Sr isotopes and unradiogenic Nd isotopes, requiring that ~50 % of the mass of these elements is derived from melts of Archean upper crust. Our data reveal distinctive features of the early erupted lower AVT melt including: variable F concentrations up to 1.4 wt%, homogenous and low Cl concentrations (~0.08 wt%), H2O contents ranging from 2.3 to 6.4 wt%, CO2 contents ranging from 79 to 410 ppm, and enrichment of incompatible elements compared to the late erupted AVT, subsequent Picabo rhyolites, SRP rhyolites, and melt inclusions from other metaluminous rhyolites (e.g., Bishop Tuff, Mesa Falls Tuff). We couple melt inclusion data with Ti measurements and cathodoluminescence (CL) imaging of the host quartz phenocrysts to elucidate the petrogenetic evolution of the AVT rhyolitic magma. We observe complex and multistage CL zoning patterns, the most critical being multiple truncations indicative of several dissolution–reprecipitation episodes with bright CL cores (higher Ti) and occasional bright CL rims (higher Ti). We interpret the high H2O, F, F/Cl, and incompatible trace element concentrations in the context of a model involving melting of Archean crust and mixing of the crustal melt with basaltic differentiates, followed by multiple stages of fractional crystallization, remelting, and melt extraction. This multistage process, which we refer to as distillation, is further supported by the complex CL zoning patterns in quartz. We interpret new Δ18O(Qz-Mt) isotope measurements, demonstrating a 0.4 ‰ or ~180 °C temperature difference, and strong Sr isotopic and chemical differences between the upper and lower AVT to represent two separate eruptions. Similarities between the AVT and the first caldera-forming eruptions of other caldera clusters in the SRP (Yellowstone, Heise and Bruneau Jarbidge) suggest that the more evolved, lower-temperature, more H2O-rich rhyolites of the SRP are important in the initiation of a caldera cluster during the onset of plume impingement.

Geology of the High Rock caldera complex, northwest Nevada, and implications for intense rhyolitic volcanism associated with flood basalt magmatism and the initiation of the Snake River Plain–Yellowstone trend

We present the geologic history of the High Rock caldera complex (HRCC; Nevada, USA), a major mid-Miocene silicic center associated with flood basalt volcanism. Based on 70 40Ar/39Ar ages and new 1:24,000- and 1:100,000-scale geologic mapping, we document that between ca. 16.5 and 15.5 Ma a minimum of ∼700 km3 of rhyolitic magma erupted from the HRCC, covering an area of ∼8300 km2 in northwestern Nevada and southern Oregon (USA). The volcanism immediately followed eruption of the Steens flood basalt in the region, and was contemporaneous with eruption of compositionally similar rhyolites from the McDermitt volcanic field (MVF) to the east. The HRCC and MVF together mark the starting point for the eastward-younging trend of voluminous rhyolitic calderas of the Snake River Plain–Yellowstone trend. The HRCC comprises 4 major calderas, 24–40 km in diameter, that young from north-northeast to south-southwest: the Virgin Valley caldera formed on eruption of the 16.38 Ma high-silica alkali rhyolite Idaho Canyon Tuff; the overlapping Badger Mountain caldera collapsed due to eruption of the 16.34 Ma crystal-rich, low-silica rhyolite Summit Lake Tuff; eruption of the moderately peralkaline high-silica alkali rhyolite Soldier Meadow Tuff at 16 Ma resulted in formation of the Hanging Rock caldera; and the Cottonwood Creek caldera formed on eruption of the newly recognized, 15.70 Ma tuff of Yellow Rock Canyon, which is zoned from high-silica alkali rhyolite to trachyte. The four calderas contain caldera fill deposits, including pumice and ash falls, lahars, phreatomagmatic deposits, and well-bedded lacustrine deposits, which preserve diverse mid-Miocene fossil fauna and flora. Au and U mineralization developed along ring fractures of the Virgin Valley and Cottonwood Creek calderas. After silicic volcanism largely ceased, trachyte, trachyandesite, and alkalic basaltic lavas erupted through the caldera lakes. Intense silicic volcanism at the HRCC during the interval 16.4–15.5 Ma overlapped the eruption of the Steens and Columbia River Basalts, strongly suggesting a petrogenetic link. We propose that the HRCC and MVF caldera centers are localized where dikes of Steens flood basalt encountered transitional crust west of the craton with a composition and thickness that allowed significant partial melting, based on the O and Nd isotopic values of the rhyolites, which require involvement of crustal melts in their origin. Steens Basalt eruptions largely ceased in the area by the time the oldest caldera-forming ignimbrites erupted at both the HRCC and MVF, indicating that once large silicic magma bodies aggregated in the crust, they intercepted flood basalt dikes. We suggest that the roots of the HRCC and MVF are composed of large volumes of gabbroic intrusions and cumulates formed by fractional crystallization of HRCC magmas, which strengthened the middle crust beneath the calderas; major basin-bounding normal faults are diverted around them, but outflow ignimbrites are prominently offset by Basin and Range faults.

Changes in magma storage conditions following caldera collapse at Okataina Volcanic Center, New Zealand

Large silicic volcanic centers produce both small rhyolitic eruptions and catastrophic caldera-forming eruptions. Although changes in trace element and isotopic compositions within eruptions following caldera collapse have been observed at rhyolitic volcanic centers such as Yellowstone and Long Valley, much still remains unknown about the ways in which magma reservoirs are affected by caldera collapse. We present 238U–230Th age, trace element, and Hf isotopic data from individual zircon crystals from four eruptions from the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand, in order to assess changes in trace element and isotopic composition of the reservoir following the 45-ka caldera-forming Rotoiti eruption. Our data indicate that (1) mixing of magmas derived from crustal melts and mantle melts takes place within the shallow reservoir; (2) while the basic processes of melt generation likely did not change significantly between pre- and post-caldera rhyolites, post-caldera zircons show increased trace element and isotopic heterogeneity that suggests a decrease in the degree of interconnectedness of the liquid within the reservoir following collapse; and (3) post-caldera eruptions from different vents indicate different storage times of the amalgamated melt prior to eruption. These data further suggest that the timescales needed to generate large volumes of eruptible melt may depend on the timescales needed to increase interconnectedness and achieve widespread homogenization throughout the reservoir.

Peralkaline felsic magmatism at the Nemrut volcano, Turkey: impact of volcanism on the evolution of Lake Van (Anatolia) IV

Nemrut volcano, adjacent to Lake Van (Turkey), is one of the most important peralkaline silicic centres in the world, where magmatism for ~570,000 years has been dominated by peralkaline trachytes and rhyolites. Using onshore and Lake Van drill site tephra samples, we document the phenocryst and glass matrix compositions, confirming a complete spectrum from very rare mafic to dominantly silicic magmas. Magma mixing has been common and, along with the multi-lineage nature of the magmas, indicates that Nemrut has been a very open system where, nevertheless, compositionally zoned caps developed during periods of relative eruptive quiescence. Geothermometry suggests that the intermediate-silicic magmas evolved in an upper crustal magma reservoir at temperatures between 1100 and 750 °C, at fO2 close to the FMQ buffer. The silicic magmas either were halogen poor or exsolved a halogen-rich phase prior to or during eruption. An unusual Pb-rich phase, with up to 98.78 wt% PbO, is interpreted as having exsolved from the intermediate-rhyolitic magmas.

The Kivalliq Igneous Suite: Anorogenic bimodal magmatism at 1.75 Ga in the western Churchill Province, Canada

The Kivalliq Igneous Suite (1.77–1.73Ga) includes the Nueltin Granite intrusions; gabbro and anorthosite intrusions; three swarms of mafic dykes; and basalt, rhyolite and pyroclastic rocks of the Pitz Formation (Wharton Group, middle Dubawnt Supergroup). It was emplaced in the reworked Archaean hinterland of the trans-Hudson orogen, in the central western Churchill Province (WCP) of the Canadian Shield. The Nueltin granites have field relations, mineral assemblages, and compositions typical of metaluminous to peraluminous, Proterozoic to Recent rapakivi granites, and the suite comprises a classic bimodal anorogenic province. Kivalliq rhyolites/granites were erupted/emplaced in a N–S band (the “Nueltin corridor”) about 700km long and 300km wide, with volcanic rocks and dykes concentrated at the north end and coarse plutonic rocks to the south. The McRae Lake dyke swarm originates among three prominent gabbro/anorthosite intrusions inside the corridor, and forms a narrow array extending a minimum of 600km to the NE along the central axis of the WCP. Early mafic rocks (intrusions and some lavas, 1.77Ga) had an alkali basalt parent, with subalkaline basalts (sills, dykes, and mafic enclaves) present at the peak time of granite emplacement (1.75Ga). Average initial ɛNd of basaltic rocks at 1.75Ga is −4.0, indicating a significant crustal component. Sm–Nd depleted mantle model ages of granites and rhyolites (average initial ɛNd=−8.7) range from 2.2 to 3Ga, consistent with melted Archaean crust variably mixed with juvenile basalt. Metallogenic prospects of the Kivalliq Suite include U–Th–Zr–Y–REE enriched granite and syenite, epithermal Ag/Au at silicic centres (including mafic/felsic ring complexes), and Pb–Cu sulphides associated with mafic intrusions. Topaz rhyolites are present, but Sn-enriched rocks have not been observed. The Kivalliq Suite is one of about 15 prominent anorogenic provinces and dyke swarms which erupted within the Nuna supercontinent at ca. 1.75Ga, and the thermal effects of the Kivalliq event are recorded in U–Pb and K–Ar data throughout most of the western Churchill Province.

Rhyolites—Hard to produce, but easy to recycle and sequester: Integrating microgeochemical observations and numerical models

Recent discoveries of isotopically diverse minerals, i.e., zircons, quartz, and feldspars, in large-volume ignimbrites and smaller lavas from the Snake River Plain (SRP; Idaho, USA), Iceland, Kamchatka Peninsula, and other environments suggest that this phenomenon characterizes many silicic units studied by in situ methods. This observation leads to the need for new models of silicic magma petrogenesis that involve double or triple recycling of zircon-saturated rocks. Initial partial melts are produced in small quantities in which zircons and other minerals undergo solution reprecipitation and inherit isotopic signatures of the immediate environment of the host magma batch. Next, isotopically diverse polythermal magma batches with inherited crystals merge together into larger volume magma bodies, where they mix and then erupt. Concave-up and polymodal crystal size distributions of zircons and quartz observed in large-volume ignimbrites may be explained by two or three episodes of solution and reprecipitation. Hafnium isotope diversity in zircons demonstrates variable mixing of crustal melts and mantle-derived silicic differentiates. The low δ 18 O values of magmas with δ 18 O-diverse zircons indicate that magma generation happens by remelting of variably hydrothermally altered, and thus diverse in δ 18 O, protoliths from which the host magma batch, minute or voluminous, inherited low-δ 18 O values. This also indicates that the processes that generate zircon diversity happen at shallow depths of a few kilometers, where meteoric water can circulate at large water/rock ratios to imprint low δ 18 O values on the protolith. We further review newly emerging isotopic evidence of diverse zircons and their appearance at the end of the magmatic evolution of many long-lived large-volume silicic centers in the SRP and elsewhere, evidence indicating that the genesis of rhyolites by recycling their sometimes hydrothermally altered subsolidus predecessors may be a common evolutionary trend for many rhyolites worldwide, especially in hotspot and rift environments with high magma and heat fluxes. Next, we use thermomechanical finite element modeling of rhyolite genesis and to explain (1) the formation of magma batches in stress fields by dike capture or deflection as a function of underpressurization and overpressurization, respectively; (2) the merging of neighboring magma batches together via four related mechanisms: melting through the screen rock and melt zone expansion, brittle failure of a separating screen of rocks, buoyant merging of magmas, and explosive merging by an overpressurized interstitial fluid phase (heated meteoric water); and (3) mixing time scales and their efficacies on extended horizontal scales, as expressed by marker method particle tracking. The envisioned advective thermomechanical mechanisms of magma segregation in the upper crust may characterize periods of increased basaltic output from the mantle, leading to increased silicic melt production, but may also serve as analogues for magma chambers made of dispersed magma batches. Although not the focus of this work, dispersed magma batches may be stable in the long term, but their coalescence creates ephemeral, short-lived eruptable magma bodies that erupt nearly completely.

Volcanic stratigraphy and geochemistry of the Soufrière Volcanic Centre, Saint Lucia with implications for volcanic hazards

The Soufrière Volcanic Complex (SVC), Saint Lucia, represents one of the largest silicic centres in the Lesser Antilles arc. It comprises extensive pumiceous pyroclastic flow deposits, lava flows as well as Peléan-style domes and dome collapse block-and-ash-flow deposits. These deposits occur within and around the Qualibou Depression, a ~10-km diameter wide sector collapse structure. To date, vent locations for SVC pyroclastic deposits and their relationship to the sector collapse have been unclear because of limited stratigraphic correlation and few radiometric ages. In this study we reconstruct the geologic history of the SVC in light of new and recently published (U–Th)/He, U–Th and U–Pb zircon chronostratigraphic data, aided by mineralogical and geochemical correlation. Compositionally, SVC deposits are monotonous medium-K, calc-alkaline rocks with 61.6 to 67.7wt.% SiO2 and display similar trace element abundances. Combined U–Th and (U–Th)/He zircon dating together with 14C ages and mineral fingerprinting reveals significant explosive eruptions at 640, 515, 265, 104, 60 and 40ka (producing deposits previously grouped together as the “Choiseul” unit) and at 20ka (Belfond unit). The mineralogically and geochemically distinct Belfond unit is a large, valley-filling pumiceous pyroclastic flow deposit distributed to the north, northeast, south and southeast of the Qualibou Depression that was probably deposited during a single plinian eruption. The unit previously referred to as ‘Choiseul tuff’ is much less well defined. The typical Choiseul unit comprises a series of yellowish-white, crystal-poor, non-welded pumiceous pyroclastic deposits cropping out to the north and southeast of the Qualibou depression; however its age is poorly constrained. A number of other units previously mapped as Choiseul can be distinguished based on age, and in some cases mineral and whole rock chemistry. Pyroclastic deposits at Micoud (640±19ka), Bellevue (264±8ka), Anse John (104±4ka) and La Pointe (59.8±2.1ka), Anse Noir and Piaye were all previously grouped with or associated with the Choiseul tuff (all uncertainties 1σ). We suggest that these units represent individual periods of activity spanning a range of ages, whereas Choiseul pumice at the type locality has yielded a (U–Th)/He zircon age of 515±19ka. Their overall geochemical and mineralogical similarities with the Choiseul at the type locality suggest that they might have all originated from the same centre. Morne Tabac (532±21ka) is a dome truncated by the depression escarpment, whereas Morne Bonin (273±15ka), Gros Piton and Petit Piton (71±3ka and 109±4ka, resp.), Belfond (13.6±0.4ka) and Terre Blanche (15.3±0.4ka) are domes within the Qualibou Depression. Belfond and Terre Blanche have whole rock geochemistry and mineral assemblages similar to the Belfond pyroclastic flow deposit, thus possibly representing late-erupted degassed portions of the magma that produced the Belfond pyroclastics. The geochemical characteristics and similar zircon age distributions of the silicic lava domes and pyroclastics of the SVC suggest that these share a common magma source beneath the Qualibou depression. The distribution of the pyroclastic flows and the wide range in their eruption ages makes it unlikely that these were erupted during caldera-forming activity, and we instead invoke a series of smaller-volume explosive eruptions from the area of the current depression, the earliest of which occurred from a large proto-Qualibou edifice that subsequently underwent sector collapse. Activity from this proto-Qualibou centre may have ceased sometime between 38 and 59ka ago, it therefore seems unlikely given our present understanding that there will be another eruption from the southern central highland region. However, the young dome-forming activity in the Qualibou depression may have occurred in or close to the Holocene, and there have been dome collapse events and explosion craters formed since then. A new dome eruption or renewed activity at a dome within the depression, growing in the style of the ongoing Soufrière Hills lava dome on the nearby island of Montserrat, is possible; as is a future plinian eruption from this area. Such an eruption would not only have a devastating impact on Saint Lucia, but would also have significant regional and global impacts.

Initial impingement of the Yellowstone plume located by widespread silicic volcanism contemporaneous with Columbia River flood basalts

During the main phase of Steens and Columbia River (United States) flood basalt eruptions between 16.7 and 15.0 Ma, ∼3900 km 3 of silicic magma erupted from centers dispersed across ∼25,000 km 2 . The largest and oldest silicic centers, High Rock, McDermitt, and Lake Owyhee caldera complexes, lie along a narrow NNE trend at the transition between the craton and accreted oceanic crust, suggesting that silicic magma generation was greatest where mafic magma intruded more-felsic crust. Less voluminous and younger silicic centers track the spread of mafic dikes to the north. The High Rock caldera complex forms the western end of the Snake River Plain–Yellowstone trend of eastward-younging silicic centers. We attribute a regional lull in volcanism between ca. 23 and 7 Ma to lifting of the Juan de Fuca slab by a mantle plume beginning at ca. 25 Ma, and the distribution of main-stage flood basalts and coeval rhyolites to breakthrough of plume material at ca. 17 Ma to form a smaller plume head centered beneath Steens Mountain.

Evolution of tuff ring-dome complex: the case study of Cerro Pinto, eastern Trans-Mexican Volcanic Belt

Cerro Pinto is a Pleistocene rhyolite tuff ring-dome complex located in the eastern Trans-Mexican Volcanic Belt. The complex is composed of four tuff rings and four domes that were emplaced in three eruptive stages marked by changes in vent location and eruptive character. During Stage I, vent clearing produced a 1.5-km-diameter tuff ring that was then followed by emplacement of two domes of approximately 0.2 km3 each. With no apparent hiatus in activity, Stage II began with the explosive formation of a tuff ring ~2 km in diameter adjacent to and north of the earlier ring. Subsequent Stage II eruptions produced two smaller tuff rings within the northern tuff ring as well as a small dome that was mostly destroyed by explosions during its growth. Stage III involved the emplacement of a 0.04 km3 dome within the southern tuff ring. Cerro Pinto’s eruptive history includes sequences that follow simple rhyolite-dome models, in which a pyroclastic phase is followed immediately by effusive dome emplacement. Some aspects of the eruption, however, such as the explosive reactivation of the system and explosive dome destruction, are more complex. These events are commonly associated with polygenetic structures, such as stratovolcanoes or calderas, in which multiple pulses of magma initiate reactivation. A comparison of major and trace element geochemistry with nearby Pleistocene silicic centers does not show indication of any co-genetic relationship, suggesting that Cerro Pinto was produced by a small, isolated magma chamber. The compositional variation of the erupted material at Cerro Pinto is minimal, suggesting that there were not multiple pulses of magma responsible for the complex behavior of the volcano and that the volcanic system was formed in a short time period. The variety of eruptive style observed at Cerro Pinto reflects the influence of quickly exhaustible water sources on a short-lived eruption. The rising magma encountered small amounts of groundwater that initiated eruption phases. Once a critical magma:water ratio was exceeded, the eruptions became dry and sub-plinian to plinian. The primary characteristic of Cerro Pinto is the predominance of fall deposits, suggesting that the level at which rising magma encountered water was deep enough to allow substantial fragmentation after the water source was exhausted. Isolated rhyolite domes are rare and are not currently viewed as prominent volcanic hazards, but the evolution of Cerro Pinto demonstrates that individual domes may have complex cycles, and such complexity must be taken into account when making hazard risk assessments.

Quaternary faulting and volcanism in the Main Ethiopian Rift

The Main Ethiopian Rift (MER) is associated with bimodal Quaternary magmatism. Field, remote sensing, and geochronology data are used to examine the relationships between axial acidic volcanoes and basaltic eruptions. Two main Quaternary magmatic episodes are recognizeable in MER: (a) basaltic flows followed by ignimbrites and silicic centers in the rift floor (2–1 Ma) and (b) axial silicic volcanoes and basalts since ∼650 Ka. The first episode consists mainly of basaltic flows related to the Afar Stratoid and outcrops in the central and northern MER. Scattered silicic centers developed subsequently along the rift floor. In the second episode, spatial and temporal correlation between rift localization and silicic centers becomes more evident. The silicic centers are located at the intersection of the WFB with earlier structures, especially E–W faults. With ageing, these centers become faulted and allow basalts to erupt right through the volcanic edifice, suggesting a decrease in the amount of differentiation in the magma chambers. This style of evolution appears to be characteristic of continental rifts prior to the onset of drifting.

Geochemical sources for the terminal Pleistocene/early Holocene obsidian artifacts of the site of Beseka, central Ethiopia

Twenty obsidian artifacts from the terminal Pleistocene/early Holocene phase of the site of Beseka, central Ethiopia, have been instrumentally characterized using EDXRF and electron microprobe. Results show that most of the artifacts were derived from the silicic centers of Fentale and Kone. A nearby source of Abadir, although with better quality raw material, does not seem to have been utilized.

Correlation of Plio–Pleistocene Tephra in Ethiopian and Kenyan rift basins: Temporal calibration of geological features and hominid fossil records

The 200-m-thick fossiliferous Konso Formation and overlying terrace deposits, which crop out at the end of the southern sector of the Main Ethiopian Rift (MER), contain more than 30 distal tephra layers. Local and regional tephra correlations of more than 20 tephra units were established using major and trace element data of discrete and purified bulk glass samples within the Konso study area. Eleven correlative marker tuffs were recognized in stratigraphic sections of both the Konso Formation and the Omo-Turkana Basin sediments in southern Ethiopia and northern Kenya. The Turoha, Hope, Ivory, Bright White, and Boleshe Tuffs in the Konso Formation, and the Upper White Tuff in the overlying terrace deposits are securely correlated with the KBS, Akait, Lokapetamoi, Chari, Lower Nariokotome, and Silbo Tuffs of the Omo-Turkana Basin, using least mobile major elements (CaO, Fe 2O 3*, and TiO 2) and geochronology. Preliminary correlations are also suggested between the Konso Formation distal tephra and proximal units of the Quaternary caldera-forming silicic centers in the central sector of the MER. The strongly peralkaline tuffs of the Konso Formation are compositionally similar to proximal eruptions mostly located along the Quaternary axial rift zone of the southern, central, and northern sectors of the MER. The tephra correlation provides information about the temporal and spatial features of the volcanic and tectonic processes recorded in the evolving basins. Thickness and sedimentation rate were determined for both the Konso Formation and the Omo-Turkana Basin sections, measured between the Turoha (=KBS; 1.91 ± 0.03 Ma) and the Bright White (=Chari; 1.40 ± 0.02 Ma) Tuffs. Although the lithostratigraphic sequence at the Konso study area is younger, sedimentation rate within the Konso Formation was comparable to that of the Koobi Fora Formation, higher in the Nachukui Formation, and lower in the Shungura Formation. Local and regional differences in thickness and sedimentation rates within the Plio–Pleistocene deposits of the Konso, Shungura, Nachukui, and Koobi Fora Formations may be attributed to erosion associated with increased topographic gradients maintained by continued subsidence mostly along axial fault zones of basins and uplift along the margins of the broadly rifted zone of the Omo-Turkana Basin and the southern sector of the MER.

Precise hypocentre relocation of microearthquakes in a high-temperature geothermal field: the Torfajökull central volcano, Iceland

The Torfaj¨okull volcanic system is one of approximately 30 active volcanoes comprising the neovolcanic zones of Iceland. The central volcano of the system is the largest silicic centre in Iceland with a caldera of approximately 12 km diameter. Its high-temperature geothermal system is one of the most powerful in Iceland. Torfaj¨okull is a source of persistent seismicity, where both high- and low-frequency earthquakes occur. To study this microseismicity in detail, a temporary array of 20 broad-band seismic stations was deployed between 2002 June and November. These temporary stations were embedded in the permanent South Iceland Lowland (SIL) network and data from nine adjacent SIL stationswere included in this study.Aminimum one-dimensional (1-D) velocity model with station corrections was computed for earthquake relocation by inverting manually picked P- and S-wave arrival times from events occurring in the Torfaj¨okull volcanic centre and its surroundings. High-frequency earthquakes from the Torfaj¨okull volcanic centre were then relocated calculating a non-linear, probabilistic solution to the earthquake location problem. Subsequently, we correlated the waveforms of these 121 events (∼2000 observations) to define linked events, calculated the relative traveltime difference between event pairs and solved for the hypocentral separation between these events. The resulting high-resolution pattern shows a tighter clustering in epicentre and focal depth when compared with original locations. Earthquakes are mainly located beneath the caldera with hypocentres between 1 and 6 km depth and lie almost exclusively within the geothermal system. A sharp cut-off in seismicity at 3 km suggests either that there is a marked temperature increase or that this is a structural boundary. No seismic activity was observed in the fissure swarms to the northeast (NE) and southwest (SW) of the volcanic centre.

Magmatic evolution and ore-forming fluids involved in the origin of the gold/base metals mineralization in the Baia Mare province, Romania

We chose one silicic center (Danesti dacitic dome) to characterize the magmas associated with hydrothermal ore-formingsystems before fluid extraction from the melt and hydrothermal alteration, in the major gold/polymetallic Baia Mare province (Romania). Interpretation of melt inclusion data indicates that the dacites were derived from potassium-enriched, metaluminous magmas. We suggest that the aluminous character of the residual melt (trapped as melt inclusions) may play a role in the oreforming processes, as the chemistry of these melts controls the composition of evolving magmatic-hydrothermal fluid. The study of the geochemical features of silicate melt inclusions and the determination of their water contents using the recently developed Raman spectroscopy method, also reveals that the decompression was the dominant driving force for the crystallization at this silicic center.

New age constraints on the timing of volcanism in central Afar, in the presence of propagating rifts

We investigate the relationship between rift propagation and volcanism in the Afar Depression in the last 4 Myr. Potassium‐argon and thermoluminescence dating allow detailed reconstruction of the temporal evolution of volcanism. Volcanic activity is almost continuous since 3.5 Ma, with intervals characterized by more intense activity, especially around 2 Ma. Spatial distribution of ages reveals that Stratoid Series volcanism migrated northward along a 200‐km trend between 3 and 1 Ma, at about 10 cm/yr, linked to northward propagation of the Gulf of Aden Ridge, after it had cut across the Danakil horst at 4 Ma. Our work underlines the role of rhyolitic volcanism in initiation of rifting. Acid volcanoes, initially formed near the axes of extensional zones, have been subsequently dissected and are presently located on both sides of active rift segments. These lavas were the first to be erupted in areas of low extensional strain and were followed by basaltic lavas as extension increased. Differentiated volcanoes acted as zones of local weakness and guided localization of fractures, then leading to fissural magmatism. This regional‐scale, composite style of rifting, including volcanic and tectonic components, can be compared to the large‐scale continental breakup process itself. Deformation occurs through propagation of faults and fissures under a regional stress field. These become localized because of weakening of the crust (or lithosphere) due to emplacement of magmas, under the influence of a plume in the large‐scale case, or of silicic centers linked to magma chambers in the regional‐scale case.