Ignimbrite Volcanism Research Articles

Spread and frequency of explosive silicic volcanism of the Carpathian-Pannonian Region during Early Miocene: Clues from the SW Pannonian Basin and the Dinarides

Explosive silicic volcanism of the Carpathian-Pannonian Region (CPR) is increasingly recognized as the primary source of tephra across the Alpine-Mediterranean region during the Early and Middle Miocene. However, the tephrostratigraphic framework for this period of volcanic activity is still incomplete. We present new multi-proxy data from Lower Miocene ignimbrites and tephra fallout deposits from the southwestern CPR and the Dinaride Lake System and integrate them into existing datasets to better resolve the regional extent and scale of these eruptions of the CPR. Volcanic glass geochemistry indicates distal fallout tuffs deposited in the Sinj Basin are correlative with the proximal Ostoros ignimbrites from the Bükkalja Volcanic Field, indicative of regionally extensive volcanism at 17.295 ± 0.028 Ma, based on CA-ID-TIMS UPb zircon geochronology. Based on integrated tephrostratigraphic data, newly identified 17.064 ± 0.010 Ma massive rhyolitic ignimbrite deposits from the Kalnik Volcaniclastic Complex located in the southwestern CPR are correlative with the 17.062 ± 0.010 Ma Mangó massive ignimbrite found in the Bükkalja Volcanic Field located in the northern CPR. Based on these new observations of its potential areal distribution and estimated thicknesses, these two widespread ~17.1 Ma ignimbrites represent intermediate to large caldera-forming ignimbrites, larger than previously suggested. Finally, volcanic glass geochemistry of fallout deposits from the Dinaridic Sinj and Livno-Tomislavgrad Basins have similar volcanic glass geochemistry as the rhyolitic pumices from the lowermost part of the Bogács ignimbrite unit of the Bükkalja Volcanic Field. However, high-precision geochronology indicates that these distal ashfalls were deposited at 16.9567 ± 0.0074 Ma, significantly predating the 16.824 ± 0.028 Ma emplacement of the fiamme-bearing part of the Bogács ignimbrite. These distinct ages suggest that the Bogács unit represents multiple eruptive events and indicating that further work is required to deconvolve this portion of the CPR volcanic record. Together, these data suggest that large volume CPR ignimbrite volcanism was more frequent and widespread than previously understood, enhancing the existing volcanic framework and history of the source region for this time period.

Detrital zircon U-Pb ages and provenance of Paleogene paleochannel strata, Sierra Nevada and western Nevada: Implications for paleotopographic evolution

The Paleogene paleotopography of the western United States, which may grant insight into Farallon subduction dynamics and North American Cordillera orogenic processes, can be constrained by reconstructing drainage networks through sediment provenance studies. In the northern Sierra Nevada, the provenance of SW-directed, Paleogene paleochannel deposits is controversial; prior studies favor either sources proximal to the deposits or distal source areas in central Nevada. These different hypothesized source areas would have contrasting implications for the paleotopography of the Sierra Nevada and western Basin and Range. We conducted a new provenance analysis of paleochannel deposits in the Sierra Nevada and western Nevada using new and compiled detrital zircon U-Pb age data from across the preserved paleochannel network, together with compiled bedrock geochronologic ages of potential sediment source areas for the paleochannel deposits. The geochro­nology compilation of potential source areas reveals systematic longitudinal variations in bedrock ages within the study area. Jurassic ages are present primarily in the western Sierra Nevada batholith, Cretaceous ages are dominant in the eastern Sierra Nevada batholith, and Eocene ages are present only in north-central Nevada. The distribution of potential source ages allows confident inference of sediment provenance from detrital zircon U-Pb ages. The distributions of detrital zircon U-Pb ages in the paleochannel depos­its can be categorized into three distinct types. The first type, found in the vicinity of Malakoff Diggins State Historic Park (SHP) and further northwest, is dominated by Jurassic, Paleozoic, and Precambrian ages reflecting derivation from local sources in the western Sierra Nevada, and also contains scattered Eocene ages. The lack of Late Cretaceous ages in these samples, despite the presence of Late Cretaceous plutons in close proximity to the east, suggests small sediment source areas with fluvial transport of zircon grains no farther than 50 km. The few Eocene ages in these samples likely reflect volcanic air fall, consistent with an overlying tuff at one sample site that is interpreted to have its volcanic source to the north, outside the paleochannel network. The second zircon age distribution type, from samples south of Malakoff Diggins SHP, includes Cretaceous and Jurassic ages representative of exposed bedrock across the entire width of the Sierra Nevada batholith but no Eocene ages. This combination of ages suggests a sediment source area that encompassed the entire batholith but did not include north-central Nevada. The third zircon age distribution type, from samples to the northeast of Malakoff Diggins SHP, contains Jurassic, Cretaceous, and Eocene ages consistent with fluvial derivation of sediment from the entire width of the Sierran batholith as well as from north-central Nevada. This third type only occurs in fluvial deposits interbedded with Oligocene ignimbrite tuffs, whereas samples older than the Oligocene tuffs belong to zircon age distribution types 1 or 2. Thus, prior to the emplacement of Oligocene ignimbrites, there is no evidence of fluvial transport of sand-sized sediment from north-central Nevada sources to Sierran paleochannel deposits. The lack of pre-Oligocene fluvial transport across the Sierra Nevada may reflect either a Paleogene drainage divide that separated the Sierra Nevada from north-central Nevada or a large-scale knick zone with a low-gradient upstream reach that trapped sand and larger sediment. The first arrival of fluvially transported Eocene zircon grains following Oligocene ignimbrite emplacement suggests that ignimbrite volcanism, and related hinterland uplift, established or promoted sediment transport from north-central Nevada across the Sierra Nevada by driving drainage reorganization or steepening existing channels. Our provenance analysis broadly confirms previous paleochannel network reconstructions, especially for the Ancestral Yuba River, and thus validates the use of paleochannel deposits as a datum by which to infer post-Paleogene tilting of the northern Sierra Nevada. Azimuthal trends in Ancestral Yuba River paleochannel gradient suggest ~0.6° SW-directed, post-Paleogene tilting of the northern Sierra Nevada, which would have resulted in ~1 km of uplift of the range crest. Such a magnitude is consistent with Eocene–Oligocene stable isotope paleoelevation estimates.

Tracing magmatic genesis and evolution through single zircon crystals from successive supereruptions from the Socorro Caldera Complex, USA

Large volume rhyolitic ignimbrite volcanism is a significant contributor to the evolving crust. The introduction of high-silica material into the upper crust, differentiation within the middle crust, and partial melting in the lower crust contributes to geochemical and isotopic evolution of the crust. Developing accurate models for the genetic evolution of these events is dependent upon geochronology to determine rates of magmatic processes as model constraints. We present new zircon high-precision CA-ID-TIMS U-Pb geochronology and MC-ICPMS Hf isotope geochemistry for four ignimbrites from the nested caldera complex near Socorro, New Mexico (USA), within the Mogollon-Datil volcanic field. In agreement with past 40Ar-39Ar data, interpretations of new U-Pb data indicate eruptions from the Socorro caldera cluster were pulsed. These pulses were intermittently spaced, and a volcanic hiatus following the Hells Mesa Tuff at 33.442 ± 0.015 Ma was interrupted by four successive eruptions, beginning with the La Jencia Tuff at 29.158 ± 0.025 Ma and finishing with the South Canyon Tuff at 28.066 ± 0.021 Ma. Zircon age spectra became more protracted with each eruption, exhibiting age dispersions ranging from 0.347 Myr in the Hells Mesa Tuff to 4.502 Myr in the South Canyon Tuff. The increased dispersion is paralleled by an increase in the proportion of normally discordant grains, indicative of xenocryst incorporation. These protracted age spectra are not necessarily a function of thermal maturation in the middle to upper crust due to long-lived magma chambers. Rather, they are likely the result of increased melting of zircon-bearing lower crust due to deep thermal maturation from repeated juvenile magma injections based on the incorporation of zircon material at the melt source. In contrast, the Hf isotope record is volumetrically dominated by autocrystic zircon domains and becomes more radiogenic through time, recording juvenile replenishment of the lower crust during progressive melting. Together, these data record the protracted evolution of the lower crust sampled by ignimbrites, lend insight into that evolution, and emphasize the need for detailed interpretation of high-precision datasets to advance volcanic models.

Volcanological evolution of Pantelleria Island (Strait of Sicily) peralkaline volcano: a review

Pantelleria volcano has a particularly intriguing evolutionary history intimately related to the peralkaline composition of its explosively erupted magmas. Due to the stratigraphic complexity, studies over the last two decades have explored either only the pre-Green Tuff ignimbrite volcanism or the post-Green Tuff activity. We here focus on the whole evolutionary history, detailing the achievements since the first pioneering studies, in order to illustrate how the adoption and integration of progressively more accurate methods ( 40 Ar/ 39 Ar, paleomagnetism, petrography, and detailed field study) have provided many important independent answers to unresolved questions. We also discuss rheomorphism, a distinct feature at Pantelleria, at various scales and possible evidence for multiple, now hidden, caldera collapses. Although the evolutionary history of Pantelleria has shown that each ignimbrite event was followed by a period of less intense explosivity (as could be the present-day case), new geochronological and geochemical data may indicate a long-term waning of volcanic activity.

Altenberg–Teplice Caldera sourced Westphalian fall tuffs in the central and western Bohemian Carboniferous basins (eastern Variscan belt)

ABSTRACT Timing of magmatic activity of the late-Variscan Altenberg–Teplice Caldera was rather vaguely constrained. In this paper, we present five new laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon ages, which along with published data indicate ~13 Myr lifetime. Formation of the Altenberg–Teplice Caldera commenced with the emplacement of the pre-caldera pluton at ~325–319 Ma, and terminated with the intrusion of the syn-collapse ring dykes and post-collapse granites at ~312 Ma. The main ignimbrite volcanism in the area of the ATC occurred at ~318–313 Ma and peaked at ~314–313 Ma. The latter corresponds to the age of the caldera-forming eruptions, which sourced the extra-caldera pyroclastic deposits. The same age yielded the rhyolite dyke swarm that fed the ignimbrite eruptions. Some pyroclastic fall deposits preserved in adjacent Carboniferous basins indicate similar ages of ~314–312 Ma including the widespread ~314 Ma Bělka tuff, which represents the main chronostratigraphic marker of this area. Its thickness and grain size progressively degrease from the ATC towards S and SW. This, together with the isopach map distribution and the available geochronological data suggest that the Bělka tuff was sourced from the Altenberg–Teplice Caldera. The Bělka tuff distribution and its association with the extra-caldera ignimbrites of the Altenberg–Teplice Caldera imply that this tuff is a co-ignimbrite fall deposit that accompanied pyroclastic density currents sourced from the caldera. The calculated minimum volume of fallout ash-tuffs and extra-caldera ignimbrite facies contribute 30 km3 to the 350 km3 (dense rock equivalent) estimates of the total Altenberg–Teplice Caldera products. Such volumes correspond well to similar modern analogues of collapse calderas of intermediate size.

Gravity modeling finds a large magma body in the deep crust below the Gulf of Naples, Italy

We analyze a wide gravity low in the Campania Active Volcanic Area and interpret it by a large and deep source distribution of partially molten, low-density material from about 8 to 30 km depth. Given the complex spatial-temporal distribution of explosive volcanism in the area, we model the gravity data consistently with several volcanological and petrological constraints. We propose two possible models: one accounts for the coexistence, within the lower/intermediate crust, of large amounts of melts and cumulates besides country rocks. It implies a layered distribution of densities and, thus, a variation with depth of percentages of silicate liquids, cumulates and country rocks. The other reflects a fractal density distribution, based on the scaling exponent estimated from the gravity data. According to this model, the gravity low would be related to a distribution of melt pockets within solid rocks. Both density distributions account for the available volcanological and seismic constraints and can be considered as end-members of possible models compatible with gravity data. Such results agree with the general views about the roots of large areas of ignimbritic volcanism worldwide. Given the prolonged history of magmatism in the Campania area since Pliocene times, we interpret the detected low-density body as a developing batholith.

Paleoproterozoic evolution of the Guiana Shield in Suriname: A revised model

AbstractThe Proterozoic basement of Suriname consists of a greenstone–tonalite–trondhjemite–granodiorite belt in the northeast of the country, two high-grade belts in the northwest and southwest, respectively, and a large granitoid–felsic volcanic terrain in the central part of the country, punctuated by numerous gabbroic intrusions. The basement is overlain by the subhorizontal Proterozoic Roraima sandstone formation and transected by two Proterozoic and one Jurassic dolerite dyke swarms. Late Proterozoic mylonitisation affected large parts of the basement. Almost 50 new U–Pb and Pb–Pb zircon ages and geochemical data have been obtained in Suriname, and much new data are also available from the neighbouring countries. This has led to a considerable revision of the geological evolution of the basement. The main orogenic event is the Trans-Amazonian Orogeny, resulting from southwards subduction and later collision between the Guiana Shield and the West African Craton. The first phase, between 2.18 and 2.09 Ga, shows ocean floor magmatism, volcanic arc development, sedimentation, metamorphism, anatexis and plutonism in the Marowijne Greenstone Belt and the adjacent older granites and gneisses. The second phase encompasses the evolution of the Bakhuis Granulite Belt and Coeroeni Gneiss Belt through rift-type basin formation, volcanism, sedimentation and, between 2.07 and 2.05 Ga, high-grade metamorphism. The third phase, between 1.99 and 1.95 Ga, is characterised by renewed high-grade metamorphism in the Bakhuis and Coeroeni belts along an anticlockwise cooling path, and ignimbritic volcanism and extensive and varied intrusive magmatism in the western half of the country. An alternative scenario is also discussed, implying an origin of the Coeroeni Gneiss Belt as an active continental margin, recording northwards subduction and finally collision between a magmatic arc in the south and an older northern continent. The Grenvillian collision between Laurentia and Amazonia around 1.2–1.0 Ga caused widespread mylonitisation and mica age resetting in the basement.

Ignimbrites to batholiths: Integrating perspectives from geological, geophysical, and geochronological data

Multistage histories of incremental accumulation, fractionation, and solidification during construction of large subvolcanic magma bodies that remained sufficiently liquid to erupt are recorded by Tertiary ignimbrites, source calderas, and granitoid intrusions associated with large gravity lows at the Southern Rocky Mountain volcanic field (SRMVF). Geophysical data combined with geological constraints and comparisons with tilted plutons and magmatic-arc sections elsewhere are consistent with the presence of vertically extensive (>20 km) intermediate to silicic batholiths (with intrusive:extrusive ratios of 10:1 or greater) beneath the major SRMVF volcanic loci (Sawatch, San Juan, Questa-Latir). Isotopic data require involvement of voluminous mantle-derived mafic magmas on a scale equal to or greater than that of the intermediate to silicic volcanic and plutonic rocks. Early waxing-stage intrusions (35–30 Ma) that fed intermediate-composition central volcanoes of the San Juan locus are more widespread than the geophysically defined batholith; these likely heated and processed the crust, preparatory for ignimbrite volcanism (32–27 Ma) and large-scale upper-crustal batholith growth. Age and compositional similarities indicate that SRMVF ignimbrites and granitic intrusions are closely related, but the extent to which the plutons record remnants of former magma reservoirs that lost melt to volcanic eruptions has been controversial. Published Ar/Ar-feldspar and U-Pb-zircon ages for plutons spatially associated with ignimbrite calderas document final crystallization of granitoid intrusions at times indistinguishable from the tuff to ages several million years younger. These ages also show that SRMVF caldera-related intrusions cooled and solidified soon after zircon crystallization, as magma supply waned. Some researchers interpret these results as recording pluton assembly in small increments that crystallized rapidly, leading to temporal disconnects between ignimbrite eruption and intrusion growth. Alternatively, crystallization ages of the granitic rocks are here inferred to record late solidification, after protracted open-system evolution involving voluminous mantle input, lengthy residence (105–106 yr) as near-solidus crystal mush, and intermittent separation of liquid to supply volcanic eruptions. The compositions of the least-evolved ignimbrite magmas tend to merge with those of caldera-related plutons, suggesting that the plutons record nonerupted parts of long-lived cogenetic magmatic systems, variably modified prior to final solidification. Precambrian-source zircons are scarce in caldera plutons, in contrast to their abundance in some peripheral waning-stage intrusions of the SRMVF, implying dissolution of inherited crustal zircon during lengthy magma assembly for the ignimbrite eruptions and construction of a subvolcanic batholith. Broad age spans of zircons (to several million years) from individual samples of some ignimbrites and intrusions, commonly averaged and interpreted as “intrusion-emplacement age,” alternatively provide an incomplete record of intermittent crystallization during protracted incremental magma-body assembly, with final solidification only when the system began to wane. Analyses of whole zircons cannot resolve late stages of crystal growth, and early growth in a long-lived magmatic system may be poorly recorded due to periods of zircon dissolution. Overall, construction of a batholith can take longer than recorded by zircon-crystallization ages, while the time interval for separation and shallow assembly of eruptible magma may be much shorter. Magma-supply estimates (from ages and volcano-plutonic volumes) yield focused intrusion-assembly rates sufficient to generate ignimbrite-scale volumes of eruptible magma, based on published thermal models. Mid-Tertiary processes of batholith assembly associated with the SRMVF caused drastic chemical and physical reconstruction of the entire lithosphere, probably accompanied by asthenospheric input.

Campanian Ignimbrite volcanism, climate, and the final decline of the Neanderthals

The eruption of the Campanian Ignimbrite at ca. 40 ka coincided with the final decline of Neanderthals in Europe. Environmental stress associated with the eruption of the Campanian Ignimbrite has been invoked as a potential driver for this extinction as well as broader upheaval in Paleolithic societies. To test the climatic importance of the Campanian eruption, we used a three-dimensional sectional aerosol model to simulate the global aerosol cloud after release of 50 Tg and 200 Tg SO2. We coupled aerosol properties to a comprehensive earth system model under last glacial conditions. We find that peak cooling and acid deposition lasted one to two years and that the most intense cooling sidestepped hominin population centers in Western Europe. We conclude that the environmental effects of the Campanian Ignimbrite eruption alone were insufficient to explain the ultimate demise of Neanderthals in Europe. Nonetheless, significant volcanic cooling during the years immediately following the eruption could have impacted the viability of already precarious populations and influenced many aspects of daily life for Neanderthals and anatomically modern humans.

Petrogeochemical aspects of the Late Cretaceous and Paleogene ignimbrite volcanism of East Sikhote-Alin

The chemical and trace-element features of the Late Cretaceous and Early Paleogene ignimbrite complexes of East Sikhote Alin are discussed. The Turonian-Campanian volcanic rocks of the Primorsky Complex compose linear structure of the Eastern Sikhote Alin volcanic belt. They are represented by crystalrich rhyolitic, rhyodacitic, and dacitic S-type plateau ignimbrites produced by fissure eruptions of acid magmas. The Maastrichtian-Paleocene volcanic rocks occur as isolated volcanic depression and caldera structures, which have no structural and spatial relations with the volcanic belt. This period is characterized by bimodal volcanism. The Samarginsky, Dorofeevsky, and Severyansky volcanic complexes are made up of basalt-andesite-dacite lavas and pyroclastic rocks, while the Levosobolevsky and Siyanovsky complexes are comprised of rhyolitic and dacitic tuffs and ignimbrites. Petrogeochemically, the felsic volcanic rocks are close to the S-type plateau ignimbrites of the Primorsky Complex. The Paleocene-Early Eocene silicic volcanics of the Bogopolsky Complex are represented by S- and A-type dacitic and rhyolitic tuffs and ignimbrites filling collapsed calderas. The eruption of A-type ferroan hyaloignimbrites occurred at the final stage of the Paleogene volcanism (Bogopolsky Complex). The magmatic rocks show well expressed mineralogical and geochemical evidence for the interaction between the crustal magmas and enriched sublithospheric mantle. It was shown that the revealed differences in the mineralogical and geochemical composition of the ignimbrite complexes are indicative of a change in the geodynamic regime of the Asian active continental margin at the Mesozoic-Cenozoic transition.

High-frequency, moderate to high-amplitude sea-level oscillations during the late Early Aptian: Insights into the Mid-Aptian event (Galve sub-basin, Spain)

This paper presents a field-based stratigraphic architecture research from the Aptian of Spain in which high-frequency (7cycles/Ma), moderate to high-amplitude (8–127m) sea-level oscillations are evidenced by the repeated incision of palaeovalleys in a carbonate platform developed during the late Early Aptian in a rift setting. Global eustacy, tectonic uplift and mantle dynamic topography are considered as potential mechanisms to explain the observed relative sea-level record. Glacioeustacy is interpreted as the most reliable mechanism to drive the observed sea-level oscillations, however, other mechanisms as oscillations of the groundwater capacity of sedimentary basins and sudden flooding of major a basin as the south Atlantic rift are also considered. A major cooling event is proposed to occur during the late Early Aptian as a response to a large-scale episode of ignimbritic volcanism, increased burial of organic carbon during the Aptian OAE-1a, and a significant demise of carbonate platforms. This cooling event is interpreted as the origin for glacioeustatic sea-level oscillations during the Mid-Aptian.

Synvolcanic crustal extension during the mid-Cenozoic ignimbrite flare-up in the northern Sierra Madre Occidental, Mexico: Evidence from the Guazapares Mining District region, western Chihuahua

The timing and spatial extent of mid-Cenozoic ignimbrite flare-up volcanism of the Sierra Madre Occidental silicic large igneous province of Mexico in relation to crustal extension is relatively unknown. Extension in the Sierra Madre Occidental has been variably interpreted to have preceded, postdated, or begun during Early Oligocene flare-up volcanism of the silicic large igneous province. New geologic mapping, zircon U-Pb laser ablation–inductively coupled plasma–mass spectrometry dating, modal analysis, and geochemical data from the Guazapares Mining District region along the western edge of the northern Sierra Madre Occidental silicic large igneous province have identified three informal synextensional formations. The ca. 27.5 Ma Parajes formation is an ∼1-km-thick succession composed primarily of welded to nonwelded silicic outflow ignimbrite sheets erupted from distant sources. The 27–24.5 Ma Temoris formation is interpreted as an andesitic volcanic center composed of locally erupted mafic to intermediate composition lavas and associated intrusions, with interbedded andesite-clast fluvial and debris flow deposits, and an upper section of thin distal silicic outflow ignimbrites. The 24.5–23 Ma Sierra Guazapares formation is composed of silicic vent facies ignimbrites to proximal ignimbrites, lavas, plugs, dome-collapse deposits, and fluvially or debris flow–reworked equivalents. These three formations record (1) the accumulation of outflow ignimbrite sheets, presumably erupted from calderas mapped ∼50–100 km east of the study area that were active during the Early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up; (2) development of an andesitic volcanic field in the study area, likely related to rocks of the Southern Cordillera basaltic andesite province that were intermittently erupted across all of the northern Sierra Madre Occidental toward the end of and following the Early Oligocene ignimbrite pulse; and (3) the initiation of explosive and effusive silicic fissure magmatism in the study area during the Early Miocene pulse of the mid-Cenozoic ignimbrite flare-up. The main geologic structures identified in the Guazapares Mining District region are NNW–trending normal faults, with an estimated minimum of 20% total horizontal extension. Normal faults were active during deposition of all three formations (Parajes, Temoris, and Sierra Guazapares), and bound half-graben basins that show evidence of synvolcanic extension (e.g., growth strata) during deposition. Normal faulting began by ca. 27.5 Ma during deposition of the youngest ignimbrites of the Parajes formation, concurrent with the end of the Early Oligocene silicic ignimbrite pulse to the east and before magmatism began in the study area. In addition, preexisting normal faults localized andesitic volcanic vents of the Temoris formation and silicic vents of the Sierra Guazapares formation, and some faults were reactivated during, as well as after, deposition of these formations. We interpret extensional faulting and magmatism in the Guazapares Mining District region to be part of a regional-scale Middle Eocene to Early Miocene southwestward migration of active volcanism and crustal extension in the northern Sierra Madre Occidental. We show that extension accompanied silicic volcanism in the Guazapares region, and overlapped with the peak of mid-Cenozoic ignimbrite flare-up in the Sierra Madre Occidental; this supports the interpretation that there is a relationship between lithospheric extension and silicic large igneous province magmatism.

Deep-sea ash layers reveal evidence for large, late Pleistocene and Holocene explosive activity from Sumatra, Indonesia

Deep-sea tephra layers sampled from sediment cores collected within, and adjacent to the Sunda trench of offshore Sumatra reveal evidence for five previously undocumented, and apparently large (minimum volume >0.6–>6.3km3; volcanic explosivity index values of 4–5) explosive eruptions over the last ~31,000years, with a presumptive source of mainland Sumatra. Chemical analysis of glass shards and 14C age constraints are used to distinguish the five tephra layers, as well as a sixth that likely correlates with the Youngest Toba tuff (YTT). The tephra layers are labeled V-1 through V-6 relative to their north-to-south positioning along the Sunda trench. The three tephra layers taken from cores west of central Sumatra (V-3, V-4, V-5) are well-constrained by 14C age determinations, whereas less reliable sedimentation-rate estimates are available for the northern (V-1, V-2) and southern (V-6) tephra layers. Deposition of the northernmost tephra, layer V-1, was likely accompanied by seismicity as two chemically indistinguishable tephras are separated by 12cm of course-grained turbidite. Layer V-2 shows a strong chemical resemblance to the YTT and age estimates do not rule out the correlation. With the exception of a likely correlation with the YTT, no other correlations were made between the tephras analyzed in this study with the marine or terrestrial record from the published literature. The most frequent, widespread, and youngest marine tephra layers were found in the central region of the study area. Layers V-3, V-4, and V-5 were all deposited within the last 17 thousand years with minimum eruptive volumes of >0.6 to >5.2km3. A complex depositional sequence of layer V-6 is estimated at ~27.5ka, and may be associated with Late Pleistocene ignimbrite volcanism of southern Sumatra. The ages and suggested minimum volumes represented by the deep-sea tephra layers are consistent with an active volcanic arc, and demonstrate the need for further terrestrial studies.

Origin of the Colorado Mineral Belt

The Colorado Mineral Belt (CMB) is a northeast-trending, ∼500-km-long, 25–50-km-wide belt of plutons and mining districts (Colorado, United States) that developed within an ∼1200-km-wide Late Cretaceous–Paleogene magma gap overlying subhorizontally subducted segments of the Farallon plate. Of the known volcanic gaps overlying flat slabs in subduction zones around the Pacific Basin, none contains zones of magmatism analogous to the CMB. I suggest that the primary control of the CMB was a northeast-trending segment boundary within the underlying Farallon flat slab. The boundary was dilated during warping of slab segments by the overriding thick (∼200 km) lithospheres of the Wyoming Archean craton and the continental interior craton during acceleration of Farallon–North American convergence beginning in mid-Campanian time (ca. 75 Ma). Because the primary control was not in the North American plate, the CMB cut indiscriminately across the geologic grain of Colorado, seemingly independent of the tectonic elements it crossed. A series of discontinuous shear zones of Proterozoic ancestry provided some local control at the district level but were not the primary control. Geologic contrasts north and south of the CMB reflect its relationship to a segment boundary in the Farallon plate. The dominant trends of Laramide basement-cored uplifts are northwestward north of the CMB but northward south of the CMB. Laramide sedimentary deposits of Late Cretaceous and Paleogene age (exclusive of the Sevier foredeep) are as much as 6 km thick north of the CMB versus only ≤3 km south of the CMB. The Farallon segment south of the CMB rolled back to the southwest and sank into the mantle beginning ca. 37 Ma with resultant major ignimbrite volcanism and generation of the large San Juan and Mogollon-Datil volcanic fields. Volcanism in the Rocky Mountains north of the CMB was sparse. Laramide plutons (ca. 75–43 Ma) are mainly alkaline monzonites and quartz monzonites in the northeastern CMB, but dominantly calc-alkaline granodiorites in the central CMB. Geochemical and isotopic studies indicate that CMB magmas were generated mainly in metasomatized Proterozoic intermediate to felsic lower crustal granulites and mafic rocks (± mantle). Late Eocene–Oligocene rollback magmatism superimposed on the CMB during waning of Laramide compression (ca. 43–37 Ma) resulted in world-class sulfide replacement ores in the Leadville area. Overprinting of the CMB by Rio Grande Rift extension beginning ca. 33 Ma resulted in intrusion of evolved alkali-feldspar granites and generation of major porphyry molybdenum deposits at Climax and Red Mountain.

The exhumed Eocene Sultepec-Goleta Volcanic Center of southern Mexico: record of partial collapse and ignimbritic volcanism fed by wide pyroclastic dike complexes

The Sultepec-Goleta area in southern Mexico hosts one of the major Eocene silicic volcanic centers that make up the Cenozoic volcanic record of the north-central part of the Sierra Madre del Sur. This center is represented by a partially exhumed NNE-SSW trending ignimbritic field that covers an area of ∼400 km2 with a preserved volume of ∼200 km3. A remarkable feature of volcanic center is the exposure of pyroclastic dike complexes up to 1 km wide that extend almost continuously along the western and southern flanks of the volcanic field. The ignimbritic record comprises four units with different textural features as well as variable proportions of the main components (i.e. phenocrysts, pumice and lithic clasts). The most extensive unit is the basal Goleta Ignimbrite, of which the middle and upper members are richer in phenocrysts than the lower member. It has a thickness that ranges from ∼200 m in the north to at least 600 m in the south of the volcanic center. The overlying ignimbrite units (Cienega, Lobera and Potrero ignimbrites) are phenocryst-poor and are distributed in the northern sector of the volcanic field. The semi-curvilinear trend of the pyroclastic dikes along the southern flank of the Goleta Range, coupled with the greater thickness of the Goleta Ignimbrite encircled by the dikes, is indicative of the development of a partial-collapse caldera. Based on spatial relationships as well as analogies in the nature and abundances of the components, it has been recognized that the central and southern dike complexes fed the relatively phenocryst-rich Goleta Ignimbrite, whereas the phenocryst-poor units of the northern sector were extruded through the pyroclastic conduits distributed around Sultepec. We suggest that the stirring produced by the partial collapse of the magma chamber roof beneath the southern sector of the study area triggered the tapping of phenocryst-rich portions of the zoned magma chamber, resulting in an increase of phenocryst content in the upper members of the Goleta Ignimbrite. The presence of breccias and lithic-rich facies dominated by wallrock fragments at the borders of the pyroclastic dikes, including those of the northern extra-caldera sector, indicate that erosion of the conduit walls above the fragmentation level contributed to widening of the conduits and the construction of wide dike complexes.

Birth of the Sierra Nevada magmatic arc: Early Mesozoic plutonism and volcanism in the east-central Sierra Nevada of California

Granitic and volcanic rocks in the east-central Sierra Nevada, western United States, record the earliest stages of magmatism in the eastern Sierra Nevada magmatic arc, allowing us to examine magma sources and connections between plutonic and volcanic processes in the initial stages of arc construction. The Scheelite Intrusive Suite is one of the largest in the Sierra Nevada region, and is composed of the Wheeler Crest Granodiorite, granite of Lee Vining Canyon, and Pine Creek Granite. The Pb/U zircon ages from each unit of the suite suggest assembly between 226 and 218 Ma. The Scheelite Intrusive Suite is a high-K calcic or calc-alkalic suite, compositionally broadly similar to the nearby Late Cretaceous Tuolumne and John Muir Intrusive Suites, though plutons of the Scheelite Intrusive Suite are consistently Ca and Fe rich and lower in Na. Although Triassic granodiorites are isotopically quite similar to nearby Late Cretaceous intrusive suites, the trend toward more isotopically primitive granites is in contrast to the constant or more whole-rock radiogenic Sr trends observed in younger intrusive suites. Along the western margin of the Scheelite Intrusive Suite, the basal Mesozoic volcanic section in the Saddlebag Lake pendant includes silicic volcanic rocks that are in part coeval and potentially comagmatic with Triassic plutonic rocks. Widespread quartz-phyric ash-flow tuffs of Black Mountain, Saddlebag Lake, and Greenstone Lake yield Pb/U zircon ages of 232, 224, and 219 Ma, indicating that felsic ignimbrite volcanism commenced earlier and continued during emplacement of the 226–218 Ma Scheelite Intrusive Suite. Ash-flow tuffs are hydrothermally altered but have high field strength element abundances and Nd isotopic compositions, suggesting affinity to the relatively felsic parts of the Wheeler Crest Granodiorite and the granite of Lee Vining Canyon.

40Ar/39Ar chronostratigraphy of Altiplano-Puna volcanic complex ignimbrites reveals the development of a major magmatic province

The Lipez region of southwest Bolivia is the locus of a major Neogene ignimbrite flare-up, and yet it is the least studied portion of the Altiplano-Puna volcanic complex of the Central Andes. Recent mapping and laser-fusion 40 Ar/ 39 Ar dating of sanidine and biotite from 56 locations, coupled with paleomagnetic data, refine the timing and volumes of ignimbrite emplacement in Bolivia and northern Chile to reveal that monotonous intermediate volcanism was prodigious and episodic throughout the complex. The new results unravel the eruptive history of the Pastos Grandes and Guacha calderas, two large multicyclic caldera complexes located in Bolivia. These two calderas, together with the Vilama and La Pacana caldera complexes and smaller ignimbrite shields, were the dominant sources of the ignimbrite-producing eruptions during the ∼10 m.y. history of the Altiplano-Puna volcanic complex. The oldest ignimbrites erupted between 11 and 10 Ma represent relatively small volumes (approximately hundreds of km 3 ) of magma from sources distributed throughout the volcanic complex. The first major pulse was manifest at 8.41 Ma and 8.33 Ma as the Vilama and Sifon ignimbrites, respectively. During pulse 1, at least 2400 km 3 of dacitic magma was erupted over 0.08 m.y. Pulse 2 involved near-coincident eruptions from three of the major calderas resulting in the 5.60 Ma Pujsa, 5.65 Ma Guacha, and 5.45 Ma Chuhuilla ignimbrites, for a total minimum volume of 3000 km 3 of magma. Pulse 3, the largest, produced at least 3100 km 3 of magma during a 0.1 m.y. period centered at 4 Ma, with the eruption of the 4.09 Ma Puripicar, 4.00 Ma Chaxas, and 3.96 Ma Atana ignimbrites. This third pulse was followed by two more volcanic explosivity index (VEI) 8 eruptions, producing the 3.49 Ma Tara (800 km 3 DRE) and 2.89 Ma Pastos Grandes (1500 km 3 DRE) ignimbrites. In addition to these large caldera-related eruptions, new age determinations refine the timing of two little-known ignimbrite shields, the 5.23 Ma Alota and 1.98 Ma Laguna Colorada centers. Moreover, 40 Ar/ 39 Ar age determinations of 13 ignimbrites from northern Chile previously dated by the K-Ar method improve the overall temporal resolution of Altiplano-Puna volcanic complex development. Together with the updated volume estimates, the new age determinations demonstrate a distinct onset of Altiplano-Puna volcanic complex ignimbrite volcanism with modest output rates, an episodic middle phase with the highest eruption rates, followed by a decline in volcanic output. The cyclic nature of individual caldera complexes and the spatiotemporal pattern of the volcanic field as a whole are consistent with both incremental construction of plutons as well as a composite Cordilleran batholith.

Aptian-Albian coaliferous sediments of Kotel’nyi Island (New Siberian Islands): New data on the section structure and ignimbrite volcanism

The Aptian-Albian sediments of Kotel’nyi Island are represented by a terrigenous coaliferous complex with the apparent thickness of approximately 700 m. The upper two thirds of their section enclose ignimbrites and rhyolitic ash tuffs. The integral thickness of volcanics is 170 m. A new sequence composed largely of acidic volcanics and sedimentary rocks is defined in the upper part of the Cretaceous section. The K-Ar age estimated for ignimbrite glasses is 110–107 ± 2.5 Ma, which corresponds to the first half of the Albian. The fossil flora list is added by several previously unknown forms. The macroflora of Kotel’nyi Island is similar to its Albian counterpart from the Kolyma-Indigirka region and allows Cretaceous sediments from the lower part the Kotel’nyi Island section to be dated back to the Aptian (?)-Albian (except for the terminal Albian). The palynological characteristic of rocks immediately contacting the dated volcanics appeared to be untypical of Albian sediments of Siberia and similar to that of the Late Neocomian palynocomplexes. This is partly explained by erosion and reworking processes. The examined continental sediments accumulated in post-orogenic extension settings. They constitute the lower strata of the Aptian(?)-Tertiary post-orogenic complex filling riftogenic depressions in the New Siberian Islands and Laptev Sea.

Petrogeochemical features of Oligocene-Early Miocene volcaniclastic rocks of the Sea of Japan

Petrographic and geochemical studies showed that the Oligocene-Early Miocene volcaniclastic rocks from the southern part of the Sea of Japan are ascribed to the high-potassium aluminous rocks of the subalkaline volcanic series of active continental margins. A comparative analysis revealed the spatiotemporal relation of Oligocene-Early Miocene subaerial volcanism of the Sea of Japan with Late Cretaceous and Eocene-Early Miocene ignimbrite volcanism of the East Eurasian margin. This allows us to refer the volcaniclastic rocks of the Sea of Japan to a stage of ignimbrite volcanism that occurred during relative quiescence against a general extension in the continental margin setting.

Evolution of the southern termination of the Taupo Rift, New Zealand

To understand the tectonic evolution of the southern termination of the Taupo Volcanic Zone (TVZ), New Zealand, we compare the late Quaternary structure and kinematics of the southern part of the Taupo Rift or Taupo Fault Belt (Mt Ruapehu Graben) with central parts of the rift (Ngakuru Graben), and with the South Wanganui Basin. We also investigate the differences between displacements of Pliocene and late Quaternary markers within the southern Taupo Rift. Comparison of fault displacement rates derived from displacements of late Quaternary and Pliocene markers yields a preliminary estimate of <400 000 yr for fault initiation of the majority of faults in the southern Taupo Rift. The timing of the onset of faulting in the southern TVZ appears to be younger than central parts of the TVZ and may indicate a step‐wise southward propagation of the rift. Faulting also appears to be less evolved in the south than in the central parts of the Taupo Rift (thicker seismogenic crust and hence wider fault spacing), reinforcing the impression of a recent incremental propagation of the Taupo Rift to the south. The onset of faulting coincides with independent observations of the initiation of volcanism in the southern TVZ. We propose that the termination of the TVZ, geographically short of the limit of the Hikurangi margin, may only be temporary, and it is most likely to propagate southward in association with future major perturbations in the subduction margin, such as the occurrence of large ignimbrite volcanism.