Silicic Volcanism Research Articles

The Ediacaran Ferani and Rutig volcano-sedimentary successions of the northernmost Arabian-Nubian Shield (ANS): New insights from zircon U–Pb geochronology, geochemistry and O–Nd isotope ratios

Late Neoproterozoic (Ediacaran) evolution of the northernmost segment of the Arabian-Nubian Shield (ANS) included vast intrusion of granitoids and development of volcano-sedimentary basins. Here we describe two of such volcano-sedimentary successions (Rutig and Ferani) exposed in southern Sinai, Egypt. The successions are dominated by intermediate to silicic volcanics of mostly high-K calc-alkaline affinity, interbedded with immature sediments. Minor andesites form the basal parts of both successions, whereas the dominant volcanics are dacites to rhyolites. The latter are highly fractionated and include A-type granite affinities. In the Rutig succession, the basal andesites have characteristics of low-silica adakitic (LSA) rocks. New zircon U–Pb dating by SIMS of dacites and rhyolites from both successions yielded ages between ca. 627 and 593 Ma. Combined with field evidence and previous age data from interbedded conglomerates, the Rutig succession is divided into a lower and an upper part recording distinct volcano-sedimentary cycles. The lower Rutig volcanic rocks of ca. 630–615 Ma and the associated sediments were folded and then unconformably overlain by the upper Rutig succession (615–590 Ma). Dacites and rhyolites from the lower and upper parts of the Ferani succession record ages of 607–593 Ma overlapping with the end of volcanic activity in the upper Rutig. SIMS δ 18O of dated zircons from dacites in the Rutig and Ferani are all within the mantle range for zircon, and show no consistent trends associated to cores and rim domains. Coupled with minor inheritance in the Rutig and Ferani volcanics this indicates little crustal contamination during evolution of the magmatic suites. Whole-rock ɛNd( t) for the volcanics of the Rutig range between +2.7 and +3.2, whereas slightly but significantly higher values of +3.5 to +4.7 are recorded in the Ferani volcanics. The values are consistent with the juvenile character of the ANS magmas, and also further corroborate models of geographical isotope provinciality in this segment of the shield. Detailed comparison of geochemical patterns and timing of volcanic activity confirms that Rutig and Ferani, as well as several other volcano-sedimentary successions of Sinai correlate with the Dokhan Volcanics of the Eastern Desert of Egypt. The Dokhan Volcanics including those exposed in the Rutig and Ferani successions compare well in terms of age and geochemical patterns with the local 635–590 Ma calc-alkaline plutonic suite, but some volcanic rocks display geochemical similarities with a younger (608–580 Ma) alkaline plutonic suite. This variance may in part reflect a change in the tectonic regime at ca. 610–600 Ma with a change to more alkaline compositions, as reflected in the plutonic suites. However, the occurrence of early (ca. 620 Ma) volcanic rocks of highly fractionated A-type granite affinity in the Rutig succession has not been documented in the calc-alkaline plutonic suite and requires a different explanation. We interpret the Ediacaran magmatic activity in the northernmost ANS to have occurred in a post-collisional regime and note for the occurrence of ca. 635–630 Ma adakitic rocks, including the andesites of the lower Rutig. These may record early manifestations of lithospheric delamination involving parts of mafic lower crust that melted in the process to form adakitic magmas. However, major asthenospheric upwelling, which heated the lithospheric mantle and the crust and triggered vast plutonic and volcanic activity, peaked much later, between 610 and 590 Ma. This later event was also associated with the transition between high-K calc-alkaline and alkaline magmatic activity in this region.

Detailed three-dimensional shear wave velocity structure of the northwestern United States from Rayleigh wave tomography

Since the mid-Miocene, the northwestern United States has experienced extensive flood basalt volcanism, followed by the formation of two time-progressive tracks of silicic volcanism: the Yellowstone/Snake River Plains (YSRP) and the High Lava Plains (HLP). The YSRP track progresses towards the northeast, parallel to North American plate motion, and has therefore often been attributed to a deep mantle plume source. However, the HLP track progresses to the northwest over the same time frame in a direction not consistent with any regional plate motion. The causes of the mid-Miocene flood basalts and the tracks of the YSRP and HLP are a matter of ongoing debate. We present results of Rayleigh wave phase velocity inversions and inversions for 3-D shear wave velocity structure of the northwestern United States using data collected from the High Lava Plains seismic experiment and the EarthScope USArray Transportable Array (TA). The large number of stations used in these inversions allows us to show an unprecedented level of detail in the seismic velocity structures of this tectonically complex area. Our velocity images indicate that low S-wave velocities in the uppermost mantle do not well match the track of HLP volcanism. While at the surface the Newberry caldera appears to anchor the NW end of the HLP hotspot track, the seismic results show that it lies in a separate, north–south trending low velocity band just east of the Cascades that is distinct from the main HLP trace. The ultra-low S-wave velocities beneath the YSRP track extend locally to at least 175 km depth and are by far the most prominent seismic anomalies in the region. Along axis, the YSRP hotspot track is characterized by a discrete low velocity channel in the upper mantle that shallows, narrows and intensifies to the northeast, but then deepens rapidly to the north beneath Yellowstone. The shallowing of the low velocity anomaly to the northeast is consistent with a stationary heat source beneath a moving plate, coupled with Basin and Range extension and heating.

Largest Phanerozoic provinces of silicic volcanism: Tectonic position and genetic prerequisities

This paper presents a brief overview of largest Phanerozoic volcanic provinces of the Earth, which comprise the substantial volumes (over 105 km3) of silicic extrusive rocks. The provinces of this kind may re- present a separate geodynamic class, having the volume comparable to that of intraplate Large Igneous Provinces (LIPs), and being formed in convergent margin settings. The thermal energy preserved in the crust after a previous major magmatic event is considered to be an important factor affecting the volume of crust-derived magmas and playing a significant role in the formation of large provinces of silicic volcanism.

Three‐dimensional electrical resistivity image of magma beneath an active continental rift, Taupo Volcanic Zone, New Zealand

Magmatic activity in regions of continental extension may result in huge (>400 km3) explosive eruptions of viscous, gas‐rich silicic‐magma. Geochemical and geological data suggest that the large volumes of magma erupted are produced by extracting interstitial liquid from a long‐lived ‘mush zone’ (a mixture of solid crystals and liquid melt) that accumulates in liquid‐dominated lenses at the top of a much thicker region of lower melt‐fraction mush. Such lenses will be highly electrically conductive compared with normal mid‐crustal rocks. Here we use results of 220 magnetotelluric (MT) soundings to construct a 3‐D electrical resistivity image of the northern (silicic) part of New Zealand's Taupo Volcanic Zone, a young continental rift associated with very high heat flow and intense silicic volcanism. The electrical resistivity image shows a plume‐like structure of high conductivity, interpreted to be a zone of interconnected melt, rising from depths >35 km beneath the axis of extension.

Facies architecture of a Triassic rift-related Silicic Volcano-Sedimentary succession in the Tethyan realm, Peonias subzone, Vardar (Axios) Zone, northern Greece; Regional implications

In northern Greece, along the western edge of the Paleozoic Vertiscos terrane (Serbomacedonian massif) and within the Peonias subzone – the eastern part of the Vardar (Axios) Zone – a Silicic Volcano-Sedimentary (SVS) succession of Permo(?)–Skythian to Mid Triassic age records the development of a faulted continental margin and the formation of rhyolitic volcanoes along a continental shelf fringed by neritic carbonate accumulations. It represents the early rifting extensional stages that eventually led to the opening of the main oceanic basin in the western part of the Vardar (Axios) Zone (the Almopias Oceanic Basin). Even though the SVS succession is deformed, altered, extensively silicified and metamorphosed in the low greenschist facies, primary textures, original contacts and facies relationships are recognized in some places allowing clues for the facies architecture and the depositional environment. Volcanic and sedimentary facies analysis has been carried out at Nea Santa and Kolchida rhyolitic volcanic centres. Pyroclastic facies, mostly composed of gas-supported lapilli tuffs and locally intercalated accretionary lapilli tuffs, built the early cones which were then overridden by rhyolitic aphyric and minor K-feldspar–phyric lava flows. The characteristics of facies, especially the presence of accretionary lapilli, imply subaerial to coastal emplacement at this early stage. The mature and final stages of volcanism are mostly represented by quartz–feldspar porphyry intrusions that probably occupied the vents. At Nea Santa area, the presence of resedimented hyaloclastite facies indicates subaqueous emplacement of rhyolitic lavas and/or lobes. Moreover, quartz–feldspar–phyric sills and a partly extrusive dome featuring peperites at their margins are inferred to have intruded unconsolidated, wet carbonate sediments of the overlying Triassic Neritic Carbonate Formation, in a shallow submarine environment. The dome had probably reached above wave-base as is indicated by the presence of reworked rhyolitic clasts in the younger mixed rhyolite–carbonate epiclastic sedimentary facies. This facies is interpreted as mass- and debris-flow of mixed provenance, deposited below wave-base. The facies architecture of the SVS succession records a change in volcanic activity from explosive to effusive and then to intrusive. The depositional environment changed from subaerial-coastal to shallow submarine as the silicic volcanism evolved and carbonate sedimentation was progressively taking over, probably compensating for the gradual subsidence of the corresponding basin. Silicic magmatism and carbonate sedimentation were contemporaneous and spatially related. The timing of the rifting, the continental crustal elements involved and the accompanying tectonic, magmatic and sedimentary processes are features of the spatially and temporally evolving western peri-Tethyan region.

Petrogenesis of a silicic magma system: Geochemical evidence from Bamenda Mountains, NW Cameroon, Cameroon Volcanic Line

The concentrations of major and trace elements and the isotopic compositions were measured in felsic lavas of the Bamenda Mountains, an extinct volcanic center of the West Cameroon Highlands (WCH), in order to assess the relative roles of mantle versus crustal magma sources in the evolution of silicic magmatic systems. These silicic volcanics are composed of benmoreites, trachytes and rhyolites associated with alkaline basaltic lavas and have characteristics of A-type magmas. Chemical diagrams show two magmatic lineages based on the ratios of HFSE/LILE (e.g. Hf/Rb: 0.19–0.34 and 0.11–0.23), the degree of fractionation (La/Sm: 5.2–6.4 and 6.5–7.5) and the contents of REE; they are respectively referred to as less-differentiated felsic lavas (LDFL) and high-differentiated felsic lavas (HDFL). Their evolution is mainly governed by alkali feldspar-dominated fractional crystallization. The K/Ar ages on trachytes range from 27.40 ± 0.50 to 12.74 ± 0.25 Ma with the HDFL being mostly younger. Their ε Nd range mostly between −0.88 and +2.43 with one value at −3.43 precluding their derivation from melting of a continental crust. Besides the most negative value (−3, 43) obtained in trachyte BA63 remains high compared to −20.28 calculated for a Bamemda Pan-African granitoid suggesting very low crustal contamination. However, the fact that this lower ε Nd in BA63 is not followed by the corresponding low Ce/Pb and high La/Nb rather indicates a selective role of crustal contamination. This feature seems characteristic of the felsic lavas of the whole Cameroon Volcanic Line. The isotopic and trace element variations observed for the Bamenda felsic lavas are most consistent with a source in the continental lithospheric mantle.

Petrogenesis of Early Cretaceous silicic volcanism in SE Uruguay: The role of mantle and crustal sources

1Dipartimento di Scienze della Terra, Universita degli Studi di Roma La Sapienza, P.le A. Moro, 5, 00185 Roma, Italy 2Istituto di Geologia ambientale e Geoingegneria (IGAG, CNR), c/o Universita degli Studi di Roma La Sapienza, P.le A. Moro, 5, 00185 Roma, Italy 3Dipartimento di Scienze della Terra, Universita degli Studi di Napoli Federico II, via Mezzocannone, 8, 80134 Napoli, Italy 4Departamento de Mineralogia e Geotectonica, Universidade de Sao Paulo, Rua do Lago, 562, Cidade Universitaria, DEP 05508-900, Sao Paulo, Brazil

Trace elements and characteristics of the sources of clastic material in the Devonian and Carboniferous sedimentary formations of the eastern zones of the Southern Urals

This paper focuses on the characteristics of possible provenances for the Devonian and Carboniferous clastic complexes of the eastern zones of the Southern and, in part, Central Urals on the basis of the investigation of trace- and rare-earth element geochemistry, accounting for petrographic evidence. It was found that the material of basic and silicic volcanics, ultrabasic rocks, and metamorphic complexes influenced the geochemical characteristics of the clastic rocks. It was shown that the main sources of the Devonian clastic material were probably subalkaline volcanic rocks from an ensialic oceanic island arc similar to the Silurian arc of the Central Urals, including displaced ones, and the main sources of the Carboniferous sediments were Devonian island-arc complexes and, in part, continental margin volcanics. An eastern source of material existed probably in the Carboniferous in the southern part of the Eastern Ural megazone. The geochemical features of the sedimentary rocks of the Irendyk Formation (upper Emsian-Eifelian) indicate a differentiated source of clastic material.

Large-volume silicic volcanism in Kamchatka: Ar–Ar and U–Pb ages, isotopic, and geochemical characteristics of major pre-Holocene caldera-forming eruptions

The Kamchatka Peninsula in far eastern Russia represents the most volcanically active arc in the world in terms of magma production and the number of explosive eruptions. We investigate large-scale silicic volcanism in the past several million years and present new geochronologic results from major ignimbrite sheets exposed in Kamchatka. These ignimbrites are found in the vicinity of morphologically-preserved rims of partially eroded source calderas with diameters from ∼ 2 to ∼ 30 km and with estimated volumes of eruptions ranging from 10 to several hundred cubic kilometers of magma. We also identify and date two of the largest ignimbrites: Golygin Ignimbrite in southern Kamchatka (0.45 Ma), and Karymshina River Ignimbrites (1.78 Ma) in south-central Kamchatka. We present whole-rock geochemical analyses that can be used to correlate ignimbrites laterally. These large-volume ignimbrites sample a significant proportion of remelted Kamchatkan crust as constrained by the oxygen isotopes. Oxygen isotope analyses of minerals and matrix span a 3‰ range with a significant proportion of moderately low- δ 18O values. This suggests that the source for these ignimbrites involved a hydrothermally-altered shallow crust, while participation of the Cretaceous siliceous basement is also evidenced by moderately elevated δ 18O and Sr isotopes and xenocryst contamination in two volcanoes. The majority of dates obtained for caldera-forming eruptions coincide with glacial stages in accordance with the sediment record in the NW Pacific, suggesting an increase in explosive volcanic activity since the onset of the last glaciation 2.6 Ma. Rapid changes in ice volume during glacial times and the resulting fluctuation of glacial loading/unloading could have caused volatile saturation in shallow magma chambers and, in combination with availability of low- δ 18O glacial meltwaters, increased the proportion of explosive vs effusive eruptions. The presented results provide new constraints on Pliocene–Pleistocene volcanic activity in Kamchatka, and thus constrain an important component of the Pacific Ring of Fire.

The magnetic fraction: A tracer of deep water circulation in the North Atlantic

We report on a mineral magnetic study of a series of Holocene marine sedimentary sequences distributed from northern to southern Bjorn and Gardar drifts (56°N to 61°N) and in the Charlie–Gibbs fracture zone (about 53°N). All the cores are located at water depths bathed by the Iceland–Scotland Overflow Water (ISOW), a precursor of the North Atlantic Ocean. Deep Water formed in the Nordic Seas and flowing into northeast North Atlantic after passing over the volcanic sills between Iceland and Scotland. The magnetic fraction is composed of low Ti-content magnetites with a grain size distribution in the pseudo-single domain range. These grains originate from a unique source in the North through the erosion by the bottom waters of the Icelandic basaltic province. The magnetic grain size remains fairly constant in each core but, on average, progressively fines towards the south and downstream. Likewise the magnetic concentration and mean magnetic flux decrease towards the south. The spatial evolution of the magnetic concentration and of the magnetic grain size suggests a transport of these magnetites by bottom currents associated with the overflow water downstream from the source with progressive deposition of this magnetic fraction along the path of the ISOW. This study provides evidence that characterization of the magnetic parameters of sediments deposited in such an oceanographic context of a unique source upstream and a well-defined water mass may be used to detect and trace deep oceanic circulation changes.

Volcanic and structural evolution of the Okataina Volcanic Centre; dominantly silicic volcanism associated with the Taupo Rift, New Zealand

The Okataina Volcanic Centre (OVC) contains the northeasternmost caldera complex in onshore Taupo Volcanic Zone (TVZ), New Zealand, sited largely within the Taupo Rift. Pyroclastic fall deposits and ignimbrites deposited in the Bay of Plenty coast area between 420 and 625 ka were probably erupted from OVC, and these provide a maximum age for the centre. The earliest ignimbrite for which there is good evidence for eruption from OVC is the c.550 ka Quartz-biotite Ignimbrite, exposed to the west of Lake Okataina and to SE of OVC. This ignimbrite probably correlates with one of the early ignimbrites found in the Kawerau geothermal wells, and is large enough to have been accompanied by caldera collapse. It was followed by extensive rhyolitic explosive eruptions of the Murupara Subgroup, culminating with eruption of the Matahina Ignimbrite at c.325 ka. This c.160 km 3 (magma volume) eruption was accompanied by caldera collapse to form the southern part of the present day Okataina caldera complex. A long duration sequence of rhyolite lavas and pyroclastics was then erupted on the southern and western sides of OVC, before eruption of the > 100 km 3 Rotoiti Pyroclastics at c.61 ka was accompanied by caldera collapse on the northern side of the centre. The Rotoiti episode was followed by an intensive period of intra-caldera volcanic activity which is still going on today. The Mangaone Subgroup pyroclastics were erupted between 40 and 31 ka, and include the c.33 ka Kawerau Ignimbrite (∼ 20 km 3), large enough to have caused further minor caldera collapse. In the last 26 ka, nine rhyolite eruption episodes have built the Haroharo and Tarawera lava and pyroclastic massifs (> 85 km 3 magma volume) within the caldera complex. The structural boundaries of the OVC calderas are buried by the products of later eruptions, but are probably controlled by regional tectonic features. Both the Matahina and Rotoiti calderas appear to have embayments which represent downsags where magma has migrated along regional structures associated with the Taupo Rift. OVC is sited at a major offset within the young Taupo Rift and represents a structurally complex transfer zone. Some early rhyolite domes are aligned north–northwest suggesting control by structures in the subvolcanic basement, while more recent domes are aligned northeastwards, reflecting the orientation of the Taupo Rift. Southwestward propagation of the axial rift of the Whakatane segment and northeastward propagation of the Kapenga segment have created two linear vent zones through OVC (Haroharo and Tarawera). At Tarawera, fissures and near surface dikes formed during the 10 June 1886 basalt eruption are oblique to the vent lineation suggesting some near surface strike-slip component consistent with OVC being in a zone of transtension.

Interaction of model F-bearing silicic melt with chloride fluid, uraninite, and columbite at 750°C and 1000–2000 bar and its implications for estimation of the ore-forming capability of the upper crustal magma chamber beneath the Strel’tsovka caldera, eastern Transbaikalia

The experimental study of an F-bearing silicic melt—U, Nb, Ta minerals—chloride-fluoride fluid system is focused on ascertaining the origin of uranium deposits spatially related to intraplate silicic volcanism. The first series of experiments on uranium solubility in silicic melts close in composition to ore-bearing rhyolite of the unique Strel’tsovka Mo-U ore field has been performed in order to determine more precisely the ore genesis. As starting solid phases, model homogeneous glass of the chemical composition (wt %) 72.18 SiO2, 12.19 Al2O3, 1.02 FeO, 0.20 MgO, 0.33 CaO, 4.78 Na2O, 3.82 K2O, 1.44 Li2O, and 2.4 F (LiF, NaF, KF, CaF2, MgF2); synthetic UO2 and UO3·0.33H2O; and natural columbite were used. The starting solutions contained 1.0 m Cl and 10−2m F. The runs were conducted in a gas vessel at a pressure of 1000 bar and in a high-pressure hydrothermal vessel at 2000 bar. The O2 (H2) fugacity was set by Ni-NiO, Co-CoO, Fe3O4-Fe2O3, and Cu-Cu2O buffers. The equilibrium between melt and solution for major elements is reached during the first day, whereas 5–7 days are required for ore elements (U, Nb, Ta) to come into equilibrium. The solubility of Nb and especially Ta in Cl-F solutions equilibrated with F-bearing melt is extremely low. The solubility of U is much higher (10−4−10−5 mol/kg H2O). The energy dispersive spectroscopy of run products allowed us to establish that columbite dissolved incongruently with formation of U- and F-bearing pyrochlores. The performed experiments have shown that a silicic melt close to the rhyolitic magma of the Strel’tsovka caldera in composition is not able to generate postmagmatic ore-forming solutions containing more than 10−6−10−5 mol U/kg H2O under the relatively low pressure necessary for the existence of the first type of fluid. The amount of uranium that could have precipitated from this fluid in the zone of ore deposition is estimated at 216–9000 t. This estimate is two orders of magnitude lower than the total uranium resources of the deposits localized in the Strel’tsovka caldera. Thus, the upper crustal silicic magma chamber hardly was a source of uranium for Mo-U deposits of the Strel’tsovka ore field.

Magmatic plumbing of a large Permian caldera exposed to a depth of 25 km

We report evidence for a >= 13-km-diameter Permian caldera associated with a renowned section through the continental crust comprising the Ivrea-Verbano Zone and Serie dei Laghi of northwest Italy. Correlation of ages of volcanic and middle to deep crustal plutonic rocks suggests that they constitute an unprecedented exposure of a subcaldera magmatic plumbing system to a depth of 25 km, and points to a cause and effect link between intrusion of mantle-derived basalt in the deep crust, and large-scale, silicic volcanism.

Pyroxene thermometry of rhyolite lavas of the Bruneau–Jarbidge eruptive center, Central Snake River Plain

The Bruneau–Jarbidge eruptive center of the central Snake River Plain in southern Idaho, USA produced multiple rhyolite lava flows with volumes of <10 km 3 to 200 km 3 each from ~11.2 to 8.1 Ma, most of which follow its climactic phase of large-volume explosive volcanism, represented by the Cougar Point Tuff, from 12.7 to 10.5 Ma. These lavas represent the waning stages of silicic volcanism at a major eruptive center of the Yellowstone hotspot track. Here we provide pyroxene compositions and thermometry results from several lavas that demonstrate that the demise of the silicic volcanic system was characterized by sustained, high pre-eruptive magma temperatures (mostly ≥950 °C) prior to the onset of exclusively basaltic volcanism at the eruptive center. Pyroxenes display a variety of textures in single samples, including solitary euhedral crystals as well as glomerocrysts, crystal clots and annealed microgranular inclusions of pyroxene ± magnetite ± plagioclase. Pigeonite and augite crystals are unzoned, and there are no detectable differences in major and minor element compositions according to textural variety — mineral compositions in the microgranular inclusions and crystal clots are identical to those of phenocrysts in the host lavas. In contrast to members of the preceding Cougar Point Tuff that host polymodal glass and mineral populations, pyroxene compositions in each of the lavas are characterized by single rather than multiple discrete compositional modes. Collectively, the lavas reproduce and extend the range of Fe–Mg pyroxene compositional modes observed in the Cougar Point Tuff to more Mg-rich varieties. The compositionally homogeneous populations of pyroxene in each of the lavas, as well as the lack of core-to-rim zonation in individual crystals suggest that individual eruptions each were fed by compositionally homogeneous magma reservoirs, and similarities with the Cougar Point Tuff suggest consanguinity of such reservoirs to those that supplied the polymodal Cougar Point Tuff. Pyroxene thermometry results obtained using QUILF equilibria yield pre-eruptive magma temperatures of 905 to 980 °C, and individual modes consistently record higher Ca content and higher temperatures than pyroxenes with equivalent Fe–Mg ratios in the preceding Cougar Point Tuff. As is the case with the Cougar Point Tuff, evidence for up-temperature zonation within single crystals that would be consistent with recycling of sub- or near-solidus material from antecedent magma reservoirs by rapid reheating is extremely rare. Also, the absence of intra-crystal zonation, particularly at crystal rims, is not easily reconciled with cannibalization of caldera fill that subsided into pre-eruptive reservoirs. The textural, compositional and thermometric results rather are consistent with minor re-equilibration to higher temperatures of the unerupted crystalline residue from the explosive phase of volcanism, or perhaps with newly generated magmas from source materials very similar to those for the Cougar Point Tuff. Collectively, the data suggest that most of the pyroxene compositional diversity that is represented by the tuffs and lavas was produced early in the history of the eruptive center and that compositions across this range were preserved or duplicated through much of its lifetime. Mineral compositions and thermometry of the multiple lavas suggest that unerupted magmas residual to the explosive phase of volcanism may have been stored at sustained, high temperatures subsequent to the explosive phase of volcanism. If so, such persistent high temperatures and large eruptive magma volumes likewise require an abundant and persistent supply of basalt magmas to the lower and/or mid-crust, consistent with the tectonic setting of a continental hotspot.

Climate forcing by iron fertilization from repeated ignimbrite eruptions: The icehouse–silicic large igneous province (SLIP) hypothesis

During middle Eocene to middle Miocene time, development of the Cenozoic icehouse was coincident with a prolonged episode of explosive silicic volcanism, the ignimbrite flare-up of southwestern North America. We present geochronologic and biogeochemical data suggesting that, prior to the establishment of full glacial conditions with attendant increased eolian dust emission and oceanic upwelling, iron fertilization by great volumes of silicic volcanic ash was an effective climatic forcing mechanism that helped to establish the Cenozoic icehouse. Most Phanerozoic cool-climate episodes were coeval with major explosive volcanism in silicic large igneous provinces, suggesting a common link between these phenomena.

Post-Laramide and pre-Basin and Range deformation and implications for Paleogene (55–25 Ma) volcanism in central Mexico: A geological basis for a volcano-tectonic stress model

At central-eastern Mexico, in the Mesa Central province, there are several ranges that were formed after the K/T Laramide compression but before the Basin and Range peak extensional episodes at middle–late Oligocene. Two important volcano-tectonic events happened during this time interval, 1) uplift of crustal blocks exhuming the Triassic–Jurassic metamorphic sequence and formation of basins that were filled with red beds and volcanic sequences, and 2) normal faulting and tilting to the NE of these blocks and fanglomerate filling of graben and half-graben structures. The first event, from late Paleocene to early Eocene, was related to NNE and NNW oriented dextral strike-slip faults. These faults were combined with NW–SE en echelon faulting in these blocks through which plutonism and volcanism occurred. The second event lasted from early Oligocene to early Miocene and coincided with Basin and Range extension. Intense volcanic activity occurred synchronously with the newly-formed or reactivated old fault systems, producing thick sequences of silicic pyroclastic rocks and large domes. Volcano-tectonic peaks occurred in three main episodes during the middle–late Oligocene in this part of Mexico, at about 32–30 Ma, 30–28 Ma, and 26–25 Ma. The objectives of this work is to summarize the volcano-tectonic events that occurred after the end of the Laramide orogeny and before the peak episodes of Basin and Range faulting and Sierra Madre Occidental Oligocene volcanism, and to discuss the influence of these events on the following Oligocene–Miocene volcano-tectonic peak episodes that formed the voluminous silicic volcanism in the Mesa Central, and hence, in the Sierra Madre Occidental. A model based upon geological observations summarizes the volcanic-tectonic evolution of this part of Mexico from the late Paleocene to the Early Miocene.

Thermodynamics of Rhombohedral Oxide Solid Solutions and a Revision of the FE-TI Two-Oxide Geothermometer and Oxygen-Barometer

A model for the thermodynamic properties of rhombohedral oxide solid solutions in the system Fe2O3-FeTiO3-MgTiO3-MnTiO3 (containing minor amounts of Al2O3) is presented. The model accounts for temperature and compositionally dependent long-range cation-order and the related high to low symmetry structural phase transition. The model is calibrated from the cation-ordering data of Harrison and others (2000; Harrison and Redfern, 2001) and experimental data on Fe+2Ti ⇔ (Fe+3)2 exchange between rhombohedral oxide and spinel from Lattard and others (2005) and Evans and others (2006). Successful calibration require introduction of an energetic contribution attributed to short-range cation-order, which reduces the configurational entropy of the solid solution. The resultant thermodynamic model for the rhombohedral oxides is internally consistent with the model for spinel solid solutions of Sack and Ghiorso (1991a, 1991b) and with the endmember thermodynamic properties database of Berman (1988); a new model equation for the isobaric heat capacity of ulvospinel (cubic Fe2TiO4) is proposed and values of the enthalpy of formation, −1490.417 kJ/mol, and third law entropy, 184.199 J/K-mol, at 298.15 K and 105 Pa are recommended. The new model forms the basis of a revised FeTi-oxide geothermometer/oxygen barometer, which is applied to a newly compiled dataset of natural two oxide pairs from silicic volcanic rocks. Results are compared to previous formulations with the general conclusion that the new model gives a better estimate of oxidation state for magmas that equilibrated under conditions more oxidizing than the nickel-nickel oxide buffer. Estimates of oxygen fugacity are fairly insensitive to analytical uncertainties in oxide compositions. By contrast, temperature estimates are especially sensitive to analytical error and to the abundances of “minor” constituents. Application of the geothermometer to oxide pairs that grew under conditions where the rhombohedral phase was cation disordered (that is high temperature or at oxygen fugacities greater by about one log10 unit than the nickel-nickel oxide buffer) results in an uncertainty due solely to analytical error of at least 50°C and sometimes as high as 100 °C. Temperature estimates from the new geothermometer can be made using either the Fe+2Ti ⇔ (Fe+3)2 exchange or Fe+2 ⇔ Mg exchange between the two oxides. Comparison of the two temperature estimates provides a means of evaluating the internal consistency of coexisting oxide compositions and assessing the extent of disequilibrium. Temperatures calculated from the new model are found to be consistent with experimental phase relations for the stability of cummingtonite in silicic volcanics. Other petrologic constraints on derived temperatures are examined including limits on the width of the miscibility gap and the development of self-reversed remanent magnetization in the rhombohedral series. Software that implements the new thermodynamic model and the two-oxide geothermometer/oxygen barometer is available from http://www.ofm-research.org/.

Geochronology of Pliocene volcanism in the Dzhavakheti Highland (the Lesser Caucasus). Part 2: Eastern part of the Dzhavakheti Highland. Regional geological correlation

The results of isotopic-geochronological study of the Pliocene volcanic rocks in reference sections and volcanic edifices of eastern part of the Dzhavakheti Highland (the northwestern Lesser Caucasus) are considered. The isotopic-geochronological data obtained here are correlated with data on western part of the Dzhavakheti Highland, which have been considered in previous part of this work. Based on correlation, time spans of principal volcanic events of the Pliocene in the study region as a whole are determined, and general trends of the young magmatism evolution within the region are established. In sum, the isotopic-geochronological dates evidence that the Pliocene magmatism of the Dzhavakheti Highland developed practically without essential breaks during the period of about 2 Ma long, from 3.75 to 1.75–1.55 Ma ago. The areal basic volcanism that was most widespread at that time is divisible into five discrete phases according to the isotopic dates obtained. Comparatively short pauses, which separated these phases of magmatic activity, were a few hundreds thousand years long, not more. Chemical composition of moderately acidic to silicic volcanics, which are of a limited distribution in the Dzhavakheti Highland, and their age relations with basic lavas of the region suggest that they are most likely the differentiation products of parental basic mantle-derived magmas. The analyzed distribution of volcanic centers, which erupted basic lavas of the Dzhavakheti Highland, evidence that first two phases of basic magmatism were connected here with volcanic activity in southwestern part of the region (northern termination of the Egnakhag Ridge), whereas activity of volcanoes situated on the east, predominantly in water-shed part and on slopes of the submeridional Dzhavakheti Ridge, controlled development of the third and fourth phases. Consequently, magmatic activity of the Pliocene stage in history of the Neogene-Quaternary magmatism of the Dzhavakheti Highland laterally migrated from the west to the east, being controlled by development of regional submeridional extension zones. Volcanic ridges marking the latter are formed by volcanic edifices, which are amalgamated at their bases and have erupted lavas of close age and composition. The migration of volcanic activity can be described in terms of the “domino effect,” when cessation of volcanism in one zone led to formation of the other submeridional zone of extension and magmatic activity displaced from the west eastward in sublatitudinal direction. In general, evolution of the Pliocene magmatism of the Dzhavakheti Highland, was similar, despite the essential regional peculiarities, to the generalized trend of magmatism evolution in the continental rifts and intraplate zones of the “hot-spot” type.

New geologic evidence for additional 16.5–15.5 Ma silicic calderas in northwest Nevada related to initial impingement of the Yellowstone hot spot

Three silicic calderas have been newly identified in northwest Nevada west of McDermitt caldera. This volcanism is interpreted to have formed during a short interval at 16.5-15.5 Ma, during the waning stage of Steens flood basalt volcanism after the initial impingement of the Yellowstone hot spot. New mapping demonstrates that the area affected by this mid-Miocene silicic volcanism is significantly larger than previously appreciated in the western U.S.

Petrology of volcanic and plutonic rocks from the Uimen-Lebed’ terrain, Gorny Altai

The Uimen-Lebed' volcanoplutonic terrane is located at the junction of the Gorny Altai, Gornaya Shoriya, and western Sayan structures and is part of the Devonian-Early Carbonaceous Salair-Altai volcano- plutonic belt. The volcanic facies of the terrane composes the contrasting Nyrnin-Sagan Group, which includes basalt-basaltic andesite and basalt-rhyolite associations. The plutonic facies makes up the multiplet Elekmonar Group, which includes two independent complexes: monzogabbro-monzodiorite-granodiorite-granite and granodiorite-granite-leucogranite. The volcanic and plutonic rocks are asymmetrically distributed: volcanic sequences fill inherited depressions in the eastern part of the terrane, whereas plutonic complexes are located in its western part at the fault system branching from the transregional Kuznetsk-Teletsk-Kurai fault zone. The basalts of the Nyrnin-Sagan Group show geochemical signatures of both suprasubduction and rift-related rocks. The evolution of basaltoid magmatism reflects the formation and development of a suprasubduction man- tle wedge in the inner part of an active continental margin accompanied by the influence of an intraplate mantle source. The silicic volcanism was generated under lower crustal conditions ( P > 10 kbar) at the expense of metabasic materials and was accompanied by the influx of potassium into the anatectic zones. The gabbroids of the Elekmonar Group show suprasubduction geochemical features and no signatures of rift-related struc- tures. The composition of the Elekmonar granitoids indicates significantly shallower (compared with the silicic volcanics) depths of their generation. The Uimen-Lebed' volcanoplutonic terrane in the northeastern part of Gorny Altai was formed in the inner part of an active continental margin of the Andean type. Its magmatic com- plexes were formed over a considerable time range, from the early Emsian, when the formation of the active continental margin began, to the end of the Eifelian or, more likely, the beginning of the Givetian stage.