Regional Volcanism Research Articles

Paleomagnetically defined brief lifespans for two large shield volcanoes in the Cascades Arc

Mafic to intermediate shield volcanoes with multi-cubic-kilometer eruptive volumes are common in the Cascades Volcanic Arc, but little is known about their eruptive histories as either singular or sustained episodes, or the total time required for their construction. Paleomagnetic data were collected from the lava flows of Ash Creek Butte (17 sites) and Crater Mountain (14 sites) in northern California; both volcanoes are large shields with total volumes of ∼11 km3 each. Tightly clustered paleomagnetic results at both volcanoes, when coupled with analysis of geomagnetic secular variation, suggest that each edifice was built in only a few centuries, possibly in as little time as 50–90 years, indicating sub-century to century scale eruptive durations for two sizeable regional shield volcanoes within the Cascades Arc. These rapidly built shield volcanoes are substantially larger than typically defined monogenetic volcanoes, yet both are wholly formed within a single ‘episode’ at the limits of temporal resolution. Paleomagnetic methods provide a high-resolution tool that can be applied to understanding the tempo of regional volcanism in arcs.

Deoxygenation and organic carbon sequestration in the Tethyan realm associated with the middle Eocene climatic optimum

The middle Eocene climatic optimum (ca. 40 Ma) stands out as a transient global warming phase of ∼400 k.y. duration that interrupted long-term Eocene cooling; it has been associated with a rise in atmospheric CO2 concentrations that has been linked to a flare-up in Arabia-Eurasia continental arc volcanism. Increased organic carbon burial in the Tethys Ocean has been proposed as a carbon sequestration mechanism to bring the middle Eocene climatic optimum to an end. To further test these hypotheses, we assessed the sedimentary and geochemical expression of the middle Eocene climatic optimum in the northern Peri-Tethys, specifically, the organic-rich Kuma Formation of the Belaya River section, located on the edge of the Scythian Platform in the North Caucasus, Russia. We constructed an age-depth model using nannofossil chronobiostratigraphy. Throughout the studied middle Eocene interval (41.2−39.9 Ma), we documented sea-surface temperatures of 32−36 °C based on the tetraether index of tetraethers consisting of 86 carbons (TEX86), depending on proxy calibration, and during the early middle Eocene climatic optimum, we observed sea-surface warming of 2−3 °C. Despite the proximity of the section to the Arabia-Eurasia volcanic arc, the hypothesized source of volcanic CO2, we found no evidence for enhanced regional volcanism in sedimentary mercury concentrations. Sedimentary trace-element concentrations and iron speciation indicate reducing bottom waters throughout the middle Eocene, but the most reducing, even euxinic, conditions were reached during late middle Eocene climatic optimum cooling. This apparent regional decoupling between ocean warming and deoxygenation hints at a role for regional tectonics in causing basin restriction and anoxia. Associated excess organic carbon burial, extrapolated to the entire regional Kuma Formation, may have been ∼8.1 Tg C yr−1, comprising ∼450 Pg C over this ∼55 k.y. interval. Combined with evidence for enhanced organic carbon drawdown in the western Peri-Tethys, this supports a quantitatively significant role for the basin in the termination of the middle Eocene climatic optimum by acting as a large organic carbon sink, and these results collectively illustrate that the closing Tethys Ocean might have affected global Paleogene climate. Moreover, this study highlights the importance of the interplay between global climate and regional oceanic gateway evolution in determining local climate and oceanographic change.

A mid-Permian mafic intrusion into wet marine sediments of the lower Shoalhaven Group and its significance in the volcanic history of the southern Sydney Basin

A mid-Permian (late Kungurian) micro-dolerite dyke/sill complex located at Kinghorn Point intruded into the then unconsolidated upper Wandrawandian Siltstone of the lower Shoalhaven Group, southeastern Sydney Basin. The dark grey siltstone was deposited in the deeper part of a coastal seaway characterised by northward-directed paleocurrents, with a Cruziana ichnofacies, crinoid stem fossils and abundant glendonites indicating a cold climate. Thin periodic storm deposits, or tempestites, of fine-grained sandstone of volcanogenic origin are scattered through the siltstone. These paler sandy beds were probably derived from island volcanoes to the south or southeast, indicating that regional volcanism was pervasive during this glacial episode. Evidence for penecontemporaneous igneous emplacement into mid-Permian wet unconsolidated shallow marine sediments includes: destruction of primary sedimentary structures caused by extensive interaction and intermingling between the sedimentary and igneous materials, the presence of brecciated contacts and peperite, angular and rounded dolerite fragments in the siltstone, fluidisation and entrainment of the sediments into cooling and flow fractures in the magma, and the presence of tube-like flow features. There are also hyaloclastite and baked sediments along the contact margins, fumaroles and vesicles within both the sediment and magma. High vesiculation is characteristic of intrusion near the surface, and possibly extrusion at the sediment–water interface. The geochemistry of the intrusion, especially key element ratios of the high-field-strength, major and trace elements, is very different from that of the shoshonites of the nearby Permian (mid Wordian to Capitanian) Gerringong Volcanics, having much lower Zr/TiO2, Nb/Y, Nb/Zr, K2O, P2O5, Rb, Th, U, Y, Zr and Nb, and higher MgO, suggesting they are unrelated. This indicates that the Kinghorn Point intrusion was associated with an earlier intermediate pre-Gerringong Volcanics phase, or it may be distinctive and unrelated. Magmatic geochemical comparisons further afield along the northern Sydney Basin and Tamworth Fold Belt boundary coasts show that volcanism was pervasive during the Wandrawandian glacial episode and that regional mid-Permian mafic volcanism was well evolved in the southern Sydney Basin during sedimentation. KEY POINTS A mid-Permian syndepositional dolerite intrusion at Kinghorn Point shows classical peperite and soft-sediment deformation features. Previously unreported mafic tuffaceous sandstone beds within the Wandrawandian Siltstone were probably derived from island volcanoes to the south or southeast. Geochemistry of the intrusion shows that it is not related to the mid Wordian to Capitanian Gerringong Volcanics. Regional volcanism was pervasive during the Wandrawandian glacial episode along the southeast Panthalassan margin of Gondwana.

Mercury isotope evidence for regional volcanism during the Frasnian-Famennian transition

Mercury (Hg) concentrations and isotopes are widely used as proxies for volcanic inputs to paleodepositional systems. Enhanced volcanism during the Late Devonian was previously mainly inferred in the western Paleo-Tethys Ocean region from high Hg enrichments around the Upper Kellwasser horizon (UKW) and Frasnian-Famennian Boundary (FFB). In this study, we generated high-resolution Hg concentration data for four UKW-FFB sections (Walnut Creek and Dupont-GHS in the U.S.A., and Yangdi and Nandong in South China) and Hg isotope data for two of them (Walnut Creek and Nandong) in order to test the geographic distribution and stratigraphic range of volcanic inputs during the Frasnian-Famennian transition. Within the uppermost Frasnian Palmatolepis linguiformis Zone and the lowermost Famennian Lower Pa. triangularis Zone, Nandong (Paleo-Tethys Ocean) exhibits enrichment factors (HgEF > 2) and mass independent fractionations (MIF) of odd isotopes (Δ199Hg ∼0 ‰) that are consistent with significant volcanic inputs. Positive Δ199Hg values and an absence of Hg enrichments in the equivalent stratigraphic intervals at Walnut Creek and Dupont-GHS (peri-Panthalassic region) indicate that Late Devonian volcanism was a regional rather than a global phenomenon, and that it may have been associated with subduction zones and/or a large igneous province (LIP) of limited size in the Paleo-Tethyan region rather than with a major continental LIP of potentially global significance.

Titanite U-Pb dating and geochemical constraints on the Paleozoic magmatic-metamorphic events and Nb-Ta mineralization in the Yushishan deposit, South Qilian, NW China

Titanite, as one significant petrogenetic tracer, occurs in the ore-bearing alkaline unit (syenite and aegirine-augite syenite dikes) and in the barren calc-alkaline unit (monzodiorite) and metamorphosed strata (called as leptynite) in the recently found large Yushishan Nb-Ta deposit from South Qilian, NW China. Most titanite grains in the alkaline unit display obvious shape-preferred orientation and zonation, coexisting with allanite, zircon and alkali feldspar. Some titanite grains from the alkaline unit have been partially or completely altered into fine-grained euxenite and/or Nb-rich rutile, but retained their original crystal shapes. Although some titanite grains in the igneous rocks show obvious core-rim textures, both of them yield almost the same U-Pb ages. Combined with U-Pb dating of zircon from these different rock units, the complex magmatic-metamorphic events in the Yushishan district can be subdivided into two different periods: the first one occurring in the Neoproterozoic (ca. 834–790 Ma, zircon U-Pb ages) that are related to the regional volcanism and metamorphic activities and the formation of leptynite during the metamorphism, while the second one occurring in the early Paleozoic (ca. 490–470 Ma, titanite and zircon U-Pb ages) that are related to the formation of granitic intrusions and subsequent metamorphic process and Nb-Ta mineralization. Geochemically, titanite from these igneous rocks is consistent with the typical features of igneous titanite, such as high Fe and REE contents, and negative Eu anomalies. The zoned titanite in both alkaline (mainly syenite) and calc-alkaline units uniformly displays a core-to-rim decreasing trend for the Fe, REE, Nb and Ta contents and Th/U ratios, corresponding to a secular variation from the magmatic to metamorphic evolution. Titanite from the leptynite is interpreted to be of metamorphic origin, with low Fe and REE contents, weakly positively sloping REE patterns, along with positive Eu anomalies. It is suggested that the titanite can be used as an indicator of metal fertility and ore potential for the host rocks/fluids. The high Nb and Ta features in titanite from the alkaline unit suggest a genetic relationship between the alkaline granitic magmatism and Nb-Ta mineralization in the Yushishan deposit. In addition, the high and variable Nb and Ta contents in titanite from the leptynites reveal an inherited signature of the metamorphic fluids. Considering the absence of titanite in the ore-bearing leptynites, we speculate that titanites with high and variable Nb and Ta contents of titanite in the barren leptynites have the potential to provide abundant metals for the precipitation of Nb-Ta-bearing minerals to form economic orebodies.

Mantle geodynamics and source domain of the East Vietnam Sea opening- induced volcanism in Vietnam and neighboring regions

The spreading of the East Vietnam Sea (EVS, also known as Bien Dong, or the South China Sea), leading to the occurrence of syn-spreading (33-16 Ma) and post-spreading (< 16 to present) volcanism. Syn-spreading magma making up thick layers of tholeiitic basalt with a geochemical composition close to the refractory and depleted mid-ocean ridge basalt (MORB) is mainly distributed inside the EVS basin. The post-spreading magma is widely distributed inside the basin and extended to South and SE China, Hainan island, Southern Laos (Bolaven), Khorat Plateau (Thailand), and Vietnam, showing the typical intraplate geochemistry. Basaltic samples were collected at many places in Indochina countries, Vietnam’s coastal and continental shelf areas, to analyze for eruption age, petrographical, geochemical, and isotopic composition to understand the similarities and differences in the mantle sources between regions. The results reveal that basalts from some areas show geochemical features suggesting they were derived subsequently by spinel peridotite and garnet peridotite melting, forming high-Si, low-Mg, and low-Ti tholeiitic basalt to low-Si, high-Mg, and high-Ti alkaline basalt with the trace element enrichment increasing over time. Other basalts have geochemical and isotopic characteristics unchanged over a long period. The post-spreading basalt’s radiogenic Sr-Nd-Hf-Pb isotopic compositions show different regional basalts distribute in the various fields regardless of eruption age, suggesting that their mantle source feature is space-dependent. The post-EVS spreading basalts expose the regional heterogeneity, reflecting the mixture of at least three components, including a depleted mantle (DM) represented by the syn-EVS spreading source, similar to the DUPAL-bearing Indian MORB source; an enriched mantle type 1 (EM1), and type 2 (EM2). The DM may interact and acquire either EM1 or EM2 in the sub-continental lithospheric mantle; as a result, different eruption at different area acquires distinct isotopic signature, reflecting the heterogeneous nature of the subcontinental lithospheric mantle. The study proposes a suitable mantle dynamic model that explains the EVS spreading kinematics and induced volcanism following the India - Eurasian collision from the Eocene based on the research outcomes.

Estimating the effective elastic thickness of the Arctic lithosphere using the wavelet coherence method: Tectonic implications

Currently, knowledge of structure and tectonic evolution of the Arctic lithosphere remains limited. The effective elastic thickness of the lithosphere (Te) is a parameter that describes the lithospheric integrated strength and reflects its thermal and rheological properties, which helps to reveal intraplate tectonic process. In this study, we present a high-resolution Te model in the Arctic region (north of 67°N), which is highly heterogeneous and strongly correlated with various regional tectonic elements. To verify the recovered Te, its pattern is compared to those obtained in previous studies that used different methods and to available heat flow measurements and seismic velocity model and shows good agreement. High Te values are found in the Greenland shield, North American craton and Siberian craton, which are compatible with results from previous studies of high seismic velocity and low heat flow, suggesting that cold, thick lithosphere remains tectonically undisturbed. Low Te values occur in the Amerasia Basin, Baffin Bay, Eurasia Basin and North Atlantic Ocean where young oceanic lithosphere is created. Moreover, the areas affected by regional volcanism in the Siberian Traps and HALIP are dominated by reductions in Te, which we attribute to thermal rejuvenation triggered by the intrusion of magma. Weak lithosphere also dominates in tectonically active orogens as well, including the Alaska-Chukotka, Novaya Zemlya and Caledonides, and is related to the stress released during collisional deformation. We find that a few deep sedimentary basins are characterized by intermediate Te, which implies mechanical support of the surface sediment loads in a non-isostatic state. Our new Te model provides potentially significant insights into various tectonic and geodynamic problems of the Arctic lithosphere.

Trace element fractionation and precipitation in submarine back-arc hydrothermal systems, Nifonea caldera, New Hebrides subduction zone

Hydrothermal activity is abundant at volcanic structures in subduction zones, including those associated with young volcanism in back-arc regions. Fluid boiling is a common process in these environments, but its fractionation and precipitation effects on trace metals and metalloids are still poorly constrained. The submarine back-arc hydrothermal system of Nifonea caldera hosts two recently discovered active vent sites with sulphide-sulphate chimneys showing a diverse mineralogy and chemistry. The focused discharge of fluids with temperatures (up to 368 °C) near the seawater boiling curve at ~ 1860 m water depth and “jets of steam” emitted from the chimney structures suggest fluid boiling. Fluid processes, as well as metal and metalloid deposition vary on a relatively small spatial-scale (<0.5 km2) and coincide with changes in sulphide-sulphate mineralogy and texture between different chimneys with zoning and dendritic intergrowths, indicating temperature gradients and fluid boiling. Boiling-induced precipitation, together with seawater mixing in the sub-seafloor led to a depletion of Zn, Ga, Ge, Ag, Cd, Sb, Au and Pb in the discharging fluids and their precipitates at the main compared to the northern vent site, also resulting in a depleted trace element signature with respect to most other back-arc hydrothermal systems in the Pacific Ocean. A magmatic-hydrothermal signature (high SO42-) in some of the discharging fluids propose a weak magmatic volatile influx to the Nifonea caldera hydrothermal system. However, S isotope data provides no evidence for a magmatic volatile component and rather suggests, in combination with the sulphide-sulphate (Zn, Ge, Se, Ag, Cd, Sb, Ba, Au and Pb) and fluid data (high K) that the Nifonea caldera hydrothermal system is dominantly controlled by a combination of boiling, mixing and water–rock interaction.

Microfacies and stable isotope features of the Lower–Middle Jurassic carbonate rocks of Western Saharan Atlas (Aïn Ouarka area, Algeria)

The Jurassic Lower Carbonate Group in the Aïn Ouarka area of Western Saharan Atlas, Algeria, is represented by six formations, from base to top: Hettangian Chemarikh Dolostone, Early Sinemurian–Pliensbachian Aïn Ouarka Limestone, Toarcian Aïn Rhezala Limestone, Aalenian–Late Bajocian Raknet El Kahla Limestone Breccia, Late Bajocian Theniet El Klakh, and Late Bajocian–Bathonian Tifkirt Limestone formations. From the microfacies analysis, six microfacies types (MF1–MF6) have been recognized and grouped into three associations developed during a transgressive–regressive cycle: (a) inner ramp facies association; (b) middle ramp facies association; and (c) outer ramp facies association. The mineralogical analysis of the carbonate rocks reveals that they contain mostly low‐Mg calcite associated with ankerite, pyrite, and other detrital minerals such as quartz, chlorite, illite, feldspar (albite), and a few clay minerals. These minerals could be related to the deep fluid and hydrocarbon circulations during deposition. The isotopic data display a variation of δ13C isotopic values between −5.14‰ and +2.21‰ (VPDB) and between −8.12‰ and −4.95‰ for δ18O values (VPDB). The set of δ13C values is similar to the signature of marine dissolved inorganic carbon. First of all, the positive values of δ13C show that the origin of carbon is not from the organic‐rich zone (microbial zone), but probably derived from pore‐water and/or biogenic carbonate precursors. On the other hand, the negative values of δ13C indicate that the carbon may result from organic sources linked to the sulphate reduction bacteria activity, or by a heightened volcanic and/or hydrothermal activities releasing light carbon (12C). The negative δ18O values are not consistent with marine water ambient temperature, but with a possible influence of diagenesis or increasing of temperature by hydrothermal water. This hydrothermal activity is controlled by synsedimentary faults during the Early Jurassic and related to a late pulse of the Central Atlantic Magmatic Province (CAMP) volcanism and by regional volcanism during the Middle Jurassic (Bajocian–Bathonian).

Sedimentary stratigraphy of Lake Chalco (Central Mexico) during its formative stages

Lake Chalco lies south of the Basin of Mexico and has been the subject of studies related to Late Quaternary climate variability. In 2016, the International Continental Scientific Drilling Program “MexiDrill Project” recovered a 520-m sediment record from Lake Chalco. Magnetic susceptibility measurements revealed substantial changes in sediment physical properties between 343 and 285 m depth, suggesting changes in composition associated with fluctuations in the depositional environment. We targeted sediments in the 343–285 m interval for high-resolution facies analysis, to develop a model of Lake Chalco formation. We identified three facies associations, using sediment composition, texture, mineralogy and micro-morphological characteristics: (1) detrital facies, consisting of laminated silt, massive sand, stratified silty sand, clast-supported gravel and matrix-supported gravel; (2) biogenic facies, which include diatom ooze and bivalve coquina; and (3) volcaniclastic facies, represented by clast-supported pumice deposits. We propose that formation of Lake Chalco occurred in four stages, which we identified by changes in sediment characteristics. The first stage was an alluvial delta environment, in which debris and hyper-concentrated flows were the main sediment transport agents. The second was characterized by turbulent flows in a fluvial deltaic environment, which alternated with laminar flows associated with floodplains. The third stage was a time of fluvio-lacustrine transition in the basin, with formation of the previously identified Paleo-Chalco-I Lake, in response to wet conditions. During the fourth stage, a deep eutrophic lake formed (Paleo-Chalco-II), with an origin that appears to have been related to regional volcanism. Our working age-depth model indicates establishment of the lake at ca. 400 ± 46 ka. This paper presents the only available record of the transition from alluvial to lacustrine sedimentation of Lake Chalco. Our results allow us to establish (1) how the lake was formed and what the phases of its development were, (2) how a major volcanic event altered and transformed the lacustrine sedimentation, and (3) which climatic conditions dominated during the lake formation. The age for the onset of the lacustrine sedimentation in Chalco is for the first time constrained to around 400 ka. This enables to expand our knowledge of the climate for a time for which there is no information from terrestrial records of tropical North America.

Mercury fluxes record regional volcanism in the South China craton prior to the end-Permian mass extinction

Abstract Enhanced regional subduction-related volcanism in the South China craton concurrent with Siberian Traps large igneous province magmatism was a likely contributor to major biotic and environmental stresses associated with the Permian-Triassic boundary (ca. 252 Ma) mass extinction. However, the timing, intensity, and duration of this regional volcanic activity remain uncertain. We analyzed mercury (Hg) concentrations in three widely separated marine sections in the South China craton (Shangsi, Ganxi, and Chaohu) as well as Hg isotopic compositions in one section (Shangsi) from the Upper Permian (Changhsingian) through the lowermost Triassic (Induan) in order to track volcanic inputs. Four mercury enrichment (ME) intervals, dating to the lowermost Changhsingian (ME1), mid–Clarkina changxingensis zone (ME2), upper C. changxingensis to lower C. yini zones (ME3), and latest Permian mass extinction (LPME) interval (ME4), were recognized on the basis of elevated Hg/total organic carbon ratios. These records provide evidence of strong volcanism in the Tethyan region starting ∼2 m.y. before the LPME, whereas only the ME4 event is recorded in extra-Tethyan sections. Mercury isotopes support the inference that pre-LPME Hg peaks were related to regional subduction-related volcanism, and that Hg emissions at the LPME were the result of Siberian Traps large igneous province intrusions into organic-rich sediments. This study demonstrates the feasibility of distinguishing flood-basalt from subduction-related volcanic inputs on the basis of marine sedimentary Hg records.

Post-rift regional volcanism in southern Santos Basin and the uplift of the adjacent South American coastal range

The Serra do Mar coastal mountain range plays a crucial role in the sedimentary budget of both the South American continent and its adjacent Atlantic margin. This 1500-km range began to rise during the Late Cretaceous. The uplift prevented the direct input of sediments from the continent to Santos Basin, thus diverting the Paraná River drainage inland and southwards. However, the cause for the uplift is still debated: distinct phases of the Andean orogeny, thermal activity of the Trindade hotspot, and a general thermal weakening caused by earlier mantle plume activity. The current study attempts to explain the uplift initiation by introducing a previously unknown 150-km wide magmatic cluster discovered in multichannel seismic reflection data across the southern Santos Basin. The newly discovered “Santos cluster” includes plutons, sills, and volcanic edifices within the Turonian-Campanian stratigraphic interval, indicating an Early Campanian magmatic phase. We suggest that the widespread magmatism uplifted the southern Santos Basin, and included lateral NNE salt migration. The marine magmatism occurred along with previously reported Campanian volcanic activity in three localities along the Serra do Mar, which uplifted the coastal range and diverted the drainage inland (westward). We suggest that both marine and continental magmatism relate to the SW sub-crustal flow of the Trindade hotspot head.

Lower Jurassic felsic diatreme volcanism recognized in central Patagonia as evidence of along-strike rift segmentation

The early Jurassic volcanism of Central Patagonia covers an extensive area of 50,000 km2 where the volcanic deposits occur as isolated systems. The volcanic records form an elongated belt in which the composition, depositional and genetic features, show several differences along with its distribution. The Cañadón Chileno Complex (CCHC), located in the Río Negro province, provides the opportunity to evaluate and improve the knowledge about the Lower Jurassic volcanic stratigraphy and the Lower Jurassic regional setting of Central Patagonia.Based on the field and laboratory data developed in the present work, 22 lithofacies were recognized and grouped into eight facies associations. The continental sedimentary environments include alluvial fan deposits (FA 1), ephemeral deposits (FA 2), braided fluvial deposits (FA 3), alluvial plains, or over banks (FA 4), and lacustrine deposits (FA 9). On the other hand, the pyroclastic facies were separated according to the pyroclastic currents involved during the deposition; into fall-out, dilute, and density currents (FA 5, FA 6, and FA 7). Effusive andesitic feeders and lava-flows (FA 8) were also recognized.The stratigraphic data obtained in the present work allow proposing the existence of two felsic diatreme volcanoes in the CCHC records - described here as Southern and Northern Zone-, developed over a local subsided area, represented by an asymmetrical basin.Seven units were recognized and described in the CCHC (Units a, b, c, d, e, f, and g: unit (a) represents an initial stage of continental sedimentation recorded throughout the entire Complex. Unit (b) represents the upper felsic diatreme facies, in Southern Zone of the Complex, where debris flows are interbedded with massive lapilli-tuff deposits. Unit (c) consists of local andesitic lavas flows and feeder dikes, and the unit (d) consists of welded lapilli tuff deposits interpreted as the growth of the volcanic system. The Northern Zone evolves similarly with the felsic intra-diatreme deposits of unit (e). The unit (f) consisting of an effusive stage represented by extended andesitic lava flows. Finally, a deep lacustrine system is installed (unit g) that includes shallow and deep facies, with intercalations of local pyroclastic deposits.Also, new geochronological data (U-Pb zircon age of 188 ± 3 Ma) was determined to confirm and reinforce the correlation criteria between the different volcanic areas in Central Patagonia, indicating that the volcanism described here is synchronous with the Northern silica-rich calderas of the Garamilla Formation as well as the large andesitic volcanoes of the Lonco Trapial Formation located southward to the CCHC. The regional volcanism changes described, in the present work, are connected with the N-S rift segment limited by E-W regional transfer fault systems developed in continuity to the strike-slip structures of the La Esperanza area.

Eruptive chronology of monogenetic volcanoes northwestern of Morelia – Insights into volcano-tectonic interactions in the central-eastern Michoacán-Guanajuato Volcanic Field, México

Twelve monogenetic volcanoes formed during the last 7 Ma within the Tarímbaro graben, northwest of the city of Morelia, in the central-eastern part of the Michoacán-Guanajuato volcanic field (México). These include four scoria cones (Pelón, Tetillas, Jamanal, and Parastaco), four lava domes (La Cruz, Divisadero, Estadio, and Tetillas), two small shield volcanoes (Quinceo and Tetillas) and two lava flows (Cuto and Cerritos). Based on a detailed geological map and stratigraphy aided by new 40Ar/39Ar and 14C radiometric dates, and whole-rock chemical analyses, we established the eruptive chronology of these monogenetic volcanoes. These volcanoes were built upon four early Miocene successions of regional volcanism named Cuitzeo lavas (18.7 Ma), Cuitzeo ignimbrite (17.4 Ma), Atécuaro ignimbrite (16.8 Ma), and Punhuato lavas (16.3 Ma). Late Miocene to Pliocene activity consisted of the La Cruz, Divisadero, and Estadio domes and the Cuto lava flow (6.7–3.1 Ma). Pleistocene activity included the Cuitzeo fallout pyroclastic sequence (1.48 Ma), the Quinceo small shield volcano (1.36 Ma), the Pelón scoria cone (0.84 Ma), the Tetillas small shield volcano, two lava domes and a scoria cone (0.56–0.34 Ma), the Jamanal and Parastaco scoria cones and the Cerritos lava flow (0.11 Ma). Magma volumes from these volcanoes vary from <0.1 to 4 km3. We ascribe their variety of magma volumes, morphology, volcanic styles, and spatio-temporal distribution to tectonic changes and evolution of the ENE Tarímbaro graben, whose faults belong to the Tarímbaro-Álvaro Obregón fault segment in the central part of the Morelia-Acambay Fault System. Quinceo (2 km3) and Tetillas (4 km3) small shield volcanoes are by far the most voluminous centers dominating the landscape of the city of Morelia. All rock samples have compositions varying from 49.3 to 61.6 wt % in silica with medium-K contents (0.43–1.83 wt %).

Multiscale petrographic heterogeneity and their implications for the nanoporous system of the Wufeng-Longmaxi shales in Jiaoshiba area, Southeast China: Response to depositional-diagenetic process

Abstract The organic matter-rich shales in Wufeng-Longmaxi Formation, Jiaoshiba area, Southeast China, are showing a notable petrographic heterogeneity characteristic within the isochronous stratigraphic framework, which lead to vast differences in the mineral composition and organic matter abundance in the adjacent sections of the shale reservoir. The studied shale has been divided into three systems tracts: a transgressive systems tract (TST), an early highstand systems tract (EHST), and a late highstand systems tract (LHST). Multiple-scale petrographic observation and detailed mineralogical and geochemical analyses were combined to investigate the manifestation, origin, and the ways by which the shale heterogeneity is affected. The results indicate that polytropic depositional environments lead to different components in sediment. Subsequently, these differences among shale sections become more apparent through different diagenetic pathways. During the deposition of the section TST, the Hirnantian glaciation and regional volcanism played a crucial role, contributing to the abundant accumulation of fine-grained intrabasinal silica and organic matter. In diagenesis stage, authigenic quartz aggregates derived from siliceous organisms are formed. They filled in primary interparticle pores, forming a rigid particle-bracing structure that provide effective resistivity against the compaction and spaces for organic matter migration and occlusion. Finally, the migrated organic matter left plenty of newly created pore spaces that constituted a great portion of the total porosity of shale reservoir. The depositional process of section EHST is strongly influenced by contour current, which brings about more extrabasinal influx and impoverishes organic matter. In diagenesis stage, the rigid particle-bracing structure could only be preserved in limited areas, since insufficient siliceous supply could not produce enough authigenic quartz. Primary interparticle pores are significantly reduced owing to compaction, leaving less space for later organic matter migration and occlusion. As a result, the total porosity of shale reservoir declines in this section. In a rapid tectonic-uplifting background, the deposition of section LHST is associated with a rapid increase in terrigenous clay minerals, which further dilutes organic matter. Ductile clay experienced strong compaction and then occupies most of the primary interparticle space. Rigid particles are wrapped by a large number of clays, which has destroyed the particle-bracing structure. As a result, the nanoporous system in the shale could not be well preserved.

Temporal relationship between the Lassen volcanic center and mafic regional volcanism

A new set of 17 unspiked K-Ar and five 40Ar/39Ar ages, together with previously published radiometric ages for the volcanic region around Lassen volcanic center (LVC) in the Southern Cascades, USA, shows that eruptive periods at LVC have occurred in close temporal relationship with mafic eruptive sequences within the broader Lassen region, up to 50 km from the LVC. The back-arc Caribou volcanic field (CVF, ~ 800–15 ka) and fore-arc volcanoes, active simultaneously with the LVC, are characterized by eruptive sequences that each persisted for 20–40 ky, with the most recent eruptions occurring during the last glacial episode (< 15 ky). Crater Mountain, a relatively young (282–395 ka) shield volcano spatially close to the CVF, confirms the presence of localized higher fluxes of mantle-derived melts that persisted for hundreds of thousands of years in the back-arc region. Crater Mountain eruptions overlap in time with LVC and CVF eruptions. Cumulatively, during the past 3.5 Ma, periods of intense regional mafic activity have occurred simultaneously with eruptive sequences at the silicic LVC. This link in eruption timing between the LVC and regional mafic volcanoes suggests that a broadly distributed increase in heat flux triggered spatially dispersed mantle-derived melt injections. The increase in regional melt injections triggered rejuvenation of crystal mushes beneath LVC, explaining the coincidence in eruption timing.

Detrital zircon provenance record of middle Cenozoic landscape evolution in the southern Rockies, USA

The high-elevation, high-relief landscape of the southern Rocky Mountains (Rockies), USA, reflects interactions between tectonics, paleoclimate, and surface processes. The southern Rockies experienced several episodes of uplift, extension, and major climate changes during the Cenozoic, but the landscape and river drainage evolution remain poorly constrained. Here we apply detrital zircon UPb geochronology to Eocene-Miocene strata in south-central Colorado to constrain the depositional ages and sediment provenance. A total of 1284 concordant UPb ages were determined and are grouped into 75–11 Ma, 1500–1300 Ma, and 1800–1500 Ma populations. Intense late Eocene-Oligocene regional volcanism provided abundant air fall zircons into the latest Eocene-Oligocene sedimentary systems where maximum depositional ages can be used to closely proximate depositional ages and improve the chronostratigraphy. The new data are integrated with interpretation of sedimentary environments, and the detrital zircon signatures of potential source terranes and age-equivalent strata in other nearby basins to interpret landscape and paleodrainage evolution. Specifically, the new provenance data show that (1) after the main phase of the Laramide deformation, the Wet Mountains, but not the Sangre de Cristo Range, was the dominant local topographic feature, and a southward-flowing river connected the Wet Mountain Valley with the Huerfano and Raton Basins to the south; (2) during the Eocene-early Oligocene, aggradation of the Wet Mountain Valley and the Huerfano Basin formed a low-relief surface, and subsequent river erosion changed the drainage pattern eastward and likely formed the modern lower Arkansas River valley; and (3) during the Miocene, dissection of the low-relief surface by the opening of the Rio Grande Rift formed the upper Arkansas River valley, and the upper valley was connected with the lower valley to establish the modern drainage pattern of the Arkansas River in the southern Rockies.

Volcaniclastic lithostratigraphy and paleoenvironment of the lower Eocene McAbee fossil beds, Kamloops Group, British Columbia, Canada

The lower Eocene McAbee fossil beds (∼53 Ma), in south-central British Columbia, Canada, represent a lacustrine sequence deposited during a time of pervasive regional volcanism. Previous studies on fossil assemblages at the McAbee fossil beds consist of amalgamated collections of plants from several disjunct and stratigraphically unconstrained exposures and horizons, with limited knowledge of the spatio-temporal variation in depositional and taphonomic setting. This study presents a high-resolution lithostratigraphic analysis of the McAbee main site to provide stratigraphic, paleoenvironmental, and taphonomic context to fossil collections. A lithostratigraphic framework was developed for the McAbee main site by correlating tuff marker beds. The sequence was divided into eight lithostratigraphic units on the basis of systematic lithologic trends, a result of varying degrees of volcanic influences. An absence of shallow water indicators, bioturbation, and evidence for fluvio–deltaic influence, and the nonrestricted presence of highly abundant and diverse well-preserved plant fossils indicates a deep water, yet relatively near shore facies, suggesting steep sided lake margins. This taphonomic regime imparts minimal transport- and degradation-induced biases in fossil plant assemblages and suggests plant fossils represent local vegetation growing near the point of deposition. The new lithostratigraphic framework coupled with a refined understanding of depositional setting and taphonomic regime demonstrates the opportunity to document conditions of forest ecology within a dynamic volcanic environment over millennial and multi-millennial time scales.

Terrestrial and marine floral response to latest Eocene and Oligocene events on the Antarctic Peninsula

ABSTRACTPalynological results from opposite sides of the northernmost Antarctic Peninsula provide insight on terrestrial vegetation and sea-surface conditions immediately before the Eocene–Oligocene transition (EOT), through Early Oligocene glacial conditions and the subsequent Late Oligocene interglacial interval. A latest Eocene sample set from the uppermost La Meseta Formation on Seymour Island, James Ross (back-arc) Basin, records a low-diversity Nothofagus (southern beech)-dominated vegetation with some podocarp conifers similar to Valdivian-type forest found today in Chile and Argentina. Marine organic-walled phytoplankton include leiospheres and Eocene dinoflagellate cysts such as Vozzhennikovia rotunda, V. apertura, Senegalinium asymmetricum and Spinidinium macmurdoense. Immediately before the EOT near the top of the section the decrease in terrestrial palynomorphs, increase in reworked specimens, disappearance of key dinocysts, and overwhelming numbers of sea-ice-indicative leiospheres plus the small dinoflagellate cyst Impletosphaeridium signal the onset of glacial conditions in a subpolar climate. Early to Late Oligocene samples from the Polonez Cove and Boy Point formations on King George Island, South Shetland Islands (magmatic arc), yielded an extremely depauperate terrestrial flora, likely resulting in part from poor vegetation cover during the Polonez Glaciation but also because of destruction of vegetation due to continued regional volcanism. The prevalence of sea-ice-indicative leiospheres in the marine palynomorph component is consistent with polar to subpolar conditions during and following the Polonez Glaciation.

40Ar-39Ar geochronology and petrogenesis of postcollisional trachytic volcanism along the İzmir-Ankara-Erzincan Suture Zone (NE, Turkey)*

The obliteration of the Neo-Tethyan Ocean and collision of the microplates along the northern part of Turkey led to the development of the Izmir-Ankara-Erzincan suture zone (IAESZ). After the collision of Pontides with the Central-Anatolian Crystalline Complex (CACC) in the Paleocene, a new phase of extension and volcanism concomitantly developed along the northern (Almus; Pontides) and southern (Yildizeli; CACC) sides and along the IAESZ during the Middle Eocene time interval. The first products of the Middle Eocene volcanism in these areas are represented by calc-alkaline to alkaline (basic-intermediate) volcanic and volcanoclastic units together with late-stage trachytic dikes, plugs, and stocks. The mantle source area of both volcanic units displays a metasomatized character, which was dominantly fluxed by sediment-sourced melts. The partial melting of the metasomatized source area gave rise to first-stage basic-intermediate volcanism in the crustal levels. Simultaneously with the generation of the first-stage volcanism, basaltic trachyandesitic shallow-seated magma mushes were also developed. The reactivation of these shallow-seated mushes by late-stage extensional tectonics gave rise to the development of trachytic volcanism in both regions, which have a high-K to shoshonitic character. Almus trachytic lavas are phenocryst-poor and have differentiated Mg# numbers (avg. 26). On the other hand, Yildizeli trachytic lavas have a broad compositional range (benmoreite to latite); they are phenocryst-rich and show more basic character (Mg# avg. 40). Trachytic volcanism in both areas is largely controlled by fractional crystallization of similar basaltic trachyandesitic parental magma with minor assimilation of the upper crustal lithologies. 40Ar-39Ar ages from sanidine phenocrysts from both areas also confirm that trachytic volcanism in both regions developed nearly coevally in different tectonic blocks (~41?40 Ma). Generation of similar volcanism on the different tectonic blocks during the postcollisional stage was probably governed by a regional-scale delamination and/or lithospheric removal-related tectonomagmatic processes.