Stages Of Volcanism Research Articles

Hydrochemistry and isotopic characteristics of non-volcanic hot springs around the Miocene Kofu granitic complex surrounding the Kofu Basin in the South Fossa Magna region, central Honshu, Japan

‐4.6‰, whereas waters from the magnetite series area have positive δ 34 S values ranging from +1.7 to +8.0‰. The hot spring water quality of the Na‐Alk, Na‐SO 4 , Na‐SO 4 ·Cl·Alk, and Ca‐SO 4 types from the volcanic rocks area were estimated to be controlled by anhydrite dissolution, plagioclase weathering, cation exchange reaction of Na‐ montmorillonite, and precipitation of calcite during the fluid flow and mixing process. Different concentrations of SO 4 2‐ ions determined for these waters have a wide range of δ 34 S values ranging from ‐4.1 to +13.6‰, which is likely attributed to the dissolution of 34 S-rich and 34 S-poor anhydrite. The 34 S-rich SO4 2‐ ions were interpreted to be derived from sulfate in sulfuric acid, which arose from the disproportionation reaction of volcanic sulfur dioxide, whereas the 34 S-poor SO 4 2‐ ions were derived from the oxidation of ascending hydrogen sulfide in shallow ground waters during the active stage of past volcanism.

Seychelles alkaline suite records the culmination of Deccan Traps continental flood volcanism

Silhouette and North Islands in the Seychelles represent an alkaline plutonic–volcanic complex, dated at 63 to 63.5Ma by U–Pb zircon and 40Ar/39Ar methods. This magmatism coincides with the final stages of the cataclysmic Deccan Traps continental flood volcanism in India (67 to 63Ma), and thus a causal link has been suggested. Recent reconstructions have placed the Seychelles islands adjacent to the Laxmi Ridge and at the western margin of the Réunion mantle plume at the time of formation of the complex. Here we present geochemical evidence in support of the notion that the Seychelles alkaline magmatism was initiated by the peripheral activity of the Réunion mantle plume and is thus part of the Deccan magmatic event. Positive εNd (0.59 to 3.76) and εHf (0.82 to 6.79) and initial Sr of 0.703507 to 0.705643 at 65Ma indicate derivation of the Seychelles alkaline magmas from a Réunion-like mantle source with an additional minor enriched component, suggesting entrainment of sub-continental lithospheric mantle. The similarity in trace element composition between the Seychelles suite and Deccan alkaline felsic and mafic rocks provides additional evidence for a common mantle source for the Seychelles and Deccan magmatism. Furthermore, we demonstrate the role of fractional crystallisation in the evolution of the alkaline suite. Modelling using major elements suggests that fractional crystallisation and varying degrees of accumulation of olivine, plagioclase, ilmenite, clinopyroxene, alkali feldspar and apatite can describe the spectrum of rock types, from gabbro, through syenite, to granite.

The Sugetbrak basalts from northwestern Tarim Block of northwest China: Geochronology, geochemistry and implications for Rodinia breakup and ice age in the Late Neoproterozoic

Zircons from two samples of the Sugetbrak basalts (SB) yield weighted mean ages of 615.2±4.8Ma and 614.4±9.1Ma. These ages, interpreted as the eruption age of the SB, provide an age constraint on the timing of the Sugetbrak Formation in Sugetbrak, northwestern Tarim Block, China. These two ages suggest that the igneous activities related to the breakup of the Neoproterozoic supercontinent Rodinia lasted until 614–615Ma in at least the northwest Tarim Block. Geochemical analysis indicates that the SB was generated in an intra-continental rifting environment. Application of the dynamic melting inversion method suggests that the Sugetbrak basalts represent 7–12% partial melts. Unlike large-volume tholeiites, these low-degree transitional basaltic melts may represent the waning stage of plume volcanism during a long-lasting continental breakup. Based on the ages of the SB and its stratigraphic relationship with the Yuermeinak diamictite, the Yuermeinak glaciation in the Sugetbrak section of the northwest Tarim Block should correlate with the Tereeken glaciation in the Qurugtagh area of the northeast Tarim Block, the Nantuo glaciation in Yangtze Block and the Elatina glaciation in South Australia.

The earliest mantle fabrics formed during subduction zone infancy

Harzburgites obtained from the oldest crust–mantle section in the Philippine Sea plate (∼52 Ma) along the landward slope of the southern Izu–Ogasawara Trench, preserve mantle fabrics formed during the infancy of the subduction zone; that is during the initial stages of Pacific plate subduction beneath the Philippine Sea plate. The harzburgites have relatively fresh primary minerals despite of their heavy serpentinizations, and show inequigranular interlobate textures, and crystal preferred orientation patterns in olivine (001)[100] and Opx (100)[001]. The harzburgites have the characteristics of residual peridotites, whereas the dunites, obtained from the same location as the harzburgites, provide evidence for the earliest stages of arc volcanism during the inception of subduction. We propose that the (001)[100] olivine patterns began forming in immature fore-arc mantle with an increase in slab-derived hydrous fluids during the initial stages of subduction in in situ oceanic island arc.

Eocene mafic volcanism in northern Anatolia: its causes and mantle sources in the absence of active subduction

Eocene mafic volcanic rocks occurring in an E–W-trending, curvilinear belt along and north of the Izmir–Ankara–Erzincan suture zone (IAESZ) in northern Anatolia, Turkey, represent a discrete episode of magmatism following a series of early Cenozoic collisions between Eurasia and the Gondwana-derived microcontinents. Based on our new geochronological, geochemical, and isotope data from the Kartepe volcanic units in northwest Anatolia and the extant data in the literature, we evaluate the petrogenetic evolution, mantle melt sources, and possible causes of this Eocene volcanism. The Kartepe volcanic rocks and spatially associated dikes range from basalt and basaltic andesite to trachybasalt and basaltic trachyandesite in composition, and display calc-alkaline and transitional calc-alkaline to tholeiitic geochemical affinities. They are slightly to moderately enriched in large ion lithophile (LILE) and light rare earth elements (LREE) with respect to high-field strength elements (HFSE) and show negative Nb, Ta, and Ti anomalies reminiscent of subduction-influenced magmatic rocks. The analysed rocks have 87Sr/86Sr(i) values between 0.70570 and 0.70399, positive ϵNd values between 2.7 and 6.6, and Pb isotope ratios of 206Pb/204Pb(i) = 18.6–18.7, 207Pb/204Pb(i) = 15.6–15.7, and 208Pb/204Pb(i) = 38.7–39.1. The 40Ar/39Ar cooling ages of 52.7 ± 0.5 and 41.7 ± 0.3 Ma obtained from basaltic andesite and basalt samples indicate middle to late Eocene timing of this volcanic episode in northwest Anatolia. Calculated two-stage Nd depleted mantle model (TDM) ages of the Eocene mafic lavas range from 0.6 to 0.3 Ga, falling between the TDM ages of the K-enriched subcontinental lithospheric mantle of the Sakarya Continent (1.0–0.9 Ga) to the north, and the young depleted mantle beneath central Western Anatolia (0.4–0.25 Ga) to the south. These geochemical and isotopic features collectively point to the interaction of melts derived from a sublithospheric, MORB-like mantle and a subduction-metasomatized, subcontinental lithospheric mantle during the evolution of the Eocene mafic volcanism. We infer triggering of partial melting by asthenospheric upwelling beneath the suture zone in the absence of active subduction in the Northern Neotethys. The geochemical signature of the volcanic rocks changed from subduction- and collision-related to intra-plate affinities through time, indicating an increased asthenospheric melt input in the later stages of Eocene volcanism, accompanied by extensional deformation and rifting.

철원분지 동막골응회암의 분출유형과 분출과정

동막골응회암은 대부분 화쇄류암으로 구성되지만 입도와 퇴적구조에 따라 암상을 분류하면 (1) 괴상응회각력암, (2) 용결 응회암 및 라필리응회암, (3) 유변상 응회암 및 라필리응회암이 있고 (4) 괴상 라필리응회암, (5) 성층화 라필리응회암, (6) 점이층리 라필리응회암, (7) 불량층리 라필리응회암과 (8) 괴상 세립질 응회암이 있다. 이 암상들은 상하로 3개 화성쇄설암군으로 구별하여 조합할 수 있다. 하부 화성쇄설암군(LI)은 괴상 라필리응회암, 용결 응회암 및 라필리응회암과 유변상 응회암 및 라필리응회암으로 조합되며, 화쇄류형성 분출로부터 끓어넘침 분출에 의한 화쇄류로부터 정치된 것이다. 중부 화성쇄설암군(LT+MI)은 하부에 성층화 라필리응회암, 점이층리 라필리응회암, 괴상 라필리응회암과 이들에 조합된 괴상 세립질 응회암으로 구성되고 중 상부에 괴상 라필리응회암, 용결 응회암 및 라필리응회암과 유변상 응회암 및 라필리응회암으로 조합된다. 이들은 외부물의 소량 유입으로 수증기마그마성 분출에 의한 화쇄써지로 시작하여 끓어넘침 분출의 화쇄류로 전환된 것이다. 상부 화성쇄설암군(lUT+uUT+UI)은 최하부에 성층화 라필리응회암, 점이층리 라필리응회암으로 조합되고, 하부에 괴상 응회각력암, 불량층리 라필리응회암, 괴상 라필리응회암과 괴상 세립질 응회암으로 조합되며 중 상부에 괴상 라필리응회암, 용결 응회암 및 라필리응회암으로 조합되며, 이들은 수증기플리니언 분출에 의한 써지로 시작해서 플리니언 분출에 의한 강하를 거쳐 끓어넘침 분출로 전환되어 화쇄류로부터 퇴적된 것이다. 결론적으로 동막골응회암에서 분출과정은 대체로 전 중 후기 화산작용이 단계별로 수증기플리니언 혹은 플리니언 분출로 시작하여 화쇄류형성 분출로 전환되고 끓어넘침 분출로 진행되는 순서를 밟았다. 그러나 전기 화산작용은 아마도 초엽의 수증기플리니언 혹은 플리니언 분출 없이 진행되었다. The Dongmakgol Tuff is divided into 8 lithofacies based on their grain size and depositional structures: massive tuff breccia(TBm), welded tuff and lapilli tuff(LTw), rheomorphic tuff and lapilli tuff(LTr), massive lapilli tuff(LTm), stratified lapilli tuff(LTs), gradedly bedded lapilli tuff(LTg), crudely bedded lapilli tuff(LTb) and massive fine tuff(Tm). They can be divided into 3 pyroclastic rock group based on their constituents of the lithofacies. The lower group(LI) is composed of LTm, LTw and LTr, which are interpreted to have resulted from emplacement of voluminous pyroclastic flows due to ignimbrite-form eruption to boiling-over eruption. The middle group(LT+MI) consists of LTs, LTg and LTm associated with Tm in the lower part, and of LTm, LTw and LTr in the middle and upper parts; these suggest that started with deposition of pyroclastic surges from phreatoplinian eruption by poor eternal water, passed through emplacement of pyroclastic flows from ignimbrite-form eruption and ended with deposition of voluminous pyroclastic flows from boiling-over eruption. The upper group(lUT+uUT+UI) is composed of LTs, LTg and Tm in the lowermost, TBm, LTb, LTb and Tm in the lower part, and LTm and LTw in the middle and upper part, suggesting that began with deposition of surges from phreatoplinian eruption, passed through deposition of pumice- and ash-fallouts from plinian eruption and transformed into emplacement of pyroclastic flows due to boiling-over eruption. As result, eruptive processes in the Dongmakgol Tuff approximately began with phreatoplinian or/and plinian eruption, transformed into ignimbrite-forming eruption and proceeded into boiling-over eruption in each volcanism, but proceeded presumably without phreatoplinian or plinian eruption in the earlier stage of 1st volcanism.

Isotopic systematics of the early Mauna Kea shield phase and insight into the deep mantle beneath the Pacific Ocean

The 3500 m deep Hawai'i Scientific Drilling Project core provides a ~680 kyr record of the magmatic history and source components of Mauna Kea volcano. We report high‐precision Pb‐Sr‐Nd isotopic compositions of 40 basalts from the last 408 m of the final drilling phase (HSDP2‐B and HSDP2‐C) and show that these lowermost basalts represent the early shield stage of Mauna Kea's growth history. Two sample groups are distinguished based on their isotopic variability compared to the rest of the core. Over a depth interval of 210 m (3098.2–3308.2 mbsl), the basalts show very restricted isotopic variation and represent sampling of a relatively homogeneous source. Samples from the bottom 192 m record the largest range of 206Pb/204Pb and 208Pb/204Pb in the core, reflecting the greater isotopic variability of the earlier stages of volcanism compared to subsequent stages. The heterogeneity of Mauna Kea lavas is explained by mixing variable proportions of four distinct components intrinsic to the Hawaiian mantle plume. One of these components, Kea, is a prevalent and long‐lived composition within the Hawaiian plume, whereas the other three components are involved at different stages of the volcano's history and contribute to the short‐term isotopic variability of Mauna Kea. The compositional similarity of the Kea component to “C” and to the super‐chondritic bulk‐silicate Earth suggests that Kea may be part of the primitive mantle of a non‐chondritic Earth. Other Pacific oceanic island basalts share Kea‐like compositions, indicating that the Kea component is a common, widespread composition within the Pacific deep mantle.

Chronological implications of the paleomagnetic record of the Late Cenozoic volcanic activity along the Moravia-Silesia border (NE Bohemian Massif)

Abstract This paper presents the results of a paleomagnetic study carried out on Plio-Pleistocene Cenozoic basalts from the NE part of the Bohemian Massif. Paleomagnetic data were supplemented by 27 newly obtained K/Ar age determinations. Lavas and volcaniclastics from 6 volcanoes were sampled. The declination and inclination values of paleomagnetic vectors vary in the ranges of 130 to 174 and -85 to -68° for reversed polarity (Pleistocene); or 345 to 350° and around 62° for normal polarity (Pliocene). Volcanological evaluation and compilation of older geophysical data from field survey served as the basis for the interpretation of these results. The Pleistocene volcanic stage consists of two volcanic phases, fairly closely spaced in time. Four volcanoes constitute the Bruntál Volcanic Field; two others are located 20 km to the E and 65 km to the NW, respectively. The volcanoes are defined as monogenetic ones, producing scoria cones and lavas. Exceptionally, the largest volcano shows a possibility of remobilization during the youngest volcanic phase, suggested by paleomagnetic properties. The oldest one (4.3-3.3 Ma), Břidličná Volcano, was simultaneously active with the Lutynia Volcano (Poland) which produced the Zálesí lava relic (normal polarity). Three other volcanoes of the volcanic field are younger and reversely polarized. The Velký Roudný Volcano was active during the Gelasian (2.6-2.1 Ma) and possibly could have been reactivated during the youngest (Calabrian, 1.8-1.1 Ma) phase which gave birth to the Venušina sopka and Uhlířský vrch volcanoes. The reliability of all available K-Ar data was evaluated using a multidisciplinary approach.

The geochronology and origin of mantle sources for late cenozoic intraplate volcanism in the frontal part of the Arabian plate in the Karacadağ neovolcanic area of Turkey. Part 1. The results of isotope-geochronological studies

This paper considers results from isotope-geochronological (K-Ar) studies of the products of Neogene-Quaternary volcanism in the Karacadag area, which is situated within the northern frontal part of the Arabian plate. It was found that magmatic activity has been evolving at this location for at least the last 11–10 Myr and was distinctly discrete in character. Three stages of volcanism have been identified: (I) Early or Miocene, ∼11–6.7 Ma; (II) Middle or Pliocene-Early Quaternary, 4–1 Ma; and (III) Late or Late Quaternary, 0.4–0.1 Ma. The most recent manifestations of magmatic activity in the region date back to about 100000 years ago.

Basin and Range volcanism as a passive response to extensional tectonics

A long-standing issue in Cordilleran geology involves the nature of Basin and Range (western USA) volcanism, and whether such magmatism provides a trigger for extensional deformation, or if volcanic activity is a passive response to extension. We use space-time-composition patterns across the central and southern Basin and Range, and where appropriate, reconstructed latitudes and longitudes of volcanic rocks, to show that volcanism is fundamentally a passive process. Our analysis suggests that Basin and Range volcanism is initiated by the transition from a subduction to a transform boundary (now manifest as the San Andreas fault), which causes a slab window to open, as the subducting Farallon plate falls away. Accordingly, volcanic activity follows the northward-migrating Mendocino Triple Junction (MTJ). In the wake of the MTJ, continental mantle lithosphere is heated over a time scale of 10–12 Ma; it then rapidly degrades (or is removed) 17–20 Ma after MTJ arrival at any given latitude, and is replaced by asthenosphere. In the central Basin and Range, MTJ migration triggers the well-documented structural migration of the Sierra Nevada away from the Colorado Plateau. Not only is volcanism triggered by the tectonic transition, but in the central Basin and Range volcanism also migrates west, following the initiation of upper crustal extensional faulting; the lag between the onset of extension and initiation of volcanism is a remarkably consistent 2 Ma. Within this framework, the initial stage of eastward-concentrated volcanism is dominated by felsic magmatism, which then gradually degrades to a more mafic composition. This pattern, and temporal relations indicate that once lithospheric extension begins, between 2 and 5 Ma are needed to develop conduits through which nonviscous magmas can transit the crust, and that especially high amounts of extensional strain favor the eruption of K 2 O-rich, low-degree partial melts. Our observations indicate that it is unlikely that mantle plume processes initiated Basin and Range volcanism within the study area, and that decreases in SiO 2 and increases in incompatible element abundances to the east within the Cordillera are best explained by continental mantle lithosphere that thickens to the east, which reduces average melt fractions.

Formation and evolution of the midlands of Venus: Geological features and structures, stratigraphic relationships and geologic history of the Fredegonde area (V-57)

The topographic midlands on Venus comprise about 80% of the surface and an understanding of their mode of formation is essential to unraveling the geologic and geodynamic history of the planet. We explore this question by undertaking a comprehensive geological mapping of the Fredegonde Quadrangle (V-57, 50–75°S, 60–120°E, 1:5M scale) that represents the transition zone from the midlands to the lowlands at the edge of Lada Terra. We report on the geologic units and structures and the sequence of events and, thus, the major stages in the evolution of this region of the midlands. At earlier stages of evolution of the long-wavelength topography, broad (hundreds of kilometers wide) and relatively low (1–1.5km high) topographic ridges formed due to sequential development of deformation zones, first of contractional ridge belts (NW orientation) and then crosscut by extensional groove belts (NE orientation). Arcuate swarms of graben within groove belts often form the rims of coronae and represent their tectonic component. This suggests that groove belts and coronae within the quadrangle formed simultaneously. Intersections of these deformation zones caused separation of the topography of the region into a series of broad, shallow equidimensional basins many hundreds of kilometers across and currently hundreds of meters up to a kilometer deep. Thus, the principal topographic features within the quadrangle were established near the beginning of its observable geological record. The basins then remained sites of accumulation of successive volcanic plains units such as shield plains (psh) and the lower unit of regional plains (rp1). The flows of the younger plains, such as upper unit of regional plains (rp2) and lobate plains (pl), are less voluminous, and flow down the current topographic gradients. This implies that the major topographic pattern of the Fredegonde quadrangle has been stable since its establishment. Further evidence for this is that the vast volcanic plains units (psh and rp1) that postdate the heavily tectonized units of the deformation zones are only mildly deformed. This suggests that since the emplacement of shield plains, volcanism has been the primary geologic process and that the time of formation of unit psh corresponds to a major change from the earlier regime dominated by tectonics to the later volcanically dominated regime. Consistent age relationships among the main volcanic units within the quadrangle from older shield plains, through regional plains, to lobate plains, documents an evolution in volcanic style. Shield plains were formed from small eruptions from ubiquitous small shield volcanoes and are interpreted to be derived from broadly distributed and shallow magmatic sources. The lower unit of regional plains is widely distributed but vents and flow fronts are rare; this unit is interpreted to represent massive and probably short-lived flood basalts-like eruptions that filled in the lowlands basins. The upper unit of regional plains (rp2) and lobate plains (pl) are associated with localized and distinctive sources, such as late-stage volcanic activity at coronae. Thus, the tectonic stage of evolution of coronae (formation of the rims) and the volcanic stage when coronae served as magmatic centers and sourced lava flows, were separated in time by the emplacement of the shield and lower regional plains. How and when did the major components of Venus midland topography form? Clearly, in the Fredegonde quadrangle, regional deformation produced the deformation belts and groove belts/coronae in the earliest phases, and this topography formed the basis for the next, volcanic stage of emplacement (filling of the basins), with coronae-associated volcanism following this phase. The broad topography resulting from this early phase has persisted until the present. We compare this tectonic-volcanic sequence and history of topography in the Fredegonde quadrangle with other areas on Venus and find that the sequence has widespread application globally, and that the history of topography may be similar planet-wide.

Age, geology, geophysics, and geochemistry of Mahukona Volcano, Hawai`i

The size, shape, and magmatic history of the most recently discovered shield volcano in the Hawaiian Islands, Mahukona, have been controversial. Mahukona corresponds to what was thought to be a gap in the paired sequence (Loa and Kea trends) of younger Hawaiian volcanoes (<4 Ma). Here, we present the results of marine expeditions to Mahukona where new bathymetry, sidescan sonar, gravity data, and lava samples were collected to address these controversies. Modeling of bathymetric and gravity data indicate that Mahukona is one of the smallest Hawaiian volcanoes (∼6,000 km3) and that its magmatic system was not focused in a long-lived central reservoir like most other Hawaiian volcanoes. This lack of a long-lived magmatic reservoir is reflected by the absence of a central residual gravity high and the random distribution of cones on Mahukona Volcano. Our reconstructed subsidence history for Mahukona suggests it grew to at least ∼270 m below sea level but probably did not form an island. New 40Ar–39Ar plateau ages range from 350 to 654 ka providing temporal constraints for Mahukona’s post-shield and shield stages of volcanism, which ended prematurely. Mahukona post-shield lavas have high 3He/4He ratios (16–21 Ra), which have not been observed in post-shield lavas from other Hawaiian volcanoes. Lava compositions range widely at Mahukona, including Pb isotopic values that straddle the boundary between Kea and Loa sequences of volcanoes. The compositional diversity of Mahukona lavas may be related to its relatively small size (less extensive melting) and the absence of a central magma reservoir where magmas would have been homogenized.

Post-collisional transition from an extensional volcano-sedimentary basin to a continental arc in the Alborz Ranges, N-Iran

The Alborz Magmatic Assemblage (AMA) is an Eocene volcanic complex in northern Iran, and is situated at the site of the closure of the Tethyan basin. The magmatic rocks of the Alborz assemblage exhibit a distinct progression in style, from shallow submarine explosive eruptions to more effusive sub-aerial eruptions. Their chemical compositions indicate that they belong to the high-K calc‐alkaline (shoshonitic) suite, and are related to either a subduction regime or continental collision. This conclusion is verified by major and trace element abundances, such as enrichments in Light Rare Earth Elements (LREEs) and Large Ion Lithophile Elements (LILEs) (e.g., K, U, and Sr) and depletion in High Field Strength Elements (HFSEs) (e.g., Nb, Ta, Ti, and Zr). However, HFSE plots suggest that the source region of the AMA magmas was affected by multiple processes, including deeply subducted lithosphere and the partial melts of extensional lithosphere in a back-arc environment.The isotopic composition of this suite and their trace element ratios suggest that the primary magmas were derived from a depleted mantle source and were subsequently affected by both fractional crystallization (ol+cpx in basic magmas and plg+bio±hbl in intermediate magmas) and assimilation during magmatic evolution. Assimilation and fractional crystallization modeling, based on isotopic and trace element ratios, indicates that the ascending magmas were contaminated by approximately 40% continental crust.The petrography and geochemical composition of the Eocene Alborz magmatic assemblage indicate that it developed in a back-arc basin, in which explosive eruptions produced various pyroclastic and epiclastic deposits. A subsequent stage of volcanism then produced more effusive sub‐aerial eruptions, as well as sporadic explosions that generated ignimbritic sheets.

Syn-eruptive alluvial and fluvial volcanogenic systems within an eroding Miocene volcanic complex, Lyttelton Volcano, Bank Peninsula, New Zealand

The Diamond Harbour Volcanic Group (DHVG; 8.1–5.8 Ma) was deposited during the last stage of volcanism on Banks Peninsula, New Zealand. Interbedded with eruptives of the DHVG are volcanogenic sequences that constrain erosional extent, processes, timing, and degradation of Lyttelton Volcano. Such phases of erosion and deposition are commonly recorded in volcanic ring plains, but exposures within the highly eroded Lyttelton Volcano provide an excellent example of degradation processes and deposition in the interior of an eroding volcano. At Black Point, eastern Lyttelton Harbour, the Hays Bay Volcanogenic Sequence of the DHVG comprises interbedded conglomerate and sandstone, formed by debris flows to stream flows in an E–W valley-controlled alluvial fan system. Volcanogenic sequences elsewhere around Lyttelton Harbour are formed from multiple alluvial fan systems depositing into a braided alluvial/fluvial, single outlet depositional basin, indicating the inception of a NNE draining proto-Lyttelton Harbour by 8.1 Ma.

Evolution of Macquarie Ridge Complex seamounts: Implications for volcanic and tectonic processes at the Australia–Pacific plate boundary south of New Zealand

The Macquarie Ridge Complex (MRC) coincides with the Australia–Pacific plate boundary south of New Zealand and is characterised by a rugged bathymetry that comprises numerous seamounts along its length. Tectonic plate reconstructions show that the Australia–Pacific plate boundary evolved from a spreading centre to a transpressional boundary from ca. 40 to 6Ma. However, limited sampling and high-resolution mapping of the region have meant that the composition and morphology of the MRC remain relatively unknown. Here we present results from multibeam mapping, photographic investigation and petrologic and geochemical analyses from 10 seamounts along the MRC. Dredge samples collected from the seamounts are alkaline to sub-alkaline basalts, containing 45.1–50.8wt.% SiO2, 3.5–12.0wt.% MgO and 1.9–4.9wt.% total alkalis. All samples are enriched mid-ocean ridge basalts that have K/Ti>0.15, which originated as low degree partial melts during the late stages of mid-ocean ridge volcanism. This oceanic crust has been sheared and accreted along the plate boundary since ca. 6Ma by cumulative transpressional relative plate motion, such that lavas from distinct magma chambers have been juxtaposed at individual seamounts along the ridge. MRC seamounts have formed as discrete elevations as a result of dip-slip and strike-slip faulting of the ridge axis. Three guyot-type seamounts have flat-top summit plateaux that were presumably formed by wave erosion via relative sea-level lowering due to uplift of the ridge. These seamounts have since subsided due to extension associated with the step-over fault geometry of the transform plate boundary along the McDougall and Macquarie ridge segments. While the seamounts are of volcanic origin, they are manifestations of tectonic and geomorphic processes. Seamounts distal from the ridge axis, however, are not subjected to plate boundary deformation and instead have morphologies formed through volcanic processes.

The Catoca kimberlite pipe, Republic of Angola: A paleovolcanological model

The Catoca kimberlite pipe is among the world’s largest primary diamond deposits. The Catoca volcanic edifice is only slightly eroded. Kimberlitic rocks of various facies compose a crater of about 1 km in diameter and a diatreme. The structure of the pipe and mining conditions of the deposit are complicated by intense intrapipe tectonic processes related to large-amplitude subsidence. Based on geological data, we propose a structural model of the deposit and a paleovolcanological model of the Catoca pipe formed during a full cycle beginning with a stage of active volcanism and completed by stages of gradually waning volcanic activity and sedimentation. It is suggested that the banded tuffisitic kimberlite of the crater zone was deposited at the stage of active volcanic eruption from specific pyroclastic suspension as a low-viscosity mixture of crystals and aqueous sol rich in serpentine.

First data on U-Pb SHRIMP-II isotopic dating based on zircons of ash sediments from Cenozoic depressions in Southwest of Primorie

U-Pb (SHRIMP-II) isotopic dating has been conducted for zircons of the ash sediments of the Ust’-Suifun Suite, which was the final stage of Late Cenozoic explosive volcanism in Southwest Primorskii krai. These pyroclastic units are widespread within sediments that fill in Cenozoic depressions including large coalfields. The concordant dates (23.7–24.6 Ma) are in line with the results of the K-Ar determinations for volcanic tephra (23.6–27.1 Ma) and correspond to the beginning of the active phase of spreading and taphrogenesis in the neighboring Trench of the Sea of Japan. These processes started as early as the Eocene and are reflected in the continental vicinity with the formation of riftogenic depressions and the occurrence of a peculiar gas volcanism.

Geochemical variations at intraplate hot spots caused by variable melting of a veined mantle plume

Three-dimensional geodynamic models of plume-lithosphere interaction were used to explore the causes of spatial patterns of magmatic compositions at intraplate hot spots. The models couple mantle flow, heat transfer, and the melting of multiple components present in the mantle as small blobs or veins with different solidi and composition. Predicted magma compositions evolve from having a strong signature from the deepest-melting component in the early stages of volcanism to a strong signature from the shallowest-melting component in the later stages. This compositional trend arises by progressive melt extraction of the different components and a horizontal displacement of their melting zones due to shear flow associated with plate motion. When three or more components are present, the composition of a volcano evolves along arrays in isotope space that trend toward mixed compositions of the components rather than the components themselves. Models explain the average Pb isotope trends in the Hawaiian Scientific Drilling Program core at Mauna Kea volcano. Observed scatter about the average trends and the distinction between the Kea and Loa subchains are explained by spatial variability in the relative proportions of the components in the mantle. Monte Carlo simulations show that linear Pb isotope arrays are unlikely to originate from nonsystematic, binary mixing if the scale of the magma capture zone is much larger than the scale of isotopic heterogeneity. However, systematic sampling by progressive melt extraction naturally generates such linear arrays if the capture zone is large compared to the scale of heterogeneity.

Relationships between Kuroko volcanogenic massive sulfide (VMS) deposits, felsic volcanism, and island arc development in the northeast Honshu arc, Japan

The northeast (NE) Honshu arc was formed by three major volcano-tectonic events resulting from Late Cenozoic orogenic movement: continental margin volcanism (before 21 Ma), seafloor basaltic lava flows and subsequent bimodal volcanism accompanied by back-arc rifting (21 to 14 Ma), and felsic volcanism related to island arc uplift (12 to 2 Ma). Eight petrotectonic domains, parallel to the NE Honshu arc, were formed as a result of the eastward migration of volcanic activity with time. Major Kuroko volcanogenic massive sulfide (VMS) deposits are located within the eastern marginal rift zone (Kuroko rift) that formed in the final period of back-arc rifting (16 to 14 Ma). Volcanic activity in the NE Honshu arc is divided into six volcanic stages. The eruption volumes of volcanic rocks have gradually decreased from 4,600 km3 (per 1 my for a 200-km-long section along the arc) of basaltic lava flows in the back-arc spreading stage to 1,000–2,000 km3 of bimodal hyaloclastites in the back-arc rift stage, and about 200 km3 of felsic pumice eruptions in the island arc stage. The Kuroko VMS deposits were formed at the time of abrupt decrease in the eruption volume and change in the mode of occurrence of the volcanic rocks during the final period of back-arc rifting. In the area of the Kuroko rift, felsic volcanism changed from aphyric or weakly plagioclase phyric (before 14 Ma), to quartz and plagioclase phyric with minor clinopyroxene (12 to 8 Ma), to hornblende phyric (after 8 Ma), and hornblende and biotite phyric (after 4 Ma). The Kuroko VMS deposits are closely related to the aphyric rhyolitic activity before 14 Ma. The rhyolite was generated at a relatively high temperature from a highly differentiated part of felsic magma seated at a relatively great depth and contains higher Nb, Ce, and Y contents than the post-Kuroko felsic volcanism. The Kuroko VMS deposits were formed within a specific tectonic setting, at a specific period, and associated with a particular volcanism of the arc evolution process. Therefore, detailed study of the evolutional process from rift opening to island arc tectonics is very important for the exploration of Kuroko-type VMS deposits.

DEM-based reconstruction of southern Basse-Terre volcanoes (Guadeloupe archipelago, FWI): Contribution to the Lesser Antilles Arc construction rates and magma production

We combine radiometric ages and geomorphologic investigations to quantify the relief creation of well preserved volcanic surfaces, applied to the volcanoes of southern Basse-Terre (Guadeloupe, Lesser Antilles Arc). The last 650 ka volcanic evolution of this island has been modeled using ten main stages constrained by K–Ar ages previously obtained on lava flows and domes from the volcanic massifs of the Axial Chain (1000–435 ka) and from the Grande Découverte Volcanic Complex (250 ka–present). Based on the construction of a 250,000-point database inferred from the analysis of the Guadeloupe Digital Elevation Model, 3D reconstructions of the successive volcanic stage landforms were calculated and the correlated geochronological maps drawn using ArcGIS software. Volumes and rates of construction were computed for each time span separating these ten stages. The average construction rates calculated here are 2.4 ± 0.3 × 10 −4 km 3/yr for the last million years, 0.9 ± 0.3 × 10 −4 km 3/yr for the last 100 ka, and 0.9 ± 0.2 × 10 −4 km 3/yr for the last 15 ka. Although Basse-Terre volcanism is characterized by a marked dominance of effusive products, our estimates should be considered as minimum values because the material that went into the sea during explosive events was not taken into account. However, we note that a relatively high construction rate of 4.5 ± 1.3 × 10 −4 km 3/yr has been obtained for the Icaques volcano, which was emplaced during the 630–600 ka time interval, within the depression formed by the first large-scale flank collapse having affected southern Basse-Terre. The sudden release of the lithostatic load induced by this mass-wasting event could explain this value, which is significantly higher than the average value of 0.8 ± 0.1 × 10 −4 km 3/yr obtained for the last 650 ka. Finally, the comparison with other analogous volcanic massifs from islands and continental arcs points to a relatively low magmatic production for southern Basse-Terre, which could be tentatively related to the relatively slow subduction rate of the Atlantic plate.