Volcanic Succession Research Articles

The Paleoarchean Buffalo River komatiites: Progressive melting of a single large mantle plume beneath the growing Kaapvaal craton

Several Archean granitoid-greenstone terranes are exposed on the southeastern Kaapvaal craton in South Africa, but they received little scientific attention compared to the archetypal greenstone belt successions of the Barberton Mountain Land at the eastern craton margin. This study reports on a detailed field and geochemical survey of the Buffalo River Greenstone Belt at the southern Kaapvaal craton margin in KwaZulu-Natal, with focus on hitherto unstudied komatiites and basaltic rocks from this volcanic succession. Cross-cutting relationships and new U-Pb zircon age determinations for several granitoid units establish a minimum age of 3.26 Ga for komatiitic volcanism, possibly as old as ca. 3.5 Ga if a 3.47 Ga granodiorite sheet is interpreted as ‘intrusive’ into the greenstone succession.Geochemical data reveal three types of Paleoarchean komatiites at Buffalo River. Spinifex textured lava flows represent Al-depleted komatiites, with subchondritic Al2O3/TiO2 ratios and enrichment of LREE over HREE. The second type comprises Al-undepleted komatiites that have chondritic Al2O3/TiO2 and flat REE patterns. The third type identified comprises Al-enriched komatiites that display suprachondritic Al2O3/TiO2 ratios, with significant LREE depletion. The Al-depleted and Al-undepleted komatiites from Buffalo River are geochemically similar to komatiites from the 3.48 Ga Komati and 3.26 Ga Weltevreden formations of the Barberton Supergroup respectively, whereas the Al-enriched komatiites resemble the 3.33 Ga Commondale komatiites on the southeastern Kaapvaal craton.To explain the co-occurrence of three discrete komatiite types within a single volcanic succession at Buffalo River, we suggest that each major komatiite magmatic pulse originated from the same upwelling mantle source, from which melt was extracted at different pressure but similarly hot temperature conditions. 187Os/188Os data for the Al-depleted komatiites suggest an ultimate magma origin from a primitive mantle reservoir. The contrasting γOs values for Kaapvaal craton komatiites (zero to positive) and peridotitic mantle xenoliths (zero to negative) support a complementary nature of these lithologies as high-degree melts and depleted residues linked by vigorous mantle plume activity at around 3.5 Ga. Such a relationship can explain the contrasting Re/Os systematics of komatiites and lithospheric mantle peridotites, which creates the contrasting γOs over time.The highly siderophile element patterns of the Al-depleted komatiites from Buffalo River are similar to those of Barberton-type komatiites, for which an origin from the deepest upper mantle with high melt retention in an upwelling plume source was suggested. We confirm that this ca. 3.5 Ga mantle source had only 60–80 % of the platinum-group element budget of the modern ambient mantle, which points indirectly to a location at great depth in the aftermath of the meteoritic late accretion. Progressive melting of such an upwelling mantle source, to the point of majoritic garnet exhaustion, may explain the Al-undepleted and Al-enriched komatiites at Buffalo River. The presence of all three major komatiite types within a single volcanic succession may be linked to deep critical melting of a large mantle plume associated with growth of the Kaapvaal ‘continent’ at 3.5 Ga.

Revised stratigraphy and first geochronology of the Miocene submarine volcanic succession at Kennaook/Cape Grim, northwestern Tasmania

Kennaook/Cape Grim in far northwestern Tasmania, Australia, was a site of submarine intraplate basaltic volcanism during the Miocene. The succession is exceptionally well preserved and is dominated by pillow lava, massive lava and pillow fragment breccia. The total volume of volcanic products (1.1 km3) is relatively small on a global scale, but the preservation is world class. The oldest unit, the Woolnorth Tuff lies unconformably on the Neoproterozoic Rocky Cape Group. The first 40Ar/39Ar dating of the volcanic rocks reveals an early Miocene (24.5–23 Ma) age for the entire sequence. The Woolnorth Tuff is composed almost entirely of devitrified basaltic glass shards and olivine crystal fragments. The Woolnorth Tuff is overlain by the Slaughter Bluff Volcanic Breccia (23.12 ± 0.19 Ma) at Kennaook/Cape Grim. The Slaughter Bluff Volcanic Breccia is dominated by diffusely bedded pillow fragment breccia. North of Kennaook/Cape Grim, the Little Trefoil Basalt (ca 24 Ma) intrudes the Woolnorth Tuff. To the south of Kennaook/Cape Grim, the Woolnorth Tuff is overlain by the Studland Bay Basalts (23.73 ± 0.08 Ma) and intruded by a newly recognised unit, the Hippo Basalt (24.52 ± 0.12 Ma). The Studland Bay Basalts comprise mounds of basaltic pillow lavas followed by a succession of diffusely bedded, matrix-dominated, pillow fragment breccia and basaltic breccia. Detailed field mapping and sampling have revealed that the environment of deposition of all Kennaook/Cape Grim units was submarine and that they were emplaced in relatively rapid succession. KEY POINTS First 40Ar/39Ar geochronology for the Kennaook/Cape Grim volcanic succession. Little Trefoil Basalt has been re-interpreted from a subaerial extrusive unit to a submarine intrusive unit. A new intrusive unit, the Hippo Basalt, has been recognised. The stratigraphy of the Kennaook/Cape Grim volcanic succession has been revised.

Late Cretaceous‐Palaeocene arc magmatism in Bumeicun, Gangdese, southern Tibet: Products of slab rollback of Neo‐Tethyan Ocean?

The most intense arc magmatism in the Gangdese arc occurred during the Palaeocene–Eocene, which is widespread across the entire southern Lhasa subterrane with Palaeocene–Eocene I‐type intrusive rocks and Linzizong volcanic successions. What drives the voluminous magmatism remains enigmatic. New geochronological and geochemical data on Late Cretaceous‐Palaeocene magmatic activity from Bumeicun, southern Tibet enables recognition of this distinct magmatic flare‐up event that we ascribe to Neo‐Tethyan Oceanic slab rollback. Bumeicun, located in the southern margin of Gangdese magmatic arc, exposes volcanic and plutonic rocks with close spatial–temporal links, and a wide range of silica content (53.0–74.7 wt.%). Zircon U–Pb dating result shows that the rocks form at 65.24 ± 0.77–67.65 ± 0.31 Ma. The Bumeicun igneous rocks are characterized by relative enrichment of large‐ion lithophile elements, depletion of high field‐strength elements (HFSEs) and enrichment of light rare earth elements, showing typical arc features. These rocks also show positive zircon εHf(t) value (+8.70 to+19.68), which are ascribed to the partial melting of arc mantle wedges. Followed by a succession of melt extraction and fractional crystallization, the coeval intermediate‐felsic volcanic and plutonic rocks in Bumeicun are formed. Previous researches note that Gangdese magmatic arc had a period of magmatic quiescence in the Late Cretaceous, and a subsequent burst of magmatism occurred in the P‐E. Our proposal of slab rollback model is consistent with the distinctly high zircon saturation temperatures (Tzr: 730–827°C) of Bumeicun magmatism. In addition, the slab rollback led to the Gangdese arc magmatism migration from north to south from the Cretaceous to the Eocene.

Magmatic evolution and metal systematics of back-arc volcanic rocks of north–east Japan and implications for deposition of massive sulphide Kuroko ores

The Hokuroku region of north-eastern Japan is endowed with important volcanic-hosted massive sulphide Zn–Pb–Cu deposits, which are considered the archetype of Kuroko (black ore) deposits worldwide. The bimodal, felsic-dominated volcanic succession that hosts the ore was deposited in a continental rift formed during continental extension in the final stages of the Miocene back-arc opening that led to the formation of the Japan Sea. In this study, we define some of the fundamental intensive parameters of this volcanism (temperature, pressure of crystallisation, fluid saturation, fO2) based on rock textures, and analyses of whole-rock samples, minerals and melt inclusions. Based on the melt inclusion analyses, we assess the behaviour of metals during magma evolution and degassing, and evaluate the possible implications for ore deposition. Plagioclase-melt geothermometry in felsic tuff and lava samples collected from both the units underlying and overlying the Kuroko indicates temperatures of 880–940 °C, and Fe–Ti oxide equilibrium indicates oxygen fugacity of ca. FMQ + 1.5. Melt inclusions have high-SiO2 rhyolite compositions (> 75 wt%, on an anhydrous basis), and the plot of normative mineral compositions in the granitic triplot indicates low pressure of magma stalling and crystallisation (< 1 kbar) at cotectic compositions. Melt inclusion metal contents plotted vs incompatible element Y suggest contrasting behaviour of different metals during fractionation and degassing. Zinc was mostly retained in the melt during crystallisation, whereas other metals, such as Pb, Cu, Sn and Mo, were released to an exsolving fluid phase. The latter may have thus been transferred to the hydrothermal system from a degassing magma. Shallow storage of relatively hot magma would have induced vigorous hydrothermal circulation on the seafloor, a precondition for ore deposition.

Eruptive history of the last-1300-years activity of Kelud volcano, Indonesia: Inferences from stratigraphy, chronology, sedimentology, componentry, and geochemistry

Kelud is considered one of the deadliest volcanoes in Indonesia due to its proximity to three cities in East Java (Malang, Kediri, and Blitar) and violent eruptions over the last century. Regardless of the frequently witnessed eruptions, the volcanic stratigraphy for the historical ages has been poorly understood. Here, we provide the first documentation of volcanic stratigraphy for the last 1300 years of activity at Kelud. Componentry and 14C data are coupled with detailed field observations to develop a comprehensive stratigraphy and characterize the main eruption types. The volcanic succession can be subdivided into ten bed-sets composed of pyroclastic density current (PDC) and tephra fallout deposits. A total of twelve fallouts and ten individual PDC units were identified between ∼1.3 and 0.1 cal. ka B.P. One of these units may be attributed to the 1481C.E. eruption. The fallout layers are generally thin, derived from short-sustained eruption columns. By contrast, PDC deposits are massive to stratified, suggesting variable particle concentrations typically resulting from low-column collapses. This complex stratigraphy suggests that Kelud is capable of producing a wide spectrum of eruptions with an increasing eruptive frequency over time and shorter dormancy periods than those between witnessed eruptions (from once /∼100 years to once/∼10 years). This stratigraphic framework, which documents extended records from the oldest witnessed eruption in Kelud (1000C.E.), sheds new insights into Kelud's activity during historical time. Since the products share similar characteristics with those from the most recent activities at Kelud (e.g., 1990, 2007–2008, and 2014), similar eruptions may also occur in the future.

Lithostratigraphy, geology and geochemistry of the Tertiary volcanic rocks on Svartenhuk Halvø and adjoining areas, West Greenland

The Palaeogene volcanic succession in the northern part of the Nuussuaq Basin in West Greenland comprises three formations: the Vaigat and Svartenhuk Formations of Paleocene age (61–58 Ma) and the Naqerloq Formation of Eocene age (57–54 Ma). In this study, we formalise and describe the volcanic stratigraphy on Svartenhuk Halvø and the areas with lavas that flowed across the basin boundary onto the adjoining basement areas in the north and east. The Vaigat Formation comprises three members. The Kakilisaat and Nerutusoq Members are of minor volume and consist of, respectively, crustally contaminated basalts and chemically enriched basalts with relatively high contents of incompatible trace elements. They are overlain by the voluminous Nunavik Member of tholeiitic picrites (MgO ≥12 wt%) and subordinate magnesian basalts. The oldest volcanic deposits are commonly foreset-bedded hyaloclastites, and the overlying subaerial lavas are mainly thin, grey, crumbling flows. Eruption sites were mainly within the basin, with depocentres in the south and hyaloclastite and lava transport directions towards the north. Thicknesses vary from up to at least 2000 m in the south to ≥380 m in the northernmost exposures close to 72°N. The Svartenhuk Formation comprises four members. The lowest, Kuugaartorfik Member, is up to 100 m thick and consists partly of quartzofeldspathic and partly volcanogenic sediments; it is restricted to northern Svartenhuk Halvø and the Innerit peninsula. The overlying volcanic Tunuarsuk, Nuuit and Skalø Members are voluminous and widespread, with a combined thickness of up to 1800 m. They consist of tholeiitic basalts with similar chemical compositions but with correlatable stratigraphic variation patterns. The Tunuarsuk Member consists of interspersed flow groups of thin, grey flows and massive, brown flows; the Nuuit Member comprises mainly massive brown flows, and the Skalø Member is dominated by light grey flows. The Svartenhuk Formation oversteps the Vaigat Formation on the basement in the north and east. In these distal areas the Tunuarsuk and Nuuit Members constitute the major volumes, and preserved thicknesses are up to 1400 m. In northern and eastern Svartenhuk Halvø and also farther to the north and east, foreset-bedded hyaloclastites indicate transport directions towards the north and possibly east from eruption sites within the basin. The Naqerloq Formation comprises one member, the Arfertuarsuk Member, consisting of flows of brown basalt with relatively enriched chemistry and a single trachyte flow. The member is only found in western Svartenhuk Halvø and on Skalø, where it conformably overlies the older lavas with up to 350 m thickness preserved after erosion. Dykes of all three formations are present. The distribution of dykes of the Naqerloq Formation suggests that this originally extended much farther east. Picrites and basalts of the Vaigat and Svartenhuk Formations are geochemically related; the picritic lavas represent erupted primitive magmas, whereas the basaltic lavas represent fractionated melts formed in deep magma chambers. The melts formed from a geochemically depleted but heterogeneous mantle; in addition melts from enriched sources were occasionally incorporated. The enriched basalts of the Naqerloq Formation arose from another mantle source. Low contents of V, Cu and Ni in some crustally contaminated lavas indicate that accumulation of these elements may be present at depth.

Katian volcanism in Central Armorican Domain (France) and Central Iberian Zone (Portugal): a single monogenetic basaltic field?

The Crozon peninsula and Buçaco syncline belong respectively to the Central Armorican Domain and the Central Iberian Zone, located to either side of the Variscan ‘Ibero-Armorican Arc’ structure at present. The Ordovician sedimentary successions at both sites are known to display identical lithofacies. The paired Upper Katian formations also exhibit comparable volcanic successions of anorogenic affinity, including volcaniclastic beds and pillow lavas. The tephra deposits are inferred to have resulted from the activity of subaqueous monogenetic basaltic tuff cones above a &lt;100 m deep calcareous soft substrate, in the context of a passive margin. The Surtseyan-style hydrovolcanic eruptions were followed by effusion of pillow lavas. Both sites also display a large range of similar peperite facies, sometimes related to magmatic segregation structures likely to derive from vapour differentiation. The deeper doleritic dykes and sills, comparable in terms of mineralogy and geochemistry, contain identical metasedimentary xenoliths, whose high-temperature phyllosilicates display a very restricted range of compositions. In the Late Ordovician, Crozon and Buçaco were probably nearby volcanic edifices belonging to a single monogenetic field inside the Armorica microplate. The sudden appearance of high-latitude Katian carbonates, associated with volcanism only at these two peri-Gondwanan sites, just preceded the Gondwanan Hirnantian glaciation. Supplementary material: Analytical data for minerals and whole-rock samples and an M + –4Si–R 2+ diagram are available at https://doi.org/10.6084/m9.figshare.c.6096737

Decoding the Emeishan Permian mantle plume in the southeastern margin of Tibet from the seismic signature of the local lithosphere

SUMMARY The Emeishan large igneous province (ELIP), which is located along the southeastern margin of Tibet, is widely regarded as the erosional remnants of voluminous Permian mafic volcanic successions that were associated with mantle plume activity. In addition, evidence for the eruption of the mantle plume and its signature of lithospheric modification should have been retained in the lithosphere after cooling of the plume. Here we use calculated compressional- and shear wave receiver functions (PRFs and SRFs, respectively) from &amp;gt;750 broad-band stations, together with heat flow and Bouguer gravity anomaly data, to investigate the seismic signature within the lithosphere beneath the southeastern margin of Tibet. Our results from the inner zone (INZ) of the ELIP indicate that the ∼60 km depth of the Moho interpreted from depth-migrated PRF images is consistent with that inferred from SRFs and is deeper than that in the region surrounding the INZ, which shows a continuous positive converted phase at ∼35–40 km depth. The crustal geometry and physical properties show a 15–20-km-thick mafic underplated layer that extends for 150–200 km in the E–W direction and ∼400 km in the N–S direction along the base of the crust in the northeastern INZ and its surrounding region. We interpret the underplated layer as the concealed Emeishan plume hotspot track and suggest that the head of the postulated mantle plume is centred approximately beneath Panzhihua City and its surroundings. We propose a new geodynamic model for the formation of the ELIP, in which the lower–middle crust has been strengthened by voluminous mafic intrusions and underplating, whereas the Yangtze Craton in the plume region has been destroyed by removing the base of the lithosphere.

Adakite generation as a result of fluid-fluxed melting at normal lower crustal pressures

Adakites with continental origin have K-rich compositions distinctive from the sodic, subduction-related adakites and are generally considered as partial melting products of thickened mafic lower crust at high pressure conditions (above 1.5 GPa; >50 km depth). In order to compare the geochemical differences and to constrain their source compositions and partial melting conditions, we compiled published data for the sodic adakites related to subduction processes in the Central Asian Orogenic Belt (CAOB) and the potassic adakite-like rocks related to continental collision in the Lhasa Terranes (LT) of the Tibetan Plateau. Based on the spatial similarity of Sr-Nd isotopes and nature of inherited zircons, the origin of the CAOB sodic adakites is best explained by fluid-fluxed melting of low-K arc basalts, while the LT potassic adakites are best explained as generated by fluid-fluxed melting of rocks similar to the voluminous high-K Linzizong mafic volcanic successions (LVS). The phase equilibria and trace-element modeling reveal that these low Mg# mafic sources would expand garnet stability to pressures as low as 7-8 kbar and fluid-fluxed melting (X(H2O) = 2-3 wt%) would significantly decrease the modal content of plagioclase, resulting in melts with “high pressure” signature (e.g., high Sr/Y and La/Yb). This paper provides an alternative model for the origin of sodic or potassic continental adakites arguing that they originate from fluid-fluxed melting at garnet amphibolite facies at normal lower crustal pressures (10-11 kbar). Our findings highlight the importance of the source composition and fluid content in generating the “high pressure” signatures of adakites, which may also explain the origin of Archean tonalite-trondhjemite-granodiorites (TTGs).

840–820 Ma Dahongshan bimodal volcanic rocks: new constraints on the Neoproterozoic arc–back-arc basin system along the northern margin of the Yangtze Block

ABSTRACT The newly identified earliest part of the Sanligang bimodal volcanic succession in the Dahongshan area of the northern Yangtze Block provides insight into the role of this block in the evolution of the Rodinia supercontinent. Field contact relationships and dating indicate that these rocks are contemporaneous and formed at 840–820 Ma. The Sanligang mafic rocks have low SiO2 (40.33–48.17 wt.%) and K2O (0.07–1.65 wt.%), high TiO2 (2.64–3.52 wt.%) contents, and low U/Th (0.33–0.38), Th/Zr (0.001–0.002), and Nb/Y (0.059–0.064) ratios with positive εNd(t) (+5.08 to +7.59). This suggests that they are formed from magmas derived from an asthenospheric mantle source that had not been modified by slab-derived materials, with a low degree of crustal contamination during their eruption. Contemporaneous felsic rocks in the study area have high SiO2 (73.1–80.0 wt.%), variable K2O (0.63–4.39 wt.%) and Na2O (1.73–6.54 wt.%), and low MgO (0.1–0.58 wt.%) contents and are light rare earth element enriched, with (La/Yb)N values of 1.37–2.65 and significantly negative Eu anomalies. They have A2-type granite affinities, as evidenced by their high 10,000 Ga/Al values (3.51–4.55) and Zr contents (285–620 ppm), and positive whole-rock εNd(t) (+6.16–+6.52), zircon εHf(t) (+9.69–+12.29) and δ18O (+4.82–+6.11) values, suggesting they formed by partial melting of juvenile crustal material caused by heating associated with contemporaneous mantle-derived mafic magmatism. Combining the data for these bimodal volcanic successions with other rock assemblages in the Dahongshan area indicates that this magmatism occurred in a back-arc extensional setting. This magmatism was caused by slab detachment and separation along a vertical tear during the Neoproterozoic evolution of the arc to arc–back-arc system of the northern margin of the Yangtze Block.

Eruptive tempo of Emeishan large igneous province, southwestern China and northern Vietnam: Relations to biotic crises and paleoclimate changes around the Guadalupian-Lopingian boundary

Abstract The Emeishan large igneous province (LIP), southwestern China and northern Vietnam, is thought to have been a potential driver for the biotic crises and paleoclimate changes around the Guadalupian-Lopingian boundary (GLB; Permian), but the lack of high-precision radiometric dates to constrain the duration and eruption rates of the volcanism has limited the assessment of their relationship. We present new chemical abrasion–isotope dilution–isotope ratio mass spectrometry U-Pb zircon geochronology of three samples from the lowermost and uppermost parts of the volcanic succession in the central portion of the Emeishan LIP. The results demonstrate that Emeishan volcanism began by 260.55 ± 0.07 Ma and persisted until at least 257.22 ± 0.37 Ma. Combined with a previously published age of 259.1 ± 0.5 Ma for silicic ignimbrites, we estimate that ~85% of Emeishan LIP volume erupted within 1.45 ± 0.50 m.y. Our new results confirm that the Emeishan volcanism began slightly prior to the initiation of the associated mass extinction event and was contemporaneous with the associated warming interval. The new data support the hypothesis that the Emeishan LIP likely triggered the biotic crises and paleoclimate changes around the GLB.

Primary Microbial Succession in the Anchialine Ecosystem

When new land is created, initial microbial colonization lays the foundation for further ecological succession of plant and animal communities. Primary microbial succession of new aquatic habitats formed during volcanic activity has received little attention. The anchialine ecosystem, which includes coastal ponds in young lava flows, offers an opportunity to examine this process. Here, we characterized microbial communities of anchialine habitats in Hawaii that were created during volcanic eruptions in 2018. Benthic samples from three habitats were collected ∼2 years after their formation and at later time points spanning ∼1 year. Sequence profiling (16S and 18S) of prokaryotic and eukaryotic communities was used to test whether communities were similar to those from older, established anchialine habitats, and if community structure changed over time. Results show that microbial communities from the new habitats were unlike any from established anchialine microbial communities, having higher proportions of Planctomycetota and Chloroflexi but lower proportions of green algae. Each new habitat also harbored its own unique community relative to other habitats. While community composition in each habitat underwent statistically significant changes over time, they remained distinctive from established anchialine habitats. New habitats also had highly elevated temperatures compared to other habitats. These results suggest that idiosyncratic microbial consortia form during early succession of Hawaiian anchialine habitats. Future monitoring will reveal whether the early communities described here remain stable after temperatures decline and macro-organisms become more abundant, or if microbial communities will continue to change and eventually resemble those of established habitats. This work is a key first step in examining primary volcanic succession in aquatic habitats and suggests young anchialine habitats may warrant special conservation status.

The eruptive chronology of the Carihuairazo volcano (Ecuador): Recurrent sector collapses of a Middle Pleistocene stratovolcano of the northern andes

The eruptive chronology of arc volcanoes consists of construction stages usually punctuated by large collapse events affecting the edifice. In this paper, we reconstruct the eruptive chronology of Carihuairazo volcano, a Middle Pleistocene edifice from the Ecuadorian segment of the Andean Northern Volcanic Zone. This study is based on extensive fieldwork on both the proximal flanks of the volcano and the medial-to-distal deposits of the nearby Ambato basin, as well as a large dataset of geochronological (40K-40Ar, 14C) and geochemical (major and trace element) data. The Basal Carihuairazo edifice is mainly composed of an andesitic lava flow succession dated at 230–200 ka. The resulting edifice suffered a first sector collapse that was responsible for a relatively large (4 ± 1 km3) debris avalanche deposit (DAD-1) that covers the entire Ambato basin. This event occurred between 206 ± 4 and 216 ± 5 ka. Then, Carihuairazo started a succession of construction (Intermediate and Terminal lava flow successions) and destruction stages lasting about 50 ky (i.e., from 200 to 150 ka), which are composed by thick block-and-ash flow deposits (L- and U-BAFD) and debris avalanche deposits (DAD-2). This volcanic succession is recorded in the Ambato basin, interlayered with several tephra fallout deposits (TFD-1 to −4) whose source is the neighbouring Huisla-Mulmul volcanic complex. The current morphology of Carihuairazo results from two additional sector collapses (DAD-3 and -4) that occurred during the past 40–50 ka, i.e., following a long period (at least 100 ka) without volcanic activity. We stress these debris avalanches were not related with magmatic activity. Samples from the Carihuairazo volcano defines a medium-K magmatic trend composed of andesites and dacites with a mineral assemblage of plagioclase, amphibole, ortho- and clino-pyroxene, and Fe–Ti oxides. The evolution of the Carihuairazo edifice, recorded in the medial-distal deposits of the Ambato basin, represents a unique example in the Ecuadorian arc of an edifice that experienced successive destruction and construction stages during a major part of its volcanic history.

The petrogenesis and emplacement mechanism of layered ultramafic-mafic complexes of the Barberton Greenstone Belt: Insights from the Stolzburg Complex, South Africa

Abstract Ultramafic-mafic layered complexes are important but not-well studied components of Archaean granitoid-greenstone terranes. In the vicinity of the Barberton Greenstone Belt (BGB), at least 27 such complexes are intimately associated with the supracrustal succession. The petrogenesis of one of these layered bodies, the Stolzburg Complex (SC), is explored, together with its relationship to the surrounding Barberton volcanic succession. Previous models for the origin of Barberton layered complexes proposed a variety of mechanisms, such as single chamber subvolcanic sills, ponded lavas, and alpine-type tectonites. In contrast, the present work suggests that emplacement mostly occurred as sheeted sills of crystal slurries into the country rocks. Unlike the subvolcanic sills model, whereby each complex grew through repetitive magma injection and differentiation in a single chamber, the preferred model regards the layered bodies as ‘stacks’ of discrete intrusions, where each magmatic unit represents a distinct sill. Through comparison of trace element geochemistry (i.e., trace element ratios and patterns), the Lower and Upper divisions of the SC are inferred to be petrogenetically related, but compositionally distinct from the enveloping Nelshoogte volcanic rocks. The trace element geochemistry of the Lower and Upper divisions of the complex is indistinguishable. While the SC ultramafic rocks display an Al-undepleted character, Nelshoogte metavolcanics can be classified as Al-depleted komatiites and komatiitic basalts.

Unraveling the origin of geothermal heat in absence of recent volcanism: The Santiago Papasquiaro hydrothermal area, Central-Eastern Sierra Madre Occidental, México

In western Mexico, the central part of the Sierra Madre Occidental (SMO) large igneous province hosts several thermal springs with groundwater temperatures up to 74 ºC within late Eocene to Oligocene extensional structures. Given that in this region the last pulse of silicic volcanism ended >30 Ma ago this hydrothermal activity cannot be associated with cooling magma chambers and/or ongoing crustal thinning as in conventional geothermal sites. A possible explanation for the anomalous water temperature includes two non-mutual excluding processes: 1) heat production by radioactive decay of abundant U, Th, K radioisotopes in the silicic rocks forming the upper crust of the SMO and accumulated below the low conductive sediments filling the tectonic depressions, and 2) water heating by convection through deeply rooted faults. To prove this hypothesis, we studied the Santiago Papasquiaro hydrothermal area, located at the intersection between the NNW trending Santiago Papasquiaro half-graben and the NW trending San Luis-Tepehuanes fault system. We provide a detailed stratigraphy of the area supported by new U-Pb ages and we estimated heat production with in-situ gamma ray measurements of radioelements (U, Th, K), and groundwater He isotopes analyses.The stratigraphy of the region consists of 1) a Late Cretaceous-Paleocene continental volcanosedimentary succession (Garame Group), 2) Early Eocene rhyolitic lavas and ignimbrites, 3) Late Eocene volcanosedimentary succession, 4) Late Eocene-Early Oligocene pyroclastic succession capped by rhyolitic lavas and domes, and 5) a siliciclastic succession (Papasquiaro Formation) filling the graben interbedded with minor basaltic lavas (∼12–11 Ma) in its upper part. We also mapped two intermediate composition intrusive bodies, a Late Cretaceous (66 Ma) granodiorite and a Late Eocene (∼39 Ma) diorite.Heat production values are in the range of 1 µWm−3 to 8 µWm−3, with extreme values up to 310 µWm−3 for carbonaceous beds within the Garame Group. Late Cretaceous to Eocene volcanosedimentary and volcanic successions have lower heat production values than the Oligocene volcanic succession and the Oligocene-Quaternary siliciclastic succession. As a whole, heat production of Santiago Papasquiaro rocks exceeds the average for the upper crust as well as that of continental granite/rhyolite, being in the range of the high heat production granites worldwide, and above plutonic and volcanic rocks from other areas of Mexico. Noble gasses isotopes show a major crustal radiogenic input (79.4–96.1%), consistent with the radiogenic heat source hypothesis for the thermal springs, and a secondary mantle input (1.9 – 9.4%), which can be related to the ascent of mantle fluids through a deep crustal zone at the intersection of the regional structures. The isotopic He composition differs from the volcanic geothermal systems of Mexico such as Los Azufres, Los Humeros and from the unconventional geothermal area of the Juchipila Graben, located 300 km southeast in the same tectonic setting. The first two nearly represent a binary mixture between mantle and air saturated water end-members, whereas the latter also includes a minor crustal signal. Modeling of He isotopes data confirm that the 3He signature is associated to the ascent of mantle fluids through a deep crustal zone of highly fractured rocks at the intersection of regional structures. The signal of mantle fluids is diluted over the time by 4He sourced from the Oligocene high-heat production rocks.

Early Devonian volcanic facies, central Lachlan Orogen, New South Wales: implications for tectonic and metallogenic models

The central Lachlan Orogen volcanic belt contains predominantly silicic and explosive eruptive products emplaced between ca 428 and 412 Ma within elongate basins over an area ∼350 km long and 20 km wide. The volcanic rocks are exposed in two parallel, south-southeast-trending belts between Cobar and Albury, NSW, and have S-, I- or A-type magmatic affinities. The southernmost sequences include subaerial rocks of the Gurragong Volcanics and Walleroobie Volcanics (herein renamed the Gurragong Group), and the newly described Culcairn Group. The Gurragong Group comprises crystal-rich Walleroobie Ignimbrite, lithic-rich Corella Ignimbrite, Mount Ariah Rhyolite coherent facies and newly recognised airfall Indarra Tuff. The Culcairn Group includes the Budginigi and Wallandoon ignimbrites. The Budginigi Ignimbrite includes crystal-rich ignimbrite, the Hadrells Hill Member hyaloclastite breccia and the Soldiers Quarry Member (coherent rhyodacite). The Wallandoon Ignimbrite is compositionally intermediate, metaluminous and amphibole-bearing. The volume estimate of southern pyroclastic units ranges from 15 to 75 km3 per succession. The northern volcanic sequences include the Canbelego-Mineral Hill Volcanic Belt, Mount Hope Group and Ural Volcanics, all deposited in a shallow- to deep-marine environment during the Cobar Basin opening. The subaerial depositional environment of the southern volcanic successions, the proximity of exposed Silurian granitoid, and contact metamorphic overprint of the Devonian volcanic successions by contemporaneous intrusions implies the volcanic pile was thicker, and synchronous uplift was occurring. Variation in the aluminium saturation index from peraluminous to metaluminous, and a variation in high-field-strength elements in the almost exclusively felsic igneous rocks may reflect variation in the source rocks and temperature of partial melting. The resulting metallogenic patterns reflect these basement differences. Metals in the south resulted in intrusion-related Sn–W; in the north, the marine environment proximity to a major transcrustal structure and variation in the fertility of the lower crustal rocks led to the development of diverse mineral systems. Key points The central Lachlan Orogen Siluro-Devonian volcanic belt is predominantly silicic, containing explosive eruptive products emplaced within a series of elongate basins between Cobar and Albury, NSW. The southernmost sequences, the Gurragong Group and the newly described Culcairn Group are subaerial; the northern sequences include the Canbelego-Mineral Hill Volcanic Belt, Mount Hope Group and Ural Volcanics and are wholly submarine. Variations in the igneous rock chemistry and regional mineralisation reflect the variation in the basement source rocks, related fertility and associated structures.

Interaction between volcanic and non-volcanic systems and its implication for prospectivity in the Faroe–Shetland Basin, NE Atlantic continental margin

Exploration in the Faroe–Shetland Basin on the Faroese Continental Shelf has revealed thick and complex volcanic successions and discovery of inter-volcanic oil-bearing siliciclastic sandstone fan deposits in the central parts of the basin. The possibility for such play types at the fringe of the North Atlantic Igneous Province requires a better understanding of the interaction between competing sedimentary and volcanic depositional transport systems. We have re-examined volcanic units in cuttings from exploration wells in the greater Judd Sub-basin area for evaluation of facies and geochemical affinity. This allows for chemostratigraphical correlation of wells to the absolute radiometrically age-constrained Faroe Islands Basalt Group. The collective well data were subsequently tied to a regional interpretation of 2D seismic data which facilitated a detailed interpretation of temporal development of the volcanic successions in the Judd Sub-basin area in terms of geometry, volcanic facies, depositional environment, and interdigitation with non-volcanic sedimentary units. The Judd Sub-basin was influenced by major volcanic phases during pre-breakup and syn-breakup. The influence was both direct, in the form of volcanic deposits, and indirect, in the form of obstructing established sedimentary transport systems and creating new provenance areas. The volcanic transport systems reached different areas of the Judd Sub-basin at different times during pre-breakup volcanism. The earliest incursion in the west was during late Mid Paleocene (T-sequence T31/T32). With at least three stratigraphically discrete incursions of volcanic material into the Judd Sub-basin, possibilities arise for sub- and inter-volcanic stratigraphic and structural traps for each incursion.

Baffin Bay Composite Tectono-Sedimentary Element

Abstract Baffin Bay formed as a result of continental extension during the Cretaceous, which was followed by seafloor spreading and associated plate drift during the early to middle Cenozoic. The formation of an oceanic basin in the central part of Baffin Bay may have begun from about 62 Ma in tandem with the Labrador Sea opening, but the early spreading phase is controversial. Plate-kinematic models suggest that from Late Paleocene the direction of seafloor spreading changed to north–south, generating strike-slip movements along the transform lineaments, e.g. the Ungava Fault Zone and the Bower Fracture Zone, and structural complexity along the margins of Baffin Bay. The Baffin Bay Composite Tectono-Sedimentary Element (CTSE) represents a 3–7-km-thick Cenozoic sedimentary and volcanic succession that was deposited over oceanic and rifted continental crust since active seafloor spreading began. The CTSE is subdivided into 5 seismic mega-units that have been identified and mapped using a regional seismic grid tied to wells and core sites. Thick clastic wedges of likely Late Paleocene to Early Oligocene age (mega-units E and D2) were deposited within basins floored by newly formed oceanic crust, transitional crust, volcanic extrusives and former continental rift basins undergoing subsidence. The middle–late Cenozoic is characterized by fluvial–deltaic sedimentary systems, hemipelagic strata and aggradational sediment bodies deposited under the influence of ocean currents (mega-units D1, C and B). The late Pliocene to Pleistocene interval (mega-unit A) displays major shelf margin progradation associated with ice-sheet advance–retreat cycles resulting in the accumulation of trough-mouth fans and mass-wasting deposits in the oceanic basin. The Baffin Bay CTSE has not produced discoveries, although a hydrocarbon potential may be associated with Paleocene source rocks. Recent data have improved the geological understanding of Baffin Bay, yet large data and knowledge gaps remain.

Periodicity of Karoo rift zone magmatism inferred from zircon ages of silicic rocks: Implications for the origin and environmental impact of the large igneous province

New U-Pb ages for zircons constrain the duration of silicic magmatism and timing of coeval mafic magmatism across the main rift zone of the Karoo large igneous province in Mozambique. Our 190 ± 2 Ma, 188.4 ± 0.9 Ma, 181.7 ± 1.0 Ma, 180 ± 3 Ma, 178 ± 2 Ma and 172 ± 2 Ma ages support periodicity of Karoo magmatism previously inferred from 40Ar/39Ar age data. The ∼ 190–188 Ma ages confirm early onset of magmatism and the ∼ 182–178 Ma ages correlate the bimodal volcanic successions of the Lower Zambezi and the Movene Formation with widespread silicic magmatism across the rift zone. The ∼ 172 Ma age corresponds to waning magmatic activity. The age range and Hf isotopic compositions of zircons indicate up to ∼9 Ma lifespan for the Jurassic silicic magma chambers and suggest that the ∼2700–400 Ma xenocrysts represent crustal sources of the host rocks. The available chronological data indicate that the ∼183 Ma main phase magmatism was largely confined within the main Karoo and Kalahari basins and that the preceding and subsequent phases were mainly associated with the Karoo rift zone. Judging from geochemical literature, different kinds of magmas were erupted during the successive magmatic phases. We calculate from published geochemical data that the mafic main phase magmas were relatively poor in CO2 and SO2 and the lava stacking patterns point to low eruption rates, which suggests that degassing of sedimentary wall-rocks of intrusions probably triggered the coeval Pliensbachian-Toarcian extinction. In contrast, the mafic late phase magmas were rich in CO2 and SO2 and at least some of the lavas indicate high eruption rates. We propose that efficient degassing from widespread mafic magmatism and explosive eruption of over 30,000 km3 of silicic magmas in the Karoo rift zone linked the ∼182–178 Ma late phase magmatism with contemporaneous global biosphere crises.

Structural setting of the Narusongduo Pb-Zn ore deposit in the Gangdese belt, central Tibet

The Narusongduo Pb-Zn deposit is located at the northern boundary of the Luobadui-Milashan fault zone (LMF) in central Tibet and is spatially associated with the Linzizong volcanic succession (LVS). Our study indicates that the regional structural setting was formed by two-stage tectonic events. The first stage, spanning from the late Mesozoic to early Paleocene, is characterized by significant N-S crustal shortening associated with the Cordilleran-type orogeny along the Gangdese arc. The region's Paleozoic-Mesozoic metasedimentary rocks were penetratively strained. Locally deformation was largely partitioned along the LMF. During the second stage (ca. 66–55 Ma), the area was affected by extensive multi-stage Linzizong volcanism, including caldera formation, as well as a coaxial N-S propagative deformation with a distinctive lower shortening rate. We recognized two types of mineralization, both were formed during the second stage. The first type of mineralization (orebody III) is governed by fractures within the extensively deformed Paleozoic carbonate rocks at the LMF's footwall. The overlying LVS, however, includes discrete but numerous mineralized sections. The terminal splays of the ore shoots typify the products of hydraulic fracturing. We propose that the propagative compressive deformation drained fluid reservoirs at depth to higher levels via the “Fault valve” effect. Episodic fluid influxes and mineral deposition formed the time-integrated mineralization. The second type of mineralization (orebody I) is hosted in the LVS in a number of breccia pipes and dykes that were controlled by the structural weaknesses generated by the intersection of the radial and ring fractures. Mineralization occurs as veinlets in the matrix and clasts inside the breccias, which are characterized by multi-stage brittle cracking, fluid injection and mineral precipitation. It is interpreted that the multi-stage magmatism (ca. 66–55 Ma) triggered repeated hydrothermal activities and incremental mineralization within the ore-bearing breccia bodies.