Volcanic Succession Research Articles

Prospecting for clay minerals within volcanic successions: Application of electrical resistivity tomography to characterise bentonite deposits in northern Sardinia (Italy)

Electrical resistivity tomography (ERT) is applied to prospect for and characterise a bentonitic clay deposit in northern Sardinia. Sardinian bentonites derived from the hydrothermal alteration of thick successions of pyroclastic flows and epiclastites are associated with the Oligo-Miocene calc-alkaline volcanic cycle. The alteration of these rocks is generally controlled by faults that control the local circulation of hydrothermal fluids. Two-dimensional ERT investigations were performed close to a faulted area to define the location, thickness and lateral continuity of the clayey body, and determine how it relates to faulting and stratigraphy. A line-based three-dimensional ERT data acquisition was carried out in a selected area to estimate the available clay reserves. The reliability of these resistivity models was assessed by comparison with local borehole data. Finally, the interpretation of the ERT results was optimised through synthetic modelling of the electrical resistivity imaging technique. The results define the extent and geometry of the bentonitic deposit with good accuracy and outline the scenarios where the ERT method may provide optimal results when prospecting for clay deposits.

The Cosmos greenstone succession, Agnew-Wiluna greenstone belt, Yilgarn Craton, Western Australia: Geochemistry of an enriched Neoarchaean volcanic arc succession

Abstract The geodynamic setting of the Neoarchaean Eastern Goldfields Superterrane (EGS) of the Yilgarn Craton is the subject of debate. Some authors propose plume models, while others advocate variants on a subduction accretion model for the origin of mineralised greenstone belt sequences. Felsic volcanism in the Kalgoorlie Terrane, the westernmost terrane of the EGS, is considered to have a tonalite-trondhjemite-granodiorite/dacite (TTG/D) geochemical affinity. The Cosmos greenstone succession, which lies in the Agnew-Wiluna greenstone belt (AWB) of the Kalgoorlie Terrane, contains several komatiite-hosted nickel sulphide deposits, the volcanic footwall to which consists of an intercalated succession of fragmental and coherent rocks ranging in composition from basaltic andesite to rhyolite. Light rare earth elements (LREEs) and large ion-lithophile elements (LILEs) are strongly enriched relative to high field strength elements (HFSEs) across all volcanic units, and the rocks display strong positive Pb and negative Nb anomalies. These geochemical characteristics resemble closely those of modern high-K calc-alkaline to shoshonite continental arc successions. Contrasting REE, LILE and HFSE concentrations, coupled with assimilation-fractional crystallisation (AFC) modelling, shows that the intercalated dacitic and andesitic volcanic rocks within the footwall succession are not co-genetic. Xenocrystic zircons within the felsic volcanic lithologies indicate that some assimilation of older continental crust contributed to the generation of the footwall volcanic sequence. The geochemical characteristics of the Cosmos volcanic succession indicate that parental melts were derived via partial melting of enriched peridotite that had been contaminated by subducted crustal material within the mantle wedge of a subduction zone. In contrast, two younger felsic porphyry intrusions, which cross-cut the volcanic succession, have a distinct TTG/D affinity. Therefore, these intrusions are considered to be generated via partial melting of a subducting slab and are related to local high-Ca granitoid intrusions. The Cosmos volcanic succession represents the first extrusive high-K calc-alkaline to shoshonitic volcanic arc sequence described in the Kalgoorlie Terrane and, coupled with age dating of the stratigraphy, is indicative of formation in a long-lived volcanic arc setting active from 2736 Ma to later than 2724 Ma. The composition and geochemical affinity of the volcanic footwall succession to the Cosmos komatiite-hosted nickel-sulphide deposits contrasts with the majority of felsic volcanic rocks within the AWB and also the wider Kalgoorlie Terrane, suggesting that the overall architecture of this region is more complex than is currently thought. Our conclusions not only have consequences for recent models of the tectonic evolution of the EGS but also contribute to the debate on the operation of plate tectonics during the late Archaean in general. The arc affinity of the Cosmos volcanic succession, containing abundant high-K calc-alkaline andesite lavas, provides further support for the operation of plate tectonics in the Neoarchaean.

The volcanological and petrogenetic origins of the basaltic Flinders Volcanic Province (49–39 Ma), Older Volcanics Provinces, southeast Australia

Eocene age basalts within the Mornington Peninsula represent a segment of volcanics constituting the Flinders Volcanic Province, one of the Older Volcanics Provinces, in Victoria, Australia. Section logging at selected coastal sites between Cape Schanck and Flinders, within the Mornington Peninsula National Park, has depicted a succession of gently southeasterly dipping basaltic pahoehoe sheet lavas and localised debris flows, emplaced in a subaerial environment, and distinguished by post emplacement altered weathering horizons. Eight dykes intrude the volcanic succession, controlled structurally by NW normal and NE-trending transform faults of the Gippsland Basin, forming swarms at the West Point Gunnery Range in Flinders, Cairns Bay and Cape Schanck. Geochemical analysis of the lava succession suggest the influence of multiple magma batches, each enriched in light rare earth elements, which based upon Sr–Nd isotopic data came from a long-term incompatible element depleted magma source.

Oligocene–Miocene geodynamic evolution of the central part of Urumieh-Dokhtar Arc of Iran

Basic volcanic rocks from Tafresh, west Kashan, and west Nain volcanic successions in the central part of Urumieh-Dokhtar Magmatic Assemblage (UDMA) of Iran yield K–Ar ages ranging from 26.8 to 18.2 Ma. These ages indicate significant Late Oligocene–Early Miocene basic volcanism in the UDMA. These ages, combined with K–Ar ages of 26.0 and 14.1 Ma, respectively, for associated low-silica and high-silica adakites, help constrain reconstructions of the UDMA geodynamic evolution. Late Oligocene–Early Miocene slab roll-back associated with an asthenospheric mantle influx are suggested as the major processes responsible for concurrent volcanism showing Nb–Ta-depleted, Nb–Ta-enriched and low-silica adakite signatures. Slab roll-back, the likely consequence of a decrease in subduction velocity, led to partial melting of the subducted slab and produced Early–Middle Miocene high-silica (dacitic) adakites. Oligocene to Miocene volcanic rocks do not conform to the Oligocene continental collisional model for the UDMA, rather they suggest a decrease in the subduction rate that prompted the asthenospheric mantle influx.

Lithofacies architecture of basaltic andesite lavas and their interaction with wet-sediment: Port a’ Chroinn, Kerrera, NW Scotland

Synopsis Lavas of the Lorne Plateau Volcanic Succession (LPVS) crop out on the island of Kerrera, NW Scotland, and were emplaced during the Devonian Period, synchronous with Old Red Sandstone (ORS) terrestrial sedimentation. These flows are discontinuously exposed for c . 3 km at Port a’ Chroinn, Kerrera, and typically comprise blocky, basal breccias, coherent, crystalline cores, and clinker flow tops, interpreted as basaltic andesite ’a’a lavas. Gradational zones are observed from coherent core, through highly vesicular core, to semi-coherent lava and normally graded clinker. Prismatically jointed, inflated pahoehoe lavas are also present, but have few characteristic morphological features. Localized domains of magma–sediment mingling occur at the bases of several lavas, with clasts of basaltic andesite within a sandstone matrix. These domains are interpreted as peperites, formed as lava flowed over and mingled with unconsolidated, wet sediment that had been deposited on the surfaces of underlying lavas during periods of volcanic quiescence.

Eocene to Quaternary mafic-intermediate volcanism in San Luis Potosí, central Mexico: The transition from Farallon plate subduction to intra-plate continental magmatism

The San Luis Potosí Volcanic Field (SLPVF) of central Mexico includes volcanic sequences of felsic, intermediate and basic compositions that were erupted as discrete episodes from the Eocene to the Pleistocene. Volcanism was dominated by widespread and voluminous rhyolitic ignimbrites of the mid-Tertiary Ignimbrite Flare-up. However, the complete volcanic history must consider basaltic and andesitic Eocene–Pleistocene volcanic successions that provide key evidence for understanding the geochemical evolution of the volcanism in the SLPVF during this time span. Five sequences are recognized according to their geochemical characteristics, each comprising a volcano-tectonic episode. The first episode comprises basaltic andesites and andesites erupted during three intervals, 45–42Ma, 36–31Ma, and 31–30Ma. The oldest was derived from subduction magmatism, whereas the youngest has an intra-plate magmatic signature and this represents the transition from the end of a long lasting subduction regime of the Farallon plate to the initiation of intra-plate continental extension in the North American plate. The second episode, at 29.5–28Ma, comprises a bimodal succession of high-silica rhyolites and alkaline basalts (hawaiites) that are interpreted as magmatism generated in an intra-plate continental extension regime during the Basin and Range faulting. The third episode, at 21Ma, is characterized by trachybasalts and trachyandesites that represent mantle basaltic melts that were contaminated through assimilation of the lower crust during advanced stage of intra-plate extension that started at Oligocene. The fourth episode includes 12Ma alkaline basalts and andesites that were erupted from fissures. These mantle derived magmas evolved to andesites by crustal anatexis and crystal fractionation within a continued, extensional, intra-plate regime. Lastly, the fifth episode comprises 5.0 to 0.6Ma alkaline basalts (basanites) containing mantle xenoliths, that were erupted from maars and tuff cones, which are the youngest manifestations of mantle-derived intra-plate extensional events. Based upon this volcanic record, the last subduction manifestations of the extinct Farallon plate occurred at about 42Ma, this was followed by a transition to intra-plate magmatism between 42 and 31Ma, and an extensional, intra-plate tectonic setting from 31Ma to almost Present.

Petrogenesis of the late Cretaceous K‐rich volcanic rocks from the Central Pontide orogenic belt, North Turkey

AbstractSubduction‐related volcanic rocks are widespread in the Central Pontides of Turkey, and represented by the Hamsaros volcanic succession in the Sinop area to the north. The volcanic rocks display high‐K calc‐alkaline, shoshonitic and ultra‐K affinities. 40Ar/39Ar age data indicate that the rocks occurred during the Late Cretaceous (ca 82 Ma), and the volcanic suites were coeval. Primitive mantle‐normalized trace element patterns of all the lavas are characterized by strong enrichments in large ion lithophile elements (LILE) (Rb, Ba, K, and Sr), Th, U, Pb, and light rare earth elements (LREE; La, Ce) and prominent negative Nb, Ta, and Ti anomalies, all typical of subduction‐related lavas. There is a systematic increase in the enrichment of incompatible trace elements from the high‐K calc‐alkaline lavas through the shoshonitic to the ultra‐K lavas. In addition, the shoshonitic and ultra‐K lavas have significantly higher 87Sr/86Sr (0.70666–0.70834) and lower 143Nd/144Nd (0.51227–0.51236) initial ratios than coexisting high‐K calc‐alkaline lavas (87Sr/86Sr 0.70576–0.70613, 143Nd/144Nd 0.51245–0.51253). Geochemical and isotopic data show that the shoshonitic and ultra‐K rocks cannot be derived from the high‐K calc‐alkaline suite by any shallow level differentiation process, and point to a derivation from distinct mantle sources. The shoshonitic and ultra‐K rocks were derived from metasomatic veins related to melting of recycled subducted sediments, but the high‐K calc‐alkaline rocks from a lithospheric source metasomatized by fluids from subduction zone.

Volcanic successions in Marquesas eruptive centers: A departure from the Hawaiian model

The temporal evolution and geochemical evolution of Marquesas hotspot volcanoes have often been interpreted with reference to the Hawaiian model, where a tholeiitic shield-building stage is followed by an alkali basaltic post-shield stage, followed after a 0.4 to 2.5Myr long quiescence period, by a rejuvenated basanitic/nephelinitic stage. Here we discuss geochemical data on 110 Marquesas lavas also dated using the unspiked 40K–40Ar method on separated groundmass (including 45 new ages measured on the southern islands of Hiva Oa, Motane, Tahuata and Fatu Hiva). Sample locations were positioned on detailed geological maps to determine their shield or post-shield position with respect to the caldera collapse event(s), without taking into account their geochemical features. A rather regular decrease of the ages towards SE, consistent with the Pacific plate motion, is observed from Eiao (5.52Ma) to Fatu Hiva (1.11Ma), and rejuvenated basanitic volcanism occurs only in Ua Huka (1.15–0.76Ma). The occurrence of intermediate and evolved lavas is restricted to the post-caldera stage, with the exception of Eiao island. However, many other features of the Marquesas chain are rather atypical with respect to those of Hawaii. Although Marquesas shields are tholeiitic, several of them (Eiao, Tahuata) contain interbedded alkali basaltic and basanitic flows. Moreover, post-shield volcanoes are either alkali basalts (Ua Huka), tholeiites (Hiva Oa, Tahuata, Fatu Hiva) or both (Nuku Hiva). This feature is consistent with the temporal continuity of the two stages and the usually short length of the post-shield period (<0.2Myr). In a given island, the trace element and isotopic compositions of shield and post-shield lavas overlap, although both display large variations. The sources of alkali basalts and basanites are more enriched than those of the contemporaneous tholeiites. These specific features support the hypothesis of an extremely heterogeneous Marquesas plume. The “weak” character of this plume led to low partial melting degrees, which in turn resulted in the preservation in the basaltic magmas of geochemical features inherited from small-size source heterogeneities.

Setting and Evolution of the Archean Synvolcanic Mooshla Intrusive Complex, Doyon-Bousquet-LaRonde Mining Camp, Abitibi Greenstone Belt: Emplacement History, Petrogenesis, and Implications for Au Metallogenesis

The synvolcanic Mooshla Intrusive Complex intrudes coeval ~2699 to 2696 Ma volcanic rocks of the Blake River Group within the southern margin of the Archean Abitibi greenstone belt. The upper Blake River Group is host to the Doyon-Bousquet-LaRonde mining camp that contains Au-rich volcanogenic massive sulfide (VMS) deposits, possible subsea-floor epithermal-style deposits, and orogenic Au deposits. In total, the camp contains to date in excess of 28 million ounces (Moz) Au, making it a world-class example of Au-rich paleosea-floor environments. The Mooshla Intrusive Complex is spatially, temporally, and most probably genetically associated with all of the above types of mineralization. It is host to parts of the Doyon (5.5 Moz Au), Mouska (0.8 Moz Au), and Mic Mac (0.11 Moz Au) Au deposits and host to the smaller Mooshla A and B Au occurrences. Host volcanic units to the Mooshla Intrusive Complex are intensely deformed, metamorphosed, altered, and mineralized, as is the intrusion itself. The Mooshla Intrusive Complex was formed by nine distinctive phases of subvolcanic dikes, sills, and stocks. These were emplaced in two stages to form a shallow, multiphase synvolcanic intrusion along the contact between the Hebecourt and Bousquet volcanic formations. The Mouska stage is represented by a preliminary swarm of thin diabase sills, intruded by a well-layered gabbroic sill, a more crudely layered quartz diorite, and tonalite. A period of devolatilization accompanied crystallization of the xenolith-rich top of the tonalite magma chamber, as evidenced by the presence of an aplite dike swarm and associated extensive alteration zones and miarolitic cavities. The younger Doyon stage comprises a series of fine-grained aphyric to porphyritic, tonalite and trondhjemite dikes and sills, which also contain evidence of in situ devolatilization. The geochemical signatures of the Mooshla Intrusive Complex indicate emplacement during formation of an evolved, extensional oceanic island arc-style succession. Primitive mantle-normalized spider plots suggest a common origin for this island-arc intrusive suite that is similar to that of the volcanic succession of the upper member of the Bousquet Formation. Various element ratio plots used to further define magma origin and emplacement history suggest that whereas the Mouska-stage magmatic phases have a relatively straightforward, coexisting fractionation history, the Doyon-stage tonalite-trondhjemite has a more complex interplay of assimilation-fractionation-contamination, suggesting midcrustal partitioning and interaction with both earlier formed, partially hydrated ~2720 Ma oceanic crust and upper Blake River host strata (~2699-2696 Ma). The protracted and mulitphased magmatic evolution of the Mooshla Intrusive Complex led to the generation of volatile-rich phases that contributed to the development of a submarine magmatic-hydrothermal system that is thought to be responsible for the formation of the Doyon Au-Cu deposit. Geologic and timing relationships suggest that this magmatic-hydrothermal system might also have contributed to the generation of Au-rich VMS deposits higher in the host volcanic succession as part of a large Archean magmatic and hydrothermal center.

The Lemoine Auriferous Volcanogenic Massive Sulfide Deposit, Chibougamau Camp, Abitibi Greenstone Belt, Quebec, Canada: Geology and Genesis

The Lemoine auriferous volcanogenic massive sulfide deposit (0.76 Mt at 4.6 g/t Au, 4.2 wt % Cu, and 9.5 wt % Zn) is part of the Chibougamau camp located in the northeastern part of the Abitibi greenstone belt. The deposit is hosted by a steeply dipping, S-facing homoclinal volcanic succession (~2729–2726 Ma Waconichi Formation, Lemoine Member) composed of effusive and intrusive tholeiitic rhyolites and andesites cut by comagmatic diorite and gabbro dikes and overlain by transitional to mildly calc-alkaline basalts, andesites, and rhyolites. Seven predominant synvolcanic alteration assemblages, now intensely deformed and metamorphosed to upper greenschist facies, were defined based on their mineralogy and position relative to ore. Albite-quartz, sericite-carbonate, sericite-chlorite, sericite-chlorite-(Zn), and chlorite-sericite-epidote-carbonate assemblages define semiconcordant zones that are stacked from the paleoseafloor to the deep footwall. In contrast, chlorite and chlorite-sericite-chloritoid assemblages overprint the other alteration zones and form subconcordant to now transposed discordant zones in the deposit footwall. Most alteration assemblages are characterized by relative SiO2, CaO, and Na2O mass losses and relative FeO, MgO, K2O, and CO2 mass gains. Whole-rock oxygen isotopes indicate that the temperatures of alteration ranged from ~100° to 150°C (sericite-carbonate assemblage) to ≥350°C (sericite-chlorite, chlorite-sericite-chloritoid, and chlorite assemblages). The chlorite-sericite-chloritoid assemblage, and to some extent the sericite-chlorite assemblage, are associated with strong to near total depletion of light rare earth elements possibly due to reaction with Cl-bearing, mildly acidic fluids at depth in the ore-forming hydrothermal system. The sulfide lens was formed in part on the seafloor and in part by subseafloor replacement. The massive sulfides are particularly rich in Bi, suggesting a possible magmatic input into the Lemoine ore-forming hydrothermal system. High Bi concentrations in the mineralizing system are likely to have enhanced Au precipitation by scavenging the precious metal from the hydrothermal fluids. The Au-rich nature of the Lemoine auriferous volcanogenic massive sulfide deposit can be explained by a combination of very efficient metal transport, highly effective capture of metals at or near the seafloor, and a possible magmatic contribution to the hydrothermal system.

Clinopyroxene chemistry in Tertiary alkaline volcanic rocks from Taleghan, central Alborz, Iran: Implications for two parental melts

In the Taleghan area of Alborz mountain range, two distinct units of alkaline rocks are identified. Whole rock major and trace element data does not provide a clue as to their common or different parental melts. However, major and trace element geochemistry of their included clinopyroxene phenocrysts, combined with the whole rock data, helped to infer that two parental magmas were involved in the evolution of volcanic succession in the Taleghan area.

Space–time evolution of monogenetic volcanism in the mafic Garrotxa Volcanic Field (NE Iberian Peninsula)

We reconstructed the evolution of the volcanic activity within the central Garrotxa monogenetic Volcanic Field, the youngest volcanic area of the Iberian Peninsula, by investigating the stratigraphy of the volcanic successions and the morphology of the monogenetic eruptive centres. Analysis of this volcanic succession has been conducted following the Unconformity Bounded Stratigraphic Units criteria. The detailed stratigraphy of the volcanic successions shows that the central Garrotxa Volcanic Field (GVF) evolved through four main periods of volcanic activity (Synthems) represented by the eruptive products of the mafic monogenetic volcanoes and associated syn-eruptive reworked deposits (Eruptive Units) and by the inter-eruptive deposits (Epiclastic Units). The distribution and the morphologies of the monogenetic volcanoes suggest that feeder dykes were emplaced under influence of the present stress filed and along pre-existing fractures of the basement. Our facies analysis of the deposits and their distribution shows that migration of volcanism toward the centre of the basin was accompanied by a trend of increasing explosivity. Episodic hydromagmatism in central Garrotxa occurred without a specific geographic locus or obvious temporal correlation. Finally, by integrating field data with the stratigraphy extracted from water wells, we determined the volume of the volcanic deposits. The small average volume of products emitted during each eruptive period, and the long quiescence separating them, allow us to classify the GVF as a low-output rate volcanic field.

Geochemical constraints on the relationship between the Miocene–Pliocene volcanism and tectonics in the Palaoco and Fortunoso volcanic fields, Mendoza Region, Argentina: New insights from 40Ar/39Ar dating, Sr–Nd–Pb isotopes and trace elements

New 40Ar/39Ar analyses constrain the formation of the volcanic succession of Sierra de Palaoco in the present back-arc of the Andean Southern Volcanic Zone (SVZ), near 36°S, to the Late Miocene and assigns them to the Huincán II Formation. The composition of major and trace elements, Sr, Nd and Pb isotopes of the Palaoco and nearby Río Grande rocks require a strong arc-like component in the mantle that is absent or weak in both Early Miocene (Fortunoso Group) and Pleistocene alkaline lavas (Llancanelo Group) erupted in the same area. We evaluate the relative roles of varying mantle source compositions and crustal contamination in the generation of geochemically very different lavas from the Palaoco, Fortunoso and Río Grande volcanic fields, north of the Payún Matrú Volcano. The source for the Early Miocene Fortunoso(I) basalts was a OIB-type mantle devoid of subduction zone input. This type of OIB-like volcanic activity terminated due to a change from an extensional to a compressional tectonic regime. Towards the end of the Miocene renewed alkaline volcanism at Fortunoso (II) display a transition to arc-type incompatible element enrichment. Shortly after the calc-alkaline Palaoco volcanism started with a very strong geochemical arc-signature including Ba/La≈60 and La/Nb=2–3. After a quiesence of 1Ma the major part of the voluminous Late Palaoco basalts were erupted around 7.5Ma over a few hundred ka. These are less enriched in Ba and Sr and have compositions like many Holocene rocks of the Southern Volcanic Zone. Isotopically the Fortunoso I and Palaoco rocks are distinct. Regional volcanism of the Charilehue, Huincán I and II mostly has a moderate arc-type enrichment indicating incipient arc developments. However, Palaoco and La Brea at (c. 35°S) show full geochemical arc-signature, and we infer that a frontal arc was established. The subsequent development in the Palaoco-Río Grande area encompasses renewed late Pliocene calc-alkaline low volume volcanic eruptions (Río Grande group) succeeded in the Late Pleistocene by alkaline OIB-type eruptions (Llancanelo group). In the light of the course of volcanism to the east, in the Nevado area, where late Miocene–Pliocene calc-alkaline volcanism was followed by Late Pliocene–Pleistocene alkaline volcanism. We propose a scenario where the Nazca plate developed an eastwards widening flat slab from which the east dipping slab before the Late Pliocene translated from Palaoco to Nevado and subsequently retreated passing Río Grande in the Late Pliocene. Alkaline back-arc volcanism was active east of the arc-volcanism and expanded westwards during the Late Pliocene and Pleistocene.

The age and composition of the pre-Cenozoic basement of the Jalisco Block: implications for and relation to the Guerrero composite terrane

The Jalisco Block is thought to be part of the Guerrero terrane, but the nature and age of the underlying crystalline basement are largely unknown. We have collected a suite of schists, granitoids, and weakly metamorphosed marine sediments from various parts of the Jalisco Block including Atenguillo and Ameca, Mascota and San Sebastian, Cuale, Puerto Vallarta, Punta Mita, Yelapa, and Tomatlan. The schists range in age from 135 to 161 Ma, with many exhibiting Proterozoic and Phanerozoic zircon ages. The granitoids range in age from 65 to 90 Ma, and are calc-alkaline compositionally—similar to granitoids from the Puerto Vallarta and Los Cabos batholiths. The Jalisco granitoids also experienced similar uplift rates to granitoids from the regions to the north and south of the Jalisco Block. The marine sediments yield a maximum depositional age of 131 Ma, and also contain a significant zircon population with ages extending back to the Archean. Granitoids from this study define two age groups, even after the effects of thermal resetting and different closure temperatures are considered. The 66.8-Ma silicic ash flow tuff near Union de Tula significantly expands the extent of this Cretaceous–Paleocene age ash flow tuff unit within the Jalisco Block, and we propose calling the unit “Carmichael silicic ash flow tuff volcanic succession” in honor of Ian Carmichael. The ages of the basement schists in the Jalisco Block fully overlap with the ages of terranes of continental Mexico, and other parts of the Guerrero terrane in the south, confirming the autochthonous origin of the Jalisco Block rather than exotic arc or allochthonous origin. Geologic data, in combination with geochronologic and oxygen isotopic data, suggest the evolution of SW Mexico with an early 200–1,200-Ma passive margin, followed by steep subduction in a continental arc setting at 160–165 Ma, then shallower subduction by 135 Ma, and finally, emplacement of granitoids at 65–90 Ma.

Stratigraphy and age of the Eocene Igtertivâ Formation basalts, alkaline pebbles and sediments of the Kap Dalton Group in the graben at Kap Dalton, East Greenland

A NE–SW-trending graben at Kap Dalton on the Blosseville Kyst contains an at least 600 m thick succession of Eocene basalt lavas and sediments. The succession has been investigated by new field work, geochemical analysis and radiometric dating by the 40Ar-39Ar incremental heating method. The results show that the volcanic succession comprises about 220 m of the uppermost plateau basalt formation, the Skrænterne Formation. This is separated from the overlying lava flows of the Igtertivâ Formation by 7 m of sediments that represent a period of around six million years. The two formations can be distinguished by different trace element ratios. The Igtertivâ Formation comprises an at least 300 m thick main succession of flows dated to 49.09 ± 0.48 Ma, overlain by sediments of the Bopladsdalen Formation. A basal conglomerate in the sediments contains pebbles of alkaline igneous rocks of which three were dated at 49.17 ± 0.35 Ma, 47.60 ± 0.25 Ma, and 46.98 ± 0.24 Ma. The sediments are thus younger than 47 Ma. Above 30 m of sediments occur two Igtertivâ Formation lava flows dated to 43.77 ± 1.08 Ma. The overlying sediments of the Bopladsdalen and Krabbedalen Formations are therefore not older than about 44 Ma and palynological evidence shows that they are also not much younger than this. Use of the Geological Time Scale 2012 has resulted in good agreement between radiometric and palynological ages. The Igtertivâ Formation lava flows were fed from a regional coast-parallel dyke swarm indicating a new rifting episode at 49–44 Ma. This coincides with a major mid-Eocene plate reorganisation event in the North Atlantic and the start of northward-propagation of the Reykjanes Ridge through the continent. The Igtertivâ rift may have been directly instrumental for the initiation of this process.

Stratigraphic, geochemical and U–Pb zircon constraints from Slieve Gallion, Northern Ireland: a correlation of the Irish Caledonian arcs

Recent Ar–Ar and U–Pb zircon geochronology from across the British and Irish Caledonides has revealed a prolonged period of arc-ophiolite formation ( c . 514–464 Ma) and accretion ( c . 490–470 Ma) to the Laurentian margin during the Grampian orogeny. The Slieve Gallion Inlier of Northern Ireland, an isolated occurrence of the Tyrone Volcanic Group, records the development of a peri-Laurentian island arc–backarc and its obduction to an outboard microcontinental block. Although a previous biostratigraphic age constraint provides a firm correlation of at least part of the volcanic succession to the Ca1 Stage of the Arenig ( c . 475–474 Ma), there is uncertainty on its exact statigraphic position in the Tyrone Volcanic Group. Earliest magmatism is characterized by light rare earth element (LREE) depleted island-arc tholeiite. Overlying deposits are dominated by large ion lithophile and LREE-enriched, hornblende-phyric and feldspathic calc-alkaline basaltic andesites and andesitic tuffs with strongly negative ϵNd t values. Previously published biostratigraphic age constraints, combined with recent U–Pb zircon geochronology and new petrochemical correlations, suggest that the Slieve Gallion Inlier is equivalent to the lower Tyrone Volcanic Group. Temporal and geochemical correlations between the Slieve Gallion Inlier and Charlestown Group of Ireland suggest that they may be part of the same arc system, which was accreted at a late stage ( c . 470 Ma) in the Grampian orogeny. A switch from tholeiitic volcanism to calc-alkaline dominated activity within the Lough Nafooey Group of western Ireland occurred prior to c . 490 Ma, some 15–20 Myr earlier than at Tyrone and Charlestown. Supplementary materials: Sampling and geochemical results (major elements, loss on ignition, trace elements, REE and Nd isotopes) are available at www.geolsoc.org.uk/SUP18640 .

Volcano-stratigraphy of the extension-related silicic volcanism of the Çubukludağ Graben, western Turkey: an example of generation of pyroclastic density currents

AbstractWestern Turkey's extension-related Cumaovası volcanic rocks (Lower Miocene, 17 Ma) are excellent examples of silicic eruptions. The sub-aerial silicic volcanism at Çubukludağ Graben between İzmir and Kuşadası in west–central Anatolia is mainly in the form of rhyolite domes, lava flows and pyroclastic deposits. The initial features of volcanism derived from phreatomagmatic explosive eruptions from silicic magma that came into contact with lake waters during Neogene times. Most of the volcanic succession represents pyroclastic density currents (PDCs), known as the Kuner ignimbrite. The deposits are fine grained and laminated at the base and pass laterally and vertically into deposits displaying well-developed traction structures, soft sediment deformation and/or erosion channels in the NE part of the region. Alternate deposits of massive, diffusely stratified lapilli and ash are the main products of the later explosive stage. Massive lithic breccias forming the top of the sequences are the proximal facies of the PDCs. The lava phase mainly consists of rhyolite extruded as dome and fissure eruptions of lavas, aligned along NE–SW-trending faults as well as from extensional cracks that are nearly perpendicular to the main graben faults. Considering the tectono-stratigraphical aspects and geochemical nature of the study area, we propose that the Cumaovası silicic volcanism was produced by extension-related crustal melting during the Late–Early Miocene period (17 Ma).

U–Pb dating constraints on the felsic and intermediate volcanic sequence of the nickel-sulphide bearing Cosmos succession, Agnew-Wiluna greenstone belt, Yilgarn Craton, Western Australia

The Cosmos succession, within the Agnew-Wiluna greenstone belt of the Yilgarn Craton, consists of ultramafic, intermediate and felsic volcanic lithologies, and contains several komatiite-hosted nickel sulphide ore bodies. The volcanic succession that forms the stratigraphic footwall to the mineralised komatiite lavas consists of an intercalated sequence of fragmental and coherent extrusive lithologies, ranging from basaltic-andesites through to rhyolites. It also contains cross cutting felsic intrusions. The stratigraphic hangingwall consists largely of reworked volcanic-derived sedimentary rocks that include polymictic conglomerates containing granitic clasts. SIMS U–Pb dating was undertaken on zircons from nine felsic lithologies spanning the established stratigraphy. The footwall succession gives emplacement ages ranging from 2736±4 to 2724±3Ma. The overlying Cosmos Ultramafic sequence consists of two units; the basal ultramafic UMu1 package and the upper ultramafic UMu2 package, which are separated by a discontinuous felsic volcanic horizon that sits approximately 30–60m above the basal contact of the Cosmos Ultramafic sequence. This discontinuous felsic horizon is dated at 2685±8Ma and represents a break in accumulation of komatiite lavas and may be associated with a major hiatus. The succession also features two felsic intrusions emplaced at 2653±7Ma and 2670±6Ma, where the latter cross-cuts the upper contact of the UMu2 package. These results bracket the age of UMu1 package between 2724±3Ma and 2685±8Ma and that of UMu2 package between 2685±8Ma and 2670±6Ma. This stratigraphic division of the Cosmos ultramafic is further strengthened by the contrasting styles of mineralisation above and below the discontinuous felsic horizon and associated unconformity; UMu1 hosts several massive nickel sulphide deposits situated at the contact with the footwall volcanic succession, while UMu2 is typified by high-tenor disseminated nickel sulphide deposits situated just above the discontinuous felsic horizon. Our data indicates that the Cosmos volcanic succession was accumulated via punctuated volcanism spanning ∼50Ma that was followed by intrusive activity lasting for at least a further 32Ma. The results also imply that construction of this relatively small, mineralised greenstone succession lasted longer than anticipated on stratigraphic considerations alone, an outcome that may have implications for the timing of emplacement of other bi-modal volcanic successions in similar greenstone terranes.

Palaeomagnetism of the Cappadocian Volcanic Succession, Central Turkey: Major ignimbrite emplacement during two short (Miocene) episodes and Neogene tectonics of the Anatolian collage

The Central Anatolian Volcanic Province in Cappadocia includes 13 high volume calc-alkaline ignimbrite sheets emplaced by plinian eruptions within a succession (the Ürgüp Formation) after ~10Ma recording the last phase of Neotethyan subduction and accompanying emplacement of the Tauride orogen in southern Turkey. To evaluate magnetostratigraphy in the context of recent revisions of the chronostratigraphy we have extended palaeomagnetic investigation to 32 new sites yielding significant ChRM directions. Integrated rock magnetic and palaeomagnetic investigations identify magnetic remanence residing predominantly in Ti-poor titanomagnetites although secondary processes within the ignimbrite sheets, notably post-emplacement oxidation, have locally produced hematisation expressed by composite IRM spectra and variable reduction in intensity of magnetisation. The ignimbrite sheets possess weak anisotropies of magnetic susceptibility (AMS, mostly <5%) describing tensors with axial distributions close to bedding and minimum axes predominantly perpendicular to this plane; collectively directions show weak imbrication correlating with palaeoflow during emplacement predominantly towards the north and east away from the Erdas Dağ, an inferred topographic palaeohigh at the southern margin of the basin. The precise control provided by magnetostratigraphy and radiometric age dating now shows that the bulk of Cappadocian ignimbrite magmatism was concentrated into two short episodes. An older Cardak Centre (Kavak Group and Zelve ignimbrites) produced in excess of 200km3 of pyroclastic deposits during polarity chron C4r.1n between 9.31 and 9.43Ma. Subsequent activity from the Acıgöl Centre further to the south west (Cemilköy, Gelveri, Gördeles, and Kızılkaya) produced in excess of 620km3 of pyroclastic deposits during polarity chrons between 5.3 and 7.1Ma. The younger İncesu ignimbrite was sourced in the Sultansazlığı pull-apart basin to the east during the Gauss Chron (2.58–3.60Ma). All pre-İncesu ignimbrites are rotated uniformly anticlockwise and the overall (reversed) group mean direction of magnetisation is D/I=170.8/−52.4° (N=9, R=8.91, α95=5.4°, k=91). The implied tectonic rotation in this sector of central Anatolia (16±4° relative to Eurasia) is young and postdates the 5.3–7.1Ma Acıgöl Centre whilst largely predating emplacement of the İncesu ignimbrite. Whilst rotational deformation within Anatolia is young, it proves to be distributed with a temporal variation from block to block. It is embraced by a complex post-Miocene tectonic regime of strike slip and extension during tectonic escape and suction towards the Hellenic Arc to the west.

Testing the ureilite projectile hypothesis for the El'gygytgyn impact: Determination of siderophile element abundances and Os isotope ratios in ICDP drill core samples and melt rocks

AbstractThe geochemical nature of the impactites from International Continental Scientific Drilling Project—El'gygytgyn lake drill core 1C is compared with that of impact melt rock fragments collected near the western rim of the structure and literature data. Concentrations of major and trace elements, with special focus on siderophile metals Cr, Co, Ni, and the platinum group elements, and isotope ratios of osmium (Os), were determined to test the hypothesis of an ureilite impactor at El'gygytgyn. Least squares mixing calculations suggest that the upper volcanic succession of rhyolites, dacites, and andesites were the main contributors to the polymict impact breccias. Additions of 2–13.5 vol% of basaltic inclusions recovered from drill core intervals between 391.6 and 423.0 mblf can almost entirely account for the compositional differences observed for the bottom of a reworked fallout deposit at 318.9 mblf, a polymict impact breccia at 471.4 mblf, and three impact melt rock fragments. However, the measured Os isotope ratios and slightly elevated PGE content (up to 0.262 ng g−1 Ir) of certain impactite samples, for which the CI‐normalized logarithmic PGE signature displays a relatively flat (i.e., chondritic) pattern, can only be explained by the incorporation of a small meteoritic contribution. This component is also required to explain the exceptionally high siderophile element contents and corresponding Ni/Cr, Ni/Co, and Cr/Co ratios of impact glass spherules and spherule fragments that were recovered from the reworked fallout deposits and from terrace outcrops of the Enmyvaam River approximately 10 km southeast of the crater center. Mixing calculations support the presence of approximately 0.05 wt% and 0.50–18 wt% of ordinary chondrite (possibly type‐LL) in several impactites and in the glassy spherules, respectively. The heterogeneous distribution of the meteoritic component provides clues for emplacement mechanisms of the various impactite units.