Volcanic Succession Research Articles

Bimodal volcanism at the western margin of the Kaapvaal Craton in the aftermath of collisional events during the Namaqua-Natal Orogeny: The Koras Group, South Africa

The Koras Group is a bimodal metavolcanic suite on the western margin of the Archean Kaapvaal Craton at its contact with the poly-deformed and highly metamorphosed Proterozoic Namaqua Sector of the Namaqua-Natal Province of southern Africa. It is composed of two temporally distinct but lithologically similar volcanosedimentary successions, the lower at ∼1.17Ga and the upper at ∼1.1Ga. Both successions are characterised by basaltic andesites and rhyolites sandwiched between immature siliciclastic metasediments.The subalkaline basalt to basaltic andesites are tholeiitic, magnesian, and calcic to calc-alkalic. Their trace element concentrations are enriched in the large ion lithophile (LIL) elements (Cs, Rb, Sr, Ba, K and Pb), along with Th and U relative to primitive mantle (PM), and show depletions in P and Ti, with a Nb–Ta ‘trough’ relative to the LILE and light rare earth elements (LREE). The basaltic andesites were extruded in a continental rift setting, but with low Nb contents (7.8–10.9ppm), along with La/Nb ratios of 2.3–3.1, have a subduction-related signature for both successions. The rhyolitic porphyries of both successions are peraluminous, magnesian and alkali-calcic to alkalic. They have similar trace element patterns to the basaltic andesites, but are strongly enriched in the LILE and LREE relative to PM, and have strong depletions in P, Ti and Sr and a prominent Nb–Ta ‘trough’. The rhyolitic porphyries were emplaced in a within-plate tectonic setting and are characterized by high Zr+Nb+Ce+Y contents (>350ppm) and Ga/Al ratios (10,000Ga/Al>2.6), typical of A-type granitoids. The upper volcanic succession, in particular, shows many similarities, both in terms of whole rock geochemistry, as well as age, to the late- to post-tectonic granitoids of the Keimoes Suite, an extensive granitoid suite that ‘stitches’ accretionary terranes together on the eastern margin of the Namaqua Sector. Chondrite-normalized REE patterns of the bimodal volcanism are characterised by LREE enrichment [(La/Sm)CN=2.7–4.96], slight HREE depletion [(Gd/Yb)CN=1.09–2.46] and negative Eu anomalies which become more pronounced from the basaltic andesites (Eu/Eu*=0.69–0.86) to the rhyolitic porphyries (Eu/Eu*=0.57–0.66). The overall coherence of both major and trace element trends, as well as the positive correlation between incompatible element contents favours a co-magmatic origin for the basaltic and rhyolitic rocks with fractional crystallization processes dominant giving rise to A-type magmas.The magmas were emplaced during trans-tensional periods on the western margin of the Kaapvaal Craton that followed two collisional events due to decompression melting of the subduction-related subcontinental lithospheric mantle following slab detachment in an area of previously thickened crust, which then incorporated varying degrees of continental crustal material. This is supported by ɛNd(t) values ranging from positive for some rhyolitic porphyries through to dominantly negative, variable TDM ages of >1.5Ga, and low Nb/La and Ta/Th ratios.The Koras Group represents an example of A-type bimodal volcanism at 1.1Ga which finds regional correlatives in similar Mesoproterozoic successions in southern Namibia and northwestern Botswana associated with accretionary collisional events.

The Unaz Formation: A Key Unit in the Western Black Sea Region, N Turkey

The Pontide magmatic belt in the Western Pontides, which developed in response to the northward subduction of the northern branches of the Tethys Ocean, consists of two different volcanic successions separated by an Upper Santonian pelagic limestone unit, the Unaz Formation. The first period of volcanism and associated sedimentation started during the Middle Turonian and lasted until the Early Santonian under the control of an extensional tectonic regime, which created horst-graben topography along the southern Black Sea region. The lower volcanic succession, the Dereköy Formation, was deposited mainly within these grabens. This extensional period probably represents the rifting of magmatic arc, giving rise to the opening of the Western Black Sea back-intra-arc basin. The Unaz Formation commonly covers horsts and grabens developed before its deposition. This formation implies sudden subsidence of the region and termination of the volcanism during the Late Santonian. This period was interpreted as the time of the beginning of the oceanic spreading in the Western Black Sea Basin. The second period of magmatism developed on the Unaz Formation was more voluminous and was active during the Campanian. Stratigraphy, contact relationships and regional correlations indicate that the deposition of the Unaz Formation and similar deep marine red pelagic sediments in the Black Sea and Eastern Mediterranean region were probably controlled by local and regional tectonic events and sea level and/or climate changes.

Sucesión volcánica Jurásica en el área de Charcas, San Luis Potosí: Contribución al entendimiento del Arco Nazas en el noreste de México

El presente trabajo reporta datos de campo, petrograficos, geoquimicos y geocronologicos de U-Pb en circones de una sucesion volcanica de decenas a cientos de metros de espesor, que sobreyace de manera discordante a sedimentos triasicos marinos de la Formacion Zacatecas y subyace en discordancia erosional a los depositos clasticos de la Formacion La Joya, del Calloviano-Oxfordiano, al poniente de Charcas, San Luis Potosi. Esta sucesion volcanica ha sido correlacionada por semejanza litologica y posicion estratigrafica con pequenos afloramientos de rocas comparables, distribuidos desde el norte, de los estados de Durango y Zacatecas y sur de Coahuila, hasta el sur de Nuevo Leon, Tamaulipas y el poniente del estado de San Luis Potosi. Las rocas estudiadas corresponden geoquimicamente a andesitas, dacitas y riolitas, y muestran un amplio rango composicional (SiO2 = 53-82%). La geoquimica de elementos traza se empleo para determinar el ambiente tectonico, y se encontro que los diagramas de elementos de tierras raras se caracterizan por enriquecimientos en las tierras raras ligeras con una pequena anomalia negativa en Eu, mientras que los diagramas multielementos presentan patrones con enriquecimiento en elementos litofilos de ion grande (LILE, large ion lithophile elements), con una clara anomalia negativa en Nb y Ta. De igual manera los diagramas de discriminacion tectonomagmatica muestran un ambiente tectonico de arco volcanico. Se reporta ademas la primera datacion isotopica para esta localidad, resultado del analisis de U-Pb en 29 circones por la tecnica de ablacion laser en un equipo ICP-MS con multicolector, la cual arrojo una edad de 179 Ma para un flujo piroclastico de composicion riodacitica. La interpretacion de estos datos sustenta la correlacion de esta sucesion volcanica, tanto en su ambiente geotectonico como en el tiempo, con las rocas expuestas en otras localidades del arco volcanico continental del Jurasico Inferior en el noreste de Mexico. Dichas rocas han sido consideradas como parte del arco volcanico continental del Jurasico Inferior situado a lo largo de la paleomargen pacifica de Pangea.

Reassessment of petrogenesis of Carboniferous–Early Permian rift-related volcanic rocks in the Chinese Tianshan and its neighboring areas

The Carboniferous−Early Permian rift-related volcanic successions, covering large areas in the Chinese Tianshan and its adjacent areas, make up a newly recognized important Phanerozoic large igneous province in the world, which can be further divided into two sub-provinces: Tianshan and Tarim. The regional unconformity of Lower Carboniferous upon basement or pre-Carboniferous rocks, the ages (360–351Ma) of the youngest ophiolite and the peak of subduction metamorphism of high pressure–low temperature metamorphic belt and the occurrence of Ni-Cu-bearing mafic-ultramafic intrusion with age of ∼352Ma and A-type granite with age of ∼358Ma reveal that the final closure of the Paleo-Asian Ocean might take place in the Early Mississippian. Our summation shows that at least four criteria, being normally used to identify ancient asthenosphere upwelling (or mantle plumes), are met for this large igneous province: (1) surface uplift prior to magmatism; (2) being associated with continental rifting and breakup events; (3) chemical characteristics of asthenosphere (or plume) derived basalts; (4) close links to large-scale mineralization and the uncontaminated basalts, being analogous to those of many “ore-bearing” large igneous provinces, display Sr-Nd isotopic variations between plume and EM1 geochemical signatures. These suggest that a Carboniferous asthenosphere upwelling and an Early Permian plume played the central role in the generation of the Tianshan–Tarim (central Asia) large igneous province.

Melilitites in the alkaline volcanic succession of the Gorringe bank, Southwestern Portugal

Melilitites were found for the first time in the alkaline volcanics of the Gorringe bank. With allowance for previous studies, this indicates the existence of a common melilitite-nephelinite-phonolite alkaline series, which is widely spread as lava fields, dikes, and diatremes in the continental magmatic provinces of Europe, Africa, Asia, and America, often accompanying the formation of carbonatite complexes. This is considered as one of the evidence in support of the continental nature of the alkaline volcanism of the bank. The fact that the volcanics contain fragments of plutonic rocks, which are usually observed in the carbonatitebearing alkaline complexes (melteigites, ijolites, nepheline syenites), makes it possible to suggest the presence of alkaline intrusions of such type at a shallow depth.

Thermomechanical maturation of the continental crust and its effects on the late Eocene–early Oligocene volcanic record of the Sierra Madre del Sur Province, southern Mexico

We interpret the voluminous late Eocene–early Oligocene volcanic successions of the north-central Sierra Madre del Sur as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts into the crust beneath the Michoacán-Puebla area promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of the Tilzapotla, Muñeca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination, and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in the Tilzapotla and Muñeca suites document an evolution through low-pressure fractional crystallization of plagioclase-dominated assemblages, simultaneous with the assimilation of middle–upper crustal materials. In contrast, marked Eu, Sr, and Ba depletions coupled with high and variable Rb/Nd at constant 143Nd/144Nd in the Goleta rhyolites suggest their derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magma emplacement and accumulation at shallow depths shifted the brittle–ductile crustal transition close to the surface, and produced an ignimbrite flare-up through caldera-forming eruptions. A different petrogenetic–volcanologic scenario developed in north-western Oaxaca, where less profuse early–middle Eocene igneous activity and an ancient lower crustal basement made up of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmatic evolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced through high-pressure fractional crystallization and crustal contamination. Specifically, increasing La/Yb and Sm/Yb with increasing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO2 at low Rb/Nd, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials.

Temporal Evolution of a High-K Andesitic Magmatic System: Taranaki Volcano, New Zealand

Taranaki (Mt. Egmont) in the western North Island of New Zealand is a high-K andesite volcano with an eruptive history extending over more than 200 kyr. In general, petrological research has concentrated on the post-10 ka record of the modern edifice. This study focuses on the earlier history, which is recorded in 11 major pre-7 ka debris avalanche deposits. Each of these formed as a result of a catastrophic collapse of the edifice of the time. The clast assemblages of these deposits provide insights into the chemical compositions of magmas erupted during the earlier stages of activity of the volcano and form the basis for a new chemo-stratigraphic analysis of the pre-10 ka volcanic succession. Sample suites from the studied debris avalanche deposits show a progressive enrichment in K2O and large ion lithophile elements (LILE), reflecting a gradual evolution to high-K andesite. The early magmatic system (pre-100 ka) produced a wide range of compositions including relatively primitive basalts and basaltic andesites.These rocks contain phenocryst assemblages that indicate crystallization within the lower crust or mantle, including a broad range of clinopyroxene compositions, high-Al2O3 hornblende, olivine and phlogopite. A higher proportion of high-silica compositions in the younger sample suites and the appearance of late-stage, low-pressure mineral phases, such as high-TiO2 hornblende, biotite and Fe-rich orthopyroxene, reflect a gradual shift to more evolved magmas with time.These new data are interpreted to reflect a multi-stage origin for Taranaki andesites. Parental magmas were generated within a lower crustal ‘hot zone’, which formed as a result of repeated intrusions of primitive melts into the lower crust. The geochemical and mineralogical evidence indicates that prior to 100 ka this zone was relatively thin and cold, so that primitive magmas were able to rise rapidly through the crust without significant interaction and modification. As the hot zone evolved, larger proportions of intruded and underplated mafic material were partially remelted, and interaction of these melts with fractionating mantle-derived magmas generated progressively more K- and LILE-enriched compositions. A complex and dispersed magma assembly and storage system developed in the upper crust where the hot-zone melts were further modified by fractional crystallization and magma mixing and mingling.

Geological map of Etna volcano, 1:50,000 scale

The new geological map of Etna volcano at 1:50,000 scale represents a significant progress in the geological studies of this volcano over the last 30 years, coming after Waltershausen’s map published around the mid of 19th century, the first geological map of a large active volcano, and the [Romano et alii (1979)][1] map published about a century later, both at 1:50,000 scale. Lithostratigraphy was used for mapping volcanic units and then Unconformity Bounded Units were applied to group lithostratigraphic units into synthems. In addition, lithosomes were exploited to better represent the spatial localization of different eruptive centres according to their morphology. On the whole, we identified 27 lithostratigraphic units, grouped into 8 synthems, and 9 volcanoes. In detail, effusive and explosive deposits generated by each eruption of Mongibello and, partially, Ellittico volcanoes were mapped as flow rank. This stratigraphic framework represents the best synthesis of the geological evolution of Etna volcano using the main unconformities recognized within its complex volcanic succession. In addition, we constrain the Etna volcanic succession and its lithostratigraphic units chronologically by radioisotope age determinations. On the basis of the outlined synthemic units, it was possible to divide Etna’s volcanic succession into 4 supersynthems, which correspond to 4 well-defined and spatially localized phases. The detailed reconstruction of the past eruptive activity allowed compiling the most accurate dataset in particular of the Holocene eruptions of Etna volcano, which will enable significantly improving the volcanic hazard assessment, together with petrological interpretation of erupted magmas and geophysical modelling of the volcano plumbing system. [1]: #ref-67

40Ar/39Ar isotopic dating of Etna volcanic succession

Since the 1970's, about 50 radio-isotopic ages have been determined on Etna volcanics using different techniques: Th-U and K/Ar.Unfortunately, these ages cannot be readily used to constrain the new stratigraphic setting of the volcano, because of the uncertainty in sample locations or, sometimes, the large errors affecting the calculated ages. For this reason a program of radio-isotopic dating applying the 40Ar/39Ar incremental heating technique to date the groundmass of basaltic samples has been carried out from 2002.Forty samples (22 of which are of new publication) were collected from key outcrops on Etna volcano, selected on the basis of their stratigraphic position, while one sample was collected from the Hyblean plateau volcanics. We have obtained reliable results from all volcanics analysed from 542 ka up to 10 ka with the MSWD's (Mean Square of Weighted Deviates) ranging from 0.03 up to 1.7 excluding IS sample (MSWD = 6.28). These new results allow us to: i) assign an age to 19 of the 25 lithostratigraphic units defined in the new geological map of Etna volcano; ii) clarify the uncertain stratigraphic position of isolated volcanic units; iii) constraint the temporal hiatus that matches the main unconformities; iv) outline the lapse of time between the end of the Hyblean volcanism and the beginning of eruptive activity in the Etna region.

Post-Eocene volcanics of the Abazar district, Qazvin, Iran: Mineralogical and geochemical evidence for a complex magmatic evolution

The style of volcanism of post-Eocene volcanism in the Alborz zone of northern Iran is different to that of Eocene volcanism (Karaj Formation). Indeed, the volcanic succession of the Abazar district, located in a narrow volcanic strip within the Alborz magmatic assemblage, is characterized by distinct mineralogical and chemical compositions linked to a complex magmatic evolution. The succession was produced by explosive eruptions followed by effusive eruptions. Two main volcanic events are recognized: (1) a thin rhyolitic ignimbritic sheet underlain by a thicker lithic breccia, and (2) lava flows including shoshonite, latite, and andesite that overlie the first event across a reddish soil horizon. Plagioclase in shoshonite (An 48–92) shows normal zoning, whereas plagioclase in latite and andesite (An 48–75) has a similar composition but shows reverse and oscillatory zoning. QUILF temperature calculations for shoshonites and andesites yield temperatures of 1035 °C and 1029 °C, respectively. The geothermometers proposed by Ridolfi et al. (2010) and Holland and Blundy (1994) yield temperatures of 960 °C and 944 °C for latitic lava, respectively. The samples of volcanic rock show a typical geochemical signature of the continental arc regime, but the andesites clearly differ from the shoshonites, the latites and the rhyolites. The mineralogical and chemical characteristics of these rocks are explained by the following petrogenesis: (1) intrusion of a hot, mantle-depth mafic (shoshonitic) magma, which differentiated in the magma chamber to produce a latitic and then a rhyolitic liquid; (2) rhyolitic ignimbritic eruptions from the top of the magma chamber, following by shoshonitic and then latitic extrusions; (3) magma mingling between the latitic and andesitic magmas, as indicated by the occurrence of andesite clasts within the latite; and (4) andesitic effusions. The youngest volcanic events in the Alborz zone show a close chemical relationship with continental arc rocks, indicating that they formed in a continental collision setting.

Geochemical and Sr–Nd isotopic constraints on the genesis of the Cenozoic Linzizong volcanic successions, southern Tibet

The Linzizong volcanic successions that crop out in the Lhasa terrane, southern Tibet have been conventionally regarded as the products of northward subduction of the Neotethyan oceanic slab beneath South Asia. This study reports geochemical data of 100+ volcanic rocks from the Lhasa terrane to better constrain the temporal–spatial distribution and petrogenesis of the Linzizong volcanism. The Linzizong volcanic successions consist dominantly of calc-alkaline rocks that erupted from ca. 69 to 43Ma and show typical arc-lava geochemical features marked with LILE enrichment and HFSE depletion in the spidergram. Their Sr and Nd isotope ratios [εNd(T)=+3.3 to −2.4; ISr=0.7048–0.7072] are generally similar to those of the associated Gangdese I-type granitoids. The Linzizong volcanism is characterized by a flare-up period (ca. 50Ma) that shows significant geochemical variations, manifested by the coexistence of five types of volcanic rocks: (1) the main suite of calc-alkaline rocks [SiO2=45–80wt.%; La=12–45ppm; εNd(T)=+3.8 to −4.9; ISr=0.7037–0.7105] that could be interpreted by partial melting of the mantle wedge followed by assimilation and fractional crystallization (AFC) processes with >10% crustal contamination and/or magma mixing; (2) the low-K, low-REE suite (SiO2=48–61wt.%; K2O=0.5–1.1wt.%; La=7–10ppm) that has the highest Nd isotope ratios [εNd(T)=+5.9 to 3.5], suggesting a juvenile mantle origin possibly related to decompression melting of the asthenosphere; (3) the shoshonitic suite [SiO2=53–71wt.%; K2O=3.8–6.7wt.%; εNd(T)=−2.8 to −6.1] from small-degree melting of the metasomatized lithospheric mantle; (4) the high-REE suite [SiO2=61–75wt.%; La=36–87ppm; εNd(T)=−2.6 to −3.2] originating from remelting of a newly underplated basaltic lower crust; and (5) the evolved suite of rhyolitic flows and ignimbrites (SiO2>65wt.%) that have the lowest εNd(T) values of −14 to −18, representing remelting products of the basement or continental crust of the Lhasa terrane. Such geochemical heterogeneities are attributed to breakoff of the subducted Neotethyan slab under southern Tibet that occurred in the early stage of the India–Asia collision.

Comment on “Geochemistry of the Early Miocene volcanic succession of Northland, New Zealand, and implications for the evolution of subduction in the Southwest Pacific” by M.A. Booden, I.E.M. Smith, P.M. Black and J.L. Mauk

In a recent paper Booden et al. (2011) present new geochemical and petrological data of Early Miocene volcanics from the Northland region (Northland volcanic belt) in New Zealand, and interpret these data to support a particular regional tectonic model. This tectonic model involves Early Miocene westward subduction of Cretaceous Pacific oceanic lithosphere below the Northland volcanic belt and the authors interpret the volcanic belt as a continental magmatic arc. Although the new data are not in disagreement with such a tectonic model, they provide more support for an alternative interpretation that involves a northeast-dipping subduction zone. Furthermore, geometric and plate kinematic data show that the west-dipping subduction model is unviable, geological and geophysical data contradict the model, while geodynamic arguments indicate that the model is implausible. Here it will be shown that a subduction model, involving a northeast-dipping southwestward retreating slab (made of the Late Cretaceous–Paleocene South Loyalty backarc basin lithosphere) that subsequently detaches, is in agreement with the local geology, geophysics and geochemistry, is geometrically, kinematically and geodynamically viable, and fits within the regional Southwest Pacific tectonic framework.

U–Pb zircon ages of orthogneisses and supracrustal rocks of the Cariris Velhos belt: Onset of Neoproterozoic rifting in the Borborema Province, NE Brazil

New geochronological and geochemical constraints on Precambrian sedimentary and volcanic successions exposed in the western part of the Central Domain of the Borborema Province, NE Brazil, indicate the presence of two distinct tectono-stratigraphic complexes: Riacho Gravatá and São Caetano. Both complexes and associated orthogneisses are referred in the literature as the Cariris Velhos belt, having depositional, extrusive, or intrusive ages within the interval 985–913 Ma. The Riacho Gravatá complex consists of bimodal (but mostly felsic) volcanic and volcanoclastic rocks, muscovite ± graphite schists, quartzites, and marble with local occurrences of banded-iron-formation. The São Caetano complex mainly consists of metagreywackes, marbles, calc-silicate rocks, and rare meta-mafic rocks. Meta-mafic rocks from both complexes have geochemical signatures similar to those of continental flood basalts, with ɛNd (1.0 Ga) values ranging from −1.0 to −2.8. Felsic volcanic rocks from the Riacho Gravatá complex show ɛNd (1.0 Ga) values ranging from −1.0 to −7.4 and geochemical signatures similar to A 2-type granitoids. New SHRIMP U–Pb zircon data from felsic volcanic rocks within the Riacho Gravatá complex yielded ages of 1091 ± 13 Ma and 996 ± 13 Ma. In contrast, meta-graywackes from the São Caetano complex show a maximum deposition age of ca. 806 Ma in the northern part and ca. 862 Ma in the southern part of the outcrop area. The orthogneisses show ɛNd (1.0 Ga) values ranging from 1.0 to −4.2 with U/Pb TIMS and SHRIMP ages ranging from 960 to 926 Ma and geochemical signatures of A 2-type granitoids. The data reported in this paper suggest at least two periods of extension within the Central Domain of the Borborema Province, the first starts ca. 1091 Ma with magmatism and deposition, creating the Riacho Gravatá basin and continued intrusion of A-type granites to 920 Ma. A second rift event, which reactivated old faults, generated a basin with a maximum deposition age of ca. 806 Ma. Furthermore, the oldest granitoids cutting these metasedimentary rocks have crystallization ages of ca. 600 Ma. This suggests that the second rift event could be early Brasiliano in age. The resulting São Caetano basin received detritus from a variety of sources, although detritus from the Riacho Gravatá complex dominated. Deposition ages of the Riacho Gravatá and the São Caetano complexes are coeval with deposits in other basins of the Borborema Province (Riacho do Tigre in the Central Domain; Macururé and Marancó in the Sergipano Belt of the Southern domain). The Macaúbas Group from SE Brazil and its counterparts in Africa, the Zadanian and Mayumbian Groups, in the western edge of the Congo Craton are also coeval. Closure of the Riacho Gravatá and São Caetano basins occurred during the Brasiliano convergence (705–600 Ma). During the last stage of convergence, ca. 612 Ma, pull-apart basins were created and filled; final basin closure took place 605–592 Ma, after deposition ceased.

The Lagoa Salgada Orebody, Iberian Pyrite Belt, Portugal

The Iberian Pyrite Belt is one of the most outstanding European ore provinces and hosts one of the largest concentrations of massive sulfide deposits today, totaling 1,850 million metric tons (Mt) in more than 90 deposits. Lagoa Salgada is a small orebody (estimated to have at least 4 Mt) and, as yet, an unexploited orebody found within this ore province. It is located 80 km northwest of Neves Corvo and occurs ~130 m beneath sediments of the Sado Tertiary basin, limiting interpretation to drill hole data. Lagoa Salgada is folded, faulted, and interpreted to occur mostly on the subvertical-overturned and intensely faulted limb of a southwest-verging anticline. It is represented by a central stockwork zone and a massive sulfide lens zone in the northwestern part of the orebody. Mineralization is mainly composed of pyrite with minor sphalerite, tetrahedrite-tennantite, arsenopyrite, chalcopyrite, galena, stannite, and supergene minerals. The orebody is hosted by a volcanic succession of rhyodacitic composition. These small orebodies and some of the other abandoned mines within the Iberian Pyrite Belt may represent interesting and feasible mining projects as a result of the added value generated by the presence of trace metals, such as In, serving a significant future demand for the high-tech industry. Lagoa Salgada is one such case. Indium is a significant trace metal in the ores of Lagoa Salgada as indicated by whole-rock analyses. This element is preferentially contained by sphalerite. Electron probe microanalyses (EPMA) of In contents within sphalerite show a large variability, ranging from below detection limits to an obtained maximum of 0.8 percent In. Discrete inclusions of In-bearing minerals have not been observed, thus favoring the idea that In occurs either dispersed or in nanodomains within the host mineral.

Eruptive response of oceanic islands to giant landslides: New insights from the geomorphologic evolution of the Teide–Pico Viejo volcanic complex (Tenerife, Canary)

Large sector collapses are a major component of oceanic islands evolution. Here we show that voluminous events such as the Icod landslide on Tenerife (Canary Islands) cause dramatic changes on the magma feeding system and control the subsequent volcanic and geomorphologic evolution of the eruptive complex over a period of more than 150 kyr. Instantaneous unloading by the Icod landslide is marked by the development of a large phonolitic explosive eruption dated at 175 ± 3 ka and interpreted as reflecting the immediate emptying of a shallow pre-existing magma chamber. Geochronological, geomorphological and geochemical analyses, carried out on the post-landslide volcanic succession sampled in a 4.4 km-long underground water-recovery gallery, provide further evidence for an enhanced extrusion of primitive lavas starting in the 10 kyr time interval following the failure. Rapid construction (< 40 kyr) of a thick basaltic volcano in the landslide scar at high eruptive rates (up to 8 km 3 kyr −1) increased the lithostatic pressure which then favored the intermittent storage of basic magma under the edifice. This resulted in more episodic construction evidenced by a significant decrease in output rates and the increasing occurrence of lavas with intermediate composition from 117 ± 7 to 52 ± 7 ka. An apparent volcanic gap is observed between 52 ± 7 and 18 ± 1 ka, after which highly differentiated lavas have been dominantly erupted. We propose that part of the gap can be explained by the individualization of a shallow magma reservoir a few kilometers below the base of the Teide volcano. During recent periods, vertical and lateral extrusions of trachytic and phonolitic viscous bodies from this storage area contributed to increase the slope of the main edifice up to 35°, overall favoring its present-day instability.

Geology and hydrothermal resources in the northern Lake Abaya area (Ethiopia)

The northern Lake Abaya area located in the southern part of the Main Ethiopian Rift (MER) evidently contains an advective hydrothermal system. Regional extensional tectonics and subsidence that began in Late Miocene was followed by rift margin rhyolitic volcanism which produced extensive ignimbrite succession and rift shoulder trachytic volcanism in Pliocene. The extensional axis of the MER became the locus of volcanic activity in the Quaternary with bimodal basalt – rhyolitic volcanic products in the rift floor. Shallow crustal magma chambers feeding the axial volcanic complexes provide heat for the hydrothermal system which reside in Tertiary volcanic succession and is capped by lacustrine and volcanoclastic graben infill sediments. Duguna Fango rhyolitic volcanic complex which has extruded as much volume of volcanic products as all the other felsic centers put together is the most important heat source for the hydrothermal system and related manifestations. The closed drainage basin centered at Lake Abaya with adjacent plateau receiving over 1500 mm annual precipitation maintains a stable recharge for the prevailing hydrothermal system. Regional ground water flow direction is to the southwest following Bilate River which is the major river draining into the lake flowing parallel to the major NNE-SSW structural pattern of MER. Analyses of remote sensing data provided some insight into how the structural fabric had a control on the distribution of the groups of hydrothermal manifestations identified as hydrothermal fields. Thermal infrared image showed distinct thermal signature over the rift floor where hydrothermal fields are situated as compared to the plateau and areas covered by the cap-rock. A hydrothermal field around Duguna Fango volcanic complex has volcanological favorable setting despite lower inferred reservoir equilibrium geothermometer temperatures on the thermal springs. About 30 km to the south two other hydrothermal fields are located at a latitudinal distance of less than 15 km between each other. The near boiling point spring #6 of Northwest Abaya discharge mature water with highest geothermal fluid – host rock equilibrium geothermometer temperatures which together with a nearby fumarolic activity may be explained as an outflow. Across Chewkare graben the Bolcho and northeast Abaya hydrothermal fields located between Bilate and Gidabo rivers geochemically appear to be from a different high-enthalpy geothermal reservoir probably related to the cluster of felsic volcanic centers such as Chericho, Kilisa, Donga and Werencha. The high temperature springs indicate the existence of at least two geochemically distinct hydrothermal reservoirs in the study area. It remains to be proven weather an up-flow of an advective hydrothermal system centered to the north around Duguna Fango have long distance concealed out flows to the south on the northern shores of Lake Abaya which is the hydrologic depocenter.

Public geoscience to reduce exploration risk: new methods to characterize the basement beneath geological cover and to address community engagement in the Cariboo-Chilcotin region of British Columbia1This article introduces a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia. James

Mineral and petroleum exploration in the Cariboo-Chilcotin region of British Columbia is hampered by widespread and extensive cover of Pleistocene glacial deposits and Tertiary volcanic successions. Seeing through this geological cover is critical to reducing exploration risk and enhancing exploration activity. Also critical to exploration is effective community engagement, hopefully resulting in endorsement and support for exploration initiatives. The forests in the Cariboo-Chilcotin region have been extensively destroyed by an infestation of the mountain pine beetle, Dendroctonus ponderosae , disrupting established communities and greatly affecting economic activity in the region. Governments of all levels recognized that geological exploration activity could provide a ready and appreciable stimulus to economic activity, but only if local communities endorsed such programs. Relatively little oil and gas exploration and research has taken place in the region, and its effects have been poorly understood locally. Consequently, an extended effort was undertaken to establish an integrated geoscience program for the Cariboo-Chilcotin region, focused on mineral and petroleum exploration-related research and coupled with engagement with local communities to inform them of exploration benefits and risks. This Introduction to the “Mountain Pine Beetle” Special Issue of Canadian Journal of Earth Sciences provides a brief overview of the scientific papers included in the issue and also a review of the community engagement process that was undertaken to establish working relationships with First Nations and other communities in the region.

Syn-eruptive/inter-eruptive relations in the syn-rift deposits of the Precuyano Cycle, Sierra de Chacaico, Neuquén Basin, Argentina

The syn-rift volcanic successions of the Upper Triassic–Lower Jurassic Precuyano Cycle (i.e. the Lapa Formation) from the Sierra de Chacaico in the Neuquén Basin, Argentina, were studied in order to address the distinctive characteristics of accumulation during syn-eruptive and inter-eruptive periods in a depocentre associated with active volcanism and extensional tectonics. In particular, the syn-rift fill in this area comprises a wide range of compositions, as well as of transport and depositional processes. Lava flows coexist with pyroclastic and epiclastic deposits in the same accumulation space. In order to analyse the complexities inherent in a volcanic environment subjected to extension, five different accumulation units were identified in the area: Lava Flow/Shallow Intrusion Units, Pyroclastic Units, Volcaniclastic Alluvial Units, Polymictic Alluvial Units, and Lacustrine Units. The analysis of each of these units and of the relationship between them provided meaningful insights into the evolution of the syn-rift sedimentary environments and the identification of different stages of effusive activity, explosive activity and relative quiescence, determining syn-eruptive and inter-eruptive rock units. The relationship between these units was examined, and two accumulation stages were defined. The underfilled stage originates when the material supplied to the depocentre during the eruptive events is not enough to level the existent topography, allowing the development of high-gradient alluvial systems during the next inter-eruptive period. The overfilled stage occurs when extensive pyroclastic density current deposits choke the accumulation space during syn-eruptive periods, causing low-gradient sedimentary systems to develop during the subsequent inter-eruptive periods.

Implications of the Nuvvuagittuq Greenstone Belt for the Formation of Earth's Early Crust

of the belt formed at � 4·28 Ga, which would make it the only known remnant of Hadean crust preserved on Earth. The dominant lithology of the belt has a mafic composition that consists of gneisses ranging from cummingtonite amphibolite to garnet^biotite schist composed of variable proportions of cummingtonite þ biotite þ quartz, � plagioclasegarnetanthophyllitecordierite. The composition of this unit ranges from basalt to andesite and it is divided into two distinct geochemical groups that are stratigraphical- ly separated by a banded iron formation (BIF). At the base of the se- quence, the mafic unit is mainly basaltic in composition and generally has relatively low Al2O3 and high TiO2 contents, whereas above the BIF, the unit is characterized by high Al2O3 and low TiO2 contents and exhibits a wider range of compositions from bas- altic to andesitic. The low-Ti unit can be further subdivided into a trace element depleted and a trace element enriched subgroup. The high-Ti unit is characterized by relatively flat REE patterns as opposed to the low-Ti gneisses, which display light REE-enriched profiles with flat heavy REE slopes. The incompatible element depleted low-Ti rocks have U-shaped REE profiles.The geochemical groups have compositional analogues in three types of ultramafic sills that exhibit the same stratigraphic succession. Generally, the mafic gneisses have low Ca, Na and Sr contents, with many samples having CaO contents51wt %. Such low Ca contents are unlikely to represent the original composition of their igneous precursors and are interpreted to reflect intensive alteration of plagioclase. These compositional characteristics along with the presence of cordier- ite þ anthophyllite suggest that the protoliths of the mafic gneisses were mafic volcanic rocks exhibiting variable degrees of hydrothermal alteration. The high-Ti compositional type shares geochemical char- acteristics with tholeiitic volcanic suites with low Al2O3 and high TiO2 contents and is consistent with crystal fractionation at low pressures under dry conditions. In contrast, the low-Ti compositional group is geochemically similar to boninitic and calc-alkaline volcanic suites.The high Al2O3 and lowTiO2 contents in the andesitic com- positions suggest the early crystallization of Fe^Ti oxides and late appearance of plagioclase, and are more consistent with fractionation at elevated water pressures. The succession from 'tholeitic' to 'calc-alkaline' magmatism seen in the Nuvvuagittuq greenstone belt is typical of the volcanic successions of many younger Archean green- stone belts. Regardless of the exact tectonic setting, this volcanic suc- cession suggests that the geological processes responsible for the formation and evolution of Archean greenstone belts were active at 3·8 Ga and perhaps as early as 4· 3G a.

Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 1. Mode of emplacement in three areas1This article is a companion paper to Ross et al. 2011. Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 2. Origin, geochemistry, and geochronology. Canadian Journal of Earth Sciences, 48: this issue.2MRNF Contribution BEGQ 8439-2010/2011-1. Natural

The Archean Blake River Group (BRG) of Ontario and Quebec is dominated by submarine mafic to intermediate lavas, with more restricted felsic volcanic rocks. Given the good quality of outcrop, and high level of preservation of some BRG rocks, the mafic to intermediate lavas were used in the 1970s and 1980s to better understand the evolution of massive and pillowed submarine flows, and their associated fragmental facies (pillow breccias, hyaloclastite). Potentially, the BRG could also represent a useful volcanic succession for the study of explosive submarine eruption products in the ancient record. Before this is possible, however, a regional inventory of the mafic to intermediate volcaniclastic units is needed to clarify their characteristics and origins. In this paper, we compare and contrast volcaniclastic rocks from three areas within the same formation of the northern BRG in Quebec: the Monsabrais area, the Lac Duparquet area, and the D’Alembert tuff area. Close examination reveals pronounced differences in terms of lateral continuity, thickness, grading, bedding, clast shapes, textures, etc. in the volcaniclastic rocks. These differences are interpreted to reflect vastly different emplacement processes, ranging from hyaloclastite generation as a result of self-fragmentation and lava contact with water (dominant in the Monsabrais and Lac Duparquet areas) to aqueous density currents likely fed directly by explosive submarine eruptions (dominant in the D’Alembert tuff).