Volcanic Range Research Articles

八ヶ岳火山群，硫黄岳・横岳鞍部におけるコマクサの分布に関する地生態学的研究

八ヶ岳火山群，硫黄岳・横岳鞍部におけるコマクサの分布に関する地生態学的研究

Combining ultraconserved elements and mtDNA data to uncover lineage diversity in a Mexican highland frog (Sarcohyla; Hylidae).

Molecular studies have uncovered significant diversity in the Mexican Highlands, leading to the description of many new endemic species. DNA approaches to this kind of species discovery have included both mitochondrial DNA (mtDNA) sequencing and multilocus genomic methods. While these marker types have often been pitted against one another, there are benefits to deploying them together, as linked mtDNA data can provide the bridge between uncovering lineages through rigorous multilocus genomic analysis and identifying lineages through comparison to existing mtDNA databases. Here, we apply one class of multilocus genomic marker, ultraconserved elements (UCEs), and linked mtDNA data to a species complex of frogs (Sarcohyla bistincta, Hylidae) found in the Mexican Highlands. We generated data from 1,891 UCEs, which contained 1,742 informative SNPs for S. bistincta and closely related species and captured mitochondrial genomes for most samples. Genetic analyses based on both whole loci and SNPs agree there are six to seven distinct lineages within what is currently described as S. bistincta. Phylogenies from UCEs and mtDNA mostly agreed in their topologies, and the few differences suggested a more complex evolutionary history of the mtDNA marker. Our study demonstrates that the Mexican Highlands still hold substantial undescribed diversity, making their conservation a particularly urgent goal. The Trans-Mexican Volcanic Range stands out as a significant geographic feature in Sarcohyla and may have acted as a dispersal corridor for S. bistincta to spread to the north. Combining multilocus genomic data with linked mtDNA data is a useful approach for identifying potential new species and associating them with already described taxa, which will be especially important in groups with undescribed subadult phenotypes and cryptic species.

Insights into magma histories through silicate-oxide crystal clusters: Linking the Hiltaba Suite intrusive rocks to the Gawler Range Volcanics, Gawler Craton, South Australia

The Roxby Downs Granite (RDG), host to the supergiant Olympic Dam deposit, and the Gawler Range Volcanics (GRV) in South Australia, both contain clusters of touching silicate and oxide minerals. Here we report on the geochemistry and petrography of these rocks and their contained mineral clusters, before comparing them to similar occurrences elsewhere in the world to gain a better understand of their genesis and the links between the RDG (and other members of the Hiltaba Suite) and the GRV. Crystal clusters in the RDG comprise magnetite + apatite ± titanite ± biotite ± zircon which are typically enveloped in a plagioclase crystal-matrix. In the GRV, crystal clusters comprise pyroxene + titanomagnetite ± plagioclase ± apatite ± zircon, display variably glomeroporphyritic, equigranular, poikilitic, and loosely packed habits, and are enclosed by texturally varied groundmass. Of the GRV lavas studied, pyroxenes and titanomagnetite show systematic trends in major and trace element compositions conformable with the stratigraphic position of their hosts. This suggests the lavas are related through differentiation in the upper crust and represent the inversion of a normally zoned reservoir. There are no compositional contrasts between phenocrysts and crystals in GRV clusters, and Mg# of pyroxenes are consistent with those predicted from whole-rock compositions, implying that all analysed crystals (including those in clusters) were formed within, or were at least equilibrated to, the host lavas’ precursors. Rather than entire histories as isolated crystal clusters entrained by melt flow, intergrowth textures and mafic magmatic enclaves suggest that the majority of GRV crystal clusters were liberated from closely packed crystal masses through magma rejuvenation and associated crystal resorption. Consistent Fe-Ti oxide + apatite + zircon + plagioclase assemblages and textures in both the GRV and RDG suggest a shared genetic scenario, although an extended model is needed to account for the transformation of GRV-type to HS-type crystal clusters. For hot and dry magmas such as those that formed the GRV and RDG, cooling causes pyroxene + Fe-Ti oxide + plagioclase in GRV-type crystal clusters to react with each other and the melt, forming HS-type magnetite + titanite + biotite + feldspar crystal clusters. Rejuvenation events implicated in the liberation of GRV-type crystal clusters may be important drivers of magmatic-hydrothermal iron-oxide deposit formation.

Geochemistry and U-Pb geochronology of 1590 and 1550 Ma mafic dyke swarms of western Laurentia: Mantle plume magmatism shared with Australia

Three new UPb ID-TIMS isotopic ages confirm previous evidence for two geochemically distinct, early Mesoproterozoic pulses of mafic magmatism along the western margin of Laurentia. The first is a UPb baddeleyite age of 1590 ± 5 Ma from a dyke swarm in the Tobacco Root Mountains of western Laurentia. It is the first evidence for magmatism of this age in west-central Laurentia. The second is a UPb baddeleyite age of 1592.4 ± 2.5 from the Western Channel Diabase, 2000 km to the north in NW Laurentia, which supports two previous UPb baddeleyite ages of ca. 1590 Ma for these intrusions. The third is a UPb baddeleyite age of 1551 ± 5 Ma, also from the Tobacco Root Mountains, and provides evidence for a distinct younger pulse of mafic magmatism.We propose that the ca. 1590 Ma mafic intrusions in northwestern and west-central Laurentia represent components of a large igneous province (LIP), which we name the “ca. 1590 Ma Mammoth-Western Channel LIP.”This 1590 Ma LIP is geochemically similar to the contemporaneous volcanic rocks in the Gawler Craton and Curnamona Province of Australia. Furthermore, the 1599 Ma Wernecke Breccias near the Western Channel Diabase are geologically similar to the 1590 Ma Olympic Dam Breccias on the South Australian craton. We propose that a mantle plume at ca. 1590 Ma centered between the Laurentian and Southern Australian cratons, located by converging dyke swarms, fed the intrusions on Laurentia plus the Gawler Range Volcanics, the Hiltaba Suite granites and the Ninnerie Supersuite on the South Australian craton. Additionally, magmatic underplating from the plume set up the hydrothermal system responsible for the formation of the Wernecke and Olympic Dam Breccias. The younger 1551 Ma magmatism in the Tobacco Root Mountains, which shows less contamination of a metasomatized subcontinental lithospheric mantle than the older 1590 Ma pulse, may represent an early stage of rifting that pre-dates the ca. 1470 Ma Belt-Purcell Basin extension of western Laurentia.Felsic magmatism, hypothesized to have accompanied the 1590–1550 Ma LIP magmatism, could be a potential source for detrital zircon, thereby reducing the requirement for a non-Laurentian source for detrital zircon ages within the 1610–1490 Ma “North American Magmatic Gap.” Additionally, the 1590 Ma and 1550 Ma ages on western Laurentia dykes provide tighter constraints for previous paleomagnetic studies.

Sedimentary basin formation associated with a silicic large igneous province: stratigraphy and provenance of the Mesoproterozoic Roopena Basin, Gawler Range Volcanics

ABSTRACTThe Mesoproterozoic Gawler Silicic Large Igneous Province (SLIP) in the Gawler Craton and Curnamona Province, southern Australia, comprises extensive felsic and lesser mafic volcanic sequences, with only limited sedimentary successions. The Roopena Basin is a rare example of a synvolcanic sedimentary basin that formed within the Gawler SLIP in the eastern Gawler Craton. It is a north–south-trending basin with a preserved area of 75 km2, bound by the Roopena and Wizzo Well faults, and contains three units of the lower Gawler Range Volcanics; the Angle Dam Dacite, Fresh Well Formation and Roopena Basalt. The Angle Dam Dacite is a porphyritic lava and the oldest part of the volcanic succession, directly overlying basement. The Fresh Well Formation overlies the Angle Dam Dacite or basement, comprises three coarsening-upwards volcaniclastic packages of claystone, siltstone, fine-grained to coarse-grained lithic sandstone and conglomerate deposited in a fluvio-lacustrine setting, and contains three tuff members. The Roopena Basalt is interlayered with the Fresh Well Formation, and comprises auto-brecciated lavas that exhibit only local interaction with water or wet sediment. Sharp basal contacts of the prograding packages within the Fresh Well Formation provide evidence of rapid flooding events within the basin. New detrital zircon geochronology of a sandstone within the Fresh Well Formation yielded a maximum depositional age of ca 1580 Ma, with provenance dominated by felsic volcanic units of the 1635–1605 Ma St Peter Suite. Sedimentation in the Gawler SLIP appears to have occurred in isolated basins with limited areal extent. It was largely restricted to the eastern Gawler Craton, and as well as the Roopena Basin, and includes similar basins at the Olympic Dam and Prominent Hill iron oxide–copper–gold ± uranium (IOCG ± U) deposits. The coincidence of sedimentation and mafic volcanism in the eastern Gawler Craton suggests that this region underwent extension at this time, although high-temperature metamorphism and compressional deformation occurred in some parts of the Gawler Craton and Curnamona Province synchronous with the Gawler SLIP. The Roopena Basin sedimentary rocks and underlying basement contain hematite–chlorite–sericite–white mica assemblages, permissive of hematite-style IOCG mineral deposits; however, no significant ore deposit has yet been discovered in the Roopena Basin.

Forest cover change in the Los Tuxtlas Biosphere Reserve and its future: The contribution of the 1998 protected natural area decree

The Los Tuxtlas mountain range harbors one of the last remnants of tropical rain forest on the coastal plain of the Gulf of Mexico. This volcanic range has a high degree of heterogeneity in its geology, climate and ecology, in addition to a very long history of human occupation. The original area covered by tropical forest has been drastically reduced by agricultural activities, and during the last four decades in particular, deforestation has been very intense. In order to protect the remaining forest, in 1998 the Mexican government created the Los Tuxtlas Biosphere Reserve (LTBR). While previous studies estimated deforestation rates and the amount of forest cover remaining in some areas of the LTBR, this is the first study to do so for the entire protected area. A retrospective analysis from 1986 to 2011 was conducted to assess the effect of the 1998 decree of the LTBR on deforestation rates, and to predict future changes in forest cover up to the year 2025 using Markov chains and cellular automata based on current deforestation patterns. The results show that shortly after the 1998 decree, deforestation rates in the LTBR not only decreased but reversed, however this trend did not continue. In recent years deforestation has again increased. Our projection shows that if current trends continue unchanged then by the year 2025 we might have lost close to 14% (ca. 9000 ha) of the forest cover that was present in 2011. The decree of the LTBR was part of the federal policy to protect biodiversity in Mexico and our results show that the strategy of establishing this protected area did work to protect tropical forest, at least temporarily in Los Tuxtlas. Also, our results show that it is not only possible to reverse forest loss within the ample buffer zone of the LTBR, it is also still relatively easy to achieve by promoting passive restoration.

Thermal history of the northern Olympic Domain, Gawler Craton; correlations between thermochronometric data and mineralising systems

Multi-method thermochronology applied to the northern Olympic Domain of South Australia reveals a complex thermal evolution with multiple thermal events. Apatite U/Pb (closure temperature ~550–350 °C) and muscovite 40Ar/39Ar ages (~400–350 °C) record post magmatic cooling from 1850 to 750 Ma for the ~1850 Ma Donington Suite, and from ~1560–1500 Ma for the ~1590 Ma Hiltaba Suite and the Gawler Range Volcanics. Potassium feldspar 40Ar/39Ar ages (~350–150 °C) record disturbed spectra that are likely related to hydrothermal alteration at ~1000–650 Ma in the Neoproterozoic Adelaide Rift Complex. Apatite fission track (~60–120 °C), zircon (U-Th-Sm)/He (~180–200 °C), and apatite (U-Th-Sm)/He (~45–75 °C) ages reveal regional low temperature thermal events at ~1000 Ma, ~430–400 Ma, 350–330 Ma, and ~200 Ma, in addition to localised thermal events during the Cretaceous. The ~1000 Ma apatite fission track ages are amongst the oldest recorded for South Australia and are only preserved near the margins of the Olympic Domain. The ~430–400 Ma and ~350–330 Ma cooling events are interpreted to be driven by the Alice Springs Orogeny. The Mesozoic thermochronometric ages are interpreted to record localised thermal perturbations, possibly related with hydrothermal activity within the northern Olympic Domain. The presence of abundant IOCG deposits near these young thermal anomalies may indicate that these zones record more favourable thermal conditions and/or exposure levels for IOCG discoveries in the study area.

Mapping geology beneath volcanics using magnetic data

Volcanic layers within sedimentary basins cause significant problems for petroleum exploration because the attenuation of the seismic signal masks the underlying geology. A test study was conducted for the South Australia Government to map the thickness of volcanics and sub-volcanic geology over a large area in the Gawler Range Volcanics province. The area is covered by good quality magnetic data. The thickness of volcanics and basement configuration was unknown as there has only been a limited amount of drilling. The Automatic Curve Matching (ACM) method was applied to located magnetic data and detected magnetic sources within different rock units, providing their depth, location, geometry and magnetic susceptibility. The magnetic susceptibilities detected by ACM allowed the differentiation of the volcanics and the underlying basement. The base of volcanics and the depth to the top of basement was mapped along 75 km NS profiles, that were spaced 1 km apart over a distance of 220 km. The volcanic and basement magnetic susceptibilities and the magnetic source distribution pattern were used as key determinants to interpret the depth to the two interfaces. The results for each interface were gridded, and images of the base of volcanics and depth to basement were generated. The mapped volcanics thickness was validated by comparison with the results from drilling, with the volcanics thickness matching very well. After project completion, a passive seismic survey was conducted in part of the test area, indicating a base of volcanics of ~4 km, which further confirmed the results.

Pómez Bosque de Tlalpan, producto de una erupción de gran magnitud en el margen suroeste de la cuenca de México

In this contribution we describe a white pumice fall deposit, informally named pómez Bosque de Tlalpan (PBT), found in several outcrops in the southern Mexico basin. The most representative sequence occurs at the Bosque de Tlalpan park, Delegación Tlalpan, in Mexico City. In this site the deposit is 3 m thick, massive, and contains 80–90 vol. % of pumice clasts, predominantly of coarse lapilli-sized and some block-sized fragments. The PBT has a dacitic composition (64.9–66 wt. % SiO2, on anhydrous basis) of calc-alkaline affinity, and a mineral assemblage represented by plagioclase > amphibole > orthopyroxene > biotite, ±Fe-Ti oxides, with quartz in lesser proportions, and zircon as accesory mineral, in a vesicular and glassy matrix. According to data collected from seven outcrops, thickness and clast size of the deposit decrease towards the NE and therefore the vent source should be located to the SW of Bosque de Tlalpan. Possible sources are the volcanoes Ajusco and San Miguel, which are part of the Sierra de Las Cruces volcanic range, and are located in this direction at ~11 km from Bosque de Tlalpan. The age of the PBT deposit was determined by radiocarbon dating of underlying paleosoils that yielded 25,730 ± 130 to 37,450 ± 330 yrs B.P. (Late Pleistocene). Additionally, thermoluminscence dating of one sample yielded an age of 30,300 ± 5,000 years, similar to the radiocarbon ages. Hence, the PBT represents the youngest reported deposit from the Sierra de Las Cruces volcanic range. Although only seven outcrops were described in this work, we interpret the PBT deposit as produced by a plinian-type eruption, based on its massive structure, its ca. 3 m thickness, and the presence of pumice clasts in coarse lapilli to block sizes, as well as 3.5 cm sized lithics, at 11 km from the possible vent source, characteristic of this kind of deposits. We discard the monogenetic volcanoes from the Chichinautzin volcanic field as the vent source of the PBT deposit, because this kind of volcanoes is relatively mafic in composition and do not produce plinian-type deposits.

Magmatic zircon Lu–Hf isotopic record of juvenile addition and crustal reworking in the Gawler Craton, Australia

New in situ zircon Lu–Hf isotopic data are presented from magmatic rocks distributed across the Gawler Craton, Australia. These rocks range in composition from granite to gabbro, with the majority being granite or granodiorite and moderately peraluminous in composition. The new Lu–Hf isotopic data, together with previously published data, provide insight into the magmatic evolution of the craton and crust and mantle interaction through time. Increased juvenile content of magmatic rocks correlate with periods of extensional tectonism, in particular basin formation and associated magmatism during the Neoarchean to earliest Paleoproterozoic (c. 2555–2480Ma), Middle Paleoproterozoic (c. 2020–1710Ma) and Late Paleoproterozoic (c. 1630–160Ma). In contrast, magmatic rocks associated with periods of orogenic activity show greater proportions of crustal derivation, particularly the magmatic rocks generated during the c. 1730–1690Ma Kimban Orogeny. The final two major magmatic events of the Gawler Craton at c. 1630–1604Ma and c. 1595–1575Ma both represent periods of juvenile input into the Gawler Craton, with εHf(t) values extending to as positive as +8. However, widespread crustal melting at this time is also indicated by the presence of more evolved εHf(t) values to −6.5. The mixing between crust and mantle sources during these two youngest magmatic events is also indicated by the range in two stage depleted mantle model ages (TDMc) between 1.76Ga and 2.51Ga. Significant mantle input into the crust, particularly during formation of the c. 1595–1575Ma Hiltaba Suite and Gawler Range Volcanics, likely facilitated the widespread crustal magmatism of this time period. Viewed spatially, average εHf(t) and TDMc values highlight three of the major shear zones within the Gawler Craton as potentially being isotopic as well as structural boundaries. Differences in isotopic composition across the Coorabbie Shear Zone in the western Gawler Craton, the Middle Bore Fault in the northern Gawler Craton and, to a lesser extent, the Kalinjala Shear Zone in the southern Gawler Craton, broadly correspond to crustal and even lithospheric-scale discontinuities evident in geophysical studies. Therefore, these shear zones may approximate some of the first order crustal domains within the Gawler Craton.

Protected Natural Values of Geoheritage Interest in the Călimani National Park, Eastern Carpathians, Romania

The Călimani National Park (CNP), legally founded in 2000 in order to ensure the highest level of protection for the unique natural and scientific values it hosts, is the largest protected volcanic area in Romania. It covers a significant part of the Călimani Mountains representing the largest, highest, and most voluminous segment of the Neogene Călimani-Gurghiu-Harghita volcanic range. The natural values under protection in the CNP include the unique high-mountain volcanic landscape hosting glacial and periglacial features, lakes and marshes, spectacular rock formations, and unique floral elements. Due to their outstanding scientific value, the botanical Science Reserve of Juniperus with Pinus cembra, the geographical Iezer Lake Science Reserve, and the geological “12 Apostles” Science Reserve are subject to the highest legal level of protection. CPN also hosts damaged natural values of geoheritage importance. This is the place where the phenomenon of volcanic karst was described for the first time, and the term “vulcanokarst” coined. Open-pit sulfur mining destroyed its original features and only poor traces of them can be found today. The high-mountain volcanic landscape was also catastrophically damaged by mining. However, remediation and re-cultivation activities in the open-pit area significantly attenuated the damage. Little attention is paid by the Park’s management to the valuation of the geological and volcanological features deserving higher levels of protection. Most of them are not even known to the wider public, with the exception of geologists. Touristic valuation of the area was recently enhanced by the remediation of the old “Maria Theresia road” linking Transylvania and Bukovina across the mountains. There is still a lot to do in order to fully take advantage of the natural values of geoheritage relevance hosted by the CNP, in teaching, research and touristic/leisure activities. Actions to be undertaken to this end suggested here include organization of thematic workshops, conferences, exhibitions, and field practice for students, professionals and the general public focused on geoheritage values, placing geoeducational boards along touristic trails, devising field guides and promotional material, and connecting the Călimani Mts. to broader touristic networks such as the proposed “Pannonian volcano route.”

Marco geológico y tectónico de la Isla del Coco y la región marítima circunvecina, Costa Rica

La Isla del Coco es la única isla oceánica y el único afloramiento subaéreo de la Cordillera Volcánica del Coco, el rasgo geográfico y geológico más extenso en aguas territoriales costarricenses. Desde el punto de vista geológico, está conformada por rocas volcánicas, predominantemente coladas de lavas basálticas y traquíticas en menor cantidad, con rocas piroclásticas y epiclásticas subordinadas. Posee suelos, coluvios y depósitos de playas (arenas y cantos) superficiales; su topografía es muy variable pero predomina el relieve quebrado y rugoso. La Isla del Coco es la parte emergida de un volcán submarino de evolución compleja, desarrollado a partir y durante el Pleistoceno Inferior (entre 2.2 y 1.5 millones de años, Ma), producto de una anomalía térmica en el manto a través de varias fisuras, que originaron varios alineamientos de volcanes submarinos. La Isla del Coco se encuentra en un estadio de erosión activo, y sus arcos y plataformas sumergidas (90-110m y 183m) son probablemente el producto de una erosión subaérea durante las dos últimas máximas glaciaciones, combinados quizás con la subsidencia debida al enfriamiento del escudo volcánico y de la corteza oceánica. Al ser la isla geológicamente joven, posee importantes implicaciones para la comprensión de la evolución y el endemismo de su biodiversidad. Las amenazas en la geodinámica principales identificadas son los deslizamientos, los tsunamis y, en menor grado, la sismicidad, poco frecuente y con magnitudes moderadas (≤ 5.8 Mw hasta la fecha) dentro de un radio de alrededor de 300km, predominantemente asociada con fallas dextrales de rumbo N-S. Los pocos datos obtenidos hasta el momento indican que hay un cierto grado de actividad sísmica en los alrededores de la Isla del Coco, relacionados con las fallas locales. Algunos pequeños tsunamis históricos y prehistóricos han afectado a la Isla del Coco y los sismos así como la precipitación pluvial elevada han generado deslizamientos. La licuefacción está restringida a las dos playas arenosas (bahías Chatham y Wafer).

The youngest volcanic eruptions in East‐Central Europe—new findings from the Ciomadul lava dome complex, East Carpathians, Romania

Violent explosive eruptions occurred between c. 51 and 29 thousand years ago—during the Last Glacial Maximum in East‐Central Europe—at the picturesque volcano of Ciomadul, located at the southernmost tip of the Inner Carpathian Volcanic Range in Romania. Field volcanology, glass geochemistry of tephra, radiocarbon and optically stimulated luminescene dating, along with coring the lacustrine infill of the two explosive craters of Ciomadul (St Ana and Mohos), constrain the last volcanic activity to three subsequent eruptive stages. The explosivity was due to the silicic composition of the magma producing Plinian‐style eruptions, and the interaction of magma with the underlying, water‐rich rocks resulting in violent phreatomagmatic outbursts. Tephra (volcanic ash) from these eruptions are interbedded with contemporaneous loess deposits, which form thick sequences in the vicinity of the volcano. Moreover, tephra layers are also preserved in the older Mohos crater infill, providing an important archive for palaeoclimate studies. Identifying the final phreatomagmatic eruption of Ciomadul at c. 29.6 ka, which shaped the present‐day landform of the 1600‐m‐wide St Ana explosion crater, we were able to correlate related tephra deposits as far as 350 km from the source within a thick loess‐palaeosol sequence at the Dniester Delta in Roxolany, Ukraine. A refined tephrostratigraphy, based on a number of newly found exposures in the Ciomadul surrounding region as well as correlation with the distal terrestrial and marine (e.g. Black Sea) volcano‐sedimentary record, is expected from ongoing studies.

Effects of hydrothermal alteration on mafic lithologies at the Olympic Dam Cu-U-Au-Ag deposit

Two groups of mafic to ultramafic lithologies occur at the Olympic Dam copper-uranium-gold-silver deposit, South Australia. The first group consists of olivine-phyric basalt and mafic dykes belonging to the ca. 1590Ma Gawler Range Volcanics; the second group is the Olympic Dam dolerite affiliated with the ca. 820Ma Gairdner Dyke Swarm. Both groups of mafic rocks occur within and adjacent to the Olympic Dam Breccia Complex, the immediate host to the deposit, and have been variably altered. They contain a number of secondary minerals, mainly including chlorite, sericite, iron oxides and carbonates. Drill core assays of the ca. 1590Ma olivine-phyric basalt reflect the variable abundance of secondary minerals such as carbonates and iron oxides (e.g. up to 26wt.% CO2 and 50wt.% Fe2O3). Positive correlations between compatible Cr and other incompatible elements (e.g. Ti and Zr) in the olivine-phyric basalt suggest whole-rock mass and/or volume loss due to hydrothermal alteration, in accordance with the previously proposed role of the olivine-phyric basalt as an important source of Cu using Sm-Nd isotope constraints. However, rigorous mass balance calculation is not feasible due to a lack of fresh equivalents. Geochemical comparisons between the least-altered ca. 820Ma Olympic Dam dolerite and the more-altered equivalents reported in this study reveal that the concentrations of some elements have been affected by hydrothermal alteration. In particular, Mg, K, Cs, Ba, Rb, Pb, U, and heavy rare earth elements have mostly been enriched; Ca, Sr and light rare earth elements have mostly been depleted; and Si, Fe, and Na have been affected both ways. Elevated Zn and Pb but depleted Cu concentrations in some Olympic Dam dolerite samples suggest that these metals were also mobilized. Petrographic observation and drill core assays are consistent with sodic alteration having been responsible for the Cu depletion in some dolerite samples. However, there is currently no evidence that this had any significant impact on the Cu content and distribution of the Olympic Dam deposit. Further, alteration of both groups of mafic rocks implies multiple hydrothermal events at the Olympic Dam deposit.

Post‐rift magmatic evolution of the eastern North American “passive‐aggressive” margin

AbstractUnderstanding the evolution of passive margins requires knowledge of temporal and chemical constraints on magmatism following the transition from supercontinent to rifting, to post‐rifting evolution. The Eastern North American Margin (ENAM) is an ideal study location as several magmatic pulses occurred in the 200 My following rifting. In particular, the Virginia‐West Virginia region of the ENAM has experienced two postrift magmatic pulses at ∼152 Ma and 47 Ma, and thus provides a unique opportunity to study the long‐term magmatic evolution of passive margins. Here we present a comprehensive set of geochemical data that includes new 40Ar/39Ar ages, major and trace‐element compositions, and analysis of radiogenic isotopes to further constrain their magmatic history. The Late Jurassic volcanics are bimodal, from basanites to phonolites, while the Eocene volcanics range from picrobasalt to rhyolite. Modeling suggests that the felsic volcanics from both the Late Jurassic and Eocene events are consistent with fractional crystallization. Sr‐Nd‐Pb systematics for the Late Jurassic event suggests HIMU and EMII components in the magma source that we interpret as upper mantle components rather than crustal interaction. Lithospheric delamination is the best hypothesis for magmatism in Virginia/West Virginia, due to tectonic instabilities that are remnant from the long‐term evolution of this margin, resulting in a “passive‐aggressive” margin that records multiple magmatic events long after rifting ended.

Insights into lithospheric architecture, fertilisation and fluid pathways from AusLAMP MT

The Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) has the goal to map the electrical resistivity of the Australian lithosphere to constrain the geodynamic framework of the continent. Data acquisition in South Australia has covered two-thirds of the state to date. Three-dimensional resistivity models of subsets of the AusLAMP grid across the Gawler Craton show a generally electrically resistive crust and lithosphere, but an area of low resistivity between depths of 100 km and 200 km beneath the Gawler Range Volcanics exists. A possible explanation is a fertilised mantle signature as a result of metasomatic events in the Proterozoic. The continuous low resistivity connection along the margins of the Gawler Craton core to the surface coincides with the prospective IOCG belt along the eastern margin of the Gawler Craton. The results support the importance of the AusLAMP project to define the lithospheric architecture of the continent and the value of primary lithospheric architecture for mineral exploration.

Spectral characteristics of the Gawler Range Volcanics in drill core Myall Creek RC1

ABSTRACTTraditional core-logging methods in conjunction with spectral scanning techniques have been used to log volcanic successions of the lower Gawler Range Volcanics. The open-file drill core Myall Creek RC 1 was re-logged and scanned using HyLogger™ core scanning technologies as a part of the Geological Survey of South Australia's Southern Gawler Rangers mapping program and the Mineral System Drilling Program. Myall Creek RC 1 is one of the key stratigraphic drill cores for the region, owing to the intersection of a large section of the Gawler Range Volcanics. Spectral characterisation of the Eucarro Rhyolite revealed differential weathering of plagioclase phenocrysts, while high-resolution imagery and spectral results were used to log new basaltic flows and small flow features in the Roopena Basalt. The composition and distribution of feldspars in the unnamed lower Gawler Range Volcanic units were used to aid traditional logging of visually similar lithologies. A spectral scalar, the felsic–mafic index, was used to identify unusual features in the unnamed sequence and was found to identify iron oxides as either fracture coatings or finely disseminated in the matrix of the sample. Iron oxides were also used to identify features within lithological units, which were difficult to discern visually, especially the layers in the deepest layered ignimbrite at the end of the drill core.

A magmatic source of hydrothermal sulfur for the Prominent Hill deposit and associated prospects in the Olympic iron oxide copper-gold (IOCG) province of South Australia

The Prominent Hill deposit is a world-class iron oxide copper–gold (IOCG) deposit in South Australia, characterized by a high Cu/S ratio of the dominant Cu-(Fe) sulfides hosted by hematite breccias. It contains a total resource of 278Mt of ore at 0.98% Cu and 0.75g/t Au. Prominent Hill is one of several IOCG deposits and numerous prospects in the Olympic IOCG province that are temporally associated with the 1603–1575Ma Gawler Range Volcanics, a large igneous province including co-magmatic granitoid intrusions of the Hiltaba Suite. Globally, IOCG deposits share many similar features in terms of their geological environment and mineral association. However, it is not yet clear whether sulfur and copper originate from the same source rocks and which hydrothermal redox processes created the characteristic iron oxide enrichment. Highly variable sulfur isotope compositions of sulfides and sulfates in IOCG deposits have previously been interpreted in terms of diverse sulfur sources that may include contributions from magmatic, sedimentary, seawater or evaporitic sulfur. In order to test these alternatives, we performed a detailed sulfur isotope study of Cu-(Fe) sulfides from Prominent Hill and IOCG prospects nearby. The Prominent Hill deposit shows a wide range in δ34SV-CDT between −33.5‰ and 29.9‰ for Cu-(Fe) sulfides, and a narrower range of 4.3‰ to 15.8‰ for barite. Iron sulfides (pyrite, pyrrhotite) show a narrow range in sulfur isotope composition, whereas Cu-bearing sulfides show a much wider range, and more negative δ34SV-CDT values on average. We propose a two-stage sulfide mineralization model for the IOCG system in the Prominent Hill area, in which all hydrothermal sulfur is ultimately derived from a magmatic source that had a composition of 4.4±2‰. The diversity in sulfur isotope composition can be produced by different fluid evolution pathways along reducing or oxidizing trajectories. A reduced sulfur evolution pathway is responsible for stage I mineralization, when intrusion-derived magmatic-hydrothermal fluids produced early pyrite and minor chalcopyrite at Prominent Hill, and iron±copper sulfides in regional magnetite skarns and in some pervasively altered volcanic rocks of the Gawler Range Volcanics. Shallow-venting magmatic-hydrothermal fluids and subaerial volcanic gases that became completely oxidized by reaction with atmospheric oxygen produced sulfate and sulfuric acid with a sulfur isotope composition equal to their magmatic source. This highly oxidized ore fluid probably consisted dominantly of water from the hydrosphere, but contained magmatic solute components, notably sulfate, acidity and Cu. Sulfate reduction produced hydrothermal Cu sulfides with a wide range in sulfur isotope compositions from very negative to moderately positive values. Partial reaction of the Cu-rich stage II fluid with earlier stage I sulfides resulted in mixing of sulfur derived from sulfate reduction and from sulfides deposited during stage I. Modeling of the sulfur isotope fractionation processes in response to reducing and oxidizing pathways demonstrates that the entire spectrum of sulfur isotope data from stage I and stage II mineralization can be explained with a single, ultimately magmatic sulfur source. Such a magmatic sulfur source is also adequate to explain the complete spectrum of sulfur isotope data of other IOCG prospects and deposits in the Olympic province, including Olympic Dam. The results of our study challenge the conventional model that suggests the requirement of multiple and compositionally diverse sulfur sources in hematite-breccia hosted IOCG style mineralization.

Extent of Cropland and Related Soil Erosion Risk in Rwanda

Land conversion to cropland is one of the major causes of severe soil erosion in Africa. This study assesses the current cropland extent and the related soil erosion risk in Rwanda, a country that experienced the most rapid population growth and cropland expansion in Africa over the last decade. The land cover land use (LCLU) map of Rwanda in 2015 was developed using Landsat-8 imagery. Based on the obtained LCLU map and the spatial datasets of precipitation, soil properties and elevation, the soil erosion rate of Rwanda was assessed at 30-m spatial resolution, using the Revised Universal Soil Loss Equation (RUSLE) model. According to the results, the mean soil erosion rate was 250 t·ha−1·a−1 over the entire country, with a total soil loss rate of approximately 595 million tons per year. The mean soil erosion rate over cropland, which occupied 56% of the national land area, was estimated at 421 t·ha−1·a−1 and was responsible for about 95% of the national soil loss. About 24% of the croplands in Rwanda had a soil erosion rate larger than 300 t·ha−1·a−1, indicating their unsuitability for cultivation. With a mean soil erosion rate of 1642 t·ha−1·a−1, these unsuitable croplands were responsible for 90% of the national soil loss. Most of the unsuitable croplands are distributed in the Congo Nile Ridge, Volcanic Range mountain areas in the west and the Buberuka highlands in the north, regions characterized by steep slopes (>30%) and strong rainfall. Soil conservation practices, such as the terracing cultivation method, are paramount to preserve the soil. According to our assessment, terracing alone could reduce the mean cropland soil erosion rate and the national soil loss by 79% and 75%, respectively. After terracing, only a small proportion of 7.6% of the current croplands would still be exposed to extreme soil erosion with a rate >300 t·ha−1·a−1. These irremediable cropland areas should be returned to mountain forest to foster environmental sustainability or further sustainable alternative erosion control techniques may be applied, such as applying Vetiver Eco-engineering Technology due to its economical soil erosion control and stabilization of steep slopes and the construction of erosion control dams to absorb and break down excess runoff from unusually intense storms in various parts of the watersheds.

Olivine-phyric basalt in the Mesoproterozoic Gawler silicic large igneous province, South Australia: Examples at the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit and other localities

The felsic-dominant Gawler Range Volcanics and cogenetic Hiltaba Suite granitoids constitute the ca. 1590Ma Gawler silicic large igneous province in the Gawler Craton, South Australia. The province includes minor occurrences of olivine-phyric basalt at Kokatha and Mount Gunson. In this study, we describe additional olivine-phyric basalts and dykes intersected by drill holes at the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit and the Wirrda Well Cu–Au prospect. Laser Ablation Inductively Coupled Plasma Mass Spectrometry U–Pb dating results (un-anchored) of apatite in the basalts and dykes at Mount Gunson (1576±33Ma), Wirrda Well (1596±17Ma), and Olympic Dam (1621±20Ma) confirm their effective temporal correlation with the ca. 1590Ma Gawler silicic large igneous province.Compositions of Cr-spinel inclusions enclosed in former olivine phenocrysts (forsterite number >80) in basalts at Mount Gunson, Wirrda Well and Olympic Dam imply derivation from a heterogeneous mantle source that may have been modified by subduction. Least-altered olivine-phyric basalts at Mount Gunson and Kokatha are characterized by negative Nb and Ta anomalies, which are typical of arc basalts but are also common in back-arc basin basalts, in accordance with the result suggested by the Cr-spinel source indicator. The mafic components of the Gawler Range Volcanics generally have higher Zr contents and Zr/TiO2 ratios than those of high-Mg basalts and picrites produced in variable tectonic settings worldwide, possibly reflecting continental crustal components involved in their mantle source.High abundances (typically ∼20vol.%) of former olivine phenocrysts in the basalt at Olympic Dam imply high-Mg whole-rock compositions and a high temperature of the primary magma. This result is consistent with previous models in which the heat flux from mantle magmas caused large-scale partial melting of crustal rocks that subsequently gave rise to silicic magmas erupted and intruded to form the Gawler silicic large igneous province.