Mafic Lava Flows Research Articles

Exploring the formation of Tangará sill: A single-pulse intrusion feeding CAMP lava flows in Parecis Basin, Brazil

The Central Atlantic Magmatic Province (CAMP) stands out as a distinctive large igneous province on the Earth. Due to the extensive outcrop area of CAMP and its complex geological setting, the links between the extrusive and intrusive units are not fully understood. In the intracratonic Parecis Basin located in Central Brazil, the CAMP rocks are represented by mafic lava flows at its south (Tapirapuã Formation) and western edges (Anari Formation). Despite the availability of substantial geophysical, petrological and geochronological data, the understanding of CAMP feeder system in this region remains unclear. In this study, our focus is on investigating the Tangará sill, a dolerite intruded into Triassic sedimentary rocks in the South region of the Parecis Basin. Our aim is to contribute to a better understanding of the plumbing system in this area by investigating the Tangará sill and its potential links to the lava flows at the surface. The sill is identified as a tabular feature in seismic data, with a discernible base and top that extend for about 100 km from northeast to southwest. The Tangará sill was drilled by the 6-ANP-MT-02 stratigraphic well, revealing a continuous igneous intrusion with a thickness of 180 m (at depths ranging from 470 to 650 m). The dolerite intrusion is composed by plagioclase, clinopyroxene, opaque minerals and mesostasis (quartz + feldspar intergrowth and apatite). It can be divided into five segments: the upper and basal margins consist of fine-grained dolerite, the central portion comprises coarse-grained gabbro, which includes pegmatoid zones. The central portion is separated from the margins by two sections of medium-grained dolerite. The dolerite is tholeiitic, with low TiO2 content (<2 wt% TiO2) and low La/Yb ratio (average of 4.4). Geochemical and petrographic data indicate that the intrusion was formed by a single magma pulse and underwent differentiation through plagioclase and pyroxene dominated crystal fractionation. Two samples dated by 40Ar/39Ar geochronology using laser step-heating technique, yield plateau ages ranging from 204.0 ± 0.4 Ma to 202.0 ± 0.5 Ma. Sr-Nd-Pb isotopic data suggest a lithospheric mantle source with possible crustal contamination. Our findings suggest an emplacement model in which a thick single-pulse magma intruded the Triassic red beds. Along a NE-SW section, the sill is truncated by faults along the southern border of the basin, near the mapped lava flows of the Tapirapuã Formation. This suggests that the magma propagated from the northeast to the southwest along the sedimentary strata and fed the Tapirapuã volcanism. This example highlights a single-pulse sill feeding lava flows of the Central Atlantic Magmatic Province (CAMP).

A temporal dissection of late Quaternary volcanism and related hazards within the Rio Grande rift and along the Jemez lineament of New Mexico, USA

Abstract New 40Ar/39Ar ages, combined with selected ages from published studies, provide detailed insight into the late Quaternary (&amp;lt;500 ka) eruptive history and related hazards of the Rio Grande rift and Jemez lineament of New Mexico, USA. Most eruptions in the region during this time were within monogenetic volcanic fields, which largely produced cinder cones and mafic lava flows. 40Ar/39Ar ages of mafic groundmass determined using the high-sensitivity ARGUS VI multicollector mass spectrometer are significantly more precise, by as much as an order of magnitude, than prior 40Ar/39Ar dates. The high-precision data permit more rigorous interpretation of age spectra and isochrons, leading to a greater confidence in assigning eruption ages, and thus allowing more accurate and thorough calculations of eruptive rates and repose periods. For most fields, comprehensive dating identifies a greater number of late Quaternary eruptions than previously known and, for some fields, determines younger-than-previously established ages for the last eruptive events. Repose periods in the fields range from too short to measure with the 40Ar/39Ar method to a few hundred thousand years, which suggests that all 12 fields in the rift and lineament with late Quaternary activity should be considered dormant rather than extinct, with the possibility of future eruptions. Average recurrence intervals for these fields during the late Quaternary range from 16.5 k.y. to 170.8 k.y. Many fields display peak periods of activity where rates spike to a recurrence interval of 5 k.y. or less. At the scale of the entire rift and lineament, 75 late Quaternary eruptions were dated, yielding an average recurrence interval of 6.5 k.y., which is a minimum estimate considering the likelihood of undated eruptions (either not studied or buried). During the last 100 k.y., the volcanic record is better preserved, and the recurrence interval is 3.2 k.y., which indicates that the current hiatus of 3.9 ± 1.2 k.y. is typical for the region. Since ca. 36 ka, the average recurrence interval decreased to 2.3 k.y., which suggests a slight increase in recent activity. When ages are compared to vent locations, a previously unrecognized migrational pattern is observed in nearly all of the fields studied. Migration vectors vary from 1.0 cm/yr to 4.0 cm/yr, and always with an eastward component—similar to migration patterns at some other late Cenozoic fields throughout the American Southwest. Volcanic migration is attributed to a combination of mechanisms including asthenospheric convection along the margin of the Colorado Plateau, North American plate motion over partial mantle melt, and extensional tectonics. Developing similar high-precision chronologies for other Quaternary fields throughout southwestern North America will be necessary to better understand the volcanic hazards of the region.

Spectral Variability of Rocks and Soils on the Jezero Crater Floor: A Summary of Multispectral Observations From Perseverance's Mastcam‐Z Instrument

AbstractNASA's Mars‐2020 Perseverance rover spent its first year in Jezero crater studying the mafic lava flows of the Máaz formation and the ultramafic cumulates of the Séítah formation, both of which have undergone minor alteration and are variably covered by coatings, dust, and/or soil deposits. Documenting the rock and soil characteristics across the crater floor is critical for establishing the geologic context of Perseverance's cached samples—which will eventually be returned to Earth—and for interpreting the deposition and modification of the Máaz and Séítah formations. Mastcam‐Z, a pair of multispectral, stereoscopic zoom‐lens cameras, provides broadband red/green/blue and narrowband visible to near‐infrared images (VNIR, 440–1,020 nm). From multispectral observations from sols 0 to 380, we compiled a database of ∼2,400 representative Mastcam‐Z spectra. We analyzed principal components, spectral parameters, and laboratory spectra of pure minerals and natural rock surfaces to interpret the spectral diversity of rocks and soils. We define eight spectral classes of rocks: Dusty, Hematite‐like, Coated, Low‐Ca Pyroxene‐like, Olivine‐like, Weathered Olivine‐like, Fe‐rich Pyroxene‐like, and Dark Oxide‐like. The variability of soil spectra in the Jezero crater floor is controlled primarily by the amount of dust and indicates a largely consistent soil mineralogy across the traverse, with the exception of the area disturbed by the landing event. In comparison to rock spectra from the Curiosity rover's Mastcam instrument in Gale crater, rocks on the Jezero crater floor are generally less spectrally diverse, but the Olivine‐like rocks within the Séítah formation represent a new spectral rock class in Mars surface exploration.

Emplacement dynamics of the plumbing system and lava pile of the Paraná Magmatic Province in Morro da Igreja, Santa Catarina, Brazil

The mechanisms of magma ascent, transport and emplacement of the volcanic pile from LIPs are key issues regarding the understanding of the complex construction of volcanic systems and magma transport through the crust. The integrated approach of morphology of the volcanic and subvolcanic bodies, whole rock geochemistry and ASM data provides a robust tool to unravel the flow dynamics of the volcanic bodies and the sequence of the magmatic events. Such approach allows building up a model of construction of the lava pile considering the role played by the plumbing system. This paper investigates the emplacement mechanism of sill and lava flows of the Serra Geral Group (SGG), in the southern part of Mesozoic Paraná-Etendeka Magmatic Province (PEMP) in Brazil. Geologic and geochemical analysis were integrated with anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and rock magnetism experiments on 14 sampling sites in the Morro da Igreja region, top of the lava pile in Santa Catarina. The area is composed of low-Ti basaltic (with predominance of rubbly pahoehoe) and silicic lava flows, both intruded by a tabular sill that reaches a few hundred meters wide. The mafic lava flows are composed of Gramado, Urubici and Esmeralda magma-type basalts and andesite basalts. The silicic lava flows are massive or foliated, classified as Palmas type dacites. The sill has regular columnar joints and is classified as Esmeralda type. Magnetic mineralogy data suggests magnetite or Ti-poor magnetite as the main magnetic mineral for all the igneous rocks, and AARM results show anomalous fabric for the lava flows and normal or intermediate fabric for the sill. Besides the anomalous fabric sites, AMS data provide reliable directional data to infer flow direction for the sill, with initial propagation towards NE, followed by a preferential SE direction of magma flow. For the silicic rocks, the presence of vertical and inclined magnetic foliation suggest a lava dome geometry. The dynamics of the magmatic flow of the sill and lava flows associated with the compositional characteristics of the rocks allows stablishing the stratigraphy of the magmatic events in the area as well as the proposition of an emplacement model for the sequence.

Classification, identification, and reservoir characteristics of intermediate mafic lava flows: a case study in Dongling area, Songliao Basin

Intermediate mafic lava is a special oil and gas reservoir. While its internal structure is an important factor affecting the reservoir properties, the identification of facies and understanding of the relationship between facies architecture and reservoir are limited. This study evaluated the intermediate mafic lava flows of the Yingcheng Formation in the Dongling area of Songliao Basin by analyzing drilling cores, corresponding thin sections, and scanning electron microscope (SEM) images, as well as well-logging and seismic attributes. We also performed helium gas experiments and high-pressure mercury intrusion (HPMI) analysis to assess the physical properties and pore structure of the reservoir, respectively. The results showed that intermediate mafic lava flows develop tabular lava flow, compound lava flow, and hyaloclastite. Three facies showed present diverse well-logging and seismic responses. The intermediate mafic lava facies architecture was divided into crater-proximal facies (CF-PF), medial facies (MF), and distal facies (DF), which were characterized by their vesicles and joints and could be identified through their seismic attributes. The reservoir spaces including vesicles, amygdale inner pores, joint fissures, and dissolution pores predominantly showed oil and gas accumulation. The results of the tests of the reservoir’s physical properties showed that the reservoir quality was best in the CF-PF, which is the main target of oil and gas exploration.

Lithostratigraphy of the Bumbeni Complex and its associated subdivisions, South Africa

Abstract The Bumbeni Complex represents an Early Cretaceous volcanic complex that developed at the southern termination of the Lebombo mountain range in northern KwaZulu-Natal, South Africa. The volcano-sedimentary complex intrudes and disconformably overlies the Lebombo Group of the Karoo Large Igneous Province. The volcano-stratigraphy is defined by an assemblage of volcanic and volcaniclastic rocks as well as late stage quartz-syenite intrusions. Based on current geological and geophysical studies the complex is considered to represent the remnants of a collapsed cauldron structure. The stratigraphic succession is intricate and laterally discontinuous, comprising mafic and felsic lava flows, welded and non-welded tuffs, ignimbrites, lahar deposits, rhyolite domes and dykes and coarse-grained syenitic rocks. Rhyolite dome formation and collapse, and associated plinian eruptions and column collapse, were largely responsible for the variety of volcaniclastic rocks, including classic air-fall tuff deposits. The complex forms part of a northeast trending volcanic basement lineament known as the Bumbeni Ridge defined by aeromagnetic data.

Geologic map of Slate Range Crossing area, California, USA

This detailed geologic map and supplemental digital data set1 examine and demonstrate the complex Neogene–Quaternary deformation in the Slate Range Crossing area (California, USA) of the active dextral transtension of the Death Valley region and Walker Lane belt. This map integrates the late Cenozoic structures and geologic units with the Mesozoic geologic units and deformation as a data set to examine the controls on reactivation of older structures. These geologic data were collected to study pre-, syn-, and post-kinematic rocks to examine the deformation history of the area and to find palinspastic markers to examine the late Cenozoic fault displacement and displacement history across Panamint Valley to the east, as reported in Andrew and Walker (2009). The study focused on defining the Miocene and Pliocene rocks and deposits and examining lateral changes and depositional sources of clasts. There are two different volcanic-sedimentary sequences in this area. A Miocene section contains mafic to felsic volcanic units, numerous debris-flow to laharic deposits, and several associated conglomerates and breccias containing exotic clasts. The exotic clasts are matched to rocks in the Panamint Range on the east side of Panamint Valley as reported in Andrew and Walker (2009) as displacement vectors for palinspastic reconstructions. These Miocene strata ubiquitously dip eastward 20–40º. A younger volcanic-sedimentary sequence contains relatively thin mafic lava flows and associated locally derived, coarse-grained mass wasting deposits. These younger basaltic lavas generally have gentle dipping lava flow features and foliation. Numerous faults cut the different age deposits allowing a chronology of Neogene to Quaternary faulting; additionally, there are numerous fabrics associated with Jurassic contraction and Cretaceous(?) dextral shear. The area near Slate Range Crossing has a conspicuous zone of earthquake foci; this study found that some of this seismic activity coincides with a zone of southwest-striking, moderately dipping to the north, sinistral-oblique normal faults, which cut across the northernmost Slate Range. These faults form a structural boundary between the Argus and Slate Ranges and link the fault networks in Panamint Valley with those in Searles Valley. This mapping and structural data demonstrate the two-stage Neogene fault history of the Walker Lane belt deformation in this area and show that regional tilting of rocks occurred after ca. 13 Ma and before ca. 4 Ma; this eastward down-tilting appears to be a discrete event and may mark the change from extension to transtension. This detailed geologic mapping and collection of structural data for the rocks in the eastern Argus and northern Slate Ranges and Panamint Valley were created using digital in-the-field geographic information systems software running on a field-hardened laptop computer. This map is a simplification of detailed geologic mapping data collected at 1:6000 scale and reduced to 1:20000 scale. Structural data includes kinematic and relative timing of deformation information.

The Castle Rock and Ironside Mountain calderas, eastern Oregon, USA: Adjacent venting sites of two Dinner Creek Tuff units—the most widespread tuffs associated with Columbia River flood basalt volcanism

Abstract The Dinner Creek Tuff is an important unit of mid-Miocene rhyolite volcanism contemporaneous to flood basalts of the Columbia River magmatic province. Field mapping along with analytical data of tuff samples identify two calderas, the Castle Rock and Ironside Mountain calderas, as the venting sites of two widespread ignimbrites of the Dinner Creek Tuff. Both calderas lie within the area of the proposed general storage sites of main-phase Columbia River Basalt magmas. The Castle Rock caldera formed during the eruption of the 16.16 Ma Dinner Creek Tuff unit 1. The northwestern boundary of the caldera is roughly defined by the juxtaposition of over 300 m of densely welded rheomorphic intra-caldera tuff and tuffaceous mega-breccia deposits against Mesozoic Weathersby Formation shale and pre-Miocene Ring Butte trachybasalt lavas. Following caldera collapse, fluvial and lacustrine volcaniclastic sediments were deposited on the caldera floor, and outflow tuffs of the Dinner Creek Tuff units 2 and 4 were deposited into the caldera. Aphyric basaltic andesite and icelandite (Fe-rich andesite), which correlate stratigraphically to upper Grande Ronde Basalt lavas, intrude the caldera floor deposits, and lavas are interbedded with sediments and Dinner Creek Tuff unit 4. The Ironside Mountain caldera formed during eruption of the 15.6 Ma Dinner Creek Tuff unit 2, which lies ~15 km north of the Castle Rock caldera. The caldera is an 11 × 6 km depression wherein over 900 m of intra-caldera, rheomorphic, and partially welded tuff are bound by Weathersby Formation shale and Tureman Ranch granodiorite. Post-caldera collapse, basaltic andesite and icelandite dikes and sills that are also stratigraphically correlative to upper Grande Ronde Basalt lavas intruded into the tuff, mostly along the margins of the caldera, which altered much of the tuff. Mafic lavas within the study area that closely pre- and post-date Dinner Creek Tuff units were correlated with regional units of the Columbia River Basalt Group. Porphyritic and aphyric mafic lava flows underlying Dinner Creek Tuff unit 1 at Castle Rock are correlated with Picture Gorge Basalt and Grande Ronde Basalt. Aphyric basaltic andesite and icelandite that intrude into and overlie the Dinner Creek Tuff units 1 and 2 are westward extensions of fractionated tholeiitic magmas as seen in late-stage Grande Ronde Basalt units such as the Hunter Creek Basalt. Finally, porphyritic basalt lava flows that overlie the Hunter Creek Basalt and volcaniclastic sediments at the Castle Rock caldera are correlative with the 13.5 Ma Tim's Peak Basalt. At Castle Rock, pre-caldera Columbia River Basalt Group lavas appear to lap onto a mid-Miocene topographic high that stretches northward and westward for tens of kilometers based on stratigraphic data, and it may be related to regional uplift at initial impingement of the mantle upwelling to produce the Columbia River Basalt Group. The Castle Rock and Ironside Mountain calderas exemplify bimodal volcanism of the Columbia River magmatic province. Eruption of rhyolites is closely pre- and post-dated by the eruption of local and regional tholeiitic lavas belonging to the Columbia River Basalt Group. The local eruption of evolved tholeiitic lavas likely concealed calderas, but these lavas also illustrate the close proximity of mafic and rhyolitic magmas at depth at these rhyolite centers. Consequently, the stratigraphy of both the Castle Rock and Ironside Mountain calderas somewhat differs from that of rhyolite calderas dominated by silicic and calc-alkaline intermediate pre- and post-caldera volcanism.

Petrogenesis and economic potential of the Obatogamau Formation, Chibougamau area, Abitibi greenstone belt

Unravelling the petrogenesis and stratigraphy of Archean mafic lava flows is essential to our comprehension of the early geodynamic evolution and economic potential of greenstone belts. This study focuses on one of the oldest and thickest sequences of lava flows observed in the Neoarchean Abitibi Subprovince (greenstone belt), that is, the Obatogamau Formation. The undated formation extends for more than 100 km in an E–W direction and consists mostly of aphyric and feldspar megacryst-bearing basaltic-andesite lava flows. These lava flows are tholeiitic, mostly Fe-rich, and have nearly homogeneous chemistry. Petrogenetic modelling carried out using MELTS software points to limited magmatic differentiation as most samples of mafic lava flows did not reach Fe-Ti-oxide saturation. Zircon U-Pb dating establishes a crystallization age of 2726.2 ± 1.6 Ma for a felsic unit located at an intermediate stratigraphic position in the sequence of lava flows. Constraints from stratigraphically overlying volcanic units suggest that the Obatogamau Formation was likely emplaced rapidly, possibly within a few million years and as a consequence of frequent replenishment of shallow magma accumulations. High eruption rates are consistent with short episodes of volcanic quiescence deduced from field observations, indicating non-optimal conditions for volcanogenic massive sulfide systems. The pressure and temperature of peak metamorphism deduced from amphibole chemistry, however, points to favorable conditions for the release of metamorphic fluids. The study area may thus be prospective for orogenic gold mineralization, provided that fluids had access to a source of gold and that structural conduits allowed for the channeling of hydrothermal fluids.

Characterization, origin, and evolution of one of the most eroded mafic monogenetic fields within the central Andes: The case of El País lava flow field, northern Chile

El País is a monogenetic lava flow field located 16 km southeast of Peine town in the Altiplano-Puna area, Antofagasta region, Chile. El País is a monogenetic lava flow field located 16 km southeast of Peine town in the Altiplano-Puna area, Antofagasta region, Chile. El País is composed by lava flows of small-to-moderate volumes, which are fed from individual point sources unassociated with a central volcano of large-volume (stratovolcano) typical of the Andean arc. This lava flow field was emplaced in an area dominated by thin-skinned deformation under a compressive tectonic regime, where multiple volcanic centers (monogenetic and polygenetic volcanoes) have been developed. The volumetric and chemical evolutions were determined by fieldwork, stratigraphic, morphometric, textural (density and vesicularity), petrographic, and geochemical analyses. El País can be divided into three principal mafic lava flows such as Northern-, Main-, and Western-lava flows, which display an E-W orientation. Besides, at least eight ephemeral vents were identified as small erosion remnants of lava emitting point-sources. The identified vents are principally aligned in an ENE-WSW direction, east of the Main lava flow. It is inferred that El País was emplaced near-simultaneously above two ignimbrite sheets: Tucúcaro Ignimbrite (3.2 Ma) and Patao Ignimbrite (3.1 Ma). Stratigraphically, the Main lava flow has a surface texture with strong similarities to an ʻaʻā lava flow, composed of three unique layers well distinguishable in the field such as i) Basal, ii) Top auto-breccia, composed of sub-angular scoriaceous and dense clasts in an oxidized matrix, and iii) A single core between both auto-breccias, composed of dense lavas, exhibiting multiple cooling jointing, such as vertical to platy jointing. The advanced stage of erosion since the formation of El País is evidenced by the poorly preserved lava flows (flat morphology), upper auto-breccia layer, and the presence of large sub-angular blocks of lava (vesicular and dense) along the axis of the lava flows. The eruptive products are geochemically basaltic andesite (Main lava flow and ephemeral vents) and andesite (specifically in the Northern lava flow), making the lava flow field one of the most mafic in the broader region. The lava presents a glomeroporphyric texture, showing at least three different glomerocrysts: i) Plagioclase + clinopyroxene having reabsorption boundaries, ii) Clinopyroxene with reabsorption boundaries, and iii) Plagioclase with sieve-texture. The groundmass is principally composed of glass, plagioclase microlites with a low orientation preference, and opaque minerals (magnetite). This study helps understand the development of dispersed-monogenetic-volcanism (e.g., Tilocálar complex and Cerro Tujle maar), and its relationship with neighboring polygenetic volcanism (e.g., Toloncha stratovolcano) in the Central Volcanic Zone of the Andes, northern Chile.

Physical volcanology of the Pavagadh rhyolites, northern Deccan Traps: Stratigraphic, structural, and textural record of explosive and effusive eruptions

Mount Pavagadh (840 m) in the northern Deccan Traps, India, is mainly composed of mafic lava flows (picrites, ankaramites, mugearites, basalts) and capped by rhyolites. Rhyolites also found at lower elevations, on and around the mountain, are thought to represent blocks of the eroded upper rhyolites or local eruptions in situ. Although the Pavagadh sequence has been studied for 150 years, the stratigraphy, structures, textures, and physical volcanology of the Pavagadh rhyolites have been poorly documented, and are discussed here at length. The upper rhyolite sequence contains tens of non-welded, crystal-bearing, glass- and pumice-rich ash beds of 47 m total thickness. These represent a primary fallout ash succession derived from distant Plinian eruptions, possibly corresponding with massive ignimbrite deposits recently recognised in Saurashtra several hundred kilometers to the southwest. The fallout ash succession is overlain by a 50 m thick lava flow showing well-developed flow banding and folding, basal and upper breccias, and a local basal vitrophyre. The lava flow is overlain by an ignimbrite with a basal vitrophyre, both these units showing excellent vitroclastic textures. The upper rhyolites thus represent alternating explosive-effusive-explosive eruptions. The lower-level rhyolites in the area, showing intense rheomorphic deformation in outcrop but lacking vitroclastic textures, are probably also lavas. However, they are not blocks of the eroded upper rhyolite lava flow but from separate eruptions. The Pavagadh rhyolites were probably far more laterally extensive than their present outcrops, with a minimum original volume of 9 km3; their source areas remain unknown. Geochemical and geochronological work on the Pavagadh rhyolites is needed, in the stratigraphic and volcanological context provided here, to understand their petrogenesis and potential relationship with the Cretaceous/Palaeogene (K/Pg) boundary mass extinction.

Bridging the gap: 40Ar/39Ar dating of volcanic eruptions from the ‘Age of Discovery’

Many volcanoes worldwide still have poorly resolved eruption histories, with the date of the last eruption often undetermined. One such example is Ascension Island, where the timing of the last eruption, and consequently, the activity status of the volcano, is unclear. Here, we use the 40Ar/39Ar dating technique to resolve ages of the three youngest lava flows on the island, which are hawaiites and mugearite with 1.5 – 1.9 wt% K2O. In dating these lavas, we provide the first evidence of Holocene volcanic activity on Ascension (0.51 ± 0.18 ka; 0.55 ± 0.12 ka; 1.64 ± 0.37 ka), determining that it should be classed as an active volcanic system. In addition, we demonstrate that the 40Ar/39Ar method can reproducibly date mafic lava flows younger than 1 ka, decreasing the gap between recorded history and geological dating. These results offer new prospects for determining patterns of late-Holocene volcanic activity; critical for accurate volcanic hazard assessment.

Regional geological setting of Guanajuato silver-gold deposits and quantitative petrophysical assessment of their hydrothermal alteration, Guanajuato, México

Geological setting of one of the world famous silver-gold deposits type is presented. The silver-gold vein system occurs in the southeastern portion of the Sierra de Guanajuato, which is a 100 km long, N45°W-trending mountain range in Central Mexico. The rocks exposed in the Sierra de Guanajuato and its surroundings can be divided into four broad stratigraphic successions: (1) the “Basal complex” which includes Mesozoic rocks and an early Eocene granitic batholith, (2) a thick (>1000 m) red bed sequence resting unconformably atop the basal complex, which was deposited in the lapse between 49 and 36 Ma, (3) a thick, faulted and tilted, Paleogene subaerial volcanic succession, and (4) Immediately north of Sierra de Guanajuato are exposed a widespread, flat-lying, sand and gravel deposits accumulated in a continental basin. These sediments contain fossil vertebrate faunas with ages between late Clarendonian and Blancan. Southeast of El Bajío fault system sand and gravel deposits, partially covered by basaltic andesites crop out. A similar relation between gravel deposits and mafic lava flows is seen at the top of Cerro El Cubilete, which is one of the highest point in Sierra de Guanajuato. Succession (1) experienced several shortening events during the Cretaceous and Paleocene, whereas successions (2) thru (4) were extended in several deformation pulses during the Cenozoic.Wall rock alteration changes in some petrophysical parameters were studied in the Veta Madre vein in the central part of the Guanajuato mining district. Hydrothermally altered rocks were characterized by their variations in the following petrophysical characteristics: (1) density, (2) effective porosity, (3) rapid volumetric saturation, and (4) constant saturation.Among the petrophysical characteristics was observed that is important for exploration at Guanajuato:1.The rock density tends to fall near the structures that present ore bodies at depth, and having little change when no hydrothermal alteration is present.2.The effective porosity (Pefec) of rocks suitable for mineralization increase their values.3.The rapid volumetric conditioned saturation (A) plays a big role when the hydrothermal fluids pass near the rocks with high values and produces mineralization.4.The constant saturation (1/B), their values tend to increase in areas with mineralization.Samples of host rocks collected above known spikes such as Las Torres and Sirena exhibit hydrothermal alteration with the following petrophysical characteristics: density tends to decrease whereas porosity, rapid volumetric and constant saturation increase. The use of these petrophysical parameters may be a low-cost technique to predict the presence of undiscovered orebodies in the Guanajuato mining district and elsewhere where similar mineralization occurs.

The magmatic evolution and genesis of the Quaternary basanite-trachyphonolite suite of Itasy (Madagascar) as inferred by geochemistry, Sr-Nd-Pb isotopes and trace element distribution in coexisting phases

The Itasy is a Pleistocene-Holocene volcanic field in central Madagascar, located to the west of the Ankaratra volcanic complex. It comprises scoria cones and lava domes (>120), with associated pyroclastic fall and mafic lava flows, covering an area of ab. 400 km2. The last volcanic episodes probably dated ca. 6000–7100 y BP; warm springs and geysers are active. The juvenile samples comprise a peculiar, almost bimodal, rock suite ranging from potassic leucite-kaersutite-bearing basanites, tephrites and phonotephrites, to benmoreites and titanite-haüyne-bearing trachyphonolites (MgO from 9–10 wt% to 0.1 wt%). These rocks show continuous and overlapping variations in the bulk-rock and phase composition (olivine, clinopyroxene, amphibole, feldspar, leucite, haüyne, nepheline, oxides, apatite, titanite, glass and other accessories). The basanites have homogeneous isotopic composition (87Sr/86Sr = 0.70366–0.70378, 143Nd/144Nd = 0.51274–0.51277, 206Pb/204Pb = 18.7–18.9, 207Pb/204Pb = 15.53–15.56; 208Pb/204Pb = 38.89–39.01), and a marked enrichment in the most incompatible elements (LILE and HFSE reach 100–215 times primitive mantle). These features are consistent with low degrees of partial melting of a volatile-, LILE- and HFSE-rich, amphibole-bearing peridotitic mantle induced by uplift during an E-W-directed extensional regime, as is found in central Madagascar. The marked changes in the geochemical composition, and small variations of the Sr-Nd-Pb isotopes in the trachyphonolites (87Sr/86Sr = 0.70425–0.70446, 143Nd/144Nd = 0.51266–0.51269, 206Pb/204Pb = 18.18–18.39, 207Pb/204Pb = 15.49–15.51; 208Pb/204Pb = 38.38–39.57) with respect to basanites and tephrites point to a limited amount of crustal contamination by the relatively low-206Pb/204Pb, low-143Nd/144Nd, high-87Sr/86Sr Precambrian basement rocks (of Middle Archean to Late Proterozoic age), and highlight the geochemical effects of titanite and anorthoclase removal on the trace element fractionation trends, a feature also shown in the trace element composition of the phenocrysts in trachyphonolites.

Directional change during a Miocene R‐N geomagnetic polarity reversal recorded by mafic lava flows, Sheep Creek Range, north central Nevada, USA

AbstractRecurring transitional field directions during three Miocene geomagnetic reversals provide evidence that lateral inhomogeneity of the lower mantle affects flow in the outer core. We compare new paleomagnetic results from a composite sequence of 15.2 Ma lava flows in north central Nevada (Sheep Creek Range; 40.7°N, 243.2°E), erupted during a polarity reversal, to published data from Steens Mountain (250 km to the northwest in Oregon) and the Newberry Mountains (650 km to the south in California) that document reversals occurring millions of years and many polarity switches earlier. Alternating field demagnetization, followed by thermal demagnetization in half the samples, clearly isolated the primary thermoremanent magnetization of Sheep Creek Range flows. We correlated results from our three sampled sections to produce a composite record that begins with a single virtual geomagnetic pole (VGP) at low latitude in the Atlantic, followed by two VGPs situated near latitude 30°N in NE Africa. After jumping to 83°N (one VGP), the pole moves to equatorial South America (one VGP), back to NE Africa (three VGPs), to high southern latitudes (two VGPs), back to equatorial South America (three VGPs), and finally to high northern latitudes (nine VGPs). The repeated visits of the transitional VGP to positions in South America and near NE Africa, as well as the similar behavior recorded at Steens Mountain and the Newberry Mountains, suggest that lower mantle or core‐mantle boundary features localize core flow structures, thereby imparting a discernible regional structure on the transitional geomagnetic field that persists for millions of years.

The stratigraphy and history of Mars' northern lowlands through mineralogy of impact craters: A comprehensive survey

AbstractThe basin‐filling materials of the northern lowlands, which cover approximately one third of Mars' surface, record the long‐term evolution of Mars' geology and climate. The buried stratigraphy was inferred through analyses of impact crater mineralogy, detected using data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars. Examining 1045 impact craters across the northern lowlands, we find widespread olivine and pyroxene and diverse hydrated/hydroxylated minerals, including Fe/Mg smectite, chlorite, prehnite, and hydrated silica. The distribution of mafic minerals is consistent with infilling volcanic materials across the entire lowlands (~1–4 × 107 km3), indicating a significant volume of volatile release by volcanic outgassing. Hydrated/hydroxylated minerals are detected more frequently in large craters, consistent with the scenario that the hydrated minerals are being excavated from deep basement rocks, beneath 1–2 km thick mafic lava flows or volcaniclastic materials. The prevalences of different types of hydrated minerals are similar to statistics from the southern highlands. No evidence of concentrated salt deposits has been found, which would indicate a long‐lived global ocean. We also find significant geographical variations of local mineralogy and stratigraphy in different basins (geological provinces), independent of dust cover. For example, many hydrated and mafic minerals are newly discovered within the polar Scandia region (&gt;60°N), and Chryse Planitia has more mafic mineral detections than other basins, possibly due to a previously unrecognized volcanic source.

Lithostratigraphy and volcanology of the Serra Geral Group, Paraná-Etendeka Igneous Province in Southern Brazil: Towards a formal stratigraphical framework

The volcanic rocks of the Lower Cretaceous Paraná-Etendeka Igneous Province, in Brazil, are grouped in the Serra Geral Group. The province can be chemically divided into low-TiO2, and high-TiO2. In southern Brazil, the low-TiO2 lava pile reaches a thickness of ~1km and is formed of heterogeneous lava packages here divided into four lava formations. Torres Formation (TF) is characterized by chemically more primitive basaltic (>5wt% MgO) compound pahoehoe flow fields; these lavas stratigraphically overly aeolian sandstones of Botucatu Formation and represent the onset of the volcanic activity. Vale do Sol Formation (VSF) groups vertically stacked sheet-like rubbly pahoehoe basaltic andesites (SiO2>51wt%; MgO<5wt%). These lavas covered the former basalts in the Torres Syncline axis and pinch out towards southwest and represent the most voluminous mafic lava flows. Dacites and rhyolites of Palmas Formation (PF) overlay VSF flows in the central and eastern outcrop area and rest directly upon TF lavas in the west. The acidic units were emplaced as lava domes and widespread tabular lava flows. Esmeralda Formation (EF) is the upper stratigraphic unit and it is formed by a basaltic pahoehoe flow field emplaced during the waning phase of volcanic activity of the low-TiO2 lava sequence. Sedimentary interbeds are preserved throughout the whole lava pile and were deposited during quiescence periods of volcanic activity, and represent important stratigraphic markers (e.g. TF-VSF contact). The newly proposed stratigraphy provides promptly recognized stratigraphic units in a regional framework of fundamental importance for future correlations and provide vital information in the understanding of how the Paraná-Etendeka Igneous Province evolved through time.

A historical overview of Moroccan magmatic events along northwest edge of the West African Craton

Located along the northwestern edge of the West African Craton, Morocco exhibits a wide variety of magmatic events from Archean to Quaternary. The oldest magmatic rocks belong to the Archean Reguibat Shield outcrops in the Moroccan Sahara. Paleoproterozoic magmatism, known as the Anti-Atlas granitoids, is related to the Eburnean orogeny and initial cratonization of the WAC. Mesoproterozoic magmatism is represented by a small number of mafic dykes known henceforth as the Taghdout mafic volcanism. Massive Neoproterozoic magmatic activity, related to the Pan-African cycle, consists of rift-related Tonian magmatism associated with the Rodinia breakup, an Early Cryogenian convergent margin event (760–700 Ma), syn-collisional Bou-Azzer magmatism (680–640 Ma), followed by widespread Ediacaran magmatism (620–555 Ma). Each magmatic episode corresponded to a different geodynamic environment and produced different types of magma.Phanerozoic magmatism began with Early Cambrian basaltic (rift?) volcanism, which persisted during the Middle Cambrian, and into the Early Ordovician. This was succeeded by massive Late Devonian and Carboniferous, pre-Variscan tholeiitic and calc-alkaline (Central Morocco) volcanic flows in basins of the Moroccan Meseta. North of the Atlas Paleozoic Transform Zone, the Late Carboniferous Variscan event was accompanied by the emplacement of 330–300 Ma calc-alkaline granitoids in upper crustal shear zones. Post-Variscan alkaline magmatism was associated with the opening of the Permian basins.Mesozoic magmatism began with the huge volumes of magma emplaced around 200 Ma in the Central Atlantic Magmatic Province (CAMP) which was associated with the fragmentation of Pangea and the subsequent rifting of Central Atlantic. CAMP volcanism occurs in all structural domains of Morocco, from the Anti-Atlas to the External Rif domain with a peak activity around 199 Ma. A second Mesozoic magmatic event is represented by mafic lava flows and gabbroic intrusions in the Internal Maghrebian flysch nappes as well as in the external Mesorif. This event consists of Middle-Upper Jurassic MORB tholeiites emplaced during opening of the Alpine Tethys ocean. The Central High Atlas also records Early Cretaceous alpine Tethys magmatism associated with the aborted Atlas rift, or perhaps linked to plume activity on the edge of the WAC.Cenozoic magmatism is associated with Tertiary and Quaternary circum-Mediterranean alkaline provinces, and is characterized by an intermittent activity over 50 Ma from the Anti-Atlas to the Rif Mountain along a SW–NE volcanic lineament which underlines a thinned continental lithosphere.

Identification of mantle peridotite as a possible Iapetan ophiolite sliver in south Shetland, Scottish Caledonides

The Neoproterozoic Dunrossness Spilite Subgroup of south Shetland, Scotland, has been interpreted as a series of komatiitic and mafic lava flows formed in a marginal basin in response to Laurentian continental margin rifting. We show that ultramafic rocks previously identified as komatiites are depleted mantle peridotites that experienced seafloor hydrothermal alteration. The presence of positive Bouguer gravity and aeromagnetic anomalies extending from the Dunrossness Spilite Subgroup northward to the Shetland Ophiolite Complex suggests instead that these rocks may form part of an extensive ophiolite sliver, obducted during Iapetus Ocean closure in a forearc setting. Supplementary material : Supplementary information, including methods, supplementary figures and tabulated data, is available at https://doi.org/10.6084/m9.figshare.c.3469161 .

Paleoproterozoic closure of an Australia–Laurentia seaway revealed by megaclasts of an obducted volcanic arc in Yukon, Canada

The Slab volcanics are a late Paleoproterozoic volcanic succession in the Wernecke Mountains of Yukon Territory in northwestern Canada. Fragments of the succession are preserved as megaclasts in km-scale zones of hydrothermal breccia. The largest clast is 160×380m and consists of 31 mafic lava flows and minor intercalated sandstone and tephra. Hydrothermal activity during the brecciation led to extensive metasomatic alteration, both sodic and potassic. Despite the alteration, an igneous geochemical signal is discernible. The rocks are mafic to intermediate and moderately alkalic. Their trace element profiles indicate derivation from a hydrated, enriched mantle source, consistent with an origin as a volcanic arc affected by a plume, rift or slab window. Major and trace element patterns demonstrate a cyclical magmatic evolution involving intervals of fractional crystallization punctuated by recharge with mafic magma. The Slab volcanics were previously thought to have been deposited on Laurentia but are herein regarded as part of the exotic terrane Bonnetia that was obducted onto the Laurentian continental margin in late Paleoproterozoic time. Bonnetia may have developed on or near the eastern margin of Australia as a continental or fringing arc. In that scenario, Bonnetia grew through arc magmatism as oceanic crust between Australia and Laurentia was consumed prior to terrane obduction and continental collision in the late Paleoproterozoic to early Mesoproterozoic. Sediment or tephra derived from the arc was transported westward into the interior of Australia, raising εNd values of late Paleoproterozoic sedimentary strata. Vigorous hydrothermal brecciation followed and led to fragmentation of the crust, including the obducted terrane Bonnetia. Giant blocks of the Slab volcanics and other units of Bonnetia foundered into breccia zones and moved downward for thousands of meters. Erosion removed the obducted terrane and the uppermost Laurentian crust, exposing the megaclasts.