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At least 703 records · Page 39Linked to original sources
USGS1997

New K-Ar and 40Ar/39Ar ages of plutonism, hydrothermal alteration, and mineralization in the central Wasatch Mountains, Utah

Twenty-one new K-Ar and 10 new 40 Ar/ 39 Ar ages are reported for igneous and hydrothermal minerals from intrusive rocks of the Wasatch igneous belt in the central Wasatch Mountains. Interpretation of our new data combined with previously published K-Ar ages and with new 40 Ar/ 39 Ar and U-Pb ages reported by Vogel et al. (1997) suggests that the Clayton Peak stock was emplaced at about 36 to 35 Ma, the Alta stock at about 35 to 33 Ma, and the Little Cottonwood stock at about 31 to 30 Ma. Biotite K-Ar ages progressively increase from west to east in the Little Cottonwood stock, which is consistent with more rapid cooling of the eastern part of the stock and with other evidence suggesting about 15 degrees of eastward tilting of the central Wasatch Mountains following emplacement of the Wasatch igneous belt. Most porphyry stocks in the Park City mining district were emplaced at about 41 to 40 Ma; the Ontario stock was emplaced at about 36 Ma. Vein deposits in the Park City mining district formed at about 36 to 33 Ma. The Park Premier stock was emplaced in several pulses between about 35 to 32 Ma. Alteration related to porphyry copper mineralization in the Park Premier stock formed at 33.5 Ma and advanced argillic alteration and gold mineralization formed at 31.4 Ma. Molybdenum mineralization in the eastern part of the Little Cottonwood stock formed between 26 to 23.5 Ma.

Utah↗
USGS1988

Aeromagnetic maps of the Colorado River region including the Kingman, Needles, Salton Sea, and El Centro 1 degree by 2 degrees quadrangles, California, Arizona, and Nevada

Aeromagnetic data for the Colorado river region have been compiled as part of the Pacific to Arizona Crustal Experiment (PACE) Project. The data are presented here in a series of six compilations for the Kingman, Needles, Salton Sea, and El Centro 1 degree by 2 degree quadrangles, California, Arizona, and Nevada, at scales of 1:250,000 and 1:750,000. The scales and map areas are identical to those used by Mariano and others (1986) to display the Bouguer and isotatic residual gravity for this region. Data were compiled separately for the Kingman quadrangle, the Needles quadrangle, and an area covering the Salton Sea quadrangle and part of the El Centro quadrangle. Aeromagnetic anomalies are produced by variations in the strength and direction of the magnetic field of rocks that include magnetic minerals, commonly magnetite. Patterns of anomalies on aeromagnetic maps can reveal structures - for example, faults which have juxtaposed magnetic rocks against non-magnetic rocks, or areas of alteration where magnetic minerals have been destroyed by hydrothermal activity. Tectonic features of regional extent may not become apparent until a number of aeromagnetic surveys have been compiled and plotted at the same scale. Commonly the compilation involves piecing together data from surveys that were flown at different times with widely disparate flight specifications and data reduction procedures. The data may be compiled into a composite map, where all the pieces are plotted onto one map without regard to the difference in flight elevation and datum, or they may be compiled into a merged map, where all survey data are analytically reduced to a common flight elevation and datum, and then digitally merged at the survey boundaries. The composite map retains the original resolution of all the survey data, but computer methods to enhance regional features crossing the survey boundaries may not be applied. On the other hand, computer methods can be applied to the merged data, but the accuracy of the data may be slightly diminished.

Arizona, California, Nevada↗
USGS1978

Reconnaissance of chemical quality of surface water and fluvial sediment in the Dirty Devil River basin, Utah

The water-quality reconnaissance in the Dirty Devil River basin, Utah, covered an area of about 4,300 square miles (11,100 square kilo-meters). Data were obtained by the U.S. Geological Survey one or more times at 104 sites during the period July 1975 to September 1976; and during visits to 34 other sites during the same period, the streams were dry. Precipitation was below normal in both 1975 and 1976 at weather stations at Emery and Capitol Reef National Park and was above normal in 1975 and below normal in 1976 at Hanksville. Streamflow was near normal in the Dirty Devil River basin in 1975 and was much below normal in 1976. Rocks that crop out in the basin range in age from Permian to Quaternary. The Carmel Formation of Jurassic age and various members of the Mancos Shale of Cretaceous age are major contributors of dissolved solids to streams in the basin. Major diversions are made from Muddy Creek for irrigation in the Emery area. Downstream reaches of Muddy Creek commonly receive only seepage and irrigation return flow. Three major irrigation diversions near Fremont commonly remove nearly all the flow from the upper Fremont River. Major flow accretions to the Fremont River in Bicknell Bottoms furnish water for additional diversions along the remaining length of the Fremont River. The most pronounced change in chemical characteristics of water in streams in the Dirty Devil River basin occurs in a 15-mile (24-kilometer) reach of Muddy Creek between the major diversions 5 miles (8 kilometers) north of Emery and the point at which Highway 1-70 crosses Muddy Creek. Dissolved-solids concentrations at the diversions are generally less than 300 milligrams per liter and at the lower end of the reach are commonly greater than 2,000 milligrams per liter. Factors that contribute to the deterioration in the chemical quality of water in the Emery area are the soluble minerals in the rocks of Cretaceous age that crop out in the area, the major or total diversion of flow of Muddy Creek, and the irrigation of and return flow from soils that have commonly developed on material derived from gypsum-bearing marine shale. Dissolved-solids concentrations remain high to the mouth of Muddy Creek near Hanksville. Only one perennial stream--Salt Wash--enters Muddy Creek between Highway 1-70 and the mouth. The discharge of Salt Wash is usually 2.0 to 2.5 cubic feet per second (0.057 to 0.071 cubic meter per second), and the dissolved-solids concentration ranges from about 5,400 to 5,900 milligrams per liter.

Utah↗
USGS1952

Geology of the Iron King Mine, Yavapai county, Arizona

The Iron King mine is about 2,000 feet west-northwest of the intersection of the 112 15 west meridian and the 34 30 north parallel in the Humboldt region in central Yavapai County , Arizona . The mine is approximately in the geographical center of the Humboldt region. Precambrian rocks form the bedrock. Late Cenozoic unconsolidated river wash and valley fill with some interbedded basalt locally mantle the Precambrian rocks, especially in the north-central part of the region. The Precambrian rocks consist of two metamorphosed volcanic formations and intrusive rocks that range in composition from quartz porphyry to gabbro. The volcanic formations originally were flows, volcanic breccias, and tuffaceous sedimentary rocks. Dynamothermal metamorphism of these rocks formed textures, structures, and mineral assemblages characteristic of low-grade ;riietamorphic rocks, but sufficient relict textures and structures remain to permit delineation of formations on the bases of their original nature. All the Precambrian rocks are foliated, except those in the interior parts of the larger intrusive masses. This foliation has two major trends, (1) north to N 20° W, and (2) N 20° E. The northeast-trending foliation is younger and locally is superimposed on the north- to northwest-trending foliation. The Precambrian rocks strike north to northwest and dip steeply, chiefly westward. Duplication of stratigraphic units and determination of tops suggest two major northeast-trending folds, which probably plunge southward. The northeast-trending foliation appears to bear an axial-plane relationship to these folds. Some masses of igneous rock probably were intruded during development of foliation. The Iron King deposit supports the only active mine in the Humboldt region. It consists of 12 steeply plunging echelon veins arranged along the footwall of a sheared and altered zone in the metamorphosed andesitic tuffaceous sedimentary rocks. Narrow zones of more intense shear probably localized the veins. Solutions first introduced quartz, pyrite, ankerite, and sericite, forming a sporadically mineralized zone (in the hanging wall of the deposit) and probably veins in the Iron King fracture system. Intra-mineralization shear strongly brecciated these early minerals and formed the structures that localized and distributed the ore minerals in the veins. After this deformation, sphalerite, galena, chalcopyrite, tennantite, arsenopyrite, pyrite, quartz, and ankerite were deposited; the last three minerals may have formed through solution and the redeposition of earlier minerals. Formation of sericite either accompanied or followed the deposition of ore-forming minerals. Silver is related closely in distribution to copper and probably is in the tennantite. Gold occurs chiefly in the pyrite. Banding, mimetic after foliation and planes developed by shearing, is pronounced in much of the vein material. Mineral zoning, generally similar in each vein, is a characteristic of the deposit. High-angle reverse faults of about 100 feet in maximum vertical separation offset the veins. These faults are nearly parallel to the veins in strike and dip.

Arizona↗
USGS2001

The geology and mineral deposits of part of the western half of the Hailey 1 degree x 2 degrees quadrangle, Idaho

Rocks in the western half of the Hailey 1°x2° quadrangle of south-central Idaho include various units of the Atlanta lobe of the Idaho batholith (biotite granodiorite to two-mica granite) of Cretaceous age and plutons and dikes of Tertiary (Eocene to Miocene) age that intrude the batholith. Eocene plutonic rocks consist of a bimodal suite of anorogenic granite and tonalite-granodiorite and hypabyssal rhyolite and rhyodacite dikes. Rocks of the Eocene Challis Volcanics are scarce in the map area but are widespread to the east. Rhyolite ash flows of the Miocene Idavada Volcanics and basalt of the Snake River Plain crop out in the southern part of the area. Lacustrine rocks of probable Eocene to Holocene age are present in the vicinity of Anderson Ranch Reservoir. Quaternary basalts and gravels are widespread on the South Fork of the Boise River, and alluvial deposits are common along active drainages. Metasedimentary rocks of unknown age crop out on House Mountain, Chimney Peak, and on the ridges east of Anderson Ranch Reservoir. Older structures in the Idaho batholith include a major fault beneath House Mountain that may be a decollement for one of the large thrust sheets in eastern Idaho or part of an extensional core complex. The southern part of the Atlanta lobe of the Idaho batholith is cut by northeast-striking faults (parallel with the Trans-Challis fault system) that are related to Eocene extension and by northwest-oriented faults that formed during basin and range extension in the Miocene. The basin and range faults have prominent scarps typical of basin and range topography. The combination of northeast and northwest faults has broken the batholith into a series of rhomboid blocks. Some of these northeast and northwest faults are older structures that were reactivated in the Eocene or Miocene, as indicated by Ar 40 /Ar 39 dates on mineralized rock contained in some of the structures. The Idaho batholith and associated rocks in the map area host several hundred mines and prospects in 18 mining districts. The deposits range in age from Cretaceous to Eocene, and many were developed for precious metals. Most of the deposits are in quartz veins in shear zones in granitic rocks of the batholith. Several districts were actively being explored for low-grade, bulk-minable, precious-metal deposits in the late 1980s and early 1990s.

Idaho↗
USGS1989

A gravity survey of parts of quadrangles 26E, 26F, 27E, and 27F, northeastern Arabian Shield, Kingdom of Saudi Arabia

A gravity survey using nearly 800 stations was conducted over an area of about 13,400 km 2 located in the northeast part of the Arabian Shield. The stations were set on spot elevations of relative high density and shown on high-quality l:50,000-scale topographic base maps. The error in a gravity reading due to uncertain elevation is estimated to be less than 0.33 mgal. Determination of station coordinates was aided by helicopter-mounted LORAN-C navigation units. The cost/time factors involved in the survey compared favorably with commercial surveys done with inertial-guidance systems but without the l:50,000-scale maps. As topographic-map coverage becomes available, gravity surveys should be run over the entire Arabian Shield. The Arabian Shield is generally comprised of Proterozoic sedimentary rocks metamorphosed to varying degrees and intruded by plutonic rocks. The northeastern Shield is traversed by the north-trending Nabitah mobile belt, a zone of flexing, faulting, shearing, and mineralization. The complete Bouguer gravity field defines the boundary of the mobile belt, as well as a tectonic platelet that has undergone especially conspicuous thrusting and other types of deformation within the mobile belt. Many plutons intruded the metamorphic rocks in the survey area. The youngest rocks are peraluminuous, contain anomalously high tin and tungsten concentrations, and are similar to other tin and tungsten-bearing granites elsewhere. Pronounced gravity lows are associated with these plutons, but not all of a pluton or its associated mineralized rocks may crop out. Thus, gravity surveys of areas with similar plutonism are important in the study of plutonic mineralization. Quaternary deposits are distributed along mountain fronts and wadis. Gravity lows are commonly associated with the wadis, but their low amplitude suggests alluvial thicknesses of less than 25 m. This implies that the valleys are not structurally controlled by extension and that the alluvial aquifers, from which nearly all domestic water is produced, are relatively thin. The greatest complete Bouguer anomaly is associated with basaltic lava flows located in the northeastern part of the survey area. The thickness of the basalt in outcrop does not account for the anomalies with the highest amplitudes, but the latter may be due to the presence of a basalt-filled vent. Those anomalies that are present do not define the basalt flows well, but the largest free-air anomaly occurs over the southwestern margin of the Salma Caldera, located about 15 km from the basalt flows. The source of the free-air anomaly is unknown, but it may be related to another hidden basaltic vent.

Open-File Report↗
USGS1952

Uranium-bearing deposits west of Clancey, Jefferson County, Montana

Nine uranium deposits occur in a small area vest of Clancey,Jefferson County, Mont. These deposits are all in or near silicifiedfracture zones in quarts monsonite and related rocks of the Boulderbatholith. The deposits contain pockets of uranium minerals in cavities in brecciated silicified rock. The primary uranium mineral pitch-blende has been found in one pod. Secondary uranium minerals occur as fracture linings and in pore spaces in and adjacent to the silicified zones. Uranium in the Clancey district was deposited probably during oneof the later of at least four periods of silicification. The quartsmonsonite, rather than the younger alaskitic dike rocks, is the host rock of the deposits. Newmont Mining Corp. has leased the properties containing theuranium deposits and has begun development of one of these deposits.

Montana↗
USGS1982

Analytical determinations from samples taken in the Ten Mile West Roadless Area, Boise and Elmore counties, Idaho

A total of 584 stream-sediment and rock samples were collected from within or near the Ten Mile West Roadless Area, Boise and Elmore Counties, Idaho, as part of a geologic study aimed at appraising the mineral resource potential of the area. Emphasis was placed on stream-sediment samples because stream sediments represent the erosional products of a drainage system and offer a rapid means of locating anomalous concentrations of ore-forming elements. An unusually high proportion of the stream-sediment samples contained concentrations of silver and molybdenum. Unaltered rocks of the area contain normal quantities of the common rock-forming elements. Some exposures of altered or mineralized rocks and some quartz veins contain significant quantities of gold and silver.

Open-File Report↗
USGS2025

Noble and base metal distribution and processes affecting ore tenors in the disrupted lower stratigraphy of the Stillwater Complex, USA

Exploration continues for contact-style Ni-Cu sulfide and chromitite-associated PGE mineralization in ultramafic rocks of the Stillwater Complex. At the Iron and Chrome Mountain areas, massive sulfides occur along the complex’s footwall contact and anomalous concentrations of PGE+Au are associated with the three lowermost chromitite seams. Southeast of Chrome Mountain, magmatic layering is highly disrupted by the presence of faults, magmatic breccias, serpentinized discordant dunites, pyroxenite pegmatoids, and disaggregated chromitite seams. The bulk rock chemistry, sulfide chemistry, and noble metal mineralogy of samples from this area were examined to determine the deportment of PGE and processes that led to enrichments in PGE, Au, Cu, Co and Ni. Results show that a sulfide liquid was the principal collector of PGE. If sulfide liquid was initially deposited with chromite, it was disaggregated or redistributed by subsequent melt or fluid infiltration, which may have resulted in the offset of peak PGE(+Cu, Ni) from peak Cr 2 O 3 concentrations, and upgraded PGE tenors. Upon cooling, PGE exsolved from sulfides to form discrete bismuth tellurides, arsenides, arsenic sulfides, antimonides, and alloys, commonly along the margins of sulfide globules. Calculated metal tenors are highest in the disseminated sulfides southwest of Chrome Mountain, whereas massive and net-textured sulfides near the Iron Mountain-Camp zone represent monosulfide solid solution cumulates. At progressively shallower levels, higher metal tenors combined with lower S/Se ratios are consistent with increasing R-factors from 100 to 100,000. Serpentinization and talc-tremolite alteration resulted in S loss through partial replacement of sulfides by secondary silicate+carbonate+magnetite+sulfide assemblages, further upgrading Ni-Cu-PGE tenors. The present work shows that processes responsible for the disruption of magmatic layering and post-magmatic fluid alteration along the intrusion’s lower contact led to noble and base metal enrichments.

Montana↗
USGS2020

Evidence for primitive magma storage and eruption following prolonged equilibration in thickened crust

In continental arcs, the exposure of primitive eruptive products at the surface is typically a result of rapid magmatic transfer through the crust. As a result, the initially primitive magma experiences minimal crustal residence and thus insignificant differentiation towards more evolved products. This rapid transfer of primitive magma through thickened crust is commonly recorded from smaller, monogenetic cinder cones. Manantial Pelado (35.5° S) is a long-lived stratocone in the Southern Andean Volcanic Zone (SVZ) overlying thick continental crust (45–50 km) that produces almost exclusively mafic material. As Manantial Pelado is surrounded by extensive silicic volcanism, the study of its mafic exposure as a stratocone can be used to further understand magmatic origins of long-lived volcanic systems. Our study uses textural, geochemical, and geochronological data from lavas collected from Manantial Pelado to characterize its magmatic petrogenesis, assess the primitive nature, and explain processes in the crust within the SVZ. A geologic description of the volcano reveals a mostly monotonous eruptive history of basaltic andesites that are now accessible through glacially carved valleys. New 40 Ar/ 39 Ar dating constrains most of the volcano’s cone constructing phase to last from ~ 220 to 190 ka. At ~ 30 ka, small-volume activity and different petrography of more intermediate magmas were present reflecting a change in the volcano’s character. A combination of the whole-rock and mineral-scale data reveals that basaltic andesites at Manantial Pelado are among the most primitive magmas in the thickened crust of the SVZ. Evidence for this primitive signature consists of textural and zonation patterns in olivine, the presence of Cr-spinel in olivine cores, and elevated Fo and Ni content within olivine cores. This data combined with elemental diffusion modeling provides evidence for a primitive signature for these lavas. Intermediate Fo olivines with uniform core compositions (Fo 80–84 ) suggest that basaltic andesites reside in the crust in quasi-closed system environments for extended storage prior to eruption (~ 25–6000 years). Diffusive equilibration in those intermediate Fo olivines masks the primitive nature of the magmas. These results suggest that mafic magmas can have a protracted storage history in the crust that does not significantly alter their primitive bulk composition before reaching the surface. We argue that these are important processes in understanding the magmatic origin of long-lived systems and the presence of compositionally homogenous olivines at intermediate Fo content may represent cryptic evidence for recharge with primitive magmas that experienced prolonged crustal storage.

Manantial Pelado↗
USGS2021

A petrological and conceptual model of Mayon volcano (Philippines) as an example of an open-vent volcano

Mayon is a basaltic andesitic, open-vent volcano characterized by persistent passive degassing from the summit at 2463 m above sea level. Mid-size (<0.1 km3) and mildly explosive eruptions and occasional phreatic eruptions have occurred approximately every 10 years for over a hundred years. Mayon’s plumbing system structure, processes, and time scales driving its eruptions are still not well-known, despite being the most active volcano in the Philippines. We investigated the petrology and geochemistry of its crystal-rich lavas (~50 vol% phenocrysts) from nine historical eruptions between 1928 and 2009 and propose a conceptual model of the processes and magmatic architecture that led to the eruptions. The whole-rock geochemistry and mineral assemblage (plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxide ± olivine) of the lavas have remained remarkably homogenous (54 wt% SiO2,~4 wt% MgO) from 1928 to 2009. However, electron microscope images and microprobe analyses of the phenocrysts and the existence of three types of glomerocrysts testify to a range of magmatic processes, including long-term magma residence, magma mixing, crystallization, volatile fluxing, and degassing. Multiple mineral-melt geothermobarometers suggest a relatively thermally buffered system at 1050±25 °C, with several magma residence zones, ranging from close to the surface, through reservoirs at ~4–5 km, and as deep as ~ 20 km. Diffusion chronometry on >200 orthopyroxene crystals reveal magma mixing timescales that range from a few days to about 65 years, but the majority are shorter than the decadal inter-eruptive repose period. This implies that magma intrusion at Mayon has been nearly continuous over the studied time period, with limited crystal recycling from one eruption to the next. The variety of plagioclase textures and zoning patterns reflect fluxing of volatiles from depth to shallower melts through which they eventually reach the atmosphere through an open conduit. The crystal-rich nature of the erupted magmas may have developed during each inter-eruptive period. We propose that Mayon has behaved over almost 100 years as a steady state system, with limited variations in eruption frequency, degassing flux, magma composition, and crystal content that are mainly determined by the amount and composition of deep magma and volatile input in the system. We explore how Mayon volcano’s processes and working model can be related to other open-vent mafic and water-rich systems such as Etna, Stromboli, Villarrica, or Llaima. Finally, our understanding of open-vent, persistently active volcanoes is rooted in historical observations, but volcano behavior can evolve over longer time frames. We speculate that these volcanoes produce specific plagioclase textures that can be used to identify similar volcanic behavior in the geologic record.

Mayon volcano↗
USGS2023

Applications of natural language processing to geoscience text data and prospectivity modelling

Geological maps are powerful models for visualizing the complex distribution of rock types through space and time. However, the descriptive information that forms the basis for a preferred map interpretation is typically stored in geological map databases as unstructured text data that are difficult to use in practice. Herein we apply natural language processing (NLP) to geoscientific text data from Canada, the U.S., and Australia to address that knowledge gap. First, rock descriptions, geological ages, lithostratigraphic and lithodemic information, and other long-form text data are translated to numerical vectors, i.e., a word embedding, using a geoscience language model. Network analysis of word associations, nearest neighbors, and principal component analysis are then used to extract meaningful semantic relationships between rock types. We further demonstrate using simple Naive Bayes classifiers and the area under receiver operating characteristics plots (AUC) how word vectors can be used to: (1) predict the locations of “pegmatitic” (AUC = 0.962) and “alkalic” (AUC = 0.938) rocks; (2) predict mineral potential for Mississippi-Valley-type (AUC = 0.868) and clastic-dominated (AUC = 0.809) Zn-Pb deposits; and (3) search geoscientific text data for analogues of the giant Mount Isa clastic-dominated Zn-Pb deposit using the cosine similarities between word vectors. This form of semantic search is a promising NLP approach for assessing mineral potential with limited training data. Overall, the results highlight how geoscience language models and NLP can be used to extract new knowledge from unstructured text data and reduce the mineral exploration search space for critical raw materials.

Natural Resources Research↗
USGS2019

Age of the dacite of Sunset Amphitheater, a voluminous Pleistocene tephra from Mount Rainier (USA), and implications for Cascade glacial stratigraphy

The dacite of Sunset Amphitheater, Mount Rainier (USA), illustrates the difficulties in establishing accurate ages of Pleistocene tephra eruptions. Nearly uniform whole-rock, glass, and mineral compositions, texture, and phenocryst assemblage establish that certain conspicuous dissected pumice exposures scattered from Mount Rainier to southern Puget Sound are products of the same Pleistocene Plinian eruption . Deposit thicknesses and pumice sizes support an eruption on the order of low Volcanic Explosivity Index (VEI) 5, atypically explosive for dominantly lava-producing Mount Rainier. Statistically permissible 40 Ar/ 39 Ar plateau ages of plagioclase phenocryst separates are 138 ± 20 ka and 101 ± 11 ka (2σ). A previously published result of 206 ± 11 ka is herein shown to result from a sample selection error. Zircon from the pumice yields a U-Th crystallization age of 147 ± 8 ka if the isochron is required to pass through the tephra U-Th isotopic composition . In contrast, pooled (U-Th)/He measurements on the zircon yield an age of 85 ± 6 ka (2σ), which accords with well-behaved 40 Ar/ 39 Ar ages of stratigraphically associated lavas high on Mount Rainier, and is the best estimate of the pumice's true eruption age. Inclusions of undegassed melt (glass) in the plagioclase separates are proposed as biasing apparent 40 Ar/ 39 Ar plateau ages to old values through coupling of undegassed magmatic excess Ar with radiogenic Ar that accumulated post-eruptively from relatively K-rich glass. U-Th ages record zircon growth prior to eruption, consistent with a possible complex history of advanced solidification followed by remobilization . The ca. 85 ka eruption age confirms that bracketing glacial tills on the flanks of Mount Rainier were products of the Penultimate Glaciation (MIS 6) and Last Glacial Maximum (MIS 2). This eruption age also provides an important time marker for glacial and other sedimentary deposits in southern Puget Sound lowland that, excepting the Vashon Drift (MIS 2), generally lack reliable age determinations.

Washington↗
USGS2024

To mix or not to mix: Details of magma storage, recharge, and remobilization during the Pacheco stage at Misti Volcano, Peru (≤21–2 ka)

We investigate ten of the most recent tephra-fall deposits emplaced between ≤21–2 ka from the Pacheco stage of Misti volcano, Peru, to elucidate magma dynamics and explosive eruption triggers related to magma storage, recharge, and remobilization. Whole-rock, glass, and mineral textures and compositions indicate the presence of broadly felsic, intermediate, and mafic magmas in a chemically and thermally stratified magma storage system (Zones 1–3) that interact to differing extents prior to eruption. Intermediate magmas are defined by plagioclase + amphibole + two-pyroxenes + Fe-Ti oxides and phase equilibria indicate they formed at ~300–600 MPa and ~950–1000 °C. Intermediate magmas dominate the Pacheco stage and either erupted alone as hybridized magmas or mingled with minor volumes of cool felsic magmas (~800 °C) in which only plagioclase + Fe-Ti oxides are stable. Felsic magmas do not exclusively comprise any tephra-fall deposit emplaced during the Pacheco stage but were remobilized by recharge and mixing with intermediate magmas in order to erupt. Furthermore, felsic-hosted amphibole cognate to the intermediate magmas are reacted despite the felsic magmas being water saturated, which suggests they are staged above the amphibole stability limit (≤200 MPa). The cryptic presence of mafic magmas is indicated by high-An plagioclase cores (An 74–88 ), rare anhedral olivine (Fo 77–80 ), and possibly high Mg# augite and amphibole (up to Mg# 84 and 77, respectively). The dearth of basalt to basaltic andesite melts recorded in erupted glasses and exclusivity of high-An plagioclase to crystal cores signals mafic magmas are staged deeper in the crust than the intermediate magmas. Periodic interactions between these magmas tracked via glass compositions and crystal exchange reveal an alternation between the production of mingled magmas and their eruption shortly after a recharge event, followed by a period of homogenization and eruption of hybridized magmas. As such, we identify magma recharge as a key mechanism by which half of the explosive eruptions were triggered in the Pacheco stage. A >100 °C increase in Misti’s fumarole temperatures from 1967 to 2018 coincident with changes in fumarolic gas composition is consistent with degassing of a mafic recharge magma, signaling that Misti could produce similar explosive eruptions in the future.

Misti Volcano↗
USGS1970

A Permian disturbance of K-Ar radiometric ages in New England: Its occurrence and cause

Approximately 200 K-Ar mineral and whole rock ages from New England, half of which are previously unpublished, are used to delineate an area of Permian thermal disturbance. The disturbed area, as outlined by K-Ar mica ages, forms a north-northeast-trending belt 60–80 mi wide that extends from the coast of Long Island Sound in southern Connecticut to southwestern Maine, where it terminates against rocks displaying older radiometric ages. Several possible mechanisms that may have affected the radio-metric systems of pre-existing rocks are examined: (1) contact metamorphism related to contemporaneous igneous activity, (2) alteration associated with major faulting, (3) regional metamorphism in late Paleozoic time, and (4) burial followed by uplift and erosion. Evidence is given that each of these mechanisms was operative locally, especially in the southern portion of the belt. The general lack of late Paleozoic tectonism in New Hampshire and Maine suggests that only burial is a likely cause of the disturbance there.

Connecticut, Maine, Massachusetts, New Hampshire, ↗
USGS1971

Detrital serpentinite from the Guajira Peninsula, Colombia

Small lenses of detrital serpentinite, completely surrounded by massive serpentinite, occur within a sequence of Mesozoic phyllites and metasiltstones in the Cerros de Parashi area, Guajira Peninsula. These lenses, which are as much as 30 m thick, consist of poorly sorted breccias that grade irregularly upward into bedded serpentinite sandstone and shale. Graded bedding is well developed in the sandstones, and channel structures and cross-bedding are also common. Two chemical analyses of serpentinite sandstone and shale show them to be of normal serpentinite composition. The original detrital serpentine minerals in these rocks were lizardite and chrysotile, but they have been partially replaced by antigorite during a period of upper greenschist facies metamorphism. The detrital serpentinites were formed by large submarine turbidity currents or mudflows, and the massive serpentinites surrounding them are believed to have been emplaced by gravity sliding.

Cerros de Parashi, Guajira Peninsula↗
USGS1990

Chapter 22: Changing patterns of extensional tectonics; Overprinting of the basin of the middle and upper Miocene Esmeralda Formation in western Nevada by younger structural basins

The middle and upper Miocene Esmeralda Formation of western Nevada was deposited in a continental basin that crops out over an area of about 2,000 km 2 . The formation consists of thin, westerly derived sedimentary rocks in the western three-quarters of the outcrop area and of thick (3+ km) easterly derived sedimentary rocks in the eastern quarter. Megabreccias along the eastern margin of the basin are interpreted as landslide deposits derived from fault scarps. The basin was probably a half-graben with a major fault or faults on the east side. The position of the basin, its size, and the inferred major syndepositional faults on its east side are all unrelated to present-day topography and the distribution of major faults in the area and indicate a change in paleogeography and structural pattern since the late Miocene. In the eastern part of the basin, this change was accompanied by deformation that includes low-angle-fault detachment of the Esmeralda Formation from underlying amphibolite-grade Late Proterozoic and lower Paleozoic rocks in the Mineral Ridge-Weepah Hills area and uplift, folding, faulting, tilting, and surface exposure of the entire 3+ km thickness of the formation. Middle and upper Miocene basins in the northern part of the Basin and Range province commonly have been attributed to the onset of basin-range tectonism. The tectonic history of the Esmeralda Formation, however, indicates that some of these basins do not occupy the same area, nor are they related to the same syndepositional faults, as present basins. The extensional basin of the Esmeralda Formation can be viewed either as an early structure in an evolving, but kinematically related, extensional terrane in which the distribution of basins and faults changed gradually with time, or as the product of an extensional event kinematically different from that which produced present structures in the same area. The latter hypothesis is favored because of the marked contrasts in the paleogeography and structures associated with the Esmeralda basin compared with modern basins.

Nevada↗
USGS1996

Geology and resource assessment of Costa Rica at 1:500,000 scale; a digital representation of maps of the U.S. Geological Survey's 1987 Folio I-1865

This CD-ROM contains digital versions of the geology and resource assessment maps of Costa Rica originally published by the U.S. Geological Survey (USGS), the Direccion General de Geologia, Minas e Hidrocarburos, and the Universidad de Costa Rica in 1987 at a scale of 1:500,000 in USGS Folio I-1865. The following layers of the map are available on the CD-ROM: geology, favorable domains for selected deposit types, Bouguer gravity, isostatic gravity, mineral deposits, and rock geochemistry sample points. Some of the layers are provided in the following formats: ArcView 1 for Windows and UNIX, ARC/INFO 6.1.2 Export, Digital Line Graph (DLG) Optional, and Drawing Exchange File (DXF). This CD-ROM was produced in accordance with the ISO 9660 and Apple Computer's HFS standards.

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