Geology topics

Amy K. Gilmer

Publications and source records attributed to Amy K. Gilmer.

At least 19 recordsLinked to original sources

USGS2026

USGS Geochron—A database of geochronological and thermochronological dates and data—Technical documentation

Geochronological and thermochronological data are essential for constraining the timing and rates of geological processes, supporting geologic mapping, natural hazard assessment, and resource exploration. The U.S. Geological Survey (USGS) Geochron Database is a centralized, relational database that integrates USGS and State geological survey data in accordance with the National Geologic Mapping Act of 1992 and its reauthorizations. This report documents the structure and development of the database, including its standardized schema, controlled vocabularies, and protocols for compiling legacy and newly published data. The database is designed to adhere to FAIR (Findable, Accessible, Interoperable, and Reusable) data principles and currently (2026) includes data from a wide range of geochronological and thermochronological methods. Public access is provided through the USGS Geochron Database Explorer, a browser-based geographic information system (GIS) interface, and through versioned data releases available on ScienceBase, which provide sample-level summary data and detailed analytical information. The USGS Geochron Database is structured to be extensible, supporting the inclusion of additional methods and data types as they are compiled. This format ensures long-term utility and aligns with the Mapping Act’s directive to create a national archive of geochronological information that adds interpretive value to geologic map data. Its design reflects a commitment to data transparency, scientific reproducibility, and national geoscience priorities.

Data Report↗

USGS2026

USGS Geochron Database

Introduction Geochronology helps us understand Earth’s history by determining when important events, like volcanic eruptions, the rise of mountains, the formation of mineral resources, and changes in the landscape, happened. Geochronological data directly support geologic mapping and can inform decisions about geologic hazard mitigation, natural resource management, and infrastructure resilience. The U.S. Geological Survey (USGS) Geochron database provides access to more than 300,000 published, publicly available age measurements from more than 40,000 geological samples. This database is the result of a collaborative effort with State geological surveys and geoscientists from across the globe. The USGS Geochron database is the most comprehensive collection of geochronological data available for the United States. Users can view data through an interactive map explorer, download datasets, and integrate data into geospatial software or other analysis tools.

Fact Sheet↗

USGS2025

Finding the hidden orogeny – The Proterozoic polymetamorphic history of northern New Mexico

Pressure–temperature–time-deformation histories provide key constraints on orogenic processes but can be affected by later overprinting. This is exemplified in the Proterozoic orogenic belts of southwestern Laurentia where competing tectonic models involve either a single progressive Mesoproterozoic event, the Picuris orogeny, or a polyorogenic history that also includes the ~1.65 Ga Mazatzal orogeny. We address this controversy with structural analysis and petrochronology in the type locality of the Picuris orogeny. Xenotime and monazite domains associated with the greenschist-facies axial planar fabric of early F 1 folds yield 1644 ± 11 Ma (xenotime) and 1641 ± 15 Ma (monazite) ages and 450°C–482°C temperatures recording metamorphism and shortening associated with the Mazatzal orogeny. This Palaeoproterozoic greenschist-facies assemblage was overprinted by higher grade, protracted (1470–1350 Ma) tectonism that included the Picuris orogeny. Our results document the complex polyphase crustal assembly of Laurentia and highlight how petrochronology can effectively see through higher grade overprints to identify a more complete orogenic evolution.

New Mexico↗

USGS2025

Re-evaluating the tectonic affinity of Proterozoic crustal provinces in the Southwest USA: Detrital zircon evidence for a Laurentian source for the Yavapai and Mojave Provinces

Models for crustal growth commonly involve the accretion of dominantly juvenile crust to continental margins. However, tracking the provenance and tectonic affinity of dominantly juvenile crustal provinces is challenging. This difficulty is highlighted by uncertainty over whether the Yavapai and Mojave Provinces, part of the >1300-km-wide system of Proterozoic orogens in southwestern Laurentia, (1) have similar crustal and tectonic histories and (2) if they formed on or near Laurentian, Australian, or Antarctic cratons. Here, we contribute new large-n detrital zircon U-Pb geochronology and Sm-Nd whole-rock isotope geochemistry to help constrain the provenance of the Yavapai Province and address these broader questions. Yavapai Province metasedimentary rocks from central Colorado in the southwestern USA have abundant pre-1.80 Ga detrital zircon grains, with ca. 1.85 Ga, 2.30 Ga, and 2.50−2.70 Ga peaks, and variable amounts of 1.79−1.78 Ga grains. Evolved whole-rock Sm-Nd isotopic compositions from these rocks, including 2.36−2.08 Ga model ages, also suggest mixing between 1.79 Ga and 1.78 Ga Yavapai Province arcs and early Proterozoic to Archean sources. Nearly identical pre-1.8 Ga detrital and inherited zircon age distributions suggest that the Yavapai and Mojave Provinces formed on and/or incorporated similar material. The Trans-Hudson orogen, and to a slightly lesser extent the Penokean orogen, provide the closest matches to the pre-1.80 Ga material in the Yavapai and Mojave Provinces. This similarity, coupled with a weaker resemblance to Australian and Antarctic sources, support a Laurentian affinity for the Yavapai and Mojave Provinces. We envision Paleoproterozoic arc formation on both oceanic crust and material of Laurentian affinity and multiple phases of arc-back-arc genesis, closure, and accretionary tectonism along the long-lived margin of the supercontinent Columbia (Nuna).

Arizona, Colorado, Idaho, New Mexico, Utah, Wyomin↗

USGS2025

Seamless Geologic Map Database for the Intermountain West, United States: A foundational dataset for mineral systems analysis

The Intermountain West has a complex geologic history, resulting in the formation of a diverse array of mineral deposits. Effective mineral exploration requires understanding the spatial and temporal relationships among geologic processes and events, a key focus of the mineral systems approach to exploration. This paper presents the Intermountain West Seamless Geologic Map Database, a unified dataset designed to support mineral exploration. Integrating geologic provinces, structural settings, and hydrothermal alteration, the database leverages the Seamless Integrated Geologic Mapping (SIGMa) extension to the USGS Geologic Map Schema (GeMS) to standardize geologic data from varied sources. SIGMa's hierarchical stratigraphic organization and feature-level metadata enhance data interoperability and reusability, enabling seamless query, analysis, and visualization of lithology, structural features, mineral deposits, geochronology, hydrothermal alteration. volcanic activity, and By providing a regionally consistent and dynamically evolving geologic map, this database provides a foundational framework for mineral exploration and geologic research. It also allows for an efficient workflow that expedites the publication of integrated geologic map databases.

Conference Paper↗

USGS2025

Enhancing mineral systems exploration through geochronology, thermochronology, and isotope analysis: USGS Geochron and USGS Isotope databases

A mineral systems approach to mineral exploration provides a comprehensive framework for understanding ore deposit formation by examining the geodynamic, magmatic, hydrothermal, and sedimentary processes responsible for mineralization, alteration, and remobilization of economic mineral deposits. Temporal and thermal constraints on ore genesis are crucial for refining mineral system models and guiding predictive exploration strategies. Geochronology and thermochronology offer invaluable insights into the timing and thermal evolution of ore-forming processes, whereas isotopic analyses provide critical information on the source and geochemical history of ore-forming fluids. Combining these methodologies have proven highly effective for mineral exploration in regions like Australia, however, their combined application has been limited in the United States. To apply these tools to mineral systems-based exploration, the U.S. Geological Survey (USGS) has developed two products: (1) The USGS Geochron Database, and (2) the USGS Isotope Database. These databases provide centralized repositories of geo/thermochronological dates and data (Geochron Database) and both radiogenic and stable isotope data (Isotope Database) generated by the USGS and partners over the past decades. Integrating these datasets together and with traditional exploration approaches provides the mineral exploration community with powerful tools for determining the temporal and thermal histories of ore systems and identifying metallogenic source provinces.

Continental United States↗

USGS2024

Paleoproterozoic reworking of Archean crust and extreme back-arc metamorphism in the enigmatic southern Trans-Hudson orogen

The crustal evolution of the southernmost ∼2000–1800 Ma Trans-Hudson orogen (THO) is enigmatic due to burial by Phanerozoic sediments. We provide new insights through petrochronologic analysis of a paragneiss drill core sample. Detrital zircon age peaks at 2625, 2340, and 1880 Ma and Hf isotopes suggest Paleoproterozoic arc development proximal to Archean source(s). Phase equilibria modeling and ternary feldspar thermometry suggest peak conditions of ≥1 GPa, ≥900°C, the first recognition of extreme, ultra-high temperature metamorphism in the THO. The largely isobaric P-T path, rapid heating rate, and ∼20 Myr duration (1872–1850 Ma) of peak conditions suggest that this metamorphism occurred in a back-arc tectonic setting. The sample records post-peak (1850–1815 Ma) mid-crustal residence, slow cooling, and exhumation. Further retrogression occurred during Proterozoic regional exhumation (1630–1470 Ma) and Phanerozoic (360–220 Ma) reheating and/or fluid influx. Evidence for Paleoproterozoic arc(s) supports geophysical data for Archean cratonic and Paleoproterozoic arc crust in this region.

Geophysical Research Letters↗

USGS2024

Heterogeneous multi-stage accretionary orogenesis — Evidence from the Gunnison block in the Yavapai Province, southwest USA

Proterozoic rocks exposed in the southwestern U.S.A. represent one of the best examples of crustal growth by arc-related magmatism and accretionary orogenesis. Within the Southwest the 1.8–1.7 Ga Yavapai Province is widely regarded as a classic example of juvenile arc crust, however 1.8–2.5 Ga inherited zircon and Nd and Hf model ages have been recognized near Gunnison in central Colorado. These data have led to questions regarding the extent and nature of pre-1.8 Ga crustal material and the genesis of the Yavapai Province. We present evidence for a geochemically distinct, spatially restricted crustal block underlain by pre-1.8 Ga crust material (referred to here as the Gunnison block) in central to western Colorado within the Yavapai Province. The Gunnison block is characterized by 1.8–1.9 and 2.4–2.6 Ga inherited zircon, Pb isotopic systematics (μ = 9.8 ± 0.1, κ = 3.7 ± 0.1) elevated relative to 1.8 Ga depleted mantle values, 1.8–2.5 Ga Nd and Hf model ages, and a distinct pressure-temperature-time history. The geochemical data are consistent with mixing between juvenile 1.8 Ga and pre-1.8 Ga sources. The older crustal component is most similar to the isotopically enriched Mojave Province of eastern California and western Arizona, suggesting greater similarities between these provinces than previously recognized. Monazite and xenotime petrochronology indicate ca. 1.75–1.74, 1.72–1.69, 1.67, and 1.47–1.38 Ga tectono-metamorphic events. These data suggest that the Gunnison block accreted to other components of the Yavapai Province outboard of Laurentia at 1.75–1.74 Ga. The composite Yavapai Province was accreted to the margin of Laurentia during the 1.72–1.69 Ga Yavapai orogeny. Later overprinting is associated with the ∼1.68–1.60 Ga Mazatzal and ∼1.47–1.37 Ga Picuris orogenies. Identification of distinct crustal terranes within the Yavapai Province supports models involving multiple arcs and back-arcs that were progressively assembled prior to their accretion to Laurentia, perhaps akin to the present-day Banda Sea in Indonesia.

Yavapai Province↗

USGS2024

Geochemical and geochronologic evidence for a contiguous northeastern Wyoming Province

The extent and nature of the Wyoming Province, an Archean craton in southwestern Laurentia, are poorly understood due to limited exposure between spatially isolated basement-cored uplifts. This lack of exposure has led to debate about whether the northeastern Wyoming Province is underlain by contiguous Archean crust or Proterozoic rocks and suture zone associated with the Trans-Hudson orogeny. To assess these models we analyzed samples recovered from drill cores in the buried northeastern Wyoming Province that straddle the proposed Proterozoic suture. Whole-rock geochemical and Nd-Pb-O isotopic data suggest the rocks formed by partial melting of > 3.5 Ga hydrated mafic to tonalitic sources similar to elsewhere in the northern Wyoming Province. Zircon U-Pb dates record Mesoarchean magmatism and metamorphism (3.0–2.8 Ga). Published whole-rock Rb-Sr, hornblende and biotite K-Ar, and apatite fission track dates suggest these rocks have not been heated above 300 °C since 2.5–2.1 Ga. Geophysical potential field data are consistent across northeastern Wyoming contrasting with major discontinuities associated with documented Proterozoic orogens on all other margins of the Wyoming Province. Hence, geochemical, isotopic, and geochronologic data along with geophysical imaging can be most simply interpreted in terms of continuous Archean crust in the northeastern Wyoming Province. A geophysically-imaged reflector east of the Bighorn Mountains may juxtapose Archean terranes with similar ages and sources, similar to the boundary between the Montana metasedimentary terrane and Beartooth-Bighorn magmatic zone in the northwestern Wyoming Province. This work emphasizes the value of integrating geologic and geophysical constraints to constrain Archean provinces and their tectonic evolution.

Colorado, Idaho, Montana, Nebraska, North Dakota, ↗

USGS2023

Precursors to a continental-arc ignimbrite flare-up: Early central volcanoes of the San Juan Mountains, Colorado, USA

Our newly acquired and recently published map, geochronologic, and compositional data for early intermediate-composition central volcanoes in the northeastern San Juan Mountains provide insights about the broad magmatic precursors to the large continental-arc ignimbrite flare-up in the mid-Cenozoic Southern Rocky Mountain volcanic field (SRMVF). Initial volcanism migrated from central Colorado to northern New Mexico ca. 40–29 Ma, as part of a more regional trend of southward-progressing mid-Cenozoic magmatism in the U.S. segment of the North American Cordillera. Within the San Juan locus, which represents the largest preserved erosional remnant of the SRMVF and site of most intense eruptive activity, new 40 Ar/ 39 Ar and U-Pb zircon ages show that eruptions at many individual edifices began nearly concurrently, at ca. 35 Ma, with peak activity at 34–32 Ma. Broadly similar precursor effusive volcanism characterizes other major loci of continental-arc ignimbrite magmatism along the western American cordilleras, but none of these sites records early volcanism as voluminous, spatially widespread, well exposed, or compositionally diverse as the San Juan locus in Colorado.

Colorado↗

USGS2023

Monazite and xenotime petrochronologic constraints on four Proterozoic tectonic episodes and ca. 1705 Ma age of the Uncompahgre Formation, southwestern Colorado, USA

The Proterozoic tectonic evolution of the south-western USA remains incompletely understood due to limited constraints on the timing and conditions of the tectono-metamorphic phases and depositional age of metasedimentary successions. We integrated multi-scale compositional mapping, petrologic modeling, and in situ geochronology to constrain pressure-temperature-time paths from samples of Paleoproterozoic basement gneisses and overlying quartzites in southwestern Colorado, USA. Basement gneiss from the western Needle Mountains records metamorphic conditions of 600 °C at 0.75 GPa at 1764 ± 9 Ma and ~575 °C at 1741 ± 10 Ma. Gneiss sampled from drill core near Pagosa Springs, Colorado, records conditions of 700 °C at 1748 ± 9 Ma, 800 °C at 1.1 GPa at 1650 ± 40 Ma, 540 °C at 1570 ± 36 Ma, and 440 °C at 1424 ± 12 Ma. The Uncompahgre Formation was deposited at ca. 1705 Ma, as constrained by detrital monazite (1707 ± 8 Ma) and xenotime (1692 ± 40, 1725 ± 50 Ma), metamorphic xenotime (1650 ± 10 Ma), and published 40 Ar/ 39 Ar and detrital zircon data. Compositions of ca. 1705 Ma detrital monazite and xenotime are consistent with derivation from a garnet-bearing source in the Yavapai orogenic hinterland. The Vallecito Conglomerate and Uncompahgre Formation record macroscopic folding and greenschist-facies metamorphism at 1650 ± 10 Ma and temperatures of 270 °C to >570 °C at 1470–1400 Ma. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon geochronology yielded dates of 1775 ± 18 Ma from the Twilight Gneiss and 1696 ± 7 Ma from the Bakers Bridge Granite, supporting previous isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dates. The Eolus Granite yielded a date of 1463 ± 6 Ma, which is older than previous 1.44–1.43 Ga ID-TIMS dates. The newly dated granite of Cataract Gulch is 1421 ± 12 Ma. In situ analysis of detrital and metamorphic monazite and xenotime, igneous zircon, and quantitative thermobarometry, integrated with previously published constraints, indicate multiple tectonic episodes after the emplacement of 1800–1760 Ma arc-related rocks. The region experienced greenschist- to amphibolite-facies metamorphism (M1) from 1760 Ma to 1740 Ma, which was followed by the intrusion of granites at 1730–1695 Ma and deposition of the Uncompahgre Formation at ca. 1705 Ma, contemporaneous with the Yavapai orogeny. Metamorphism at 1680–1600 Ma was characterized by greenschist-facies conditions near Ouray, Colorado, and granulite-facies conditions near Pagosa Springs (M2) during the Mazatzal orogeny. From 1470 Ma to 1400 Ma, greenschist- to amphibolite-facies metamorphism (M3) and largely granitic plutonism occurred during the protracted Picuris orogeny. These results demonstrate the power of monazite and xenotime analyses to constrain depositional ages, provenance, and pressure-temperature-time ( P - T - t ) paths to resolve the compound orogenic history that is characteristic of most mountain belts.

Colorado↗

USGS2023

Community for data integration 2019 project report

The U.S. Geological Survey Community for Data Integration annually supports small projects focusing on data integration for interdisciplinary research, innovative data management, and demonstration of new technologies. This report provides a summary of the 14 projects supported in fiscal year 2019 and outlines their goals, activities, and accomplishments. Proposals in 2019 were encouraged to address the optional disciplinary theme of biosurveillance of emerging invasive species and health threats.

Open-File Report↗

USGS2023

Geochemical evidence for evolving Proterozoic crustal thickness and orogenic styles in southwestern Laurentia

It has long been challenging for researchers to track the crustal thickness and mode(s) of crustal modification in ancient convergent margins, limiting evaluation of the tectonic styles and processes that modify continental crust during orogenesis. We present trace element igneous geochemical crustal thickness proxies that quantitatively track the crustal thickness evolution of the long-lived Proterozoic active margin in the southwestern U.S.A. We integrate these results with geobarometric data to constrain the mode of crustal modification. The data indicate a complex record of crustal thickness change in space and time and evolving orogenic styles. Geochemical proxies at 1.84–1.72 Ga are consistent with 20–40 km thick magmatic arcs that were locally thickened to ∼50 km during ∼1.75 Ga tectonism. During the Yavapai orogeny, 1.72–1.69 Ga, a ∼200-km-wide belt of 50-60 km thick crust extended from southern California to northern Colorado and was rapidly thinned and exhumed by ∼1.68 Ga. Crustal thickening and thinning during the Yavapai orogeny largely occurred by shortening and exhumation, respectively, in the upper 25 km of the crust. Subsequent 1.68–1.60 Ga tectonism involved crustal growth, local crustal thickening, and low-P, high-T metamorphism, consistent with extensional accretionary orogenesis. The 1.47–1.37 Ga Picuris orogeny was associated with 50–60 km thick crust across much of the Southwest and involved crustal shortening with ∼10 km of magmatic underplating. Advective heat from the emplacement of ferroan granites in the mid-crust likely contributed to elevated geothermal gradients and rheologically weakened the crust. Our results suggest evolving orogenic styles in the Southwest from 1.75–1.69 Ga short-lived crustal thickening associated with terrane accretion to 1.69–1.60 Ga largely extensional accretionary orogenesis, and regional, long-lived crustal thickening at 1.47–1.37 Ga involving extensive basaltic underplating. Contrasting with some recent hypotheses, our data document a complex middle Proterozoic record for the Southwest that was not orogenically quiescent or tectonically stagnant but involved complex mountain building styles.

Earth and Planetary Science Letters↗

USGS2022

Revised age and regional correlations of Cenozoic strata on Bat Mountain, Death Valley region, California, USA, from zircon U-Pb geochronology of sandstones and ash-fall tuffs

Basin analysis and tectonic reconstructions of the Cenozoic history of the Death Valley region, California, USA, are hindered by a lack of volcanic (tuff) age control in many stratigraphic successions exposed in the Grapevine and Funeral Mountains of California, USA. Although maximum depositional ages (MDAs) interpreted from detrital zircon U-Pb data may be a promising alternative to volcanic ages, arguments remain regarding the calculation of MDAs including, but not limited to, the number of “young” grains to consider (i.e., the spectrum of dates used to calculate the MDA); which grains, if any, should be ignored; which approaches yield results that are statistically rigorous; and ultimately, which approaches result in ages that are geologically reasonable. We compare commonly used metrics of detrital zircon MDA for five sandstone samples from the Cenozoic strata exposed on Bat Mountain in the southern Funeral Mountains of California—i.e., the youngest single grain (YSG), the weighted mean of the youngest grain cluster of two or more grains at 1σ uncertainty (YC1σ(2+)) and of three or more grains at 2σ uncertainty (YC2σ(3+)), the youngest graphical peak (YPP), and the maximum likelihood age (MLA). Every sandstone sample yielded abundant Cenozoic zircon U-Pb dates that formed unimodal, near-normal age distributions that were clearly distinguishable from the next-oldest grains in each sample and showed an apparent up-section decrease in peak age. Benchmarked against published K/Ar and 40 Ar/ 39 Ar ages and five new zircon U-Pb ages of ash-fall tuffs, our analysis parallels prior studies and demonstrates that many MDA metrics—YSG, YC1σ(2+), YC2σ(3+), and YPP—drift toward unreasonably young or old values. In contrast, the maximum likelihood estimation approach and the resulting MLA metric consistently produce geologically appropriate estimates of MDA without arbitrary omission of any young (or old) zircon dates. Using the MLAs of sandstones and zircon U-Pb ages of interbedded ash-fall tuffs, we develop a new age model for the Oligocene–Miocene Amargosa Valley Formation (deposited ca. 28.5–18.5 Ma) and the Miocene Bat Mountain Formation (deposited ca. 15.5–13.5 Ma) and revise correlations to Cenozoic strata across the eastern Death Valley region.

California↗

USGS2022

The Seamless Integrated Geologic Mapping (SIGMa) extension to the Geologic Map Schema (GeMS)

Geologic maps are the fundamental building blocks of surface and subsurface three-dimensional geologic framework models of the Earth’s crust. However, as the production and availability of geologic map databases continues to increase, inconsistent data models and the lack of synthesized, national geologic map data at scales appropriate for informed decision making negatively affect the functional integration of geologic map data with other national datasets. The Geologic Map Schema (GeMS) is the publication and archive database standard for geologic map data funded by the U.S. Geological Survey National Cooperative Geologic Mapping Program, and standardizes the organization and content of a single map database. However, synthesizing multiple databases into a seamless geologic map database creates a different set of challenges and database needs than GeMS was designed to accommodate. The Seamless Integrated Geologic Mapping (SIGMa) extension is designed to expand the capabilities of GeMS by enabling integration of map-based geoscience data. In particular, the SIGMa extension enables capturing a diverse and ever-changing set of map units, produced by many contributors operating independently, and by incremental and noncontiguous assembly and publication. Feature-level metadata fields allow data sources and digital compilation methods to be attributed separately and a relational structure is designed to support the link between data sources and features attributed with multiple data sources. Instead of paragraph-style map-unit descriptions that can be highly inconsistent, SIGMa parses fundamental map-unit attributes, including material, genetic process, and age, into thematically specific fields. The SIGMa extension uses a hierarchical map-unit organization to facilitate a dynamic and evolving, formation-level stratigraphic framework. The hierarchy is developed around geologic provinces that represent temporally restricted geologic events, processes, and settings. Geologic provinces can include magmatic events, depositional settings associated with tectonic processes or stable continental margins, and processes that are actively shaping the modern landscape. A geologic province hierarchy places map units into a geologic context at subregional to continental scales and provides the flexibility to support incremental assembly of the stratigraphy.

Scientific Investigations Report↗

USGS2021

Postcaldera intrusive magmatism at the Platoro caldera complex, Southern Rocky Mountain volcanic field, Colorado, USA

The Oligocene Platoro caldera complex of the San Juan volcanic locus in Colorado (USA) features numerous exposed plutons both within the caldera and outside its margins, enabling investigation of the timing and evolution of postcaldera magmatism. Intrusion whole-rock geochemistry and phenocryst and/or mineral trace element compositions coupled with new zircon U-Pb geo-chronology and zircon in situ Lu-Hf isotopes document distinct pulses of magma from beneath the caldera complex. Fourteen intrusions, the Chiquito Peak Tuff, and the dacite of Fisher Gulch were dated, showing intrusive magmatism began after the 28.8 Ma eruption of the Chiquito Peak Tuff and continued to 24 Ma. Additionally, magmatic-hydrothermal mineralization is associated with the intrusive magmatism within and around the margins of the Platoro caldera complex. After caldera collapse, three plutons were emplaced within the subsided block between ca. 28.8 and 28.6 Ma. These have broadly similar modal miner-alogy and whole-rock geochemistry. Despite close temporal relations between the tuff and the intrusions, mineral textures and compositions indicate that the larger two intracaldera intrusions are discrete later pulses of magma. Intrusions outside the caldera are younger, ca. 28–26.3 Ma, and smaller in exposed area. They contain abundant glomerocrysts and show evidence of open-system processes such as magma mixing and crystal entrainment. The protracted magmatic history at the Platoro caldera complex documents the diversity of the multiple discrete magma pulses needed to generate large composite volcanic fields.

Colorado↗

USGS2021

Duration of hydrothermal alteration and mineralization of the Don Manuel porphyry copper system, central Chile

The Don Manuel porphyry copper system, located in the Miocene–Pliocene metallogenic belt of central Chile, contains spatially zoned alteration styles common to other porphyry copper deposits including extensive potassic alteration, propylitic alteration, localized sericite-chlorite alteration and argillic alteration but lacks pervasive hydrolytic alteration typical of some deposits. It is one of the youngest porphyry copper deposits in the Andes. Timing of mineralization and the hydrothermal system at Don Manuel are consistent with emplacement of the associated intrusions (ca. 4 and 3.6 Ma). Two molybdenite samples yielded consistent ages of 3.412 ± 0.037 and 3.425 ± 0.037 Ma. 40 Ar/ 39 Ar ages on hydrothermal biotites (3.57 ± 0.02, 3.51 ± 0.02, 3.41 ± 0.01, and 3.37 ± 0.01 Ma) are associated with potassic alteration. These ages are younger than the youngest intrusion by ~300 k.y. recording the cooling of the system below 350 °C. Such a time gap can be explained by fluxing of hot magmatic fluids from deeper magmatic sources.

central Chile↗

USGS2020

Geologic map of Petroglyph National Monument and vicinity, Bernalillo County, New Mexico

This geologic map depicts and briefly describes geologic units underlying Petroglyph National Monument and immediately adjacent areas in Bernalillo County, New Mexico. The Monument is underlain dominantly by Quaternary basalts of the Albuquerque Volcanoes volcanic field, a series of basin-filling volcanic flows and associated vents from a monogenetic volcanic highland along the eastern margin of the Llano de Albuquerque. This compilation builds on data of previously published geologic maps and reports but includes new interpretive synthesis of volcanic stratigraphy and a unified representation of Quaternary surficial deposits overlying volcanic deposits within the Monument and areas immediately adjacent. This geologic map emphasizes the distribution of Quaternary volcanic vent areas and lava flow deposits which were incompletely mapped on previous publications. Surficial deposits are simplified, but uniformly mapped and described in contrast to varying map unit distributions, names and descriptions presented in the references above. Underlying deposits of the upper Santa Fe Group are exposed in the western part of the map area and described briefly. North-trending, syn- and post-eruption faulting is well preserved in the volcanic field and reflected in the subsurface models of aeromagnetic data. These faults are dominated by dip-slip displacement and are interpreted as extensional faults of the central Albuquerque Basin of the northern Rio Grande rift. Elongate distribution of vents for most of the volcanic deposits are spatially associated with the easternmost of these faults and are interpreted to reflect eruptions from fissures paralleling the regional extensional fault trends of the rift.

New Mexico↗