Geology topics

Amanda Kate Souders

Publications and source records attributed to Amanda Kate Souders.

16 recordsLinked to original sources

USGS2026

Re-Os dating of paleozoic organic-rich black shales by LA-ICP-MS/MS

We present a new in-situ method for Re- Os geochronology of black shales using LA-ICP-MS/MS that is rapid, cost-effective, and sufficiently accurate and precise to address many geological questions. The method is tested on Paleozoic organic-rich black shales with independently constrained stratigraphic or radiometric ages. Results demonstrate that LA-ICP-MS/MS analysis yields Re- Os dates and initial Os isotopic compositions generally within uncertainty of published values using a laser spot size of 100 to 120 μ m in diameter. Despite low Os abundances in many samples, high-density microscale sampling compensates through large numbers of analyses, enabling robust inverse isochron regressions. The approach also allows contemporaneous acquisition of trace- element data (V, Ni, Mo and Re), providing insight into metal distribution in a sample, degree of metal enrichments, and serve as a proxy for open-system behavior for Re- Os. As a result, LA-ICP-MS/MS Re- Os analysis can serve both as a stand-alone geochronological tool and as an efficient screening method prior to conventional ID-N-TIMS, helping to identify optimal intervals and exclude isotopically disturbed material. Together, our study demonstrates that in-situ Re- Os geochronology of organic-rich shales is technically viable, geologically informative, and broadly applicable, opening new opportunities to investigate basin evolution, metallogenesis, hydrocarbon systems, and Earth-system processes across deep time.

Chemical Geology↗

USGS2026

Unraveling protracted modification of Archean and Paleoproterozoic crust in central Laurentia, Penokean orogen, with garnet and accessory mineral geochronology and microstructural analysis

Proterozoic metamorphism and deformation of the southern margin of the Superior craton in the Lake Superior region is attributed to the Penokean orogeny (1890−1830 Ma). This model includes a period of crustal inversion in which Archean basement blocks were exhumed through overlying Paleoproterozoic strata, producing the corridor of gneiss domes that parallels the trend of the Penokean orogen across the northern Midcontinent, USA. However, recent geologic mapping and 40 Ar/ 39 Ar geochronology challenge this interpretation, suggesting instead that the gneiss dome structures reflect younger episodes of tectonic activity along the southern margin of Laurentia. In absence of integrated pressure-temperature-time-deformation constraints for these rocks, interpretations are largely limited to their final cooling history, making it difficult to both identify the tectonic forces that shaped the architecture of the Penokean orogenic belt and assess the extent to which later Proterozoic tectonism modified the southern Superior craton. We address this problem with an approach joining thermodynamic modeling, garnet and accessory mineral geochronology, and microstructural analysis for several metamorphic rocks across the gneiss dome corridor. The U-Pb ages of titanite reveal that the Proterozoic geometries of exhumed basement gneiss domes are governed by preexisting Archean structures. Garnet Lu-Hf geochronology constrains the timing of prograde-to-peak metamorphism in the Penokean orogenic belt. Granulite facies metamorphism is related to the final stages of the Penokean orogeny at 1837 Ma and localized in a belt of high-grade rocks near a major Penokean suture. Garnet Lu-Hf ages of samples adjacent to gneiss domes reflect regional metamorphism following the accretionary phase of the Penokean orogeny, between 1825 Ma and 1782 Ma, which we suggest reflects continued crustal thickening related to convergence farther south during this time interval. Combination of garnet microstructures and Sm-Nd ages reflects later exhumation of gneiss domes and buried metasedimentary rocks by ca. 1750 Ma, consistent with previously published 40 Ar/ 39 Ar cooling ages across the region. Reset Lu-Hf and Sm-Nd garnet ages and U-Pb ages of syn-kinematic titanite reflect reactivation of primary Penokean structures during this period of basement uplift. These data document significant modification of the Penokean orogen and the Archean crust of the southern Superior province between 1800 Ma and 1700 Ma. Tectonic activity during this interval coincides with collisional events recognized in western Laurentia, suggesting that the period immediately following the Penokean orogeny may be a broadly important time for crustal growth and modification in proto-North America.

Michigan, Wisconsin↗

USGS2026

Geophysical architecture and geochronology of the Neoarchean Mentor anorthosite intrusive complex, northwestern Minnesota: Largest anorthosite complex of the Superior Province?

The Neoarchean Mentor anorthosite intrusive complex (MAIC) lies within the Wawa subprovince in northwestern Minnesota, in a region where the Wawa, Quetico, and Wabigoon subprovinces are juxtaposed in close proximity. Archean rocks are entirely concealed, and interpretations are developed from aeromagnetic, gravity, and borehole samples. The MAIC includes both anorthosite (dense, weakly magnetized) and oxide-rich gabbro (strongly magnetized) lithologies. Anorthosite is coarse-grained to megacrystic, intensely altered, and locally brecciated. Pervasive epidote alteration enhanced the density of the anorthosite via introduction of higher-density mineral assemblages, explaining why the MAIC produces a significant gravity high. Oxide-rich gabbro forms a border phase of the MAIC and has potential for vanadium, chromium, and titanium mineralization, and produces a strong aeromagnetic high. The MAIC is interpreted to extend over an area of 640 km 2 , making it the largest known anorthosite complex of the Superior Province, as measured by preserved areal extent. Modeling indicates the MAIC extends more than 6 km into the subsurface. A new Pb–Pb zircon age of 2737.2 ± 4.5 Ma is interpretated as the crystallization age of anorthosite within the MAIC, showing that the MAIC formed well before the ca. 2690 Ma Shebandowanian orogeny, and raising new questions about correlations with other parts of the Wawa subprovince. A low-density batholith, here informally called the Fertile batholith, is interpreted to intrude the southern part of the MAIC. A new Pb–Pb zircon age of 2701.1 ± 6 Ma is interpreted as the magmatic age of the Fertile batholith.

Minnesota↗

USGS2026

In-situ geochronology using LA-ICP-MS/MS: Application of the Lu-Hf system in carbonate, apatite and fluorite

Geochronology is a fundamental tool for reconstructing earth history and constraining the timing of ore deposit formation. Traditionally, U-rich minerals like zircon, titanite, monazite and apatite have been analysed to determine the timing of mineralization and alteration events. However, not all deposits or alteration assemblages contain these minerals, and the U-Pb system can be disturbed due to open system behaviour. We present a methodology for obtaining in-situ Lu-Hf dates from various minerals to demonstrate the potential for dating ore deposits that lack U-rich minerals. Carbonate, apatite and fluorite minerals from different tectonic settings and geological environments were analysed. We acquired Lu-Hf and REE (rare earth element) data from carbonate minerals from a carbonatite orebody and dike samples from the Mountain Pass REE carbonatite deposit in California, USA. We also tested a ‘double-dating’ method, where U-Th-Pb dates and Lu-Hf dates were obtained from the same analysis of apatite from rocks in the Michigan’s Great Lakes Tectonic Zone. Preliminary results for Lu-Hf dating of fluorite from carbonatite rocks in the Lemitar Mountains of New Mexico are also included. The analytical methodology shows promising results in each of these cases where there is sufficient parent Lu and U and measurable daughter Hf and Pb respectively.

Conference Paper↗

USGS2025

New constraints on location and timing of the Great Lakes tectonic zone, central Upper Peninsula, Michigan, USA

The Great Lakes tectonic zone (GLTZ) forms the boundary between the Wawa–Abitibi and Minnesota River Valley subprovinces within the Archean Superior Province. The GLTZ is concealed for all of its 1100 km length, except for a segment in the central Upper Peninsula of Michigan. There, it is exposed as a northwest-striking mylonite zone along a 11 km segment, extending to the onlap of Paleozoic rocks to the east. Farther east, its location has been unknown. Here, we use aeromagnetic and gravity data to develop interpretations of the expression of the GLTZ and to define its extent under cover. Aeromagnetic gradients over the mylonite zone are interpreted to be produced by structurally juxtaposed rocks with varying magnetizations. Gravity data show a regional gradient along the GLTZ, produced by the juxtaposition of a dense greenstone belt on the north against lower-density gneisses and granites on the south. The GLTZ is interpreted to extend ∼55 km under cover to the east. The GLTZ is terminated on the east by the buried eastern arm of the ca. 1100 Ma Midcontinent Rift. An undeformed granitic dike that cuts the mylonitic foliation produces a U–Pb apatite age of 2523 ± 33 Ma, implying no major post-Archean shearing occurred, and is at odds with previous interpretations of major Proterozoic reactivation. A granite intrusion in the Minnesota River Valley subprovince produces a Pb–Pb zircon age of 2606.9 ± 3.6/7.4 Ma. This suggests that magmatism related to the Sacred Heart orogeny, previously known in Minnesota, extended to Michigan.

Michigan↗

USGS2025

Accuracy and precision of U–Pb zircon geochronology at high spatial resolution (7–20 μm spots) by laser ablation-ICP-single-collector-sector-field-mass spectrometry

Use of small spots (≤20µm) for laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology is of increasing interest in the Earth sciences because the temporal record of geologic processes is often preserved on a fine-scale within zircon grains. However the systematic biases and external sources of uncertainity of U-Pb ages is poorly defined when measured on small spots by LA-ICP-single-collector-sector-field (SF)-MS instrumentation. This study addresses the accuracy and precision for small spots and specifically the extent to which short ablation times limit Pb/U Down-Hole Fractionation (DHF), which largely controls the accuracy of the U-Pb ages. Six zircon reference materials (91500, FC-1, R33, Temora 2, Plešovice and Fish Canyon Tuff) were measured on spot sizes of 20, 15, 10 and 7 µm diameter. Laser fluence was increased from 3 to 6 J/cm2 with decreasing spot size to compensate partially for decreasing U and Pb signals. 91500 zircon was the calibration reference material. Raw count rate data were processed using Iolite version 3.63 software with the U-Pb Common Approach data reduction scheme and smoothed cubic spline DHF correction model. Samples were ablated for 30 seconds and results processed for the first 28, 15, 10 and 7 seconds of ablation (masking the initial 2 seconds) in order to assess the accuracy and precision of U-Pb ages as a function of ablation time. Measured 206Pb/238U ratios for the six zircon reference materials increase steadily with ablation time, reflecting DHF, but exhibit somewhat different patterns of increase for different zircons, producing the major source of uncertainty for the U-Pb ages. A secondary source of uncertainty is differences between the 206Pb/238U (normalized to their accepted values) for different zircons near the start of ablation, which may reflect matrix-dependent instrumental mass bias in the ICP. Nonetheless, processing data from only the first 10 to 15 seconds of ablation (50 to 75 laser pulses) restricts the extent of DHF and time-resolved Pb/U variations between different zircons to a sufficient degree to give concordant U-Pb ages on 20 to 7 µm spots that are accurate and precise to better than 1.4% using LA-ICP-single-collector-SF-MS instrumentation.

Journal of Analytical Atomic Spectrometry↗

USGS2025

In situ Re-Os geochronology of Re-rich Palaeogene molybdenite by LA-ICP-MS/MS

In situ Re–Os geochronology by LA-ICP-MS/MS was previously demonstrated by reacting Os with CH 4 or N 2 O reaction gasses. However, for both reactions, a minor proportion of the Re parent isotope also reacts, potentially leading to significant isobaric interferences of 187 Re on 187 Os, especially for young samples with little radiogenic in-growth. Here we present an interlaboratory comparison and compare three reaction gas mixtures (CH 4 + H 2 + He, N 2 O and N 2 O + He) with the aim to robustly date Palaeogene (66–23 Ma) molybdenite from the Bingham Canyon and Henderson deposits. CH 4 mixed with H 2 gas gives the highest sensitivity, while N 2 O and He gas buffer Re reaction. On balance, the analytical method involving N 2 O + He reaction gas is most suitable for dating Palaeogene molybdenite, resulting in age precision of 2.6% for Bingham and 5.8% for Henderson. For older, >1 Ga molybdenite, CH 4 + H 2 + He may give comparatively better age precision.

Colorado, Utah↗

USGS2025

Re-Os geochronology of molybdenite by LA-ICP-MS/MS

Re-Os geochronology is a powerful tool to directly date sulphide mineralisation using phases such as molybdenite, pyrite and chalcopyrite. We present a snapshot of a method in development utilizing laser ablation inductively coupled-plasma ‘triple quad’ mass spectrometry (LA-ICP-MS/MS) for Re-Os geochronology of molybdenite. While traditional LA-(MC)-ICP-MS measurements are limited by the >90% isobaric interference correction of parent radioactive 187 Re on radiogenic daughter 187 Os, the MS/MS technology minimizes this correction through careful instrument tuning and use of a (CH 4 + H 2 + He) reaction gas mixture to mass shift 187 Os away from 187 Re. We demonstrate the potential of the MS/MS technique by presenting robust Re-Os dates for three different molybdenite samples ranging in age from ~28 to 1520 Ma.

Conference Paper↗

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

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

Geophysical architecture of the Neoarchean Mentor anorthosite intrusive complex, northwestern Minnesota

The ca. 2737 Ma (Souders, 2023) Mentor anorthosite intrusive complex (MAIC) lies near the northern margin of the Wawa subprovince of the Archean Superior Province, in an area of northwestern Minnesota where the Wawa, Quetico, and Wabigoon subprovinces are juxtaposed in close proximity (Fig. 1). The rocks of interest are entirely concealed by 10s to >100 m of unconsolidated Quaternary sediments and localized Cretaceous strata and saprolite. The MAIC comprises a large volume of megacrystic anorthosite, with a lesser volume of oxide-rich gabbros. The gabbros are known, from a single borehole intersection at ~70 m depth, to be enriched in vanadium (see http://minarchive.dnr.state.mn.us), and have further potential for chromium and titanium mineralization. New interpretations are based on data from an Earth Mapping Resources Initiative (MRI)-sponsored aeromagnetic survey flown in 2021 and pre-existing ground gravity data, constrained by approximately ten boreholes in the area.

Minnesota↗

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

Significance of U-Pb detrital zircon geochronology for mudstone provenance

Detrital zircon U-Pb studies of mudstone provenance are rare but may preferentially fingerprint distal zircon sources. To examine this issue, Pierre Shale and Trinidad Sandstone deposited in a Late Cretaceous deltaic environment in the Raton Basin, Colorado (USA), were measured for detrital zircon U-Pb age by laser ablation–inductively coupled plasma–mass spectrometry. Two major detrital zircon age peaks at ca. 70 and 1690 Ma are found in both Pierre Shale and Trinidad Sandstone but in inversely varying proportions: 68% and 16%, respectively, for the finest zircon fraction (~15–35 μm) in the shale, and 25% and 32%, respectively, for the coarsest zircon fraction (~60–80 μm) in the sandstone. Proximal sources in the Sangre de Cristo Mountains, directly west of the Raton Basin, contain coarse-grained, ca. 1690 Ma zircon, whereas distal sources in Laramide uplifts and basins in Colorado, New Mexico, and Arizona contain fine-grained, ca. 70 Ma zircon. The results indicate that U-Pb zircon provenance of mudstone reflects availability of volcanic and other fine-grained source rocks rather than simply distal sources. U-Pb zircon provenance studies should routinely include mudstone units because these units may identify fine-grained zircon sources more reliably than sandstones alone.

Arizona, Colorado, New Mexico, Utah↗

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↗