Geology topics

Christopher S. Holm-Denoma

Publications and source records attributed to Christopher S. Holm-Denoma.

At least 55 records · Page 3Linked to original sources

USGS2019

Petrographic, geochemical, and geochronologic data for cenozoic volcanic rocks of the Tonopah, Divide, and Goldfield Mining Districts, Nevada

The purpose of this report is to summarize geochemical, petrographic, and geochronologic data for samples, principally those of unmineralized Tertiary volcanic rocks, from the Tonopah, Divide, and Goldfield mining districts of west-central Nevada (fig. 1). Much of the data presented here for the Tonopah and Divide districts are for samples collected by Bonham and Garside (1979) during geologic mapping in and around those districts, whereas much of that for samples from the Goldfield district were obtained by Ashley (1974; 1979; 1990a). Additional data were derived from samples collected between 2012–2015, as part of the Mineral Resources Program funded project titled: “Magmatic-tectonic history and component sources of major precious metal deposits in the southern Walker Lane”. A small amount of additional geochemical data for samples from each of the districts were compiled from other sources. Individual sample collectors are identified by appropriate entries in the “Collector” field (appendix 1) and published sources of geochemical data are defined by entries in the “Chem_Src” data field (appendix 1).

California, Nevada↗

USGS2019

A stratigraphic approach to inferring depositional ages from detrital geochronology data

With the increasing use of detrital geochronology data for provenance analyses, we have also developed new constraints on the age of otherwise undateable sedimentary deposits. Because a deposit can be no older than its youngest mineral constituent, the youngest defensible detrital mineral age defines the maximum depositional age of the sampled bed. Defining the youngest `defensible' age in the face of uncertainty (e.g., analytical and geological uncertainty, or sample contamination) is challenging. The current standard practice of finding multiple detrital minerals with indistinguishable ages provides confidence that a given age is not an artifact; however, we show how requiring this overlap reduces the probability of identifying the true youngest component age. Barring unusual complications, the principle of superposition dictates that sedimentary deposits must get younger upsection. This fundamental constraint can be incoporated into the analysis of depositional ages in sedimentary sections through the use of Bayesian statistics, allowing for the inference of bounded estimates of true depositional ages and uncertainties from detrital geochronology so long as some minimum age constraints are present. We present two approaches for constructing a Bayesian model of deposit ages, first solving directly for the ages of deposits with the prior constraint that the ages of units must obey stratigraphic ordering, and second describing the evolution of ages with a curve that represents the sediment accumulation rate. Using synthetic examples we highlight how this method preforms in less-than-ideal circumstances. In an example from the Magallanes Basin of Patagonia, we demonstrate how introducing other age information from the stratigraphic section (e.g., fossil assemblages or radiometric dates) and formalizing the stratigraphic context of samples provides additional constraints on and information regarding depositional ages or derived quantities (e.g., sediment accumulation rates) compared to isolated analysis of individual samples.

Frontiers in Earth Science↗

USGS2019

Links between tectonics, magmatism, and mineralization in the formation of Late Cretaceous porphyry systems in the Yukon-Tanana upland, eastern Alaska, USA

Cretaceous-Paleocene porphyry Cu(±Mo±Au) occurrences are scattered throughout the Yukon-Tanana upland in eastern Alaska. Known occurrences in eastern Alaska are poorly characterized, despite a resurgence in exploration. Porphyry deposits in the upland are emplaced into structurally complex metamorphic rocks representing a variety of tectonic environments, resulting in diverse alteration and mineralization assemblages. New mapping, drill core logging, petrography, geochemistry, geochronology, and structural analysis allow improved characterization of the parameters of porphyry systems and identify key linkages to regional tectonic and magmatic events. New sericite 40 Ar/ 39 Ar and zircon U/Pb dates constrain porphyry systems to the Late Cretaceous-earliest Paleocene (ca. 71-63 Ma). Zircon Hf-isotope ratios and Ce and Eu concentrations indicate that Late Cretaceous-Paleocene intrusions emplaced into basement dominated by Triassic and Jurassic plutons are more isotopically juvenile, reflecting more oxidized conditions. In contrast, those emplaced into basement dominated by mid-Cretaceous plutons are more reduced crustal geochemical-affinity. Diversity in mineral assemblages in contrasting systems may reflect emplacement into crustal domains of varying compositions and oxidation states. Those formed within a domain containing more-oxidized Triassic and Jurassic plutons are molybdenite-rich and apparently lack gold. In contrast, systems formed within domains dominated by more reduced mid-Cretaceous plutons contain lower-sulfidation state mineral assemblages with reported gold.

Alaska, Yukon↗

USGS2018

Review of the geochemistry and metallogeny of approximately 1.4 Ga granitoid intrusions of the conterminous United States

The conterminous United States hosts numerous volumetrically significant and geographically dispersed granitoid intrusions that range in age from 1.50 to 1.32 billion years before present (Ga). Although previously referred to as A-type granites, most are better described as ferroan granites. These granitoid intrusions are distributed in the northern and central Rocky Mountains, the Southwest, the northern midcontinent, and a swath largely buried beneath Phanerozoic cover across the Great Plains and into the southern midcontinent. These intrusions, with ages that are bimodally distributed between about 1.455–1.405 Ga and 1.405–1.320 Ga, are dispersed nonsystematically with respect to age across their spatial extents. Globally, although A-type or ferroan granites are genetically associated with rare-metal deposits, most U.S. 1.4 Ga granitoid intrusions do not contain significant deposits. Exceptions are the light rare-earth element deposit at Mountain Pass, California, and the iron oxide-apatite and iron oxide-copper-gold deposits in southeast Missouri. Most of the U.S. 1.4 Ga granitoid intrusions are composed of hornblende ± biotite or biotite ± muscovite monzogranite, commonly with prominent alkali feldspar megacrysts; however, modal compositions vary widely. These intrusions include six of the eight commonly identified subtypes of ferroan granite: alkali-calcic and calc-alkalic peraluminous subtypes; alkalic, alkali-calcic, and calc-alkalic metaluminous subtypes; and the alkalic peralkaline subtype. The U.S. 1.4 Ga granitoid intrusions also include variants of these subtypes that have weakly magnesian compositions. Extreme large-ion lithophile element enrichments typical of ferroan granites elsewhere are absent among these intrusions. Chondrite-normalized rare-earth element patterns for these intrusions have modest negative slopes and moderately developed negative europium anomalies. Their radiogenic isotopic compositions are consistent with mixing involving primitive, mantle-derived components and evolved, crust-derived components. Each compositional subtype can be ascribed to a relatively unique petrogenetic history. The numerically dominant ferroan, peraluminous granites probably represent low-degree, relatively high-pressure partial melting of preexisting, crust-derived, intermediate-composition granitoids. The moderately numerous, weakly magnesian, peraluminous granites probably reflect similar partial melting but at a higher degree and in a lower pressure environment. In contrast, the ferroan but metaluminous granites may be the result of extensive differentiation of tholeiitic basalt. Finally, the peralkaline igneous rocks at Mountain Pass have compositions potentially derived by differentiation of alkali basalt. The varying alkalic character of each subtype probably reflects polybaric petrogenesis and the corresponding effect of diverse mineral stabilities on ultimate melt compositions. Mantle-derived mafic magma and variably assimilated partial melts of mainly juvenile Paleoproterozoic crustal components are required to generate the relatively low initial strontium (87Sr/86Sr) and distinctive neodymium isotope compositions characteristic of the U.S. 1.4 Ga granitoid intrusions. The characteristics of these intrusions are consistent with crustal melting in an extensional/decompressional, intracratonic setting that was triggered by mantle upwelling and emplacement of tholeiitic basaltic magma at or near the base of the crust. Composite magmas, formed by mingling and mixing mantle components with partial melts of Paleoproterozoic crust, produced variably homogenized storage reservoirs that continued polybaric evolution as intrusions lodged at various crustal depths.

Scientific Investigations Report↗

USGS2018

Processes and facies relationships in a Lower(?) Devonian rocky shoreline depositional environment, East Lime Creek Conglomerate, south‐western Colorado, USA

Rocky shorelines are relatively common features along modern coastlines, but few have been recognized in the geological record. The hard substrates of rocky shorelines telescope the width of offshore marine environments, thus the diagnostic deposits observed in such settings today have a low preservation potential due to small accommodation space and high‐energy conditions. This study recognized previously overlooked, laterally extensive Lower(?) Devonian rocky shoreline deposits in the San Juan Mountains of south‐western Colorado. The newly defined lithostratigraphic unit, the East Lime Creek Conglomerate (ELCC), is 0–23 m thick, unconformably overlying Proterozoic crystalline rocks and unconformably overlain by the Upper Devonian Ignacio Formation and/or Elbert Formation. The unit mostly consists of clast‐supported cobble‐boulder conglomerate with rounded quartzite clasts up to 1.4 m in length interbedded with thin sandstone layers and lenses. Sandstones in the ELCC are distinguished from unconformably overlying Upper Devonian sedimentary rocks because they have sericite cements. Most importantly, there are buttressing relationships between the ELCC and underlying Proterozoic crystalline rocks interpreted as palaeo‐sea cliffs, palaeo‐wave‐cut platforms and palaeo‐tombolos. A proposed rocky shoreline facies model includes headlands with upper shoreface‐beachface tabular cobble‐boulder gravels sourced from rock fall talus, nearshore subaqueous debris‐flow deposits and intervening pocket beaches with imbricated, stratified pebble‐cobble gravel sheets. Palaeocurrent data ( n = 338) from clast long‐axis orientations, imbrication and cross‐bedding indicate south‐to‐north transport roughly onshore‐offshore to a coastline consisting of alternating rocky headlands and pocket beaches. This Lower(?) Devonian unit documents a previously unrecognized episode in the geological history of south‐western Colorado.

Colorado↗

USGS2018

In situ LA-ICPMS U–Pb dating of cassiterite without a known-age matrix-matched reference material: Examples from worldwide tin deposits spanning the Proterozoic to the Tertiary

Cassiterite (SnO 2 ), a main ore mineral in tin deposits, is suitable for U–Pb isotopic dating because of its relatively high U/Pb ratios and typically low common Pb. We report a LA-ICPMS analytical procedure for U–Pb dating of this mineral with no need for an independently dated matrix-matched cassiterite standard. LA-ICPMS U-Th-Pb data were acquired while using NIST 612 glass as a primary non-matrix-matched standard. Raw data are reduced using a combination of Iolite™ and other off-line data reduction methods. Cassiterite is extremely difficult to digest, so traditional approaches in LA-ICPMS U-Pb geochronology that utilize well-characterized matrix-matched reference materials (e.g., age values determined by ID-TIMS) cannot be easily implemented. We propose a new approach for in situ LA-ICPMS dating of cassiterite, which benefits from the unique chemistry of cassiterite with extremely low Th concentrations (Th/U ratio of 10 −4 or lower) in some cassiterite samples. Accordingly, it is assumed that 208 Pb measured in cassiterite is mostly of non-radiogenic origin—it was initially incorporated in cassiterite during mineral formation, and can be used as a proxy for common Pb. Using 208 Pb as a common Pb proxy instead of 204 Pb is preferred as 204 Pb is much less abundant and is also compromised by 204 Hg interference during the LA-ICPMS analyses. Our procedure relies on 208 Pb/ 206 Pb vs 207 Pb/ 206 Pb (Pb-Pb) and Tera-Wasserburg 207 Pb/ 206 Pb vs 238 U/ 206 Pb (U-Pb) isochron dates that are calculated for a ~1.54 Ga low-Th cassiterite reference material with varying amounts of common Pb that we assume remained a closed U-Pb system. The difference between the NIST 612 glass normalized biased U-Pb date and the Pb-Pb age of the reference material is used to calculate a correction factor (F) for instrumental U-Pb fractionation. The correction factor (F) is then applied to measured U/Pb ratios and Tera-Wasserburg isochron dates are obtained for the unknown cassiterite analyzed in the same analytical session. This allows for U-Pb dating of cassiterite of any age with no need for an independently dated matrix-matched reference material, nor assumptions about the isotopic composition o f common Pb. Results for cassiterite from tin deposits in Bolivia, Brazil, China, Russia, Saudi Arabia, South Africa, Spain, and the United Kingdom, with ages ranging from ~20 Ma to ~2060 Ma, demonstrate the applicability of this approach across a broad range of geologic time. These ages are in good agreement with published geochronology of the host rocks assoc iated with the tin deposits and with previously published U-Pb ages of some cassiterites from the same deposits. Thus, our in situ LA-ICPMS methodology verifies the use of cassiterite as a reliable U-Pb mineral-geochronometer with the advantages of fast and relatively low cost in situ analyses with moderate spatial resolution.

Chemical Geology↗

USGS2018

Structural evolution of a gold-bearing transtensional zone within the Archean Porcupine-Destor deformation zone, southern Abitibi greenstone belt, eastern Ontario, Canada

The Garrison camp comprises four structurally distinct orogenic gold deposits that formed in different host lithologies during progressive deformation. Detailed field mapping, drill core logging, and geochronological constraints suggest that the 2678 ± 2 Ma Garrison granitic stock played a fundamental rheological role in the location of the four deposits. Initial local shear movement occurred along the southwestern margin of the stock leading to the development of the NW-trending sinistral NE-side-up Buffonta shear zone, which hosts the Buffonta deposit. Subsequently, a transtensional zone formed between the NE-trending sinistral Porcupine-Destor and Munro deformation zones, which host the 903 and Jonpol deposits, respectively. Finally, a local change in shortening orientation from NE to NNW caused westerly directed extension resulting in the formation of the younger gold-bearing veins composing the Garrcon deposit. In situ U-Pb laser ablation-inductively coupled plasma-mass spectrometry performed on monazite grains formed within the shear fabric of the Munro deformation zone indicates that transtension occurred at 2657 ± 15 Ma. Therefore, at least three of the four deposits formed subsequent to crystallization of the Garrison granitic stock. The reported U-Pb dates represent the first direct age constraints on the movement along a gold-bearing deformation zone in the southern Abitibi greenstone belt.

Ontario↗

USGS2018

Igneous and detrital zircon U-Pb and Lu-Hf geochronology of the late Meso- to Neoproterozoic northwest Botswana rift: Maximum depositional age and provenance of the Ghanzi Group, Kalahari Copperbelt, Botswana and Namibia

New igneous and detrital zircon laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb geochronology and Lu-Hf isotopic data are presented for the Mesoproterozoic Kgwebe Formation and the unconformably overlying Ghanzi Group in northwestern Botswana. The Makgabana Hills porphyritic rhyolite flow from the Ghanzi area yielded a U-Pb concordia age of 1085.5 ± 4.5 Ma and provides a new maximum depositional age for the unconformably overlying Ghanzi Group. Detrital zircon (n = 448) from the Ghanzi Group yielded a 207 Pb/ 206 Pb age distribution with a dominant (70 to 90%) Mesoproterozoic population (∼1450 to ∼1050 Ma), a smaller (5 to 20%) Paleoproterozoic (∼2200 to ∼1700 Ma) population, and a few (n = 4) older (∼3000 Ma to ∼2450 Ma) grains. A maximum depositional age constraint of ∼1060 to ∼1050 Ma was obtained for middle and upper Ghanzi Group based on the weighted-mean 207 Pb/ 206 Pb age of the youngest clusters of overlapping zircon ages for each sample. Initial hafnium ratios (εHf i ) and corresponding crustal residence model ages (T C DM ) for the Paleoproterozoic zircon populations indicate either fractionation from a chondritic uniform reservoir (CHUR) or mixing between juvenile mantle and older crustal components. Mesoproterozoic zircon with εHf i values between −20 and +15 and T C DM model ages between 3000 and 1200 Ma suggest that the source terrane(s) contained magmatic rocks including both juvenile material and substantially reworked Paleoproterozoic and possibly Archean crust. Comparison with a compilation of published U-Pb, Lu-Hf, and Sm-Nd data from the Kalahari Craton suggests that the predominant Mesoproterozoic zircon population was derived from the Namaqua Sector, Rehoboth Basement Inlier, Kwando Complex, and Choma-Kalomo Block; some zircon may have had distal sources in adjacent Rodinia landmasses. Both Archean cratonic components and juvenile ∼1200 to ∼1000 Ma magmatic rocks of the Natal Sector and the Maud and Mozambique belts on the eastern margin of the craton are unlikely sources for the detrital zircon based on isotopic composition. Sediment transported from the western margin of the Kalahari Craton entered the northwest Botswana rift and mixed with sediments from the Rehoboth Basement Inlier and Paleo- to Mesoproterozoic terranes that bound the northwest Botswana rift.

Precambrian Research↗

USGS2017

Detrital zircon geochronology of quartzose metasedimentary rocks from parautochthonous North America, east-central Alaska

We report eight new U-Pb detrital zircon ages for quartzose metasedimentary rocks from four lithotectonic units of parautochthonous North America in east-central Alaska: the Healy schist, Keevy Peak Formation, and Sheep Creek Member of the Totatlanika Schist in the northern Alaska Range, and the Butte assemblage in the northwestern Yukon-Tanana Upland. Excepting 1 of 3 samples from the Healy schist, all have dominant detrital zircon populations of 1.9–1.8 Ga and a subordinate population of 2.7–2.6 Ga. Three zircons from Totatlanika Schist yield the youngest age of ca. 780 Ma. The anomalous Healy schist sample has abundant 1.6–0.9 Ga detrital zircon, as well as populations at 2.0–1.8 Ga and 2.7–2.5 Ga that overlap the ages from the rest of our samples; it has a minimum age population of ca. 1007 Ma. Detrital zircon age populations from all but the anomalous sample are statistically similar to those from (1) other peri-Laurentian units in east-central Alaska; (2) the Snowcap assemblage in Yukon, basement of the allochthonous Yukon-Tanana terrane; (3) Neoproterozoic to Ordovician Laurentian passive margin strata in southern British Columbia, Canada; and (4) Proterozoic Laurentian Sequence C strata of northwestern Canada. Recycling of zircon from the Paleoproterozoic Great Bear magmatic zone in the Wopmay orogen and its Archean precursors could explain both the Precambrian zircon populations and arc trace element signatures of our samples. Zircon from the anomalous Healy schist sample resembles that in Nation River Formation and Adams Argillite in eastern Alaska, suggesting recycling of detritus in those units.

Alaska, British Columbia, Yukon Territory↗

USGS2017

The Valmy thrust sheet: A regional structure formed during the protracted assembly of the Roberts Mountains allochthon, Nevada, USA

A synthesis of field, biostratigraphic, detrital zircon geochronologic, and remote sensing data across north-central Nevada, United States, defines a thick, regionally extensive sheet of Middle–Upper Ordovician Valmy Formation quartzite that structurally overlies deformed early Paleozoic units of the Roberts Mountains allochthon. Late Paleozoic regional unconformities that record tectonic disruptions have been recognized in the foreland of central and eastern Nevada and locally within the Roberts Mountains allochthon; these identify multiple, regional tectonic events between the Devonian–Mississippian initiation of the Antler orogeny and the Permian–Triassic Sonoma orogeny. However, few studies have documented the regional kinematic history of the Robert Mountains allochthon sensu stricto. In the Independence Mountains of northern Nevada, emplacement of the Roberts Mountains allochthon is restricted to the Mississippian. In the Tuscarora Mountains, the range west and southwest of the Independence Mountains, several deformation events have been identified, and emplacement of the thrust sheet containing the Valmy Formation is restricted to the Late Pennsylvanian–Early Permian. These structural and temporal relations, reflected in the Antler foreland basin adjacent to the Roberts Mountains allochthon and overlap sequences, suggest that the Roberts Mountains allochthon is a composite stratigraphic terrane assembled along the Cordilleran margin during two or more late Paleozoic contractional events. Valmy Formation deposits likely represent the development of coalescing submarine fans below or within bypass channels in a deep slope or rise environment. Petrographic characteristics, biostratigraphy, and detrital zircon U-Pb age populations of the Valmy Formation link it to coeval slope and rise turbidites of the Vinini Formation and shelfal Eureka Quartzite; Valmy Formation detrital zircon age populations are dissimilar to the rift-to-drift facies of the Neoproterozoic–Cambrian Prospect Mountain Quartzite. Throughout north-central Nevada, the Valmy Formation is in fault contact with units of the Roberts Mountains allochthon, including the Devonian–Mississippian Slaven Chert, Silurian–Devonian Elder Sandstone, and Cambrian(?)–Ordovician Vinini Formation, which were deformed prior to, or during, emplacement of the thrust sheet containing Valmy Formation quartzite. Our mapping and data synthesis, guided by regional quartz maps based on remote sensing (Advanced Spaceborne Thermal Emission and Reflection Radiometer [ASTER]) data, delineate similar structural relationships discontinuously for >200 km along strike of the Roberts Mountains allochthon. Exploration for concealed gold deposits within reach of drilling requires knowledge of the relative thicknesses of the Roberts Mountains allochthon and the Valmy Formation. Overall thicknesses of deformed Roberts Mountains allochthon units between the Valmy Formation and underlying carbonate rocks, which host large, world-class Carlin-type gold deposits, vary by hundreds of meters, but are generally less than 700 m in three of the areas studied here. Recognition of windows through and klippen of the Roberts Mountains allochthon is essential for identification of areas where deposits may be at or near the surface. Correspondingly, most ongoing exploration for Carlin-type gold deposits subjacent to the Roberts Mountains allochthon targets concealed deposits. The model proposed in this study is applicable to determining depth to rocks prospective for undiscovered deposits.

Nevada↗

USGS2016

High spatial resolution U-Pb geochronology and Pb isotope geochemistry of magnetite-apatite ore from the Pea Ridge iron oxide-apatite deposit, St. Francois Mountains, southeast Missouri, USA

The Pea Ridge iron oxide-apatite (IOA) deposit is one of the major rhyolite-hosted magnetite deposits of the St. Francois Mountains terrane, which is located within the Mesoproterozoic (1.5–1.3 Ga) Granite-Rhyolite province in the U.S. Midcontinent. Precise and accurate determination of the timing and duration of oreforming processes in this deposit is crucial for understanding its origin and placing it within a deposit-scale and regional geologic context. Apatite and monazite, well-established U-Pb mineral geochronometers, are abundant in the Pea Ridge orebody. However, the potential presence of multiple generations of dateable minerals, processes of dissolution-reprecipitation, and occurrence of micrometer-sized intergrowths and inclusions complicate measurements and interpretations of the geochronological results. Here, we employ a combination of several techniques, including ID-TIMS and high spatial resolution geochronology of apatite and monazite using LA-SC-ICPMS and SHRIMP, and Pb isotope geochemistry of pyrite and magnetite to obtain the first direct age constraints on the formation and alteration history of the Pea Ridge IOA deposit. The oldest apatite TIMS 207 Pb*/ 206 Pb* dates are 1471 ± 1 and 1468 ± 1 Ma, slightly younger than (but within error of) the ~1474 to ~1473 Ma U-Pb zircon ages of the host rhyolites. Dating of apatite and monazite inclusions within apatite provides evidence for at least one younger metasomatic event at ~1.44 Ga, and possibly multiple superimposed metasomatic events between 1.47 and 1.44 Ga. Lead isotop analyses of pyrite show extremely radiogenic 206 Pb/ 204 Pb ratios up to ~80 unsupported by in situ U decay. This excess radiogenic Pb in pyrite may have been derived from the spatially associated apatite as apatite recrystallized several tens of million years after its formation. The low initial 206 Pb/ 204 Pb ratio of ~16.5 and 207 Pb/ 204 Pb ratio of ~15.4 for individual magnetite grains indicate closed U-Pb system behavior in this mineral and are consistent with derivation of the Pb from a mantle-like source.

Missouri↗

USGS2016

The Permian–Triassic transition in Colorado

The Lykins Formation and its equivalents in Colorado are a stratigraphically poorly constrained suite of redbeds and intercalated stromatolitic carbonates, which is hypothesized to span the Permian-Triassic boundary. Herein we present a preliminary detrital zircon geochronology, new fossil occurrences, and δ 13 C chemostratigraphy for exposures along the Front Range and in southeastern Colorado, to refine understanding of the unit's age and depositional history. Detrital zircons from the uppermost Lykins Formation and an overlying eolianite consist of a complex and highly diverse primary and multi-cycle grain population transported from Laurentian and Gondwanan terranes, potentially both by wind and water. Youngest concordant zircons do not rule out deposition of the uppermost Lykins Formation during a portion of Early Triassic time. Conodonts from the lower Lykins Formation require Middle Permian (Guadalupian) deposition. Conodont alteration indices of 1 indicate the unit has a shallow burial history and is amenable to paleomagnetic inquiry. Conodonts, together with other vertebrate, invertebrate, microfossil, and trace fossils, suggest a very shallow to emergent marine origin for the unit's most substantial carbonates, and hint at a marine origin for the unit's intercalated gypsum-anhydrite members. Chemostratigraphy corroborates field evidence of emergence and karst development capping certain units, like the Forelle Limestone Member of the Lykins Formation, where potential sequence boundaries appear to be punctuated by a short-lived meteoric signature. Results presented here are a progress report of ongoing work in these successions. This field trip consists of a brief tour through exposures of the Lykins Formation, in which we will examine well-known localities as well as view new ones for which we seek insights.

GSA Field Guides↗

USGS2016

Geology of the Mount Rogers area, revisited: Evidence of Neoproterozoic continental rifting, glaciation, and the opening and closing of the Iapetus ocean, Blue Ridge, VA–NC–TN

Recent field and geochronological studies in eight 7.5-minute quadrangles near Mount Rogers in Virginia, North Carolina and Tennessee recognize important stratigraphic and structural relationships for the Neoproterozoic Mount Rogers and Konnarock formations, the northeast end of the Mountain City window, the separation of Mesoproterozoic rocks of the Blue Ridge into three age groups, and timing and emplacement of the Blue Ridge thrust sheet. The study area includes folded and faulted Paleozoic strata of the Valley and Ridge to metamorphic and igneous rocks of the Blue Ridge. In the Valley and Ridge, Cambrian to Middle Ordovician carbonate and clastic rocks are exposed in a syncline on the Pulaski thrust sheet; these rocks are overridden by the Blue Ridge thrust sheet. The northeast end of the Mountain City window is interpreted as a simple window; the Stone Mountain fault is folded and continues as the Iron Mountain fault on the NW-side of the window. The Stone Mountain fault does not exist to the NE near the Razor Ridge volcanic center. Instead a continuous section of Proterozoic gneisses, Mount Rogers Formation, Konnarock Formation and Chilhowee Group is now recognized. Rhyolites of the Mount Rogers Formation range from 760–749Ma, with detrital zircon age populations from associated volcaniclastic rocks indicating magmatism and rifting began by ~780 Ma. Rhyolite blocks in the Konnarock Formation and a change from rift-related clastic rocks of the Mount Rogers Formation transitioning to maroon laminites and laminites with dropstones, suggest that the Konnarock Formation may be as old as ~749 Ma. Mesoproterozoic crystalline rocks of the Blue Ridge, previously referred to as the Cranberry Gneiss, are separated based on field relationships and SHRIMP U–Pb geochronology: (1) pre-Grenvillian crust,1.33 Ga; (2) 1190–1140 Ma granitoids; and (3) 1075–1030 Ma granitoids. Multiple greenschist-facies high-strain zones, including the 2–11 km wide Fries high-strain zone, occur in the Blue Ridge thrust sheet. Fabrics across the Fries and Gossan Lead faults have similar orientations and NW–directed contractional deformation. 40Ar/39Ar hornblende, muscovite, and K-feldspar ages indicate the western and eastern Blue Ridge had different thermal histories. The eastern Blue Ridge (Gossan Lead thrust sheet) experienced a 360–340 Ma amphibolite facies event prior to juxtaposition with the western Blue Ridge. 40Ar/39Ar muscovite ages in western Blue Ridge rocks document greenschist facies metamorphism and deformation and emplacement of the Blue Ridge thrust sheet at ~340 Ma; the Catface and Fries faults are tentatively interpreted to be contemporaneous. After initial emplacement of the Blue Ridge thrust sheet at ~340 Ma, shortening was accommodated by westward translation along the basal decollement, which carried the Blue Ridge thrust sheet to its final position.

Book chapter↗

USGS2015

Geochemical, modal, and geochronologic data for 1.4 Ga A-type granitoid intrusions of the conterminous United States

Introduction The purpose of this report is to present available geochemical, modal, and geochronologic data for approximately 1.4 billion year (Ga) A-type granitoid intrusions of the United States and to make those data available to ongoing petrogenetic investigations of these rocks. A-type granites, as originally defined by Loiselle and Wones (1979), are iron-enriched granitoids (synonymous with the ferroan granitoids of Frost and Frost, 2011) that occur in an anorogenic, within-continent setting. Relative to other granitic rocks, A-type granites have high FeO*/(FeO*+MgO), high K 2 O and K 2 O/Na 2 O, are metaluminous to weakly peraluminous, and are enriched in incompatible trace elements. Loiselle and Wones (1979) further suggested that A-type granites are relatively anhydrous. Anderson (1983) provides an early compilation of data for the products of 1.4 Ga magmatism in North America and notes the spatial and temporal association of a trio of rock types, which includes gabbro to anorthosite, intermediate composition mangerite, and granitic rapakivi rocks. In North America, the majority of known A-type intrusions were emplaced between 1.5 and 1.3 Ga and are predominantly of the granitic variety (Anderson, 1983). This report addresses the broadly Mesoproterozoic-age granitic rocks of the conterminous United States. Constituents of this group of intrusive rocks were defined using a variety of spatial, compositional, and geochronologic metrics. Thomas and others (2012) provided an updated synthesis, largely based on new isotopic and geochronologic data (for example, Fisher and others, 2010), for the large-scale geologic and tectonic evolution of the eastern United States. Their findings suggest that the basement rocks of the central and southern Appalachian region are allochthonous relative to the remainder of Laurentia and were accreted along the Grenville front between 1.25 and 1.0 Ga. Accordingly, Mesoproterozoic rocks east of the Grenville front and south of the approximate latitude of New York City do not represent North American magmatism. Consequently, geochemical, modal, and geochronologic data for these rocks are not included in the compilation described herein. Further, the structural styles and compositions of granitoid rocks east of the Grenville front, mostly highly deformed gneissic rocks, are dissimilar to those characteristic of the A-type granitoid rocks described herein. A variety of compositional and age information further characterizes the 1.4 Ga A-type granitoid rocks in the conterminous United States. Most samples included in this compilation have felsic compositions, although some extend to intermediate compositions. SiO2 contents range from 56 to almost 78 weight percent, and median and mean SiO2 contents are 72.0 and 71.1 weight percent, respectively. The majority of these rocks for which modal data are available are composed of monzogranite (Streckeisen, 1976), although the dataset also contains many samples composed of granodiorite and syenogranite. A smaller group of the granitoid rocks in this dataset are composed of quartz monzodiorite and quartz monzonite, and a very small subset of samples is composed of alkali-feldspar granite, tonalite, alkali-feldspar quartz syenite, and quartz syenite (fig. 1). Many of the 1.4 Ga granitoid rocks are further characterized by medium- to coarse-grain size and are also conspicuously porphyritic; alkali feldspar phenocrysts or megacrysts (2–10 cm), often with rapakivi overgrowths, are a common feature of many of these rocks (Anderson, 1983; Anderson and Bender, 1989; Anderson and Cullers, 1978; Condie and Budding, 1979). The age of A-type magmatism in North America ranges from about 1.8 to 1.0 Ga, although Anderson (1983) suggests that more than 70 percent (by volume) of A-type magmatism in this region occurred between 1.49 and 1.41 Ga. In the conterminous United States, ages of A-type granitoid rocks are restricted to the period between about 1.49 and 1.33 Ga (Anderson, 1983; Bauer and Pollock, 1993; Bickford and Mose, 1975; Bickford, Harrower, and others, 1981; Bickford and others, 1989; Dewane and Van Schmus, 2007; Hoppe and others, 1983; Peterman and Hedge, 1968; Van Schmus and Bickford, 1981; Van Schmus and others, 1975). Using these recognition criteria, we identified A-type granitoid intrusions of the conterminous United States; for those intrusions, we compiled available geochemical, modal, isotopic (Sr and Nd) and geochronologic data for inclusion in the databases described herein. The significance of 1.4 Ga granitoid rocks relative to the geologic evolution of the conterminous United States remains unclear, despite Anderson’s (1983) compilation and synthesis of compositional data pertinent to these rocks. The large-volume magmatic events indicated by these rocks, as well as their broad geographic distribution, tectonic significance, and association with mineral deposits, underscore their importance. The broad distribution of these rocks, from the northern mid-continent to the southwestern United States (in New Mexico, Arizona, California, and southernmost Nevada), throughout the Rocky Mountains in New Mexico and Colorado (and sporadically in southern Wyoming and central Idaho), and beneath much of the Plains region (as indicated by drilling), has led to the large-scale tectonic and magmatic processes responsible for genesis of the associated magmas being actively studied. In addition, Kisvarsanyi (1972) suggests that iron-copper deposits in the St. Francois Mountains of southeastern Missouri are petrogenetically associated with 1.4 Ga A-type granitoids that occur in that region. Similarly, Dall’Agnol and others (2012) summarize important global associations between A-type granitoid rocks and a variety of important ore deposit types, particularly tin, high-field-strength elements (Zr, Hf, Nb, Ta), rare-earth elements, and iron oxide-copper-gold deposits. Consequently, the need to better understand relations between A-type granitoid rocks, tectonic setting, and magma petrogenesis, as well as their genetic associations with important types of ore deposits, suggests that developing a definitive geochemical, modal, and geochronologic database for these rocks in the conterminous United States is of considerable value.

Data Series↗

USGS2015

A Laurentian margin back-arc: the Ordovician Wedowee-Emuckfaw-Dahlonega basin

Independent researchers working in the Talladega belt, Ashland-Wedowee-Emuckfaw belt, and Opelika Complex of Alabama, as well as the Dahlonega gold belt and western Inner Piedmont of Alabama, Georgia, and the Carolinas, have mapped stratigraphic sequences unique to each region. Although historically considered distinct terranes of disparate origin, a synthesis of data suggests that each includes lithologic units that formed in an Ordovician back-arc basin (Wedowee-Emuckfaw-Dahlonega basin—WEDB). Rocks in these terranes include varying proportions of metamorphosed mafic and bimodal volcanic rock suites interlayered with deep-water metasedimentary rock sequences. Metavolcanic rocks yield ages that are Early–Middle Ordovician (480–460 Ma) and interlayered metasedimentary units are populated with both Grenville and Early–Middle Ordovician detrital zircons. Metamafic rocks display geochemical trends ranging from mid-oceanic-ridge basalt to arc affinity, similar to modern back-arc basalts. The collective data set limits formation of the WEDB to a suprasubduction system built on and adjacent to upper Neoproterozoic–lower Paleozoic rocks of the passive Laurentian margin at the trailing edge of Iapetus, specifically in a continental margin back-arc setting. Overwhelmingly, the geologic history of the southern Appalachians, including rocks of the WEDB described here, indicates that the Ordovician Taconic orogeny in the southern Appalachians developed in an accretionary orogenic setting instead of the traditional collisional orogenic setting attributed to subduction of the Laurentian margin beneath an exotic or peri-Laurentian arc. Well-studied Cenozoic accretionary orogens provide excellent analogs for Taconic orogenesis, and an accretionary orogenic model for the southern Appalachian Taconic orogeny can account for aspects of Ordovician tectonics not easily explained through collisional orogenesis.

Alabama, Georgia, North Carolina, South Carolina↗

USGS2015

Basement domain map of the conterminous United States and Alaska

The basement-domain map is a compilation of basement domains in the conterminous United States and Alaska designed to be used at 1:5,000,000-scale, particularly as a base layer for national-scale mineral resource assessments. Seventy-seven basement domains are represented as eighty-three polygons on the map. The domains are based on interpretations of basement composition, origin, and architecture and developed from a variety of sources. Analysis of previously published basement, lithotectonic, and terrane maps as well as models of planetary development were used to formulate the concept of basement and the methodology of defining domains that spanned the ages of Archean to present but formed through different processes. The preliminary compilations for the study areas utilized these maps, national-scale gravity and aeromagnetic data, published and limited new age and isotopic data, limited new field investigations, and conventional geologic maps. Citation of the relevant source data for compilations and the source and types of original interpretation, as derived from different types of data, are provided in supporting descriptive text and tables. The tectonic settings for crustal types represented in the basement domains are subdivided into constituent geologic environments and the types of primary metals endowments and deposits in them are documented. The compositions, architecture, and original metals endowments are potentially important to assessments of primary mineral deposits and to the residence and recycling of metals in the crust of the United States portion of the North American continent. The databases can be configured to demonstrate the construction of the United States through time, to identify specific types of crust, or to identify domains potentially containing metal endowments of specific genetic types or endowed with specific metals. The databases can also be configured to illustrate other purposes chosen by users.

Data Series↗

USGS2014

Early to Middle Ordovician back-arc basin in the southern Appalachian Blue Ridge: characteristics, extent, and tectonic significance

Fault-dismembered segments of a distinctive, extensive, highly allochthonous, and tectonically significant Ordovician (ca. 480–460 Ma) basin, which contains suites of bimodal metavolcanic rocks, associated base metal deposits, and thick immature deep-water (turbiditic) metasediments, occur in parts of the southern Appalachian Talladega belt, eastern Blue Ridge, and Inner Piedmont of Alabama, Georgia, and North and South Carolina. The basin's predominantly metasedimentary strata display geochemical and isotopic evidence of a mixed provenance, including an adjacent active volcanic arc and a provenance of mica (clay)-rich sedimentary and felsic plutonic rocks consistent with Laurentian (Grenvillian) upper-crustal continental rocks and their passive-margin cover sequences. Geochemical characteristics of the subordinate intercalated bimodal metavolcanic rocks indicate formation in a suprasubduction environment, most likely a back-arc basin, whereas characteristics of metasedimentary units suggest deposition above Neoproterozoic rift and outer-margin lower Paleozoic slope and rise sediments within a marginal basin along Ordovician Laurentia's Iapetus margin. This tectonic setting indicates that southernmost Appalachian Ordovician orogenesis (Taconic orogeny) began as an extensional accretionary orogen along the outer margin of Laurentia, rather than in an exotic (non-Laurentian) arc collisional setting. B-type subduction polarity requires that the associated arc-trench system formed southeast of the palinspastic position of the back-arc basin. This scenario can explain several unique features of the southern Appalachian Taconic orogen, including: the palinspastic geographic ordering of key tectonic elements (i.e., back-arc, arc, etc.), and a lack of (1) an obducted arc sensu stricto on the Laurentian margin, (2) widespread Ordovician regional metamorphism, and (3) Taconic klippen to supply detritus to the Taconic foreland basin.

Alabama, Georgia, North Carolina, South Carolina↗

USGS2012

Integration of new geologic mapping and satellite-derived quartz mapping yields insights into the structure of the Roberts Mountains allochthon applicable to assessments for concealed Carlin-type gold deposits

Geologic mapping and remote sensing across north-central Nevada enable recognition of a thick sheet of Middle and Upper Ordovician Valmy Formation quartzite that structurally overlies folded and faulted Ordovician through Devonian stratigraphic units of the Roberts Mountains allochthon. In the northern Independence Mountains and nearby Double Mountain area, the Valmy Formation is in fault contact with Ordovician through Silurian, predominantly clastic, sedimentary rocks of the Roberts Mountains allochthon that were deformed prior to, or during, emplacement of the Valmy thrust sheet. Similar structural relations are recognized discontinuously for 200 kilometers along the strike of the Roberts Mountains allochthon in mapping guided by regional remote-sensing-based (ASTER) quartz maps. Overall thicknesses of deformed Roberts Mountains allochthon units between the base of the Valmy and the top of underlying carbonate rocks that host large Carlin-type gold deposits varies on the order of hundreds of meters but is not known to exceed 700 meters. The base of the Valmy thrust sheet is a complimentary datum in natural resource exploration and mineral resource assessment for concealed Carlin-type gold deposits.

Nevada↗