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C. Mark Fanning

Publications and source records attributed to C. Mark Fanning.

15 recordsLinked to original sources
USGS2017

Petrology and geochronology of Mesoproterozoic basement of the Mount Rogers area of southwestern Virginia and northwestern North Carolina: Implications for the Precambrian tectonic evolution of the southern Blue Ridge province

Results from new geologic mapping, SHRIMP U-Pb geochronology, and petrologic studies indicate that Mesoproterozoic basement in the northern French Broad massif near Mount Rogers consists of multiple, mostly granitic plutons, map- and outcrop-scale xenoliths of pre-existing crustal rocks, and remnants of formerly overlying meta-sedimentary lithologies. Zircon and titanite ages demonstrate that these rocks collectively record nearly 350 m.y. of tectonic evolution including periods of igneous intrusion at ca . 1190 to 1130 Ma (Early Magmatic Suite) and ca . 1075 to 1030 Ma (Late Magmatic Suite) and three episodes of regional metamorphism at ca . 1170 to 1140, 1070 to 1020, and 1000 to 970 Ma. The existence of ca . 1.3 Ga age crust is indicated by (1) orthogranofels of ca . 1.32 Ga age in a map-scale xenolith, (2) inherited zircons of ca . 1.33 to 1.29 Ga age in Early Magmatic Suite plutons, and (3) ca . 1.36 to 1.30 Ga age detrital zircons in meta-sedimentary lithologies. Mineral assemblages developed in amphibolites and granofelses indicate that metamorphism during both Mesoproterozoic episodes occurred at upper amphibolite- to lower granulite-facies conditions. Syn-orogenic Early Magmatic Suite plutons emplaced at ca . 1190 to 1145 Ma are characterized by high-K, variably magnesian, dominantly calc-alkalic compositions, and have trace-element characteristics indicative of continental-arc magmatic origin involving melting of thick continental crust. In contrast, ca . 1140 Ma age quartz syenite displays A-type features indicating derivation from depleted crustal sources with increased mantle input during waning stages of regional contraction. Plutons of the compositionally bimodal Late Magmatic Suite include (1) ca . 1060 Ma meta-granite with geochemical characteristics transitional between silicic rocks of arc systems and post-collisional granites of A-type lineage, and (2) ca . 1055 Ma monzodioritic rocks with A-type compositional characteristics that likely reflect derivation from fertile, mafic sources in the lower crust. Collectively, these data suggest that Mesoproterozoic rocks of the study area preserve evidence of multiple orogenic episodes that likely involved continental-arc development and deformation at ca . 1150 Ma followed by crustal thickening at ca . 1060 Ma. Field relations and geochronologic data indicate that regional uplift and sedimentation occurred at ca . 1100 Ma between the two episodes of overlapping magmatism and orogenesis. The nature and timing of Mesoproterozoic events recorded in basement rocks of the study area illustrate significant differences in the lithologic assemblages and geologic history preserved by Mesoproterozoic basement of the adjacent Shenandoah and French Broad massifs, suggesting that the Blue Ridge massifs occupied different locations within the regional Grenville-age orogen until about 1070 Ma when the effects of Ottawan-age tectonics began to affect both areas. The near ubiquity of Ottawan-age orogenic activity recorded in Mesoproterozoic rocks of the Blue Ridge, other Appalachian inliers such as the New Jersey Highlands, and the Grenville province of Canada, including the Adirondacks, suggests that these formerly disparate terranes were amalgamated to form a common, regional orogen by this time.

North Carolina, Virginia↗
USGS2016

U-Pb, Re-Os, and Ar/Ar geochronology of rare earth element (REE)-rich breccia pipes and associated host rocks from the Mesoproterozoic Pea Ridge Fe-REE-Au deposit, St. Francois Mountains, Missouri

Rare earth element (REE)-rich breccia pipes (600,000 t @ 12% rare earth oxides) are preserved along the margins of the 136-million metric ton (Mt) Pea Ridge magnetite-apatite deposit, within Mesoproterozoic (~1.47 Ga) volcanic-plutonic rocks of the St. Francois Mountains terrane in southeastern Missouri, United States. The breccia pipes cut the rhyolite-hosted magnetite deposit and contain clasts of nearly all local bedrock and mineralized lithologies. Grains of monazite and xenotime were extracted from breccia pipe samples for SHRIMP U-Pb geochronology; both minerals were also dated in one polished thin section. Monazite forms two morphologies: (1) matrix granular grains composed of numerous small (<50 μ m) crystallites intergrown with rare xenotime, thorite, apatite, and magnetite; and (2) coarse euhedral, glassy, bright-yellow grains similar to typical igneous or metamorphic monazite. Trace element abundances (including REE patterns) were determined on selected grains of monazite (both morphologies) and xenotime. Zircon grains from two samples of host rhyolite and two late felsic dikes collected underground at Pea Ridge were also dated. Additional geochronology done on breccia pipe minerals includes Re-Os on fine-grained molybdenite and 40 Ar/ 39 Ar on muscovite, biotite, and K-feldspar. Ages (±2 σ errors) obtained by SHRIMP U-Pb analysis are as follows: (1) zircon from the two host rhyolite samples have ages of 1473.6 ± 8.0 and 1472.7 ± 5.6 Ma; most zircon in late felsic dikes is interpreted as xenocrystic (age range ca. 1522–1455 Ma); a population of rare spongy zircon is likely of igneous origin and yields an age of 1441 ± 9 Ma; (2) pale-yellow granular monazite—1464.9 ± 3.3 Ma (no dated xenotime); (3) reddish matrix granular monazite—1462.0 ± 3.5 Ma and associated xenotime—1453 ± 11 Ma; (4) coarse glassy-yellow monazite—1464.8 ± 2.1, 1461.7 ± 3.7 Ma, with rims at 1447.2 ± 4.7 Ma; and (5) matrix monazite (in situ)—1464.1 ± 3.6 and 1454.6 ± 9.6 Ma, and matrix xenotime (in situ)—1468.0 ± 8.0 Ma. Two slightly older ages of cores are about 1478 Ma. The young age of rims on the coarse glassy monazite coincides with an Re-Os age of 1440.6 ± 9.2 Ma determined in this study for molybdenite intergrown with quartz and allanite, and with the age of monazite inclusions in apatite from the magnetite ore ( Neymark et al., 2016 ). A 40 Ar/ 39 Ar age of 1473 ± 1 Ma was obtained for muscovite from a breccia pipe sample. Geochronology and trace element geochemical data suggest that the granular matrix monazite and xenotime (in polygonal texture), and cores of coarse glassy monazite precipitated from hydrothermal fluids during breccia pipes formation at about 1465 Ma. The second episode of mineral growth at ca. 1443 Ma may be related to faulting and fluid flow that rebrecciated the pipes. The ca. 10-m.y. gap between the ages of host volcanic rocks and breccia pipe monazite and xenotime suggests that breccia pipe mineral formation cannot be related to the felsic magmatism represented by the rhyolitic volcanic rocks, and hence is linked to a different magmatic-hydrothermal system.

Missouri↗
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↗
USGS2015

Mid-Cretaceous oblique rifting of West Antarctica: Emplacement and rapid cooling of the Fosdick Mountains migmatite-cored gneiss dome

In Marie Byrd Land, West Antarctica, the Fosdick Mountains migmatite-cored gneiss dome was exhumed from mid- to lower middle crustal depths during the incipient stage of the West Antarctic Rift system in the mid-Cretaceous. Prior to and during exhumation, major crustal melting and deformation included transfer and emplacement of voluminous granitic material and numerous intrusions of mantle-derived diorite in dikes. A succession of melt- and magma-related structures formed at temperatures in excess of 665 ± 50 °C based on Ti-in-zircon thermometry. These record a transition from wrench to oblique extensional deformation that culminated in the development of the oblique South Fosdick Detachment zone. Solid-state fabrics within the detachment zone and overprinting brittle structures record translation of the detachment zone and dome to shallow levels. To determine the duration of exhumation and cooling, we sampled granite and gneisses at high spatial resolution for U–Pb zircon geochronology and 40 Ar/ 39 Ar hornblende and biotite thermochronology. U–Pb zircon crystallization ages for the youngest granites are 102 Ma. Three hornblende ages are 103 to 100 Ma and 12 biotite ages are 101 to 99 Ma. All overlap within uncertainty. The coincidence of zircon crystallization ages with 40 Ar/ 39 Ar cooling ages indicates cooling rates > 100 °C/m.y. that, when considered together with overprinting structures, indicates rapid exhumation of granite and migmatite from deep to shallow crustal levels within a transcurrent setting. Orientations of structures and age-constrained crosscutting relationships indicate counterclockwise rotation of stretching axes from oblique extension into nearly orthogonal extension with respect to the Marie Byrd Land margin. The rotation may be a result of localized extension arising from unroofing and arching of the Fosdick dome, extensional opening within a pull-apart zone, or changes in plate boundary configuration. The rapid tectonic and temperature evolution of the Fosdick Mountains dome lends support to recently developed numerical models of crustal flow and cooling in orogenic crust undergoing extension/transtension, and accords with numerous studies of migmatite-cored gneiss domes in transcurrent settings.

LITHOS↗
USGS2015

SHRIMP U–Pb and REE data pertaining to the origins of xenotime in Belt Supergroup rocks: evidence for ages of deposition, hydrothermal alteration, and metamorphism

The Belt–Purcell Supergroup, northern Idaho, western Montana, and southern British Columbia, is a thick succession of Mesoproterozoic sedimentary rocks with an age range of about 1470–1400 Ma. Stratigraphic layers within several sedimentary units were sampled to apply the new technique of U–Pb dating of xenotime that sometimes forms as rims on detrital zircon during burial diagenesis; xenotime also can form epitaxial overgrowths on zircon during hydrothermal and metamorphic events. Belt Supergroup units sampled are the Prichard and Revett Formations in the lower Belt, and the McNamara and Garnet Range Formations and Pilcher Quartzite in the upper Belt. Additionally, all samples that yielded xenotime were also processed for detrital zircon to provide maximum age constraints for the time of deposition and information about provenances; the sample of Prichard Formation yielded monazite that was also analyzed. Ten xenotime overgrowths from the Prichard Formation yielded a U–Pb age of 1458 ± 4 Ma. However, because scanning electron microscope – backscattered electrons (SEM–BSE) imagery suggests complications due to possible analysis of multiple age zones, we prefer a slightly older age of 1462 ± 6 Ma derived from the three oldest samples, within error of a previous U–Pb zircon age on the syn-sedimentary Plains sill. We interpret the Prichard xenotime as diagenetic in origin. Monazite from the Prichard Formation, originally thought to be detrital, yielded Cretaceous metamorphic ages. Xenotime from the McNamara and Garnet Range Formations and Pilcher Quartzite formed at about 1160– 1050 Ma, several hundred million years after deposition, and probably also experienced Early Cretaceous growth. These xenotime overgrowths are interpreted as metamorphic–diagenetic in origin (i.e., derived during greenschist facies metamorphism elsewhere in the basin, but deposited in sub-greenschist facies rocks). Several xenotime grains are older detrital grains of igneous derivation. A previous study on the Revett Formation at the Spar Lake Ag–Cu deposit provides data for xenotime overgrowths in several ore zones formed by hydrothermal processes; herein, those results are compared with data from newly analyzed diagenetic, metamorphic, and magmatic xenotime overgrowths. The origin of a xenotime overgrowth is reflected in its rareearth element (REE) pattern. Detrital (i.e., igneous) xenotime has a large negative Eu anomaly and is heavy rare-earth element (HREE)-enriched (similar to REE in igneous zircon). Diagenetic xenotime has a small negative Eu anomaly and flat HREE (Tb to Lu). Hydrothermal xenotime is depleted in light rare-earth element (LREE), has a small negative Eu anomaly, and decreasing HREE. Metamorphic xenotime is very LREE-depleted, has a very small negative Eu anomaly, and is strongly depleted in HREE (from Gd to Lu). Because these characteristics seem to be process related, they may be useful for interpretation of xenotime of unknown origin. The occurrence of 1.16–1.05 Ga metamorphic xenotime, in the apparent absence of pervasive deformation structures, suggests that the heating may be related to poorly understood regional heating due to broad regional underplating of mafic magma. These results may be additional evidence (together with published ages from metamorphic titanite, zircon, monazite, and garnet) for an enigmatic, Grenville-age metamorphic event that is more widely recognized in the southwestern and eastern United States

Alberta, British Columbia, Idaho, Montana, Oregon,↗
USGS2014

U-Pb zircon geochronology of plutonism in the northern Peninsular Ranges batholith, southern California: Implications for the Late Cretaceous tectonic evolution of southern California

Utilizing both sensitive high-resolution ion microprobe (SHRIMP) and conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) methods, crystallization and/or emplacement ages have been obtained for a suite of Cretaceous intermediate-composition plutonic samples collected along a roughly E-W–trending traverse through the northern Peninsular Ranges batholith. Previously noted petrologic, mineralogic, and textural differences delineated four major zonations from west to east and raised the need for detailed geochemical and isotopic work. U-Pb zircon geochronology establishes that these zonations are essentially temporally separate. Mean 206 Pb/ 238 U ages date the three older zones from west to east at 126–107 Ma, 107–98 Ma, and 98–91 Ma. Despite petrologic differences, a relatively smooth progression of magmatism is seen from west to east. A fourth zone is defined by magmatism at ca. 85 Ma, which represents emplacement of deeper-level plutons east of the Eastern Peninsular Ranges mylonite zone in an allochthonous thrust sheet in the northeastern Peninsular Ranges batholith. The age data presented here differ slightly from those presented in earlier work for similar rocks exposed across the middle and southern portions of the Peninsular Ranges batholith in that our data define a relatively smooth progression of magmatism from west to east, and that the transition from western-type to eastern-type plutonism is interpreted to have occurred at ca. 98 Ma and not at ca. 105 Ma. The progressive involvement of older crustal components in the enrichment of eastern Peninsular Ranges batholith–type magma sources is documented by the occurrence of Proterozoic zircon inheritance within samples of the eastern part of the batholith.

Book chapter↗
USGS2012

SHRIMP U-Pb ages of xenotime and monazite from the Spar Lake red bed-associated Cu-Ag deposit, western Montana: Implications for ore genesis

Xenotime occurs as epitaxial overgrowths on detrital zircons in the Mesoproterozoic Revett Formation (Belt Supergroup) at the Spar Lake red bed-associated Cu-Ag deposit, western Montana. The deposit formed during diagenesis of Revett strata, where oxidizing metal-bearing hydrothermal fluids encountered a reducing zone. Samples for geochronology were collected from several mineral zones. Xenotime overgrowths (1–30 μm wide) were found in polished thin sections from five ore and near-ore zones (chalcocite-chlorite, bornite-calcite, galena-calcite, chalcopyrite-ankerite, and pyrite-calcite), but not in more distant zones across the region. Thirty-two in situ SHRIMP U-Pb analyses on xenotime overgrowths yield a weighted average of 207 Pb/ 206 Pb ages of 1409 ± 8 Ma, interpreted as the time of mineralization. This age is about 40 to 60 m.y. after deposition of the Revett Formation. Six other xenotime overgrowths formed during a younger event at 1304 ± 19 Ma. Several isolated grains of xenotime have 207 Pb/ 206 Pb ages in the range of 1.67 to 1.51 Ga, and thus are considered detrital in origin. Trace element data can distinguish Spar Lake xenotimes of different origins. Based on in situ SHRIMP analysis, detrital xenotime has heavy rare earth elements-enriched patterns similar to those of igneous xenotime, whereas xenotime overgrowths of inferred hydrothermal origin have hump-shaped (i.e., middle rare earth elements-enriched) patterns. The two ages of hydrothermal xenotime can be distinguished by slightly different rare earth elements patterns. In addition, 1409 Ma xenotime overgrowths have higher Eu and Gd contents than the 1304 Ma overgrowths. Most xenotime overgrowths from the Spar Lake deposit have elevated As concentrations, further suggesting a genetic relationship between the xenotime formation and Cu-Ag mineralization.

Montana↗
USGS2010

Ordovician volcanic-arc terrane in the Central Appalachian Piedmont of Maryland and Virginia: SHRIMP U-Pb geochronology, field relations, and tectonic significance

U-Pb zircon geochronology and field relations provide insights into metavolcanic and associated rocks in the Central Appalachian Piedmont of Maryland and northern Virginia. Ordovician ages were determined for volcanic-arc rocks of the James Run Formation (Churchville Gneiss Member, 458 ± 4 Ma; Carroll Gneiss Member, 462 ± 4 Ma), Relay Felsite (458 ± 4 Ma), Chopawamsic Formation (453 ± 4 Ma), and a Quantico Formation volcaniclastic layer (448 ± 4 Ma). A previously dated first phase of volcanism in the Chopawamsic Formation was followed by the second phase dated here. The latter suggests a possible source for contemporaneous volcanic-ash beds throughout eastern North America. Dates from the Chopawamsic and Quantico Formations constrain the transition from arc volcanism to successor-basin sedimentation. Ordovician metatonalites of the Franklinville (462 ± 5 Ma) and Perry Hall (461 ± 5 Ma) plutons are contemporaneous with the James Run Formation, whereas granitoids of the Bynum Run (434 ± 4 Ma) and Prince William Forest (434 ± 8 Ma) plutons indicate an Early Silurian plutonic event. The Popes Head Formation yielded Mesoproterozoic (1.0–1.25 Ga, 1.5–1.8 Ga) detrital zircons, and metamorphosed sedimentary mélange of the Sykesville Formation yielded Mesoproterozoic (1.0–1.8 Ga) detrital zircons plus a minor Archean (2.6 Ga) component. A few euhedral zircons (ca. 479 Ma) in the Sykesville Formation may be from granitic seams related to the Dalecarlia Intrusive Suite. A Potomac orogeny in the Central Appalachian Piedmont is not required, but the earliest Taconic orogenesis remains poorly constrained.

Maryland, Virginia↗
USGS2008

Isotopic evidence for the diversity of late Quaternary loess in Nebraska: Glaciogenic and nonglaciogenic sources

Pb isotope compositions of detrital K-feldspars and U-Pb ages of detrital zircons are used as indicators for determining the sources of Peoria Loess deposited during the last glacial period (late Wisconsin, ca. 25&ndash;14 ka) in Nebraska and western Iowa. Our new data indicate that only loess adjacent to the Platte River has Pb isotopic characteristics suggesting derivation from this river. Most Peoria Loess in central Nebraska (up to 20 m thick) is non-glaciogenic, on the basis of Pb isotope ratios in K-feldspars and the presence of 34-Ma detrital zircons. These isotopic characteristics suggest derivation primarily from the Oligocene White River Group in southern South Dakota, western Nebraska, southeastern Wyoming, and northeastern Colorado. The occurrence of 10&ndash;25 Ma detrital zircons suggests additional minor contributions of silt from the Oligocene-Miocene Arikaree Group and Miocene Ogallala Group. Isotopic data from detrital K-feldspar and zircon grains from Peoria Loess deposits in eastern Nebraska and western Iowa suggest that the immediate source of this loess was alluvium of the Missouri River. We conclude that this silt probably is of glaciogenic origin, primarily derived from outwash from the western margin of the Laurentide Ice Sheet. Identification of the White River Group as the main provenance of Peoria Loess of central Nebraska and the Missouri River valley as the immediate source of western Iowa Peoria Loess indicates that paleowind directions during the late Wisconsin were primarily from the northwest and west, in agreement with earlier studies of particle size and loess thickness variation. In addition, the results are in agreement with recent simulations of non-glaciogenic dust sources from linked climate-vegetation modeling, suggesting dry, windy, and minimally vegetated areas in parts of the Great Plains during the last glacial period.

Nebraska↗
USGS2004

U-Pb geochronology and evolution of Mesoproterozoic basement rocks, western Connecticut

Geologic mapping and U-Pb geochronology by ion microprobe on zircon, titanite, and monazite in the New Milford quadrangle, western Connecticut indicate Meso-proterozoic events at ca. 1.3, 1.05, and 0.99 Ga in the Laurentian basement rocks. Pink granite gneiss (1311 ± 7 Ma) intruded a paragneiss sequence during the early stages of the Elzevirian orogeny. During the Ottawan orogeny, syn-tectonic anatexis produced a belt of stromatic migmatite at 1057 ± 10 Ma. Ottawan igneous activity included syn-tectonic intrusion of abundant sills of biotite granite gneiss, dated at 1050 ± 14 and 1048 ± 11 Ma, and intrusion of the Danbury augen granite at 1045 ± 8 Ma. Overgrowths on igneous zircon and metamorphic zircon in hornblende gneiss indicate that terminal Grenville metamorphism occurred at ca. 993 ± 8 Ma. Late Ordovician syn-tectonic events included intrusion of a leucogranite dike into the Brookfield Gneiss at 453 ± 6 Ma and intrusion of the Candlewood Granite at 443 ± 7 Ma. A monazite age from the Candlewood Granite of 445 ± 9 Ma agrees with the zircon age. A second phase of migmatization in the basement rocks is associated with the injection of numerous granitic leucosomes at 444 ± 6 Ma along the margin of the Candlewood Granite. Titanite ages from 431 to 406 Ma indicate several high-grade heating events from the Silurian to the Early Devonian. The lack of Grenville-age titan-ite in the basement suggests that post-Taconian heating was sufficient to completely reset old titanite in the massif.

Connecticut↗
USGS2000

Geochronology of the proterozoic basement of southwesternmost North America, and the origin and evolution of the Mojave crustal province

The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 ± 12 and 1675 ± 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at ∼1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf-slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc-continent collision along the western margin of Laurentia.

Tectonics↗
USGS2000

SHRIMP U-Pb zircon ages for Big Creek gneiss, Wyoming and Boulder Creek batholith, Colorado: Implications for timing of Paleoproterozoic accretion of the northern Colorado province

Sensitive, high-resolution, ion microprobe (SHRIMP) U-Pb zircon ages from a sample of the high-grade, hornblende-feldspathic Big Creek gneiss of the southeastern Sierra Madre, along with samples of a quartz monzonitic phase of the Boulder Creek batholith, help define timing of three major Paleoproterozoic thermo-tectonic events within the northern Colorado province at approximately 1810, 1710, and 1610 Ma. Previous ages determined for these key rock units were problematic; they hindered regional tectonic interpretations of the Paleoproterozoic crustal accretion history of the Colorado province that extends from the Cheyenne belt of southern Wyoming to north-central New Mexico. The Colorado province has been popularly modelled as a series of accreted oceanic volcano-plutonic arc systems and associated sediments, although alternative interpretations suggest that the series represents continental-margin arc rocks. The Big Creek gneiss has been interpreted as a high-grade basement equivalent of the oldest arc volcanic rocks exposed within the Green Mountain arc terrane, but it also has been suspected of being either an older block of pre-arc basement or perhaps an allochthonous piece of crust from slightly older orogens to the east and north. Previous ID-TIMS work on mg-size zircon fractions yielded U-Pb concordia upper-intercept ages of 1618 ± 22 and 1684 ± 5 Ma as well as negative lower-intercept ages, indicating complex U-Pb isotopic systematics involving at least two ages of zircon growth overprinted by at least one episode of Pb-loss. Zircons from this gneiss were analyzed using the SHRIMP, and a total of 32 spot analyses on both centers and rims produced a range of different 207 Pb/ 206 Pb ages between ∼1840 and ∼1560 Ma. The weighted mean of the oldest 207 Pb/ 206 Pb ages is 1812 ± 12 Ma and is interpreted to estimate the age of the protolith that appears to be slightly older than lower-grade metabasalts and associated plutons at ∼1790–1775 Ma. This protolith age of 1812 Ma further implies that significantly older crust (> 1820 Ma; e.g., Penokean orogeny) is not found in the Green Mountain magmatic arc. The youngest 207 Pb/ 206 Pb ages of ∼1610 Ma are interpreted to represent a time of new zircon growth during highly localized high-grade metamorphism—an event that also produced local granitic magmatism at ∼1625 Ma. The Boulder Creek batholith had been dated previously using the ID-TIMS, U-Pb zircon technique that yielded ages at ∼1670 and ∼1714 Ma, a 45-m.y. discrepancy that left the true age of the batholith in doubt. Zircons from two samples, previously dated using the ID-TIMS method, were analyzed using SHRIMP, and yielded concordia upper-intercept ages of 1713 ± 10 and 1721 ± 15 Ma. These results, combined with two earlier U-Pb zircon determinations, help to establish the age of the Boulder Creek batholith at 1714.4 ± 4.6 Ma (weighted mean), an age more compatible with those for the other large, tonalitic to quartz monzonitic, syntectonic plutons within the northern Colorado province. The new Boulder Creek age helps to establish a discrete period of plutonism (∼1735–1705 Ma) that is syn- to post-tectonic with respect to major regional structures of deformation and metamorphism in the northern Colorado province. Assuming the multiple oceanic arc accretion model, the new age for the mid-crustal emplacement of this batholith into a deforming composite back-arc basin may date the closure of that basin during crustal shortening.

Colorado, Wyoming↗
USGS1999

Distinguishing Grenvillian basement from pre-Taconian cover rocks in the Northern Appalachians

Distinguishing Grenvillian basement rocks from pre-Taconian cover sequences in the Appalachians is a first-order problem essential for accurate structural interpretations. The Cavendish Formation in southeastern Vermont presents a classic example of this problem. Doll and others (1961) showed the Cavendish Formation as younger than the Middle Proterozoic Mount Holly Complex but older than the lithologically similar Cambrian Tyson and Hoosac Formations. More recently, the name Cavendish Formation has been informally abandoned, and its metasedimentary units have been mapped as the Tyson and Hoosac Formations of Late Proterozoic to Cambrian age. In a radical departure from these interpretations, Ratcliffe and others (1997) reassigned metasedimentary rocks of the Cavendish Formation to the Mount Holly Complex based on an inferred intrusive relationship between them and a 1.42 Ga tonalite. This new age assignment, if correct, requires a completely new structural interpretation of the region. SHRIMP and Pb evaporation ages of detrital zircons extracted from a quartzite layer from Cavendish Gorge near the proposed intrusive contact with the tonalite constrain the time of deposition of the Cavendish Formation. Grain shapes of the zircons vary from euhedral to nearly spherical. Virtually all the grains have pitted surfaces and show at least some rounding of edges and terminations; grains exhibit oscillatory zoning typical of zircons that crystallized from a magma. Single-grain Pb evaporation analyses of ten zircons and SHRIMP analyses of 15 zircons all yield ages less than 1.42 Ga. Seven of the grains are consistent with derivation from the Bull Hill Gneiss that postdates the Grenville orogenic cycle and predates deposition of the Cavendish Formation. Thus, the metasedimentary units of the Cavendish Formation should not be assigned to the Mount Holly Complex.

northern Appalachians↗
USGSDate not supplied

Scouting craton’s edge in Paleo-Pacific Gondwana

The geology of the ice-covered interior of the East Antarctic shield is completely unknown; inferences about its composition and history are based on extrapolating scant outcrops from the coast inland. Although the shield is clearly composite in nature, a large part of its interior has been represented by a single Precambrian block, termed the Mawson block, that includes the Archean-Mesoproterozoic Gawler and Curnamona cratons of Australia. In Australia, the Mawson block is bounded on the east by Neoproterozoic sedimentary rocks and the superimposed early Paleozoic Delamerian Orogen, marked by curvilinear belts of arc plutons, and on the west by the unexposed Coompana block and Mesoproterozoic Albany-Fraser mobile belt. In Antarctica, these crustal elements are inferred to extend across Wilkes Land and south to the Miller Range region. Aero- and satellite magnetic data provide a means to see through the ice, helping to elucidate the broad composition of the shield. Rocks of the Mawson block in Australia produce distinctive magnetic anomalies; Paleoproterozoic granites and Meso- to Neoproterozoic mafic igneous rocks are associated with high-amplitude, broad-wavelength positive aero- and satellite-magnetic anomalies. The same types of magnetic anomalies can be traced to ice-covered Wilkes Land, Antarctica, and are interpreted to signify similar rocks. However, the diagnostic satellite magnetic high ends ∼800 km south of the Antarctic coast, suggesting that the Mawson block is smaller than first proposed and that the remaining East Antarctic shield is composed of several Precambrian crustal blocks of largely undetermined composition and age. Nonetheless, the coincident eastern borders of these magnetic highs and high seismic-velocity anomalies characteristic of the Precambrian shield, together define the edge of thick cratonic lithosphere. East of this boundary, magnetic lows are explained by magnetite-poor upper Neoproterozoic and lower Paleozoic sedimentary rocks, and their metamorphic equivalents, which crop out discontinuously along the Ross margin of Antarctica and in eastern Australia. These rocks are inferred to overlie a Neoproterozoic rift margin, which transects older basement provinces. The coincidence of this cratonic rift boundary with the western limit of Paleozoic and Jurassic magmatism suggests that, although tectonically modified by younger events, the composite Antarctic-Australian shield comprised thick lithosphere that was not penetrated by Paleozoic and younger convergent-margin magmas.

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