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J. C. Reed

Publications and source records attributed to J. C. Reed.

At least 19 recordsLinked to original sources
USGS1999

40Ar/39Ar thermochronology of mesoproterozoic metamorphism in the Colorado Front Range

A low-pressure metamorphic episode in the Colorado Front Range has been identified by the presence of staurolite, andalusite, cordierite, and garnet porphyroblasts overprinting earlier assemblages. The overprinting assemblages and reaction textures are most consistent with porphyroblast growth on a prograde metamorphic path with peak temperatures exceeding ~525??C. Twenty-eight 40Ar/39Ar dates on hornblende, muscovite, biotite, and microcline were used to infer the age and thermal conditions of metamorphism. Muscovite and biotite 40Ar/39Ar ages fall mainly in the interval 1400-1340 Ma, consistent with cooling through the closure temperature interval of micas (~400??-300??C) after about 1400 Ma. In contrast, hornblende apparent ages (T(c)~500??-550??C) between 1600 and 1390 Ma reflect variable retention of radiogenic argon. Forward modeling of argon diffusion shows that the distribution of hornblende and mica ages is consistent with the partial resetting of argon systematics ca. 1400 Ma by a thermal pulse reaching maximum temperatures around 550??C and decaying within <20 m.yr. These temperatures match the conditions inferred from the overprinting assemblage; thus, muscovite and biotite ages are interpreted to date the cooling phase of this metamorphic event. This late metamorphism is broadly coeval with the intrusion of ca. 1400-Ma granitic plutons in the study area and throughout the southwestern United States. However, thermal effects are observed far from pluton margins, suggesting pervasive, regional crustal heating rather than restricted contact metamorphism. Our results suggest that ca. 1400-Ma metamorphism and plutonism are manifestations of a regional thermal episode that both partially melted the lower crust and pervasively metamorphosed middle crustal rocks.

Journal of Geology↗
USGS1998

Zircon geochronology of the Webb Canyon Gneiss and the Mount Owen Quartz Monzonite, Teton Range, Wyoming: Significance to dating late Archean metamorphism in the Wyoming craton

The Webb Canyon Gneiss is a strongly foliated and lineated orthogneiss intercalated with layered Archean gneisses in the northern part of the Teton Range in northwestern Wyoming. The Mount Owen Quartz Monzonite is a non-foliated or weakly flow foliated rock which forms a discordant pluton exposed in the central part of the range and that cuts the Webb Canyon Gneiss and the associated layered gneisses. U-Pb zircon geochronology reported here indicates that euhedral pink zircon grew in the Webb Canyon Gneiss at about 2680 Ma, probably during the peak of regional metamorphism and that the Mount Owen was emplaced at 2547??3 Ma. These dates provide the best constraints so far reported on the age of Late Archean regional metamorphism in the western part of the Wyoming craton.

Mountain Geologist↗
USGS1993

Lead isotopic evidence for the origin of Paleo- and Mesoproterozoic rocks of the Colorado Province, U.S.A.

Lead isotopic ratios of K-feldspars and whole-rocks from 1.7- and 1.4-Ga plutonic rocks of the Colorado Province are relatively non-radiogenic for 207Pb 204Pb, plotting below the average crust model curve of Stacey and Kramers (1975), indicating that the terrane was derived primarily from juvenile, mantle material. Slightly more radiogenic ratios in the northern part of the terrane, near the Archean Wyoming Province, suggest minor inclusion of an older component. The data from 1.7-Ga plutons plot in a broad field suggesting two episodes of re-equilibration with whole-rock Pb, probably related to heating events in the Mesoproterozoic (1.4 Ga) and Cretaceous (70 Ma). Possible differences in calculated whole-rock Th U, coupled with slight Pb isotopic variations, along the north-south transect suggest either a terrane boundary through central Colorado (near Salida and Gunnison), or fundamental differences in source rocks (metasedimentary vs. metavolcanic). UPb analyses of multigrain splits of detrital zircons from quartzites throughout the Colorado Province have failed to identify Archean detritus. The oldest 207Pb 206Pb ages found (in two samples of quartzite from northern Colorado) are about 2.0 Ga (perhaps derived from rocks of the Trans-Hudson orogen), in contrast to 2.75-Ga detrital zircon in a Paleoproterozoic quartzite from the southern part of the Wyoming Province. While we are not yet able to discern if these ages are true provenance ages or mixtures of Archean and Paleoproterozoic components, the absence of easily recognizable Archean zircons supports other isotopic data and a conclusion that most of the Paleoproterozoic crust of the Colorado Province was ultimately derived from a juvenile (at 1.8 Ga) mantle reservoir. ?? 1993.

Precambrian Research↗
USGS1993

The Mount Evans batholith in the Colorado Front Range: Revision of its age and reinterpretation of its structure

The Mount Evans batholith, in the central Front Range of Colorado, is composed of a main phase of massive to conspicuously foliated monzogranite and granodiorite and undeformed aplite and pegmatite. The Mount Evans batholith was previously considered to be part of the 1.7 Ga Routt Plutonic Suite. New U-Pb zircon ages on four samples (granodiorite, monzogranite, and granite), however, indicate that the batholith was emplaced at 1,442 ± 2 Ma and belongs to the Berthoud Plutonic Suite. Most of the batholith has igneous textures and structures, except in the vicinity of the Idaho Springs-Ralston shear zone where those features are tectonically recrystallized and foliated. Foliation elsewhere in the batholith is a flow structure. Zircons in two granodiorite samples, collected near the shear zone (just south of the Colorado Mineral Belt), are reversely discordant by about 0.8%-2.1%, with a considerable spread in 207 Pb/ 206 Pb ages. Many of the zircons from these samples contain apatite, K-feldspar, and quartz inclusions that appear to replace zircon along cracks and imperfections from rim to core. We suggest that these inclusions formed during a Laramide ore-forming event and incorporated Early and Middle Proterozoic radiogenic lead scavenged from the country rock. The excess radiogenic lead caused the scatter and reverse discordance in the data. The Mount Evans batholith is anomalous in composition and structure compared to most other 1.4 Ga plutons of the southwestern United States. The differences probably reflect different sources of partial melting; the specific tectonic setting where rocks of such disparate origin are temporally and spatially juxtaposed is not understood.

Colorado↗
USGS1989

Geologic map of the Latir Volcanic Field and adjacent areas, northern New Mexico

This map was first published as a printed edition in 1989. The geologic data have now been captured digitally and are presented here along with images of the printed map sheet and component parts as PDF files. This map encompasses all or parts of ten 7.5 minute quadrangles in the Taos Range of the Sangre de Cristo Mountains in northern New Mexico. Geologic mapping was initiated in this area by the U.S. Geological Survey in response to its mandate under the Wilderness Act of 1964 to evaluate the mineral resource potential of the Latir and Wheeler Peak Wildernesses and the Columbine-Hondo Wilderness Study Area in the Carson National Forest. The mapping was later extended to adjacent areas in order to better understand the regional geology and geologic history of the range. The present map focuses on the early Tertiary (largely Oligocene) Latir Volcanic Field and plutonic rocks associated with it. The basement rocks in the map area are Paleoproterozic amphibolite-facies metasedimentary and metavolcanic rocks intruded by large bodies of quartz monzonite, granodiorite and gabbro dated at between 1,750 and 1,690 Ma. The basement rocks are locally overlain by Mississippian limestone or by Pennsylvanian and Permian redbeds. The basement rocks were thrust eastward across a thick section of similar late Paleozoic rocks along low-angle faults during the Laramide Orogeny. Post-Laramide erosion largely removed the sedimentary cover from the basement rocks and reduced the topography to a low-relief surface locally overlain by scattered lenses of Eocene to Oligocene shale, sandstone, and conglomerate. It was on this surface that the earliest rocks of the Latir Volcanic Field were deposited, beginning in the Oligocene, about 30 Ma. The early volcanic rocks comprise a thick sequence of andesite, dacite, and minor rhyolite, deposited as flows, breccia, and volcaniclastic sediments from numerous local volcanic centers; thin layers of rhyolite tuff are from distant eruptions, some probably in the San Juan Mountains. These volcanic rocks probably record early growth of an upper-crustal batholith. At 25 Ma enormous eruptions of peralkaline rhyolite ash-flow tuff were accompanied by collapse of the roof of the growing batholith to form the Questa Caldera, a volcanic depression at least 14 km across. The ash flows traveled for tens of kilometers from the caldera rims to form a widespread sheet of densely welded tuff; ash also fell back to fill the subsiding caldera depression. Concurrent extension along northwest-trending faults segmented the volcanic edifice and rotated the originally sub-horizontal layers, so that some are nearly vertical. Formation of the caldera was accompanied by further batholith growth, involving intrusion of large plutons of granite, granodiorite, and related rocks into both caldera fill and the adjacent rocks, including both the earlier volcanic rocks and the basement rocks. The large molybdenum deposit along the Red River east of Questa is related to the emplacement of one of these late granitic plutons. Intrusive activity continued into the Miocene. Development of the Rio Grande Rift beginning at about 15 Ma was accompanied by development of the north-south trending normal faults along the western foot of the Taos Range. The rift is filled with thousands of meters of clastic sediments interleaved with basalt flows, some as young as 3.6 Ma. The bounding faults of the rift cut the Questa Caldera so that the western part of the original structure is now deeply buried beneath the rift fill.

New Mexico↗
USGS1987

Precambrian geology of the U.S.A.

Although exposures of Precambrian rocks in the USA are widely scattered, recent studies have provided much information about their subsurface distribution and tectonic history. This article reviews the geological development of the Archean craton, orogenic belts and cratonic cover of Proterozoic age, the Midcontinent rift system, and Precambrian rocks caught up in Phanerozoic terranes. -Author

Episodes↗
USGS1972

Oil development and conservation in Arctic America

As in his earlier article to which reference is made, the author stresses the need for more background information and a much greater research effort before problems of environmental protection from oil developments in northern Alaska can be effectively tackled. Meanwhile, there are indications that the earlier estimates of around ten thousand million barrels should be raised—perhaps to about five times as much. There has moreover been a great increate also in the known and probable gas reserves in northern Canada—particularly in the Arctic Archipelago. In spite of planned automation, people will be needed for development—including Eskimos and Indians, whose interests will be widely protected under the Alaska Native Claims Settlement Act of December 1971. The International Biological Programme ‘Tundra Biome’ projects have accumulated much new information, including the results to be expected from disturbing the tundra seriously, and means of preventing or repairing damage to it. There is much interest in Alaska and Canada in protection of the northern environment, and in the former the US National Environmental Policy Act is already leading to stricter control of some developments.

Biological Conservation↗