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S. S. Harlan

Publications and source records attributed to S. S. Harlan.

23 records · Page 2Linked to original sources
USGS1996

Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Mountains, southwestern Montana: A paleomagnetic and 40Ar/39Ar study

Paleomagnetic results from Early Proterozoic metabasite sills and Middle Proterozoic diabase dikes from the southern Highland Mountains of southwestern Montana give well-defined, dual-polarity magnetizations that are statistically identical to those from a small Late Cretaceous pluton that cuts the dikes. The concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the Proterozoic rocks were remagnetized, probably during Late Cretaceous time. Paleomagnetic, rock magnetic, and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low-Ti magnetite. Combining virtual geomagnetic poles from metabasite sills, diabase dikes, and the Late Cretaceous pluton, we obtain a paleomagnetic pole at 85.5°N, 310.7°E ( K = 19.9, A 95 = 9.1°, N = 14 sites) that is similar to a reference pole from the 74 Ma Adel Mountain Volcanics of western Montana. Biotite and hornblende 40 Ar/ 39 Ar isotopic dates from host basement gneiss and a hornblende from a remagnetized metabasite sill yield ages of ca. 1800 Ma; these dates probably record cooling of the southern Highland Mountains following high-grade metamorphism at 1.9–1.8 Ga. The gneiss and metabasite age spectra show virtually no evidence of disturbance, indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems. Thus, the overprint magnetization of the Highland Mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby Late Cretaceous plutons. Remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of Late Cretaceous hydrothermal fluids related to epithermal mineralization. The absence of significant disturbance to the 40 Ar/ 39 Ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may go unrecognized and undated, even if 40 Ar/ 39 Ar dating is applied.

Montana↗
USGS1996

The Eocene Big Timber stock, south-central Montana: Development of extensive compositional variation in an arc-related intrusion by side-wall crystallization and cumulate glomerocryst remixing

The Eocene Big Timber stock in the Crazy Mountains of south-central Montana is an elliptical, 8 by 13 km, compositionally and texturally diverse composite intrusion with a well-developed radial dike swarm. A sharp intrusive contact separates its two phases: the core of the intrusion is fine-grained quartz monzodiorite, and the volumetrically dominant remainder is composed of medium-grained diorite and gabbro. Differentiation-related major oxide variation within the stock is extensive and spatially nonsystematic. However, abundances of most trace elements were not strongly influenced by differentiation; late zircon and apatite fractionation caused moderate heavy and slight light rare earth element abundance depletions, respectively. Mineral compositions and assemblages indicate crystallization between ≈950 and 700 °C at a pressure of ≈0.8 kbar (3 km). Mixing models indicate that fractionation of varying amounts of plagioclase, orthopyroxene, clinopyroxene, magnesio-hastingsite, hornblende, biotite, titanite, apatite, and magnetite (the stock's principal constituents, with quartz and potassium feldspar) and remixing of these minerals and residual liquids controlled compositional evolution in the reservoir. Crystals apparently nucleated at the reservoir wall while residual silicate liquid was displaced inward and remixed. Some crystals were plucked from the solidification front, as indicated by glomerocrysts present throughout the stock, and also remixed with residual liquid. Solidification of the reservoir represented by the stock involved heat loss to enclosing wall rock, side-wall crystallization, and subsequent, variably effective, crystal-liquid remixing. This process is an important variant of conventionally invoked models pertaining to solidification of intrusions and explains extensive, relatively nonsystematic compositional variation. The genesis of compositional evolution in other intrusions characterized by extensive, spatially nonsystematic variation may result from the important process documented herein. Compositional and geologic relationships are consistent with magma genesis related to subduction and magmatic-arc processes inboard from the western edge of the early Cenozoic North American plate. Arc magmatism in south-central Montana during Eocene time is consistent with models pertaining to early Cenozoic southward sweep and westward retreat of magmatism. Magmatism represented by the Big Timber stock provides significant new support for steepening subduction, westward retreat of the subduction hinge line, and development of an asthenospheric mantle wedge that fueled renewed magmatism beneath the western edge of the North American continent following early Cenozoic shallow subduction.

Geological Society of America Bulletin↗
USGS1996

40Ar/39Ar geochronology and paleomagnetism of Independence volcano, Absaroka volcanic supergroup, Beartooth mountains, Montana

Independence volcano is a major volcanic complex in the lower part of the Absaroka Volcanic Supergroup (AVS) of Montana and Wyoming. Recently reported Rb–Sr mineral dates from the complex give apparent ages of 91 and 84 Ma, whereas field relationships and the physical and compositional similarity of the rocks with other dated parts of the AVS indicate an Early to Middle Eocene age for eruption and deposition. To resolve the conflict between age assignments based on stratigraphic correlations and Rb–Sr dates, we report new paleomagnetic data and 40 Ar/ 39 Ar dates for Independence volcano. Paleomagnetic data for the stock and an andesite plug that cuts the stock are well grouped, of reverse polarity, and yield a virtual geomagnetic pole that is essentially identical to Late Cretaceous and Tertiary reference poles. The reverse polarity indicates that the magnetization of these rocks is probably younger than the Cretaceous normal superchron, or less than about 83.5 Ma. Hornblende from a volcanic breccia near the base of the volcanic pile gives a 40 Ar/ 39 Ar age of 51.57 Ma, whereas biotites from a dacite sill and a granodiorite stock that forms the core of the volcano give dates that range from 49.96 to 48.50 Ma. These dates record the age of eruption and intrusion of these rocks and clearly show that the age of Independence volcano is Early to Middle Eocene, consistent with stratigraphic relations. We suggest that the Rb–Sr mineral dates from the Independence stock and related intrusions are unreliable.

Montana, Wyoming↗
USGS1990

Brunhes chron excursion/polarity episode recorded during the late pleistocene, Albuquerque Volcanoes, New Mexico, USA

All basaltic lava flows of the Albuquerque Volcanoes (lat.: 35.2°N, long.: 253.2°E), Albuquerque-Belen Basin, New Mexico, record a short excursion/polarity episode. K-Ar isotopic age determinations (weighted average: 155 ± 47 ka) and evaluation of soil profiles on flow surfaces suggest the late Pleistocene ( circa between 250 and 80 ka) as the time of extrusion. Results from 63 sites in a minimum of eight flows yield a mean direction of D = 101.1°, I = -36.1°, α 95 = 1.2° (α 1 95 = 0.7, α 2 95 = 1.2), k = 2219 ( N = 8 flows) and a corresponding virtual geomagnetic pole position (VGP) of 354.1°E, 20.2°S, dp = 0.8°, dm = 1.4° ( A 1 95 = 0.5, A 2 95 = 1.4), and VGP angular standard deviation (ASD) = 1.8°. The unusual magnetization in flows of the Albuquerque Volcanoes is carried by fine-grained [single-domain (SD) and/or pseudo-single-domain (PSD)] low-Ti magnetite and is apparently not a function of complex sub-solidus alteration of magnetic phases. Underlying baked soils contain a relatively low coercivity magnetization with directions comprising two groups. One is roughly antipodal to the lava remanence; the other intermediate between the lava remanence and present field direction. Because all flows yield statistically indistinguishable directions, a field directional path clearly cannot be defined. At the precision level of our age determinations, the Albuquerque feature may correlate with other short polarity episodes or excursions of late Pleistocene age (e.g., Blake or Jamaica). Though characteristically limited in directional morphology, well-dated polarity episodes (i.e. sub-chrons) and excursions recorded in volcanic rocks may provide information on the frequency of significant dynamo instabilities.

New Mexico↗