Biotite Dates Research Articles

40Ar/ 39Ar incremental-release ages of biotite from a progressively remetamorphosed Archean basement terrane in southwestern Labrador

Gneisses within an Archean basement terrane adjacent to the southwestern portion of the Labrador Trough were variably retrograded during a regional metamorphism of Grenville age (ca. 1000 Ma). Biotites from non-retrograded segments of the gneiss terrane record 40Ar/ 39Ar plateau and isochron ages which date times of cooling following an episode of the Kenoran orogeny (2376–2391 Ma). A suite of gneiss samples displaying varying degrees of retrograde alteration was collected across the Grenville metamorphic gradient. Biotites in these samples show no petrographic evidence of retrograde alteration, however they do record internally discordant 40Ar/ 39Ar age spectra. Although the extent of internal discordance is variable, the overall character of the release patterns is similar with younger apparent ages recorded in intermediate-temperature gas fractions. The total-gas dates range from 2257±27 Ma (northwest) to 1751±23 Ma (southeast), suggesting that variable quantities of radiogenic argon were lost from the Archean biotites during Grenville metamorphism. The “saddle-shaped” nature of the discordant spectra indicates that argon loss was not accomplished through single-stage, volume diffusion processes. Biotites in portions of the gneiss terrane which were completely recrystallized during Grenville metamorphism are petrographically and texturally distinct. A representative of this phase records a 40Ar/ 39Ar plateau age of 2674±28 Ma. This date is markedly inconsistent with regional constraints on the timing of Grenville metamorphism, and indicates the presence of extraneous argon components. Both the extraneous and radiogenic argon components must have been liberated in constant proportions during experimental heating because the argon isotopic data yield a well-defined 40Ar/ 36Ar vs. 39Ar/ 36Ar isochron corresponding to an age (2658±23 Ma) similar to that defined by the plateau portion of the spectrum. The 40Ar/ 39Ar biotite dates suggest that the effects of Grenville metamorphism extent 15–20 km northward into the Superior Province. The limit of this overprint is approximately coincident with the northernmost development of Grenville age thrust faults in the Archean terrane. Therefore, it is proposed that the northern margin of the Grenville Province in southwestern Labrador should be located along the northernmost Grenville thrust fault because this represents both a structural and a thermal discontinuity.

Preliminary investigations of U-Th-Pb systematics in uranium-bearing minerals from two granitic rocks from the Granite Mountains, Wyoming

Analyses of mineral separates from granites from the Granite Mountains, Wyoming, suggest that most of the thorium and at least half of the uranium are associated with biotite, epidote, and accessory minerals. Biotite and epidote contain most of the uranium and thorium in a drill core sample of granite that did not incur any appreciable amount of uranium loss during Tertiary time. These two minerals also form the dominant sites for thorium in a surface sample of granite that incurred a large uranium loss during Tertiary time but only account for 35 percent of the whole-rock uranium in this sample. Concentration and isotopic data for biotite and epidote from the surface sample suggest that these minerals lost enough uranium in Tertiary time to account for the uranium loss by the whole-rock system. None of the mineral ages, calculated from the uranium-lead, lead-lead, and thorium-lead systems, seem to be geologically significant.

Rubidium-strontium geochronology of a transect of the Chilean Andes between latitudes 45° and 46° S

Plutons of quartz diorite to adamellite composition give Rb-Sr total-rock and biotite calculated ages of 12 to 176 m.y. along a transect of the Chilean Andes between 45° and 46° S. Miocene biotite dates of quartz diorite from the Puerto Aisen area are interpreted as minimum ages related to probable local structural deformation (faulting). Along a north-south traverse of the Argentine Andes between 40° and 44° S, plutons of similar composition to the Chilean Andes give K Ar and Rb Sr total-rock and mineral dates which range from late Paleozoic to Cretaceous. No systematic unidirectional migration of the radiometric ages is observed.

Potassium–argon ages of intrusion and scheelite mineralization, Cantung, Tungsten, Northwest Territories

K–Ar dating of magmatic biotite, and of hydrothermal biotite and muscovite, demonstrates that quartz monzonite intrusion and exoskarn scheelite mineralization at Cantung, N.W.T., took place over a brief interval in the Upper Cretaceous (ca. 91 Ma). The regional age relationships of magmatic and ore-forming activity in the Logan–Mackenzie Mountains are poorly defined, but it is tentatively inferred that tungsten mineralization may have been related to a late stage in the plutonic development of the area.

Pre‐ to syn‐tectonic emplacement of early Palaeozoic granites in southeastern South Australia

Abstract Stratigraphic and structural observations indicate that the Encounter Bay Granites concordantly intruded the youngest formations of the Kanmantoo Group in the Mount Lofty Ranges metamorphic belt prior to the culmination of the first phase of folding and associated schistosity development recorded during the early Palaeozoic Delamerian Orogeny. Metamorphic textures in the metasediments of the Kanmantoo Group suggest that cordierite crystallized locally near the granites prior to and during the F 1 folding, whereas andalusite crystallized on a regional scale during the F 1 folding and in the post‐F 1 and pre‐F 2 static phase. Rb‐Sr isotope data for total‐rock, feldspar, and muscovite samples of the meta‐sediment‐contaminated border facies and the uncontaminated inner facies of the Encounter Bay Granites indicate that the granites were emplaced between 515 ± 8 m.y. and 506 ± 6 m.y. ago in the Late Cambrian epoch. Rb‐Sr and K‐Ar data for biotite from the granites record variable radiogenic Sr loss un...

Mid-tertiary igneous activity in east Greenland — The Kialineq complex

The igneous rocks of the Kialineq area on the East Greenland coast at 67 ° N comprise, gabbro, acid-basic breccia and net-vein complexes, syenite and granite and minor volcanics. Rb-Sr whole-rock and mineral isochrons for the epizonal syenites, granites and acid-basic complexes indicate a consanguineous mantle origin at 35±2 m.y. B.P. in agreement with the age obtained by K-Ar dating of biotites and hornblendes. This igneous activity may be associated with major reorganisation of plate geometry in the North Atlantic and Arctic oceans at this time.

$$^{40}Ar/^{39}Ar$$ Age Spectra of Minerals from the Fleur de Lys Terrane in Northwest Newfoundland: Their Bearing on Chronology of Metamorphism within the Appalachian Orthotectonic Zone

$$^{40}Ar/^{39}Ar$$ age spectra have been determined for hornblende, muscovite, and biotite from the Fleur de Lys metamorphic terrane. Age spectra of all samples are undisturbed, and indicate that total-gas ages date times of post-metamorphic cooling. Within the western division of the Fleur de Lys total-gas ages are variable (hornblende 420-386 m.y., muscovite 411-390 m.y., and biotite 386-368 m.y.), but for each mineral phase show a consistent geographic distribution (older ages recorded in the southwest). At each locality examined, hornblende and muscovite have similar total-gas ages which average ~20 m.y. older than biotite dates. Although distinctly younger dates are recorded in the eastern division of the Fleur de Lys (hornblende 364 m.y., biotite 346-342 m.y.), a similar age discordance is observed. The regional distribution of total-gas ages suggests diachronous southwest-northeast post-metamorphic cooling. This diachronisim may be a result of earlier and, in total, more extensive vertical displacement of the southwestern Fleur de Lys terrane. Relatively rapid post-metamorphic uplift is indicated by the limited discordancy of hornblende and biotite total-gas dates at each locality. When combined with regional stratigraphic relations, the data suggest that in the south and southwest, uppermost structural levels of the Fleur de Lys terrane may have been exhumed by ~460-470 m.y. However, the present erosional level diachronously cooled and was exhumed later. The data do not support previous suggestions of Acadian metamorphism within the Fleur de Lys terrane.

Mineral age patterns in ca. 3700 my old rocks from West Greenland

K Ar, 40Ar 39Ar and Rb Sr dates are reported for minerals from the ca. 3700 my-old Amîtsoq and Isua gneisses of the Godthaabsfjord area of West Greenland. K Ar dates on biotites and hornblendes range from about 1900 to 3500 my, with hornblendes having a much narrower range (ca. 2250–2750 my) than biotites. One biotite from Isua gives an impossibly high K Ar date of 4940 my. 40Ar 39Ar mineral dates are in close agreement with conventional K Ar dates over the entire range of apparent age values. The presence of minor amounts of excess argon is observed in the hornblendes, but radiogenic and excess argon in the biotites are completely homogenised and cannot be differentiated. Rb-Sr measurements on biotites are closely concordant and show that all biotites were completely open to diffusion of radiogenic 87Sr at about 1600–1700 my. This is the first proof of a regional thermal event at this time in the Archaean of West Greenland, although similar dates are well known from the Proterozoic belts to the north and south. The evidence suggests that those K Ar biotite dates greater than about 2700–2800 my result from excess radiogenic argon incorporated during a thermal event of about this age or, more probably, during the 1600–1700 my Sr isotope homogenisation event. Scatter of mineral dates below about 2700 my could also be due, at least in part, to overprinting by the 1600–1700 my event. K Ar mineral dates and an Rb-Sr mineral isochron from a pegmatite associated with the last major rock-forming event in the Godthaabsfjord area, namely the Qoˆrqut granite, indicate an age of emplacement of 2580 ± 30 my.

Rb-Sr dating and microclinization in dioritic rocks of the Haparanda group, northern Sweden

Abstract Recent Rb-Sr data on the Haparanda plutonites, Northern Sweden, are discussed in relation to field evidence of post-intrusion microclinization. It is concluded that the plutonites did not remain closed systems after emplacement and that the whole-rock data represent mixed ages, whereas the biotite data may record the time of microclinization. The minimum age of primary crystallization is 2.25–0.10 • 109 y and the maximum time since the resetting of biotite is 1.85 + 0.035 • 109 y. The microcline data are anomalous. Routine statistical treatment of such Rb-Sr data does not yield the ages of primary intrusion. Criteria for the recognition of rejuvenation in the Rb-Sr data are discussed, and geological comparisons with other areas in Sweden and Finland are made.

Potassium-Argon Geochronometry of Mesozoic Igneous Rocks in Nevada, Utah, and Southern California

Research Article| April 01, 1973 Potassium-Argon Geochronometry of Mesozoic Igneous Rocks in Nevada, Utah, and Southern California RICHARD LEE ARMSTRONG; RICHARD LEE ARMSTRONG 1Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 Search for other works by this author on: GSW Google Scholar JOHN SUPPE JOHN SUPPE 2Department of Geology, University of California, Los Angeles, California 90024 Search for other works by this author on: GSW Google Scholar Author and Article Information RICHARD LEE ARMSTRONG 1Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 JOHN SUPPE 2Department of Geology, University of California, Los Angeles, California 90024 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (4): 1375–1392. https://doi.org/10.1130/0016-7606(1973)84<1375:PGOMIR>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation RICHARD LEE ARMSTRONG, JOHN SUPPE; Potassium-Argon Geochronometry of Mesozoic Igneous Rocks in Nevada, Utah, and Southern California. GSA Bulletin 1973;; 84 (4): 1375–1392. doi: https://doi.org/10.1130/0016-7606(1973)84<1375:PGOMIR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract K-Ar dates for 123 mineral separates from 91 samples of granitic rocks, and 3 samples of Orocopia and Vitrefax Schist from Nevada, western Utah, the Mojave Desert, the Colorado Desert, and the northern and eastern Peninsular Ranges of southern California, range in age from 218 to 19 m.y.; however, most of the dates lie between 160 and 50 m.y. A majority of the hornblende and muscovite dates are nearly concordant with, but generally a few m.y. older than, coexisting biotite dates; some are highly discordant.The oldest plutons gave Late Triassic to Early Jurassic dates (El Paso and San Bernardino Mountains, 218 to 194 m.y.). Jurassic igneous activity reached a culmination slightly before 150 m.y. ago over a large area in northern Nevada and western Utah and supplied ash to the Morrison Formation of the Colorado Plateau. Several plutons of the northeastern Mojave Desert yielded similar dates.A similar major culmination of Mesozoic pluton emplacement in early and middle Late Cretaceous time is represented by scattered plutons in Nevada and probably by most of the plutons in the Peninsular Ranges and southern deserts of California. Earliest Tertiary dates were obtained from far-southeastern California (50 to 66 m.y.).Most dates in the Cordilleran batholith belt of the Southwest, which parallels the edge of the late Mesozoic continent, are between 160 and 75 m.y. and are essentially coeval with metamorphism in the parallel high-pressure, low-temperature Franciscan belt to the west (approximately 150 to 75 m.y.). About 75 m.y. ago, this pattern abruptly changed to a Laramide one with abundant 75 to 50 m.y. dates in a belt running through southern Arizona into southeastern California at a high angle to the edge of the continent. Only very sparse dates of this age are found in Nevada. Abundant Laramide dates are not encountered again until the Idaho, Boulder, and Coast Range batholiths. These important changes in Cordilleran tectonics may be effects of a change from dominantly dip-slip subduction during Franciscan time to dominantly north-northeast strike-slip subduction during Laramide time. Furthermore, the initiation of rifting of eastern North America in Triassic time, changes in spreading rate, and reorientation of spreading directions in the Atlantic Ocean appear to correlate with the beginning of widespread pluton emplacement in western North America, changes in the rate of magma generation in the Cordillera, and shifts of the Cordilleran tectonic pattern, respectively, during Mesozoic time. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Variance of Precambrian K-Ar biotite dates, Bighorn Mountains, Wyoming

Biotite K-Ar dates for rocks from throughout the Bighorn Mountains are systematically related to rock unit and degree of alteration of the biotite. Fresh biotite from granitic rocks in the northern half of the range averages 2.75 ( σ = 0.05) by, biotite from gneiss, which forms the southern half of the range, averages 2.50 ( σ = 0.07) by, regardless of degree of alteration, and chloritized biotite from the northern granites averages ( 2.53 σ = 0.08) by. K-Ar systematics in Precambrian rocks may be obscured by uncritical grouping of samples and between-lab and within-lab variances that are greater than the best precision attainable in a single laboratory.

Paleomagnetism of the Triassic Guichon Batholith and Rotation in the Interior Plateau, British Columbia

The Guichon Batholith, located near the south end of the Interior Plateau in south–central British Columbia, is composed of unmetamorphosed massive felsic intrusive rocks in several distinct phases (Northcote 1969). Stratigraphic and radiometric evidence indicate that the batholith was emplaced during the Late Triassic (198 ± 8 m.y.) and unroofed by Early Jurassic. Analysis of the remanence of 92 cores (184 specimens) from 19 representative sites led to the isolation of a stable primary remanent magnetism at 15 sites after alternating-field demagnetization. Variance ratio analysis of the remanence directions indicates that the phases cannot be distinguished by the paleomagnetic method. This supports the evidence from contact relationships and K–Ar isotopic dating of biotites that the phases cooled nearly contemporaneously. The pole position determined for the Guichon Batholith (12.9° E, 65.6° N) is discordant with other Upper Triassic pole positions determined for North American formations. The discordance may be explained by a clockwise rotation 40° ± 10° of the batholith and surrounding rocks in the southern end of the Interior Plateau, with most of the Plateau to the north acting as a stable non-rotated tectonic block. Other evidence is cited which is consistent with this hypothesis.

Potassium‐argon dates from the Arltunga Nappe complex, northern territory

Abstract Twenty‐four mineral separates from the Arunta Complex, four from the metamorphosed Heavitree Quartzite (White Range Quartzite), and one whole rock sample of metamorphosed Bitter Springs Formation, all from the western part of the White Range Nappe of the Arltunga Nappe Complex, and two samples from the autochthonous basement west of the nappe have been dated by the K‐Ar method. The samples from the basement rocks form two groups. Those in the southern or frontal part of the nappe are of Middle Proterozoic (Carpentarian) age (1660–1368 m.y.), determined on hornblende, biotite, and muscovite. In the northern or rear part of the nappe, all but one of the muscovite samples and two biotites are of Middle Silurian to Early Carboniferous age (431–345 m.y.); the remainder of the biotite dates range from 1775 to 548 m.y. (including the two samples from the autochthon), and two hornblendes gave dates of 1639 and 2132 m.y. respectively. All the muscovite samples from the Heavitree Quartzite, and the whole r...

The Isotopic Ages of the Oak Point and Stonington Granites, Eastern Penobscot Bay, Maine

Previously reported K-Ar dates on biotites from the Oak Point rapakivi granite range from 343 m.y. to 354 m.y., and a K-feldspar has been dated by the same method at 264 m.y. Previous biotite determinations from the Stonington rapakivi granite yield dates of 350 and 360 m.y. It is presumed that the K-feldspar date is low because of normal diffusion of radiogenic , and that the biotites date the time of formation of the granites. However, field evidence (Stewart 1956) indicates that the Stonington granite is younger than the Oak Point granite; but the K-Ar dates do not allow distinction between them. Further, there is one granite from the surrounding area (Wallamatogus granite) where a biotite date is significantly younger than a muscovite date from the same sample. To test the reliability of the previous K-Ar dates and the field relationships, Rb-Sr whole-rock analyses have been determined for both granites with the following results: Oak Point granite: . with initial ; Stonington granite:. with initial . Although the analytical uncertainties overlap, the data suggest that the Stonington granite is slightly younger than the Oak Point granite and, on the basis of their initial ratios, that both were derived from the same parent magma. Further, the reliability of the K-Ar dates on the biotites is confirmed.

Interpretation of Precambrian K-Ar biotite dates in the bighorn mountains, wyoming

The K-Ar age distribution pattern for biotite in the Precambrian core of the Bighorn Range suggests that contours of equal K-Ar mineral age in small geographic areas may represent fossil contours of equal argon retention rather than fossil thermochrons.

Fission-Track Ages of Accessory Minerals from Granitic Rocks of the Central Sierra Nevada Batholith, California

Ages of apatite, sphene, allanite, epidote, and garnet from plutonic rocks of the central Sierra Nevada and Inyo Mountains have been determined by the fission-track method. Ages of 44 specimens of apatite range from 54 to 128 m.y. Oldest apatites generally occur in rocks from the western portion of the batholith; youngest are from granitic rocks along the eastern slope of the Sierra Nevada. Thirty-four specimens of sphene have been dated and range in age from 71 to 118 m.y. Within experimental error, all sphenes are as old as, or older than, coexisting apatites. The oldest sphene is from granitic rock of the Inyo Mountains; however, sampled rocks from the western Sierra Nevada do not contain sphene. The youngest sphenes are generally from rocks slightly west of the Sierran crest. Fission-track ages of two allanites are 86 and 87 ± 9 m.y. Two epidotes yielded ages of 84 ± 8 m.y. and a third is 140 ± 14 m.y. Age of a garnet was determined to be 114 ± 11 m.y. Several of the apatites and sphenes used for fission-track dating are from the same samples that previous workers had used for potassium-argon dating of biotites and hornblendes. Hornblende K-Ar ages appear to be the most insensitive to heat, apatite fission-track ages, the least, and biotite K-Ar and sphene fission-track ages fall between the two extremes and show the closest concordance of all pairs.

Field evidence of very old Precambrian rocks in the Godthåb area, West Greenland

The Godthåb area, situated in the old central gneiss complex of West Greenland, has yielded two of the oldest radiometric dates yet obtained from Greenland. They are both K/Ar biotite dates, one of 2710 ± 130 m.y. on granodiorite gneiss from Godthåb (Armstrong, 1963), and the other of 2610± 50 m.y. on gneiss collected 55 km to the north and on the opposite side of Godthåbsfjord (Larsen and Møller, this report). The latter gneiss shows evidence of having undergone retrograde metamorphism from granulite facies to amphibolite facies, and its date is presumed to be related to this retrogression (Windley, in press).

Effects of weathering on the Rb [sbnd]Sr and K [sbnd]Ar ages of biotite from the Morton Gneiss, Minnesota

Weathering has drastically reduced the Rb Sr and, to a lesser extent, the K Ar age of biotite from the Morton Gneiss of southwestern Minnesota. The ages are approximately 75% and 25% lower than the corresponding ages for biotite from the fresh gneiss. The effects of even incipient weathering cannot be neglected in Rb Sr dating of biotite and, by analogy, of feldspar and whole-rock samples.

A GEOCHRONOMETRIC STUDY OF THE SUPERIOR PROVINCE NEAR RED LAKE NORTHWESTERN ONTARIO

Nine samples of granites and gneisses collected along a 125 mi traverse running south from Red Lake have been analyzed in a Rb–Sr whole-rock study. It is concluded that an isochron may meaningfully be drawn through points representing these sample on a Nicolaysen (1961) plot. A slope equivalent to an age of 2.45 ± 0.10 b.y. results. If the gneisses represent the residue of an ancient crust then the Rb–Sr data yield little evidence of this and the effect of the Kenoran orogeny is clearly the dominant feature. K–Ar and Rb–Sr dating of biotites separated from two of the gneisses indicates that the area has been more mildly affected by a later metamorphism.

Isotopic Age Determinations on Upper Devonian Rocks from Victoria, Australia: A Revised Estimate for the Age of the Devonian-Carboniferous Boundary

A thick sequence of felsic volcanic rocks in the Cerberean Cauldron of Victoria, Australia, is Upper Devonian as evidenced from a fossil fish fauna found in intercalated sedimentary rocks (Hills, 1958). Radiometric dating of the lavas was undertaken to provide further control as to the age of the Devonian-Carboniferous boundary. Potassium-argon dates on biotites from a nevadite and a biotite rhyodacite range from 340 to 365 million years; from this data, we estimate a minimum value of 362 ± 6 m.y. for their true age. Rubidium-strontium dates on several of these biotites are systematically lower at 352–357 m.y. (λRb 87 = 1.47 × 10 −11 yr −1 ), and this difference seems best resolved by decreasing the value selected for λRb 87 to 1.43 × 10 −11 yr −1 . The Rb-Sr isochron for the basal rhyolite in the Cerberean Cauldron, based on whole-rock and feldspar samples, is significantly older at 367 ± 22 m.y. than the Rb–Sr biotite dates and older than an (apparent) isochron for the nevadite. Its relationship to the K–Ar date of 362 m.y. is obscured by the uncertainty in the Rb 87 decay constant. We conclude that either the extrusion of the lavas occurred over an unexpectedly long period or that loss of radiogenic strontium has occurred from the biotites and that the nevadite Rb–Sr data do not correctly define an isochron. In either case, these results show that the radiometric age for the top of the Devonian should be increased by 5 per cent from 345 ± 10 m.y. (Kulp, 1961) to 362 ± 6 m.y. and possibly by more than 10 per cent, depending on the correct value for the decay constant for Rb 87 . Rb–Sr data for the Barjarg Granite and the Ryans Creek Rhyolite from the nearby Mansfield-Tatong area yield an age of 369 ± 11 m.y., using λRb 87 = 1.47 × 10 −11 yr −1 . Although these rocks are thought to be the same age as the lavas of the Cerberean Cauldron, their stratigraphic position within the Devonian is not conclusively established.