Total Gas Ages Research Articles

Hercynian/Alleghanian overprinting of an acadian terrane: 40Ar/ 39Ar studies in the Meguma zone, Nova Scotia, Canada

40Ar/ 39Ar age spectrum data are presented on 34 micas, 3 hornblendes and 6 (whole rock) slate/siltstones, all from the metamorphic rocks of the Meguma zone, Nova Scotia, one of the “suspect” terranes of the Appalachian collage. Most of the age spectra are discordant, probably due to the combined effects of primarily two tectonothermal events - the intrusion of the large South Mountain batholith (385-360 Ma ago), and secondly, a major transcurrent or transpressive reactivation of suture zones in the Appalachian-Hercynian belt ( ∼ 320-300 Ma ago). Apparent ages in the northern part of the study area, along with existing geophysical data, suggest that these rocks were disturbed primarily by the earlier event. In the southern region, the age data suggest that the later event was the more prominent, an event apparently directly recorded by post-deformational vein muscovites and dynamically recrystallized micas and associated with mineralization in the terrane. The majority of the biotite spectra throughout the terrane exhibit low-age “saddle” structures. The size of these features inversely correlates with apparent K20 as determined by microprobe scans across apparently unaltered grains. We suggest that the existence of saddles in overprinted biotites is due to the presence of submicroscopic lamellae of some low-K phase, probably chlorite, the latter a product of retrograde metamorphism. Furthermore, we suggest that saddle formation results from irradiation-induced recoil of 39Ar into these other phases and the resulting internal redistribution of Ar isotopes. In spite of the pervasive thermal overprinting, some of the mineral data, in conjunction with the slate/siltstone total gas ages, constrain the timing of regional metamorphism in this terrane to the interval 405-390 Ma.

40Ar/39Ar dating of cleavage formation in tuffs during anchizonal metamorphism

40Ar/39Ar incremental-release analyses were carried out on whole-rock and constituent white mica (illite)-rich size fractions (0.63–1 to 6.3–20 μm) within two very-low grade, penetratively cleaved metatuffs of contrasting anchizonal metamorphic grade (northeastern Rheinisches Schiefergebirge, Federal Republic of Germany). One sample from the upper anchizone displays internally concordant 40Ar/39Ar spectra with plateau ages ranging between ca. 316 and 325 Ma. These are similar to conventional K-Ar ages determined for the whole-rock and size fractions. Together the isotopic results suggest that cleavage formed at ca. 320 Ma during a concomitant very-low grade metamorphism. This is consistent with biostratigraphic controls which suggest that metamorphism and cleavage formation occurred during the Westphalian. A metatuff sample from the middle anchizone records more internally discordant 40Ar/39Ar age spectra with total-gas ages ranging from 366 to 372 Ma. These are ca. 35–45 Ma older than corresponding conventional K-Ar ages, indicating marked recoil-loss of 39Ar occurred during irradiation. Transmission electron microscopy reveals that white mica grains within size fractions from the upper anchizone sample have clearly defined, straight edges whereas those within the middle anchizone samples are embayed and diffuse. This results in an increase in surface/volume ratio and therefore greater susceptibility for recoil-loss of 39Ar in the middle anchizone sample. Grain-edge morphology appears to be a major factor in determining the extent of recoil-loss of 39Ar during 40Ar/39Ar analysis of fine-grained size fractions.

Significance of [formula omitted] age spectra of whole-rock and constituent grain-size fractions from anchizonal slates

Constituent grain-size fractions and whole-rock samples of “middle” and “upper” anchizone slates from the north-eastern Rheinisches Schiefergebirge (Federal Republic of Germany) were analyzed with 40 Ar 39 Ar incremental heating techniques. Comparison of the total-gas ages and previously determined KAr ages indicates variable recoil-loss of 39Ar, reflecting differences in grain size and/or grain-edge morphology. Grains with poorly defined edges characterize the middle anchizone size fractions. They have relatively large surface/volume ratios and are more strongly affected by recoil-loss of 39Ar than grains in the upper anchizone size fractions which have clearer, more well-defined edges. The strongly discordant age and K Ca spectra reflect differences in diffusive characteristics of constituent mineralogical phases, including: relatively non-retentive chlorite, white mica, and relatively refractory albite. White mica records anomalously old apparent ages as a result of recoil-loss of 39Ar. Gas fractions evolved from chlorite and albite record anomalously young ages because of recoil-gain of 39Ar. These effects are more prominent in the middle anchizone size fractions.

Timing of Acadian deformation in northeastern Maine

40Ar/39Ar ages determined for 24 mineral separates from the Pleasant Lake, Hunt Ridge, and Cochrane Lake plutons bear on the timing of the main phase of the Acadian orogeny in northern Maine. Six hornblende samples and one clinopyroxene sample have only minor disturbances in their release spectra. The increments with concordant ages form overlapping plateaus at 403 ± 4 Ma. Many of the biotite samples have discordant release spectra. The average of their total gas ages is 400 ± 6 Ma. Calculated closure temperatures for the hornblendes and biotites are 550 °C and approximately 350 °C, respectively. The difference between closure temperatures and concordancy in ages demonstrates that the plutons cooled very rapidly upon emplacement. Therefore, the 40Ar/39Ar ages date both the time of argon retention in the minerals and the time of emplacement of the plutons. The best estimate for their time of emplacement is the average of the combined ages of the micas, hornblendes, and clinopyroxene, 402 ± 6 Ma. The plutons are undeformed and crosscut upright, open, Acadian folds in the Carys Mills, Smyrna Mills, and Madrid(?) Formations. The age of the Pleasant Lake, Hunt Ridge, and Cochrane Lake plutons therefore places a lower limit of 402 ± 6 Ma on the time of Acadian folding in northern Maine.

Dating of deformation phases using K-Ar and 40Ar/ 39Ar techniques: results from the northern apennines

Paleozoic to Oligocene metasedimentary rocks present in the Alpi Apuane region of the Northern Apennines, Italy, have been sequentially deformed during a Tertiary progressive deformation. In an attempt to date the individual deformation episodes, over 50 conventional K-Ar and 11 40 Ar/ 39Ar incremental gas release analyses have been carried out on fine grained white micas separated from samples whose structural settings were well known. Mineralogy, X-ray diffractometry, and thin-section analyses indicate that the constituent muscovite and phengite formed under metamorphic conditions of 3–4 kbars and 300–400°C during all deformational phases. Pre-existing micas were variably crenulated during each subsequent deformational phase. Both K-Ar and 40Ar/ 39Ar analyses were carried out on 0.6-2μm, 2–6 μm and 6–20 μm size separates of the phengitic white mica. Although the K-Ar apparent ages range from 11 to 27 Ma and are consistent with available stratigraphic constraints, the 40Ar/ 39Ar age spectra display variable internal discordancy. These isotopic data indicate that: (1) both the K-Ar and 40Ar/ 39Ar total-gas ages decrease as the degree of crenulation increases; (2) the K-Ar and 40Ar/ 39Ar total-gas ages decrease as grain size decreases; (3) for each sample, characteristics of the 40Ar/ 39Ar age spectra depend upon grain size, with fine sizes yielding discordant patterns which systematically increase in apparent age from low to high temperature and (4) phengitic micas associated with earliest structures yield generally older ages than micas associated with later structures. The isotopic results are interpreted to indicate that the major deformation phase ( D 1) occurred at approximately 27 Ma with subsequent pulses ending by c. 10 Ma. These results may be combined with finite strain data to suggest that the region was deformed at strain rates between 10 −15 and 10 −14 s −1. A 27 Ma age indicates Mid-Oligocene initiation of plate tectonic activity in the Western Mediterranean and concomitant deformation in the Northern Apennines.

Recognition of extraneous argon components through incremental-release [formula omitted] analysis of biotite and hornblende across the Grenvillian metamorphic gradient in southwestern Labrador

40Ar 39Ar incremental-release ages have been determined for 15 hornblende and 20 biotite concentrates separated from rocks collected across the garnet and kyanite zones of Grenvillian metamorphism in southwestern Labrador. Most hornblende spectra from the kyanite zone are slightly discordant, with low-temperature increments yielding ages older than the ca 1000 Ma date suggested for culmination of Grenvillian metamorphism in the area. However, all the hornblende concentrates record well-defined plateau ages. These range from 968 to 905 Ma across the kyanite zone and date times of diachronous post-metamorphic cooling. The discordant spectra are interpreted to result from low-temperature liberation of excess 40Ar components from grain margins. Two hornblende concentrates from the garnet zone display very discordant spectra (total-gas ages of 2100 and 3017 Ma) in which incremental dates systematically decrease during analysis. This pattern of discordance suggests that excess argon components are inhomogeneously distributed throughout these hornblende grains. Most biotites from the garnet and kyanite zones record total-gas or plateau ages in excess of 1000 Ma (2066-857 Ma), reflecting the widespread presence of excess argon components. Because most of the 40Ar 39Ar age spectra are internally concordant, the ratios of excess 40Ar relative to radiogenic 40Ar must have been uniform in the various gas fractions liberated from each sample. This is also reflected in the inability of isotope correlation diagrams to differentiate between excess, radiogenic, and atmospheric argon components. The biotite total-gas or plateau dates show marked local variation. This is interpreted to indicate that the biotite grains were in contact with a post-metamorphic intergranular vapor phase that was characterized by large and variable 40Ar 36Ar ratios. Such ratios most likely resulted from widespread diffusion of the argon liberated from adjacent Archean basement gneisses during the Grenvillian metamorphic overprint.

40Ar/39Ar ages from the Botwood – Mount Peyton region, Newfoundland: possible paleomagnetic implications

Hornblende and biotite from the freshest available sample of the gabbroic (early) phase of the Mount Peyton batholith have both yielded 40Ar/39Ar apparent age plateaus. An age of 420 ± 8 Ma is suggested for this phase. This means that there is a resolvable age difference between the mafic and the acidic rocks of the batholith (Rb/Sr age of the latter is 390 ± 15 Ma). Biotite and hornblende from a gabbroic inclusion and biotite from the latter's granitic matrix have all yielded very disturbed age spectra and variable total gas ages. Previously published paleomagnetic data suggest that these inclusions were remagnetized at the time of granite intrusion. The present radiometric data give some support to this interpretation, but do not clearly indicate the timing of any such event. A whole-rock sample from the (Lower Silurian?) Botwood Group rhyolite has also yielded a disturbed age spectrum; its total gas age is 385 ± 13 Ma. The Botwood pole is distinct from the granite pole, so either remagnetization did not occur in this case or it occurred at a later time, a time not clearly revealed in the present radiometric data.On the basis of previously published magnetic data and the now improved age control on the Mount Peyton complex, it appears that the 400 Ma paleopole for Newfoundland may differ substantially from other North American poles.

40Ar/39Ar age spectra for biotite and hornblende from plutonic rocks in the Victorville region, California

40Ar/39Ar data from plutonic rocks in the Mojave Desert near Victorville, California, provide preliminary insight into the complex thermal history of this region and suggest that some plutons here may be significantly older than any plutons in the Sierra Nevada to the north. Near Victorville, multiple deformation and metamorphism of platform-miogeoclinal strata is inferred to postdate deposition of Permian rocks and predate intrusion of monzonite plutons. Monzonite that intrudes multiply deformed Paleozoic marble and that is unconformably overlain by the Mesozoic (Triassic or Jurassic) Fairview Valley Formation has hornblende with an age spectrum typical of severe 40Ar loss. We interpret the age of the highest temperature increment (fusion), 233 ± 14 m.y., as a minimum age for this pluton. The total gas age for the hornblende is only 169.9 m.y. Biotite from the same rock exhibits a plateau age of 76.59 ± 1.24 m.y. The Fairview Valley Formation is overlain by volcanic rocks of the Sidewinder sequence. These rocks were folded and intruded by quartz monzonite that postdates all penetrative deformation in the region. Biotite-hornblende tonalite at Quartzite Mountain is part of one of these younger quartz monzonite plutons, and its biotite yields a plateau age of 73.16 ± 1.11 m.y. This age is the same at the 95% confidence level as the age of biotite from the older monzonite; thus, this younger thermal event was at least the partial cause for the argon loss pattern displayed by the monzonite hornblende. Hornblende gabbro that intrudes Paleozoic marble at Sidewinder Mountain and is in turn contact metamorphosed by later plutonic rocks has hornblende with a disturbed 40Ar/39Ar age spectrum. The 1025, 1050, and 1075 °C steps contain 44.64% of the total argon and yield a common age of 119.2 ± 3.4 m.y., which we interpret as the best estimate for the age of the hornblende.

40Ar/39Ar dating of dikes from the Frontenac Axis and implications for Grenville paleomagnetism

Stepwise outgassing experiments have been carried out on nine whole rock samples of material from these diabase dikes. None of the samples is entirely free of alteration and all of the age spectra exhibit at least some degree of discordance. Over the intermediate temperature interval (775–1025 °C, arbitrarily defined), apparent age plateaus (or plateau-like features) appear to be the rule. The mean apparent age of gas released at these temperatures for six of the samples is ~900–910 Ma; the average apparent Ca/K ratio is ~3. These samples have a mean total gas age of ~910 Ma and total gas apparent Ca/K ratios ranging from ~5 to ~7. The above value is interpreted as a probable minimum age for K-rich phases (biotite and sericite).The dikes were intruded into the host gneisses probably while they were still above the temperature at which argon is quantitatively retained in biotites. Although the magnetic blocking temperatures of the dikes are not known, the Frontenac paleopole is probably in the vicinity of ~900 Ma old. This value is consistent with other recent estimates of the age of this middle portion of the Grenville apparent polar wander track.

40Ar/39Ar incremental-release ages of hornblende and biotite from Grenville basement rocks within the Indian Head Range complex, southwest Newfoundland: their bearing on Late Proterozoic – Early Paleozoic thermal history

Hornblende and biotite from autochthonous basement rocks within the Indian Head Range complex of southwest, insular Newfoundland record undisturbed 40Ar/39Ar release spectra with average total-gas ages of 880 Ma (hornblende) and 825 Ma (biotite). These gas-retention ages date the times when this segment of the western Appalachian basement terrane cooled below hornblende and biotite argon retention temperatures (~500 °C and ~300 °C respectively) following culmination of the ~ 1150–1100 Ma Grenville metamorphism. Although these results indicate that elevated temperatures were maintained for a prolonged period following the Grenville thermal peak, once initiated, cooling must have been relatively rapid because hornblende and biotite record generally similar total-gas dates.The undisturbed release spectra of minerals within the Indian Head Range complex indicate that this segment of the western Appalachian basement terrane was not affected by Paleozoic metamorphism. This is consistent with recent tectonic models that indicate that the overlying Humber Arm allochthon was emplaced into its present position as a cold, already assembled structural unit. Lack of Paleozoic metamorphism within the Indian Head basement rocks is also compatible with suggestions that the obduction site of the Bay of Islands ophiolite lay considerably east of the Early Paleozoic continental margin of North America.

$$^{40}Ar/^{39}Ar$$ 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.

Timing of Kenoran Metamorphism in the Eastern Abitibi Greenstone Belt, Quebec: Evidence From 40Ar/39Ar Ages of Hornblende and Biotite From Post-Kinematic Plutons

The Opemisca and La Ronde Lake plutons are typical of post-Kenoran felsic plutons which intrude the Archean greenstone terrane of the eastern Abitibi orogen. Hornblendes from the plutons record undisturbed 40Ar/39Ar release spectra with total-gas ages of 2692 ± 30 and 2656 ± 30 m.y. (Opemisca) and 2650 ± 30 and 2623 ± 30 m.y. (La Ronde Lake). Two additional K–Ar ages of 2653 ± 64 and 2647 ± 61 m.y. have been determined for hornblendes from the Opemisca intrusive. Release spectra of biotite from the La Ronde Lake pluton are also undisturbed but define discordantly younger total-gas ages of 2544 ± 30 and 2559 ± 30 m.y. These are similar to a 2505 ± 30 m.y. 40Ar/39Ar total-gas age for biotite (undisturbed spectra) from the pre-Kenoran Opawica River pluton located immediately south of La Ronde Lake.It is suggested that the hornblende dates are close to the actual time of intrusion of both plutons. This range in age is similar to that of other post-kinematic plutons in the eastern Abitibi belt and suggests that Kenoran metamorphism must have occurred prior to ~2650–2700 m.y. Available age determinations for pre-kinematic plutons within the area are slightly older (~2780–2820 m.y.), and a bracket on the time of Kenoran metamorphism between 2650–2700 m.y. and 2780–2820 m.y. is defined.The 40Ar/39Ar biotite ages are similar to published K–Ar biotite ages (~2500–2550 m.y.) in the eastern Abitibi belt and are interpreted as dating the time during post-Kenoran metamorphic cooling when temperatures dropped below those required for argon retention in biotite.

Incremental 40Ar/39Ar ages of biotite and hornblende from the northeastern Reading Prong: Their bearing on late Proterozoic thermal and tectonic history

K-Ar and 40 Ar- 39 Ar incremental-release ages have been determined for biotite and hornblende from Grenville basement gneiss units of the northeastern Reading Prong west of the Hudson River. Biotite ages range from 768 ± 15 to 819 ± 18 m.y. (average of 790 m.y.), and those for unaltered hornblende range from 869 ± 20 to 949 ± 24 m.y. (average of 900 m.y.). Individual biotite and hornblende release spectra show negligible variations in 40 Ar/ 39 Ar ratios, and total-gas and incremental-release ages are similar. These ages are discordantly younger than zircon ages of about 1,060 m.y. from this area; however, the consistent correlation of K-Ar, total-gas, and incremental-release ages argues against the discordancy resulting from partial argon loss during a distinct post-Grenville thermal event. The correlation suggests that the ages date times during post-Grenville metamorphic cooling when temperatures dropped below those required for argon retention in the minerals. The ages place constraints on post-metamorphic cooling history and allow development of a model for the late Proterozoic uplift of this segment of the Appalachian Grenville terrane.

40Ar/39Ar age spectra of biotite from Grenville basement gneisses in northwest Georgia

Biotite from two samples of Grenville gneisses exposed near Cartersville, Georgia (Corbin and Salem Church units), record undisturbed 40 Ar/ 39 Ar release spectra with total-gas ages of 735 and 702 m.y. (both ± 15 m.y.). The ages are discordant with >1 b.y. Rb-Sr whole-rock and U-Pb zircon ages from the same units. It is suggested that the biotite ages date the time of cooling below temperatures required for argon retention following Grenville metamorphism. Release spectra of biotite from samples along the southeastern border of the gneissic terrain are variably disturbed, reflecting the overprinting of a Paleozoic metamorphic event. The regional distribution of disturbed release spectra permits a more definitive evaluation of the extent of Paleozoic thermal effects in this area. The data also help to resolve late Precambrian–early Paleozoic stratigraphic relations.

40Ar/39Ar Incremental Release Ages of Biotite and Hornblende from Pre-Kenoran Gneisses between the Matagami-Chibougamau and Frotet-Troilus Greenstone Belts, Quebec

Biotite and hornblende from high-grade, granitic gneisses exposed between the Matagami-Chibougamau and Frotet-Troilus greenstone belts in Quebec have been affected by Kenoran metamorphism. Biotites record total gas 40Ar/39Ar ages of 2308 ± 30 m.y. and 2338 ± 30 m.y. Incrementally released gas fractions yield similar plateau ages, suggesting that the biotites have been totally degassed as a result of the thermal event. The ages are interpreted as reflecting the time of post-metamorphic cooling when radiogenic 40Ar began to be retained within biotite. Hornblendes record total gas 40Ar/39Ar ages of 2517 ± 40 m.y. and 2610 ± 40 m.y. Incrementally released gas fractions show a wide deviation from the total gas ages, with a continuous increase in age from low to high temperature release fractions. This lack of correlation suggests that the hornblendes have been only partially degassed by Kenoran metamorphism. However, lack of a high-temperature release plateau indicates that original meramorphic crystallization was older than the ages recorded by the highest temperature release fractions (2599 ± 40 and 2801 ± 40 m.y.). Recognition of an older sialic terrain between these greenstone belts supports recent models proposed for the tectonic evolution of the supracrustal orogenic belts in the Superior Province.

Precision in manufacture improves lamp quality

Due to their minute size, 40Ar/39Ar analysis of illite faces significant analytical challenges, including mineral characterization and, especially, effects of grain size and crystallography on 39Ar recoil. Quantifying the effects of 39Ar recoil requires the use of sample vacuum encapsulation during irradiation, which permits the measurement of the fraction of recoiled 39Ar as well as the 39Ar and 40Ar∗ retained within illite crystals that are released during step heating. Total-Gas Ages (TGA) are calculated by using both recoiled and retained argon, which is functionally equivalent to K–Ar ages, while Retention Ages (RA) only involve retained Ar in the crystal. Natural applications have shown that TGA fits stratigraphic constraints of geological processes when the average illite crystallite thickness (ICT) is smaller than 10 nm, and that RA matches these constraints for ICTs larger than 50 nm. We propose a new age correction method that takes into account the average ICT and corresponding recoiled 39Ar for a sample, with X-ray Corrected Ages (XCA) lying between Total-Gas and Retention Ages depending on ICT. This correction is particularly useful in samples containing authigenic illite formed in the anchizone, with typical ICT values between 10 and 50 nm.In three samples containing authigenic illite from Cretaceous carbonates in the Monterrey Salient in northern Mexico, there is a range in TGAs among the different size-fractions of 46–49, 36–43 and 40–52 Ma, while RAs range from 54–64, 47–52 and 53–54 Ma, respectively. XCA calculations produce tighter age ranges for these samples of 52.5–56, 45.5–48.5 and 49–52.5 Ma, respectively. In an apparent age vs ICT or %2M 1illite plot, authigenic illite grains show a slope that is in general slightly positive for TGA, slightly negative for RA, but close to zero for XCA, with thinner crystallites showing more dispersion than thicker ones. In order to test if dispersion is due to a different formation history or the result of retention capability, degassing spectra were modeled for site XCA averages and overall XCA average. Modeling shows that local site age average best match the measured spectra, instead of a global average age, indicating that illite growth reflects local deformation, and is not the result of regional metamorphism. Modeling also shows that Ar-degassing spectra are very sensitive to grain size, such that age interpretation based on Ar-plateaus is meaningless for most fine-grained clays.