Ar Isotopic Ages Research Articles

A New Calibration Point for the Late Cretaceous Time Scale: The 40Ar/39Ar Isotopic Age of the C33r/C33n Geomagnetic Reversal from the Judith River Formation (Upper Creataceous), Elk Basin, Wyoming, USA

We present a new calibration point for the latest Cretaceous time scale, an interval that at present contains no well-dated polarity reversals. The magnetostratigraphy of Campanian-aged sediments in the northern Bighorn basin of Wyoming documents a geomagnetic reversal in the upper part of the Judith River Formation that occurs in direct association with ash fall layers dated by the method. Extrapolation based on three new laser fusion dates from sanidines extracted from the ash fall layers dates the reversal at 79.34 Ma; thus the geomagnetic reversal in the Judith River formation can be confidently correlated to the C33r/C33n boundary of the Geomagnetic Reversal Time Scale. This age estimate is in good agreement with recent time scales that date this polarity interval by interpolation between few and widely spaced calibration points. The Judith River Formation is nonmarine and cannot be correlated regionally by traditional ammonite biostratigraphic methods. This study shows that in northern Wyoming the formation was deposited from 80 to 79 Ma, and can be correlated both magnetostratigraphically and by isotopic age to the Campanian-aged ammonite zones of the marine record that range from the top of Baculites obtusus to the lower part of B. perplexus (early form). The Judith River is underlain by the Claggett Shale, which contains one of the ash fall levels of the Ardmore bentonite. This ash fall is an important datum level found throughout the Western Interior U.S. in the range zone of B. obtusus, one we have dated at .

Paleozoic deformation and isotopic disturbance in the southeastern Arunta Block, central Australia

Proterozoic metamorphic rocks of the Arunta Block, central Australia, were thrusted southward over the northern margin of the Neoproterozoic to mid-Paleozoic Amadeus Basin during the 400-300 Ma Alice Springs Orogeny. The extent and nature of mid-Paleozoic ductile deformation in the southeastern Arunta Block is examined in light of available UPb, 40Ar 39 Ar , KAr, RbSr and SmNd isotopic age data. Most of the Proterozoic amphibolite and granulite facies rocks north of the zone of basement-cover interaction were exhumed and cooled from above ∼ 500 to below ∼ 125°C as a result of mid-Paleozoic nappe formation. This suggests that deformation early in the cooling history occurred at amphibolite facies conditions and progressed to greenschist facies conditions later in the cooling history. In general, shear zone rocks yield the youngest isotopic ages, indicating that deformation-induced recrystallization, grain size reduction, or localized thermal perturbations are responsible for the relatively young ages. We conclude that shear zones of mid-Paleozoic age were active under greenschist and amphibolite facies conditions, though further work is needed to differentiate whether amphibolite facies shear zones are Paleozoic or Proterozoic. Some Proterozoic shear zones were uplifted passively in the mid-Paleozoic, resulting in final closure and mixed ages in key minerals like hornblende.

The chronology of Cenozoic volcanism and deformation in the Yerington area, western Basin and Range and Walker Lane

High-precision 40 Ar/ 39 Ar isotopic ages obtained from Cenozoic volcanic rocks and subvolcanic intrusions document the age of initiation and the temporal evolution of extensional and strike-slip faulting in the western Basin and Range Province. In the northern Wassuk Range, faulting began between ca. 26 and 24.7 Ma; both normal and strike-slip faults are bracketed between 23.1 and 22.2 Ma, and between 15 and 14 Ma. These ages document inception of the Ancestral Walker Lane, a northwest-trending zone of right-transtensional faulting in western Nevada that separated extending crust on the east from the unextended Sierra Nevada block on the west at lat 39°N. We speculate that the southwesterly migrating, episodic Oligocene–early Miocene, east-west extensional faulting in the Basin and Range thinned and weakened the crust, allowing right-slip faults to develop in the Walker Lane in response to San Andreas right-shear in California. Southwest of the Walker Lane there was no faulting prior to 15 Ma. Here, in the Yerington district, andesitic magmatism began at ca. 15 Ma and was followed by >150% east-west extension along closely spaced (1–2 km) normal faults with up to 4 km of offset each (Proffett, 1977). These faults tilted older Cenozoic rocks 35°–40°W. Our new 40 Ar/ 39 Ar ages substantially revise earlier K-Ar ages of the timing of extension and establish that andesite lava flows cut by normal faults are 13.8–15 Ma, and that these faults are intruded by 12.6–13.0 Ma dacites. Rapid extension is thus bracketed to a 0.7–1.7 m.y. interval at 95% confidence, indicating local, east-west strain rates of 2–4 × 10 −14 /s (5–10 mm/yr). Following this period, lower rates of extension prevailed near Yerington along more widely spaced normal oblique-slip faults that localized clastic sedimentation of the Wassuk Group between 11 and 8 Ma. These faults and sedimentary rocks are more abundant southwest of Yerington in a belt parallel to the Walker Lane in previously little-extended crust. From 7 Ma to present, normal right-oblique slip faults with a lower rate of extension than the previous two periods produced the modern ranges near Yerington and extend 100 km southwest of the Walker Lane, which continues to be the locus of strike-slip faulting. Thus, since 15 Ma the margin of the Basin and Range has moved progressively 100 km west creating the broad Walker Lane belt and lower strain rates near Yerington.

Preliminary results of a palaeomagnetic and rock magnetic study of the Proterozoic Tsuomasvarri intrusions, northern Fennoscandia

Preliminary palaeomagnetic results are presented for the Proterozoic Tsuomasvarri central ultramafic intrusion and surrounding, older gabbro-diorite intrusion in the northeastern Fennoscandian shield. In the ultramafic intrusion, five remanence components, reflecting different geological processes, were isolated in the alternating field (AF) and thermal demagnetization of 55 samples from ten sites. A reversed polarity remanence component C (pole: Plat=26°N, Plon=245°E, dp=5°, dm=9°, n =32 samples) dominates in the ultramafic intrusion. The normal polarity pole A (Plat=44°N, Plon=232°E, dp=4°, dm=6°, n =9 samples) corresponds to an age of ∼ 1.88 Ga on the Apparent Polar Wander Path (APWP), and is probably a thermochemical remanence acquired during the Svecokarelian orogeny or in the late stages of magma cooling. Pole B (Plat=35°N, Plon=158°E, dp=9°, dm=15°, n =5 samples, normal polarity) corresponds to the ∼ 1.75 Ga old post-orogenic activity in the area, and is also seen in results of KAr (whole rock) isotopic age determinations from the vicinity. The normal polarity pole D (pole: Plat=25°S, Plon=285°E, dp=12°, dm=19°, n =11 samples) has a palaeomagnetic age of ∼ 2.44 Ga which is in contrast with the isotopic age determination of 1.93 Ga (UPb age on zircon) of the gabbro-diorite intrusion. Pole F (Plat=3°N, Plon=212°E, dp=10°, dm=18°, n =5 samples) indicates an age of 0.5-0.4 Ga on the Palaeozoic APWP segment of Fennoscandia and probably reflects a weak Caledonian overprint in northeastern Finland. In the gabbro-diorite intrusion, two remanence components were isolated in 46 samples from eight sites. The palaeomagnetic age, ∼ 1.93 Ga, of the dominant normal polarity component, A′ (pole: Plat=40°N, Plon=247°E, dp=5°, dm=8°, n =28 samples) is consistent with the isotopic age of 1.93 Ga (UPb determination on zircon). The normal polarity component D (pole: Plat=20°S, Plon=285°E, dp=14°, dm=23°, n =5 samples), which was also isolated in the ultramafic intrusion, contradicts the UPb isotopic age determination.

Episodic caldera volcanism in the Miocene southwestern Nevada volcanic field: Revised stratigraphic framework, 40Ar/39Ar geochronology, and implications for magmatism and extension

The middle Miocene southwestern Nevada volcanic field (SWNVF) is a classic example of a silicic multicaldera volcanic field in the Great Basin. More than six major calderas formed between >15 and 7.5 Ma. The central SWNVF caldera cluster consists of the overlapping Silent Canyon caldera complex, the Claim Canyon caldera, and the Timber Mountain caldera complex, active from 14 to 11.5 Ma and centered on topographic Timber Mountain. Locations of calderas older than the Claim Canyon caldera source of the Tiva Canyon Tuff are uncertain except where verified by drilling. Younger peralkaline calderas (Black Mountain and Stonewall Mountain) formed northwest of the central SWNVF caldera cluster. We summarize major revisions of the SWNVF stratigraphy that provide for correlation of lava flows and small-volume tuffs with the widespread outflow sheets of the SWNVF. New laser fusion 40 Ar/ 39 Ar isotopic ages are used to refine and revise the timing of eruptive activity in the SWNVF. The use of high-sensitivity mass spectrometry allowed analysis of submilligram-sized samples with analytical uncertainties of ∼0.3% (1σ), permitting resolution of age differences as small as 0.07 Ma. These results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the SWNVF. Major caldera episodes (Belted Range, Crater Flat, Paintbrush, Timber Mountain, and Thirsty Canyon Groups) erupted widespread ash-flow sheets within 100-300 k.y. time spans, and pre- and post-caldera lavas erupted within 100-300 k.y. of the associated ash flows. Peak volcanism in the SWNVF occurred during eruption of the Paintbrush and Timber Mountain Groups, when over 4500 km 3 of metaluminous magma was erupted in two episodes within 1.35 m.y., separated by a 750 k.y. magmatic gap. Peralkaline and metaluminous magmatism in the SWNVF overlapped in time and space. The peralkaline Tub Spring and Grouse Canyon Tuffs erupted early, and the peralkaline Thirsty Canyon Group tuffs and Stonewall Flat Tuff erupted late in the history of the SWNVF, flanking the central, volumetrically dominant peak of metaluminous volcanism. Magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data, particularly in the overlapping Grouse Canyon and Area 20 calderas of the Silent Canyon caldera complex. Volcanism in the SWNVF coincided with the Miocene peak of extensional deformation in adjoining parts of the Great Basin. Although regional extension was concurrent with volcanism, it was at a minimum in the central area of the SWNVF, where synvolcanic faulting was dominated by intra-caldera deformation. Significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the SWNVF between the Paint-brush and Timber Mountain caldera episodes. Larger magnitude detachment faulting in the Bullfrog Hills, southwest of the central SWNVF caldera cluster, followed the climactic Timber Mountain caldera episode. Postvolcanic normal faulting was substantial to the north, east, and south of the central SWNVF caldera cluster, but the central area of peak volcanic activity remained relatively unextended in postvolcanic time. Volcanism and extension in the SWNVF area were broadly concurrent, but SWNVF area were broadly concurrent, but in detail they were episodic in time and not coincident in space

Brittle shear deformation in northern Kordofan, Sudan: late carboniferous to triassic reactivation of precambrian fault systems

Analysis of folding and faulting in the basement of the Umm Badr and Sodiri areas of Northern Kordofan, Sudan, revealed the existence of four major deformational episodes. A late Proterozoic deformation (D1) produced isoclinal folds in high-grade migmatitic-gneissic basement and in the metasedimentary belt of Umm Badr. A phase of intense NNE—SSW ductile shearing (D2) in the Umm Badr belt refolded the earlier structures in the metasediments and is of late Pan-African age. Dextral ENE—WSW brittle shearing (D3) of latest Pan-African age also affected the metasedimentary belt of Umm Badr.A late Carboniferous to Triassic brittle shearing (D4) reactivation event in the Umm Badr and Jebel Nehud areas (dextral?) and in the Sodiri area (sinistral) extensively reactivates structures of the previous episodes of deformation, exploiting part of the anisotropic zones of weakness. Age limits for brittle shear deformation in the Umm Badr and Sodiri areas were established from KAr isotopic ages of igneous rocks with varying field relationships to the shearing.

The Anvil plutonic suite, Faro, Yukon Territory

The Anvil plutonic suite consists of three phases: a peraluminous muscovite–biotite granite (Mount Mye phase) and two metaluminous to peraluminous hornblende–biotite granodiorite and minor granite intrusions (Orchay and Marjorie phases). The suite is massive or foliated, equigranular or seriate, and contains alkali-feldspar megacrysts. The Marjorie phase is characteristically porphyritic.Geochemical trends are irregular for the suite and for individual phases. High-K2O, low-CaO, and low-MgO compositions typify the silicic, calc-alkaline suite. Hornblende-bearing phases contain less SiO2, K2O, and Rb and more cafemic oxides, TiO2, Sr, Ba, and Y than the Mount Mye phase and are compositionally similar to coeval South Fork volcanics.Isochrons from some of the Orchay phase whole-rock samples (t = 99 ± 2.5 Ma; 87Sr/86Sri = 0.7161 ± 0.0001) and from whole rocks and minerals of the Mount Mye phase (t = 100 ± 2 Ma; 87Sr/86Sri = 0.7405 ± 0.0001) indicate they are coeval but not comagmatic, accounting for the lithologic, petrographic, and geochemical distinctions. Similar K–Ar isotopic ages (81–102 Ma) suggest rapid cooling and therefore high-level emplacement. Together, the isotopic ages provide a minimum (youngest) age for the main deformation of the surrounding metasediments and a maximum (oldest) age for movement along the Tintina Fault.A petrographically and geochemically distinct sample from the Orchay phase yielded a Rb–Sr isochron age of 61 ± 1.5 Ma and an initial 87Sr/86Sr ratio of 0.7090 ± 0.0001, implying intrusive activity in the Paleocene.Field relations, lithology, petrography, geochronometry, and geochemistry suggest that the Orchay and Marjorie phases are plutonic equivalents of the South Fork volcanics. Similarities in plutonic style characterize the extensive mid-Cretaceous igneous event in southeast Yukon.

Age of volcanism and rifting in southwestern Ethiopia

It has been suggested that volcanism in the Ethiopian region of the Afro–Arabian Rift System has migrated with time, both laterally towards the present axial zone1–3 and longitudinally southwards from the Red Sea4,5. Field data and K–Ar isotopic ages from southwestern Ethiopia, summarised below, indicate that volcanism in this area began earlier than previously suspected, and that Quaternary volcanism was not restricted to the axial zone. That major rifting in this region did not happen until the late Miocene6,7 is corroborated.

K—Ar isotopic age determinations from some Lake District mineral localities

SummaryForty-seven new K—Ar analyses are reported from 15 mines in the Lake District. Analyses of clay mineral concentrates from vein gouges and altered wallrock support a hypothesis of at least 3 distinct isotopic events. Caledonian ages (388 Ma) are reported from the Coniston Copper deposits, while the extensive mineralization in and around the Vale of Newlands indicates a metasomatic event occurring at around 360 Ma. The ages of the sporadic mineral localities to the E of the area (i.e. Greenside, Threlkeld, etc.) are grouped around 325 Ma while those of the Caldbeck Fell deposits are close to 230 Ma. An epidosic model may be advanced for the Carrock Fell Tungsten deposits with a range from 197 to 282 Ma. The geochronological significance of these and other previously published results is considered.