Extraneous Argon Research Articles

Chronology of late Paleozoic tectonothermal activity in the southeastern Bohemian Massif, Austria (Moldanubian and Moravo-Silesian zones): 40Ar/ 39Ar mineral age controls

New 40Ar/ 39Ar hornblende and muscovite ages from the Moldanubian and Moravian zones of the southeastern Bohemian Massif indicate rapid cooling associated with continental underplating during Early Carboniferous plate collision and closure of the Moravo-Silesian foredeep. 40Ar/ 39Ar ages have been determined for hornblende concentrates from: (1) amphibolite from the Moldanubian Nappe Complex (including the Dobra gneiss, Rehberg amphibolite and Loosdorf complex: all part of the “Variegated unit”); (2) micaschist structurally overlying the Bittesch gneiss; and (3) amphibolite within the Bittesch gneiss (Moravian Zone). The five hornblende concentrates display variably discordant 40Ar/ 39Ar age spectra because of contamination with extraneous argon components. However, 36Ar/ 40Ar vs. 39Ar/ 40Ar isotope correlations are well defined for the five hornblende concentrates ( MSWD < 1.5). Using the inverse abscissa intercepts ( 40Ar/ 39Ar ratio) in the age equation yields dates which range between 328.7 ± 3.3 and 341.1 ± 1.4 Ma. There are no detectable differences in age between the Moldanubian and Moravian samples. Two muscovite concentrates have been analyzed from schist collected within the Moravian allochthon (Bittesch and Weitersfeld gneiss units). These display internally concordant 40Ar/ 39Ar apparent age spectra which define plateau ages of 328.7 ± 0.8 and 325.5 ± 0.7 Ma. The hornblende and muscovite ages are interpreted to date post-Variscan metamorphic cooling through ca. 500°C and ca. 400°C, respectively. No record of pre-Variscan thermal events is preserved in either the hornblende or muscovite argon systems; however, the widespread occurrence of extraneous argon components suggest a pre-Variscan stage of tectonothermal development. The near concordancy of the hornblende and muscovite cooling ages suggest relatively rapid cooling through the contrasting argon closure temperatures, and indicate that relatively high crustal levels had likely been attained during the Early Carboniferous.

40Ar/ 39Ar mineral dates related to Devonian extension in the southwestern Scandinavian Caledonides

Middle Devonian sedimentary basins exposed in west-central Norway formed during post-Caledonian extension. They developed along west-dipping, low-angle, normal faults which effected ductile shearing of pre-Caledonian basement units of the Western Gneiss Region (WGR). Kinematic indicators suggest that the west-vergent ductile deformation overprinted older, east-vergent Caledonian fabric elements. Ductile extensional deformation led to development of a 2-km-thick shear zone beneath the Devonian basins. Five amphibole and nine muscovite concentrates from rocks within the shear zone and from unaffected gneisses of the Western Gneiss Region have been analyzed with 40Ar/ 39Ar incremental-release dating techniques in an attempt to calibrate the timing of extensional events. Well-defined plateau ages are recorded by muscovite concentrates within the shear zone. Amphiboles display disturbed plateau ages clearly related to extraneous argon. Outside the shear zone, muscovite records Caledonian ages. Results suggest that a late tectonometamorphic event affected the rocks within the shear zone. This event, which not exceed ca. 500°C, completely rejuvenated muscovite argon systems and perturbed argon distribution within amphiboles. This metamorphism is correlated with the extensional deformation which effected exhumation of the Caledonian high-pressure rocks and controlled formation of Devonian basins.

$$^{40}Ar/^{39}Ar$$ Mineral Age Constraints on the Paleozoic Tectonothermal Evolution of High-Grade Basement Rocks within the Ross Orogen, Central Transantarctic Mountains

The Precambrian to early Paleozoic orogenic history of the central Transantarctic Mountains is obscured by uncertainties in. the regional extent, relative intensities, and tectonic significance of three or more events: the Nimrod, Beardmore, and Ross orogenies. Significant problems exist in correlation of structural styles, structural geometry, and lithotectonic units across major tectonic boundaries, as well as in conflicting geochronological results. In an effort to resolve these disparities, ages were determined for hornblende and muscovite from several lithotectonic subunits of the Nimrod Group exposed in the Miller Range, including amphibolitic gneiss, ca. 1.7 Ga orthogneiss, and metasedimentary mica schist. These rocks display ductile deformation fabrics that formed prior to intrusion of ca. 500 Ma, post-kinematic granitic plutons. Hornblende concentrates from eight gneiss samples in the Miller Range display internally discordant apparent age spectra due to variable intracrystalline contamination with extraneous argon components (total-gas ages range between 510 and 738 Ma). vs. isotope correlations are generally well defined (MSWD < 2.0) and yield ages between ca. 525 and 487 Ma. Two muscovite concentrates from mica schist samples in the Miller Range record plateau ages of 508 and 486 Ma, which are interpreted to date cooling through temperatures required for intracrystalline retention of argon. The data indicate that conventional K-Ar dates (up to 1100 Ma) previously reported for hornblende from the Miller Range probably have no geologic significance because of the widespread presence of extraneous argon. They cannot, therefore, be regarded as dating any "pre-Ross" event(s). Intracrystalline argon systems in samples from this study appear to have been completely rejuvenated during granite emplacement associated with the Ross orogeny. Structural fabric elements were partially annealed in many dated Nimrod Group samples, likely reflecting contact effects imposed by emplacement of post-tectonic granite plutons at ca. 500 Ma.

Chronology of eclogite retrogression within the Seve Nappe Complex, Råvvejaure, Sweden: Evidence from40Ar/39Ar mineral ages

Amphibole concentrates from four samples of variably retrogressed eclogite from the Tsakkok Lens of the Seve Nappe Complex display discordant40Ar/39Ar age spectra suggestive of extraneous argon contamination.36Ar/40Ar vs.39Ar/40Ar isotope correlations are well-defined for two samples and yield ages of 464.7 ± 1.3 Ma and 463.2 + 6.3 Ma. Inverse ordinate intercepts are very much larger than 295 and confirm significant extraneous argon components. Two white mica concentrates from metasedimentary units of the Tsakkok Lens display internally concordant40Ar/39Ar age spectra which define plateau ages of 448.2 ± 1.6 Ma and 468.4 ± 0.9 Ma. An amphibole concentrate from a metamorphosed basic dike within the Sarek Lens of the Seve Nappe Complex yields an isotope correlation age of 463.8 ± 12.3 Ma, suggesting a post-metamorphic thermal evolution similar to that of the Tsakkok Lens. The amphibolite and white mica ages are interpreted to date post-metamorphic cooling through appropriate temperatures for intracrystalline retention of argon following amphibolite facies retrogression of the eclogite facies assemblages. The Seve Nappe Complex appears to have originated within distal facies of the late Proterozoic-early Paleozoic miogeocline of Baltica. The present40Ar/39Ar results provide a record of early Paleozoic (pre-Middle Ordovician) tectonothermal activity and therefore are compatible with previous suggestions of significant early Caledonian orogenesis within the Scandinavian Caledonides.

Formula omitted] mineral age record of a polyorogenic evolution within the Seve and Köli nappes, Trøndelag, Norway

In the central Scandinavian Caledonides, eugeoclinal terranes represented in the Köli Nappes were thrust > 500 km onto the Baltoscandian platform during the early to middle Paleozoic. Immediately underlying thrust sheets of the Seve Nappe Complex are suspected to have been derived from the outer margin of continent Baltica. Hornblende within amphibolite at one locality within westernmost exposures of Seve Nappe units in Trandelag, Norway, displays an internally discordant 40Ar 39Ar age spectrum which suggests a polyphase thermal evolution including: 1. (1) initial post-metamorphic cooling through argon retention temperatures at c. 475–480 Ma; 2. (2) partial rejuvenation during a Silurian (c. 400–425 Ma) thermal overprint. Muscovite within the exposure was more extensively rejuvenated during the Silurian overprint. At twelve other Trandelag sample locations, hornblende, muscovite and biotite within both the Seve and Köli Nappe Complexes record post-metamorphic cooling ages between 420 and 435 Ma and display no direct evidence of a pre-Silurian thermal record. However, the widespread occurrence of intracrystalline extraneous argon components and textural complexities clearly point toward a pre-Silurian tectonothermal history. Comparison with 40Ar 39Ar mineral ages reported elsewhere in the central Scandinavian Caledonides suggests a polyorogenic evolution consisting of: 1. (1) variable high-pressure metamorphism of distal Baltoscandian miogeoclinal sequences during entrainment in an accretionary wedge which developed in the Late Cambrian-Early Ordovician over a west-dipping subduction zone; related arc volcanic activity occurred in more outboard, western sectors; 2. (2) ductile imbrication of high- and intermediate-pressure sequences within the accretionary wedge (diachronous cooling and/or chronologically distinct tectonic episodes are indicated throughout the Early and Middle Ordovician); 3. (3) ultimate accretion of a variety of eugeoclinal terranes along the Baltica continental margin; 4. (4) imbrication and metamorphism of the eugeoclinal terranes and previously deformed portions of the miogeocline during eastward translation onto the Baltoscandian platform in the Silurian. Crystalline Baltica basement and overlying structural cover units were depressed to significant crustal depths during eastward nappe transport with resultant formation of high-temperature eclogues. Previously reported crystallization ages of c. 425 Ma suggest that the eclogites formed at approximately the same time as minerals within the superencumbent nappe units cooled through argon closure temperatures. Following eastward nappe transport, the basement experienced rapid uplift and resultant adiabatic decompression. This was followed by rapid cooling through argon retention temperatures at 390–400 Ma.

Effects of excess argon within large diffusion domains on K-feldspar age spectra

Unusual shapes of 40Ar/ 39Ar age spectra have long been attributed to the presence of excess argon. We recognize a distinctive age spectrum type brought about by extraneous argon trapped in large diffusion domains. Spectra of this type are characterized by little or no excess argon in low temperature steps, sensible apparent ages over the first ~40 to 50% 39Ar released, an abrupt increase to ages exceeding the permissible age of the sample, followed by either a gradual decrease in age until fusion or an anomalously old plateau segment. Detailed study of one such orthoclase reveals that the excess argon within these samples is likely located in radiogenic argon sites within large diffusion domains. Argon contained within these features is trapped when the domains close to argon loss at ~400°C. Thus, there exists a genetic difference between large-domain-trapped-argon-effect spectra and those saddle-shaped age spectra which result from excess argon trapped in anion vacancies at temperatures below ~350°C.

40Ar/ 39Ar polyorogenic mineral age record in the northern Mauritanide orogen, West Africa

Northern segments of the Mauritanide orogen in the Akjoujt region are dominated by suprastructural nappe complexes. These structurally overlie other tectonic elements, including: (1) autochthonous/parautochthonous foreland units composed of late Proterozoic-late Paleozoic (Frasnian) sedimentary cover sequences, and unconformably underlying crystalline basement (late Archean-middle Proterozoic) of the West African Shield, (2) parautochthonous successions largely represented by low-grade metasedimentary rocks, and (3) allochthonous infrastructural complexes with characteristics similar to those of late Proterozoic-early Paleozoic (“Pan-African”) tectonic elements exposed in central and southern parts of the Mauritanide orogen. Amphibole from various structural levels of the Ijibiten and Akjoujt suprastructural nappe complexes generally display internally discordant, “saddle-shaped” 40Ar/ 39Ar release spectra suggestive of extraneous (excess) argon contamination. One sample records a well-defined, intermediate-temperature plateau of ~2420 Ma. Muscovite within lower and intermediate structural levels of the complexes displays variably discordant 40Ar/ 39Ar age spectra which suggest that variable thermal rejuvenation of older (up to ~2600 Ma) mineral systems occurred during a ca. 310-300 Ma metamorphism. Uppermost structural levels of the complexes are represented, in part, by penetratively mylonitic metagranite. Muscovite within these rocks displays internally concordant 40Ar/ 39Ar age spectra which define plateau ages of ~300–310 Ma. There is no indication in either amphibole or muscovite results of any late Proterozoic-early Paleozoic (Pan-African) thermal record. Together with field relationships, the 40Ar/ 39Ar results suggest that structural units of the suprastructural nappe complexes are, in part, composed of variably overprinted basement rocks. These structurally overlie allochthonous Pan-African sequences and therefore must have been derived west of and now largely conceal, the Pan-African orogen. The complexes were transported eastward to higher structural levels during late Paleozoic orogenesis. The extent of late Paleozoic thrusting within the Akjoujt region contrasts with more limited transport distances documented southward in the orogen. This contrast probably reflects a kinematic influence of the Reguibat promentory of the West African Shield, which during the late Paleozoic collision of Gondwana and Laurentia effected extensive compression in proximal sequences.

40Ar/39Ar polyorogenic mineral age record within the central Mauritanide orogen, West Africa

The central Mauritanide orogen in the vicinity of Magta Lahjar is characterized by several internally imbricated, polydeformed and variably metamorphosed infrastructural allochthons, which include (1) a calc-alkaline igneous complex (Kelbe intrusive and El Hneikât extrusive sequences); (2) portions of an ophiolitic sequence (El Aoueija and Oued Amour Units); and (3) continental-margin rift sequences (Farkâka Association), which include amphibolite (low-Ti continental tholeiite dike protoliths), mylonitic quartzite, and kyanite schist. Several variably meta-morphosed, quartzitic supracrustal nappes (Gâoua and Sangarafa allochthons) structurally overlie the infrastructural units. 40 Ar/ 39 Ar incremental-release ages have been determined for 10 hornblende and 22 muscovite concentrates prepared from various units of the infrastructural and supra-structural allochthons. Hornblende from Farkâka amphibolite is characterized by internally discordant spectra, reflecting wide-spread contamination with extraneous argon. 36 Ar/ 40 Ar versus 39 Ar/ 40 Ar isotopic correlations are well defined for several concentrates, and the resultant ages suggest that post-metamorphic cooling occurred following distinct Late Proterozoic tectonothermal events at ca. 700-720 Ma and ca. 640-650 Ma. Muscovite within Farkâka lithologies initially cooled through argon closure temperatures between ca. 570 and 595 Ma. Muscovite records similar post-metamorphic cooling ages in the other infrastructural allochthons and in the Gâoua suprastructural allochthon. These muscovite ages are interpreted to reflect cooling associated with widespread post-orogenic uplift and are consistent with deposition of upper Proterozoic-lower Paleozoic flysch in the proximal foreland (Tichilit el Beida Group). The Tichilit el Beida Group and correlative parautochthonous sequences (Djonâba Group) were penetratively deformed prior to the Early Ordovician. 40 Ar/ 39 Ar incremental-release analyses of 6 very fine-grained white mica concentrates from mylonitic quartzite along thrust faults within the parautochthon indicate that this deformation occurred prior to ca. 540-550 Ma. There is no clear thermal record of this orogenic activity in either infrastructural or suprastructural allochthons, suggesting that it must have occurred at relatively high structural levels and was probably largely of intracontinental character. All foreland units were affected by post-Emsian folding. Effects of this tectonic activity are widespread throughout the parautochthon and western metamorphic sequences. These effects include emplacement of the suprastructural allochthons into their present structural positions at ca. 300-325 Ma with an associated partial rejuvenation of muscovite argon systems proximal to basal thrust faults. Local reactivation of older thrust faults within the parautochthon and infrastructural allochthons is also indicated by variable 300-325 Ma rejuvenation of muscovite argon systems within some fault zones. Muscovite within westernmost exposures of the infra-structural allochthons was totally rejuvenated during the early Carboniferous and records ∼300-m.y. plateau ages. The early tectonic history recorded within the central Mauritanide orogen appears to have been related to restricted rifting of a western crustal block from the West African craton and formation of a limited, intervening oceanic domain. Subsequent closure resulted in compression and imbrication of continental crust with resultant development of an ensialic volcanic arc (calc-alkaline sequences within the infrastructural allochthons) together with widespread metamorphism and deformation between ca. 700 and 650 Ma. Later orogenesis at ca. 550 Ma could have reflected distal effects of collisional events which dominate the Rokelide orogen in Sierra Leon and Guinea. Late Paleozoic tectonotherma activity within the central Mauritanides is interpreted to have developed as a result of a renewed translation between the western continental block and the West African craton during final collision of Laurentia arc Gondwana.

Late Paleozoic tectonothermal evolution of the western Piedmont and eastern Blue Ridge, Georgia: Controls on the chronology of terrane accretion and transport in the southern Appalachian orogen

Hornblende within the Helen and Tallulah Falls nappes of the eastern Blue Ridge and portions of the western Piedmont (higher grade sequences of the Chauga- Walhalla complex and eastern portions of the Six-Mile nappe) record 322-333-m.y. 40 Ar/ 39 Ar plateau ages which date post metamorphic cooling through temperatures required for intracrystalline retention of argon. Hornblende within lower-grade portions of the Chauga-Walhalla complex and western portions of the Six-Mile nappe (Alto allochthon) records older plateau ages (322-362 m.y.). None of these release spectra suggests a super-posed metamorphic history, and the plateau ages are interpreted to date regionally diachronous cooling of the allochthon following a Late Devonian or earlier high-grade metamorphism. Muscovite within all structural units of the eastern Blue Ridge and western Piedmont records 302-318-m.y. 4O Ar/ 39 Ar plateau ages which date post meta-morphic cooling through lower closure temperatures (∼350 °C?). Biotite from these structural units yields 312-348-m.y. 40 Ar/ 39 Ar plateau ages. The geologic significance of the biotite ages is uncertain because of variable contamination with extraneous argon. The Alto allochthon appears to have originated within the western Piedmont following a Late Devonian or earlier high-grade metamorphism which may have developed during initial terrane accretion to North America. Diachronous postmetamorphic cooling of the allochthon likely occurred during its movement to higher crustal levels associated with emplacement over lower-grade rocks of the Chauga-Walhalla complex. Emplacement occurred prior to local development of retrogressive mylonitic fabrics related to tectonic activity within the Brevard fault zone and prior to regional cooling through muscovite closure temperatures.

40Ar/39Ar mineral age record of variably superimposed Proterozoic tectonothermal events in the Grenville Orogen, central Labrador

Penetratively cleaved slate–phyllite within supracrustal sequences (Seal Lake and Letitia Lake groups) in northernmost portions of the Grenville parauthochthon in central Labrador record 1018 ± 15 and 985 ± 14 Ma whole-rock 40Ar/39Ar plateau ages, which are interpreted as closely dating deformation and a concomitant low-grade metamorphism. The 40Ar/39Ar signature of Grenvillian tectonothermal effects is variably recorded in more southerly portions of the parautochthon. Locally within the extensive North Pole Brook igneous suite, there is no significant metamorphic retrogression. In these areas, igneous hornblendes record ca. 1650 Ma plateau ages similar to those suggested for emplacement of most of the suite by previously reported Rb–Sr whole-rock isochron and U–Pb zircon analyses. Low-temperature retrogression of igneous assemblages is observed throughout most of the North Pole Brook igneous terrane. Hornblende concentrates from five relatively undeformed samples are characterized by internally discordant 40Ar/39Ar spectra reflecting variable extraneous argon contamination and minor development of retrogressive blue–green amphibole. Biotite spectra are generally more concordant but display a wide range in total-gas and plateau ages (1534–2419 Ma) as a result of variable extraneous argon contamination. The North Pole Brook suite is progressively more deformed southwestward in the Grenville parautochthon, where it is largely represented by blastomylonitic orthogneisses that record hornblende plateau ages of 991–1013 Ma. Where not contaminated with extraneous argon, biotite records ages identical to those of coexisting hornblende, suggesting rapid post-Grenvillian metamorphic cooling. Paragneiss exposed in northeastern portions of the parautochthon displays a heterogeneous Grenvillian tectonothermal record. Muscovite within mylonitic rocks, developed along internal imbrication zones, yields plateau ages of 1046 and 1088 Ma. Hornblendes within amphibolite horizons in the paragneisses show little rejuvenation and record pre-Grenvillian thermal histories (1873 and 1315 Ma plateau dates). Hornblende within statically metamorphosed amphibolite near the structural base of the Wilson Lake allochthon records an early Grenville (1112 Ma) plateau age. Biotite within the allochthon has been previously shown to be regionally contaminated with extraneous argon components. The 40Ar/39Ar results support previous hypotheses that suggest that northwestward emplacement (at ca. 1100 Ma?) of the Wilson Lake and other allochthons was partially responsible for the burial and associated polydeformation and metamorphism of the Grenville parautochthon. The 985–1018 Ma low-grade metamorphism and penetrative deformation recorded in supracrustal sequences in the northernmost parautochthon are markedly younger and must reflect late Grenvillian orogenic activity with different tectonic implications.

K-Ar ages of carbonate- and mantle nodule-bearing lamprophyre dikes from Shingu, central Shikoku, Southwest Japan.

K-Ar age determinations were carried out on two lamprophyre dikes intruded into the Sambagawa metamorphic rocks at Shingu in the central part of Shikoku Island, Southwest Japan. These dikes are very distinctive among Neogene igneous rocks in Shikoku because of their abundant mantle xenoliths and carbonate minerals in the fresh groundmass, and of their unique chemical compositions. In order to eliminate the effect of extraneous argon from xenoliths and carbonates, removal of large crystals and acid leaching experiments were done for three grain size fractions. Apparent ages ranging from 18.6±0.7Ma to 21.6 ±2.0Ma were obtained for both rocks. However, positive correlation between apparent ages and atm. 40Ar% suggests that samples contain initial argon which is not atmospheric. The well defined isochron age of 17.7±0.5Ma is obtained from all samples with the initial 40Ar/36Ar ratio of 310±5 in the 40Ar/36Ar-40K/36Ar diagram. As the initial 40Ar/36Ar ratio is higher than that of atmosphere, apparent ages do not show any geologic meaning, and the isochron age of 17.7±0.5Ma gives the intrusive age of dikes. The Shingu lamprophyres are much older than other Miocene igneous rocks in Shikoku ranging in age from 14 to 12Ma, and both magmatisms could be genetically independent. As the Shikoku Basin kept opening in a northeast-southwest direction at 18Ma, it could not subduct significantly beneath Southwest Japan from the south when the Shingu lamprophyres intruded into the Sambagawa metamorphic belt. Petrogenesis of the Shingu magma could be related to the rifting of the back arc basin, the Japan Sea.

40Ar/39Ar mineral dates from retrogressed eclogites within the Baltoscandian miogeocline: Implications for a polyphase Caledonian orogenic evolution

Late Proterozoic, rift-facies dolerite dikes within Baltoscandian rocks of the Seve Nappe Complex locally underwent eclogite metamorphism during Caledonian orogenesis. Hornblende from retrograde amphibolite selvages developed around two eclogite boudins exposed at Grapesvare, Norrbotten County, Sweden, record identical 40Ar/39Ar plateau dates of 491 ± 8 Ma. Phengitic muscovite from host schists records plateau dates of 447 ± 7 Ma and 436 ± 7 Ma. Coexisting biotite yields plateau dates of 594 ± 10 Ma and 808 ± 13 Ma. The biotite dates are interpreted to reflect the presence of extraneous argon components. The hornblende and phengitic muscovite ages are interpreted to date times of postmetamorphic cooling through argon retention temperatures. Together with previous 40Ar/39Ar mineral ages from Jamtland, Sweden, they confirm that a significant pre-Scandian tectonothermal event is recorded regionally in allochthonous sequences which originated within the Baltoscandian miogeocline. The eclogite assemblages are interpreted to have formed during westerly subduction of distal portions of the miogeocline with attendant development of an accretionary wedge. The latter was subsequently uplifted and eroded, providing a source for Middle Ordovician through Lower Silurian clastic successions which accumulated in both eastern and western basins. These, together with previously metamorphosed older portions of the miogeocline, were imbricated, folded, and variably metamorphosed during Late Silurian to Early Devonian transport onto the Baltoscandian Platform.

Polymetamorphism in the Panamint Mountains, California: A 39AR‐40AR study

Metamorphosed upper Precambrian rocks in the Panamint Mountains show textural evidence for polymetamorphism. Previous isotopic studies have established the age of metamorphism as middle to late Mesozoic. This study was undertaken to try to determine the ages of the two metamorphic events using the 39Ar‐40Ar stepwise degassing technique of the K‐Ar method on biotite, muscovite, and hornblende separates. The resulting spectra indicate that metamorphism occurred during the intervals 170–150 Ma ago and 80–70 Ma ago. Many analyses yield ambiguous results because of slow cooling following metamorphism, contamination of the separates by refractory phases containing extraneous argon, and in some cases, the superposition of the two metamorphic events. The ages of metamorphism correspond to periods of emplacement of granitic rocks in the vicinity of the Panamint Mountains, suggesting that the high heat flux responsible for metamorphism was associated with the emplacement of the Sierra Nevada batholith. The ages of metamorphism and granite emplacement are diachronous with the development of major thrust faults in the western Great Basin, which suggests that metamorphism might have been triggered by the formation of nappes rather than causing the formation of nappes.

40 Ar/ 39 Ar mineral age record of early Caledonian tectonothermal activity in the Baltoscandian Miogeocline, central Scandinavia

In central portions of the Scandinavian Caledonides, greenschist facies volcanosedimentary successions within the Koeli Nappe Complex have been thrust several hundred kilometers eastward onto the Baltoscandian platform. These were derived from eugeoclinal terranes situated outboard (west) of the Baltica continent during the early Paleozoic. The Koeli Nappe Complex is tectonically underlain by higher grade units within the Seve Nappe Complex. These are composed of amphibolite and granulite facies rocks and locally contain eclogites. The Seve Nappes tectonically separate Koeli units from structurally lower allochthons derived from more inboard environments along the Baltoscandian miogeocline. Previous mineral isotopic age-determinations from Seve and Koeli units have been in the 430 to 390 Ma range and have been interpreted to presumably date cooling following Scandian (Middle Silurian to Early Devonian) metamorphism. However, incremental-release /sup 40/Ar//sup 39/Ar dates recorded by minerals within some of the Koeli and Seve Nappes exposed in Jaemtland, Sweden (Taennforsen and Are districts) provide evidence of earlier tectonothermal activity. Hornblendes from the Seve and Koeli Nappe Complexes display variably discordant age spectra as a result of low-temperature, experimental evolution of loosely bound extraneous argon components. However, in most analyses plateau ages of 510 to 475 Ma (Koeli) and 465 to 455more » Ma (Seve) are defined. In contrast, muscovite and biotite from all tectonic units record Scandian cooling ages between 245 and 410 Ma. The older events recorded by hornblende within these Seve and Koeli units are evidence of early Caledonian tectonothermal activity and subsequent diachronous cooling during the Early-Middle Ordovician.« less

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 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.

K-Ar dating young rhyolitic rocks: A case study of the Sierra La Primavera, Jalisco, Mexico

This study demonstrates the feasibility of dating by the K-Ar method an eruptive sequence of silicic volcanic rocks with ages close to the limits of conventional 14 C dating. Forty-six conventional K-Ar analyses of sanidine and glass from rhyolites of the Sierra La Primavera, Mexico, yielded dates of 5 to 150 Ka (thousand years). One-sigma errors range from 3 to 39 Ka. Although the samples contain small amounts of extraneous argon, most of the dates are consistent with relative ages determined in the field. The exceptions are dates on immediately postcaldera eruptive units, which are older than the dates for the caldera-producing ash-flow tuff. A similar result has been obtained on dating tuffs and lavas of some other Pleistocene calderas. This effect is attributed to incomplete outgassing of the larger amount of extraneous argon dissolved in magma that occupied a position deeper in the chamber than the ash-flow tuff magma and that erupted immediately after caldera collapse. We present two new error equations: One, a simple extension of the commonly used equation, demonstrates the importance of including a term for the uncertainty in the spectrometer mass-discrimination factor in the error equation. The second equation is a more rigorous formulation of the propagation of errors in the calculation of the age. It yields meaningful error estimates for all proportions of radiogenic 40 Ar, and, because it expresses the error in absolute rather than relative terms, it can be used to put an upper limit on the age of a sample when no radiogenic 40 Ar has been resolved.

Origin of hydrous cordierite and isotopically light oxygen in granulite-facies rocks, Strangways Range, Central Australia

AbstractThe origin of hydrous cordierite from a granulite-facies terrain in the Strangways Range, Central Australia, reported by Wilson (1978) is reviewed. In the light of geochemical and microstructural evidence for a metasomatic episode which affected the terrain subsequent to the growth of the cordierites, it is contended that the hydrous character of the cordierites results from hydration of the cordierite at this time. Extraneous argon also recorded in these cordierites may either have been incorporated into the cordierites at this time or at a later stage in the history of the terrain due to partial loss of radiogenic argon from micas which grew during the metasomatic episode. Furthermore, in view of the history of these cordierites, oxygen isotope values for the cordierites reported by Wilson are likely to be anomalous, and the isotopically light oxygen in the host granulites recorded by Wilson and Baksi (1978) probably reflects exchange with the metasomatic fluids which permeated the terrain.

Geochronology and thermal history of the Coast Plutonic Complex, near Prince Rupert, British Columbia

Fission track, K–Ar, and Rb–Sr mineral dates for the Ecstall and Quottoon plutons were determined to resolve the different cooling histories indicated by Rb–Sr whole-rock and mineral isochrons and published K–Ar dates.Comparison of mineral dates with previously assigned closure temperatures for the various isotopic chronometers has allowed temperature–time plots for the thermal history of individual samples and sample suites to be constructed.The Quottoon pluton, emplaced approximately 51 Ma ago, cooled rapidly during initial uplift that ended ~46 Ma ago with the present-day land surface at a depth of ~4 km. A second episode of uplift at an average rate of 0.05 cm year−1 (measured in Kasiks pluton that lies immediately to the east) began in the late Miocene. Cooling of the Ecstall pluton, following emplacement approximately 80 Ma ago, was disrupted by a thermal event approximately 65 Ma ago. Biotite in the Ecstall pluton appears to have incorporated extraneous argon during this event so that the calculated K–Ar dates lie between the time of emplacement and time of cooling through the closure temperature for Ar in biotite.The cooling curves and observed dates yield estimates for closure temperatures, at an intermediate cooling rate, for Ar in plagioclase and K-feldspar of ~260 °C and ~160 °C, respectively. For fission tracks in epidote the closure temperature estimate is ~240 °C.

Rubidium‐Strontium dates and extraneous argon in the Arltunga Nappe complex, northern territory

Rubidium‐Strontium dates and extraneous argon in the Arltunga Nappe complex, northern territory