Plateau Dates Research Articles

Incremental heating of hornblende in vacuo: Implications for 40Ar/39Ar geochronology and the interpretation of thermal histories

Research Article| September 01, 1991 Incremental heating of hornblende in vacuo: Implications for 40Ar/39Ar geochronology and the interpretation of thermal histories J.K.W. Lee; J.K.W. Lee 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar T. C. Onstott; T. C. Onstott 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar K. V. Cashman; K. V. Cashman 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar R. J. Cumbest; R. J. Cumbest 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar D. Johnson D. Johnson 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar Author and Article Information J.K.W. Lee 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 T. C. Onstott 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 K. V. Cashman 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 R. J. Cumbest 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 D. Johnson 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (9): 872–876. https://doi.org/10.1130/0091-7613(1991)019<0872:IHOHIV>2.3.CO;2 Article history First Online: 02 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 J.K.W. Lee, T. C. Onstott, K. V. Cashman, R. J. Cumbest, D. Johnson; Incremental heating of hornblende in vacuo: Implications for 40Ar/39Ar geochronology and the interpretation of thermal histories. Geology 1991;; 19 (9): 872–876. doi: https://doi.org/10.1130/0091-7613(1991)019<0872:IHOHIV>2.3.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 SocietyGeology Search Advanced Search Abstract Increasing evidence of the incompatibility between the geologic thermal histories deduced from the 40Ar/39Ar incremental-heating technique and those derived from other methods has necessitated a careful evaluation of the assumptions inherent in the former approach. 40Ar/39Ar age spectra are frequently used to infer the spatial distribution of Ar in hydrous minerals and, as such, require that the mineral remains stable while heated in vacuo. Aliquots of Mmhb-1, an inter-laboratory hornblende standard used for K-Ar and 40Ar/39Ar dating, were heated in vacuo at temperatures of 500, 700, 930, and 1050 °C for 30 min. Up to 700 °C, no physical changes in the hornblende were observed with the scanning electron microscope, although X-ray diffraction data indicate that it had dehydroxylated to oxyhornblende at this temperature. The onset of the structural decomposition of the oxyhornblende, as revealed by backscattered- and secondary-electron images, occurs at ∼930 °C and is directly correlated with the initial release of 6% of its radiogenic Ar (40Ar*). The breakdown of the oxyhornblende by the internal nucleation of incongruent melt is almost (93%) complete by 1050 °C—the reaction products being titano-magnetite, melt, a "pyribole" phase, and internal voids. Because Ar is released simultaneously from these products and is subsequently homogenized in vacuo, this breakdown can yield plateau age spectra, regardless of any original Ar spatial gradients in the mineral. Consequently, independent evidence may be required to establish whether a plateau date from hornblende is truly a geologically significant age. This content is PDF only. Please click on the PDF icon to access. 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.

The Strength of the Geomagnetic Field at the Cretaceous-Tertiary Boundary: Palaeointensity Results from the Deccan Traps(India)and the Disko Lavas(Greenland).

The lavas of the Deccan Traps (India) and Disko Island (West Greenland) were both erupted close to the Cretaceous-Tertiary (K-T) boundary. Ar-Ar dating has provided plateau dates of 67.1±1.1Ma and 67.4±0.4Ma for samples respectively near the base and near the top of the Deccan succession, and the uppermost flow of the Disco succession has a preliminary age of 64.7±1.4Ma. Palaeointensities have been obtained from orientated samples from 47 Indian and 38 Greenlandic sites, using the SHAW (1974) technique with the ROLPH and SHAW (1985) correction for laboratory alteration. The intensity values show considerable scatter, but the mean virtual dipole moment calculated for the Deccan is 4.2±0.3×1022Am2 and for Disko is 5.8±0.6×1022Am2 compared to the present value of about 8×1022Am2. Studies of the variation of low-field susceptibility between -196°C and 700°C shows that the proportion of samples containing multidomain magnetite is somewhat higher than in some younger Lavas, suggesting hydrothermal alteration, which could account for some of the scatter in the palaeointensity values.

Alloys developed and computer-aided tomography used to study damage in aging aircraft: Mater. Eval. Feb. 1990 48, (2), 293–294

New apatite U–Pb and multiphase 40Ar/39Ar data constrain the high to medium temperature (~ 500 °C–~ 300 °C) thermal histories of igneous and metamorphic rocks exposed in the Mérida Andes of Venezuela, and new apatite and zircon fission track data constrain the ~ 500 °C–~ 60 °C thermal histories of pre-Jurassic igneous and metamorphic rocks of the adjacent Santander Massif of Colombia. Computed thermal history envelopes using apatite U–Pb dates and grain size information from an Early Palaeozoic granodiorite in the Mérida Andes suggest that it cooled from > 500 °C to < 350 °C between ~ 266 Ma and ~ 225 Ma. Late Permian to Triassic cooling is also recorded in Early Palaeozoic granitoids and metasedimentary rocks in the Mérida Andes by numerous new muscovite and biotite 40Ar/39Ar plateau dates spanning 257.1 ± 1.0 Ma to 205.1 ± 0.8 Ma. This episode of cooling is not recognised in the Santander Massif, where 40Ar/39Ar data suggest that some Early Palaeozoic rocks cooled below ~ 320 °C in the Early Palaeozoic. However, most data from pre-Jurassic rocks reveal a regional heat pulse at ~ 200 Ma during the intrusion of numerous shallow granitoids, resulting in temperatures in excess of ~ 520 °C, obscuring late Palaeozoic histories.The generally accepted timing of amalgamation of Pangaea along the Ouachita–Marathon suture pre-dates Late Permian to Triassic cooling recorded in basement rocks of the Mérida Andes by > 30 Ma, and its effect on rocks preserved in north-western South America is unknown. We interpret late Permian to Triassic cooling in the Mérida Andes to be driven by exhumation. Previous studies have suggested that a short phase of shortening and anatexis is recorded at ~ 253 Ma in the Maya Block, which may have been adjacent to the basement rocks of the Mérida Andes in the Late Permian. The coeval onset of exhumation in the Mérida Andes may be a result of increased coupling in the magmatic arc, which was located along the western margin of Pangaea. Triassic extension is documented in the Central Cordillera of Colombia and Ecuador between ~ 240 Ma and ~ 215 Ma, although extension at this time has not been clearly identified in the Mérida Andes or the Santander Massif. Permian to Triassic cooling is not recorded in the structurally isolated Caparo Block in the southern Mérida Andes, suggesting that it may have constituted a distinct fault block in the Triassic.New fission track data from the Santander Massif suggest that it started exhuming at ~ 40 Ma during a period of accelerated convergence between the Nazca/Farallòn Plate and the western margin of South America. Exhumation in the Santander Massif occurred diachronously since ~ 18 Ma in distinct fault blocks at rates of 0.5–1 km/Ma, and may have been driven by east–west compression as a result of the indentation of the Panama–Chocó terrane to western Colombia.

Initial argon in amphiboles from the Chugach Mountains, southern Alaska

The 40Ar/39Ar step‐heating analyses of three hornblende samples from the Border Ranges ultramafic‐mafic assemblage yield plateaus with slightly discordant dates of 180.2±0.7, 177.5±0.7, and 181.1±1.7 Ma. A plot of the plateau temperature data for all three hornblende samples on an 36Ar/40Ar versus 39Ar/40Ar diagram reveals the presence of a non‐atmospheric “initial” argon component with an apparent 40Ar/36Ar of 379±16 and yields a date of 175.7±1.0 Ma. A potassium‐poor hastingsite from the same complex yields a complicated spectra with dates as high as ∼900 Ma. These older dates reflect contamination by argon with an isotopic composition similar to that found for the initial argon in the hornblende and do not record a Paleozoic component in the age of the Border Ranges ultramafic‐mafic complex. Hornblende from an Eocene tonalite sill in the nearby Chugach Metamorphic Complex also contains an initial argon component (40Ar/36Ar = 314 ± 4). The composition of this initial argon is presumably dictated by the argon isotopic composition of the host rock fluid at the time of amphibole/fluid equilibration. Approximately 2×10−7 cm3/g of this initial argon is sufficient to perturb the 40Ar*/39ArK “model” plateau dates. Although 40Ar/39Ar stepwise degassing at low temperatures removes an additional “atmospheric” argon component, the initial argon in these samples is released at a rate that closely coincides with the degassing rate of radiogenic argon from the amphibole, and only through application of the 36Ar/40Ar versus 39Ar/40Ar diagram can this nonatmospheric argon component be detected. For the Border Ranges ultramafic‐mafic and Chugach Metamorphic complexes, the range in K‐Ar amphibole dates could be interpreted as recording differential uplift and an extended duration of intrusive activity, respectively. The results suggest, however, that nonatmospheric initial argon contributes significantly to the observed age variations.

40Ar/ 39Ar thermochronometry of the Imataca Complex, Venezuela

The Imataca Complex of Venezuela, an Archean metamorphic belt located on the northern margin of the Amazonian Craton, underwent upper amphibolite to granulite facies metamorphism during the 2.0 Ga Trans-Amazonian Orogeny. 40Ar/ 39Ar plateau dates on hornblende separates from the Imataca Complex vary from 1972 to 1760 Ma and record post Trans-Amazonian Orogeny cooling. Biotite and feldspar pairs, which were separated from the same samples or from samples collected at the same exposure and should record identical thermal histories, yield distinct results. The biotite 40Ar/ 39Ar spectra are primarily plateau profiles with dates ranging from 2044 to 1254 Ma, whereas perthitic potassium feldspar yield stair-step 40Ar/ 39Ar spectra and integrated dates ranging from 1395 to 1166 Ma. The internally discordant perthite spectra appear to reflect cooling at rates of 0.1–0.2°C Ma −1, consistent with the cooling rates estimated from the blocking temperature and age differences between biotite-feldspar pairs from the same sample. Two plagioclase samples yield saddle-shaped 40Ar/ 39Ar spectra with 1235 and 1221 Ma minimum dates. These internally discordant spectra are caused by degassing of non-atmospheric, trapped Ar from the plagioclase and of radiogenic Ar sited in K-rich inclusion phases. The 2.0–1.7 Ga biotite dates are confined to the margins of the Imataca Complex and record the waning stages of Trans-Amazonian Orogeny uplift and cooling. The 1.4-1.2 Ga biotite dates, which are confined to the center of the Imataca Complex, are correlated with their 200–350°C Ar blocking temperatures as estimated from their average grain size and Fe/(Fe + Mg) composition. The 1.3-1.1 Ga feldspar ages are correlated with their 200–300°C Ar blocking temperatures as determined from analysis of the 40Ar/ 39Ar step-heating data. The extremely slow cooling rates implied by the biotite-feldspar pairs suggest that between 1.7 and 1.1 Ga, the Imataca Complex cooled isobarically at intermediate crustal levels. All Ar mineral systems closed by 1.1 Ga, perhaps reflecting a renewed phase of uplift associated with the mild tectonism of the Nickorie Episode.

40Ar/ 39Ar and U-Pb evidence for late proterozoic (Grenville-age) continental crust in north-central Cuba and regional tectonic implications

Central Cuba is composed of fault-bounded tectonostratigraphic terranes juxtaposed and deformed during plate collision and subsequent transform motion between the Caribbean and North American plates in the Late Mesozoic-Cenozoic. One of these, the Las Villas terrane, contains crystalline basement rocks thought to be pre-Upper Jurassic on stratigraphic grounds. The Socorro Complex occurs in the northwestern Las Villas terrane, and consists of marbles and siliciclastic metasedimentary rocks, and the Río Caña Granite. An 40Ar/ 39Ar plateau date of 903.5 ± 7.1 Ma for phlogopite from a marble corroborates previous K-Ar dates from this unit, and establishes unambiguously a Late Proterozoic age for high-grade metamorphism. Discordance of the 40Ar/ 39Ar age spectrum can be reliably attributed to diffusive Ar loss, which if modeled as an episodic thermal event implies a reheating age that closely coincides with the Late-Cretaceous-Paleogene collision between the Caribbean and North American plates. U-Pb zircon data indicate an intrusive age of 172.4 Ma for the Río Caña Granite, and reveal an inherited zircon component with an age of ∼900 Ma. Radio-isotopic data from the Socorro Complex display no evidence of Pan-African age thermal overprinting. These observations, combined with constraints provided by published results from nearly Pangean landmasses, suggest that the complex lay substantially to the southwest (present-day co-ordinates) during the Early Paleozoic. Following its genesis in the mid-Mesozoic, the Caribbean plate evidently transported fragments of an extensive Grenville-age belt that spanned the Americas during the Late Proterozoic.

An 40Ar/39Ar geochronological study of the Elzevir batholith and its bearing on the tectonothermal history of the southwestern Grenville Province, Canada

Results of 40Ar/39Ar step heating for muscovite, potassium feldspar, and plagioclase from the ca. 1240 Ma Elzevir trondhjemite in the Central Metasedimentary Belt of the Canadian Grenville Province have been combined with earlier data and used to deduce a thermal history for the eastern Elzevir terrane following the Ottawan orogeny (1050–950 Ma). Muscovite yielded precise plateau dates of ca. 900 Ma, whereas the potassium feldspar displayed disturbed age spectra with ca. 730–760 Ma dates for broad "plateau" regions. This age difference could be explained by slow cooling or by thermal overprinting at 760–800 Ma that has updated the microcline. The plagioclase spectra provide evidence for a low-temperature hydrothermal event ca. 380 Ma ago that has generated sericite in saussurite and has caused partial argon loss from the microcline. This result agrees with the other plagioclase thermochronometry and paleomagnetism on the Cordova gabbro 25 km to the west but provides a lower estimate for the upper age bracket of this event. It is suggested that earlier models of protracted, post-700 Ma cooling of this part of the Grenville orogen may be a consequence of variable updating of plagioclase by this Devonian alteration event.Results of 40Ar/39Ar step heating for muscovite and microcline from the Bark Lake diorite in the Bancroft terrane are in agreement with earlier work in this area, and the cooling path of the Elzevir trondhjemite is seen to be indistinguishable, within the limitations of the method, from that of the Bark Lake diorite in the time period 1050–750 Ma.

Polyorogenic40Ar/39Ar Mineral Age Record in the Seve and Köli Nappes of the Gäddede Area, Northwestern Jämtland, Central Scandinavian Caledonides

In the 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. Contrasting tectonothermal histories are recorded in Seve and Köli Nappes exposed in the Gäd-dede area, northern Jämtland County, Sweden. Eclogite-bearing Seve rocks underwent an initial amphibolite-grade regional metamorphism (SMI) with different structural units experiencing contrasting high-to intermediate-pressure conditions. Subsequent ductile imbrication (SM2) of high-and intermediate-pressure structural units occurred prior to 460-470 Ma cooling through temperatures required for intracrystalline retention of argon in hornblende. A later thermal overprint (SM3) everywhere rejuvenated Seve biotite and muscovite (and locally hornblende) argon systems at plateau ages of hornblende suggest attainment of peak metamorphic conditions (KM1) in the Lower and Middle Köli Nappes at ca. . Plateau dates of muscovite and biotite in both Seve and Köli units range down to 417 Ma, suggesting relatively rapid post-metamorphic cooling. Initial outboard assembly (KD1) of the Lower and Middle Köli Nappes preceded the regional metamorphic peak. Ductile translation (KD2) of the assembled Köli units occurred during maintenance of near peak KM1 metamorphic conditions. Although this thrusting was clearly later than SM2 ductile imbrication within the underlying Seve Nappe Complex, a widespread thermal overprint (SM3) appears to have accompanied structural emplacement of the Köli Nappes. Comparison with mineral ages reported elsewhere in the central Scandinavian Caledonides suggests a polyorogenic evolution consisting of: (1) variable high-pressure metamorphism of distal Baltoscandian miogeoclinal sequences during their entrainment in an accretionary wedge that developed in the Late Cambrian-Early Ordovician over a westdipping subduction zone; related arc volcanic activity occurred in more outboard, western terranes; (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) ultimate accretion of a variety of eugeoclinal terranes along the Baltic continental margin; and (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 and Devonian.

Tectonic implications of 40Ar/ 39Ar ages from a pre-Mesozoic metamorphic basement penetrated on Leg 77 of the deep sea drilling project in the southern Gulf of Mexico

Drilling penetrated pre-Mesozoic crystalline basement beneath abbreviated sedimentary sequences overlying fault blocks in the south-eastern Gulf of Mexico. At Site 538A located on Catoche Knoll, a foliated, regional metamorphic association of variably mylonitic felsic gneisses and interlayered amphibolite is intruded by post-tectonic diabase dikes. Hornblende from the amphibolite records 40Ar/ 39Ar plateau dates of c. 500 Ma, indicating early Paleozoic metamorphism. Biotite from the gneiss yields a plateau age of c. 350 Ma which likely dates a superposed late Paleozoic thermal event. A whole-rock diabase sample records a crystallization date of 190.4 ± 3.6 Ma. A phyllitic metasedimentary sequence was penetrated at Site 537 drilled 30 km northwest of Catoche Knoll. Whole-rock phyllite samples display variably discordant 240Ar/ 39Ar age spectra, but plateau segments clearly document an early Paleozoic metamorphism at c. 500 Ma. The age and lithologic character of the basement terrane penetrated at Sites 537 and 538A suggest that the drilled fault block are underlain by attenuated fragments of continental crust of “Pan-African” affinity. This support pre-Mesozoic tectonic reconstructions which locate Yucatan in the present Gulf recess during amalgamation of Pangea.

Revised 40 AR– 39 AR age for granites of the Mourne Mountains, Ireland

The British Tertiary Igneous Province (BTIP) consists of many geographically separate areas throughout the British Isles and the question arises whether activity occurred contemporaneously throughout the province. In the absence of palaeontological evidence, this is best answered using radiometric dating, and many potassium-argon analyses have been carried out. However, experience has shown that conventional K–Ar dating in the BTIP is prone to error, often giving spuriously high or low dates. For this reason, Rb–Sr isochron and 40Ar–39Ar step–heating plateau dates are preferred, for internal acceptance criteria can be applied to them. The Mourne Mountain granites consist of five successive granite intrusions, G1 to G5, and lie to the NE of the Carlingford Complex and Slieve Gullion central intrusive complexes; together these form the only major masses of Tertiary intrusive rocks in Ireland. They are described in Meighan et al. (1984). Table 1 lists available dates. The most reliable are the 40Ar–39Ar plateau dates of 59.6 ± 1.6 Ma for G2 and 59.5 ± 1.6 Ma for G5; though acceptance criteria were not applied to establish the validity of the plateaus chosen, the steps selected for each plateau, although increasing in apparent age, are concordant at the 1σ level and contain the bulk of released 39Ar. Thus they appear to establish the age of the Mournes as about 59.5 Ma. However, three new 40Ar–39Ar step-heating analyses are reported here that indicate that the age is significantly younger. An analysis has been performed for a biotite from a mafic-rich facies within G2 (Meighan . . .

Delimitation of a cryptic Eocene tectono-thermal domain in the Eastern Cordillera of the Bolivian Andes through K–Ar dating and 40 Ar– 39 Ar step-heating

New K–Ar ages and 40 Ar– 39 Ar age spectra for samples from granitoid rocks and meta-sediments of the Cordillera Real, Bolivia, reveal a complex mid-Tertiary tectono-thermal history for this segment of the Central Andean eastern cordillera. Previous radiometric dating of the Cordillera Real intrusions defined two, areally-distinct, plutonic domains: in the N, Upper Triassic–Lower Jurassic; in the S, Upper Oligocene–Lower Miocene. Our new K–Ar ages, many from deformed intrusions, fall between these two episodes. For the variably foliated Zongo massif, there is a marked discordance between biotite and muscovite K–Ar dates, each decreasing consistently from SW to NE across this 8 km wide plutonic centre. The age gradients are developed within the foliated and unfoliated facies of the Zongo intrusions, but do not continue NE into the Palaeozoic meta-sedimentary rocks, for which K–Ar dates are erratic. 40 Ar– 39 Ar step-heating analyses of muscovites and most biotites from granitoid rocks reveal that the oldest and youngest micas yield plateaux (210 and 39 Ma), whereas those with intermediate ages exhibit disturbed spectra. A plateau date for a biotite from the NE margin of the Zongo massif shows that final cooling through c. 300 °C occurred in the Late Eocene. The new data reveal that a marked thermal discontinuity existed 39 Ma ago at the NE contact of the Zongo intrusion. Additional discordant dates for the Yani and Illampu intrusions, to the NW, indicate that the discontinuity was of regional scale. There is no direct relationship between this tectono-thermal event, which affected at least 2000 km 2 , and the Upper Palaeozoic foliation of several of the plutons involved, and thus it is cryptic on outcrop and microscopic scales. We propose that the thermal discontinuity resulted from Late Eocene crustal shortening, SW-verging thrust faulting, and attendant cordilleran uplift at the inner (eastern) boundary of the Andean orogen, coincident with the Incaic deformation defined in Peru.

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.

40Ar/39Ar mineral age record of polyphase tectonothermal evolution in the southern Mauritanide orogen, southeastern Senegal

Research Article| May 01, 1987 40Ar/39Ar mineral age record of polyphase tectonothermal evolution in the southern Mauritanide orogen, southeastern Senegal R. D. DALLMEYER; R. D. DALLMEYER 1Department of Geology, University of Georgia, Athens, Georgia 30602 Search for other works by this author on: GSW Google Scholar MICHEL VILLENEUVE MICHEL VILLENEUVE 2Laboratoire Associé Au CNRS no 132, “Etudes Géologiques Ouest-Africaines,” Université d'Aix-Marseille III, 13397 Marseille, Cedex 13, France Search for other works by this author on: GSW Google Scholar Author and Article Information R. D. DALLMEYER 1Department of Geology, University of Georgia, Athens, Georgia 30602 MICHEL VILLENEUVE 2Laboratoire Associé Au CNRS no 132, “Etudes Géologiques Ouest-Africaines,” Université d'Aix-Marseille III, 13397 Marseille, Cedex 13, France Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1987) 98 (5): 602–611. https://doi.org/10.1130/0016-7606(1987)98<602:AMAROP>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 R. D. DALLMEYER, MICHEL VILLENEUVE; 40Ar/39Ar mineral age record of polyphase tectonothermal evolution in the southern Mauritanide orogen, southeastern Senegal. GSA Bulletin 1987;; 98 (5): 602–611. doi: https://doi.org/10.1130/0016-7606(1987)98<602:AMAROP>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 40Ar/39Ar incremental-release mineral ages have been determined for several tectonostratigraphic units in southernmost portions of the Maiuritanide orogen in Senegal. Here, middle Proterozoic crystalline basement rocks of the West African craton are unconformably overlain by an upper Proterozoic (ca. 1100 Ma) continental sedimentary succession (Madina Kouta Group) transitional into volcanic and intercalated deep-marine sediments (Tennesse Group). Western portions of the volcano-sedimentary unit were variably deformed and metamorphosed prior to deposition of an unconformably overlying, upper Precambrian-Lower Cambrian sedimentary succession (Mali Group). Muscovite within cleavage domains in the low-grade metamorphic terrane records 40Ar/39Ar plateau dates of ca. 655–665 Ma and are interpreted to closely date the Pan African I phase of orogenic activity. The Mali Group itself was locally folded during a second phase of Pan African tectonothermal activity, which also affected a western, calc-alkaline, volcanic-plutonic sequence (Niokolo-Koba Group). Igneous hornblende from several granites within this sequence displays internally discordant 40Ar/39Ar release spectra which record variably developed plateaus of ca. 640–660 Ma. These are interpreted to approximately date initial postmagmatic cooling through ∼500 °C and are consistent with a previously published Rb-Sr whole-rock iso-chron crystallization age of 683 ± 18 Ma for one of the plutons. Associated calc-alkaline volcanic and volcaniclastic rocks are more penetratively deformed. Muscovite that is aligned along a regional cleavage records 40Ar/39Ar plateau ages of ca. 550–560 Ma, which are interpreted to closely date the Pan African II orogenesis.Widespread deposition of a molassic sequence (Youkoukoun Formation) occurred in the Middle Cambrian-Early Ordovician. This was disconformably overlain by Upper Ordovician-Upper Devonian sedimentary rocks (Bové basin succession), which are locally folded. More significant late Paleozoic tectonothermal activity is recorded in north-westernmost portions of the study area where the calc-alkaline, upper Proterozoic granites locally display penetrative mylonitic fabrics. Development of these fabrics is interpreted to be closely dated by the 40Ar/39Ar 270- to 280-Ma plateau ages recorded by dynamically recrystallized muscovite developed along fluxion foliations. These results allow demarcation of a late Paleozoic tectonic “front” and permit correlations with genetically related events in southeastern portions of the Appalachian orogen. This content is PDF only. Please click on the PDF icon to access. 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.

40Ar/ 39Ar and paleomagnetic results from Liberia and the Precambrian APW data base for the West African Shield

Lower amphibolite to granulite facies metamorphic rocks in Nimba County, Liberia have yielded 2.2–2.9 Ga RbSr whole rock ages, indicating that they are part of the Archean Liberian age province. We report a 2040 Ma 40 40Ar/ 39Ar plateau date on hornblende from an amphibolite in this region, and suggest that these rocks were also severelyreworked during the Eburnean (∼2.0 Ga) metamorphic episode. 40Ar/ 39Ar analyses of biotite and feldspars from neighboring schists also indicate the presence of two mild thermal events, at 1.5 Ga and 0.6 Ga. Paleomagnetic analyses of samples from these same metamorphic rocks reveal three components of magnetization. The predominant and most stable component (273°E, 21°N) is considered to have been acquired as a result of pos Eburnean uplift and cooling at ∼ 2.0 Ga, whereas the two less stable components with poles at 235°E, 43°N and 16°E, 36°N, probably correlate with the 1.5 Ga and 0.6 Ga thermal pulses, respectively. Rock units from southern Liberia also yield two secondary magnetizations, one at 247°E, 37°N and the other at 104°E, 5°N, and a 1.5 Ga 40Ar/ 39Ar date on plagioclase. Comparison of the paleomagnetic poles corresponding to the ∼2.0 Ga Eburnean component with published paleomagnetic data for West Africa is not consistent with prior interpretations of the polar wander path for West Africa. Our paleomagnetic data, when compared to poles of comparable age from the Kalahari Shield, still suggest that some form of displacement has occurred between the Kalahari and West African Shields since 2.0 Ga.

Dating blueschist metamorphism: A combined 40Ar/ 39Ar and electron microprobe approach

40Ar 39Ar and electron microprobe examination of blueschist samples from the Iceberg Lake schist, southern Alaska suggest that phengite inclusions are the source of 40Ar in crossite. Because such finegrained inclusions may be susceptible to argon loss, caution should be exercised in interpreting K-Ar ages from this phase, and possibly other low-K amphiboles from blueschist suites. The estimated blocking temperature for phengite in the matrix (314° to 450°C), however, is close to the estimated peak metamorphic temperatures (325° ± 50° C), suggesting that phengite 40Ar 39Ar plateau dates may coincide closely with the time of blueschist metamorphism.

Geochronology of Early to Middle Paleozoic Tectonic Development in the Southwest Newfoundland Gander Zone

Southwest Newfoundland's Gander Zone was once considered part of the collided eastern margin of an early Paleozoic Iapetus ocean, distinguished from the rest of the Gander Zone by the extensive exposure of a polydeformed amphibolite to greenschist facies sequence considered Devonian in age, overlying presumed Precambrian basement. It was later found that truly Devonian strata were confined to the Cape Ray Fault system, the local northwestern boundary of the Zone. U-Pb zircon isotopic ages of and on metavolcanic rocks in the upper part of the sequence southeast of the fault system indicate that Ordovician and possibly older rocks underlie the Gander Zone itself. A temporal framework for the three main stages of regional deformation, corresponding to pre-metamorphic recumbent folding and transpression ( and ), is presented using U-Pb and dates of granitoid and metamorphic rocks. ) is bracketed as late Ordovician or Silurian by means of U-Pb zircon isotopic ages on the metavolcanic rocks and a 412 Ma U-Pb minimum age on a post- granitoid intrusion. Spatial domains of contrasting biotite and hornblende cooling ages reflect the and episodes, which led to post-metamorphic cooling following uplift of different segments of the amphibolite facies metamorphic terrane. Early Devonian differential uplift on the Cape Ray Fault in the northeast was associated with and caused rapid post-metamorphic cooling through Ar retention temperatures to yield nearly concordant biotite and hornblende ages between 374 and 388 Ma. Reverse and oblique-slip faulting during produced oblique northwesterly uplift of the still-buried portion of the amphibolite facies terrane against Devonian strata in the southwest part of the Cape Ray Fault zone. Late Devonian to early Carboniferous post-metamorphic cooling in this segment is confirmed by plateau dates of 345-353 Ma for biotite and 361 Ma for hornblende. The Gander Zone as a whole is imprinted with a strong penetrative structural grain similar in pattern to and structures in the southwest Newfoundland Gander Zone. K-Ar mineral dates are likewise earliest Devonian and younger, in contrast to numerous pre-Devonian K-Ar ages in the Humber and the Dunnage Zones. The Gander Zone, already identified as a wide, Acadian ductile shear belt and noted for widely variable metamorphic grade, might represent a mobile zone where coherent relics of the "Iapetus" or prior stages of Appalachian history were exhumed during Devonian orogeny, many after Silurian or earlier tectonic burial.

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.

Time and duration of Tertiary igneous activity of Rhum and adjacent areas

Synopsis Two new 40 Ar– 39 Ar plateau dates are given for the Rhum central complex (58.4 ± 0.8 (2σ) Ma) and lavas (60.1 ± 1.0 Ma). These are considered in conjunction with available geological, radiometric and palaeomagnetic evidence to show that the formation of the central intrusive complex of Rhum, its subsequent unroofing, followed by the intrusion of lavas in Rhum, Canna and Skye, together with the development of most or all of the Cuillins centre of Skye probably occurred within the single reversed polarity interval 26r lasting about 2.8 Ma about 59 Ma ago. In all three areas igneous activity extended into the following interval of normal magnetisation, though only just in the cases of Rhum and Canna.

Geochronology of the Baie Verte Peninsula, Newfoundland: Implications for the Tectonic Evolution of the Humber and Dunnage Zones of the Appalachian Orogen

U-Pb analyses of zircon from the Burlington Granodiorite suggest intrusion at c. 460-465 Ma. Hornblende and biotite from northern border phases of the pluton record similar plateau dates suggesting rapid post-magmatic cooling. Hornblende and biotite from central portions of the pluton record markedly younger plateau dates (410-420 Ma) which are interpreted to date contact metamorphic effects associated with the widespread emplacement of Silurian-Devonian igneous suites. Northern portions of the Burlington Granodiorite are polydeformed and regionally metamorphosed. Hornblende and biotite from this terrane yield plateau ages of 345-350 Ma. U-Pb analyses of zircon from the Dunamagon Granite indicate emplacement at c. 440-460 Ma, thereby providing an upper limit for tectonic juxtapositioning of the Humber and Dunnage Zones along the Baie Verte Line. plateau ages of 345-350 Ma are recorded by biotite from the polydeformed and variably metamorphosed stock. Similar ages are also recorded by hornblende and biotite throughout northerly portions of the Mings Bight (Humber Zone) and Pacquet Harbour (Dunnage Zone) Groups. These results indicate that the tectonic evolution of the Baie Verte Line was poly genetic, and involved: (1) regionally significant tectonothermal activity prior to the Middle Ordovician, likely a result of the amalgamation of the Humber and Dunnage Zones following the obduction and westward transport of oceanic crust and mantle, which was previously rooted along the Baie Verte Line; and (2) Middle to Late Paleozoic (dissipating by 345-355 Ma) tectonothermal activity centered along easterly segments of the Baie Verte Line. The origins of the later tectonism are uncertain, but could, in part, have resulted from initiation of either a large-scale Variscan megashear system or through processes of lithospheric detachment and crustal shortening. The regional metamorphism associated with this orogenic activity altered primary U-Pb and Rb-Sr isotopic systems within various igneous suites exposed in northeastern portions of the Burlington Peninsula, which may explain some inconsistent geochronological results previously obtained.

U-Th-Pb, Rb-Sr, and Ar-Ar mineral and whole-rock isotopic systematics in a metamorphosed granitic terrane, southeastern California

Mesozoic structural domes are developed in an older Proterozoic crystalline basement of granitic to granodioritic foliate metaplutonic rocks in the Halloran Hills, southeastern California. Isotopic analyses of whole rocks and mineral separates from these rocks by U-Th-Pb, Rb-Sr, and Ar-Ar techniques yield a complex pattern of discordance that is the result of a fairly simple geologic history. Individual mineral isotopic systems have variably equilibrated with each other in response to Mesozoic regional metamorphism and locally to later heating during Mesozoic batholith emplacement. Discordant U-Th-Pb zircon data indicate that the granitic core rocks are 1,710 Ma and that one dioritic phase may be slightly older. Rb-Sr whole-rock model dates scatter about 1,700 Ma Rb-Sr amphibole–whole-rock and U-Th-Pb amphibole dates are also Proterozoic. Potassium feldspars retain a 207 Pb/ 206 Pb signature of their Proterozoic age. Ar-Ar amphibole spectra from the flank of the main dome reveal disturbed dates of 1,450 Ma to 1,100 Ma, and the dates become younger toward the structurally deeper core of the dome. All remaining isotopic determinations yield Mesozoic or younger dates for mineral–whole-rock systems. Rb-Sr whole-rock–apatite–feldspar–biotite analyses show nonequilibration of strontium isotopes, with resultant mineral pair dates from 4 foliate plutonic rocks ranging from 200 to 50 Ma. No single metamorphic age is indicated by the Rb-Sr data. Rb-Sr whole-rock–biotite dates are consistently younger than any other determinations and may be reduced by weathering or gain of nonradiogenic strontium from ground water. U-Pb sphene and apatite analyses from rocks that yield 1,710-Ma zircon dates are nearly concordant at 140 Ma. An amphibole from the structurally deepest rocks of the main dome that yield 140- to 150-Ma U-Pb sphene dates has an Ar-Ar plateau date of 144 Ma. The U-Pb sphene and Ar-Ar amphibole analyses are believed to be the best age estimate for the end of the highest-temperature phase of regional metamorphism. Th-Pb sphene and apatite dates and Ar-Ar biotite dates cluster at 90 ± 5 Ma as a consequence of regional cooling during Late Cretaceous time following extensive Mesozoic plutonism in the region at 97 to 90 Ma. We interpret the discordant mineral date patterns to have resulted from metamorphism of ∼1,700-Ma plutonic rocks during the Jurassic (≥ 140–50 Ma) and subsequent uplift and cooling to ∼200 °C at about 90 Ma. On the basis of this study, the isotope dating systems ranked in decreasing order of resistance to resetting are: U-Th-Pb zircon (concordia intercept) ≥ Rb-Sr whole rock ∼Rb-Sr amphibole ∼U-Th-Pb amphibole ∼Pb-Pb whole rock > Ar-Ar amphibole ≥ Rb-Sr sphene ≥ U-Pb sphene and apatite > Rb-Sr plagioclase-potassium feldspar-apatite > Th-Pb sphene and apatite ∼Ar-Ar biotite ∼U-Pb feldspars > Rb-Sr biotite.