Hornblende Ages Research Articles

New hornblende and muscovite 40Ar/39Ar cooling ages in the central Rinkian fold belt, West Greenland

The Palaeoproterozoic Rinkian fold belt in West Greenland consists of reworked Archaean basement, mainly orthogneiss, and the unconformably overlying Palaeoproterozoic Karrat Group. Both parts were intensely deformed and metamorphosed at around 1.87 Ga, at which time the crustal anatectic Prøven igneous complex was emplaced into the northern part of the belt. Seven new hornblende and muscovite 40Ar/39Ar cooling ages are presented from the central–northern parts of the Rinkian fold belt. Four 40Ar/39Ar hornblende ages ranging from 1795 ± 3 to 1782 ± 3 Ma were obtained from amphibolite and hornblendite enclaves in the Archaean orthogneiss, and two from relict dyke fragments in the latter that may be of Palaeoproterozoic age. Three 40Ar/39Ar muscovite ages of 1681 ± 6 Ma, 1686 ± 3 Ma and 1676 ± 3 Ma were obtained from samples of Karrat Group metagreywacke, andalusite schist and metasiltstone. The new 40Ar/39Ar ages, from hornblende and muscovite respectively, are very uniform and probably unrelated to local metamorphic grade and structural history, and are interpreted as regional late orogenic cooling ages. The new hornblende ages are significantly older than those previously obtained from the central and northern parts of the adjacent Nagssugtoqidian orogen to the south, and point to different uplift histories, which may suggest that the orogeny was not synchronous in the two regions.

Isotopic constraints on the thermal history of the Wind River Range, Wyoming: implications for Archean metamorphism

Precise U–Pb monazite and 40Ar/39Ar hornblende ages have been obtained from three locations in the high-grade Archean core of the Wind River Range, Wyoming. Monazites from metapelites in the Paradise Basin, Medina Mountain, and Crescent Lake have U–Pb ages of 2718 ± 1, 2633 ± 5, and 2657 ± 2 Ma, respectively. Hornblendes from amphibolites and granulites from the same locations yield plateau 40Ar/39Ar isotope ages of 2652 ± 11, 2572 ± 9, and 2527 ± 8 Ma, respectively, and are interpreted as cooling ages from the last thermal event. The three localities experienced similar peak pressure–temperature conditions. The timing of high-grade metamorphism in the Paradise Basin is older than the emplacement of large subjacent batholiths at 2.63–2.67 Ga. Calculated cooling rates based on monazite–hornblende pairs of 3.4 ± 1.0 °C/Ma for Paradise Basin, 3.8 ± 1.2 °C/Ma for Medina Mountain, and 1.7 ± 0.4 °C/Ma for Crescent Lake cannot be used to rule out reheating during subsequent pluton emplacement. The markedly slower cooling rate inferred for Crescent Lake may indicate early differential uplift or may demark another regional metamorphic event. The difference in 40Ar/39Ar ages between hornblende (2652 ± 11 Ma) and biotite (2637 ± 11 Ma) suggests a more rapid cooling rate, 11 °C/Ma, for Paradise Basin between 2.65 and 2.63 Ga, which may be related to the time of large-scale batholith emplacement elsewhere in the terrane. Combining new data with other ages in the Wind River Range reveals an extended metamorphic history, punctuated by thermal events over a time interval of at least 700 Ma.

Potassium-argon ages of gneisses from the basin granitoid in South-Eastern Ghana

Biotite and hornblende from gneiss samples from the basin granitoid in South-eastern Ghana were separated and analyzed for potassiumargon (K / Ar) ages. The results indicated that the gneisses have average biotite and hornblende ages of 1942 ± 11 Ma and 2044 ± 17 Ma, respectively. The highest single hornblende age of 2092 Ma was recorded for the amphibolite gneiss, which is a protolith of tholeiitic basalt. These ages fall within the main phase of the Eburnean thermo-tectonic event. The defined ages correspond to the metamorphic event, and are expectedly lower than the average ages of 2270 Ma (Pb/Pb), 2116 Ma (U/Pb), 2216 Ma (Rb/Sr, WR Isochron), and 2240-2290 Ma (Sm/Nd, Model) determined for the basin granitoid in other parts of Ghana and the reported Birimian age (2000- 2300 Ma). This confirms that the basin granitoid was derived from older basement rocks, and that its emplacement was contemporaneous with the activities that resulted in the emplacement of the Birimian volcanic rocks and sediments. A period of 70 to 250 Ma separates the period of emplacement of the basin granitoid and subsequent metamorphic activities. However, the period of metamorphism of the basin granitoid (1922-2045 Ma) was contemporaneous with the intrusion by the Nsawam microgranite (1996 ± 43 Ma), and that of the Tarkwaian sediments by the K-rich granitoid (1968 ± 49 Ma). Ghana Journal of Science Vol. 45, 2005: 27-33

Apparent partial loss age spectra of Neoarchean hornblende (Murmansk Terrane, Kola Peninsula, Russia): the role of biotite inclusions revealed by 40Ar/39Ar laserprobe analysis

AbstractMetamorphic hornblende frequently yields spectra with progressively increasing 40Ar/39Ar age steps, often interpreted as caused by partial resetting due to thermally activated radiogenic argon loss by solid‐state diffusion. Yet, in many cases rising Ca/K ratio spectra for such samples imply the presence of minor inclusions of K‐contaminant minerals. To avoid parts of grains with mineral inclusions or compositional zoning we drilled tiny discs from thin sections under a petrographic microscope. Laser step‐heating of drilled biotite‐free hornblende discs yielded flat age and ratio spectra. In contrast, furnace step‐heated hornblende separates from the same samples produced apparent loss age spectra. Moreover, biotite‐free samples yielded flat spectra by laser and furnace dating. Consequently, apparent loss spectra result from degassing of included substantially younger biotite before its hornblende host during laboratory step‐heating; c. 2640 Ma hornblende ages constrain the Murmansk Terrane's cooling.

Magmatic relationships and ages of Caribbean Island arc tholeiites, boninites and related felsic rocks, Dominican Republic

Located in the Cordillera Oriental of the Dominican Republic, the Early Cretaceous Los Ranchos Fm (LRF) comprises a > 3-km thick sequence of volcanic and volcaniclastic rocks with variable geochemical characteristics, which is intruded by tonalite batholiths, minor gabbro/diorite plutons and mafic dykes. From top to bottom, three main stratigraphic units have been mapped: upper basaltic, intermediate rhyodacitic and lower basaltic. Combined detailed mapping, stratigraphy, geochemistry, Rb–Sr/Sm–Nd isotopic studies and U–Pb/Ar–Ar geochronology show that the mafic rocks of the LRF include boninites and LREE-depleted island arc tholeiites (IAT) in the lower unit, both which appear genetically related, whereas normal IAT occur in the upper unit. The source for these rocks is thought to reflect variably depleted mantle, overprinted by a subduction zone component. Contemporaneous Aptian U–Pb zircon ages were obtained for a rhyodacite from the intermediate unit (116.0 ± 0.8 Ma) and a tonalite of the Zambrana batholith (115.5 ± 0.3 Ma) that intrudes the LRF. The similarity of trace element signatures in both units argues for genetic link between the felsic volcanics of the LRF and the tonalite plutonism. Low-K rhyolites and tonalite batholiths are interpreted as products of secondary melting at the base of thickened early arc crust. 40Ar/ 39Ar plateau ages of hornblende in most tonalites are Albian (109–106 Ma) and interpreted as final cooling ages, prior to unroofing and growth of unconformable overlying reef limestones of the Hatillo Fm (112–100 Ma). The LREE-depleted IAT and boninites of lower basaltic unit are interpreted to have formed during subduction zone initiation in the Caribbean Island arc, and the normal IAT of the upper unit are thought to represent the subsequent establishment of the volcanic front.

40Ar-39Ar Thermochronological Constraints on the Exhumation of Ultrahigh-Pressure Metamorphic Rocks in the Sulu Terrane of Eastern China

In order to better understand the uplift history of the Sulu ultrahigh-pressure metamorphic terrane, 40Ar-39Ar thermochronological analysis was conducted on minerals extracted from both metamorphic (gneiss and retrograded eclogite) and igneous (pegmatite, granite, and mafic intrusion) rocks from this terrane. Samples of the major lithotectonic units were collected from key localities, including Rongcheng, Laoshan, Yangkou, Taohang, and Rizhao. The analyses of metamorphic rocks yielded ages for hornblende ranging from 222 to 186 Ma, biotite from 130 to 119 Ma, and K-feldspar from 112 to 108 Ma. Hornblende, biotite, and K-feldspar from igneous rocks yielded exclusively late Mesozoic ages from 137 to 97 Ma. A two-stage cooling path for the metamorphic rocks were revealed. A slow cooling (1.9 to 3.8°C/m.y.) during the period of ∼220 to ∼130 Ma was followed by fast cooling (6.8 to 16.7°C/m.y.) during ∼130 to ∼100 Ma. The time span and cooling rate of the second stage coincide with those of the analyzed post-collisional igneous rocks as well as with the formation of rift basins in the region. The study strongly indicates that extensional tectonics played an important role in the final stage of rapid exhumation of the ultrahigh-pressure rocks in the Sulu terrane.

Time constraints on exhumation of the East African Orogen from field observations and 40Ar/ 39Ar cooling ages of low-angle mylonites in Eritrea, NE Africa

Neoproterozoic high-grade metamorphic rocks of the lower to middle crust are exposed along parts of the East African Orogen (EAO). However, the mechanisms and rates of exhumation of these deep orogenic rocks remain unclear. In eastern Eritrea, the rocks of the EAO comprise two lithotectonic domains: Ghedem and Bizen. The Ghedem domain consists of low-angle mylonites among lower to middle crustal gneisses and schists that were metamorphosed at amphibolite facies conditions before being retrogressed locally. Here we quantitatively constrain time increments in the exhumation history of the low-angle Ghedem mylonites using 40Ar/ 39Ar-cooling ages for metamorphic hornblende and white mica samples extracted from them. The hornblende samples gave an average plateau age of about 579 ± 6 Ma and the white mica samples an average plateau age of about 567 ± 5 Ma. Geothermobarometry had already shown that these rocks experienced progressive syn-deformation metamorphism that peaked when P– T conditions were near 12 kbar and 650 °C at 593 ± 5 Ma. These P– T conditions and the new 40Ar/ 39Ar cooling ages indicate that cooling and exhumation of the low-angle mylonites of the Ghedem domain were accomplished in three increments of different rate as they progressively rose from as deep as 45 km while the orogen was collapsing during the late Neoproterozoic. In the first increment, the rocks cooled ∼11 °C/Ma as they rose at about 1.07 km/Ma to cool through 500 °C at a depth of 30 km at 579 ± 5 Ma. In the second increment, the rocks cooled about ∼10 °C/Ma after their exhumation rate increased to about 1.25 km/Ma so that they cooled through 300 °C at 567 ± 5 Ma. A third increment, poorly constrained by unconformable Permo-Carboniferous sediments, implies minimum cooling rates of 1.4 °C/Ma and exhumation rates of only 0.06 km/Ma. The first two increments of exhumation occurred when the EAO in eastern Eritrea was undergoing gravitational collapse probably much faster and later than in the southern EAO. Such regional differences in gravitational collapse rates map local differences in maximum lithospheric thickness built by the final convergence of East and West Gondwana.

Timing of subduction and exhumation along the Cambrian East Gondwana margin, and the formation of Paleozoic backarc basins

Research Article| January 01, 2005 Timing of subduction and exhumation along the Cambrian East Gondwana margin, and the formation of Paleozoic backarc basins David A. Foster; David A. Foster 1Department of Geological Sciences, PO Box 112120, University of Florida, Gainesville, Florida 32611, USA Search for other works by this author on: GSW Google Scholar David R. Gray; David R. Gray 2School of Earth Sciences, University of Melbourne, Melbourne, Victoria 3010, Australia Search for other works by this author on: GSW Google Scholar Catherine Spaggiari Catherine Spaggiari 3Department of Applied Geology, Curtin University of Technology, Perth, Western Australia 6845, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (2005) 117 (1-2): 105–116. https://doi.org/10.1130/B25481.1 Article history received: 08 Sep 2003 rev-recd: 10 Jun 2004 accepted: 15 Jun 2004 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David A. Foster, David R. Gray, Catherine Spaggiari; Timing of subduction and exhumation along the Cambrian East Gondwana margin, and the formation of Paleozoic backarc basins. GSA Bulletin 2005;; 117 (1-2): 105–116. doi: https://doi.org/10.1130/B25481.1 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 The inversion of the Neoproterozoic-Cambrian passive margin of East Gondwana occurred during the early Paleozoic Delamerian-Ross orogeny. We present 40Ar/39Ar and structural data from deformed and metamorphosed Neoproterozoic clastic rocks beneath the Tasmanian ophiolite and the footwall of a high-pressure metamorphic complex in northern Tasmania. These data reveal the timing of accretionary deformation and the initiation of backarc extension along the Australian margin of Gondwana. 40Ar/39Ar analyses of muscovite from lower greenschist facies fault slices bounding the Forth metamorphic complex give plateau ages of 521.4 ± 2.5 and 520.7 ± 1.6 Ma. These data suggest that deformation within an accretionary prism off the margin of Tasmania, and possibly ocean arc collision, were under way by ca. 521 Ma. Muscovite from upper amphibolite and upper greenschist facies rocks in five locations of the Forth metamorphic complex, including retrograde shear zones, give 40Ar/39Ar cooling ages that average 508.1 ± 2.6 Ma. Identical muscovite cooling ages from rocks originally at very different metamorphic P-T conditions suggest rapid cooling of the Forth complex at ca. 508 Ma, due to the juxtaposition of higher-grade against lower-grade rocks. Rapid cooling is also indicated by concordant 40Ar/39Ar ages of hornblende and muscovite in the high-grade core. Cooling was probably due to rapid exhumation along extensional shear zones within a regional extensional setting that also produced the Mount Read–Mount Stavely volcanic complexes (505–495 Ma) along with rift basins in Tasmania and southeast Australia. This continental rift magmatism and extension were caused by west-dipping subduction under the Australian margin of Gondwana after the collisional phase of the Delamerian-Ross orogeny. Rollback of subduction in the Australian sector of the margin between ca. 508 and 460 Ma produced a backarc basin >1000 km wide that became the basement for the Lachlan orogen turbidites. Similar amounts of subduction rollback seem not to have occurred in Antarctica at this time (unless the record is lost), suggesting significant along-strike differences in the early Paleozoic geodynamics of the Delamerian-Ross orogenic system. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Discrete ultrahigh‐pressure domains in the Western Gneiss Region, Norway: implications for formation and exhumation

AbstractNew eclogite localities and new 40Ar/39Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh‐pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km2; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. 40Ar/39Ar mica and K‐feldspar ages show that this outcrop pattern is the result of gentle regional‐scale folding younger than 380 Ma, and possibly 335 Ma. The UHP and intervening high‐pressure (HP) domains are composed of eclogite‐bearing orthogneiss basement overlain by eclogite‐bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc‐silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15‐cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near‐isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12–17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low‐pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New 40Ar/39Ar hornblende ages of 402 Ma document that this decompression from eclogite‐facies conditions at 410–405 Ma to mid‐crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20–30 km. 40Ar/39Ar muscovite ages of 397–380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr−1), perhaps because of continued tectonic unroofing.

Grampian and late Grenville events recorded by mineral geochronology near a basement–cover contact in north Mayo, Ireland

The Palaeoproterozoic to early Neoproterozoic Annagh Gneiss Complex structurally underlies the Dalradian sequence in north Mayo, Ireland, and has been proposed as the depositional basement to Dalradian metasediments. The Annagh Gneiss Complex was deformed, metamorphosed and migmatized during the Grenville Orogeny and later reworked under amphibolite-facies conditions. This paper focuses on the timing of the post-Grenville events and particularly on the possible presence of post-Grenville, pre-Grampian deformation that could be attributed to the Knoydartian Orogeny. Seven U–Pb titanite analyses from Annagh Gneiss Complex gneisses have a weighted mean 207 Pb/ 206 Pb age of 963 ± 8 Ma, which dates cooling after the main Grenville metamorphism. Locally, a later phase of titanite growth at 943 ± 8 Ma post-dates the last phase of Grenville deformation. The weak discordance of the titanite data suggests that post-Grenville events had little effect on the U–Pb system in titanite. If the discordance was caused by a tectonic event, this is likely to have occurred during the early Ordovician Grampian Orogeny rather than in the Neoproterozoic. Within the Annagh Gneiss Complex, cross-cutting metadolerites provide a structural marker allowing post-Grenville deformation to be distinguished. In contrast, correlative metadolerites cutting the adjacent Dalradian metasediments share all Grampian deformation events affecting their host. Ar–Ar hornblende ages from the post-Grenville metadolerites indicate that reworking of the Annagh Gneiss Complex and the first episodes of Dalradian deformation occurred during the Grampian Orogeny in this part of Ireland. One sample yields a 475 ± 4 Ma Ar–Ar plateau age, which is interpreted to date Grampian deformation. Younger Ar–Ar hornblende and Rb–Sr mica ages record post-Grampian cooling. Neither field nor isotopic evidence for the Knoydartian Orogeny has been found in this part of Ireland.

Geochronological constraint on the brittle-plastic deformation along the Hatagawa Fault Zone, NE Japan

K-Ar ages and fission-track ages of granitic rocks in the Hatagawa Fault Zone (HFZ), NE Japan were measured to examine the cooling history of the HFZ. The HFZ is an NNW-SSE trending fault zone in the Cretaceous granitic rocks, and consists of a conspicuous cataclasite and two types of mylonite with a sinistral sense of shear. The cataclasite zone is NNW-SSE trending and continuous over at least 40 km with a maximum thickness of 100 m. One type of mylonite is low-T mylonite, which is mainly developed for a length of 6 km along the HFZ. The other is high-T mylonite, which is widely distributed in the HFZ. Most of K-Ar ages of hornblende and biotite from granitic rocks are about 110 Ma and show no obvious differences along the strike of the HFZ or among different granite bodies. This implies that the granitic rocks in the HFZ have a similar cooling history and cooled rapidly from closure temperature of hornblende to that of biotite. Zircon fission-track analysis shows little possibility of reheating of the granitic rocks. This supports the formation of cataclasite and mylonite during the cooling of the granitic bodies. Fission-track ages of zircon and apatite from the samples in and near the areas where the low-T mylonite is developed are older than those for other areas. Infiltration of near-surface derived water into the low-T mylonite after plastic deformation may account for the accelerate cooling of granitic bodies.

Significance of the Nova Brasilândia metasedimentary belt in western Brazil: Redefining the Mesoproterozoic boundary of the Amazon craton

The Nova Brasilândia metasedimentary belt (NBMB) of western Brazil marks a fundamental crustal boundary in the Amazon craton. The metasedimentary rocks of the NBMB (calc‐silicates, metapelites, quartzites, metabasites) contrast strongly with the older, polycyclic granitoid rocks of the adjacent Amazon craton. Aeromagnetic anomalies indicate that the belt is continuous for at least 1000 km in an E‐W direction, although the easternmost extent of the NBMB is covered by the Cretaceous sediments of the Parecis Formation. Additional geologic evidence suggests that the belt extends along an E‐W trend for ∼2000 km. The northern portion of the NBMB preserves vestiges of an early high pressure‐temperature (P‐T) assemblage (kyanite + staurolite) overprinted by sillimanite during prograde metamorphism. A higher metamorphic grade is observed in the southern portion of the belt, with peak conditions calculated to be 800 MPa and 800°C for granulitic assemblages. The combined P‐T path demonstrates that the competing processes of imbrication (northern domain) and magma generation (southern domain) are responsible for regional metamorphism and crustal thickening. Cooling from peak metamorphic conditions is recorded by U‐Pb monazite ages of 1090 Ma and titanite ages of ∼1060 Ma. Integrated cooling rates of 2°–3°C/Myr from regional metamorphism are calculated from these U/Pb ages combined with 40Ar/39Ar ages of hornblende (∼970 Ma) and biotite (∼910 Ma). The NBMB marks the Mesoproterozoic limit of the SW Amazon craton. The discordance of the NBMB to the NNW structural trend of the younger Aguapeí belt (200 km SE of NBMB), together with marked differences between the two belts in sedimentary environment, metamorphic grade, and timing of deformation, signify that these two belts are not geologically continuous. The “Grenvillian” deformation recorded by the NBMB belt marks the final docking of the Amazon craton and Paragua craton within the Rodinia framework. The Aguapeí belt, in contrast, seems to record only limited deformation internal to the Paragua craton.

Slow post-orogenic cooling in a deeply eroded orogen: Reindeer Zone, Trans-Hudson Orogen, Saskatchewan

We present 40Ar/39Ar hornblende and muscovite data from a 200 km transect across the northern flank of the Paleoproterozoic Trans-Hudson Orogen (THO) exposed along Reindeer Lake in Saskatchewan, Canada. The transect crosses a series of distinct lithotectonic domains that experienced pressure–temperature (P–T) conditions of 600–730 °C at 4.5–6.0 kbar (1 kbar = 100 MPa) during D2 peak metamorphism at 1820–1790 Ma. The distribution of 40Ar/39Ar hornblende ages shows that most of the transect cooled through approximately 500 °C by ca. 1765 Ma, with two significant exceptions. At the north end of the transect, older ages in the Peter Lake Domain (up to 2200 Ma) suggest that this region did not experience high-grade metamorphism that affected the rest of the transect and may indicate a structural break between these Archean rocks and the Cree Lake Zone to the north. In the south-central part of the transect, U–Pb (monazite, titanite) and 40Ar/39Ar (hornblende, muscovite) ages indicate that rocks in the vicinity of the Duck Lake Shear Zone, a ductile thrust zone separating the Kisseynew and La Ronge domains, cooled at a rate of 6 °C/Ma and that cooling was delayed by ca. 20–25 million years relative to that of adjacent regions. Based on generic thermal–tectonic models for large hot orogens, we suggest that delayed cooling in the vicinity of the shear zone reflects post-orogenic thermal relaxation of lower crustal isotherms that were perturbed by the effects of convergence.

The Middle to Late Devonian Høybakken detachment, central Norway: 40Ar–39Ar evidence for prolonged late/post-Scandian extension and uplift

The regionally significant 0.5–2 km thick Høybakken detachment in central Norway bounds the southern margin of the Central Norway Basement Window and exhibits a well-developed top-to-the-WSW fabric characteristic of late Scandian, Devonian ductile extension. 40Ar–39Ar data obtained from hornblende, mica and K-feldspar mineral separates of rocks collected in a transect through the Høybakken detachment yield well-defined plateau and isochron mineral ages. Early Devonian exhumation and cooling of the Høybakken detachment footwall is recorded by hornblende ages of ∼ 400 Ma and mica ages of ∼ 390 Ma. The mylonitic fabric overlying the footwall records younger Middle Devonian mica crystallization ages of 384–381 Ma that are among the youngest extensional ductile fabrics dated in the Caledonides and suggest prolonged extensional activity on the Høybakken detachment. After inferred cessation of ductile extension at 381 Ma, the rate of uplift and cooling was reduced, and the footwall records Late Devonian to Early Carboniferous K-feldspar ages of 371–356 Ma. Prolonged extensional activity at Høybakken is compatible with recent U–Pb ages of deformed titanite crystals and established Rb–Sr ages of white mica in shear-related pegmatites, both from the southwestern part of the Fosen Peninsula, and 40Ar–39Ar ages of syn-tectonic mica overgrowth from the adjacent Hitra–Snåsa Fault. Together, these ages suggest the onset of ductile extension soon after ∼ 401 Ma, and with the Middle Devonian crystallization ages determined here, suggest that ductile extension on the Høybakken detachment had a duration of 11–20 Ma. The youngest age of 320 Ma was obtained from a K-feldspar in a cataclastic granite of the Høybakken detachment's hangingwall and is considered to date a phase of post-Scandian brittle deformation that overprinted the mylonitic shear fabric.

An intercalibration study of the Fish Canyon sanidine and biotite 40Ar/ 39Ar standards and some comments on the age of the Fish Canyon Tuff

The age of the Fish Canyon Tuff (FCT) has been the subject of intense study due to its potential significance as a geochronological test to establish age concordance between the K/Ar and U/Pb isotopic systems. Minerals from the FCT (notably sanidine and biotite) are in widespread use as 40Ar/ 39Ar fluence monitors (“standards”) and have been employed as both primary and intercalibrated standards. In attempting to constrain the true age of FCT, the distinction between primary vs. intercalibrated ages is important, as only the primary age determinations are valid. An investigation of both Fish Canyon sanidine (FCs) and Fish Canyon biotite (FCT-3) was undertaken to assess the chemical and isotopic homogeneity of the minerals on a grain-to-grain scale. Electron microprobe results show that FCs is chemically homogeneous, verifying previous studies, although there are some very small zones of minor K depletion; FCT-3 biotite commonly contains numerous and small mineral inclusions. 40Ar/ 39Ar experiments further show that FCs is remarkably homogeneous, confirming previous observations, whereas FCT-3 shows much greater isotopic variability due to contamination from the mineral inclusions. By setting the age of FCs equal to 27.98±0.15 Ma (2 σ), as intercalibrated to the U/Pb system by Villeneuve et al. (2000), the resultant intercalibration factor between FCs and FCT-3 is 1.005±0.017 yielding an age of 28.13±0.47 Ma (2 σ) for FCT-3, although the significant intragrain chemical and isotopic variability precludes its use as an effective 40Ar/ 39Ar fluence monitor. The true age of the FCT is problematic; a survey of all primary ages for FCT shows that mean K/Ar ages for plagioclase, hornblende, biotite, and sanidine are consistently younger than mean U/Pb ages from titanite and zircon. In conjunction with recently published petrological results which indicate that the FCT contains material from multiple crystallization events, this significant discrepancy between the K/Ar and U/Pb data sets demonstrates that the Fish Canyon Tuff cannot serve as a golden standard between the K/Ar and U/Pb systems.

Regional metamorphism of the Appalachian Humber zone of Gaspé Peninsula: 40Ar/39Ar evidence for crustal thickening during the Taconian orogeny

Geochronological studies on the timing of deformation and metamorphism along the Laurentian margin have shown that the ages of metamorphic events change along-strike within the Newfoundland – southern Quebec segment of the Canadian Appalachians. The Gaspé Peninsula is located at mid-point of the two extremities of this segment. New single-grain laser 40Ar/39Ar plateau ages solely reflect latest Middle–Late Ordovician metamorphism. Samples taken within the internal Humber zone in the Shickshock Group rocks yield 40Ar/39Ar muscovite and hornblende ages ranging from 457 to 454 Ma. Samples from the Amphibolite du Diable, the metamorphic sole of the Mont Albert ophiolite, yield 40Ar/39Ar muscovite and hornblende ages ranging from 459 to 457 Ma. Ordovician ages of the internal Humber zone are consistent with 40Ar/39Ar ages from southern Quebec and are interpreted as the result of the emplacement onto the margin of both the ophiolite and its metamorphic sole.

Travels of the Cache Creek Terrane: a paleomagnetic, geobarometric and 40Ar/ 39Ar study of the Jurassic Fourth of July Batholith, Canadian Cordillera

The Middle Jurassic Fourth of July Batholith and cross-cutting mafic dikes have been studied geochronologically, geobarometrically and paleomagnetically to estimate subsequent tectonic motion of the Cache Creek Terrane (CCT) in the northern Canadian Cordillera. 40Ar/ 39Ar hornblende ages from a granodiorite phase are similar to U–Pb zircon ages and indicate rapid cooling of the batholith upon intrusion, suggesting that the magnetization age is coincident with the 173-Ma crystallization age. Argon ages of biotite from the granodiorite and two mafic dikes have similar ages of ∼165 Ma, which dates cooling through ∼280 °C. Aluminum-in-hornblende geobarometry indicates differential uplift of the batholith across a north–south fault zone along Atlin Lake with >6 km more uplift on its eastern side. Also, the eastern side has been tilted downward to the south–southwest by ∼9°. Combined paleomagnetic data from 20 granitoid and 11 mafic dike sites yield an in situ paleopole at 55°W, 63°N ( d p=5°, d m=5°) and a tilt-corrected paleopole at 81°W, 55°N ( d p=5°, d m=6°). Compared to the 173-Ma reference pole for the North American craton, the tilt-corrected pole suggests a significant southward translation of 16.1±3.7° and a significant clockwise rotation of 107±7°. The translation estimate is similar to the Jurassic Teslin Crossing pluton in the Stikine Terrane, however, the rotation estimate is very different. This could indicate that the Cache Creek Terrane was at a similar latitude of the Stikine Terrane, but the two were not yet amalgamated.

Development of the Hatagawa Fault Zone clarified by geological and geochronological studies

The occurrence of mylonite and cataclasite, mineral assemblages of cataclasite, and the K-Ar ages of surrounding granitic rocks and dikes were studied to examine the possibility that the Hatagawa Fault Zone (HFZ), NE Japan was experienced under the conditions of the brittle-plastic transition. The Hatagawa Fault Zone is divided into three structural settings: mylonite zones with a sinistral sense of shear and a maximum thickness of 1 km, a cataclasite zone with a maximum thickness of about 100 m, and locally and sporadically developed small-scale shear zones. Occurrence of epidote and chlorite, lack of montmorillonite in cataclasite, and the coexistence of cataclasite and limestone mylonite suggest that the cataclasite was deformed at temperatures higher than 220°C. Crush zones in the mylonite near the cataclasite zone were recognized in one outcrop; they have a structure concordant with the surrounding mylonite and some fragments in them are dragged plastically. Granodiorite porphyry dikes near the HFZ intruding into cataclasite and mylonite with a sinistral sense of shear exhibit no deformational features. K-Ar ages of hornblende from host granitic rocks and from one granodiorite porphyry dike are 126 ± 6 to 95.7 ± 4.8 and 98.1 ± 2.5 Ma, respectively. These indicate that the fault activity gradually changed from mylonitization to cataclasis within 28 m.y., and suggest that the HFZ underwent a brittle-plastic transition during its activity.

Posttectonic Cooling of the Nagssugtoqidian Orogen and a Comparison of Contrasting Cooling Histories in Precambrian and Phanerozoic Orogens

Abstract 40Ar/39Ar and U‐Pb mineral ages provide important constraints for the cooling history of the Paleoproterozoic Nagssugtoqidian Orogen (NO), southwest Greenland. In the central segment of the orogen, hornblende yields late Nagssugtoqidian 40Ar/39Ar cooling ages of ca. 1700–1740 Ma. North and south of the orogen, hornblende ages predate Nagssugtoqidian orogenesis. This relationship indicates that metamorphic temperatures exceeding ∼580°C were restricted to a region broadly coinciding with the Nagssugtoqidian Orogen as defined on structural elements. Following peak metamorphism, the southern and central segments of the Nagssugtoqidian Orogen cooled contemporaneously, initially with a rate of 3°C/m.yr. (1762 Ma) that later decreased to 1°–3°C/m.yr. (1400 Ma). The northern segment of the orogen cooled originally slightly faster, with a rate of 5°–7°C/m.yr. at 1740 Ma and subsequently at 1°–3°C/m.yr. Such slow and regionally homogeneous cooling, as observed in the Nagssugtoqidian Orogen, is common to ma...

Geochronological constraints on the timing of migmatization in the Dabie Shan, East-central China

Combined isotopic dating techniques reveal a complicated tectonothermal history for the migmatites at Fenghuangguan and Zongluzui in the Northern Dabie Unit (NDU). The zircon U-Pb data indicate that the rocks were migmatized about 131.7 ± 1.1 Ma ago. Two whole-rocks (leucosome and melanosome) and three mineral separates yield an internal Rb-Sr isochron age of 115 ± 2 Ma. The 40 Ar/ 39 Ar step-heating analyses for four leucosomes give isochron ages of 114 ± 1 Ma and 111 ± 1 Ma for hornblende, 107 ± 0.5 Ma for biotite and 95 ± 2 Ma for K-feldspar. These data indicate that the NDU was uplifted after the migmatization at a cooling rate of 10–12°C/Ma. Biotite 40 Ar/ 39 Ar step-heating analyses for a felsic granulite at Huangtuling give an isochron age of 194 ± 2 Ma and a plateau age of 195 ± 2 Ma, suggesting that the granulite facies metamorphism predated the regional migmatization. Decoupling between the Rb-Sr age for biotite and 40 Ar/ 39 Ar ages for hornblende is interpreted as recrystallization induced by a fluid action. The migmatization is considered as one result of the extensive thermal event in east China, also resulting in the Yanshanian magmatic activities. The previously published isotopic ages of about 130 Ma for the orthogneisses in the NDU may simply record this thermal event. The Nd and Sr isotopic data indicate the existence of pre-Cretaceous, or even Precambrian rocks in the NDU.