Potassium‐argon dates from the Arltunga Nappe complex,northern territory

Twenty‐four mineral separates from the Arunta Complex, four from the metamorphosed Heavitree Quartzite (White Range Quartzite), and one whole rock sample of metamorphosed Bitter Springs Formation, all from the western part of the White Range Nappe of the Arltunga Nappe Complex, and two samples from the autochthonous basement west of the nappe have been dated by the K‐Ar method. The samples from the basement rocks form two groups. Those in the southern or frontal part of the nappe are of Middle Proterozoic (Carpentarian) age (1660–1368 m.y.), determined on hornblende, biotite, and muscovite. In the northern or rear part of the nappe, all but one of the muscovite samples and two biotites are of Middle Silurian to Early Carboniferous age (431–345 m.y.); the remainder of the biotite dates range from 1775 to 548 m.y. (including the two samples from the autochthon), and two hornblendes gave dates of 1639 and 2132 m.y. respectively. All the muscovite samples from the Heavitree Quartzite, and the whole rock sample from the Bitter Springs Formation gave Early to Middle Carboniferous dates (358–322 m.y.). The findings support the identification of the White Range Quartzite as the metamorphosed part of the Heavitree Quartzite, which in turn supports the interpretation of the structure of the area as a large, basement‐cored fold nappe. In addition, they date the time of the Alice Springs Orogeny as pre‐Late Carboniferous, which agrees with fossil evidence from elsewhere in the area. The Alice Springs Orogeny was accompanied by widespread greenschist facies meta‐morphism that progressively metamorphosed the Heavitree Quartzite and Bitter Springs Formation, and retrogressively metamorphosed the Arunta Complex. However, the basement rocks in the southern part of the nappe escaped this metamorphism and retain a Middle Proterozoic age, thus dating the time of the Arunta Orogeny in this region as Carpentarian or older.     

Alice Springs age shear zones from the southeastern Reynolds Range,central Australia

The southeast Reynolds Range, central Australia, is cut by steep northwest‐trending shear zones that are up to hundreds of metres wide and several kilometres long. Amphibolite‐facies shear zones cut metapelites, while greenschist‐facies shear zones cut metagranites. Rb–Sr and ⁴⁰Ar–³⁹Ar data suggest that both sets of shear zones formed in the 400–300 Ma Alice Springs Orogeny, with the sheared granites yielding well‐constrained ⁴⁰Ar–³⁹Ar ages of ca 334 Ma. These data imply that the shear zones represent a distinct tectonic episode in this terrain, and were not formed during cooling from the ca 1.6 Ga regional metamorphism. A general correlation between regional metamorphic grade and the grade of Alice Springs structures implies a similar distribution of heat sources for the two events. This may be most consistent with both phases of metamorphism being caused by the burial of anomalously radiogenic heat‐producing granites. The sheared rocks commonly have undergone metasomatism implying that the shear zones were conduits of fluid flow during Alice Springs times.     

Timing of Cadomian and Variscan tectonothermal activity,La Hague and Alderney,North Armorican Massif: Evidence from 40Ar/39Ar mineral ages

The La Hague region of northwest France exposes Palaeo-Proterozoic Icartian gneisses which were reworked and intruded by calc-alkaline plutonic rocks during the Cadomian Orogeny (about 700–500 Ma). ⁴⁰Ar/³⁹Ar mineral cooling ages have been determined to clarify the timing of the regional metamorphism of orthogneisses and the emplacement of quartz diorite plutons in this region. Metamorphic amphiboles within Icartian gneisses display discordant ⁴⁰Ar/³⁹Ar apparent age spectra interpreted to result from limited Variscan (about 350–300 Ma) overprinting of intracrystalline argon systems which initially cooled through post-metamorphic hornblende closure temperatures during the Cadomian at about 600 Ma. Igneous hornblendes from the weakly foliated Jardeheu and Moulinet quartz diorites record isotope correlation ages of 599 ± 2 and 561 ± 2 Ma, respectively. Igneous hornblende and biotite from foliated quartz diorite on the nearby Channel Island of Alderney record isotope correlation ages of about 560 Ma. The results imply that metamorphic and plutonic events in the La Hague-Alderney region were approximately contemporaneous with those recorded on Guernsey and Sark, which are thus likely to have formed part of the same tectonic block during the Cadomian Orogeny.     

Assessing Ar transport paths and mechanisms in the McClure Mountains hornblende

 We have investigated the mechanisms and pathways by which Ar diffuses through the McClure Mountains hornblende (ferroan pargasite), selected as a good example of material normally dated during Ar-Ar studies. A coarse-grained hornblende separated from the same hand specimen as the MMhb-1 age standard was subjected to a hydrothermal cold-seal bomb experiment and characterized by TEM. Heated and unheated crystals were subjected to four different ³⁹Ar/⁴⁰Ar dating extraction techniques: conventional stepwise heating, infra-red laser spot, ultra-violet laser depth profiling, and closed-system stepwise etching. The stepwise heating age spectrum reproduces the features often interpreted as resulting from a concentric diffusive zonation, but the other three techniques yield results that are not compatible with such a simple picture. The IR laser data indicate that the dependence of laboratory Ar loss on grain size, predicted by Fickian diffusion, is at best poor and instead is related mainly to mineralogical variations. The depth profiles show the importance of planar zones (spaced between <1 and >150 μm from TEM evidence) in providing fast pathways for inward diffusion of atmospheric Ar from the capsule, but showed no evidence of diffusive profiles in the bulk of the hornblende lattice. The data from closed system stepwise etching underscore the role of zones rich in planar defects both for Ar loss and for nucleation of etching. The age spectra obtained by stepwise heating suffer from the differential breakdown of impurity phases, whose presence can be diagnosed with several isotope correlation plots; particularly revealing are Cl-Ca-K trends. In addition to the problems of mineral decomposition during in-vacuo laboratory degassing, an equally important decomposition occurs during many hydrothermal experiments which, combined with problems of mineral purity, have led to an overestimation of the rate of argon diffusion in hornblende. The response of hornblende to thermal disturbance in a hydrothermal environment can be every bit as complex as breakdown in vacuo. Laboratory experiments on bulk samples have not succeeded in quantitatively constraining volume diffusion. Received: 12 October 1995 / Accepted: 11 July 1996     

An 40Ar/39Ar investigation of the contact effects of a dyke intrusion,Kapuskasing Structural Zone,Ontario

An ⁴⁰Ar/³⁹Ar study on biotite and hornblende from baked tonalite gneiss adjacent to a 14-m-wide Kapuskasing dyke in the Chapleau Block of the Kapuskasing Structural Zone employed resistance-furnace step-heating, of bulk-mineral separates, single-grain laser step-heating, and laser spot dating of individual grains in thick section. All three methods yielded concordant results for the biotite, indicating that it has been uniformly overprinted at ca. 2.05 Ga. This date is probably a close estimate of the age of the dyke intrusion. The argon data from the hornblende are more difficult to interpret. All three methods suggest that the hornblende was partially overprinted and then affected by a later influx of excess argon from the external environment. Laser spot dates record an apparent age gradient, extending radially outward from 2.25 Ga in the grain cores to 2.4–2.7 Ga in the grain rims. Corresponding ³⁷Ar/³⁹Ar values show no systematic variation across the hornblende, although ³⁸Ar/³⁹Ar ratios display a similar pattern to the age distribution, with lower values in the cores than in the rims. The bulk separate and single grain step-heating spectra mimic a volume diffusion Ar-loss profile that would appear to be inconsistent with the laser-spot dates. Three possible interpretations to explain this apparent discrepancy may involve either experimental difficulties, an effective radius of diffusion which is much smaller than the actual grain radius, or an effective radius of diffusion that is of the order of the grain radius. It is clear that for the ⁴⁰Ar/³⁹Ar dating of samples that have experienced complex geologic histories, it is important to utilize both resistance-furnace and laser techniques.     

Basement and cover relations on the northern margin of the Amadeus Basin, central Australia

A major, linear, west-trending deformed zone (The Redbank Zone), 350 km long and up to 20 km wide, can be identified in the Arunta Block immediately north of the Amadeus Basin. The marked linearity of this zone and of the coincident gravity anomaly probably result from thrust-fault movement during the Carboniferous Alice Springs Orogeny. However, in the Ormiston area, there is evidence that the zone originated prior to 1070 m.y. and acted as a major crustal feature controlling the later orogenic event.The Alice Springs Orogeny affected the overlying Proterozoic and Lower Palaeozoic cover rocks as well as the Arunta Block basement. During the orogeny, steep north-dipping thrusts within the Redbank Zone were reactivated causing uplift to the north. These faults penetrated the Heavitree Quartzite—the basal unit of the cover sequence—to drive wedges of basement, with attached veneers of Heavitree Quartzite, for up to 20 km southward within the overlying Bitter Springs Formation. The nappes did not reach the surface or penetrate formations above the Bitter Springs. Accompanying nappe emplacement the Basin to the south rapidly deepened to receive a thick wedge of synorogenic molasse sediments.Gravity, sedimentary and structural features combine to suggest that the Alice Springs orogeny movements reached their maximum on the central part of the northern margin of the Amadeus Basin, in the Ormiston area.     

40Ar/39Ar geochronology of a high-grade polymetamorphic terrain,northeastern Strangways Range,Central Australia

Minerals from the northeastern Strangways Range have been dated by ⁴⁰Ar/³⁹Ar total degassing and incremental heating methods. Four periods of metamorphism are indicated: M1 > 1710 Ma, M2 = 1470 Ma, M3 = 700–1050 Ma, and M4 = 326–353 Ma. The two older events are recognised as distinct granulite facies metamorphic episodes, the third event as a complex reheating of the terrain, and the youngest event is the Alice Springs Orogeny.     

40Ar/39Ar age spectra of some undisturbed terrestrial samples

⁴⁰Ar/³⁹Ar age spectra and ⁴⁰Ar/³⁶Ar vs ³⁹Ar/³⁶Ar isochrons were determined by incremental heating for 11 terrestrial rocks and minerals whose geology indicates that they represent essentially undisturbed systems. The samples include muscovite, biotite, hornblende, sanidine, plagioclase, dacite, diabase and basalt and range in age from 40 to 1700 m.y. For each sample, the ⁴⁰Ar/³⁹Ar ratios, corrected for atmospheric and neutron-generated argon isotopes, are the same for most of the gas fractions released and the age spectra, which show pronounced plateaus, thus are consistent with models previously proposed for undisturbed samples. Plateau ages and isochron ages calculated using plateau age fractions are concordant and appear to be meaningful estimates of the crystallization and cooling ages of these samples. Seemingly anomalous age spectrum points can be attributed entirely to small amounts of previously unrecognized argon loss and to gas fractions that contain too small (less than 2 per cent) a proportion of the ³⁹Ar released to be geologically significant. The use of a quantitative abscissa for age spectrum diagrams is recommended so that the size of each gas fraction is readily apparent. Increments containing less than about 4–5 per cent of the total ³⁹Ar released should be interpreted cautiously. Both the age spectrum and isochron methods of data reduction for incremental heating experiments are worthwhile, as each gives slightly different but complementary information about the sample from the same basic data. Use of a least-squares fit that allows for correlated errors is recommended for ⁴⁰Ar/³⁶Ar vs ³⁹Ar/³⁶Ar isochrons. The results indicate that the ⁴⁰Ar/³⁹Ar incremental heating technique can be used to distinguish disturbed from undisturbed rock and mineral systems and will be a valuable geochronological tool in geologically complex terranes.     

Hornblende Ar/Ar geochronology across terrane boundaries in the Sveconorwegian Province of S. Norway

Hornblende incremental heating ⁴⁰Ar/³⁹Ar data were obtained from augen gneiss and amphibolite of the Sveconorwegian Province of S. Norway. In the Rogaland-Vest Agder and Telemark terranes, four pyroxene-rich samples, located close (≤ 10 km) to the anorthosite-charnockite Rogaland Igneous Complex, define an age group at 916 + 12/ − 14 Ma and six samples distributed in the two terranes yield another group at 871 + 8/ − 10 Ma. The first age group is close to the reported zircon U---Pb intrusion age of the igneous complex (931 ± 2 Ma) and the regional titanite U---Pb age (918 ± 2 Ma), whereas the second group overlaps reported regional mineral Rb---Sr ages (895-853 Ma) as well as biotite K---Ar ages (878-853 Ma). In the first group, the comparatively dry parageneses of low-P thermal metamorphism (M2) associated with the intrusion of the igneous complex are well developed, and hornblende ⁴⁰Ar/³⁹Ar ages probably record a drop in temperature shortly after this phase. In other hornblende + biotite-rich samples, with presumably a higher fluid content, the hornblende ages are probably a response to hornblende-fluid interaction during a late Sveconorwegian metamorphic or hydrothermal event. A ca 220 m.y. diachronism in hornblende ⁴⁰Ar/³⁹Ar ages is documented between S. Telemark (ca 870 Ma) and Bamble (ca 1090 Ma). Differential uplift between these terranes was mostly accommodated by shearing along the Kristiansand-Porsgrunn shear zone. The final stage of extension along this zone occurred after intrusion of the Herefoss post-kinematic granite at 926 ± 8 Ma. On the contrary, the southern part of the Rogaland-Vest Agder and Telemark terranes share a common cooling evolution as mineral ages are similar on both sides of the Mandal-Ustaoset Line the tectonic zone between them. The succession within 20 m.y. of a voluminous pulse of post-tectonic magmatism at 0.93 Ga, a phase of high-T-low-P metamorphism at 0.93-0.92 Ga, and fast cooling at a regional scale ca 0.92 Ga, suggests that the southern parts of Rogaland-Vest Agder and Telemark were affected by an event of post-thickening extension collapse at that time. This event is not recorded in Bamble.     

Ar/Ar thermochronologic constraints on deformation, metamorphism and cooling/exhumation of a Mesozoic accretionary wedge, Otago Schist, New Zealand

Structural thickening of the Torlesse accretionary wedge via juxtaposition of arc-derived greywackes (Caples Terrane) and quartzo-feldspathic greywackes (Torlesse Terrane) at 120 Ma formed a belt of schist (Otago Schist) with distinct mica fabrics defining (i) schistosity, (ii) transposition layering and (iii) crenulation cleavage. Thirty-five ⁴⁰Ar/³⁹Ar step-heating experiments on these micas and whole rock micaceous fabrics from the Otago Schist have shown that the main metamorphism and deformation occurred between 160 and 140 Ma (recorded in the low grade flanks) through 120 Ma (shear zone deformation). This was followed either by very gradual cooling or no cooling until about 110 Ma, with some form of extensional (tectonic) exhumation and cooling of the high-grade metamorphic core between 109 and 100 Ma. Major shear zones separating the low-grade and high-grade parts of the schist define regions of separate and distinct apparent age groupings that underwent different thermo-tectonic histories. Apparent ages on the low-grade north flank (hanging wall to the Hyde-Macraes and Rise and Shine Shear Zones) range from 145 to 159 Ma (n=8), whereas on the low-grade south flank (hanging wall to the Remarkables Shear Zone or Caples Terrane) range from 144 to 156 Ma (n=5). Most of these samples show complex age spectra caused by mixing between radiogenic argon released from neocrystalline metamorphic mica and lesser detrital mica. Several of the hanging wall samples with ages of 144–147 Ma show no evidence for detrital contamination in thin section or in the form of the age spectra. Apparent ages from the high-grade metamorphic core (garnet–biotite–albite zone) range from 131 to 106 Ma (n=13) with a strong grouping 113–109 Ma (n=7) in the immediate footwall to the major Remarkables Shear Zone. Most of the age spectra from within the core of the schist belt yield complex age spectra that we interpret to be the result of prolonged residence within the argon partial retention interval for white mica (430–330 °C). Samples with apparent ages of about 110–109 Ma tend to give concordant plateaux suggesting more rapid cooling. The youngest and most disturbed age spectra come from within the ‘Alpine chlorite overprint’ zone where samples with strong development of crenulation cleavage gave ages 85–107 and 101 Ma, due to partial resetting during retrogression. The bounding Remarkables Shear zone shows resetting effects due to dynamic recrystallization with apparent ages of 127–122 Ma, whereas overprinting shear zones within the core of the schist show apparent ages of 112–109 and 106 Ma. These data when linked with extensional exhumation of high-grade rocks in other parts of New Zealand indicate that the East Gondwana margin underwent significant extension in the 110–90 Ma period.     

Exhumation of the lower crust during crustal shortening: an Alice Springs (380 Ma) age for a prograde amphibolite facies shear zone in the Strangways Metamorphic Complex (central Australia)

Foliated garnet-bearing amphibolites occur within the West Bore Shear Zone, cutting through granulite facies gneisses of the Strangways Metamorphic Complex. In the amphibolites, large euhedral garnet (up to 3 cm) occurs within fine-grained recrystallized leucocratic diffusion haloes of plagioclase–quartz. The garnet and their haloes include a well-developed vertical foliation, also present in the matrix. This foliation is the same as that cutting through the unconformably overlying Neoproterozoic Heavitree Quartzite. The textures indicate syn- to late kinematic growth of the amphibolite facies mineral assemblages.
All mineral assemblages record an arrested prograde reaction history. Noteworthy is the growth of garnet at the expense of hornblende and plagioclase, and the breakdown of staurolite–hornblende to give plagioclase–gedrite. These dehydration reactions indicate increasing P – T  conditions during metamorphism, and suggest heating towards the end of a period of intense deformation. Temperature estimates for the garnet–amphibolite and related staurolite–hornblende assemblages from the shear zone are about 600 °C. Pressure is estimated at about 5 kbar.
An Sm–Nd isochron gives an age of 381±7 Ma for the peak metamorphism and associated deformation. This age determination confirms that amphibolite facies conditions prevailed during shear zone development within the Strangways Metamorphic Complex during the Alice Springs Orogeny. These temperature conditions are significantly higher than those expected at this depth assuming a normal geothermal gradient. The Alice Springs Orogeny was associated with significant crustal thickening, allowing exhumation of the granulite facies, Palaeoproterozoic, lower crust. Along-strike variations of the tectonic style suggest a larger amount of crustal shortening in the eastern part of the Alice Springs Orogeny.     

Dating deformation using crushed alkali feldspar: 40Ar/39Ar geochronology of shear zones in the Wyangala Batholith,NSW, Australia

Diffusion parameters have been estimated for K-feldspar in and adjacent to mylonite shear zones in the Wyangala Batholith. The parameters obtained suggest that deformation during mylonitisation would have caused argon systematics to reset because diffusion distances were reduced by cataclasis, deformation and/or recrystallisation. However, the mineral lattice remained sufficiently retentive to allow subsequently produced radiogenic argon to be retained. ⁴⁰Ar/³⁹Ar geochronology is thus able to constrain operation of these biotite-grade ductile shear zones to the period from ca 380 Ma to ca 360 Ma, at the end of the Tabberabberan Orogeny.     

40Ar/39Ar dating of white micas from an Alpine high-pressure metamorphic belt on Naxos (Greece): the resetting of the argon isotopic system

Overprinting of white micas from high pressure, low to medium temperature (M ₁) metamorphic assemblages in pelitic schists on Naxos during subsequent thermal dome (M ₂) metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome. The original (M ₁) minerals are phengites (Si⁴⁺=6.7–7.0) and the overprinting minerals are muscovites (Si⁴⁺=6.0–6.45). ⁴⁰Ar/³⁹Ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra, characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between. These upward-convex age spectra are shown to be caused by mixing of two generations of micas, each of which has a different age spectrum and argon release pattern. Seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages. Further, the upward-convex age spectra have been used to trace the isotopic signature of phengites toward increasing M ₂ metamorphic grade, and suggest that as long as phengites can be observed in the rocks upward-convex age spectra occur. On Naxos, crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas. However, there is also good evidence for argon loss by volume diffusion from phengites. Simple diffusion calculations suggest that the M ₂ metamorphism was caused by a shortlived heat source.Now at Department of Geology, University of Alberta, Edmonton T6G 2E3, Canada     

40Ar/39Ar ages of detrital muscovite and whole-rock slate/phyllite,Narragansett Basin,RI-MA,USA: implications for rejuvenation during very low-grade metamorphism

Late Pennsylvanian sedimentary rocks in the Narragansett basin were metamorphosed (lower anchizone to sillimanite grade) during late Paleozoic regional metamorphism at ca. 275–280 Ma. Twenty-five variably sized concentrates of detrital muscovite were prepared from samples collected within contrasting low-grade areas (diagenesis — lower greenschist facies). Microprobe analyses suggest that the constituent detrital grains are not chemically internally zoned; however, some grains within several concentrates display very narrow (<25 m), compositionally distinct, low-grade, epitaxial peripheral overgrowths. Detrital muscovite concentrates from the lower anchizone are characterized by internally concordant ⁴⁰Ar/³⁹Ar age spectra which define plateau ages of ca. 350–360 Ma. These are interpreted to date post-Devonian (Acadian) cooling within proximal source areas. Concentrates from lower grade sectors of the middle anchizone display slightly discordant spectra in which apparent ages systematically increase from ca. 250–275 Ma to define intermediate- and high-temperature plateaus of ca. 360–400 Ma. Detrital muscovite within samples from higher grade sectors of the middle anchizone and the upper anchizone are characterized by systematic low age discordance throughout both low-and intermediate-temperature increments. High-temperature ages only range up to ca. 330 Ma. Six size fractions of detrital muscovite from a sample collected within the lower greenschist facies have similarly discordant spectra, in which, apparent ages increase slightly throughout the analyses from ca. 250 Ma to 275 Ma. The detrital muscovite results are interpreted to reflect variable affects of late Paleozoic regional metamorphism. However, it is uncertain to what extent the systematic low age spectra discordance reflects intracrystalline gradients in the concentration of ⁴⁰Ar and/or experimental evolution of gas from relatively non-retentive epitaxial overgrowths. However, low age discordance occurs regardless of the extent of epitaxial overgrowth. Intermediate-temperature increments evolved during ⁴⁰Ar/³⁹Ar whole-rock analyses of five slate/phyllite samples are characterized by internally consistent apparent K/Ca ratios. These are attributed to gas evolved from constituent, very fine-grained white mica. Samples from lower grade portions of the middle anchizone are characterized by intermediate-temperature apparent ages which systematically increase from ca. 275–300 Ma to ca. 360–375 Ma before evolution of a high-temperature contribution from detrital plagioclase feldspar. This age variation may reflect partial late Paleozoic rejuvenation of very fine-grained detrital material with a source age similar to that for the detrital muscovites. Slate/phyllite samples from upper sectors of the middle anchizone and from the upper anchizone were completely rejuvenated during late Paleozoic metamorphism and record intermediate-and high-temperature plateau ages of ca. 270–290 Ma. These data document that metamorphic conditions of the lower to middle biotite zone (ca. 325–350 °C) are required to completely rejuvenate intracrystalline argon systems of detrital muscovite. Therefore, the ⁴⁰Ar/³⁹Ar dating method may be useful in determination of detrital muscovite provenance and in resolution of the metamorphic evolution of low-grade terranes.     

Geology and geochronology of the Arunta complex north of Ormiston Gorge,Central Australia

A major west‐trending lineament marked by a wide belt of highly deformed rocks (the Redbank Zone), lies in the Arunta Complex, north of the Amadeus Basin. Along its southern margin the Zone has been progressively affected by, and is hence older than, migmatization and granite intrusion. The migmatization event yields a Rb‐Sr isochron age of 1076 ± 50 m.y. Within the migmatite complex, relicts of a pre‐migmatite metasedimentary sequence around the Chewings Range yield a Rb‐Sr isochron age of 1620 ± 70 m.y. The migmatites are unconformably overlain by the basal unit of the Amadeus Basin sequence, the Heavitree Quartzite. The 1076 ± 50 m.y. date thus provides a maximum age for the start of sedimentation along the northern margin of the Basin. The existence of a major zone of weakness in the basement probably exerted a strong control on basement and cover deformation during the Palaeozoic Alice Springs Orogeny.     

Post-orogenic (<1500 Ma) thermal history of the Palaeo-Mesoproterozoic, Mt. Isa province, NE Australia

We present hornblende, white mica, biotite and alkali feldspar ⁴⁰Ar/³⁹Ar data from Paleo-Mesoproterozoic rocks of the Mt. Isa Inlier, Australia, which reveal a previously unrecognised post-orogenic, non-linear cooling history of part of the Northern Australian Craton. Plateau and total fusion ⁴⁰Ar/³⁹Ar ages range between 1500 and 767 Ma and record increases in regional cooling rates of up to 4 °C/Ma during 1440–1390 and 1260–1000 Ma. Forward modelling of the alkali feldspar ⁴⁰Ar/³⁹Ar Arrhenius parameters reveals subsequent increases in cooling rates during 600–400 Ma. The cooling episodes were driven by both erosional exhumation at average rates of 0.25 km/Ma and thermal relaxation following crustal heating and magmatic events. Early Mesoproterozoic cooling is synchronous with exhumation and shearing in the Arunta Block and Gawler Craton. Late Mesoproterozoic cooling could have either been driven by increased rates of exhumation, or a result of thermal relaxation following a heat pulse that was synchronous with dyke emplacement in the Arunta, Musgrave and Mt. Isa province, as well as Grenville-aged orogenesis in the Albany–Fraser Belt. Latest Neoproterozoic–Cambrian cooling and exhumation was probably driven by the convergence of part of the East Antarctic Shield with the Musgrave Block and Western Australia (Petermann Ranges Orogeny), as well as collisional tectonics that produced the Delamerian–Ross Orogen. Major changes in the stress field and geothermal gradients of the Australian plate that are synchronous with the assembly and break-up of parts of Rodinia and Gondwana resulted in shearing and repeated brittle reactivation of the Mt. Isa Inlier, probably via the displacement of long-lived basement faults within the Northern Australian Craton.     

Rb-Sr whole-rock and 40Ar/39Ar mineral ages of the Togus and Hallowell quartz monzonite and Three Mile Pond granodiorite plutons,South-Central Maine: Their bearing on post-Acadian cooling history

Generally synmetamorphic granitic stocks intrude high-grade, Silurian-lowermost Devonian metasedimentary rocks near Augusta, Maine. Rb-Sr whole-rock isochrons (8 points each) define mutually overlapping crystallization ages of 394±8 m.y. (Togus quartz monzonite), 387±11 m.y. (Hallowell quartz monzonite), and 381±14 m.y. (Three Mile Pond biotite granodiorite), thereby providing a narrow chronologic bracket for Acadian tectonothermal activity in the area. Igneous hornblende, muscovite, and biotite display internally concordant ⁴⁰Ar/³⁹Ar age spectra with plateau dates of 350 m.y. (hornblende) and 300-265 m.y. (muscovite and biotite), with an overall southwestward younging trend. The mineral dates are similar to those recorded in adjacent portions of the regional metamorphic terrain and suggest a prolonged postmagmatic cooling which closely followed the diachronous northeast-southwest post-Acadian cooling of the country rocks. No evidence for a distinct Permian thermal overprint of older isotopic systems has been observed.     

Argon retentivity and argon excess in amphiboles from the garbenschists of the Western Tauern Window,Eastern Alps

22 hornblende K-Ar ages and 10 ³⁹Ar-⁴⁰Ar spectra were obtained for hornblende garbenschists from the Western Tauern Window. The post-kinematic amphiboles were produced during the late Alpine prograde metamorphism (6–10 kb and 500–570° C). Two nearly potassiumfree cummingtonites rimming hornblende yield K-Ar ages of 120 Ma, while the 20 tschermakitic hornblendes scatter between 17 and 37 Ma. The reason for this scatter is excess Ar, possibly incorporated into amphiboles during healing of fractures, now traceable by trails of fluid inclusions. Excess Ar is semiquantitatively corrected for by combining cogenetic hornblende and cummingtonite with K-Ar isochrons. It can be quantified in 4 out of 10 hornblendes by ³⁹Ar-⁴⁰Ar stepwise heating experiments. Ages of 18–20 Ma result for corrected hornblendes. The retentivity of ⁴⁰Ar, after correction for excess, shows no correlation with chemistry within the narrow compositional range observed; rather, it shows intriguing correlations with irregularities in Ca/K spectra, pointing to a microstructurally controlled mechanism for Ar loss. This observation leads to a critical evaluation of the closure temperature constant, which apparently depends on an incompletely known number of mineralogical and environmental parameters. In particular those ³⁹Ar-⁴⁰Ar release spectra which yield low temperature steps with younger ages than the plateaus are not interpretable in terms of a synchronous closure. This gives evidence that loss of radiogenic isotopes proceeds by a more complex mechanism than simple volume diffusion through isotropic media.     

The Harry Creek Deformed Zone,a retrograde schist zone of the Arunta Block,central Australia

The Harry Creek Deformed Zone, a retrograde schist zone of epidote amphibolite facies grade, which separates the granulite facies Utralanama Block from the amphibolite facies Ankala Block in the southeastern Strangways Range, N.T., is typical of the retrograde schist zones transecting the Arunta Block. Associated with the deformed zone is a small deformed granitic pluton and its various offshoots—the Gumtree Granite Suite—which provides structural and geochrono‐logical evidence that the Harry Creek Deformed Zone has had a polyphase deforma‐tional history. Early movements within the deformed zone pre‐dated intrusion of the Gumtree Granite Suite and resulted in the movement of the Utralanama and Ankala Blocks into their present juxtaposition. Reactivation of much of the zone during the Alice Springs Orogeny brought about the schistose character of the zone and the deformation of the granitic rocks. Further minor reactivation of the zone, subsequent to the main phase of the Alice Springs Orogeny, resulted in limited development of pseudotachylytes.

The age of the granite (990 ± 13 m.y.) gives a minimum age for initiation of the zone, and evidence for the nature of the structures associated with the early movements is presented. It is suggested that the Harry Creek Deformed Zone represents a post‐orogenic wrench fault which has been reactivated. Early movements, which were of a brittle transcurrent nature, brought about major uplift (up to 10 km) to the north, and lateral movements may have been of the order of 60 km in a sinistral sense. Comparison with the Redbank Zone indicates many similarities, suggestive of a similar history.     

40Ar/39Ar spectrum ages for biotite,hornblende and muscovite in a contact metamorphic zone

Biotite, hornblende and muscovite from 2700 m.y. old rocks in northeastern Minnesota near the contact of the 1150 m.y. Duluth Complex have been analyzed by ⁴⁰Ar/³⁹Ar technique to determine whether spectrum ages can be used to distinguish partial loss of radiogenic argon due to a reheating event. Biotite and hornblende give plateau ages comparable to the ordinary K-Ar ages for all samples including those with intermediate ages. Muscovite gives plateau ages for the samples with less than 11% argon loss. An intermediate muscovite with a conventional K-Ar age of 1850 m.y. gives progressively older ⁴⁰Ar/³⁹Ar ages for higher temperature fractions.Microprobe analysis reveals no systematic correlation between biotite chemistry and loss of argon in the contact zone. This suggests that the rate-controlling process for the loss of argon from biotite in the contact zone may be volume diffusion or recrystallization without a measurable change in major element composition. Biotites with intermediate ages give plateaus because the rate-controlling processes in the vacuum furnace are related to dehydroxylation and delamination and are unrelated to the process causing loss of argon in the contact zone.The data for the muscovites are not easily interpreted, in part because of the limited number of samples. The hornblende data show a correlation between argon loss and change in major element composition suggesting that recrystallization may be a rate-controlling process for the loss of argon from hornblende in the contact zone. The small number of samples precludes a definitive statement.