Rb-Sr Mineral Ages Research Articles

T-t Evolution of the Early Proterozoic Rocks in the Northern Ladoga Region from the Data on U-Pb, Rb-Sr and Sm-Nd Systems in Minerals

This paper presents the results of a study of isotopic systems in minerals and rocks in southern margin of the epi-Archean Karelian craton in the zone of its junction with the Svecofennian mobile belt. U-Pb, Sm-Nd and Rb-Sr mineral ages of metamorphic rocks allowed reconstructing a T-t trend during ~1.88–1.61 Ga, which reflects a wide-ranging cooling history of metamorphic rocks from the peak values of about 650–700 °C at 1.88–1.79 Ga (U-Pb age of monazites and apparent oldest Sm-Nd age of amphiboles) to 300–400 °C at 1.61 Ga (model Rb-Sr age of biotites) in zones of low- and medium-temperature metamorphism. The specificity of removal of deep-seated rocks to the present-day erosion surface and the reconstructed T-t trend comply with the development of thrust-nappe structures during the exhumation of the Svecofennids. It is also assumed that differential vertical block movements played a significant role during the post-orogenic extensional collapse and neorifting.

Petrography and Rb-Sr mineral age of mafic dyke rock from Niban Iwa, Lützow-Holm Complex, East Antarctica

Petrography and Rb-Sr mineral age of mafic dyke rock from Niban Iwa, Lützow-Holm Complex, East Antarctica

Paleo-Mesoproterozoic arc-accretion along the southwestern margin of the Amazonian craton: The Juruena accretionary orogen and possible implications for Columbia supercontinent

The southwestern portion of the Amazonian craton, between the Ventuari-Tapajós province and the Andean chain, has been ascribed to a succession of orogenic events from 1.81 to 0.95 Ga, culminating with widespread anorogenic magmatism. Southwestward of the Serra do Cachimbo graben occurs the Juruena accretionary orogenic belt (ca. 1.81–1.51 Ga), previously included in the Rio Negro-Juruena and Rondonian/San Ignácio geocronological provinces or Rondônia-Juruena geologic province. The Juruena orogen proposed here includes the Jamari and Juruena tectonostratigraphic terranes, products of convergence which culminated in the soft collision of the Paraguá protocraton and the Tapajós-Parima arc system (Tapajós Province) ca. 1.69–1.63 Ga ago. Geophysical, geochemical, petrological and geochronological data and systematic geological mapping suggest that the convergent event resulted in a single orogenic system with two continental margin arcs, namely the Jamari and Juruena arcs. Modern geological and tectonic approaches, combined with aerogeophysics data, enable to interpreting this wide region of the Amazonian craton as a Paleoproterozoic orogen with well defined petrotectonic units and tectonoestructural framework. The Juruena orogen is an E-W trending belt, about 1100 km long and 350 km wide, inflecting to NW-SE, in Mato Grosso, Amazonas and Rondonia, Brazil. The general direction of the belt, its inflections and internal geometric and kinematic aspects of its macrostructures do not corroborate the general NW-SE trend of the originally proposed geocronological provinces.The Juruena accretionary orogen has been the site of repeated reactivation with renewed basin formation, magmatism and orogeny during the Mesoproterozoic and the early Neoproterozoic. U-Pb and whole-rock Sm-Nd ages, Ar-Ar and Rb-Sr mineral ages suggest that the older high grade tectonometamorphic events in the Juruena accretionary orogen took place between 1.69 and 1.63 Ga, defining the metamorphic peak of the Rio Negro-Juruena orogeny and the original structural architecture of the orogen. The main structures of the Rio Negro-Juruena orogeny were almost completely obliterated between 1.42 and 1.37 Ga, due to the superimposed Rondonian-San Ignácio orogeny. Later on, between 1.20 and 1.12 Ga, these structures were affected by a ductile event represented by the large Ji-Paraná transpressive shear system and by the Tarilândia transcurrent shear zone developed under low-T amphibolite facies conditions, both related to the Sunsás orogeny. The evolution of Jamari and Juruena terranes as a Paleoproterozoic/Mesoproterozoic orogen developed between 1.81 and 1.51 Ga and their thorough reworking in Mesoproterozoic times is instrumental to establishing the position of the Amazonian craton in the proposed Paleo-Mesoproterozoic Columbia (Nuna) supercontinent, as well as in the Rodinia reconstruction at the end of the Mesoproterozoic.

Evidence from Rb–Sr mineral ages for multiple orogenic events in the Caledonides of Shetland, Scotland

Shetland occupies a unique central location within the North Atlantic Caledonides. Thirty-three new high-precision Rb–Sr mineral ages indicate a polyorogenic history. Ages of 723–702 Ma obtained from the vicinity of the Wester Keolka Shear Zone indicate a Neoproterozoic (Knoydartian) age and preclude its correlation with the Silurian Moine Thrust. Ordovician ages of c . 480–443 Ma obtained from the Yell Sound Group and the East Mainland Succession constrain deformation fabrics and metamorphic assemblages to have formed during Grampian accretionary orogenic events, broadly contemporaneously with orogenesis of the Dalradian Supergroup in Ireland and mainland Scotland. The relative paucity of Silurian ages is attributed to a likely location at a high structural level in the Scandian nappe pile relative to mainland Scotland. Ages of c . 416 and c . 411 Ma for the Uyea Shear Zone suggest a late orogenic evolution that has more in common with East Greenland and Norway than with northern mainland Scotland. Supplementary material: Detailed appraisal of biotite petrography is available at http://www.geolsoc.org.uk/SUP18887 .

Pressure–temperature–deformation–time of the ductile Alpine shearing in Corsica: From orogenic construction to collapse

Definition of the Tertiary tectono-metamorphic history of Alpine Corsica is a key task to decipher the space–time linkage between the Alpine and Apennine subduction systems in the Mediterranean region. Alpine Corsica exposes a nappe stack of oceanic- and continental-derived units, structurally juxtaposed onto the former European continental margin (Hercynian Corsica). Still uncertain is the timing of involvement of the continental-derived units in orogenic construction and shift to regional extension. This paper focuses on reconstruction of the pressure–temperature–deformation–time evolution of selected ductile shear zones activated during transition from the tectonic underplating to the extensional reworking stages. New Rb–Sr mineral age data, integrated with structural and thermobarometric investigations constrain the waning stages of the high-pressure (from blueschist to greenschist facies metamorphic conditions) top-to-the-W thrusting of the HP, oceanic-derived realm (Schistes Lustrés Complex) onto the Hercynian Corsica along the East Tenda Shear Zone in the early Oligocene (from ~32 to ~27Ma). This early compressional evolution is overprinted by a major phase of retrogressive, syn-greenschist top-to-the-E extensional shearing in the Schistes Lustrés Complex with the last episode of deformation-related ductile recrystallization recorded during the early Miocene at ~20–21Ma, in a continuum transition from ductile to brittle shearing. The same early Miocene Rb–Sr deformation ages are recovered from the ductile-to-brittle top-to-the-E reactivation domains within the East Tenda Shear Zone, documenting that transition from compression to extension in Alpine Corsica occurred during the late Oligocene–early Miocene time lapse. Implications of these data are discussed in the broader context of the Tertiary geodynamic evolution of the Central Mediterranean region.

Evaluation of the 87Rb decay constant by age comparison against the U–Pb system

The Rb–Sr decay system is one of the most widely used geochronometers for obtaining ages and cooling rates of terrestrial magmatic, metamorphic, and hydrothermal events. It has also been extensively applied to date extraterrestrial, early solar system events. The accuracy of Rb–Sr ages, however, strongly depends on the accuracy of the 87Rb decay constant (λ87Rb). We determined λ87Rb relative to the decay constants of 235U and 238U by comparing Rb–Sr ages of minerals with U–Pb ages obtained from the same intrusion. Comparison of U–Pb emplacement ages with high-precision Rb–Sr mineral ages from three rapidly cooled igneous rocks covering an age range of ca. 2.5Ga yields an unweighted mean λ87Rb of 1.393±0.004×10−11yr−1 (i.e., ±0.3%), corresponding to a half-life of 49.76×109years. Because this decay constant is 2% lower than the presently recommended one, many previously published ages are 2% too young and the resulting geologic interpretations may need revision.

On the origin and tectonic significance of the intra-plate events of Grenvillian-type age in South America: A discussion

Abstract The objective of this article is to examine the available evidence of intra-plate tectonic episodes of “Grenvillian-type age”, affecting the South-American continent, assessing their possible causal correlation with the tectonic processes occurring within the orogenic belts active at their margins. For the Amazonian Craton, the active margin is represented by the Rondonian-San Ignacio and Sunsas belts. However, active margins of similar age are not recognized for the Sao Francisco and the Rio de La Plata Cratons, and the intra-plate events over them could be reflections of the Kibaran, Irumide or Namaqua orogenic collisions in Africa. Grenvillian-type age events over the Amazonian Craton can be described in four different aspects: shearing and tectonic reactivation along zones of weakness, cratogenic granitic magmatism, alkaline ring complexes, and pervasive regional heating in some localized regions. The first of them may reflect the compressional stresses at active margins, however the others may have different origins. Within the type-region of the K’Mudku tectono thermal episode, mylonites and pseudotachylites cut across the regional granitoid and metamorphic rocks. These shear belts developed under low-to-moderate temperature conditions, that induced resetting of K–Ar and Rb–Sr mineral ages. In the Sao Francisco Craton, extensional and compressional events of Grenvillian-type age are well registered by the structural features exhibited by the sedimentary rocks of the Espinhaco Supergroup. For example, in Bahia state, an Appalachian-style structure is observed, with large synclines and anticlines extending along hundreds of kilometers. The major difference between the Amazonian and the Congo-Sao Francisco Cratons is related to heat originated from the Earth’s interior. Amazonia exhibits very large areas heated up to 350–400 °C, where the K’Mudku thermo-tectonic episodes were detected. In addition, Amazonia comprises a large amount of cratogenic granitic intrusions, and some alkalic complexes of Mesoproterozoic age, whose origin could be attributed, at least partially, to deeper sources of heat. This is not reported for the Sao Francisco Craton, and also for its African counterpart, the Congo Craton. Moreover, the Grenvillian-type age intra-plate features over South America demonstrate that while many cratonic fragments were colliding to build Rodinia, rifting was already occurring in parts of the Amazonian and the Congo-Sao Francisco Cratons.

Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction–collision system, Bergen Arcs, Norway

Rb–Sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes. Granulite facies rocks of the Lindas nappe, Bergen Arcs, Norway, were subducted and exhumed during the Caledonian orogeny. The rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation, along fractures and shear zones. The first occurred at eclogite facies (15–21 kbar, 650–750°C) and a later one at amphibolite facies conditions (8–10 kbar, 600°C). The presence of fluids enabled local metamorphic equilibration only near fluid pathways. In fluid-absent domains, preexisting assemblages were metastably preserved. This resulted in a heterogeneity of metamorphic signatures on meter to μm-scales. Well-preserved granulite facies rocks preserve their Proterozoic Rb–Sr mineral ages, as does the U–Pb system of zircon in most lithologies. Six Rb/Sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid-induced eclogitization at 429.9 ± 3.5 Ma (2σ, weighted average, MSWD = 0.39). An eclogite facies vein has yielded metamorphic zircon with concordant U–Pb ages of 429 ± 3 Ma, identical to the U–Pb age of 427.4 ± 0.9 Ma for zircon xenocrysts in an amphibolite facies vein. Seven Rb/Sr mineral isochron ages date amphibolite-facies fluid infiltration at 414.2 ± 2.8 Ma (MSWD = 1.5), an age value testifying to residence of the rocks in the deep orogenic crust at temperatures >600°C for nearly 15 Ma. The new data show that Rb–Sr mineral isochron ages effectively date fluid-induced (re)crystallization events rather than stages of cooling. The direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the P–T-reaction-time space, which is essential for understanding exhumation histories and the internal dynamics of orogens in general.

470 Ma granitoid magmatism associated with the Grampian Orogeny in the Slishwood Division, NW Ireland

Ion-microprobe U–Pb zircon geochronology demonstrates that magmatic rocks that cut the Slishwood Division are early Ordovician in age and relate to the earliest stages of the Grampian Orogeny. These tonalite and granite intrusions yield ages of 474 ± 5 Ma, 472 ± 6 Ma, 471 ± 5 Ma and 467 ± 6 Ma. A Sm–Nd mineral isochron of 457 ± 36 Ma confirms an Ordovician age for one of the tonalite bodies and demonstrates that they were affected by Ordovician metamorphism whereas biotite Rb–Sr mineral ages from two of the tonalite bodies show that the Slishwood Division had cooled below c. 350 °C at 449 ± 7 Ma. The early orogenic tonalite and granite intrusions are mylonitized in structures that overthrust the Dalradian Supergroup onto the Slishwood Division during major nappe formation and, thus, date the onset of such deformation at or before 471 ± 5 Ma, the age of the youngest intrusion where such field relations can be demonstrated.

Dating deformation and cooling in the Caledonian thrust nappes of north Sutherland, Scotland: insights from 40 Ar/ 39 Ar and Rb–Sr chronology

40 Ar/ 39 Ar and Rb–Sr mineral ages have been determined from various lithologies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb–Sr muscovite ages of c . 428, c . 421 and c . 413 Ma obtained from Moine Thrust Zone mylonites are interpreted to date closely regional thrusting during the Late Silurian to Early Devonian. 40 Ar/ 39 Ar muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb–Sr analyses of muscovites from the same samples; it is likely that this discrepancy results from a component of extraneous or ‘excess’ argon. 40 Ar/ 39 Ar hornblende ages and Rb–Sr and 40 Ar/ 39 Ar muscovite ages obtained from structurally higher metamorphic units in the Caledonian thrust nappes generally range between c . 440 Ma and c . 410 Ma. These ages are interpreted to date cooling during and following ‘D 2 ’ regional thrusting and folding within internal sectors of the nappe sequence. A possible tectonic model involves the Silurian collision of Baltica with Scottish segments of Laurentia resulting in the Scandian orogeny and broadly coeval Moine Thrust Zone. D 2 structures were superimposed on structures and metamorphic fabrics formed during a regional Mid-Ordovician tectonothermal event dated previously at c . 470–460 Ma. Syn-D 2 temperatures were generally >600°C and sufficient to achieve more or less complete thermal rejuvenation of Rb–Sr and 40 Ar/ 39 Ar isotopic systems in muscovite and hornblende, even in areas of low D 2 strain.

Th–U–Pb isotopic systems and internal structures of complex zircons from an enderbite from the Pium Complex, Carajás Province, Brazil: evidence for the ages of granulite facies metamorphism and the protolith of the enderbite

SHRIMP U–Pb micro-analyses are reported for parts of complex zircons from a granulite facies enderbite from the Pium Complex in the Carajás Metallogenic Province of the Amazon Craton of Brazil. Zircons from the granulite consist of two types of grains — (a) those with oscillatory zoned cores surrounded by rims composed of two or more curved, overlapping, nebulously zoned domains distinguished by differences in cathodoluminescence intensity, and (b) grains which are composed entirely of such domains and have no obvious cores. The SHRIMP U–Pb age of 3002±14 Ma, determined for oscillatory-zoned zircon cores is interpreted as dating the parent rock of the enderbite. This age is resolved from the U–Pb age of 2859±9 Ma, determined on zircon rims, which is interpreted as a time point within the evolving granulite facies metamorphism. SHRIMP U–Pb ages of cores and rim-type zircon are generally concordant to slightly discordant where uranium contents are less than about 820 ppm, but once this value is exceeded the zircon becomes strongly discordant suggesting that a threshold value of radiation damage exists above which zircon stability with respect to Pb loss is sharply reduced. The results provide the first substantial age determination of the granulite metamorphism in the Carajás Province. The zircon U–Pb ages have not been influenced by the Transamazonian metamorphism dated at ca. 2.0–1.85 Ga from Ar–Ar and Rb–Sr mineral ages.

A reassessment of the timing of early Archaean crustal evolution in West Greenland

In last year’s Review of Greenland activities, Kalsbeek (1997) divided the recent history of geochronology into three successive periods: 1. single-sample K-Ar and Rb-Sr mineral or whole-rock age determinations; 2. Rb-Sr and Pb/Pb whole-rock isochrons and multigrain zircon U-Pb isotope data; 3. the present, where ‘single’ zircon U-Pb data are predominantly used. To these three, we would propose adding a fourth, namely a combination of all three, in order to achieve the maximum age information within complex terrains. For an early Precambrian terrain like that of West Greenland, we consider that the combined use of at least the last two approaches is essential (to which should be added the Sm-Nd method). In recent years, study of the geochronological evolution of the Godthåbsfjord and Isua regions has been dominated by rapid and precise ion-probe U-Pb dating of complex-structured zircons, and it has become fashionable to regard the wide range of zircon dates, and particularly the oldest, as giving the age of rock formation. Dates obtained from whole-rock Rb-Sr, Sm-Nd and Pb/Pb regressions have been regarded as too imprecise for adequate age resolution, whilst constraints on crustal evolution imposed by initial Sr, Nd and Pb isotope ratios have been summarily dismissed or totally ignored. We consider that this sole dependence on ion-probe dating of zircon can lead (as, indeed, in the early Archaean of West Greenland) to a potential misinterpretation of the timing of crustal evolution, especially in those cases where little or no information regarding the relationship between measured date and internal grain structure is available. Figure 1 shows the localities mentioned in the text.

Timing of an extensional detachment during convergent orogeny: New Rb-Sr geochronological data from the Zanskar shear zone, northwestern Himalaya

The Zanskar shear zone of northwest India forms the western segment of the South Tibetan detachment system, a north-dipping normal fault and shear zone that unroofed high-grade metamorphic rocks during contraction of the Himalayan orogen. New Rb-Sr mineral ages from the shear zone and its footwall show that ductile deformation was ongoing at 26 Ma, and continued to 16 Ma in some sections. The nature of the deformation varies along strike; displacement at 25 Ma or earlier in the western section did not result in a large thermal offset. By contrast, the sections to the east display substantial ductile thinning at midcrustal levels that was superseded by brittle detachments as the system continued to exhume rocks of the High Himalayan crystalline sequence. The implication is that the Zanskar shear zone may have had a smaller offset in the west and cut down-section to the east. The time of the bulk of the deformation of this structure is determined to be similar to that of the equivalent structure in most of the eastern and central Himalaya.

Argon Geochronology and Phengites: A Story of Excess Argon, Diagrams and Paradoxes

Among the most debated questions in the world of metamorphic dating is certainly that of high pressure metamorphism which ultimately underpins most geodynamical reconstructions and mechanisms of orogenesis. Applying argon dating to high pressure material brings both good news and possibly bad. The good news is that during the HP peak potassium rich phases are likely to crystallise in most pelitic but also in many magmatic systems. This is often represented by the crystallisation of the silica rich white mica: phengites. On the other hand the bad news is that the dating of such a ubiquitous and highly valuable phase has lead to a growing controversy. Many authors suspect that large quantities of excess argon can be incorporated during the growth of phengite leading to anomalously old ages. Moreover several flaws in the analyses protocol or the graphical tools usually used often prohibit the identification and correction of the incorporated excess argon. However only rarely can clear evidence of that artefact be isolated and the controversy rises from contradictory interpretations of age data. Below we expose a geological case in which field data and independent age constraints lead to the proposal of either the presence of excess argon, or very unusual diffusion characteristics for phengite. This in turn implies geological consequences which can be tested. Finally we discuss the possible directions of research which should lead to a definitive assessment of the role of phengite in dating HP terrains. metamorphic rocks which have yielded 40Ar-39Ar phengite and glaucophane ages of 88 to 108 Ma (Okay and Kelley, 1994; Harris et al., 1995) and 72 to 124 Ma A single sample, 96/134, has yielded a range of isotopic ages for the single high-pressure metamorphic event, with a Rb-Sr mineral age of 80.2 -41.6 Ma. for the same sample ultra-violet laser ablation microprobe (UVLAMP) white mica analyses are in the range 78 • 2 to 109 + 3 Ma and classical furnace step heating yielded a convincing plateau age of 107 _+ 2 Ma (Fig. 1), consistent with many of the oldest UVLAMP ages but 25 Ma older than the RbSr age for the same mineral.

Age determination of Precambrian rocks from Greenland: past and present

Age determination is an essential part of many geological investigations. The ages of Phanerozoic sedimentary rocks are typically determined by palaeontological studies (often with high precision), but for Precambrian rocks as well as for younger igneous intrusions, precise ages can only be obtained by isotopic analysis of minerals or rocks. Isotopic (radiometric) age determination of Greenland rocks began in the early 1960s and has continued since with gradually improving methods. In this contribution, the development of geochronological knowledge of the Precambrian of Greenland is described in historical perspective, and an outline of new results is given. The history of geochronology can be roughly divided into three periods: 1) a period of single-sample K-Ar and Rb-Sr mineral or whole-rock age determinations; 2) a time when most ages were determined with the help of Rb-Sr and Pb-Pb whole-rock isochrons and multi-grain zircon U-Pb isotope data; 3) the present, where ‘single’ zircon U-Pb data are the preferred method to obtain rock ages. These stages in the development of radiometric dating methods partly overlap in time, and each has yielded very significant contributions to the knowledge of Precambrian evolution in Greenland.

RbSr microchrons in the Manaslu granite: implications for Himalayan thermochronology

Samples of minerals from the Manaslu leucogranite in the central Himalaya were microsampled and measured for RbSr mineral ages. The ages obtained are used in conjunction with models for RbSr closure temperatures, to reconstruct the thermochronological history of the Manaslu granite. The RbSr results indicate a rapid change in temperature between 21 Ma and 17 Ma of 60–70°C/million yr, which, in turn, indicates unroofing at 2 mm/yr. The temperature-time curve inferred from RbSr data is in excellent agreement with existing ArAr data on the Manaslu granite and its thermal aureole. The results confirm that parts of the Himalaya, in addition to the Gangdese belt, were eroded rapidly in the 20-17 Ma time period, and this erosion could account for the observed rapid increase in seawater 87Sr 86Sr . A tourmaline-whole rock age of 29.5 Ma was obtained on a sample from the Chokkang Arm, but may be unreliable because of post-crystallization alteration of feldspar. Microsampling approaches are demonstrated that allow high precision isotopic data to be obtained on minerals sampled at scales of 100 μm or less. Microanalysis, combined with models for Sr redistribution, can be a useful thermochronological tool that can augment the 40Ar 39Ar approach.

Isotopic constraints on Pan-African metamorphism in Dronning Maud Land, Antarctica

Garnet SmNd isotopic data are presented for selected samples from the metamorphic terrane of the H.U. Sverdrupfjella, western Dronning Maud Land, and yield Pan-African ages ranging from 680 to 415 Ma. Reproducibility of the garnet data is outside analytical error, but this does not affect the consistency of ages obtained. The garnet ages are considerably younger than the inferred initial crystallization age, based on petrographical and fabric analysis, structural, and whole-rock and zircon isotopic data which support initial growth during the Grenvillian orogeny (∼ 1200-1100 Ma). The garnet ages, however, are indistinguishable from a large number of RbSr mineral ages. The possibility that the garnets remained above the blocking temperature until Pan-African exhumation is discounted. Polymetamorphism of the metamorphic terrane associated with both the Grenbillian and Pan-African orogenes is favoured. The garnet ages are interpreted as reset ages, but the possibility that the garnets crystallized during the Pan-African event cannot be discounted. In this case, the data imply that the Pan-African orogeny attained lower granulite- to amphibolite-facies metamorphic conditions, as indicated by geothermometry and geobarometry calculations involving garnet. In either case, the H.U. Sverdupfjella area was subjected to significant Pan-African heating to temperatures of the order of 600–650°C.

Rb-Sr redistribution during amphibolite-grade mylonitization: An example from the Hope Valley Shear Zone, Massachusetts, U.S.A.

The Sr and Pb isotopic systematics of minerals and whole rock slabs cut from three large samples of the Ponaganset Gneiss near the Hope Valley Shear Zone (HVSZ) in south-central Massachussets indicate that isotopic redistribution along a Late Paleozoic mylonitic zone in the gneiss occurred in two distinct episodes, via two discrete mechanisms. The Rb-Sr data also provide additional geochronologic evidence for high-grade Alleghanian metamorphism in SE New England. At the sample locality, the Ponaganset Gneiss was sheared at the contact between two lithologically distinct gneiss units. A narrow (<10 cm) band of mylonitic mafic schist also occurs intermittently at the contact. A whole rock age from thin slabs of the mylonitized gray gneiss (274 ± 14) agrees well with a mineral age from the mylonitic gray gneiss (268 ± 5), and with other published age data on Alleghanian metamorphism in southeastern New England. The mafic schist Rb-Sr mineral age (306 ± 4) is significantly older, and may represent the age of the initial episode of deformation and recrystallization. The age-corrected Sr and Pb isotopic signatures of the mylonitized gray gneiss are consistently more radiogenic than those of the adjacent units. Therefore, it is possible that the isotopic alteration of the mylonitic zone involved Sr and Pb from an outside source. Closer examination of the mylonitic gneiss-mafic schist contact also revealed a zone of less radiogenic Sr within 2–3 cm of the contact. The shapes of Rb-Sr isotopic profiles across the contact are indicative of diffusive redistribution which occurred during, or after, the foliation-parallel alteration of Sr and Pb isotopic signatures in the mylonitized gray gneiss. Nonlinear least-squares fits of the data to a diffusion equation yield Dt of approx. 8 × 10 5 m 2 ( D = effective diffusivity of Sr in m 2/s, t = time in s). Attempts to use this result to estimate the duration of Rb-Sr diffusion produced results that are not consistent with geochronological data, which indicate that these rocks experienced high temperatures for up to 10 7 yr.

Tectonothermal chronology within a blueschist-eclogite complex, west-central Spitsbergen, Svalbard: Evidence from [formula omitted] and Rb[sbnd]Sr mineral ages

A high-pressure, relatively low-temperature metamorphic complex is exposed at Motalafjella, Spitsbergen. White mica concentrates from the complex record variably discordant 40Ar39Ar age spectra in which apparent ages systematically increase throughout low-temperature portions of the analyses and define intermediate- and high-temperature plateaux. Phengitic concentrates record plateau ages of c. 470 Ma whereas paragonitic concentrates yield c. 460 Ma plateaux. These ages are interpreted to date diachronous cooling through different argon closure temperatures following the high-pressure metamorphism. The slight discordance displayed in low-temperature portions of the experiments is interpreted to reflect a partial rejuvenation of intracrystalline argon systems during an c. 400–425 Ma thermal overprint associated with late Caledonian tectonothermal activity. White mica concentrates and associated whole-rocks yield RbSr mineral + whole-rock ages from 457 ± 11 Ma to 474 ± 11 Ma. These are interpreted to date post-metamorphic cooling through Sr blocking temperatures. Because similar ages are recorded by both KAr and RbSr isotopic systems, relatively rapid post-metamorphic cooling is implied. This and relatively rapid depressurization during uplift indicate that the 40Ar39Ar and RbSr mineral ages likely closely date the peak metamorphism.The high pressure complex is unconformably overlain by variably cleaved, Upper Ordovician-Lower Silurian flysch which was deformed into regional recumbent folds prior to deposition of unconformably overlying Early Devonian molasse. Two penetratively cleaved slate samples display internally discordant 40Ar39Ar whole-rock age spectra with ages increasing from c. 100 Ma to c. 470 Ma. The spectral discordance is interpreted to reflect the combined effects of: (1) a polymineralic character: (2) a detrital source similar in age to the Motalafjella complex; and (3) a partial, post-Paleozoic thermal rejuvenation of the detrital mica argon systems. No thermal overprint associated with Late Silurian cleavage formation appears to be recorded. This agrees with textural characteristics which suggest that the cleavage largely developed through pressure-solution assisted, grain-boundary sliding.

Tectonothermal evolution of contrasting metamorphic complexes in northwest Spitsbergen (Biskayerhalvøya): Evidence from 40Ar/39Ar and Rb-Sr mineral ages

Contrasting metamorphic sequences are exposed within a fault-bounded horst on Biskayerhalvoya, northwestern Spitsbergen. The Biskayerfonna Group is composed of variably pelitic metasedimentary rocks together with interlayered amphibolite. Amphibole within this sequence records 40 Ar/ 39 Ar isotope correlation ages of 429 and 437 m.y., which are interpreted to date cooling through appropriate argon closure temperatures following a late Caledonian amphibolite-facies metamorphism. Muscovite and biotite record slightly younger 4O Ar/ 39 Ar and Rb-Sr ages (400-430 m.y.), which are interpreted to date cooling through lower closure temperatures. The Biskayerfonna Group is tectonically imbricated with an older metamorphic complex (Richarddalen Group) composed of variably retrogressed eclogite, metagranite, foliated amphibolite, and various metasedimentary rocks. Amphibole within massive retrogressive assemblages derived from eclogite and wihin foliated amphibolite records 4O Ar/ 39 Ar isotope correlation ages between 500 and 542 m.y. These are interpreted to date cooling through appropriate closure temperatures following the initial high-pressure metamorphism. Rb-Sr ages recorded by muscovite and most biotite within the Richarddalen Group are markedly younger, ranging between 410 and 430 m.y. These ages likely record cooling following a late Caledonian metamorphic overprint that occurred at approximately the same time as initial prograde metamorphism of the Biskayerfonna Group; however, maximum overprint temperatures in the Richarddalen Group must have been less than prograde conditions in the Biskayerfonna Group because amphibole in the former shows no evidence of any isotopic rejuvenation. These relationships suggest that structural imbrication of the contrasting metamorphic sequences must have occurred during or immediately following late Caledonian metamorphism at ca. 429-437 Ma but prior to regional cooling between ca. 402 and 430 Ma. Phengitic muscovite within the Richard-dalen Group records 4O Ar/ 39 Ar plateau ages between 440 and 480 m.y. and displays no spectral evidence of subsequent rejuvenation. This suggests that the late Caledonian metamorphism must have been of variable intensity within the Richarddalen Group.