Intracrystalline Argon Systems Research Articles

Variscan vs Cadomian tectonothermal activity in northwestern sectors of the Teplá-Barrandian zone, Czech Republic: constraints from 40 Ar/ 39 Ar ages

Five muscovite concentrates from high-grade, pelitic metasedimentary basement rocks exposed in northwestern sectors of the Tepla-Barrandian zone (Czech Republic) record 40Ar/39Ar mineral plateau ages which range between ca. 376 and 362 Ma. Hornblende concentrates from metagabbro (Marianske Lanzě complex) and fine-grained basement amphibolite display plateaux which define 36Ar/40Ar vs 39Ar/40Ar isotope-correlation ages of ca. 370 and ca. 375 Ma. The mineral ages are interpreted to date relatively rapid cooling through appropriate argon retention temperatures following early phases of Variscan (Early Devonian) regional metamorphism. A slate/phyllite basement sample collected within lower-grade metasedimentary rocks in southeastern portions of the Tepla-Barrandian zone is characterized by an internally discordant 40Ar/39Ar whole-rock age spectrum which suggests partial Variscan rejuvenation of intracrystalline argon systems which had cooled through appropriate argon retention temperatures following an initial regional metamorphism at or prior to ca. 500 Ma (Cadomian). Hornblende from undeformed diorite of the Kdynž complex records a well-defined 40Ar/39Ar age plateau which corresponds to an isotope-correlation age of ca. 516 Ma. This is interpreted to date post-magmatic cooling following emplacement.

40Ar/ 39Ar muscovite dates from the nappe region of southwestern Norway: dating extensional deformation in the Scandinavian Caledonides

The Caledonian thrust nappes and basal décollement zone above the Baltic shield record penetrative Caledonian deformation related to ESE-directed nappe transport, and subsequent WNW-directed ductile shearing during extensional collapse of the orogen. Muscovite within Proterozoic quartzite conglomerates in the Lower Bergsdalen Nappe that record only ESE-vergent Caledonian deformation display internally concordant 40Ar/ 39Ar age spectra which yield plateau ages of 403 Ma and 398 Ma. Similar plateau ages (402 and 399 Ma) were obtained from muscovite from mica schists which record additional penetrative WNW-directed shearing, suggesting that muscovite intracrystalline argon systems did not cool below appropriate closure temperatures until during or after extensional WNW-vergent movements. Quartz microfabrics indicate that temperatures were higher than those required for argon retention in muscovite until the end of the WNW-directed nappe translation (extension), and that lower temperatures were maintained during subsequent movements along W- and NW-dipping extensional ductile shear zones. The 40Ar/ 39Ar muscovite ages are therefore interpreted to post-date the extensional WNW-directed shearing along the décollement zone, and closely date the initiation of W- and NW-dipping extensional shear zones. When compared to the age of contractional deformation, the results confirm that extension followed orogenic contraction very shortly after the main, Scandian orogenic phase.

Diachronous Variscan tectonothermal activity in the NW Iberian Massif: Evidence from 40Ar/ 39Ar dating of regional fabrics

Multigrain concentrates of hornblende and muscovite together with whole-rock slate/phyllite samples have been dated (27 analyses) using 40Ar/ 39Ar incremental-release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the Variscan Iberian Massif. Hornblende concentrates from amphibolites in the allochthonous Ordenes Complex yield plateau isotope-correlation ages of 425 Ma and 377 Ma. Muscovite concentrates and whole-rock slate/phyllite from this and the Cabo Ortegal Complex yield plateau ages which range from 367 Ma to 295 Ma. Analyses of similar material from the relative autochthon yield plateau ages between 359 Ma and 275 Ma. Muscovite concentrates from three late- to post-kinematic granitic stocks yield plateau ages between 309 Ma and 274 Ma. At least seven of the 40Ar/ 39Ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks. The remaining analyses may be used to constrain the local age of various Variscan tectonothermal events. The oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones. Middle Devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon, where Variscan deformation initiated in the Upper Devonian and diachronously prograded eastward. The first deformational events recorded in the limit within the internal and external zones occured ca. 20–25 Ma later (lower Namurian). Variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge. However, the entire orogen remained tectonically active with different structural features forming at different times and at different places. An average propagation rate of ca. 5 km/m.y. is suggested by consideration of a 20–25 Ma difference in correlative fabric ages and present separations.

Tectonothermal evolution of the Badajoz-Cordóba shear zone (SW Iberia): characteristics and 40Ar/ 39Ar mineral age constraints

Field characteristics indicate that the Badajoz-Cordóba shear zone has had a long, polyphase tectonothermal evolution. 40Ar/ 39Ar cooling ages recorded within central, ductile sectors of the Badajoz-Cordóba shear zone range from ca. 370 to 360 Ma (amphibole) and 340 to 330 Ma (muscovite). These are interpreted to date post-metamorphic cooling through contrasting temperatures required for intracrystalline retention of argon following a regionally significant Variscan tectonothermal overprint. These results combined with field relationships indicate that the Badajoz-Cordóba shear zone experienced at least 15 km of uplift relative to adjacent areas during late Paleozoic sinistral transpression. The data also indicate that individual structural units within the shear zone experienced contrasting uplift histories (both in total amount and rate). The 40Ar/ 39Ar cooling ages indicate complete Variscan rejuvenation of all older intracrystalline argon systems within central sectors of the Badajoz-Cordóba shear zone.

40Ar/39Ar mineral age constraints on the timing of deformation and metamorphism, North-East Greenland

40Ar/39Ar mineral ages have been determined from basement gneisses and cover sequences exposed in the Caledonides of North-East Greenland. These range between c. 438 Ma and c. 370 Ma (Early Silurian – Middle Devonian). They are interpreted as dating cooling folIowing polyphase Caledonian metamorphism which completely rejuvenated intracrystalline argon systems within the (early Proterozoic) basement gneisses. The 40Ar/39Ar results indicate that thrust-related regional deformation.

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

AbstractThe 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.

Variable Variscan thermal rejuvenation in the St Malo region, Cadomian Orogen, France: evidence from 40Ar/39Ar mineral ages

Abstract The St Malo region in north‐west France contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during Cadomian orogenesis at c. 540 Ma. Previously reported Rb–Sr model ages for muscovite and biotite range from c. 550 to c. 300 Ma, and suggest variable resetting of mineral isotopic systems. These rocks display microscopic evidence for variably intense Cadomian intracrystalline plastic strain but record no obvious evidence of penetrative Palaeozoic regional deformation. 40Ar/39Ar mineral ages have been determined to evaluate better the extent, timing and significance of Palaeozoic overprinting.Eleven muscovite concentrates and one whole‐rock phyllite have been prepared from various units exposed in the St Malo and adjacent Mancellian regions. In the Mancellian region, muscovite from two facies of the Bonnemain Granite Complex record 40Ar/39Ar plateau ages of c. 527 and 521 Ma. An internally discordant 40Ar/39Ar release spectrum characterizes muscovite from protomylonitic granite within the Cadomian Alexain‐Deux Evailles‐Izé Granite Complex, and probably records the effects of Variscan displacement along the North Armorican Shear Zone. Muscovite concentrates from anatectic granite and from Cadomian mylonites along ductile shear zones within the north‐western sector of the St Malo region exhibit internally discordant 40Ar/39Ar release spectra which suggest variable and partial late Palaeozoic rejuvenation. By contrast, muscovite concentrates from samples of variably mylonitic Brioverian metasedimentary rocks exposed within the south‐eastern sector of the St Malo region display internally concordant apparent age spectra which define plateaux of 326–320 Ma. A whole‐rock phyllite sample from Brioverian metasedimentary rocks exposed along the eastern boundary of the St Malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late Palaeozoic thermal overprint. Muscovite from the Plélan granite, part of the Variscan Plélan‐Bobital Granite Complex, yields a 40Ar/39Ar plateau age of c. 307 Ma.The 40Ar/39Ar results indicate that Cadomian rocks of the St Malo region have undergone a widespread and variable Palaeozoic (Carboniferous) rejuvenation of intracrystalline argon systems which apparently did not affect the Mancellian region. This rejuvenation was not accompanied by penetrative regional deformation, and was probably of a static thermal–hydrothermal origin. The heat source for rejuvenation was probably either the result of heating during Variscan extension or advection from Variscan granites which are argued to underlie the St Malo region.

Cadomian vs. Variscan evolution of the Ossa-Morena zone (SW Iberia): field and 40Ar/ 39Ar mineral age constraints

Six hornblende and ten muscovite concentrates have been dated from three contrasting tectonic units exposed within the Ossa-Morena zone of the Iberian Massif. These include: (1) the Obejo-Valsequillo domain (north of the Badajoz-Córdoba shear zone); (2) the Sierra Albarrana structural unit (immediately south of the Badajoz-Córdoba shear zone); and (3) the thermal some exposed within the Olivenza-Monesterio antiform (Zafra-Monesterio domain). Hornblende from foliated amphibolite in the Obejo-Valsequillo domain (Siera Negra unit) displays internally discordant 40Ar/ 39Ar age spectra as a result of both intracrystalline contamination with extraneous argon and slight rejuvenation. 36Ar/ 40Ar vs. 39Ar/ 40Ar isotope-correlations are generally well-defined and yield ages of c. 550–560 Ma. These are interpreted to date cooling following late Precambrian-early Paleozoic (Cadomian) tectonothermal activity. This was followed by slight Variscan rejuvenation at c. 375–400 Ma. Muscovite from this unit displays internally discordant age spectra which reflect more extensive Variscan rejuvenation of intracrystalline argon systems which had initially cooled below appropriate blocking temperatures at c. 560 Ma. Hornblende and muscovite from the Sierra Albarrana domain (Sierra Albarrana Group) record 40Ar/ 39Ar plateau and isotope-correlation ages which range between c. 392 Ma and 351 Ma. These results indicate that complete Variscan rejuvenation was followed by relatively rapid post-metamorphic cooling. Hornblende separated from various lithologie elements exposed in the Olivenza-Monesterio Antiform yield variable results. Hornblende within a mafic xenolith in the Monesterio Granodiorite records an isotope-correlation age of c. 553 Ma which is interpreted to date the last cooling through appropriate argon retention temperatures. Hornblende within amphibolite of the Montemolin Series (lower Serie Negra) records internally discordant age spectra which reflect extensive Variscan (c. 390–400 Ma) rejuvenation of intracrystalline argon systems which had initially recorded ages > c. 500 Ma. Muscovite was concentrated from several lithologie elements in this area, including: (1) mylonitic Monesterio Granodiorite; (2) metasedimentary rocks of the Montemolin succession (displaying high-grade, sillimanite-potassium feldspar-bearing assemblages); and (3) migmatitic schist of the Siere Negra Group. These muscovite concentrates display similarly discordant 40Ar/ 39Ar age spectra which suggest significant but incomplete Variscan rejuvenation occurred at c. 400 Ma. This appears to have affected intracrystalline argon systems which had initially cooled through appropriate closure temperatures sometime prior to c. 450 Ma following an initial high-grade (Cadomian?) metamorphism. The 40Ar/ 39Ar results clearly reflect a variable, complex, and polymetamorphic evolution for the Ossa-Morena zone. Internal consistency of data within each regional tectonic unit compared with marked contrast between the units provides evidence of the extremely heterogeneous Variscan tectonothermal overprint. The results demonstrate that the present tectonic architecture of the Ossa-Morena Zone was not solely the result of late Variscan wrench tectonics, but, instead, a consequence of processes active since at least the Lower-Middle Devonian. Juxtaposition of regional structural units with contrasting tectonothermal histories, and recording variable rates and extents of uplift are consistent with maintainence of an overall transpressional Variscan tectonic regime.

40Ar/' 39Ar mineral ages within the Western Gneiss Terrane, Troms, Norway: evidence for polyphase Proterozoic tectonothermal activity (svecokarilian and sveconorwegian)

Hornblende concentrates from units of the Western Gneiss Terrane exposed on Kvaløy and northeastern Senja, Troms, Norway, display variably discordant 40Ar/ 39Ar age spectra. They suggest a complex Precambrian tectonothermal evolution involving: (1) middle Proterozoic (∼ 1700–1800 Ma) partial rejuvenation of older (> 2000 Ma) intracrystalline argon systems; and (2) variable rejuvenation of argon systems by a late Proterozoic tectonothermal event. Samples completely rejuvenated during the lastest event record plateau isotope correlation ages of 941 ± 6 Ma and 916 ± 5 Ma. There is no record of any Caledonian rejuvenation in the 40Ar/ 39Ar results. This contrasts markedly with southwestern Senja where previously reported 40Ar/ 39Ar mineral ages indicate a penetrative, polyphase Caledonian history. This indicates that the intervening Svanfiellet shear zone may have profound tectonic significance. The record of late Proterozoic tectonothermal history provided by the 40Ar/ 39Ar results suggests that at least local sectors of the Western Gneiss Terrane must be comprised of relatively high-level Caledonian structural units, which include elements of Baltic continental crust derived west (present reference) of the Sveconorwegian-Grenville ‘front’ extrapolated from southern Scandinavia.

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 μm), 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.

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

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

Middle Paleocene terrane juxtaposition along the Median Tectonic Line, southwest Japan: Evidence from 40Ar/ 39Ar mineral ages

The Median Tectonic Line (MTL) is a regional transcurrent fault separating anadalusite-sillimanite-type metamorphic rocks and granitic rocks within the Ryoke terrane from glaucophanic-type metamorphic rocks within the Sambagawa terrane in southwest Japan. The late Cretaceous Izumi Group unconformably overlies Ryoke metamorphic rocks and is tectonically separated from the Sambagawa terrane by the MTL. Recent 40Ar/ 39Ar mineral ages reported for the Sambagawa terrane in this area suggest extensive Late Cretaceous uplift; however, no Sambagawa erosional debris has been described from the Izumi Group. 40Ar/ 39Ar ages have been determined for hornblende and biotite separated from samples systematically collected along a traverse from the Ryoke terrane into the MTL (Kashio area, Chubu district). In addition whole-rock samples of protomylonite within the MTL have been analyzed. Similar 68–70 Ma isotope correlation ages are recorded by hornblende within massive (Ikuta) and foliated (Minakata) granite and protomylonite within the MTL. Biotite from these samples records ages ranging between 66 and 68 Ma. Together the mineral ages suggest relatively rapid post-magmatic cooling through appropriate argon closure temperatures. These also indicate that temperatures maintained during protomylonite formation within the MTL were less than those required to effect even partial rejuvenation of intracrystalline argon systems within hornblende. Protomylonite samples yielded well-defined whole-rock plateau ages of 62–63 Ma which are interpreted to closely date the last phase of ductile flow within the MTL. These results suggest that a significant ductile phase of movement occurred within the MTL in the Middle Paleocene. This may have been associated with significant sinistral displacement and resultant tectonic juxtaposition of previously separated portions of the Sambagawa and Ryoke belts (with unconformably overlying Izumi Group).

Polyphase Variscan emplacement of exotic terranes (Morais and Bragança Massifs) onto Iberian successions: Evidence from 40Ar/ 39Ar mineral ages

The Bragança and Morais Nappe Complexes comprise internally imbricated continental tectonic units (including schists, deformed metagranite, granulite and local eclogite) and a structurally underlying ophiolite nappe. High-pressure assemblages in some of the continental sequences were variably retrogressed during an amphibolite facies metamorphic overprint. By contrast, rocks within the ophiolite nappe display prograde amphibolite facies assemblages. Hornblende concentrates from both tectonic units exposed in the Bragança Nappe Complex record 40Ar/ 39Ar plateau and isotope correlation ages of c. 385–390 Ma. A similar date is recorded by hornblende within mylonitic amphibolite (ophiolite protolith) along a ductile thrust which separates the two major nappe units. Muscovite within blastomylonitic metagranite (Lagoa augen gneiss) in upper structural levels of the Morais Massif records 370–375 Ma 40Ar/ 39Ar plateau ages. Muscovite within phyllinite developed along a ductile thrust separating the gneiss from the structurally underlying ophiolite nappe records a 375 Ma age. Together the 40Ar/ 39Ar data suggest that a regionally significant, early Variscan amphibolite facies metamorphism occurred prior to c. 385–390 Ma (Early Devonian). This was associated with ductile imbrication of continental and ophiolite structural units with resultant formation of a composite nappe complex. The composite nappe complex must have been maintained at relatively high structural levels throughout the remaining Variscan evolution because there is no evidence for post-370 Ma rejuvenation of intracrystalline argon systems in any of crystalline structural units examined. This is in marked contrast to a significant Late Devonian (c. 330 Ma), locally high pressure metamorphism which is recorded in structurally underlying elements of the Lower Allochthon (part of a foreland thrust complex composed of metasedimentary units with Iberian protolith affinities). Together available chronologic constraints indicate emplacement of the previously assembled composite nappe complex into its present relative structural position at c. 330 Ma with resultant deformation and metamorphism of structurally underlying components of the Iberian thrust complex.

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.