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An exposure near Gassetts, Vermont, contains lithologies varying from staurolite-kyanite grade aluminous schists with paragonitic muscovite to potassic gneiss with phengitic muscovite. Single-crystal laser fusion [sup.40]Ar/[sup.39]Ar ages for paragonitic and phengitic muscovite yield similar distributions with ranges between 366 [+ or -] 4 and 326 [+ or -] 4 Ma. Intracrystalline ages vary from ca. 394 [+ or -] 4 to 330 [+ or -] 4 Ma. Thus, we find that the intracrystalline (core-rim) age distribution of relatively large, single crystals essentially encompasses the range of ages obtained through total fusion of smaller crystals, consistent with models for development of diffusion profiles and [sup.40]Ar-closure during cooling with a diffusion dimension controlled by the physical grain size. However, some of the larger crystals studied, particularly those with prominent microscopic defects (features readily evident such as internal grain boundaries and twin planes), yield relatively young ages and lack significant core-rim age discordance. Furthermore, the overall distribution of single-crystal ages in the two samples is bimodal, and we suggest that this age distribution reflects metamorphic deformation and recrystallization event(s) superimposed on early generation muscovite. Thus, the mean age of muscovite in these samples (typical of K/Ar and [sup.40]Ar/[sup.39]Ar incremental heating analysis of bulk mineral separates) has little relationship to any single, hypothetical closure temperature. In view of the similar results we obtain for muscovite of contrasting composition, the net effects of variations in grain size, deformational character, and growth history are interpreted to be more important in forming the observed variations in age than are the chemical substitutions in these samples. Keywords: Muscovite, paragonite, phengite, [sup.40]Ar/ [sup.39]Ar, geochronology, laser analysis, Appalachians, Acadian

[1] The Lofoten archipelago exposes Precambrian Baltic basement and Caledonian allochthonous sequences within a 1000 km long chain of gravity and magnetic highs and structural culminations along the extended, British and Norwegian continental shelf. Previous regional geophysical studies indicate that post-Caledonian extension and development of the northern Norwegian shelf occurred during broadly defined Carboniferous-Permian, Cretaceous-Jurassic, and early Tertiary events. Structures related to these events are known to young westward. We report field, structural, and [sup.40Ar/.sup.39] Ar thermochronologic data from rocks exposed in Lofoten that further define the history and style of post-Caledonian extension. The islands of southwest Lofoten also represent the most outboard exposures of Caledonian basement in northern Norway that presumably formed the middle to deep crustal core of the orogen. Metasedimentary rocks and penetratively deformed basement in Lofoten record high-grade Silurian-Devonian metamorphism and top-to-the-east (hereinafter tops-east) thrusting followed by episodes of Late Devonian to Early Carboniferous, tops-west, ductile extension which progressed into oblique left-slip movements. The structural style and timing of Silurian contraction in this area are remarkably similar to that determined for the more forelandward areas on the mainland, ~120 km to the east, supporting the inference that distal parts of the Baltic continental margin that were once deeply subducted are presently exposed in Lofoten. The timings of post-Devonian structural events that affected rocks in Lofoten are partially constrained by the ages of unconformities and strata known to be preserved in graben flanking the Lofoten culmination. The radiometric age and structural data presented in this study, in combination with stratigraphic constraints, suggest a westward progression through time of extensional deformation over a protracted interval of Silurian to Permian time. The latest, Permian extension in Lofoten is largely characterized by brittle structures that formed at conditions substantially less than 300[degrees]C. Compared to the exhumation history of the southern Western Gneiss Region, the depth of Caledonian, continental (A-type) subduction and subsequent unroofing of Lofoten are of lesser magnitude, and the present erosional level remained in the middle crust for a much longer interval of time. The Permian [sup.40Ar/.sup.39] Ar mineral ages documented in this study are the youngest such ages yet identified in Scandinavia. These ages relate to episodes of deformation and cooling in response to extensional tectonic events that occurred roughly 100 m.y. after comparable effects identified on the Caledonian mainland. Our preferred explanation for the Carboniferous-Permian radiometric ages, structural evolution, and stratigraphic data for Lofoten is that they all developed in the context of a long-lived Cordilleranstyle metamorphic core complex. INDEX TERMS: 8105 Tectonophysics: Continental margins and sedimentary basins (1212); 8110 Tectonophysics: Continental tectonics--general (0905); 8115 Tectonophysics: Core processes (1507); 9335 Information Related to Geographic Region: Europe; KEYWORDS: Norwegian shelf, Caledonides, Lofoten, extension, Permian. Citation: Steltenpohl, M. G., W. E. Hames, and A. Andresen (2004), The Silurian to Permian history of a metamorphic cole complex in Lofoten, northern Scandinavian Caledonides.

Field observations and [sup.40]Ar/[sup.39] Ar isotopic dating indicate that eclogites exposed in the Lofoten Islands, north Norway, formerly presumed to be Proterozoic features, most likely formed as a result of early to middle Paleozoic, i.e., Caledonian, metamorphism. The Lofoten eclogites occur in shear zones that cut Baltic Precambrian continental basement. This unusual style of occurrence is shared only with Caledonian shear-zone eclogites of the allochthonous Bergen arcs of western Norway. Our findings help to link Scandinavian eclogites with those on the Laurentian side of this collisional zone in East Greenland. Ordovician to Silurian eclogites also are found locally throughout the southern, Appalachian continuation of the orogen in eastern North America. We compare the pressures, ages, and tectonic and structural settings of the eclogites along the ~10,000 km length of the Appalachian-Caledonian system. Our synthesis supports the idea that Lanrentian Taconic elements may be preserved in high-level nappes in Norway. The rare, deep-crustal metamorphic relicts also appear to be shared between Baltica and Laurentia, offering a new perspective in which to view the geodynamic evolution of this once-Earth-spanning orogenic system. Keywords: eclogites, Caledonian, Appalachian, 40Ar/39Ar isotopes, Laurentian, Baltica.

Research was conducted to examine the ages of arc magmatism, metamorphism and collisional tectonics in the Taconian orogen of western New England. Geochronologic ages of intrusive and volcanic rocks indicate that the Bronson Hill and Shelburne Falls arcs are not spatially or temporally discrete. Results provide evidence of a long-lived volcanic arc terrane that prograded oceanward and a collision with this time-composite art terrane that produced the Taconian orogeny.

Metamorphic rocks of Lofoten, Norway, represent a leading edge of the Baltic craton that was partly subducted beneath Laurentia during Caledonian orogeny. We find that muscovite porphyroblasts in Lofoten record a remarkably complete history of cooling and unroofing and yield data about Lofoten's tectonic evolution in the interval from 425 to 265 Ma. These data confirm that culminating metamorphism of basement and allochthonous rocks in Lofoten occurred during the Scandian phase of orogeny. Permian ages for muscovite are the youngest obtained for regional metamorphic rocks anywhere in Scandinavia and appear to record significant basement unroofing during extensional evolution of the Norwegian-Greenland seaway. Regional age variations of muscovite in Lofoten can be related to present crustal thickness, Devonian to Permian sedimentation in the Norwegian seaway, and extensional structures, and they indicate the timing of multiple extension events in the evolution of Norway's passive margin. The results of this study show that 40Ar closure profiles in porphyroblasts can form even in active tectonic settings characterized by rapid and episodic unroofing.

U-Pb dates of Zircon and titanite, and 40Ar/39Ar dates of hornblende which constrain the intrusion and incipient mylonitization of the Symvolon or 'Kavala' granodiorite within the Symvolon shear zone in northeastern Greece at nearly 300 degrees for 5-7 my suggest that the coaxial deformations have continued within a widening Symvolon rupture. The Symvolon shear zone in middle Miocene time was succeeded by the Styrmon valley detachment. The unroofing of the Rhodope metamorphic core complex was facilitated by southwestward displacement of the Serbo-Macedonian gneiss complex by 80 km.

Estimates of the time-temperature history of geologic samples are often based on 40Ar/39Ar ages obtained through incremental-heating experiments on minerals with different Ar retentivities. Laser incremental heating of single crystals of muscovite and biotite from an approximately 1700 Ma monzogranite from a low-pressure-high-temperature metamorphic terrain in central Arizona suggested an essentially uniform distribution of radiogenic 40Ar in each crystal and yielded ages of 1412 [+ or -] 5 Ma for the muscovite and 1410 [+ or -] 10 Ma for the biotite. The similarity in these ages implies rapid (approximately 25 K/m.y.) cooling of the monzogranite after a reheating episode slightly before 1412 Ma. In contrast, detailed age mapping of single crystals from the same samples with the laser microprobe revealed large radiogenic 40Ar gradients indicative of very slow ([less than]1 K/m.y.) cooling and a closure interval for Ar diffusion lasting [less than]400 m.y. Evidence for such low cooling rates has important implications for the longevity of high thermal gradients in the Early-Middle Proterozoic lithosphere. In addition, inconsistency between the laser mapping and incremental-heating results for these micas suggests caution when interpreting incremental-heating data for slowly cooled samples.

The tectonic configuration and evolution of southern Appalachian orogenic episodes are constrained, to a large extent, by knowledge of the timing and environment of pre- or syn-orogenic sediment deposition along with their subsequent thermal and deformational histories. The orogenic events that gave rise to regional folding and Barrovian metamorphism of the Western Blue Ridge (WBR) terrane are a classic example. We present new U-Pb and 40Ar-39Ar results bearing on the timing of magmatism and subsequent metamorphism of the Marble Hill Hornblende Schist (MHHS) of the WBR (southeastern USA). The schist is a key marker unit in the Murphy synclinorium, a central structure in the tectonic configuration of the WBR. The MMHS originated as basaltic volcanics overlying the Murphy Marble at the base of the Mineral Bluff Group. Ocean island basalt geochemistry and a zircon U-Pb age of ca. 437 Ma for the MHHS is consistent with plume-related volcanism in a Silurian remnant ocean basin of the Iapetus Ocean. Hornblende 40Ar-39Ar and rutile U-Pb ages for the MHHS are ca. 380-340 Ma and ca. 320 Ma, respectively, and record subsequent amphibolite facies metamorphism during Devonian Acadian orogeny and the deformation through regional folding and thrusting during the Carboniferous Alleghanian orogeny. The new age constraints for the MHHS require that "isograds" mapped in the region of the Murphy Belt are actually polymetamorphic mineral distributions that developed at least in part through post-Silurian tectonic events. [ABSTRACT FROM AUTHOR]

A study on the prograde garnet zoning patterns of schists from central Vermont and western Connecticut is conducted. The results indicate that the compositional zoning patterns in both sites resulted from the metasomatic dissolution and reprecipitation of the preexisting garnets. This is shown by the chemical analyses of fluid and mineral inclusions in the garnets and its associated abrupt compositional changes. It is also shown that the mineral assemblages produced by the metasomatic processes are more hydrous than the original prograde composition.

Nonlinear unroofing rates from 10 to

Paul Karabinos and associates conducted an experiment on Taconian orogeny in the New England Appalachians. They came to the conclusion that the igneous activity in the Bronson Hill in Massachusetts is too young to be contemporaneous with Taconian dynamothermal metamorphism. They also provided geochronologic and geochemical proof for 485 to 447 Ma arc-related volcanic and intrusive activity within the Rowe-Hawley zone of western Massachusetts and southern Vermont. However, the study is misleading due to its reliance on an apparent conflict between Taconian ages of metamorphism and volcanism.

The differences in magnitude and style of late- to post-Caledonian extension and basement exhumation between the Lofoten archipelago in northern Norway and the Western Gneiss Region (WGR) in the south are the result of differences in crustal architecture. It is attributed to basement dewatering on the archipelago's subduction depth that occurred during Caledonian orogeny. The relative stability of the WGR region from 380 to 330 Ma, as opposed to the tectonic and thermal instability of Lofoten, has also done much to make the differences.