Willis E. Hames

New evidence for geologically instantaneous emplacement of earliest Jurassic Central Atlantic magmatic province basalts on the North American margin

Dikes in the southeastern United States represent a major component of the Central Atlantic magmatic province and record kinematics of Pangean breakup near the critical, predrift junction of three major continental masses. Until now, the age of these dikes had not been determined with the same precision as those of Central Atlantic magmatic province basalts on other parts of the circum-Atlantic margin. Our new results for three dike samples from the South Carolina Piedmont yield plateau ages of 198.8 ± 2.2, 199.5 ± 1.8, and 199.7 ± 1.5 Ma. For comparison, we present new age determinations of the benchmark Watchung flows I and III of the Newark basin: 201.0 ± 2.1 and 198.8 ± 2.0 Ma, respectively. Collectively, these data suggest that basaltic volcanism responsible for the dikes, flows, and sills of eastern North America occurred within ∼1 m.y. of 200 Ma. The timing, brief duration, and extent of the Central Atlantic magmatism imply that it may have been causally related to Triassic-Jurassic mass extinctions. The distribution and timing of this magmatism and the absence of regional uplift or an identifiable hotspot track lead us to favor strong lithospheric control on the origin of the Central Atlantic magmatic province, consistent with the modern generation of plume incubation or edge-driven convection models.

990 and 1100 Ma Grenvillian tectonothermal events in the northern Oaxacan Complex, southern Mexico: roots of an orogen

Inliers of f1.0–1.3 Ga rocks occur throughout Mexico and form the basement of the Oaxaquia microcontinent. In the northern part of the largest inlier in southern Mexico, rocks of the Oaxacan Complex consist of the following structural sequence of units (from bottom to top), which protolith ages are: (1) Huitzo unit: a 1012F12 Ma anorthosite–mangerite– charnockite–granite (AMCG) suite; (2) El Catro´n unit: z1350 Ma orthogneiss migmatized at 1106F6 Ma; and (3) El Marquez unit: z1140 Ma para- and orthogneisses. These rocks were affected by two major tectonothermal events that are dated using U–Pb isotopic analyses of zircon: (a) the 1106F6 Ma Olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds; and (b) the 1004–978F3 Ma Zapotecan event produced at least two sets of structures: (Z1) recumbent, isoclinal, Class 1C/3 folds with gently NW-plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism; and (Z2) tight, upright, subhorizontal WNW- to NNE-trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions. Cooling through 500 jC at 977F12 Ma is documented by 40 Ar/ 39 Ar analyses of hornblende. Fold mechanisms operating in the northern Oaxacan Complex under Zapotecan granulite facies metamorphism include flexural and tangential–longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes. Subsequent Phanerozoic deformation includes thrusting and upright folding under lower-grade metamorphic conditions. The Zapotecan event is widespread throughout Oaxaquia, and took crustal rocks to a depth of f25–30 km by orogenic crustal thickening, and is here designated as Zapotecan Orogeny. Modern analogues for Zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens. Contemporaneous tectonothermal events took place throughout Oaxaquia, and in various parts of the Genvillian orogen in Laurentia and Amazonia. D 2003 Elsevier Science B.V. All rights reserved.

ON THE LOSS OF 40AR* FROM MUSCOVITE DURING POLYMETAMORPHISM

Abstract Recent laser studies have documented 40Ar concentration gradients in micas that correspond to age variations of tens to hundreds of millions of years. A fundamental problem to interpretation of such variations is to evaluate whether they represent loss via lattice diffusion or, alternatively, loss during chemical reactions and deformation. New data of this study and comparisons with previous work suggest that muscovite is far less prone to lose accumulated 40Ar via lattice diffusion during overprinting events than has been inferred previously. Kyanite-muscovite assemblages in Proterozoic schist along the eastern flank of the Green Mountain massif, central Vermont Appalachians, formed during Proterozoic metamorphism at ca. 1.1 Ga and were remetamorphosed in the greenschist facies during both Taconian (ca. 465 Ma) and Acadian (ca. 390 Ma) events. The overprinting events resulted in deformation of earlier kyanite and muscovite, growth of chemically and isotopically distinct rims on earlier muscovite, and growth of neoblastic muscovite and chloritoid, with each event at inferred minimum temperatures of ca. 425°C. Laser spot analyses of the Grenville muscovite range from 988 ± 11 Ma in the centers of porphyroclasts to 397 ± 7 Ma along rims. Ages younger than 750 Ma occur only on the deformed edges of porphyroclasts and in highly deformed areas within crystals. Parts of early-formed muscovite that escaped late deformation and chemical reaction seem remarkably retentive of accumulated 40Ar; laser spot analyses along the outer 100 μm of an undeformed growth face yield ages close to the timing of Proterozoic metamorphism and cooling. The present study suggests episodic heating events are ineffective for producing measurable, diffusive argon loss along the edges of large muscovite crystals (ca. 1 mm in diameter), particularly in comparison with deformation and reaction mechanisms, and are unlikely to result in complete 40 Ar 39 Ar age resetting of such crystals. The results of this study further emphasize that the argon diffusivity in muscovite is lower than commonly considered in previous work.

TIMING OF PALEOZOIC OROGENY AND EXTENSION IN THE CONTINENTAL SHELF OF NORTH-CENTRAL NORWAY AS INDICATED BY LASER 40AR/39AR MUSCOVITE DATING

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 Lofoten9s 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 Norway9s passive margin. The results of this study show that 40 Ar closure profiles in porphyroblasts can form even in active tectonic settings characterized by rapid and episodic unroofing.

New Caledonian eclogite province in Norway and potential Laurentian (Taconic) and Baltic links

Field observations and 4 0 Ar/ 3 9 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 Laurentian 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.

Mid-Miocene rhyolite volcanism in northeastern Nevada: The Jarbidge Rhyolite and its relationship to the Cenozoic evolution of the northern Great Basin (USA)

We present new physical, geochemical, geochronologic, and oxygen isotope constraints on the mid-Miocene Jarbidge Rhyolite in northeastern Nevada (USA), providing new constraints on the tectonomagmatic evolution of the Cenozoic northern Great Basin. Widespread extension due to rapid collapse of the Nevadaplano began at ca. 17–16 Ma across the northern Great Basin. Coeval with this event was compositionally bimodal basalt-rhyolite volcanism that is often attributed to the inception of the Yellowstone hotspot. The most widespread mid-Miocene volcanic units in northeastern Nevada are lavas and domes of the Jarbidge Rhyolite. The thickest and most areally extensive exposures of these lavas include, and are found just west of, the Jarbidge Mountains, Nevada. This study focuses on Jarbidge Rhyolite directly south of the central Snake River Plain, adjacent to the thickest exposures in the vicinity of Jarbidge, Nevada. Textures on a range of scales indicate that the Jarbidge Rhyolite consists primarily of phenocryst-rich lavas. Laser 40 Ar/ 39 Ar ages for sanidine are consistent with effusive eruption of metaluminous to slightly peraluminous ferroan calc-alkalic rhyolite from 16.1 to 15.0 Ma; prior K-Ar ages suggest that some activity occurred over a slightly longer duration. Major and trace element data, coupled with new stable and prior radiogenic isotope measurements, suggest that Jarbidge Rhyolite magmas formed primarily via melting of quartzofeldspathic crust. The Jarbidge Rhyolite lavas are geochemically dissimilar from younger Snake River Plain rhyolites (e.g., lower MgO, lower Nb, higher Rb/Nb) and are more similar to coeval rhyolites erupted to the west on or adjacent to the Oregon Plateau. The distribution of the Jarbidge Rhyolite lavas in northeastern Nevada reflects an intimate association with temporally and spatially coincident extension rather than the Yellowstone hotspot.

Eclogitization and exhumation of Caledonian continental basement in Lofoten, North Norway

U-Pb and 40 Ar/ 39 Ar isotopic data combine with structural and petrological information to allow insights into the timing of Caledonian tectonic burial and exhumation of lower crustal rocks now exposed in the Lofoten Islands of North Norway (latitude 68° N). Severely retrogressed eclogites occur in rare lenses or, even more rarely, hydrated shear zones within Archaean and Proterozoic granulite-facies Baltic basement gneisses. The timing of high-pressure metamorphism in Lofoten has been difficult to determine because retrogression has disturbed mineral isotopic systems and zircon apparently was not generated, leaving its tectonic significance uncertain. Recently discovered pre- and post-kinematic felsic injections provide the opportunity to bracket the age of eclogitization. U/Pb analyses of zircon and xenotime from a prekinematic syenogranite dyke that cuts the mafic host to one of the retro-eclogite lenses intruded at 1800 ± 5 Ma, and we interpret the strong disturbance of its U-Pb system at 478 ± 41 Ma to approximate the age of eclogitization. Omphacite breakdown textures imply rapid isothermal decompression during initial uplift, followed by slow uplift. Syn-upper-amphibolite-facies thrust emplacement of the overlying Leknes Group at ca. 464 Ma implies that the Lofoten basement resided in the lower crust for ca. 14 m.y. 40 Ar/ 39 Ar cooling dates from the retro-eclogites trace their exhumation path into the middle crust (hornblende ca. 433 Ma) where they resided before being slowly elevated to levels at the base of the ductile-brittle transition in the early Carboniferous (muscovite ca. 343 Ma). The Middle Ordovician Lofoten eclogites likely formed at a time similar to the “early” group of Eocaledonian eclogites (ca. 505–450 Ma) found in the Tromso, Seve, and Bergen Arcs allochthons that were later thrust onto Baltica during the main Scandian (Siluro–Devonian) collision. Lofoten eclogites appear to be ca. 50 m.y. older than the “late” group of autochthonous, Scandian (ca. 425–400 Ma) high-pressure and ultrahigh-pressure (UHP) eclogites of the Western Gneiss Region (WGR), and the former preserve a much longer (ca. 100 m.y.) exhumation history. To date, there is no evidence to indicate either UHP or Scandian eclogite-facies metamorphism in Baltic basement along coastal Norway north of latitude 64° N. Middle Ordovician eclogites of Scandinavia are contemporaneous with Taconic eclogites found in thrust nappes of the Appalachian Orogen. If the Lofoten eclogites correlate to those in the Tromso Nappe Complex (Uppermost Allochthon), then both terranes might represent Laurentian relics emplaced during the Scandian and left orphaned on the conjugate side of the orogen when the North Atlantic began to open in the Eocene. Lithologic, petrologic, kinematic, provenance, and palinspastic information favor, however, correlation with eclogites in transitional Baltic-Iapetan crust of the Seve Nappe Complex (Upper Allochthon), which provides a piercing point linking eclogites in autochthonous Baltic crust now exposed in the most internal parts of the orogen with those transported and preserved in the Swedish foreland. We suggest a broad twofold subdivision for the Eocaledonian eclogite provinces into those that are (1) Baltic derived, and (2) exotic with respect to Baltica. Middle Ordovician eclogites falling under category (1) can further be subdivided into those occurring in autochthonous Baltic basement (i.e., Lofoten), and those in thrust translated terranes. Apparently, continental crust in a collisional setting can be subducted to mantle depths and show only very sparse evidence of this tectonic history.

Shallow-marine impact origin of the Wetumpka structure (Alabama, USA)

The Wetumpka structure, an arcuate, 7.6 km diameter, rimmed feature of the inner Coastal Plain, Alabama, is a Late Cretaceous shallow-marine impact crater.In this paper, we show unequivocal evidence of Wetumpka’s impact origin.Within and about this structure, pre-existing Upper Cretaceous stratigraphy was resedimented and(or) deformed, thus creating distinctive intra-structure and extra-structure terrains.These terrains are located, respectively, within Wetumpka’s crystalline-rim terrain and adjacent to the structure on the southern side.Core drilling near the structure’s geographic center revealed that Wetumpka’s basin-filling sequence has two distinctive units, suggestive of a two-stage filling process consisting of (1) fall-back plus resurge followed by (2) a later secondary seawater resurge event.Wetumpka’s lower subsurface unit includes polymict impact breccias, which contain quartz grains displaying shock-characteristic multiple sets of planar deformation features.Selected subsurface samples of this breccia also contain elevated Ir, Co, Ni and Cr concentrations indicative of a minor extraterrestrial component.9 2002 Elsevier Science B.V. All rights reserved.

Kinematics, U-Pb geochronology, and 40Ar/39Ar thermochronology of the Gold Hill shear zone, North Carolina: The Cherokee orogeny in Carolinia, Southern Appalachians

The Gold Hill shear zone is the most prominent pre-Carboniferous structure in the Southern Appalachian peri-Gondwanan tract of Carolinia. Common perception, based on indirect evidence, holds that it is an Acadian, dextral strike-slip shear zone. However, our recent structural and geochronologic studies directed at the shear zone indicate that it is a complex structure in both time and space. Structural studies in central North Carolina kinematically link deformation in the shear zone to regional shortening structures in both the hanging wall and footwall and indicate that there was a sinistral component to the deformation. Collectively, these structures constitute a regional sinistral transpressional system. We obtained nine new U-Pb zircon ages (Ediacaran–Devonian) and 12 new 40 Ar/ 39 Ar muscovite ages (Late Ordovician–Middle Mississippian); these data, in conjunction with the regional geology indicate that the shear zone has ∼12 km of stratigraphic throw and that the main motion on the zone was Late Ordovician. Collectively, geologic relations, structures, and the distribution of 40 Ar/ 39 Ar ages indicate that the shear zone was reactivated in the Late Devonian and the Middle Mississippian. The regional Late Ordovician–Silurian sinistral transpressive system, of which the Gold Hill shear zone is part, represents the most widespread tectonism documented in Carolinia; it overlaps in time with the Ordovician–Silurian Cherokee unconformity in Laurentian strata and Late Ordovician–Silurian suprasubduction-zone magmatism and metamorphism in peri-Laurentian rocks and consequently is considered a manifestation of the Southern Appalachian Cherokee orogeny, marking the accretion of Carolinia to Laurentia.

Single-crystal 40Ar/39Ar age variation in muscovite of the Gassetts Schist and associated gneiss, Vermont Appalachians

Abstract An exposure near Gassetts, Vermont, contains lithologies varying from staurolite-kyanite grade aluminous schists with paragonitic muscovite to potassic gneiss with phengitic muscovite. Singlecrystal laser fusion 40Ar/39Ar ages for paragonitic and phengitic muscovite yield similar distributions with ranges between 366 ± 4 and 326 ± 4 Ma. Intracrystalline ages vary from ca. 394 ± 4 to 330 ± 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 40Ar-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 40Ar/39Ar 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.