Detrital White Mica Research Articles

Two-stage erosion and deposition in a continental margin setting: an 40 Ar/ 39 Ar laserprobe study of offshore detrital white micas in the Norwegian Sea

Detrital white micas from a restricted region of the Norwegian Sea have been analysed by the 40 Ar/ 39 Ar laserprobe. Examining the ages of individual white mica grains from offshore Triassic, Jurassic and Cretaceous sandstones reveals a narrow range of detrital white mica ages (369–424 Ma). Moreover, there is an inverse relationship between sediment age and detrital white mica age, with the Triassic sands containing a larger proportion of younger white micas, and the Cretaceous sands containing the oldest white micas. In addition, residence times of the detritus, which is the age of the sediment subtracted from the age of the detrital white mica, range from c. 150 to c. 300 Ma (for the Triassic and Cretaceous sands respectively). The inverse age relationships combined with the large residence times for these stratigraphic levels can best be explained by an erosion-deposition model involving at least two stages. Taking into account previous provenance studies in the Lofoten region of the Norwegian Sea, these new data suggest that detritus shed initially from the exhuming Caledonian nappes of NW Norway in late Silurian-early Devonian times was deposited in intramontane basins that were themselves, subsequently unroofed and re-eroded from Permo-Triassic times. This caused the recycling of the Caledonian detritus from the Devono-Carboniferous basins and led to the inverted detrital age distribution ovserved in the Triassic-Cretaceous sediments examined in this study. While sedimentary recycling is not a surprising feature for Mesozoic basins in the North and Norwegian Seas, this study documents a previously untapped method, 40 Ar/ 39 Ar laserprobe provenance studies, by which to identify its effect and extent.

Contrasting provenance signals in Riphean and Vendian sandstones in the SW Urals (Russia): constraints for a change from passive to active continental margin conditions in the Neoproterozoic

Two contrasting provenance areas for Neoproterozoic sandstones can be distinguished in the southwestern Urals by light and heavy mineral analyses as well as by mineral chemistry. These reflect a dramatic change of geotectonic conditions at the eastern border of the Baltica protocontinent at around 610–620 Ma. Detritus from Riphean and Lower Vendian sandstones representing about 1 Ga of continuous sedimentation is characterised by a ‘continental platform provenance’ reflecting its derivation from the Proterozoic basement of the East European platform. Source rocks include mainly minerals and few lithic clasts from granitoids and high-grade metamorphic rocks as well as reworked clastic sediments and silicic volcanic rocks. The provenance signal changes abruptly with Upper Vendian detritus representing a ‘recycled orogenic provenance’. This includes mineral and lithic clasts of low-grade siliciclastic metasediments and mylonites containing phengites with a high-pressure signature as well as clasts of bimodal volcanic rocks and reworked siliciclastic sediments. The composition of phengites contrasts strongly with those of detrital white mica in the Riphean rocks. Also, the composition of tourmalines derived from a metapsammopelitic source indicates a mainly Al-poor metasediment provenance, whereas Riphean tourmalines were derived from mainly Al-rich metapsammopelites. Zircon morphology, tourmaline zoning and a reduced heavy mineral spectrum provide evidence for polycyclic sedimentation during Riphean and Vendian. Upper Vendian sedimentary rocks were deposited in a foreland basin derived from a proximal uplift to the east. Provenance characteristics of the Upper Vendian detritus are consistent with areas affected by a high-pressure/low-temperature metamorphism during a pre-Uralian orogenic event, most likely from the metamorphic complex of Beloretsk, which was emplaced and exhumed during the Upper Vendian. The change of geotectonic conditions in the Upper Vendian reflects a change from a passive continental margin in the South Urals throughout the entire Riphean since at least 1350 Ma to a convergent continental margin within a presumed transpressional setting.

A central European Variscide source for Upper Carboniferous sediments in SW England: 40 Ar/ 39 Ar detrital white mica ages from the Forest of Dean Basin

40 Ar/ 39 Ar detrital white mica ages from a Westphalian C/D sediment in the Varisan foreland Forest of Dean Basin (SW England) range from 309±12 to 371±2 Ma. Combining these ages with chemical data from the detrital white micas constrain their provenance to two sources: reworked Westphalian A to C, and, more controversially, the central European Variscides. Taking into account the palaeogeography of the Late Devonian to Late Carboniferous, the sediment derived from the central European Variscides must have resided in a holding basin proximal to their source for 30–40 Ma. During the Mid- to Late Carboniferous sediment was transported longitudinally from east to west along the Variscan front. Deposition and mixing of the two source components took place in the Forest of Dean Basin at approximately 307 Ma.

Exhumation of the Central Alps: evidence from 40Ar/39Ar laserprobe dating of detrital white micas from the Swiss Molasse Basin

40Ar/39Ar single‐grain laserprobe dating of detrital white micas from early Oligocene to middle Miocene (31–14 Ma) sedimentary rocks of the central Swiss Molasse basin reveals three distinct clusters of cooling ages for the hinterland. Two Palaeozoic age clusters reflect cooling after the Variscan orogeny with only limited reheating during the Alpine orogeny. The third Tertiary age cluster reflecting late Alpine cooling is restricted to sediments younger than 20 Myr old. Micas with cooling ages < 30 Myr are interpreted to originate from the footwall of the Simplon detachment fault, thus representing formerly exposed upper levels of the present‐day Lepontine metamorphic dome. Erosion of these levels is reflected by an increase of low‐grade metamorphic lithic grains in the sandstones. This interpretation puts constraints on the timing of exhumation as well as on the evolution of the drainage pattern of the Central Alps.

Provenance of Cretaceous synorogenic sandstones in the Eastern Alps: constraints from framework petrography, heavy mineral analysis and mineral chemistry

The detrital components of Cretaceous sedimentary rocks in the Northern Calcareous Alps reflect the early Alpine geodynamic evolution of the Austroalpine microplate. Two contrasting source areas are distinguished on the base of light and heavy mineral analysis. The first source area is located at the southeastern margin of the Austroalpine and is composed of Palaeozoic sediments and metamorphic rocks, Mesozoic carbonate rocks, and ultrabasic rocks derived from the suture zone of the Vardar/Meliata Ocean. The second source area is located in the northwest in a Lower Austroalpine position near the transpressive plate margin that juxtaposes Austroalpine and Penninic units. This source area comprises Palaeozoic low-grade metamorphic rocks including high-pressure (HP) rocks, late Palaeozoic (meta)sediments, Mesozoic carbonate rocks, and ultrabasic rocks from obducted slices of Penninic oceanic crust. Chemical analyses of detrital white mica, amphibole and garnet support the discrimination between the two source areas. Tourmaline chemistry calls for a significant amount of metasedimentary rocks in the source area. Granitoid rocks and high-grade metamorphic rocks are rare. Blue amphibole, phengite, and chloritoid composition suggest the erosion of lower blueschist facies rocks in the northwestern source area. We suggest a modified model of Austroalpine Valanginian to Coniacian tectono-sedimentary evolution which is based on (1) an onset of subduction of the Penninic Ocean no earlier than Late Cretaceous, (2) deposition of the analyzed sedimentary rocks in piggyback basins, and (3) a reconstruction of provenance as proposed in this study.

Rapid exhumation of medium to high-pressure metamorphic rocks at the Saxothuringian - Moldanubian boundary: Evidence from detrital white micas in Saxothuringian synorogenic sediments (E-Variscides, Germany)

Rapid exhumation of medium to high-pressure metamorphic rocks at the Saxothuringian - Moldanubian boundary: Evidence from detrital white micas in Saxothuringian synorogenic sediments (E-Variscides, Germany)

40 Ar/ 39 Ar ages of detrital white mica from Upper Austroalpine units in the Eastern Alps, Austria: Evidence for Cadomian and contrasting Variscan sources

Five detrital white mica concentrates from very low-grade, metaclastic sequences within pre-Variscan basement and post-Variscan cover units of the Upper Austroalpine Nappe Complex (Eastern Alps) have been dated with 40Ar/39Ar incremental heating techniques to constrain the age of tectonothermal events in their respective source areas. Two samples from early Palaeozoic sandstone exposed within the same Alpine nappe record slightly discordant age spectra. The maximum age recorded in one is 562.2±0.7 Ma, whereas the other yielded a 40Ar/39Ar plateau age of 607.3±0.3 Ma. These results indicate a source area affected by Cadomian tectonothermal activity. Three detrital muscovite concentrates from post-Variscan, Late Carboniferous and Permian cover sequences exposed within three different Alpine nappes yielded 40Ar/39Ar plateau ages of 359.6 ± 1.1 Ma, 310.5±1.2 Ma, and 303.3±0.2 Ma. The contrasting detrital white mica ages are interpreted to reflect different source areas. Detrital muscovite from a post-Variscan Carboniferous molasse-type sequence and from a Permian Verrucano-type sequence record ages which indicate “late” Variscan (e.g. 330–300 Ma) metamorphic sources. By contrast, detrital white mica from another Permian Verrucano-type sequence suggests a source area affected by “early” Variscan (e.g. 400–360 Ma) metamorphism. These results help clarify palinspastic relationships and tectonic correlations between pre-Late Carboniferous metamorphic basement sequences and Carboniferous to Permian cover sequences.

Laser 40Ar/39Ar dating of single detrital muscovite grains from early foreland-basin sedimentary deposits in India: Implications for early Himalayan evolution

Research Article| June 01, 1997 Laser 40Ar/39Ar dating of single detrital muscovite grains from early foreland-basin sedimentary deposits in India: Implications for early Himalayan evolution Y. M. R. Najman; Y. M. R. Najman 1Department of Geology and Geophysics, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, United Kingdom2Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, United Kingdom Search for other works by this author on: GSW Google Scholar M. S. Pringle; M. S. Pringle 3Scottish Universities Research and Reactor Centre, Scottish Enterprise Technology Park, East Kilbride G75 0QF, United Kingdom Search for other works by this author on: GSW Google Scholar M. R. W. Johnson; M. R. W. Johnson 1Department of Geology and Geophysics, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, United Kingdom Search for other works by this author on: GSW Google Scholar A. H. F. Robertson; A. H. F. Robertson 1Department of Geology and Geophysics, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, United Kingdom Search for other works by this author on: GSW Google Scholar J. R. Wijbrans J. R. Wijbrans 4Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081HV Amsterdam, The Netherlands Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (6): 535–538. https://doi.org/10.1130/0091-7613(1997)025<0535:LAADOS>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Y. M. R. Najman, M. S. Pringle, M. R. W. Johnson, A. H. F. Robertson, J. R. Wijbrans; Laser 40Ar/39Ar dating of single detrital muscovite grains from early foreland-basin sedimentary deposits in India: Implications for early Himalayan evolution. Geology 1997;; 25 (6): 535–538. doi: https://doi.org/10.1130/0091-7613(1997)025<0535:LAADOS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In India, the Dagshai and overlying Kasauli Formations represent the oldest exposed continental foredeep sediments eroded from the Himalayan orogen. 40Ar/39Ar dating of individual detrital white micas from these sedimentary units has provided maximum depositional ages of <28 Ma for the Dagshai Formation at one locality and <25 Ma at a second locality, whereas deposition of the Kasauli Formation occurred after 28 Ma at two localities and after 22 Ma at a third locality. This timing suggests that, in India, the start of substantial exhumation and erosion from the rising Himalayan orogen was delayed until 28 Ma. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

40Ar/39Ar laser probe dating of detrital white micas from Cretaceous sedimentary rocks of the Eastern Alps: Evidence for Variscan high-pressure metamorphism and implications for Alpine orogeny: Comment and Reply

The detritus of Cretaceous synorogenic sandstones of the northern margin of the Austroalpine microplate contains evidence for a high-pressure metamorphic basement and obducted oceanic crust exposed in early Alpine time. 40 Ar/ 39 Ar laser-probe data of detrital white micas give excellent plateau ages in a narrow range from 320 to 360 Ma. White micas cover the whole range from muscovites up to phengites (3.04 to 3.48 Si per formula unit). Heavy mineral spectra contain chrome spinel, glaucophane, chloritoid, epidote, and garnet, as well as zircon, tourmaline, and rutile. Glaucophane, chloritoid, and phengite correlate in their abundance. These minerals also correlate positively with the stable minerals zircon, tourmaline, and rutile derived from continental rock and negatively with chrome spinel, which represents oceanic crustal provenance. We propose that both glaucophane and phengite come from the same source rock, Variscan high-pressure metamorphic rocks of the Austroalpine basement. The Early Carboniferous age gives constraints for paleogeographic models concerning the collision between Gondwana and Laurussia. Furthermore, detrital glaucophane in the early Alpine sedimentary rocks cannot be used as proof for Cretaceous subduction at the Austroalpine-Penninic plate boundary.

40Ar/39Ar laser-probe dating of detrital white micas from Cretaceous sedimentary rocks of the Eastern Alps: Evidence for Variscan high-pressure metamorphism and implications for Alpine orogeny

The detritus of Cretaceous synorogenic sandstones of the northern margin of the Austroalpine microplate contains evidence for a high-pressure metamorphic basement and obducted oceanic crust exposed in early Alpine time. 40 Ar/ 39 Ar laser-probe data of detrital white micas give excellent plateau ages in a narrow range from 320 to 360 Ma. White micas cover the whole range from muscovites up to phengites (3.04 to 3.48 Si per formula unit). Heavy mineral spectra contain chrome spinel, glaucophane, chloritoid, epidote, and garnet, as well as zircon, tourmaline, and rutile. Glaucophane, chloritoid, and phengite correlate in their abundance. These minerals also correlate positively with the stable minerals zircon, tourmaline, and rutile derived from continental rock and negatively with chrome spinel, which represents oceanic crustal provenance. We propose that both glaucophane and phengite come from the same source rock, Variscan high-pressure metamorphic rocks of the Austroalpine basement. The Early Carboniferous age gives constraints for paleogeographic models concerning the collision between Gondwana and Laurussia. Furthermore, detrital glaucophane in the early Alpine sedimentary rocks cannot be used as proof for Cretaceous subduction at the Austroalpine-Penninic plate boundary.

Ages of Detrital White Mica from Devonian-Pennsylvanian Strata of the North Central Appalachian Basin: Dominance of the Acadian Orogen as Provenance

Detrital micas (mostly white mica) from Devonian-Middle Pennsylvanian strata of the north central Appalachian Basin at 12 sites were dated by conventional K/Ar. Three small pilot samples were less precisely dated than the rest. The youngest stratigraphic sample of Pennsylvanian depositional age (Alleghenian) produced 414/Ma muscovite and 361 Ma biotite. Excluding this sample, a narrower 35 m.y. range of Early to Middle Devonian detrital dates (from 406-371 Ma) were obtained. The precise dates from the proximal and distal Catskill wedge represent an even narrower 19 m.y. span (406-387 Ma), which is entirely Early Devonian and only approximately 30 m.y. older than the Late Devonian-Early Mississippian time of deposition. All these detrital dates from pre-Alleghenian samples correspond to reported primary ages of crystallization, or cooling therefrom, of regional metamorphism and plutonism during the Acadian orogeny, believed to be the principal Paleozoic orogeny in New England and eastern Canada. The dominance of the Acadian Orogen as a source region is evident. Even though these are conventional K/Ar dates of multi-grain separates, the mean provenance ages from so many individual samples are so close to the age of deposition to preclude almost any Precambrian contribution from either the nearby Canadian Shield or the uplifted cores of the orogen. They imply Acadian sources dominated considerably over Taconic ones. Furthermore, our results for the Mauch Chunk (377 Ma), Pottsville (371 Ma), and our youngest (Alleghenian) sample suggest that the Acadian Orogen persisted as the major provenance for post-Catskill clastic pulses that prograded into the central Appalachian Basin at least up until Middle Pennsylvanian time.

Phengite K‐Ar ages of schists from the Sanbagawa southern marginal belt, central Shikoku, southwest Japan: Influence of detrital mica and deformation on age

Abstract K–Ar age determinations were carried out on phengite separates from pelitic schists collected systematically from the Sanbagawa southern marginal belt and the associated area. The petrography and phengite chemistry by electron probe micro‐analyzer (EPMA) revealed the existence of detrital white micas in the schist that have an extremely older age (108 Ma) in comparison with the neighboring schists (88 Ma) without any detrital mica. The ages become gradually older from the north (ca 78 Ma) to the south (ca 90 Ma) except for some samples that contain detrital micas and/or have been reactivated thermally by intrusives. The age is interpreted as an exhumation‐cooling age that has been controlled by the ductile deformation of the host rocks that have never experienced a culmination temperature higher than 350°C which corresponds to the closure temperature of the K–Ar phengite system. The southward aging of the recorded ages in the extensive chlorite zone of the central Shikoku, from the Dozan river area of the north (ca 65 Ma) to the study area of the south (ca 85 Ma) through the Asemi river area (ca 75 Ma), is explained in terms of increasing exhumation/cooling rates of the host rocks from north to south. The phengite K–Ar ages in the pelitic schists from the Kyomizu tectonic zone, which is classically considered as a remarkable thrusting shear zone, have no significant difference in comparison with that of the neighboring schists. This fact suggests that the latest stage of brittle deformation during exhumation/uplift has not significantly affected the ages of phengite in the schists.

弱変成付加体のＫ‐Ａｒ年代測定における砕屑性白雲母の混入と接触変成作用の影響 山口県東部ジュラ紀付加体の例

弱変成付加体のＫ‐Ａｒ年代測定における砕屑性白雲母の混入と接触変成作用の影響 山口県東部ジュラ紀付加体の例

Isotopic provenance of Paleogene sandstones from the accretionary core of the Olympic Mountains, Washington

Conventional modal sandstone data from Paleogene units of the Pacific Northwest are not precise enough to pinpoint source areas and constrain displacement histories of accreted sedimentary terranes. Isotopic provenance study, used in conjunction with traditional basin-analysis techniques, provides a powerful means of identifying source areas. Analysis of Rb-Sr data in both wholerock and detrital white micas of Paleogene sandstones from allochthonous units in the eastern core of the Olympic Mountains and coeval autochthonous sandstones from coastal Pacific Northwest shows that the Needles-Gray Wolf and Grand Valley lithic assemblages of the core came from the same source as sandstones of the Chuckanut Formation and Puget Group in northern Washington. The source of these units is isotopically distinct from the source for units farther south, such as the Tyee Formation in Oregon. Chemical compositions, conventional K-Ar age determinations, and oxygen- and hydrogen-isotope compositions of white micas support this conclusion. Similar analyses of sandstones from the Western Olympic lithic assemblage and the Yakutat terrane of southeastern Alaska suggest that these two units have a similar source, but that they differ slightly from sandstones of the eastern Olympic core and autochthonous Washington units. The overall composition of sandstones (lithic arkosic), and the very high initial-Sr values (>0.710), moderately high δ 18 O values (∼+9) and Mesozoic age for detrital white mica of Olympic core rocks and sandstone of the Yakutat terrane suggest a source in the high-rank metamorphic and plutonic rocks from the eastern part of the Omineca Crystalline Belt in southeastern British Columbia. Furthermore, rapid uplift of this source area during Eocene time is consistent with the depositional age of the Olympic rocks. Sediment derived from this source area was transported westward by major river systems that crossed the low-lying North Cascade Range and supplied the deposits of the autochthonous units of the northern Washington coast and the offshore equivalents before the latter were accreted to form the Olympic core. Limited data from the Yakutat terrane suggest that it lay offshore of southern British Columbia sometime during middle Eocene to early Oligocene time, consistent with paleomagnetic and some paleontologic interpretations, and subsequently migrated northward by plate motions.

Terrigenous input to the Moffat Shale sequence, Southern Uplands

The Moffat Shale, of Llandeilo to Llandovery age, has been widely described as of pelagic open-ocean origin (e.g. Leggett 1987 and references therein). Within this shale sequence are many interbedded grey metabentonite ash horizons; these are well developed in the classic Dob’s Linn section, 15 km NE from Moffat which includes the international stratotype for the base of the Silurian. A recent detailed study of the Dob’s Linn section by Merriman and Roberts (1990) recorded 135 metabentonite horizons, ranging in age from Ashgill to Llandovery and together representing the accumulation of about 6 m o f ash over 25 Ma. The large volume of ash present, together with geochemical data and the presence in the shale of detrital white micas led Merriman and Roberts (op. cit.) to conclude “Lithological, petrological and REE characteristics suggest that the Moffat Shale Group is not exclusively pelagic in origin, . . . . .” Such a conclusion is of great importance in placing the Moffat Shale Group in its correct palaeogeographical setting which, in turn, constrains the applicability of the alternative genetic models proposed for the Southern Uplands thrust belt (e.g. McKerrow 1987 and references therein). We stress that the interpretation of Merriman and Roberts is supported by the presence, within the Moffat Shale, of greywacke interbeds. These are most extensive in the Ettrickbridge section, 20 km ENE from Dob’s Linn. Peach and Horne (1899, p. 122) reported that at least 50 m of greywacke occur above shale horizons yielding Dicellograptus anceps and are . . .

Chlorite crystallinity: an empirical approach and correlation with illite crystallinity, coal rank and mineral facies as exemplified by Palaeozoic and Mesozoic rocks of northeast Hungary

Abstract Fairly strong (r= 0.75–0.85) positive linear correlations were found between crystallinity indices (peak widths) measured on the first two basal reflections of chlorite and those of illite–muscovite in &lt;2‐μm fractions of a representative shale–slate–phyllite series from Palaeozoic and Mesozoic formations of northeast Hungary. The metamorphic grade ranges from late or deep diagenesis through anchizone to epizone conditions. Chlorite crystallinity values measured on air‐dried and ethylene‐glycol‐solvated samples suggest that the effects of expandable interlayers are negligable, especially in the higher grade (∼temperature) part of the series. However, the greater scattering of crystallinity values for the chlorite 001 reflection compared to those of the 002 reflection may be related to the effects of minor amounts of interlayered and/or discrete smectite and/or vermiculite. With increasing metamorphic grade and advancing equilibrium recrystallization, the chlorite compositions in different samples become more homogenous. No correlation exists between crystallinity and changes in chlorite composition as estimated from the intensity ratios of basal reflections. Hence an increase of domain size and a decrease of lattice distortion with increasing grade (∼temperature) may be decisive factors affecting chlorite crystallinity.Chlorite crystallinity can be applied as a reliable regional, statistical technique complementary with, or instead of, the illite crystallinity method. The illite and chlorite crystallinity scales used here are related to Kübler's epi‐, anchi‐ and diagenetic zones and correlated with coal rank, conodont colour alteration and mineral facies data. As the effects of the detrital white mica can be observed even in the &lt;2‐μm fractions of anchizonal metapelites, the anchizone boundaries determined solely on the base of ‘fixed’illite crystallinity values may vary with amounts of detrital and newly formed muscovite–illite. Hence a complex approach utilizing more than one method for determination of grade is preferred for petrogenetic purposes, even if relationships between crystallinity scales, coal rank and mineral facies also vary strongly in different tectonic settings and lithologies.

The metamorphic history of the Damara Orogen based on K/Ar data of detrital white micas from the Nama Group, Namibia

Abstract Detrital white micas from molasse sediments of the Nama Group were dated by the K/Ar method. The Kuibis Subgroup and the underlying pre-Damara Sinclair Sequence contain detrital muscovite with ages of 1100-1000 Ma and thus cannot be derived from the Damara Orogen, but prove to have originated from a basement probably affected by the Kibaran orogeny. Muscovite size-fractions of 250-100 μm from the upper Nama Group (upper Schwarzrand and Fish River Subgroups) yielded K/Ar ages 670-570 Ma. Because these sediments are regarded as molasse series of the Damara Orogen, these K/Ar ages can be interpreted as cooling ages of a first, early tectono-thermal event in the Damara Orogen. A later Damaran metamorphic overprint, which peaked at about 530 Ma, is not documented in the detrital content of the Nama Group at the present level of erosion, because detrital micas with K/Ar ages over 600 Ma, have been found up to the uppermost Fish River Subgroup. It can be concluded that, at least, two tectono-metamorphic events followed by uplift and erosion, have taken place during the development of the Damara Orogen. Thermal alterations of the Nama sediments were detected by means of illite crystallinity and K/Ar dating of clay size-fractions. These data, as well as the almost exclusive occurrence of the 2M mica polytype in these size-fractions point to a diagenetic to very low grade metamorphic alteration of the area investigated at about 530-500 Ma and younger. The deposition of the upper Nama Group is younger than the 570 Ma detrital white micas but occurred before the 530-500 Ma thermal alteration of the Nama deposits. The deposition of the lower Nama Group probably took place between 635 and 570 Ma.

Stratigraphic Control of Chemistry and Mineralogy in Metamorphosed Witwatersrand Quartzites

Within metamorphosed quartzites of the Witwatersrand Supergroup bulk chemistry is the major control on the distribution of metamorphic mineral phases and on their chemistry. Bulk chemistry is in turn controlled by stratigraphic position. Pyrophyllite distribution varies directly with, whereas feldspar distribution varies inversely with, bulk Al. Chloritoid distribution is more complex and varies with both bulk Al and bulk Fe/Mg. Despite the overprint of low-grade metamorphism and, possibly, of other alteration processes, both detrital and metamorphic phyllosilicates can be recognized and characterized. Al in metamorphic white mica and metamorphic chlorite varies with bulk Al; whereas the composition of detrital white mica is unrelated to bulk chemistry. Similarly, Mg in metamorphic chlorite varies with bulk Mg. Bulk chemical variation, particularly in Al, Mg, and Na, and thus metamorphic mineral chemistry, is controlled by stratigraphic position, both within individual stratigraphie units and in the stratigraphie succession as a whole. K content increases upward and varies more strongly with position in the entire section than with position in individual units. The quartzites display a high degree of chemical maturity, as measured by the Chemical Index of Alteration, which increases upward both within individual stratigraphie units and in the section as a whole. The strong stratigraphie control on bulk chemistry suggests that intense weathering occurred in the source area and that alteration events associated with faults and other structural features play no more than a minor role in controlling variations in bulk chemistry. The observation that these rocks are strongly leached, yet retain detrital pyrite and uraninite, indicates that the source area weathering occurred under reducing conditions.

Formula omitted] whole-rock dating and the age of cleavage in the Finnmark autochthon, northernmost Scandinavian caledonides

Four variably cleaved samples of red shale/slate were collected from the Nyborg Formation (Vestertana Group) exposed in the Finnmark autochthon, northernmost Scandinavian Caledonides. Quartz-normalized illite crystallinity values suggest metamorphism varied between upper diagenesis/lower anchizone (east) and lower anchizone/middle anchizone (west). The samples display similarly discordant 40 Ar 39 Ar whole-rock age and K Ca spectra with total-gas ages ranging between ∼606 and ∼661 Ma. Spectra complexity is a result of evolution of gas from relatively non-retentive chlorite, white mica, and relatively refractory detrital plagioclase. Apparent ages defined by intermediate-temperature increments are relatively constant within each sample but show marked intersample variations (550–575 to 635–650 Ma). Three size fractions (1–2, 2–4, and 4–8 μm) were separated from the sample containing the least chlorite. The two coarser fractions contain numerous rock fragments and display 40 Ar 39 Ar age and K Ca spectra similar to the whole rock. The 1–2 μm fraction is largely comprised of individual mineral grains. Gas increments evolved between 375° and 495°C record similar K Ca ratios but systematically increasing apparent ages between ∼637 and ∼783 Ma. These results suggest that the 1–2 μm fraction is composed of detrital white-mica grains which were affected by a diagenetically related geologic disturbance at ∼635 Ma. Although the affects of a Late Cambrian-Early Ordovician orogenic event (Finnmarkian Orogeny) have been locally described from portions of the overlying Caledonian allochthons, no clear record of correlative tectonothermal activity has been documented within samples from the Finnmark autochthon.

Deformation of chlorite-mica aggregates in cleaved psammitic and pelitic rocks from Islesboro, Maine, U.S.A.

Chlorite—white mica aggregates are large porphyroblasts found in weakly metamorphosed and deformed pelitic and psammitic rocks throughout the world. They are usually considered to originate either as detrital white mica layers surrounded by crystallized chlorite or as synkinematic porphyroblasts. In chlorite—muscovite aggregates observed at Islesboro, the chlorite layers were found to represent prekinematic porphyroblasts that have been strongly deformed and rotated during cleavage development. The muscovite layers in the aggregates formed by crystallization along split and dislocated (001) surfaces in the chlorite, or by alteration within intensely deformed sections of chlorite grains, such as kink-bands.