Age, origin and development of blanket mires in Sør-Trøndelag, Central Norway

Pollen analysis, ¹⁴C datings and peat stratigraphy from blanket mires overlying two of the six drumlins in the Momyr area NW of the Trondheimsfjord in Sør-Trøndelag county are presented in order to trace the peat development and vegetational history. ¹⁴C datings of the mineral soil/peat transition in 9 of the II profiles indicate that peat formation started about 7,800 years ago on the drumlin plateaux which at that time had a vegetation of scattered birch trees. From the plateaux the peat formation spread slowly down the slopes. Eventually, the mire surface bore a cover of pines which disappeared about 4,900 years ago, at the same time as peat formation commenced at the foot of the drumlins. This was probably a result of a change in the water table and onset of erosion in the already existing peat on the drumlin plateaux. During a period of about 1,000 years this new peat buried a dense birch vegetation existing at the lower part of the drumlins. Peat growth then spread upslope, and the drumlins' overall blanket of peat was completed well before the Subatlantic chronozone.     

Diatom changes in sediments of basins possessing marine/lacustrine transitions in Frosta, Nord-Trøndelag, Norway

Diatoms in lower sediments from eight basins of the Frosta peninsula, Nord-Trødelag, Norway, have been analysed. All the basins have been isolated from the sea, and a succession from a flora influenced by marine and brackish water to a freshwater flora is always found. An attempt is made to establish a relative salinity range for the fjord basin around the investigated area, from the Younger Dryas chronozone to the end of the Atlantic chronozone. These changes seem to be controlled by the accumulation and ablation of the ice shield and also by major climatic changes in the early and middle Holocene. It is not possible to make a general list of lagoon taxa, because of great variation from basin to basin. The isolation contact (marine/brackish boundary) must be established at the end of the brackish water flora in the absence of lagoon types. The first period after isolation is characterized by a maximum in productivity with dominance of alkaliphilous types among the diatoms. A pioneer flora with dominance of Fragilaria taxa can be traced in the uppermost basins which were isolated just after the deglaciation of the area     

The age and origin of Neoglacial moraines in Jotunheimen, southern Norway: new evidence from weathering-based data

This paper considers the controversial issue of the existence of pre-'Little Ice Age' Neoglacial moraines in southern Norway. Schmidt hammer rebound values are combined with measures of boulder roundness and weathering rind thickness in an attempt to isolate moraines that include weathered boulders. A critical approach is used in distinguishing sites where boulders have weathered in situ from those where previously weathered clasts have been incorporated into relatively young moraines. The results confirm that possible pre-'Little Ice Age' Neoglacial moraines seem to be restricted to small, high-altitude glaciers in eastern Jotunheimen. It is concluded that at these glaciers a particularly large response to a short-lived earlier Holocene climatic event is more likely to explain the survival of such moraines than a particularly subdued response to the climatic deterioration of the 'Little Ice Age'. More refined dating techniques are required to determine the age of formation of the anomalous moraines, but before the palaeoclimatic significance of such dates can be assessed, a critical test is required to establish whether the moraines mark former ice-front positions, and therefore reflect lowering of equilibrium line altitudes, or whether they have been displaced forwards by later and more extensive glacier advances.     

Mid‐ and late‐Holocene climatic changes: a test of periodicity and solar forcing in proxy‐climate data from blanket peat bogs

A wide range of palaeoenvironmental evidence from the Holocene has suggested periodicities in the Earth's climate of 10s to 1000s of years. Identifying these millennial‐, century‐ and decadal periodicities, and their impacts, is critical in developing a fuller understanding of natural climate variability. Any solar‐induced climatic change needs to be distinguished from other causes of natural climate variability and from short‐term catastrophic events induced either by external or internal processes. Such events might themselves generate a periodicity, or in combination with other forcing factors they may contribute towards a periodicity (and so spuriously imply a universal and continuing periodicity in the climate record), or they may resonate with a solar‐induced periodicity. Here, evidence from peat records for periodicity in climate change over the mid to late Holocene is reviewed and this is followed by a test of the replicability of claimed periodicities using blanket peat data covering the past 2000 yr from four sites in the British Isles. Results suggest that the mires studied do go through phases of being responsive to periodic forcing factors, with ca. 200, ca. 80 and 60–50 yr wavelengths reflected in some data sets. However, the patterns shown are not consistent. This could be the result of local conditions at individual mires (human impact, sensitivity and vegetation succession) or of changes in the strength or nature of global forcing factors. Assessing a solar–mire link remains difficult because the century‐scale variations of the Sun show different intervals between solar minima, the durations of which are themselves unequal, and because the proxy‐climate data‐sets from peat profiles may themselves not be dated with sufficient precision and/or accuracy. Copyright © 2001 John Wiley & Sons, Ltd.     

Age determination of intermoraine areas, Austerdalen, southern Norway

This paper is concerned with the size variation of thalli of the lichen Rhizocarpon geographicum agg. on intermoraine surfaces in the proglacial valley of Austerdalen, southern Norway, and with the use of thallus dimensions for both relative and absolute dating of intermoraine deposits. Measurements of the five largest lichens in 200 m² search areas at 255 sites show significant cross-valley and down-valley variations, with the smallest thalli located towards the centre of the valley and at up-valley sites. This variation has been examined in greater detail by dividing the intermoraine areas into twelve morphological zones, for which geomorphological and stratigraphical relations enable the construction of a relative time sequence of formation. This relative time sequence was then found to he confirmed by mean values of measurements of the largest lichen thalli in 100 m square quadrats within each morphological zone. This approach has allowed not only the relative age of morainic and intermorainic deposits to he established from geomorphological evidence but also, using tentative dates for the moraines, the construction of a chronology of moraine and intermoraine depositional events.     

Age and origin of anorthosites,charnockites, and granulites in the Central Virginia Blue Ridge: Nd and Sr isotopic evidence

Rb-Sr isotopic data for anorthosites, charnockites, ferrodioritic to quartz monzonitic plutons, and high-grade gneisses of the Blue Ridge of central Virginia show evidence of post-emplacement metamorphism, but in some cases retain Grenville ages. The Pedlar River Charnockite Suite yields an isochron age of 1021 +/-36 Ma, (initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7047 +/-6), which agrees with published U-Pb zircon ages. Five samples of that unit which contain Paleozoic mylonitic fabrics define a regression line of 683 Ma, interpreted as a mixing line with no age significance. Samples of the Roseland Anorthosite Complex show excessive scatter on a Rb-Sr evolution diagram probably due to Paleozoic (475 m.y.) metamorphism. Data from the ferrodioritic to quartz monzonitic plutons of the area yield an age of 1009 +/-26 Ma (inital ratio=0.7058 +/-4), which is in the range of the U-Pb zircon ages of 1000–1100 Ma. The Stage Road Layered Gneiss yields an age of 1147 +/-34 Ma (initial ratio of 0.7047 +/- 5).Sm-Nd data for the Pedlar River Charnockite Suite reflect a pre-Grenville age of 1489 +/-118 Ma ( _Nd=+6.7 +/-1.2). Data for the Roseland Anorthosite Complex and the ferrodioritic to quartz monzonitic plutons yield Grenville isochron ages of 1045 +/44 Ma ( _Nd=+1.0 +/-0.3) and 1027 +/-101 Ma ( _Nd=+1.4 +/-1.0), respectively. Two Roseland Anorthosite samples plot far above the isochron, demonstrating the effects of post-emplacement disturbance of Sm-Nd systematics, while mylonitized Pedlar River Charnockite Suite samples show no evidence of Sm-Nd redistribution.The disparity of the Sm-Nd age and other isotopic ages for the Pedlar River Charnockite Suite probably reflects a Sm-Nd source age, suggesting the presence of an older crust within this portion of the ca. 1 Ga old basement.     

Age and origin of clayey silt ‚brickearth’︁ in west London,England

The loamy ?brickearth’? deposits overlying coarse cold climate Kempton Park, Lynch Hill and Taplow Gravels of the R. Thames in west London are dated by thermoluminescence and mineralogical comparisons with loesses of three known ages. Although much of the ?brickearth’? resembles loess, it often contains more sand or clay than typical loess, and the mineralogical studies suggest that even the coarse silt (16–63 μm) fraction is rarely similar in composition to any known English loess. The thermoluminescence measurements suggest that most of the ?brickearth’? on the Taplow and Lynch Hill Gravels was deposited in the late Devensian, whereas that on the Park Gravel is mainly Flandrian. It is suggested that the latter is mainly floodloam, whereas the earlier ?brickearth’? is a mixed aeolian sediment derived from various local sources in and close to the Thames Valley.     

U-Pb Age, Setting and Tectonic Significance of the Anorthosite-Mangerite-Charnockite-Granite Suite, Lofoten-Vesteralen, Norway

Mangerites, charnockites, anorthosites, gabbros and granitesoccur within a high-grade metamorphic complex in the Lofoten–Vesterålenislands of northern Norway. U–Pb dating of zircon, titaniteand monazite indicates a three-stage magmatic history beginningat 1870–1860 Ma with the emplacement of the Lødingenand Hopen plutons, followed by a dominant stage at 1800–1790Ma that formed the bulk of the suite, and concluded by the emplacementof pegmatites, local rehydration and retrogression between 1790and 1770 Ma. On the scale of the Baltic Shield the 1870–1860Ma episode corresponds to contraction, amalgamation of arcs,and regional deformation. By contrast, the episode at 1800–1790Ma was characterized by major shifts in plate convergence, byintraplate deformation, and by a diversity of magmatic associationsincluding suites derived from the subcontinental mantle andwidespread granitoid rocks extracted from the continental crust.The diversity of concurrent magmatic events across the Svecofennianorogen, and the temporal coincidence with collisional eventsin coeval orogenic belts, suggests that the genesis of the suiteof magmatic rocks may have been related to tectonically drivenmechanisms of magma generation. KEY WORDS: anorthosite–mangerite–charnockite–granite; lithospheric processes; Lofoten–Vesterålen; Svecofennian orogen; U–Pb geochronology     

Sedimentology and origin of deposits from a small ice-dammed lake, Leirbreen, Norway

This paper describes recent proglacial lacustrine sediments exposed by the drainage of a small (probably never more than 0·03 km²) ice-dammed lake basin at Leirbreen, central Jotunheimen, Norway. The dominant facies include ripple-laminated, massive and horizontally-stratified sands, massive and horizontally-laminated silts, and irregularly-laminated fine sands and silts. The major control on lake circulation and the nature and distribution of these facies was an underflow driven by a subglacial meltwater stream which formed the major lake input. Although much of the sedimentary sequence indicates a pulsatory input, the proximal character of this small lake prevented the development of classic varved silts. Compressional deformation of shoreline sediments was due to winter lake ice push. Other deformational processes included the grounding of icebergs, water escape and syn-sedimentary downslope collapse. Observations from an adjacent small ice-marginal lake at Leirbreen provide support for several of the inferences drawn from the sediments of the former ice-dammed lake.     

Age and origin of charoitite,Malyy Murun massif,Siberia, Russia

Сharoitite consists of gem-quality mineral charoite and subordinate quartz, aegirine, K-feldspar, tinaksite, canasite, and some other minerals. This rock type is known only from one locality in the world associated with the Early Cretaceous (131.3 ± 2.4 Ma, K–Ar age) Malyy Murun syenite massif, Siberia, Russia. Although charoitite mineralogy is well known, there is disagreement whether it reflects metasomatic or magmatic activity. In order to understand when the charoitites formed we attempted to date it by ⁴⁰Ar/³⁹Ar incremental step-heating and laser ablation techniques. Our results show that the fibrous structure of water-bearing charoite does not retain radiogenic argon. Laser ablation ⁴⁰Ar/³⁹Ar for K-feldspar and tinaksite from the charoitite yielded several age clusters even from the same mineral grain. The oldest cluster of 134.1 ± 2.9 Ma for the K-feldspar agrees with the age of the Malyy Murun syenites. The youngest age of 113.3 ± 3.4 Ma for charoitite K-feldspar overlaps with the youngest of published K–Ar ages (112 ± 5 Ma) for one K-feldspar sample of the Malyy Murun syenite. Tinaksite is characterized by a similar spread of ages (from 133.0 ± 3 Ma to 115.7 ± 4.3 Ma) within a single grain. We suggest that charoitites originated due to the interaction of metasomatic agents derived from the Malyy Murun magma and country rocks. Timing of magma emplacement and charoitite crystallization is reflected by the older cluster of ages, whereas the younger ages are due to a secondary process.     

The origin of oceanic plagiogranites from the karmoy ophiolite,western Norway

Both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the Karmoy ophiolite of western Norway. The plutonic zone of this ophiolite can be subdivided into three complexes; the East-Karmoy Igneous Complex, the Visnes High Level Complex and the Veavagen Igneous Complex and plagiogranitic rocks are well developed in the first two of these.Within the East-Karmoy Igneous Complex, plagiogranites are associated with high temperature, pre-basic dyke, shear zones. Rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite (hydrated diabase) assuming a starting material consisting of 40% hornblende and 60% plagioclase and that batch melting occurred within the stability field of hornblende.In comparison, plagiogranite occurs in a number of bodies in the upper part of the Visnes High Level Complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes, microgabbros and magnetite gabbros, and a floor assemblage of layered and non-layered gabbros. The R.E.E. modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari-textured gabbros, although the distribution of highly hygromagmatophile elements such as K, Rb, Ba, etc. cannot be explained satisfactorily by this model alone. Depletion in these elements appears to be an autometasomatic effect.     

Common origin for De Geer moraines of variable composition in Raudvassdalen,northern Norway

The study of De Geer moraines in Raudvassdalen shows that most De Geer moraines are likely to have a common origin at the grounding line of glaciers despite variability in composition of the ridges. Pebble fabric, grain‐size analysis and structures within exposures of De Geer moraines in the Raudvassdalen area, with compositions ranging from mostly till to mostly sorted sediment, indicate that the ridges all formed at the grounding line of a tidewater glacier by common processes: deposition of sorted sediments beyond the grounding line followed by deformation of pre‐existing sediments and deposition of till as the glacier overrode the ridges. The compositional variation of the ridges is probably related to the position of the section studied relative to the location of the outlet of subglacial streams. Ridges composed entirely of till form at locations remote from the outlet of subglacial streams, and ridges with a component of sorted sediments form in closer proximity to these streams. This unifying theory of De Geer moraine formation, along with theoretical and geological evidence showing that there are limited physical conditions where basal crevasses can form, suggests that the number of De Geer moraines interpreted to have formed in basal crevasses is probably unrealistic. Copyright © 2000 John Wiley & Sons, Ltd.     

Age and origin of the Kitoi sillimanite schist deposit,eastern Siberia

Doklady Earth Sciences -     

The origin and significance of metamorphosed tectonic blocks in mélanges: evidence from Sulawesi, Indonesia

Metamorphosed tectonic blocks (or ‘knockers’) are widespread but volumetrically minor constituents of many circum-Pacific mélange belts, Due to the common lack of an exposed in situ provenance and to the seemingly chaotic field disposition of most block-bearing mélanges, their origin and uplift history are problematic and controversial. On the Indonesian island of Sulawesi a block-bearing mélange is overlain by an ophiolite nappe, the base of which is characterized by a metamorphic sole sequence. Petrological, geochemical and geochronological data indicate a direct genetic relationship between high-grade tectonic blocks in the mélange and amphibolites in the metamorphic sole. Amphibolite precursors to lower temperature blueschist assemblages are virtually ubiquitous in the tectonic blocks and subdivisions based on the nature of the overprinting relationships can be systematically correlated with block distribution patterns orientated subparallel to the strike of the mélange belt. It is suggested here that the high-grade tectonic blocks originated in a thin, thermally zoned metamorphic sheet welded to the oceanic hangingwall plate at the inception of subduction. Break-up of this sequence at depth, by tectonic erosion, led to dispersal of fragments into a newly developed serpentinite mélange wedge. Blocks experienced abrupt changes in P-T-X conditions due to a combination of hydration in the new fluid-rich environment, gradual cooling of the hangingwall over time and continuing underflow dragging sheared blocks deeper into the subduction zone, prior to upflow. Blocks plucked from the hangingwall at different depths and at different times evidently experienced uplift in different flow channels, resulting in block concentrations, with P-T-t paths characteristic of their source and flow trajectory, at systematically greater distances from the subduction zone hangingwall. The elucidation of the origin and significance of tectonic blocks in Sulawesi has important implications not only for the tectonometamorphic evolution of similar inclusions in other mélange belts, but also for models of the inception and early stages of subduction.     

Late‐glacial to Holocene environmental changes and climate variability: evidence from Voldafjorden,western Norway

Sedimentological, micropalaeontological (benthic foraminifers and dinoflagellate cysts), stable isotope data and AMS ¹⁴C datings on cores and surface samples, in addition to acoustic data, have been obtained from Voldafjorden, western Norway. Based on these data the late glacial and Holocene sedimentological processes and variability in circulation and fjord environments are outlined. Glacial marine sedimentation prevailed in the Voldafjorden between 11.0 kyr and 9.2 kyr BP (radiocarbon years). In the later part of the Allerød period, and for the rest of the Holocene, there was deposition of fine‐grained normal marine sediments in the fjord basin. Turbidite layers, recorded in core material and on acoustic profiles, dated to ca. 2.1, 6.9–7.6, ca. 9.6 and ca. 11.0 kyr BP, interrupted the marine sedimentation. The event dated to between 6.9 and 7.6 kyr BP probably corresponds to a tsunami resulting from large‐scale sliding on the continental margin off Norway (the Storegga Tsunami). During the later part of the Allerød period, Voldafjorden had a strongly stratified water column with cold bottom water and warm surface water, reaching interglacial temperatures during the summer seasons. During the Younger Dryas cold event there was a return to arctic sea‐surface summer temperatures, possibly with year‐round sea‐ice cover, the entire benthic fauna being composed of arctic species. The first strong Holocene warming, observed simultaneously in bottom and sea‐surface temperature proxies, occurred at ca. 10.1 kyr BP. Bottom water proxies indicate two cold periods, possibly with 2°C lowering of temperatures, at ca. 10.0 (PBO 1) and at 9.8 kyr BP (PBO 2). These events may both result from catastrophic outbursts of Baltic glacial lake water. The remainder of the Holocene experienced variability in basin water temperature, indicated by oxygen isotope measurements with an amplitude of ca. 2°C, with cooler periods at ca. 8.4–9.0, 5.6, 5.2, 4.6, 4.2, 3.5, 2.2, 1.2 and 0.4–0.8 kyr BP. Changes in the fjord hydrology through the past 11.3 kyr show a close correspondence, both in amplitude and timing of events, recorded in cores from the Norwegian Sea region and the North Atlantic. These data suggest a close relationship between fjord environments and variability in large‐scale oceanic circulation. Copyright © 2001 John Wiley & Sons, Ltd.     

Age and origin of the corundum-bearing rocks of Khitostrov Island, Northern Karelia

The Chupa nappe of the Belomorian Complex contains aluminous silica-undersaturated rocks with corundum, which are characterized by extremely low ¹⁸O/¹⁶O (whole-rock δ¹⁸O up to −21‰). Revealed isotopic anomalies are explained by the influence of meteoric waters that were modified through evaporation-precipitation cycles (Rayleigh distillation) under cold climatic conditions. In order to estimate whether the decrease in δ¹⁸O occurred prior to metamorphism of the protoliths of the Chupa Sequence or during water percolation in the course of metamorphic or postmetamorphic transformations, we studied oxygen composition in the rocks and minerals and conducted U-Pb dating on single zircons from corundumbearing rocks of Khitostrov.     

Cupriferous bogs in the Sackville area, New Brunswick, Canada

Two cupriferous bogs in the Sackville area of New Brunswick, Canada, are described. The organic matter (muck) in these bogs contains from 2 to 6% Cu, derived from cupriferous springs that carry copper in amounts ranging from 0.005 to 1 ppm. The source of the copper in the springs is unknown, but the metal probably originates from the leaching of cupriferous (chalcocitic) deposits in grits and conglomerates at the base of the Boss Point Formation of Carboniferous (Pennsylvanian) age.The copper occurs in the muck as copper humate(s), the precise chemical characterization of which is unknown.Examples of copper bogs related to copper mineralization in many parts of the world are reviewed, and it is pointed out that such bogs are good geochemical indicators of cupriferous deposits.The moss, Pohlia nutans, grows in the springs in the copper bogs and in other wet sites in the area where large concentrations of copper occur. This moss is tolerant to large amounts of available copper and markedly accumulates the metal. When slightly chlorotic Pohlia nutans may indicate the presence of higher than normal amounts of copper.     

Pb---Nd---Sr isotopic constraints on the origin of the Caledonian Bindal Batholith, central Norway

The Bindal Batholith is the largest granitoid batholith in the Scandinavian Caledonides, emplaced prior to or during the Scandian collision in a complex scenario of Ordovician to Middle Silurian nappe assembly. The Bindal Batholith ranges in compositon from mafic gabbro to leucogranite, but granites and granodiorites are by far the most abundant rock types.

Pb---Pb, Sm---Nd and Rb---Sr isotopic results from plutons of the batholith constrain the origin of the Bindal Batholith magmas. The isotope results suggest the presence of several source reservoirs, giving rise to the granitoid magmas. Both a source relatively depleted in U, Th and Rb and enriched in Sm, a source enriched in U and Rb and depleted in Sm, a source enriched in Th and Rb, but depleted in Sm, and, finally, a source enriched in Th and Sm, but depleted in Rb, is indicated by the initial compositions of the radiogenic isotope ratios. It is suggested that the depleted source reservoirs were contemporaneous depleted mantle and mantle derived rocks in the nappe sequences, that the enriched source reservoir was sediments derived from Proterozoic upper crust of Baltic Shield affinity and that the Th-enriched source reservoir was various Proterozoic rocks, in a lower crustal position, of either Baltic or Laurentian affinity.     

Sub‐arctic Holocene climatic and oceanographic variability in Stjernsund,northern Norway: evidence from benthic foraminifera and stable isotopes

A high‐resolution record, covering 9.3–0.2 ka BP, from the sub‐arctic Stjernsund (70°N) was studied for benthic foraminiferal faunas and stable isotopes, revealing three informally named main phases during the Holocene. The Early‐ to Mid‐Holocene (9.3–5.0 ka BP) was characterized by the strong influence of the North Atlantic Current (NAC), which prevented the reflection of the Holocene Climatic Optimum (HCO) in the bottom‐water temperature. During the Mid‐Holocene Transition (5.0–2.5 ka BP), a turnover of benthic foraminiferal faunas occurred, Atlantic Water species decreased while Arctic‐Polar species increased, and the oxygen isotope record showed larger fluctuations. Those variations correspond to a period of global climate change, to spatially more heterogeneous benthic foraminiferal faunas in the Nordic Seas region, and to regionally diverging terrestrial temperatures. The Cool Late Holocene (2.5–0.2 ka BP) was characterized by increased abundances of Arctic‐Polar species and a steady cooling trend reflected in the oxygen isotopes. In this period, our record differs considerably from those on the SW Barents Sea shelf and locations farther south. Therefore, we argue that regional atmospheric cooling triggered the late Holocene cooling trend. Several cold episodes centred at ～8.3, ～7.8, ～6.5, ～4.9, ～3.9 and ～3.3 ka BP were identified from the benthic foraminiferal faunas and the δ¹⁸O record, which correlated with marine and atmospherically driven proxy records. This suggests that short‐term cold events may result from reduced heat advection via the NAC or from colder air temperatures.