Eastern Iceland Research Articles

Precise dating of the Holmatindur cooling event in eastern Iceland: Evidence for mid‐Miocene bipolar glaciation

A succession of basaltic lavas, volcaniclastic sediments, and lignite beds in the Holmatindur region of eastern Iceland provides the means for tying magnetic reversal stratigraphy and a record of major North Atlantic cooling to a precise radiometric timescale. A prominent part of the section is the Holmatindur clastic bed, which is up to 66 m thick and can be traced 80 km along strike. The geological significance of this unit lies in its composition and associated thin lignite seams. The volcaniclastic composition is basaltic (in contrast to the common rhyolitic/dacitic composition of tuff units in the area), and hyaloclastic material is dominant. Hydroexplosive volcanism most likely produced the hyaloclastite, through subglacial eruptions within the paleovolcanic zone. Pollen assemblages from the lignite beds indicate a dramatic climatic deterioration, from subtropical to cool temperate conditions, through these sediments. Feldspar crystals separated from the Holmatindur clastic bed were determined to be from 10.72 (± 0.16) Ma, from 40Ar‐39Ar incremental heating experiments. We correlate the Holmatindur cooling event with a relative maximum in benthic foraminiferal δ18O (Zone Mi6 of Miller et al. [1991]), which has been interpreted as an episode of ice accumulation within the mid‐Miocene cooling period, as well as with pulses in ice‐rafted debris supplied to deep water sites in the North Atlantic Ocean, a global sea level drop of 70 m, and submarine canyon cutting, all of which occurred within the early part of magnetic anomaly 5 normal (C5n). While strong evidence exists for mid‐Miocene ice sheets in Antarctica, northern hemisphere glaciation is thought to have started much later. The Iceland sites studied here demonstrate that significant ice accumulation occurred, albeit intermittently, as early as late middle Miocene time in the North Atlantic Ocean. From radiometric dating of Tertiary lavas that lie below and above the well‐established lower boundary C5n, we conclude that the onset of this long normal polarity period occurred at 10.94 (± 0.16) Ma. The new data are compatible with the Cande and Kent [1995] interpolated estimate for this boundary (10.95 Ma) but are significantly older, or more precise, than previous direct dating by conventional K‐Ar methods.

A model for estimating meal frequency and meal size from stomach data with an application to Atlantic cod (Gadus morhua) feeding on capelin (Mallotus villosus)

The feeding of a predator on prey that occurs in schools is modelled as a stochastic process involving two random variables: (i) the number of schools that the predator finds per unit time and (ii) the number of prey items captured in each encounter. The number of encounters is governed by a Poisson process and the number of prey items can be either a Poisson random variable or more generally a negative binomial. The principal parameters in the model can be interpreted as the average meal frequency and the average meal size. This model is applied to Atlantic cod (Gadus morhua) feeding on capelin (Mallotus villosus). Cod stomachs from six length-groups were sampled off eastern Iceland and the number of capelin in individual stomachs recorded. The parameters in the stochastic feeding model are estimated by the method of maximum likelihood using the observed frequency distribution of capelin numbers in cod stomachs. The data fit the model well where the meal size is modelled by a negative binomial distribution, but less successfully where the meal size has a Poisson distribution. Finally, using the estimates of average meal frequency and average meal size the average and the standard deviation of consumption rate are calculated.

Changes in the Western Boundary Undercurrent Outflow since the Last Glacial Maximum, from smectite/illite ratios in deep Labrador Sea sediments

High‐resolution mineralogical studies were performed on late glacial and deglacial sediments from two deep piston cores from the Labrador Sea, located at the inlet (SW Greenland Rise) and outlet (Labrador Rise) of the Western Boundary Undercurrent (WBUC) gyre. At the two sites, smectites transported from the eastern Iceland and Irminger basins by the WBUC are observed. Clay mineral changes are used as proxies for the paleointensity reconstruction of the WBUC. On the Greenland Rise, a clay mineral index (smectite/illite (S/I) ratio) is defined. A S/I ratio of ∼1 characterized the Last Glacial Maximum. It increased after ∼17 ka. and reached a maximum value of 4 during the early Holocene. The mineralogical changes are gradual and do not show any reversal during the Younger Dryas. This pattern, which is confirmed by first‐order estimations of smectite and illite fluxes, suggests gradually increasing sedimentary fluxes and WBUC intensity since the Last Glacial Maximum. A peak in the velocity of the WBUC at ∼9 ka, as recorded by clay assemblages, is consistent with other regional studies based on pollen, foraminifera, or grain‐size measurements. A massive dilution of smectites by illite and chlorite (S/I ≈ 3) occurs at ∼8.5 ka. It corresponds to a period of rapid sediment accumulation and reflects an intensified illite‐rich detrital supply by meltwaters from the southern Greenland Ice Margin. On the Labrador Rise, the smectite content varies between 20 and 60% with no obvious trend through time. The mineralogical composition is strongly influenced by ice‐rafted deposition and by the abundance of fast deposit units (cf. Heinrich layers in the North Atlantic) which contain abundant detrital carbonates spilled‐over from the North‐West Atlantic Mid‐Ocean Channel. In such layers, smectites are present but are diluted by the addition of illites, chlorites, and kaolinites. This provides evidence for a discrete and continuous WBUC supply of fine particles from the Irminger and Iceland Basins as far as the southeastern part of the Labrador Basin. Early deglacial smectite‐rich layers (up to 60%) are also observed at this site. They indicate an increase in the outflow of the WBUC at ∼13.5 ka. (Bölling‐Alleröd), as previously reported from grain size or foraminiferal assemblage studies.

Hummock fields in the Norway Basin and Eastern Iceland Plateau: Rayleigh-Taylor instabilities?

Side-scan imagery and 3.5 kHz profiles in the Norway Basin reveal ∼3–4 × 10 4 km 2 of sea floor covered by regularly spaced (500–2000 m), low-relief hummocks previously attributed to bottom currents. Extrapolating from nearby borehole data, I attribute these mound fields to gravitational (Rayleigh-Taylor) instabilities resulting from superposition of 50–100 m of thick Pliocene-Pleistocene, glacigenic (glacier-derived) sandy clays (density 1800 kg/m 3 ) on a several-hundred-metre thickness of Miocene biosiliceous oozes having a density of 1300 kg/m 3 . This hypothesis predicts or explains similar hummock fields in many oceanic regions where such density inversions exist.

New and little known mollusca from Iceland and Scandinavia. Part 3

Abstract This paper is a continuation of Warén's articles in Sarsia volumes 74, 76, and 78. Mikro globulus gen. et sp.n. (Archaeogastropoda, provisionally in Skeneidae) is described from the Icelandic shelf and Anekes giustii Nofroni & Bogi, 1989 from the Mediterranean is transferred to this genus. Protolira thorvaldsoni sp.n. (Archaeogastropoda, Skeneidae) is described from decaying whale bone found off southwestern Iceland. Coccopigya lata sp.n. (Archaeogastropoda, Cocculinidae), Alvania angularis sp.n. and A. incognita (Meso gastropoda, Rissoidae) are described from s'unken drift wood from deep water off Iceland. Onoba improcera sp.n. and O. torelli sp.n. (Mesogastropoda, Rissoidae) are described from northern Iceland. ‘Cingula’ globuloides Warén', 1972 (formerly in Rissoidae) is transferred to Elachisina (Mesogastropoda, Elachisinidae). Alvania alaskana (Dall, 1887) and Alvania dinora (Bartsch, 1917), both from Alaska, are synonymised with Onoba mighelsi (Stimpson, 1851) (Mesogastropoda, Rissoidae). Brookesena turrita sp.n. (Heterobranchia, Mathildidae) is described from several localities around the Icelandic upper continental slope and Turritellopsis stimpsoni Dall, 1919 (fonnerly known as T. acicula (Stimpson, 1851), Mesogastropoda, Turritellidae) is transferred to the Mathildidae. All northeast Atlantic species ofRissoidae (Mesogastropoda) except the genera Rissoa and Pusillina are reviewed, and their distribution and habitat is given. Substantial extension of the distributional range is given for the following Gastropoda: Granigyra inflata (Warén, 1992), off southwestern Iceland ca 1000 m. Lissotesta turrita (Gaglini, 1987), off southwestern Iceland ca 1200111. Xyloskenea naticiformis (Jeffreys, 1883), on sunken driftwood, off southwestern Iceland ca 1200–1800m. Basilissopsis watsoni Dautzenberg & Fischer, 1897, off southwestern Iceland ca 1000 m. Cingula trifasciata (J. Adams, 1800), Swedish west coast, Gullmarsfjorden (empty shells). Pseudosetia turgida (Jeffreys, 1870), off southeastern Iceland. Boreocingula castanea (Møller, 1842), Varangerfjord, northern Norway. Boreocingula globulus (Møller, 1842), restricted to southwestern Greenland. Benthonella tenella (Jeffreys, 1867), off southern Iceland. Alvania moerchi (Collin, 1886), northern and eastern Iceland. Alvania subsoluta (Aradas, 1847), northern and southwestern Iceland. Alvania pseudosyngenes Warén, 1973, northern Iceland, 400-200 m. Manzonia crassa (Kanmacher, 1798), Swedish west coast (empty shells) and Norway to 60.5° N Crisilla semistriata (Montagu, 1808), Swedish west coast (empty shells) and western Norway to 63.5°N. Onoba exarata (Stimpson, 1851) is recorded from northern Iceland.

Ecological relations in historical times: human impact and adaptation

Ecological relations in historical times - an introduction, N. Roberts and R.A. Butlin. Part 1 Scotland: the Machair vegetation of South Uist and Barra in the Outer Hebrides of Scotland - its interacting ecological, geomorphic and historical dimensions, D.D. Gilbertson et al a lake-sediment record of changing land use intensity on the Isle of Lewis, Scotland, J.A. Entwistle et al a Scottish broad - historical, stratigraphic and numerical studies associated with polleniferous deposits at Kilconquhar Loch, G. Whittington and K.J. Edwards a rising cloud of midges in the Scottish Highlands?, A. Roberts. Part 2 England and Wales: Medieval land use, agriculture and environmental change on Holy Island, Northumbria, K. Walsh et al a rough and cold country - farming and landscape changes in Northern Snowdonia since c.1600, M. Brayshay and A. Williams social, economic and environmental change in the fen and fen-edge region of East Cambridgeshire in the 19th century, R.A. Butlin woodland history and plant indicator species in North-East Yorkshire, England, R. Gulliver. Part 3 The North Atlantic region: Ancient Saami in Finnish Lapland and their impact on the forest vegetation, C. Carpelan and S. Hicks introductions, extinction or continuity? faunal change in the North Atlantic islands, J.P. Sadler and P. Skidmore by-pasing Ellis Island - insect immigration to North America, P. Buckland et al a question of Landnam - pollen, charcoal and insect studies on Papey, Eastern Iceland, P. Buckland et al. Part 4 North America and the Caribbean: ecological responses to ecosystem shock in the island Caribbean - the aftermath of Columbus, 1492-1992, D. Watts human impact on the wetlands of Belize, Central America, P. Furley et al invasion and transformation of California's valley grassland, a Mediterranean analogue system, M. Blumler. (Part contents)

Stratigraphy and paleomagnetism of a 3-km-thick Miocene lava pile in the Mjoifj�rdur area, eastern Iceland

We have carried out stratigraphic mapping in the Upper Miocene basalt lava pile around the fjords Mjoifjordur and Seydisfjordur, eastern Iceland. The mapping is based on conventional methods including the use of interbasaltic clastic horizons and petrographically distinct lava groups. These units are also used to provide correlations with the Nordfjordur area south of Mjoifjordur. We present a 3-km composite stratigraphic column for the area between Mjoifjordur and Seydisfjordur. The geology of this area shows some differences from the classical model of Walker for the structure of eastern Iceland partly due to the fact that most of Mjoifjordur is not in the vicinity of central volcanoes. Detailed laboratory measurements of remanent magnetization were carried out on oriented core samples from 363 lavas in 10 selected profiles. The local paleomagnetic polarity stratigraphy supports correlations made on the basis of other criteria. Over 20 geomagnetic reversals are recorded in the eastern Iceland lava pile in a period approximately 13-10 Ma ago. The geomagnetic field during this period averages to a central axial dipole field, and its overall statistical properties resemble those obtained in earlier surveys in Iceland.

Stratigraphy and paleomagnetism of a 3-km-thick Miocene lava pile in the Mjoifjördur area, eastern Iceland

Stratigraphy and paleomagnetism of a 3-km-thick Miocene lava pile in the Mjoifjördur area, eastern Iceland

Zircon thermometry of high-temperature rhyolites near volcanic-associated massive sulfide deposits, Abitibi subprovince, Canada

Research Article| February 01, 1995 Zircon thermometry of high-temperature rhyolites near volcanic-associated massive sulfide deposits, Abitibi subprovince, Canada C. Tucker Barrie C. Tucker Barrie 1Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar Geology (1995) 23 (2): 169–172. https://doi.org/10.1130/0091-7613(1995)023<0169:ZTOHTR>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation C. Tucker Barrie; Zircon thermometry of high-temperature rhyolites near volcanic-associated massive sulfide deposits, Abitibi subprovince, Canada. Geology 1995;; 23 (2): 169–172. doi: https://doi.org/10.1130/0091-7613(1995)023<0169:ZTOHTR>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract High-temperature, high-silica rhyolites are commonly found near Cu-Zn volcanic-associated massive sulfide (VMS) deposits in the Archean Abitibi subprovince. They are characterized by anhydrous phenocrysts (feldspar, quartz) in concentrations of generally <15 modal percent and have >73% SiO(2), high incompatible element contents, Rb/Sr > 1, Zr/Y < 5, La(N)/Yb(N) < 3.5, and negative Eu anomalies. They have zircon saturation temperatures of 840–940 ° C, as calculated by using the zircon solubility model, which are higher than most other Abitibi rhyolites. Considering their high zircon saturation temperatures, their composition, and their spatial and temporal associations with tholeiitic mafic and ultramafic rocks, Abitibi high-temperature rhyolites are similar to silicic volcanic rocks found in hotspot-related oceanic settings with anomalously high heat flow, such as in eastern Iceland. The association of high-temperature rhyolites with VMS deposits may provide a useful exploration guide where zircon inheritance is minimal, such as in many of the primitive Archean and Early Proterozoic greenstone belts. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Growth of an Intermittent Ice Sheet in Iceland during the Late Pliocene and Early Pleistocene

Deep-sea paleoclimatic records show that the Quaternary climate around Iceland was, and probably still is, very sensitive to rapid shifts in North Atlantic oceanic circulation. Studies of several key sections in Iceland indicate that similar oscillations are reflected in the late Pliocene and early Pleistocene terrestrial stratigraphy. Correlations between six rock sequences in western, northern, eastern, and southern Iceland show a fairly distinct trend during the Pliocene-Pleistocene transition indicating the progressive growth of an ice sheet from southeast toward the north and west. The correlations are based on K/Ar dates and paleomagnetic studies. A total of 11 glacial horizons are recorded in a stratigraphic column from eastern Iceland extending back 6.5 myr. In western Iceland, 7 glacial horizons are preserved in a rock section dated from 7.0 to 1.8 myr, and in northern Iceland 14 glacial horizons are identified in a section that extends back to 9.0 myr. Well over 20 glacial horizons have been identified in the stratigraphic column in Iceland. Full-scale glacial-interglacial cyclicity with regional ice cover is indicated at approximately 2.6 myr. A further amplification leading to islandwide glaciations is identified at 2.2-2.1 myr. These results show that changes in the ice cover in Iceland correlate with the deep-sea oxygen isotope records from benthic and planktonic foraminifera as well as IRD studies from the North Atlantic.

Iceland hotspot track

We use a model of plate motions relative to major hotspots underneath the African, Indian, North American, South American, and Australian plates to compute the track of the Iceland hotspot after 130 Ma. The present-day hotspot is located under eastern Iceland offset about 240 km east of the Reykjanes and Kolbeinsey ridges. At 40 Ma, the Kangerlussuaq region of East Greenland would have been directly above the hotspot. The anomalous postdrift uplift of the East Greenland margin can also be explained by passage of the rifted margin over a hotspot. At 60 Ma, the Umanak Fjord region of the west coast of Greenland was above the hotspot, where picrites and hyaloclastites of nearby Disko Island are dated at ∼64 to 59 Ma. Our reconstruction shows Ellesmere Island above the hotspot between 130 and 100 Ma. Latest Albian to early Cenomanian volcanic rocks on Axel Heiberg Island and northern Ellesmere Island indicate a nearby hotspot at that time. At 130 Ma, our model locates the hotspot near the northern margin of Ellesmere Island, close to the intersection of the Alpha Ridge with the coast. The hotspot would have been located beneath the Arctic Alaska-Chukotka plate when it formed the Mendeleyev Ridge, and as the spreading center migrated over the hotspot, it transferred to the North American plate, where it formed the Alpha Ridge. Our model suggests that the initiation of the Iceland hotspot predates the opening of the North Atlantic by at least 70 m.y. and that the massive early Tertiary volcanism along the North Atlantic plate margins reflects the effect of rifting in the vicinity of existing thinned crust, rather than the arrival of a plume head.

The benthic algal vegetation of the Mjóifjörđur, Eastern Iceland

The benthic algal vegetation of Mjoifjorđur, situated in the central area of the East Icelandic coast, is described regarding the species distribution from the head to the mouth of the fjord, the main algal associations and zonation patterns. The vegetation gradient along the fjord coast is related to the hydrographic conditionsviz. the influence of freshwater discharge in the inner area and that of the cold water masses of the East Icelandic Current in the outer area. Comparisons are made with the vegetation of other Icelandic fjords investigated during surveys of the Icelandic algal vegetation as a whole.

Patterns in species composition of rocky shore communities in sub-arctic fjords of eastern Iceland

Quantitative data on plants and animals from 20 transects of the eulittoral zone in 8 fjords of sub-arctic east Iceland were analysed by a number of classification and ordination techniques. Field work was carried out during two periods of spring tides in June and July 1987. The study was restricted to gently sloping stable bedrock, but transects were otherwise randomly chosen. Species composition was strongly influenced by the relative position of the transects along the length of the fjords. Species richness increased from heads towards mouths. Increased temperature fluctuations towards inner parts of fjords are suggested as an explanation for these patterns, while salinity is unlikely to be a critical factor. The analyses also indicate that degree of exposure, as measured both by a physically derived index and by a simple biological index, is an important factor in structuring these eulittoral communities, although affecting species richness only slightly, if at all. Classification of transects results in a number of community types which, however, are not well defined. A classification based on the presence/absence of three canopy-forming fucoids yields groups of transects differing in species composition and in degree of exposure. The rocky eulittoral communities of the fjords of eastern Iceland resemble those of northern Norway, but differ in that several key species of the latter communities are missing, probably largely due to the isolation of Iceland.

Gradient in seaweed vegetation patterns along the North Icelandic coast, related to hydrographic conditions

The benthic algal vegetation of the North Icelandic coast exhibits particular features, different from those found in southern and western Iceland on the one hand and in eastern Iceland on the other. It is characterised by low-eulittoral belts of Devaleraea ramentacea, Petalonia species and Chordaria flagelliformis as well as by meadows of diverse Acrosiphonia species. In the tide pools Atlantic and typical North Icelandic associations occur side by side. This area thus represents an intermediate region between Atlantic and subarctic growth conditions. A certain gradient in the west-east direction was observed within the North Icelandic vegetation type and is possibly related to the gradual cooling and dilution of the water masses which pass the North Icelandic shelf. The hydrographic discontinuity in the extreme NW creates a sharp floristic and vegetational boundary for benthic algae, whereas in the NE a gradual transition towards the subarctic East Icelandic vegetation was observed. The Atlantic character of the vegetation was more pronounced in the western than in the eastern part of the north coast. An enclave of warm water vegetation was interposed along the western banks of the Tjornes peninsula.

Geochemical constraints on the petrogenesis of the Kamiskotia gabbroic complex and related basalts, Western Abitibi Subprovince, Ontario, Canada

The Kamiskotia gabbroic complex (KGC) is a large (170 km 2), late Archean synvolcanic intrusion located in the western Abitibi Subprovince, Canada. It is divided into four stratigraphic units: a Lower Zone (LZ), comprised of adcumulus and mesocumulus peridotites, troctolites and gabbro-norites (Mg number = 73–86); a Middle Zone (MZ) comprised of mesocumulus gabbro-norites (Mg numbemr = 60–74); mesocumulus and orthocumulus gabbro-norites and ferroan gabbro-norites of the Upper Zone (UZ, Mg number=37–64) which are overtain and partly along strike with granophyric rocks of intermediate to felsic composition. The cumulates are characterized by flat REE patterns with La N/Yb N = 0.4–2.6, and range from 0.6–2× chondrites for olivine-bearing LZ adcumulates to 10–25× chondrites for upper UZ meso-to orthocumulates. Europium anomalies are strongly positive for the LZ and and diminish upsection to the upper UZ which has slight negative Eu anomalies. Chilled rocks are found in mixed magma and contact chill outcrops, and have a Mg number of 54–58, La N/Yb N = 0.8–1.2 and slight negative Eu anomalies. Phase relationships and mass balance calculations using the REE indicate that LZ cumulates may be derived from the average chill composition. Similarly upper UZ cumulates may be derived from evolved basalts which overlie the KGC. Assimilation-fractional crystallization modeling indicates that the evolved basalts may be derived from the average chill composition by 70–80% fractionation at low pressures; or lower degrees of fractionation accompanied by recharge of the averagr chill composition into the system. The basalt compositions do not reflect significant assimilation of nearby felsic rocks or average Archean crust during fractionation. Primitive KGC liquids are estimated by adding an average olivine-bearing adcumulate composition to the KGC chill composition in equal proportions to obtain a liquid with an Mg number of 70. The REE content of such a liquid is consistent with a derivation by 34% partial melting of a chemically primitive mantle. Neodymium isotope ratios for KGC cumulates, mineral separates and overlying volcanic rocks plot on a 2710 Ma isochron with ϵ Nd( t) = 2.5 ± 0.8, in close agreement with U-Pb zircon ages for the KGC and Kamiskotia felsic volcanic rocks at 2707 ± 2 Ma and 2702 ± 2 Ma, respectively. The initial value is identical to the isotopically depleted, MORB-like Abitibi mantle signature at 2.7 Ga, and provides supporting evidence for fractionation-dominated supracrustal processes, with no detectable assimilation of an isotopically enriched crustal component. Kamiskotia mafic magmatism is similar to more recent, rift-related systems represented by the Skaergaard Intrusion, East Greenland, and differentiated tholeiitic volcanic fields in Eastern Iceland and along the Galapagos Spreading Center.

Tertiary silicic rocks in the area of the Kækjuskörð rhyolitic volcano, eastern Iceland

Tertiary silicic rocks in the area of the Kækjuskörð rhyolitic volcano, eastern Iceland

Physical volcanology of mid-Proterozoic plateau lavas: The Keweenawan North Shore Volcanic Group, Minnesota

The North Shore Volcanic Group, a thick sequence of plateau lavas of Keweenawan (Proterozoic, 1,100 m.y.) age, was erupted during the development of the Midcontinent Rift of North America. It contains many well-exposed physical features that imply a range of eruptive styles and flow mechanisms. These include (a) high-volume and high-effusion-rate floods of basalt and basaltic andesite, which ponded and crystallized largely after flow had ceased; (b) moderate- to small-volume and -effusion-rate flows and small floods, some probably building low shields; and (c) small flows to very large flows and ignimbrites of icelandite and rhyolite lacking visible vent areas. Viscosities calculated from 183 chemical analyses correlate well with the physical features. The style of volcanism combined characteristics of the Columbia Plateau and the Snake River Plain (plains volcanism) and was very similar to that of the Tertiary of eastern Iceland.

The Fjallgardar volcanic ridge in NE Iceland: an aborted early stage plate boundary or a volcanically dormant zone?

Summary The extinct Fjallgardar volcanic ridge (FVR) in NE Iceland is approximately 15–20 km wide and extends for about 190 km from the Vatnajökull glacier toward the Slétta peninsula in the far N. The FVR was mainly formed subglacially and strikingly parallels the curvature of the present median line (Askja to Theistareykir centres) of the neovolcanic zone (NVZ) which lies about 40 km to the W. The basalts from the Fjallgardar and Slétta regions are dominantly evolved tholeiites as shown by major- and trace-element compositions of 42 samples. The average magnesium number is 47 (range 37–58). 90 per cent of the samples can be grouped into either ‘low-’ potassium (0.1%–0.4% K 2 O) or ‘high-’ potassium (0.5%–0.7% K 2 O) units. The high-potassium lavas in the FVR are chemically similar to Tertiary tholeiites of eastern Iceland. A test for temporal geochemical variations for three stratigraphic formations was carried out for the Mödrudalur area of the FVR. The oldest of the three formations (M3) consists of low-potassium basalts only, whereas the two younger formations (M4 and M5) consist predominantly of more evolved high-potassium basalts. Formation M5 is most probably fed into Mödrudalur from the Kverkfjöll volcanic system some 75 km further S. Volcanism in the FVR S of Slétta was short-lived and lasted for perhaps 200 000–300 000 years. Subsurface lateral flow of magma from the NVZ is probably responsible for the Recent subaerial volcanism in Slétta. In the Slétta area the older subglacially formed ridges have only a low-potassium basalt composition. The FVR may be viewed as ‘abortive’ and as the precursor to the NVZ in northeastern Iceland. During the Brunhes magnetic polarity chron (less than 0.7 Ma) a rift jump is postulated to have occurred from the FVR to the present NVZ, approximately 40 km to the W. E.-W lateral rift jumps (20–40 km) may have been common in the eastern half of Iceland in the vicinity of the Iceland hot-spot located 150–200 km E of the Reykjanes and Kolbeinsey mid-ocean ridges.

Age of the Austurhorn intrusion; a net-veined complex in southeastern Iceland

Abstract Previous attempts to date the rocks of eastern Iceland by radiometric means have resulted in a considerable scatter in ages. This was probably the result of hydrothermal alteration, affecting the investigated material, particularly lavas, to a varying degree. This study reports the results of a combined Rb-Sr and K-Ar whole-rock investigation of granophyric material from the Austurhorn complex, southeastern Iceland. The Rb-Sr isochron age obtained for four samples is 12.8 ± 5.3 Ma, consistent with the K-Ar ages which average 11.8 Ma.

Geomorphology of Fljótsdalshérað, eastern Iceland, and its implications

Geomorphology of Fljótsdalshérað, eastern Iceland, and its implications