DATERING

Zircon fractions extracted from a metatrondhjemite sheet interbanded with
mafic volcanites on the island of Ytterøy in the Støren Nappe, Trondheims-
fjorden, have yielded a U-Pb age of 495.3 +- 2.5 Ma. This is interpreted as
the crystallisation age of this subvolcanic felsic intrusion. Field
relationships indicate that the trondhjemite and greenstone volcanites are
more or less coeval; both rock-types have participated in all the Scandian
deformation phases recorded on the island. A nearby metalimestone with
gradational contacts into mafic volcanites is also considered to be roughly
the same age as the trondhjemite.
Based on comparisons with other U-Pb dated trondhjemitic sheets associated
with metabasalts in this part of Norway, the Ytterøy greenstone pile appears
to be one of the oldest volcanite units in the Trondheim Nappe Complex. Both
the metabasalts and the metalimestone are probably of latest Cambrian to
very earliest Ordovician age.

Pyroxene separates from two dolerite dykes from eastern Varanger Peninsula (at Komagnes and on the island St.Ekkerøya) have been analysed by the 40Ar- 39Ar method. The Komagnes dykes shows steps converging on a 'plateau' with an age of 344+- 17 Ma. The data from the St.Ekkerøya dyke do not yield any conclusive result; a total fusion age of c.640 Ma is shown to be meaningless, and cannot be considered as a crystallisation age for the dyke. The data are compared with results from (1) an earlier K-Ar study and (2) a palaeomagnetic investigation on these same dykes; the former which yielded dates of c.350 Ma, and the latter a loosely Vendian age. The 40Ar-39Ar results have not provided us with any new reliable informationon the age of these dykes which will resolve the conflict between the radiometric and palaeomagnetic 'ages'. The true crystallisation ages of the dykes remain enigmatic. Other isotopic methods may eventuelly help to provide a clearer indication of the true age of these dykes.

In this report we present evidence for the occurrence of at least two generation of authigenic illite in diagenesis-grade Upper Riphean and Vendian shales of the Stangenes, Nyborg and Stappogiedde Formations, Varanger Peninsula, North Norway. Dating of illite subfractions by the Rb-Sr methods has yielded ages for redeposited clastic material and for the diagenetic events. As well as dating actual events, an objective of this study has been to contribute to the intense and long-lasting research and discussion on the absolute age of the Riphean - Vendian boundary, and the age ranges of the 'Varangerian glaciation' and the Late Vendian (=Ediacaran) period of time. The results suggest that the age of the Riphean - Vendian boundary is >630 Ma, and that the age of the Early Vendian 'Varangerian glaciation' is bracketed between <630 Ma and c. 560 Ma, and perhaps even between <630 and c.590 Ma. The age of burial diageneses at 560+-10Ma may be related to a regional compaction of the sedimentary succession arising from Baikalian deformation which is recognised in NW Russia. The c.530 Ma minimum age for burial disgenesis of the Stappogiedde Formation does not contradict the location of the Precambrian - Cambrian boundary at the top of this formation. The younger ages, ranging from 440 to 390 Ma, appear to reflect phases of Scandian deformation and uplift.

Trondhjemite dykes forming part of a swarm cutting the Gula Group metasediments
and tectonically overlying Støren Group metabasalts in the district south of
Støren, Sør-Trøndelag, have been investigated in road-cuts in the Snøan area.
Geochemically, the fine-grained dykes are high-Al2O3 trondhjemites which show
clear trace and rare-earth elements signatures indicative of a continental
margin setting. An attempt at Rb-Sr radiometric dating produced a best-fit
isochron of 465 +- Ma, interpreted as a minimum age for the dyke swarm. The
dyke has also probably been affected by thermal resetting.
The dykes do not penetrate the Lower Hovin Group succession which unconformably
overlies the Støren Group volcanites, and contains fossils of Late Arenig age.
The Rb-Sr minimum age is thus in reasonable accord with the faunal evidence.
The trondhjemite dyke swarm was evidently emplacement into a thickened conti-
nental margin crust following obduction of the Støren ocean-floor basalts upon
the Gula, with coeval deformation, metamorphism and subsequent uplift, probably
in earliest Ordovician time.

A dolerite dyke cutting very low-grade metasandstones of the Løkvikfjellet group near Hamningsberg, NE Varanger Peninsula, has yielded a U-Pb zircon age (upper intercept) of 554+20/-14 Ma; and a lower intercept of c.375 Ma. This dyke is one of several in this area that have previously given K-Ar dates of c.360 Ma. These dykes are similar in field appearance, petrography and chemistry to dolerite dykes cutting Baikalian (Late Vendian) NW-SE-trending folds and cleavage on the Rybachi Peninsula, NW Russia; and all trend between NE-SW and c.N-S. It is suggested that the Hamningsberg dyke, and the other, comparable dykes with published Late Devonian K-Ar 'ages', probably intruded along 'ac' extensional, master fractures either during the terminal stages of, or shortly after the Baikalian deformation in Late Vendian time. The lower intercept date of c.375 Ma is interpreted as reflecting a Mid Devonian isotopic disturbance related to a Caledonide (Scandian) tectono- thermal event.

The report is a guidebook for a one day excursion in connection to the COPENA meeting held at NGU 18-22 August 1997. The excursion route is Trondheim- Orkanger-Lensvik-Agdenes-The report is a guidebook for a one day excursion in connection to the COPENA meeting held at NGU 18-22 August 1997. The excursion route is Trondheim- Orkanger-Lensvik-Agdenes-Ørlandet-Hasselvika-Rissa-Rørvik-Trondheim. The purpose of the excursions is to demonstrate lithologies in the Proterozoic basement in the Western Gneiss Region and the Caledonian nappe sequences that are down folded into the basement in thight synclines. The effect of the Scandian deformation and metamorphic overprinting on the Proterozoic gneisses will also be emphasised. The following tectonic units will be studied: Protero- zoic gneisses of the Western Gneiss Region; The Risberget Nappe consisting of augen gneisses; The Sætra Nappe consisting of flagstones; the Blåhø Nappe with mica schist, hornblende schist, amphibolite and marble; The lowgrade Støren Nappe with greenstones, schists, marbles and gabbroic to felsic intrusions: Devonian sediments.

Cores from two stations in the Tampen area, in the Norwegian Trench west of Florø, have been analysed (geochemistry, sedimentology, physical properties) and dated (210Pb-dating). One station was located 1000 m downstream, northeast of the Snorre Field, while the other (reference) station was located 9 km to the east-northeast of the Snorre field. The downstream station was clearly influenced by drilling mud discharges, with elevated levels of Ba and Pb in the top sediments. Discrepanices between reported discharges and contaminant contents at dated sediment intervals indicated that resuspension or changing oceanographic conditions may influence the sedimentary record of contaminant discharges. Elevated levels of Ba and Pb in the reference station indicate spreading of drilling-related contaminants over considerable areas. Comparatively high levels of trace metals in old sediments not influenced by anthropogenic inputs demonstrates the importance of geological control in environmental monitoring.

The Hellevik-Flekke area comprises Precambrian crystalline rocks that have experienced a complex history of deformational and metamorphic events through both Precambrian and Caledonian times. The area can be divided into four different lithological units. The Basal Gneisses, the Flekke Unit, the Gjøl- anger Unit and the Vardeheia Unit. During the Caledonian Orogeny all units were deformed and metamorphosed under eclogite facies conditions, but relicts of undeformed and unmetamorphosed protholiths from the units can be identified. To the south, granodioritic and monzonitic gneisses occur together with granites, pegmatites, and minor amphibolitic and eclogitic bands and lenses. This assembly of lithologies is similar to those of the Western Gneiss Complex in the middle part of the Sognefjord transect. The Flekke Unit is interpreted to represent an allocthonous nappe containing a layered mafic-ultramafic complex. The gabbroic rocks of th complex are dated to 1522+-55 Ma (Sm-Nd age). The Gjølanger Unit consists of grey and green orthogneisses of dioritic to granitic composition, with geochemical character- istics of rocks formed at an active continental margin. Rb-Sr isotopic data indicate a minimum age for the gneisses of 155 Ma, with subsequent Sr-isotope disturbance. This unit is also interpreted to be an allochthonous nappe. The Vardeheia Unit in the north experienced the strongest Caledonian deformation and is dominated by heterogeneous banded gneisses and mylonites with small lenses of amphibolites and eclogites.

Sommeren 1997 ble det tatt 200 vibrokjerneprøver på Egersundbanken, lokalisert mellom 4o og 6o øst og 57o og 58o nord. Kjernelengden varierte mellom 0 og 3.6 m, med en gjennomsnittlig lengde på 2.2 m. Høsten 1997 ble 38 av disse kjernene åpnet og beskrevet. I denne rapporten er resultatene oppsummert. Morene ble funnet i 4 av kjernene. Resten av kjernene besto av overkonsoliderte glasimarine leirer, ofte med et sandlag på toppen. Kun 3 av de 38 Kjernene inneholdt sand gjennom hele kjernen. 20 kjerner hadde mindre enn 12 cm sand over de glasimarine leirene, mens 6 kjerner hadde mer enn 50 cm sand på toppen. Dette viser at deler av Egersundbanken mangler eller har et meget tynt dekke med sand av antatt Holocen alder. De glasimarine leirene består enten av massiv leire eller leire med lag eller linser av finsand/silt. Linsene er som regel tynnere enn 1 cm, mens sandlagene varierer fra mindre enn 1 mm til flere cm. De glasimarine leirene inneholder spredte klaster opp til 2 cm i diameter, men oftest mindre enn 0.5 cm. Deformasjonsstrukturer (forkastninger og folder) samt skyveplan i mange av kjernen viser at leirene (sannsynligvis) har vært over- lagret av is. De glasimarine leirene er uten skjell eller skjellfragmenter, noe som gjør det vanskelig å datere dem. Disse kjernebeskrivelsene vil sammen med tolkning av sidesøkende sonar data, seismikk og andre prøver danne grunnlag for å lage kart som viser sedimenttyper i området. Sammenstillingen vil bli utgitt i en separat rapport.