in the Linacre College Department of Earth Sciences University of Oxford 1985

314 Pages·2012·5.24 MB·English

Thesis submitted for the Degree of
Doctor of Philosophy
in the
University of Oxford
RADIOGENIC ISOTOPE STUDIES OF
CRUST-FORMING PROCESSES IN
THE LOFOTEN-VESTER&LEN PROVINCE
OF NORTH NORWAY
(Volume One: Text)
by
Stephen James Rochfort Wade
Linacre College
and
Department of Earth Sciences
University of Oxford
1985
Short Abstract
The Lofoten-Vesteralen province of North Norway consists almost
exclusively of Precambrian granulite-facies rocks. The oldest rocks
in the province are monzonitic and dioritic migmatitic gneisses, the
protoliths of which were formed at 2.7 Ga.
The migmatites are overlain by a series of supracrustal gneisses,
from 2.1 Ga, largely volcanogenic in origin, but with interbedded marbles
and banded ironstones. The first occurrence of marble in western Lofoten
is reported. Deposition in a subsiding back-arc basin or in an Andean-
type environment on a thin continental margin is inferred.
Both gneiss sequences were intruded by basic rocks at 1.8 Ga. The
basic rocks could not have been formed simply by extraction from the mantle
at 1.8 Ga. The required contribution from 2.7 Ga migmatites could be as
much as 37%, but less if contamination took place via anatectic melts.
The first report of eclogitic rocks from the Lofoten-Vesteralen province
is made in this study; their formation is associated with shear deformation
Both gneiss sequences and the basic rocks were intruded by mangeritic
rocks at 1.8 - 1.7 Ga. Their chemical compositions can be explained by
fractional crystallization from magmas formed from 2.7 Ga and 2.1 Ga
gneisses and 1.8 Ga mantle-derived magmas. Parameters derived from Rb-Sr,
Sm-Nd and U-Pb systems to express the relative proportions of crust and
mantle contributions to the mangerites mutually correlate, supporting
the crust-mantle source model for the mangerites. Mixing calculations
suggest that the late Archaean contributes in excess of 50% by mass for
almost all mangerites.
Anatectic veins present, especially in the Moskenes^y supracrustal
gneisses, are inferred to represent partial melts which coalesced to
form the mangerites at higher structural levels. Anatexis was caused
by basaltic underplating associated with limited crustal extension.
Later rock-forming events were the emplacement of dolerite dykes;
the 1.65 Ga Mdingen Granite; the Leknes Group metasediments and the
Caledonian granite pegmatites.
Long Abstract
Crust-forming processes in the Lofoten-Vesteralen province of
North Norway have been investigated using detailed fieldwork, petrography,
geochemical and isotopic methods.
The Lofoten and Vesteralen islands form an archipelago off the west
coast of Norway opposite Narvik.
The oldest rocks in the province are a series of migmatitic gneisses
which are most extensive on Moskenestfy and Lang^y, at the deepest structural
level in the area. These migmatites are monzonitic or dioritic in
composition, they are foliated in some places but not in others and have
granulite or amphibolite-facies mineral assemblages. The development of
amphibolite-facies assemblages is a retrogressive stage. Sm-Nd model ages
have been used to date the age of formation of the protoliths of the
migmatites. A late Archaean age, 2.7 Ga has been obtained for migmatites
from Lang^y and the existence of Archaean gneisses on AustvSg^y has been
demonstrated for the first time. The Moskenestfy migmatites are distinctly
younger. The Rb-Sr isotopic systems of the late Archaean rocks at Festvag,
Austvag^y and Sandset, Lang^y have been disturbed subsequent to the formation
of the gneiss protoliths. The present scattered distribution of data-
points for these gneisses can be explained as due to Rb-depletion during
granulite- facies metamorphism at 1.8 Ga.
The migmatitic gneisses are overlain by a series of supracrustal
gneisses. The bulk of these gneisses are volcanogenic in origin, ranging
from basaltic trachyandesites to rhyolites. Marble layers are present in
this gneiss sequence. The present study has revealed the first reported
marble layer in western Lofoten. The mineral assemblages in this zoned
marble layer have been used to obtain a minimum temperature for the
granulite-facies metamorphism at 01kona, Moskeaes^y of 750 C. Other
work (Krogh, 1975) has shown that the temperature reached 950 C in the
structurally deeper parts of the province, so a relatively small temperature
increase could have produced granitic melts. The migmatitic and supracrustal
gneisses of Moskenes^y and Flakstadtfy are more basic than those at higher
structural levels which could be the result of extraction of relatively
acid melts.
The mineral assemblages and the reactions in the 01kona Marble show
decreased in the layer with time. The presence of marbles and
banded ironstones in the thick volcanogenic pile suggests that the supra
crustal gneisses were deposited in a back-arc basin in which the rate of
basin subsidence was similar to the rate of infill from the andesitic lavas,
limestones and haematitic cherts formed when the marginal sea was at the
appropriate depths. The formation of the protoliths of the rhyolitic
gneisses at Vik, western Vestvagtfy has been dated at 2.1 Ga using Sm-Nd
model ages. A supracrustal gneiss sample from Flakstad^y yields a Sm-Nd
T age of 2.3 Ga suggesting that a component of its Nd may have been derived
from the migmatitic gneisses. The Rb-Sr isotopic systems of the Vik gneisses
have been disturbed. The scattered distribution of data-points can by
explained, in a similar way to that for the late Archaean migmatites, by
disturbance during the retrograde metamorphism at 1.1 Ga.
Both the migmatitic and supracrustal gneisses are intruded by basic
rocks, either small gabbroic bodies or larger intrusions containing anorthosites
The basic intrusion on Flakstad^y has well-developed cumulate textures and
magmatic sedimentation features. Rb-Sr and Sm-Nd isochron ages for the
Flakstad^y Basic Complex are both within error of 1.8 Ga. The precision
of the Rb-Sr age is very poor because of the very low Rb concentrations
in the basic rocks. Evolution diagrams for the Sm-Nd system show
that the basic rocks could not have been derived entirely from the
depleted (or undepleted) mantle at 1.8 Ga. Estimates of the proportion
of late Archaean migmatite - derived material in the mantle-derived magmas
to produce the observed isotopic characteristics in the basic rocks have
been made using typical values for Nd concentrations and isotopic comp
ositions of the components involved. Similar calculations are made using
Pb isotopes. An origin from the mantle at 1.8 Ga can be ruled out for
any finite present day y value in the basic rocks, since the samples with
the least radiogenic Pb plot very close to the 1.8 Ga first-stage isochron
^ 207T3, /204T., 206_ ,204,,. ,. _ . n XTJ , ,
on the Pb/ Pb v. Pb/ Pb diagram. For typical Nd and Pb concen
trations in an ancient lower crustal rock the crustal contribution by mass
may be as much as 37%. If contamination took place via anatectic melts,
the necessary crustal contribution by mass would be reduced. The high
crustal contribution to the Napp Gabbro implied by Pb isotopes is inter
preted as due to marginal assimilation of the adjacent South-West Lofoten
Mangerite, because of the clear relationship between the marginal basic
samples and a mangerite component in chemical variation diagrams for the
Flakstad^y Basic Complex.
The first report of eclogitic rocks from the Lofoten-Vesteralen province
is made in this study. Eclogite mineral assemblages were developed in
shear zones in the gabbroic rocks. Grain size reduction, surface area
increase and an increase in pathways causes reactions to eclogite para-
geneses to take place in parts of the rock that have undergone shear
deformation while the undeformed parts remain in granulite facies. The
eclogitic rocks have subsequently been retrogressed to amphibolite facies.
Glaucophane is present in some samples. A qualitative P,T path has been
constructed, which involves essentially isobaric cooling after crystallization
at P < 8kb and a temperature of about 1150 C.
The exposed basic intrusions are subordinate in extent compared to
the mangeritic intrusions emplaced into the migmatitic and supracrustal
gneisses at 1.8 Ga. The mangeritic rocks are coarsely crystalline rocks
which are only rarely foliated. Some of the mangerites, especially the
Raftsund Mangerite, show magmatic sedimentation features and well-developed
cumulate textures. The Raftsund Mangerite is concentrically zoned on the
basis of mafic phase assemblages. The Hopen Mangerite/Charnockite has an
apatite-rich layered gabbroic margin adjacent to the basement gneisses.
The mangerites are clearly intrusive into the basic intrusions on Vestvag^y,
but the relationship on Flakstad^y and Lang^y is uncertain, but mangeritic
or charnockitic veins in the basic intrusions on these islands suggest that
the mangerites are the later intrusions. The mangerites, especially Sund,
Hopen, Engenykken and parts of Raftsund, contain abundant xenoliths, some
of which resemble parts of the migmatitic gneisses. The xenoliths are
typically well-rounded, zoned and may have reaction rims; all are more
mafic than the host mangerite, suggesting that they may be restites after
removal of leucocratic melts.
The mangerites show a wide range in chemical compositions (e.g. SiO,., =
51-77%); the widest range is for the South-West Lofoten Mangerite, the
smallest is for the Raftsund Mangerite. The mangerites, Hopen and
Raftsund in particular, are extremely Fe-enriched. They have high K90
concentrations and most are shoshonitic in composition. The chemistry
of the mangerites, by analogy with Phanerozoic andesites, suggests that
they were formed in the orogenic zone of a continental region. The
evolution of the mangerite magmas has been modelled using variation diagrams
to compare their compositions to those of possible end-member components
contributing to the magmas, and to the composition of possible early
fractionating phases. These diagrams are consistent with the formation
of the mangerites from anatexis of average 2.7 Ga migmatitk and 2.1 Ga
supracrustal gneisses, and also of more acid gneisses, and from a mantle-
derived basic magma. The variation diagrams are consistent with the magmas
formed from these components subsequently evolving by fractional crystallization
of pyroxenes, olivine, apatite, ilmenite and magnetite. These minerals are
concentrated in the marginal gabbro to the Hopen intrusion.
Rb-Sr whole-rock isochron ages of 1.73 to 1.81 Ga have been obtained
for the Sund-01kona, Hopen, Austvag^y Grey and Raftsund Mangerites. An
older imprecise age was obtained for the South-West Lofoten Mangerite
87 86
due to the wide scatter of data-points. Initial Sr/ Sr ratios range
from 0.7032 to 0.7048 which implies that the mangerites were derived from
either upper mantle or granulite-f acies depleted lower crust.
The narrow range in Sm/Nd ratios for the mangeritic intrusions
effectively rules out dating by Sm-Nd whole-rock isochron methods. Sm-Nd
T_1TTTT> and T_^_ model ages have been calculated and are shown on evolution
LriUK L)M
diagrams, e , values at 1.8 Ga preclude an origin for the mangerites by
crystallization from a magma derived solely from the mantle at 1.8 Ga.
The model age, T , of a mangerite sample can be used to estimate the
proportions of different components present in that sample.
In tuh e mP,b // T>Pub v. mPb,/ /mP,b pli ot<- s ff or t^he mangerites, a number
of whole-rock and alkali feldspar separates plot to the left of the 1 . 8 Ga
first-stage isochron. A single-stage evolution for these rocks until
1.8 Ga (i.e. in the mantle) is therefore totally impossible. These
unradiogenic samples have been used to constrain y, for the source of an
ancient crustal component to between 7.85 and 8.1. The Pb isotope comp
ositions have been explained using a model in which most of the Pb in
the mangerites has evolved in three stages: in the mantle until 2.7 Ga,
in the migmatitic gneisses from 2.7 Ga until 1.8 Ga, and in the mangerites
subsequent to 1.8 Ga. The second component in the model is Pb derived
o r\ 7 o c\/
from the mantle at 1.8 Ga. The linear array of data in the Pb/ Pb
v. Pb/ Pb diagram is a consequence of the different y values for the
two components in the interval 2.7 - 1.8 Ga being reflected in the magmas
resulting from mixing different proportions of those components. "Plumbo-
tectonics" confirms that the mangerites were derived from upper mantle and
lower crustal sources. Apparent y. values for individual samples can be
used to estimate the total contribution of Pb from lower crustal sources
to those samples in the same way as T can be used to estimate the total
contribution of Nd from the lower crust. Both have also been used to
estimate total mass contributions from the lower crust. Mixing calculations
for Nd suggest that the maximum contribution by mass from the 2.7 Ga crust
to the mangerites is 60-80%. Fractional crystallization of mantle-derived
magmas would progressively raise Nd concentrations in the residual magmas.
Hence if mixing of crust- and mantle-derived magmas took place after such
fractionation, a much smaller contribution by mass is required from the
mantle-derived component. Pb too would be concentrated in residual magmas
after fractional crystallization. If it were concentrated to as much as
20ppm prior to mixing, all except three samples would require a contribution
by mass from the 2.7 Ga lower crust of over 50%.
Correlations of isotopic parameters derived from Rb-Sr (apparent
87 8fi
initial Sr/ Sr) , Sm-Nd (T ) , and U-Pb (apparent y.) systems to express
the relative proportions of crust and mantle contributions to the mangerites
show that the crust-mantle source model for the mangerite is fully self-
consistent. The wide variations in isotope ratios in three systems and
in chemistry, for the South-West Lofoten Mangerite, are all correlated.
This strongly suggests that the scatter of data about the Rb-Sr isochron
87
for this intrusion is due to variations in initial Sr/ Sr ratios rather
than to metamorphic disturbances. The samples with the largest mantle
contribution for the South-West Lofoten Mangerite are from the side of
the islands where the Moho is shallowest.
The mangerites crystallized at about 1000 C and llkb. Retrograde
reactions have subsequently taken place.
Anatectic veins and dykes of mangeritic and charnockitic composition
are present in the structurally deepest parts of the Lofoten-Vesteralen
province. In-situ anatexis has only been seen in one locality (Festvag,
Austvig^y) in the late Archaean migmatites, but is common in the early
Proterozoic supracrustal gneisses where it is commonly associated with the
emplacement of basic veins and dykes. The most convincingly in-situ
anatectic melt, from 01kona, Moskenes^y, is of intermediate composition.
Its field relations, petrography and T value suggest that this melt is
derived solely from supracrustal gneisses. These gneisses are isotopically
similar to mantle-derived magmas at 1.8 Ga, hence part of the "mantle-
derived" component calculated for the mangerites is due to assimilation
of juvenile continental crust. The estimates for total crustal contribution
by mass to the mangerite intrusions are minimum values.
The anatectic melts are chemically similar to the mangeritic intrusions.
High K?0 concentrations suggest derivation of the melts by reworking of
continental crust.
Nd evolution diagrams are consistent with formation of the Sund-01kona
and South-West Lofoten Mangerites by coalescing melts similar to those of
mangeritic composition on Moskenes^y and Flakstad^y. The charnockite
veins on Vestvag^y have chemical and isotopic similarities to the Flakstad^y
Charnockite.