by Brian Robins1 and J. Richard Wilson2
2Department of Earth Sciences, University
of Aarhus, 8000 Aarhus C, Denmark
Figure 1.
Localities in the northern part of the Bjerkreim-Sokndal Intrusion
described in the guide
DESCRIPTION OF LOCALITIES
By
Brian Robins.
Description: The traverse commences in plagioclase cumulates (pC) (mainly leuconorite) southeast of the farm at Holmen, near the base of MCU IB. They contain plagioclase with a bimodal size distribution and intercumulus orthopyroxene and ilmenite. Both the megacrystic and smaller plagioclase crystals are laminated. Xenoliths of anorthosite similar to that found in the nearby Egersund - Ogna Massif are common. Note the substantial thickness of pC. To the southwest this zone gradually thins and eventually disappears near the Berse lake. Further southwards along the traverse, pC is succeeded stratigraphically by plagioclase-ilmenite cumulates (piC) with faint modal layering and numerous rafts of mafic granulite and then strongly-layered plagioclase-orthopyroxene-ilmenite cumulates (phiC). The succession reflects the sequence of crystallisation (plagioclase ® ilmenite ® orthopyroxene) of the parental magma. In the upper part of the zone of piC there is a horizon that is particularly notable for its content of anorthosite blocks. In the phiC at the south end of the traverse there are abundant, thin but laterally-persistent modally-graded layers, in which plagioclase megacrysts are concentrated in the plagioclase-rich layer tops. The modally-graded layers are separated by thicker isomodal phiC layers, and the basal contacts of some modally-graded layers are disconformities. The more mafic cumulates which form a knoll on the opposite (west) side of the river are more-evolved plagioclase-orthopyroxene-Ca-rich pyroxene-ilmenite cumulates (phciC) that occur higher up in MCU IB. They are preserved in a downthrown fault block.
Mineral compositions. Plagioclase in pC towards the base of MCU IB along this traverse is around An45, significantly more calcic than at the top of the underlying phiC (An41). Variation within the pC zone is slight, but in the phiC and overlying phciC the An content decreases from 45 to 42. No clear systematic variation in the Mg# of Ca-poor pyroxenes has been detected in this traverse; it varies in the range of 68.5-67. The lowest of these values are, however, found in the phciC towards the top of the unit.
Figure 2. Veiw of the northernmost
part of the Bjerkreim-Sokndal Intrusion. The left middle ground and the dark ridge in the background are
country-rock gneisses. The foreground and the central middle ground
(illuminated by the sun) are plagioclase-rich cumulates belonging to MCU
1A. Note the low escarpments and
dip slopes marking the orientation of
the plagioclaes lamination. Stratigraphic up is to the right
(south).
Locality 2: Xenolithic pC (MCU
IB).
South side of Odlandsholen (1212
II, LK323984).
By
Brian Robins.
Description:
A major NE-SW fault (with a downthrow to the north) repeats the succession seen
in the previous traverse on this southern side of the Ørsdalen valley. In this
recently excavated surface numerous large blocks and slabs of anorthosite and
leuconorite are clearly visible enclosed in leuconorite (pC) belonging to the
lowermost unit of MCU IB. These xenoliths appear to have been derived from the
adjacent Egersund-Ogna Anorthosite Massif, and are particularly abundant in the
lowermost parts of MCUs IA & IB.
Locality 3: Boundary between MCU
IB and II.
Roadside exposures between Netland
and Hytland (1212 II, LK333982).
By
Brian Robins.
Description: Proceeding
down the hill, road cuts expose the transition from phiC, forming the central
zone of MCU IB, to plagioclase-orthopyroxene-Ca-rich pyroxene-ilmenite
cumulates (phciC) (hand lenses are essential!), the most evolved cumulates in
MCU IB in this area. Further to the west, the phciC is overlain by a thin layer
of gabbronorite in which magnetite and apatite are cumulus minerals. In the
lowest exposures, a 2m thick ilmenite-rich transition zone between the top of
MCU IB and pC forming the lower part of the thick MCU II can be examined. This
regressive zone of phiC reflects the entry of more-primitive magma into the chamber
in which the cumulates were crystallising. The overlying pC characteristically
contains magnetite, as do the troctolites that occur near the bases of MCUs III
and IV, but evidence of cumulus olivine has only been discovered at a single
locality near the base of MCU II.
Mineral
compositions. The upper part of MCU IB at this
locality contains An43 and pyroxenes
with Mg#s of 64-68 (Ca-poor) and 72-78 (Ca-rich) . The pC at the base of MCU II
contains An49-51.
Locality 4. Cm-scale layering and
cross bedding.
Roadside exposure above a small
stream, W. of Netlandsvatnet (1212 II, LK320968).
By
Brian Robins.
Description: At this
locality phiC in the central part of MCU II exhibits several sets of
cross-lamination. Such structures are fairly rare in the intrusion, but are
important in indicating the ability of currents moving along the floor of the
magma chamber to erode unconsolidated cumulates and to transport and redeposit
cumulus crystals elsewhere.
Locality 5. Boundary between MCUs
II and III.
Eptelandsvatnet - Short traverse
from north to south along the bluffs above the NE-end of Eptelandsvatnet (1212
II, LK325957-LK327954).
By
Brian Robins & Kristine Krogh Jensen.
Description:
The traverse starts in rather massive phiC at the top of MCU II. This is
followed stratigraphically by a thin sulphide-bearing subunit that occurs at
the base of MCU III. This subunit consists of rusty, massive ilmenite norite,
strongly- to intensely-layered mafic ilmenite norite or massive
orthopyroxenite. This subunit can be traced discontinuously around the whole of
the Bjerkreim lobe of the BKSK, a distance of more than 30 km. The main
sulphides are pyrrhotite, chalcopyrite and pentlandite (Cu/Cu+Ni »
0.5). Sparse secondary pyrite occurs locally. Assays have shown that the
sulphides contain insignificant concentrations (< 40ppb) of PGE + Au. The
sulphide-bearing subunit defines the base of zone IIIa. It is overlain by
leuconorites that in places are characterised by pronounced modal layering in
which layers rich in cumulus plagioclase alternate with ilmenite and
orthopyroxene-rich layers (intruded in this traverse by a basalt sheet probably
belonging to the Egersund dyke swarm). These are followed by rather massive pC
or phiC which grades(?) into coarse-grained, plagioclase-olivine, zone IIIb
cumulates. Cumulus olivine is not always visible in the poC; it is commonly
replaced by orthopyroxene (due to reaction with intercumulus melt) or
fine-grained, symplectitic intergrowths of orthopyroxene and magnetite (due to
subsolidus oxidation). Olivine can, however, be identified with a hand lens on
glaciated pavements along the sides of the stream at the southern end of the
traverse. Further to the south, the troctolites are succeeded by
modally-layered phiC with rare slump structures. The traverse demonstrates a
stratigraphic sequence different from that of MCU IB & II suggesting that
the magmas which periodically flowed into the chamber varied in composition.
We
can return by walking to the northeast and then following the sulphide-rich
subunit southwestwards back to Eptelandsvatnet. This subunit is generally
underlain by up to 10 meters of rusty leuconorite with disseminated sulphides,
and overlain by a thinner zone of sulphide-bearing norite. The subunit itself
is up to 1m thick and consists of either pyrrhotite pyroxenite or
modally-graded layers of pyrrhotite norite, in places containing norite
xenoliths. It varies considerably in thickness and in places is totally absent,
although the norite with disseminated sulphides persists. The subunit appears to be the result of the
collection of droplets of an immiscible sulphide melt in depressions on the
floor of the magma chamber. The subunit probably reflects the initial response
to the inflow of new magma into the chamber, mixing of new and more-evolved,
resident magma and the formation of a hybrid magma that was saturated only in
othopyroxene and sulphides. Continued magma inflow (and a reduction in the
efficiency of mixing) eventually led to the crystallisation of poC, that are
among the highest-temperature cumulates
present in the intrusion. The thickness of the regressive zone a cumulates here
is ~120m, showing that magma replenishment persisted for a considerable period
of time.
Mineral
compositions. The An-content of plagioclase in
a 30m thick interval beneath the sulphide-rich subunit increases from An47 to
An49. Immediately above the subunit it varies from An50-49 and in poC of zone
IIIb from An52-48. The Mg# of Ca-poor pyroxene decreases rapidly from 76 to ~71
through 20m of phiC forming the upper part of MCU II and increases to 75 just
above the sulphide-rich subunit. The unusually Fe-rich pyroxenes at the top of
MCU II may be a result of subsolidus re-equilibration with the sulphides
present, particularly in connection with the oxidation of pyrrhotite to pyrite.
Above this subunit the Mg# of Ca-poor pyroxene remains fairly constant but
increases slightly (to 77-76) in the poC. Olivine varies between Fo77-74 in
poC. Although the minerals show irregular and uncorrelated changes in
composition (possibly due to magma mixing and disequilibrium), there appears to
be a slight reverse cryptic variation in An% through the zone a cumulates.
Figure 3. Cryptic variation in
plagioclase and orthopyroxene across the MCU II/III contact at
Eptelandsvatnet (loc. 5). Based on work by Kristine Krogh Jensen.
Locality 6: Contact of MCUs II and
III.
Hågåsen (1212 II, LK355886).
By
Brian Robins & Kristine Krogh Jensen.
Description: Hågåsen is a hill located
on the southern flank of the Bjerkreim
lobe of the Bjerkreim Sokndal intrusion, on which the layered sequence across
the boundary between MCUs II and III is well exposed. The sequence of cumulates
is similar to that at Eptelandsvatnet (see loc. 5) but condensed in thickness.
We shall walk from the road northeastwards up the layered sequence to the top
of Hågåsen.
The
stratigraphically-lowest cumulates exposed along the traverse are massive to
weakly modally-layered ilmenite leuconorites (phiC) forming the uppermost part
of MCU II. These are succeeded by 20cm of intensely modally-layered melanorite
and a massive layer of sulphide-bearing pyroxenite up to 2m thick that mark the
base of MCU III. The orthopyroxenite is exceptionally thick here and as it dies
out in both directions along strike it appears to occupy a shallow and >200m
broad trough. The orthopyroxenite is overlain by phiC with laterally-persistent
and remarkably rythmic modal layering that gradually dies away upwards into
massive phiC about 15-20m above the orthopyroxenite. The base of the succeeding
poC is encountered a few meters further up the sequence (25-30m above the
orthopyroxenite).
Figure 4. Comparison of the
stratigraphic sequences across the MCU II/III contact at Eptelandsvatnet (loc.
5), Hågåsen (loc. 6) and Teksetjørni (loc. 7). Note the thicker sequence of
zone a cumulates in the axial region of the intrusion at Eptelandsvatnet
compared the equivalent in the southern flank (at Hågåsen), and the lack of
zone b (poC) at Teksetjørni in the eastern flank of the intrusion.
Mineral
compositions. Mg#s of orthopyroxenes in the upper part of MCU II
are ~74, decreasing rather abruptly to 70 immediately beneath the
orthopyroxenite, as at Eptelandsvatnet.
The orthopyroxenite itself contains
pyroxene with a Mg# of 72.5 and in
the overlying phiC the Mg# shows a slight regressive trend from 72 to 74. The
poC contains olivine of Fo74-76. Plagioclase exhibits no systematic variations
in composition. PhiC in the upper part of MCU II contains plagioclase of fairly
constant composition (An47) while in the sequence above the orthopyroxenite
plagioclase compositions vary irregularly between An46 and 49.
Locality 7: Contact of MCUs II and
III.
Between Teksevatnet and
Teksetjørni (1312 III, LK433935).
By
Brian Robins
Description:
This
locality lies on the eastern flank of the Bjerkreim lobe of the
Bjerkreim Sokndal intrusion, to the east of the
Teksevatnet re-entrant. Cumulates in this part of the intrusion are believed to
have crystallised on an elevated portion of the floor of the magma chamber and
as a result the Layered Series is condensed relative to the axial regions of
the intrusion and certain stratigraphic zones are absent. The cumulates along a
short N-S traverse at this locality can be compared with those present at
Eptelandsvatnet and Hågåsen, illustrating some of the lateral stratigraphic
variations that occur in the Layered Series. The locality also demonstrates the
relationship that exists between ilmenite-rich cumulates and replenishment of
the magma chamber.
Figure 5. Minor unconformity in modally-layered ilmenite norite in the
uppermost part of MCU II near Teksetjørni.
Stratrigraphic up is to the left (south).
The
traverse starts from the track leading to Teksetjørni in ilmenite norites
(phiC) belonging to the upper part of MCU II. These cumulates exhibit
steeply-dipping to overturned small-scale modal layering, minor unconformities
and abundant, generally tabular xenoliths. They are succeeded by ~5m of
ilmenite-rich melanorite with sulphides and sparse olivine at the base of MCU
III, then rather massive leuconorite containing discontinuous ilmenite-rich
layers and a thin sequence of modally-layered ilmenite norite. The latter are
overlain by magnetite-bearing norite enclosing numerous blocks and slabs up to
100m across of massive norite or leuconorite. Note that the prominent poC seen
at Eptelandsvatnet and Hågåsen is absent here. The unit of olivine-bearing
melanorite is the lateral equivalent of the layer of sulphide-bearing
orthopyroxenite developed locally elsewhere.
Figure 6. Modally-layered magnetite-ilmenite norite in MCU III near
Teksetjørni. Stratrigraphic up is
to the left (south).
Mineral
compositions. The compositional variations in
plagioclase, orthopyroxene and olivine in the sequence developed at this
locality are illustrated in Fig. 7. Note the cryptic regression through the
uppermost 5m of MCU II as well as the presence of more calcic plagioclase, more
magnesian orthopyroxene and magnesian olivine in the melanorite, all suggesting
that the formation of the melanorite was a response to magma-chamber
replenishment. Orthopyroxene (and to a lesser degree plagioclase) near the base
of the overlying leuconorite returns to compositions similar to those in the upper
part of MCU II, and it exhibits a slight but consistent cryptic regression
through the leuconorite into the phiC. This trend does not persist further upwards in the sequence, the
majority of MCU III in this region being characterised by a normal cryptic
variation (Fig. 8)
Figure 7. Cryptic variation in
plagioclase, orthopyroxene (green) and olivine (violet) across the MCU
II/III boundary at Teksetjørni.
Interpretation.
Recently we have studied the lithostratigraphic
relationships and cryptic layering in a series of sections across the boundary
between MCUs II and III spaced over a distance of 25km along strike. This
boundary is particularly instructive with respect to processes during
replenishment since lateral variations in the thicknesses of MCU II and III are
pronounced and reflect the topography that existed on the chamber floor at the
time of magma replenishment. In the central region of the BKSK, around the
hinge of the deep syncline defined by the layering, MCU III has a thickness
of 900-1050m while in the SW limb of
the syncline its thickness is reduced but fairly constant at ~800m. To the east
of the axial region MCU III decreases to <350m in thickness over a step of
gneiss in the substrate, then increases slightly to ~450m before thinning and
wedging out further to the SE against the base of the intrusion. The thickness
of MCU II varies in a similar but even more dramatic way. These variations are
considered to be due to crystallisation of the megacyclic units in a central
trough on the magma-chamber floor and on an elevated «shelf» or shallow trough
to the east. In addition, differentiation of the resident magma was arrested at
a relatively early stage by the influx of magma marked by the MCU II/III
transition. MCU II consists exclusively of a thin basal sequence of plagioclase
cumulates and a thick series of phiC. Cumulus magnetite does not make an
appearance in MCU II, and it is likely that the resident jotunitic magma was
differentiating with increasing density during its crystallisation.
Figure 8. Cryptic variation in
plagioclase through MCU II, III and the lower part of IV in the eastern
flank of the Bjerkreim lobe as displayed in a series of samples collected
along a traverse from the margin of the intrusion, through the Teksetjørni
locality (loc. 7) to Lauvneset (loc. 8).
The stratigraphically lowest cumulates in MCU III are a thin sequence of strongly-layered melanocratic, orthopyroxene- and ilmenite-rich norite (phiC), or a discontinuous layer of orthopyroxenite up to 3m thick, all characterised by elevated amounts of disseminated sulphides (pyrrhotite, pentlandite, chalcopyrite and pyrite). The layer of orthopyroxenite is unique in the Bjerkreim Layered Series. Although it shows considerable lateral variations both in thickness and modal composition, this sequence can be recognised everywhere at the base of MCU III. A distinctive feature of the sequence as developed on the «shelf» is the local occurrence of sparse cumulus olivine in melanocratic ilmenite norite and the correlative layer of orthopyroxenite.
The
basal sulphide-bearing cumulates are succeeded in the majority of the Bjerkreim
lobe by 25-130m of massive to strongly-layered phiC (zone a cumulates), then a
massive unit of troctolite (pomC, zone b) up to 100m thick, that constitute the
highest-temperature cumulates in MCU III.
The troctolites reside beneath lower-temperature phiC, phimC and
eventually phcmiaC that form the remainder of MCU III. In the eastern part of
the lobe the sequence immediately above the basal cumulates of MCU III is much
thinner (25-45m) and consists of either rather massive leuconorite with
discontinuous thin layers of ihC (resembling zone a cumulates elsewhere in the
intrusion), or modally-layered, melanocratic ilmenite norite. With the
exception of a locally-developed 1m-thick layer, troctolite (zone b) is
conspicuously absent in this shelf region and the leuconorite or melanocratic
norite is succeeded by «normal» phiC.
The
cryptic variation across the MCU II/III boundary is characterised by a
regression in mineral compositions (and 87Sr/86Sr) from the
phiC forming the upper part of MCU II
to the most primitive compositions that are found either in the troctolites
forming the zone b cumulates of MCU III in the central and western part of the
lobe or in the cumulates above the sequence of leuconorite/melanocratic norite
in the eastern, «shelf» area. This is consistent with prolonged magma-chamber
replenishment associated with progressive mixing of the inflowing and resident
jotunitic magmas. The sulphide-enriched orthopyroxenite and related melanocratic
ilmenite norite are explained by crystallisation of hybrid magmas residing in
the pyroxene phase volume during the initial stages of replenishment. Their
«global» distribution is inferred to result from mixing taking place some
distance above the chamber floor at a level where the plume formed by the
inflowing magma reached a level of neutral buoyancy in the
compositionally-stratified magma column and spread laterally throughout the
chamber. As the influx proceeded the resident magma was stripped from the base
of the chamber and mixed into the ascending plume as the hybrid layer increased
in thickness and became compositionally stratified. Eventually the lower
boundary of the hybrid layer reached the floor of the magma chamber. The
highest-temperature cumulates (poC, zone b) crystallised from the lowest part
of this hybrid layer and were restricted to the central trough on the chamber
floor, while lower-temperature cumulates crystallised simultaneously on the
eastern «shelf» from magma higher up in the hybrid layer.
Locality 8: Magnetite, ilmenite
and apatite-bearing cumulates (MCU IV) .
Lauvneset, nr. Tekse (1312
III, LK428931).
By
Brian Robins
Description:
Cumulates
containing apatite together with magnetite, ilmenite and Ca-rich pyroxene occur
in the upper parts of MCUs IB, III and IV. Apatite in the Bjerkreim-Sokndal
Intrusion generally makes its entry as a cumulus mineral at about the same
stratigraphic level as Ca-rich pyroxene but may preceed or postdate it
stratigraphically by some tens of meters. The late appearance of Ca-rich
pyroxene is a reflection of the unusually Ca-poor composition of the parental
jotunite magmas. Apatite is most abundant immediately after its appearance as a
cumulus mineral when it may constitute as much as 10% of the rocks. This
locality on the shore of Teksevatn exhibits apatite- and oxide-rich
gabbronoritic cumulates with pronounced modal layering immediately above the
apatite-in phase contact within MCU IV. The sequence of MCU IV cumulates in
this area is different from the axial region of the intrusion: The thickness of
zone c (phiC) is reduced and zone d (phimC) seems to be absent (Fig. 4).
Mineral
compositions. In keeping with the evolved cumulus assemblage, the
minerals in this part of the Layered Series have relatively low-temperature
compositions: Plagioclase is ~An40; Ca-poor and Ca-rich pyroxene have Mg#s of
~63 and ~70 respectively.
Locality 9: Basal contact of MCU
IV.
Storeknuten (1212 II,
LK342905).
By
J. Richard Wilson.
Figure 9. Storeknuten seen from the
south. The lower part of the hill is composed of plagioclase‑rich
zone a cumulates with dark orthopyroxene- and
ilmenite-rich layers. These are overlain by brownish-weathering zone b
troctolites with thick diffuse layers.
Description:
Storeknuten is a rounded hill, located on the southern flank of the Bjerkreim lobe
of the Bjerkreim Sokndal intrusion (Fig. 1), where the boundary between
Megacyclic Units III and IV has been studied in detail (Jensen et al.
1993, Nielsen et al. 1996, Barling et
al. 2000). There is a good view of the Bjerkreim lobe from the summit of
Storeknuten. The strike of the layering here is NW‑SE and the dip is
about 50‑60° to the NE into the core of the Bjerkreim synform. Olivine‑bearing
rocks in the Layered Series are restricted to two zones just above the bases of
MCUs III and IV and form pronounced topographic ridges. Storeknuten belongs to
the uppermost olivine‑bearing zone which Paul Michot called the Svalestad
unit (referred to as zone IVb here).
We
shall approach Storeknuten from the southeast, along a farm track. There are
sporadic exposures of gabbronorite (phcimaC) belonging to the uppermost part of
MCU III (zone e) in the fields. On the left of the track (at LK344902) is a
particularly instructive exposure of modally-layered gabbronorite in which
several gneissic xenoliths are embedded. Structures due to blocks of gneiss
impacting and slicing into partly-crystallised cumulates are preserved and
allow discussion of layering‑forming processes.
Figure 10. Gabbronorite (phcimaC)
within the uppermost part of MCU
III close to the path to Storeknuten. The cumulates enclose a large slab
of granulite-facies gneiss. Stratigraphic up is to the left.
Figure
11. Geological map of the Storeknuten area showing the distribution
of major outcrops, lithologies, strike and dip of layering, and sample
locations. The legend explains the abbreviations used for the cumulate
nomenclature. From Jensen et
al. (1993)
At
the base of Storeknuten, the MCU IIIe cumulates are overlain by a zone (about
30m thick) of plagioclase‑rich cumulates belonging to MCU IVa (Fig. 11).
MCU IVa consists dominantly of pC and piC, with magnetite and Ca‑poor
pyroxene occurring as sporadic early phases. A characteristic feature is the
presence of thin ilmenite‑rich layers, many of which are discontinuous
and deformed by slump folds. These ilmenite-rich layers, particularly abundant
in the axial region of the intrusion at this stratigraphic level, suggest that
magma-chamber replenishment and magma mixing were a prerequisite for ilmenite
concentration.
Figure 12. Cryptic variation through
the upper part of MCU III and the lower part of MCU IV at Storeknuten. The
sample numbers (J1-15) correspond to those marked in Fig. 6. Based on data from Jensen et
al (1993) and Nielsen et al. (1966).
The
entry of olivine as a cumulus phase defines the base of MCU IVb. This boundary
is exposed on the southern slopes of Storeknuten. MCU IVb consists dominantly
of massive leucotroctolite containing oikocrysts of Ca‑poor pyroxene. MCU
IVb is about 100m thick and contains sporadic cumulus magnetite (and possibly
ilmenite and Ca‑poor pyroxene), as well as small quantities of biotite
and brown hornblende. Most olivines are partly or completely replaced by
orthopyroxene-oxide symplectites. The summit of Storeknuten is composed entirely
of rocks belonging to MCU IVb.
The disappearance of olivine and magnetite, and the entrance of cumulus Ca‑poor pyroxene in the slopes to the north define the base of MCU IVc (phiC). This phase contact is accompanied by the development of an igneous lamination and modal layering. Modal layering becomes increasingly well developed up through MCU IVc.
Mineral
compositions and Sr-isotope ratios. There is a cryptic regression in
mineral compositions through the uppermost part of zone IIIe that continues through
zone IVa (Fig. 12). The upper part of MCU IIIe has Ca‑poor pyroxene with
En68‑70, plagioclase with An44-46 and an initial Sr‑isotope ratio
of 0.7061; the base of MCU IVb has En76, An53 and Sro
= 0.7049, together with olivine Fo74. There is an extremely
systematic upward decrease in Sr‑isotope ratios through the upper part of
MCU IVa (Fig. 13). The variation in initial Sr isotope ratios appears to be
delayed relative to the regression in mineral compositions.
Plagioclases show a rather erratic trend to more evolved compositions upwards through MCU IVb, while olivines first become more Fe‑rich and then more Mg‑rich (Fig. 12). The olivine trends may have been influenced by trapped-liquid shift. Sr-isotope ratios increase systematically from 0.7049 at the base of zone IVb to 0.7053 at its top, reach 0.7058 in zone IVd and returns to 0.7061 in zone Ive (Figs. 12 & 13).
Figure 13. Cryptic variation through the basal, regressive zone of MCU IV at
Storeknuten. From Nielsen et al. (1966).
The
significance of the MCU III/IV boundary for magma mixing.
The cryptic variations across the boundary between MCUs III and IV clearly
indicate the operation and importance of magma mixing during magma-chamber
replenishment. The magma residing in the chamber when the influx marked by the
base of MCU IV took place was compositionally zoned (Fig. 14), and assimilation
of gneissic country rock at the roof had resulted in an elevated Sr‑isotope
ratio that may have increased upwards through the magma column. The inflowing
magma had an Sr‑isotope ratio of about 0.7049 while the resident magma
had a ratio of 0.7061 at the floor in the Storeknuten area. The inflowing magma
mixed with the basal layer(s) of the resident magma as a result of the new
magma fountaining into the chamber. A decreasing degree of mixing between the
inflowing and resident magma with time led to hybrid magmas with decreasing Sr‑isotope
ratios. Crystallisation of these hybrid magmas during replenishment produced
the isotopic regression in the upper part of MCU IVa. Initially, influx led to
elevation of the zoned magma column, exposing the base of the chamber in the
Storeknuten area, located some distance up the inwardly-sloping floor, to
progressively more primitive magma. It took some time before the hybrid magma
flooded this point on the floor, causing the delay in the regression in isotope ratios relative to that defined
by the mineral compositions. When the magma inflow ceased, olivine‑bearing
rocks of MCU IVb began to crystallise at the base of the chamber. The
leucotroctolites at the base of MCU IVb are amongst the most primitive rocks in
the entire intrusion.
Calculations
based on geochemical modelling, the thickness of cumulate stratigraphy repeated
(from the top of zone IIIe to the appropriate part of zone IVe) and Sr‑isotope
ratios indicate that the layer of hybrid magma generated during replenishment
had a thickness of 350‑500m in the Storeknuten area and that the
leucotroctolites of MCU IVb represent about 20‑30% crystallisation of
this layer.
Figure 14. Sketches of the Bjerkreim‑Sokndal magma chamber during
formation of the MCU III/IV boundary. From Jensen et al. (1993). A. Crystallisation of the
upper part of MCU III. The magma layer parental to zone IIId is only
present in the central, lowest part of the saucer‑shaped chamber.
Zone IIIe is crystallising from the overlying magma layer(s) towards the
margins. The Storeknuten profile is located near the margin, where zone
IIIe cumulates are present. B. Magma replenishment
elevated the residual magma and produced a hybrid magma layer at the floor.
Zone IIIa crystallised during influx to produce a modal and cryptic
regression. Note that the vertical scale has been greatly exaggerated in these
sketches.