The Oslo Rift: P-T relations and lithospheric structure

TECTONOPHYSICS ELSEVIER

Tectonophysics 240 (1994) 159-172

The Oslo Rift: P-T relations and lithospheric structure Else-Ragnhild Mirterulogisk-Geologisk

Museum,

Neumann

Uttic~ersity of Oslo, Sursgt. I, 0562 Oslo. Norwq

Received 15 December 1992: revised version accepted 24 June 1993

Abstract Extrusion temperatures for basaltic lavas in the Permo-Carboniferous Oslo Rift, estimated from whole rock major clement compositions, are estimated to be 1270 to 1340°C. This means that magmatism during the Oslo rifting event was not associated with a large temperature anomaly in the underlying upper mantle. Partial melting is believed to be caused by a combination of crustal extension, a weak temperature anomaly in the underlying asthenosphere, and/or high fluid-contents in the mantle source region (“wet-spot”). Pctrological and gcochemical data imply that large masses of cumulate rocks were deposited in the deep crust during the Oslo rifting event. The densities and seismic velocities (V,) of these cumulate rocks are estimated to be 2.X-3.5 g/cm’ and 7.5-X.0 km/s. A rough estimate suggests that cumulus minerals alone account for a net transfer of at least 2 X 10” kg of magmatic material from the mantle into the deep crust. In addition comes material representing (a) cumulate mineral5 corresponding to eroded magmatic surface and subsurface rocks, (b) intercumulus material. and (c) magma5 crystallized to completion in the deep crust. Estimates based exclusively on geophysical data tend to underestimate the true transfer of mass into the lower crust as gabbroic cumulate rocks, and melts crystallizing to completion in the lower crust have densities and seismic velocities similar to those of lower crustal wallrocks.

1. Introduction Increased understanding of rifts and rifting processes requires careful integration of data from different fields of speciality for a large number of rifts and rift systems (e.g. geological, gravity, seismic, structural, petrological, and geochemical data). In order to facilitate such integration it is very important that petrological and geochemical data on each rift, and conclusions drawn from these, are translated into information that can be used directly in numerical models for extensional basin formation. The aim of this paper is to use petrological and geochemical data to extract additional information on the P-T relations in the lithosphere under the Oslo Rift in Permo-Carboniferous

time, and the mass transfer mantle-crust during the rifting event. This paper is concerned with information deduced from the magmatic rocks in the Oslo Graben (the northern part of the Oslo Rift).

2. Geological

setting and magmatic

processes

The main part of the Permo-Carboniferous Oslo Rift, southeastern Norway, extends northnortheast from the Sorgenfrei-Tornquist Zone to lake Mjtisa. A possible continuation north of lake Mjosa is suggested by tectonic activity which may be of Permian age, and by the 282 Ma old %rna complex in Sweden, 150 km north-northeast of

0040- I95I /94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0040- 195 I (94)00102-F

Gfanitfc and syenitic rocks Larvikite etc.

Lavaflows Cambro-Silurian sed rocks Precambrian gneisses

t Fig. 1. Simplified

geological

maps by Oftedahl

(l%O),

(1978)

map of the Oslo Rift,

Larsen (1975).

and K. Buer (pers.

commun.,

cates the position of the profile estimates

Ramberg

1989).

based on

and Larsen

Profile

IX

indi-

used as a basis for the mass

shown in Fig. 5.

lake Mj@sa (Sundvoll et al., 1990; Sundvoll and Larsen, 1994). The northern part of the Oslo Rift, including the Oslo Graben, is exposed on land, whereas its southern part, the Skagerrak Graben, is submerged. Erosion has removed the upper l-3 km of the crust in the Oslo Graben, exposing a wide variety of rift-related extrusive and intrusive rocks (Fig. 1). Together with Cambro-Silurian sedimentary rocks, these magmatic rocks make up a ca. 200 km long and 35-65 km wide zone (the Oslo Region) along the floor of

the Oslo Graben. However. faults which were active/ reactivated during the Oslo rifting event and Permo-Carboniferous dykes in the Prccambrian terrain on both sides of the Oslo Region (e.g. Larsen. lY75; Rambcrg and Larsen, 197X: Neumann et al., lY92: Sundvoll and Larsen. lY9.3, 1994), illustrate that a much wider Lone was affected by rift-related magmatism and tectonic activity. The width of the active zone appears to increase southwards, and thus to be inversely related to the crustal thickness (see map of Moho depths by Kinck ct al., 1991). In the southern. submerged part, the Skagerrak Graben, rift-related sedimentary and magmatic deposits are buried under younger sediments. The extent of rift-related magmatic activity in this part of the Oslo Rift is not yet known (Ro and Faleide, 1992). Magmatism and tectonic activity in the Oslo Graben started about 300-305 million years ago, and lasted till about 240 million years ago (e.g. Sundvoll and Larsen, 1990; Sundvoll et al., 1990, 1992). From petrological modelling based on a combination of major element. trace element and isotopic data (e.g. Neumann, 1980; Neumann et al., 1985, 1988b, 1990) we have extensive information about the evolutionary history of the different rock types exposed in the Oslo Graben. These data imply that mantle-derived magmas collected in magma-chambers in the deep crust, where they underwent different degrees of fractional crystallization and contamination before ascent to the surface, or shallow crust: (1) The majority of the magmatic rocks in the Oslo Rift have lower mg# (cation proportions Mg/(Mg + Fe,,,,,) than expected by primitive mantle melts (e.g. Fig. 2). This implies that they have been subjected to fractional crystallization and/or contamination. The major and trace element variations among Oslo Rift rocks of gabbroic and intermediate composition cannot be explained by removal of their low-pressure mineral assemblages (plagioclase f olivine + clinopyroxene), but require removal of assemblages stable at pressures of 7-10 kbar (Neumann. 1980: Neumann et al., 1985). (2) The Rb-Sr, Sm-Nd and U-Th-Pb isotopic compositions of Oslo Rift magmatic rocks

E.-R. Neumann / Tectonophysics 240 (1994) 159-l 72

Oslo Rift basaltic lavas ,

8

,

12

Fe0 mole % Fig. 2. Extrusion temperatures for Oslo Rift basaltic lavas estimated on the basis of whole rock major elements (SchouJensen and Neumann, 1988: Anthony et al., 1989; Neumann et al., 1990; Neumann. unpublished data) assuming a Fe’+/ (Fe’+ +Fe3+ ) ratio of 0.17, combined with the olivine-melt geothermometer of Roeder and Emslie (1970). The grey field shows the total range covered by basaltic lavas; squares and triangles indicate aphyric lavas from Jeloya and Vestfold, respectively; dots show groundmass compositions of nephelinitic lavas from the Skien area.

of mafic to intermediate compositions imply interaction between two lithospheric mantle sources and a deep crustal source (deep crustal contamination). The lowermost, nephelinitic lavas in the Skien area appear to originate in a nearly undepleted mantle reservoir with initial isotopic ratios Of ‘Nd about + 1, and lSr - 10 to - 15 (HIMUtype source; Anthony et al., 1989). With time, the EN,-values of the Skien lavas increase to about +4. The majority of the basaltic lavas from Vestfold and Krokskogen, together with intrusive rocks of mafic to intermediate compositions define a mixing trend in the Sr-Nd and U-Th-Pb isotopic systems. One source is characterized by 20hPb/204Pb)3,,0 Ma of ( 2 + 4, < 5, ESr, tENd3 > 19.2>, the other by (< - 1, 2 + 18, < 17.7). This mixing trend requires the involvement of (at least) two isotopically distinct sources. Neumann et al. (1988b, 1990) proposed that these sources correspond to a mildly depleted (PREMA-type) mantle source and the lower crust, respectively.

161

The HIMU-type source may represent local, metasomatized domains within a lithospheric mantle which was generally dominated by PREMA-type isotopic composition (Anthony et al, 1989). The metasomatized domains would have relatively low solidus temperatures and become exhausted during the early stages of partial melting. With progressive partial melting the magmatic products became dominated by melts derived from the mildly depleted PREMA-source. (3) The isotopic data are supported by Th/ La-Th/Ta relations which imply that many of Vestfold basal&, larvikites and RP (rhomb porphyry) lavas are uncontaminated by crustal matcrials. and have retained the chemical characteristics of their mantle source, whereas others have slightly higher Th/Ta ratios (and lower Ed’,) which strongly suggests that a lower crustal component is involved. (4) Trace element and isotopic compositions of syenitic and granitic rocks indicate that also in these rocks the contribution of old crustal components is quite low, implying formation either by extensive crystal fractionation of clinopyroxeneand amphibole-rich assemblages from mantle-derived melts, or, alternatively, partial melting of clinopyroxene k amphibole-rich Permian gabbros (Neumann et al., 1988b; Rasmussen et al., 1988: Sundvoll et al., 1990; Trannes and Brandon. 1992). (5) Some upper crustal contamination is indicated by the high initial Sr isotopic ratios exhibited by some granitic and syenitic rocks, and by a few basaltic lavas and dykes (Neumann et al.. 1988b; Trannes and Brandon. 1992: Sundvoll et al., 1992). (6) The only ultramafic xenoliths found so far in the Oslo Rift are olivine clinopyroxenites (Neumann et al., 1988a). These xenoliths represent cumulate rocks formed at a temperature of about 1100°C and a minimum pressure of 5.5-h kbar (corresponding to a minimum depth of 16- 17 km). The Sr-Nd isotopic compositions of clinopyroxene in these xenoliths fall on the VestfoldKrokskogen mixing trend, reflecting minor amounts of lower crustal contamination. The estimated pressure and isotopic compositions indicate that these cumulates formed in lower crustal magma chambers.

(7) The presence of dense, mafic cumulate rocks in the deep crust under the Oslo Graben is supported by a broad, regional gravity high along the Oslo and Skagerrak Grabens which indicate a high-density (about 3.00-3.05 g/cm’) layer in the lower crust between about 20 km depth to Moho at about 32 km (e.g. Ramberg and Smithson. 1971; Ramberg, 1976; Wessel and Husebye, 1087: Ro et al., 1990a, b; Neumann et al.. 1992; Ro and Faleide, 1992). Some local gravity highs suggest that minor volumes of cumulate rocks may also have been deposited in shallow magma chambers (maximum depths 1.9-5.1 km; Ramberg, 1976). (8) Also seismic data (Tryti and Sellevoll, 1977: Cassell et al., 1983; Gundem, 1984) suggest some differences in crustal structure between the rift area and the adjacent Precambrian Shield which are in agreement with the presence of mafic cumulates and residues in the deep crust. The Precambrian Shield appears to consist of distinct upper and lower crustal layers each with small or no internal velocity gradients, a Conrad discontinuity at 14-17 km depth (velocity contrast 0.250.40 km/s), and a marked Moho discontinuity (velocity increase from 6.7/6.8 to 8.1/8.2 km/s). Under the Oslo Graben, the Conrad discontinuity seems to be less well defined, the lowermost crust beneath the Permian volcanic areas is characterized by an approximately 12 km thick layer with relatively high (7.0-7.1 km/s) P-wave velocity, and the velocity contrast at Moho is < 0.9 km/s.

3. Potential temperatures

estimated

from basaltic

lavas in the Oslo Rift

lnformation about the P-T conditions in the mantle melt region may be obtained from mafic magmas which have followed a predictable P-T path during ascent from the mantle melt region. Following the argumentation of McKenzie and Bickle (1988) that the temperature loss of melts ascending quickly through the lithosphere is due to adiabatic decompression (about l’C/km), the extrusion temperatures (T,) of primitive lavas may be used to estimate the temperature of their mantle source region. As it is difficult to compare

temperatures at different depths, McKenzie and Bickle (1988) introduced the term “potential temperature” (T,,), which is the tempcraturc at ;L given depth in the mantle. corrected to I bar along a solid adiabate (about 0.6X/ km). In general, geothermometers based on mineral compositions are more reliable than the ones involving whole rock compositions. However. mineral assemblages suitable for gcothermometry are not found in the most primitive Oslo Rift lavas (olivine has been oxidized and altered, orthopyroxene is not present). Extrusion temperatures for mafic lavas in the Oslo Rift have therefore been estimated on the basis of whole rock major element compositions published by SchouJensen and Neumann (19881, Anthony et al. (19891, Neumann et al. ( 19901, and unpublished data, combined with the olivine-melt geothermometer of Roeder and Emslie (1970). In order to obtain reliable temperatures by the Roeder and Emslie (1970) method, it is important to have information about the Fe’ ‘/(Fe’+ + Fe”.‘) ratios of the lavas at the time of extrusion. The Fc3’/ (Fe” + Fe3’) ratio of a melt is a function of temperature, oxygen fugacity (fo,), and pressure. T-f{,: determinations on continental basalts in general give values close to the QFM (quartzfayalite-magnetite) buffer (e.g. Basalt Volcanism Study Project, 1981). Oxygen fugacities close to the QFM-buffer are also indicated for Oslo Rift magmas. T-fo, estimates for intrusive rocks of intermediate compositions in the Oslo Rift indicate oxygen fugacities between about 0.5 log units below, and 1 log unit above, the QFM-buffer (Neumann, 1976). A Fe”‘/(Fe’* + Fe” ’ 1 ratio of 0.17, corresponding to f,,>-values close to the QFM-buffer (Roeder and Emslie, 1970) for mafic lavas at 12oo”C, was used in the calculations. Aphyric, mildly alkaline basalts from the Vcstfold and Jel@ya sequences, and separated groundmass from nephelinites in the Skien area, give temperatures between 1060 and 127o”C, and are in equilibrium with olivine of compositions between Fo,~ and Fo,, (Fig. 2). The highest temperatures, ca. 13OO”C,were obtained on basalts with a high volume proportion of augite and olivine phenocrysts. The whole rock compositions of these samples therefore probably do not repre-

if3

E.-R. Neumanrt / Tecfonophysics 240 (1994) 1.59-I 72

or (b) the results of fractionation processes at mantle or crustal depths. A relatively Fe-rich mantle source is in agreement with the groundmass compositions of xenoc~st-bearing Skien nephelinites (Fig. 2). Case (a) gives an extrusion temperature CT,) of 1270-1300°C. Assuming case (b) to be true, we get T, in the range 1330- 1340°C by extending the trend defined by the most mafic lavas in Fig. 2 towards forsterite contents which are common for mantle Iherzolites, that is Fox+,), (e.g. Boyd, 1989).

sent melt compositions. This means that the highest temperatures indicated in Fig. 2 are least reliable. A decrease or increase in oxygen fugacity of one log unit would lead to a horizontal shift of + 0.4 or - 1.0 mole% FeO, respectively, in Fig. 2. The corresponding temperature change is within 20°C. The relatively low mg# of the majority of the basaltic rocks in the Oslo Rift (Fig. 2) indicates that they represent melts of which compositions have been modified during ascent by processes such as fractional crystallization and contamination. It is not clear, however, if this applies to all the rocks. The lowermost (oldest) nephelinites from the Skien area carry xenocrysts of olivine and Cr-diopside (mg# 0.85-0.90), the latter coated by Al-rich augite (mg# 0.71-0.84). The compositions of the most Mg-rich Cr-diopside xenocrysts strongly suggest a mantle origin. The presence of mantle xenoliths in a lava are generafly taken to indicate that the host magma ascended quickly from mantle depths to the surface. However, the xenocrysts in the Skien nephelinites represent only the partly dissolved remnants of mantle xenoliths. A residence period in crustal magma chambers can therefore not be excluded. It is thus not clear whether the reiatively Fe-rich compositions of the Skien nephelinites (corresponding to a mantle source with Fo xs_xx) is (a) a primary feature resulting from partial melting of relatively Fe-rich mantle rocks,

Table 1 Cumulate

and residual

Cumulate

rock types formed I LOO-90%

Stage: F: assemblages

Type Modal composition Density (g/cm’) V, (km/s) Surface and subsurface Rock types

- 3.3 - 8.1

The petrological and geochemical data summarized above imply extensive fractionation processes at pressures of 6.5-10 kbar, which must have left considerable volumes of cumulate rocks at depth in the crust under the Oslo Rift. Fractionation processes in the mantle cannot be excluded, but are likely to be of minor importance compared to the well documented fractionation processes which took place in the lower crust. Mantle xenoliths, which indicate fast ascent from mantle depths to the surface, have not been found in Oslo Rift lavas. The discussion below is therefore based on the assumption that fractional crystallization at mantle depths was insignificant. The generation of basaltic rocks involved formation of cumulus assemblages of type I f II in

as the result of progressive 11 90-70%

formed in the deep crust: dunite 01. clinop. 01

4. Cumulate rocks in the deep crust under the Oslo Rift

fractional

crystallization

in the deep crust under the Oslo Rift

III 70-45%

IV < 45%

01. clinop.

Ol2oCPX,,,

ol,,c~x~~mt~

- 3.3-3.4 - 8.0-S. 1

- 3.5 - 7.5

gabbro/ diorite plag + cpx + amph i 01 + mt + ap < 2.8 < 7.5

RP lava, larvikite

syenitejgranite

rocks formed from residual - basalt, gabbro -

liquid:

F = mass ratio of residual to initial magma; 01 = olivine, cpx = clinopyroxene, mt = Ti-rich magnetite; plag = plagioclase, amph = amphibole, ap = apatite, 01. clinop. = olivine clinopyroxenite, RP lava = rhomb porphyry lava. Densities and seismic velocities (V,,) for the different cumulate assemblages have been estimated on the basis of mineral and modal compositions, corrected for thermal expansion and compressibility to assumed present P-T conditions of 6 kbar and 250” C. See text for further explanation,

E.-R. Neumunn / Tectonophysics 240 (1994) 159-I 72

164

Table 1, formation of the residual magmas which gave rise to RP lavas and larvikites involved separation of cumulus assemblages I + II + III from the primary magma(s). The formation of syenites and granites involves formation of cumulus or residual assemblages of types I + II + III + IV before ascent. For representative compositions of the cumulus phases at each stage of crystallization, the reader is referred to Neumann et al. (1986). Densities representative of cumulus assemblages of types I, II, III and IV have been estimated on the basis of modal and mineral compositions inferred from petrogenetic modelling (Neumann et al., 19861, using data on mineral densities from the literature (Robie et al., 1966; Johnson and Olhoeft, 1984). The results have been adjusted to 6 kbar and 250°C (which is the assumed present state of the deep crust under the Oslo Rift) using correction methods and data on compressibility and thermal expansion given by Birch (1966), Skinner (19661, and Sumino and Anderson (1984). Seismic velocities (I’,) have been estimated on the basis of methods described by Simmons (19641, using equations for tempera-

Basalt

Residual magma

Rhomb porphyry/ larvikite

ture and pressure corrections by Anderson and Sammis (1970). The results are listed in Table 1. The densities and seismic velocities listed in Table 1 do not take the effects of intercumulus material (trapped magma between cumulus minerals) into consideration. The minimum space between densely packed cumulus minerals which would be taken up by trapped magma, is estimated to about 6 vol.% (Cox et al., 1979). Trapped magma making up less than about 10 vol.% of a cumulate rock is only expected to lead to additional growth of the cumulus minerals, and will not have any significant effect on the densities or seismic velocities listed in Table 1. Higher volume proportions of trapped liquid may result in formation of intercumulus material consisting partly of additions to the cumulus phases, partly of nucleation and growth of new phases (e.g. feldspar). In the latter case the presence of intercumulus material may cause somewhat reduced densities and seismic velocities. The effects of intercumulus material may be estimated from weighted proportions of cumulus assemblages (using data for cumulus assemblages I, 11or III in Table l), and gabbro IV. The estimated densities

Granite/ syenite

Gabbro

Surface

Cumulates MOHO

Mantle Initial magma

Fig. 3. A schematical presentation of extent of crystallization necessary to form some of the most common rock types in the Oslo Rift from a given mass of initial mantle melt. In each column the box volumes indicate mass proportion of residual magraa and cumulates relative to mass of initial magma. Darker grey indicates more mafic compositions, higher densities and higber seismic velocities, lighter grey more silicic compositions, lower densities, etc. See text for further information.

E.-R. Neumann

/ Tectonophysics

and seismic velocities fall within published values for corresponding rocktypes (e.g. Daly et al., 1966; Jackson et al., 1981; Johnson and Olhoeft, 1984). We see from Table 1 that cumulate rocks formed at stages I (dunite) and II (olivine clinopyroxenite) have mantle characteristics with respect to densities (2 3.3 g/cm”) and seismic velocities (V, 2 8.0 km/s) if the proportion of trapped liquid is relatively low. The onset of crystallization of feldspar causes a marked decrease in the seismic velocity towards crustal values, whereas a corresponding decrease in density follows at a somewhat more advanced stage of the crystallization process. Table 1 also gives information about the mass proportions of cumulus minerals to surface and subsurface (shallow intrusive) rocks. The extent of crystallization (mass proportion of cumulates/ initial magma) needed to derive some of the most common rock types (representing residual magmas) in the Oslo Rift, are demonstrated schematically in Fig. 3. Table 1 implies that if we know the volume, composition and mass of a given type of rock

240 (1994)

165

72

exposed at the surface, we may estimate the corresponding mass of cumulus minerals associated with its formation. The magmatic rocks in the Oslo Graben comprise a wide range of compositional types (Fig. 1). The mean densities, area1 extents, volumes and masses of the main rock types are listed in Table 2. All densities and areal extents are from Ramberg (1976). Volume and mass estimates for gabbroic and syenitic/ granitic rock bodies are mainly based on gravity interpretations (Ramberg, 1976). The volumes of basaltic rocks are based on Ramberg’s (1976) estimate of 259 km’ for the nephelinites and basalts in the Skien area, adjusted to 300 km’ to account for the smaller volumes of basalts found in other parts of the rift. The volume of the larvikites is estimated from surface exposure. assuming an average thickness of 5 km, as proposed by Oftedahl (1952). The thickness of the RP lava sequences ranges from about 2000 m in Vestfold, to about 300 m in Brumunddal. in the northernmost part of the Oslo Graben (Ramberg and Larsen, 1978; Bverli, 1985; Tollefsrud. 1987; Schou-Jcn-

Table 2 Areal distribution, mean density, estimated volumes and masses of the assemblages in the deep crust, as indicated by petrogenetic modeling Mean density (g/cm’)

159-l

main

Area (km’)

rock

types

in the Oslo

Volume (km’)

Graben.

and

Mass (lO’J kg)

E,urusrc~e rocks Basaltic laws RP laws Rhyolitic lavas

2.91 (’ 2.72 “ 2.63 “

220 ,’ 1153 ‘I 31 ,’

300 - 1200 so ~’

x.7 54 I.3 ”

Inrrusii~ rocks Gabbroic rocks Larvikite. etc. Syenites. granites,

3.06 .’ 2.71 ” 2.61 a

IS ‘I 1990 “ 3100 ,’

- 5 “ IO 000 1404.5 .’

- 0. I IN 271 368 ,I

6509 LJ

?S 600

6’,3

E.yposed magmatic Oslo Grahen,

etc. rocks, total

x573 “

total

Other magmatic

cumulus

rocks

Dense, subsurface bodies Deep crustal cumulus assemblages Gabbroic rocks in the deep crust

- 2.9 h 3.0-3.1

4610 I’ 65 000 ‘I’,

TOTAL “.h Data from Ramberg (1976); h mafic subsurface 1.9-5.1 km. Other data are discussed in the text.

> 9.5 200 intrusions

only identified

by gravimetric

data:

I34 ,’ 2000 ‘1’) > 2x30

estimated

maximum

depths

are

166

E.-R. Neumann / Tectonophysics 240 (1994) 159-172

sen and Neumann, 1988). An average thickness of 1 km has been used to calculate the total volume of RP lavas (Table 2). If we assume that the known volumes of basaltic and gabbroic rocks formed by about 20% fractional crystallization on the average (Tables 1, 21, the corresponding mass of cumulus minerals is l/4 of the combined mass of gabbros and basalts. The formation of the larvikites in the Vestfold Graben segment (Fig. 1) by fractional crystallization, on the other hand, requires the presence of cumulus assemblages of a similar, or larger, mass at depth in the crust. The formation of syenites and granites may have involved cumulus and/or residual assemblages corresponding to 8-9 times the mass of exposed syenites and granites. A rough estimate suggests that cumulus minerals alone account for at least 2000 X lOi kg. In addition come intercumulus material and magmas crystallized to completion in the deep crust. A quantitative method for estimating excess mass of cumulus assemblages from a combination of petrological modelling and volumes of different types of rocks, was presented by Neumann et al. (1986).

s g

30

a 40

60 0

400

1200

800

1600

T ( C) Fig. 4. Estimated magmas,

adiabat

corresponding

(Takahashi

to extrusion

intersection

peridotite between

gests a lithospheric Carboniferous Oslo

and Eggler,

km (A-B).

corresponding

(OOB),

mantle with a potential

by McKenzie

Information about the regional P-T conditions in the upper mantle is important to understand rifting processes and, ultimately, the causal mechanisms of rifting. Intraplate or excess magmat&m is commonly interpreted in terms of rising hot plumes which deflect laterally at the base of a somewhat eroded lithosphere (e.g. Courtney and White, 1986; McKenzie and Bickle, 1988; Richards et al., 1989; Griffiths and Campbell, 1990; Watson and McKenzie, 1991). Large temperature anomalies, 200-300°C above the average potential temperature of 1280°C for the MORB source, have been estimated for the mantle source regions of some intraplate magmatic areas, for example Hawaii (Watson and McKenzie, 19911, which thus appear to represent true

on estimates further

by Watson

Mantle

to extrusion by dotted

The

adiabats for

temperatures

of

lines. For compari-

for old oceanic basins

and the adiabatic

temperature

gradient

for

of 128fPC (dashed line),

and Bickle

line shows the solid adiabat

and mantle during

1983).

thickness under the Oslo Rift in Permo-

son are shown the convective geotherm

5.1. P-T conditions in the cwt the Oslo rzfting event

of 1270-

the solidus of dry peridotite

(Olafsson

1270 and l.340°C are indicated

as proposed

Oslo Rift

the grey band and the dry solidus sug-

time of 60-85

Rift,

temperatures

19831, and solidus + 1’ the solidus of

and Kushiro,

H,O-saturated

the

fgrey band) for primitive

Dry solidus represents

1340°C.

and young continents

5. Discussion

2000

(1988).

of the Hawaii

and McKenzie

The dash-dot

“hot-spot”.

(1991).

based

See text for

explanation.

“hot-spots”. In other areas, such as northwest Spitsbergen and the Azores, no or only minor potential temperature anomalies have been found, and partial melting has been explained as the result of decreased solidus temperatures due to H,O and CO, in the mantle source regions (“wet-spot”) (Bonatti, 19y0, Vignes and Amundsen, 1992). The presence of fluids in mantle rocks lowers the melting temperature, and the effect increases with increasing proportion of Hz0 as compared to CO, (e.g. Olafsson and Eggler, 1983; Fig. 4). For northwest Spitsbergen the “wet-spot” hypothesis is supported by the presence of amphibole, phlogopite, and apatite in mantle-wall-

rock (spine1 Iherzolites) and mantle-cumulate xenoliths (pyroxenites and wehrlites) (Amundsen et al., 1987). The adiabatic temperature gradient for Oslo Rift basalts, assuming T, between 1270 and 1340°C is shown as a grey band in Fig. 4. So far no crustal or mantle wallrock xenoliths which might be used to determine regional P-T conditions in the lithosphere during the Oslo rifting event have been recovered. It is of the greatest importance to find deep crustal xenoliths (for example garnet granulites) which might be used to establish points on the geothermal gradient. As we do not know the geothermal gradient in the Oslo Rift area in Pcrmo-Carboniferous time, it is difficult to estimate the depth to the melt region, and thus also the potential temperature. Case (al (T, 1270-1300°C) gives an intersection between the melt adiabat and the dry solidus at a pressure of 19-2 1 kbar (A in Fig. 4). This indicates a minimum thickness of the lithosphere under the Oslo Rift in Permo-Carboniferous time of 60 km. The corresponding T, is about 1300°C (dotted line corresponding to point A in Fig. 4). Case (b) (T, 133O-1340°C) gives an intersection bctwetn the melt adiabat and the solidus at about 28 kbar (point B in Fig. 4). This corresponds to a lithospheric thickness of about 85 km, and a T, of about 1370°C. The large volumes of magmatic rocks exposed along the floor of the Oslo Graben (Fig. 1) place the Oslo Rift among the highly magmatic contincntal rifts. However, the estimated potential temperature for the upper mantle beneath the Oslo Rift lies only between about 20 (case (a)) and about 100°C (case (b)) above the average mantle T, of 1280°C. The extensive Oslo Rift magmatism does thus not appear to be associated with any major temperature anomaly in the underlying mantle, although the presence of a minor thermal anomaly of > 100°C cannot be excluded. The Oslo Rift can thus not be categorized as a classical “hot-spot”. Alternative modes of formation are: (I) a “wet-spot”; (11) melting by decompression due to stretching of the lithosphere; or (III) a combination of causal mechanisms. Ro and Faleide (1992) proposed that partial melting resulted from the combination of a

moderate thermal anomaly and heating by decompression. GOfer f. There is no conclusive evidence that the mantle source region for the Oslo Rift magmatism was unusually rich in fluids. Amphibole is present in some Oslo Rift gabbros (Neumann et al., 19X.51, and amphibole and/or biotite are found in a number of diabase dykes (e.g. ðer, 1962; Scott. 1980; Scott and Middleton, 1983). However, H,O-bearing minerals have not been reported from basaltic lavas (e.g. Segalstad, 1479; 0verli, 1985; Tollefsrud, 1987; Schou-Jensen and Neumann, 1988). Furthermore, it is not clear if the ‘H,O-rich fluids which gave rise to amphiboIe and biotitc in gabbros and mafic dykes originate in the mantle source region. or if they represent a crustal component, added to the melts through contamination. Carbonate segregations and CO,-inclusions are common in basaltic rocks, gabbros and dykes. Radiogcnic and stable isotope compositions indicate that calcite in nephelinites from the Skien area is magmatic and formed by segregation from the mantle melts (Anthony ct al., 1989). However, CO,-fluids are frequently associated with basaltic lavas in many different geological environments, inclLlding typical hotspots such as Hawaii (e.g. Roedder. 1984: Greenland, 1984). It is thus questionable if the Oslo Rift may be characterized as a “wet-spot”. Mode/ II. According to McKenzie and Bicklc (1988) extension of the lithosphere generates littie melt unless p > 2 and T,, > i3XO”C.Stretching in the Oslo Rift is cstimatcd to a &factor of only 1. I - 1.2 (Ro and Faleide, 1992). Stretching alone can thus not account for the extensive magmatism observed in the Oslo Rift. MO&/ Ill. Available informati~~n on the Oslo Rift magmatism seems to agree best with model (III 1, a combination of stretching, a weak thermal anomaly in the underlying asthenosphere, and/or decreased solidus temperatures due to high fluid contents in the mantle source region.

Models on rifting and graben formation arc frequently based on the simple assumptions that the lithosphere in the rift area is a layered block

lb8

E-R. Nertmunn / Tectonophysics 240 (1994) 159-l 72

where the mass and volume of each layer (upper crust, lower crust, upper mantle) remains constant during the rifting process. However, petrological and geochemical studies of rift-related magmatic rocks have revealed that for many rifts this assumption is false. Mass may be transferred between mantle and crust, and between lower and upper crust, and heating and metamorphism may cause changes in mass/volume relations in parts of the crust. In general, rifting processes thus lead to permanent changes in the structure of the lithosphere. This is also true for the Oslo Rift. The total volume of exposed magmatic rocks in the Oslo Graben is estimated to be about 26,000 km3, and the mass about 700 X lOI kg (Table 2). This gives an average of 130 km3/km of surface and subsurface magmatic rocks along the axis of the Oslo Graben. However, the exposed rocks represent only part of the total mass of magmas transferred from mantle to crust during the Oslo rifting event. Estimates outlined above suggest that the generation of the exposed magmatic rocks in the Oslo Graben required the formation of dense cumulus assemblages (and residues after partial melting) corresponding to a mass of least 2000 x 1014 kg. The total volume of shallow, dense rock bodies is estimated to only 134 x 1014 kg (Table 2; Ramberg, 19761, which is insignificant as compared to the total volume of rift-related cumulate rocks. The major part of the cumulate rocks is thus concentrated in the deep crust, implying that gravity filtering and formation of deep crustal intrusions represent important rift processes. Assuming an average density of 3.0-3.1 g/cm” for the cumulus assemblages (Table 11, this corresponds to a volume of cumulus (and/or residual) minerals of roughly 65,000 km3, or about 320 km”/km in the deep crust under the Oslo Graben. The total estimated volume of magmatic rocks is about 95,000 km3 (Table 2). This corresponds to a more than 11 km thick layer of melt covering the floor of the Oslo Graben. However, the above estimates represent minimum values with respect to transfer of material from the mantle into the deep crust. One reason is that the large masses of lavas and subsurface

F

I

Southern Vestfold Graben Segment

B

0.1

0.2

Volume proportion

0.3

of trapped liquid

0.4

(VR)

Fig. 5. Range in excess mass per metre along the rift axis in the Vestfold Graben segment. Field A shows excess mass estimated from petrological/geological data. VR = (Vr)/( VT + I’(..), where VT = volume of melt trapped in interstices behveen cumulus minerals plus volume of melts crystallized to completion in the deep crust (forming gabbros); V,. = volume of cumulus minerals. KQ = 0.06 represents the lower limit of trapped liquid, as indicated by studies of natural cumulates and porosity in packing experiments (Cox et al., IY79). Line B shows excess mass along profile IX (Fig. I) as estimated from the gravity inversion of Neumann et al. (lY9II). See text for additional information.

rocks which clearly have been removed by erosion have not been taken into consideration in the estimates. Another reason is that at least 6% of a cumulate rock consists of magma which has been trapped in the interstices between cumulus minerals (the proportion of trapped magma may be considerably higher). Additional masses of magma may have crystallized to completion within the lower crust, giving rise to gabbroic rocks. In addition to the cumulate rocks, the lower crust may contain dense residues after anatexis and/or dehydration reactions (Rasmussen et al., 1988; Sundvoll et al., 1990). Fig. 5 shows excess mass per metre along the rift axis in the southern Vestfold Graben segment (Field A), as estimated from petrological and geological data, using the method of Neumann et al. (1986). Excess mass is plotted against the parameter P7I (VR = /, where V-r = volume of melt trapped in interstices between

E.-R. Neumann / 7‘ectonophysic.s ,740 (1994) 159-172

cumulus minerals plus volume of melts crystallized to completion in the deep crust (forming gabbros), and V, = volume of cumulus minerals). The upper limit of Field A is based on the assumption that all magmatic rocks in the Vestfold Graben segment have formed by fractional crystallization from mantle magmas; the lower limit allows for all syenites and granites to have formed by anatexis of Precambrian lower crust. Information given above indicates that true cxcess mass lies closer to the upper than the lower limit of Field A. There is a very good agreement between the magnitudes of estimated excess masses in the crust under the southern Vestfold Graben scgment using petrological and gravimetric methods, for low volume proportions of trapped liquid (Fig. 5). This agreement lends strong support to Ramberg’s (1976) suggestion that the positive gravity anomaly along the Oslo Rift reflects dense cumulate rocks at depth in the crust. However, as discussed above, it seems likely that considerable volumes of surface and subsurface rocks have been removed by erosion, and that the true value of VK may be relatively large. It may therefore be concluded that the true excess mass reflecting cumulate and gabbroic rocks formed during the Oslo rifting event is considerably higher than indicated by the density interpretations. One reason for the discrepancy between gravity data and petrological estimates indicated in Fig. 5 may be that gabbroic cumulate rocks (type IV) and melts crystallizing to completion in the lower crust have densities and seismic velocities similar to those of lower crustal wallrocks, and will therefore not be detected in gravity data. Estimates based exclusively on geophysical data therefore tend to underestimate the true transfer of mass into the lower crust. An additional/ alternative expianation for the discrepancy is that crystallization and formation of cumulates started in the upper mantle. Upper mantle cumulates associated with basaltic volcanism have been reported from some c~)ntinental rifts. such as the East African Rift System (e.g. Dautria and Girod, 1987) and the Rio Grande Rift (e.g. Frey and Prinz, 1978). The above estimates imply that the Oslo rifting event involved transfer of large masses of magma

16’)

from the mantle into the lower crust. It is to be expected that this magma formed numerous magmachambers with a larger horizontal than vertical extension. The present lower crust is therefore expected to consist of a mixture of stratified, dense cumulates, and less dense gabbros and Precambrian countryrocks. However, we may obtain a better understanding of the volumes of mantle-crust mass transfer if we assume that cumulate rocks and residues form a uniform layer in the deep crust under the Oslo Graben; a layer made up by cumulus (and/or residual) minerals alone would be about 3.5 km. This estimate is based on a total volume of 65,000 km’ cumulus minerals (Table 2), a length of the Oslo Graben of 200 km, and a width of the zone affected by rift-related magmatic activity, which increases from about 75 km near lake Mjosa, to about I20 km across the Skien-Larvik area (Fig. If. Taking into account (a) cumulate minerals c~~rresp~~nding to eroded magmatic surface and subsurface rocks. (b) intercumulus material, and (cl magmas crystallized to completion in the deep crust, the total thickness of the cumulate rock layer must bc B 3.5 km. A more mafic lower crust inside the Oslo Grahen than in the adjacent Precambrian Shield is also indicated by higher compressional wave velocities (7.0-7.1 and 6.7-6.8 km/s, rcspectively) (Neumann et al.. 1992. and references therein). Wendlandt et al. (1991) found a negative correlation between the volumes of erupted volcanic rocks associated with continental rifts and the seismic compressional wave velocity contrast across the Moho. The largest seismic velocity contrast, > 1.8 km/s, was found for the Rhine Graben with an average lava volume of 6.7 km3/ km along the rift axis (Rhine Grabenf, whereas the highly magmatic Kenya Rift (230 km3/ km) shows a velocity contrast across the Moho of only about 0.9 km/s. These results, which are valid for both active rifts and pnlaeorifts. arc taken as evidence that gravity filtering and accumulati~)ll of dense, mafic magmas in the lower crust play an important part in continental rifts. Interaction between the lower crust and mantle-derived magmas causes igneous and metamorphic processes (fractional crystallization, anatexis, dehydration,

etc.) which modify the lower crust to more mafic bulk compositions, resulting in higher compressional velocities and densities. The presence of large masses of hot melts in the lower crust over long periods of time represents a significant heat source within the crust which must significantly affect the crustal P-T-relations during the rifting event. The extent of magmatic activity in the Skagerrak Graben is not known. However, the gravity high along the Skagerrak Graben suggests the presence of dense intrusions in the crust, or a 6-10 km thick dense layer at the base of the crust (Ro and Faleide, 1992). The gravity high along the Skagerrak Graben continues ESE-wards along the Sorgenfrei-Tornquist Zone, and passes Skdne in southernmost Sweden which exhibits PermoCarboniferous dyke-intrusions (Ro and Faleide, 1992; Sundvoll and Larsen, 199.3).

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