Polydiapirism of granitic rocks in the Svecofennian of Central Sweden

Precambrian Research, 2 (1975) 189--214 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

POLYDIAPIRISM OF GRANITIC CENTRAL SWEDEN

ROCKS IN THE SVECOFENNIAN

OF

O. STEPHANSSON*

Department of Geology, University of Uppsala, Uppsala (Sweden) (Received June 4, 1974; revised and accepted October 25, 1974)

ABSTRACT Stephansson, O., 1975. Polydiapirism of granitic rocks in the Svecofennian of Central Sweden. Precambrian Res., 2: 189--214. A classification of igneous intrusions from orogenic zones is presented. Depending on the tectonic appearance of the intrusions (size, structure, contacts, ect.) these are divided into epi-, meso- and catatectonic diapirs. The Svecofennian of Central Sweden has been divided into tectonic units such as batholiths and subprovinces. The division is supported by the Bouguer anomalies. The geological situation of several of the batholiths in Central Sweden suggests a mechanism of formation which is described as polydiapirism or diapirism within diapirs. This means that younger granite diapirs have forced their way up through diapirically formed granitoid rocks and supracrustal septa. The driving force is the density contrast between the diapir and the surrounding rocks. A positive correlation exists for the different granitoid massives and the Bouguer anomalies. An interpretation of the gravity anomaly for one of the granite domes in a twin dome structure suggests that the late-kinematic granites extend to depths of about 18 km. Finally the theory of polydiapirism is applied to the evolution of the Svecofennian orogeny in Central Sweden.

INTRODUCTION The study of intrusive rocks has followed two general lines. The first, and e a r l i e s t , is b a s e d o n d i r e c t f i e l d o b s e r v a t i o n s , e m p h a s i z i n g t h e c o n t a c t c o n d i t i o n s b e t w e e n t h e i n t r u s i v e b o d y a n d its s u r r o u n d i n g s . I n r e c e n t t i m e s , t h i s a p p r o a c h h a s b e e n e n l a r g e d b y g e o p h y s i c a l m e t h o d s , m a i n l y g r a v i m e t r i c a n d seis m i c , f o r d e t e r m i n a t i o n o f t h e f o r m a n d size o f t h e i n t r u s i v e b o d i e s . T h e s e c o n d method, originally based on petrographic examinations of the intrusive rock, h a s in m o d e r n t i m e s b e e n e x t e n d e d b y g e o c h e m i c a l a n d r a d i o m e t r i c m e t h o d s . During the last few years more interest has been devoted to the mechanisms of i n t r u s i o n s a n d t h e s t r u c t u r e s w h i c h axe f o r m e d t h e r e b y in t h e i n t r u s i v e b o d y i t s e l f a n d its s u r r o u n d i n g s ( R a s t , 1 9 7 0 ) . B e c a u s e o f t h e m a g n i t u d e o f t h e e c o -

*Present address: Department of Rock Mechanics, University of Lule~, Lule~ (Sweden)

190 nomic values involved in petroliferous diapiric structures, these have been studied in detail. Much of this knowledge can be applied to analogous structures in granitoid rocks. The similarities in structure and mode of formation of salt and granite diapirs was p u t forward by Wegmann (1930) and applied to the Fennoscandian shield. A comparative study of salt and granite structures was recently presented by Sorgenfrei (1971). In this paper an a t t e m p t is made to classify igneous intrusions. The term batholith is proposed for a large composite body of granitic diapirs and narrow septa of supracrustal rocks in a region of orogenic deformations. The upward migration of the granitic diapirs involves gravity as the principal driving force. Depending on the tectonic setting they are divided into epi-, meso- and catatectonic diapirs. The Precambrian terrain of Central Sweden is composed of batholiths and subprovinces. The batholiths mainly contain synorogenic intrusions, ranging in composition from quartz-dioritic to granitic, with narrow septa of metavolcanics and metasediments forming a pattern similar to the structure of greenstone belts. A mechanism of polydiapirism or diapirism within diapirs is suggested for the emplacement of the granitoid rocks in the batholiths. The serorogenic or late-kinematic granites are most frequent in the subprovinces, where they have pierced the supracrustal rocks in the form of ridges and domes. The diapirism was initiated at the discontinuity in the contact region between the bathotith and the subprovince. An interpretation of the gravity anomalies is given for one of the domes at the twin-dome structure in the M~ilar subprovince. T H E S T R U C T U R E , D I M E N S I O N S A N D C O M P O S I T I O N O F I N T R U S I V E BODIES IN AN OROGENIC ZONE

There is at present no concentrated and exhaustive description and classification of granitoid intrusions or plutons from orogenic zones. According to Cloos (1936), a pluton is a b o d y or mass of plutonic rocks, irrespective of m o d e of formation, size or shape. Depending on the shape, size and composition of the pluton, we distinguish between batholiths and diapirs. The latter may be 'mushrooms', stocks, ridges, phaccoliths, etc. This proposed division of plutons is based mainly on the work of Buddington (1959), Raguin (1965) and Mehnert (1968), and on the experimental diapirs produced in the tectonic laboratory at Uppsala by Ramberg (1967, 1970), Stephansson (1972) and Dixon (1974). Batholiths

The term 'batholith' is used to denote large, more or less continuous areas of mainly granitoid bedrock. A number of authors have followed R.A. Daly's division, where batholith designates a pluton with a surface area in excess of 100 km 2, the b o t t o m of which could not be studied. The designation 'stock' (Buddington, 1959) has been used for plutons with a surface area of less than

191

100 km 2. More recent investigations of the large, classic batholiths, e.g. Idaho, Sierra Nevada and Boulder, show that these are made up of a number of smaller plutons or diapirs, which differ form each other in composition (Hamilton and Meyers, 1967}, and that supracrustal rocks, gneisses and migmatites comm o n l y occur in the batholiths. In this paper the designation 'batholith' is used for a large, continuous area of granitoid bedrock, which is composed of several diapirs, such as domes, stocks and phaccoliths. The plutons may be of the same or different compositions. The composition may also vary within a single diapir. One of the characteristic features of the batholith is that the diapirs contained within it have pierced a roof of older rocks. The supra- and infracrustal rocks which surround the individual plutons in the batholith are described as 'septa'. The contacts between the different diapirs and between the diapirs and surrounding bedrock or septa range from sharp, via various transitional forms, to conformable. The contact conditions are largely controlled by the tectonic level of the intrusion. A gradual intrusion of the diapirs into the batholiths often occurs, beginning with the basic quartz-diorites and granodiorites and ending with the acid granites. In the case of certain batholiths this process has been confirmed by radiometric datings (Knopf, 1964). A hypothetical batholith with different stages of diapirism and narrow septa is shown in Fig.1.

Diapirs The variation in the tectonic appearance of diapirs in orogenic zones results from a series of factors, such as the stage of formation during the orogenesis, type of rock and metamorphosis. Depending on the tectonic appearance of the diapir, e.g. size, structure, contact with surrounding bedrock, these are divided into epi-, meso- and catatectonic diapirs. A similar threefold division has long been used in the classification of plutons in fold mountains, with respect to their depth of formation in the earth's crust. Depending on the tectonic level, i.e., the depth and the intensity at which the deformation occurred, Wegmann (1935} divides the styles of deformation into infrastructures, transitional structures and superstructures. Infrastructures are characterised by intense deformation of rocks in a plastic conditio~ with consequent granitization. The infrastructure is the core of an orogenic zone and the deformations occurring here are passed on to other parts, where they give rise to the superstructures. In ordeI to designate plutons along the Cordillera, Hutchison (1970) divided them into allochthonous, parautochthonous and a u t o c h t h o n o u s plutons, based of the degree of deformation and metamorphism as a function of time. The autochthonous plutons display their maximal deformation at an early stage of the orogenesis. A third division of the plutons into epi-, meso- and catazonal has been presented by Buddington (1959). The relations between diapirs and the orogenic phases are shown in Table I. The catatectonic diapirs are connected with the synorogenic phase of the orogenesis with consequent granitization. The granitoid material forms diapirs,

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Fig.l. A hypothetical model of a batholith consisting of igneous diapirs and narrow septa of metavolcanics and metasediments: 1, gneisses and migmatites; 2, quartz-diorite; 3, granodiorite; 4, granite; 5, metavolcanics and metasediments; 6, volcanics and sediments. which rise to the surface because of their lower density and greater plasticity. This group of diapirs also includes gneiss domes which are formed in remobilized basements of earlier orogeneses. Diapirism during the serorogenic phase of the orogenesis, with consequent regional metamorphism, tends to y i e l d diapirs of mesotectonic type. These are in their turn closely associated with eugeosynclinal rocks in the fold mountains and appear as elongated d o m e s or ridges in the long axes of these mountains. This diapirism reveals the closest resemblance to salt diapirism. The rock c o n t e n t of the orogenic diapirs may be homogeneous, but is generally composite. The latter applies, for example, to the epitectonic diapirs, where the composition often varies from syenite to granite, or from diorite via quartzdiorite and granodiorite to granite (Buddington, 1959). Mesotectonic diapirs often have zonal structure with the most basic variants at the contacts and the granitic and aplitic rocks in the centre. As a rule it is possible to trace the relationship between the older rocks at the contact and the latest stage at the centre

Examples from the Svecofennian orogeny of Central Sweden

Associated dykes and veins Others

Contact structures

Implacement in orogeny Phase of orogeny Composition of diapir Metamorphism of surrounding rock Contact metamorphism Geometry of diapirs Size of diapirs Viscosity contrast diapir/surrounding Internal structures

Plutonic styles

diffuse contact, massive or gneissic core concordant with gneissic foliation

Late Svecofennian

Stockholm granite Fellingsbro granite

Early Sveeofennian

Uppsala quartz-diorite A m 6 granodiorite V~/nge granite

formation of gneiss domes in remobilized basement of early orogenies

cataclastic foliation at contact, massive core discordant or concordant

homophan or primary linear structure discordant no lineation or foliation basalt, lamprophyre and granophyre dykes closely associated with volcanic rocks emplacement guided by fracturing Latest Dala granites Svecofennian

50--1,000 km 2 10--0.1

associated with belts of eugeosynclinal rocks similarity to salt diapirism

100--500 km 2 100--0.01

10---100 km 2 0.01--0.001

migrnatite zones

replacement structures

dome, ridge

stock, laccolith

migmatite zones

amphibolite--granulite facies

granitization composite, mostly granodiorite

infrastructural emplacement* autochthonous plutons*2, .3 catazonal plutons .4 synorogenic

Catatectonic diapirs

pegmatites and aplites

regional metamorphism composite, the youngest most alkalic or siliceous green schist to epidote-amphibolite facies aureoles

volcanism composite syenite--granite diorite--granite unmetamorphosed -- low grade

chill margins

transition level emplacement *~ parautochthonous plutons*:, .3 mesozonal plutons .4 ser-synorogenic

Mesotectonic diapirs

superstructural emplacement* allochthonous plutons .2 epizonal plutons .4 postorogenic

Epitectonic diapirs

Tectonic setting of diapirs

TABLE I

194

(Mehnert, 1968). This relationship provides very strong support for a granito~. dal m e t h o d of formation of granite diapirs, and restricts the possibilities of a purely magmatic process. If the diapir was formed from a differentiated magma we would expect to find the most basic parts in the centre. The contact conditions between diapir and the surrounding rock provide important information on the mode of intrusion. Sharp and discordant contacts and the appearance of angular blocks from the surrounding rock in the contact zone have often been cited as arguments in favour of the intrusion of viscous, granitoid magmas. A study of equivalent structures in salt domes and in model experiments allows us to establish that corresponding structures occur on plastic intrusion under crystalline conditions. Sharp contacts and moderate contact metamorphism around, primarily, the epitectonic diapirs suggest a moderate temperature gradient right across the contact during formation. This also contradicts the theory that fluid magmas intrude in a cold, rigid bedrock. The discordant contact conditions and moderate size of the epitectonic diapirs result in a greater viscosity contrast between the intrusive rock and its surroundings than is the case for the catatectonic diapirs. The connection between the size and form of the intrusive bodies as a function of the contrast in viscosity between the diapir and the surrounding rock has been clearly shown in tectonic model experiments (Ramberg, 1970). If we apply these results to large catatectonic diapirs with their concordant, diffuse and gneissified contacts and their generally plastic deformations, this indicates a formation with minor contrasts in viscosity between diapir and surrounding rock. The deformation structures which are formed in the mantled rocks at diapirism are often deposited on top of already deformed rocks. This leads to complex structures. The picture is the more complicated as the contact areas undergo a severe migmatization. The mode of intrusion and consequent deformation of surrounding rock in the three types of diapir is shown schematically in Fig.2. Many epitectonic diapirs are directly associated with volcanic rocks and can often be classified as subvolcanic formations. Their size, shape, and way of intrusion frequently suggest an intrusion mechanism with great contrast in viscosity between diapir and surrounding bedrock. The driving force here, as in other diapirs, is the density contrast between intrusive body and surrounding rock. The epitectonic diapirs generally have the smallest cross-section and the intrusions are often in the shape of stocks or laccoliths. The meso- and catatectonic diapirs are assumed to emanate from more or less extensive granitoid layers in the earth's crust. A description of the possibilities as to the generation of this layer, for example by anatexis and metasomatism of sialic bedrock or phase transformation in the earth's upper mantle, is beyond the scope of this paper. Domes, more of less elongated, and ridges dominate among the intrusion forms in the mesotectonic diapirs. Their association with the serorogenic phase in the orogenesis, with resultant regional metamorphism, involves intrusion via synorogenic plutons, migmatite formation and formation of pegmatites and aplites. The catatectonic diapirs display the widest variation in shape and are at the same time the most irregular. This

195

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Fig.2. Igneous intrusions. A, epitectonic diapir. B, mesotectonic diapir. C, catatectonic diapit. 1, quartz-diorite; 2, granodiorite; 3, granite; 4, migmatite; 5, gneiss; 6, metasediments; 7, metavolcanics; 8, sediments; 9, volcanics; 10, basic intrusions; 11, pegmatite.

group of diapirs is characterized by their concordant appearance with varying degrees of metamorphism and foliation along the contacts. Table I gives the most characteristic features of the different diapirs, with examples from the Svecofennian of Central Sweden. A TECTONIC DIVISION OF THE SVECOFENNIAN OF CENTRAL SWEDEN

The term 'Svecofennian' is used to designate the division of the bedrock of Central Sweden proposed in this paper. In his attempts to refer the geological course of events to a time scale based on geology, Welin (1970) distinguished between distinct periods in the development of the Svecofennian orogenic zone, which he found to be between 2,100 and 1,500 million years old. He described an early period of deposition, with the laying down of volcanics and sediments, followed by a period of intrusion, with deformation and intrusion of granodiorites and granites, then a new period of deposition of volcanics and sediments, which were finally permeated by intrusions of granites and syenites. In the pro-

196 TABLE II A division of the Svecofennian in Central Sweden

Tectonics* ~

Chronostratigraphy *:

Lithostratigraphy*

Early Svecofennian D~

2,000

supracrustal rocks metavolcanics metasedimentary an limestone, dometavolcanic lomite supergroup sulphide ores iron ores metasediments

D:

1,950--1,850

synorogenic intrusions and diapirism gabbro Roslag-type quartz-diorite Uppsala-type monzonite Akersberga-type granodiorite Arno-type granite V~inge-type intraorogenic intrusives metabasites Herr~ingtype

Middle Svecofennian Late Svecofennian D~

D4

1,800

regional metamorphism, migmatization veined gneisses S0rmland gneisses migmatites

1,785

early serorogenic diapirism fine-grained Stockholm-type granites coarse-grained Feilingsbro-type granites (pegmatites) late serorogenic diapirism granite Sm,~land-V~irmland type

1,775--1,740

Late Sveeokarelian 1,670 D~

1,500

volcanism and deposition porphyries and Dala-type metasediments late serorogenic diapirism granites and Dala-type syenites

,1 Wikstr6m (1974); ,2 Welin (1970), Welin et al. (1970a, 1970b); ,3 Magnuason (1962), Lundqvist (1968), Lundeg~dh (1971), Koark (1973), Welin (1970), Gorbatschev (1971). p o s e d division o f t h e S v e c o f e n n i a n p r e s e n t e d here, t h e s e c o n d p e r i o d o f intrusion b e l o n g s t o t h e L a t e S v e c o k a r e l i a n ( 1 , 6 7 0 - - 1 , 5 0 0 m . y . ) , T a b l e II. T h e p r o . p o s e d division o f t h e S v e c o f e n n i a n c o n t a i n s m i n o r d e v i a t i o n s f r o m , a n d a d d i t i o n s t o , Welin's t i m e - t a b l e . T h e d e p o s i t i o n o f s u p r a c r u s t a l r o c k s a n d t h e

197 an epeirogenetic period with basic intrusions, here called Middle Svecofennian. The Late Svecofennian regional metamorphism, with migmatization and vein n gneiss formation, affects both the supracrustal rocks and the Early Svecofennian granitoids before concluding with the serorogenic granitic diapirism. The proposed lithostratigraphic division of the Svecofennian is based on data from Magnusson (1962), Lundqvist (1968), Welin {1970), Gorbatschev (1971), Lundeg~rdh (1971) and Koark (1973). The chronostratigraphy was devised with the help of age determinations presented by Welin (1970) and Welin et al. (1970 a,b). A division of the Svecofennian phases of deformation from an area in northeastern Ostergbtland has recently been presented by WikstrSm (1974). The Svecofennian of Central Sweden has been divided into a number of tectonic units, such as batholiths and subprovinces. The batholiths consist of mostly granitoid intrusives of catatectonic type and septa of supracrustal rocks whic~ surround the intrusives. The Vast--C)stgOta, the Roslag, the Vdala--Uppland, the Dala, the H~ilsinge and the J~imtland--Vhsterbotten batholiths are assigned to this t y p e (see Fig.3). The term 'province' is applied to a region which is dominated by granitic intrusives and where the intrusive is surrounded by, mainly, effusive rocks. The Dala--H/irdal batholith is assigned to this type. Metasediments, metavolcanics and migmatites predominate over the intrusior in the subprovinces; serorogenic granite diapirs predominate in the intrusions. The H~rn~5 subprovince is assigned to this group. Synorogenic granites may also be included in the subprovinces, and are often more or less foliated and migmatitizated. The Leptite, the Miilar and the Dala--Ghstrike subprovinces are counted in this latter group. The map of Sweden's bedrocks, with accompanying description by Magnusson (1962), together with the litho- and chrono-stratigraphic table presented, form the basis of the tectonic division. A comparative gravimetric investigation of Sweden was carried o u t in the years 1943--1948 under the direction of Wideland (1946, 1951). Fig.4 shows the Bouguer anomalies for every 10 mgals, together with the proposed division of the Svecofennian of Central Sweden into subprovinces and batholiths. The proposed division is supported by the relative gravity, apart from the Leptite and Miilar subprovinces, where the densities of the rocks closely coincide. The belt of postorogenic granites round the Dala and Vdala--Uppland batholiths leaves a considerable mass deficiency. The variation in the relative gravity within the batholiths is ascribed to changes in the composition of the rocks in the granitic layer in the earth's crust, with the exception of the Dala--H~irdal province and the J{imtland--V~isterbotten batholith, where the variations in relative gravity are also assumed to be related to changes in the position of the Moho discontinuity (Seguin, 1972). THE VDALA--UPPLAND BATHOLITH This is the largest and most characteristically formed of the batholiths of Central Sweden. The batholith forms an almost circular structure with a belt of serorogenic granites and foliated granitic gneisses, which concentrically sur-

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200

round a core composed of granitoid bodies. In the core area and the surrounding belt, there occur septa of metavolcanics and metasediments which are concor. dantly attached to, or encircle, the massif. The limit to the Dala batholith is governed by a change in the structures from almost E--W foliation and fold axis east of the boundary to NE--SW structures to the west thereof. The NE--SW striki area of vein gneisses, migmatites and serorogenic granite diapirs between Stockholm and Uppsala forms the boundary between the Vdala--Uppland and the Roslag batholiths. The area between the batholiths belongs to the Miilar subprovince. A detailed map of the Vdala--Uppland batholith and neighbouring areas is given in Fig.5. The map is based in essence on a part of the geological survey map of the Rocks of Central Sweden, which was published by A.E. TSrnebohm in the years 1880--1882, and which was far in advance of its time. The present map has been complemented with data from recent mapping by Lundeg~rdh (1956) and S~lh6s (1972). Massifs of basic rocks in the central parts of the Vdala--Uppland batholiths are irregular in shape, with certain tendencies to extension at the contacts of the batholith. The younger, acid variants consist of smaller, but more evenly rounded massifs. This geometric fact may be taken as an indication of the reciprocal age where the intermediary and acid types, i.e., granodiorite and granite, are the youngest massifs. The ultrabasic rocks are strongly associated with the Early Svecofennian intrusives (Stfllhiss, 1972). In the Vdala--Uppland batholith, they are concentrated in an almost E--W area between the Central region and the Hedesunda massif. Only smaller bodies occur in the central region. St~lhSs (1972) interprets the ultrabasics as synorogenic intrusives and folded sills and interlayers, thus excluding the possibility of each massif constituting a separate intrusion into its present site. This interpretation appears to be correct. I may add that the ultrabasic rocks contributed, at an early stage of the orogenesis, to the formation of the heavy overburden through which the lighter granitoid masses have forced their way. Such an interpretation is supported to some extent by the fact that the ultrabasic rocks are to be found, for the most part, in the zone of gneisses round the central massif. Diapirism of ultrabasic rocks in a granitoid bedrock is a physical impossibility. The majority of ultrabasic bodies in the actual granite massif must therefore be interpreted as elements withdrawn during the diapirism. The quartz-dioritic (I) massif is often irregular in shape. In connection with the regional metamorphism and the folding and the intrusion of the serorogenic granites (V), a severe deformation occurs in what now forms the zone of gneisses between the serorogenic granites and the central massif. This process has seriously distorted the quartz-dioritic bodies in the zone of gneisses. The quartz-diorite (I) -- Uppsala-type -- contains large numbers of deformed xenoliths of ultrabasic material. The granodiorites (11) are the predominant rock type in the central massif. The rocks in the gneiss zone which surrounds the central massif also consist to a great extent of granodiorites. The porphyric granodiorites (III) of A m 6 type form clearly delimited bodies in the southern parts of the batholith.

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Fig.5. Geological map of the Vdala--Uppland batholith and adjacent areas. The core of the batholith consists of massive synorogenic diapirs of different age and composition. The deformation increases towards the boundaries, where light serorogenic granites ( V ) h a v e intruded in the form of a ridge around the batholith. From the crest of the ridge the diapirism continued to form domes. Compiled mainly from TSrnebohm (1880), Lundeg~irdh (1956), and St~ilhos (1972)

202

Bodies which border closely on the serorogenic granites (V) display an elongation, with the longest axis parallel to the boundary of the batholith. The deformation of the porphyric granodiorites, like the formation of granitic gneisses in this vicinity, is a direct result of the intrusion of the younger granites. This interpretation has also been proposed by SthlhSs (1972, p.140) for the region north of Stockholm. The salic granites (IV) are the youngest synorogenic intrusives. Their contact cone with respect to the older, intermediate granitoids is more or less cross-cutting, with conformable structures and replacement nearest the contacts. The granites which are situated outside the granite region in the middle of the batholith are usually foliated, and should really be described as granitic gneisses. The information available on the contact relations between the youngest salic granites (IV) and the intermediate types suggest a mode of formation which is best compared with mesotectonic diapirs. In 1916 Geijer offered an important contribution to the understanding of the intrusion mechanism of the granites of Central Sweden. He based his analysis on a comparative study of intrusives of Daly type, i.e., fractures and depression of the roof in a magma chamber (stoping) and intrusives of Turner-Tigerstedt type, where the surrounding bedrock has been forced aside by the intrusion (roof doming). Geijer classifies the mode of formation of the serorogenic granites as Daly type, and assigns the synorogenic Early Svecofennian granitoids to the Turner--Tigerstedt type. Recent investigations from different parts of the Svecofennian have revealed that the time sequence in the Early Svecofennian granitoids follows the usual pattern, with ultrabasics and diorites as the oldest and granites as the youngest (Gorbatschev, 1971). Information in the literature {with the exception of TSrnebohm, 1880}, m y own observations, and to some extent the reciprocal localisation of the various granitoid massifs as they appear in Fig.5, all support such a sequence. However, as yet radiometric age determinations to confirm this are lacking, and the evidence is wholly based on the contact conditions. The time sequence of the intrusion of the rocks of the Early Svecofennian granitoid group in the Vdala--Uppland batholith indicates a mode of formation which may be characterized as diapirism within diapirs. First the intermediate quartz- and granodiorites (I a n d / / ) intrude through the heavier supracrustal rocks and associated ultrabasic massifs, sills and dykes. Thereafter, or simultaneously, an intrusion of the intermediate and acid types (III and IV) occurs through the earlier intrusions of types I and H. GRAVITY

I N V E S T I G A T I O N S IN T H E S T O C K H O L M - - U P P S A L A

REGION

The interpretation of gravity anomalies m a y cause problems, since an anomaly may result from an infinite number of different distributions of mass, Bearing in ming the mass distributions physically possible and the prevailing fringe conditions, the anomalies may be used as an aid to a tectonic division of the bedrock into different units, such as batholiths and subprovinces, and to esti-

203

mate possible depth and density contrast in individual bedrock units (Bott et al., 1967). In studies of granite diapirism, gravity investigations may be used as a petrographic cum tectonic tool in determining the form of the diapirs, their density distribution and their rock composition. The investigations may also contribute to the interpretation of the possible modes of formation of granite diapirs. The Geodetic Institute at Uppsala University has been carrying on gravity investigations in the surroundings of Uppsala for more than ten years. Parts of the map of the Bouguer anomalies (see Fig.6) were published by TengstrSm ~o

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204

{1971). The map includes more than 800 points and covers an area of about 10,000 km 2. As stated above, the Vdala--Uppland batholith is limited by the 20 mgals anomaly, which follows the structure of the granitic gneisses in a wide arc near the boundaries of the batholith. From the south and from the north the relative gravity declines, until we reach the homogeneous granite region in the centre of the batholith. Here as in the other parts of the mapped area, there is close agreement between the anomalies and the rock distribution. This is most clearly shown for the heaviest massifs of Uppsala type, e.g. at Harbo, north of Uppsala, and at Norrtalje, and for the light massifs of Stockholm type, such as the Vallentuna and Rimbo domes, Fig.6. The large mass deficiencies in the massifs at Vallentuna and Rimbo attract special interest. Both lie within a zone of mass deficiencies which can be followed southwestward from the Baltic Sea toward Lake M~laren where it bears sharply west-northwest. The direction of the isoanomaly curves coincides with the position of the postorogenic granites south of the Vdala--Uppland batholith. Where the granites are exposed, the curves form concentric circles with Bouguer anomalies of around -- 40 mgals in the middle. The anomalies, together with the geological information, show that the area consists of a continuous ridge of granite which surrounds the batholith. Separate domes have been formed from the top of the ridge of granites, which pierce the rocks above and give rise to conformable structures in their vicinity. The plastic movement of granite material to the twin domes has led to a strong attraction in the area between them. In agreement with this, the bedrock here also displays subvertical foli ation as well as subhorizontal fold axes and elongation cf. the map Uppsala NO (St~t hSs, 1972). On formation of the twin domes by diapirism, a convergent movement t o o k place in the granite material between the domes, which led in its turn to a foliation and folding. This state of affairs has been illustrated in model experiments with the centrifuge m e t h o d and applied to culmination structures along the Norwegian west coast by Ramberg (1967}. The granite ridge with accompanying domes is directly analogous to structures in Cornwall in southwestern England. Here six granite domes form a long, curved structure running E--W. The domes have a diameter of between 15 and 30 km. The contours for the largest mass deficiency from the gravity investigation agree with the axis binding the granite domes together, which indicates an underground connection between the individual domes (Bott et al., 1967 ).

Rock density In the description to the bedrock maps of the Stockholm--Uppsala region, St~lh~s (1969, 1972} presented a large number of planimetric analyses to illustrate the mineralogical composition of different rocks. The analyses show the percentage volume of the main minerals, quartz, potash feldspar, plagioclase (giving the An in %), biotite and hornblende. In addition, they give the incidence or c o n t e n t of a dozen other minerals. With knowledge of the density of the

205 minerals, and the variation of the density in plagioclase in cases of changed An content, the density of the rocks were determined for 353 published analyses. The different rock types were then combined in groups and classes. The values are presented on the geological map (Fig.5) and in some parts for the gravity map of Uppland (Fig.6), The densities, calculated on the basis of planimetric analysis, show close agreement with values determined by weighing the rock in air and in water and also with the 675 density determinations on rocks from the Uppsala region published by Henkel (1970).

Interpretation of the gravity anomaly at the Vallentuna dome The granite dome at Vallentuna is the more southerly of the serorogenic twil domes in the border zone between the Vdala--Uppland and the Roslag batholit The geological position (Fig.5) together with the Bouguer anomaly (Fig.6) suggest that the Vallentuna dome belongs to the rid:,~e of serorogenic granite which surrounds the Vdala--Uppland batholith, from the top of which the granite continued to work its way up in the form of separate domes. The dome has a breadth of 18 km, measured from NW to SE. Its northeaster parts belong to the ridge between the twin domes, and its length along the long axis of the ridge may be estimated at 25 km. The contact of the dome with the surrounding rock is conformable and steeply dipping except for the northeastel parts, where the strong pull between the twin domes has created cylindrical fob structures with subhorizontal fold axes. The observed Bouguer anomaly with its clearly defined, closed structure, ant the outcrops of the granite in the dome, are shown in Fig.7. The interpretation of gravity anomalies was carried out by an automatic, thr, dimensional model adjustment method, developed by Dyrelius and Bina (19731 The model is made up of rectangular prisms with know densities. The depth of the prisms represents the parameters sought, and the depth is iteratively adapte in the computer until the gravity effect in the entire model is associated with the observed anomaly. Density determinations along the profile gave a mean value of 2.63 g/cm 3 for the granite and 2.68 g/cm 3 for the surrounding bedrock which corresponds to a density contrast of 0.05 g/cm 3. The final model, established after successive tests with different distributions of the prisms and different depths to their lower surface consists of four prisms The Early Svecofennian granitoids are assumed to have been shallow where the me gives way to the ridge, so that they were replaced by material which corresponds to serorogenic granite. The gravity effect from the model is closely related to the observed anomaly, both on the level and along the profile, (Fig.8), A cross-section through the model shows that the central part of the dome (prism no.3) reaches a depth of 18 km. Two-dimensional model adjustments for the rapakivi granites in the Viipuri and Laitila massifs in southern Finland gave a depth of 18--20 km (Lauren, 1970). This result, like the model adjustment for the Vallentuna dome, suggests that the granites emanate from a granitic layer, and that this layer may coincide with the position of the Conrad dis-

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c o n t i n u ity , i.e., the b o u n d a r y between the granitic and basic layers in the earth' crust (Seguin, 1972). The structure o f the granite d o m e has been studied. As a rule foliation and lineation are most distinct in the contacts with the surrounding rock, but these structures, although very indistinct, can also be traced in the central parts of the dome. The structures which are m ar ke d on the bedrock maps (St&lhSs, 1969, 1972), together with new field observations, strongly support gravitational form a t i o n o f the dome. The m o r e n o r t h e r l y of the two arms branching out from tt d o m e to the NE has a cylindrical structure with subhorizontal foliation in the middle, which gradually becomes steeper toward the flanks. These structural features, to g eth er with a strong vertical lineation, can only be explained if the granite t o o k up its position by means of a vertical, plastic flow. The driving force of this flow was the density contrast between the serorogenic granite and the surrounding Early Svecofennian granitoids and metasediments.

208 DISCUSSION The geological situation of several of the batholiths in Central Sweden suggests a mechanism of formation which may be described as polydiapirism, or diapirism within diapirs. This means that litho- and chronostratigraphically younger granite diapirs have forced their way up through diapirically formed granitoid rocks with intervening septa of metavolcanics and metasediments. The driving force of the diapirism is the density difference between the intruding body and the surrounding rock. Such a mechanism of formation is supported by the picture of gravity variation displayed by the Vdala--Uppland batholith, and the gravity anomalies around the serorogenic granite ridges and granite domes in, for example, the area between the Vdala--Uppland and the Roslag batholiths. The sequence in Fig.9 A--E shows one possible application of polydiapirism in the formation of the bedrock in the southern parts of the Svecofennian of Central Sweden. Diapirism can only begin as a result of a gravitational instability, i.e., a light layer of rock must be overlain by a heavier layer. There are several ways of creating such an instable configuration. In the model chosen here, sediments and volcanics are intruded by basic rocks, diabases and basalts, Fig.9 A The model was introduced by Ramberg {1945) to generate a gravitational instability in connection with the origin of folded mountains. On their way up to the sediments and the volvanics, the basic rocks pierce a layer of granitoid rocks, the composition of which presumably corresponds to quartz-diorite--granodiorite. The composition of the layer is thought to be retained, on the whole, t h r o u g h o u t the Svecofennian period; its dykes and feeder channels of basic rocks are nowadays to be found as xenoliths. The quartz-diorite of Uppsata t y p e (I) is an example of such a xenolith-bearing rock. The supply of basic material to the sediment and volcanics induces the gravitational instability which fosters the diapirs of the granitoid rock layer, which, in the form of ridges of domes, pierce the surface. The diapirism is followed by metamorphism and folding, together with deformation of the basic rocks, Fig.9 B.

Fig.9. Polydiapirism as a basic mechanism in the evolution of the Svecofennian orogeny in Central Sweden. A, gravity instability by intrusion of basic rocks into sediments and volcanics. B, diapirism of quartz-diorite (I) and granodiorite (H). C, gravity instability of a granitic layer of mantle origin; two different wavelengths are formed. D, diapirism of granodiorite (III) and granite (IV) and a regional metamorphism in the synclinorium between the batholiths. E, diapirism of serorogenic granite (V) along the border of the batholiths. 1, sediments and volcanics; 2, basic intrusions; 3, synorogenic quartz-diorite (I) and granodiorite (H); 4, metasediments and metavolcanics; 5, synorogenic granodiorite (III) and granite (IV); 6, serorogenic granite (V); 7, pegmatite and veined gneiss.

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The difficulties in explaining the sequence of intrusions of different types of granite in the same pluton or batholith were discussed earlier. In most granite provinces the youngest and most acid (lightest) variants are found in the middle of the massif. This is one of the reasons for trying to explain the incidence of granitic layers in the lower parts of the earth crust as formed of material from the upper mantle. Such a theory has also been applied to the model of the Svecofennian to explain the intermediary and acid variants of the synorogenic intrusions (cf. the lowest layer in Fig.9 C). Theoretical investigations of gravitational instability have shown that the wavelength of an instable layer is controlled by the thickness, density and viscosity of the layer. An application of the theory that the mantle consists of different layers has indicated the possibilities of producing instabilities with different wavelengths, i.e., a light granitic layer may form ridges or domes with a wavelength of hundreds of kilometres, on top of which lies, in turn, an instability with a wavelength of tens of kilometres (Ramberg, 1973). Such a situation is assumed to have existed in the Svecofennian. An instability with a wavelength of approx. 100 km is thought to have given rise to the tectonic division into batholiths and subprovinces, where the anticlinal parts are those which today constitute the batholiths, e.g. the Vdala--Uppland and the H~ilsinge batholiths, while the synclinal parts belong to the subprovinces, e.g. the Dala--G/istrike subprovince (cf. Fig.3). This explanation of the division into batholiths and subprovinces may also be applied to the parts of the Svecofennian which lie outside the area discussed in this paper. This phase in the development corresponds to synorogenic diapirism during the Early Svecofennian (1,950--1,850 million years The intrusion from the granitic layer corresponds to the porphyric granodiorites of Arn~ type (III) and the granites of V~inge type (IV). The diapirism throu~ the quartz-dioritic layers has led to a further compression and deformation of the metasediments and metavolcanics. The m o d e of intrusion may best be referred to as meso- and/or cata-tectonic diapirism in the proposed division. Field investigations have indicated the occurrence of intra-orogenic dykes of metabasites belonging to the Middle Svecofennian. These dykes, together with the introductory regional metamorphism and migmatization of the synclinal region between the batholiths, are shown in Fig.9 D. In this model of the Svecofennian, the postorogenic granites are considered to derive from palingenesis and anatexis of deformed metasediments and metavolcanics. The granitization processes are assumed to lead to a homogenization and the formation of a granitic layer with a lower density than the original rocks. A gravitational instability again exists which allows diapirism of the serorogenic granites. The distribution and the position of the granites suggests that with a few exceptions these intruded into the boundary zones between the batholiths and the subprovinces. The model experiments have shown that diapirism is initiated by irregularities and discontinuities (Ramberg, 1967, Stephansson, 1972). In the area of synorogenic diapirs there are differences in b o t h density and competence which could initiate the serorogenic granite diapirism. This emerges most distinctly in the circle of domes which surrounds the Vdala--Upp-

211 land batholith. The intrusion of the granites has further emphasized the deformation in .the metasediment, the metavolcanics and the basic variants of the serorogenic intrusives. This influence is re-echoed in the central parts of the batholiths. The effect of the granite intrusions is also clear in bodies which belong to the porphyric granodiorites and granites of Arn6 and V~inge types (III and IV). This course of events, like the occurence of pegmatite in association with the serorogenic diapirs, is illustrated in Fig.9 E, which also shows the present erosion level and the limits of the Malar subprovince and the batholiths north and south of it. According to the litho- and chrono-stratigraphic scheme of the Svecofennian (Table II), the serorogenic intrusions were followed by an early postorogenic diapirism with granite of the Sm~land--V~irmland type. Their origin is difficult to interpret, although there is much to suggest a close affinity to the serorogenic granites. According to Welin (1970), the Svecofennian ended with a period of volcanics and deposition together with a late postorogenic diapirism. This period is exclusively confined to the Dala--H~dal province in Central Sweden. An a t t e m p t to fit the granite intrusions of Dala type, via the volcanics and sediments of this type, into the suggested course of events would best correspond to a somewhat more advanced stage of diapirism than that shown in Fig.9 B, although with the difference that the intruding layer here has a granitic composition (Dala type). A polydiapirism of the type here applied to the Svecofennian of Central Sweden involves a dominance of subvertical structures in the deep erosion level which may be studied. Comprehensive field investigations within the Vdala-Uppland batholith have confirmed this: moreover it has emerged that subvertical lineation is usually more clearly formed and of more frequent occurrence than foliation, which provides further support for diapirism as an intrusion mecll anism. The investigations also show the usual condition with severe deformation~ in the contacts of the intrusions and moderate structure with massive rocks in the central parts. According to classic granite tectonics, a mineral orientation in the granite which is parallel to the contact is interpreted as evidence that the mineral is oriented with respect to some form of strain: moreover, since the mineral is rarely affected by external deformations it is t h o u g h t to have been included in a liquid (magma), which completed its crystallization after the m o v e m e n t ceased. This model can hardly be applied to the development within the Svecofennian of Central Sweden. The rock distribution, the size, shape and contact condition of the intrusions together with the deformation of the granitoid rocks at the polydiapirism point to moderate contrasts in the competence (viscosity) of the rocks and suggest that the granitic material, which has been mobile, has been deformed in a solid state. Model experiments by Ramberg (1967, 1970) and Dixon (1974) clearly show that the intruding body and the surrounding rock can only display moderate difference in viscosity in order to reach realistic dimensions of intrusions, and that intrusion by diapirism of solid, plastic material in an equiva.

212 lent e n v i r o n m e n t creates s t r u c t u r e s which d i r e c t l y r e s e m b l e those we find in granitoid b e d r o c k . T h e t h e o r y t h a t the intrusion o f granitic material o c c u r r e d w h e n the r o c k was c o m p o s e d o f solid c o m p o n e n t s has also b e e n p r o p o s e d b y structural geologists, g e o c h e m i s t s , a n d petrologists, such as M e h n e r t ( 1 9 6 8 ) and Pitcher and Berger {1972). T h e r e are at p r e s e n t n o results f r o m e x p e r i m e n t a l r o c k d e f o r m a t i o n s which c o u l d illustrate plastic d e f o r m a t i o n s and c o n s e q u e n t mineral o r i e n t a t i o n in granitoid rocks. T h e reason is n o t t h a t granitoid r o c k s are invulnerable to plastic d e f o r m a t i o n b u t t h a t we have n o t y e t achieved the technical skill necessary to carry o u t this t y p e o f e x p e r i m e n t . ACKNOWLEDGEMENTS This w o r k was initiated b y managers for the p r o j e c t G e o t r a v e r s e t h r o u g h Bergslagen and has b e e n carried o u t at the I n s t i t u t e o f G e o l o g y , University o f Uppsala, h e a d e d b y Prof. H. Ramberg. The writer wishes to t h a n k Prof. E. T e n g s t r S m for permission t o use t h e graw ity data for the U p p s a l a - - S t o c k h o l m region. S. J o n z o n and C. E l l w y n m e a s u r e d relative gravity and d e t e r m i n e d r o c k d e n s i t y in t h e V a l l e n t u n a region. Dr. D. Dyrelius carried o u t a m o d e l i n t e r p r e t a t i o n o f the gravity anomalies a r o u n d t h e V a l l e n t u n a d o m e . Prof. W. S c h w e r d t n e r p a r t i c i p a t e d in the geological field reconnaissance in the Uppsala region and g e n e r o u s l y c o m m u n i c a t e d his experiences f r o m investigations o f the Canadian shield. Valuable discussions with Prof, H. Ramberg, Prof. R. G o r b a t s c h e v and Drs. T. EkstrSm, A. Lind, K. R S s h o f f and A. WikstrOm are gratefully a c k n o w l e d g e d . This research was s p o n s o r e d b y the Swedish B o a r d for T e c h n i c a l Developm e n t t h r o u g h G r a n t 7 1 - - 3 2 2 / U 2 3 7 and 7 3 3 2 1 2 .

REFERENCES

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