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Basic Intrusion, Charnockite.Rapakivi Granite Plutonism Aod Crusta! Depletion, S

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Basic Intrusion, Charnockite.Rapakivi Granite Plutonism Aod Crusta! Depletion, S RENolCONTI DELL,\ SOCIETA lTAUANA 01 MINERALOG IA E PETROLOGIA, 1988, \bI. 4}.2, pp. '4).,,4 Basic intrusion, charnockite.rapakivi granite plutonism aod crusta! depletion, S. W. Sweden FREDERIK H. HUBBARD Depanment of Geolosy, Dundee University, Dundee, Scotland AUSTRACT. - The southern segment of the South-West South-West Swedish Province by a series of Swedish Province (SWSP) of the Precambrian of the major thrust zones, it has characteristics Baltic Shield has features which distinguish it from the northern sesment of the same province and the peculiar to this region pf the Swedish Svecofennian sequences to the east. Repeated cycles of Precambrian. These include an abundance of syn and late·orogenic b..sic magma injection maimained meta-basic intrusions, the occurrence of high crusta! temperatures and led to progressive granulite fades assemblages within the general depletion of the rode! at, and adjacent to, the conduits. This is shown by the development of loc:::aI ~u1ile facies upper amphibolite facies terrain and zones in the seneral amphibolite facies terrain and the distinctive plutonic associations of alkaline occurrence of plutonic charnockite Jrulsses. granite and charnockite (e.g. QUENSEL P., The syn-orogenic: basic magmas caused depletion largely 1951; CALDENruS et al., 1966, MOHREN E., by the generation of hydrous solutions derived from dehydration of mafic minerals in the rountry rocks LARSSON W., 1968, HUBBARD F.H., 1975, ronsequent on the heat influx associated with the 1978). A Rb-Srisochron age of 1420Ma was intrusion. These hydrothermlll solutions metasomatised obtained by WELIN and GORBATSCHEV (1978) and migmatised the basic rocks, formed ap!ite from a composite charnock.ite and granite segregatioO$, or were exhausted to higher level. pIu ton (the Charnockite-Granite Association The late orogenic basic magmalism, which was associated with a shearing deformation, led to the of the Varberg region, HUSBARD F.H., 1975). development of granitic plutons which segregated during This piu ton was affected by post­ I15CCnt to leave residual-testite charnockile and released emplacement, hot shearing and thrusting and REE and RE-enriched alkali granites with. rapakivi the date may reflect isotope resetting during granite signature. While a COl volatile phase is indicated as importam this tectonic activity rather than the age of in the llte·tectonic episode, it is not evident as a faclor the intrusion. A similar age is, however, found in the earlier syntectonic crustal depletion. for amphibolites and granitoids in the Key words: Basic magma, depletion, dJarnockile, sranite. northern segment of the South-West Swedish Province (e.g. SKIOLD T. , 1976; DALY].S., PARK R.G., CUFF R.A. , 1979; SAMUELSSON L. , AHALL K.-L, 1985). The add plutonic rocks were emplaced in metamorphic Introduction sequences which show evidence of a long and The southern segment of the South West complex earlier development history. The Swedish Province (Fig. 1) occupies a impression gained is one of an old sequence somewhat enigmatic position in the hierarchy of crustal rocks which has undergone of development of the Precambrian rock extensive, and perhaps repeated, reworking. sequences of the Baltic Shield. Separated from Unfortunately no firm time scale has been the Svecofennian Province to the east by a established for the period preceding the late, major mylonite zone and its associated hot deformation and recrystallisation. granites and from the northern segment of the However, the rocks of south-west Sweden are ~44 HUBBARD EH . clearly old and may represent an Early DALY j.S. et al., 1983), they lack similar Proterozoic, or even Archaean, province with prominence in the southern segmenl. a history of Proterozoic reworking. The differences between the rock associations of General geology of the Varberg Region the South-West Swedish Province and those of the Svecofennian Province to the east may The best exposure of the rock sequences merely be a consequence of the exposure of of the southern segmenl of the South-West differing crustallevels but the possibility that Swedish Province is to be found around the the South-West Swedish Province is exotic town of Varberg, on the Kattegat coast south cannot be ignored. The rocks of south-west of Gothenburg (Fig. 1), and contiguous with Sweden most closely resemble those of the the limiting northen thrust zone. The geology Pee-Caledonian rocks of southern Norway is dominated by an intrusive plutonic complex (e.g. TORSKE T., 1985), which have a strong which comprises both charnockitic and non­ Sveconorwegian (GrenvilIian) signature charnockitic granitoid components in intimate thought, to some extent, to mark overprinting association - the Varberg Charnockite­ of a younger age on older rocks (PRlEM Granite Association (CGA) - (HUBBARD RN.A. et aI., 1973; VERSTEEVE A., 1975; F.H ., 197.5). The country-rocks to this TORSKE T., 1977; SMALLEY p,e. et al., 1983). association are, in large pare, upper While c. 1000Ma ages are fairly common in am phibolite facies grade granitic para-gneisses the northern segment of the South-West which have locally developed charnockitic Swedish Province (e.g. SAMUfJ..SSON 1., 1980; characters (HUBBARD F.H., 1978). Horizons I' Imet.basit, Okm Fig. 1. - Location map (S.A ... study area; G .. Gothc:nburg; S. Stockholm; 0 .. Oslo; SWSP(S)·South-We$\ Swedish Province (southern segment); SWSP(N) .. South-We$t Swedish Province (northern segment); SOA .. Svcwfennian Orogenic A15ociation; cross panelD" Protogene woe granite$; dot panern .. Caledonide$) and geological sketch map of the Varberg area (black .. mela·basic rocks; VC, AGe and TC .. charnockite and sub-charDOCkite components and TG . the mator granite component of the Oumockite-Granite Association, CGA). BASIC INTRUSlON, CHARNOCKJTE..RAPAKlVI GRANITE PWIONISM AND CRUSTAL DEPLETION ETC. 5 4 .5 of more varied lithology, which include meta· occurring as enclaves, xenoliths and intrusive basic layers, occur within these rather sheets in the charnockites and as sub­ monotonous granitic gneisses. It is in these conformable sheets in the country rocks. compositionally-layered horizons that the Several generations of basic intrusion are structural complexity of the country rock indicated (Figs. 2, 3), which are all now Fig. 2. - Folded metabuites cut by younger metabasile in depleted (charnockitic) granite gneiss. Triislovsliige. Fig. }. - Agmatite of hornblendite blocks in scapolite·metabaggro. N. Honen. ~nciss is most clearly expressed. Metabasic represented by metamorphic derivatives. They rocks are a feature 01 both the plutonic may be grossly divided into those which complex and the surrounding country rocks; predate the granitoid plutonic activity and 546 HU88ARO F.H. The Period of Granitoid Plutonic Activity Although rather severely disrupted by post­ emplacement hot shearing and thrusting the general form of the charnockite and granite complex of the Varherg region can be reconstructed with reasonable confidence due to the excellence of the outcropping along this coastal region (Fig. 4). The composite intrusion was emplaced during a period of shearing and shear folding, synchronous with a further basic magma influx into country rocks which already had well-established major structural and metamorphic patterns. The form of the pluton follows the general structural grain of the country rocks with the narrow, deeper, neck lying in a structural steep zone, which was concurrently being further accentuated by shearing. The lateral spread Fig. 4. - Idealised 5e(:tion of the Chamockitc:·Granite of the pluton at higher levels coincides with AS$odation. Fine dots .. charnockite cumulate (VC of a similar change in country rock structure. A Fig. I); coarse 001$ .. the active fractionatins zone (AGe of Fig. 1); heavy c;ros5eS" charnockitc and sub­ parallel distribution can be observed in the charnockite raiduum (Te of Fig. 1); fWit crosses .. the products of the accompanying basic activity. evolved granite (r G of Fig_ 1). This discontinuity of structural grain continued to play a fundamental role during those sub-sYhchronous with the emplacement the subsequent major thrusting which of the granitoids. The early group of meta­ established the northern limit of the southern basic rocks are amphibolites and garnet­ segment of the South-West Swedish Province. amphibolites and afe commonly linked with migmatisation. The younger group rocks, on The Charnockite-Granite Plutonic Activity the other hand, characteristically occur as garnet-pyribolite, although there are The plutonic granitoid association of the associated local developments of coarse garnet­ Varberg region has less well , and less amphibolite. completely, exposed counterparts throughout the province. In some instances, the granite has become detached from its charnockitic associates, in others, there is arrested The Pre-CGA Meta-basic Rocks development, as displayed at Varherg (the The earliest recognisable basic magmatism Charnockite-Granite Association or CGA of is that associated with the lithologically­ HUBBARD F. H ., 1975), where the granite­ layered gneiss/granulites within the granitic charnockite link remains intact, thus revealing gneisses, represented by tight, strongly the mechanism of formation. flattened, fold remnants. These may ~prese:nt The massive charnockite phases of these a volcanic component of the early supracrustal plutons are intcpreted as «cumulate-restites» system of the the gneiss complex. The from the fractionation of anatectic melt­ following
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