The tectonothermal evolution of the Kirwanveggen—H.U. Sverdrupfjella areas, Dronning Maud Land, Antarctica

Precambrian Research Precambrian Research 75 ( 1995) 209-229

The tectonothermal evolution of the Kirwanveggen-H.U. Sverdrupfjella areas, Dronning Maud Land, Antarctica G.H. Grantham ‘, C. Jackson b, A.B. Moyes ‘, P.B. Groenewald b, P.D. Harris ‘, G. Ferrar b, J.R. Krynauw b ’ Department of Geology, University of Pretoria, Pretoria 0002, South Africa h Department of Geology, University of Natal, P.O. Box 375, Pietennaritzburg 3200, South Africa ’ Bernard Price Institute, University ofthe Witwatersrand. P/Bag 3, P.O. Wits 2050, South Africa

Received 18January 1994; revised version accepted 18 September 1994

Abstract Three episodes of deformation spanning a period of N 900 Ma are recognised. The first deformation at - 1000 Ma involved progressive, yet distinct stages within a protracted, composite event involving recumbent folding, low-angle thrust faulting and locally, highly oblique, transpressive strike-slip shearing. These structures suggest tectonic transport from the south and southeast during D,. D2 at - 500 Ma is variable in the different areas and involved thrust faults and folding with locally developed axial-planar foliations. The orientations of the fault planes and axial-planar structures suggest transport from the west and northwest in western Sverdrupfjella and from the southeast in Kirwanveggen. D3 involved normal faulting and jointing, adjacent and parallel to the Jutulstraumen Glacier in the west. The joints affect the alkaline complexes, some of which are Jurassic in age and consequently D3 is related to the breakup of Gondwana. Four phases of metamorphism, related to the deformation, are recognised. The dominant mineral assemblages M,_, are typical of medium- to high-grade metamorphism. Discordant mafic intrusions provide evidence of a long history of metamorphism. Mafic nodules from the eastern part of the area contain high-pressure assemblages representing M,. Assemblages and textures indicate that MI was related to isothermal decompression related to the uplift caused by thrust faulting during D,. The period between MI and M, appears to have involved annealing at mid-crustal levels. M3 mineral development is dominated by biotite which is oriented axial-planar to D, folds. M4 assemblages are typically low-grade and involve hydrothermal alteration resulting in chloritisation and sausseritisation related to D3.

1. Introduction

This paper summarises our understanding of the tectonothermal evolution of the H.U. Sverdrupfjella (hereafter called Sverdrupfjella) and Kirwanveggen areas (SKA) of western Dronning Maud Land (Fig. 1) New data are included with some of the conclusions being based on data published in Grantham et al. ( 1988)) Groenewald et al. ( 199 1) , Groenewald and 0301.9268/95/$09.50

0 1995 Elsevier Science B.V. All rights reserved

SSDlO301-9268(95)00031-3

Hunter ( 1991)) Grantham and Hunter ( 1991)) Grantham ( 1992) and Moyes et al. ( 1993a, b). A companion paper in this issue (Groenewald et al. 1995) concentrates on the tectonic setting and a model for the area. 2. Stratigraphic

framework

Grantham et al. (1988) suggested a revised stratigraphic framework for the SKA, which incorporated

210 Table 1 Lithologies

G.H. Grantham et al. /Precambrian

in the H.U. Sverdrupfjella

Research 75 (1995) 209-229

and Kirwanveggen

Western H.U. Sverdrupfjella

Eastern H.U. Sverdrupfjella

Central Kirwanveggen

Northern Kirwanveggen

Fuglefellet Formation: Carbonates, quartzofeldspathic and talc-silicate gneisses and subordinate malic rocks.

Rootshorga Formation: Almandine-bearing gneisses, locally containing sillimanite ( + kyanite in the south). Pelitic to semipelitic, with subordinate quartzofeldspathic rocks and boudinaged mafic rocks.

MjGllfdyke Banded Gneiss: QtzFspBt-)Grt)-( Hbl) gneiss with fine compositional banding. Lenses of talc-silicate and marble present locally.

Quartzofeldspathic gneiss: Qtz-FspBt-( Grt)(Hbl) gneiss with fine compositional banding. Lenses of talc-silicate and marble present locally.

Gre.v Gneiss Complex of the Jutulriira Formation: Epidotebiotite-hornblende tonalitic quartzofeldspathic gneiss with subordinate biotite-bearing granitic gneiss. Banded Gneiss Complex of the Jutulriira Formation: Interlayered mafic and quartzofeldspathic gneisses with subordinate Mg-rich schists and talc-silicate gneisses. Syn- to post-tectonic units: Include leucocratic orthogneiss, Grt amphibolite, metadiorite, amphibolite dykes, granitic veins and sheets, Jurassic dolerite and syenitic intrusions.

SW- to post-tectonic unifs: Include leucocratic orthogneiss, Qtz-Fsp-Bt-( Grt)-( Ky) lenticular granite, Grt amphibolite, metagabbro and -diorite, amphibolite dykes, granitic veins and sheets and Jurassic dolerite dykes.

and expanded the proposals of Roots ( 1953, 1969), Hjelle ( 1972) and Wolmarans and Kent ( 1982). We make use here of the stratigraphic nomenclature used by these authors, namely divisions into groups and formations. However, subdivision into lithodemic units based on the recommendations by Ricci et al. (1993) is in progress. The lithologies and stratigraphy of the different areas discussed are summarised in Table 1 with the different areas being defined in Fig. 1. The geology of the Kirwanveggen has been described by Wolmarans and Kent ( 1982) who did not further sub-

Tverregga Banded Gneiss: HblBt-PI gneisses interlayered with Hbl amphibolite and Qtz-Fsp-Bt ( f Hbl) granitic gneisses; locally Qtz-sericite schists. Migmatitic veining, granite and pegmatite dyke intrusion. Retrogressed Di-Ep-PI calcsilicate gneiss, Chl-phyllite, ActEp-Chl-Cc schist and Tr-Serp schist locally. Syn- to post-tectonic units: Kvemelnatten Granite Gneiss: Composite intrusive, wellfoliated Hbl-Bt granite gneiss with subordinate Hbl f Bt amphibolite gneiss. Kirwanveggen Megacrystic Orthogneiss: Concordant, tabular bodies, mainly of augen-textured Qtz-FskBt f Grt f Hbl granite gneiss; local chamockitic remnants. Wide compositional variation. Include leucocratic orthogneiss, Qtz-Fsp-Bt-(Grt)(Ky) lenticular granite, wellfoliated, tabular bodies of orange-weathering, granite leucogneiss, amphibolite, amphibolite dykes, granitic veins and Jurassic dolerite dykes.

Amphibolite gneiss: Hbl-Bt gneiss with minor, interlayered Qtz-Fsp gneiss. Biotite garnet migmatite gneiss: Well-foliated banded Bt-GrtHbl-Fsp gneiss. Minor Grt amphibolite, KY-bearing granite. Amphibolite boudins occur throughout the above units. Syn- to post-tectonic units: Kirwanveggen Megactystic Orthogneiss: As for Central Kirwanveggen. Include leucocratic orthogneiss, Qtz-Fsp-Bt-(Grt-(Ky) lenticular granite, Grt amphibolite, metagabbro and -diorite, amphibolite dykes, granitic veins and Jurassic dolerite dykes.

divide the Sverdrupfjella Group stratigraphically. No direct correlation between the two areas, other than noting similarities within some lithodemic units, has been attempted at this stage. Aucamp et al. (1972) and Wolmarans and Kent ( 1982) have described a succession of quartzites and conglomerates, termed the Urfjell Group, in the southwestern Kirwanveggen, which is in tectonic contact with the metamorphic rocks of the Sverdrupfjella Group. Near-vertical dips are common, the bedding is overturned in places and the grade of metamorphism is

G. H. Grantharn et al. /Precambrian

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Research 75 (I 995) 209-229

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1.Location

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212

G.H. Grantham et al. /Precambrian Research 7.5 (1995) 209-229

low. Wolmarans and Kent ( 1982) have correlated the Urfjell Group with the Cambro-Ordovician Blaicklock Glacier Group of the Shackleton Range and the Neptune Group of the Pensacolla Mountains, whereas Roots ( 1969) included these rocks with his ‘Ahlmannrygg Group’, i.e. as part of the Ritscherflya Supergroup. The age of these rocks is therefore uncertain, but the age of the Urfjell Group is critical to an understanding of the evolution of western Dronning Maud Land (see later). The only certainty is that they are older than the Mesozoic rocks of the southwestern Kirwanveggen, which overlie them unconformably.

independently in different parts of Sverdrupfjella and Kirwanveggen concur favourably for the early part of the deformation history and differ significantly only with regard to the nature and timing of late deformation. Furthermore geometrical analyses of regionally penetrative fabric elements and principal fold structures demonstrate that fabric geometries are extremely simple and remarkably similar in most areas (Figs. 3 and 4). A highly consistent kinematic framework must have been operative during much of the tectonic evolution of the belt to maintain such primitive fabric patterns. 3.2. Geometrical analysis

3. Structural

history

3.1. Overview

Recent structural investigations in Kirwanveggen and Sverdrupfjella in western Dronning Maud Land have shown that these geographically discrete portions of the Meso-Neoproterozoic mobile belt of East Antarctica share a common deformation history dominated by two major erogenic episodes at - 1000 Ma and - 500 Ma. Although structural correlations between the two areas are hindered by insufficient geochronological control, comparison of the structural data from both terrains reveals a broadly coherent deformation sequence with local variation reflecting differences in the style and intensity of shared deformation episodes. Groups of kinematically equivalent structures are recognised in both areas and these demonstrate tectonic continuity across the intervening splay of the Jutulstraumen glacier (Fig. 1) . Discrimination of structures and fabrics associated with the two erogenic periods is complicated by their near-identical kinematic expression and it remains unclear to what extent early fabrics have been reworked during subsequent deformation. A summary of the available structural data for Sverdrupfjella and Kirwanveggen (Fig. 2) accommodates local deformation schemes within a single regional framework. Since detailed structural investigations in Kirwanveggen are still at an early stage and no studies of this nature have yet been attempted in Sverdrupfjella, regional correlations have been made using limited structural, kinematic and geochronological data and must be regarded as preliminary. The structural-intrusive sequences recognised by researchers working

To facilitate integration of structural data with the local deformation schemes presented in Fig. 2, geometrical analyses were performed in five domains which broadly correspond with the field areas mapped by different researchers. These are western Sverdrupfjella (W HUS), northeastern Sverdrupfjella (NE HUS), southeastern Sverdrupfjella (SE HUS) , northern Kirwanveggen (N KIRW) and central Kirwanveggen (C KIRW). Poles to composite regional gneissic foliations define girdle distributions consistent with folding about NWSE axes on stereographic projections (Figs. 3 and 4). Average foliation planes generally dip moderately towards the southeast. Statistical fold axes (sfa), calculated using foliation data, correspond closely with the hemispherical vector means (vm) derived from measured fold axes. Most significantly, pronounced collinearity is observed between mineral elongation lineations and fold axes. Mineral elongation lineations usually define cluster-like distributions with limited spread within the mean foliation plane. Measured fold axes define girdle distributions within the plane of flattening, but exhibit point-maxima which coincide closely with those obtained for the elongation lineations. This geometry may be a function of primary noncylindricity related to rotation by shearing of newly nucleated fold axes towards a SE-NW-plunging elongation direction. The uniformity of fabric geometries is particularly striking in the NE HUS, SE HUS and C KIRW domains where collinearity on a SE-plunging (127/30) lineation direction is observed. Deviations from this prevalent geometry are observed in the W HUS where patterns are much more weakly developed

213

G.H. Gruntham et (11./ Precumbrian Research 75 (1995) 209-229

SE HUS Dl (GG) lsocl~nal lotdIng Fabric development

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Fig. 2. Structural summary for the H.U. Sverdrupfjella and Kirwanveggen accommodating various structural schemes within a single regional framework. The structural notation used throughout this paper is that indicated in the shaded boxes on the left side of the table. Abbreviations of study areas: W HU.S= western H.U. Sverdrupfjella; NE HUS= northeastern H.U. Sverdrupfjella; SE HUS= southeastern H.U. Sverdrupfjella; N KIRW = northern Kirwanveggen; schemes in each area: (CC)

C KIRW= central Kirwanveggen. Subscripted initials in brackets indicate the sources of the local structural

= Grantham, 1992; (BG) =GroenewaJd,

PhD thesis, in prep.; (K) = Krynauw, unpubl. data; (H) = Harris et al.,

in prep.; (J) = Jackson et al., in prep. Vertical lines join progressive deformation events. Ages are only approximate and are derived from geochronoiogical data for various structurally well-constrained intrusions.

G.H. Grantharn et al. /Precambrian

214

Poles

togn.

fed.

Research 75 (1995) 209-229

Elong.

lin.

TVOlZl 0

Poles

togn.fd.

Elong.

lin.

Fold axes

Fig. 3. Structural map for the H.U. Sverdrupfjella indicating the position of D, thrust zones and the dominant orientation of L, mineral elongation lineations within each nunatak group, The geometrical relationships between regional composite gneissic foliations (pfol.), mineral elongation lineations (elong. /in.) and principal fold axes in each area are illustrated stereographically. All stereoplots are equal-area, lower-hemisphere projections. Domain boundaries are shown schematically with dashed lines. Domains correspond with the areas indicated in Fig. 2: W HIT/S= western H.U. Sverdrupfjella; NE HUS= northeastern H.U. Sverdrupfjella; SE HUS= southeastern H.U. Sverdrupfjella.

DYE.:-.

Svaibandufsa

Poles

to gn. fol.

Post-D, pm-D, dykes . poles to dykes (n = 40, o poles to dyke fol. (n = 25) x dyke elong. lin. (n = 22)

I

3.00’W

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Fold axes

Poles to gn. fol.

,

-“-,-

--

,,~,,

-

Fig. 4. Structural map for the Kirwanveggen indicating the position of composite thrust zones and the dominant orientation of L, mineral elongation lineations within each nunatak group. The geometrical relationships between regional composite gneissic foliations (gn.fol.), mineral elongation lineations (elong. lin.) and principal fold axes in each area are illustrated stereographically. The inset block of stereoplots presents Dz fabric elements from the Tverregga-Tverreggtelen area and demonstrates reworking of post-D,-pre-DZ mafic and felsic dykes by D2. All stereoplots are equal-area, lower-hemisphere projections. Domain boundaries are shown schematically with dashed lines. Domains correspond with the areas indicated in Fig. 2: N KIRW= northern Kitwanveggen; C KIRW= central Kirwanveggen.

Skappehabben

73’20’s

zones

/

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L, elong. lin.

Poles to s, fOl.

216

G.H. Grantham et al. /Precambrian

and occur on more easterly trends, and in the N KIRW where collinearity along a shallow NNW-plunging lineation direction is observed. The variation in basic fabric geometries in the latter two domains is mirrored by differences in structural style, lithology, metamorphic grade and aeromagnetic signature. The structural patterns and fabric geometries described above were imparted during intense deformation and high-grade metamorphism that occurred between - 1100 Ma and - 1000 Ma. Although most workers in the area have recognised multiple highgrade deformation events using classic fold-fabric transposition criteria (Fig. 2), these events may be grouped with confidence since they are kinematically indistinguishable and simply represent different stages within a single protracted, progressive deformation episode (D,). 3.3. D, episode The D, event represents the earliest discernible deformation in Sverdrupfjella and Kirwanveggen and is characterised by intense fabric development and collinear refolding parallel to regionally developed, SEplunging mineral elongation lineations (see Figs. 3 and 4). Throughout the area and on all scales, refolding relationships are dominated by Ramsay type-3, convergent-divergent interference patterns attesting to continuous refolding on SE-trending axes. Locally, up to five superimposed fold generations may be recognised and F, fold sequence geometries show a typical progression from early intrafolial relics, through isoclinal folds to late upright folds. Although early macroscopic isoclinal folds are rarely encountered, local repetitions of subunit stratigraphy and regional subparallelism between lithological contacts and the regional gneissic foliation endorse the existence of extensive Fi nappes. D, penetrative gneissic fabrics are superficially simple, but the local occurrence of foliation truncations and numerous examples of multiply overprinting foliations containing identical mineral elongation lineations, indicate that regional foliations are the result of serial transposition (see Dirks and Wilson, 1995) and are likely to be composite in nature. Within eastern Sverdrupfjella and central Kirwanveggen numerous SE-dipping, D, high-strain zones

Research 75 (1995) 209-229

have been recognized. These zones are tens to hundreds of metres wide and are defined by high-temperature mylonites and straight gneisses exhibiting generally down-dip (SE-plunging) elongation lineations. The zones are laterally continuous and individual discontinuities may be traced along strike for up to 20 km. Deformation fabrics inside and outside of the zones differ only in terms of intensity. Independent shear sense criteria (a and 6 porphyroclast systems, S-C composite planar fabrics, shear band foliations, asymmetrical microfolds, offset of passive markers etc.) within the mylonites and straight gneisses indicate that D, high-strain zones represent ductile thrusts characterised by top-to-NW movement sense. Along the boundary between western and eastern Sverdrupfjella, D, thrusting resulted in a metamorphic inversion with the emplacement of granulites above amphibolite facies rocks (Grantham et al., 1988). Tectonic interleaving of lithologies and emplacement of tabular bodies of foliated megacrystic granites (Sveabreen Granites, Kirwanveggen Megacrystic Orthogneiss Complex; Fig. 2) occurred along D, thrusts in eastern Sverdrupfjella and central Kirwanveggen. D, thrusts have not been recognised in western Sverdrupfjella or northern Kirwanveggen. In the Neumayerskarvet-Armalsryggen area of northern Kirwanveggen (Fig. 4)) the regional gneissic foliation changes strike by almost 90” and becomes arched into a 4-km-wide zone characterised by NNWSSE-trending upright folds, steeply inclined planar fabrics and intense subhorizontal elongation lineations. Although shear sense criteria are equivocal, translation paths are similar to those in eastern Sverdrupfjella and central Kirwanveggen, with elongation lineations prescribing a SSE-NNW shear direction. Although the structural significance of this zone is not yet fully understood, it is clear that strike-slip movements occurred within a D,-compatible kinematic framework and it is possible that the zone may reflect a deep-seated lateral ramp to the thrusts observed in central Kirwanveggen and eastern Sverdrupfjella. The timing of Di deformation is poorly constrained by the isotopic ages of pre-D, gneisses and syn- and post-D, intrusive rocks. Isotopic work by Moyes and Barton ( 1990) on the pre-D, Grey Gneiss Complex of the Jutulrora Formation has yielded Rb-Sr whole-rock ages of 114 1 + 43 Ma and 1065 + 8 1 Ma. These data may be interpreted as either depositional or early meta-

G. H. Gmntham

et al. /Precambrian

morphic reset ages. Rb-Sr ages of 1161 k 98 Ma obtained from early granites (Fugitive Granites, Fig. 2; Moyes and Barton, 1990; Groenewald et al., 1995) are within error of the above dates, suggesting that high-temperatures in the period 1200-l 100 resulted in granite generation and extensive isotopic homogenisation. Regionally widespread megacrystic sheet granites (Sveabreen Granites/Kirwanveggen Megacrystic Orthogneiss Complex, Fig, 2) are intimately associated with D, thrust zones and these have been dated at 1028 k94 Ma (Rb-Sr) (Groenewald et al., 1995). Preliminary Rb-Sr dating of a pegmatite dyke which exploits a late-D, strike-slip shear zone in the Neumayerskarvet area, has yielded an age of - 1000 Ma (P.D. Harris, unpublished data). A mafic dyke at Roerkulten in western Sverdrupfjella has been dated at 789 + 90 Ma (Rb-Sr) and 85 l* 220 Ma (Sm-Nd; Moyes and Barton, 1990) and provides a minimum age for the D, episode. This dyke clearly truncates the early composite gneissic foliation, yet it bears a foliation which is refolded about NE-trending D, (F3(oGj, Fig. 2) fold axes. 3.4. D, episode The nature and extent of deformation during the second tectonothermal episode at - 500 Ma remains controversial. In the past, the 500 Ma episode has been largely dismissed as a purely thermal overprint causing widespread resetting of isotopic systems, with no significant deformation. Notwithstanding this, new geochronological data from Sverdrupfjella and recent detailed structural mapping in Kirwanveggen have identified structural and intrusive events which clearly postdate much of the deformation described above. The regional correlation of ‘late’ structures is extremely difficult since it appears that the style and intensity of 500 Ma deformation varied considerably from area to area. In central Kirwanveggen this problem is accentuated by the finding that 1000 Ma and 500 Ma fabrics are probably parallel and collinear, making them indistinguishable in the absence of structurally well-constrained time markers such as intrusive dykes. In Sverdrupfjella, D2 produced horizontal to shallow NE-SW plunging asymmetrical folds, commonly with well-developed axial-planar foliations defined by growth of new biotite. K-Ar dating of the biotite has yielded ages of - 500 Ma (Ravich and Solo’vev, 1966,

Research 75 (I 995) 209-229

211

p. 120). F2 folds and related fabrics are best developed in the north and western Sverdrupfjella, where S2 axial surfaces dip towards the northwest, consistent with a top-to-SE sense of vergence. Steep, reverse-sense shear zones with similar orientations displace the margins of sheet-like intrusions of Dalmatian Granite in the central Sverdrupfjella, but do not foliate the granite itself, suggesting syn-tectonic emplacement (Grantham et al., 199 1). The Dalmatian Granite has been dated at 469 + 5 Ma ( Rb-Sr) (Grantham et al., 199 1) . In northeastern Sverdrupfjella, tight folding is observed within the Brattskarvet Monzonite which has yielded ages of 518+ 18 Ma (Rb-Sr) and 522k 120 Ma (Sm-Nd) (Moyes et al., 1993b). D2 age folding on NE-SW axes is notably absent in central Kirwanveggen but is recognised on a regional scale in northern Kirwanveggen, where long wavelength (km-scale), open folds and monoclines are responsible for the double plunge (NW and SE) of L, mineral elongation lineations in this area (Fig. 3). D2 fabric development is limited to the development of a biotite crenulation in micaceous lithologies. In both Sverdrupfjella and northern Kirwanveggen high-angle superimposition of NE-SW trending, upright F, folds upon shallow-inclined, SEplunging F, folds resulted in complex Ramsay type- 1 and type-2 interference patterns (Ramsay, 1967). The breakdown of regional fabric geometries in western HUS (see Fig. 4) from the striking collinear patterns exhibited in all other areas, may reflect greater transposition of the D, form surface by more extensive D2 folding in this area. In central Kirwanveggen, numerous discrete, brittleductile shear zones transect and rework the Di regional gneissic fabrics and several generations of anatectic veins and dykes. These shear zones are marked by narrow ( <5 m wide) zones of fine-grained mylonite and ultramylonite. Mylonite elongation lineations within these zones are collinear with respect to regionally developed L, mineral elongation lineations and kinematic analyses reveal a complex history of both extensional (top-to-SE) and thrust/reverse (top-toNW) movements. Textures within the mylonites are broadly consistent with development in a declining thermal regime, although equivocal brittle+luctile overprinting relationships suggest that temperatures may have fluctuated considerably. Petrographically the mylonites are characterised by microstructures consistent with deformation under lower amphibolite to upper

218

G. H. Grantham et al. /Precambrian

greenschist conditions. Cataclastic feldspar and amphibole porphyroclasts are ubiquitous. Quartz occurs as highly elongate polycrystalline ribbons exhibiting significant crystal substructure. Variation in structural geometries and styles are extreme and a range from low-angle thrusts to high-angle, reverse and normal shear zones is observed. Despite the apparent complexity of this group of structures, they are characterised by a common SE-plunging, mylonite elongation lineation direction. Furthermore, extensional (top-to-SE) zones are relatively uncommon, and the discrete shear zones are dominated by top-to-NW shear sense. Similar discrete brittle-ductile shear zones occur in the Kottasberge of the Heimefrontfjella to the southwest, where they truncate discordant 1000 Ma pegmatites and 500 Ma mafic dykes. Despite enormous differences in structural style and P-T environments, these structures may be correlated with the D, structures of the Sverdrupfjella on the basis of their inferred 500 Ma minimum age and feasible co-development within a common SENW-directed compressive strain field. At Tverregga and Tverreggtelen in central Kirwanveggen, detailed mapping of regionally transgressive, penecontemporaneous mafic and felsic dyke swarms has shown that these represent critical time markers separating two periods of essentially collinear deformation characterised by top-to-NW shear sense. The subvertical, SE-NW-striking dykes truncate regional D, fabrics at a high angle, but are reworked within composite high-strain zones that are practically indistinguishable from early D, thrust zones. Only through these dykes can reactivation of composite shear zones be recognised since no variation in the orientation of planar and linear fabric elements or shear sense is discernible between D, thrusts and composite D,-D, highstrain (compare D, and D2 fabric elements in the stereoplots of Fig. 3). Within the reactivated zones, the dykes become severely attenuated and boudinaged and exhibit intense fabric development. Outside of the D2 high-strain zones the dykes are not folded suggesting that D2 shear strain was strongly partitioned into discrete domains. Notwithstanding this, D2 strain was strongly partitioned into the dykes themselves and strong L-S fabrics are developed. Elongation lineations within the dykes and the wall-rock gneisses are collinear (Fig. 3). Furthermore, the fold axes of reoriented dykes and dyke foliations are parallel to the SE-plunging elongation lineations and fold axes within the

Research 75 (1995) 209-229

gneisses (Fig. 3). By using the dykes to isolate zones of D, reworking it is apparent that structural sections several hundreds of metres thick may have been affected by late penetrative deformation. Although dating of the dyke swarms is still in progress, the supposition that at least two kinematically equivalent deformation episodes are represented in Kirwanveggen has far-reaching implications. It is not known to what extent 1000 Ma fabrics have been reworked during subsequent deformation, which raises the question: Are the regional gneissic foliations simple or complex? This question is critical to the interpretation of P-T-t paths for the area, since failure to recognise the complexity of composite gneissic fabrics is likely to result in the grouping together of mineral assemblages of temporally unrelated episodes (see also Dirks and Wilson, 1995).

3.5. D3 episode

The D, episode represents the termination of tectonic activity in the area and reflects Mesozoic brittle extension in response to the rifting of Gondwana. D3 structures comprise steep to vertically inclined ENE-striking fractures, joints and normal faults. The faults are commonly defined by zones of brecciation, cataclasis and pervasive secondary alteration. The occurrence of D3 structures decreases eastwards away from the Jutulstraumen-Pencksiikket discontinuity suggesting a relationship with the major tectonic structure interpreted by various workers (Ravich and Solo’vev, 1966; Grantham and Hunter, 1991) to underlie that area. The orientation of joints and faults is parallel to the orientation of the spreading centre proposed for the second stage of the break-up of Gondwana (Cox, 1992). The timing of D3 is constrained by strong cleavage development within the Tvora Alkaline Complex indicating that D, is younger than the Complex. Although it is uncertain whether the Tvora and Straumsvola Complexes are of similar age, the Straumsvola Alkaline Complex has yielded ages of approximately 170-l 80 Ma ( 170 &-9 Ma, A.R. Allen, pers. commun.; and 182 Ma Ar-Ar, I. Evans, pers commun.; 170-200 Ma KAr, Ravich and Solo’vev, 1966). No age data for the Tvora Complex or Mt Sistenup (Fig. 1) are available.

G.H. Grantham et al. /Precambrian Research 75 (1995) 209-229

4. Metamorphic

219

history

4.1. Introduction and petrography Mineral abbreviations used are those of Kretz (1983). The metamorphic history presented here is based largely on the petrology of samples from western and central Sverdrupfjella, the metamorphic petrology of Kirwanveggen being in a preliminary stage. The metamorphic history is based on microscopic textural evidence and field relationships. Four episodes or stages of metamorphism have been distinguished in the SverdrupfjellaM,_, involved medium to high-grade metamorphism. The separation of the metamorphically similar M2 and M3 is based on the distinction, on structural grounds, of several generations of mafic intrusions. M4 is represented by late-stage retrogression related either to D, faulting and jointing. D, folds defined by anatectic veins with axial-planar foliations are seen throughout the area to varying degrees providing evidence of early high-grade metamorphism. In the east, garnet of various ages is present in most lithologies with unfoliated garnet + clinopyroxene granulite assemblages being preserved in mafic boudins. In the west, garnets are uncommon being restricted to some amphibolites and thin layers of semipelitic gneisses. No garnet + clinopyroxene assemblages have been recognised in mafic lithologies in the west. In the west, garnets typically show porphyroblastic, poikiloblastic pre-tectonic textures. In Sverdrupfjella aluminosilicates have only been recorded in the Rootshorga Formation (Table 1) in the east with sillimanite being present north of - 72”25’S and kyanite partially replaced by sillimanite south of this into Kirwanveggan. M2 is distinguished from rocks in which new minerals were produced during D2 or those mineral assemblages developed in intrusions recognisably discordant to S,. These intrusions are dominantly mafic and now consist of amphibolites, many of which are garnetiferous. The M2 conditions were typically high grade and approached levels of anatexis indicated by epidotebearing lenticular pegmatites, commonly developed in the tonalitic Grey Gneisses of the Jutulrora Formation, and oriented axial-planar to F, folds. Locally poikiloblastic garnets preserve mylonitic S, fabrics defined by hornblende. These mylonitic fabrics are considered to be related to the thrust faulting during the latter stages

Fig. 5. (a) Garnet and clinopyroxene separated by a symplectitic intergrowth of plagioclase and amphibole. Hornblende, plagioclase and quartz arc also present. The symplectite suggests the reaction of Cpx + Grt = Hbl + Pl. Field of view is 4 mm across (plane light) (b) Garnet breaking down to a symplectite of hornblende, plagioclase and magnetite. Field of view is 4 mm across (plane light).

of D, which elevated eastern Sverdrupfjella to shallower levels. The hornblende in the groundmass outside the garnets is far coarser, locally showing idiomorphic grain shapes suggesting a long period of annealing and grain growth after the thrust faulting. Also related to the thrust faulting are the development of decompression textures defined by the reactions Grt=Hbl+Pl+Mt (Fig. 5b) and cpx + Grt = Hbl + Pl (Fig. 5a) M, is recognised largely by planar fabrics commonly defined by biotite which is commonly developed axialplanar to D, folds. M, assemblages also include amphibolitised mafic intrusions which locally show no planar fabric. The M, mineral formation is characterised by the local development of chlorite and/or saussurite commonly replacing biotite/hornblende and plagioclase, respectively. This retrogressive alteration is commonly developed along brittle structures and is related to D,.

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G.H. Granthatn et al. /Precambrian Research 75 (I 995) 209-229

The following typically retrogressive reactions have been recognised in eastern and western Sverdrupfjella (Table 2). Modelling of the reactions using THERMOCALC (Powell and Holland, 1988) shows that they are steep with positive slopes with all involving hydration during cooling. Because of the steepness of the reaction slopes, they do not constrain pressures and without a more precise knowledge ofp,,, they do not constrain temperatures. Magnesian and grossular-bearing assemblages in the Jutulriira Formation (from W HUS) are consistent with mixed (HZ0 +CO,) but H,O-rich fluids (Grantham, 1992). In ArmHlsryggen (Fig. 1) zoisite-dominated talc-silicate assemblages indicate an H,O-rich fluid phase. In carbonate rocks of the Fuglefjellet Formation it may be assumed that the fluids were CO,-rich, supported by scapolite-bearing talc-silicate assemblages which suggest that the fluid in these rocks contained significant CO* (possibly X,-o, > 0.55) (Grantham, 1992). The partial replacement of the scapolite by zoisite/epidote suggests that during retrogression the fluid phase became increasingly H,O enriched. In the Fuglefjellet Formation in western Sverdrupfjella, the retrogressive formation of brucite, serpentine and talc from tremolite, monticellite and olivine suggests that the fluid composition during the retrogressive stages was almost pure HZ0 (Grantham, 1992). Mineral assemblages in the Dalmatian Granite suggest that plluid =pload at least during D2 (Grantham et al., 1991). In conclusion, the above discussion suggests that during the retrogressive stages of metamorphism in the study area the fluid phase was dominantly H,O-rich.

5. Thermoharometry 5.1. Introduction The following activity models and mineral formula recalculation schemes were used where necessary for the various thermobarometers. Clinopyroxene end members were calculated after Cawthorn and Collerson ( 1974), Fe3? contents being calculated after Droop (1987) and amphibole cation values after Spear and Kimball ( 1984). Albite activities were calculated after Newton ( 1986)) anorthite and clinopyroxene activities after Newton and Perkins ( 1982) and garnet activities after Ganguly and Saxena ( 1984).

Table 2 Retrogressive

reactions recognised

in Sverdrupfjella

I. 4Bt + 6Zo + 15Qtz + 6PI + 3Hbl+ 4Kfs + 4 H,O 2. MS + Ann + 3 Qtz + Aim + 2Kfs + 2HZ0 3. Qtz + MS + Sil + Kfs + Hz0 4. Bt + Qtz + Opx + Kfs + HZ0 5.2Hbl + 2PI + 2Cpx + 3Kfs + H,O 6. 19Qtz + 3Grs + IOPrg + I3An + 2Ab + 8Ed + 2H20 7.4Zo + Qtz + 5PI + Grs + 2Hz0 8.3Qtz + Hbl + 620 --f IOPI + 4Cpx + 4H20 9. Hbl + 4Spn + 26Zo --) 411m + 9Grt + 3 1Pl + 14H,O 10.2Hbl+ 3Spn + 311m + 5Cpx + 2P1+ 3Qtz + 2HZ0

In Table 3 a summary of the thin section mineralogy is given with the interpreted metamorphic parageneses used in the thermobarometry. For the purposes of thermobarometry the area is divided into eastern and western domains (Table 3) with samples SLKl, SLK50, SA 10, and SA13 being from the eastern domain and JE39, S2, BK8, SV9 and SK1 from the westerndomain. The analytical data used for the thermobarometry are summarised in Table 4 and the results of the thermobarometry in Fig. 6. The origins of the curves in Fig. 6 are tabulated in Table 5 with other thermobarometric data. 5.2. Thermometry Assemblages used for thermometry in this study include garnet-biotite, garnet-clinopyroxene, and garnet-hornblende. In addition to these thermometers, an idea of thermal conditions is gained from geochronological studies based on a variety of isotopic mineral/ whole-rock data. The isotopic data (except the garnet data) are from Moyes et al ( 1993a) and Grantham et al. ( 1991) and are summarised in Fig. 7, which shows the ages calculated from different isotopic systems plotted against the closure temperatures for those systems. The closure temperatures are taken from Hodges ( 199 1) and Mezger ( 1990). The temperature for RbSr in chlorite in Fig. 7 is taken as the assumed temperature of chlorite formation rather than a closure temperature for Rb-Sr diffusion in chlorite. Chlorite clearly replaces biotite in many samples. Fig. 7 shows that the isotopic systems, on a mineral scale, only began closing or were reset at -500 Ma, temperatures of > u 600°C being indicated for this resetting or blocking event.

G.H. Gruntharn et al. /Precambrian

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221

Table 3 Table of localities of samples used in thermobarometry Sample No.

Sample locality

and the interpreted relative ages of the mineral assemblages Minerals developed during various phases M,

JE39 BK8 s2 sv9 SKI SAIO SAl3 SLKl SLKSO

72”14’S, o”23’W 72”13’S, O”19’W 72”lO’S. O”l5’W 72”08’S, O”16’W 72”12’S, 0”17’E 72”19’S, O”59’E 72”19’S, O”59’E 72”19’S, IDE 72”19’S, l”E

Mz

MY

Grt Hbl PI Qtz Bt Kfs Ilm Ru

Bt

Cpx Hbl Pl Qtz

Bt Grt Hbl Pl

Ilm Qtz Ru

Cpx Hbl Pl Qtz

Bt

Grt Pl Qtz Kfs Ilm

Bt

Grt Hbl Bt Qtz PI Cpx

Bt

Grt Hbl Bt Qtz PI Ilm

Bt

Grt Pl Qtz Kfs Ilm Bt Grt Cpx PI Qtz f Hbl

One SLKSO, contains garnet + sample, clinopyroxene and is therefore suitable for garnet-clinopyroxene thermometry. Although the garnet and clinopyroxene grains are separated by symplectites of Hbl + Plag, it is assumed that they were initially in contact prior to the symplectite formation. The core compositions were used to calculate curves 1a and 1b (Fig. 6) for the Fe-Mg exchange thermometer between garnet and clinopyroxene using the calibration of Krogh (1988) and the program THERMOCALC (Powell and Holland, 1988)) respectively. The garnet-hornblende thermometer (Graham and Powell, 1984) was calibrated against the garnet-clinopyroxene geothermometer of Ellis and Green ( 1979), which yields values higher than those derived from the Krogh calibration (Krogh, 1988; Ghent and Stout, 1986). Samples SA13, SLKSO, S2 and JE39 contain both garnet and hornblende in contact with one another. Samples SLKSO and SA13, from the eastern Sverdrupfjella, suggest that temperatures of at least - 650°C prevailed during M,_, (curves 8 and 9, Fig. 6, respectively). In western Sverdrupfjella, samples JE39 and S2 provide estimates from 580 to 680°C (curves 13 and 12, Fig. 6, respectively). A survey of various garnet-biotite thermometers (Chipera and Perkins, 1988) suggested that the most

Bt Hbl Pl Qtz

reliable calibration was that of Perchuk and Lavrent’eva ( 1983). Consequently, the temperature values in Table 5 are calculated using their calibration. Four samples containing garnet and biotite were analysed (Table 5). Temperatures were calculated for biotite inclusions in garnet and groundmass biotite located at garnet grain margins. Generally the latter values are marginally higher than those from core/inclusion compositions; however, the values do not differ from each other by more than 10%. Most of the values calculated are ~600°C and are at variance, firstly with the anatectic conditions required by pre-F, anatectic veins and syn-FZ pegmatitic lenses and secondly with other thermometers used. The temperatures for garnet-biotite pairs are lower than other calibrations and are considered to be the temperatures at which Fe-Mg diffusion ceased. Alternatively, they may reflect retrogressive biotite formation during M,. 5.3. Barometry Assemblages used for barometry include clinopyroxene-plagioclase-quartz, garnet-rutile-plagioclase-ilmenite-quartz (GRIPS), garnet-plagioclase+linopyroxenequartz and garnet-hornblende-plagioclase-quartz.

G.H. Grantham et al. /Precambrian Research 75 (I 995) 209-229

222 Table

4

Table

of representative

mineral

chemical

data from the samples analyzed

PLAGIOCLASE

JE39 SLKSO s2 BK8 sv9 SAlO SA13

X /\n MEAN 0.34 0.70 0.43 0.23 0.38 0.30 0.39

: 2 4 2 4 3 5

RANGE 0.30-0.4 0.41-0.46

0.28-0.35 0.39-0.41

X *b MEAN 0.65 0.22 0.57 0.76 0.59 0.69 0.60

X or MEAN 0.01 0.004 0.003 0.01 0.03 0.01 0.006

RANGE 0.60-0.70 0.59-0.54

0.63-0.71 0.56-0.64

RANGE 0.001-0.053

GARNET CORE

RIM

CORE

RIM

CORE

RIM

CORE

RIM

BIOTITE INCLUSION

x

xm

xs,

XSD

X,”

XP,

x,,,

x0,,

&lrm

X

0.13 0.34 0.28 0.11 0.12 0.27 0.31 0.10

0.42

0.44

0.72 0.52 0.45

0.75 0.52 0.51

0.51

0.52

Ah

JE39 SLK50 SAlO SAlO SLKl SA13 s2 SK1

0.68 0.40 0.62 0.74 0.74 0.60 0.56 0.75

0.66 0.47 0.62 0.74 0.73 0.60 0.56 0.74

0.04 0.01 0.06 0.05 0.03 0.06 0.02 0.06

X,” 0.390 0.390 0.364 0.378 0.385 0.214

Xk 0.080 0.120 0.131 0.122 0.085 0.275

JE39 42.72 15.56 16.78 0.21 9.12 10.77 1.60 0.50 0.88 0.19 98.33

SLK 50 51.52 5.90 9.80 0.12 16.79 11.79 0.60 0.25 0.37 0.11 97.25

SLKBO 48.86 8.63 10.91

6.310 2.709 2.073 0.026 2.008 1.705 0.458 0.094 0.098 0.022 15.503

7.367 0.995 1.172 0.015 3.578 1.806 0.166 0.046 0.040 0.012 15.197

0.508 1.690 1 ,019 0.072 0.386 0.481

0.247 0.633 0.361 0.028 0.138 0.184

0.03 0.01 0.06 0.06 0.06 0.06 0.02 0.05

0.15 0.24 0.04 0.11 0.12 0.07 0.11 0.09

0.18 0.18 0.04 0.09 0.09 0.07 0.11 0.10

0.18 0.35 0.28 0.10 0.11 0.27 0.31 0.11

MATRIX A””

CLINOPYROXENE SLK50 CORE SLK50 RIM BK8 CORE BK8 RIM sv9 SAlO

XW0 0.441 0.429 0.467 0.479 0.481 0.462

X 0~~00 0.000 0.000 0.001 0.000 0.000

X.l, 0.025 0.013 0.032 0.020 0.015 0.012

AMPHIBOLES SiO,

4% Fe0 MnO MgO CaO Na,O K,G TiO ? Cr,O, TOTAL SiO, AW, Fe0 MnO MgO cao Na,O K,O TiO C$, TOTAL Fe’+/Fe’+ Al,” Al”, NAM4 NA-A SUMA

+Mg

SAlO 41.40 13.59 21.14

14.30 11.99 0.87 0.49 0.76

S2 42.92 13.54 18.64 0.27 8.84 11 .J9 1.57 0.86 1.15

96.81

99.31

98.27

7.087 1.476 1.324

6.370 2.369 2.314

3.091 1.864 0.245 0.091 0.083

1.955 1.875 0.452 0.163 0.128

6.308 2.441 2.694 0.035 1.476 1.896 0.387 0.198 0.164

15.261

15.626

15.599

0.300 0.913 0.563 0.076 0.169 0.260

0.542 1.630 0.738 -0.010 0.462 0.625

0.646 1.692 0.750 -0.014 0.401 0.600

6.50 11.61 1.31 1.02 1.43

X CITl 0.005 0.005 0.001 0.000 0.001 0.002

Xw. 0.000 0.000 0.001 0.000 0.026 0.032

X CaTr 0.059 0.043 0.005 0.000 0.007 0.003

G. H. Granthatn

et al. /Precambrian

Research

75 (1995)

209-229

223

EAST Ml-3 16

n

WEST M,,

12 2 B 6

‘6 M %

400

450

550 TEMPEZL

500

700

750

600

!!I0

Fig. 6. Summary of P-T values from the eastern and western domains. The shaded areas show the ranges of estimates. The aluminosilicate triple point of Bohlen et al. ( 1991) is shown with the fields for kyanite (K), sillimanite (S) and andalusite (A). The amow shows the transition from M, to Mz_? in the east. The fine dashed line represents a geothermal gradient of 30”C/km.

Table 5 Table relating the curves in Fig. 6 with their sample numbers, mineral assemblages, mineral data in Table 3 CURVE la lb 2a 2b 3 4 5 6 6 7 B 9 10 11 12 13 14 15 16

SAMPLE NO. SLKBO SLK50 ,I II II II II II II II SLK50 SA13 SLK50 SA13 52 JE39 BK8 JE39 s2 SK1 II

DOMAIN WEST 1, II II ,! II II II 11 1, 11 v 1, 11 EAST 4, II II II

JE39 11

WEST II II II

SLKl 1,

EAST II

JE39 s2

WEST

* represents the calculation using THERMOCALC

ASSEMBLAGE Grt-Cpx” Grt-Cpx# Grt-PI-Cpx-Qtz(Mg)’ II (Mg) + Grt-PI-Cpx-Qtz(Fe) Jd-Ab-Qtz Cats-An-Qtz Grt-HbCPI-Qtz Grt-Hbl-PI-Cltz Cats-An-& Grt-Hbl Jd-Ab-Qtz Grt-Ru-!lm-PI-Cltz Grt-Hbl Grt-Hbl Cats-An-Qtz Grt-Ru-llm-PI-Qtz II Grt-8t

CORE/RIM CORE CORE

l

CORE CORE RIM CORE CORE

PRESSURE (kb) 11-13 lo-16 10.15-15.0 9.6-14.5 11.75-l 5.5 7.8-l 1.6 10.4-9.8 8.62-9.12 8.65-9.14 10.4-9.8

RIM

5.0-8.0 6.5-l 0.0

CORE

3.1-6.0 5.5-10.2 6.5-10.0 2-8

CORE RIM CORE RIM CORE RIM

Grt-Hbl-PI-Qtz

(Powell and Holland, 1988);

1988); # represents the calculation after Krogh (1988).

domain, and the physical conditions

11

3.15-3.9 2.4-3.4 3.23-2.75

calculated

from the

TEMPERATURE (“Cl 622-761 601-638 500-700 500-900 500-700 500-700 500-700 500-800 500-800 500-700 627 655 500-700 500-800 682 583 550-800 500-900 500-800 510-525 554-569 600-563 617-579 527-543 520-535 400-700 400-700 500-700

+ represents the calculation using GEOCALC

(Brown et al.,

224

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et al. /Precambrian

Research 75 (1995) 209-229

1000

33

900 -

30

U-Pb i!r

800

Rb-St’ WV

700 2 E

600

zd

500 -

g

400 -

z 18 y E 15 a x 12

MS

300

9

Rb-Sr Bt & Fld

200

6

100 0 -0

24 21

-

[Rb-Sr

F

27

n

3

FTAp

0 0.1

0.2

0.3

0.4

0.5

ftE

1)67a, 0.8

0.9

1

1.1

1.2

1.3

Fig. 7. Temperature-time diagram for the various mineralogical and whole-rock data determined from the H.U. Sverdrupfjella: w/r= whole rock; FT= fission track; Fld= feldspar; MS = muscovite; Bt= biotite; Ap = apatite; Zr= zircon; Grr= garnet; U-Pb = uranium-lead; SmNd= samarium-neodynium. The depth column (right-hand axis) assumes a geothermal gradient of 30”C/km.

Sample SLK 50, taken from a mafic boudin, contains garnet, clinopyroxene, quartz, plagioclase and hornblende, the garnet and clinopyroxene never being in contact and being separated by symplectitic intergrowths of anorthitic plagioclase and hornblende (Fig. 5a). Clinopyroxene, plagioclase, quartz and horn-

blende are in contact and show granoblastic textures as do hornblende and garnet. The formation of hornblende + plagioclase + quartz symplectites between garnet and clinopyroxene is interpreted as an M2 reaction resulting from partial hydration of an M, assemblage consisting of Cpx + Grt.

I

60

16 i 50 i

0

100

200

300 400 500 TEMPERATURE ( Cl

600

700

600

Fig. 8. P-T time loop for the Sverdrupfjella. The dashed lines represent geothermal gradients of 30”C/km and 40Wkm. triple point is from Bohlen et al. ( 199 1). The rounded labels represent the timing along the P-T-t path.

The aluminosilicate

G.H. Grantham et al. /Precambrian

Only sample SLKSO has the necessary minerals for the application of the barometer which is based on the reaction of Cpx + PI = Grt (Grs + Alm + Py) + Qtz. Various authors have provided calibrations for this barometer (Newton and Perkins, 1982; Moecher et al., 1988), dependent on activity models for clinopyroxene, plagioclase and garnet. The data were calculated and modelled using the computer programs THERMOCALC (Powell and Holland, 1988) and GEOCALC (Brown et al., 1988). The latter program does not include data for hedenbergite. The reaction curves shown in Fig. 6 involve the following equilibria: 3Qtz + Alm + 2Grs = 3An + 3Hed (curve 3, Fig. 6, Powell and Qtz+ and Holland, 1988); Py + 2Grs = 3An f 3Di (curve 2a in Fig. 6, Powell and Holland, 1988; curve 2b in Fig. 6, Brown et al., 1988). The reactions were calculated for the core compositions of the garnet and clinopyroxene; however, the core and rim values do not vary significantly. At the temperatures indicated by the garnet-clinopyroxene and garnet-hornblende thermometers above pressures of approximately 1 l- 14 kbar are suggested (Fig. 6). using clinopyroxene-plagioclaseBarometers quartz are based on increasing solution of jadeite molecules (NaAlSiO,) and Ca-Tschermaks (CaAl,SiO,, CaTs) in clinopyroxene with increasing pressure. The CaTs and jadeite contents of the clinopyroxenes in the samples studied (SLKSO, BK8, SV9 and SA 10, Table 4) are low and are therefore assumed to approximate ideal mixtures. Possible sources of error with these barometers are analytical because the CaTs and jadeite molecules are calculated assuming charge balance and thus minor analytical errors can result in errors. The computer program THERMOCALC by Powell and Holland ( 1988) was used to calculate the physical conditions of reaction for the reactions: anorthite + Ca - Tschermaks

+ quartz( 1)

albite + quartz --$jadeite( 2) Fig. 6 shows the reactions ( 1) and (2) for sample SLKSO using average values for the core (curves 4 and 5, Fig. 6) and rim (curves 7 and 10, Fig. 6). P-T conditions for reaction (2) were calculated for sample BK8 (curve 14, Fig. 6). Using plagioclase and clinopyroxene compositions from sample SAlO and SV9 did not yield realistic values. The discrepancy between these samples may arise from the possibility that the

Research 75 (1995) 209-229

225

porphyroclastic granitoid, from which SAlO was sampled, was emplaced subsequent to the higher-pressure assemblages preserved in SLK50. Kohn and Spear (1989) calibrated an empirical barometer utilising tschermakitic exchange in the assemblage garnet + hornblende + plagioclase + quartz. Four different models are presented by these authors. An average of the four values from the different models is reported here in accordance with their recommendations (Table 5). The values calculated show widely varying estimates for the different samples. The Mg-based calibrations yield values which appear to be more consistent with the other barometers applied in this study. Samples collected from the western Sverdrupfjella (S2 and JE39) show pressures of - 1 kbar to -4.5-5.0 kbar with a mean of 2.75 kbar at 700°C (Table 5). Using the core compositions of minerals from sample SLKSO, pressures of between - 5.4 and - 12 kbar with a mean of - 9 kbar at 700°C are estimated (curve 6, Fig. 6). Pressures of 0.0 to 4.76 kbar with a mean of 2.7 kbar at 700°C are estimated (Table 5) from sample SA13, in accordance with the low pressures suggested by the plagioclase + clinopyroxene barometry. The GRIPS barometer (Bohlen and Liotta, 1986) based on the reaction of 3Pl+3Qtz+611m= Grs +2Alm+6Ru has a relatively flat slope in P-T space and is therefore accurate with little temperature influence. Samples which contain the assemblage are JE39, S2 and SA13. Ilmenite compositions were assumed to be stoichiometric with ideal-mixing. Using the activities calculated, the reaction curves for samples S2, JE39 and SA13 are shown in Fig. 6 (curves 16, 15 and 11, respectively). The curves were calculated using the computer program THERMOCALC by Powell and Holland ( 1988). At temperatures of - 650°C (suggested by garnet-hornblende thermometry above) pressures of - 7 kbar are indicated for sample SA 13 (Fig. 6). At 650°C pressures of - 7-8 kbar are suggested for samples JE39 and S2 (Fig. 6). These values are significantly higher than those calculated for the garnet-hornblende-plagioclasequartz estimates from the same samples. 6. Discussion and definition of a P-T-t loop The data pertinent to the construction of a P-T-t loop are discussed in relation to Figs. 6,7 and 8. In the

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eastern domain, the rocks were characterised by an early M, anhydrous high-pressure ( - 12k2 kbar) assemblage of Grt + Cpx + Pl + Qtz which probably developed at temperatures between - 675 and 750°C at - 1000 Ma. The pressures reported here support Groenewald and Hunter ( 199 1) . The temperature estimates for the eastern area are slightly lower than those reported by Groenewald and Hunter ( 1991); however, the differences may result from their use of the Ellis and Green ( 1979) garnet-clinopyroxene thermometer. These temperatures are, however, lower than might be expected from the presence of sillimanite and may suggest that they represent diffusion closure temperatures. The minimum age of this stage is partially constrained by the emplacement of megacrystic granites at - 1000 Ma, in which the clinopyroxene-plagioclase barometry does not record the relatively high pressures preserved in the boudins. The later ( M2) P-T conditions involve rim compositions of minerals and partially hydrated assemblages containing Cpx + Hbl + Grt + Pl + Qtz and appear to indicate temperatures of - 600°C to 700°C and pressures of between 7 and 9.5 kbar. These values are defined by the intersections of curves 6,7,8,9, 10 and 11 (Fig. 6, Table 4). The lower pressures shown here are consistent with thrust-fault related uplift during the latter stages of D,. The pressure reduction is supported by the partial replacement of kyanite by sillimanite in southern Sverdrupfjella and northern Kirwanveggan; however, the thermometry yields values which, at the pressures indicated, are dominantly in the kyanite field. This may suggest that the thermometry values represent diffusion closure temperatures. The timing of this stage is not well constrained. The highly sheared diorite at Midbresrabben (Grantham and Hunter, 1991) and folded mafic dyke at Midbresrabben yield whole-rock Rb-Sr ages of - 800 Ma (Moyes et al., 1993a). Both intrusions were clearly emplaced after significant deformation episodes and both were subsequently highly deformed (Grantham and Hunter, 1991; Grantham, 1992). The intersections of curves 12, 13,14 and 15 (Table 5, Fig. 6) define physical conditions in the western part of the study area of 620°C to 700°C and 3 to 9 kbar. No clear distinction between M, and M3 is possible but the P-T conditions for sample S2, from an amphibolite dyke discordant to the layering and planar foliation and therefore formed after M,, suggest that P and T may

have been higher during M2 than M,. This would be consistent with the depression of the western domain by the overriding thrust faults from the southeast. This is reflected by the upward pointing arrow in Fig. 8. The data for the western domain show that the high-pressure assemblages, characteristic of the east, are absent in the west. No evidence has been recorded to suggest that such conditions were reached at any time. Barometers which have yielded very low pressures in some cases have been ignored because the physical conditions represented by them suggest a geothermal gradient of > 6O”C/km. Geothermal gradients are generally in the range 15-30”C/km with extreme values from 5 to 6O”C/km being recognised (Yardley, 1989, pp. 1415). During D,/Ms, temperatures and pressures in the east and west appear to have been similar: - 650°C and 6.5 kbar, respectively (Grantham et al., 1991) . The period between D,/M,_* an D*/M, involved significant time at mid-crustal levels (Fig. 7). In amphibolite from northeastern Sverdrupfjella, mylonitic textures, defined by 0.01 mm grains of hornblende and opaques, are preserved in garnets ( 15 cm in diameter). Outside the garnets, hornblende (1 mm grain size) forms a gneissic granoblastic texture across which idiomorphic hornblende upto 15 mm in width has grown, suggesting significant annealing after the thrust-faulting. The closure of the garnet-biotite and possibly other thermometers at temperatures significantly lower than might be expected, and the closure of the mineral isotopic systems significantly later than the whole-rock systems, also suggest a significant residence time at mid-crustal levels. Alternatively this may reflect resetting of the isotopic and thermometric systems by the thermal event (M,) associated with the syn-tectonic intrusion of Dalmatian Granite and Brattskarvet Intrusive Suite in the Ma. In the latter alternative it is period -450-550 possible in the Sverdrupfjella that the crust was thinned or extended between D, and D, (suggested by the intrusion of the Roerkulten mafic dyke and Midbresrabben Diorite) and subsequently thickened during syntectonic emplacement of the Dalmatian Granite and Brattskarvet Monzonite in the period - 450-550. It is uncertain as to the extent of the thickening during this period, but an estimate at Robinheiea in the central Sverdrupfjella indicates that sheets of broadly concordant Dalmatian Granite comprise approximately 10% of the stratigraphy.

G.H. Granthatn et al. /Precambrian Research 75 (1995) 209-229

During the latter stages of D, and after D, many of the reactions recognised reflect hydration by a fluid dominated by H,O during cooling. Chlorite developed during this stage has yielded Rb-Sr ages of - 300-400 (Grantham et al., 199 1) (Fig. 7). Significantly, curves for the reactions described in Table 5 are relatively steep and are dominantly hydrothermally driven with subordinate influence of pressure. Texturally, the reaction products are all located on the low-temperature side of the curves indicating a cooling trajectory crossing the steep reaction curves to give a cooling path broadly parallel to a geothermal gradient of 30-4O”C (Fig. 7). The correlation of the Urfjell Group with the Cambro-Ordivician Blaicklock Glacier Group of the Shackleton Range (Wolmarans and Kent, 1982) would necessitate its deposition at approximately this time, the deformation recognised in it possibly reflecting latestage D2 structures or an, as yet unrecognised, D, deformation stage. Alternatively, the correlation of the Urfjell Group with the Ahlmannryggen Group by Roots ( 1969) would require it being tectonically emplacement into its present position. The final stages involve the faulting and jointing (D,) related to the break up of Gondwana at - 170-l 80 Ma (Cox, 1992). The presence of amygdales in dykes of the - 180 Ma old (Harris et al., 199 1) Kirwanveggan Dolerite Suite provides evidence that at the time of emplacement, the cover was probably of the order of Q 0.5 kbar (Grantham, 1992). Evidence from Kirwanveggan that lends support to indications of shallow depths includes the Kirwanveggan basalts which erupted - 180 Ma ago. In Heimefrontfjella ( -200 km to the SW of the Kirwanveggan, Fig. 1) fission-trackdating on apatite (effectiveclosure - 100°C) shows two groups of ages temperature (Jacobs, 1991) . One group has ages around 100 Ma and the other ages from 130 to 200 Ma (Jacobs, 199 1) . Jacobs ( 199 1) interprets the younger ages to represent rapid uplift and associated cooling during the Late Cretaceous. The older ages are interpreted as mixed ages where temperatures were insufficient to anneal (reset) the apatite.

7. Conclusions In conclusion the tectonothermal evolution of the SKA area may be described as involving D,/M,_, at

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- 1000 Ma during which folding and thrust-faulting emplaced rocks from lower to higher crustal levels. The extent of D*/M, at - 500 Ma is less clear. Following D,/M, thermal relaxation through erosion followed until - 180 Ma when Gondwana fragmentation occurred. Aspects which require further attention in SKA revolve around the superimposition of the Early Cambrian event on the Mesoproterozoic event. This superimposition involved similar structural orientations. Characterisation of the metamorphism is similarly complicated by the superimposition. The knowledge of the timing of the deformation events is imprecise due to a lack of single zircon geochronology. The contradiction of - 1100-1000 Ma whole-rock ages and the 450-550 Ma mineral ages needs to be reconciled. The structural orientation of the western Sverdrupfjella differs from the eastern Sverdrupfjella in that the strong thrust-fault-related overprint appears to be absent and consequently the pre-thrust fault history may be preserved in this area. Perhaps one of the most important aspects needing attention is to constrain the depositional and deformational histories of the Urfjell Group.

Acknowledgements We gratefully acknowledge the financial and logistical support provided by the Department of Environmental Affairs, the helicopter personnel of the South African Air Force and the technical personnel at the three Universities represented by the authors of this paper. We would also like to acknowledge the constructive criticism of the paper by L. Kriegsman and an anonymous referee. This paper contributes to IGCP 348 on the Mozambique and Related Metamorphic Belts. Note added in proof

Preliminary whole-rock Rb/Sr data from the Urfjell Group indicate an age of 54Ok29 Ma with detrital muscovites from the sandstones yielding an age of f 620 Ma.

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