Tectonic significance of Proterozoic ductile shortening and translation along the Antarctic margin of Gondwana

Earth and Planetary Science Letters, 102 (1991) 58-70

58

Elsevier Science Publishers B.V., A m s t e r d a m

[DTI

Tectonic significance of Proterozoic ductile shortening and translation along the Antarctic margin of Gondwana J o h n W. G o o d g e a, Scott G. Borg

b, Brad

K. Smith c a n d Victoria C. B e n n e t t d

a Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USA b Berkeley Center for Isotope Geochemistry, Department of Geology and Geophysics, University of California, Berkeley, CA 94720, USA c Institute of Earth Sciences, State University of Utrecht, Postbus 80.021, 3508 TA Utrecht, The Netherlands d Research School of Earth Sciences, Australian National University, Canberra, A C T 2601, Australia Received March 3, 1990; revised version accepted August 20, 1990

ABSTRACT The central Transantarctic Mountains of Antarctica are divided into distinct lithotectonic packages of variable age, deformation style and metamorphic character. In the Miller Range (83°S, 155°E), the boundary between two such lithotectonic units is a low-angle, thrust-type ductile shear zone that carried high-grade gneisses of the East Antarctic craton to the southeast over a lower-grade metasedimentary sequence. Broad constraints limit the age of ductile deformation as late Early Proterozoic to Late Cambrian. In addition to a reverse component of movement, this zone also records along-strike motions in a direction subparallel to the present orientation of dominant Beardmore- and Ross-age orogenic trends. We postulate that major pre-Ordovician crustal shortening which occurred along this part of the G o n d w a n a continental margin began in the Middle to Late Proterozoic as an early phase of long-lived convergence. Furthermore, orogen-parallel displacements indicate that oblique plate interactions m a y have played an important role in the early evolution of this active margin.

I. Introduction The Precambrian basement of East Antarctica represents a large segment of the ancient Gondwana supercontinental nucleus, and it plays an important role in our understanding of the Archean and Proterozoic evolution of continental lithosphere in Gondwana [1-9]. The evolution of the Antarctic component of Gondwana is particularly important with respect to various models of Proterozoic crustal evolution, because both interior and marginal portions of Gondwana are exposed across Antarctica [10]. This contribution addresses the Proterozoic tectonic evolution of the western Antarctic cratonal margin within the central Transantarctic Mountains, which is one of only a few regions where it is possible to study the active Proterozoic cratonal margin of Gondwana. The central Transantarctic Mountains orogenic belt preserves a complex and poorly resolved Pro0012-821X/91/$03.50

© 1991 - Elsevier Science Publishers B.V.

terozoic and early Phanerozoic tectonic history. Structural and isotopic evidence from the Miller Range, on the cratonward edge of the orogenic belt in the vicinity of Nimrod Glacier (Fig. la), shows that a possibly large amount of tectonic shortening across the range occurred as a result of mid-crustal shear along a low-angle, thrust-type drcoUement. In this paper we show that Proterozoic deformation in the central Transantarctic Mountains took place, in part, by movement along a major zone of shortening, placing crystalline rocks from cratonal East Antarctica above supracrustal continental-margin rocks. In addition to contractional displacements, however, ductilelydeformed rocks in the Miller Range also record a component of movement at a low angle to principal tectonic trends in the central Transantarctic Mountains, indicating possible along-strike shear. In fight of these relations, it is necessary to reevaluate existing geologic and geochemical evi-

PROTEROZOIC D U C T I L E S H O R T E N I N G A N D T R A N S L A T I O N A L O N G T H E A N T A R C T I C M A R G I N O F G O N D W A N A 4

(b)

NORTHER VICT~

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ICE SHEE

83000' I+

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\s*

/

OUIN POLE

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~iet ~n$

20 km

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Granite

o krO0 Glacief

(Camb.-Ord.) ~

Argosy Fro.

~

WorsleyFm.

(metasediments)

83°30'S + 156°E

d" , r c J ~

(metacarbonate)

Aurora Fm. (orthogneiss)

Polar Plateau

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(c)

A

A' Polar Plateau

5,000 m --]

o

Marsh Glacier

~L~

~

[- 5,000 m

-

,-'-~,iI,?I-o

Fig. 1. Geology of the Miller Range, central Transantarctic Mountains, Antarctica. Inset map (a) of the Transantarctic Mountains (shaded) shows location of the Miller Range (black). Geologic sketch map (b) and cross section (c) of the Miller Range (simplified from [14,36,54], and this report). Nimrod Group is divided into four metasedirnentary and meta-igneous units, as described in text. Post-tectonic granite plutons are - 5 0 0 Ma in age. Movement within Endurance shear zone (dashed thrust symbol) is thrust-type in a top-to-the-southeast direction; continuity of shear zone is poorly constrained in the vicinity of Argo Glacier.

dence concerning the Early Proterozoic to earliest Phanerozoic orogenic evolution of the central Transantarctic Mountains, and to consider alongstrike displacements in new tectonic models. Here we focus on petrologic and structural relations of metamorphic rocks in the Miller Range. Our ongoing isotopic investigations will provide new constraints on the age of crustal materials involved in this orogen, the timing of tectonic events, and crustal growth in this portion of Antarctica in general.

59

2. Tectonic setting of metamorphic rocks in the central Transantarctic Mountains The Transantarctic Mountains extend for more than 3000 km between the Weddell Sea and the northern Victoria Land coast of the Ross Sea. Since the 1950s, several groups have established a framework for pre-Devonian tectonism and magmatism in the central sector adjacent to the Ross Ice Shelf [11-19]. From these studies it is evident that this mountain belt was the site of profound structural shortening, as shown by contrasting lithologic associations, abrupt metamorphic transitions, and large-scale fold-belt structures across the range. These features and the development of a regionally extensive CambroOrdovician magmatic province indicate that rocks of the central Transantarctic Mountains evolved in a convergent plate--margin environment in earliest Phanerozoic time. New sedimentological and isotopic evidence suggests that the central Transantarctic Mountains orogen developed as a result of accretion of parautochthonous or allochthonous terranes [20,21], as has been argued for segments of northern Victoria Land [22-24]. The central Transantarctic Mountains between the Beardmore and Byrd Glaciers contain a variety of upper Precambrian and lower Paleozoic rocks that record a complex orogenic history prior to overlap by Devonian and younger Gondwanan continental sediments. The pre-Devonian basement system consists of at least four principal components: (1) The Nimrod Group [25], exposed in the Miller and Geologists Ranges adjacent to the polar plateau, consists of a lithologically diverse sequence of Precambrian high-grade gneisses and schists. (2) The more outboard Beardmore Group consists of low-grade, folded clastic sediments dominated by a thick succession of unfossiliferous turbiditic greywacke and slate (Goldie Formation of Gunn and Walcott [11]), and the conformably underlying Cobham Formation [15], an areally restricted sequence of meta-carbonate and quartzite. Grindley and Laird [26] inferred that Beardmore strata unconformably overlie the Nimrod Group, but Stump et al. [27] correlated the Cobham with part of the Nimrod. Borg et al. [21] suggested that the Beardmore and Nimrod

60

groups may require reorganization and argued for three different tectono-depositional units. Thus, the stratigraphic relationship between the Nimrod and Beardmore Groups is equivocal. (3) Byrd Group sedimentary rocks include Lower Cambrian carbonate (Shackleton Limestone) and an overlapping sequence of Middle to Upper Cambrian clastic units [20,53]. These deformed rocks unconformably overlie folded Goldie Formation in several places [28,29] and indicate the Goldie is pre-Early Cambrian in age. (4) Cambrian to Ordovician plutonic rocks occur throughout the Transantarctic Mountains between northern Victoria Land and the Thiel Mountains [12,21,24,30-34]. In the central Transantarctic Mountains these - 500 Ma granites show lithologic and isotopic variation indicative of a subduction origin [21,30,35], and a lack of deformation features indicates they post-date tectonic events within the older basement complex. Metamorphic grade across the central Transantarctic Mountains in the vicinity of Nimrod Glacier increases generally from east to west (toward the polar plateau). Metasedimentary Goldie Formation rocks contain biotite-zone greenschistfacies assemblages regionally, but higher-grade hornblende-hornfels facies assemblages occur near margins of plutons. Cobham Formation rocks also contain garnet-zone greenschist-facies assemblages, overprinted by younger biotite-hornfels assemblages [15]. Nimrod Group gneisses and schists contain assemblages indicative of upper greenschist- to upper amphibolite-facies regional metamorphism [13,36,37]. Field, stratigraphic, and geochronologic data provide evidence for at least three pre-Devonian episodes of deformation in the central Transantarctic Mountains. The youngest and most evident of these, the Ross orogeny [12], formed tight upright folds in Beardmore and Byrd Group rocks, and preceded emplacement of - 5 0 0 Ma granite plutons. The age of Ross deformation is therefore constrained to be Middle Cambrian to Early Ordovician in age. Rowell et al. [18] also provide evidence for a younger, pre-Devonian deformation that may be tectonically linked to the preceding Ross event. An angular unconformity between the Goldie Formation and Shackleton Limestone defines a Late Proterozoic to early Paleozoic episode of deformation, the Beardmore orogeny. Early

J.W. G O O D G E

ET AL.

study of the Nimrod Group indicated they were deformed during high-grade metamorphism not evident in younger sedimentary units [25]. However, evidence that such an event, called the Nimrod orogeny, occurred in Middle to Late Proterozoic time [36,38] was questioned by Adams et al. [39] and Stump et al. [27], who doubt the occurrence of a separate Nimrod event. The focus of this report is on metamorphic tectonites of the most inboard segment of the central Transantarctic Mountains in the Miller Range (Fig. la), which have been described previously by Gunn and Walcott [11], Grindley and Warren [25], Grindley [36], and Gunner [14]. In the Miller Range, rocks of the Nimrod Group are nowhere observed in contact with lower-grade Beardmore or Byrd rocks, but they are intruded by several large, - 500 Ma granite plutons (Fig. lb). We recognize four distinct metamorphic rock units within the Miller Range, based on the descriptive nomenclature of Grindley et al. [13]. Exposed from east to west, these are: (1) the Worsley Formation, a relatively minor unit consisting of calc-silicate and calcareous schist; (2) the Argosy Formation, the most areally extensive unit, including phyllite, mica schist, mica quartzite, pelitic schist and micaceous gneiss; (3) the Aurora Formation, a quartzofeldspathic gneiss containing conspicuous K-feldspar augen; and (4) the Miller Formation, a banded gneiss characterized by garnet-mica schist, amphibolite, and calc-silicate. The first three of these units were interpreted by Grindley et al. [13] as an originally intact stratigraphic sequence because of their general concordance; they were viewed as separated from structurally higher rocks of the Miller Formation along the Endurance thrust [36]. Thin compositional layering and the presence of pelite, quartzite, marble, and calcareous schist within the Argosy and Worsley units affirms their sedimentary origin. However, we suggest new interpretations concerning the origin of Aurora and Miller protoliths as follows. The Aurora is a homogeneous gneiss that contains prominent augen-shaped K-feldspar megacrysts. Earlier workers interpreted the elongate K-feldspar "lenticles" as the product of feldspathization of a sedimentary protolith during high-grade metamorphism and associated Kmetasomatism [13,14,26,36,39,40]. Gunner [14] de-

PROTEROZOIC

DUCTILE

SHORTENING

AND TRANSLATION

ALONG

scribed textures in this unit as m part cataclastic, citing undulatory extinction in matrix minerals and fine-grained textures near the margins of feldspar megacrysts as evidence of granulation. Field and petrographic relations of the augen gneiss unit (see discussion of structural relations, p. 63) indicate that this unit is a mylonitic gneiss that formed by ductile deformation of a plutonic, rather than a sedimentary, protolith [37,41]. Primary contact relations among the Nimrod subunits are obscured by deformation, but the Aurora gneiss intrudes Argosy schists locally. Thus, the coarse-grained, augen-shaped K-feldspar porphyroclasts are relict igneous phenocrysts altered in shape by ductile strain, rather than by metasomatic infiltration * Goodge and Borg [37] concluded on lithologic, structural, and petrologic grounds that the Miller gneiss is notably distinct from other units in the Nimrod Group. Evidence in support of this interpretation includes generally uniform bulk compositions, higher metamorphic grade, polyphase folds, common anatectic textures, and the presence of detrital zircons that give anomalous old ages (see below). Nonetheless, we favor retaining within the Nimrod Group all metamorphic rocks of the Miller Range, although all units do not necessarily share a common origin or tectonometamorphic history, until more definitive evidence warrants formal reorganization. Field and petrographic relations indicate that subtle metamorphic gradients exist from east to west across the Miller Range, and that these gradients may be related to structures described below. Along the south side of Argosy Glacier, Argosy metasedimentary rocks include micaceous phyllite

* Borg et al. [21] used the term Camp Ridge granodiorite to describe an outcrop of what is referred to in this paper as Aurora orthogneiss. The latter term is preferred for all metamorphosed plutonic rocks intruded into metamorphic rocks of the Miller Range. In addition, Borg et al. [21] assumed that metamorphic rocks intruded by this m a g m a were Miller Formation. Recent field observations by us allow a distinction to be made between these intruded rocks and Miller Formation, so these rocks are included with Argosy Formation here. Because of remaining uncertainties in the broader coherence of Argosy Formation mapped by Grindley, the above relations neither confirm nor deny possible correlations proposed by Stump et al. [52], and discussed by Borg et al. [21].

THE ANTARCTIC

MARGIN

OF GONDWANA

61

and schist containing muscovite + biotite _ chlorite _+ garnet indicative of greenschist-facies metamorphism. Progressing westward, the Argosy consists chiefly of garnet-mica schist containing muscovite + biotite + garnet + epidote +_ chloritoid _ Al-silicate, and calcareous schist in the underlying Worsley contains biotite + muscovite + calcite + plagioclase. This gradation in lithology and mineral paragenesis provides evidence of a weak metamorphic gradient increasing to the west. Phase assemblages in eastern parts of the Miller Range thus record lower-greenschist- to amphibolite-facies conditions. In general, these metamorphic assemblages are compatible with those described from the Cobham Range to the northeast [15], which is, based on this criteria alone, supportive of the correlation proposed by Stump et al. [271. In western and southern parts of the range, layered Miller gneisses contain notably different amphibolite- to transitional granulite-facies assemblages consisting of hornblende + plagioclase + biotite _+ garnet _+ epidote + clinopyroxene. Pelitic schists contain kyanite + sillimanite, and calc-silicate interlayers contain calcite + quartz + scapolite + clinopyroxene + epidote + tremolite. No metamorphic gradations within the Miller rocks are evident from phase assemblages. Preliminary garnet-biotite and garnet-hornblende geothermometry [42-44] on five samples of Miller gneiss indicate peak metamorphic temperatures in the range of 680-750°C. These high metamorphic temperatures are consistent with the presence of scapolite-clinopyroxene phase assemblages in calc-silicate rocks [45,46]. In summary, a metamorphic gradation exists from east to west across the range that represents a minimum temperature difference of up to 200°C between the main Argosy and Miller units. Estimates of pressure are as yet poorly constrained-the presence of kyanite in Miller rocks indicates moderate minimum pressures--but the sharp temperature gradient may reflect differences in crustal levels represented by these contrasting rock units. The juxtaposition of terranes representing different crustal levels may furthermore be attributed to tectonic processes involving crustal shortening, as the higher-grade Miller rocks now occur at a higher structural level above a wide, low-angle ductile shear zone.

62

J.W. GOODGE ET AL.

3. Ductile deformation in the Miller Range

of the Nimrod Group, and that this tectonic boundary formed during regional contraction. However, recent advances in the interpretation of ductilely deformed rocks [47-49] allow us to add new constraints on the scale and kinematic history of this zone. In the western Miller Range, the Endurance thrust is exposed as a complex ductile shear zone with an exposed minimum thickness of 1 km that separates layered Miller gneisses from structurally lower Nimrod units (Fig. lc). Herein we refer to this structural zone as the Endurance shear zone, in keeping with Grindley's [36] original name, and in order to emphasize its nature as a distributed zone of ductilely deformed mylonitic gneisses. This zone is characterized by porphyroclastic mylonitic gneiss and ultramylonite that contain a southwestdipping foliation and a well-defined, gentlyplunging mineral elongation lineation that trends approximately 330 ° . At least three principal zones of high strain occur in the southern part of the range, although it is unclear if they represent bifurcations of the principal zone exposed to the north, or different exposures of the same undulating primary zone. Ductile shear fabrics are best preserved in the Aurora granodioritic orthogneiss, which occupies the structurally highest position in the lower-plate assemblage, and basal calc-silicate gneisses of the Miller with which the Aurora is interleaved. Nimrod rocks above and below the main ductile zone also contain moderately welldeveloped L - S tectonite fabrics, and ductilely deformed tectonites in the western Miller Range

Metamorphic rocks in the Miller Range display regionally-formed L - S tectonite fabrics that are particularly evident in layered Argosy Formation schists and Miller Formation gneisses. Metamorphic foliation, expressed by compositional layering and coplanar mineral schistosity, in general dips southwest and contains a mineral alignment lineation of variable orientation. Ubiquitous isoclinal folds of 10 cm to 10 m amplitude trend subhorizontally to the northwest or southeast, the axes of which plunge generally less than 25 ° . Locally there is evidence of polyphase folding [14,25,36]. Grindley [36] also described field relations indicating that the Miller Formation lies structurally above lower-plate units along a reverse fault he termed the Endurance thrust; he attributed three discrete phases (D~_ 3) of pre-Ross deformation to the formation of large-scale folds during high-grade metamorphism and displacement along this structure. Grindley [36] postulated a reverse sense of displacement for the Endurance thrust, based on its geometry and fold orientations in lower-plate rocks, in which upper-plate Miller rocks were emplaced to the northeast over lowerplate units during early stages of regional metamorphism. He also cited evidence for minor postmetamorphic folding (D4) during deformation of younger units to the east during the Ross orogeny. We concur with Grindley that a major structural zone exists in the Miller Range which separates structurally higher Miller gneisses from lower units SW

NE calc-silicate layer

sheared hbl-bio gneiss

~ 500

, ,o0oe,ss

+ amphibolite

E

ca,.,,,oa,e teetonite

normal fault

rn

m

ultramymonite

+ mylonitic gneiss

[]

mylonitic

orthogneiss

Fig. 2. Sketch of structural relations along Camp Ridge, western Miller Range, taken from photomosaic. Location (CR) near head of Argosy Glacier shown in Fig. 1. Note patterns are not the same as in Fig. 1. Miller Formation in these exposures is principally biotite-hornblende gneiss with subordinate amphibolite and calc-silicate layers; tectonite fabrics in the Miller become increasingly strong toward the northeast. Miller gneisses pass structurally downward abruptly into marble tectonite and sihceous ultramylonite, which are structurally interleaved in detail. Beneath the main zone of mylonitic deformation lie mylonitic orthogneisses (Aurora Formation) that show progressively weakening ductile deformation fabrics toward underlying Nimrod metasedimentary units.

63

P R O T ER OZ OIC D U C T I L E S H O R T E N I N G A N D T R A N S L A T I O N A L O N G T H E A N T A R C T I C M A R G I N O F G O N D W A N A

may comprise a zone several kilometers in thickness. Textures of the mylonitic rocks are best displayed near the head of Argosy Glacier (Fig. 2). Aurora granodiorite of weakly anisotropic fabric grades structurally upward to well-foliated mylonitic gneiss. Coarse-grained tectonites are interrupted in places by narrow seams of black ultramylonite immediately below a wider zone ( - 5 m) of continuous ultramylonite. This zone of intense shear deformation is intimately interleaved with calc-silicate tectonites at the base of the upper-plate Miller gneiss. The structural sequence of orthomylonite-ultramylonite-calc-silicate tectonite is repeated at least once in the vicinity of Spur A by a high-angle normal fault showing down-to-the-northeast displacement. Above the calc-silicate tectonites are strongly foliated and

lineated Miller layered gneisses, which exhibit less strong shear fabrics structurally upward away from the main zone. The planar shear zone fabrics also change progressively upward to irregularly folded Miller gneisses. Microscopic strains are annealed in some samples of mylonitic orthogneiss and calc-silicate gneiss, as a result of continued metamorphism or later heating during intrusion of granitic magmas. Nonetheless, a variety of smallscale shear sense indicators are preserved, such as S - C fabrics, rotated porphyroclasts and boudins, and asymmetric folds (Fig. 3), which consistently indicate a top-to-the-southeast sense of shear in a direction parallel to mylonitic mineral elongation lineation. Based on these relations, our interpretations of Miller Range deformation include the following: (1) the Endurance thrust is a distributed ductile

T a

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Fig. 3. Sketches from photographs of mesoscopic structures used as criteria to determine shear displacement directions within the Endurance shear zone. All SW-directed views are sections in the motion plane as defined by the intersection of mineral elongation lineation and the pole to foliation. (a) S - C fabrics formed in Aurora granodioritic orthogneiss containing K-feldspar augen (horizontal S and left-dipping C in this view define sinistral shear sense). Compass is 8 cm. (b) a-structure formed by K-feldspar megacryst in Aurora granodioritic orthogneiss, showing sinistral shear sense. (c) Rotated boudins of marie layer (mottled) in calc-silieate tectonite (white) below siliceous ultramylonite (black), showing sinistral shear sense. (d) Asymmetric folds along boundary between calc-silicate (white) and siliceous ultramylortite (black), with intervening epidote-amphibole reaction zone (fined); shear sense is sinistral. Pen is 14 cm long.

64

J.W. GOODGE ET AL.

shear zone, rather than a discrete thrust fault; (2) augen textures of the lower-plate Aurora gneisses formed during penetrative ductile deformation, rather than metasomatism; (3) displacement during shear was top-to-the-southeast, rather than to the northeast; (4) ductile deformation along this zone must have occurred after regional metamorphism and at least one episode of folding, and after emplacement of the Aurora granodiorite, but before emplacement of the - 500 Ma granite plutons; and (5) the mylonitic shear zone is not

1M8 400 - -

DEPOSITION

4. Timing of events

Constraints on the timing of metamorphic, plutonic, and structural events within basement rocks exposed in the Miller Range are provided by K-Ar, Rb-Sr, and U-Pb data collected during previous studies. U-Pb data on zircon (presumed to

MAGMATISM

TECTONISM

S 0

500

isoclinally folded by late-stage, post-metamorphic structures, but rather it is structurally repeated by high-angle normal faults.

empiacement of -500 Ma granites

--

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Ross Orogeny m

deposition of Byrd Group

l

/

600 - -

.......................................................................Beardmore

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deposition of Beardmore Group

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deposition of Cobham Fro.

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intrusion of Aurora granodioriUc orthognoiss Deformation of Argosy

2400 deposition of Nimrod Group

3 0 0 0 ~

partial melting of part of Nimrod Group (Miller Fm.)

t

Nimrod source

Fig. 4. Timing of principal geologic events in the central Transantarctic Mountains (see text for sources of information). Arrows show estimated range of activity, and shaded bars show ages of well-constrained tectonic events. Age of Beardmore orogeny is constrained only to be post-750 Ma and pre-Early Cambrian; its actual age or duration m a y be different than shown here.

PROTEROZOIC DUCTILE SHORTENING

AND TRANSLATION

ALONG THE ANTARCTIC MARGIN OF GONDWANA

be of detrital origin) from Nimrod Group gneisses provide a minimum age of Nimrod source materials of 2.8 Ga [40,50]. Grindley and McDougall [38] cited K-Ar evidence for a 1100 Ma age of the Nimrod orogeny, but both K-Ar and Rb-Sr data in Nimrod Group rocks reflect disturbance during the Ross orogeny and emplacement of - 500 Ma granite plutons, by incorporation of excess radiogenic Ar and resetting of the Rb-Sr system [30,38,39,51]. Thus, it is difficult to assign a certain age to Nimrod deformation and metamorphism. Some K-Ar data, however, may indicate a minimum age of uplift and cooling of about 580 Ma following the Beardmore orogeny, and concordant biotite-hornblende ages of Nimrod samples of 500-560 Ma may date cooling and uplift during the Ross orogeny [39]. New Rb-Sr, U-Pb, and Sm-Nd isotopic data have been collected from metamorphic and plutonic basement rocks in the Nimrod to Shackleton Glacier region, and relevant results from the Miller Range are presented here. A 3.03 + 0.02 Ga U-Pb age on zircon from Miller layered gneisses [V.C. Bennett, unpubl, data], and Nd depleted-mantle model ages (T~M) Nd of 2.8-3.0 Ga on Miller rocks [21], reflect a Late Archean minimum age of detrital source materials, as suggested earlier [40,50]. A T~M NO model age for Argosy schist of 2.72 Ga [21] indicates that the Miller and Argosy probably shared a similar source terrane and cannot be distinguished isotopically. Zircon from leucosome pods in Miller gneisses gives a U-Pb age of 2.40 Ga [V.C. Bennett, unpubl, data], indicating a period of high-grade metamorphism and partial melting, possibly in the middle to deep crust. The Aurora granodioritic orthogneiss gives single-crystal Pb-Pb zircon ages of - 1.7 Ga [V.C. Nd Bennett, unpubl, data], and a T~M model age of 2.73 Ga [21], suggesting its igneous protolith was derived from similar crustal components as the other Miller Range metamorphic units during a late Early Proterozoic melting event. These data indicate that protoliths of the major Nimrod Group metamorphic rocks were stabilized as part of the ancient Antarctic craton some 2.6-2.8 Ga ago, and that at least one of the Nimrod subunits (i.e. Aurora) also was generated during a mid-Proterozoic crust-formation event. Post-tectonic granites give U-Pb zircon ages of - 500 Ma [J.M. Mattinson, unpubl, data] and Tiara Nd model ages of

65

- 2 . 0 Ga, indicating their magmas were not derived from material like that exposed presently in the Miller Range. Thus, the following pre-Devonian events may be accepted with someconfidence (Fig. 4). Nimrod Group sedimentary protoliths were derived in part from an Archean source terrain. Nimrod orthogneisses provide evidence for at least one late Early Proterozoic magmatic event, and they provide a maximum age limit on ductile deformation in the Endurance shear zone, which is constrained to have occurred between - 1.7 and - 0.50 Ga. These meta-igneous rocks also provide a minimum age of an early contractional deformation within the Nimrod gneisses that they intrude. The actual number and age of metamorphic/orogenic events affecting rocks of the Nimrod Group remains unclear, but at least by about 580 Ma they began cooling from metamorphic conditions indicative of lower crustal residence and were uplifted prior to emplacement of the Cambro-Ordovician granite phitons. At this time, sediments of the latest Precambrian Beardmore Group, which show evidence of Ross deformation, were heated and recrystallized in contact aureoles of the - 500 Ma plutons [39]. 5. Discussion

Nimrod Group rocks in the Miller Range are an important geological component with respect to Precambrian orogenic activity within the central Transantarctic Mountains. The geologic development of these rocks is clearly complex and our understanding is rudimentary at present. Nevertheless, structural fabrics in tectonites of the Miller Range and isotopic data available so far yield considerable insight to the tectonic evolution of the Antarctic Gondwana margin. This is possible despite differences in the tectono-stratigraphic correlations that have been proposed for various constituents of the Nimrod and Beardmore Groups. Based on lithologic, petrologic, and structural relations the Nimrod Group is divisible into at least two distinct lithotectonic packages, a structurally high layered gneiss unit (Miller Formation) and structurally lower metasedimentary and meta-igneous rocks (Argosy, Worsley, and Aurora Formations). We infer both packages to represent

66

an isolated exposure of the East Antarctic cratonal shield; these rocks may in fact be the only recognized re-worked Archean basement exposed in the entire Transantarctic Mountains. Miller rocks give Archean source ages and evidence of an early Early Proterozoic anatectic event. Upper amphibolite- to granulite-facies metamorphism and complex fold deformation may pre-date post-late Early Proterozoic movement along the Endurance shear zone that juxtaposes these rocks against lower-grade metamorphic units of the lower plate. With present data, we cannot rule out the possibility that the Miller Formation and lower-plate metasedimentary rocks underwent temporally and spatially independent histories prior to their juxtaposition along the Endurance shear zone. However, Nd isotopes in the lower- and upperplate rocks are indistinguishable, indicating similar source terrains and the possibility of common depositional tectonic settings. In this case, the difference in metamorphic grade may indicate that the lower-plate rocks were a higher-crustal equivalent of the Miller Formation. Contact relations, rock textures, and zircon morphology all indicate the lower-plate Aurora orthogneiss was emplaced into the older deformed metasedimentary sequence as a plutonic granodiorite. Its crystallization age of 1.7 Ga places an important constraint on the age of pre-magmatic dynamothermal event(s) and post-magmatic ductile deformation. Thus, at least some of the high-grade rocks were metamorphosed and deformed before about 1.7 Ga, and they may record an Early to Middle Proterozoic shortening event. The Endurance shear zone is viewed as a significant crustal-scale structure exposed in the central-western Miller Range, which separates gneisses of the cratonal interior from lower-grade rocks that developed closer to the Late Proterozoic plate margin. Displacement of upper-plate rocks along this low-angle ductile zone was to the southeast (present-day coordinates), rather than to the east or northeast. We estimate crustal contraction of the order of - 2 5 km along the main ductile zone based on: (1) the regional dip of fabrics within the shear zone; and (2) differences in pre-shear peak metamorphic temperatures of > 200°C recorded in gneisses of the upper plate relative to greenschist-facies rocks of the lower plate (assuming a normal continental geothermal

J.W. G O O D G E

E T AL.

gradient). A thin upper plate and low angle of movement is consistent with Nd isotopic compositions in post-tectonic granites exposed in the Miller Range, which s h o w no isotopic contribution from the veneer of Archean material exposed at the surface. Nonetheless, as noted earlier by Grindley and Laird [26] and Grindley [36], the presence of eclogite blocks within the Miller tectonites indicates structural involvement of deep-crustal material. Other ductile shear zones of potentially large displacement within the Miller and Geologists Ranges are the object of on-going study [55]. The significance of the Endurance shear zone in terms of Early Proterozoic to early Paleozoic central Transantarctic Mountains orogenesis remains uncertain, but two likely alternatives are: (1) the Endurance shear zone represents a mid- to deep-crustal expression of contractional fold-belt structures recorded in supracrustal rocks to the east, either those within the Beardmore Group (formed during the Late Proterozoic Beardmore orogeny) or the Byrd Group (formed during the early Paleozoic Ross orogeny); or (2) the Endurance shear zone may significantly pre-date Beardmore-age structures. With presently available age data we are unable to discriminate between these two interpretations. Nevertheless, ductile deformation features in these rocks may be a mid- to deep-crustal response to protracted Proterozoic to early Paleozoic basement-involved orogenesis. The southeast-directed displacements recorded by mylonitic rocks in the Endurance shear zone add a new perspective to the Proterozoic and early Paleozoic tectonic patterns along the western Antarctic margin of Gondwana. The general disposition of Cambro-Ordovician plate-margin tectonic elements is now fairly well-established. In the present region of the central Transantarctic Mountains between the Byrd and Scott Glaciers, the early Paleozoic Antarctic margin was characterized by subduction of oceanic lithosphere westward beneath cratonal and more outboard, allochthonous comporients. Furthermore, the Cambro-Ordovician convergent boundary was probably oriented subparallel to the present trend of the mountain belt, based on the facies distribution of Cambrian sedimentary rocks [18,23,29], the distribution and isotopic variation of granite emplaced at - 5 0 0 Ma [21], and orogenic trends

PROTEROZOIC

DUCTILE

SHORTENING

AND TRANSLATION

ALONG

be of detrital origin) from Nimrod Group gneisses provide a minimum age of Nimrod source materials of 2.8 Ga [40,50]. Grindley and McDougall [38] cited K-Ar evidence for a 1100 Ma age of the Nimrod orogeny, but both K-Ar and Rb-Sr data in Nimrod Group rocks reflect disturbance during the Ross orogeny and emplacement of - 500 Ma granite plutons, by incorporation of excess radiogenic Ar and resetting of the Rb-Sr system [30,38,39,51]. Thus, it is difficult to assign a certain age to Nimrod deformation and metamorphism. Some K-Ar data, however, may indicate a minimum age of uplift and cooling of about 580 Ma following the Beardmore orogeny, and concordant biotite-hornblende ages of Nimrod samples of 500-560 Ma may date coohng and uplift during the Ross orogeny [39]. New Rb-Sr, U-Pb, and Sm-Nd isotopic data have been collected from metamorphic and plutonic basement rocks in the Nimrod to Shackleton Glacier region, and relevant results from the Miller Range are presented here. A 3.03 _ 0.02 Ga U-Pb age on zircon from Miller layered gneisses [V.C. Bennett, unpubl, data], and Nd depleted-mantle model ages (TDNd) of 2.8-3.0 Ga on Miller rocks [21], reflect a Late Archean minimum age of detrital source materials, as suggested earlier [40,50]. A T~M Nd model age for Argosy schist of 2.72 Ga [21] indicates that the Miller and Argosy probably shared a similar source terrane and cannot be distinguished isotopically. Zircon from leucosome pods in Miller gneisses gives a U-Pb age of 2.40 Ga [V.C. Bennett, unpubl, data], indicating a period of high-grade metamorphism and partial melting, possibly in the middle to deep crust. The Aurora granodioritic orthogneiss gives single-crystal Pb-Pb zircon ages of - 1.7 Ga [V.C. NO model age of Bennett, unpubl, data], and a T~M 2.73 Ga [21], suggesting its igneous protolith was derived from similar crustal components as the other Miller Range metamorphic units during a late Early Proterozoic melting event. These data indicate that protoliths of the major Nimrod Group metamorphic rocks were stabilized as part of the ancient Antarctic craton some 2.6-2.8 Ga ago, and that at least one of the Nimrod subunits (i.e. Aurora) also was generated during a mid-Proterozoic crust-formation event. Post-tectonic granites give U-Pb zircon ages of - 500 Ma [J.M. Mattinson, unpubl, data] and T~M Nd model ages of

THE ANTARCTIC

MARGIN

OF GONDWANA

67

- 2 . 0 Ga, indicating their magmas were not derived from material like that exposed presently in the Miller Range. Thus, the following pre-Devonian events may be accepted with some confidence (Fig. 4). Nimrod Group sedimentary protoliths were derived in part from an Archean source terrain. Nimrod orthogneisses provide evidence for at least one late Early Proterozoic magmatic event, and they provide a maximum age limit on ductile deformation in the Endurance shear zone, which is constrained to have occurred between - 1.7 and - 0.50 Ga. These meta-igneous rocks also provide a minimum age of an early contractional deformation within the Nimrod gneisses that they intrude. The actual number and age of metamorphic/orogenic events affecting rocks of the Nimrod Group remains unclear, but at least by about 580 Ma they began cooling from metamorphic conditions indicative of lower crustal residence and were uplifted prior to emplacement of the Cambro-Ordovician granite plutons. At this time, sediments of the latest Precambrian Beardmore Group, which show evidence of Ross deformation, were heated and recrystallized in contact aureoles of the - 500 Ma plutons [39]. 5. Discussion

Nimrod Group rocks in the Miller Range are an important geological component with respect to Precambrian orogenic activity within the central Transantarctic Mountains. The geologic development of these rocks is clearly complex and our understanding is rudimentary at present. Nevertheless, structural fabrics in tectonites of the Miller Range and isotopic data available so far yield considerable insight to the tectonic evolution of the Antarctic Gondwana margin. This is possible despite differences in the tectono-stratigraphic correlations that have been proposed for various constituents of the Nimrod and Beardmore Groups. Based on hthologic, petrologic, and structural relations the Nimrod Group is divisible into at least two distinct lithotectonic packages, a structurally high layered gneiss unit (Miller Formation) and structurally lower metasedimentary and meta-igneous rocks (Argosy, Worsley, and Aurora Formations). We infer both packages to represent

68

(2) The two Nimrod lithotectonic packages are juxtaposed along a distributed ductile shear zone that records significant eastward contraction, as indicated by differences in metamorphic grade and inferred crustal level. The age of movement within this zone is constrained to be between 1.7 and 0.5 Ga. (3) The Endurance shear zone also records southeast-directed displacements subparallel to the paleogeographically reconstructed Late Cambrian plate margin in the region. By extrapolation, possible Proterozoic along-strike displacement may reflect the presence of an obliquely subducting oceanic plate beneath the western Antarctic margin of Gondwana in Middle Proterozoic to early Paleozoic time. (4) Not all deformation of Nimrod Group rocks is required to be synchronous with ductile deformation. Gneisses containing mesoscopic folds that pre-date the ductile fabrics are intruded by a 1.7 Ga granodiorite, providing evidence for a pre-late Early Proterozoic contractional deformation.

Acknowledgements We thank the UH-1N and LC-130 flight crews of the U.S. Navy Antarctic Development Squadron Six (VXE-6) for getting us into a number of tight spots (and out again) during field work conducted in the upper Nimrod Glacier area during the austral summers between 1985-1987. We also thank Eugene Mikhalsky and David Edgerton for comradeship in the field, and Don DePaolo, Vicki Hansen, Jim Mattinson, Simon Peacock, Peg Rees, Bert RoweU, and Ed Stump for discussion of ideas. Jim Mattinson (University of CaliforniaSanta Barbara) kindly provided preliminary U-Pb age data on the granites. We are grateful to Vicki Hansen, Bert Rowell, and an anonymous reviewer, who provided valuable critiques of the manuscript. This work was supported by the National Science Foundation under grants DPP83-16807 (Borg) and DPP88-16807 (Goodge). References 1 T. Van Autenboer and W. Loy, Recent geological investigations in the Sor Rondane Mountains, Belgicafjella and

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