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Metamorphic rocksin the Geologists and Miller ranges,

Nimrod Glacier area,central Transantarctic Mountains

JOHN W. GOODGE and VICKI L. HANSEN

Department of Geological SciencesSouthern Methodist University

Dallas, Texas 75275

SIMON M. PEACOCK

Department of GeologyArizona State UniversityTempe, Arizona 85287

BRAD K. SMITH

Institute of Earth SciencesState University of Utrecht

Utrecht, Netherlands

This article summarizes results of geologic field work con-ducted in the Nimrod Glacier area of the central TransantarcticMountains during 1989-1990. Our field party established twocamps in the Geologists Range and Miller Range to study base-ment metamorphic rocks of the Precambrian Nimrod Group(figure 1). The principal goal of the project is to improve ourunderstanding of the tectonic evolution of these metamorphicrocks with respect to development of the ancient continentalmargin along the eastern edge of the antarctic craton. Our workduring the 1989-1990 season focused on field relations betweenlithotectonic units, mega- and mesoscopic structural relations,and collection of samples for quantitative petrologic and ther-mochronometric analysis. A New Zealand field party recon-noitered the Geologists Range in 1961-1962 (Grindley 1962),but no detailed report of the geology in this range has previ-ously been published.

Field relations in the Geologists Range. Metamorphic rocks ofthe Nimrod Group are exposed in the Geologists Range alongthe Endurance and Quest Cliffs areas (figure 1). They may bedivided into four principal units based on lithological andstructural grounds. The first, exposed in the southern end ofthe range, consists of complexly folded layered amphiboliticgneiss, rich in modal hornblende and plagioclase. This unit isstructurally higher than other Geologists Range units and maybe correlative with the Miller Formation as regarded by Goodgeet al. (in press). The other units, exposed in both areas of theGeologists Range, include metasedimentary schists, a biotite-hornblende granodioritic orthogneiss, and calc-silicate gneiss.These three units are correlative with units of the NimrodGroup exposed in the Miller Range exclusive of the MillerFormation. The metasedimentary schists constitute a lithol-ogically varied assemblage of interlayered pelitic schist, mi-caceous quartzofeldspathic schist and gneiss, micaceousquartzite, amphibolite, and caic-silicate gneiss. A metasedi-mentary origin for the bulk of this unit is reflected by com-positional layering on a scale of centimeters to meters.

Figure 1. Geologic sketch map of the Geologists Range. Inset showslocation in the Nimrod Glacier area. The Precambrian Nimrod Groupis divided into four metasedimentary and meta-igneous units, asdescribed in text, shown in legend generally in ascending structuralposition. These rocks are tentatively correlated with Nimrod unitsin the Miller Range. Post-tectonic granite of the Mount Summersonpluton is probably correlative with other approximately 500 million-year-old granites in the central Transantarctic Mountains. Tecton-ites of the Nimrod Group display uniformly gently plunging north-west-trending elongation lineations. These tectonites enclose severaltectonic blocks of mafic and ultramafic rocks in the Mount Albrightarea. (km denotes kilometer.)

In general, rocks in the Geologists Range were metamor-phosed in at least the middle amphibolite facies. Represen-tative metamorphic assemblages include garnet + aluminum-silicate + mica pelitic schist, garnet + hornblende amphibol-ite, and diopside + wollastonite calc-silicate gneiss. Alumi-num silicates include both kyanite and sillimanite. Structurallyenclosed within metasedimentary tectonites in several places(particularly in the Mount Albright area) are tectonic blocks(!^ 10 meters) of mafic and ultramafic rocks (figure 1). Rims ofretrograde(?) amphibolite around the mafic blocks suggest thatthe block interiors may not be isofacial with the enclosingmatrix.

Metamorphic rocks in the Geologists Range exhibit pene-trative L-S tectonite fabrics consisting of a generally southwest-dipping mylonitic foliation and gently northwest- or southeast-plunging mineral elongation lineation. Nimrod tectonites forma continuous structural zone of ductilely deformed rocks atleast 13 kilometers in thickness. Field assessment of strainvariation through this zone was not possible. Mesoscopic and

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microscopic structures record a consistent top-to-the-southeastsense of shear (figure 2), similar to kinematic indicators intectonites of the western Miller Range (Goodge et al. in press).Large-scale folds in the layered metasedimentary sequence,particularly in the Mount Albright and Mount Ronca areas,plunge north to northwest subparallel to mylonitic lineation.

Metamorphic rocks in the northern part of the EnduranceCliffs area are intruded by the Mount Summerson granite (fig-ure 1), which is correlative with a suite of approximately 500million-year-old post-tectonic granites exposed throughout thecentral Transantarctic Mountains (Borg, DePaolo, and Smith1990).

Field relations in the Miller Range. Due to an abbreviated fieldseason, we concentrated our field work in the Miller Range inthe northern region near the head of the Argosy Glacier. Geo-logic relations in this area are described by others (Grindley,McGregor, and Walcott 1964; Gunner 1969; Grindley 1972;Gunner 1976; Adams, Gabites, and Grindley 1982; Goodge etal. in press), and we will not elaborate on them here. Lithol-ogically varied metamorphic rocks in this area are similar tothose exposed in the Geologists Range, and they are stronglyductilely deformed in a shear zone originally mapped by Grin-dley (1972) as the Endurance thrust.

We completed a detailed transect normal to strike of tectonitefoliation in the vicinity of Argosy Glacier. Metamorphic rocksalong this approximately 30-kilometer transect exhibit pene-trative ductile shear fabrics of similar type and orientation tothose that occur in the Geologists Range. We, thus, view theEndurance thrust as a distributed zone of ductile shear withan exposed structural thickness of approximately 15 kilome-ters. Kinematic indicators throughout the zone record tectonicdisplacement in a top-to-the-southeast direction parallel to my-lonitic elongation lineation. Based on arguments outlined by

Figure 2. Photo of metasedimentary unit in Geologists Range,showing thin layering of tectonite foliation and southeast-dippingshear bands indicative of top-to-the-southeast shear. View is to thesouthwest with elongation lineation in the plane of the photo. Penis 14 centimeters long.

Goodge et al. (in press), this shear zone may represent a struc-tural juxtaposition between Miller Formation and the otherNimrod units. High-temperature pelitic tectonites containingkyanite enclose tectonic blocks of possible eclogite near thehead of Argosy Glacier. From the coexistence of kyanite andmuscovite in the ductile tectonites, we infer that the shear zoneformed at depths of at least 24 kilometers (Goodge et al. 1990).

We consider the ductilely deformed rocks in the Geologistsand Miller ranges to constitute a single tectonic zone of con-siderable structural thickness. Ductilely deformed tectonites inboth of these ranges are an important element of the Precam-brian evolution of the Transantarctic Mountains. Petrologicfeatures of these rocks and their structural fabrics reflect a mid-to deep-crustal setting during Late Proterozoic to early Paleo-zoic continental-margin orogenesis. Ongoing research in-cludes structural and kinematic analysis, analysis of pressureand temperature conditions during synkinematic metamorph-ism, and thermochronometry of metamorphic and other events.

We thank the LC-130 crews of the U.S. Navy VXE-6 squad-ron for assistance in completion of our field work, and MichaelRoberts for his contributions to the field party. This work wassupported by National Science Foundation grant DPP 88-16807.

References

Adams, C.J.D., J.E. Gabites, and G.W. Grindley. 1982. Orogenic his-tory of the central Transantarctic Mountains: New K-Ar age data onthe Precambrian-Lower Paleozoic basement. In C. Craddock (Ed.),Antarctic geoscience. Madison: University of Wisconsin Press.

Borg, S.G., D.J. DePaolo, and B.M. Smith. 1990. Isotopic structureand tectonics of the central Transantarctic Mountains. Journal of Geo-physical Research, 95, 6,647-6,669.

Goodge, J.W., S.G. Borg, B.K. Smith, and V.C. Bennett. In press.Tectonic significance of Proterozoic ductile shortening and transla-tion along the Antarctic margin of Gondwana. Earth and PlanetaryScience Letters.

Goodge, J.W., V.L. Hansen, S.M. Peacock, and B.K. Smith. 1990.Deep-crustal ductile deformation within the central TransantarcticMountains. EQS. Transactions of the American Geophysical Union,71, 643.

Grindley, G.W. 1962. New Zealand Antarctic Expedition 1961-62.Beardmore-Nimrod region. New Zealand Alpine Journal, 49, 41-46.

Grindley, G.W. 1972. Polyphase deformation of the Precambrian Nim-rod Group, central Transantarctic Mountains. In R.J. Adie (Ed.),Antarctic geology and geophysics. Oslo: Universitetsforlaget.

Grindley, G.W., V.R. McGregor, and R.I. Walcott. 1964. Outline ofthe geology of the Nimrod-Beardmore-Axel Heiberg Glaciers region,Ross Dependency. In R.J. Adie (Ed.), Antarctic geology. Amsterdam:North-Holland Publishing.

Gunner, J.D. 1969. Pet rogra phy of metamorphic rocks from the Miller Range,Antarctica. (Institute of Polar Studies, Report No. 32.) Columbus:Ohio State University.

Gunner, J.D. 1976. Isotopic and geochemical studies of the pre-Devonianbasement complex, Beardmore Glacier region, Antarctica. (Institute ofPolar Studies, Report No. 41.) Columbus: Ohio State University.

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