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Geological and Geophysical Investigations in the Northern Ford

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Geological and Geophysical Investigations in the Northern Ford Fosdick metamorphic complex. The Fosdick Mountains provide Geological and geophysical an opportunity for direct observation of the deeper levels of investigations in the continental crust in this segment of Marie Byrd Land. The Fos- northern Ford Ranges, dick metamorphic complex consists primarily of a variety of interlayered migmatitic orthogneisses and paragneiss. The Marie Byrd Land, West Antarctica complex is subdivided into three structural-lithologic assem- blages: • predominantly metapelitic gneisses with a steep 5, and BRUCE P. LUYENDYK, STEVE M. RICHARD, strong crenulation imposed by development of subhorizontal and CHRISTINE H. SMITH S2 (Avers assemblage); • predominantly granitoid orthogneisses, well layered in S, Institute for Crustal Studies with subsidiary layers of paragneiss (Bird Bluff assemblage); University of California and Santa Barbara, California 93106-1100 • transitional metamorphic/plutonic rocks (Neptune-Griffiths transitional rocks). We interpret that the protolith for the paragneiss component of DAVID L. KIMBROUGH the Fosdick metamorphic complex was a relatively homogene- ous graywacke-argillite, probably correlative with the Cambro- Department of Geological Sciences Ordovician Swanson Formation (Bradshaw, Andrews and Field San Diego State University 1983), intruded by granodiorite plutons (Devonian-Carbonifer- San Diego, California 92182 ous Ford granodiorite?). The mixed orthogneiss and paragneiss of the northern part of the complex pass southward, structur- The Phillips, Fosdick, and Chester Mountains are located in ally upward, into mostly granodioritic orthogneiss. Biotite, sil- the northern Ford Ranges of western Marie Byrd Land. They limanite, garnet, cordierite, quartz and potassium-feldspar are trend east-west and are separated by about 15-20 kilometers present in pelitic gneisses throughout the range. Petrologic ob- (figure 1). Plutonic rocks crop out in the Phillips and Chester servations indicate a clockwise pressure-temperature path with Mountains while the Fosdick Mountains comprise high-grade an episode of near-isothermic decompression at the time of metamorphic rocks. The Ford granodiorite is a Devonian-Car- migmatization. Leucogranitic neosome bodies range from boniferous granodiorite-tonalite found in the Phillips and homotropic and crosscutting to gneissic and concordant, dem- Chester Mountains and the Denfield Mountains immediately onstrating that migmatization and the development of penetra- to the south; the Byrd Coast granite is a Cretaceous epizonal tive gneissic foliation were coeval. Peak metamorphic condi- granite which is ubiquitous throughout the Ford Ranges (Wade, tions are estimated at 4-5 kilobars and 725-800 °C. Preliminary Cathey, and Oldham 1977a, 1977b, 1978). The Fosdick meta- uranium-lead data indicate that migmatization occurred during morphic complex comprises migmatized paragneiss and or- Mesozoic time, overprinting older metamorphic events. Argon- thogneiss found in the Fosdick Mountains (Wilbanks 1972). We 40/argon-39 cooling studies show evidence of rapid exhumation conducted field studies during the 1989-1990 and 1990-1991 of the Fosdick Mountains in Late Cretaceous time. austral summers in these ranges. The field studies included The Fosdick Mountains constitute a large-scale deformation mapping, structural measurements, and sampling for isotopic, zone. Older, steeply dipping gneissic foliation is preserved in paleomagnetic, and petrological investigations (Project FORCE; low-strain, highly crenulated zones in the north-central part of Ford Ranges Crustal Exploration). (See also Kimbrough et al. the range. Straight gneisses in much of the rest of complex are 1990.) The field team included the authors and mountaineers interpreted to be transposed from this older foliation. This Alasdair Cain and Steve Tucker for 1989-1990 and John Roberts younger foliation dips predominantly south to southeast. Mig- and Terry Schmidt for 1990-1991. matization was accompanied and followed by intrusion of mafic The Phillips, Fosdick, and Chester Mountains form one or dikes. Advanced partial melting led to the formation of block more south-dipping tilted blocks. The north face of the Fosdick gneiss, in which lenses of paleosome and boudinaged dikes Mountains is defined by spectacular cliffs up to 500 meters were engulfed in a matrix of massive granitic neosome. Block high. A subhorizontal geomorphic surface seen in the Phillips gneiss emplacement was largely post-kinematic. Retrograde (Mount Paige) and the eastern Fosdick Mountains, may be the pegmatites and extensional veins trend east-southeast, consis- Late Cretaceous/Early Tertiary erosion surface described tent with north-northeast extension during initial cooling of the throughout Marie Byrd Land by LeMasurier and Rex (1983). complex. Our field mapping resulted in revisions to the maps of Wade, The deformation of the metamorphic complex was largely a Cathey, and Oldham (1977a, 1978). In the Phillips Mountains, flattening, as indicated by the lack of a well-defined lineation outcrops at 145°W are Ford granodiorite not Byrd Coast granite, and the highly variable orientation of syn-metamorphic mafic and Herrmann Nunatak is Ford granodiorite not Byrd Coast dikes. One discrete, high-strain zone, about 50 meters wide granite. In the Fosdick Mountains, some outcrops mapped as and dipping to the south is located on the southeast ridge of Fosdick metamorphic rocks in the east are Cenozoic volcanics. Mount Richardson; shear indicators show top-to-the-west shear A transitional plutonic-metamorphic rock unit (Neptune-Grif- in this zone. Major plastic deformation was synchronous with fiths transitional rocks) has been defined for rocks exposed on peak metamorphic conditions. Progressive overprinting by Neptune and Griffith Nunataks and on the south side of Mount lower temperature deformation features, typical of mid-crustal Richardson. It comprises foliated hornblende-biotite tonalite or ductile deformation zones, is not observed. granodiorite (Ford granodiorite?) with metamorphic enclaves. Plutonic and hypabyssal rocks. Ford granodiorite outcrops south It is interpreted to represent the transition between the Fosdick of the Fosdick Mountains in the Chester Mountains consist of Complex and the Ford granodiorite of the Chester Mountains. granodiorite plutons (approximately 353 million years old, 1991 REVIEW 37 Firmann Nunatak illi %f I I cMou ntai Ford Ranaes . SCIRM Hutcheson ( -- BlockBay >1 Bachefl Glacier, • • •,; U •.SS - .- W d1G ' F p ) S)( (( Nu, ".5 %4...yMOufltaU: mt:: in \ mt. P rkins ve Lockhart .............................................................. .. .r, A 1? :::( \ : : : .........:•:•: •:•:.•:•: Map Units Tertiary basalt Chester WW Bird Coast Granite Mountains N Ford granodiorite inferred fault foliated granodiorite and granite • camp Fosdick Gneiss mixed gneiss o km 10 20 outcrop mostly paragneiss I I I I - - - - - travel routes Figure 1. Revised geologic map of the northern Ford Ranges showing routes of the Ford Ranges Crustal Exploration (FORCE) project trail parties during the 1990-1991 austral field season. based on uranium-lead zircon studies) with pendants of upper Lithologic gradation from mixed migmatite (Fosdick meta- greenschist facies metasedimentary rocks. Uranium-lead dis- morphic complex) to granodioritic orthogneiss (Neptune-Grif- cordia on zircons show an inherited age of 1,300 million years fiths transitional rocks) to unfoliated granodiorite (Ford grano- suggesting Precambrian contributions to source magmas. A diorite) between the northern Fosdick and Chester Mountains similar tonalite to granodiorite plutonic complex crops out in suggests that the Fosdick complex originated in a sub-bathol- the western Phillips Mountains north of the Fosdick Moun- ithic domain related to the Ford granodiorite (figure 2). Ura- tains; these are correlated with Ford granodiorite. Most of the nium-lead isotopic data indicating Mesozoic metamorphism Phillips Mountains, the eastern Fosdick Mountains, and iso- suggests migmatization was coeval with the intrusion of Byrd lated nunataks east of the Chester Mountains comprise epi- Coast granites. Chemical data from Byrd Coast granite zonal biotite granite (approximately 103 million years old, based (Weaver, Bradshaw, and Adams 1991) preclude derivation of on uranium-lead zircon studies) correlated with the Byrd Coast these plutons from anatectic melts derived from migmatite granite. Both Ford granodiorite and Byrd Coast granite are complexes like the Fosdick metamorphic complex. Heating as- intruded by mafic dikes. Argon-40/argon-39 age spectra on sociated with the plutonism may have resulted in minor mid- hornblende from a dike in the Chester Mountains are complex crustal melting without generating discrete magma bodies. De- but indicate an Early Cretaceous age. Undeformed biotite-mus- formation of the complex probably reflects some combination covite granite intrudes the Fosdick metamorphic complex west of flattening to accommodate pluton intrusion and large-scale of Mount Avers. Two-mica granites also intrude the Ford grano- deep-crustal flow related to initial breakup of Gondwanaland. diorite in the Chester Mountains and the Neptune-Griffiths Volcanic rocks. Cenozoic volcanic centers (with a potassium- transitional rocks at Neptune Nunataks. Uranium-lead zircon argon age of 39 to 3.4 million years (LeMasurier and Rex 1982)) studies from the Neptune granite suggest a Permian age. crop out along an east-west trend in the eastern Fosdick Moun- 38 ANTARCTIC
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