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Geochemistry of the Cenozoic Volcanic Rocks of Ross Island and Vicinity, Antarctica

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Geochemistry of the Cenozoic Volcanic Rocks of Ross Island and Vicinity, Antarctica University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Earth and Atmospheric Sciences, Department Papers in the Earth and Atmospheric Sciences of 7-1975 GEOCHEMISTRY OF THE CENOZOIC VOLCANIC ROCKS OF ROSS ISLAND AND VICINITY, ANTARCTICA S. S. Goldich Northern Illinois University S. B. Treves University of Nebraska-Lincoln N. H. Suhr Pennsylvania State University - Main Campus J. S. Stuckless US Geological Survey, Denver, CO Follow this and additional works at: https://digitalcommons.unl.edu/geosciencefacpub Part of the Earth Sciences Commons Goldich, S. S.; Treves, S. B.; Suhr, N. H.; and Stuckless, J. S., "GEOCHEMISTRY OF THE CENOZOIC VOLCANIC ROCKS OF ROSS ISLAND AND VICINITY, ANTARCTICA" (1975). Papers in the Earth and Atmospheric Sciences. 332. https://digitalcommons.unl.edu/geosciencefacpub/332 This Article is brought to you for free and open access by the Earth and Atmospheric Sciences, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Papers in the Earth and Atmospheric Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. VOLUME 83 NUMBER 4 THE JOURNAL OF GEOLOGY July 1975 GEOCHEMISTRY OF THE CENOZOIC VOLCANIC ROCKS OF ROSS ISLAND AND VICINITY, ANTARCTICA1 STUCKLESS2 S. S. GOLDICH, S. B. TREVES, N. H. SUHR, AND J. S. Northern Illinois University, DeKalb, Illinois 60115; University of Nebraska, Lincoln, Nebraska 68508; and Pennsylvania State University, University Park, Pennsylvania 16802 ABSTRACT The Cenozoic volcanic rocks of Ross Island and vicinity, Antarctica, are surface flows, tuffs, breccias, and small intrusives. The prominent rock is basanitoid. Alkali-basalt magma reached the surface over an appreciable length of time and was differentiated to produce a rock series: basanitoid -. trachybasalt -, phonolite. The alkalic, silica-undersaturated rocks are part of a larger petrologic province composed of volcanic centers in a belt nearly 2,000 km long roughly parallel to the Trans- antarctic Mountains. Two trends of magmatic differentiation are apparent. At depth the fractional crystallization of olivine, clinopyroxene, and opaque oxide minerals, all of which are abundant as phenocrysts in the basanitoid flows, produced trachybasalt magma. At crustal levels, fractional crystallization of clinopyroxene, apatite, opaque oxides, kaersutite, plagioclase, and anorthoclase developed the phonolitic rocks. Alkali enrichment is marked, and the end-member phonolites contain 11% NazO, 5.6% K20, and 57% SiO2. The basanitoids contain relatively large amounts of Ba (~ 400 ppm), Sr (~ 1,000 ppm), and Rb (~ 30 ppm). The concentrations of these and other trace elements in the various rock types support the model of small degrees of partial melting of peridotitic mantle for the derivation of the basanitoid magma, and of magmatic differentiation for the subsequent development of the rock series. INTRODUCTION Locations are shown on figures 2 and 3, Twenty-nine samples of volcanic rocks except for sample 5 (fig. 1). The volcanic rocks of the Ross Island from Ross Island and vicinity in Antarctica region are part of a larger province that (fig. 1) were analyzed for major, minor, extends for a distance of 2,000 km and trace constituents. This study is part nearly from Mt. Weaver to the Islands of the Dry Valley Drilling Project (DVDP), Balleny and is parallel to the Trans- a cooperative program sponsored by roughly antarctic Mountains (fig. 1). Hamilton Japan, New Zealand, and the United suggested that the volcanic belt States, and was undertaken to obtain (1972) information on the exposed volcanic rocks might be called the Ross Sea petrologic that might be encountered in drilling. The province with three major subdivisions: and samples were collected by Treves during (1) McMurdo, (2) Hallett, (3) Balleny five field seasons from 1960 to 1971. volcanic provinces. He gave excellent descriptions of the larger features of the major volcanic centers and noted that the 1 Manuscript received September 5, 1974; McMurdo volcanic province is character- revised October 29, 1974. 2 Present address: U.S. Geological Survey, ized by high subaerial volcanoes. To the Denver, Colorado 80225. north in the Hallett province, the volcanic rocks are subglacial domes and low sub- [JOURNAL OF GEOLOGY, 1975, Vol. 83, p. 415-435] © 1975by the Universityof Chicago.All rightsreserved. aerial volcanoes. 415 416 S. S. GOLDICH, S. B. TREVES, N. H. SUHR, AND J. S. STUCKLESS FIG. 1.-Index map of Antarctica showing location of Ross Island area (fig. 2), Mt. Discovery area (fig. 3), and sample no. 5. The Ross Sea petrologic province is in the Transantarctic Mountains, and characterized by alkalic, silica-under- xenoliths in the volcanic rocks of the Ross saturated Neogene lavas and pyroclastic Island vicinity (Prior 1907; Thompson rocks. The volcanic rocks rest on older 1916; Forbes 1963) indicate that the Ross rocks of considerable diversity and range Sea region (fig. 1) is underlain by similar in age. These rocks include granite, gneiss, rocks. This interpretation is supported by and metasedimentary rocks of late Pre- aeromagnetic data (L. D. McGinnis, cambrian and early Paleozoic age, the verbal communication, 1974). The large arenites of the Beacon Supergroup of topographic relief on the western edge of Devonian to Jurassic age, and diabase sills the Ross Sea is interpreted as a zone of of the Ferrar Dolerite, dated as Jurassic by normal faulting or sharp flexures McDougall (1963) and Compston et al. (Hamilton 1972). (1968). The rock names used in this paper are Outcrops of these old rocks range from based on modal compositions supple- sea level to altitudes of 2,000 to 4,000 m mented by chemical and normative data. GEOCHEMISTRY OF CENOZOIC VOLCANIC ROCKS 417 Cape ROSS SEA -77015' -N- -77°30' 2 Cape Roy Crozier Cape -77045' ROSS ICE SHELF Contour Interval : 1000 meters MlcMurdo 8\ 0 10 20 30 Km. C- ,Cape Armit - 168 I I 169 166° 167° 168' 169' I I I i FIG. 2.-Map of Ross Island area showing locations of analyzed samples At each of the major centers in the Ross exposure of volcanics in the Ross Island Island vicinity there appears to be a vicinity, and regardless of their relative succession that ranges from basanitoid to ages, they are chemically and petro- phonolite followed by younger basanitoids graphically similar. All display textures (Treves 1967). Pyroclastic deposits of tuff from intersertal to granular, with augite, and breccia underlie the lavas as has been plagioclase, nepheline, and opaques form- confirmed by the core from DVDP drill ing the groundmass. Olivine and clino- holes recently completed at McMurdo pyroxene are the dominant phenocrysts Station (locality 25, fig. 2). with most samples containing subhedral to euhedral olivine (Foso) and titaniferous PRINCIPAL ROCK TYPES augite. Less ubiquitous phenocrysts in- Basanitoid.-Basanitoids are alkalic clude plagioclase and kaersutite. Labrador- olivine basalts which contain more than ite phenocrysts occur in only one of the 5% normative nepheline, but no pheno- analyzed basanitoids (sample 7) and are crystic nepheline (Macdonald and Katsura not abundant (less than 5%) in other 1964). With the exception of sample 12, basanitoids in the Ross Island vicinity. which contains 3.7% of normative nephe- They are generally anhedral and resorbed. line, the range is from 7.6 to 18%, Kaersutite in three of the analyzed averaging 12% normative nepheline. basanitoids is generally subhedral to Basanitoids occur in nearly every anhedral and rimmed with, or totally 418 S. S. GOLDICH, S. B. TREVES, N. H. SUHR, AND J. S. STUCKLESS FIG. 3.-Map of Mt. Discovery area showing locations of analyzed samples replaced by, opaque oxides. Sparse pheno- augite are the dominant phenocrysts, each crysts of opaque oxides occur in most of ranging from subhedral to euhedral. the samples. Plagioclase, commonly labradorite, makes Rocks of basanitoid composition occur up 4%-39% of each sample and is both as thin flows and pyroclastic cones. normally zoned with a range from An65 to The latter often contain ultramafic, mafic, An35. Kaersutite in the trachybasalts is and sedimentary rock inclusions from not as strongly reacted as that in the which small amounts of xenocrystic basanitoids, but is commonly rimmed by material, such as labradorite, appear to opaque oxides. Accessory phenocrysts have been derived. include olivine, opaque minerals, and Trachybasalt.-The trachybasalts are trace amounts of apatite. The groundmass rocks of intermediate composition, be- consists of plagioclase, augite, finely dis- tween basanitoid and phonolite. These seminated opaques, and, in some samples, rocks are fairly diverse in chemistry and a brown glass. At Half Moon Crater, 2 km mineralogy. They are characterized by north of McMurdo Station, a trachybasalt pilotaxitic to trachytic texture and by contains abundant mafic and sedimentary plagioclase in the phenocryst assemblages. rock inclusions. Like the basanitoids, the trachybasalts are Anorthoclase phonolite.-Anorthoclase undersaturated with respect to silica. phonolites, or antarctic kenytes (Prior Normative nepheline ranges from 6.3 to 1907; Smith 1954; Treves 1967), are 15%, averaging 11%. remarkably uniform in chemistry and Plagioclase, kaersutite, and titaniferous mineralogy. Flows crop out on the summit GEOCHEMISTRY OF CENOZOIC VOLCANIC ROCKS 419 of Mt. Erebus and on the flanks of the intrude and flows overlie basanitoids and mountain. All are coarsely porphyritic and locally are partially
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