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Geology and Petrology of the Mcmurdo Volcanic Group at Rainbow Ridge, Brown Peninsula, Antarctica

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Geology and Petrology of the Mcmurdo Volcanic Group at Rainbow Ridge, Brown Peninsula, Antarctica Geology and petrology of the McMurdo Volcanic Group at Rainbow Ridge, Brown Peninsula, Antarctica PHILIP R. KYLE Institute of Polar Studies, Ohio State University, Columbus, Ohio 43210 JOHN ADAMS Department of Geology, Victoria University, Wellington, New Zealand PETER C. RANKIN Soil Bureau, Department of Scientific and Industrial Research, Lower Hütt, New Zealand ABSTRACT Strongly undersaturated rocks of the McMurdo Volcanic Group of late Cenozoic age at Brown Peninsula form three eruptive cycles of basaltic to salic lavas. Two cycles at Rainbow Ridge show a basanite to nepheline-hawaiite to nepheline-benmoreite and a basanite to nepheline-benmoreite eruptive sequence. Fractional crystallization processes for the sequence have been modeled using major-element least squares mass-balance models. Two models can ex- plain the basanite to nepheline-hawaiite transition. Model 1 involves fractionation of olivine, clinopyroxene, plagioclase, opaque oxides, and apatite; model 2 is simi- lar, but also includes kaersutite. The nepheline-hawaiite to nepheline-benmoreite transition involves fractionation of clinopy- roxene, kaersutite, plagioclase, opaque oxides, and apatite. Trace-element (includ- ing rare-earth element) contents calculated using solutions from the mass-balance models are compatible with both models for the formation of nepheline-hawaiite. Excellent agreement is shown between cal- culated and observed trace-element data for the nepheline-benmoreite. Small pods of basanite, derived from a garnet peridotite mantle by a low degree of partial melting, have been intruded into the crust where subsequent fractional crystallization formed the intermediate and salic rocks. This process has apparently occurred re- peatedly at many eruptive centers in the McMurdo Sound area. INTRODUCTION Undersaturated alkalic volcanic rocks of late Cenozoic age in the McMurdo Sound area constitute the Erebus volcanic province of the McMurdo Volcanic Group (Kyle and Figure 1. Generalized geological map of Brown Peninsula, showing location of Rain- Cole, 1974). The lavas range in composi- bow Ridge (after Cole and Ewart, 1968). Inset shows location map of McMurdo Sound tion from basanite (basanitoid) to phono- area. Geological Society of America Bulletin, Part I, v. 90, p. 676-688, 9 figs., 8 tables, July 1979, Doc. no. 90710. 676 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/7/676/3419086/i0016-7606-90-7-676.pdf by guest on 01 October 2021 McMURDO VOLCANIC GROUP, ANTARCTICA 677 lite. Trachyte has been reported at one lo- dertaken in January 1973 as part of tatively proposed were (youngest to oldest) cation (Smith, 1954; Goldich and others, Victoria University of Wellington Antarctic (1) Trachyte Hill Formation, (2) Nubian 1975) but is volumetrically insignificant. Expedition (VUWAE) 17. Although Rain- Basalt Formation, (3) Aurora Trachyte Two lava lineages have been recognized on bow Ridge was almost completely snow- Formation, and (4) Melania Basalt Forma- the basis of mineralogical and detailed free, geologic contacts were commonly tion. Black Island was the type area for the geochemical data (Kyle and Rankin 1976; obscured by loose debris. A geological map three older formations, Cape Bird the type Sun and Hanson, 1976). The Dry Valley (Fig. 2) has been compiled from field obser- area for the youngest. Cole and Ewart Drilling Project (DVDP) lineage of basanite, vations and petrological examination of 54 (1968, p. 802) qualified their tentative nepheline-hawaiite, nepheline-mugearite, samples. proposal by stating that it was "quite likely nepheline-benmoreite, mafic phonolite that each formation was erupted over a evolved by fractional crystallization of VOLCANIC STRATIGRAPHY period and eruptions in one area do not olivine, clinopyroxene, kaersutite, spinel, necessarily exactly correspond in time to feldspar, and apatite (Kyle, unpub.), Cole and Ewart (1968) mapped alternat- eruptions in another area." Cole and others whereas evolution of the Erebus lineage of ing basalt-trachyte-basalt-trachyte eruptive (1971) extended the use of these formation basanite(P) nepheline-hawaiite, nepheline- sequences at Black Island, Brown Peninsula, names to eruptive events at Hut Point benmoreite, anorthoclase phonolite (ke- and Cape Bird in the McMurdo Sound area. Peninsula, Cape Crozier, and White Island. nyte) involved fractionation of the minerals Because of the morphological similarity of They did comment (p. 534) that "prelimi- listed above, with the exception of kaersu- the eruptive sequences, a series of four for- nary K/Ar dates indicate that formations in tite. DVDP lineage lavas are best exposed at mations were proposed and correlated be- different areas within McMurdo Sound Hut Point Peninsula in surface exposures tween the three areas. The formations ten- were erupted at different times." and drill cores from DVDP holes 1, 2, and 3 (Kyle and Rankin, 1976; Kyle, 1976; Kyle and Treves, 1974a). m A petrogenetic model for the origin of 2.7± 0.09 m.y x \ Brown Peninsula lavas is developed here \ 24087 \ and shown to be similar to that for lavas in \ the Hut Point Peninsula area. A model in- Inclusionx | : volving the intrusion of small pods of ..: Flow-. -v mantle-derived basanite magma into an Vl-v.;? upper mantle and/or crustal magma chamber, followed by crystal fractionation, j|:-yx\24090Íy would account for the eruptive sequence at m Rainbow Ridge. The model is believed to be generally applicable to mafic phonolite ISpít- * - . • o f y lavas in the Erebus volcanic province but 'vIV'V-VV.'A;-» will not explain the eruptive history of the anorthoclase phonolite lavas from Mt. wà.f" Erebus. m Rainbow Ridge is situated at the north- western end of Brown Peninsula (Fig. 1). It is 300 m wide and trends north-northeast lOOm. for 1 km (Fig. 2); the highest point is about 380 m above sea level and 150 m above KEY nearby Lake Auger. (Topographic features Inferred stratigraphy at Rainbow Ridge have been given informal (youngest to oldest) names to aid discussion of the geology; 17;:;:] Ne- benmoreite these names, shown in Figure 2, have not ÎÏÏT1 Basanite been approved by the New Zealand Geo- K;-1 Ne-benmoreite graphic Board.) Cole and Ewart (1968) Ne- hawaiite mapped the geology of Rainbow Ridge and considered a flow on the north side to be H3 Basanite one of the youngest flows on Brown Penin- m Moraine sula. K-Ar age determinations (available in X Sample location 1972 and later published by Armstrong, Contact obscured 1978) indicated possible inconsistencies in the inferred eruptive events. A detailed B Grey Cone removed by ice- study of Rainbow Ridge was therefore un- Inclusion Flow Mound Figure 2. Geological map and cross section of Rainbow Ridge, Brown Peninsula. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/7/676/3419086/i0016-7606-90-7-676.pdf by guest on 01 October 2021 678 KYLE AND OTHERS It is apparent from the extensive K/Ar Figure 3 gives suggested stratigraphie Antarctica. Its present status (Fig. 3) is re- dating by Armstrong (1978) that there is nomenclature for the McMurdo Volcanic tained, however, but with caution. little age overlap in volcanic activity among Group, which has been subdivided into four K-Ar ages (Armstrong, 1978) indicate the many eruptive centers in McMurdo informal volcanic provinces (Kyle and Cole, that the timing of an eruptive sequence at Sound. The volcanic centers probably acted 1974; Hamilton, 1972). Inclusion of the any major eruptive center is largely inde- individually, so correlation of these eruptive Balleny volcanic province within the pendent of eruptive sequences at other cen- events is erroneous and misleading. There- McMurdo Volcanic Group can be ques- ters. Therefore, formal or informal strati- fore, it is proposed to discontinue the use of tioned, particularly as it occurs in an graphic names can be applied only locally. formation names in areas other than their oceanic environment and appears to be un- Unfortunately, this will lead to a prolifera- type locality (Kyle, unpub.). related to volcanism within continental tion of names, but this apparently is un- McMurdo Volcanic Group (1,2) Balleny volcanic province (3) Erebus ^volcanic province (4) Hut Point Peninsula Black Island volcanic Bird volcanic volcanic center (6) center (6) center (6) I I I Roberts Cliff Tuffite Twin Crater Nubian Basalt Trachyte Hill (Formation) (7) sequence (8) Formation (9) Formation (9) Herschel Tuffaceous Moraine Half Moon Crater Aurora Trachyte (Formation) (7) sequence (8) Formation (9) Castle Rock Melania Basalt sequence (8) Formation (9) Observation Hill sequence(8) Crater Hill sequence (8) References (1) Harrington (1958) (6) This paper (2) Nathan and Schulte (1968) (7) Harrington and others (1964, 1967) Figure 3. Suggested stratigraphie (3) Hamilton (1972) (8) Kyle and Treves (1974 b) hierarchy for McMurdo Volcanic (4) Kyle and Cole (1974) (9) Cole and Ewart (1968) (5) Kyle (1976); Kyle and Rankin (1976) Group rocks. TABLE 1. WHOLE-ROCK K-Ar AGE DETERMINATIONS OF McMURDO VOLCANIC GROUP ROCKS FROM BROWN PENINSULA, ANTARCTICA Sample Location1 Rock type+ and occurrence Former formation name* Age no. (Cole and Ewart, 1968) (m.y.) 21068 3 km west-northwest Kaersutite basanite flow Melania Basalt Formation 2.1 ± 0.4 of Mt. Wise (hornblende basalt) 21094 1.5 km southeast of Olivine-augite basanite flow Nubian Basalt Formation 2.2 ± 0.09 Mt. Wise (olivine-augite basalt) 21047 2 km northeast of Kaersutite flow Aurora Trachyte Formation 2.25 ± 0.05 Mt. Wise (hornblende
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