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Miocene-Pliocene Ice-Volcano Interactions at Monogenetic Volcanoes Near Hobbs Coast, Marie Byrd Land, Antarctica T.I

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Miocene-Pliocene Ice-Volcano Interactions at Monogenetic Volcanoes Near Hobbs Coast, Marie Byrd Land, Antarctica T.I U.S. Geological Survey and The National Academies; USGS OF-2007-1047, Short Research Paper 074, doi10.3133/of2007-1047.srp074 Miocene-Pliocene ice-volcano interactions at monogenetic volcanoes near Hobbs Coast, Marie Byrd Land, Antarctica T.I. Wilch1 and W.C. McIntosh2 1Department of Geological Sciences, Albion College, Albion MI 49224 2Department of Earth and Environmental Science, New Mexico Tech, Socorro NM 87801 Abstract Paleoenvironmental reconstructions and 40Ar/39Ar geochronology of seven eroded monogenetic volcanoes near the Hobbs Coast, Marie Byrd Land, West Antarctica provide proxy records of WAIS paleo-ice-levels in Miocene- Pliocene times. Interpretations, based on lithofacies analysis, indicate whether the volcanoes erupted below, near, or above the level of the ice sheet. Our interpretations differ significantly from previous interpretations as they highlight the abundant evidence for ice-volcano interactions at emergent paleoenvironments but limited evidence of higher-than- present syn-eruptive ice-levels. Evidence for subglacial volcanic paleoenvironments is limited to Kennel Peak, a ~8 Ma volcano where a pillow lava sequence extending 25 m above current ice level overlies an inferred glacial till and unconformity. A major complication in the Hobbs Coast region is that the volcanism occurred on interfluves between regions of fast-flowing ice. Such a setting precludes establishing precise regional paleo-ice-levels although the presence or absence of ice at times of eruptions can be inferred. Citation: Wilch, T. I., and W. C. McIntosh (2007), Miocene-Pliocene ice-volcano interactions at monogenetic volcanoes near Hobbs Coast, Marie Byrd Land, Antarctica, in Antarctica: A Keystone in a Changing World-- Online Proceedings of the 10th ISAES, edited by A. K. Cooper and C. R. Raymond, USGS Open-File Report 2007-1047, Short Research Paper 074, 7 p.; doi:10.3133/of2007-1047.srp074 Introduction Volcanism and glaciation have been active geological previous interpretations and suggest that there is abundant forces in Marie Byrd Land since middle Cenozoic time evidence for ice-volcano interactions but limited evidence (Wilch and McIntosh, 2002). The Marie Byrd Land of higher-than-present syn-eruptive ice-levels. Volcanic Province consists of about 50 middle to late Cenozoic alkaline volcanic centers, including nineteen large, polygenetic volcanoes (2364-4181 m above sea level), exposed as nunataks in the marine-based West Antarctic Ice Sheet (WAIS). Many of the volcanoes preserve records of syn-eruptive interactions with ice that can be used to infer paleo-ice-levels of the WAIS. LeMasurier (1972) pioneered the use of volcanic records to infer glacial history in West Antarctica, based on regional reconnaissance field work and K/Ar geochronology. The premise of the volcanological approach is that when volcanoes erupt below, at, or above the level of an ice sheet, the resulting rocks exhibit specific textural features and structures that are diagnostic of their eruptive environments. Subglacially erupted rocks imply local paleo-ice-levels higher than the outcrops; subaerially erupted rocks imply local paleo-ice-levels lower than the outcrops. Thus, records of the age and eruptive environment of volcanic rocks provide snapshot views of the syn-eruptive levels of the ice sheet at each Figure 1. Satellite image map of Hobbs Coast nunatak outcrop locality. study area. Image from Modis Mosaic of Antarctica Our study combines new 40Ar/39Ar dating and detailed (Haran, et al., 2005; http://planet.sr.unh.edu/MOA/). field work to reconstruct a volcanic record of the WAIS in 40Ar/39Ar geochronology the Hobbs Coast region, building upon previous work (see Twenty-one mafic to intermediate alkaline volcanic summary in LeMasurier et al., 1990). There are seven rocks were dated by the 40Ar/39Ar resistance-furnace volcanic nunataks located on the east side of the Berry incremental heating method at the New Mexico Glacier trough (Figure 1). The Miocene-Pliocene volcanic Geochronology Research Laboratory at the New Mexico deposits overlie basement rocks on what appear to be Institute of Mining and Technology, Socorro, NM. uplifted horst blocks adjacent to the downdropped, Sample preparation and analyses were conducted glacier-filled graben troughs. The regional significance of according to methods described in Wilch et al. (1999). the paleo-ice-levels is complicated by the fact that many Groundmass concentrates from all twenty-one samples of these volcanoes are located on glacial interfluves. In and plagioclase phenocrysts from two samples were general, our interpretations differ significantly from 10th International Symposium on Antarctic Earth Sciences analyzed and plotted on age spectrum and isochron isochron intercept exceeded 295.5, isochron intercept ages diagrams. Table 1 summarizes the dating results; were selected as the best estimates of eruption age. Two complete data are available at NM open-file report (Wilch samples (Table 1) yielded disturbed age spectra and and McIntosh, in press). poorly correlated isochrons that precluded accurate determinations of eruptions age. For each locality, a Table 1. 40Ar/39Ar Dating Results weighted mean age was calculated by pooling all Sample Best Notes available analyses. In all cases where multiple samples WCM93- MSWD Age±2s.d. (Rock Type) from one locality were dated, the narrow range of ages, Coleman Nunatak combined with stratigraphic and geochemical data, 199 1.5 2.55±0.06 S.end, lava (bn) suggests a brief monogenetic eruptive history, as opposed 185 0.2 2.62±0.28 S. end, lava (hw) to extended or polygenetic activity, as further discussed 184 0.0 2.68±0.14 S. end, lava (bn) below. The seven dated volcanic centers ages range from 192 0.8 2.59±0.20 S. end, lava( bn) 11.38±0.23 to 2.58±0.12 Ma. 190 2.1 3.37±0.251 S. end, lava (bn) Volcanic lithofacies analysis 1 198 3.9 7.30±0.72 S. end, bomb (bn) Recent Antarctic field-based lithofacies studies have 188 2.3 2.47±0.12 S. end, lava (bn) led to major advances in reconstruction of 208 5.8 2.57±0.12 N. end, clast (bn) paleoenvironments associated with ice-volcano 207 3.4 2.60±0.09 N. end, dike (bn) interactions (e.g., Smellie, 2001; Wilch and McIntosh, 205 0.5 2.77±0.27 N. end, dike(bn) 2000, 2002). In lithofacies analysis, paleoenvironmental 202 2.0 2.82±0.23 N. end, dike (bn) interpretations are built upon detailed, field-based, non- 1.6 2.58±0.05 Mean (n=9) genetic descriptions of rock units and their relationships Cousins Rock to one another. Non-genetic volcanic lithofacies 218 2.8 4.89±0.10 bomb (pt) 2 designations are typically based on rock type (lava or 219 1.5 4.99±0.04 lava (pt) clastic), grain size, sedimentary structures and clast 3.3 4.97±0.07 Mean (n=2) characteristics (morphology, vesicularity, componentry) Shibuya Peak (following McPhie et al., 1993). Depositional processes 221 0.8 5.09±0.12 bomb (hw) and environments are interpreted by comparison of 223 1.3 5.32±0.20 lava crusts (hw) features of lithofacies and lithofacies assemblages to 229 2.0 4.78±0.11 dike (hw) modern analogues. In many volcanological studies, partly 14.5 4.97±0.28 Mean (n=3) genetic lithofacies designations that define the original Patton Bluff 2 clast-forming mechanism, such as pyroclastic or 307 3.8 11.38±0.23 lava (bs) autoclastic, are used along with non-genetic designations Kouperov Peak 3 (e.g., Sohn, 1996). The mixing of genetic and non- 303 1.6 9.26±0.58 lava (bs) genetic terminology in the description of volcanic 303 3.4 9.58±0.36 lava (bs(p)) 4 sequences has led to some confusion. 304 1.3 9.02±0.49 lava (bs) In previous reconnaissance studies (e.g. LeMasurier et 304 1.7 8.99±0.31 lava (bs(p)) al., 1990), the term hyaloclastite was broadly defined as a 2.3 9.21±0.30 Mean (n=4) fracture-bounded, glassy, fragmental rock, which included Kennel Peak 2 other forms of palagonitized fragmental rock. The 299 62.4 7.96±0.87 pillow (hw) definition of hyaloclastite included a distinct genetic Holmes Bluff interpretation as an indicator of subglacial environments 292 0.5 6.32±0.07 lava (bn) and higher paleo-ice-levels. This broad definition of Notes:1Inaccurate age determination, not used in mean hyaloclastite has two major weaknesses. First, the age. 2Weighted mean age. 3isochron age; 40Ar/36Ar = definition does not differentiate clasts produced by 301 ± 2. 4isochron age, 40Ar/36Ar = 300 ± 3; Rock type: passive granulation in a subaqueous setting from those bs: basanite, bs(p): plagioclase in basanite; bs: basalt; produced by hydromagmatic explosivity in an emergent hw: hawaiite; pt: phonotephrite (from Wilch, 1997). to subaerial environment. Second, palagonite is a product of hydration and alteration of quenched sideromelane Most step-heating analyses yielded relatively flat age glass and is common in a variety of hydrovolcanic spectra for which plateau ages (weighted mean of three or environments ranging from deep subaqueous to emergent more steps that differ by <2 sigma) are interpreted as the to subaerial (see review by Stroncik and Schmincke, best estimates of eruption age. In a few cases, where age 2002). Palagonitized deposits have also been identified in spectra were slightly more disturbed, weighted-mean ages dry, strombolian subaerial volcanoes, where they are were calculated for select steps that approached but failed attributed to post-eruptive alteration by steam or to meet plateau criteria. In two cases, where the 40Ar/36Ar 2 Wilch and McIntosh: Miocene-Pliocene ice-volcano interactions at monogenetic volcanoes near Hobbs Coast, Marie Byrd Land, Antarctica a. b. d. c. Figure 2. Field photos: a. Nested pillow lava at Kennel Peak.
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