AN ABSTRACT OF THE THESIS OF Randall A. Keller for the degree of Master of Science in Oceanography presented on October 6. 1989. Title:Geochemistry of Ouaternary. Rift-Related Volcanism in the Bransfield Strait. Antarctica Redacted for privacy Abstract approved: Martin R. Fisk The Bransfield Strait is the narrow, late Tertiary to Quaternary marginal basin separating the South Shetland Islands from the northern end of the Antarctic Peninsula magmatic arc. Quaternary volcanism in the strait is tholeiitic to mildly alkaline, and contrasts chemically with the pre-Quaternary caic-alkaline arc volcanism. Geochemical evidence presented here shows that the Quaternary volcanism is related to active rifting in the strait.All of the rift- related volcanoes are chemically related by different extents of partial melting of garnet peridotite variably enriched in alkali and alkali earth elements relative to rare earth elements.This enrichment is characteristic of subduction zones, where fluids from the dehydrating subducted slab concentrate alkalies and alkali earths that can then be mixed into, and partially melted with, the surrounding mantle.The lavas that erupted during the formation of the Bransfield Strait provide evidence that subduction zone processes influence the chemistry of marginal basin volcanism. The Bransfield Strait lavas are chemically similar to published analyses from other marginal basins, especially the Cretaceous marginal basin that is now preserved as the Sarmiento ophiolite of southern Chile.This confirms the interpretation that, prior to obduction, the Sarmiento was a narrow, immature marginal basin analogous to the present day Bransfield Strait. Geochemistry of Quaterilary, Rift-Related Volcanism in the Bransfield Strait, Antarctica by Randall A. Keller A THESIS submittedto Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed October 6, 1989 Commencement June 1990 V1'J J Redacted for privacy Associate Professor of Oceanography in charge of major Redacted for privacy Dean ol/ College of Oceanography Redacted for privacy Dean of Sch Date thesis is presented October 6. 1989 Typed by Randall A. Keller ACKNOWLEDGEMENTS This thesis would not have been possible without the support and inspiration of my advisor, Martin Fisk.J owe its completion to his helpful ideas and encouragement.I could not have asked for a better boss, nor a more enjoyable traveling companion. Bob Duncan and Anita Grunder made numerous helpful comments on improving the content and style of this thesis. Krzysztof Birkenmajer and Szczepan Porebski provided precious samples and expertise from their fieldwork in Antarctica. Many generous people made the drudgery of laboratory work possible, and occasionally even enjoyable.Thanks to the crews at OSU (Andy, Jim, and Greg) and Cornell (Bill, Mike, and Bill S.). The best part of graduate school is the treasured friendships that carry one through.Jay and Jan kept me sane between battles with Lucifer-the-anti-christ-mass spec.I cherish the good times in Oregon on bikes, skis, and barstools with Kelly, Mark, Susan, Sallie, Dana, Joe, Catherine, Margaret, Dan, Doug, Kevin, Mike, Jim, and John. Brigitte merits special thanks for sustaining me during the final stages of this work. This research was supported by grants DPP85-12395 and DPP86-14022 from the Division of Polar Programs of the National Science Foundation. TABLE OF CONTENTS INTRODUCTION 1 Tectonic Setting 1 Geological Background 6 Geochronology 1 2 PRESENTwox 16 Introduction 1 6 Methods 42 Results 8 DISCUSSION 54 Chemical variation within the Bransfield Strait samples 5 8 Comparisons with other locations 66 CONCLUSIONS 77 BIBLIOGRAPHY 78 APPENDICES I. Keller and Fisk, l989b 8 6 II. Birkenmajer and Keller, submitted 92 III. Complete Petrographic Descriptions 1 07 IV. Results of Normative Calculations 1 2 1 LIST OF FIGURES Figure Page 1. Map of interpreted ocean floor magnetic anomalies in the southeastern Pacific. 2 2.Tectonic sketch map of the Bransfield Strait region. 4 3.Location and bathymetry map of South Shetland Islands and Bransfield Strait. 7 4.Silica versus total alkalies plot of published analyses from Deception, Bridgeman, and Penguin Islands. 11 5.Bathymetry and dredge locations in the eastern end of Bransfield Strait. 1 9 6. Geologic sketch map of Deception Island. 20 7.Geologic cross-section of Penguin Island. 2 1 8. Silica versus total alkalies plot of samples analyzed in this study. 22 9.Geologic cross-section of Low Head, King George Island. 43 10.Plot of MgO versus Ti02, Fe203, and Na20/K20. 49 11.Plot of 87Sr/86Sr versus 143Nd/144Nd of samples analyzed in this study. 50 12.Plot of 2O7Pb/2O4Pb versus 208Pb/204Pb of samples analyzed in this study. 5 1 13.Plot of trace element data from Sun (1980). 5 6 14.Plot of trace element data for Andean and Patagonian rocks. 57 15.Chondritenormalized rare earthelement patterns for two MelvillePeak samples. 5 9 16.Chondritenormalized rare earthelement patterns for two PenguinIsland samples. 60 17.Chondritenormalized rare earthelement patterns for two dredgedseamount samples. 6 1 18.Results ofbatch melting modelsof garnet lherzolite and spinel lherzolite (from Lin et al.,1989). 6 2 1 9.Chondrite normalized rareearth element patterns for representative samples from Melville Peak, Penguin Island, and a dredged seamount. 63 20.Chondrite normalized rareearth element patterns for three Deception Island samplesand a dredged sample. 64 21.Plot of trace element datafor Scotia Rise spreading center and South Sandwich Islands volcanic arc. 67 22.Plot of trace element datafor dredged samples. 6 9 23.Plot of trace element datafor Deception Island samples. 7 1 24.Plot of trace element datafor Melville Peak samples. 7 2 25.Plot of trace element datafor Penguin Island samples. 7 3 26.Plot of rare earth elementdata for a dike in the Sarmiento ophiolite. 7 6 27.Sketch map of BransfieldStrait area. 90 28.Silica versus alkalies plotof Bransfield Strait samples. 9 1 29.Volcanoes in the BransfieldStrait area. 100 30.Map of the Melville Peakvolcano. 10 1 3 1.Cross-section of Cape Melville. 1 02 32.Exposure in Melville Peakabove Sherratt Bay. 103 33.Exposure in eastern ridgeof Melville Peak. 1 04 LIST OF TABLES Table Page 1. Representative analyses of South Shetland Island Arc rocks (data from Smellie, 1983). 9 2.Coordinates and results of dredge stations. 1 7 3.Major and trace element concentrations and loss on ignition data. 23 4.Trace and rare earth element concentrations by isotope dilution. 27 5.Isotopic compositions of samples from the Bransfield Strait.28 6.Microprobe analyses of glasses. 29 7. Microprobe analyses of olivines. 3 0 8.Microprobe analyses of oxides. 33 9.Microprobe analyses of plagioclases. 3 7 10.Microprobe analyses of pyroxenes. 3 9 11.Bransfield Strait dredge stations and results. 8 9 12.Summary of evolution of Melville Peak volcano. 105 13.Potassium-argon ages of Melville Peak samples. 106 Geochemistry of Quaternary, Rift-Related Volcanism in the Bransfield Strait, Antarctica INTRODUCFION The Quaternary volcanic rocks of the Bransfield Straitand South Shetland Islands are chemically very different fromthe pre- Quaternary volcanic rocks.Jurassic to late Tertiary volcanic rocks intermediate between caic-alkaline and tholeiitic types make upthe bulk of the South Shetland Islands arc (Smellie et al., 1984).In contrast, the Quaternary volcanic rocks aretholeiitic to mildly alkaline, and are associated with the rifting that has createdthe Bransfield Strait marginal basin (Weaver et al., 1979).There was clearly a major transformation in the style and source ofvolcanism on the South Shetland Islands duringthe late Tertiary, from subduction-related arc volcanism to rift-related marginal basin (back-arc?) volcanism.This thesis presents geochemical evidence that the source of the Quaternary, rift-related volcanism waspartial melting of a garnet bearing mantle source that has beenenriched in alkali and alkali earth elements by subduction zone processes. TectonicSetting From Jurassic to Tertiary time the western margin ofthe Antarctic Peninsula was the site of continuous subduction ofoceanic crust of the Aluk Plate (Barker, 1982; Barkerand Daiziel, 1983).At approximately 50 Ma, a segment of the Aluk Ridge spreading center entered the trench at the base (southern end) of theAntarctic Peninsula, causing a cessation of spreading on that ridge segmentand a cessation of subduction in that segmentof the trench (Figure 1). 2 ( .J2 S I SOUTH1 /43 I8// /'7/ / .,..- ,.-I8 / \ ' / (27 /20'/J7 \ '!2N c'b\ / "J8/ 9 3 / - --...,/ IS-... II 26 ..20 i 24 -._.___ Ii; " I, / ° 60°S / 8 . 1/! N7 r i'cMyr 2( 2f',, 20 25 - ,i /5 pf' ? \ ZO'S c Figure 1. Map of interpreted ocean floor magnetic anomalies in the southeastern Pacific (from Barker and Dalziel,1983).Numbers are magnetic anomalies, with lessernumbers (younger crust) closer to the margin of the Antarctic Peninsula,showing ridge crest-trench collisions of progressively younger ages northward upthe coast of the Antarctic Peninsula. 3 Because the oceanic crust on the northwest side of the subducted Aluk Ridge was part of the same plate as the Antarctic Peninsula, that part of the western margin of the Antarctic Peninsula became a passive margin (Barker and Daiziel, 1983).Ridge-trench collisions then occurred progressively northward along the margin of the peninsula as other segments of the Aluk Ridge arrived at their respective segments of the trench.This process ended when the ridge segment directly south of