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Petrochemical Evolution of High Cascade Volcanic Rocks in the Three Sisters Region Oregon

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Petrochemical Evolution of High Cascade Volcanic Rocks in the Three Sisters Region Oregon AU ABSTRACT OF THE THESIS OF SCOTT STEVENS HUGHES for the degree of DOCTOR OF PHILOSHY in GEOLOGY presentedon December 8, 1982 Title: PETROCHEMICAL EVOLUTION OF HIGH CASCADE VOLCANIC ROCKS IN THE THREE SISTERS REGION OREGON Abstract approved: Redacted for Privacy Multi-element abundances and petrographic dataare compiled for a suite of 50 volcanic rocks and selected mineralseparates located within the Three Sistersarea. Major element oxides, ob- tained by x-ray fluorescence and atomic absorptionspectrophoto- metry, and trace element concentrations, obtained by sequential instrumental neutron activation analyses, enableclassifications of normal basalts, divergent basalts, Mount Washington (MW)and North Sister (US) type basaltic andesites, dacitesand rhyodacites. Petrochemical types and geochemical modelsare evaluated in light of field studies (E.M. Taylor, Oregon State University),regional geologic environment and comparisons with similarsystems. High-alumina olivine tholeiites exhibit low Fet values and fractionated abundances of K, Ba, Sr, REE and Sc which producenon- choridritic monotonic patterns relative to ionic radii. Primary basalts are modeled as 14% melts, withminor olivine crystalliza- tion, from a LIL element-enriched spinel lherzolitesource. Diver- gent basalts represent contaminatedmagmas or different sources. Basaltic andesites havingup to 62 wt.% Si02 also exhibit low Fe' and fractionated monotonic trace elementpatterns and are modeled as 10% primary melts from similar source regions. Basalts and basaltic andesites containsource-equilibrated olivine and plagio- clase phenocrysts attesting to theirprimary origin. One andesite sample (Si02 = 58.6 wt.%) is derivedby significant fractionation of a basalt parent and representsa low-Si02 and high-Ti02 member of an early Pleistocene silicicsystem. Dacites and rhyodacites (Si02 = 62.2-75.8 wt.%)have strongly fractionated non-monotonic trace element patterns andare derived by extensive (>60%) frac- tionation from primary NSbasaltic andesite magmas. A petrochemical hiatus is recognized betweenmafic and silicic compositions. A comprehensive model ofHigh Cascades volcanic evolution is presented which incorporatesa series of events in a subduction zone-mantle-crust system: Hydrous fluids, expelled froma dehy- drating subducted slab,become enriched in incompatible elements through processes of liquidextraction and small amounts of partial melting. These fluids ascend through theoverlying mantle wedge contaminating and catalyzingniafic melts which accumulate and frac- tionate in upper mantle andlower crust regions. Mafic magmas erupt as near-primary liquidsor are intruded into upper crust regions where extensive fractionationproduces siliceous dacites and rhyo- dacites. The magmatic eventsare conceptualized as vertical sequences of basalt-basaltic andesite-dacite-rhyodacitemagmas pro- duced in response to theadvancing front of hydrous fluids and erupted during tensionalreadjustments of a thermally weakened crust. PETROCHEMICAL EVOLUTION OF HIGH CASCADE VOLCANIC ROCKS IN THE THREE SISTERS REGION, OREGON by Scott Stevens Hughes A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Completed December 8, 1982 Commencement June, 1983 Approved: Redacted for Privacy Associate Professor of Geology in Charge of Major Redacted for Privacy Head of Department of,4ology Redacted for Privacy Dean of Grad,(e SchooY Date Thesis Presented'i -- Thesis Typed by Donna Lee Norvell-Race for ott .ftus ACKNOWLEDGEMENTS AND DEDICATION I wish to express gratitude to my major professor, Dr. Edward M. Taylor, for introducing me toan extraordinary region of the High Cascades and providing continual guidance during the last few years. His contributions to my knowledge of the Three Sisters are definitely innumerable. The basic research for this paper could not have been performed without the superb guidance and support I received from Dr. Roman A. Schmitt and Dr. Monty R. Smith. These two researchers, along with Dr. Maw-Suen Ma, have provided assis- tance far greater than what was expected. Technical assistance was provided by personnel of the O.S.U. Radiation Center, especially T.V. Anderson, W.T. Carpenter and A.G. Johnson, during the data acquisition. The help and concern of all individuals participating in the analytical phase of this study is greatly appreciated. The quality of this manuscriptwas significantly im- proved by discussions and helpful criticisms fromnumerous friends in the Geology Department and the Radiation Center. I would like to especially thank Karl Wozniak for the significant discussions on South Sister eruptions and fcr providing two samples of the South Sister rhyodacites. This study was facilitated by reactor and detectortime grants from Dr. C.H. Wang, a computer time grant from the O.S.U. Computer Center and partial funding by the Parker Scholarship in volcanology. I dedicate this thesis to my wife, Susan,my parents, Steve and Jean Hughes, and my in-laws, John and Betty Aitheide. The multi-faceted support I received from thesefriends and relatives has not gone unnoticed anda mere word of thanks is hardly sufficient toknowledge their kindness. November 24, 1982 TABLE OF CONTENTS Chapter INTRODUCTION 3. 1.1 Physiography and Geologic Setting 1 1.2 Concepts of Caic-alkaline Systems 5 1.3 Caic-alkaline Magma Genesis 8 1.4 Research Objectives 13 2 BASALTS 23 2.1 Sample Descriptions and Petrography 23 2.2 Geochemistry 30 Normal Basalts 38 MB-134 40 WR-225 45 TFJ-440 TS-688 47 BT-382 48 2.3 Geochemical Modeling 50 2.4 Magma Genesis 54 Normal Basalts 58 MB-134 61 WR-225 and TFJ-440 63 3 INTERMEDIATE UNITS 65 3.1 Sample Descriptions and Petrography 65 Basaltic Andesites 65 Andesites 76 3.2 Geochemistry 82 MW Type Basaltic Andesites 91 NS Type Basaltic Andesites and Andesites 95 Andesites of Silicic Association 98 3.3 Magma Genesis 101 Constraints on Source Composition 101 Volatile and Pressure Effects on Bulk Composition 105 Geochemical Models 109 Chapter 4 SILICIC UNITS 121 4.1 Sample Descriptions and Petrography 121 Dacites 121 Rhyodacites 124 4.2 Geochemistry 130 4.3 Silicic Magma Evolution 142 4.4 Circum-Pacific Dacite Comparisons 145 5 HIGHCASCADES EVOLUTION 151 5.1 Petrogenetic Sequence 151 5.2 Source Contamination 155 5.3 Tectonic Implications 158 5.4 Evolutionary Model 163 6 SUMMARY AND CONCLUSIONS 168 REFERENCES 172 APPENDIX A 182 APPENDIX 8 184 APPENDIX C 192 LIST OF FIGURES Figure 1-1 Generalized map showing extent of High Cascades 2 system and location of study area. 1-2 High Cascade sample locations. 15 2-]. Iron-titanium oxide tetrahedron illustrating 29 Ulvospinel content of titaniferous magnetites. 2-2 Photomicrographs of High Cascades basalts. 31 2-3 Least-squares fit and variations in CaO/A19O3 39 vs. SiO, and Fe' vs. MgO for normal basalt and divrgent basalts. 2-4 Variations and least-squares fit of Ni/Co ratios 41 with Fe' values for normal and divergent basalts. 2-5 Composite plot of chondrite-normalized trace 42 element patterns for normal basalts. 2-6 Chondrite-normalized trace element abundances 44 of MB-134 divergent basalt. 2-7 Chondrite-normalized trace element abundances 46 of TFJ-440 and WR-225 divergent basalts. 2-8 Chondrite-normaljzed trace element abundances 49 of divergent basalts BT-382 and TS-688. 2-9 Effects of partial melting and olivine fractiona- 56 tion on La and Fe' in liquids derived from a source of La=1 ppm and Fe'=O.l. 2-10Trace element and F& model calculated for the 59 range of High Cascade normal basaits. 2-11 Trace element and Fe' model determined for the 62 generation of MB-134, a high Fe basalt. 3-1 Photomicrographs ofHigh Cascades basaltic 73 andesites. 3-2 Photomicrographs of High Cascades units having 77 'andesite" Si02 contents. Figure 3-3 Variation of CaO/A1203 with Si02 for basalts and 89 basaltic andesites. 34 Fe' vs. MgO comparison of basalts and basaltic 90 andesi tes. 3-5 Variations in Ni and Co with Fe' for basalts 92 and basaltic andesites. 3-6 Chondrite-normalized trace element patterns of 93 MW basaltic andesites erupted on the west side of the High Cascade crest. 3-7 Chondrite-normalized trace element patterns for 94 MW basaltic andesites eruptedon the east side of the High Cascades crest. 3-8 Chondrite-normalized trace element patterns for 96 MS basaltic andesite samples of the Collier lava flow. 3-9 Chondrite-normalized traceelementpatterns for 97 MS basaltic andesites erupted fromventsnear North Sister and Hogg Rock. 3-10Chondrite-normalized traceelementpatterns of 99 rocks which exhibit "andesite' Si02 compositions. 3-11 Comparison of incompatible elements Hf and Ta 100 vs. Si02 for MW and MS basaltic andesites. 3-12 Chondrite-normalized trace element patterns for 102 two rocks which exhibit "andesite' Si02 compo- sitions, BT-386 and 31-383. 3-13 Variations in MgO and FeO vs. Si02 for Collier 104 lava samples. 3-14 Generalized P-X diagram indicating conditions 108 of nepheline-, olivine- and quartz-normative magmas produced by melting peridotite sources. 3-15 Trace element and Fe' model determined for MW 111 basaltic andesites. 3-16 Trace element and Fe' model calculated for MS 112 basaltic andesites. Figure 3-17 Trace element and Fe' model determined for high 114 silica basaltic andesites of NS association. 3-18Mixing model forhigh silica basaltic andesite 116 of Hogy Rock. 3-19 Trace element andFe' model calculated for the 118 BT-383 sample. 3-20Trace element andFe' model for evolution of 119 BT-386 magma andcomparison with the normal basalt field. 4-1 Photomicrographsof High Cascades dacites and 125 rhyodaci tes. 4-2 Silica variationdiagrams for major elements 134 in mafic and silicicsamples. 4-3 Silica variation diagrams for High Cascade 135 major elements in mafic and silicic rock types. 4-4 Variations in MgO/(Ti02+FeO) vs. SiO., and 137 CaO/A1203 vs. La for mafic and silic'1c units. 4-5 Ternary diagram of normative feldspar compositions. 138 4-6 Chondrite-normalized trace element patterns for 140 dacites in the Three Sister region. 4-7 Chondrite-normalized trace element patterns for 141 rhyodacites in the Three Sisters region.
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