Geochemical Evolution of the Roseburg Formation Basaltic Rocks
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AN ABSTRACT OF THE THESIS OF Douglas G. Pyle for the degree of Master of Science in Geology presented on January 8. 1988 Title: Geochemical Evolution of the Roseburg Formation Basaltic Rocks. Southern Oregon Coast Range Redacted for Privacy Abstract Approved ur. vascn The Paleocene-lower Eocene volcanic rocks of the southern Oregon Coast Range Roseburg Formation are the oldest remnant eruptive sequence of an oceanic island- seamount province which is presently exposed along a north-south lineament that stretches from Vancouver, B.C. into southwestern Oregon. This seamount terrane is thought to have formed on the Farallon plate and, been subsequently accreted into its present position along the Oregon and Washington coast during subduction of this oceanic plate beneath the North American Plate during the early Tertiary The Roseburg volcanic sequence is composed of a basal submarine tholeiitic section which grades upward into a subaerially erupted sequence of highly undersaturated alkalic basalts. Fields studies show that the morphological evolution of eruptive styles within the volcanic pile (i.e. sequences of pillowed and massive flows, pyroclastic beccias and hyaloclastite deposits, subareal flows, and erosional surfaces) closely follow successive stages of growth observed at present day oceanic islands such as Hawaii. The regional distribution of basalt types reveal a progressive increase in differentiation from exposures of tholeiitic basalts in the south to exposures of alkalic basalts in the north, and demonstrate a coherencey in the geochemical evolution between basalts which cropout on the east and west side of an intervening Eocene basin. The Roseburg basaltic sequence displays the greatest amount of chemical diversity yet reported for the early Tertiary volcanic centers which compise the Oregon-Washington Coast Range province. The tholeiitic basalts have trace element compositions which are typical of E-type MORB and tholeiites from hotspot generated oceanic islands. The alkalic suite is highly enriched in incompatible element constituents typically found in the late stage eruptive cycles of oceanic island volcanism. The generation of these two basaltic suites are related to a common mantle source composition which has undergone variable degrees of partial at different depths. Differentiation of the tholeiitic basalts results from uniform degrees of partial melting, whereas compositional variations in the alkalic basalts can be explained only by variable degrees of partial melting at deeper depths in the mantle. Fractional crystalization does not seem to be an important control on the differentiation of these two basaltic suites. In contrast, compositional variations of basalts from other volcanic centers in the Oregon-Washington Coast Range province indicate a more dominate control of differentiation by fractional crystalization. The trace element variations in the province is related to varying influences of mid-ocean ridge spreading and hotspot volcanism. The trace element data reported here does not support models for the generation of these seamount volcanoes by marginal basin spreading possibly caused by the highly oblique subduction of the Farallon plate during this time. The regional geochemical variations exhibited by basalts from different volcanic centers in the Oregon-Washington Coast Range province is shown to be geographically controlled rather than time dependent. Generally, the available data shows a decrease in the highly incompatible trace element constituents from the southern end of the Coast Range (Roseburg basalts) to the northern volcanic centers (Metchosin and Crescent Fm. basalts). This geochemical trend is thought to be caused by a declining influence of hotspot-type volcanism on the magma compositions of basalts erupted in the northern regions of the Coast Range and/or the increasing influence of spreading ridge magmatic activity in that area. GEOCHEMICAL EVOLUTION OF THE ROSEBURG FORMATION BASALTIC ROCKS, SOUTHERN OREGON COAST RANGE by DOUGLAS G. PYLE A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed June 10, 1988 Commencement June 1989 APPROVED: RedactedforPrivacy Professor ofe logy in charge of Major Redacted for Privacy Department of tojgy Redacted for Privacy Dean of Graduate Obi Date thesis is presented January 8, 1988 Typed by Douglas G. Pylefor Douglas G. Pyle TABLE OF CONTENTS Page CHAPTER 1: INTRODUCTION 1 I. Purpose 1 II. Objectives 3 CHAPTER 2: PREVIOUS WORK 8 I. The southern Oregon Coast Range 8 Basaltic Rocks 8 Stratigraphy 9 Structure 12 II. Oregon-Washington Coast Range, regional overview 13 Geologic Setting 13 Nature of the Crust 14 Geochemistry and Petrology 16 Age of Volcanism 17 Paleomagnetism and Plate Tectonic Configuration 19 Summary 24 CHAPTER 3: FIELD AND LABORATORY INVESTIGATIONS 28 I. Field Relationships 28 II. Methods 34 Sampling Procedure 34 Sample Preparation 36 Analytical Methods 37 CHAPTER 4: MAJOR ELEMENT GEOCHEMISTRY 40 I. Basalt Classification 40 II. Major Element Variation 51 SiO2 vs total alkalies 51 AFM diagram 51 MgO scatter plots 54 Primary and Parental Magamas 62 CHAPTER 5: TRACE ELEMENT GEOCHEMISTRY 64 I. Introduction 64 II. Trace element variation 74 Incompatible elements 74 Table of contents (cont.) Page Rare-earth element variation 83 Spider Diagrams 98 CHAPTER 6: DISCUSSION 113 I. Summary of Results 113 II. Plate Tectonic models for petrogenesis 115 Model 1: Marginal basin/Continental rifting 116 Model 2: Oceanic ridge/Hotspot 118 III Conclusions 124 BIBLIOGRAPHY 126 Appendix A Sample collection localities 140 Appendix B Normalization values 142 LIST OF FIGURES Figures Page 1.1 Geologic provinces of Oregon and Washington 2 1.2 Geologic map of the study area (southern Oregon Coast Range) 4 2.1 Oregon-Washington stratigraphic correlation chart 11 2.2 Oregon Coast Range crustal profiles 15 2.3 Oregon-Washington early Tertiary age progressive volcanism 18 2.4 Tectonic rotation models of Simpson and Cox (1977) 20 2.5 Model 1: subduction-marginal basin origin of Coast Range 25 2.6 Model 2: mid-ocean ridge/hotspot origin of Coast Range 27 3.1 Geologic evolution summary of Hawaiian volcanoes 32 4.1 Ne-01-Di-Hy-Qtz normative mineral system 50 4.2 SiO2 vs Na20+K20 scatter plot a) Oregon-Washington Coast Range 52 b) Roseburg basalts 52 4.3 Roseburg basalt AFM diagram 53 4.4 Major element scatter plots a) SiO2 and A1203 vs MgO 55 b) FeO(t) and CaO vs MgO 56 c) TiO2 and Mn0 vs MgO 57 d) Na20 and K20 vs MgO 58 e) P205 vs MgO and CaO/A1203 vs Mg* 59 5.1 La vs Th 75 a) Oregon Coast Range b) Washington Coast Range 5.2 Ce vs La 76 a) Oregon Coast Range b) Washington Coast Range 5.3 La vs Ta 77 a) Oregon Coast Range b) Washington Coast Range 5.4 Th vs Ta 78 a) Oregon Coast Range b) Washington Coast Range 5.5 Hf vs Ta 82 a) Oregon Coast Range b) Washington Coast Range 5.6 Chondrite normalized REE diagrams 86 a) Roseburg exposure b) Sugarloaf Mountain 5.7 Chondrite normalized REE diagrams 87 a) Red Hill anticline b) Sutherlin Mobil Well #1 5.8 Chondrite normalized REE diagrams 89 a) Blue Ridge Mountain b) Coquille-Myrtle Point List of Figures (cont.) Page 5.9 Chondrite normalized REE diagrams 90 a) Jack Creek Anticline b) Coos River-Kentucky Slough 5.10 Chondrite normalized REE diagrams 91 a) Drain anticline 5.11 Chondrite normalized REE diagrams 92 a) Siletz River basalts 5.12 Chondrite normalized REE diagrams 93 a) Crescent Fm. basalts b) Black Hills basalts c) Grays River basalts 5.13 Chondrite normalized REE diagrams 94 a) Metchosin Fm. basalts 5.14 (Ce/Yb)n vs (Ce)n plot of representative oceanic basalts 96 5.15 (Ce/Yb)n vs (Ce)n plot 97 a) Roseburg basalts b) Oregon-Washington Coast Range 5.16 Spider Diagrams 100 a) North Atlantic MORB b) Average MORB 5.17 Spider Diagrams 101 a) Roseburg exposure b) Sugarloaf Mountain 5.18 Spider Diagrams 102 a) Red Hill anticline b) Sutherlin Mobil Well # 1 5.19 Spider Diagrams 103 a) Blue Ridge Mountain b) Coquille-Myrtle Point 5.20 Spider Diagrams 104 a) Jack Creek anticline b) Coos River-Kentucky Slough 5.21 Spider Diagrams 105 a) Hawaiian alkali basalts 5.22 Spider Diagrams 106 a) Drain anticline 5.23 Spider Diagrams 107 a) Hawaiian basanites 5.24 Spider Diagrams 108 a) Siletz River basalts b) Average Hawaiian tholeiites 5.25 Spider Diagrams 109 a) Black Hills basalts b) Grays River basalts 5.26 Spider Diagrams 110 a) Metchosin Fm. basalts b) Crescent Fm. basalts 6.1 Th-Hf/3-Ta ternary plot (representative oceanic basalt systems) 120 6.2 Th-Hf/3-Ta ternary plot (Roseburg basalts and Coast Range) 121 6.3 Th/Yb vs Ta/Yb (Roseburg basalts and Coast Range) 123 LIST OF TABLES Tables Page 3.1 Analytical Errors and STD Results 38 4.1 Roseburg Basalt Major Elements Abundances a. Roseburg and Sugarloaf Mountain 43 b. Coquille-Myrtle Point and Red Hill 44 c. Blue Ridge, Coos River, and Kentucky Slough 45 d. Jack Creek Anticline and Drain Anticline 46 4.2 Mobil Sutherlin Well #1, Major and Trace Element Abundances, 47 (Core samples) 4.3 Washington-British Columbia Coast Range Major Element Abundances, 48 (Black Hills, Grays River, and Metchosin basalts) 5.1 Roseburg Basalt Trace Element Abundances a. Roseburg and Sugarloaf Mountain 68 b. Coquille-Myrtle Point and Red Hill 69 c. Blue Ridge, Coos River, and Kentucky Slough 70 d. Jack Creek Anticline and Drain Anticline 71 5.2 Mobil Oil Sutherlin Well #1, Trace elements Abundances, 72 (Drilling chip samples) 5.3 Washington-British Columbia Coast Range Trace Element Abundances, 73 (Black Hills, Grays River, and Metchosin basalts) Geochemical Evolution of the Roseburg Formation Basaltic Rocks, Southern Oregon Coast Range CHAPTER 1: INTRODUCTION I.Purpose The basaltic rocks of the Oregon-Washington Coast Range represent a significant section of oceanic crust that was emplaced and uplifted to its present position by subduction of the Farallon plate beneath the North American plate during the early Tertiary.