Portland State University PDXScholar Dissertations and Theses Dissertations and Theses Fall 1-22-2014 Revisiting Volcanology and Composition of Rhyolites and Associated REE Rich Mafic Clasts of the Three Fingers Caldera, SE Oregon Phillip Ira Marcy Portland State University Follow this and additional works at: https://pdxscholar.library.pdx.edu/open_access_etds Part of the Volcanology Commons Let us know how access to this document benefits ou.y Recommended Citation Marcy, Phillip Ira, "Revisiting Volcanology and Composition of Rhyolites and Associated REE Rich Mafic Clasts of the Three Fingers Caldera, SE Oregon" (2014). Dissertations and Theses. Paper 1543. https://doi.org/10.15760/etd.1542 This Thesis is brought to you for free and open access. It has been accepted for inclusion in Dissertations and Theses by an authorized administrator of PDXScholar. Please contact us if we can make this document more accessible: [email protected]. Revisiting Volcanology and Composition of Rhyolites and Associated REE Rich Mafic Clasts of the Three Fingers Caldera, SE Oregon by Phillip Ira Marcy A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology Thesis Committee: Martin J. Streck, Chair Michael L. Cummings Robert B. Perkins Portland State University 2013 Abstract Two adjacent caldera systems, the Mahogany Mountain and the Three Fingers caldera constitute voluminous rhyolitic volcanic deposits on the eastern margin of the Oregon-Idaho graben during the middle-Miocene. Both calderas are part of the Lake Owyhee volcanic field that in turn is part of widespread rhyolite deposits associated with the Columbia River Basalt province. We focus on establishing relationships between intracaldera units of Three Fingers caldera and caldera-forming tuff of Spring Creek and surveying the distribution of entrained mafic clasts which often display anomalous concentrations of rare earth elements. Previous mapping identified two intra-caldera facies and one outflow facies of the tuff of Spring Creek, in addition to a younger rhyolite within the caldera (Trp). New 40Ar/39Ar dates show these units are nearly time equivalent at 15.64 ± 0.08 Ma for Trp and 15.64 ± 0.09 Ma for tuff of Spring Creek. Field evidence shows extensive coverage of Trp and associated facies emplaced after a period of sedimentation within the caldera. The main reinterpretations are: i) the mostly devitrified units of Trp are time equivalent to flows and domes of glassy, vesicular, or brecciated rhyolite previously mapped as intra-caldera tuff of Spring Creek; and ii) mafic clasts present in dense glass i and porous rhyolite are fragments of mafic lava flows entrained by the subsequent eruptions. New geochemical and mineralogical evidence clearly distinguish the outflow tuff of Spring Creek and intracaldera rhyolites. Compared to the outflow tuff, intracaldera rhyolite flows are less Fe-rich, (2 vs. 3 wt.% FeO), and higher silica (77 vs. 74 wt.% SiO2) rhyolites that lack vitrophyric texture. I interpret the investigated area as a rhyolite dome field, erupted subsequent to caldera collapse. The proximity of vents resulted in a complex stratigraphic overlap of rhyolite flows and clastic debris issued from coalescing domes. The predominance of high-standing dome interiors reflects the more resistant nature of dense devitrified rhyolite as compared to pumiceous, glassy, or brecciated facies of intra-caldera rhyolite. Enrichment of REE in mafic clasts is highly variable, and does not correlate with their entrainment in a specific facies of intra-caldera rhyolite. Individual clasts contain up to 2400 ppm Nd, 1800 ppm Ce, and 1400 ppm La in the most enriched samples. Linear regression shows these highly anomalous concentrations are not correlated with variations in major element chemistry between enriched and un-enriched clasts. The geographic extent of mafic clast-bearing units is limited to less than 5 percent of the ii area mapped, and their distribution within these units is typically volumetrically insignificant, limiting their economic potential. Mechanisms for enrichment of REE within these rocks is however significant to our understanding of a yet unexplained phenomenon and may lead to further discoveries with greater economic potential. iii ACKNOWLEDGEMENTS I would especially like to thank my advisor, Dr. Martin J. Streck for his patience and guidance throughout this study, and for providing me with the spark of curiosity since my arrival at PSU. I would also like to thank Dr. Robert Perkins for his assistance in the laboratory, and Dr. Michael Cummings for keeping a watchful eye on my use of the English language. The field portion of this study was made possible by patience and endurance of my field assistants, Kathy Breen, Adam Large, and Matthew Gibson, who did not hesitate to join me in a harsh and unfriendly landscape. I am grateful for the hard work of fellow students Ellie Brown and Brian Webb, who provided me with microprobe data and the insight to understand it. Nancy Eriksson deserves special thanks for her detailed and personal assistance, without which I and so many others would be lost. Nobody has been more important to me in the pursuit of this project than the members of my family. I would like to thank my parents and my brother, who continue to support and encourage me in all that I pursue. Most of all, I wish to thank my loving and supportive wife, Amy, and my three wonderful children, Evan, Alex, and Abby, for their continued love and inspiration. iv Table of Contents Abstract .................................................................................................................................................. i List of Figures ................................................................................................................................. viii List of Tables ....................................................................................................................................... x Introduction ........................................................................................................................................ 1 Location ................................................................................................................................................ 4 Chapter 1 – Geology of Three Fingers Caldera .................................................................... 6 1.1 - Background ............................................................................................................................. 6 1.1.1 - Regional Geology .......................................................................................................... 6 1.1.2 - The Oregon-Idaho Graben ....................................................................................... 8 1.1.3 - Calderas of the Lake Owyhee volcanic field ................................................... 10 1.1.4 - Three Fingers Caldera .............................................................................................. 12 1.2 - Methods.................................................................................................................................. 13 1.2.1 - Field Mapping and Sampling ................................................................................ 13 1.2.2 - Analysis Methods ....................................................................................................... 14 1.3 - Results ..................................................................................................................................... 16 1.3.1 - Lithology of units from Three Fingers caldera ............................................... 16 1.3.1.1 - Intra-caldera Rhyolite ..................................................................................... 17 1.3.1.2 - Glassy rhyolite autobreccia and pseudo-breccia ................................. 26 1.3.1.4 - Pumice-bearing ash-flow tuff ..................................................................... 31 1.3.1.5 - Tuffaceous sediments ....................................................................................... 32 1.3.1.6 - Matrix supported polymict rhyolite breccia .......................................... 36 1.3.1.7 - Mafic clasts .......................................................................................................... 37 1.3.2 - Petrography of units within Three Fingers Caldera ..................................... 39 1.3.2.1 - Intra-caldera rhyolite ...................................................................................... 39 1.3.2.2 - Dense glassy intra-caldera rhyolite ........................................................... 40 1.3.2.3 - Dense devitrified rhyolite............................................................................... 41 1.3.2.4 - Porous devitrified rhyolite ............................................................................. 42 1.3.2.5 – Sparsely phyric basalt ...................................................................................... 43 1.3.2.6 - Pumice-bearing ash-flow tuff ..................................................................... 44 1.3.2.7 – Tuffaceous sedimentary rocks ..................................................................... 45 1.3.3 - Geochemistry of units within Three Fingers Caldera.................................. 46 1.3.3.1 – Major elements................................................................................................... 47 v 1.3.3.2 - Effects