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United States Department of the Interior Geological Survey UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Intracaldera Tuffs and Central-Vent Intrusion of the Mahogany Mountain Caldera, Eastern Oregon Dean B. Vander Meulen Open-File Report 89-77 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey. l- U.S. Geological Survey Menlo Park, California 94025 1989 ACKNOWLEDGEMENTS I would like to express my appreciation to several people at the U.S. Geological Survey for their willing assistance during the field-mapping phase of the study. Special thanks are extended to Mike Grubensky and Tom Vercoutere for their helpful field observations and comments. I am particularly grateful to Jim Rytuba who conceptualized the problem, formalized and focused my efforts, and provided insightful guidance throughout the study. 11 TABLE OF CONTENTS page ABSTRACT...................................................................................................... vii INTRODUCnON ..................... 1 Location and Access ....................................................................... 3 Topography and Climate .............................................................. 4 Previous and Present Investigations ..................................... 4 VOLCANIC SETTING................................................................................... 7 Regional Kinematics....................................................................... 7 Topographic Margin of the Caldera ......................................... 8 Resurgent Dome and Central Graben ...................................... 1 0 Evolution of the Mahogany Mountain Caldera ................... 12 Central Vent Volcanism ............................................................... 1 4 GEOPHYSICAL INVESTIGATIONS .......................................................... 1 7 INTRACALDERA STRATIGRAPHY ......................................................... 2 0 Leslie Gulch Ash-Flow Tuff ........................................................ 2 2 Outflow Facies ...................................................................... 2 2 Intracaldera Facies ............................................................. 2 3 Surge Deposit............................................................... 2 3 Ash-Flow Tuff............................................................. 2 7 Air Fall Facies ....................................................................... 3 1 Lower Air Fall Facies ................................................ 3 1 Upper Air Fall Facies ................................................ 3 4 Vent Complex ................................................................................... 3 4 Vent Breccia.......................................................................... 3 5 Rhyolite Dikes ....................................................................... 3 6 Post-Caldera Ash-Flow Tuffs ..................................................... 3 6 Tuff of Spring Creek .......................................................... 3 6 Tuff of Birch Creek ............................................................. 38 Rhyolite and Basalt Flows and Intrusions ............................ 3 9 Rhyolite Dikes ....................................................................... 3 9 Rhyolite Ring-Domes and Flows ................................... 40 in Basalt Flows and Shallow Intrusions .......................... 42 Sedimentary Rocks ......................................................................... 4 2 GEOCHEMISTRY............................................................................................ 44 Sampling and Analytical Methods ........................................... 44 Analytical Data................................................................................. 4 8 Major- and Trace-Element Composition ............................... 50 Major-Element Content.................................................... 5 0 Trace-Element Content..................................................... 5 2 Trace-Element Gradient................................................... 57 Discussion of Magma Chamber Model .................................... 57 SUMMARY...................................................................................................... 62 REFERENCES CITED...................................................................................... 6 3 IV LIST OF ILLUSTRATIONS Figure Page 1. Index Map of the Study Area ............................................... 2 2. Location Map of Cenozoic Calderas Showing Their Relation to Selected Geologic Structures .............. 5 3. Mahogany Mountain Caldera ................................................. 1 1 4. Cross Sections Illustrating the Central Vent Complex ............................................................................... 1 3 5. Geologic Map of the Central Vent Complex ..................... 1 6 6. Bouguer Gravity Anomaly Map of the Owyhee Volcanic Field ............................................................. 1 8 7. Schematic Stratigraphic Section Illustrating the Tuffs that Fill the Mahogany Mountain Caldera ........... 21 8. Near-Vent Surge Deposit Showing Flow Direction ....... 25 9. Near-Vent Surge Deposit Showing Sand-Wave Beds and Lithic Breccia Deposit............................................ 2 6 10. Near-Vent Intracaldera Ash-Flow Tuff with Pumice Blocks .................................................................... 2 8 11. Near-Vent Intracaldera Ash-Flow Tuff with a Banded Rhyolite Block ............................................... 2 9 12. Cross-Beds within the Intracaldera Facies ...................... 30 13. Bedded Air-Fall Tuff Deposit................................................. 3 2 14. Near-Vent Air-Fall Tuff with Rhyolite Blocks ............... 33 15. Intrusive Autobreccia .............................................................. 37 16. Rhyolite Dike Exposed Along the Northeast Margin of the Caldera ............................................................... 4 1 17. A12O3 Versus FeO* for Samples of Intracaldera Tuff and Vent Intrusion .......................................................... 4 9 18. SiO2 Versus AhO3 for Samples of Intracaldera Tuff and Vent Intrusion .......................................................... 5 1 19a. Nb Versus Zr for Samples of Intracaldera Tuff and Vent Intrusion .................................................................... 5 3 19b.Nb Versus Zr for Samples of Intracaldera Ash-Flow Tuff.............................................................................. 5 4 19c. Nb Versus Zr for Samples of Intracaldera Air-Fall Tuff.................................................................................. 5 5 19d. Nb Versus Zr for Samples of Vent Intrusion .................. 55 19e. Nb Versus Zr for All Samples of Intracaldera Tuff and Vent Intrusion .......................................................... 5 6 20. Schematic Model of the Mahogany Mountain Caldera Magmatic System ....................................................... 5 8 21. Cartoon Sketch Illustrating the Initial Development of the Mahogany Mountain Caldera......................................................................... 6 0 Plate 1. Geologic Map of Mahogany Mountain Caldera ......in pocket 2. Partial Strati graphic Section Illustrating the Intracaldera Ash-Flow and Air-Fall Tuffs ..............in pocket Table 1. Major-Element Analyses of the Intracaldera Tuffs and Vent Intrusion ........................................................ 45 2. Minor-Element Analyses of the Intracaldera Tuffs and Vent Intrusion ........................................................ 46 VI ABSTRACT The Mahogany Mountain caldera is one of several volcanic centers in southeastern Oregon that erupted silicic peralkaline tuffs during the middle Miocene. The 15- by 20-km caldera structure coincides with a 25 mGal gravity low that is filled with an estimated 280 km3 of ash-flow and air-fall tuff. An unusually well-preserved central-vent complex is exposed in the resurgent dome of the caldera. Facies changes within the caldera indicate the majority of the tuffs erupted from the central vent. Near-vent facies include, surge deposits, lag breccias, and ash-flow and air- fall tuffs containing abundant blocks. Major- and trace-element data indicate that both the intracaldera tuffs and the vent intrusion erupted from a bimodal magmatic system consisting of high-silica comendite, and zoned intermediate comenditic trachyte. The high-silica melt that formed the upper part of the magma chamber appears to have evolved as an open system with a small compositional gradient. Unusually low Nb, Zr, and Y values are characteristic of rocks that erupted from this part of the magma chamber. In contrast, an extreme range of Nb, Zr, and Y values is characteristic of rocks erupted from the underlying intermediate-silica melt. This melt evolved in a closed magmatic system that developed a steep compositional gradient. The central-vent eruption tapped both parts of the magma chamber simultaneously. Pyroclastic fragments that comprise the intracaldera tuffs and vent intrusion are a composite of both melts. vn INTRODUCTION Vents for major ash flow eruptions rarely have been identified in the geologic record (Smith, 1960). The lack of identifiable ash flow vent structures within most
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