AN ABSTRACT OF THE THESIS OF Suhail Alhejji for the degree of Master of Science in Geology presented on June 3, 2019. Title: Timing and Composition of Volcanism at Harrat Ithnayn, Western Saudi Arabia. Abstract approved: ______________________________________________________ Adam J.R. Kent Western Saudi Arabia hosts a number of young volcanic fields, known as “Harrats”. Harrats cover a significant proportion of western Saudi Arabia and are associated with significant volcanic hazards. However, the ultimate cause of volcanic activity remains unclear. Younger volcanism (<12 Ma) is clearly represented by the north-south-trending region known as the Makkah-Madinah-Nafud (MMN) line, which consists of three moderate sized volcanic fields: Harrat Rahat, Harrat Khaybar, and Harrat Ithnayn. Harrat Ithnayn is the northern-most and the least studied volcanic field of the MMN line, and it has been suggested that Ithnayn represents the youngest field produced by age progressive volcanism along the MMN line. Harrat Ithnayn is thus a critical piece in the puzzle in determining the causes of the volcanic activity in the MMN line region. This research focuses primarily on investigating the age and composition of the volcanic activity at Harrat Ithnayn and how it changes through time. I apply geochronological, geochemical, and petrological methods to understand the origin and tectonic controls on volcanism in this region. I report new age determinations on 10 lava flows, one sample from the northern part of Harrat Khaybar and 9 samples from Harrat Ithnayn, by the 40Ar-39Ar laser step heating method. All ages are younger than 2 Ma and most of these lavas range in age between ~500 and 120 Ka. These ages constrain the timing and chemical variations of volcanic activity at Harrat Ithnayn. Unlike older Harrats Rahat and Khaybar, the volcanism at Harrat Ithnayn has also undergone less magmatic evolution, suggesting a lack of shallow crustal magma bodies. Similar to Harrat Hutaymah and other younger volcanic fields peripheral to the MMN line, olivine ± clinopyroxene dominated crystal fractionation at a range of upper mantle and crustal pressures, with some evidence of crustal assimilation. The new age constraints agree with the hypothesis of south-to- north volcanic progression of volcanism along the MMN line. In addition, active mantle upwelling, decompression melting and possible asthenospheric flow from the Afar mantle plume, appears to have been the source of this volcanic activity, produced from ~ 2-14% partial melting of a shallow garnet peridotite mantle source for magmas. ©Copyright by Suhail Alhejji June 3, 2019 All Rights Reserved Timing and Composition of Volcanism at Harrat Ithnayn, Western Saudi Arabia by Suhail Alhejji A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented June 3, 2019 Commencement June 2019 Master of Science thesis of Suhail Alhejji presented on June 3, 2019 APPROVED: Major Professor, representing the Geology Dean of College of Earth, Ocean, and Atmospheric Sciences Dean of the Graduate School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. Suhail Alhejji, Author ACKNOWLEDGEMENTS I would first like to express my sincere thanks to my major professor advisor Dr. Adam Kent for his great support and help throughout my master’s degree journey. I would also like to thank my committee members, Dr. Robert Duncan, Dr. David Graham, and Dr. Clare Reimers for their encouragement and guidance. My sincere thanks also go to Dr. Abdullah Al-Amri for his helpful academic advising and funding a part of the fieldwork expanse. I also greatly thank the Saudi Arabian Cultural Mission (SACM) for the fully funding of my graduate school expanses. I am grateful to the National Science Foundation (NSF) and King Abdulaziz City for Science and Technology (KACST) for supporting this research. Also, I would like to thank King Saud University (KSU) and the Saudi Society for Geosciences (SSG) for funding a major part of the fieldwork expense. I thank Dr. Victor Camp for kindly providing the geochemical data of lavas from Harrat Ithnayn, Dr. Dan Miggins provided valuable assistance with 40Ar-39Ar age analysis, Dr. Saeed Al-Shaltoni for his field assistance, Mr. Mufleh Aldossari for his valuable academic advising and his assistance in shipping the rock samples from Saudi Arabia to OSU. Last but not least, my special thanks to my parents, my wife, and my friends for being there when I need them and their support throughout this experience. TABLE OF CONTENTS Page 1 Introduction...…………………………………………………………………………… 1 1.1 Regional Background …………………………………………………. 6 1.2 Makkah-Madinah-Nufud (MMN) line Volcanism……………………… 7 2 Research Problems and Specific aims……………………………………………………. 12 3 Materials and Methods …………………………………………………………………… 13 3.1 Sampling Collection and Categorization…...…………………………… 13 3.2 Whole-rock XRF and ICP-MS Geochemistry……………………...…… 14 3.3 40Ar/39Ar Geochronology ………………………………......................... 16 3.4 Conditions and Compositions Modelling……………………………… 17 4 Results …………………………………………………………………………………… 19 4.1 Geochemistry …………………………………………………………. 19 4.2 Geochronology ……...…………………………………………………. 27 5 Discussion……………………………………………………………………….…….…. 30 5.1 Timing of magmatism at Harrat Ithnayn………………………………... 30 5.2 Causes of chemical variationsin Harrat Ithnayn lavas…………………. 33 5.2.1 Crystal fractionation …………………………………………. 33 5.2.2 Variations in mantle sources and degree of melting…………. 36 5.2.3 Crustal assimilation…………………………………………... 39 5.3 Regional Implications…………………………………………………… 41 6 Conclusions………………………………………………………………………………. 43 TABLE OF CONTENTS (Continued) Page 7 Bibliography ……………………………………………………………………………. 44 8 Appendices ……………………………………………………………………………… 50 LIST OF FIGURES Figure Page 1. Geologic map of western Arabia ………………………………………………… 1 2. Geologic map of the Arabian plate………………………………………………. 6 3. Geologic map of Harrat Rahat ………………………..…………………………. 9 4. Total Alkali vs. silica diagram for Harrat Rahat………………………………… 10 5. Map of samples collected at Harrat Ithnayn ……………………………………. 15 6. Total Alkali vs. silica diagram for Harrat Ithnayn ………………………………. 23 7. Bivariate diagram of MgO vs. K2O wt. % ……………………………………….24 8. Bivariate diagram of MgO vs. SiO wt. % ………………………………………. 24 9. Bivariate diagram of MgO vs. CaO wt. % ……………………………………… 25 10. Bivariate diagram of MgO wt. % vs. Ni ppm ………………………………….. 25 11. REE profiles for lava flows from Harrat Ithnayn ……………………………… 26 12. Evidence of excess 40Ar is shown in the 40Ar-39Ar age spectrum of a sample. 29 13. Evidence of Ar loss is shown in the 40Ar-39Ar age spectrum of a sample.......... 29 14. Age vs. latitude bar chart ………………………………………………………. 33 15. Bivariate diagram of MgO vs. CaO/Al2O3 wt. %................................................ 35 16. The result of the REEBOX MATLAB calculation …………………………… 39 17. A scatter diagram of K/Nb ratio vs. MgO concentrations……………………. 41 18. A scatter diagram of age vs. latitude, including helium isotopes ratio values … 42 LIST OF TABLES Table Page 1. The estimated area and volume of volcanism in MMN volcanic fields … 8 2. Parental magma for Harrat Ithnayn lavas ………………………………. 18 3. Geochemical data for Ithnayn lava flows ………………………………. 20 4. Summary table for new 40Ar-39Ar age determination for Ithnayn lavas… 28 LIST OF APPENDICES Appendix Page A. Petrographic data ……………………………………………………………… 50 B. Geochemical data………………………………………………………………. 54 1) Previous geochemical data of Camp et al. (1991) ……………… 54 2) REEBOX model and sensitivity analysis ………………………. 56 3) Important bivariate diagrams …………………………………… 57 C. Geochronological data…………………………………………………………… 59 1) Previous stratigraphic classification and K/Ar age results ………. 59 2) New 40Ar–39Ar age spectra and isochron plots for Ithnayn lavas … 60 LIST OF APPENDIX FIGURES Figure Page 1. Thin section photo of sample HI-5.………………………………………………….50 2. Thin section photo of sample HI-17.………………………………………………. 51 3. Thin section photo of sample HI-12.………………………………………………...52 4. Thin section photo of sample HI-2.………………………………………………….53 5. The input pop-up in the MATLAB script of the REEBOX model ………………....56 6. Sensitivity model used in the REEBOX model …………………………………… 56 7. P2O5 (wt.%) vs. Zr (ppm) bivariate diagram ……………………………………… 57 8. Assimilation fractional crystallization (AFC) calculated for K ppm ……………… 58 9. Previous stratigraphic classification and K/Ar age results ………………………… 59 10.40Ar–39Ar age spectra and isochron plots for HI-14 ………………………………. 60 11.40Ar–39Ar age spectra and isochron plots for HI-4 ………………………………. 61 12.40Ar–39Ar age spectra and isochron plots for HI-15 ………………………………. 62 13.40Ar–39Ar age spectra and isochron plots for HI-3A ……………………………….63 14.40Ar–39Ar age spectra and isochron plots for HI-19 ………………………………. 64 15.40Ar–39Ar age spectra and isochron plots for HI-12 ………………………………. 65 16.40Ar–39Ar age spectra and isochron plots for HI-11 ………………………………. 66 17.40Ar–39Ar age spectra and isochron plots for HI-13 ………………………………. 67 18.40Ar–39Ar age spectra and isochron plots for HI-7 ………………………………. 68 19.40Ar–39Ar age spectra and isochron plots for HI-20 ………………………………. 69 LIST OF APPENDIX TABLES Table Page 1. Previous geochemical data of Camp et al. (1991) ………………………………… 54 1 1- Introduction: Volcanic lava fields, locally known as Harrats, cover a large area (about 180,000 km2) in the western part of the Arabian