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Beil & Howell Information and Learning 300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA 800-521-0600 UMT PETROGENESIS OF CHROMITE AND ASSOCIATED MINERALS IN THE UPPER MANTLE PERIDOTITE OF THE NORTHERN SEMAIL OPmOLITE (UAE) “Volume I” DISSERTATION l*resented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of the Ohio State University by Solaiman Al-Aabed, MS. ********* The Ohio State University 2000 Dissertation Committee: Approved by Professer Michael Barton, adviser Professer Hallen Noltimeir Professer Douglas Pride Adviser Professer Terry Wilson Department of Geological Sciences UMI Number 9982513 Copyright 2000 by Al-Aabed, Solaiman Ali All rights reserved. UMI* UMI Microform9982513 Copyright 2000 by Bell & Howell Information and Leaming Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. Bell & Howell Information and Leaming Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 ABSTRACT The northern part of the Semai I (Oman) ophiolite extends along the northeastern border of the UAE. The upper mantle peridotites of this ophiolitic section contain chromite deposits in three main localities: Al’Hel, Siji and Zikt. The study is focussed in the Al’Hel and Zikt areas. Siji is not considered in detail because it is chemically similar to Al’Hel. Field, pétrographie, and geochemical evidence is used to divide the chromite fields into three major zones: Al’Hel, E. Zikt, and S. Zikt. The Al’Hel zone is very close to the mantle-crust transition, and is characterized by less depleted peridotites than those in Zikt. Al’Hel chromitites contain minor modal pyroxene, along with low-grade, Al-rich chromites. In contrast, the Zikt zone is located deeper in the mantle sequence, with E. Zikt closest to the mantle-crust transition, and S. Zikt furthest from the transition. The S. Zikt fields contain the highest-grade, Cr-rich chromites, whereas, the chromite composition at E. Zikt is between that of Al’Hel and S. Zikt. Chromite is always associated with olivine (sometimes with trace amounts of pyroxene) in chromitite and dunite lensoid bodies. The chromite/olivine ratio varies from 1 to zero, from massive chromitite to dunite. Olivine and chromite show a variety of textures such as net and circular texture. Some textures are characteristic for a specific zone (e.g. nodular texture at Zikt). Pré-, syn-, and post-emplacement deformation obliterated most of the original textures and formed new deformation fabrics. However, some original magmatic textures such as rhythmic layering are preserved. A magmatic origin for the formation of chromite and associated minerals is indicated by whoie-rocks and mineral geochemistry. The geochemical characteristics of the Al’Hel bodies are different from those of Zikt. Disequilibrium interaction between rising melts and the wall rocks was the main driving mechanism for chromite precipitation, particularly the massive chromitites. This process leads to Mg and Si enrichment in the melts by dissolution of some peridotite materials (e.g pyroxene). Increasing silica content increases polymerization and decreases Cr (and probably other heavy metals) solubility in the melts, and thus triggers chromite precipitation. It is concluded that there were three genetic stages of formation of chromite bodies in the UAE Semai 1 ophiolite. First, partial melting at a MOR spreading occurred center during drifting of Afro-Arabia from Eurasia (~ 200Ma ago). No significant chromite accumulation is expected during this phase. Second, island arc magmatism occurred above an intra-oceanic subduction zone that formed due to northeastward movement of Arabia toward Asia (begining at ca. 110 Ma). At this stage, the Zikt chromite orefields originated by interaction of calc-alkaline melts with upper mantle harzburgites. Third, back-arc (marginal basin) spreading began, and the Al’Hel orefields formed through interaction of MORB-like melts with the relatively less depleted peridotites. In other words, Al’Hel chromite bodies formed from MORB-like tholeiite melts in a marginal basin environment, whereas, the Zikt chromite bodies originated fiom calc-alkaline to boninite melts in an island arc to fore arc environment. Ill DEDICATION To my motherfor her love and patience To my wife fo r ahvays being with me To my sisters and my brothers for their encouragement and support IV ACNOWLEDGMENTS I thank my advisor Michael Barton for his assistance and patience, and for beneficial discussion. I would like to recognize Hallen Noltimeir, Douglas Pride, and Terry Wilson for serving in my committee, and for valuable guidance. I am grateful to Dr. Henery Clemmey from Derwent Company for his assistance and discussion in the field work. I would like to thank my family for their continuous support and encouragement, while 1 am at home and abroad. I greatly thank the UAE University and all people in the Department of Geological Sciences at the Ohio State University for the comprehensive support to make this study possible. VITA December, 21, 1965 ................................................ Bom -Dibba, Ai Fujairah,UAE 1984 .........................................................................Graduate, Uqba Bin Nafi High School, Diba A1 Fjairah 1988 .........................................................................B.S., Geology Department, UAE University, AI Ain 1988-1990 ............................................................... General Science and Geology Teacher, A1 Khalidiah School, Dibba AlFfujairah 1995 .........................................................................M.S. in Geological Science. The Ohio State University FIELD OF STUDY Major Field: Geological Sciences VI TABLE OF CONTENTS Page Dedication ..........................................................................................................................iv Acknowledgments ...............................................................................................................v Vita...................................................................................................................................tdi List of Tables ....................................................................................................................vii List of Figures ....................................................................................................................ix List of Plates ................................................................................................................... xxi Chapters; 1. Introduction ...................................................................................................................1 1.1 Preface................................................................................................. I 1.2 Historical synopsis ..............................................................................2 1.3 Oman Chromites ............................................................................... 12 1.4 The present study .............................................................................. 14 1.5 Research organization ....................................................................... 15 2. Oman mountains, ophiolites & the Semail ophiolite ...................................................17 2.1 Regional geology ..............................................................................17 2 .1.1 The Arabian (Persian) Gulf and Zagros Fold Belt ........................... 20 2.1.2 The Gulf of Oman and the Makran platform ................................... 23 2.1.3 The Arabian platform ....................................................................... 24 2.2 Tectonostratigraphy of The Oman mountains ................................. 36 2.3 Ophiolites .........................................................................................42 2.4 Oman (Semail) ophiolite: .................................................................44 2.4.1 Tectonic setting of the Semail ophiolite ..........................................49 \T l 3. Chromite fields in the UAE ........................................................................................59 3.1 Introduction ...................................................................................