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Genesis and geodynamic evolution of serpentinized ultramafics and associated magnesite deposits in the Al-Wask ophiolite, Arabian Shield, Saudi Arabia

Gahlan, Hisham A. and Azer, Mokhles K. and Asimow, Paul D. and Al-Kahtany, Khaled M. (2020) Genesis and geodynamic evolution of serpentinized ultramafics and associated magnesite deposits in the Al-Wask ophiolite, Arabian Shield, Saudi Arabia. American Journal of Science, 320 (3). pp. 236-279. ISSN 0002-9599. doi:10.2475/03.2020.02.

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Situated along the Yanbu Suture Zone, the Al-Wask ophiolite is one of the largest and best-preserved ophiolite sequences in the Proterozoic Arabian shield. A mantle section of serpentinized ultramafics is structurally overlain by a crustal section of gabbros and pillow lavas. The whole ophiolite sequence is capped by pelagic sedimentary cover, and tectonically emplaced over a metamorphosed island-arc volcano-sedimentary succession. The Al-Wask ultramafic rocks are strongly deformed, metamorphosed, and altered by carbonatization and silicification. Samples dominated by antigorite indicate upper greenschist to lower amphibolite facies peak metamorphic grade, whereas samples dominated by lizardite and magnesite preserve lower grade conditions that we interpret as a cooling path buffered to low CO₂ activity by the increasing stability of magnesite with decreasing temperature. Nearly all the primary silicate minerals have been replaced by serpentine minerals, leaving only relics of primary olivine and chromian spinel. Petrographic observation of relict olivine and spinel and of mesh and bastite textures in the serpentines suggest that the peridotite protoliths were mainly harzburgite with minor dunite. Whole-rock compositions of serpentinites show low CaO (<0.1 wt.%), Al₂O₃ (<1.5 wt.%), and Y (<0.4 ppm) combined with high Mg# (0.90–0.92), Ni, Co, and Cr contents; all these indicate a highly refractory mantle protolith. The mineral chemistry of relict primary spinel and olivine provides additional petrogenetic and geodynamic indicators. The high Cr# (> 60) and low TiO₂ (≤0.2 wt. %) of spinel and high forsterite contents (90–92) of associated olivine indicate residual mantle that underwent extensive partial melt extraction. The whole-rock and mineral chemistry of the serpentinized ultramafic rocks are both consistent with extracted melt fractions from ∼32 to 38 percent. This extent of melting is typical of fore-arc supra-subduction zone settings, which is the most likely tectonic environment for formation and preservation of the Al-Wask ophiolite. Two types of magnesite deposits can be distinguished in the Al-Wask mantle section: an early generation of massive magnesite and a later generation of magnesite veins. Hence the Al-Wask ophiolite underwent multiple stages of carbonatization, likely involving different sources of CO₂-bearing fluids. The massive magnesite likely formed at relatively high temperature during cooling from peak metamorphic condition from CO₂-bearing fluid probably derived from decomposition of subducted carbonates. Using thermodynamic calculations in the simple MgO-SiO₂-H₂O-CO₂ system, we constrain the path of the reaction boundary where lizardite and magnesite can coexist at equilibrium. On the other hand, the cryptocrystalline magnesite veins fill tectonic fractures and likely formed at low temperature and shallow levels, after serpentinization and ophiolite obduction.

Item Type:Article
Related URLs:
URLURL TypeDescription
Azer, Mokhles K.0000-0002-3711-8630
Asimow, Paul D.0000-0001-6025-8925
Additional Information:© 2020 American Journal of Science. Special thanks are paid to King Saud University, Deanship of Scientific Research, Research Group No. RG-1436-036, for their support. PDA acknowledges support from the US NSF, award EAR-1550934. The authors are grateful to Peter Kelemen and an anonymous reviewer for their insightful and constructive comments.
Funding AgencyGrant Number
King Saud UniversityRG-1436-036
Subject Keywords:Al-Wask ophiolite, mantle, magnesite, supra-subduction zone, Arabian Shield
Issue or Number:3
Record Number:CaltechAUTHORS:20201009-131404911
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Official Citation:Hisham A. Gahlan, Mokhles K. Azer, Paul D. Asimow, and Khaled M. Al-Kahtany. Genesis and geodynamic evolution of serpentinized ultramafics and associated magnesite deposits in the Al-Wask ophiolite, Arabian Shield, Saudi Arabia. Am J Sci March 2020 320:236-279; doi:10.2475/03.2020.02
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:105968
Deposited By: Tony Diaz
Deposited On:09 Oct 2020 20:37
Last Modified:16 Nov 2021 18:48

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