Listwaenite Genesis in Upper Crustal Extensional Fault Zones at the base of the Semail Ophiolite, Oman
Christopher Bailey, William & Mary
Eric Nubbe, William & Mary
Andreas Scharf, Sultan Qaboos University
Robert , University of Witwatersrand
Abstract
Listwaenite is a distinctive reddish-orange rock composed primarily of carbonate + quartz with accessory siderite, magnesite, fuchsite, hematite and goethite that is interpreted to form by carbonation of serpentinized peridotite. In northern Oman, listwaenite is localized near/at the base of the Semail Ophiolite, thereby leading researchers to posit that listwaenite formed in the mantle wedge during Late Cretaceous obduction and ophiolite emplacement.
Our field studies indicate that listwaenite occurs as tabular sheets and dike-like bodies that are associated with post-obductional extensional fault zones. Multiple generations of listwaenite are exposed in 1) extensional duplexes bound by shallowly-dipping normal faults, 2) younger moderately- to steeply-dipping extensional faults, and 3) as klippen that overlie rocks of the metamorphic sole and unmetamorphosed platform carbonates. Dike-like bodies of listwaenite and associated carbonate veins cut the post-obductional conglomerates of the Al-Khod Formation (Maastrichtian, ~70 Ma).
Meso- and micro-scale analysis reveals well-developed cataclastic textures indicative of multiple generations of brittle deformation in the listwaenite. Strongly brecciated samples appear to have undergone further 'listwaenization' after deformation ceased. Other microstructures include undulose extinction, spherulites, deformation lamellae, and extremely fine-grained recrystallized quartz. Kinematic evidence at the micro-scale is consistent with extensional deformation and positive dilational strains during listwaenite formation.
LA-ICP-MS U-Pb ages from three samples of carbonate in listwaenite and associated veins yield a range of well-constrained ages from 55 to 64 Ma, while six samples yield imprecise ages that range from 3 to 33 Ma. We suggest that hydrothermal circulation of carbon-rich fluids in active extensional fault zones facilitated multiple episodes of listwaenite formation in the upper crust during Cenozoic exhumation of the north Oman Mountains. These results are incompatible with listwaenite genesis in the mantle wedge during obduction of the Semail Ophiolite and require reassessment of some geochemical models for carbon fluxes in the mantle.
Session
Session 1: Fault Zones from Top to Bottom