Abstract. Numerous studies have revealed genetic similarities between Tethyan
ophiolites and oceanic “proto-arc” sequences formed above nascent subduction
zones. The Semail ophiolite (Oman–U.A.E.) in particular can be viewed as an
analogue for this proto-arc crust. Though proto-arc magmatism and the
mechanisms of subduction initiation are of great interest, insight is
difficult to gain from drilling and limited surface outcrops in marine
settings. In contrast, the 3–5 km thick upper-crustal succession of the
Semail ophiolite, which is exposed in an oblique cross section, presents an
opportunity to assess the architecture and volumes of different volcanic
rocks that form during the proto-arc stage. To determine the distribution of
the volcanic rocks and to aid exploration for the volcanogenic massive
sulfide (VMS) deposits that they host, we have remapped the volcanic units
of the Semail ophiolite by integrating new field observations, geochemical
analyses, and geophysical interpretations with pre-existing geological maps.
By linking the major-element compositions of the volcanic units to rock
magnetic properties, we were able to use aeromagnetic data to infer the
extension of each outcropping unit below sedimentary cover, resulting in
a new map showing 2100 km2 of upper-crustal bedrock. Whereas earlier maps distinguished two main volcanostratigraphic units, we
have distinguished four, recording the progression from early spreading-axis
basalts (Geotimes), through axial to off-axial depleted basalts (Lasail), to
post-axial tholeiites (Tholeiitic Alley), and finally boninites (Boninitic
Alley). Geotimes (“Phase 1”) axial dykes and lavas make up ∼55 vol % of the Semail upper crust, whereas post-axial (“Phase 2”) lavas
constitute the remaining ∼45 vol % and ubiquitously cover
the underlying axial crust. Highly depleted boninitic members of the Lasail
unit locally occur within and directly atop the axial sequence, marking an
earlier onset of boninitic magmatism than previously known for the
ophiolite. The vast majority of the Semail boninites, however, belong to the
Boninitic Alley unit and occur as discontinuous accumulations up to 2 km
thick at the top of the ophiolite sequence and constitute ∼15 vol % of the upper crust. The new map provides a basis for targeted
exploration of the gold-bearing VMS deposits hosted by these boninites. The
thickest boninite accumulations occur in the Fizh block, where magma ascent
occurred along crustal-scale faults that are connected to shear zones in the
underlying mantle rocks, which in turn are associated with economic
chromitite deposits. Locating major boninite feeder zones may thus be an
indirect means to explore for chromitites in the underlying mantle.
Abstract. Recent studies have revealed genetic similarities between Tethyan ophiolites and oceanic proto-arc sequences formed above nascent subduction zones. The Semail ophiolite (Oman–U.A.E.) in particular can be viewed as an analogue for this proto-arc crust. Though proto-arc magmatism and the mechanisms of subduction-initiation are of great interest, insight is difficult to gain from drilling and limited surface outcrops in submarine fore-arcs. In contrast, the Semail ophiolite, in which the 3–5 km thick upper-crustal succession is exposed in an oblique cross-section, presents an opportunity to assess the architecture and volumes of different volcanic rocks that form during the protoarc stage. To determine the distribution of the volcanic rocks and to aid exploration for the volcanogenic massive sulphide (VMS) deposits that they host, we have re-mapped the volcanic units of the Semail ophiolite by integrating new field observations, geochemical analyses and geophysical interpretations with pre-existing geological maps. By linking the major element compositions of the volcanic units to rock magnetic properties, we were able to use aeromagnetic data to infer the extension of each outcropping unit below sedimentary cover, resulting in in a new map showing 2100 km2 of upper-crustal bedrock. Whereas earlier maps distinguished two main volcanostratigraphic units, we have distinguished four, recording the progression from early spreading-axis basalts (Geotimes) through to axial to off-axial depleted basalts (Lasail), to post-axial tholeiites (Tholeiitic Alley) and finally boninites (Boninitic Alley). Geotimes (Phase 1) axial dykes and lavas make up ~55 vol% of the Semail upper crust, whereas post-axial (Phase 2) lavas constitute the remaining ~ 45 vol % and ubiquitously cover the underlying axial crust. The Semail boninites occur as discontinuous accumulations up to 2 km thick at the top of the sequence and constitute ~ 15 vol % of the upper crust. The new map provides a basis for targeted exploration of the gold-bearing VMS deposits hosted by these boninites. The thickest boninite accumulations occur in the Fizh block, where magma ascent occurred along crustal-scale faults that are connected to shear zones in the underlying mantle rocks, which in turn are associated with economic chromitite deposits. Locating major boninite feeder zones may thus be an indirect means to explore for chromitites in the underlying mantle.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.