A revised map of volcanic units in the Oman ophiolite: insights into the architecture of an oceanic proto-arc volcanic sequence

Abstract

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.

Paleobathymetry of Submarine Lavas in the Samail and Troodos Ophiolites: Insights From Volatiles in Glasses and Implications for Hydrothermal Systems

“…Pristine volcanic glasses are exceedingly rare in the Samail lavas (Kusano et al., 2017). Our field sampling throughout the Samail ophiolite (Belgrano et al., 2019) confirmed that glass is rare, but is most likely to be preserved in areas with sub‐greenschist facies hydrothermal alteration, hence mostly within the upper Alley group volcanic units (Figure 2). All the glasses we collected from the axial Geotimes lavas were subtly affected by secondary hydration, though one pristine glass was recovered from Axial Lasail lavas intercalated within the uppermost axial Geotimes sequence, near similar intercalations along Wadi Fizh (Belgrano & Diamond, 2019).…”

“…Magmatism along a seafloor spreading axis formed much of the lower crust as well as the sheeted‐dike complex and comagmatic “ Geotimes ” basal lava unit (Pearce et al., 1981; Umino et al., 1990). These dikes and lavas are similar in trace element composition to mid‐ocean ridge basalts (MORB), but show a broader range of SiO 2 compositions and are subtly contaminated by a hydrous subduction component (Belgrano et al., 2019; MacLeod et al., 2013; Pearce et al., 1981). Locally, axial magmatism was more substantially contaminated by subduction fluids, resulting in the eruption of high Th/Nb “ Axial Lasail ” basalts depleted in incompatible trace elements (Belgrano & Diamond, 2019).…”

“…Some areas of the axial crust (Figure 1a) were then immediately overlain by off‐axial “ Lasail ” unit basalts (Alabaster et al., 1982; Belgrano et al., 2019). Post‐axial lavas buried this entire sequence, and can be split into lower “ Tholeiitic Alley ” and upper “ Boninitic Alley ” units based on stratigraphy and composition (Belgrano et al., 2019; Gilgen et al., 2014; Ishikawa et al., 2002; Kusano et al., 2014). Where preserved, the top of the volcanic sequence is conformably capped by umbers and pelagic sediments of the Suhaylah formation (Robertson & Fleet, 1986).…”

“…(a) Bedrock geological map and upper crustal stratigraphy of the northeastern (Batinah Coast) region of the Samail ophiolite, simplified after Belgrano et al. (2019). Inset shows entire ophiolite (dark grey) on Arabian Peninsula.…”

High‐Precision U‐Pb Zircon Dating of Late Magmatism in the Samail Ophiolite: A Record of Subduction Initiation

“…Belgrano et al. (2019) recently published a comparison of trace element data from the two localities. They note differences in the trace element compositions of the Samail and IBM forearc lavas—including higher Ti/V in Samail V1 versus IBM forearc basalts and elevated Zr in IBM boninites—which they argue may be related to prior depletion of the mantle wedge in the IBM and differences in the composition of subducted crust.…”