Sub-seafloor epidosite alteration: Timing, depth and stratigraphic distribution in the Semail ophiolite, Oman

Abstract

Pervasive epidotization of igneous rocks is a common feature in the ophiolite record of hydrothermally altered oceanic crust. Current genetic models view epidosites as markers of focussed upflow of hydrothermal fluid beneath oceanic spreading ridges. The epidosites are envisaged to form at the base of the sheeted dike complex (SDC) during active plate spreading. Our mapping of the Semail ophiolite in Oman has revealed abundant epidosites in the volcanic sequence, some exceeding 1 km 2 in extent.They are more frequent and far larger than the mineralogically identical epidosites in the SDC. We have also found epidosites that traverse the entire SDC from bottom to top. Thus, rather than being restricted to the base of the SDC, as implied by current models, epidosites in fact occur throughout the SDC and dominantly within the overlying volcanic pile. We report the occurrence of 19 epidosite bodies and their crosscutting relations with respect to host lava units, dikes, intrusive stocks and also seafloor umbers. The volcanostratigraphic affiliation of the dikes is identified by their whole-rock and clinopyroxene compositions. The relations set constraints on the timing of epidotization with respect to igneous activity in the ophiolite. At least one of the epidosites in the SDC formed during Lasail offaxis volcanism. Another epidosite in the SDC and many in the volcanic units formed later during postspreading, Alley and Boninitic Alley supra-subduction zone volcanism. Only permissive, not compelling, evidence allows just two of the epidosites to have formed within the main-stage SDC during or shortly after its emplacement. We conclude that epidotization of the oceanic crust is not necessarily coupled to spreading ridges and that it can occur during fore-arc volcanism. This finding is consistent with evidence from the modern seafloor and it requires a different hydrothermal environment to that traditionally associated with alteration beneath spreading axes. The timing and stratigraphic constraints show that some of the epidosites formed within the top 1.4 km of the crust, others between 1.2 and 1.7 km depth, whereas the deepest formed between 2.6 and 3.8 km below the paleo-seafloor. Finally, the timing constraints do not prescribe a strict temporal relationship between epidotization and the formation of the numerous seafloor massive-sulphide (VMS) deposits known in the ophiolite. However, a role for epidosites as source rocks for the metals in the deposits remains permissible.

Constraints on Cooling of the Lower Ocean Crust From Epidote Veins in the Wadi Gideah Section, Oman Ophiolite

“…It was collected from the base of the sheeted dike complex in the Wadi Amadhi, just 79 m above the isotropic gabbros (i.e., former melt lens). The sample is a monomineralic epidote precipitate and not equivalent to epidosites that have been observed in the sheeted dikes of the northern Oman Ophiolite (Gilgen et al, ; Nehlig et al, ). The average fluid inclusion entrapment temperature of 330 °C and the seawater‐like salinities of the fluid inclusions are similar to results of earlier workers.…”

Reply to referee Y. Kusano's comment

“…Variations in volcanostratigraphy can then be used to test previously proposed along-strike variations in subduction-zone influence (e.g. Goodenough et al, 2010;Python et al, 2008) and hydrothermal processes (Alabaster et al, 1982;Gilgen et al, 2016;MacLeod and Rothery, 1992), as well as the overall significance of the "Phase 2" magmatic Nicolas et al (2000) with the additional differentiation of the structurally separate western (W.) and eastern (E.) Fizh blocks, showing the area mapped in this study. Inset: map of Tethyan ophiolites adapted after Dilek et al (2007) with U-Pb zircon ages of initial accretion indicated for the Albian-Turonian ophiolite chain of 1 Troodos, 2 Kizildag, 3 Nain, 4 Deshir, and 5 Semail after Shafaii Moghadam et al (2013) and references therein.…”

“…4a). Gilgen et al (2016) sampled similar lavas and their dyke equivalents (pillow lava sample RAM010 was remeasured with ICP-MS for this study), assigning them to Lasail on the basis of their clinopyroxene compositions. also reported similar lavas within a lens of "Axial Lasail" intercalated within Geotimes near Wadi Hawarim.…”

“…Tabular intrusions intruded along faults, as well as epidotecemented fault-breccias, were taken as evidence for "synmagmatic" deformation, similarly to Reuber (1988). Unlike chlorite, epidote is not part of the overprinting Q 0 quartzhematite vein assemblage, and pervasive epidote alteration in the upper crust occurred mostly during Tholeiitic Alleyphase magmatism (Gilgen et al, 2016).…”

“…Red circles: field mapping locations (this study). Reference samples include triangles: volcanic glasses (this study); filled diamonds: dykes and sills (this study); unfilled diamonds: dyke samples from Adachi and Miyashita (2003); squares: lava and volcanic glass samples from Gilgen et al (2014Gilgen et al ( , 2016, Kusano et al (2017), andMacLeod et al (2013); unfilled squares: P1 and P2 intrusive samples from Haase et al (2016) and de Graaff et al (2019). Numbered outlines: coverage of previous publications used to support our mapping: (1) BRGM (1993b), (2) BRGM (1993a), (3) BME (1987a), (4) BME (1987c), (5) BME (1987b), (6) BRGM (1986b), (7) BRGM (1986a) and local maps, (8) Reuber (1988), (9) Lower crustal and mantle rocks (copied and simplified from the regional map set) are shown for context.…”