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 1km2 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 off-axis volcanism. Another epidosite in the SDC and many in the volcanic units formed later during post-spreading, 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 1.4km of the crust, others between 1.2 and 1.7km depth, whereas the deepest formed between 2.6 and 3.8km 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.