The Northern Samail Ophiolite: An Oxygen Isotope, Microprobe, and Field Study

Geological, petrological, and axygcn isotopic data are pn:sentecl for 228 whole rock and mineral samples collected from a 100x20 km area of the nortbem Samail opbiolite in Oman. MOlt of these samples arc from three detailed profiles tluough the pillow IIIYBI, sbeetecl dikes, and .red pbbi'OI of this laterally heterogeneous fratpnent of Cretaceous oceanic crust, down to and 8C1'0I& the petroaop: Moho. The profiles encompass a l'llDF of petrologic and tectonic styles, and each profile cmibit& distinctive llloJ16o variations compared to one another and to mid-ocean ridge basalt&, as a result of ~ seawater-h)'drothermal interaction that varied in intensity along strike in the ophiolite. In general, 1Bo depletions arc observed in the layered gabbi'OI and 1llo enrichments in most of the sheeted dikes and pillow lavas, similar to results previously observed in the southem part of the ophiolite (Ibra area), where 1Bo depletions within the gabbroic section arc quantitatively balanced by 1llo enrichments in the sballower parts of the oceanic crust. The Wadi Hilti profile, selected as an example of relatively intact crust, differs from lbra in having more uniform and slightly bigher6 1Bo in the gabbi'OI ( + S.4 to +6.3), as well as in containing more h)'drous alteration minerals (amphibole, epidote, cblorite, and prcbnite ). The profiles in the Wadi Kanut-Shafan and Wadi Rajmi sections arc much more complex and reveal the impact of off-axis intrusions and deep crustal shearing. Plagiogranitewebrlite intrusions in the Sbafan-Kanut area superimposed a local h)'drothermal aureole on the ophiolite, evident in dikes highly depleted in 1Bo, quartz-sulfide veins, abundant epidote, thullite, and cblorite in shallower rocks, and low-temperature h)'drous alteration of deeper gabbroic rocks; the latter produced an overall increase in wbole rock 6 1Bo ( +6.2 to +6.9). Such late stage intrusions arc found throughout the northem half of the Samail ophiolite. The Wadi Rajmi area, which is a possible fossil transform or propagating rift, represent& the most complex of the three profiles; it also contains the most abundant highly deformed and h)'drothermally altered rocks, together with the deepest and largest zone of 1Bo depletion yet found in any ophiolite (locally61llo < + 2.0). Conduits for large volumes of high-temperature h)'drothermal fluids were provided by fractures now occupied by low-1Bo gabbro pegmatitcs and Iow-lllo dikes. Material balance estimates for the regional samples and from the various transects through the ophiolite give crustal bulk 61Bo averages ( +S.9 to +6.3) that arc, within sampling error, almost identical to the average MORB basalt value of about + S.8, if both vertical and lateral crustal heterogeneities are integrated into a threedimensional model. This supports and amplifies the conclusion of earlier workers that the 6 1Bo of seawater is buffered and controned by h)'drothermal interaction with oceanic crust, as long as the cumulative effects (both spatial and temporal) of all seafloor magmatic/bydrothermal processes arc considered. The very slight out-of-balance enrichment of the integrated crustal average 6 1Bo compared to MORB may be explained by the ubiquitous mineralogical and isotopic evidence for a late, low-temperature alteration event in the basal gabbros; these effects arc prominent in the vicinity of the petrologic Moho and may indicate exchange with low-temperature aqueous fluids during or after detachment of the obductecl slab.