The Northern Samail Ophiolite: An Oxygen isotope, microprobe, and field study

Abstract

Geological, petrological, and oxygen isotopic data are presented for 228 whole rock and mineral samples collected from a 100×20 km area of the northern Samail ophiolite in Oman. Most of these samples are from three detailed profiles through the pillow lavas, sheeted dikes, and layered gabbros of this laterally heterogeneous fragment of Cretaceous oceanic crust, down to and across the petrologic Moho. The profiles encompass a range of petrologic and tectonic styles, and each profile exhibits distinctive ^(18)/^(16)O variations compared to one another and to mid-ocean ridge basalts, as a result of pervasive seawater-hydrothermal interaction that varied in intensity along strike in the ophiolite. In general, ^(18)O depletions are observed in the layered gabbros and ^(18)O enrichments in most of the sheeted dikes and pillow lavas, similar to results previously observed in the southern part of the ophiolite (Ibra area), where ^(18)O depletions within the gabbroic section are quantitatively balanced by ^(18)O enrichments in the shallower parts of the oceanic crust. The Wadi Hilti profile, selected as an example of relatively intact crust, differs from Ibra in having more uniform and slightly higher δ^(18)O in the gabbros (+5.4 to +6.3), as well as in containing more hydrous alteration minerals (amphibole, epidote, chlorite, and prehnite). The profiles in the Wadi Kanut-Shafan and Wadi Rajmi sections are much more complex and reveal the impact of off-axis intrusions and deep crustal shearing. Plagiogranite-wehrlite intrusions in the Shafan-Kanut area superimposed a local hydrothermal aureole on the ophiolite, evident in dikes highly depleted in ^(18)O, quartz-sulfide veins, abundant epidote, thullite, and chlorite in shallower rocks, and low-temperature hydrous alteration of deeper gabbroic rocks; the latter produced an overall increase in whole rock δ^(18)O (+6.2 to +6.9). Such late stage intrusions are found throughout the northern half of the Samail ophiolite. The Wadi Rajmi area, which is a possible fossil transform or propagating rift, represents the most complex of the three profiles; it also contains the most abundant highly deformed and hydrothermally altered rocks, together with the deepest and largest zone of ^(18)O depletion yet found in any ophiolite (locally δ^(18)O < +2.0). Conduits for large volumes of high-temperature hydrothermal fluids were provided by fractures now occupied by low-^(18)O gabbro pegmatites and low-^(18)O dikes. Material balance estimates for the regional samples and from the various transects through the ophiolite give crustal bulk δ^(18)O averages (+5.9 to +6.3) that are, within sampling error, almost identical to the average MORB basalt value of about +5.8, if both vertical and lateral crustal heterogeneities are integrated into a three-dimensional model. This supports and amplifies the conclusion of earlier workers that the δ^(18)O of seawater is buffered and controlled by hydrothermal interaction with oceanic crust, as long as the cumulative effects (both spatial and temporal) of all seafloor magmatic/hydrothermal processes are considered. The very slight out-of-balance enrichment of the integrated crustal average δ^(18)O 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 are prominent in the vicinity of the petrologic Moho and may indicate exchange with low-temperature aqueous fluids during or after detachment of the obducted slab.