The high temperature reaction zone of the Oman ophiolite: new field data, microthermometry of fluid inclusions, PIXE analyses and oxygen isotopic ratios


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Abstract

The present study is focused on the so-called High Temprature Reaction Zone of the Oman ophiolite, a thin zone located between the roots of the sheeted dyke complex and the high-level gabbros marking the roof of the fossil magma chambers. The distribution of diabases, chloritised dykes, spilitized dykes and epidosites (in the order of increasing hydrothermal alteration) was studied along continuous outcrops of the lower sheeted dyke complex in three selected areas. The Muwaylah section, in the Haylayn massif, representing a fossil axial discontinuity, is the most massively epidotised area, with epidosite zones of 3–15 m wide and an average spacing of 12 m. In this area, there are two directions of dyking, and a massive sulfide deposit (Daris prospect). The wadi Andam area (Samail massif), representing a much less tectonised site, close to a mantle diapir and located probably midway between the tip and the center of an accretion segment, is also well epidotised, with epidosites spacing of 10–25 m. The wadi Salahi area probably represents the central part of a fossil accretion segment and is by far the least altered site, in spite of being located quite beneath the Zuha sulfide prospect. This confirms that the hot ascending fluids in major discharge zones are strongly focused. Fluid inclusions from about 50 samples collected in the plagiogranites, gabbros and sheeted dykes of the Muwaylah and wadi Falah areas (SE Haylayn massif), consist of: (1) monophase (liquid) inclusions, generally stretched and deformed; (2) vapour-dominated, low-salinity, 2-phase inclusions, with average salinity of 3.8 wt% eq. NaCl and an average Th of 370 °C (3) liquid-dominated, low-salinity, 2-phase inclusions, with an average salinity of 4.2 wt% eq. NaCl and an average Th of 325 °C, and (4) liquid-dominated, high-salinity inclusions, containing a solid halite daughter phase, dissolving at higher temperature (292–441 °C ) than homogenisation of the fluid phases (230–403 °C ). One plagiogranite sample collected in the Muwaylah area (Haylayn massif) is particularly rich in quartz-epidote hydrothermal veins. Xenomorphic globular quartz is rich in high-salinity, brine-rich inclusions, with halite cubes dissolving at very high temperatures (358–496 °C ) yielding very high salinities (43 to 59.2% NaCl eq.), and with liquid-vapour homogenisation of 275 °C on average. Molecular Raman Spectroscopy analyses have confirmed the aqueous nature of these inclusions and the absence of detectable CO2, NH4, N2 and SO2. Daughter solid phases other than NaCl were determined as hematite and anhydrite, and a third phase is hydroxyl-bearing (amphibole?). PIXE analyses on six brine-rich inclusions allowed to detect significant but variable contents in Cl, Fe, Mn, K, Ca, Zn and Br. Copper, remarkably, was never detected, and two measured Cl/Br ratios are close to that of seawater. The measurement of oxygen isotopic ratios of the fluids extracted from some fluid inclusions and of associated host-minerals (quartz, epidote) suggest that both seawater-derived and magma-derived fluids have mixed in the High Temperature Reaction Zone. PIXE data yield a similar conclusion, based on the contrasted Fe and Mn-contents and Cl/Br ratios of the analysed inclusions. However, the oxidised and Cu-depleted nature of the brine-rich inclusions suggests that the magma-derived or seawater-derived brines are residual liquids that have degassed. Present V-X properties of most NaCl-saturated inclusions do not keep the record of the boiling process, as they homogenise by halite disappearance and not by vapour disappearance. Probably they have been modified by several post-trapping changes, for instance by necking.

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