Oxygen isotopic evidence for meteoric hydrothermal alteration of the Jabal at Tirf igneous complex, Saudi Arabia

Abstract

The ^(18)O/^(16)O ratios of rocks and minerals from the 22 m.y. old Jabal at Tirf complex are very low and highly variable. Layered gabbro: δ^(18)O plagioclase (An_(62)-An_(74)) =-1.7 to +5.2, δ^(18)O clinopyroxene = +1.3 to +5.0, Δ^(18)O plag-px =-4.6 to 0.1. Granophyres, δ^(l8)O whole rock =-0.5 to +1.8, quartz =+0.2 to +3.9. Diabase dike swarm: δ^(18)O whole rock =-1.4 to +3.7. The negative Δ^(18)O plag-px values indicate that the entire 1800 meter-thick layered gabbro has been depleted in ^(18)O subsequent to crystallization, with the most extreme depletion occurring in the plagioclase from the upper 600m and at the base. Comparing nearby samples within the stratigraphic sequence, slightly higher No contents tend to occur in the more ^(18)O-depleted feld-spars, but the extreme ^(18)O-depletions are not accompanied by any major chemical changes in the plagioclase. These data imply that large volumes of heated meteoric ground waters circulated through this complex, which represents a spreading center associated with the opening of the Red Sea. The Jabal at Tirf complex has many features in common with ophiolite complexes, the major differences being the presence of on overlying section of non-marine sediments end the lock of pillow lavas, the much lower δ^(18)O values, and the fact that the granophyres and silicic dike rocks are strongly potassic rather than sodic . We propose that these differences are basically because low-^(18)O meteoric ground waters, rather than NaCI-rich ocean waters, were involved in the hydrothermal convective system associated with the emplacement of this complex. The features in this complex are similar to those observed in the East Greenland dike swarm in the vicinity of the Skaergaard intrusion.