Epidotisation of diorites at Al Hadah,Saudi Arabia: Fluid influx into cooling plutons

Diorite plutons at Al Hadah Saudi Arabia, which constitute part of the pan-African magmatic sequence (ca. 600 Ma), exhibit extensive development of epidote. The epidote alteration is concentrated along veins and dyke margins, and is characterised by transformation of plagioclase (Ab 67)+hornblende+biotite+quartz to epidote+hornblende+tremolite−actinolite+plagioclase (Ab 99)±quartz. The reactions involve addition of CaO and total Fe₂O₃, depletion of MgO, Na₂O and K₂O, with variable gains or losses of SiO₂. Epidotised alteration products are hydrated and oxidised relative to the diorite precursor. The whole rock δ¹⁸O of fresh diorite is + 8.2‰ to + 8.8‰, whereas epidote domains have δ¹⁸O whole rock of +9.8‰ to +10.48‰ and negative Δ¹⁸O_{quartz-plagioclase}, implying oxygen isotope exchange with fluids at low temperatures. Epidotisation is considered to have accompanied influx of fluids into plutons during cooling and contraction. The fluids were probably deep formation waters with relatively high Ca²⁺/Na⁺ ratios, moving up thermal gradient.