Epigenetic alteration of the Precambrian igneous complex at Mount Timna, southern Israel: Oxygen-isotope studies

The Precambrian Mount Timna Igneous Complex (TIC) of southern Israel covers about 20 km² and is composed of mafic to felsic plutonic and hypabyssal rocks. The surrounding Cambrian sedimentary rocks were altered by hydrothermal fluids at about 13-15 Ma. In this study, oxygen isotopes were used to evaluate whether the rocks of the TIC also were affected. Many TIC rocks show mineralogical signs of alteration: olivine normally is serpentinized; amphibole and pyroxene have been corroded and altered to chlorite; and plagioclase commonly is clouded and altered to epidote, sericite, and/or kaolinite. Least-altered olivine norite has typical igneous δ¹⁸O values (+6.5 to +7.0‰), as do least-altered granites (+8.6 to +9.1‰). More highly altered rocks are enriched in ¹⁸O; altered olivine norite, for example, has whole-rock δ¹⁸O values as high as +12.6‰. Primary quartz has δ¹⁸O values (+7.6 to +8.5‰) typical for "normal" granitoid rocks, but the δ¹⁸O values of feldspars, especially plagioclase, are markedly higher (+8.3 to +11.3‰). Even for least-altered granitic rocks, Δ¹⁸O_{quartz-K-feldspar} (-0.7 to -0.1‰) and Δ¹⁸O_{quartz-plaginolase} values (-1.0 to -1.4‰) are negative. Preservation of primary quartz δ¹⁸O values indicates that alteration temperatures were no higher than 250-300°C. Oxygen isotopic results for a plagioclase-chlorite pair suggest that alteration occurred at ∼260°C from a hydrothermal fluid with δ¹⁸O = +3‰. The TIC is surrounded by sedimentary rocks, which probably were a source and conduit for basinal brines of such isotopic composition. The high emplacement level of the TIC ensured that it interacted with heated brines during the regional hydrothermal fluid movement that affected the sedimentary rocks. An analogous migration of warm basinal brines mobilized by tectonism altered large areas of mid-continental North America, including granitic basement rocks, and caused abundant Pb-Zn mineralization, and hydrocarbon generation and migration. We propose that the TIC participated in a higher-temperature version of a similar regional process, with movement of basinal brines perhaps being triggered by the early stages of Dead Sea rifting.