⁴⁰Ar ³⁹Ar dating of combustion metamorphism ("Mottled Zone", Israel)

Tertiary surface combustion of Cretaceous bituminous sediments in Israel produced high-T, low-P metamorphic assemblages, which were dated by conventional KAr and ⁴⁰Ar ³⁹Ar techniques. Meaningful ages were obtained from samples of high metamorphic grade and K content higher than 0.8%, containing sanidine, anorthite, gehlenite and zeolites. In these minerals the KAr system has been reset, and equilibrated with atmospheric Ar, yielding a mean age of 3 ± 1 Ma. ⁴⁰Ar ³⁹Ar plateau and isochron ages for four whole-rock samples range between 2.3 and 4.0 Ma. One gehlenite rock yielded an ⁴⁰Ar ³⁹Ar age of 16 ± 0.2 Ma (1σ). These results indicate a major combustion metamorphic event around 3 Ma, probably preceded by an event around 16 Ma. The early event took place upon initial exposure of the protolith in the Miocene, whereas the second followed the removal of Neogene sediments and re-exposure of the bituminous rocks.