AR-40 AR-39 DATING OF COMBUSTION METAMORPHISM (MOTTLED ZONE,ISRAEL)/

Tertiary surface combustion of Cretaceous bituminous sediments in Isra el produced high-T, low-P metamorphic assemblages, which were dated by conventional K-Ar and Ar-40/Ar-39 techniques. Meaningful ages were ob tained from samples of high metamorphic grade and K content higher tha n 0.8%, containing sanidine, anorthite, gehlenite and zeolites. In the se minerals the K-Ar system has been reset, and equilibrated with atmo spheric Ar, yielding a mean age of 3 +/- 1 Ma. Ar-40/Ar-39 plateau and isochron ages for four whole-rock samples range between 2.3 and 4.0 M a. One gehlenite rock yielded an Ar-40/Ar-39 age of 16 +/- 0.2 Ma (1 s igma). These results indicate a major combustion metamorphic event aro und 3 Ma, probably preceded by an event around 16 Ma. The early event took place upon initial exposure of the protolith in the Miocene, wher eas the second followed the removal of Neogene sediments and re-exposu re of the bituminous rocks.