Dike-resembling features on the Yir'on Plateau, northern Israel

Dike-resembling structures exposed in the Yir'on plateau area are interpreted as volcanic conduits that did not emerge to the surface during the time of volcanic activity. In this study we investigated one such structure which is exposed along an artificial road-cut leading to Kibbutz Yir'on. The width of the structure is about 2.5 m and its height, 12 m. Even though the structure is placed in Eocene limestone of the Bar-Kochva Formation, carbonate clasts are rare, and more than 95% of the fill of the structure is composed of basalt, chert clasts, and quartz geodes. The carbonate clasts include limestone, chalk, dolomitic limestone, and dolomite, derived from the Judea, Mt. Scopus, and Avedat groups. The Judea and Mt. Scopus groups are not exposed in the immediate vicinity. Thirteen cores taken from basaltic pebbles were used to conduct a paleomagnetic conglomerate test. Results show a random magnetic direction (K = 2) indicating that the pebbles were placed at temperatures lower than the magnetic blocking temperatures (T_b) of sub-aerial alkali-olivine basalt (520-580 °C). Petrographic analysis of some pebbles from the fill and of the country rock at the dike wall indicate solution and deposition processes in the presence of hot fluids which remagnetized the carbonatic wall-rock but did not affect the magnetization of the basalt pebbles. Therefore, it seems that the fill of the structure was emplaced at temperatures higher than the surrounding country-rock temperature (which could affect the carbonates but not the basalts) and lower than 520 °C, and that the filling process was associated with volcanic activity. It is suggested in this study that the investigated structure is the front of a volcanic conduit that did not reach the surface. This suggestion explains the temperatures that existed in the structure during the time of the fill. The scarcity of carbonatic component in the fill is explained by the solubility of carbonates in hot solutions in the closed system of the structure. The roundness of the pebbles is caused by chemical dissolution, movement of the particles in the dike in a high energy medium, and original roundness as in the case of the quartz geodes.