40Ar/ 39Ar dating of the Aptian–Albian igneous rocks in Makhtesh Ramon (Negev, Israel) and its stratigraphic implications

Abstract

40Ar/ 39Ar and K-Ar ages of whole rock samples and kaersutite (amphibole) crystals obtained from basanite and tephrite flows, lava lake series and pyroclastic rocks in Makhtesh Ramon (central Negev, southern Israel) define a Late Aptian–Early Albian time span of the Cretaceous magmatic event (Ramon Volcanics) and the order of the local volcanic activity, which had only been partly resolved by stratigraphic relations. The igneous activity started with volcanic flows that originated from several centres between 116.4 ± 3.4 Ma and 112.5 ± 1 Ma, continued with a lava lake series at 110.3 ± 6.1 Ma and terminated by explosive activity at 108.8 ± 1.2 Ma. The Late Aptian–Early Albian magmatic event is one of five events that are attributed to Cretaceous mantle plume activity and mark the termination of a geodynamic process involved in the evolution of the passive continental margin and the Levantine Basin of the Eastern Mediterranean. The Cretaceous plume head is postulated to be beneath northern Israel and the central Palmyrides (Syria), where fault-controlled depressions with volcanic fill of the first Cretaceous magmatic event were developed. Subsequent expansion of magmatic melts to more marginal areas (the Negev and the Coastal Range in Syria) resulted in the emplacement and eruption of igneous rocks during the second (Barremian) and third (Aptian–Albian, this paper) magmatic events, whereas sporadic and small volcanic eruptions of the fourth (Cenomanian) and fifth (Campanian) Cretaceous magmatic events were still active in the head area of the plume. The Late Aptian–Early Albian magmatic activity followed the differential uplifting and denudation of the central and southern Negev terrain, and the igneous rocks occupy a common stratigraphic interval bounded by continental formations. The lower boundary of this interval is set at the unconformable surface, which represents different levels of truncation, whereas the overlying formation covers all volcanic rocks and structures. The radiometric ages define the sedimentary gap and put constraints on the upper and lower age limits of the bounding sandstone formations that yielded only non-diagnostic fossils. The age of the volcanic interval matches the biostratigraphic data of a contemporaneous fossiliferous shallow marine intercalation (Deragot marine tongue) which occupies a similar stratigraphic position in the northern Negev. The extension of the Late Aptian–Early Albian marine invasion to the central and southern Negev was barred by the relative elevation of that terrain, which had been uplifted prior to the local volcanism and by the depositional slope. Whole rock basanite samples from the lower part of the lava lake series in Mount Arod yielded a 40Ar/ 39Ar age of 110.3 ± 6.1 Ma. Since some samples studied from this series display a reverse magnetic polarity, and the whole series was cooled simultaneously, the age obtained represents the entire series as well as the reversal event. This exclusively apparent Early Albian reversal event within the long Cretaceous Normal Polarity Superchron (C34n) adds new data to the global magnetostratigraphy.