Provenance of the great Cambrian sandstone succession of northern Gondwana unravelled by strontium, neodymium and lead isotopes of feldspars and clays

摘要

The great Cambro-Ordovician sandstone succession of northern Gondwana is one of the most voluminous siliciclastic deposits on Earth. Its deposition took place following the Neoproterozoic Pan-African Orogeny and the consolidation of Gondwana, a crucial episode in Earth history. Its provenance is of great importance for reconstructing palaeogeographic and tectonic settings and determining sediment dispersal patterns. In Israel and its environs, Palaeozoic sandstones overlie the juvenile Neoproterozoic basement of the Arabian-Nubian Shield. These basement rocks were considered as an important sediment source to the Cambro-Ordovician succession. However, recent studies established that a dominant portion of detrital zircons in the succession was derived from Archean-Proterozoic remobilized terranes, suggesting transportation distances of thousands of kilometres beyond the Arabian-Nubian Shield, and thus question the provenance of other, less weathering-resistant constituents, which were not necessarily derived from the same localities. Feldspars and clays were separated from 13 samples and analyzed for mineralogical and geochemical compositions, strontium, neodymium and lead isotopes, in order to further constrain the provenance of the Cambro-Ordovician succession of northern Gondwana. Detrital feldspars have epsilon Nd values significantly different from feldspars of the Arabian-Nubian Shield, ranging from -8 to -14, with model ages of 950 to 1750Ma, more radiogenic lead compositions, and (U-238/Pb-204) values ranging from 98 to 1026. Clays are dominated by illite, smectite and kaolinite, with epsilon Nd values ranging between -14 and -9, with Sr-87/Sr-86 ratios up to 0757. These findings point to a significant contribution of remote ancient terranes from outside the Arabian-Nubian Shield to the Cambro-Ordovician sandstone of northern Gondwana. The possible sources for these sediments are the Saharan Metacraton and the southern regions of the East African Orogen.