Unshaped basalt blocks from archaeological sites along the border of the Roman Empire (limes) in the lower Rhine area near Vleuten-De Meern (Utrecht) have been studied petrographically, analysed by XRF for major and trace elements, and dated by the 40Ar/39Ar method. The blocks are from a revetment in the bank of a fossil branch of the Rhine and a contiguous ship De Meern 4, both built around 100 AD. All nineteen blocks are alkali olivine basalt (AOB) with xenoliths of peridotite derived from the upper mantle and quartz xenocrysts from the continental crust; eighteen blocks contain resorbed plagioclase xenocrysts as well. Abundances of major and trace elements show that those eighteen samples form a chemically coherent group. The outlier, different in chemistry and without plagioclase xenocrysts, is from the ship. A basalt block from ship De Meern 1 (148 AD) conforms compositionally to the defined group. AOB lumps from a limes watchtower (2nd-3rd century) form a chemically distinct group.
Low SiO2 contents (<46 wt.%) and high abundances of Mg, Ti, Ni, and Sr indicate a within-plate origin, directly from primitive melts; proportions of selected trace elements point at a continental rift setting. In the archaeological context, the most likely source region for the blocks is the Cenozoic European Volcanic Province, upstream along the Rhine and its tributaries.
The petrographic and analytical data of the blocks have been compared with 432 published analyses of German AOB. On petrographic grounds, the Eifel can be ruled out as a source area since typical Eifel basalt minerals, amphibole, biotite, K-feldspar and feldspatoids, are absent in the blocks. Applying seven geochemical criteria, based on abundances of major elements in the Roman blocks, twelve sites with matching AOB were found in the Siebengebirge, seven in the Vogelsberg, and one in the Westerwald.
The ages of the blocks (26.3 - 28.5 Ma) are compatible with ages determined for AOB from the Siebengebirge (27.4 - 29.9 Ma), and preclude their provenance from the Vogelsberg (< 18 Ma). The matching Westerwald sample is from 60 km beyond the limes, a prohibitive distance from the perspective of Roman logistics.
AOB quarries of optimal logistic position are located adjacent to the Rhine, between Bonn and Remagen, a zone with significant Roman settlements from the first century AD. Geochemical correlation indicates AOB bodies at Rolandsbogen and Godesburg (S of Bonn) as potential sources of the blocks from the 100 AD revetment and ships. Similarly, the Erpeler Ley (E of Remagen) is indicated as the likely source for the blocks from a 2nd-3rd century AD watchtower.
As the Godesburg basalt is at 1.6 km from the Rhine today, it is not obvious how the blocks were transported from there. However, it may be that the adjacent, now sanded, branch of the old Rhine river system, was navigable for flat-bottomed vessels in Roman times.
Our study demonstrates that substantial detailed information regarding ancient mining and trading activities can be retrieved from seemingly indistinctive basalt blocks.