The Volcanic Rocks from the Rhön Mountains:
Geochemical and Petrological Investigations

G. Neumeier Mineralogisch-Petrographisches Institut der Universität München,

Theresienstr. 41/III, 80333 München, Germany

H. G. Huckenholz Mineralogisch-Petrographisches Institut der Universität München,

Theresienstr. 41/III, 80333 München, Germany

The Rhön Mountains in Central Germany contain abundant volcanic rocks, erupted in the late Oligocene and early Miocene. These rocks attracted the attention of geologists for several centuries. The geologic map from Bücking (1914) shows large areas covered with basalt, predominantly in the bavarian part of the Rhön mountains, the center of this study.

The volcanic field of the Rhön is part of the central european volcanic belt which was formed by collision of the European and African continent and extends from the Mediterranean in the west to Tschechia in the east.

The bavarian part of the Rhön mountains and neighbouring regions of Hessia and Thuringia were extensivly sampled and geochemical investigations on major and trace elements were carried out. Additional information was obtained by REE, Sr and Nd-isotopes and electron microprobe analysis on several samples. In contradiction to Bücking's opinion field studies revealed that the volcanic rocks are predominantly dykes and necks in Mesozoic sediments (limestones, sandstones) exposed by erosion. Larger areas covered with basalt are rare. The large amount of data was processed with multivariate statistical analysis techniques.

Distinct correlations of major and trace elements reveal enrichment and depletion effects typical for continental alkali basalts.

The main rock types are mafic (Mg/Mg+Fe2+ >0.73) and primary (Mg/Mg+Fe2+ 0.73-0.69) nephelin-normative olivine nephelinites (ON), nepheline basanites (NB), alkali olivine basalt (AOB), hypersthene-normative olivine basalt (OB) and rare quartz-normative basalts. Fractionated basalts are restricted to few AOB and OB. Differentiated volcanic rocks could not be found in the study area, these are obviously restricted to the eastern parts of the Rhön.

The REE-pattern show an enrichment of the light REE over the heavy REE and indicate an origin from a metasomatized spinel peridotite. 87Sr/86Sr- and 143Nd/144Nd-isotope data of the basalts confirm an origin from the upper mantle.

Different degrees of partial melting of primitive mantle produced the primary volcanics. The melt fraction calculated, increases from 5.9 (ON) to >10% (OB). The fractionated AOB und OB can easily be explained by the fractionation of small amounts of olivine (2-5%).

The alkali basalts contain abundant mantle xenoliths. According their geochemistry they represent a metasomatized upper mantle. Orthopyroxene/clinopyroxene thermometry records temperatures of equilibration about 995-1025šC for xenoliths in nepheline basanites and 940-970šC for xenoliths in alkali basalts.