Kaiserstuhl volcanic complex

The Kaiserstuhl volcanic complex has a long history of geological investigation. The name of the complex is not derived from the armchair-like shape of the complex. Rather it commemorates the place of an open-air tribunal of the Karolinger Emperor Otto III. in 994.

The Kaiserstuhl volcanic complex, is situated within the Upper Rhine Graben (Southern Germany, Fig.1) and is one of numerous manifestations of Tertiary to Quaternary volcanism within western and central Europe. The complex has a roughly elliptical outline with axes of 16 and 12 km and covering an area of 92 km². The main phase of volcanic activity in the Kaiserstuhl occurred in Miocene times between 18 and 13 Myr ago. Major rock types emplaced during this period are olivine nephelinite and basanite (including type-locality limburgite), tephrite and essexite, phonolite, sovite and alvikite carbonatite. Volumetrically insignificant carbonate-rich melilite dike rocks (bergalites) are also present.

Fig.1. a) geological map of the Kaiserstuhl volcanic complex after Wimmenauer (1966). b) Schematic profile of the Upper Rhine-graben. Modified from Walter, B. F et., al (2016).

The various rock types comprising the Kaiserstuhl volcanic complex are supposed to be derived from two different parental magmas (Keller 1984, 2001; Schleicher et al. 1991). A primary olivine-nephelinite magma, exposed at the limberg near Sasbach, resembles those found at several places in the Upper Rhine Graben. This olivine-nephelinite magma generates slightly fractionated basanites (limburgite), melilite-bearing rocks (bergalite), and carbonatites.

The second primary magma is a hypothetical, initially fractionated K-basanite, which shows signs of crustal contamination. It is not exposed in the Kaiserstuhl volcanic complex. Fractionation of this second source material led to two petrographically distinctive rock clans. The tephritic-essexitic clan (Wimmenauer 1957, 1959a; Kim 1985) comprises leucite and olivine tephrites, and phonolitic tephrites, which form the major part of the exposed Kaiserstuhl volcanic complex. Continued fractionation caused the formation of rocks belonging to the phonolitic clan (Wimmenauer 1962), e.g., phonolites, syenites, and evolved leucocratic dikes. The alternating deposition of tephritic and phonolitic tuff beds indicates simultaneous activity of these magma systems (Wimmenauer 1962).

Two thirds of the igneous rocks outcropping today are lava flows and pyroclastic rocks. The remaining rock types are subvolcanic breccias and intrusions of silicate and carbonatite melts. Most of the lava flows are less than 10 m thick and can only be traced on the scale of the outcrop. However, some flows, such as the Limburgite flow, can be followed for 1.5 km and can be up to 50 m thick. This lava flow shows locally pahoehoe structures, others represent Aa- or block lavas. Tephrites and their subvolcanic equivalents form by far the largest volume of igneous rocks. Carbonatites are generally grouped with the phonolitic family, because of similarities in their trace element patterns. There is a general age sequence with more evolved intrusive rock types at the final stages. Carbonatites appear late in the evolution.

Evolution of the Kaiserstuhl volcanic complex

The magmatic activity of the Kaiserstuhl volcano is summarized as follows (Keller, 1984):

1. The volcanological evolution starts about 17-18 Ma with the formation of a large, complex tephritic cone. Its lava flows and pyroclastic rocks overly tertiary sediments in the eastern part of the Kaiserstuhl. Tephrite volcanism lasted all along the time of igneous activity of the complex.

2. Large subvolcanic intrusions of tephritic magma into the volcanic edifice formed the coarser crystalline rocks, especially essexites and ledmorites (sodalite-syenite). Subsequently, polygene diatreme breccias formed and established pathways for the carbonatitic rocks (Sigmund 1996).

3. Carbonatite intrusions occur towards the end of the evolution in the center of the complex. They are accompanied by the formation of ultramafic breccias which fill pipe structures at the subvolcanic level. Small alvikitic dikes are the final magmatic intrusions in the center of the complex.

4. The youngest volcanic activity in the volcanic complex took place in the parasitic Limberg volcano with its olivine nephelinites, limburgites and final tephrites.

The processes that generate carbonatite melts are still under debate. Based on isotopic studies, Schleicher et al. (1990) found evidence for a genetic linkage between the carbonatites and the Na-series magma of the Kaiserstuhl (phonolitic clan). Also, the rare bergalite dykes (which dominantly consist of melilite, haüyne, nepheline, calcite, perovskite, magnetite, biotite and apatite) are considered to represent a genetic link to carbonatites (Keller 1990). They are the most fractionated rocks of the Kaiserstuhl according to their very low Mg-values and the highest P, Nb, Sr, Ba, Y and REE contents (Na₂O/K₂O >1). Chemical of apatite from bergalites support the interpretation of a transitional rock that could be formed by fractional crystallization of a melilite nephelinite magma or mixing of two independent magmas, a silicate and a carbonatitic one (Wang et al. 2014).

Carbonatites

Carbonatites appear in the final stage of the magmatic evolution; they occur in three types of geological setting and petrographic appearance:

1. Subvolcanic intrusion in the center of the Kaiserstuhl complex. They are middle to coarse grained sövites. Their outcrop area is about 1 km².
2. Alvikitic dyke, generally less than 1 m, often only 1-2 cm thick. Some of these dikes have porphyritic textures.
3. Extrusive carbonatites with droplet-shaped lapilli (Pele's tears).

Calcite, magnetite and apatite are the essential mineral in the carbonatites. Carbonates are almost exclusively calcitic, only a few late-stage veins with ankeritic dolomite have been described. Small amounts of the following accessories may be present: forsterite, diopsidic augite, melanite, Nb-perovskite, pyrochlore, baryte, pyrite and rarely zirconolite, baddeleyite, zircon. Several micas are known: phlogopite, and Ba-phlogopite.

kaiserstuhl2020(1).png

Schelingen carbonatites outcrop. From La lithothèque

Schelingen sövite. From La lithothèque

Volcanic calcite-carbonatite (alvikite). Badberg. From Sand Atlas

Calciocarbonatite lapillistone (extrusive calciocarbonatite). From James St. John

Sample of Limburgite. From Geologische Streifzüge

Sample of Limburgite with vesicles filled by philipsite. From Mineral forum.

Volcanic breccias. From Geologische Streifzüge

Tephrite with vesicles filled by zeolites. From Geologische Streifzüge

Bibliography

• Ulianov, A., Müntener, O., Ulmer, P., & Pettke, T. (2007). Entrained macrocryst minerals as a key to the source region of olivine nephelinites: Humberg, Kaiserstuhl, Germany. Journal of Petrology, 48(6), 1079-1118.
• Walter, B. F., Marks, M. A. W., & Markl, G. (2016). The Kaiserstuhl natural laboratory-an introduction and first results. Abstract Malawi Expert Council.
• Weisenberger, T. B., Spürgin, S., & Lahaye, Y. (2014). Hydrothermal alteration and zeolitization of the Fohberg phonolite, Kaiserstuhl Volcanic Complex, Germany. International Journal of Earth Sciences, 103(8), 2273-2300.
• Weisenberger, T., & Spuergin, S. (2009). Zeolites in alkaline rocks of the Kaiserstuhl Volcanic Complex, SW Germany–new microprobe investigation and the relationship of zeolite mineralogy to the host rock. Geologica Belgica