Kaiserstuhl volcanic complex

Since more than 200 year the Kaiserstuhl volcanic complex is the subject of geological and mineralogical studies. 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, with a maximum elevation of about 270 m above the surrounding alluvial plain of the River Rhine, is located about 20 km NW of Freiburg. It has a roughly elliptical outline with axes of 16 and 12 km and covering an area of 92 km2. The Kaiserstuhl volcanic complex consist of a sequence of pyroclastic rocks and lava flows which encloses a basin with intrusive rocks, including the carbonatite at the Badberg massif (Badloch).What we see today are the last ruins of a strato-volcano, which was several hundreds of meter higher than today and surrounded by several smaller parasitic volcanoes, such as the Limberg (Sasbach) or the Humberg (Burkheim).

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Simplified geological map of the Kaiserstuhl volcanic complex



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. The olivine nephelinites are regarded as primitive mantle-derived magmas, whereas limburgites are interpreted as their differentiates.

The Kaiserstuhl is a Miocene alkaline volcanic complex belonging to the larger Tertiary eruptive province of Central Europe. It is a unique feature because it is the only large volcano within the Rhine Graben and because of the occurrence of carbonatite rocks. The prominent geophysical feature of the area, the rise of the Moho to only 24 km depth right underneath the Kaiserstuhl is interpreted as expression of a diapiric mantle dome centered beneath the volcanic complex.

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Geological map of the southern part of the Upper Rhine graben showing the principal geological units and the occurrences of alkaline volcanic rocks (after Keller et al., 2002).



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. The large variety of rock types has been grouped into two petrographic families: Tephritic-essexitic family:

• olivine nephelinite
• limburgite (basanite)
• leucite tephrite
• essexite, theralite
• mondhaldeite (obsolete local name for a camptonite-like dyke rock)

Phonolitic family

• phonolite
• tinguaite (a variety of phonolite)
• hauynophyre
• ledmorite
• sodalite syenite
• leucitophyre

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.

Bibliography



• Exkursion zur Vulkanologie und Tektonik des Oberrheingrabens. Gerhard Brügmann und Dieter Mertz
• Lixian-xun W; Michael A.W. Marsk; Thomas W.; Annette Von der Handt; Jorg. K.; Holger T.; Gregor M. (2004): Apatites from the Kaiserstuhl Volcanic Complex, Germany: new constraints on the relationship between carbonatite and associated silicate rocks. Eur. J. Mineral. 2014, 26, 397–414
• Ulianov. A; Muntener. O; Ulmer. P; Pettke.T (2007): Entrained Macrocryst Minerals as a Key to the Source Region of Olivine Nephelinites: Humberg, Kaiserstuhl, Germany. J. Of Petrology, vol.48, N.6. 1079-1118 (2007)