Articles

 

BOLETÍN DE LA SOCIEDAD GEOLÓGICA MEXICANA

Vol 64, Núm. 1, 2012, p. 21-35

http://dx.doi.org/10.18268/BSGM2012v64n1a2

Mineralogía magnética de suelos volcánicos en una toposecuencia del valle de Teotihuacán< Evidence for isovolumetric replacement in some Terra Rossa profiles of northern Jordan

Bernhard Lucke1,*, Helga Kemnitz2, Rupert Bäumler1

1 Institute of Geography, Friedrich-Alexander University Erlangen-Nürnberg, Kochstr. 4/4, 91054 Erlangen, Germany
2 Helmholtz-Zentrum Potsdam, German GeoResearchCentre (GFZ), Section 3.1, Telegrafenberg, 14473 Potsdam, Germany

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Abstract

We investigated the rock-soil transition zones of three different Terra Rossa profiles near the ancient site of Abila, in northern Jordan, in order to demonstrate the possibility of a metasomatic origin for these soils using a scanning electron microscope (SEM) equipped with an energy-dispersive system (EDS). The field interpretation suggested that the first Terra Rossa might partially have formed by isovolumetric replacement, the second appeared to represent mainly the residue of the underlying limestone, and the third seemed to be derived from colluvial material. However, all investigated rock-soil transition zones showed microfossils that had partially been replaced by clay, which could be geochemically confirmed using EDS. The replacement matrix was free of detrital clay and revealed a jelly-like, amorphous structure. Large phyllosilicates, some rare feldspars, and heavy minerals that occurred throughout the investigated rock-soil transition zones are probably remains of sandy interlayers in the weathered limestone. We conclude that isovolumetric repla­cement contributes to Terra Rossa formation in northern Jordan although it is not yet possible to quantify the extent of its contribution or the sources that supply the elements triggering the pressure-driven growth of clay minerals.

Keywords: isovolumetric replacement, Terra Rossa, metasomatism, micromorphology, rock-soil transition.