Abstract
The hornfels first described by Ramdohr (1927) contains the rare, if not unique, mineral assemblage olivine-cordierite-spinel-garnet-gedrite-biotite(-tourmaline)-FeTi oxides. Mieroprobe analyses of the constituting phases show that their Fe/(Fe+Mg)-ratios increase in the order (tourmaline-) cordierite-biotite-gedrite-olivine-garnet-hercynite-rich spinel. On the basis of the experimental results of Hsu and Burnham (1969) in the system Fe-O-MgO-Al2O3-SiO2-H2O the metamorphic history of the hornfels is subdivided into an earlier high-temperature stage 1 characterized mainly by the assemblage olivine-cordierite-spinel, and a subsequent, lower-temperature stage 2, in which the additional phases garnet and gedrite grew as porphyroblasts and equilibrated with the other phases. At an estimated mean confining pressure of 500 bars for both hornfels stages the temperatures during stage 1 may have ranged from 600° to about 800° C, whereas during stage 2 only some 550–580° C were attained. It is possible that these two metamorphic stages are due to the consecutive intrusions, first of the gabbroic and then of the granitic magmas.
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Abraham, K., Schreyer, W. Petrology of a ferruginous hornfels from Riekensglück, Harz Mountains, Germany. Contr. Mineral. and Petrol. 40, 275–292 (1973). https://doi.org/10.1007/BF00371019
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DOI: https://doi.org/10.1007/BF00371019