Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Roman Rönick, Axel D. Renno

Clinopyroxene from basaltic rocks of the Erzgebirge - Krušné hory Mts. - implications for modelling the magmatic plumbing system

Journal of Geosciences, volume 55 (2010), issue 3, 231 - 249

DOI: http://dx.doi.org/10.3190/jgeosci.077



The Erzgebirge - Krušné hory area is part of the NW flank of the Eger - Ohře Rift. Within this rift-related area, basaltic rocks with low (< 3.5 wt. %) and high contents (> 3.5 wt. %) of TiO2 occur. The distribution of these basaltic rock types shows strong spatial dependency. Ti-rich basalts are concentrated in the western Erzgebirge - Krušné hory Mts. with a sharp change at longitude 13.3 °E. Clinopyroxene and olivine are the most abundant phenocrysts. The zoning and chemistry of the clinopyroxene act as a recorder of the different stages of the magmatic plumbing systems. Seven different types of clinopyroxene phenocrysts are defined with four distinct chemical types: Cr-rich clinopyroxene, green-core pyroxene, Mg-rich clinopyroxene and finely zoned Ti-rich clinopyroxene with sector zoning.
Chemical and petrographic analysis of individual zones within clinopyroxenes allow the qualitative description of distinct steps of the multistage, regionally diverse processes yielding to the formation of both Ti-rich basalts in the W as well as Ti-poor basalts in the E. Two different types of asthenospheric melts are suggested, with increased activities of H2O and potassium in the western Erzgebirge Mts. During ascent they incorporated xenoliths and xenocrysts both of the mantle (peridotite, olivine and Cr-rich clinopyroxene) and the crustal (granitic and gneissic rocks, quartz and feldspar) origin. Furthermore, green cores of clinopyroxene were introduced into the magmas. Fractionation processes took place within lithospheric magma chambers, having been accompanied by reaction of xenocrysts and xenoliths with the melt. The residence time in the magma chambers associated with the eastern and western part of the Erzgebirge appears to have been totally different: short in the eastern and long in the western parts. Within the latter, no mantle-derived xenoliths, xenocrysts, or magmatic olivines are found. It is inferred that these xenoliths and crystals settled to the bottom of the magma chambers forming cumulates and therefore did not come to the surface. In contrast, the basaltic rocks of the East still contain mantle-derived xenoliths and xenocrysts as well as magmatic olivine. The size and amount of clinopyroxene phenocrysts in the West significantly exceed that of the East. Moreover, the crystallization of giant clinopyroxenes was restricted to the West. Mixing of the magmas with Ti-rich melts, presumably derived from eclogitic source, took place in crustal magma chambers. In any case, the proportion of Ti-rich melts in the West significantly exceeded that of the East.

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Policy: Open Access

ISSN: 1802-6222

E-ISSN: 1803-1943