Multiple magmatic pulses of the Eastern Volcano-Plutonic Complex, Krušné hory/Erzgebirge batholith, and their phosphorus contents


Authors: Štemprok M, Holub FV, Novák JK

Published in: Bulletin of Geosciences, volume 78, issue 3; pages: 277 - 296; Received 8 April 2002; Accepted in revised form 28 March 2003;

Keywords: granite, ignimbrite, mafic magmas, volcano-plutonic complex, Variscan igneous activity, magma pulses, whole rock geochemistry, phosphorus, petrogenesis, Krušné hory/Erzgebirge, Bohemian Massif,

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Granites of the eastern part of the Krušné hory/Erzgebirge (Czech Republic/Germany) outcrop in separate minor stocks formed by Late Palaeozoic CaO-rich biotite monzogranites of the Older Intrusive Complex (OIC) and syenogranites or alkali feldspar granites of the Younger Intrusive Complex (YIC). These, along with abundant rhyolites and granite porphyry dykes, constitute the Eastern Volcano-Plutonic Complex (EVPC). Using new analyses as well as older unpublished and literature data, we evidenced that these rocks display a wide spectrum of phosphorus contents ranging from very low (well below 0.1 wt% P2O5) to intermediate contents (from 0.1 to 0.4 wt% P2O5). No equivalent of the high-phosphorus fractionated granites with P2O5 > 0.4 wt% known from the western Krušné hory/Erzgebirge is present in the EVPC.
We have demonstrated that the OIC granite bodies comprise a number of petrographic varieties and some of them represent individual magmatic pulses. Relatively elevated phosphorus contents in the evolved members of the Fláje granite massif may reflect their origin as individual magma batches from heterogeneous crustal sources. We observed the marginal Preisselberg granite enriched in P among the predominant P-poor YIC granites; this granite probably originated also as a separate magma batch.
On the contrary, the rhyolitic ignimbrites, granite porphyries and YIC granites were derived from a younger, huge and long-standing magmatic system which presumably was formed and maintained as a consequence of significant thermal input from mantle-derived mafic magmas. These mafic magmas homogenized granitic partial melts from lower crustal sources and produced large volumes of highly evolved magma portions via fractional crystallization. The granite magmas with I-type or transitional I/S-type characteristics evolved towards the low-P2O5 compositions. This process was disturbed by episodic hybridization of the granite magmas with the mantle-derived melts or products of their fractionation at depth.
The YIC granites probably represent residual, late-stage melts rich in volatiles, with strongly increased incompatible element abundances due to prolonged crystal fractionation. Their composition was strongly affected by interaction with an aqueous fluid phase that changed the phosphorus distribution in only a minor way. Late magmatic and post-magmatic processes obscured the primary chemical characteristics namely in the Li-rich granites. This fact could be the reason for the absence of any correlation between the generally low P2O5 and highly variable alumina-saturation indices.
The phosphorus contents thus reflect various aspects of rather complex genetic histories of Late Variscan igneous rocks in the EVPC, particularly within the lower and middle crust. In contrast with the role of phosphorus, the contents of lithium and fluorine are more dependent on subsolidus alterations of granites induced by hydrothermal fluids in late magmatic and post-magmatic stages at shallow crustal levels.