Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Temporal relations of melagranite porphyry dykes and durbachitic plutons in South Bohemia

 

František Holub, Kryštof Verner, Mark Schmitz

Geoscience Research Reports 45, 2012 (GRR for 2011), pages 23–25

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Abstract

Numerous dykes of K-feldspar-phyric melagranite porphyries of Variscan age were emplaced post-tectonically into relatively older granitoids of the Central Bohemian Plutonic Complex (CBPC) and adjacent high-grade rocks of the western Moldanubian Zone. Geochemical composition of these rocks well corresponds to durbachitic amphibole-biotite melagranite of the Čertovo břemeno type. The latter rock is a member of the dominantly more mafic ultrapotassic durbachite suite occurring as plutons and small intrusions in a NNE-SSW trending belt that borders the area of melagranite porphyry occurrences at E. Crystallization age of porphyry dykes was assessed by the U-Pb TIMS dating of single zircon grains separated from the typical melagranite porphyry from the active quarry close to village Nihošovice near Volyně. The new age of c. 338 Ma (337.92 ± 0.17 Ma, MSWD = 1.5) is slightly younger compared with durbachitic plutons dated at 339-341 Ma. Emplacement of melagranite porphyries thus represents the final stage of durbachitic igneous activity in the S Bohemian Massif but preceded another intrusive pulse of ultrapotassic magmas giving origin to petrographically distinct biotite-two-pyroxene syenitoids of the Tábor and Jihlava plutons.