Ultrapotassic magmatism in the heyday of the Variscan Orogeny: the story of the Trebic Pluton, the largest durbachitic body in the Bohemian Massif
Abstrakt
The Třebíč Pluton (TP) comprises plutonic rocks of the 'durbachite series', i.e., ultrapotassic biotite-amphibole quartz melasyenites-melagranites with conspicuous K-feldspar phenocrysts and abundant mafic enclaves. The TP with its satellite bodies intruded the eastern Moldanubian Zone (orogenic root of the Variscan Bohemian Massif) at c. 338 Ma.
Overall, the entire TP durbachite series (SiO2 = 52-67 wt%) remains highly magnesian, reduced, and (except when strongly contaminated) metaluminous. The durbachitic rocks are markedly enriched in lithophile elements (esp.
Cs, Rb, Th, U, K and Pb) and LREE; in contrast, the contents of high-field strength elements (Nb, Ta, and Ti) and Sr are much lower, resulting in conspicuous negative anomalies in Primitive mantle-normalized multielement plots. The proposed petrogenetic model requires re-enrichment of the depleted, harzburgitic lithospheric mantle by deeply subducted crustal material of the Saxothuringian lower plate, directly and/or via (U)HP melt/fluid.
This process produced phlogopite-rich metasomes (phlogopite harzburgites and/or glimmerites). The metasomes re-melted soon thereafter by a short thermal pulse caused by slab break-off or decompression melting of rising diapirs of the relatively light metasomatized mantle, yielding primary ultrapotassic melts (87Sr/86Sr337 > 0.712 and ε337Nd - 6.5).
K-feldspar-driven fractional crystallization +- accumulation represented only a late modifying influence. The occurrence of such ultrapotassic enriched/crustally contaminated mantle-derived magmatism complicates any conventional mass-balance assessments of relative roles of crustal recycling vs. growth in hot collisional orogens, e.g., Variscides or Himalayas.