International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

High-temperature to ultrahigh-temperature metamorphism related to multiple ultrapotassic intrusions: evidence from garnet-sillimanite-cordierite kinzigite and garnet-orthopyroxene migmatites in the eastern part of the Moldanubian Zone (Bohemian Massif)

Pages: 415 - 425

Authors: JAROMIR LEICHMANN, MILAN NOVAK, DAVID BURIANEK ,DAVID BURGER

Abstract: The garnet-sillimanite-cordierite kinzigite from Petrovice, Jihlava Pluton, Moldanubian Zone, Czech Republic has a hyperaluminous chemical composition. Its mineral assemblage consisting of garnet and prismatic sillimanite I, relics of cordierite, hercynite, rutile I, ilmenite and quartz, retrograde minerals — cordierite II, fibrolitic sillimanite II and Ti-rich biotite and leucocratic portions with abundant K-feldspar, quartz and plagioclase which closely matches a restite. The prograde part of the reaction history was virtually obliterated and the mineral reactions: (1) Crd+Hrc=Grt+Sil; (2) Hrc+Qtz=Grt+Sil; (3) Grt+Sil+Qtz+L=Crd+Bt; (4) Grt+L=Crd+Bt+Qtz; (5) Grt+Crd+L=Bt+Sil; (6) Crd+L=Bt+Sil+Qtz took place during cooling. The relic assemblages cordierite+hercynite and hercynite+quartz, which represent a peak of metamorphism, are stable for P~0.5 GPa at T>~900 ºC. The P-T conditions determined by geothermobarometry yield: T=900±40 ºC (garnet–ilmenite) and P=0.73–0.49 GPa (spinel–garnet); concentrations of Zr in rutile I enclosed in garnet and sillimanite I yielded T=840–970 ºC. We assume, on the basis of the hyperaluminous chemistry and mineral assembly that the garnet-sillimanite-cordierite kinzigite from Petrovice is a restite, where a large portion of melt was lost from the rock. The sequence of mineral reactions, and garnet composition with low and constant Ca in the profile across the grain, indicate an isobaric cooling path. A moderate heat input from the Jihlava Pluton, manifested by garnet-orthopyroxene-cordierite migmatites developed at the direct contact of durbachites, supported by a local but large heat input of gabbro–monzogabbro associated directly with kinzigites seem probable heat sources. The chemical U-Pb ages of monazite 319±24 Ma from kinzigite, 329.8±9.5 Ma from garnet-orthopyroxene migmatites, and 335.8±6.9 Ma from gabbros are very close to U-Pb ages of zircon 335.2±0.5 Ma from durbachites (Kotkova et al. 2003). They suggest a Variscan age of the HT (to UHT) metamorphism and support affiliation to the durbachite intrusions.

Keywords: Moldanubian Zone, Jihlava Pluton, mineral reactions, HT metamorphism, heat source, garnet-sillimanite rock, restite

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