Dating granulite-facies structures and the exhumation of lower crust in the Moldanubian Zone of the Bohemian Massif

Abstract

Deformation of granulite-facies rocks in the Moldanubian Zone of the southern Bohemian Massif is expressed in two intersecting planar fabrics -steeply disposed (S 1 ) and flat-laying (S 2 ) -which correspond to two deformation stages (D 1 ) and (D 2 ). The existing Sm-Nd garnet ages from banded granulite gneisses, new U-Pb zircon data from deformed granite intrusions within the granulite gneisses, and the P-T and field structural relations constrain the ages and P-T conditions of the two deformation phases. The early deformation (D 1 ) was associated with a HP-HT metamorphic stage with a minimum age of ca. 354 Ma which was followed by a nearisothermal decompression. A concordant U-Pb zircon age of 318±1 Ma dates the emplacement of intrusions of deformed granite into the granulite gneisses and constrains deformation phase (D 2 ). This phase was associated with an LP-HT metamorphism dated in the region at ca. 340-330 Ma. The available structural and isotopic data indicate that granulites in the southern Bohemian Massif were exhumed from lower to middle crust during compression. The structural relations and P-T-t data for the studied granulites are consistent with their exhumation by near-vertical extrusion of the softened orogenic root.

Hyperpotassic granulites from Blanský les (Moldanubian Zone, Bohemian Massif) revisited

“…The temperature estimates for the hyperpotassic granulites and peak of the high-grade metamorphism in South Bohemian granulite massifs are also grossly comparable (Vrána 1989;Owen and Dostal 1996;Svojtka et al 2002). Finally, the intrusion age for the hyperpotassic granulites is close to, if not slightly younger than, the best age estimates for this HP-HT metamorphic climax.…”

“…Arguably the best chronological constraint for the HP event is the mean of SHRIMP ages (339.8 ± 2.6 Ma) reported for multifaceted zircon grains from calc-alkaline South Bohemian granulites (Kröner et al 2000). On the other hand Svojtka et al (2002) placed, based on Sm-Nd ages for garnet cores with prograde zoning, the HP-HT metamorphic peak c. 10-15 Ma earlier. However the previous Sm-Nd and U-Pb Grt-WR dating of the Moldanubian granulites yielded a broad spectrum of (mostly spurious) ages scattered between the likely protolith intrusion and HP metamorphism (Janoušek et al 1996;Chen et al 1998).…”

Determination of SiO2 Raman spectrum indicating the transformation from coesite to quartz in Gföhl migmatitic gneisses in the Moldanubian Zone, Czech Republic

“…In contrast, thermodynamic modeling by Štípská and Powell (2005) has yielded a metamorphic peak temperature of 750 -850 °C at 1.6 -1.8 GPa that is distinctly lower than that mentioned in previous studies. Prince et al (2000) and Svojtka et al (2002) reported the Sm -Nd garnet ages of 354 ± 6 Ma as the high -P and high -T phases of granulite evolution. Sm -Nd dating of the prograde zoned garnets obtained from felsic granulite also constrains their minimum crystallization age to 354 Ma (Faryad and Košler, 2007).…”

Age determination of thorianite in phlogopite-bearing spinel-garnet peridotite in the Gföhl Unit, Moldanubian Zone of the Bohemian Massif

“…The studied spinel -garnet peridotite occurs as decimeter -size lenticular body in the Blanský les granulite massif (Fig. 1, Svojtka et al, 2002), the largest granulite massif in the Bohemian Massif, which was interpreted as the disrupted fragment of the Gföhl Unit that forms the metamorphic core of the Moldanubian Zone (Carswell and O'Brien, 1993;Kröner et al, 2000). Previous studies have revealed that most of these ultramafic lenses in the Gföhl granulite have whole rock chemistries similar to those of mantle -derived peridotites (Becker, 1997;Medaris et al, 2005), but the precise timing of the incorporation of such mantle lenses into the metamorphic rocks derived from crustal materials has been a matter of substantial controversy.…”

“…Geochronology of the garnet -bearing ultramafic lenses and the host Gföhl granulite also give constraints on the timing of their juxtapositions. The numerous U -Pb zircon and monazite ages of the Gföhl granulite clustering around 340 Ma are interpreted by most authors as reflect-ing the granulite facies metamorphic climax (Kröner et al, 2000), although some authors considered 340 Ma reflecting the retrograde amphibolite -facies metamorphism (e.g., Carswell and O'Brien, 1993;Svojtka et al, 2002). Isotopic ratios of Sm and of Nd in both garnets and whole rocks have been determined for both ultramafic lenses and the host Gföhl granulite to determine their Sm -Nd isochron ages [325 -344 (± ~ 10) Ma] (Becker, 1997).…”