Two-pyroxene syenitoids from the Moldanubian Zone of the Bohemian Massif: Peculiar magmas derived from a strongly enriched lithospheric mantle source

Abstract

Abstract In the Variscan Bohemian Massif, (ultra-)potassic plutons are conspicuously associated with the high-grade, lower crustal/upper mantle Gfohl Unit (Moldanubian Zone). They can be subdivided into two contrasting groups: (1) coarse amphibole–biotite melagranite to quartz syenite with conspicuous K-feldspar phenocrysts (the ‘durbachite suite’), and (2) essentially equigranular biotite–two-pyroxene quartz syenites to melagranites (the ‘syenitoid suite’). The latter suite, represented by the Tabor and Jihlava plutons, is characterized by an originally ‘drier’ mineral assemblage orthopyroxene\u202f+\u202fclinopyroxene\u202f+\u202fMg-biotite\u202f+\u202fplagioclase\u202f+\u202fK-feldspar\u202f+\u202fquartz, with accessory zircon, apatite, ilmenite, monazite and/or rutile\u202f+\u202fchromite. The rich assemblage of both rock-forming and accessory minerals allows testing of numerous geothermobarometers. The resulting P–T data are mutually consistent; they document emplacement of the parental magmas at mid-crustal levels (~5 kbar/19 km for Tabor and 7–8 kbar/26–30 km for Jihlava) and record near-isobaric crystallization from at least 1170\u202f°C to the solidus. New Isotope-Dilution Thermal Ionization Mass-Spectrometry (ID-TIMS) U–Pb ages for zircon (336.9\u202f+\u202f0.6 Ma) and rutile (336.8\u202f+\u202f0.8 Ma) from the Tabor Pluton, together with previously published ages from the Jihlava Pluton, provide temporal constraints for the emplacement and rapid cooling of the syenitoids below c. 600 °C. This supports the hypothesis of post-tectonic emplacement of a hot and dry melt (>1200 °C?) into an essentially consolidated orogenic crust. The two syenitoid plutons have comparable, crust-like radiogenic isotope signatures (87Sr/86Sr337\u202f=\u202f0.7114–0.7133, e337Nd\u202f=\u202f−6.8 to −8.0). This, in context of whole-rock geochemical variation and K-feldspar Pb isotopic compositions, is consistent with generation from a strongly enriched lithospheric mantle source. Shortly before melting, the local orogenic mantle was most likely modified by deep subduction and relamination of felsic crustal material of the Saxothuringian provenance, transformed to the felsic high-pressure granulites common in the Moldanubian Zone. In the subsequent evolution of the two-pyroxene syenitoid plutons, crystal fractionation and accumulation played a key role, unlike in the durbachite suite itself, where magma mixing with leucogranitic melts was much more important. Structural relations inside and around the (ultra-)potassic plutons (~343–335 Ma) suggest that – besides different depths and specific processes of magma emplacement – these plutons track different kinematic histories and evolutions of regional strain fields in their high-grade country rocks in this crucial period of the Variscan Orogeny.