Tholeiitic- and boninite-series metabasites of the Nové Město Unit and northern part of the Zábřeh Unit (Orlica–Śnieżnik Dome, Bohemian Massif): petrogenesis and tectonic significance

Abstract

The Nové Město Unit and the northern part of the Zábřeh Unit comprise back-arc basin mafic rocks metamorphosed during Variscan times. In both units, nearly identical metabasites comprise variously enriched tholeiites (from N-MORB to transitional E-MORB-OIB), high-Ca, low-silica boninites and low-Ti tholeiites. The tholeiites (Ti/V: 22-58, εNd 540 : + 7.6 to − 4.7) represent 10-15% melting at ca. 30-60 km and temperatures of 1380-1230 °C of a depleted MORB mantle-type (DMM) wedge heterogeneously modified shortly before fusion by OIB-like melts (enriched mantle possibly of EM1-EM2 type) presumably derived from decompression melting of upwelling asthenosphere. Much less common meta-boninites (Ti/V: 6-23, εNd 540 : + 6.7 to − 2.9) formed by 15-25% re-melting of residual mantle (DMM after ~ 15% melt extraction) at depths of 40-65 km and temperatures of ~ 1420-1300 °C. Scarce low-Ti meta-tholeiites (Ti/V: 18-19, εNd 540 : + 7.1) resulted from < 10% melting of an unenriched DMM-type source or re-melting of residual mantle (after < 15% of former melting). Trace elements and Nd isotope compositions imply random fluxing of tholeiitic-and boninitic magma sources by components released from a subducted slab. The metasomatic enrichment (Th, LREE-MREE) was induced not only by sediment-derived melts but also by fluids supplied by subducted sediments or juvenile crust. The Nové Město-Zábřeh association of metabasites points to an easterly prolongation (in present-day coordinates) of the Cadomian subduction system of the Teplá-Barrandian. Boninite-type magmatism and OIB-like input into mantle beneath an extensional back-arc basin suggest an upflow of hot asthenosphere through subducted ridge (slab window) followed by a cessation of subduction zone activity.