The blueschist-associated perovskite-andradite-bearing serpentinized harzburgite from Dobšiná (the Meliata Unit), Slovakia

Abstract

*Corresponding author Unique, perovskite(Prv)–andradite(Adr)-bearing serpentinized harzburgite fragments occur in Late Jurassic–Early Cretaceous subduction–accretionary wedge complex of the Meliata tectonic Unit (Slovakia, Western Carpathians). These rocks reflect magmatic (E1) and partial melting (E2) mantle events followed by metamorphic blueschist-facies (E3) and fluid–rock interaction (E4) events accompanied by crystallization of Prv and/or Adr. The incompletely solidified slightly depleted (residual) harzburgite composed of early-magmatic assemblage of Ol, Opx 1 , Cr-Spl ± Cpx 1 (E1 event) underwent partial melting (E2 event) under subsolidus mantle conditions. The late-magmatic aggregates of Cpx 2 , Cr-Spl, scarce Opx 2 , due to this batch melting, occur around the Opx 1 (±Cpx 1 ) porphyroclasts. The fractures and cleavage planes of the kinked Opx 1 porphyroclasts are filled by Cpx 3 as a result of subsolidus recrystallization of the Opx 1 . Cloudy lower Al Cpx 4 associated with pargasite in reaction rims between the Opx 1 and Cpx 2 , or antigorite replacing Ol, could represent the E3 high-pressure metamorphic event (dated at 160 to 150 Ma, according to published 40 Ar– 39 Ar data for “phengitic” white mica from blueschists) in serpentinites. Serpentinized zones of harzburgite contain Prv, which occurs in serpentinized Opx 1 porphyroclasts. Orthopyroxene (Opx 1 ) breakdown and formation of Prv, (Ti-rich) Adr and chrysotile appears to be postdating the closure of the Meliata back-arc basin (c. 200 to 160 Ma) and HP metamorphism. Perovskite genesis can therefore be bound to the exhumation-related serpentinization in subduction channel and emplacement of serpentinites into subduction–accretionary wedge (E4 event at 300–500 oC/5–15 kbar, at 150 to 130 Ma?). The bulk-rock trace-element patterns suggest enrichment in LREE, Ba, Cs, U, Pb, Sb and Li in comparison with HREE, HFSE, Rb and Sr. Such element mobility indicates fluid–rock interaction. Pervading aqueous fluids rich in CO 2 were transporting Ca, Ti, Fe, LREE partitioned into Prv grown along the Cpx exsolution lamellae in Opx 1 . The sources of such fluids might have been the blueschist- to greenschist-facies rocks from the Meliata melange type subduction–accretionary wedge. The late-metamorphic H 2 O–CO 2 /Si–Fe–Mn-rich fluids caused partial replacement of Prv by pyrophanite and crystallization of (Ti-rich) Adr at the expense of Cpx and Prv.