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Alpine ophiolite-related amphibolites from the metamorphic sole of the Krivaja-Konjuh ultramafic massif (Dinaride Ophiolite Zone, Bosnia)

The large Krivaja-Konjuh ultramafic massif (KKUM) within Dinaride Ophiolite Zone (DOZ), which is composed of spinel lherzolite, represents sheet thrust onto the ophiolite mélange and is underlained by a metamorphic sole composed mainly of amphibolites. Emplacement of the KKUM was related to processes of Late Jurassic/Early Cretaceous intra-oceanic NE-dipping subduction accompanied by the generation of metamorphic sole largely composed of amphibolites originating from cumulate ultramafic-gabbro protolith. The prograding metamorphic zonation of sole amphibolites is preserved. Age of amphibolites is spanned between 174&plusmn ; ; 14 and 157&plusmn ; ; 4 Ma. The common varieties of these amphibolites and calculated P-T conditions are: (1) Amphibolite with amphibole (Si 6.48-7.09, Mg 3.04-3.54, Fe2+ 0.29-1.31 p.f.u.) and plagioclase (21-47 % An), calculated T values are 670-730 °C ; (2) Garnet-amphibolite and clinopyroxene-garnet amphibolite comprises garnet (XAlm=0.384, XSps=0.020, XPrp=0.370, XGrs=0.224), amphibole (Si 5.90-6.36, Mg 2.75-3.28, Fe2+ 0.23-0.26 p.f.u.) and plagioclase (55-91 %An). Many of samples show kelyphite texture and some include clinopyroxene (diopside, Fs=0.108, En=0.409, Wo=0.484). Garnet-bearing amphibolites yielded 745-800°C and 9.5-11.8 kbar (kelyphite texture samples) and 630°C and 6.7-7.3 kbar (kelyphite-free samples), clinopyroxene-garnet amphibolite yielded 830°C (3) Corundum-bearing pargasite amphibolite comprises amphibole (Si 6.34, Mg 3.42, Fe2+ 0.55 p.f.u.) and anorthite-rich plagioclase (up to 91 % An), limiting P- T conditions are 620-830°C and 6-10 kbar calculated on amphibole and plagioclase inclusions inside corundum porphyroblast and 4.5-8 kbar for the matrix metamorphic assemblage ; (4) Lowermost-grade amphibole schist comprises actinolite (Si 7.65, Mg 3.12, Fe2+ 1.65 p.f.u.) and plagioclase (up to 40 %An) yielded T up to 550°C. Exhumation of the higher grade amphibolite masses started after emplacement under changed geothermal gradient giving rise to gradual metamorphic zonation of the metamorphic sole amphibolites. Breaking and dismembering of obducting oceanic crust with its metamorphic sole started at near surface parts of the obducting system. Processes of the final emplacement of the KKUM amphibolites must have been terminated by Late Jurassic/Early Cretaceous as indicated by amphibolite fragments found in adjacent Cretaceous Pogari Formation overstepping the Jurassic ophiolite mélange.

Alpine; ophiolite; amphibolite; Dinaride

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