Mineral Assemblages of Amphibolites Associated with Alpine-Type Ultramafics in the Dinaride Ophiolite Zone (Yugoslavia)

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Abstract
Amphibolites are frequently and characteristically associated with alpine-type ultramafics within ophiolite zones of different geological ages. Ultramafics of the Dinaridic ophiolite zone are predominantly lherzolite, and are associated with different varieties of gabbro, dolerite, diabase, spilite, and amphibolite. Amphibolites commonly form narrow, interrupted zones around larger ultramafic massifs, and, in some areas, they exceed the ultramafics in size. All these rocks are members of the Jurassic(?) volcanic-sedimentary complex (‘Diabas-Hornstein Formation’) consisting predominantly of graywacke type sandstone and shale. The amphibolites characteristically have crystalloblastic textures and are commonly banded and foliated, which distinguishes them from associated gabbros and dolerites. Different kinds and varieties of metamorphic rocks can be found within the Dinaridic amphibolite complexes: amphibolite schists containing plagioclase, pyroxenite schists containing diopside and plagioclase, monomineralic amphibole schists, and eclogites. Samples of each of these rock types were collected from several localities and monomineralic fractions of amphiboles, plagioclases, garnets, and clinopyroxenes were separated and examined in detail. Amphiboles range from kaersutite, pargasite, and edenite through transitional varieties enriched in tschermakite and ‘common’ hornblende molecules. Plagioclase varies from anorthite to sodic oligoclase. Associated garnets are enriched either in pyrope or in almandine. Clinopyroxene is hedenbergitic diopside with about 10–20 per cent of jadeite molecule and hypersthene contains about 40 per cent FeSiO3. Variations in mineral composition of the amphibolites demonstrates their metamorphic origin. Although there is a continuous change in chemical composition, three mineral assemblages can be distinguished. Data from experimental petrology indicate that the amphibolites associated with alpine-type ultramafics can be stable under upper mantle PT conditions.

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