engleski

Significance of compositional zoning in pyroxenes from spinel lherzolites and spinel websterites of Banovina ophiolite suite, Croatia

Spinel lherzolites and websterites of Banovina ophiolite suite (central Croatia) represent the NW end of the Central Dinaridic Ophiolite Belt. This belt is part of the Tethyan ophiolites that exhibit an age progression from Jurassic, in the western Alpine-Apennines to Cretaceous in the Tibetan- Himalayan belts to the east. To better understand the genesis of the ultramafic rocks of the tectonically dismembered Banovina ophiolite suite, we performed detailed petrographic and geochemical analyses of 4 spinel lherzolites and 1 spinel websterite. All samples have granular to porphyroclastic textures. The spinel lherzolites consist of olivine (Mg#core= 86.0- 90.8 ; Mg#rim=87.0-91.0), clinopyroxene (Mg#core= 87.6-91.9 ; Mg#rim= 88.6-92.5), orthopyroxene (Mg#core= 85.8-90.84 ; Mg#rim= 85.8-91.2) and Mg-Al chromite to pleonaste. The websterite consists of clinopyroxene (Mg#core= 87.6 ; Mg#rim= 88.6-89.3), orthopyroxene (Mg#core= 86.6-87.9 ; Mg#rim= 85.9-88.0) and large pleonaste grains (up to 3.2 mm). Pyroxenes in both rock types occur either as large porphyroclasts (up to 10.4 mm) or as smaller grains in variably sized cataclastic to partially recrystallized matrix (~ 0.1-0.005 mm). The porphyroclasts exhibit different types of deformation textures, including kink banding and undulose extinction. Most contain extensive exsolution lamellae of complementary pyroxene. In orthopyroxene, Al and Ca contents decrease at the grain rims, whereas in clinopyroxene a rimward decrease in Al, but an increase in Ca is observed. Smaller, matrix pyroxenes are characterized by compositions similar to those of the rims of the larger grains. Textures, exsolution lamellae and documented chemical disequilibrium of pyroxenes reveal a sustained cooling of the mantle rocks during uplift. Geothermobarometric calculations [1, 2] suggest that the rocks were derived from < ~ 60 km depth, with pyroxene rim compositions recording temperatures of 800-900ºC. These preliminary results are in accordance with data for spinel lherzolites from the neighboring Krivaja-Konjuh ophiolite suite in Bosnia, interpreted as a mantle column that experienced adiabatic ascent in a mid-ocean ridge environment [3]. [1] Webb & Wood (1986) Contrib Mineral Petrol 92, 471-480 [2] Brey & Köhler (1990) J Petrol 31, 1353-1378 [3] Šegvić (2010) Doctoral thesis, Uni. Heidelberg, 313 p

pyroxene ; spinel lherzolites ; websterites ; Banovina ; ophiolite

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