engleski

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

The metamorphic sole of Dinaridic ophiolites is frequently built up from compositionally different groups of amphibolites originated from ultramafic and mafic cumulates. In metamorphic sole of Jurassic Krivaja-Konjuh ultramafic massif (Dinaride Ophiolite Zone), several types of amphibolites commonly show granoblastic, porphyroblastic and nematoblastic textures of various grain size. Structure is mainly shown in foliation rarely in layering. Age of amphibolites is spanned between 174&plusmn ; ; 14 and 157&plusmn ; ; 4 Ma. The varieties of these amphibolites 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) ; (2) garnet-amphibolite and clinopyroxene-garnet amphibolite comprises garnet with composition 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). Some samples show kelyphite texture and some include clinopyroxene (diopside, Fs=0.108, En=0.409, Wo=0.484) ; (3) peculiar corundum-bearing edenite– pargasite amphibolite positioned in the most internal parts of the complex comprises amphibole (Si 6.34, Mg 3.42, Fe2+ 0.55 p.f.u.) enriched in edenite and pargasite components and anorthite-rich plagioclase (up to 91 % An). The assemblage also includes xenomorphic corundum in the matrix and large centimeter sized corundum porphyroblasts with amphibole and plagioclase inclusions. Thermobarometric calculations are performed using assemblages Grt+Hbl+Pl, Grt+Cpx and Hbl+Pl with reaction edenite + albite = richterite + anorthite in quartz-free rocks and also using Al and Si isopleths in Hbl. In the amphibolites (1) calculated T values for rims are 670-730 °C for the edenite-richterite reaction. The garnet-bearing amphibolites (2) yielded 745-800°C and 9.5-11.8 kbar for samples with kelyphite texture and 630°C and 6.7-7.3 kbar for kelyphite-free samples while clinopyroxene-garnet amphibolite yielded 830°C. Geothermobarometric estimations on the corundum-bearing amphibolites (3) yielded limiting P– T conditions of 620-830°C and 6-10 kbar for amphibole and plagioclase inclusions with presumed equilibrium inside corundum porphyroblast and 4.5-8 kbar for the main metamorphic assemblage. These estimates are thought to reflect the metamorphic conditions achieved during the Late Jurassic obduction of the hot upper mantle part of Krivaja– Konjuh ultramafic massif onto ophiolite mélange. The hot obducted ultramafic fragments acted as a heat source for metamorphism that transformed cumulate gabbro protolith into high-grade amphibolites.

geothermobarometry; amphibolite; Dinaride

nije evidentirano