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Dimitrina Dimitrova, Geological Institute, Bulgarian Academy of Sciences (Bulgaria)
Vassilka Mladenova, Sofia University "St. Kliment Ohridski" (Bulgaria)
Lutz Hecht, Natural History Museum (Germany)
Thomas Kerestedjian, Geological Institute, Bulgarian Academy of Sciences (Bulgaria)
Philip Machev, Sofia University "St. Kliment Ohridski" (Bulgaria)
Tsvetoslav Iliev, Geological Institute, Bulgarian Academy of Sciences (Bulgaria)
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The Balkan-Carpathian ophiolite fragments, which outcrop in Bulgaria, Serbia and Romania, are of Late Precambrian age (563 ± 5 Ma, Quadt et al, 1998). They are not strongly deformed and metamorphosed like the metasedimentary basement sequences. They are unconformably overlain by Cambrian-Early Ordovician volcano-sedimentary island-arc and olistostrome sequences (Haydoutov, 1991, Savov et al, 2001). These low-grade metamorphosed rock sequences are related to the evolution of the Prototethys ocean ? its spreading, subduction (island- and back-arc formation) and obduction onto the Protomoesian plate. Pilatovets gabbro (493.0 ± 6.6 Ma, Carrigan et al, 2003) belongs to the island-arc sequence (so-called Berkovitsa group in Bulgaria or Vlasinski complex in Serbia) and is located to the south of the town of Chiprovtsi. It represents a 750 m thick and about 6 km long medium-grained magmatic body. The main observed textures are ophitic and poikilitic. The primary mineral assemblage is represented by clinopyroxene (augite, pigeonite), olivine, amphibole (edenite, magnesiohornblende), plagioclase (An38.9- An48.4), titanite, titanomagnetite. The alteration products resulting of the low-grade metamorphism in greenschist facies are actinolite, chlorite (chamosite), albite (?).
The preliminary field observation revealed rich Fe-Ti oxide mineralization forming limited in size layers, lenses or nests in the gabbro intrusion. The main accessory mineral is titanomagnetite, which is exsolved to magnetite, ilmenite, hematite and rutile, forming typical fine to ultrafine lamellar exsolution textures. Ultrafine hematite lamellae (up to 1m) are observed in rutile grains. Fe-Ti oxides embrace altered olivine and pyroxene or randomly fill the interstitial spaces. The co-existence of rutile and hematite suggests rather advanced oxidation of the primary titanomagnetite. Some of the analyses of magnetite revealed up to 16 wt. % TiO2. MnO in the ilmenite rarely reaches 5.15 wt.%, usually is ranging from 0.20 to 0.32 wt.%. Small amounts of Cr2O3 in ilmenite are also recorded. The Ilmenite-magnetite geothermobarometer of Spencer and Lindsley (1981) will probably reveal a wide range of temperatures due to the different rate of exsolution of the primary titanomagnetite.
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