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The Kamenjača olivine gabbro from Moslavačka Gora (South Tisia, Croatia)

The Kamenjača olivine gabbro, in detail described by Kišpatić (1887), has been (re)found as blocks of as yet not defined primary geological position in the southern part of Moslavačka Gora crystalline complex (MGC). Based on petrology and the microfabrics these rocks exhibit three distinct mineral assemblages. The magmatic assemblage is well preserved and consists of olivine, plagioclase, clinopyroxene and minor orthopyroxene. Magmatic minerals yielded a Sm-Nd isochrone age of 109 &plusmn ; 8 Ma. The high initial epsilon value (ε Nd=+5.50), the low initial Sr isotopic ratio (0.70359), together with the high Mg number of the rock (Mg' = 0.76) and the mafic minerals (Mg' ol = 0.75 ; Mg' opx = 0.77 ; Mg' cpx = 0.85) point to upper mantle related melt. After formation the olivine gabbro underwent subsolidus and metamorphic changes, which caused the development of reaction margins along the grain boundaries, especially on contact between plagioclase and olivine. Margin mineral paragenesis comprises orthopyroxene II, amphibole I &plusmn ; phlogopite and &plusmn ; opaque phases. The second event is indicated by formation of the symplectite paragenesis extending from plagioclase margins inwards. It consists of plagioclase II, amphibole II and &plusmn ; orthopyroxene III. Thermometric calculations on clinopyroxene-orthopyroxene pairs and barometry based on clinopyroxene crystal-structure modeling yielded a magmatic temperature of 920 &ordm ; C and a pressure of 8.7&plusmn ; 1.8 kbar. According to hornblende and hornblende-plagioclase pairs thermobarometric calculations the first reaction paragenesis was formed at 610-670 &ordm ; C and 4-6 kbar and the second one at 590-670 &ordm ; C and 2-5.5 kbar.

gabbro; Moslavačka Gora; Tisia; Croatia

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