Fayalite and ferrohortonolite in post-island-arc plagiogranites of the Mt. Kastel in Mountain Crimean cimmerides

Spiridonov E. M.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 2. P. 57-68

https://doi.org/10.31857/S0869605521020118

Full text is available on eLIBRARY.RU

Language: Russian

Abstract

Post-island-arc plagiogranites (SiO₂ 72.02 wt %, Na₂О 4.57 wt %, K₂O 1.26 wt %, Na₂О/K₂O 3.6, Fe/(Fe + Mg) 92.4 %) of the dome-shaped hypabyssal intrusive of the Mt. Kastel contain high ferriferous olivine, rhombic pyroxene (eulite) and biotite (annite). Ferrohortonolite (Fo 14.5–10.4, MnO 2.32–3.02 wt %) in assotiation with low-potassiс labrador–andesine is observed in sharply porphyric endocontact plagiogranites. At a distance away from the contact, plagiogranites contain ferrohortonolite (Fo 10.6) and predominant fayalite (Fo 9.6–7.7, MnO 2.66–2.99 wt %); the mineral contains up to 0.1 wt % of CaO, notably, Ni is not found. Plagioclase which composition varies from low potassic Labrador and predominant andesine – to moderately potassic oligoclase associates with them. At a distance further from the contact, fayalite (Fo 9.3–5.9, MnO 2.90–3.20 wt %) is predominate as a component of plagiogranites; the mineral contains up to 0.1 wt % of CaO, whereby Co prevails over Ni. Low potassic andesine – potassic oligoclase associates with this fayalite. The evolutionary trend of zircon composition is correlated with the same trends of plagioclase and olivine. Specifically, zircon contains 1.17 wt % of HfO₂ with ZrO₂/HfO₂ 56.6 (n = 5) within endocontact rocks, 1.28 wt % of HfO₂ with ZrO₂/HfO₂ 52.5 (n = 6) at a distance away from the contact, and 1.56 wt % of HfO₂ with ZrO₂/HfO₂ 43.2 (n = 5) at a distance further from the contact. Consequently, there was an accumulation of Fe and Mn in olivine composition, Na and K in plagioclase composition and Hf in zircon composition during the crystallization differentiation of plagiogranites of the Mt. Kastel. Apparently, the emergence of high ferriferous olivine was facilitated by the presence of graphite in the contact zone of the intrusive; the preservation of fayalite and ferrohortonolite is contingent on hypabyssal intrusive rocks conditions.

Keywords: post-island-arc Kastel plagiogranite complex, plagiogranite with fayalite and ferrohortonolite, Mountain Crimea