Biotit a amfibol z ultradraselných hornin jihlavského plutonu – produkty sekundární hydratace pyroxenu


Syenites from the Jihlava pluton contain a broad spectrum of ferromagnesian minerals. Clinopyroxene (Cpx), Orthopyroxene (Opx), Biotite (Bt) Cummingtonite (Cum) and Actinolite (Act). Clino- and orthopyroxenes exhibit structures eg. zoning, admixtures typical for magmatic phases. Cummigtonite is partly product of hypersthene hydration, whereas actinolite originated on the expense of augite. Biotite grows at the contact between hypersthene and K-feldspar via reaction 3 Opx + Kfs = Bt + 3 Qtz. Because of hypersthene contains common admixtures of Cpx and K-feldspar contains myrmekites of andesine composition, therefore small amount of actinolitic hornblende originated during this reaction in addition. Much larger amount of actinolite, together with plagioclase was produced during the reaction between augite and K-feldspar – apart from prevailing biotite and quartz. Observed reactions indicate that the magma was dry and intrudes probably in the lower crust conditions, and was partly hydrated and reequilibrated during subsequent rapid exhumation.

Klíčová slova:
Jihlava pluton; syenites; ferromagnesian minerals; hydration

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