ACCESSORY TITANITE: AN IMPORTANT CARRIER OF ZIRCONIUM IN LAMPROPHYRES

Abstract

The chemical composition of minerals from a suite of Variscan lamprophyres (kersantite, minette, spessartite and vogesite) from the Saxothuringian Zone of the Variscan orogen, Germany, were investigated by electron microprobe. These lamprophyres usually contain small grains (Incorporation of Zr into titanite apparently takes place as a coupled substitution (Ti4++Al3+⇔Zr4++Fe3+) rather than as a simple 1:1 replacement of Ti by Zr. A ZrO2 content of about 6 wt.% (equivalent to ∼0.10 atoms per formula unit Zr) is implied as the upper concentration limit in titanite. Locally occurring higher values are attributed to tiny inclusions of secondary Zr phases. In carbonate-rich portions of lamprophyres, titanite is transformed into an unindentified TiO2 phase. High-(Al,F) titanite (XAl up to 0.54), restricted in occurrence to the contact between tonalite porphyry and kersantite, formed under the conditions of high fluorine activity connected with the rhyolitic magmatism.The results show that the Zr budget of lamprophyres, and possibly of other mafic igneous rocks lacking zircon, is strongly governed by titanite. Element distribution patterns in titanite are a key to the better understanding of magmatic evolution and petrogenesis of lamprophyres.