REE-, Zr-, and Th-rich titanite and associated accessory minerals from a kersantite in the Frankenwald, Germany

Summary The mineral chemistry of a Variscan lamprophyre (kersantite) from the Frankenwald, Germany, has been investigated by electron microprobe. This potassic, Si-saturated, mafic rock contains an assemblage of different generations of titanite and allanite-(Ce), Th-rich zircon, and metamict REE–Ti–Zr–Th silicates. The primary ferroan-ceroan titanite contains unusually high contents of REE₂O₃ (max. (ΣLa to Sm)+Y = 36.8 oxide wt.%), ZrO₂ (max. 5.4 wt.%), and ThO₂ (max. 3.1 wt.%). Its empirical formula averages to (Ca_0.31 La_0.17 Ce_0.30 Pr_0.03 Nd_0.08 Sm_0.01 Y_0.01 Fe²⁺_0.06 Th_0.02 Mn_0.01)_Σ1.00 (Ti_0.60 Fe²⁺_0.22 Al_0.06 Zr_0.07 Mg_0.04 Nb_0.01)_Σ1.00 O_1.00(Si_0.93 Al_0.07)_Σ1.00 O₄. Element correlations reveal operation of the complex substitution Ca²⁺+Ti⁴⁺+Th⁴⁺ ⇔ REE³⁺+Al³⁺+Zr⁴⁺. In comparison to allanite-(Ce), ferroan-ceroan titanite preferentially incorporated the LREE and Th. This finding is inconsistent with previous experimental studies and suggests that both minerals are not cogenetic. High Zr contents in titanite, usually known only from Si-undersaturated alkaline rocks, and the predominance of Fe²⁺ suggest that the ferroan-ceroan titanite crystallized from an alkali-rich, low-fO₂ residual melt.