Origin of titanite in metarodingite from the Zagros Thrust Zone, Iraq
-
Yousif Osman Mohammad
and
Hirokazu Maekawa
Abstract
Three types of metamorphic titanite have been noted in a metarodingite pod within the tectonized and serpentinized peridotite portion of the Penjwin ophiolite sequence within the Iraqi Zagros Thrust Zone (IZTZ), northeastern Iraq. Type I metamorphic titanite occurs as subhedral to anhedral finegrained disseminated crystals within chlorite that was formed during chloritization of biotite as a result of low-temperature (T = 330-340 °C) ocean-floor metamorphism or rodingitization of plagiogranite under a reducing environment. It is characterized by intermediate Al2O3 (Avg: 3.61 wt%), high FeOT (Avg: 0.89 wt%), and intermediate TiO2 (Avg: 34.7 wt%). Type II metamorphic titanite occurs as a thin rim around ilmenite and has high Al2O3 (Avg: 4.8 wt%), intermediate FeOT (Avg: 0.6 wt%), and low TiO2 (Avg: 33.7 wt%); it represents a reaction product between grossular and ilmenite, which resulted from an oxidizing high-pressure-high-temperature (P = 1.4-1.6 GPa and T > 750 °C) metamorphic event involving plagiogranite. Type III metamorphic titanite occurs as very coarse, highly fractured grains up to 0.5 mm, with inclusions of ilmenite, and surrounded by albitic plagioclase, analcime, and chlorite. This titanite is characterized by low Al2O3 (Avg: 1.23 wt%), low FeOT (Avg: 0.30 wt%), and high TiO2 (Avg: 36.98 wt%). It formed during extensive titanitization of ilmenite by a reaction with Ca-plagioclase during moderate pressure and temperature (P < 1.6 GPa and T < 750 °C) conditions, as a result of albitization of Ca-plagioclase in plagiogranite.
© 2015 by Walter de Gruyter Berlin/Boston
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