APATITE VS. TITANITE: WHICH IS BETTER FOR U-PB DATING? A CASE STUDY FROM THE CIESZYN MAGMA PROVINCE, SOUTHERN POLAND, CENTRAL EUROPE

On the area of the Cieszyn magma province (CMP) in Poland, igneous rocks appear mostly as a bodies, emplaced in mechanically weakened zones in Createous sediments. The main products of the magmatic activity are sills, represented by: picrite, teschenite, syenite, lamprophyre and dolerite. Previous ⁴⁰Ar/³⁹Ar dating of amphiboles (ca. 120 Ma) and K-Ar dating of teschenitic rocks in the Polish Outher Wester Carphatians, performed on amphiboles, biotite and whole rocks (“amphibole” age ca. 112-90 Ma; “biotite” age ca. 138-133 Ma; whole rock age ca. 122 Ma) as well as apatite (128-120 Ma) confirmed their Cretaceous age. In this project we would like to focus on a new data on titanite U-Pb dates of teschenite and syenite from Boguszowice from the CMP.

The investigated rocks, both syenite and teschenite comes from a small closed quarry found in the Boguszowice Valley near Cieszyn. Apatite crystals from the investigated rocks from the Cieszyn igneous province of the Polish Western Carpathians can be classified as fluoroapatite. The apatite crystals are 0.1 to 0.8 mm long and 0.1-0.2 mm wide. Titanite crystals from the investigated syenite and teschenite are represented by crystals which are up to 1.5 mm long and ca. 0.3 mm wide. Three main population of titanite may be into three populations/generations: 1) the first type is characterizade by fine internal zoning, that is partialy disturbed by parts rich in Nb; 2) the second type has internal zones rich in Zr, and the last type (3) show Al-rich external zone which are common overgrown by hydrogarnet. The anchored concordia intercept apatite and titanites ages are 124.5±4.7 Ma, 124.2±4.9 Ma and 147.7±4.0 Ma, 144.3±2.5 Ma for syenite and teschenite, respectively.

The weighted mean LA-ICP-MS U-Pb titanite age for two samples is ca. 146 Ma. According to the present literature it is the first probe of titanite dating from the rocks from the CMP and is the highest obtained age up to now. The weighted mean U-Pb apatite age for two samples is ca. 125 Ma and is similar to previously published ages. The anomalous high titanite age is probably caused by secondary alternation of the studied phase. In studied case, titanite which has been subjected to (late/early?) hydrothermal activity would not remain a closed system in respect to either U or Th decay series and therefore cannot be used for U/Pb geochronology.