Ordovician Albitites Generated by Magmatic Fluids within a Volcano-Sedimentary Sequence from NW Sardinia

Two albitite layers were discovered near Lula village within a volcano-sedimentary sequence of likely Ordovician age in the Internal Nappe Zone (NE Sardinia). The albitites consist of up to 95% albite, with small amounts of quartz, biotite, chlorite, Fe-oxides, apatite, and monazite. They are the first albitites to be found in NE Sardinia, and the first known case of Ordovician albitites. In central and southern Sardinia the albitites occur as late Variscan dykes and small intrusions. The albitization process is attributed to hydrothermal fluids produced during the late stages of middle Ordovician volcano-plutonic activity. A comparison of the albitite compositions with the non-albitized samples reveals that albitites are characterized by strong gains of Na, slight gains of Al, Si, and Sr, and remarkable losses of K, Rb, Ba, Ca, Mg, Fe, Ti, Ni, and V. These albitites are also enriched in LREE and depleted in HREE.