CHEMICAL CONTROLS ON THE PRESENCE OF MONAZITE AND/OR ALLANITE IN GRANITOIDS AND METAMORPHIC ASSEMBLAGES

Allanite (A₂M₃Si₃O₁₂OH) and monazite (CeThPO₄) are radiogenic, rare-earth bearing minerals commonly used for geochronology. Their presence in igneous rocks is thought to be controlled by magma chemistry, as peraluminous granites with lower CaO contain monazite and not allanite, whereas those with higher CaO contain allanite only. In pelitic schists, monazite and allanite are thought to coexist in rocks only with low (<0.5 wt%) CaO. To explore these commonly held assumptions, granitoids and garnet-bearing pelites and gneisses from the Menderes Massif in western Turkey were chemically analyzed. All granitoids analyzed are S-type, subalkalic, and peraluminous. Granitoids with allanite only contain 3.8-4.1 wt% CaO, whereas those with allanite + monazite contain 2.1-2.6 wt% CaO, and those with monazite only contain 0.7-1.8 wt% CaO. Granitoids with higher Al₂O₃ (>15.4 wt%) contain allanite and/or monazite, whereas those with lower Al₂O₃ (<14.8 wt%) contain monazite only. No correlations exist between the presence of monazite or allanite and P₂O₅, TiO₂, K₂O, Na₂O, MnO, Fe₂O₃, MgO, or SiO₂. Rocks with higher BaO (>522 ppm), Sr (>231 ppm), and Zr (>116 ppm) contain allanite and/or monazite, whereas those containing less Ba (<510 ppm), Sr (<180 ppm), and Zr (<108 ppm) contain monazite only. No correlations exist between REE, Th, or U and the presence of these accessory minerals.

The observation of higher CaO contents stabilizing allanite at the expense of monazite is restricted to igneous rocks, as garnet-bearing metamorphic rocks with higher CaO contain monazite. Allanite and monazite coexists in pelitic assemblages with CaO ranging from 1.13-1.68 wt%, whereas those with monazite only contain 0.29-7.04 wt%. No correlation exists between the presence of these accessory minerals and any trace elements (e.g., REE, Th, U, Ba, Sr, Zr, etc) analyzed. The presence of allanite or monazite is related to the metamorphic history of the rock, instead of its whole rock chemistry.