CHEMICAL COMPOSITION AND TH-U-PB CHEMICAL AGES OF MONAZITE IN APPALACHIAN GRANITES, NEW BRUNSWICK, CANADA

There are numerous Ordovician to Devonian intermediate to highly-evolved felsic intrusions in the Appalachian Orogen of New Brunswick that are directly associated with various types and styles of mineralization. In order to generally interpret the relationships between mineralizing episodes and the associated intrusions, it is important to ascertain their age relationships and assess the degree of fractionation of the particular intrusions. Monazite is an important accessory mineral in many of these intrusions, as it enables the determination of an emplacement age, as well as an assessment of the relative and absolute degree of fractionation of the rock. Monazite in the samples studied exhibits a wide compositional range (ThO₂=1.5-17.0 wt.%; UO₂=0.1-1.0 wt.%; Y₂O₃=0.5-4.0 wt.%). Th contents are negatively correlated with total LREE concentrations. Th-rich monazite is relatively Si-rich, yet Ca-poor, indicative of huttonite substitution. Individual monazite grains may also be compositionally heterogeneous. Chondrite-normalized LREE distribution patterns typically exhibit a steady decrease from La to Sm, but some monazite is relatively depleted in La and enriched in Nd and Sm, resulting in concave-downward REE patterns. As monazite is a major host for the LREE, Th, and U, its composition is an important facet of the REE and actinide systematics of the rock, and is reflective of the partial melting and fractionation history of any particular intrusion. Th-U-Pb chemical monazite ages were obtained with the electron microprobe, and are comparable to conventional ages determined by isotopic methods. Each intrusion is typically characterised by a single monazite age population, interpreted as its age of emplacement. Older, inherited ages are rare, but indicate that monazite is not always completely soluble in granitic melts.