REE-SIO2 Systematics in Mor Gabbros and Associated Plagiogranites from the Fournier Oceanic Fragment, New Brunswick, Canada

Seawater influx into hot, dry MOR gabbro can initiate hydration-induced melting and the generation of intermediate to felsic partial melts collectively referred to as plagiogranite. In a recent modeling study, Brophy (2009) suggested that the REE abundances of partial melts generated in this fashion should be characterized by unique patterns of REE-SiO2 variation. Specifically, REE abundances (modeled as enrichment factors, Cl/Co) should show a positive correlation with increasing SiO2 up to around 60 wt. % followed by a steady decrease in abundance as liquid SiO2 increases to around 76%. For liquids of around 55% SiO2 the degree of enrichment is around 2 for all of the REE. However, Cl/Co in the intermediate liquids of around 60 % SiO2 steadily decreases from ~5 for La to ~3 for Yb. Simarlarly, Cl/Co in the high SiO2 liquids of around 76% SiO2 decrease from ~3 for La to ~1 for Yb. If these model predictions are correct, the REE-SiO2 systematics of any naturally occurring suite of plagiogranite and MOR gabbro could be used to assess a partial melting as opposed to crystal fractionation origin. To test the model predictions, a suite of MOR gabbros and intrusive veins of plagiogranite were collected from the Fournier Oceanic Fragment, a middle Ordovician ophiolite sequence located along the northern shore of New Brunswick, and the type exposure for plagiogranites generated by hydration-induced MOR gabbro melting (Flagler and Spray, 1991). The MOR gabbros range from 48 to 55 % SiO2 while the intrusive plagiogranites range from 57 to 78 % SiO2 (anhydrous basis). When REE abundances are plotted against whole rock SiO2 they show all of the model features described above, though the absolute abundances require an initial gabbroic source rock that is more enriched in the REE than the host gabbros themselves. This correspondence between modeled and observed REE- SiO2 variations confirms the model predictions of Brophy (2009) and suggests that REE- SiO2 systematics represent an important tool for helping to decipher the origin of natural plagioganites in MOR settings.