GEOCHEMICAL EVIDENCE OF MAGMA RECHARGE AND MIXING IN LATE-STAGE DIKES ASSOCIATED WITH THE NAIN ANORTHOSITES, LABRADOR, CANADA

A suite of 1.28 Ga. alkaline basaltic dikes, previously named the LP dikes, crosscut the 1.36-1.295 Ga. anorthosites of the Nain Plutonic Suite.  These dikes have compositions that form trends of decreasing SiO₂, CaO, Al₂O₃, and vanadium with decreasing MgO.  Whereas, TiO₂, FeO (as total iron), Na₂O, and P₂O₅ all increase with decreasing MgO.  A closed system fractional crystallization model using proportions of 11:40:45:4 for olivine, clinopyroxene, plagioclase, and spinel, respectively, produces good fits to the dike data for all of the major elements.  However, the model does not reproduce the enrichment displayed by incompatible minor and trace elements (e.g. P₂O₅).  The P₂O₅ in the dike data array increases from 0.18 to 1.54 wt.% as MgO decreases from 8.0 to 4.25 wt.%.  Whereas in the model, the P₂O₅ only increases from 0.18 to 0.41 wt.% over the same MgO range.  An analysis of the P₂O₅ contents of the Nain host rocks, including the anorthosites and the Archean granitic gneisses, show that crustal contamination is not a suitable mechanism of increasing the P₂O₅ contents of the dikes, because the host rocks have an average P₂O₅ content of 0.14 (±0.11) wt.% and MgO content of 0.59 (±0.36) wt.%.  Another hypothesis to explain the significant enrichment of incompatible minor and trace elements is magma mixing associated with recharge events.  A model to test this hypothesis uses seven recharge and mixing events where the starting and mixed compositions are allowed to fractionate to a final liquid proportion of 0.28.  The residual liquid is then mixed with the starting liquid in varying proportions ranging from 15-25% and 85-75%, respectively.  The resulting trends formed by the newly mixed liquids provide good fits to the enrichment trends displayed by the incompatible minor and trace elements, while still giving good fits for all the major elements.  These results suggest that recharge and magma mixing were important processes in the differentiation history of the LP dikes.