TRACE ELEMENT AND ISOTOPIC CONSTRAINTS ON CRUSTAL CONTAMINATION AND THE ORIGIN OF COMPOSITE DIKES WITHIN THE KANGÂMIUT DIKE SWARM, WEST GREENLAND

Composite dikes with basaltic margins and andesitic centers constitute approximately 5% of the 2.04 Ga Kangâmiut dike swarm. Comparison of primitive mantle normalized incompatible trace element compositions shows that the andesitic centers are enriched in large ion lithophile elements (LILE) and light rare-earth elements (LREE) and are depleted in heavy rare-earth elements (HREE) and Sr compared to the margins of composite dikes and the majority of single-injection Kangâmiut dikes. Many of these characteristics are similar to those of the dikes' host rocks, suggesting the andesitic centers may represent more contaminated magma consanguineous with basaltic margins. This interpretation, however, is not supported by Sr and Nd isotopes. Isotope data do show that the Kangâmiut dike swarm, as a whole, is contaminated with up to 8% Archean crust, while there is little variation in the isotope compositions within composite dikes. Fractional crystallization modeling, using low pressure cotectic proportions of olivine, clinopyroxene, plagioclase and Fe-Ti oxides, shows that this assemblage can account for the enrichment of LILE and LREE and the negative anomalies of HFSE and Sr, but cannot explain the depletions of HREE. To explain the HREE behavior of the andesitic centers requires fractionation of a phase such as apatite, zircon or garnet. Fractional crystallization modeling including apatite and zircon leads to extreme depletions of P and Zr, inconsistent with the dike centers. Alternatively, garnet fractionation would lead to the observed HREE depletions without extreme P and Zr depletions, but garnet fractionation at crustal conditions is not considered likely given previous major elements and phase equilibria work showing that the Kangâmiut dikes fractionated at pressures less than 1.0 GPa. An alternative explanation is that the garnet signature is related to the magma source, leading us to speculate that the late-stage magmas forming the centers of composite dikes of the Kangâmiut dike swarm originated by low-degrees of partial melting from the garnet stability field during the waning staging of rifting. Passage of these melts through refractory subcontinental mantle may have avoided dilution of the garnet signature.