GEOCHEMISTRY AND ORIGIN OF THE PROTEROZOIC ULTRAPOTASSIC ROCKS OF THECHURCHILL PROVINCE, CANADA

Early Proterozoic ultrapotassic dikes, lava flows, and pyroclastic roc ks of the Christopher Island Formation (CIF) erupted throughout an are a 600 x 300 km within the Churchill Province of the Canadian Shield at 1.84 Ga. The rocks range from mafic lamprophyres (mg # greater than o r equal to 60; SiO2 approximate to 47-54%, mean K2O/Na2O > 4) with phe nocrysts of phlogopite + diopside + apatite +/- olivine +/- magnetite, to phenocryst-poor felsic rocks and sanidine porphyries (SiO2 approxi mate to 55-69%). All samples have high incompatible element contents a nd display large depletions of high field strength elements relative t o K, Rb, Sr, Ba, and Th. The CIF has geochemical and petrographic char acteristics of both minettes and lamproites, but overall most closely resembles young Mediterranean lamproites. Felsic rocks of the CIF were produced by crystal fractionation and crustal contamination of mafic ultrapotassic magma, and include both high-silica lamproites strongly enriched in Zr, U, and Th, and weakly potassic to sodic rocks of trach ytic composition. Flows and feeder dikes have relatively homogeneous e psilon(Nd, 1840 Ma) (-6 to -11) but highly variable epsilon (Sr, 1840 Ma) (-40 to +100); samples classified as lamproites have higher averag e E(Sr). Dike samples have highly variable present-day Pb isotope comp ositions, ranging from moderately to strongly nonradiogenic. Geochemic al and isotopic data are consistent with contributions from depleted A rchean lithospheric mantle, and GIB-type convecting mantle, both metas omatized by subduction-related processes during the Early Proterozoic. The lithospheric mantle probably contained Archean enriched domains a s well. Proterozoic enrichment may have accompanied shallow underplati ng of subducted oceanic lithosphere beneath the Churchill Province dur ing amalgamation of the Laurentian supercontinent. There are strong an alogies in isotopic composition, and interpreted source region history , between the CIF and lamproites and minettes of the Wyoming Province and western Greenland, which suggest the existence of a Laurentian ult rapotassic ''superprovince''