Archean lamprophyre dikes of the Superior Province, Canada: Distribution, petrology, and geochemical characteristics

Abstract

Shoshonitic lamprophyre dikes of late Archean age are a volumetrically minor, yet widespread component of magmatism in the Superior Province, and preferentially occur along major structures that delineate subprovince boundaries. Lamprophyre swarms are most prominently developed in fault graben, where they are associated with alkali gabbro and trachytes, and molasse sediments. Mineralogically, the dikes are characterized by zoned phlogopite or hornblende phenocrysts, restriction of feldspar to the groundmass, globular segregations of K‐feldspar and calcite, panidiomorphic texture, and the sporadic presence of crustal xenoliths. Compositionally, fresh dikes have contents of SiO2 (47–51 wt %), TiO2 (0.55–0.60), and P2O5 (0.36–1.58), and K2O/Na2O ratios (1.5–2.3) and Mg numbers (78–80), comparable to Phanerozoic shoshonitic lamprophyres. Unaltered dikes are characterized by extreme enrichments of the lithophile elements K, Rb, Ba, Cs, Th, U, and light rare earth elements (LREE), and enhanced abundances of Cr (650–920 ppm), Co (37–42), Ni (264–355) and Sc (22–28). Mid‐ocean ridge basalt normalized plots display (1) consistent enrichments of K, Rb, and Ba relative to Sr and LREE, (2) variable enrichments of Rb and Ba relative to K, and (3) troughs at Ta‐Nb and Ti. Compared with fresh rocks, altered samples have systematically lower K, Rb, Ba, and Sr contents, but similar REE distributions and profiles from Th‐Yb and Sc‐Ni on normalized diagrams. Suites of post‐Archean ultramafic lamprophyres in the Superior Province are enriched in large ion lithophile elements (LILE), LREE, and compatible elements, relative to their Archean counterparts and lack troughs at Ta‐Nb and Ti. Emplacement of the lamprophyres was diachronous from north (2705 Ma) to south (2674 Ma), probably being related to localized extension during the successive accretion of individual subprovinces. Archean lamprophyre magmas were derived from a depleted mantle source, variably enriched in LILE and LREE by sediments and altered oceanic crust, with small variable degrees of crustal contamination, whereas the post‐Archean lamprophyres lacked a plate margin setting.