JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. B4, PAGES 4667-4696, APRIL 10, 1989 ARCHEAN LAMPROPHYRE DIKES OF THE SUPERIOR PROVINCE, CANADA: DISTRIBUTION, PETROLOGY, AND GEOCHEMICAL CHARACTERISTICS D. A. Wyman and R. Kerrich Department of Geological Sciences, University of Saskatchewan, Saskatoon Canada Abstractß Shoshonitic lamprophyre dikes of late Archean age are a volumetrically minor, yet widespread component of magnetism in the Snperior 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 sediment,•. Mineralogically, the dikes are characterized by zoned phlogopite or hornblende phenocrysts, restriction of feldspar to the groundmass, globular segregations of K-feldspar and calcite, panidiomorpbic textore, and the sporadic presence of crustal xenoliths. Compositionally, fresh dikes have contents of SiO (47-51 wt %), TiO? (0.55-0 60•, and P-O ß ' 2 5 (0.36-1.58), and K90/Na?O ratios (1.522.3) and Mg numbers (78-80), c•mpar•ble to Phanerozoic .•hoshonitic lamproph•yres. Unaltered dikes are characterized by extreme enrichments of the lithophile elements K, Rb, ha, 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 bays. systematically lower K, Rb, ha, and Sr contents, but similar REE distr[b,•tions and profiles from Bruce, 1936; Smith, 1986; for a summary see McNeil and Kerrich, 1986]. Extensive geological mapping has documented the fact that lamprophyres and associated alkaline rocks are a volumet•lcally minor but widespread component of magmatism [Hopkins, 1924; Hurst, 1936; Burwash, 1937; Moore, 1937; Thompson and Griffis, 1944; Abraham, 1951; Satterley, 1952; Pye, 1956; Prest, 1957; Lawton, 1959], which systematically displays a preferred spatial association with faults of •egional extent, and was active during the terminal stages of the tectonic evolution of greenstone belts [Cooke and Moorehouse, 1969; Jensen, 197ga, b, 1980; Wyman and Kerrich, lq86, 19R7a, 1988]. Archean shoshonitic lamprophyres have also been reported from the Yellowknife greenstone belt [Webb, 1986; Webb and Kerrich, 1988] and the 2.7 Ga Norseman-Wiluna belt of westeen Australia [Rock et al., 1987; Rock and Groves, 1988], where they share both the spatial relationship to major structures and late tectonic emplacement of their counter. parts in the Superior Province. There are, however, few modern petrologic and geochemical studies of these Archean alkaline rocks. This paper describes the geological setting and distribution of lamprophyre dikes •rom greenstone belts in the Archean Superior Structural Province in northern Ontario, Canada (Figure 1) and documents the petrographic characteristics and geochemistry of the dikes. The study is part of a larger project on the characteristics and Th-Yb and Sc-Nl on normalized diagramsß Suites of significance of Archean lamprophyres and other post-Archean ultramaflc lamprophyres in the Superior Province are enriched in large ion lithophile elements (LILE), LREE, and compatible elements, relative to their Arcbean co,•nterparts and lack tronghs 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 d,•ring the successive accretion of individual sobprovinces. Arcbean lamprophyre magmas were derived from a depleted mantle source, variably enriched in LILE and LREE alkaline rocks in the Superior Province, which to date has focussed on the Abitibt greenstone belt (Figure 1) [Wymanand Kerrich, 1986, 1987a, b, 1988; Kerrich and Watson, 1984; McNeil and Kerrich, 1986]. Although Streckeisen [1979] uses the term "calc-alkaltne" for this type of lamprophyre, we adopt the nomenclature "shoshonitlc" lamprophyre in recognition of the association of Archean mtnettes with shoshonitic magmatism and following the usage of Rock (cf. "Association B" of Rock [1984]). margin setting. by sediments and altered oceanic crust, with small variable degrees of crustal contamination, whereas Methodology the post-Archean lamprophyres lacked a plate The lamprophyres described in this study occur Background Introduction and Scope Shoshonitic lamprophyre dikes of Archean age are present in all of the greens tone belts within the Superior Structural Province [Dyer, 1936a, b; Copyright 1989 by the American Geophysical Union. Paper number 88JB03733. O148-0227/89/88JB-373350 5. O0 over a wide area, approximately 650km x 500km, and were emplaced into the Uchi, Wabigoon, and Wawa subprovinces, and at or near the Uchi-Engltsh River, Wabigoon-Quettco and Wawa-Quetico subprovince boundaries (Figure l). Samples of 93 dikes were collected from 15 localities in the Superior Province. Of the total population, 33 representative samples were selected for detailed petrographic and geochemical analyses. At most localities, several samples were obtained, in order to determine the degree of compositional uniformity, within and between different dike swarms. Studies of Archean lamprophyres and associated alkaline rocks from the Abitibt greenstone belt 4667