Origins of Layering and Mineralization in Archean Megacrystic Anorthosites in Manitoba

*Manitoba Energy and Mines, Geological Services, Winnipeg, MB, dpeck@em.gov.mb.ca;

+Department of Geological Sciences, University of Manitoba, Winnipeg, MB, nm_halden@umanitoba.ca;

^Department of Earth Sciences, University of Western Ontario - Laurentian University, Sudbury, ON, scott.jb@sympatico.ca;

The best documented and preserved of several Late Archean megacrystic anorthosite bodies in Manitoba is the Pipestone Lake anorthosite complex (PLAC). The PLAC comprises a core of rhythmically-layered units engulfed by megacrystic anorthosite. Cyclic units in the PLAC display a consistent lithostratigraphy including a lower oxide-rich ferrogabbro, a central rhythmically-layered leucogabbro and an upper megacrystic anorthosite. Field and geochemical observations indicate that relatively dense ferrogabbroic residual liquids, which developed following prolonged plagioclase crystallization, migrated towards the temporary floor of the PLAC. Concurrently, the largest and most buoyant plagioclase crystals (megacrysts) ascended to the tops of the cyclic units. Textural features suggest that most of the megacrystic feldspar in the PLAC is xenocrystic and was carried into the chamber by hyperfeldspathic basaltic parental magmas that originated in deeper-level chambers. The ferrogabbroic magmas ultimately became saturated in oxides and gave rise to stratiform, cumulus oxide layers at several stratigraphic levels in the PLAC. An upward, cryptic increase in incompatible element abundances in the PLAC could reflect changes in the composition of the magma entering the chamber and/or larger amounts of mixing between later influxes of parental magma and evolved, low density, quartzo-feldspathic liquids formed during the crystallization of the oxide-rich layers.

All of the megacrystic anorthosite bodies investigated are believed to represent juvenile crust formed within major ensialic rifts or a rifted continental margin. One of the anorthosite bodies has consistently evolved geochemical compositions best explained by a sialic crust-modified mantle source. There is a recurring spatial association between the Late Archean megacrystic anorthosite bodies and komatiite-tholeiite volcanic sequences. In one area, idiomorphic plagioclase megacrysts occur in basaltic komatiite flows but are strongly resorbed in associated megacrystic basalts. In the Bird River belt of southeastern Manitoba, megacrystic anorthosite occurs with coarse-grained leucogabbro in the upper half of the Bird River Sill. The upper, anorthositic part of the sill is believed to have crystallized from residual magmas produced during the formation of underlying olivine cumulates. Elsewhere in the Bird River area, plagioclase megacrysts are present in mafic dykes, pillow basalt sequences and a relatively large mafic intrusion (Mayville intrusion). We attribute these occurrences to different levels of emplacement of xenocrystic plagioclase in low density basaltic liquids that escaped from mafic-ultramafic intrusions such as the Bird River Sill.

In addition to stratiform Ti-V-Fe oxide deposits, some of the Archean megacrystic anorthosite bodies contain contact-style Cu-PGE mineralization that warrants further exploration.