EARLY GABBROIC ROCKS OF THE PROTEROZOIC MIDCONTINENT RIFT: A "LAYERED" INTRUSION RESULTING FROM POLYBARIC MAGMA EVOLUTION AND UNDERPLATING

The Midcontinent Rift (MCR) of North America was active during the Late Proterozoic. Large volumes of a wide variety of materials, both intrusive and extrusive, were emplaced during two periods of activity (i.e., 1108-1105 Ma, and 1100-1094 Ma; James D. Miller, pers. comm.).

Gabbroic intrusives representing the earliest materials (~1108 Ma) were studied by H.D. Nathan for his dissertation (1969). Based on mineralogy and field expression, 27 separate units were identified. Most were emplaced as sheets with occasional apparent repetition, yielding the appearance of a layered mafic intrusion. Extensive felsic rocks are present in the MCR, in significant abundance relative to the mafic material (~30% of total rocks). Whatever their origin, it has been thought that most of the felsic rocks were emplaced during the most active rifting, between 1100 and 1094 Ma. Recent U-Pb dating of zircons indicates, however, that many of the felsic rocks were produced early (~1108 Ma; Karl Wirth, pers. comm.). Some of this early felsic material is adjacent to and stratigraphically above the early gabbroic rocks.

This investigation reexamined the field relations, and included chemical analyses. Rocks range from troctolites to gabbros and gabbronorites. High amounts (>30%) of oxides characterize some units (oxide gabbros). The only scenarios successful in producing the chemical range of magmas involve polybaric fractionation. This is consistent with a picture of a fracturing and foundering crust. Mineral analyses demonstrate that sheets originally grouped by Nathan (1969) as a single magma are often distinct, perhaps representing temporally separate intrusions. High-Al, Fe-basalt magmas (common in the MCR) fractionated at >9kb can produce materials not enriched in silica, while those fractionated at <5kb lead to pronounced enrichment (Scoates and Lindsley, 2000 AGU Fall Meeting). Thus, shallow, high-Si material may have produced a density barrier to rising mafic material, causing magmas to pond, resulting in broadly similar sheets with only subtle variations.