LAYERING IN THE EARLY GABBROIC SERIES OF THE MIDCONTINENT RIFT SYSTEM: THE EFFECT OF MAGMA DENSITY

The Early Gabbroic Series of the Midcontinent Rift System (MCR) is comprised of numerous tabular and sheetlike intrusions in extreme northeastern Minnesota. U-Pb systematics in zircon provide a date of 1106.9 ± 0.6 Ma for one of the units in this series (James Miller, pers. comm.), making them among the oldest materials associated with the MCR. One of the notable characteristics of the Midcontinent Rift is the presence of large amounts of felsic material, properly termed granophyre. The Early Gabbro Series layers are below this granophyre stratigraphically. This felsic rock was noted and described by H.D Nathan in his dissertation (1969) as a very late-stage material, although several observations indicate that the gabbro was emplaced later than the granophyres. These include gradational contacts, with some chilling of the gabbro. In addition, an abundance of material of an intermediate nature is found locally. This material is always found between the gabbro and granophyre, and is presumably the result of assimilation of granophyre during the intrusion of the hotter gabbro. A recent investigation into possible origins of the granophyres (Karl Wirth, pers. comm.) included radiometric age determinations, and revealed that the granophyres adjacent to the Early Gabbro Series are, like the gabbros, among the earliest rocks of the rift (~1107 Ga). This early age for the granophyres suggests that due to their low density, the granophyric material acted as a barrier that retarded the rise of more buoyant gabbroic material coming up from below. These rising liquids would have been forced to spread laterally, resulting in the apparent layering that is observed. Densities were calculated using the model of Bottinga and Weill (1970; Am. J. Sci.), with partial molar volume data from Nelson and Carmichael (1979; Cont. Min. Pet.) for representative samples from the Early Gabbro Series. These density values show a progression to higher values with depth, consistent with the stratigraphic sequence of magmas being the result of density effects rather than strict compositional ones.