LOWER CRUSTAL XENOLITHS FROM LAKE ELLEN AND SITE 69 DIATREMES, UPPER MICHIGAN PENINSULA: TRACE ELEMENT AND ISOTOPIC EVIDENCE FOR BASALTIC UNDERPLATING DURING MID-CONTINENT RIFTING AT 1.1GA

Jurassic kimberlites intruding the >3.5 Ga southern Superior Province in northern Michigan have entrained a variety of crustal lithologies, including unusual mafic lower crustal granulites with strong positive Nb and Ta anomalies on trace element “spider diagrams”. Such compositions are uncommon among granulite xenoliths worldwide, but typical of plume-related magmatism. Isotopic data suggest the rocks are similar in age to the peak of Keweenawan plume activity: whole-rock Sm-Nd errorchron of 1046± 140Ma and ages of 1104 ± 42 (2σ) Ma for rounded and colorless zircons from a unique peraluminous granulite xenolith within the suite. Other trace-element characteristics are also similar to some Mid-Continent Rift basalts, e.g. high La/Yb ratios, negative Sr and K, suggesting that the granulite xenoliths are the underplated, lower-crustal equivalent of Keweenawan plume basalts. Basaltic underplating in response to a plume event is also consistent with the layered and strongly reflective seismic structure of the lower 20-25 km of the crust beneath the region.

The plume-related granulites have unusually radiogenic Pb isotope compositions, plotting above the Stacey & Kramers (1975) growth curve and displaced to the right of the 4.55 Ga geochron—compositions that are unusual for lower crustal granulite xenoliths worldwide, but overlapping the range of Pb isotopes for Keweenawan basalts. Whole rock εNd values of -7.5 to -8.3 at 1.1 Ga, however, are inconsistent with a plume source that has an εNd = ~0, as has previously been assumed for the Keweenawan plume [1]. Significant crustal contamination is ruled out by high Nb/La ratios and negative Pb anomalies, suggesting that the Keweenawan plume already had an enriched Nd isotope signature and contained a component of recycled oceanic crust and/or sediments.

We therefore propose that these mafic granulite xenoliths are the product of underplating of trace-element-enriched, Keweenawan plume-derived basaltic magmas. The granulite protoliths retained high U/Pb and Th/Pb ratios relative to average crust, indicating that the lower crust, particularly in regions affected by plume activity, can have Pb that is strongly enriched rather than depleted in radiogenic isotopes.

[1] Paces and Bell (1989). Geochimica et Cosmochimica Acta 53, 2023–2035.