U-PB ZIRCON AGES IN LOWER CRUSTAL XENOLITHS FROM THE SOUTHERN SUPERIOR PROVINCE: EVIDENCE FOR ONSET OF SUPERCONTINENT BREAKUP AT 1.4 GA BENEATH THE UPPER MICHIGAN PENINSULA?

Zircons separated from a Middle to Early Archean-age lower crustal granulite xenolith from northern Michigan were dated by high-resolution ion microprobe. Two distinct populations of zircons were identified and they provide evidence for at least two major thermal events in the lower crust of the southern Superior Province. Abundant rounded zircons have morphologies that are similar to zircons found in lower-crustal rocks that have undergone granulite-facies metamorphism. They yield ages of 1104 ± 42 (2s) Ma, which coincides with eruption of Mid-Continent Rift flood basalts. Their 1.1 Ga age is also considered to represent the age of a compositionally distinct group of mafic granulites within the suite that have low La/Nb ratios and trace element characteristics similar to some Keweenawan plume basalts; they are interpreted as evidence for basaltic underplating in the lower crust during Mid-Continent Rifting at 1.1Ga.

Also present are less abundant elongate zircon grains that yield a mean age of 1387 ± 32 (2s) Ma. These zircons have high U contents, are elongate in shape and have zoned morphologies that are most consistent with crystallization from a melt or hydrothermal fluid. A definitive geologic event responsible for growth of the 1.4 Ga elongate zircon crystals has been difficult to establish. Although the age is strongly recorded in the zircon population, it is not evident among the ages determined by Pb-Pb, Rb-Sr or Sm-Nd geochronology for any xenolith within the suite (neither internal nor whole rock isochrons). There is also no obvious surface expression for an event of this age in the region. The Wolf Creek Batholith (1470 Ma), located approximately 100km to the south, is 80 m.y. older, which suggests that the zircons did not grow in response to melt generation and/or migration associated with this particular pluton.

Nonetheless, this 1.4 Ga age does coincide with a period of anorogenic granite emplacement along the southern margin of Rodinia that extends from California to Scandinavia between 1.3 to 1.5 Ga. These enigmatic Mesoproterozoic A-type granites, have been associated with the break-up of the Nuna/Columbia supercontinent (1.5-1.2 Ga). The xenolith zircon data may thus provide evidence for smaller thermal pulses that contributed to the break-up of Nuna, but that affected only the lower crust in this area.