CONTRASTING LOWER CRUSTAL EVOLUTION ACROSS AN ARCHEAN-PROTEROZOIC SUTURE: PHYSICAL, CHEMICAL AND GEOCHRONOLOGIC STUDIES OF LOWER CRUSTAL XENOLITHS IN SOUTHERN WYOMING AND NORTHERN COLORADO

Lower crustal xenoliths from southern Wyoming and northern Colorado have been examined to assess the evolution of lower crust beneath the Cheyenne Belt. North of the Cheyenne Belt, in the southern Wyoming craton, lower crustal xenoliths in ~ 1 Ma ultrapotassic volcanic rocks at Leucite Hills are mafic (3.5 to 13.5 wt % MgO), one or two-pyroxene (garnet absent) hornblende granulites. Estimates of peak metamorphic conditions (1.1 to 1.3 GPa and 800-1000oC) and the xenolith densities (2.7 to 3.1 g/cm3), suggest that these rocks are lower crustal in origin. U-Pb zircon dates (~2.6 Ga) and Late Archean Nd (TDM) model ages all indicate that the mafic lower crust is Late Archean. Physical property measurements from mafic granulites yield Vp=6.2 to 6.9 km/s and Vs=3.5 to 3.9 km/s, at 1 GPa. The lack of a thick high-velocity layer beneath this portion of the Wyoming craton suggests that the lower crust here is garnet-free mafic hornblende granulite throughout. South of the Cheyenne Belt, in Paleoproterozoic crust, mafic granulite xenoliths entrained in Devonian “State Line” kimberlites are dominantly one or two pyroxene, garnet granulites that lack hornblende or other hydrous phases. Peak metamorphic temperatures range from 625 – 831°C and pressures are generally 1.1 – 1.2 GPa, similar to the values determined for the Leucite Hills xenoliths. The State Line xenoliths yield a complex spectrum of Paleoproterozic U-Pb zircon dates and some Archean grains as old as 3.1 Ga. Whole rock Nd isotopic compositions are those expected for Paleoproterozoic crust. The State Line xenoliths also can be divided into high La/YbN and low La/YbN groups (La/YbN=3.0 to 14.6 vs. 0.4 to 0.9). The latter xenoliths likely represent restite remaining after partial melting of mafic lower crust at ~1.4 Ga. Seismic velocities for the State Line mafic granulites are high (Vp=6.6 to 7.2 km/s, Vs=3.7 to 3.9 km/s, at 1 GPa) and suggest that high velocity (7.xx) lower crustal layer beneath northern Colorado is garnet bearing and mafic in composition. Overall, our data reveal that mafic lower crust north of the Cheyenne Belt has remained hydrous and largely unperturbed thermally since Late Archean while Paleoproterozoic lower crust in northern Colorado is anhydrous, garnet-rich, and shows evidence for heating and partial melting after the crust originally formed.