Contrasting lower crustal evolution across an Archean-Proterozoic suture: Physical, chemical and geochronologic studies of lower crustal xenoliths in southern Wyoming and northern Colorado

Abstract

Crustal xenoliths from Quaternary ultrapotassic volcanic rocks at Leucite Hills in southern Wyoming and Devonian State Line kimberlite diatremes in northern Colorado have been examined to compare and contrast Archean and Paleoproterozoic lower continental crust across the Proterozoic Cheyenne Belt. The Leucite Hills xenoliths are dominantly mafic (3.5 to 13.5 wt % MgO), one or two-pyroxene (garnet absent) hornblende granulites. Peak metamorphic conditions (1.1 to 1.3 GPa and ca. 800°C) and the high xenoliths densities (2.7 to 3.1 g/cm 3 ) reveal that the xenoliths are lower crystal. U-Pb zircon and Nd model ages both indicate that the mafic crust is Late Archean (∼2.6 Ga). The State Line xenoliths are Paleoproterozoic, two pyroxene garnet granulites with major element compositions and peak metamorphic conditions (1.1 - 1.2 GPa, 625 - 830°C) similar to the Leucite Hills xenoliths, but with higher Vp and Vs (6.6 to 7.2 km/s and 3.7 to 3.9 km/s vs. 6.2 to 6.9 km/s and 3.5 to 3.9 km/s) due to presence of garnet. Some State Line xenoliths also have low La/Yb N (0.4 to 0.9) and likely represent restite remaining after partial crustal melting at ∼1.4 Ga. These observations reveal that mafic lower cryst north of the Cheyenne Belt has remained hydrous and largely unperturbed thermally since the Late Archean while Paleoproterozoic, less hydrous, lower crust in northern Colorado underwent heating and partial melting after its formation. We speculate that the Archean lower cryst was shielded from the thermal events that extensively affected the Paleoproterozoic cryst by thick, buoyant Archean mantle lithosphere.