LU-HF GEOCHRONOLOGY APPLIED TO DATING CENOZOIC EVENTS AFFECTING LOWERCRUSTAL XENOLITHS FROM KILBOURNE HOLE, NEW-MEXICO

Melt-extraction or crystal accumulation events that affected garnet-be aring, granulite xenoliths from Kilbourne Hole, New Mexico, have been dated using the Lu-Hf isotope system. Two garnet-bearing granulites fr om Kilbourne Hole have extreme Lu-176/Hf-177 ratios of 0.95 and 1.3 (L u/Hf approximate to 28 and 39 X chondritic), but relatively 'normal' e psilon(Hf) values (+5, and +12) necessitating either garnet accumulati on or melt-extraction from a garnet-bearing protolith in the Cenozoic. Hf isotope evolution curves for these two samples intersect those of depleted mantle and Proterozoic crust at high angles and at similar ti mes, demonstrating the potential of Hf isotope model ages to yield tru e age significance even if the initial Hf isotope composition is not w ell constrained. A third garnet-granulite xenolith (CKH63; Lu-176/Hf-1 77 = 0.025) contains zircon, which buffered this sample against change s in Lu/Hf during the Cenozoic differentiation event. The three garnet granulites lie closely about a 25 Ma Lu-Hf reference line, demonstrat ing the potential of the Lu-Hf system for detecting garnet-controlled differentiation events in the Cenozoic; given the range in Lu/Hf ratio s measured, events as young as 5 Ma may be detected using the Lu/Hf sy stem. Conventional U-Pb zircon data from sample CKH63 reveal both a ca . 1.4 Ga inherited component and a component of recent Pb-loss (or new zircon growth), supporting the Cenozoic event documented by Hf isotop e model ages. (C) 1997 Elsevier Science B.V.