Burial rates during prograde metamorphism of an ultra-high-pressure terrane: an example from Lago di Cignana, western Alps, Italy Thomas J. Lapen a;  , Clark M. Johnson a , Lukas P. Baumgartner b , Nancy J. Mahlen a , Brian L. Beard a , Je¡rey M. Amato c a Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706, USA b Institute of Mineralogy and Petrology, BFSH2, CH-1015 Lausanne, Switzerland c Department of Geological Sciences, New Mexico State University, P.O. Box 30001, Las Cruces, NM 88003, USA Received25February2003;receivedinrevisedform30July2003;accepted4August2003 Abstract Estimation of burial rates and duration of prograde metamorphism of ultra-high-pressure (UHP) rocks (T =590^ 630‡C, P =2.7^2.9GPa)oftheZermatt^SaasophiolitefromLagodiCignana,Italy,maybemadethroughcombined Lu^Hf and Sm^Nd garnet geochronology in conjunction with petrologic estimates of the prograde P^T path. We report a Lu^Hf garnet^omphacite^whole-rock isochron age of 48.8þ2.1 Ma from the UHP locality at Lago di Cignana, which stands in contrast to the Sm^Nd age of 40.6þ2.6 Ma [Amato et al., Earth Planet. Sci. Lett. 171 (1999)425^438]obtainedfromthesamesampleandmineralmaterial.TheSm^NdandLu^Hfages,aswellasother agesdeterminedonmetamorphicgarnet,zirconandwhitemica[Amatoetal.,EarthPlanet.Sci.Lett.171(1999)425^ 438;Mayeretal.,Eur.UnionGeosci.10(1999)Abstr.809;Rubattoetal.,Contrib.Mineral.Petrol.132(1998)269^ 287;DalPiazetal.,Int.J.EarthSci.90(2001)668^684]fromLagodiCignanaandelsewhereintheZermatt^Saas ophiolite, lie within a range of V50^38 Ma, which we interpret to encompass the duration of prograde metamorphism,andpossiblythedurationofgarnetgrowth.ThedifferenceinmeasuredSm^NdandLu^Hfagesfrom Cignana can be accounted for by expected core to rim variations in Lu, Hf, Sm, and Nd. The measured yttrium content in garnet, which may be a proxy for Lu, is highest in garnet core and lowest in the mineral rim, generally followingaprofilethatispredictedbyRayleighfractionation.PreferentialenrichmentofLuinthecoreproducesa Lu^Hfagethatisweightedtowardtheoldergarnetcore.Sm^Ndages,aspredictedbyRayleighfractionationofSm andNdduringgarnetgrowth,however,reflectlatergrowngarnetascomparedtoLu^Hfages.ThedifferenceinSm^ NdandLu^Hfagesfromasinglesampleshouldthereforebeaminimumestimateforthedurationofgarnetgrowth and prograde metamorphism so long as Sm^Nd and Lu^Hf blocking temperatures were not exceeded for a long periodoftime.Basedonthe12Myrdurationofprogradegarnetgrowthestimatedinthisstudy,burialratesforrocks atLagodiCignanawereontheorderof0.23^0.47cm/yr.Thesevaluescorrelatewithcontinuousshorteningratesof 0012-821X/03/$^seefrontmatterß2003ElsevierB.V.Allrightsreserved. doi:10.1016/S0012-821X(03)00455-2 * Corresponding author. Tel.: +1-608-262-8960. E-mail addresses: lapen@geology.wisc.edu (T.J. Lapen), clarkj@geology.wisc.edu (C.M. Johnson), lukas.baumgartner@img.unil.ch (L.P. Baumgartner), mahlen@geology.wisc.edu (N.J. Mahlen), beardb@geology.wisc.edu (B.L. Beard), amato@nmsu.edu (J.M. Amato). Earth and Planetary Science Letters 215 (2003) 57^72 R Available online at www.sciencedirect.com www.elsevier.com/locate/epsl