DURATION OF PROGRADE METAMORPHISM AS CONSTRAINED BY HIGH-PRECISION LU-HF GEOCHRONOLOGY OF HP/UHP ECLOGITES FROM THE WESTERN ALPS

Eclogites of the Zermatt-Saas ophiolite, western Alps, represent oceanic crust that was subducted to HP/UHP conditions (2.0-2.9 GPa; Barnicoat and Fry, 1986; van der Klauw et al., 1997) during Tertiary collision of Europe and Africa/Apulia. The age of peak metamorphism of the Zermatt-Saas ophiolite has been estimated to be between 38-50 Ma using Rb-Sr, Sm-Nd, and U-Pb geochronology (Dal Piaz et al., 2001, and references therein). Recent work has suggested that Lu-Hf geochronology may constrain the earlier periods of garnet growth that occur during prograde metamorphism because of strong partitioning of Lu into garnet cores. Here we report a Lu-Hf garnet-bulk rock age of 55.6 ± 4.3 Ma (MSWD=0.81) that was determined on eclogite from pillow cores collected near the classic pillow lava locality from Pfulwe in the Zermatt region, Switzerland. Attempts to decrease the uncertainty of the age by partial leaching were inconclusive. HF and HCl treatments were used, but the increases in Lu/Hf ratios following leaching were much less than those seen for Sm/Nd ratios, resulting in little advantage in increasing precision of Lu-Hf isochrons. Moreover, it is possible that acid leaching may fractionate Lu/Hf ratios due to their contrasting chemical behavior in HF and HCl, which could produce erroneous isochrons; fractionation of Sm/Nd ratios during leaching does not likely occur, reflecting their similar chemical behavior. Rutile and zircon as inclusions may have a negative effect on the precision of Lu-Hf isochrons and are unlikely to be affected by HCl and HF leaching. Errors introduced by old inherited rutile or zircon are minimal given the young basaltic protoliths of this ophiolite.

Based on modeling Lu/Hf partitioning during garnet growth, onset of garnet growth at the Pfulwe locality most likely began significantly earlier than 55.6 Ma, perhaps at ~60 Ma. This timing is compared with the younger Lu-Hf age of 48.8 ± 2.1 Ma, from the coesite-bearing UHP locality at Lago di Cignana, Italy (Lapen et al., 2003), where garnet growth is estimated to have begun at ~50 Ma. Our preferred model for Cignana places peak HP/UHP metamorphism at ~40 Ma, suggesting garnet growth over a period of as much as 20 m.y. at the Pfulwe area during subduction to HP/UHP conditions, or alternatively, diachronous subduction and garnet growth over the ophiolite sheet.