PROVENANCE OF SUBDUCTED OCEANIC SEDIMENTS IN THE UHP TERRAINS OF THE WESTERN ALPS

Rare Earth Element (REE) concentrations and Nd-isotope compositions are used to constrain pre-collision Tethys paleogeography in the Alpine orogen. The metasedimentary protoliths are manganese-rich, pelagic deep-sea sediments that were deposited during Jurassic/Early Tertiary divergence of the neoTethys ocean and subsequently underwent ultra-high-pressure metamorphism during Eocene subduction. The relative immobility of REEs and resistance of Sm-Nd fractionation during erosion, re-deposition, or metamorphism make the Nd-isotope system well-suited for provenance determination in high-grade metasedimentary rocks. Isotopic and REE data can be broken into three provenance groups: 1) An old upper continental crust source is suggested for the first set (Group 1) based on overall REE patterns, including flat but depleted HREE trends, Eu/Eu*=0.65, and (Sm/Nd)_N=0.57. Group 1 samples yield T_CHUR of 1.16 to 1.38 Ga and present-day epsilon Nd of -12.5 to -14.8. 2) Mixed provenance of old upper continental crust including recycled crust and juvenile mafic material is indicated for a second suite (Group 2). These samples have lower concentrations of LREE and HREE than Group 1, Eu/Eu* from 0.57 to 0.73, and (Sm/Nd)_N varying from 0.61 to 0.66. Group 2 samples have T_CHUR of 0.82 to 1.01 Ga and present-day epsilon Nd varying from -7.8 to -8.8. 3) An oceanic crustal source is suggested for the third set (Group 3), based on REE patterns that are similar to MORB, including depleted LREE and enriched HREE relative to LREE, Eu/Eu*=0.95, and (Sm/Nd)_N=0.9988. Nd isotope data provide no T_CHUR age solutions due to the high present-day epsilon Nd of +8.1. Source terrains during neoTethys divergence included European crust and African/Adriatic crust. These crustal sources are represented as various nappes in the Western Alps and are currently being studied as controls for provenance determination.