EXTREME ND ISOTOPIC VARIATION IN THE TRINITY OPHIOLITE COMPLEX AND THE ROLE OF MELT ROCK REACTIONS IN THE OCEANIC LITHOSPHERE

Peridotites, dykes and gabbros from the 470-420 Ma Trinity Ophiolite Complex of northern California exhibit large geochemical rare earth element (REE) and Nd isotopic variations on the small scales which are indicative of a complex history. The Trinity Ophiolite, which covers an area of approximate to 1600 km(2), consists of three distinct units: (1) a similar to 2-4 km-thick sheet of plastically deformed peridotites, including various ultrabasic lithologies (plagioclase and spinel Iherzolite, harzburgite, wherlite and dunite); the peridotite unit is a fragment of mantle lithosphere of oceanic affinity; (2) a series of small (similar to 1 km diameter) undeformed gabbroic massifs; (3) several generations of basic dykes. The peridotites display the largest geochemical and isotopic variations, with epsilon(Nd)(T) values ranging from + 10 down to 0. In the gabbroic massifs and intrusive dykes, the variation in model epsilon(Nd)(T) values is reduced to 7 epsilon(Nd) units: 0 to + 7. As a general rule, peridotites, gabbros and dykes with epsilon(Nd)(T) values around 0 or + 3 give less depleted L(light)REE patterns than do those with epsilon(Nd)(T) values in the range + 7 to + 10. In the peridotites, the Nd isotopic variations take place over very short distances, with jumps as large as 7 epsilon(Nd) units occurring on scales of less than 20 m. Comparison with available age data indicates that the peridotites with epsilon(Nd)(T) approximate to + 10 could be slightly older than the intrusive gabbro massifs and basic dykes (470 Ma vs. 420 Ma). Strontium isotopic data used in connection with Sm-Nd results demonstrate that the 10 epsilon(Nd) units variation displayed by the Trinity Peridotite is a primary feature and not an artefact due to REE mobility during seawater interaction. The variable Nd isotopic signatures and variable LREE patterns in the Trinity Peridotite cannot represent mantle source characteristics as there is evidence that this unit was partially melted when it rose as part of the upwelling convecting mantle. Field, petrographic, geochemical and isotopic data rather suggest that the observed heterogeneity is due to local reactions between a 470 Ma proto-peridotite with epsilon(Nd)(T) = + 10 and younger (420 Ma) basaltic melts with lower epsilon(Nd)(T) values (i.e. the gabbroic massifs and the dykes). The gabbros and basic dykes of the Trinity Complex have geochemical and isotopic compositions similar to the are basalts from the adjacent Copley Formation, so it is proposed that the younger melts are related to are magmatism.