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 C omplex of northern California exhibit large geochemical rare earth ele ment (REE) and Nd isotopic variations on the small scales which are in dicative 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 Ihe rzolite, harzburgite, wherlite and dunite); the peridotite unit is a f ragment of mantle lithosphere of oceanic affinity; (2) a series of sma ll (similar to 1 km diameter) undeformed gabbroic massifs; (3) several generations of basic dykes. The peridotites display the largest geoch emical and isotopic variations, with epsilon(Nd)(T) values ranging fro m + 10 down to 0. In the gabbroic massifs and intrusive dykes, the var iation in model epsilon(Nd)(T) values is reduced to 7 epsilon(Nd) unit s: 0 to + 7. As a general rule, peridotites, gabbros and dykes with ep silon(Nd)(T) values around 0 or + 3 give less depleted L(light)REE pat terns than do those with epsilon(Nd)(T) values in the range + 7 to + 1 0. 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 indic ates that the peridotites with epsilon(Nd)(T) approximate to + 10 coul d be slightly older than the intrusive gabbro massifs and basic dykes (470 Ma vs. 420 Ma). Strontium isotopic data used in connection with S m-Nd results demonstrate that the 10 epsilon(Nd) units variation displ ayed by the Trinity Peridotite is a primary feature and not an artefac t due to REE mobility during seawater interaction. The variable Nd iso topic signatures and variable LREE patterns in the Trinity Peridotite cannot represent mantle source characteristics as there is evidence th at this unit was partially melted when it rose as part of the upwellin g convecting mantle. Field, petrographic, geochemical and isotopic dat a rather suggest that the observed heterogeneity is due to local react ions 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 t he Trinity Complex have geochemical and isotopic compositions similar to the are basalts from the adjacent Copley Formation, so it is propos ed that the younger melts are related to are magmatism.