Extreme Nd isotopic variation in the Trinity Ophiolite Complex and the role of melt/rock reactions in the oceanic lithosphere |
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| Authors: | G Gruau J Bernard-Griffiths C Lécuyer O Henin J Macé M Cannat |
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| Institution: | (1) Géosciences Rennes, CNRS UPR 4661, Université de Rennes, F-35042 Rennes Cedex, France, FR;(2) CNRS URA 736, Université de Paris VII, 4 place Jussieu, F-75252 Paris Cedex 05, France, FR |
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| Abstract: | 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 ≈1600 km2, consists of three distinct units: (1) a ∼2–4 km-thick sheet of plastically deformed peridotites, including various ultrabasic
lithologies (plagioclase and spinel lherzolite, harzburgite, wherlite and dunite); the peridotite unit is a fragment of mantle
lithosphere of oceanic affinity; (2) a series of small (∼1 km diameter) undeformed gabbroic massifs; (3) several generations
of basic dykes. The peridotites display the largest geochemical and isotopic variations, with ɛNd(T) values ranging from +10 down to 0. In the gabbroic massifs and intrusive dykes, the variation in model ɛNd(T) values is reduced to 7 ɛNd units: 0 to +7. As a general rule, peridotites, gabbros and dykes with ɛNd(T) values around 0 or +3 give less depleted L(light)REE patterns than do those with ɛNd(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 ɛNd units occurring on scales of less than 20 m. Comparison with available age data indicates that the peridotites with ɛNd(T)≈+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 ɛNd 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 ɛNd(T)=+10 and younger (420 Ma) basaltic melts with lower ɛNd(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 arc basalts from the adjacent Copley Formation, so it is proposed that the younger melts
are related to arc magmatism.
Received: 13 January 1995/Accepted 5 May 1995 |
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