Sr,Nd and Pb isotopic and REE geochemistry of St. Paul's Rocks: the metamorphic and metasomatic development of an alkali basalt mantle source

Surface samples of peridotites and hornblendite mylonites from St. Paul's Rocks, and dredge samples from the flanks of the massif, have been analyzed for Sr, Nd and Pb isotopic ratios and Rb, Sr, and REE concentrations. This data, coupled with previous K and REE data, are used to develop a self-consistent model for the genesis of these ultramafic rocks. This model involves metasomatism of an ocean island-type mantle about 155 m.y. ago by a strongly light-REE-enriched metasomatic fluid, probably derived from the same mantle. This metasomatism produced light-REE-enriched materials which were isotopically homogeneous on a small scale (100 m), and isotopically heterogeneous on a large (km) scale. The geochemical relationships between the peridotites and the hornblendites were established by metamorphic equilibration on a relatively small scale (<10 m). The average mantle produced by these events is characterized by⁸⁷Sr/⁸⁶Sr=0.7034,¹⁴³Nd/¹⁴⁴Nd=0.51291,²⁰⁶Pb/²⁰⁴Pb=19.33 and 207/204=15.63. An alkali basalt which postdates the mylon-itization of the ultramafic massif has an isotopic character which is identical to the average ultramafic massif; it also lies on the five-dimensional isotopic mantle plane of Zindler et al. (1982). With respect to major elements, trace elements, and Sr, Nd and Pb isotopes, the average ultramafic rock of the St. Paul's massif is an ideal candidate for a mantle source from which alkali basalts can be derived by partial melting; the St. Paul's massif is in fact the first such example of an ultramafic rock which meets all the requirements to be an alkali basalt source.