Petrology, geochemistry and Sr-Nd isotopes of the Trindade and Martin Vaz volcanic rocks (Southern Atlantic Ocean)

Geochemical, mineralogical and Sr-Nd isotopic analyses were performed on th e strongly undersaturated volcanic rocks of the Trindade and Martin Vat isl ands, located in the South Atlantic Ocean (Brazil). The magmatic activity a t Trindade was essentially bimodal, generating various ultrabasic rocks and mesocratic to leucocratic phonolites. The youngest volcanic events are mad e up exclusively of ultrabasic rocks, whereas the older episodes are charac terized by abundant phonolites. Basic rocks (45 < SiO2< 52 wt.%) are rare, and commonly carry cumulate xenoliths of phaneritic alkline rocks (e.g., cl inopyroxenites, ijolites, malignites). The ulrabasic rocks belong to two di stinct groups requiring the participation of different mantle sources. The more primitive group of olivine and pyroxene melanephelinites (SiO2 = 41-42 wt.%, MgO = 10-12 wt.%) is distinguished by significantly higher contents of K (K2O = 2-3 wt.%) than the more evolved one (basanites and tephrites; S iO2 = 43-44 wt.%, MgO = 8-9 wt.%, K2O = 0.4-1.0 wt.%). Fractional crystalli zation processes may explain the differentiation from melanephelinites or b asanites-tephrites to phonolites. The Trindade stratigraphy and clinopyroxe ne mineral chemistry data strengthen the hypothesis that the extrusive mate rials probably came from stratified magma chambers, replenished in the late stages of magmatic activity with ultrabasic magmas. The phonolites display a wide range of compositions, also caused primarily by fractional crystall ization, as indicated by mass-balance modelling, evolving in the extreme to peralkaline phonolites. According to Sr-Nd isotopic systematics, mixing be tween predominant (70%) depleted mantle and subordinate EMI components can explain the isotopic variations. The decoupling between Sr-Nd isotopic comp ositions (depleted relative to the Bulk Earth) and the Rb/Sr-Sm/Nd elementa l ratios indicate that metasomatic events occurred in the mantle source(s) prior to melting processes. The enrichment episode took place either recent ly (in the last tens of millions of years) or at about 450 Ma as indicated by Nd model ages. This age agrees with those of the Brasiliano Cycle, sugge sting the involvement of detached continental lithosphere, related to the b reakup of Western Gondwanaland. (C) 1999 Elsevier Science B.V. All rights r eserved.