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

Abstract

Geochemical, mineralogical and Sr–Nd isotopic analyses were performed on the strongly undersaturated volcanic rocks of the Trindade and Martin Vaz islands, located in the South Atlantic Ocean (Brazil). The magmatic activity at Trindade was essentially bimodal, generating various ultrabasic rocks and mesocratic to leucocratic phonolites. The youngest volcanic events are made up exclusively of ultrabasic rocks, whereas the older episodes are characterized by abundant phonolites. Basic rocks (45<SiO 2<52 wt.%) are rare, and commonly carry cumulate xenoliths of phaneritic alkaline rocks (e.g., clinopyroxenites, ijolites, malignites). The ultrabasic rocks belong to two distinct groups requiring the participation of different mantle sources. The more primitive group of olivine and pyroxene melanephelinites (SiO 2=41–42 wt.%, MgO=10–12 wt.%) is distinguished by significantly higher contents of K (K 2O=2–3 wt.%) than the more evolved one (basanites and tephrites; SiO 2=43–44 wt.%, MgO=8–9 wt.%, K 2O=0.4–1.0 wt.%). Fractional crystallization processes may explain the differentiation from melanephelinites or basanites–tephrites to phonolites. The Trindade stratigraphy and clinopyroxene mineral chemistry data strengthen the hypothesis that the extrusive materials 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 crystallization, as indicated by mass-balance modelling, evolving in the extreme to peralkaline phonolites. According to Sr–Nd isotopic systematics, mixing between predominant (70%) depleted mantle and subordinate EMI components can explain the isotopic variations. The decoupling between Sr–Nd isotopic compositions (depleted relative to the Bulk Earth) and the Rb/Sr–Sm/Nd elemental ratios indicate that metasomatic events occurred in the mantle source(s) prior to melting processes. The enrichment episode took place either recently (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, suggesting the involvement of detached continental lithosphere, related to the breakup of Western Gondwanaland.