Geochemical modeling and Nd–Sr data links nephelinite–phonolite successions and xenoliths of Trindade Island (South Atlantic Ocean, Brazil)

Abstract

Abstract The Trindade Island corresponds to the eastern end of the submarine E–W Vitoria–Trindade Chain as part of the Trindade plume track on the South American plate. It is a suitable site for petrogenetic investigations, since a series of unusual rock compositions crops out within nephelinite–phonolite successions and scarce xenoliths. Software-based geochemical modeling and Nd–Sr analyses, coupled with field work, petrography and literature data were used to evaluate and model the petrogenetic processes that led to the formation of variable rock compositions. Results show the formation of: (1) nephelinites at 1490 °C and 3 GPa, from 0.1 to 7% of partial melting of an enriched garnet–lherzolitic source or from 1 to 5% partial melting of TiO2-rich garnet–phogopite lherzolite (up to 2.5 wt.% of CO2). Nephelinites represent low viscosity (18 Pa s), high-temperature (1170 °C) and high-density (2.79 g/cm3) lavas; (2) pyroxenite, jacupirangite and melteigite cumulates at 900 °C and 0.5 GPa, after 46%, 49% and 56% fractional crystallization of nephelinites, and leaving phonotephrites as residual liquids; (3) monchiquites, from fractional melting of enriched, CO2-bearing garnet–amphibole–phlogopite peridotites or from hydrated nephelinite magmas. They may evolved to sannaite via fractional crystallization or may generate phonolites through extraction of pyroxene and amphibole cumulates at 1080 to 970 °C and 1.2 to 0.9 GPa; (4) sannaites may represent ‘hydrous’ phonotephrites, and can evolve to phonolites and phonolitic foidites via fractional crystallization; and (5) bebedourite cumulates at 850 °C and 0.38 to 0.4 GPa, from phonotephritic or sannaitic magmas after 20% and 26% of fractional crystallization, leaving phonolites and phonolite–foidites as residual liquids. Phonolites and phonolitic foidites, the most evolved rocks of the nephelinite–phonolite association, represent higher-viscosity (1.1 × 106 Pa s), lower-temperature (950 °C), and lower-density (2.48 g/cm3) lavas. Both the geochemical modeling and the Nd–Sr isotopic data show that the studied rocks are genetically associated. They evolved via fractional crystallization after partial melting of a very homogeneous reservoir with inherited asthenospheric heterogeneities. Our work refines and validates some of the assumptions of previous contributions, and it can be used as a basis for further petrogenetic investigations on oceanic islands.