Oceanic Lithospheric Mantle evolution and plumbing system roots beneath Madeira Island (Portugal): evidence from petrology and geochemistry of ultramafic xenoliths

Ultramafic xenoliths brought to the surface by primitive magmas in the Macaronesian archipelagos (Canary, Cape Verde, Madeira and Azores islands) provide important constraints on the structure and chemical composition of the oceanic lithospheric mantle, its record of geodynamic processes, and the evolution of deeper portions of plumbing systems beneath active volcanic areas (Munha et al., 1990).

Here we present a detailed study of n. 16 ultramafic xenoliths collected from basaltic to basanitic magmas in the NW part of Madeira Island (Portugal). The xenoliths are mostly protogranular to porphyroclastic spinel harzburgites and dunites, with subordinate cumulate-textured wehrlites and olivine clinopyroxenites.

Two groups are recognized based on the mineralogy and major and trace element compositions. The first group is represented by harzburgites, which are composed of highly forsteritic (90-92) olivine, high Mg# (90-93) pyroxene and high Cr# (0.3-0.7) spinel, suggesting their derivation from a refractory mantle which experienced a high degree (20-25%) of melt extraction. The second group is represented by dunites, wehrlites and olivine clinopyroxenites, with low forsterite (82-86) olivine, low Mg# (82-86) pyroxene and low Cr# (0.2-0.4) spinel, suggesting formation either as cumulates of basaltic/basanitic magmas (Munha et al., 1990) or by extensive mantle metasomatism. The equilibration temperatures obtained from the two-pyroxene geothermometer and the spinel-olivine geothermometer are 1009-1073°C for the harzburgites and 896-1056°C for the cumulates, assuming a pressure of 11 kbar. The oxygen fugacity varies between +0.94 and +1.90 ΔlogƒO₂ [FMQ].

The chondrite-normalised REE patterns of clinopyroxenes from harzburgite xenoliths are broadly consistent with those of residua after 10-18% partial melting of primitive mantle under spinel-facies conditions, with LREE typically ranging from ~0.2 to 0.8 × chondrite and Ce from ~0.5 to 1.5 × chondrite. The REE patterns of clinopyroxenes from dunite, wehrlite and olivine clinopyroxenite xenoliths exhibit strongly elevated LREE–MREE abundances, with La and Ce typically ranging from ~2 to 20 × chondritic and Nd from ~4 to 15 × chondritic, corroborating the evidence that these xenoliths crystallized from, or re-equilibrated with, infiltrating basaltic to basanitic melts.

 

References

Munhá, J., Palácios, T., MacRae, N. D., & Mata, J. (1990). Petrology of ultramafic xenoliths from Madeira Island. Geological Magazine, 127(6), 543-566.