Magmatic evolution of Graciosa (Azores, Portugal)

Abstract

Graciosa is the westernmost island of theTerceira Rift, located to the east of the Mid-Atlantic Ridge, in the Azores archipelago. The three volcanic complexes recognized in the island have been characterized, including the study of lava flows, gabbroic xenoliths (alkaline and subalkaline), syenites and dunites. A detailed study of mineral compositions and whole-rock major and trace element concentrations has allowed the characterization of volcanic units and the investigation of the magmatic and temporal evolution of the island. Lava flows from the three volcanic complexes, alkaline gabbros (cumulate origin) and syenites (bulk trachyte liquid origin) can be related by polybaric fractional crystallization starting at ~15 km depth; to explain the alternation of mafic and evolved products with time, magma chamber replenishment by primitive melts is needed. In contrast, the origin of the subalkaline gabbros, found for the first time in the Azores, appears to be related to fractionation of highly refractory melts at shallower levels (~3 km).The lack of significant crustal assimilation in most Graciosa samples suggests that the range in Sr-Nd-Pb isotope compositions probably reflects those of a predominantly FOZO-like mantle source. Enriched (EMI- and EMII-type) mantle source components, similar to those of São Miguel and Faial, have also been recognized for the first time in Graciosa.