Magmatic Processes and the Role of Antecrysts in the Genesis of Corvo Island (Azores Archipelago, Portugal)

Abstract

Corvo island is one of two Azorean islands located to the west of the Mid-Atlantic Ridge. Its small size and the existence of a single main eruptive center make it an ideal island for an in-depth study of the magmatic processes controlling its evolution. This study presents detailed petrographic and geochemical data for representative samples of the complete volcanostratigraphic sequence (Pre-, Synand Post-caldera stages), including lava flows, dikes and cumulate gabbroic xenoliths hosted in a Pre-caldera lava flow. Some lava flows and dikes contain large crystals that are not in equilibrium with the host magma. These crystals are antecrysts crystallized from progenitor magmas that have been reincorporated into the host lava before eruption. We note for the first time that the accumulation of antecrysts in Azorean magmas strongly affects the bulk composition of the rocks. Antecryst contents as high as 50^60% can result in whole-rocks with MgO contents of 15 wt %. This accumulation process obscures the magmatic processes responsible for the differentiation of the volcanic products. Accordingly, only antecryst-free samples are considered to identify the magmatic processes involved in the generation of the island. The least evolved antecryst-free rock was used as the closest composition to the primary magma for modeling of major element (MELTS) and trace element variations; our results establish a petrogenetic relationship between Corvo antecryst-free lava flows, dikes and cumulate xenoliths, all of which are products of polybaric fractional crystallization. Pre- and Post-caldera products are related to an � 15 km deep magma chamber, whereas the Syn-caldera evolved products are related to a shallower magma chamber. These results suggest that Corvo island developed from a complex magmatic plumbing system in which magmas and their crystal cargo, derived from variable depths, have interacted and been tapped throughout the evolution of the island.