GEOCHEMISTRY AND PETROLOGY OF SCORIA CONES MONTANA AMARILLA AND MONTANA DE MALPASITO, BANDAS DEL SUR FORMATION, TENERIFE (CANARY ISLANDS)
The Central composite volcano, Las Canadas, violently erupted into a caldera three times (Arana, 1971; Bryan, 1998). The three caldera-forming eruptions around 1.07, 0.57, and 0.17 Ma are separated by erosional unconformities (Bryan, 1998). Lava from the most recent volcanic episode formed the Tiede-Pico Viejo complex, which overlies some slopes of the Las Canadas caldera. This peak has risen to a height of 3718 m (Bryan, 1998).
Southern Tenerife contains outcrops of every stage of the ocean island building events. The best exposed yet understudied outcrops are the scoria cone chains flanking the volcano and trending in many different directions (Bryan, 1998). As they are the most frequent eruptive activity occurring on the island in the last 1 Ma, the understanding of composition and growth of these scoria cones will enlighten us about the past and future development of the Canary Islands (Doniz, 2007).
Samples from two scoria cones, Montana Amarilla and Montana del Malpasito, in the Bandas del Sur region of southeast Tenerife, have been analyzed geochemically and petrographically in order to learn more about how, when and why they were formed. These cones are part of a linear trend that extends north-northeast from the Atlantic Ocean near the Costa de Scilencio. The scoria is tephrite and correlates strongly with the lava from the third cycle of the Las Canadas caldera. These tephrites have higher silica contents and lower CaO, TiO, Fe2O3, and MgO than the basalts from the early stage of cycle three (Bryan, 2002). Bryan (2002) attributes the difference in the cycle three basalts to the appearance of microphenocryst apatite and the removal of olivine. In thin section, the scoria cone samples are lacking olivine and are rich in plagioclase. The chemical analyses of the cycle three basalts and tephrites, compared to earlier and later eruptions, show fractional crystallization between the earlier and later lavas and ignimbrites. While other authors (Wolff, 1999; Gurenko, 2006) support assimilation and/or recycling of crust in the Canary Islands, these processes are not evident in the data from this study or earlier work (Bryan, 2002).