103 macla nº 9. septiembre ‘08 revista de la sociedad española de mineralogía Composition of Gabbroic Xenoliths in Flores Island (Azores, Portugal) / ZILDA FRANÇA (1), MARCELIANO LAGO (2,*), CARLOS GALÉ (2), TERESA UBIDE (2), ELISABETH WIDOM (3), ENRIQUE ARRANZ (2), VICTOR HUGO FORJAZ (1) (1) Departamento de Geociências, Universidade dos Açores. Ap. 1422, 9501-801, Ponta Delgada (Portugal) (2) Departamento de Ciencias de la Tierra. Ciudad Universitaria. Universidad de Zaragoza. C/ Pedro Cerbuna 12. 50009, Zaragoza (España) (3) Departament of Geology, Miami University, Oxford, 45056, Ohio (USA) INTRODUCTION. The western group of the Azores Archipielago includes the Flores and Corvo islands. Both are aligned north- south within the North American Plate. This alignment is parallel to the M.A.R. (Middle Atlantic Ridge) and nearly perpendicular to the general alignment of the other Azores islands (Fig. 1). fig 1. Location of Flores Island within the Azores Archipielago (Azevedo et al., 2006). The Flores Island is formed by volcanic basic rocks (basalts and hawaiites) whereas evolved rocks are minoritary. Azevedo et al. (1986) group the volcanic lavas and deposits into two major volcanic complexes: • Basal Volcanic Complex (BVC): includes products and structures from both submarine and subarerial volcanism. This complex is formed by pyroclastic deposits and interbedded flows of alkali basalts (Azevedo, 1999). • Upper Volcanic Complex (UVC): represents the main subaerial activity of the island. Includes three main volcanostratigraphic units. Basaltic to trachytic lava flows with interbedded pyroclastic deposits form the first two units. The most recent products are exclusively pyroclastic. The main structural elements of the island are N30 – 40ºW strike-slip and N20 – 30ºE trending normal faults. These structures are likely related to the M.A.R. and associated transform faults. In this work we present the first compositional data on the basic xenoliths included within the basalts of UVC. PETROLOGY OF XENOLITHS. Xenoliths were sampled from the base of the second unit of the UVC (Fig. 2). Six representative xenoliths were selected from a group of 21, for electron microprobe analysis. The analyses were carried out with a JEOL JXA-8900M electronic microprobe of the UCM. Mineral formulae were calculated according to the IMA recommendations. The Fe 3+ contents of anhydrous minerals were estimated applying the Droop (1987) algorithm. fig 2. Geological map of Flores Island and location of sampling zone (modified from Azevedo et al., 2006). The xenoliths have rounded shapes and sharp contacts. Lava permeations within the xenoliths were observed in some of them. According to their petrological and mineralogical composition, two groups of basic xenoliths can be defined: • Olivine bearing gabbros. • Olivine bearing gabbronorites. Olivine bearing gabbros. These xenoliths are fine to medium- grained phaneritic rocks and show a hypidiomorphic granular and seriate texture. Their mineral assemblage consists of olivine (7-25%), clinopyroxene (35-60%), plagioclase (20- 30%) and Fe-Ti oxides (∼1%) and indicates a composition of olivine- bearing gabbros. Some samples have alkaline amphibole (∼7%) and accesory apatite. Different types of discontinuous mineral zoning are recognized in the crystals. Olivine usually is altered to opaque minerals and iddingsite. When unaltered, displays a wide fractionation range (Fo88 - Fo69) and low MnO (0.2– 0,55 %) and CaO (0.2–0.45 %) contents. The more magnesian compositions have up to 0.3 % of NiO. Pyroxene is titanian diopside and ranges from Fs5 to Fs13 (mg#: 0.9–0.7). Their TiO2 content rises until 2.3% in the ferrosilite-rich terms. TiO2, Al2O3, FeO and MgO show an oscillatory zoning. The Ti, Ca and Na (per formula unit – p.f.u.) content of these clinopyroxenes is typical from alkaline magmatic assemblages (Fig. 3). Plagioclase ranges from bytownite to andesine (An86–An53). In detail, three compositional groups of plagioclase are palabras clave: Gabro, Enclaves, Azores. key words: Gabbro, Enclaves, Azores. resumen SEM/SEA 2008 * corresponding author: mlago@unizar.es