Lajes-Angra Ignimbrite (LAI) is the most recent (around 21 ka) caldera-forming event produced by Pico Alto volcano at Terceira Island (Azores). Lajes-Angra Ignimbrite Formation comprises two members closely spaced in time: Lajes and Angra. The Lajes member, the most widely distributed throughout the island, was sampled at two sites: 1) at Lajes (type location), in the northern part of the island, where the succession is characterized by a thin crystal-rich basal layer (TERS 17.1), overlaid by a fine-grained welded lapilli-tuff layer and an upper partially welded coarse clast-bearing layer (TERS 17.2); 2) at São Mateus, in the south coast, where the upper layer of the succession is also partially welded (TERS 62.1). Juvenile clasts are comenditic-trachyte in composition and coarsely porphyritic, ranging from highly vesicular pumice to dense vitrophyric clasts. Mineral assemblage mainly consists of anorthoclase (Ab64-70, An<7), and less abundant olivine (Fo28-45), clinopyroxene (En60-80Fs20-40), ilmenite and magnetite. Phenocrysts of anorthoclase range in size from <2mm in the crystal-rich basal layer to 4-5 mm in the partially welded upper layer. We found that, except for the crystals from the basal layer, in the other two samples, large crystals of anorthoclase include pockets of highly vesicular melts (fig.1). Large crystals with similar textures are quite common in trachytic magmas at Pantelleria and in other silicic magmas. In addition to vesicular glass, these crystals also include vesicles not rimmed by glass. The borders of glassy pockets can be both smooth and angular, and usually marked by a line of Fe-rich melt. This melt is enriched in elements that are not admitted into anorthoclase (Ca, Fe, Mg, Ti) and depleted in Si, Al, K and to a lesser extent in Na. The vesicular glass in the pockets is a comendite-trachyte with composition similar to that of the matrix glass. Despite the large phenocrysts appear compositionally homogeneous at SEM, the cathodoluminescence imaging spectroscopy reveals complex growing textures (fig.2). Crystals are patchy-zoned, with resorbed cores and inclusions-free rims (100-300 micron in size). Preliminary LAM-ICP-MS analyses of anorthoclases sampled from base to top of the eruptive succession also highlight wide trace-element compositional variations, which is also evident at the scale of the single crystal. Our interest is to understand HOW and WHY the large glass pockets-bearing anorthoclases were formed and if they were related to dissolution or rapid growth, during pre- or syn-eruption degassing processes. Textural and geochemical data are used here to obtain information on: i) the evolution of the shallow magma reservoir; ii) relation between magma and crystal mush; iii) syn-eruptive processes of crystallization-induced degassing. Results are discussed in terms of growth-time of crystals and magma dynamics immediately before or during the eruptive event.