STRATIGRAPHY OF THE BANDAS-DEL-SUR FORMATION - AN EXTRACALDERA RECORDOF QUATERNARY PHONOLITIC EXPLOSIVE ERUPTIONS FROM THE LAS-CANADAS EDIFICE, TENERIFE (CANARY-ISLANDS)

Explosive volcanism has dominated the large phonolitic shield volcano of Tenerife, the Las Canadas edifice, for the last 1.5 m.y. Pyroclasti c deposits of the Pandas del Sur Formation are exposed along the south ern flanks, and record the last two of at least three long-term cycles of caldera-forming explosive eruptions. Each cycle began with flank f issure eruptions of alkali basalt lava, followed by minor eruptions of basanite to phonotephrite lavas. Minor phonotephritic to phonolitic l ava effusions also occurred on the flanks of the edifice during the la tter stages of the second explosive cycle. Nonwelded plinian fall depo sits and ignimbrites are the dominant explosive products preserved on the southern flanks. Of these, a significant volume has been dispersed offshore. Many pyroclastic units of the second explosive cycle exhibi t compositional zonation. Banded pumice occurs in most units of the th ird (youngest) explosive cycle, and ignimbrites typically contain mixe d phenocryst assemblages, indicating the role of magma mixing/mingling prior to eruption. At least four major eruptions of the third cycle b egan with phreatomagmatic activity, producing lithic-poor, accretionar y lapilli-bearing fallout and/or surge deposits. The repeated, brief p hase of phreatomagmatism at the onset of these eruptions is interprete d as reflecting an exhaustive water supply, probably a small caldera l ake that was periodically established during the third cycle. Accident al syenite becomes an increasingly important lithic clast type in igni mbrites up-sequence, and is interpreted as recording the progressive d evelopment of a plutonic complex beneath the summit caldera. Successiv e eruptions during each explosive cycle increased in volume, with the largest eruption occurring at the end of the cycle. More than ten majo r explosive eruptions vented moderately large volumes (1-greater than or equal to 10 km(3)) of phonolitic magma during the last two cycles. Culminating each explosive cycle was the emplacement of relatively lar ge volume (> 5-10 km(3)) ignimbrites with coarse, vent-derived lithic breccias, interpreted to record a major phase of caldera collapse. In the extracaldera record, explosive cycles are separated by similar to 0.2 m.y. periods of non-explosive activity. Repose periods were charac terized by erosion, remobilization of pyroclastic deposits by discharg e events, and pedogenesis. The current period of non-explosive activit y is characterized by the construction of the Teide-Pico Viejo stratov olcanic complex within the summit caldera. This suggests that eruptive hiatuses in the extracaldera record may reflect effusive activity and stratovolcano or shield-building phases within the summit caldera. Al ternating effusive and explosive cycles have thus been important in th e volcanic evolution of the Las Canadas edifice.