The Pliocene seamount series of La Palma/Canary Islands

Abstract

A Pliocene submarine series of alkali basaltic pillow lavas, hyaloclastites, and breccias (A), a sheeted dike swarm (B), and a basal suite of gabbro and ultramafic rocks (C) from La Palma (Canary Islands) is interpreted as a cross section through an uplifted seamount. This series has been tilted to its present orientation of 50°/230° (plunge and azimuth), probably by upwarping due to intrusions in the central portion of the island. The basal plutonic complex (C) also includes intrusives coeval with up to 2000 m of younger subaerial alkali basaltic lavas unconformably overlying the submarine series. The plutonic suite (C) is overlain abruptly by more than 1800 m of sills (B), 0.4–1 m thick on average, with minor screens of lavas and breccias. Extrusives (A) form a 1750 m thick sequence of pillow lavas, breccias, and hyaloclastites. The clastic rocks increase in abundance upward and are of four main types: (1) breccias, consisting of partly broken pillows, formed nearly in situ, (2) heterolithologic pillow fragment breccias, (3) hyaloclastites composed dominantly of highly vesicular lapilli and ash sized shards, the latter thought to have formed by near surface explosive eruptions and been subsequently transported downslope by mass flows, (2) and (3) being interpreted to have been resedimented, and (4) pillow scoria breccias from the upper 700 m of the extrusive section consisting of amoeboidal, highly vesicular “pillows” and lava stringers and local bombs, probably formed by cracking and “bleeding” of gas‐rich expanding pillow lava and some shallow submarine/subaerial lava fountaining. The extrusive series is chemically and mineralogically crudely zoned, with the most differentiated rocks (metatrachytes and mugearites) at the base and most picritic lavas occurring near the top of the series. Subsequent to emplacement, the entire extrusive and intrusive series has been hydrothermally altered, the lower part to greenschist and the upper part to smectite—zeolite facies mineral assemblages. The La Palma succession, combined with evidence from surface studies of seamounts, suggests that seamounts are formed by intrusive and extrusive processes in approximately equal portions. The nature of eruptive clastic and depositional mechanisms changes drastically during growth of a seamount if the critical depth for major magmatic degassing is surpassed and especially if magmatic explosive processes can occur at very shallow water depth, the critical depth depending on magma and thus volatile composition. Changes in slopes of a seamount influence depositional processes. Based on these factors, at least three major depositional sites develop as a seamount grows: summit, flank, and apron facies. Nonexplosive, extrusive processes prevail in the Deep Water Stage, dominantly producing pillow lavas (75%). These consist of individual pillow volcanoes up to 200 m high, with large pillows near the base and decreasing pillow size toward the top of a volcano. Pillow breccias, and pillow fragment breccias comprise approximately 20% of this facies. The deep water flank and apron facies are characterized by debris flow deposits with possibjy rather dense matrix material. The Shallow Water (shoaling) Stage is reached when the seamount top reaches the critical depth for drastic increase in exsolution of magmatic volatiles, about 800 m for the alkali basaltic seamount of La Palma, resulting in formation of mainly clastic rocks (70%): in situ pillow rind breccias and scoriaceous amoeboidal breccias and pillows are formed in the summit regions of the seamount, by repeated expansion and leaking of frothy pillow lava possibly by lava fountaining. Resedimented, heterolithologic pillow fragment breccias, lapilli breccias, and hyaloclastites are deposited on the flanks and aprons of the seamount. Pillow lavas comprise < 30% of these deposits.