12. SHALLOW SUBMARINE TO EMERGENT BASALTIC SHIELD VOLCANISM OF GRAN CANARIA: EVIDENCE FROM DRILLING INTO THE VOLCANIC APRON 1Hans-Ulrich Schmincke 2 and Beate Segschneider 2 |
ABSTRACTAlmost 500 m of basaltic hyaloclastite tuffs, hyaloclastite lapillistones, and lithic breccias were drilled in the northern, southeastern, and southwestern flank of Gran Canaria, Canary Islands during Leg 157 (Hole 953C, total penetration 1159 meters below seafloor [mbsf], basal 293 m; Hole 954B, total penetration 446 mbsf, basal 38 m; and Hole 956B, total penetration 704 mbsf, basal 140 m). These deposits represent (1) mostly moderate to shallow water (<< ~500 m) eruptions, (2) transition to the emergence, and (3) the fully subaerial island shield stage. The volcaniclastic rocks are interlayered with minor thin layers of nannofossil ooze and clay. Volcanic clasts comprise blocky to vesicular, generally altered former sideromelane shards, tachylite crystallized basalt, round glassy shards, lapilli, and single crystals, chiefly titanaugite. Dominantly filled foraminifers, thick-walled shallow-water skeletal debris, and nannofossil ooze make up <5 vol% of the volcaniclastic rocks. Most of the basaltic volcaniclastic deposits are interpreted to have been deposited as debris flows resulting from (1) destabilization of hyaloclastites generated during voluminous moderate (<500 m?) to shallow-water explosive volcanic activity and temporarily accumulated prior to episodic failure and transfer to the deep basins, fragmentation of subaerial lava flows that entered the sea and collapse of lava deltas and by flank collapse. About 16 debris-flow units (lithologic Unit VII) in Hole 953C range in thickness from ~1 to 50 m. Most are composed of well-sorted massive lapillistone to coarse hyaloclastite tuff consisting of blocky, poorly vesicular shards, minor tachylite, and crystallized basalt. The top 5%–10% or so show laminar bedding to minor cross-bedding, the grain size rarely decreasing to fine sand (ash) size in the top beds. Basalt clasts up to 25 cm in diameter are common in the coarse-grained basal parts. Most particles in the stratigraphically highest deposits are vesicular to highly vesicular ash to lapilli-size clasts suggesting decreasing water depth. Coarse breccias at Hole 953C (lithologic Unit VI) consist of basalt clasts of diverse composition, angularity, and vesicularity, and some contain pillow rind fragments. Only the upper of three debrites at Site 956 (Cores 157-956B-43R through 45R) and underlying turbidites consist dominantly of highly vesicular formerly glassy ash to lapilli-sized clasts. Lithic-rich debris-flow deposits at Site 956 (Cores 157-956B-45R through 48R, and 49R through 57R) consist chiefly of poorly vesicular, angular tachylite, crystallized basalt, and minor formerly glassy shards set in ~30-50 vol% brown clay matrix. About 300 very thin turbidite beds, 1-40 cm thick, deposited prior to the first ignimbrite-related ash deposit at Hole 953C are composed of variable amounts of dominantly silt- to sand-sized tachylitic and lesser amounts of vesicular to blocky altered shards and minor biogenic debris. They are interpreted to represent chiefly the subaerial growth stage of the basaltic shield and to have been derived dominantly from erosionally fragmented scoria and lava flows. The phenocryst assemblage in clasts and matrix of all deposits, mainly titanaugite and olivine (Fo83-88 ) and minor plagioclase, changes with depth at both sites. Most of the submarine basaltic clasts and clastic rocks from Hole 953C are more primitive mineralogically and chemically than the subaerial shield stage basalts. Ratios of incompatible trace elements are practically indistinguishable between mafic and moderately evolved rocks, between holes and between the bulk volcaniclastic rocks, basalt clasts, and subaerial rock shield basalts, suggesting that the source for the basalt magmas was fairly homogeneous during the late submarine and subaerial evolution. More evolved compositions (plagioclase-phyric hawaiites) are restricted to Site 956 coincident with the eruption of late-stage evolved subaerial shield lavas in southwestern Gran Canaria. Fresh glass in the center of large lapilli in the uppermost of three debris flows (Core 157-956B-44R) is intermediate in composition, MgO ranging from 3.9 to 4.9 wt%. Major and trace element concentrations of some 101 hyaloclastite bulk rocks and 20 clasts document major mobile element transfer. Basaltic glass is generally replaced by smectite. Zeolite phases, mostly phillipsite, and minor carbonate, are common pore-filling phases. The vertical and lateral growth and changes in eruptive activity of the shield volcano are well reflected in the lithologic and compositional contrasts within and between the sections drilled at Hole 953C and Hole 956B (45 km southwest of Gran Canaria). Decreasing water depth of eruption is especially well documented in the increase in vesicularity in shards at Site 953 and extremely vesicular shards near the top of the basaltic section of Site 956. Eruptive activity had clearly decreased in the east of Gran Canaria during deposition of the late-stage turbidites at Hole 953C, as shown by the dominance of epiclastic particles in these rocks. Cores 157-956B-44R and 45R of the basaltic shield sequence at Hole 956B are considered to record younger active volcanism in western Gran Canaria that was taking place while largely epiclastic material was supplied to Site 953 in the north-east. A recovered 3.75-m-thick interval consisting of layers of an unusual sandstone interbedded with turbidites of mixed volca-niclastic-biogenic lithology, on top of the basal 85-m-thick lithoclast-rich debrite in the lower part of Hole 956B (interval 157-956B-48R-3, 23 cm, to 49R-1, 26 cm), consists dominantly of alkali amphibole, partially chloritized phlogopite and apatite, and minor Cr-spinel, sphene, and zircon mixed with foraminifers. We speculate that the huge basal debrite, underlying these sands and which we interpret as having been formed by collapse of the flanks of southwestern Gran Canaria, has caused major tsunamis that washed up beach sands on La Gomera, ~115 km west of Gran Canaria. |
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