Shallow submarine to emergent basaltic shield volcanism of Gran Canaria: Evidence from drilling into the volcanic apron

Abstract

Almost 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.