27. VOLCANIC EVOLUTION OF GRAN CANARIA RECONSTRUCTED FROM APRON SEDIMENTS: SYNTHESIS OF VICAP PROJECT DRILLING 1Hans-Ulrich Schmincke 2 and Mari Sumita 2 |
ABSTRACTThe ages, types, and compositions of volcaniclastic sediments, physical properties, and downhole logs obtained during Leg 157 drilling at Sites 953 and 954 north and 955 and 956 south of Gran Canaria, with a cumulative penetration of almost 3000 m and an overall recovery rate of 75%, correlate well compositionally and stratigraphically to major volcanic and nonvolcanic phases on the island. Volcaniclastic sediment contributions from neighboring islands are volumetrically insignificant. The highest rate of volcaniclastic sedimentation (>150 m/m.y.) corresponds to the middle Miocene basaltic late seamount stage of Gran Canaria, the lowermost deposits drilled being thick, graded, hyaloclastite tuffs and debris flows, dominated by poorly vesiculated, altered sideromelane clasts, suggesting eruption at a water depth of more than 500 m. Large basalt clasts, some of subaerial origin, at the base of the strongly graded deposits suggest synchronous subaerial activity. A lithic-rich debris-flow deposit (Site 956), at least 80 m thick, is interpreted to have been generated by a major collapse of southwestern Gran Canaria, thereby triggering tsunami waves, whose deposits are now represented by 2 m of amphibole, phlogopite, apatite, and Cr-spinel–rich sandstones overlying the debrite. Breccias, lapillistones, and tuffs containing vesicular shards record the transition from shallow submarine to emergent environment. The erosion of the older subaerial shield basalts in eastern Gran Canaria is reflected in hundreds of thin, fine-grained turbidites at Site 953, whereas submarine activity continued in the southwest (Site 956). Nearly identical Nb:Zr and Nb:Y ratios in all submarine and subaerial basalts suggest fairly constant source conditions. A precisely dated (13.9 Ma) rhyolitic to basaltic syn-ignimbrite turbidite separates the basal hyaloclastites at three sites from 50- to 10-m-thick, dominantly rhyolitic volcaniclastic turbidite series correlated to the Mogán Group (14-13.3 Ma) and 130- to 250-m-thick, dominantly trachyphonolitic volcaniclastic turbidite sections correlated to the Fataga Group (13.3-9.5 Ma). Near-unique mineral phases or assemblages, glass, feldspar, amphibole and clinopyroxene, and bulk-rock compositions represent robust criteria for unequivocally correlating at least seven syn-ignimbrite volcaniclastic turbidites between three sites, which are 160 km (Sites 953 and 956) and 170 km (Sites 953 and 955) apart from each other. Syn-ignimbrite turbiditic tuffs and lapillistones are characterized by brown angular glass shards, of strongly welded tuff, interpreted to have been generated when hot ash flows entered the sea. They quickly welded and were fragmented by quench granulation and steam explosions. Synignimbritic depositional units are as much as 2 m thick and many consist of a coarse-grained, massive graded basal part containing shallow-water faunal debris overlain by thin turbidites of fine-grained vitric ash, dominantly reflecting low-density waning turbidites. Broad concentric deposition of turbidites south of the island contrasts with the more channelized sediment transport off northeastern Gran Canaria. Tholeiitic to alkali basaltic, blocky, fresh sideromelane shards in many volcaniclastic turbidites are interpreted to represent the parent magmas to the rhyolites and phonolites and to have been erupted on the lower submarine flanks below the level of the low-density magma column. At all four sites, the accumulation, rate based on integrated bio-and magnetostratigraphy, is at a minimum (<~20 m/m.y.) during the major nonvolcanic phase on Gran Canaria between ~9 and 5 Ma. This clearly indicates that the rate of supply of volcanogenic sediment to the apron closely corresponds in time to major volcanic phases. Major debris avalanches generated by collapse of the Pliocene Roque Nublo stratocone occur as coarse breccias at Holes 953C, 954B, and 956B. Erosion differed significantly throughout the entire 16-m.y. history of the island between the wind-exposed, more strongly eroded northern vs. the dry, leeward southern side of the island, and this is reflected in the higher abundance of epiclastic sediments at Site 953. Pleistocene trachytic to phonolitic ash layers reflect many more explosive phases of the Las Cañadas volcanic complex on the island of Tenerife than those preserved on land. A shift with time toward more silica-undersaturated composition is interpreted as a change toward lower degrees of partial melting. The sediment supply to the sediment basins north and south of the island differed drastically because post-Miocene volcanic activity was almost completely absent in southern Gran Canaria. The sediment supply from Africa was almost completely restricted to the apron south of the island because the east Canary Ridge prevented transport to the west. High alkalinity in the absence of significant organic matter diagenesis and high magnesium and lithium contents in the CO2 -charged pore waters at Site 954, the drill site closest to the island, are explained by the proximity to the belt of Holocene mafic volcanism in northern Gran Canaria. |
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