18. GEOCHEMISTRY AND PETROLOGY OF PLEISTOCENE ASH LAYERS ERUPTEDAT LAS CAŅADAS EDIFICE (TENERIFE)1Uta Rodehorst,2, 3 Hans-Ulrich Schmincke,2 and Mari Sumita 2 |
ABSTRACTSixty-eight fallout trachytic to phonolitic ash layers recovered from Sites 953, 954, and 956 during Leg 157 are interpreted as a result of explosive eruptions of the Las Caņadas edifice on Tenerife. Their biostratigraphic ages range from 0.3 to 3.8 Ma. Tephra layers consist dominantly of highly vesicular pumice and 1–10 vol% phenocrysts. Alkali feldspar occurs in almost all layers; but plagioclase, clinopyroxene, amphibole, phlogopite, sphene, Fe-Ti oxides, and hauyne appear in some. Nepheline, hauyne and sodalite occur together in one of the younger (0.4 Ma) layers at Site 953 (Sample 157-953A-2H-5, 148–150 cm). The chemical composition of glass shards, analyzed by electron microprobe in 20 samples changes from trachytic to phonolitic in the time interval between 2 and 0.4 Ma. The concentration of trace elements (Ce, La, Mn, Fe, and Zr) in sphene, analyzed in four layers, is variable. Bulk ash analyses by X-ray fluorescence and inductive coupled plasma mass spectrometry show similar major and trace element patterns for all tephra layers. Although eruptive gaps have been postulated from distribution of volcanic rocks on land, the stratigraphic distribution of submarine ash layers suggests that explosive volcanic activity of the Caņadas volcano on Tenerife was fairly continuous. None of the noneruptive hiatuses postulated between 0.37 and 0.65 Ma, 0.85 and 1.2 Ma and 1.6 and 2.0 Ma in the Caņadas series on land were confirmed. Apparently fallout ash layers in the marine environment provide a more reliable record of volcanic activity on land. |
|