Oxygen isotope composition of xenoliths from the oceanic crust and volcanic edifice beneath Gran Canaria (Canary Islands): consequences for crustal contamination of ascending magmas

摘要

Xenolith samples of marine terrigenous sediments and altered Jurassic MORB from Gran Canaria (Canary Islands) represent samples of sub-island oceanic crust. These samples are postulated to define end-members for crustal contamination of basaltic and felsic ocean island magmas. The meta-igneous rocks show great heterogeneity in oxygen isotope compositions (delta(18)O 3.3-8.6 parts per thousand), broadly correlating with their stratigraphic position in the oceanic crust. Gabbros interpreted as fragments of oceanic crust layer 3 have delta(18)O values of 3.3-5.1 parts per thousand, which is lower than MORB (5.7-6.0 parts per thousand). Layer 2 lavas and dykes show a broader range of delta(18)O of 4.1- 8.6 parts per thousand. Therefore, high-temperature metamorphism seems to have been the dominant process in layer 3, while both high- and low-temperature alteration have variably affected layer 2 rocks. Siliciclastic sediments have high delta(18)O values (14.1-16.4 parts per thousand), indicating diagenesis and low-temperature interaction with seawater. The oxygen isotope stratigraphy of the crust beneath Gran Canaria is typical for old oceanic crust and resembles that in ophiolites. The lithologic boundary between older oceanic crust and the igneous core complex at 8-10 km depth-as postulated from geophysical data-probably coincides with a main magma stagnation level. There, the Miocene shield phase magmas interacted with preexisting oceanic crust. We suggest that the range in delta(18)O values (5.2-6.8 parts per thousand) [Chem. Geol. 135 (1997) 233] found for shield basalts on Gran Canaria, and those in some Miocene felsic units (6.0-8.5 parts per thousand), are best explained by assimilation of various amounts and combinations of oceanic and island crustal rocks and do not necessarily reflect mantle source characteristics. (C) 2003 Elsevier Science B.V. All rights reserved. [References: 42]