Journal of Petrology, Volume 39, Issue 5: May 1 1998.

Geochemistry of Jurassic Oceanic Crust beneath Gran Canaria (Canary Islands): Implications for Crustal Recycling and Assimilation

KAJ HOERNLE

DEPARTMENT OF VOLCANOLOGY AND PETROLOGY, GEOMAR, WISCHHOFSTR. 1-3, D24148 KIEL, GERMANY

Trace elements and Sr-Nd-Pb isotopes have been analyzed on sedimentary and igneous (metabasalt, metadiorite and metagabbro) samples from the Jurassic oceanic crust beneath Gran Canaria (Canary Islands). The igneous crust exhibits extreme heterogeneity in ⁸⁷Sr/⁸⁶Sr (0·7029-0·7052), ²⁰⁶Pb/²⁰⁴Pb (18·2-20·8) and ²⁰⁸Pb/²⁰⁴Pb (38·1-41·3). Leaching experiments indicate that seawater alteration has elevated the ⁸⁷Sr/⁸⁶Sr ratio but has not appreciably affected ¹⁴³Nd/¹⁴⁴Nd (0·51295-0·51306). An Sm-Nd isochron gives an age of 178 ± 17 Ma, which agrees with the age predicted from paleomagnetic data. Hydrothermal alteration near the ridge axis has increased ²⁰⁷Pb/²⁰⁴Pb (15·59-15·73), ²⁰⁸Pb/²⁰⁴Pb (as well as [Delta]7/4Pb and [Delta]8/4Pb), ²³⁸U/²⁰⁴Pb (µ) and Ce/Pb but has not appreciably changed ²⁰⁶Pb/²⁰⁴Pb. The large range in ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb reflects radiogenic ingrowth with µ being as high as 107. Portions of the Jurassic ocean crust have trace element and isotope characteristics within the range found at St Helena, the Atlantic type locality for the HIMU (high µ) mantle end-member. Evaluation of the published isotopic data for Gran Canaria volcanic rocks indicates that the isotopic composition of these melts primarily represents the composition of their mantle sources rather than crustal assimilation.

Keywords: Canary Islands; oceanic crustal recycling; seafloor hydrothermal alteration-metamorphism; HIMU; crustal assimilation in ocean island basalt

Pages 859-880

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