Gabbroic xenoliths from La Palma, Tenerife and Lanzarote, Canary Islands: evidence for reactions between mafic alkaline Canary Islands melts and old oceanic crust

Gabbroic and homblendite xenoliths from La Palms, Tenerife and Lanzarote fa ll into three main groups based on petrography and chemistry. One group (co mprising all xenoliths from Lanzarote and some from La Palma) consists of h ighly deformed orthopyroxene-bearing gabbroic rocks that show a strong affi nity to N-MORB and oceanic gabbro cumulates in terms of mineral chemistry a nd REE relations. However. they show mild enrichment in the most incompatib le elements (particularly Rb + Ba +/- K) relative to intermediate and heavy REE, and their Sr-Nd isotope ratios fall within or close to the N-MORB hel d. The second group (60% of the xenoliths from La Palma) are gabbroic cumul ates with zoned clinopyroxenes (Ti-Al-poor cores, Ti-Al-rich rims) and reac tion rims of hornblende, biotite and clinopyroxene on other phases. Their t race-element and Sr-Nd isotope relations are in general transitional betwee n N-MORB cumulates and Canary Islands alkali basalts, but they show strong enrichment in Rb +/- Ba +/- K relative to other strongly incompatible eleme nts. The third group (comprising some xenoliths from La Palma and all those from Tenerife) are undeformed gabbroic acid hornblendite rocks in which ho rnblende and biotite appear to belong to the primary assemblage. These rock s show strong affinities to Canary Islands alkali basaltic magmas with resp ect to mineral, trace-element, and St-Nd isotope chemistry. The first two g roups are interpreted as fragments of old oceanic crust which have been mil dly to strongly metasomatized through reactions with Canary Islands alkalin e magmas. The reaction process is a combination of enrichment in elements c ompatible with biotite (and hornblende), and simple mixing between N-MORB c umulates and trapped alkaline magmas. The third group represents intrusions /cumulates formed from mafic alkaline Canary Islands magmas. Modeling indic ates that locally up to 50% new material has been added to the old oceanic crust through reactions with ocean island basalts. Reactions and formation or cumulates do not represent simple underplating at the mantle/crust bound ary, but have taken place within the pre-existing oceanic crust, and are li kely to have significantly thickened the old oceanic crust. (C) 2000 Elsevi er Science B.V. All rights reserved.