Abstract

The two main islands of the Juan Fernandez group, some 180 km apart are intra-plate volcanoes related to a single hot spot. K-Ar determinations suggest an age difference of c. 3 Ma which is in accord with a spreading rate of c. 6 cm a−1 in this part of the Nazca plate. The younger island of Alexander Selkirk is a relatively recent volcano (<1 Ma) composed of olivine tholeiites, many of them picrites, together with occasional quartz tholeiites and rare trachytes. Robinson Crusoe is an older, deeply dissected island with several separate volcanic centres. The older lavas are predominantly olivine tholeiites, again often picritic, with an occasional quartz tholeiite flow. Alkali basalt flows, never abundant, become more common in the higher part of the succession. Basanite flows are restricted to younger parasitic centres, but basanite dykes and sills cut the older formations. The tholeiites of Alexander Selkirk have a low pressure phenocryst assemblage and their low Ba contents, low Nb/Zr ratios and REE patterns suggest higher degrees of mantle partial melting than their closest counterparts on Robinson Crusoe. The basanites of the latter island have a higher pressure phenocryst assemblage and have higher La/Sm ratios than the tholeiitic lavas. The basanites are not always distinct from the Robinson Crusoe tholeiites on the basis of incompatible trace elements. 86Sr/87Sr ratios are fairly uniform (c. 0.7035) in all the rock types but 143Nd/144Nd ratios are more variable, attaining their lowest values in some of the basanites. Trace element and isotopic data suggest remelting of source regions modified by migration of small degree partial melts over a long period.

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