RIFT RELOCATION - A GEOCHEMICAL AND GEOCHRONOLOGICAL INVESTIGATION OFA PALAEO-RIFT IN NORTHWEST ICELAND

A dominant process in the evolution of Iceland is the repeated eastwar d relocation of the spreading axis in response to westward migration o f the plate boundary relative to the plume centre, Two major former ri fts can be identified in western Iceland: the Snaefellsnes rift zone, which last erupted tholeiitic lavas at about 7 Ma, and an older spread ing system, lava flows from which can be traced some 100 km along a SW -NE strike in the extreme northwest of Iceland. The extinction of the latter is marked by a 14.9 Ma unconformity with a laterite-lignite hor izon representing a maximum 200 ky. hiatus in the lava succession. Lav as below the unconformity dip northwest towards the older axis from wh ich they were erupted, whereas lavas above the unconformity dip southe ast towards their source in the younger Snaefellsnes axis, Thus, two n early complete rift relocation cycles are preserved in western Iceland , each lasting about 8 m.y. as measured between rift extinction events , and for around 12 m.y. from initial propagation to extinction. In th is paper we present major-and trace-element analyses, Sr, Nd and Pb is otope data, and Ar-40/Ar-39 dates on basalt samples from above and bel ow the unconformity in northwest Iceland, The Icelandic Tertiary and Q uaternary plateau basalts are remarkably homogeneous in composition, i n contrast to the much more diverse compositions found in the presentl y active rift zone. However, basaltic lava flows beneath the unconform ity in northwest Iceland show a wider range of incompatible element an d radiogenic isotope ratios than do the younger plateau basalts, At le ast two mantle components, one depleted and the other less depleted wi th respect to bulk Earth, are required to explain the composition of p ost-15 Ma Icelandic basalt. The depleted end-member is chemically and isotopically distinct from the N-MORB source. Basalt from the northwes t palaeo-rift, however, contains a significant North Atlantic N-MORB c omponent, suggesting that depleted upper mantle can influence the comp osition of Icelandic basalt in a dying rift that is too far from the p lume centre to be dominated by plume mantle, This may account for the periods of low magma productivity represented by troughs between the V -shaped ridges on the Reykjanes Ridge. We suggest that temporal variat ion in the composition of Icelandic basalt is better explained by crus tal accretion and rift relocation processes than by variations in plum e composition and temperature. (C) 1997 Elsevier Science B.V.