|
Mantle and Magmatic Processes |
Center for Volcanology, University of Oregon, Eugene, Oregon, 97403 USA
Virtually all the MORB-like tholeiites and differentiated rocks of the Galapagos Islands are concentrated along the central axis of the archipelago within 100 km of its leading island. Both have a remarkable symmetry. Those close to the centre are strongly tholeiitic, but towards the north and south they become increasingly alkaline.
Most of the differentiated suites are readily explained by crystal fractionation at shallow depths, but voluminous, aphyric rhyolites erupted from a large active volcano immediately downstream from the hotspot are more difficult to explain. Although the geochemical criteria are consistent with crystal fractionation, these extreme differentiates are not accompanied by expected amounts of intermediate compositions, such as those found on older islands; instead, they have been erupted concurrently with basalts that have undergone only minor amounts of differentiation.
The central region in which tholeiitic rocks and their differentiates have been erupted lies west of a transform fault that separates 10 million-year-old lithosphere on the west from thinner 5 million-year-old lithosphere on the east. It is also the region of thickest crust: the MOHO reaches a maximum depth of about 18 km below sea-level directly underneath the same region.
Three possible mechanisms of differentiation have been considered for the voluminous rhyolites: (a) inward crystallization of a large reservoir; (b) side-wall crystallization with convective segregation of a light felsic liquid; and (c) melting of the base of the crust. Although the first two are consistent with the geochemical nature of the rhyolites, they are not compatible with the geological relations. Moreover, they do not explain why such rocks are found only in very restricted settings. In the case of the Galapagos, these extreme differentiates are found only in the region where the crust reaches a maximum thickness near the centre of the Galapagos Platform and directly downstream from the hotspot. On a global scale, oceanic rhyolites are found close to spreading ridges and nowhere else.
These relations indicate that oceanic rhyolites are generated only where the lithospheric mantle is thin enough to permit the thermal effect of hotspots to reach the base of unusually thick crust. Remelting of pockets of felsic differentiates produced by earlier differentiation when the gabbroic layer was close to the ridge would result in trace element compositions consistent with crystal fractionation.
