THE OSLO RIFT - P-T RELATIONS AND LITHOSPHERIC STRUCTURE

Extrusion temperatures for basaltic lavas in the Permo-Carboniferous O slo Rift, estimated from whole rock major element compositions, are es timated to be 1270 to 1340 degrees C. This means that magmatism during the Oslo rifting event was not associated with a large temperature an omaly in the underlying upper mantle. Partial melting is believed to b e caused by a combination of crustal extension, a weak temperature ano maly in the underlying asthenosphere, and/or high fluid-contents in th e mantle source region (''wet-spot''). Petrological and geochemical da ta imply that large masses of cumulate rocks were deposited in the dee p crust during the Oslo rifting event. The densities and seismic veloc ities (V-p) of these cumulate rocks are estimated to be 2.8-3.5 g/cm(3 ) and 7.5-8.0 km/s. A rough estimate suggests that cumulus minerals al one account for a net transfer of at least 2 X 10(17) kg of magmatic m aterial from the mantle into the deep crust. In addition comes materia l representing (a) cumulate minerals corresponding to eroded magmatic surface and subsurface rocks, (b) intercumulus material, and (c) magma s crystallized to completion in the deep crust. Estimates based exclus ively on geophysical data tend to underestimate the true transfer of m ass into the lower crust as gabbroic cumulate rocks, and melts crystal lizing to completion in the lower crust have densities and seismic vel ocities similar to those of lower crustal wallrocks.