The Oslo Rift: P-T relations and lithospheric structure

Extrusion temperatures for basaltic lavas in the Permo-Carboniferous Oslo Rift, estimated from whole rock major element compositions, are estimated to be 1270 to 1340°C. This means that magmatism during the Oslo rifting event was not associated with a large temperature anomaly in the underlying upper mantle. Partial melting is believed to be caused by a combination of crustal extension, a weak temperature anomaly in the underlying asthenosphere, and/or high fluid-contents in the mantle source region (“wet-spot”). Petrological and gcochemical data imply that large masses of cumulate rocks were deposited in the deep crust during the Oslo rifting event. The densities and seismic velocities (V_p) of these cumulate rocks are estimated to be 2.8–3.5 g/cm³ and 7.5–8.0 km/s. A rough estimate suggests that cumulus minerals alone account for a net transfer of at least 2 × 10¹⁷ kg of magmatic material from the mantle into the deep crust. In addition comes material representing

1. (a) cumulate minerals corresponding to eroded magmatic surface and subsurface rocks

2. (b) intercumulus material, and

3. (c) magmas crystallized to completion in the deep crust.

Estimates based exclusively on geophysical data tend to underestimate the true transfer of mass into the lower crust as gabbroic cumulate rocks, and melts crystallizing to completion in the lower crust have densities and seismic velocities similar to those of lower crustal wallrocks.