The Oslo Rift: A review

Abstract

The tectonomagmatic history of the Oslo Rift may be subdivided into 5 main periods. Stage 1 (> 300 Ma): development of a shallow depression. Stage 2 (300-295 Ma): roughly simultaneous onset of widespread basalt volcanism (B1) and vertical movement along NNW-SSE- to N-S-trending faults. Stage 3 (295-275 Ma): main rifting period, accompanied by volcanism dominated by rhomb porphyry lavas (latites). Stage 4 (275-240 Ma): change of magmatic style from basaltic shield volcanism to central volcanoes of mixed compositions, many of which went through caldera collapses; intrusion of composite batholiths of intermediate to granitic compositions. Stage 5 (< 240 Ma): dike intrusions may have continued into Triassic time. Both the crust and total lithosphere under the rift are thinned relative to the Precambrian basement on both sides. A 12-km-thick high-density and high-velocity body occupies the lower crust along the rift axis and extends under the eastern and western rift-shoulder. Petrological and geochemical data imply that this massive layer represents dense cumulates and gabbroic rocks formed from mantle-derived magmas in deep crustal magma-chambers. Two mantle sources, one nearly undepleted (age corrected ϵ Nd of about + 1 and ϵ Sr of −10 to −15), the other mildly depleted (ϵ Nd of about + 4, ϵ Sr of −10), contributed melts to the Oslo Rift magmatism. The two sources are interpreted as different parts of a heterogeneous upper mantle which, before the rifting event, belonged to the subcontinental lithosphere. On their way to the surface, the mantle-derived magmas were retained in deep crustal magma chambers Where they underwent extensive fractional crystallization, some also suffered moderate contamination. Anatectic melts, and melts formed by mixing between mantle-derived and anatectic melts, gave rise to syenitic and granitic intrusions.