The 40Ar‐39Ar analyses of 28 groundmass separates from volcanic rocks from the islands of Santiago, Sal, and São Vicente, Cape Verde archipelago, are presented. The new age data record the volcanic evolution for Santiago from 4.6 to 0.7 Ma, for Sal from around 15 to 1.1 Ma, and for São Vicente from 6.6 to 0.3 Ma. The major submarine constructional phase of Santiago was erupted within a few hundred thousand years interval around 4.6 Ma. Most of the subaerial Santiago volcanic rocks were erupted in a second episode from 3.3 to 2.2 Ma and late volcanism occurred at 1.1–0.7 Ma. Volcanism on Sal evolved in five stages: (1) poorly constrained early Miocene activity, (2) 16–14 Ma, (3) 12–8 Ma, (4) around 5.4 Ma, and (5) 1.1–0.6 Ma. São Vicente was constructed during three active periods: (1) >6.6–5.9 Ma, (2) 4.7–4.5 Ma, and (3) ∼0.3 Ma. Sr isotope analyses of carbonates from Maio confirm an Early Cretaceous age for limestones deposited on the seafloor and later uplifted. The Cape Verde Rise is indicated to have fully formed in the early Miocene around 22 Ma, accompanied by the initial alkaline volcanism. Considerable volcanism on Sal, Boa Vista, and Maio took place in the Miocene and Pliocene and extended over much larger areas than the present islands, whereas volcanism of the southwestern and northwestern island groups developed mainly during the Pliocene and Pleistocene and was mostly confined to the present island areas. The periods of volcanic activity may be broadly correlated between the northwestern and southwestern groups of islands. Young volcanism (0.3–0.1 Ma) throughout the northwestern group extends along a 150 km long NW‐SE trending lineament. A relatively moderate average melting rate for the hot spot over the 22 Ma period is estimated at ∼0.026 km3/a, corresponding to a total volume of 570 × 103 km3 of magma emplaced in the crust and a mantle volume flux of 28 m3/s, much lower than Iceland or Hawaii. The archipelago is situated to the south and SW of the center of the mantle plume anomaly and ahead of its relative movement. The timing and location of volcanism suggest that mantle melting takes place in three channels, an eastern one that has been active for 22 Ma and in southwestern and northwestern channels since late Miocene.

No abstract

The Bjerkreim-Sokndal Layered Intrusion is a large (~230 km 2 ), discordant, Late Protero-zoic, post-orogenic pluton in the Egersund-Farsund Igneous Province. The intrusion was em-placed shortly after massif-type anorthosite plutons and is cut by jotunite dykes. It contains a >7000 m thick Layered Series consisting of rocks belonging to the anorthosite kindred: andes-ine anorthosite, leuconorite, troctolite, norite, gabbronorite, mangerite, and quartz mangerite. Cumulates in the Layered Series are organized in 6 megacyclic units (MCU 0 to IV), individually up to 1800 m thick, but varying considerably in thickness and development along strike. The highest-temperature cumulates are troctolites containing plagioclase of ~An 54 and olivine of ~Fo 77 · Phase contacts in the macrocyclic units reflect crystallization of the silicate minerals in the order plagioclase (± olivine), orthopyroxene, Ca-rich pyroxene, pigeonite. Il-menite crystallized early and apatite appeared as a cumulus mineral at about the same time as Ca-rich pyroxene. Cumulus magnetite followed orthopyroxene and preceded Ca-rich pyroxene in MCU III and ΓV, but crystallized after Ca-rich pyroxene in MCU IB. MCUs 0, IA and II do not contain cumulates with cumulus magnetite or Ca-rich pyroxene. Olivine (~Fo 50 ) reappears in the uppermost part of the Layered Series where there is a rapid stratigraphic transition to mangerite and quartz mangerite. The basal parts of MCUs III and ΓV are characterized by thin sequences of plagioclase, plagioclaseorthopyroxene-ilmenite and orthopyroxene-ilmenite cumulates in which there are systematic upward decreases in initial Sr isotope ratios. They are overlain by troctolite (plagioclase-olivine cumulate) and are believed to have crystallized from hybrid magmas. The MCUs, the discordant geometry of phase contacts, the stratigraphic variations in initial 87 Sr/ 86 Sr ratio (0.7049-0.7085), and the abundance of xenoliths suggest crystallization of the cumulates at the base of a periodically-replenished, compositionally-zoned magma chamber that was continually assimilating country rocks. The parent, as indicated by medium-grained jotunite along country-rock contacts, appears to have been an evolved, Ti-rich magma similar to ferrobasalt, but poor in diopside components. Systematic stratigraphic variations in initial 87 Sr/ 86 Sr ratio at the base of MCU IΠ and MCU IV suggest that influx of magma into the chamber was accompanied by mixing with resident, contaminated magma.

No abstract

No abstract