J. Richard Wilson

The Bjerkreim-Sokndal Layered Intrusion, Southwest Norway

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, plagioclase- orthopyroxene-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.

An 40Ar‐39Ar study of the Cape Verde hot spot: Temporal evolution in a semistationary plate environment

The 40 Ar- 39 Ar analyses of 28 groundmass separates from volcanic rocks from the islands of Santiago, Sal, and Sao 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 Sao 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. Sao 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 km 3 /a, corresponding to a total volume of 570 x 103 km 3 of magma emplaced in the crust and a mantle volume flux of 28 m 3 /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.

Cooling and crystallization at an inclined plane

Experiments have been carried out on the cooling and crystallization along an inclined plane of aqueous solutions of Na2CO3 in order to investigate convective effects and the development of compositional zonation. The formation of crystals of Na2CO3.10H20 causes adjacent fluid to become relatively light because it is depleted in Na2CO3. When crystallization occurs at an inclined roof the behaviour is similar to previous results for crystallization and the release of light fluid at a vertical boundary. A compositional boundary layer of light residual fluid flows along the sloping roof and forms an upper region of strong chemical stratification. This gradient region increases in depth with time and is separated by a sharp interface from a region of homogeneous solution below. Eventually the whole depth of fluid becomes chemically stratified. When crystallization occurs along an inclined floor thin plumes of light residual fluid rise vertically from the slope with strong compositional boundary layer flows up the slope. The combination of this boundary flow and the horizontal variation in the length of the plumes leads to development of compositional stratification throughout the solution. A steep gradient develops at the top which grades down into a weaker gradient in the central and lower region. The shape and magnitude of the chemical gradients depend on the geometry of the tank adjacent to the slope. Stronger compositional gradients are developed in tanks with larger ratios of width to depth. The experiments suggest that compositional zoning can develop in magma chambers by crystallization along an inclined floor even if the angle is quite small. Differentiated residual magma should move laterally up the slope and margins to accumulate at the roof. We suggest that this process was the main cause of strong compositional zoning in the sill-shaped Hyllingen Series, part of the Caledonian Fongen-Hyllingen layered intrusion, Norway, where crystallization occurred entirely along an inclined floor.

Helium, argon and lead isotopic composition of volcanics from Santo Antão and Fogo, Cape Verde Islands

Abstract Helium, argon and lead isotopic ratios have been determined for volcanics from two of the youngest Cape Verde Islands, Santo Antao and Fogo. Helium isotopic ratios range from radiogenic 4He/3He values of 224,000 (3.2 R/Ra) to more primitive values of 52,000 (13.8 R/Ra), which suggest a contribution from different reservoirs to the magmatism at the Cape Verde Islands. 40Ar/36Ar isotopic ratios range from slightly higher values than atmospheric up to around 1250. Pb isotopic ratios are relatively radiogenic: 206Pb/204Pb=18.90–19.63, 207Pb/204Pb=15.526–15.621 and 208Pb/204Pb=38.694–39.272, with volcanics from Santo Antao being the most radiogenic. Several factors like high 4He/3He ratios, relatively radiogenic Pb isotopes, high Ce/Pb, Zr/Hf, low Ba/La, LILE/Nb and low 40Ar/36Ar indicate a significant contribution from a HIMU-type source. A lower mantle contribution is indicated from the low 4He/3He ratios in some samples. Moreover, relatively low 40Ar/36Ar ratios, low 4He/3He ratios, negative Δ7/4 Pb values and HIMU-characteristic trace element ratios preclude the DMM as a significant contributor to the Cape Verde magmatism. The HIMU component is concluded to represent subducted oceanic crust recycled to a boundary layer separating the lower mantle from the upper degassed mantle, from where new plumes originate. The observed isotopic variation can be explained by a lower mantle contribution to the recycled component either through rising of small plumes, entrainment of lower mantle material due to instabilities in the boundary layer or He migration from the lower mantle.

Crystallization processes in the Bjerkreim-Sokndal layered intrusion, south Norway: evidence from the boundary between two macrocyclic units

The Bjerkreim-Sokndal (BKSK) layered intrusion belongs to the Rogaland anorthosite province in southern Norway. The northwestern part of BKSK consists of a ca. 6 km-thick Layered Series, made up of macrocyclic units (MCU) arranged in a syncline. Each MCU, which resulted from the crystallization of a major-magma influx, can be subdivided into a series of cumulate zones. The MCU III/IV boundary has been studied in seven profiles across its strike length of 24 km. Massive piC1 at the base of MCU IV overlies laminated and modally layered phimC in the central part of the chamber and phimacC towards the flanks; there is a discordance of between 2 and 6° between the base of MCU IV and phase layering in MCU III. The MCU IV piC is overlain by 75–100 m of massive poiC (the Svaalestad unit of Michot 1960; a similar olivine-bearing unit occurs near the base of MCU III) which has more primitive compositions than the underlying piC. This is followed by laminated and modally layered phiC, phimC and phimacC. The reversal to more primitive mineral assemblages across the MCU III/IV boundary is accompanied by a cryptic reversal; plagioclase and Ca-poor pyroxene have compositions of about An 44/Mg no. 71 at the top MCU III and about An 52/Mg no. 77 near the base of MCU IV. Olivines in the MCU IV poiC vary unsystematically from Fo 66 to 76. Macrocyclic units III and IV crystallized from monzonoritic parental magma. The BKSK magma chamber had a broad saucer-like shape with a small thickness to breadth ratio. The magma in the chamber during crystallization of MCU III was compositionally zoned and crystallized on an inward-sloping floor by down-dip accretion. Just before the major-magma influx at the base of MCU IV, phimC was crystallizing from the basal-magma layer at the centre of the chamber, while phimacC was crystallizing towards the flanks. The new, dense magma fountained into and mixed with the basal-magma layers already in the chamber. This hybrid magma crystallized during continued influx to produce massive piC at the base of MCU IV. This hybrid unit is thickest near the centre of the chamber and smoothed out the floor to an essentially horizontal surface. Continued influx resulted in the dense, primitive magma ponding on the floor; this crystallized fairly rapidly to produce the massive poiC unit. The return of normal fractional crystallization conditions is marked by the overlying sequence of modally and cryptically layered cumulates which duplicate the succession in MCU III. The variation in thickness of the upper part of MCU IV indicates that crystallization of the BKSK Layered Series was accompanied by sinking of the floor at a greater rate near the centre of the chamber than towards the flanks. This was accompanied by compaction of the underlying cumulates, promoting the development of lamination and the expulsion of intercumulus melt to encourage the development of adcumulates.

The Southern Part of the Fongen-Hyllingen Layered Mafic Complex, Norway: Emplacement and Crystallization of Compositionally Stratified Magma

The 160km2 Fongen-Hyllingen complex is a syn-Caledonian, layered mafic intrusion, located south-east of Trondheim, Norway. It crystallized at 5–6kb from a basaltic parent magma under elevated PH2O. In the 1500–4000m thick Hyllingen Series modal layering strikes directly towards the wall where layered diorites interfinger with country rock metabasalts and metapelites. The lower part of the layered sequence consists of dioritic layered rocks with restricted cryptic variation, followed by a gradual, major regression to more primitive compositions. In the upper part the rocks become increasingly evolved, reaching quartzbearing syenite at the roof. There are abundant metabasaltic rafts in the layered sequence. Cryptic and modal layering are discordant: at the at the top of the regressive sequence olivine and plagioclase become increasingly evolved along the strike of modal layering towards the wall, varying over 7km from Fo75:An63 to Fo13:An42. The apparent angle between cryptic and modal layering is usually less than 20°. The start of the major regression, reflecting the influx of fresh magma at the floor of the chamber, is concordant with modal layering which implies that modal layering represents the crystallization front. Crystallization of compositionally zoned magma along an inclined floor is envisaged, with modal layering representing the crystallization front and cryptic layering reflecting the composition of the stratified magma. This is supported by laboratory experiments involving the crystallization of stratified aqueous solutions along inclined surfaces.

Laboratory Experiments with Aqueous Solutions Modelling Magma Chamber Processes II. Cooling and Crystallization along Inclined Planes

Experiments have been carried out on the cooling and crystallization of aqueous solutions of Na2CO3, along an inclined plane. Compositional stratification is generated in homogeneous solutions both below an overhanging roof and above an inclined floor. Experiments with a ternary system (FeSO4 — Na2SO4 — H2O) along a 45° slope showed the same general behaviour.In addition, layering was produced in the solid. Experiments with initially stratified solutions of Na2CO3 showed that the density gradient suppressed vertical mixing as light fluid was released. The stratified solutions developed thermally driven double-diffusive layers and caused a cusped structure to develop in the crystallization front. Contours of constant Na2CO3 content in the solid product were discordant to the positions of the crystallization front. In experiments involving large volumes for which the fluid at any height remains essentially constant in composition, the Na2CO3 content of the solid increased outwards away from the plane. In experiments involving smaller volumes of fluid, slow cooling rates or more complete crystallization of the solution, the Na2CO3 content of the solid decreased outwards in later stages causing areversal. One experiment involved replenishment of the tank by an 8 cm layer of concentrated Na2CO3 solution emplaced at the base of the container. The resident stratified solution was passively lifted and caused the solid to temporarily melt back for a few hours before crystallization recommenced. The replenishment caused the Na2CO3 content of the solid to increase on the below slope side at all levels.

Olivine-rich units in the Fongen–Hyllingen Intrusion, Norway: implications for magma chamber processes

Abstract The Fongen–Hyllingen Intrusion (FHI) is considered to have crystallised from stratified magma residing in a bowl-shaped magma chamber. Seven olivine-rich units, representing the most primitive cumulates in the central part of the intrusion, are associated with compositional reversals and are interpreted as having formed at the lowest part of the magma chamber floor. Based on phase-relationships, the crystallisation order is explained in terms of magma mixing and fractional crystallisation. Repeated influxes of small volumes of dense, primitive magma at the base of the chamber had a major impact on the crystallising assemblage on the local floor and a decreasing effect towards the flanks of the chamber. This was due to the small volume of replenishing magma, the geometry of the chamber and the consequent restriction of magma mixing to the deepest part of the chamber where the new magma was emplaced. It is estimated that the chamber floor sloped as little as 1–2°, but this was sufficient to give widely different cumulate sequences near the bottom of the chamber and on the flanks.

The age of the synorogenic Fongen-Hyllingen complex, Trondheim region, Norway

Abstract Syenitic differentiates from the Fongen-Hyllingen intrusion, which grade downwards into layered gabbroic rocks, give a Rb-Sr whole-rock isochron age of 405 ± 9 Ma and an initial 87Sr/86Sr ratio of 0.7047 ± 0.0002. The age is interpreted as dating the time of magmatic crystallisation of these final differentiates. Emplacement at 500–600 MPa, followed by amphibolite facies metamorphism, occurred during the Middle-Upper Silurian (Scandinavian) orogenic event and supports a date of around 400 Ma for the Silurian-Devonian boundary.