Brian Robins

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.

Re-Os isotopic evidence for a lower crustal origin of massif-type anorthosites

Massif-type anorthosites are large igneous complexes of Proterozoic age. They are almost monomineralic, representing vast accumulations of plagioclase with subordinate pyroxene or olivine and Fe–Ti oxides—the 930-Myr-old Rogaland anorthosite province in southwest Norway represents one of the youngest known expressions of such magmatism. The source of the magma and geodynamic setting of massif-type anorthosites remain long-standing controversies in Precambrian geology, with no consensus existing as to the nature of the parental magmas or whether these magmas primarily originate in the Earths mantle or crust. At present, massif-type anorthosites are believed to have crystallized from either crustally contaminated mantle-derived melts that have fractionated olivine and pyroxenes at depth or primary aluminous gabbroic to jotunitic melts derived from the lower continental crust. Here we report rhenium and osmium isotopic data from the Rogaland anorthosite province that strongly support a lower crustal source for the parental magmas. There is no evidence of significantly older crust in southwest Scandinavia and models invoking crustal contamination of mantle-derived magmas fail to account for the isotopic data from the Rogaland province. Initial osmium and neodymium isotopic values testify to the melting of mafic source rocks in the lower crust with an age of 1,400–1,550 Myr.

The olivine-plagioclase reaction: Geological evidence from the seiland petrographic province, northern Norway

The experimentally determined equilibrium curves for the subsolidus olivineplagioclase reaction fall into two groups: those with positive slopes (dP/dT) that suggest reaction during cooling, and those with zero slopes that imply reaction by increase of pressure. Corona structures were developed in the mafic cumulates of northern Norway at different times during Caledonian almandine amphibolite facies metamorphism, as individual intrusions cooled from solidus temperatures. Unless pressure increases coincided fortuitously with subsolidus cooling, these relationships suggest that the positive-slope models are most analogous to natural systems.

The Bjerkreim-Sokndal Layered Intrusion, Rogaland, SW Norway: Evidence from marginal rocks for a jotunite parent magma

Abstract The Late-Proterozoic Bjerkreim-Sokndal Layered Intrusion (BKSK) consists of andesine anorthosite, leuconorite, troctolite, norite, gabbronorite, jotunite, mangerite, quartz mangerite and charnockite. The sequence of appearance of cumulus minerals and their compositions suggest a parent magma that was evolved, had plagioclase (±olivine) on the liquidus, was sufficiently TiO2-rich for hemo-ilmenite to crystallise early, and low in CaO and CaO Al 2 O 3 compared to basalts as reflected by the sodic plagioclases and the delayed appearance of cumulus augite. Fine- to medium-grained jotunites found along the northern contact of the BKSK consist of plagioclase (An45–53), inverted pigeonite (Mg# = 55-50), sparse augite (Mg# = 69-59), Fe-Ti oxides, K-feldspar, quartz and apatite. They are basic to intermediate rocks with relatively high FeOtotal, high TiO2, low MgO/MgO + FeO, moderate Al2O3 and low CaO and normative diopside. The jotunites have compositions that are consistent with the parental magma for the lower part of the BKSK Layered Series, and are interpreted as being marginal chills. Similar, but slightly more differentiated, jotunite magmas were subsequently emplaced into the BKSK and the surrounding region as broad dykes and small plutons. Jotunite is a minor rock type in most massif-type anorthosite provinces but may have an important petrological significance.

Finger structures in the Lille Kufjord layered intrusion, Finnmark, Northern Norway

Highly irregular contacts are developed between peridotite and troctolite in layers forming the uppermost part of a transition zone between two of the cyclic units of the Lille Kufjord intrusion. The upwardly-directed peridotite “fingers” crosscut both the feldspar lamination and the feldspathic xenoliths in the troctolite and are interpreted as the result of the replacement of troctolite by peridotite. Similar structures are developed in the Rhum ultrabasic pluton. Replacement may have been caused by the migration of a more hydrous and magnesian magma trapped initially in olivine or olivine-clinopyroxene cumulates into plagioclase-olivine cumulates precipitated from a basaltic liquid.

Reactions involving hydration of orthopyroxene in anorthosite-gabbro

Abstract Hydration of orthopyroxene (En 60 ) in contact with plagioclase (An 36 ) leads to oronas of the type orthopyroxene-cummingtonite-edenite + quartz-edenitic hornblende. The compositions of the minerals involved indicate that kyanite needles in the adjacent plagioclases form as part of the reaction: orthopyroxene (En 60 ) + plagioclase (An 36 ) + H 2 O→ cummingtonite + edenite/edenitic hornblende + plagioclase ( 36 ) + kyanite + quartz. The reaction proceedings consumption first of the orthopyroxene and later of the cummingtonite to produce a corona structure consisting of a core of intergrown edenite and quartz surrounded by edenitic hornblende. Corona evolution of this type is controlled by diffusion.

A sulphide-bearing orthopyroxenite layer in the Bjerkreim–Sokndal Intrusion, Norway: implications for processes during magma-chamber replenishment

The Bjerkreim–Sokndal Layered Intrusion contains an up to 3-m-thick layer of sulphide-bearing orthopyroxenite or melanorite that can be followed for about 30 km along the boundary between megacyclic units II and III. The layer of orthopyroxenite is developed within a sequence of ilmenite leuconorites and defines the base of zone IIIa. The leuconorites are generally succeeded by ilmenite–magnetite leucotroctolite, which defines the base of zone IIIb. The orthopyroxenite layer has an initial Sr-isotopic ratio similar to the leucotroctolites (ca. 0.7051) and lower than the enveloping ilmenite leuconorites of zones IIc and IIIa (ca. 0.7054). Variations of tetrahedrally coordinated Al in orthopyroxene and elevated Cr/TiO2 in the orthopyroxenite layer and the leucotroctolite imply that the orthopyroxenite is genetically more related to the leucotroctolite than the enveloping leuconorites. On the basis of field observations and analytical results, we conclude that the orthopyroxenite crystallised as a result of mixing between relatively primitive magma and differentiated resident magma, both of which were sulphide-saturated, during chamber replenishment. The injected magma formed a buoyant plume that spread out laterally at its level of neutral buoyancy within the compositionally zoned resident magma, some distance above the magma-chamber floor. Mixing in the plume resulted in a hybrid saturated in orthopyroxene, ilmenite and sulphide melt. Batches of relatively dense magma containing crystals of orthopyroxene and ilmenite and droplets of sulphide melt sank to the floor of the chamber from the hybrid magma layer and formed a layer of orthopyroxenite on the magma-chamber floor. Removal of orthopyroxene from the hybrid melt resulted in the sporadic crystallisation of plagioclase at the orthopyroxene–plagioclase cotectic. Plagioclase joined orthopyroxene and sulphide droplets in the dense melt batches which sank to the floor, resulting in the local development of melanorite instead of orthopyroxenite. Crystallisation of ilmenite leuconorite from the resident magma below the hybrid magma layer resumed after formation of the orthopyroxenite layer. This continued as the hybrid layer thickened and eventually came into contact with the chamber floor when leucotroctolites of zone IIIb started to crystallise on the deeper parts of the floor. Elsewhere, lower-temperature cumulates formed on elevated portions of the floor from magma higher in the stratified column. The sulphide-bearing orthopyroxenite layer is not associated with PGE mineralisation because both the replenishing and differentiated resident magmas had lost PGE through earlier sulphide saturation. Zone IIIb is followed by a sequence of lithologies that reflect progressive fractional crystallisation with the appearance of cumulus orthopyroxene at the expense of olivine (zone IIIc), and the successive entries of magnetite (zone IIId) and clinopyroxene plus apatite (zone IIIe). A new major influx of magma took place at this time to produce megacyclic unit IV.

Picrite sills and crystal-melt reactions in the Honningsvag intrusive suite, Northern Norway

Abstract Field relations in the upper part of Intrusion II of the Caledonian Honningsvåg Intrusive Suite show that some peridotite sheets transgress, and include in situ rafts of, the adjacent gabbroic cumulates. Modal and textural analyses of three olivine melagabbro sheets show non-cotectic mineral proportions that are likely to result from crystal-melt reactions. Discordant, replacive fingers and pipes of feldspathic peridotite along interfaces between peridotite and overlying olivine melagabbro also suggest crystal-melt reactions. It is proposed that several picritic sills intruded porous gabbroic cumulates in the upper part of Intrusion II. Lateral infiltration of picritic magma led to crystal-melt reactions, mainly assimilation of plagioclase and precipitation of olivine, resulting in the formation of olivine melagabbro and peridotite sheets, and replacive fingers and pipes of feldspathic peridotite.

The mode of emplacement of the Honningsvag Intrusive Suite, Mageroya, northern Norway

The Honningsvag Intrusive Suite consists of several layered mafic/ultramafic intrusions and a transgressive body of igneous breccia that appears to represent a magma conduit. It is emplaced into a Silurian, flysch-type sedimentary sequence that is thermally metamorphosed to spotted slate, cordierite–andalusite or pyroxene hornfels and agmatitic migmatite. Folds and flattened reduction spots in the hornfelses suggest that emplacement took place after Caledonian deformation and development of a slaty cleavage. Tectonic rotation subsequent to emplacement has led to exposure of the Honningsvag Intrusive Suite in a natural cross-section corresponding to ∼10 km of crustal depth. Basaltic magma was initially emplaced as a several-kilometre-tall pipe that crystallized to form Intrusion 1. A second magma chamber was initiated alongside this pipe and subsequently expanded laterally into a sill-like magma body as batches of olivine-saturated basalt were added. A later magma chamber, represented by Intrusion 4, developed largely within the cumulates forming the upper part of Intrusion 2 and appears to have been accompanied by opening of a broad inclined feeder into which blocks and slabs of older cumulates collapsed. The resulting igneous breccias of Intrusion 3 are chaotic and largely clast-dominated in the lower part of the conduit, but enclosed slabs are matrix supported and orientated parallel to an originally subhorizontal banding in the feldspathic peridotite matrix in the upper part. The core of the breccia body has a troctolite matrix and contains blocks of older breccia, suggesting re-opening of the conduit, either during the crystallization of Intrusion 4 or possibly during the development of chambers represented by the younger layered intrusions. The cumulates in Intrusion 4 subsided sufficiently to invert marginal parts of the Layered Series before a further magma chamber was initiated in its roof rocks. The last major magma chamber opened alongside Intrusion 5 and extended upwards as a pipe or broad dyke to the highest structural levels exposed. Cross-cutting relationships show that the Honningsvag magma chambers were not active simultaneously but were emplaced sequentially, generally at successively higher structural levels. Olivine tholeiite magma initially pooled in a crustal zone where it had neutral buoyancy. Subsequent chambers are suggested to have been initiated by emplacement of magma along the density discontinuities that existed above and around crystallized intrusions and their associated hornfelses. Chambers evolved by fractional crystallization, assimilation of country rocks and periodic replenishment. The abandonment of magma chambers may have resulted from the expulsion of low-density residual melts.

Evolution of lavas in the Late Ordovician/Early Silurian Solund‐Stavfjord Ophiolite Complex, West Norway

The geochemistry of the volcanic sequence of the Late Ordovician/Early Silurian Solund-Stavfjord Ophiolite Complex (SSOC) of the Western Norwegian Caledonides has been investigated along 12 stratigraphic sections spaced over a lateral distance of ∼60 km. The metabasalts commonly show a cyclic organization, comprising sheet flows followed by pillow lavas and with volcanic breccias at the top of some volcanic units. The proportions of the volcanic rock types vary considerably along-axis, even within short distances. The robust nature of the sheeted dike complex, the proportion and regional distribution of the various volcanic rocks, the aphyric nature of the metabasalts, as well as the predominance of Fe-Ti basalts suggest that the SSOC formed at an intermediate to fast spreading ridge, probably within a back-arc basin. The variations in the concentrations of the incompatible (e.g., Zr) and compatible (e.g., Cr) elements in the lavas are substantial, i.e., from 49–384 ppm and 66–443 ppm, respectively. Nd isotopic ratios show only minor variations, suggesting that the lavas were generated from an isotopically uniform source. The variations in the incompatible elements (represented by Zr) define stratigraphic units ∼25 to 150 m thick, with either gradual decreases or increases in Zr concentrations, while Cr may show different trends. The different Zr-Cr covariations are interpreted in the light of a numerical model in which fractional crystallization and mixing of various magmas are the principal processes. Through upwardly Zr-increasing units, Cr generally decreases and estimated magma densities increase, compatible with progressive fractional crystallization. However, through upwardly Zr-decreasing units, Cr and estimated magma densities either increase or decrease, trends that are attributed to hybridization in frequently replenished magma chambers. Within short lateral distances (<1 km), the geochemical stratigraphy changes dramatically, excluding eruption from a homogeneous, axially continuous magma chamber. Instead, we propose that the volcanic sequence was fed from small, separate magma lenses that evolved independently of each other.