Jean-Clair Duchesne

The short duration and anorogenic character of anorthosite magmatism: U-Pb dating of the Rogaland complex, Norway

Abstract U Pb dating of zircon and baddeleyite extracted from orthopyroxene megacrysts yield emplacement ages of 929 ± 2, 932 ± 3, and 932 ± 3 Ma for the Egersund-Ogna, Helleren, andAna-Sira massif-type anorthosites, respectively. This surprisingly short period of magmatic activity also includes the injection of 931 ± 5 Ma old jotunite dikes. Intrusion of the ilmenite norite, which constitutes the Tellnes Fe Ti deposit took place about 10 Myr later at 920 ± 3 Ma. In the margin of massif anorthosites secondary baddeleyite was formed at 915 ± 4 Ma, in association with later deformation produced by the diapiric mode of emplacement. Zircon cannot have formed in the primary magmas and more differentiated, quartz-mangeritic liquid within mega-orthopyroxene aggregates seems to be the most plausible liquid to precipitate zircon. The magmatic event at 931 ± 2 Ma postdates latest regional deformation by about 60 Myr and, therefore, anorthosite genesis is clearly anorogenic in nature, representing a thermal pulse that followed the Sveconorwegian-Grenville orogeny. Inherited zircon components and initial Pb isotopic compositions substantiate that the anorthosite parent magmas were generated to a large degree by melting of old continental material having ages of about 1690, 1450, and 1240 Ma. The most likely source lithology for these crustal melts are gabbroic rocks that were successively emplaced into the lower crust during the Meso-Proterozoic, prior to anorthosite formation.

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.

Kibaran A-type granitoids and mafic rocks generated by two mantle sources in a late orogenic setting (Burundi)

In the Mesoproterozoic Northeastern Kibaran Belt of Burundi (Central Africa) two distinct late Kibaran magmatic suites coexist, both including A-type granitoids. They are located along the Boundary Zone between the Kibaran mobile belt (Western Internal Domain) and the Archaean Tanzanian craton overlain by Mesoproterozoic foreland deposits (Eastern External Domain). Intense deformation, high-temperature metamorphism and intrusion of abundant peraluminous anatectic crustal granites occur only in the former domain whereas the Mesoproterozoic sedimentary cover of the latter is much less or even nearly undeformed nor metamorphosed. The first late Kibaran magmatism (350 km long Kabanga-Musongati with an emplacement age of 1275 _+ ~ Ma; U-Pb on zircon) is mainly composed of mafic and ultramafic layered rocks with subordinate A-type acidic differentiates moderately enriched in incompatible elements. Initial isotopic ratios (SrlR = 0.708; ~Nd = --8 ) indicate an old continental lithospheric mantle origin. The emplacement of these late Kibaran magmatic rocks was controlled by late lateral shear, possibly contemporaneous with the latest intrusions of the Kibaran peraluminous synkinematic granites of the Western Internal Domain ( ~ 1330-1260 Ma). The second late Kibaran magmatism (40 km long Gitega-Makebuko and Bukirasazi alignment with an emplacement age of 1249_+ s Ma; U-Pb on zircon) is limited in volume. It is mainly granitic in composition (A-type), can be strongly enriched in incompatible elements, and comprises both syenites and mafic rocks. Initial isotopic ratios (SrlR=0.702; ~Na= +4.5 to -1.4) point to an OIB-type asthenospheric/lower continental lithospheric mantle origin, with only slight contamination by the lower crust during differentiation. This group was also intruded during the late lateral shear. In both groups liquid lines of descent can be reconstructed, although some of the rocks have been strongly albitized. This indicates that the granites are produced by differentiation of less evolved magmas and not by crustal anatexis. Upwelling of the asthenosphere along the Tanzanian craton can generate by adiabatic pressure release the OIBtype basic melts and provide the heat necessary to melt the continental lithospheric mantle sources. This mechanism assigns a major role to a lithosphere-scale late Kibaran shear event occurring at the end of the regional compressive deformation between two rheologically contrasted domains. Ascent of the asthenosphere, continental lithospheric mantle delamination and late orogenic extensional collapse of the Western Internal Domain are suggested as a possible geodynamic model for the entire Northeastern Kibaran Belt. Additional work is however necessary to test this model. Finally, our results indicate that in the Northeastern Kibaran Belt the Kibaran orogeny ended at ~ 1250 Ma, despite various reactivation events occurring later (e.g. at ~ 1137 Ma).

TRACE ELEMENTS AND ANORTHOSITE GENESIS

Abstract Terrestrial massif anorthosites have gained new interest for the understanding of the deep zones of the crust and for the reconstitution of its history in Proterozoic time. The purpose of this paper is to show how trace elements can enlighten two controversial questions in the problem of anorthosites, namely the nature of the parental magma and the process which gives rise to related acidic rocks. Data obtained on rocks and minerals coming from the Rogaland anorthositic province, South Norway, are presented together with those available in the literature. The Sr and Ba in plagioclase and their relationship with Ca and K, K and Rb in rocks and plagioclases, rare earth elements (REE) in cumulate minerals and in various liquids, 87Sr/86Sr initial ratios on rocks and minerals as well as a few data on transition elements and on 18O/16O ratios are discussed in the different sections. Quantitative modelling of the behaviour of trace elements is realized mainly by graphical methods in the Bjerkrem-Sogndal layered lopolith and in the Hidra body, both andesine-type massifs. The major conclusions are as follows: (1) The parental magma of the andesine-type massifs has a jotunitic (hypersthene-monzodioritic) composition characterized by variable K/Rb ratios (from 480 to 1700), by the absence of an Eu anomaly, by variable REE contents (from 50 to 220 for La chondrite-normalized content) and by La/Yb ratios almost constant (from 6 to 12), as well as by high Ti and Fe contents, by transition elements indicating calc-alkaline affinities and by 87Sr/86Sr initial ratios similar to those of rocks derived from the upper mantle or the deep crust. (2) A jotunitic composition appears not to be compatible with the parental magma of the labradorite-type massif anorthosite. The relationship between andesine anorthosite and labradorite anorthosite cannot be described simply in terms of fractional crystallization. (3) Fractional crystallization can explain the succession of rocks from andesine anorthosite to leuconorite, to norite and finally to acidic rocks. In some cases however, namely the Bjerkrem-Sogndal lopolith in Rogaland, contamination by supracrustal material must be invoked and seems to have superimposed its effects on those of fractional crystallization. (4) Deformation, granulation and recrystallization do not appear to fractionate both the major and trace elements of the plagioclase, except Ti which is lowered.

Derivation of the 1.0-0.9 Ga ferro-potassic A-type granitoids of southern Norway by extreme differentiation from basic magmas

Major and trace elements, Sr and Nd isotopic data as well as mineral compositions are presented for a selection of the 1.0–0.9 ferro-potassic A-type granitoids (Bessefjellet, Rustfjellet, Verhuskjerringi, Valle, Holum, Svofjell, Handeland-Tveit, Aseral, Lyngdal gabbronorites) that occur close to the Mandal-Ustaoset Line (MUL) of southern Norway. These hornblende biotite granitoids (HBG) define an extensive differentiation trend ranging from gabbronorites (50 wt.% SiO 2) to granites (77 wt.% SiO2). This trend is interpreted as resulting from extreme fractional crystallization of several basaltic magma batches with similar major and trace elements compositions. At 930 Ma, the HBG suite displays a narrower range in ISr (0.7027–0.7056) than in eNd(t) (+1.97 down to −4.90) suggesting some assimilation of a Rb-depleted lower crust (AFC process) or/and source variability. An age of 929 ± 47 Ma is given by a Rb-Sr isochron on the Holum granite (Sri = 0.7046 ± 0.0006, MSWD = 1.7). Geothermobarometers indicate a low pressure of emplacement (1.3–2.7 kbar) and an oxygen fugacity close to NNO. High liquidus temperatures are given by the apatite saturation thermometer (1005–1054 ◦ C) and are in agreement with results from other studies. The basaltic parent magmas of the HBG suite are partial melts of an hydrous mafic, potassic source lying either in the lithospheric upper mantle or in the mafic lower crust derived from it. This contrasts with the 930 Ma anorthosite–mangerite–charnockite suite (AMC suite) of the Rogaland Province for which a depleted lower crustal anhydrous gabbronoritic source has been indicated. The present data imply the penecontemporaneous melting of two contrasting sources in southern Norway. The source duality could result from an increasing degree of metamorphism (amphibolite to granulite) from East to West, an horizontal stratification of the lower crust or from the stratification of the lithosphere (melting of the lower crust or upper mantle). It may also indicate that the AMC and HBG suites formed in two distinct crustal segments. The linear alignment of the HBG suite along the Mandal-Ustaoset shear zone suggests that a linear uprise of the asthenosphere, following a lithospheric delamination under this structure, could be the vector of the mantle heat.

The Neoproterozoic Pan-African basement from the Alpine Lower Danubian nappe system (South Carpathians, Romania)

Abstract The South Carpathians, which were thrust to the Moesian platform in the Alpine orogeny (Late Cretaceous to Tertiary), include the Danubian nappe system. The Danubian pre-Alpine basement comprises two Variscan nappes, each one made up of partially retrogressed amphibolite facies rocks intruded by granitoids and capped by an Ordovician-Devonian volcano-sedimentary cover. No lithological correlation can be established between the pre-Ordovician basements of these two units. The metamorphic basement from the first Variscan nappe, the Drăgsan Group, is composed of banded amphibolites with some augen and aluminous gneisses intruded by granodioritic to tonalitic plutons. The banded structure of the amphibolites, together with their geochemistry, suggests a volcano-sedimentary sequence. Zircon UPb data on an intercalated augen gneiss have given an age of 777 ± 3 Ma for the emplacement of the protolith of this gneiss. Nd model ages ( T DM ) for the amphibolites range from 717 Ma to 817 Ma. At 777 Ma, ϵ Nd values cluster is at +8.3 to +9.8 and Sr initial ratios range between 0.7007 and 0.7023, indicating an oceanic origin without continental crust contamination. Major and trace elements from the Drăgsan amphibolites consistently display an island arc signature, with three differentiation trends evolving from an early tholeiitic trend to a more differentiated low-K calc-alkaline one. The Drăgsan terrane is similar to the early Pan-African juvenile terranes of the Sahara. The basement of the second Variscan nappe, the Lainici-Păius Group, is made up of metasedimentary rocks (mainly quartzites, marbles and graphitic mica gneisses) cut by early leucogranitic dykes, medium-K calc-alkaline and alkali-calcic (mainly granitic) plutons, and late medium-K porphyry diorite dykes. This magmatism can be bracketed between 588 Ma and 567 Ma (UPb zircon ages). The best preserved pluton (Tismana, 567 Ma old), displays a composite alkali-calcic (very high-K calc-alkaline) magmatic sequence, ranging from gabbro-diorite to monzogranite, including an ultramafic pod. Ages and geochemical signatures resemble the Saharan late Pan-African granitoids. The existence of Late Precambrian partly juvenile terrains is thus confirmed within the basement of South Carpathians, which renders then a segment of the European Alpine belt that can be successfully compared to the Pan-African Trans-Saharan belt.

The Rogaland Anorthosites: Facts and Speculations

The massif-type anorthosites of the province were emplaced as crystal mushes of plagioclase, lubricated by noritic liquids cyrstallizing along a P-T gradient, and containing aggregates of plagioclase and/or Al-rich orthopyroxene megacrysts, formed in a magma chamber at the base of a thickened crust. Synemplacement deformations were produced in the envelope and within the plutons, where they started in the magmatic stage and ended in the solid stage. The anorthositic suite can be accounted for by three magma types, the compositions of which are basaltic, jotunitic and charnockitic. Each magma generates part of the suite, with some overlapping. The basaltic magma is mantle-derived in an undefined geodynamic environment. The alkali to alkali-calcic jotunitic magma is generated as distinct batches with variable crustal signatures, due to contamination by deep-crustal material or direct partial melting of this material. The charnockitic magma can be produced by fractionation of the jotunites but can also result from direct partial melting in granulite facies conditions. For both magma types partial melting is triggered by the hot anorthositic diapirs en route to their final level of emplacement.

Rare-earth data on monzonoritic rocks related to anorthosites and their bearing on the nature of the parental magma of the anorthositic series

Abstract Major and trace elements have been determined in monzonoritic rocks (hypersthene-monzodiorite or jotunite) from two intrusions belonging to the South Rogaland anorthositic complex (Norway). The rare-earth abundance pattern reveals no Eu anomaly, or only a very small one. This fact together with field observations suggest that these rocks represent the parental magma of the anorthositic suite. High Ti and P abundances, low Si content, high Fe/Mg and K 2 O/SiO 2 ratios are characteristics of the major element geochemistry. Absolute amounts of some trace elements abundances vary distinctly between the two intrusions. K/Rb ratios as high as 1700 are observed. Partial fusion of upper mantle kaersutite is proposed as a possible mechanism of magma generation. Partition coefficients between plagioclase phenocrysts and liquid are determined.

Massif Anorthosites: Another Partisan Review

This review mainly concerns Proterozoic deformed massif anorthosites occurring in the Grenville Province and its South Norwegian extension (Rogaland). Evidence of syn-emplacement deformation during Grenvillian times contradicts the hypothesis of tectonic reworking. Long cooling histories at depth, or rejuvenation by partial melting of older solid masses can explain that ages older than Grenvillian are measured in the anorthosite. No unique magma is parental to the anorthosite suite. Acidic rocks can result from anatexis or from fractionation of jotunitic liquids. Most jotunites are not comagmatic with massif anorthosites, but are produced by partial melting of crustal rocks basic to intermediate compositions. Massif anorthosites contain giant Al-opx (7-9% Al 2 O 3 , plagioclase exsolutions) produced by cotectic crystallization from basic liquids at depth. They are carried to the level of emplacement either by an hyper-feldspathic liquid, which intrudes shallower magma chambers in atectonic conditions; or, by a plagioclase crystal mush, which rises diapirically and deforms its envelope and its own mass. 1. FOREWORD A few characteristics of the scientific community concerned with anorthosites are here put forward for the benefit of those not familiar with the subject. There is a relatively small number of geologists involved. Their publications, though not numerous, are usually important contributions (books or memoirs) covering the field data, geological maps, petrology, geochemistry, etc. about the particular massif they have been studying for many years. These contributions are milestones in the development of our knowledge of anorthosites and moreover have a bearing on many topics in igneous and metamorphic petrology, structural geology and geodynamics. They provide models of high internal consistency which make the most of the data collected. These models are however very vulnerable to new approaches and techniques such as trace element geochemistry or Sm/Nd isotope geochemistry and it takes a long time for these new constraints to be taken into account and to modify the old model. In the intervals of these major publications the scientific community has to fall back on short abstracts presented at Geological Society meetings, unpublished theses and internal reports of restricted diffusion, which only indicate the matters into consideration rather than providing evidence ( 1 ). Only readers capable of reading

Monzonorites from Rogaland (Southwest Norway): a series of rocks coeval but not comagmatic with massif-type anorthosites

Duchesne, J.C., Wilmart, E., Demaiffe, D. and Hertogen, J., 1989. Monzonorites from Rogaland (Southwest Norway ): a series of rocks coeval but not comagmatic with massif-type anorthosites. Precambrian Res., 45:111-128. Monzonorites, members of the anorthositic suite of rocks, are usually considered as residual after the formation of massif-type anorthosites. In Rogaland (S.W. Norway) they occur as large dykes, intrusions and chilled margins to differentiated massifs, emplaced during and soon after the main anorthosite massifs, in granulite facies conditions. Ti, P and Fe are enriched in monzonorites and steadily decrease towards quartz mangerites, the FeO/FeO + MgO ratios varying slightly during alkali enrichment (Bowen trend). Trace element spidergrams show deep troughs in Rb, Th, Nb-Ta, Sr, Zr-Hf and Ti. The REE slightly decrease in the evolution with (La/Yb)S ratios about 9 and neutral to positive Eu anomalies. Several occurrences, however, show highly contrasted trace element features. Sr isotope ratios (/st) show a wide interval of variation (0.704-0.710) between the various dykes and intrusions without any correlation with the elements indicative of crustal contamination. In the Tellnes main dyke, evolution towards acidic rocks occurs without contamination and variation in Isr. Fractional crystallization with subtraction of apatite-bearing noritic cumulates can account for the major and trace element evolution from monzonorite to quartz mangerite, but is unable to explain the large differences between occurrences. It is concluded that monzonorites cannot be comagmatic (though coeval) with massif-type anorthosites. They result from the crystallization of distinct magma batches, possibly formed through partial melting of basic to intermediate rocks in the lower crust.