Bernard Bingen

Redistribution of rare earth elements, thorium, and uranium over accessory minerals in the course of amphibolite to granulite facies metamorphism: The role of apatite and monazite in orthogneisses from southwestern Norway

The amphibolite to granulite facies transition has been studied in a high-K calc-alkaline, hornblende-biotite, K-feldspar megacrystic augen gneiss series from the Rogaland-Vest-Agder (Rog-VA) sector of the Sveconorwegian province (southwest Norway). Hornblende begins to break down mainly to clinopyroxene at the Cpx-in isograd and biotite mainly to orthopyroxene at the Opx-in isograd (abbreviations for minerals following Kretz, 1983). The magmatic accessory mineral association comprises titanite and allanite, which begin to break down before the Cpx-in isograd. Titanite is preserved as relict inclusions in other minerals at higher grade. Monazite and thorite formed in the breakdown of allanite. Monazite abundance reaches a maximum between the Cpx-in and Opx-in isograds. The middle and heavy rare earth element (M-HREEs, except Eu) content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite, hornblende, and biotite. The U contents of apatite are low and do not increase with metamorphic grade. The light rare earth elements (LREEs) and Th content of apatite are not correlated to the breakdown of allanite around the Cpx-in isograd but increase around the Opx-in isograd. A simplified equation is proposed for monazite crystallisation in the vicinity of the Cpx-in isograd that accounts for the allanite, titanite, and hornblende breakdowns and the M-HREE substitution in apatite: 3 (M-HREE)2O3in hornblende and litanite+3(LREE)2in allaniteO3+2Ca5(PO4)3apatite(F,OH)+6SiO2quartz⇔6(LREE)PmonaziteO4+2Ca2(M-HREE)3(SiO4)3(F,OH)lessingite in apatite+6CaOin plagioclase. At the Opx-in isograd, the increase of LREEs and Th content of apatite results either from the breakdown of some monazite: 3(LREE)PO4monazite+3SiO2quartz+4CaOin plagioclase+(F2,H2O)fluid⇔Ca5(PO4)3(F,OH)+apatiteCa2(LREE)3(SiO4)3(F,OH)less ingite in apatite. or the breakdown of the remaining allanite: 3(LREE)2O3in allanite+6SiO2quartz+4CaOin plagioclase+(F2,H2O)fluid⇔2Ca2(LREE)3(SiO4)3(F,OH)lessingite in apatite. The release of fluorine from the breakdown of biotite at the Opx-in isograd may increase apatite stability relatively to monazite in the granulite facies. when compared to amphibolite facies, the granulite facies augen gneisses do not show any U or Th depletion. This means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite-granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution. The coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that (high-K) calc-alkaline orthogneisses are a suitable material to date (with the U-Pb method) prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite.

The 616 Ma Old Egersund Basaltic Dike Swarm, Sw Norway, and Late Neoproterozoic Opening of the Iapetus Ocean

The Egersund dike swarm of SW Norway is made up of 11 basaltic dikes trending ESE‐WNW. Two groups are defined: porphyritic dikes with bytownite phenocrysts of tholeiitic affinity and aphyric dikes of tholeiitic to alkaline affinity. Baddeleyite in the most alkaline dike gives a U‐Pb intrusive age of 616 ± 3 Ma. The swarm is parallel to the Neoproterozoic southwestern (present‐day orientation) Tornquist margin of Baltica. It is coeval with the Long Range swarm of Labrador which is parallel to the southeastern proto‐Appalachian margin of Laurentia. Both swarms are related to rifting that resulted in the opening of Iapetus ocean. Chronocorrelation of Neoproterozoic rift‐related magmatic suites in western Baltica and eastern Laurentia suggests diachronic opening of Iapetan oceanic basins at ca. 610 Ma along the northwestern Baltoscandian margin of Baltica and at ca. 550 Ma along the proto‐Appalachian margin of Laurentia and Tornquist margin of Baltica.

Correlation of supracrustal sequences and origin of terranes in the Sveconorwegian orogen of SW Scandinavia: SIMS data on zircon in clastic metasediments

Abstract Ion probe U–Pb data on detrital zircons were acquired in five pre-Sveconorwegian clastic metasedimentary units in the Sveconorwegian orogen in S Norway. Turbidite-type sediments in the Veme complex (Begna sector, west of the Oslo rift) display zircons between 1.67 and 1.53 Ga (9 grains). This complex is interpreted as a westward, younger extension of the Stora Le–Marstrand metasedimentary belt (≤1.58 Ga) of the Idefjorden terrane (east of the Oslo rift). The belt formed in an outboard setting, and the detritus in the sediments was probably mainly derived from subduction-related complexes exposed in the Idefjorden terrane. To the west, a quartzite of the Hallingdal complex in the Telemark sector has zircon populations at 3.13–2.68 and 2.03–1.71 Ga (34 grains). It was deposited between 1.7 and 1.5 Ga and attests the presence of differentiated continental crust older than 1.5 Ga in S Norway. Two samples of the Hettefjorden and Festningsnutan Groups from the Hardangervidda plateau have zircon populations at 3.25–2.42 and 2.03–1.54 Ga (47 grains). The 1.90–1.85 Ga frequency maximum of the age distribution in these three epicontinental sediments corresponds with the peak of Svecofennian magmatic activity in central Sweden, and to the age of the main zircon populations in late-Svecofennian sediments. This is an indication of a Fennoscandian origin for the Telemark–Bamble and Rogaland–Hardangervidda terranes, at the margin of the Svecofennian domain; alternative exotic provenances are possible. A quartzite sample of the Modum complex in the Kongsberg sector displays 2.07–1.48 Ga Proterozoic zircon populations (28 grains) and minor Archaean populations. The Modum complex represents, probably together with the Kragero complex in the Bamble sector and the Seljord Group in the Telemark sector, parts of a quartzite-rich epicontinental cover deposited after 1.48 Ga on the Telemark–Bamble terrane. The juxtaposition of the Telemark–Bamble and Rogaland–Hardangervidda terranes with the subduction-related Idefjorden terrane, can be explained by a transpressive translation of these terranes at the margin of the Fennoscandian shield during the Sveconorwegian orogeny. Sveconorwegian sinistral strike-slip shearing probably took place along an amphibolite-facies banded gneiss unit situated at the boundary.

Precise eclogitization ages deduced from Rb/Sr mineral systematics: The Maksyutov complex, Southern Urals, Russia

Abstract The Maksyutov complex (Southern Urals, Russia) is a well-preserved example of subduction-related high-pressure metamorphism. One of its two litho-tectonic units consists of rocks that experienced eclogite-facies conditions. Published 40Ar/39Ar data on phengite, U/Pb data on rutile, and Sm/Nd mineral data define a cluster of ages around 370 to 380 Ma. Nevertheless, no consensus exists as to the detailed interpretation of data and the exact age of eclogitization. We present new, high-precision internal mineral Rb/Sr isochrons for eclogite-facies metabasites, felsic eclogites, and eclogite-facies quartz veins. Nine isochrons, mainly controlled by omphacite and white mica phases, give concordant ages with an average value of 375 ± 2 Ma (2σ). Microtextural features, such as prograde growth zoning in eclogite-facies phases, suggest that the assemblages dated formed at a stage of prograde metamorphism. Sr-isotopic equilibria among eclogite-facies phases, and among eclogite-facies fluid veins and the host rocks, indicate that our ages reflect crystallization ages, related to the prograde-metamorphic, probably fluid-mediated eclogitization reactions. This interpretation is reinforced by data from fluid-precipitated quartzitic eclogites, whose modal composition, together with intergrowth relationships, conclusively imply closed-system behavior after crystallization. The possible occurrence of a pre-375 Ma event of ultra-high-pressure metamorphism (UHPM) in the Maksyutov complex is disproved by isotope systematics, microtextures, and mineral zoning patterns.

Molybdenite Re–Os dating of biotite dehydration melting in the Rogaland high-temperature granulites, S Norway

Abstract The ability of the Re–Os system in molybdenite to record and preserve the age of granulite-facies metamorphism in polymetamorphic belts is tested using the Orsdalen W–Mo district, Rogaland, S Norway. A low-pressure high-temperature granulite-facies domain, displaying osumilite and pigeonite isograds, is exposed around the 931±2 Ma Rogaland anorthosite complex. Available U–Pb monazite and zircon data show that a 0.93 Ga contact metamorphism overprints a 1.03–0.97 Ga regional Sveconorwegian metamorphism in the gneiss basement. Molybdenite and scheelite in the Orsdalen district occur in orthopyroxene-bearing leucocratic veins parallel to the regional foliation. The veins are interpreted as migmatic leucosomes formed by fluid-absent incongruent melting of the biotite-rich host rock above ca. 800°C, producing a granulite-facies orthopyroxene±garnet residual assemblage. Molybdenite is interpreted as a product of the melting reaction, crystallized from trace amounts of Mo released from biotite in the host rock during partial melting. Four Re–Os analyses of molybdenite from three samples representing two mines yield an isochron age of 973±4 Ma. The isochroneity of the data indicates that the precipitation of molybdenite and the partial melting event are recorded on the district scale. The Re–Os system in molybdenite was not affected by subsequent 0.93 Ga contact metamorphism, corresponding to formation of garnet+quartz coronitic textures around molybdenite and other minerals in the deposits. The results indicate that granulite-facies conditions prevailed at 973±4 Ma, near the end of a protracted event of regional metamorphism (1.03–0.97 Ga). Biotite dehydration melting recorded in Orsdalen took place at a pressure of ca. 5.5 kbar (orthopyroxene–garnet–plagioclase–quartz thermobarometry), possibly in association with regional decompression. The study shows that granulite-facies metamorphism (0.97 Ga) took place before intrusion of massif-type anorthosites in Rogaland (0.93 Ga). The study suggests that the high-temperature osumilite-bearing assemblage may be related to the 0.97 Ga event, and that massif-type anorthosites may not be a cause for the high-temperature thermal anomaly in the crust but a late expression of it.

Tectonic regimes and terrane boundaries in the high-grade Sveconorwegian belt of SW Norway, inferred from U-Pb zircon geochronology and geochemical signature of augen gneiss suites

Six units from three suites of gneissic megacrystic granitoids in the Sveconorwegian Province of SW Norway were dated by the U–Pb zircon method. The Gjerstad suite crystallized between 1.19 and 1.15 Ga in the Bamble, Telemark and Rogaland–Vest Agder terranes; it has an A-type geochemical signature suggesting intrusion in a tensional setting. The Vennesla unit of this suite yields an age of 1166+61−21 Ma. The Feda suite intruded in Rogaland–Vest Agder during a short period at 1.05 Ga (four units between 1051+2−8 and 1049+2−8 Ma) and has a high-K calc-alkaline trend suggesting a subduction- related setting. The Fennefoss augen gneiss of Telemark (1035+2−3 Ma) possesses a geochemical signature transitional between high-K calc-alkaline and A-type. The spatial association of tensional plutonism with the Kristiansand–Porsgrunn shear zone between Bamble and Telemark suggests that crustal thinning occurred along this axis between 1.19 and 1.13 Ga, preceding Sveconorwegian compressive shearing. The boundary between Rogaland–Vest Agder and Telemark, the Mandal line, probably separated, at 1.05 Ga, a mobile belt (Rogaland–Vest Agder) and a colder, more rigid terrane (Telemark). Major strike-slip shearing along the southern section of the Mandal line was probably associated with intrusion of an elongate pluton of the Feda suite and ductile amphibolite-facies deformation along a banded gneiss unit.

Relations between 1.19–1.13 Ga continental magmatism, sedimentation and metamorphism, Sveconorwegian province, S Norway

The Sveconorwegian and Grenville orogenic belts display widespread 1.19–1.13 Ga Early Grenvillian continental magmatism including A-type granitoids. In the Sveconorwegian province, S Norway, bimodal 1.17–1.14 Ga metavolcanic rocks of the Telemark sector are part of this magmatism. Volcanic rocks in low- to medium-metamorphic grade are interlayered with immature and locally conglomeratic clastic metasediments and covered by a thick metasedimentary sequence. Minor unconformities are reported. New zircon U–Pb data are presented and integrated in a revised stratigraphy of the Telemark supracrustal rocks. A metarhyodacite at the base of the Nore group yields a crystallisation age of 1169±9 Ma and displays 1.7–1.5 Ga inherited zircon grains (SIMS data). A metarhyolite situated below sandstone of the Heddal group yields a crystallisation age of 1159±8 Ma. In the cover sequence, a metasandstone of the Heddal group has detrital zircon grains in the intervals 2.86–2.41 and 1.94–1.11 Ga (34 analysed grains) and a metasandstone of the Kalhovd formation in the intervals 2.85–2.74 and 2.00–1.05 Ga (41 analysed grains). These metasediments were deposited after 1121±15 Ma and 1065±11 Ma, respectively and were transformed by 1.01 Ga Late Sveconorwegian deformation and metamorphism. The metasedimentary rocks contain a significant amount of regionally derived clasts. Two deformed A-type granite metaplutons yield zircon U–Pb intrusion ages of 1146±5 Ma (Eiddal) and 1153±2 Ma (Haglebu, ID–TIMS data). The 1.19–1.13 Ga magmatism is distributed in the western part of the Sveconorwegian province, in the Telemark, Bamble and Rogaland–Vest Agder sectors, indicating that these sectors were part of a single plate at that time, which is characterised by a thin lithosphere today. The A-type geochemical signature of the felsic magmatism and the continental lithosphere signature of the associated mafic volcanism point to a continental non-compressional tectonic regime. The overlap in time between widespread 1.19–1.13 Ga continental magmatism, intermontane basin formation and Early Sveconorwegian 1.15–1.12 Ga granulite-facies metamorphism recorded in the Bamble sector suggest a thermal pulse linked to upflow of asthenospheric mantle. Deposition of the cover of clastic sediments between 1.12 and 1.01 Ga possibly reflects thermal subsidence after the 1.19–1.13 Ga event and before the Late Sveconorwegian (1.03–0.95 Ga) orogenic phases. An analogy between the 1.19–1.13 Ga evolution of the Sveconorwegian province and the Cenozoic formation of the Basin and Range province in USA is discussed.

A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic-ultramafic layered xenoliths from Beaunit (French Massif Central)

The Puy Beaunit volcano vent, French Massif Central, displays a population of plutonic mafic to ultramafic xenoliths, commonly showing asymmetric, millimetre to centimetre thick, layering. Layers are pyroxenitic to gabbroic, and less commonly peridotitic (lherzolite, dunite, websterite) and anorthositic. These xenoliths are interpreted as samples of a layered intrusion, located at the crust-mantle boundary. Primary cumulate phases are olivine and orthopyroxene, followed by clinopyroxene and plagioclase; rare intercumulus accessory phases (apatite, rutile and zircon) are observed in the most differentiated layers. Homogeneous xenoliths, interpreted as single cumulate layers, have a calc-alkaline geochemistry with LREE and large ion lithophile elements (LILE) enrichments relative to Nb, Ta and Ti. The negative Eu anomaly of pyroxenite can be related to earlier plagioclase fractionation, as observed in the gabbroic layers. Trace element laser ablation inductively coupled plasma emission mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS) analyses of plagioclase, orthopyroxene and zircon from layered rocks suggest equilibrium and cogenetic relations between the silicate phases. U-Pb SIMS dating of a 1.5 mm zircon crystal gives a magmatic or sub-solidus equilibration age of 257 ± 6 Ma. The Beaunit layered intrusion belongs to the large Permian within-plate magmatic episode commonly of calc-alkaline geochemical signature observed over Europe and North Africa. It probably corresponds to a mafic underplating event spatially controlled by post-Variscan trans-tensional to trans-pressional basin tectonics in an intracontinental setting. The subduction-related geochemical signature of the magmatic suite is interpreted as resulting from the passive remobilisation of a mantle source, which was previously metasomatised during the Variscan subduction.

Growth and collapse of a deeply eroded orogen: Insights from structural, geophysical, and geochronological constraints on the Pan‐African evolution of NE Mozambique

This paper presents results of a large multidiciplinary geological mapping project in NE Mozambique, with a focus on the structural evolution of this part of the East African Orogen (EAO). It integrates field structural studies with geophysical interpretations and presents new geochronological data. The tectonic architecture of NE Mozambique can be subdivided into five megatectonic units on the basis of lithology, structure and geochronology: unit 1, Paleoproterozoic Ponta Messuli Complex in the extreme NW corner of NE Mozambique, which represents the local NW foreland to the EAO; unit 2, a collage of Mesoproterozoic metamorphic complexes, which forms the basement to unit 3, a stack of Neoproterozoic, NW directed imbricate thrust nappes named here the ‘‘Cabo Delgado Nappe Complex’’ (CDNC); unit 4, restricted Neoproterozoic metasedimentary basins; and unit 5, two exotic Neoproterozoic granulite me´ lange complexes. The units were assembled during a long and complex history of NWdirected shortening, which commenced with nappe stacking and emplacement of the CDNC over the Mesoproterozoic basement terranes toward the NW foreland. It is proposed that the CDNC and the Eastern Granulites farther north in Tanzania are remnants of Neoproterozoic volcanic arcs and microcontinents formed ‘‘outboard’’ of the Mesoproterozoic continent after 596 ± 11 Ma. Field and potential field geophysical data show that the nappes were folded by regional-scale NE–SW trending folds that formed in response to a later stage of the same shortening episode and this episode gave rise to the Lurio Belt, a prominent structural feature of northern Mozambique and a key element (often as suture zone) in many Gondwana reconstructions. The Lurio Belt is here interpreted as a structure generated during folding of the CDNC during later stages of the progressive shortening event. It is, however, a repeatedly reactivated shear zone, probably at the site of an older (Mesoproterozoic?) discontinuity, with an intense pure shear deformation history. It is cored by strongly attenuated lenses of a granulitic tectonic me´lange, the Ocua Complex (megatectonic unit 5) and is intruded by Late Pan-African granitoids of the Malema Suite. The compressional phase of the orogen was postdated by NW–SE directed extension. New U-Pb zircon and monazite dates show that extension was initiated at circa 540 Ma in the eastern Lurio Belt. It is argued that extension was the result of a major episode of orogenic collapse of the EAO, initiated by gravitational instabilities resulting from crustal thickening during the shortening phase.

Hornblende 40Ar/39Ar geochronology across terrane boundaries in the Sveconorwegian Province of S. Norway

Abstract Hornblende incremental heating 40 Ar/ 39 Ar data were obtained from augen gneiss and amphibolite of the Sveconorwegian Province of S. Norway. In the Rogaland-Vest Agder and Telemark terranes, four pyroxene-rich samples, located close (≤ 10 km) to the anorthosite-charnockite Rogaland Igneous Complex, define an age group at 916 + 12/ − 14 Ma and six samples distributed in the two terranes yield another group at 871 + 8/ − 10 Ma. The first age group is close to the reported zircon UPb intrusion age of the igneous complex (931 ± 2 Ma) and the regional titanite UPb age (918 ± 2 Ma), whereas the second group overlaps reported regional mineral RbSr ages (895-853 Ma) as well as biotite KAr ages (878-853 Ma). In the first group, the comparatively dry parageneses of low- P thermal metamorphism (M2) associated with the intrusion of the igneous complex are well developed, and hornblende 40 Ar/ 39 Ar ages probably record a drop in temperature shortly after this phase. In other hornblende + biotite-rich samples, with presumably a higher fluid content, the hornblende ages are probably a response to hornblende-fluid interaction during a late Sveconorwegian metamorphic or hydrothermal event. A ca 220 m.y. diachronism in hornblende 40 Ar/ 39 Ar ages is documented between S. Telemark ( ca 870 Ma) and Bamble ( ca 1090 Ma). Differential uplift between these terranes was mostly accommodated by shearing along the Kristiansand-Porsgrunn shear zone. The final stage of extension along this zone occurred after intrusion of the Herefoss post-kinematic granite at 926 ± 8 Ma. On the contrary, the southern part of the Rogaland-Vest Agder and Telemark terranes share a common cooling evolution as mineral ages are similar on both sides of the Mandal-Ustaoset Line the tectonic zone between them. The succession within 20 m.y. of a voluminous pulse of post-tectonic magmatism at 0.93 Ga, a phase of high- T -low- P metamorphism at 0.93-0.92 Ga, and fast cooling at a regional scale ca 0.92 Ga, suggests that the southern parts of Rogaland-Vest Agder and Telemark were affected by an event of post-thickening extension collapse at that time. This event is not recorded in Bamble.