Arne P. Willner

Single zircon ages, PT evolution and Nd isotopic systematics of high-grade gneisses in southern Malawi and their bearing on the evolution of the Mozambique belt in southeastern Africa

The high grade gneiss assemblage of central and southern Malawi belongs to the Neoproterozoic Mozambique belt of East Africa, and reached peak metamorphic conditions at 900±70°C and 9.5±1.5 kbar, followed by an isobaric cooling path. We report single zircon U–Pb and Pb–Pb ages and Nd isotopic data for orthogneisses and metapelites collected around Lilongwe and farther south in the region around Blantyre and Zomba. The ages document three distinct events, (1) a Kibaran-age period of intrusion of calc-alkaline granitoids around 1040–929 Ma; Nd isotope data indicate overall juvenile compositions consistent with a magmatic arc environment, or emplacement into thinned continental crust with little involvement of older basement; (2) a Pan-African period of intrusion of calc-alkaline granitoids around 710–555 Ma; Nd isotopes for most samples indicate crustal residence ages of 1.0–1.5 Ga and suggest either remelting of ∼1.0–1.5 Ga (Kibaran) protoliths or mixing of juvenile material with subordinate amounts of older crust; (3) a long-lasting thermal peak of Pan-African high grade metamorphism around 571–549 Ma. The chemical composition of the dated orthogneisses is compatible with an origin of their protoliths in an Andean-type active continental margin setting. Penetrative deformation occurred prior to the thermal peak of metamorphism and also affected the youngest intrusions. It must, therefore, have been of Pan-African age as was the late, retrograde amphibolite facies overprint. Rocks with presumably rather different histories prior to the thermal peak were stacked together. An earlier Kibaran granulite metamorphism, postulated by previous authors, could not be verified. Rather uniform and widespread maximum PT-conditions of the granulite facies in central and southern Malawi, followed by near-isobaric, slow cooling indicate a transient metamorphic gradient of about 26°C/km and suggest stacking of a relatively hot crust and subsequent slow exhumation. The late Kibaran and Pan-African magmatic events in Malawi may be linked to granitoid magmatism of similar ages in northwestern Mozambique and the central Zambezi belt of northern Zimbabwe, but the geodynamic relationships between these terrains remain obscure. The variation in age of peak metamorphism between ∼640 and 520 Ma across the Mozambique belt from Malawi to Antarctica suggests that this belt was assembled from a number of terranes that accreted at different times over a period of ca. 100 million years. The available data, therefore, suggest that East Gondwana was not a coherent block colliding with West Gondwana but consisted of individual terranes before being amalgamated into the supercontinent Gondwana some 550–530 Ma ago.

The Puncoviscana trough — a large basin of Late Precambrian to Early Cambrian age on the pacific edge of the Brazilian shield

During the Late Precambrian to Early Cambrian a large basin with a homogeneous psammitic-pelitic sediment fill existed in the area of the NW-Argentine Andes. It is now exposed in different tectonic levels. This basin of meridional elongation was situated on a stable continental margin at the western edge of the Brazilian shield. It was underlain by segmented older continental crust. According to the modal and chemical composition of the greywackes and subgreywackes a predominant metasedimentary source may be supposed. Four clastic facies types indicate a transport by gravity currents within a submarine fan system, while massive red pelites represent cut-off periods, and rare carbonates deposition on rises. The gravity currents derived from easterly directions. The sedimentation age is proved by a rich ichnofauna and some medusoid impressions.ZusammenfassungIm Jungpräkambrium bis Unterkambrium existierte im Bereich der NW-argentinischen Anden ein ausgedehnter Trog mit homogener psammitisch-pelitischer Füllung, die heute in verschiedenen Gebirgsstockwerken aufgeschlossen ist. Dieses Becken lag an einem stabilen Kontinentalrand im Westen des Brasilianischen Schildes und war von segmentierter älterer, kontinentaler Kruste unterlagert. Nach der modalen und chemischen Zusammensetzung der Grauwacken und Subgrauwacken sind überwiegend metasedimentäre Herkunftsgesteine anzunehmen. Vier klastische Faziestypen lassen auf Transport durch Trübeströme in submarine Fächer schließen, während mächtige Rotpelite Ruhezeiten und die seltenen Karbonate Ablagerungen auf Schwellen anzeigen. Die Schüttungen stammen aus östlichen Richtungen. Das Sedimentationsalter ist durch eine reichhaltige Ichnofauna und einige Medusenabdrücke dokumentiert.RésuménDurante el Precámbrico superior hasta el Cámbrico inferior en los Andes del NW Argentino se extendía una larga cuenca rellena de sedimentos psamo-pelíticos homogéneos, que hoy afloran en distintos pisos tectónicos. Esta cuenca de elongación meridional estaba situada en un margen continental estable al lado occidental del escudo brasileño, sobre corteza continental segmentada más antigua. Según su composición modal y química, las grauvacas y subgrauvacas derivaron predominantemente de rocas metasedimentarias. Cuatro tipos de facies clásticas indican su transporte en corrientes de gravedad, provenientes del este, y sedimentación en conos submarinos. Pelitas rojas macizas representan períodos de sedimentación pelágica, mientras que los escasos carbonatos se depositaron en dorsales. Una icnofauna amplia y algunas impresiones de medusoides documentan la edad de sedimentación.Краткое содержаниеВ регионе северо-запа да аргентинских Анд о т позднего докембрия до нижнего кембрия существовал обширный трог, заполн енный гомогенными псаммито-пелитовыми отложениями, которые сегодня обнажаются в составе различных тектониче ских этажей. Этот бассейн н аходился на стабильн ом материковом крае на западе бразил ьского щита и его подстилала более дре вняя осалочная матер иковая кора. По модальному и х имическому составу г раувак и субграувак они явля ются породами преиму щественно осадочно-метаморфич еского происхождени я. Наличие четырех типо в кластических отлож ений позваляет предполаг ать перенос их мутевн ыми потоками, расходивши мися под водой веером, в то время, как мощные крас ноцветные пелиты ука зывают на период спокойстви я, а редке отложения ка рбонатов на порогов. Область сн оса, вероятно, находил ась на востоке. Возраст от ложений определяют здесь с помощью богат ой ихнофауны и по отде льным отпечаткам медуз.

Numerical modelling of PT-paths related to rapid exhumation of high-pressure rocks from the crustal root in the Variscan Erzgebirge Dome (Saxony/Germany)

Abstract The Bohemian Massif in the Central European Variscides contains many crustal slices with (ultra-)high-pressure rocks related to continent-continent collision. After closure of pre-existing oceans during the Devonian, excess crustal thickness was maintained for about 50 Ma until at around 340 Ma large volumes of high-pressure rocks from the crustal root were exhumed within a few million years. We relate this event to delamination and complete detachment of the lithospheric mantle, causing a crustal-scale isostatic instability. In the Erzgebirge dome, a model region in the northern part of the massif, an array of interrelated PTtd-paths with “decompression/cooling” and “decompression/heating” trajectories in juxtaposed tectonometamorphic units has been established. Numerical 2D-experiments using a rheologically, thermally and dynamically consistent convection technique show three stages of the crustal evolution related to delamination and detachment of mantle lithosphere under the crustal root: (1) During delamination a rapid overthickening of the crust can occur with the crust penetrating down to >160 km depth. (2) After detachment extensional crustal thinning controlling exhumation occurs with escape of rocks from the crustal root towards the margins of the orogen through tectonically weak zones. Horizontal displacement exceeds vertical by a factor of ∼3. (3) Forced circulation in the weak zones follows and upward flow of lower crustal rocks is compensated by subduction of upper crustal rocks in the footwall of these zones. One-dimensional modelling was used in order to further understand basic processes and to simulate the rock record in detail. According to 2D and 1D modelling, strongly decelerating exhumation rates with decreasing overburden and a late increase in the geothermal gradient due to upward heat transfer are necessary corollaries of this scenario, in keeping with observations from the Erzgebirge dome. Exhumation PT-paths do not conform to one single uniform exhumation trajectory; rather, assemblages of interrelated PTtd-paths are characteristic.

Phase relations and dehydration behaviour of psammopelite and mid-ocean ridge basalt at very-low-grade to low-grade metamorphic conditions

P–T pseudosections were calculated in the system Na–Ca–K–Fe–Mg–Al–Si–Ti–H–O with the PERPLE_X software package for the pressure–temperature range 1–25 kbar and 150–450 °C to gain a better understanding of the phase relations and the dehydration behaviour of psammopelite and mid-ocean ridge basalt (MORB) during prograde metamorphism at very-low-grade and low-grade. For this purpose, the thermodynamic data set of Holland & Powell was enlarged by end-member data for Fe2+- and Fe3+-pumpellyite, Fe2+- and Mg-stilpnomelane, actinolite, and magnesioriebeckite. In addition, a three-component solid-solution model for pumpellyite, a two-component model for stilpnomelane, and four-component models for amphibole and sodic pyroxene were created. Studied metamorphosed MORB and psammopelite contain around 6 wt.% structural H2O bound in minerals at 150 °C and pressures up to 5 kbar. Prograde metamorphism causes dehydration patterns, which are, for example, important for an understanding of the formation of accretionary-wedge systems: along a relatively high geotherm of 15 °C/km, significant dehydration (1.5 wt.% H2O release) of metapsammopelite can be noted in the temperature range 220–240 °C. We believe that this process leads to softening of the sedimentary cover of oceanic crust during early subduction so that this material can be scraped off the basic crust, which then would dehydrate at higher T , to form frontal accretionary prisms. Basal accretionary prisms are generated at lower geotherms ( e.g ., 12 °C/km) by dehydration of metapsammopelite between 260 and 300 °C. Again, metabasic material would dehydrate at significantly higher T and is, therefore, only subordinately involved in accretionary wedges. Along geotherms lower than 7 °C/km, almost no water is released up to temperatures of 400 °C and more. Thus, the corresponding material is subducted to mantle depths as in the subduction/exhumation channels of collision zones. We also hypothesize that accretionary-wedge complexes of the hot subduction zones in Precambrian times should mainly have formed by frontal accretion.

Inherent gravitational instability of thickened continental crust with regionally developed low- to medium-pressure granulite facies metamorphism

Abstract Petrological arguments show that regionally developed low- to medium-pressure, high-temperature granulite facies metamorphism may critically enhance the lowering of crustal density with depth. This leads to gravitational instability of homogeneously thickened continental crust, mainly due to changes in mineral assemblages and the thermal expansion of minerals in conjunction with the exponential lowering of the effective viscosity of rocks with increasing temperature. It is argued that crustal processes of gravitational redistribution (crustal diapirism) contributing to the exhumation of granulite facies rocks may be activated in this way.

Latest precambrian (Cadomian) zircon ages, Nd isotopic systematics and P-T evolution of granitoid orthogneisses of the Erzgebirge, Saxony and Czech Republic

Single zircons from two orthogneiss complexes, the ‘Grey Gneiss’ and ‘Red Gneiss’, the lowermost tectonic units in the Erzgebirge, were dated. The grey Freiberg Gneiss is of igneous origin and has a 207Pb/206Pb emplacement age of 550±7 Ma. A quartz monzonite from Lauenstein contains idiomorphic zircons with a mean 207Pb/206Pb age of 555±7 Ma as well as xenocrysts ranging in age between ∼ 850 and ∼ 1910 Ma. Red gneisses from the central Erzgebirge contain complex zircon populations, including numerous xenocrysts up to 2464 Ma in age. The youngest, idiomorphic, zircons in all samples yielded uniform 207Pb/206Pb ages between 550±9 and 554±10 Ma. Nd isotopic data support the interpretation of crustal anatexis for the origin of both units. ɛNd(t) values for the grey gneisses are −7.5 and −6.0 respectively, (mean crustal residence ages of ∼1.7–1.8 Ga). The red gneisses have a wider range in ɛNd(t) values from −7.7 to −2.8 (TDM ages of 1.4–1.8 Ga). The zircon ages document a distinct late Proterozoic phase of granitoid magmatism, similar in age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism (∼ 8 kbar/600–650° C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and red gneisses were overthrust by units with abundant high-pressure relicts and a contrasting P-T evolution. Zircon xenocryst and Nd model ages in the range 1000–1700 Ma are similar to those in granitoid rocks of Lusatia and the West-Sudetes, and document a pre-Cadomian basement in parts of east-central Europe that, chronologically, has similarities with the Sveconorwegian domain in the Baltic Shield.

Inherent gravitational instability of hot continental crust: Implications for doming and diapirism in granulite facies terrains

Modeling of in situ rock properties based on a Gibbs free energy minimization approach shows that regional metamorphism of granulite facies may critically enhance the decrease of crustal density with depth. This leads to a gravitational instability of hot continental crust, resulting in regional doming and diapirism. Two types of crustal models have been studied: (1) lithologically homogeneous crust and (2) heterogeneous, multilayered crust. Gravitational instability of relatively homogeneous continental crust sections is related to a vertical density contrast developed during prograde changes in mineral assemblages and the thermal expansion of minerals with increasing temperature. Gravitational instability of lithologically heterogeneous crust is related to an initial density contrast of dissimilar intercalated layers enhanced by high-temperature phase transformations. In addition, the thermal regime of heterogeneous crust strongly depends on the pattern of vertical interlayering: A strong positive correlation between temperature and the estimated degree of lithological gravitational instability is indicated. An interrelated combination of two-dimensional, numerical thermomechanical experiments and modeling of in situ physical properties of rocks is used to study the processes of gravitational redistribution within a doubly stacked, heterogeneously layered continental crust. It is shown that exponential lowering of viscosity with increasing temperature, in conjunction with prograde changes in metamorphic mineral assemblages during thermal relaxation after collisional thickening of the crust, provide positive feedback mechanisms leading to regional doming and diapirism that contribute to the exhumation of high-grade metamorphic rocks.

Thermal regime and gravitational instability of multi-layered continental crust : Implications for the buoyant exhumation of high-grade metamorphic rocks

Large-scale crustal thickening by tectonic and/or magmatic processes can lead to various complex patterns of multi-layered continental crust. It is well-known from one-dimensional thermal modeling that variations in total crustal thickness, mantle lithospheric thickness, thermal conductivities of the crust, and bulk radiogenic heat production of the crust will lead to variable geotherms in such heterogeneously stacked crust. By systematically changing the above parameters, we illustrate that variations on the order of 100–500°C will result at a depth of 30 km. Specifically, we show that geotherms are also strongly dependent on the pattern of vertical interlayering. Assuming a crustal structure composed of idealized granodioritic/gabbroic or granodioritic/dioritic compositional layer sequences, it can be shown that such gravitationally unstable, stacked, multi-layered continental crust can lead to temperature variations in geotherms of comparable magnitude as for the above parameters. Geotherms exhibiting the highest temperatures at a given depth are characteristic for gravitationally unstable structures in which the bulk of the granodioritic rocks underlie dioritic or gabbroic rocks. Thus a strong positive correlation between temperature and the estimated degree of gravitational instability of the multilayered crust is indicated. It is argued that the lowering of the viscosity of rocks with increasing temperature after tectonic or magmatic stacking will set the stage for processes of gravitational redistribution and buoyant exhumation of high-grade metamorphic rocks. Prograde changes in metamorphic mineral assemblages and partial melting during thermal relaxation after stacking provide positive feed-back mechanisms to enhance the possibility of gravitational redistribution. In keeping with the published results of Babeyko & Sobolev (2001) and Arnold et al. (2001), we find that gravitational overturn can be triggered only when external tectonic forces are active after stacking. Time-scales of 10 to 100 Myr are indicated for differential movement of rock masses on a kilometer-scale when the viscosity of crustal rocks is lowered to n × 10 21 Pa·s, but may be considerably less if zones of tectonic weakness in the crust lead to a further local decrease in effective rock viscosity.

Structural contacts in subduction complexes and their tectonic significance: the Late Palaeozoic coastal accretionary wedge of central Chile

Understanding the contact between the very low-grade metagreywacke of the Eastern Series and high-pressure metamorphosed schist of the Western Series in the Late Palaeozoic accretionary wedge of central Chile is fundamental for the understanding of the evolution of ancient accretionary wedges. We show the progressive development of structures and finite strain from the least deformed rocks in the eastern part of the Eastern Series of the accretionary wedge to high-pressure schist of the Western Series at the Pacific coast. Upright chevron folds of sedimentary layering are associated with an axial-plane foliation, S1. As the F1 folds became slightly overturned to the west, S1 was folded about west-vergent open F2 folds and an S2 axial-plane foliation developed. Near the contact between the Western and Eastern Series S2 represents a penetrative subhorizontal transposition foliation. Towards the structurally deepest units in the west the transposition foliation becomes progressively flattened. Finite-strain data as obtained by Rf/ϕ analysis in metagreywacke and X-ray texture goniometry in phyllosilicate-rich rocks show a smooth and gradual increase in strain magnitude from east to west. Overturned folds and other shear-sense indicators show a uniform top-to-the-west shear sense in moderately deformed rocks, whereas the shear sense is alternating top-to-the-west and top-to-the-east in the strongly flattened high-pressure rocks of the Western Series near the Pacific coast. We interpret the progressive structural and strain evolution across the contact between the two series to reflect a continuous change in the mode of accretion in the subduction wedge. Initially, the rocks of the Eastern Series were frontally accreted to the pre-Andean margin before c. 300 Ma. Frontal accretion caused horizontal shortening, and upright folds and subvertical axial-plane foliations developed. At c. 300 Ma the mode of accretion changed and the rocks of the Western Series were underplated below the Andean margin. This basal accretion caused a major change in the flow field within the wedge and gave rise to vertical shortening and the development of the penetrative subhorizontal transposition foliation. Subsequent differential exhumation was resolved gradually over a wide region, implying that exhumation was not tectonically controlled.

Origin and evolution of Avalonia: evidence from U–Pb and Lu–Hf isotopes in zircon from the Mira terrane, Canada, and the Stavelot–Venn Massif, Belgium

Laser ablation inductively coupled plasma mass spectrometry U–Pb and Lu–Hf isotope analyses of detrital zircon from Neoproterozoic–Cambrian clastic sedimentary rocks in the Mira terrane (Cape Breton Island, Nova Scotia, Canada; West Avalonia) and the Stavelot–Venn Massif (East Belgium; East Avalonia) support deposition on an originally coherent microcontinent. Crustal evolution trends defined by ϵHf(t) values varying with age reflect juvenile magma production in the source continent at 1.2–2.2 and 2.4–3.1 Ga. Mixing of juvenile and recycled crust in continental magmatic arcs is recognized at 0.5–0.72, 1.4–1.7, 1.8–2.2 and 2.4–2.7 Ga. These results concur with the crustal evolution in Amazonia, the likely parent craton. Crustal evolution in Avalonia is recorded in detrital and magmatic zircon from Neoproterozoic arcs (680–550 Ma). Positive ϵHf(t) values suggest juvenile input and mixing with recycled crust. Most negative ϵHf(t) values represent recycling of predominantly Mesoproterozoic underlying crust. Avalonian arc magmatism was followed by late Neoproterozoic–early Cambrian sedimentation in various belts in West Avalonia. These belts were juxtaposed by strike-slip during late early Cambrian deposition in a rift basin. The youngest detrital zircon population (c. 517 Ma) probably represents synrift magmatism before break-up of Avalonia. Migmatization at 406 ± 2 Ma in a xenolith from the East Avalonian crust reflects post-collisional heating. Supplementary materials: Details of sample locations and analytical results are available at www.geolsoc.org.uk/SUP18641.