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. _Nd(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 _Nd(t) values from –7.7 to –2.8 (T _DM 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.     

Tectono-thermal evolution of a part of a Variscan magmatic arc: The Odenwald in the Mid-German Crystalline Rise

Geothermobarometric studies of plutonic and medium to high grade metamorphic rocks of the Odenwald (SW Germany) provide data for the construction of PT-paths of four different crustal sections within a part of a magmatic arc of the Mid-European Hercynian orogen.The evolution of the thermal structure of the Odenwald is characterized by an early geotherm of 35–40°C/km after crustal stacking involving rocks with medium pressure relics. Peak PT-conditons reached 4–5 kbar and 650°C. Only rocks of the northernmost Odenwald show evidence for PT conditions related to a higher geotherm at this time. Early uplift occurred during normal oblique sinistral strike shear. The central Odenwald was uplifted into a level corresponding to 3 kbar during intrusion of calcalkaline magmas. The northernmost Odenwald subsided at the same time, as proved by an anticlockwise PT-path. Subsequently, in the southern and eastern Odenwald, rapid uplift caused a further increase of the geotherm to 60–80°C/km. This is concomitant with extension and granitoid intrusions.Crustal thickening in the Odenwald is restricted to an early phase during the Devonian. The magmatic arc developed within a »pull-apart«-like structure during the Lower Carboniferous.

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Zusammenfassung Geothermobarometrische Untersuchungen an Plutoniten und mittelbis hochgradigen Metamorphiten des Odenwaldes ermöglichen die Konstruktion von PT-Pfaden für vier Krustenabschnitte in einem Teil eines variszischen magmatischen Gürtels.Zu Beginn der Entwicklung der thermischen Struktur des magmatischen Gürtels stellte sich ein Geotherm von 35–40°C/km nach einer Krustenstapelung ein, in die Gesteine mit Mitteldruckrelikten einbezogen wurden. Die PT-Bedingungen zum Höhepunkt der Metamorphose erreichten 4–5 kbar und 650°C. Nur Gesteine des nördlichen Odenwaldes belegen PT-Bedingungen, die auf einen höheren Geotherm zu dieser Zeit hinweisen. Frühe Hebung fand entlang von Schrägabschiebungen statt. Der zentrale Odenwald wurde bei gleichzeitiger Intrusion kalkalkaliner Magmen in ein Niveau entsprechend 3 kbar gehoben. Der nördliche Odenwald sank gleichzeitig ab, wie durch einen gegen den Uhrzeigersinn gerichteten PT-Pfad belegt wird. Im südlichen und östlichen Odenwald bewirkte danach schnelle Hebung einen weiteren Anstieg des Geotherms auf 60–80°C/km verbunden mit Dehnung und Intrusion granitoider Magmen.Während Krustenverdickung auf eine frühe Phase im Devon beschränkt ist, entwickelt sich der magmatische Gürtel im Bereich des Odenwaldes im Unterkarbon innerhalb einer » pull apart «-ähnlichen Struktur.

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Résumé Dans l'Odenwald (sud-ouest de l'Allemagne), l'étude géothermobarométrique de roches plutoniques et de roches métamorphiques de degré moyen à élevé fournit les données qui permettent la construction de trajets (P, T) relatifs à quatre sections crustales dans une partie d'un arc magmatique de l'orogène varisque d'Europe.Le début de l'évolution de la structure thermique de l'Odenwald a été marqué par un gradient géothermique de 35–40°C/Km, en suite à un empilement crustal impliquant des roches qui contiennent des reliques de pression moyenne. Les conditions (P, T) du métamorphisme maximal furent de 4–5 Kbar et 650°C. Seules les roches de l'Odenwald septentrional témoignent d'un gradient plus élevé à cette époque. Un soulèvement hâtif s'est produit le long de zones de cisaillement sénestres. Concomitamment à l'intrusion de magmas calcoalcalins, l'Odenwald central a été le siège d'un soulèvement jusqu'à un niveau correspondant à 3 Kbar. En même temps, l'Odenwald septentrional s'affaissait, comme en témoigne uu trajet (P, T) anti-horlogique. Par la suite, dans l'Odenwald méridional et oriental, une montée rapide associée à un processus d'extension et à l'intrusion de magmas granitoïdes a amené le gradient géothermique à des valeurs de 60 à 80° /Km.L'épaississement crustal est cantonné à une phaseprécoce d'âge dévonien, tandis que l'arc magmatique s'est développé dans une structure ra pullapart « au cours du Carbonifère inférieur.

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T- . , , , 35–40°/ . T- 650° 4–5 , T- , . . , 3 , - . , T-, . 60–80°/, . , , «pullapart» .

     

Time of formation and peak of Variscan HP-HT metamorphism of quartz-feldspar rocks in the central Erzgebirge,Saxony, Germany

The Variscan Erzgebirge represents an antiform with a core of gneisses and mica schists, surrounded by a phyllitic mantle. The Gneiss-Eclogite Unit (GEU), in the central part, is a composite tectonometamorphic assemblage characterized by a HP-HT imprint and comprises migmatitic para- and orthogneisses, HT mylonites, HP granulites, eclogites and garnet peridotites. It is tectonically sandwiched between two major units with distinctly lower PT histories. The GEU experienced a characteristic “kinked” retrograde PT path after HP-HT equilibration with: (1) strong near-isothermal decompression at high temperatures; (2) extensive re-equilibration at medium pressures, followed (3) by rapid cooling during continued uplift. We dated zircons (Pb-Pb evaporation) from granitoid orthogneisses and metapelites of the GEU. The orthogneisses contain euhedral, long-prismatic zircons of igneous origin that provided protolith ages between 470 and 524 Ma. Metapelites retain well-preserved granulite-facies mineral assemblages and contain spherical, multifaceted metamorphic zircons that grew near the peak of HP/HT metamorphism. Inclusions of prograde HP phengite (∼15 kbar) and rutile are included in one such zircon. Metamorphic zircons of three samples from different localities yielded identical ²⁰⁷Pb/²⁰⁶Pb ages of 340.5 ± 0.7 Ma, 341.2 ± 0.5 Ma and 341.6 ± 0.5 Ma respectively. Consideration of these zircon ages with published ³⁹Ar/⁴⁰Ar white mica ages suggests fast cooling and uplift rates in excess of 50 °C/Ma and 4 km/Ma. This is typical for large-scale extensional tectonic unroofing of the ultra-deep part of a fossil, thickened Variscan continental crust (>60 km) during continuing continental collision and orogenic collapse. Received: 5 June 1997 / Accepted: 7 January 1998     

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.

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Zusammenfassung Im 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.

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Résumén Durante 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.

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The metamorphic complex of Beloretzk, SW Urals, Russia — a terrane with a polyphase Meso- to Neoproterozoic thermo-dynamic evolution

An integrated geological study of the tectono-metamorphic evolution of the metamorphic complex of Beloretzk (MCB) which is part of the eastern Bashkirian mega-anticlinorium (BMA), SW Urals, Russia shows that the main lithological units are Neoproterozoic (Riphean and Vendian age) siliciclastic to carbonate successions. Granitic, syenitic and mafic intrusions together with subaerial equivalents comprise the Neo- and Mesoproterozoic magmatic rocks. The metamorphic grade ranges from diagenetic and very low grade in the western BMA to high-grade in the MCB. The N–S trending Zuratkul fault marks the change in metamorphic grade and structural evolution between the central and eastern BMA. Structural data, Pb/Pb-single zircon ages, ⁴⁰Ar/³⁹Ar cooling ages and the provenance signature of Riphean and Vendian siliciclastic rocks in the western BMA give evidence of Mesoproterozoic (Grenvillian) rifting, deformation and eclogite-facies metamorphism in the MCB and a Neoproterozoic (Cadomian) orogenic event in the SW Urals. Three pre-Ordovician deformation phases can be identified in the MCB. The first SSE-vergent, isoclinal folding phase (D₁) is younger than the intrusion of mafic dykes (Pb/Pb-single zircon: 1350 Ma) and older than the eclogite-facies metamorphism. High P/low T eclogite-facies metamorphism is bracketed by D₁ and the intrusion of the Achmerovo granite (Pb/Pb-single zircon: ≤970 Ma). An extensional, sinistral, top-down-to-NW directed shearing (D₂) is correlated with the first exhumation of the MCB. E-vergent folding and thrusting (D₃) occurred at retrograde greenschist-facies metamorphic conditions. The tremolite ⁴⁰Ar/³⁹Ar cooling age (718±5 Ma) of amphibolitic eclogite and muscovite ⁴⁰Ar/³⁹Ar cooling ages (about 550 Ma) of mica schists indicate that a maximum temperature of 500±50 °C was not reached during the Neoproterozoic orogeny. The style and timing of the Neoproterozoic orogeny show similarities to the Cadomian-aged Timan Range NW of the Polar Urals. Geochronological and thermochronological data together with the abrupt change in structural style and metamorphism east of the Zuratkul fault, suggest that the MCB is exotic with respect to the SE-margin of the East European Platform. Thus, the MCB is named the ‘Beloretzk Terrane’. Recognition of the ‘Beloretzk Terrane’ and the Neoproterozoic orogeny at the eastern margin of Baltica has important implications for Neoproterozoic plate reconstruction and suggests that the eastern margin of Baltica might have lain close to the Avalonian–Cadomian belt.     

IRAC photometric analysis and the mid-IR photometric properties of Lyman-break galaxies

We present photometric analysis of deep mid-infrared (mid-IR) observations obtained by Spitzer /IRAC covering the fields Q1422+2309, Q2233+1341, DSF2237a,b, HDFN, SSA22a,b and B20902+34, giving the number counts and the depths for each field. In a sample of 751 Lyman-break galaxies (LBGs) lying in those fields, 443, 448, 137 and 152 are identified at 3.6-, 4.5-, 5.8-, 8.0-μm IRAC bands, respectively, expanding their spectral energy distribution to rest-near-IR and revealing that LBGs display a variety of colours. Their rest-near-IR properties are rather inhomogeneous, ranging from those that are bright in IRAC bands and exhibit  [ R ]−[3.6] > 1.5  colours to those that are faint or not detected at all in IRAC bands with  [ R ]−[3.6] < 1.5  colours and these two groups of LBGs are investigated. We compare the mid-IR colours of the LBGs with the colours of star-forming galaxies and we find that LBGs have colours consistent with star-forming galaxies at   z ∼ 3  . The properties of the LBGs detected in the 8-μm IRAC band (rest-frame K band) are examined separately, showing that they exhibit redder  [ R ]−[3.6]  colours than the rest of the population and that although in general, a multiwavelength study is needed to reach more secure results, IRAC 8-μm band can be used as a diagnostic tool, to separate high z , luminous AGN-dominated objects from normal star-forming galaxies at   z ∼ 3  .