Fernando Corfu

Atlas of Zircon Textures

The mineral zircon is extremely variable both in terms of external morphology and internal textures. These features reflect the geologic history of the mineral, especially the relevant episode(s) of magmatic or metamorphic crystallization (and recrystallization), strain imposed both by external forces and by internal volume expansion caused by metamictization, and chemical alteration. The paper presents a selection of both the most typical, but also of the less common, features seen in zircon, categorized according to the different geological processes responsible for their formation. The atlas is intended as a general guide for the interpretation of zircon characteristics, and of related isotopic data. Zircon has become one of the most widely used minerals for the extraction of information on the prehistory and genesis of magmatic, metamorphic and sedimentary rocks. Much of the geological usefulness of zircon stems from its suitability as a geochronometer based on the decay of U (and Th) to Pb, but in addition it is also the major host of the radiogenic isotopic tracer Hf, and it is used to determine oxygen isotopic compositions and REE and other trace element abundances, all of which yield useful clues concerning the history of the host rock, and in some case, the parent rock in which the precursor zircon crystallized. One of the major advantages of zircon is its ability to survive magmatic, metamorphic and erosional processes that destroy most other common minerals. Zircon-forming events tend to be preserved as distinct structural entities on a pre-existing zircon grain. Because of this ability, quite commonly zircon consists of distinct segments, each preserving a particular period of zircon-formation (or consumption). A long experience and modern instrumentation and techniques have provided the “zircon community” the means to image and interpret preserved textures, and hence to decipher the history and evolution of a rock. One …

U-Pb ages from SW Poland: evidence for a Caledonian suture zone between Baltica and Gondwana

Faunal and palaeomagnetic evidence suggests the existence of a c. 3000 km-wide Tornquist Sea between Gondwana and Baltica in Early Ordovician times, which narrowed to <1000 km by the Late Ordovician. The inferred suture zone between sequences with ‘Baltic’ faunas in Poland and the ‘Gondwana’ faunas in Czechoslovakia is characterized by a collage of six terranes which have distinct Cambrian to Carboniferous histories. They have geochemical characteristics and sedimentary associations of volcanic rocks in marginal basins, island arcs, ophiolites and volcanic arc granites, give Tremadoc and Arenig protolith U–Pb zircon ages, and have been regionally metamorphosed. A syn-metamorphic granite is dated as pre-Lower Ashgill (461−50–2 Ma). An unmetamorphosed ophiolite gives a 420−20–2 Ma age, whilst in two other terranes amphibolite-facies island-arc lavas have fossiliferous Lower Ludlow (424–415 Ma) sequences: all are unconformably overlain by unmetamorphosed Upper Devonian conglomerates. A sinistral transpressive regime is observed in regionally extensive mylonite zones. Titanite and zircon ages (338−2–3 and 339 ± 4 Ma) record Variscan magmatism. The data suggest considerable narrowing of the Tornquist Sea during the Ordovician, continuing ocean floor and island arc activity in the Silurian, and final sinistral transpressive closure by the Mid-Devonian.

Evolution of the southern Abitibi greenstone belt based on U–Pb geochronology: autochthonous volcanic construction followed by plutonism, regional deformation and sedimentation

Abstract New mapping and U–Pb zircon geochronological results in the southern Abitibi greenstone belt (SAGB) support an autochthonous regional stratigraphy comprised of nine supracrustal assemblages, rather than the collage of allochthonous terranes proposed in recent publications. Based on lithological and geochronological criteria these supracrustal assemblages also encompass the Swayze, Shining Tree and Montcalm greenstone belts indicating that, despite separation by large granitic intrusions, they are in fact part of one extensive greenstone belt. In conjunction with the mapping, the geochronological results support coherent, upward-facing stratigraphic sections with about 20% of the samples containing inherited zircons with ages similar to those found in underlying assemblages. The seven oldest assemblages represent semi-continuous volcanism from 2750 to 2697 Ma. The volcanism is compositionally diverse ranging from komatiite and tholeiitic basalt to calc–alkaline mafic to felsic lavas. Intimate intermingling of the different magma clans occurs throughout much of the stratigraphic section. Regional fault control on the distribution of the volcanic assemblages provides evidence for early dip–slip movement associated with volcanic extension dating back to at least 2725 Ma. The two youngest assemblages are dominantly sedimentary and were unconformably deposited on the volcanic assemblages in close proximity to regional faults. The earlier sedimentary assemblage consists of turbidites and minor iron formation deposited from 2696 to 2692 Ma, and the youngest one of subaerial conglomerates, fluvial sandstones and alkalic to shoshonitic volcanic rocks ranging in age from 2687 to 2675 Ma. The sedimentary assemblages are broadly contemporaneous with the emplacement of syntectonic granitic plutons, regional folding and reactivated movement on the regional faults probably related to accretion of the Abitibi to the Superior Province craton. Autochthonous repetition of different geodynamic environments over the 50 million years of volcanic activity with five dominantly tholeiitic ± komatiitic assemblages and two dominantly calc–alkaline assemblages, as well as the extensive intermingling of these different magma clans in a number of the assemblages, appears to be unique to the Archean. It suggests complex, large-scale and long-lived interaction between mantle plumes and subduction zone magmas. The Nd isotope data indicate a derivation of the diverse volcanic magmas from homogeneously depleted sources with eNd=2.5±0.5 demonstrating a lack of contamination by ancient enriched components.

Genesis of the southern Abitibi greenstone belt, Superior Province, Canada: Evidence from zircon Hf isotope analyses using a single filament technique

Hafnium isotopic analyses, together with U-Pb geochronology, have been performed on zircons from the main units of the Late Archean southern Abitibi greenstone belt in the Superior Province. A new analytical method in which Hf is loaded and run with a mixture of Ir-Mo and C on single filaments was used. The results indicate a relatively homogeneous isotopic composition of the greenstone belt, with ϵHf values of 6.3–4.2 for the 2730-2700 Ma preorogenic assemblages and 4.7-3.6 for the 2690-2675 Ma synorogenic intrusions. Hafnium isotopes and U-Pb ages indicate that the greenstone belt formed in a relatively short time interval within an ensimatic setting, with minimal interaction with older sialic crust. Primary magmas were derived from mantle sources with a depleted isotopic signature of ϵHf ≈ 5.5 ± 0.5, corresponding to published ϵNd ≈ 2.5, 87Sr86Sr ≈ 7011, 207Pb204Pb ≈ 14.4, and 206Pb204Pb ≈ 13.2. Slightly lower ϵHf values in some rocks may record moderate degrees of source heterogeneity; in the case of the alkalic plutons, the lower ϵHf can be interpreted in terms of mantle wedge contamination by subducted components isotopically similar to sedimentary rocks in the adjacent Pontiac Subprovince. The consistent, positive ϵHf signature confirms that the Archean mantle was already depleted prior to formation of 3.0–2.7 Ga Superior Province crust.

Differential response of U-Pb systems in coexisting accessory minerals, Winnipeg River Subprovince, Canadian Shield: implications for Archean crustal growth and stabilization

The U-Pb isotopic systems of zircon, monazite, titanite and some apatite and the Pb isotopic composition of K-feldspar have been investigated in three areas of the Winnipeg River Subprovince (WRS) of the Superior Province, Canada, in order to define the timing of magmatic and metamorphic processes in this Archean gneissic-granitoid terrain.The new data together with published results define the following stages in the evolution of the WRS: (1) an extended period of early crustal growth punctuated by the episodic generation of tonalite. New ages include 3170+20/s-5 Ma, 2875+20/s-5 Ma and 2840+20/s-5 Ma for tonalitic gneisses at Cedar Lake, Kenora and Daniels Lake, respectively. (2) This early evolution was concluded by about 2760 Ma after emplacement of tonalite-granodiorite at Cliff Lake and was followed by a period of magmatic quiescence between about 2760 and 2710 Ma that contrasts with the intensive igneous activity characterizing the evolution of neighbouring greenstone belts. (3) A major episode of magmatism, deformation and metamorphism affected the Kenora and Daniels Lake areas between about 2710 and 2700 Ma. (4) A younger event caused deformation, metasomatism and amphibolite to granulite grade metamorphism at Cedar Lake and Daniels Lake at about 2680 Ma. (5) A subsequent, protracted period of low grade activity reset or (re-)crystallized titanite and apatite defining ages that scatter between about 2640 and 2520 Ma at Cedar and Daniels Lake but not in Kenora where titanite closed by about 2690 Ma. The 2680 Ma metamorphism may have been triggered in part by crustal thickening due to nappe thrusting but the subsequent period of lower grade activity requires the protracted addition of heat and/or fluids probably derived from magmatic and metamorphic processes continuing deep in the crust.The isotopic compositions of K-feldspars are relatively homogeneous and indicate mixing of Pb evolved in different reservoirs. The general enrichment in 207Pb with respect to normal terrestrial Pb reflects the protracted Archean evolution of the terrain.Now-coexisting minerals were formed and closed isotopically at different stages of the complex evolution and were selectively involved or excluded from isotopic equilibration with each other or with external systems such as hydrothermal fluids. This cautions against the indiscriminate interpretation of isotopic values obtained from whole rock systems in such complex terrains.

Uranium-lead zircon and titanite ages from the northern portion of the Western Gneiss Region, south-central Norway

Abstract The geochronologic history of the northern portion of the Western Gneiss Region, south-central Norway has been investigated by U Pb analyses of zircon and titanite from various basement units and one supracrustal gneiss. A six-point (four zircon and two titanite) discordia line from a migmatite melt-pod and its host-gneiss (Astfjord migmatite gneiss) defines upper- and lower-intercept ages of 1659.1 ± 1.8 Ma and 393.6 ± 3.6 Ma, respectively. The upper-intercept age is interpreted as a time of tonalite emplacement and migmatite formation in the region. A seven-point (three zircon and four titanite) discordia line from the Ingdal granite gneiss has similar upper- and lower-intercept ages (1652.9 ± 1.7 Ma and 396.1 ± 4.9 Ma, respectively) and the upper-intercept age is interpreted as the time of granite crystallization. The lower-intercept age from both theAstfjord migmatite gneiss and the Ingdal granite gneiss is interpreted as the time of regional metamorphic resetting and is evidence for Caledonian influence in the region. Titanite from these two gneiss units, as well as from other tonalite, granite, and supracrustal gneisses throughout the Western Gneiss Region displays a remarkably uniform pattern of discordance. Titanite discordance is variable from 6% to 100% and, in general, the degree of discordance is proportional to the Caledonian metamorphic grade of nearby supracrustal schists. Episodic, diffusional lead loss from titanite during Caledonian metamorphism probably caused the discordance pattern, but a combined mechanism of mixing and diffusional lead loss cannot be ruled out. In any case, the seventeen-point titanite and zircon discordia line clearly indicates that: (1) the bulk of the granitoid terrane in this portion of the Western Gneiss Region was emplaced, migmatized, and cooled in a short time interval about 1657 Ma ago; (2) a second, short-lived thermal event that exceeded the blocking temperature of titanite occurred about 395 Ma ago; and (3) titanite and zircon in the region were not isotopically disturbed by comparable geologic events in the period from 1657 to 395 Ma, or at any time after 395 Ma.

Early Archean crust in Bastar Craton, Central India—a geochemical and isotopic study

Abstract High-A12O3 trondhjemitic gneisses occur in enclaves within granites at Markampara in the Bastar Craton, Central India. These gneisses exhibit a highly fractionated rare earth element (REE) pattern, [(frsol|La/Yb)n=60–83] and may have been produced by partial melting of an amphibolitic protolith. A zircon UPb age of 3509+14−7 Ma dates primary crystallization of the trondhjemites. A granite surrounding the gneissic enclaves yields a zircon UPb age of 2480 ±3Ma. RbSr wholerock data for the trondhjemitic gneisses are somewhat disturbed and indicate initial 87 Sr 86 Sr ratios of ∼0.7016. Pb isotopic data are poorly correlated and suggest a three-stage evolution with an early period of high UPb growth (μ≈9.5–10.5) after emplacement at 3.5 Ga, followed by isotopic disturbance and U depletion at 2.5 Ga during widespread magmatism and metamorphism. Loss of Rb and other mobile elements may also have occurred at this time.

Polymetamorphic evolution of the Lewisian complex, NW Scotland, as recorded by U-Pb isotopic compositions of zircon, titanite and rutile

U-Pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the Lewisian complex and bordering Laxford Front reveal complex discordance patterns indicating multiple Late Archean and Early Proterozoic crystallization, overgrowth and Pb-loss events. The earliest stages in the evolution of the complex remain poorly resolved. Zircon ages of ≥2710 Ma date high-grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting. A distinct event at 2490–2480 Ma, possibly initiated by metamorphism and deformation at high-grade conditions, caused amphibolitization of the granulites and emplacement of granitic pegmatites. This event can be correlated with development of Inverian shear zones and formation of granitoid layers along the Laxford Front. The emplacement of a younger generation of granitoid sheets during the Laxfordian event fromed hydrothermal titanite at ≥1754 Ma in gneisses south of the Laxford Bridge and partially reset older titanite at Scourie. Growth of secondary titanite and rutile also occurred during subsequent low-grade metamorphism at 1690–1670 Ma.

The Vila de Cruces Ophiolite: A Remnant of the Early Rheic Ocean in the Variscan Suture of Galicia (Northwest Iberian Massif)

The Vila de Cruces ophiolite is one of the ophiolitic units involved in the Variscan suture of the northwest Iberian Massif. This ophiolite consists of a tectonically repeated succession of greenschist facies volcanic rocks, common alternations of metasediments of pelitic or siliceous character, and scarce orthogneisses, metagabbros, and serpentinites. The protolith age of a granitic orthogneiss that intruded the mafic rocks is dated at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape