Paleoproterozoic Metamorphism Research Articles

U-Pb isotope and REE composition of zircon from the pyroxene crystalline schists of the Irkut terrane, Sharyzhalgai Uplift: Evidence for the Neoarchean magmatic and metamorphic events

The Early Precambrian granulitegneissic com� plexes are typically characterized by a long evolution� ary history, which involves formation of their pro� toliths and repeated metamorphism related mainly with collisional events. Zircons from highgrade meta� morphic rocks usually consist of cores inherited from the protolith, and one or several mantles and rims that were formed during later metamorphism or partial melting. Highresolution methods of U-Pb dating make it possible to determine the age of cores and rims. However, knowledge of the traceelement com� position of zircon is required in order to interpret cor� rectly the obtained dates, because its zones that were grown or recrystallized at hightemperature metamor� phism significantly differ in terms of REE distribution from their magmatic analogues (1-3). This paper reports the results of study of zircons from highgrade basic rocks, i.e., pyroxene crystalline schists, which allowed us to determine the formation time of their magmatic protoliths and metamorphism, as well as to estimate the conditions of zircon formation. The available data indicate that hightemperature metamorphism and granite formation in the Irkut block occurred in several stages. It was reliably estab� lished that Paleoproterozoic metamorphism and related granite and charnockite formation spanned a time range of 1.88-1.85 Ga. These events were recorded in the formation of metamorphogenic zir� cons, their rims, and baddeleyite in the metamorphic rocks, as well as magmatic zircons in the granitoids (4-7). The lower age boundary of the Neoarchean stage of granulite metamorphism is constrained by the crystallization age of magmatic zircon from gabbro (2649 ± 6 Ma) metamorphosed under the granulite� facies metamorphism (7). Vein granites cutting across the crystallization schistosity in metagabbro have an age of 2562 ± 20 Ma. The Neoarchean granulite meta� morphism and collisional granite formation of close age (~2.6 Ga and 2.53 Ga) were also established in the adjacent Kitoi granulite gneiss block (6).

Paleoproterozoic, High‐Metamorphic, Metasedimentary Units of Siberian Craton

Abstract:Sensitive, high‐resolution ion microprobe zircon U–Pb ages of Paleoproterozoic, high‐grade, metasedimentary rocks from the south‐western part of the Siberian Craton are reported. Early Precambrian, high‐grade complexes, including garnet–biotite, hypersthene–biotite, and cordierite‐bearing gneisses compose the Irkut terrane of the Sharyzhalgay Uplift. Protoliths of studied gneisses correspond to terrigenous sediments, ranging from greywacke to shale. The paragneiss model Nd ages of 2.4–3.1 Ga indicate Archean‐to‐Paleoproterozoic source provinces. Zircons from gneisses show core‐rim textures in cathodoluminescence (CL) image. Round or irregular shaped cores indicate detrital origin. Structureless rims with low Th/U are metamorphic in origin. The three age groups of detrital cores are: ≥2.7, ∼2.3, and 1.95–2 Ga. The ages of metamorphic rims range from 1.86 to 1.85 Ga; therefore, the sediments were deposited between 1.95 and 1.86 Ga and derived from Archean and Paleoproterozoic source rocks. It should be noted that Paleoproterozoic metasedimentary rocks of the Irkut Block are not unique. High‐grade metaterrigenous sediments, with model Nd ages ranging from 2.3 to 2.5 Ga, are widely distributed within the Aldan and Anabar Shields of the Siberian Craton. The same situation is observed in the North China Craton, where metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 to 2.1 Ga (Wan et al., 2006). All of these sedimentary units were subjected to Late Paleoproterozoic metamorphism. In the Siberian Craton, the Paleoproterozoic sedimentary deposits are possibly marked passive margins of the Early Precambrian crustal blocks, and their high‐grade metamorphism was related to the consolidation of the Siberian Craton.

SHRIMP U–Pb zircon dating of the Neoproterozoic Penglai Group and Archean gneisses from the Jiaobei Terrane, North China, and their tectonic implications

Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541 ± 5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga. The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866 ± 4 Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at ∼1865 Ma.

How High Are the P-T Conditions for Paleoproterozoic Metamorphism of the Huangtuling Felsic Granulite, North Dabieshan, Central China?

The 2. 34 cm-wide garnet porphyroblast in the Paleoproterozoic felsic granulite from the Huangtuling area, North Dabieshan, has been reinvestigated for compositional variation in light of Cacomposition X-ray mapping to obtain peak P-T conditions of granulite-facies metamorphism. A new core-rim traverse was conducted through where there is little influence on Ca-profile and slight modification in Mn-, Mg- and Fe-profiles with the highest Mg/(Mg+Fe) value of 0. 467. Reasonable peak P-T conditions were estimated to be 1.50–1.70 GPa and 1 100–1 150 °C according to TWQ-based garnet-Al-orthopyroxene thermobarometry. These estimations suggest that the Huangtuling granulite once was subjected to ultrahigh-temperature (UHT) granulite-facies metamorphism following a high-pressure granulite-faices metamorphic stage, implying that a deep subdaction and collision process relevant to the Yangtze block occurred in the Paleoproterozoic time, probably as a response to the global assembly event of the Columbia supercontinent.

Sulfur Isotope Characteristics of Mesothermal Gold Deposits: Results of an Investigation of the Rio Itapicuru Region, Bahia, Brazil

Stable isotope analyses were carried out on sulfides from three mesothermal gold deposits of the Rio Itapicuru greenstone belt (Fazenda Brasileiro, Maria Preta, and Ambrosio mines) to constrain the source of the hydrothermal solutions. The ores are hosted in a volcanic-sedimentary sequence that evolved in an Archean cratonic area reworked during the Transamazonic event (ca. 2.1 Ga). The ore bodies were deposited by hydrothermal solutions in polycyclic shear zones generated by Paleoproterozoic dynamothermal metamorphism. In the Fazenda Brasileiro deposit, δ34S values for arsenopyrite from mineralized quartz veins range from 5.5‰ to +1.0‰; pyrites from Au-rich mineralized veins from the Maria Preta deposit yielded δ34S values ranging from +8.9‰ to +6.9‰, in agreement with published δ34S data (+0.1‰ to +6.6‰). A narrow spread in the δ34S values for Au-rich sulfides from both deposits indicates that gold deposition occurred under conditions involving a small range of fO2 and fH2O. A barren, galena-rich vein from the Ambrosio deposit yielded δ34S values in the -7.6‰ to -9.9‰ range, probably reflecting mixing of different sources. These results, compared to stable sulfur isotope modeling of lode gold deposits, indicate that the studied deposits formed from hydrothermal solutions of metamorphic origin followed by duluite solutions of magmatic origin.

Multi-Stage Modification of the Northern Slave Mantle Lithosphere: Evidence from Zircon- and Diamond-Bearing Eclogite Xenoliths Entrained in Jericho Kimberlite, Canada

The Jericho kimberlites are part of a small Jurassic kimberlite cluster in the northern Slave craton, Canada. A variety of dating techniques were applied to constrain the nature and age of two Jericho kimberlites, JD-1 (170·2 ± 4·3 Ma Rb–Sr phlogopite megacrysts, 172·8 ± 0·7 Ma U–Pb eclogite rutile, 178 ± 5 Ma U–Pb eclogite zircon lower intercept) and JD-3 (173 ± 2 Ma Rb–Sr phlogopite megacryst; 176·6 ± 3·2 Ma U–Pb perovskite), and all yielded identical results within analytical uncertainty. As there is no discernible difference in the radiometric ages obtained for these two pipes, the composite Rb–Sr phlogopite megacryst date of 173·1 ± 1·3 Ma is interpreted as the best estimate for the emplacement age of both Jericho pipes. The initial Sr isotope composition of 0·7053 ± 0·0003 derived from phlogopite megacrysts overlaps the range (0·7043–0·7084) previously reported for Jericho whole-rocks. These strontium isotope data, combined with the radiogenic initial 206Pb/204Pb ratio of 18·99 ± 0·33 obtained in this study, indicate that the Jericho kimberlites are isotopically similar to Group 1 kimberlites as defined in southern Africa. The Jericho kimberlites are an important new source of mantle xenoliths that hold clues to the nature of the Slave craton subcontinental mantle. A high proportion (30%) of the Jericho mantle xenolith population consists of various eclogite types including a small number (2–3%) of apatite-, diamond-, kyanite- and zircon-bearing eclogites. The most striking aspect of the Jericho zircon-bearing eclogite xenoliths is their peculiar geochemistry. Reconstructed whole-rock compositions indicate that they were derived from protoliths with high FeO, Al2O3 and Na2O contents, reflected in the high-FeO (22·6–27·5 wt %) nature of garnet and the high-Na2O (8·47–9·44 wt %) and high-Al2O3 (13·12–14·33 wt %) character of the clinopyroxene. These eclogite whole-rock compositions are highly enriched in high field strength elements (HFSE) such as Nb (133–1134 ppm), Ta (5–28 ppm), Zr (1779–4934 ppm) and Hf (23–64 ppm). This HFSE enrichment is linked to growth of large (up to 2 mm) zircon and niobian rutile crystals (up to 3 modal %) near the time of eclogite metamorphism. The diamond-bearing eclogites on the other hand are characterized by high-MgO (19·6–21·3 wt %) garnet and ultralow-Na2O (0·44–1·50 wt %) clinopyroxene. Paleotemperature estimates indicate that both the zircon- and diamond-bearing eclogites have similar equilibration temperatures of 950–1020°C and 990–1030°C, respectively, corresponding to mantle depths of 150–180 km. Integration of petrographic, whole-rock and mineral geochemistry, geochronology and isotope tracer techniques indicates that the Jericho zircon-bearing eclogite xenoliths have had a complex history involving Paleoproterozoic metamorphism, thermal perturbations, and two or more episodes of Precambrian mantle metasomatism. The oldest metasomatic event (Type 1) occurred near the time of Paleoproterozoic metamorphism (∼1·8 Ga) and is responsible for the extreme HFSE enrichment and growth of zircon and high-niobian rutile. A second thermal perturbation and concomitant carbonatite metasomatism (Type 2) is responsible for significant apatite growth in some xenoliths and profound light rare earth element enrichment. Type 2 metasomatism occurred in the period 1·0–1·3 Ga and is recorded by relatively consistent whole-rock eclogite model Nd ages and secondary U–Pb zircon upper intercept dates. These eclogite xenoliths were derived from a variety of protoliths, some of which could represent metasomatized pieces of oceanic crust, possibly linked to east-dipping subduction beneath the Slave craton during construction of the 1·88–1·84 Ga Great Bear continental arc. Others, including the diamond-bearing eclogites, could be cumulates from mafic or ultramafic sill complexes that intruded the Slave lithospheric mantle at depths of about 150–180 km.

Petrology, geochemistry, and geochronology of Proterozoic granitoid and related rocks of the northern Mummy Range, north-central Colorado

Detailed geologic mapping in the northern Mummy Range, north-central Colorado, reveals seven distinguishable igneous granitoid rock units intruded into a sequence of high-grade metamorphic host rocks. U-Pb dating of zircons from twelve samples indicates that these granitoids were emplaced during two separate periods of magmatism about 300 m.y. apart, one in the Paleoproterozoic and another in the Mesoproterozoic. Zircon dating confirms that all of this magmatic activity post-dated the peak of Paleoproterozoic metamorphism in this area. Based on field, geochemical and mineralogical analyses, four distinct Paleoproterozoic granitoid rock units are interpreted to represent three separate, but geochronologically indistinguishable, magmatic events at ∼1.7 Ga. The earliest igneous event is recorded by a body of leucocratic tonalite which yields a zircon age of 1702 ± 6 Ma. The second event is recorded by small bodies of granodiorite which are mineralogically and geochemically very similar to the approximately contemporaneous Boulder Creek Granodiorite exposed to the south. The third Paleoproterozoic igneous event produced foliated granitoid rocks that yield a composite zircon age of 1695 ± 20 Ma and which comprise two distinct lithologies: a melanocratic roof facies and a strongly foliated monzogranite. This foliated Paleoproterozoic monzogranite, which is geochemically and mineralogically indistinguishable from the approximately contemporaneous Rawah batholith exposed to the northwest, is the most widespread rock unit exposed in the northern Mummy Range, accounting for over 65% of the area of the Comanche Peak quadrangle north of the Skin Gulch shear zone. The earliest Mesoproterozoic igneous event in the northern Mummy Range is represented by small bodies of coarse-grained quartz diorite. The second Mesoproterozoic event is represented by the strongly porphyritic granodiorite of the Hagues Peak pluton. The third and final Mesoproterozoic igneous event in the area produced widespread bodies of equigranular “Silver-Plume-type” peraluminous monzogranite that yields a zircon age of 1393 ± 25 Ma. Most of our zircon ages for the Mummy Range granitoids are consistent with the most recent synthesis of models for the tectonic evolution of the Proterozoic Colorado province. Our dates for certain individual samples of the Paleoproterozoic granodiorite and foliated Paleoproterozoic monzogranite suggest that Paleoproterozoic syntectonic plutonism may have continued slightly longer in this area than previously recognized. The mineralogy and geochemistry of most of the Mummy Range granitoids are consistent with current tectonic syntheses of the region. The composition of the foliated Paleoproterozoic monzogranite in the Mummy Range suggests a source that included pre-existing continental crustal material, consistent with recent suggestions of

Paleoproterozoic Metamorphism in the Northern Wyoming Province: Implications for the Assembly of Laurentia

U‐Pb ages measured on zircons from the Tobacco Root Mountains and monazite from the Highland Mountains indicate that the northwestern Wyoming province experienced an episode of high‐grade metamorphism at ∼1.77 Ga. Leucosome emplaced in Archean gneisses from the Tobacco Root Mountains contains a distinctive population of zircons with an age of 1.77 Ga but also contains zircons to ∼3.5 Ga; it is interpreted to have been derived primarily by anatexis of nearby Archean schist. A granulite facies mafic dike that cuts across Archean gneissic banding in the Tobacco Root Mountains contains two distinct populations of zircons. A group of small (<50 μm) nonprismatic grains is interpreted to be metamorphic and yields an age of 1.76 Ga; a group of slightly larger prismatic grains yields an age of 2.06 Ga, which is interpreted to be the time of crystallization of the dike. Monazite from a leucogranite from the Highland Mountains yields a well‐defined age of 1.77 Ga, which is interpreted as the time of partial melting and emplacement of the leucogranite. These results suggest that the northwestern Wyoming province, which largely lies within the western part of the Great Falls tectonic zone, experienced a metamorphic maximum at 1.77 Ga. This age is ∼100 m.yr. younger than the proposed time of Wyoming‐Hearne collision in the central Great Falls tectonic zone (1.86 Ga) and suggests that the northwestern Wyoming province may have been involved in a separate, younger collisional event at ∼1.77 Ga. An event at this time is essentially coeval with collisions proposed for the eastern and southeastern margins of the province and suggests a multiepisodic model for the incorporation of the Wyoming craton into Laurentia.

U–Pb geochronological constraints for Paleoproterozoic evolution of the Core Zone, southeastern Churchill Province, northeastern Laurentia

The Core Zone of the Southeastern Churchill Province, northeastern Laurentia, is a composite of Paleoproterozoic lithotectonic domains assembled in the vise between obliquely colliding Archean Superior and North Atlantic cratons between ca. 1860 and 1810 Ma. Detailed geological and U–Pb geochronological studies of domains in the southern Core Zone highlight significant differences in Archean and Paleoproterozoic geology between domains, and constrain timing and models for Paleoproterozoic assembly. In the south-central part of the Core Zone, Crossroads and Orma domains consist of Late Archean granite–greenstone terrane crust. Crossroads domain also contains a significant number of granite–charnockite intrusions belonging to the 1840–1810 Ma De Pas batholith; magmatism pre-dates and overlaps with ca. 1820–1775 Ma high-grade metamorphism and attendant deformation. In marked contrast, Orma domain, which occurs to the east of Crossroads domain, contains only local evidence of Paleoproterozoic (De Pas related?) intrusions and appears to have mainly escaped Paleoproterozoic metamorphism and penetrative deformation. A model compatible with available data suggests the De Pas batholith is a continental magmatic arc formed above an east-dipping subduction zone. Paleoproterozoic metamorphism and deformation is concentrated in regions contiguous with arc magmatism. West of Crossroads domain, and separated from it by the Lac Tudor shear zone, McKenzie River domain is dominated by Archean orthogneiss (>80 m.y. older than gneisses in Crossroads and Orma domains), minor amounts of Paleoproterozoic supracrustal rocks (absent in domains to the east) and 1815 Ma tonalite. It does not contain De Pas intrusions. U–Pb geochronology suggests juxtaposition of the McKenzie River domain with domains to the east may have occurred around 1810 Ma. One possible model envisages the McKenzie River domain as a piece of reassembled Superior craton crust, incompletely rifted from the Superior margin during ca 2.17 Ga rifting. Domains occurring east of the Lac Tudor shear zone are interpreted to be exotic with respect to the Superior craton.

Paleoproterozoic lithotectonic divisions of the southeastern Churchill Province, western Labrador

The southeastern Churchill Province (SECP) is a Paleoproterozoic system of orogens that formed during collision of the Nain and Superior cratons with a composite lithotectonic terrane that now forms the medial, metamorphic–plutonic core zone of the SECP. In western Labrador, the core zone consists of reworked Archean gneisses, Paleoproterozoic supracrustal rocks, and variably deformed 1.83–1.81 Ga granitic plutons. It is subdivided into three Paleoproterozoic lithotectonic domains (McKenzie River, Crossroads, and Orma), which are separated from each other by dextral transpressive high-strain zones. Crossroads and Orma domains are thought to be derived from Archean high-grade granite–greenstone terrane crust, whereas McKenzie River domain is inferred to have been part of an Archean orthogneiss terrane dominated by the ca. 2776 Ma Flat Point gneiss. U–Pb geochronological data indicate that the igneous precursor of the Flat Point gneiss is &gt;80 Ma older than the oldest tonalité–granite intrusions in Crossroads and Orma domains. The three domains were variably reworked during dextral oblique convergence of the Superior and Rae cratons. Field and geochronological data demonstrate that in McKenzie River and Crossroads domains, 1.83–1.80 Ga tectono-thermal reworking included medium- to high-grade metamorphism and the formation of north-trending structures and regionally persistent high-strain zones. Crossroads domain also contains a significant amount of 1.83–1.81 Ga granitic intrusions, including the southern part of the 500 km long De Pas batholith. Orma domain appears to have escaped Paleoproterozoic metamorphism and deformation, suggesting that a "Hudsonian" tectono-thermal front separates it from Crossroads and McKenzie River domains.