Syenite Intrusions Research Articles

Circum-Iapetus paleogeography of the Precambrian–Cambrian transition with a new paleomagnetic constraint from Laurentia

The paleogeography of the Precambrian–Cambrian transition is still poorly known, but is fundamental for understanding Late Neoproterozoic climate extremes and the spatial associations of the rapidly evolving Ediacaran and Cambrian fauna. This period has also been proposed to host unusual geodynamic events such as rapid plate motions and/or the bulk tumbling of the Earth with respect to its spin axis, called true polar wander (TPW). New paleomagnetic and 40Ar– 39Ar geochronologic results have been obtained from a pair of shallowly emplaced syenitic intrusions in western Quebec. The Mont Rigaud and the Chatham-Grenville stocks are related to a failed rift arm of the Iapetus Ocean. Both intrusions bear hornblendes which provide overlapping 40Ar– 39Ar plateau ages of 533.2 ± 1.1 Ma (Mont Rigaud) and 531.4 ± 3.4 Ma (Chatham-Grenville), corresponding to Early Cambrian (Nemakit-Daldynian) time. Characteristic paleomagnetic (reversed) directions from both intrusions are easterly and shallow ( D = 89.3°, I = 30.1°; α 95 = 6.2, k = 38.8; N = 15 sites), and the Mont Rigaud intrusion also carries antipodal normal-polarity directions. The combined result yields a north paleopole at location 11.9°S 184.6°E; d p = 3.8° d m = 6.9°, placing the Montreal region at 16 ± 4°S paleolatitude in the Early Cambrian, in good agreement with Late Ediacaran and Middle to Late Cambrian paleomagnetic results from Laurentia. Laurentia likely resided at low southerly paleolatitudes throughout the Cambrian, ruling out a major inertial-interchange TPW event for Early to Middle Cambrian time. A comparison of Late Ediacaran to Middle Cambrian paleomagnetic results from Laurentia, Baltica, Siberia and elements of West Gondwana implies that by Early Cambrian time a large paleolatitudinal gap existed between Laurentia and its presumed conjugate rift margins, thus placing a wide Iapetus Ocean between them. The position of Baltica within the circum-Iapetus paleogeography is contentious, due to differing interpretations of the few paleomagnetic results. During Ediacaran time, Laurentia underwent large apparent polar wander that is manifested as motion from low latitude at 615 Ma to the south polar region at 590–575 Ma, and then rapidly back to low latitude by 565–550 Ma. The large and perhaps rapid apparent polar wander of Laurentia and other continents in the Ediacaran remains enigmatic and controversial.

Petrogenesis of the Leo Lake and Lyndhurst plutons, Frontenac terrane, Central Metasedimentary Belt, southeastern Ontario, Canada

The 1167 ± 2 Ma Leo Lake pluton, located at the southern part of the Frontenac terrane in Central Metasedimentary Belt of southeastern Ontario, is a bimodal pluton that is dominated by olivine gabbro, gabbro, syenite, and lesser monzonite. The adjoining 1166 ± 3 Ma Lyndhurst pluton is composed of syenite and granite, and exhibits magma mingling. Even though geochemical data from both plutons suggest fractional crystallization as the main crystallization process, magma mingling is evident in the Lyndhurst pluton. Hornblende from the Leo Lake syenite yields a 40Ar–39Ar age of 1120 ± 6 Ma and titanite from the adjacent St. Lawrence skarn yields a U–Pb age of 1147 ± 8 Ma. The 40Ar–39Ar age indicates that the pluton cooled to 550 °C at a rate of ca. <2 °C/Ma following emplacement. The two plutons are similar in petrology to several other granitic, syenitic, and monzonitic intrusions of the Frontenac suite in the Frontenac terrane. Moreover, the spatial distribution, textural features, and geochemistry indicate that the plutons are coeval. The present data indicate that the two plutons may have been produced from evolved mantle-derived melts, and may have assimilated varying proportions of crustal melts.

Late Paleozoic granitoids of western Transbaikalia: magma sources and stages of formation

Geochemical and geochronological studies of the main types of granitoids of the Angara-Vitim batholith (AVB) and granites of the Zaza complex in western Transbaikalia were carried out. U-Pb (SHRIMP-II) and Rb-Sr dating yielded the age of autochthonous gneiss-granites of the Zelenaya Griva massif (325.3±2.8 Ma), quartz syenites of the Khangintui pluton (302.3±3.7 Ma) and intruding leucogranites of the Zaza complex (294.4±1 Ma), monzonites of the Khasurta massif (283.7±5.3 Ma), and quartz monzonites of the Romanovka massif (278.5±2.4 Ma). The U-Pb and Rb-Sr dates show that the Late Paleozoic magmatism in western Transbaikalia proceeded in two stages: (1) 340–320 Ma, when predominantly mesocratic granites of the Barguzin complex, including autochthonous ones, formed, and (2) 310–270 Ma, when most AVB granitoids formed. We suggest that at the early stage, crustal peraluminous granites formed in collision geodynamic setting. At the late (main) stage, magmatism occurred in postorogenic-extension setting and was accompanied by the formation of several geochemical types of granitoids: (1) typical intrusive mesocratic granites of the Barguzin complex, similar to those produced at the first stage; (2) melanocratic granitoids (monzonitoids, quartz syenites), which were earlier dated to the early stage of the AVB evolution; (3) leucocratic medium-alkali (peraluminous) granites of the Zaza intrusive complex; and (4) some alkali-granite and syenite intrusions accompanied by alkaline mafic rocks. The diversity of granitoids that formed at the late stage of magmatism was due to the heterogeneous composition of crust protoliths and different degrees of mantle-magma participation in their formation.

Intrusion mechanisms in a turbidite sequence: the Voetspoor and Doros plutons in NW Namibia

Two syntectonic plutons of Cambrian age intruded Neoproterozoic metaturbidites in Namibia at the junction of the NS trending Kaoko and EW trending Damara belts. Sinistral transpression in the Kaoko Belt produced km-scale upright D1 folds overprinted by minor D2 folds. D3 is associated with N–S shortening in the Damara Belt. The plutons show two main pulses of intrusion: hornblende syenite intruded late during D1 or during D2 and biotite granite during D3. Each tectonic event produced a strain shadow defined by the shape of folds and the foliation trend around the plutons. The internal igneous fabric and the arrangement of wall rock xenoliths that locally make up 50% of the intrusion mass suggest that the plutons have a disk or wedge shape. A marginal shear zone indicates that one of the syenite intrusions descended during emplacement with respect to the wall rock. Emplacement is therefore inferred to be at least partly accommodated by descent of the intrusion floor. The biotite granite intruded into mega-strain shadows and tension gashes alongside and in the syenite during D3. The plutons show evidence of sinistral solid state rotation with respect to the wall rocks in response to D1–D2 transpression.

Petrogenesis of mica-amphibole-bearing lamprophyres associated with the Paleoproterozoic Morro do Afonso syenite intrusion, eastern Brazil

Mica-amphibole-lamprophyres, identified as vogesites, are associated with the Paleoproterozoic Morro do Afonso syenite intrusion in northeastern Brazil. The lamprophyres occur mainly as dykes that crosscut the syenitic rocks and occasionally as enclaves. Lamprophyric rocks are formed by the early magmatic paragenesis amphibole-clinopyroxene-apatite-phlogopite-ilmenite; feldspars are found in the groundmass. Near liquidus amphibole is edenite, close to the boundary with pargasite, which is enriched in alkalis relative to the other amphiboles (Mg-hornblende and actinolite). Clinopyroxene is diopside, and inclusions of phlogopite are analyzed in both clinopyroxene and amphibole phenocrysts. The chemical evolution of the mafic minerals is consistent with increasing oxygen fugacity during late magmatic stages. Whole-rock geochemical data suggest a metaluminous, ultrapotassic parental liquid, with silica saturation close to the limit of undersaturation. Trace element concentrations, such as enrichment in large ion lithophile and strong depletion of some high field-strength elements, indicate a mantle source that was partially modified by a subduction event. In this metasomatic mantle, it is important to emphasize the strong enrichment of light rare-earth elements, which is higher than those typically associated with basaltic rocks from active continental margins, and corresponding concentrations similar to those determined in lamproitic rocks. Major element modeling shows that fractional crystallization and magma flow segregation are the main petrogenetic processes involved in the magmatic evolution of lamprophyre magma, and it is possible to generate syenite magma by these mechanisms.

Temporal evolution of a long-lived syenitic centre: The Kangerlussuaq Alkaline Complex, East Greenland

We present new mineral and whole-rock compositions and Sr–Nd–Hf–Pb–O–H isotope data on samples from the Kangerlussuaq Alkaline Complex (∼ 1000 km 2) in central East Greenland, part of the North Atlantic Igneous Province. This complex mainly consists of the Kangerlussuaq Intrusion but includes at least 13 separate satellite intrusions emplaced in the uppermost crust close to the unconformity between Archaean gneisses and overlying Palaeogene flood basalts. The complex is divided into (i) older satellite intrusions (∼ 55–53 Ma) composed of multiple syenites and granites and minor gabbros and peridotites, (ii) the voluminous Kangerlussuaq Intrusion (∼ 50 Ma), which displays a gradual transition from quartz syenites (nordmarkites) at the margin to nepheline syenites (foyaites) in the centre, and (iii) younger satellite intrusions (∼ 47–45 Ma) of minor syenites, granites and diorites concentrated southeast of the Kangerlussuaq Intrusion. The complex displays a temporal evolution in which SiO 2 decreases (74–56 wt.%) and total alkalis (6–16 wt.%), amphibole Na + K content, 206Pb/ 204Pb meas, ε Ndi and ε Hfi (+ 3 to + 11) increase from the older intrusions through the nepheline syenites. This is followed by a reversal to higher silica (62–73 wt.%) and lower total alkalis (9–12 wt.%), amphibole Na + K content, 206Pb/ 204Pb meas, ε Ndi and ε Hfi (− 13 to + 2) in the younger satellite intrusions. Temporal changes in the location of magma plumbing systems and in magma production rates played a profound role in controlling silica content, alkalinity and degree of crustal contamination during development of the complex. Phonolitic magma was only generated after prolonged magmatism had shielded the conduits from interaction with country rock. The parental magmas were probably basanitic to alkali olivine basaltic in composition. The older satellite intrusions and the Kangerlussuaq Intrusion have low δ 18O magma values (− 1 to + 6‰) compared to the younger, more crustally contaminated, satellite intrusions (+ 4.5 to + 7‰). It appears that the magmas only had sufficient over-pressure to intrude the basalt cover when larger volumes of less contaminated trachyte magma were produced, resulting in the generation of low-δ 18O magmas due to dehydration of hydrothermally altered basalt xenoliths.

Fluid–rock interaction in autoliths of agpaitic nepheline syenites in the Ilímaussaq intrusion, South Greenland

Within the 1.16 Ga old Ilímaussaq intrusion, up to 700 m large autoliths occur in one stratigraphic unit of the layered floor series of agpaitic nepheline syenites (kakortokites). These autoliths consist of two different rock types: augite syenite and naujaite (agpaitic nepheline syenite). All three rock types show a number of alteration features related to the entrapment of the autoliths in the kakortokite magma caused by the interaction with a fluid phase. In the kakortokites, the oxidation of primary arfvedsonite to aegirine and fluorite is restricted to the close proximity to the autoliths. Close to the surrounding kakortokite, the primary mafic phases of the augite syenites (augite, fayalite, Fe–Ti oxides) are completely replaced by arfvedsonite, aenigmatite, biotite, aegirine and fluorite. The decomposition of primary hastingsite to spectacular aegirine–augite–nepheline–aenigmatite symplectites can be observed up to several meters inside the autoliths. Additionally, fluorite formed at grain boundaries of primary nepheline. In the naujaite autoliths, primary arfvedsonite is replaced by aegirine–biotite intergrowths and abundant aenigmatite is occasionally replaced by Ti-rich aegirine and Fe–Ti oxides. The mineral reactions in the autoliths are used to decipher details of the late to post-magmatic processes in a peralkaline syenitic intrusion. Mineral equilibria record an evolution governed by falling temperature (620 to ca. 500 °C) and increasing relative oxygen fugacity from FMQ + 1 to above FMQ + 4. Quantification of the observed mineral reactions reveals the infiltration of the autoliths with an oxidizing fluid phase rich in Na and F and minor addition of K. Volatiles (H and F) and in some cases also Fe, Ti and Ca (± Mg) released from primary autolith phases were mainly just relocated within the autoliths.

The Himalayan collision zone carbonatites in western Sichuan, SW China: Petrogenesis, mantle source and tectonic implication

Major and trace element compositions, and Sr–Nd–Pb and O–C isotope data for Cenozoic carbonatites (WSC) in western Sichuan, east Tibet, China are presented in this paper. The WSC occur in the eastern Indo-Asian collision zone (EIACZ), and occur as sills or dykes in the contemporaneous syenitic intrusions, which form a 270-km long, NS-trending belt of carbonatite–alkalic complexes, controlled by strike-slip faults that accommodated and adjusted to the collision strain. These carbonatite–alkalic complexes with Cenozoic potassic felsic rocks and calc-alkaline (shoshonitic) lamprophyres form a semi-continuous potassic igneous province with magmatism peaking at ∼35 Ma in the EIACZ. The WSC are characterized by low SiO 2 (< 10.22 %), FeO (< 1.20 %) and MgO (< 0.73 %), and a wide range of CaO content (40.7∼55.4 %) that distinguish them from primary magnesiocarbonatites. Overlapping emplacement ages and Sr–Nd isotopic compositions and similar mantle-normalized trace element patterns to spatially associated syenites suggest an origin for the carbonatites by liquid immiscibility. The WSC are extremely enriched in LILE (Sr, Ba) and light REE, but relatively depleted in high-field strength elements (Nb, Ta, P, Zr, Hf, Ti), suggesting a metasomatized mantle source. Their δ 18O V-SMOW (6.4∼10.5‰) and δ 13C V-PDB (− 3.9 to − 8.5‰) values are similar to those of primary, mantle-derived carbonatites, also suggesting a mantle genesis. However, they have extremely low å Nd( t) of − 3.2 to − 18.7 and relative high ( 87Sr / 86Sr) i of 0.706020∼0.707923, as well as a wide range of 207Pb / 204Pb (15.362 ∼ 15.679) and 208Pb / 204Pb ratios (38.083∼39.202), which distinguish them from most carbonatites around the world. Their Sr–Nd, Sr–Pb and Nd–Pb isotopic signatures indicate that some carbonatites underwent Sr–Nd–Pb contamination by crustal materials, but the least-contaminated carbonatites were derived from a transitional source between EMI and EMII components. The T DM and modeling on Sr–Nd isotopic compositions indicate that the formation of this source probably was related to recycling of oceanic crust and pelagic/terrigenous sediments with various mass ratios. Partial melting of the mantle source was most likely triggered by a Cenozoic asthenospheric mantle diapir related to Indian–Asian continent collision at 65–45 Ma. Rising and emplacement of carbonatitic magmas with coeval potassium-rich magmas took place in the tectonic regime of the transition from transpression to transtension at Eocene/Oligocene boundary in the EIACZ.

Laacher See revisited: High-spatial-resolution zircon dating indicates rapid formation of a zoned magma chamber

High-spatial-resolution ( 230 Th)/( 238 U) disequilibrium dating reveals rapid zircon crystallization prior to the 12,900 yr B.P. Laacher See (Germany) phonolite eruption. Zircons from Lower Laacher See tephra (LLST) and syenitic subvolcanic nodules share REE and low U/Th characteristics and define an isochron age of 17.1 ± 1.3 ka (1σ). Zircons also show higher initial ( 230 Th)/( 232 Th) 0 and oxygen isotopic disequilibrium compared to their host phonolite. Thus, LLST zircons shortly predate the eruption and yet are not strictly part of the phonolite crystallization sequence. Based on their similarity to zircons in subvolcanic nodules, they are instead interpreted to be scavenged from a precursor syenite intrusion, which implies short (a few k.y.) filling and differentiation timescales for the Laacher See magma chamber. Thorium and oxygen isotopes in zircon as well as evidence for abundant crustal xenocrysts further indicate extensive crustal contamination, which allows for considerably faster basanite-phonolite differentiation than the ∼100 k.y. time span previously estimated from bulk U-series data.

Hydrothermal alteration of chevkinite-(Ce) in the Shuiquangou syenitic intrusion, northern China

A rare-earth iron titanosilicate mineral closely associated with amphibole was found in two altered syenite samples from the Shuiquangou syenitic complex. Based on electron microprobe analyses, the mineral is chevkinite-(Ce). The mineral is altered and mantled by a corona of allanite–ilmenite–titanite–epidote–quartz. Mass balance calculations reveal that the former chevkinite was replaced directly by an allanite and ilmenite association (allanite–ilmenite zone) in the inner part of the corona texture. It appears that during the replacement, Si and Ti were immobile, whereas Al, Fe, Mn, Mg and Ca were externally supplied and a proportion of REE and Th were transferred out of the former chevkinite. The released REE and Th become incorporated in allanite and titanite in the allanite–ilmenite–titanite mantle where allanite contains higher Nd, Sm and Y but lower La and Ce than that in the allanite–ilmenite zone, suggesting that redistribution of these elements took place and the mobility of Nd, Sm and Y is enhanced during hydrothermal alteration.

Geology and Age of the Late Archean Keivy Alkaline Province, Northeastern Baltic Shield

Abstract The Keivy alkaline province, Kola Peninsula, consists of six peralkaline granite bodies, alkaline syenogranite dikes, and two nepheline syenite intrusions emplaced between the tonalite‐trondhjemite‐granodiorite basement and the supracrustal Keivy terrane. U‐Pb zircon ages for the Keivy alkaline province range from 2613 to 2682 Ma and, for the spatially associated gabbro‐anorthosite complexes, from 2659 to 2668 Ma. The Keivy alkaline granitoids are geochemically different from other Late Archean alkaline rocks inferred to have been emplaced in subduction zone settings. The Keivy alkaline province may represent the earliest example of magmatism associated with continental rifting and the involvement of a mantle plume with oceanic island basalt–like characteristics.

Petrology and geochemistry of the Shuiquangou syenitic complex, northern margin of the North China Craton

The Palaeozoic Shuiquangou complex is a major syenitic intrusion along the northern margin of the North China Craton that hosts a number of gold deposits. Despite the heterogeneity of the complex in terms of texture and modal mineralogy, mineral compositions, whole-rock chemistry as well as radiogenic isotope compositions show a well-constrained magmatic evolutionary trend. The whole-rock major element and REE trends are broadly consistent with fractional crystallization dominated by clinopyroxene, amphibole and accessory minerals. Initial Nd–Sr isotopic compositions suggest that the parent magmas to the complex were ultimately derived from an enriched mantle with relatively high initial 87 Sr/ 86 Sr ratios and negative ϵ Nd values. The enrichment of the mantle source was probably inherited from an older lithospheric mantle beneath the North China Craton. Sr isotopic data indicate that the Rb–Sr system was disturbed by later alteration processes, whereas Nd isotopes are essentially unaffected and exhibit a broad trend consistent with simultaneous fractional crystallization and country-rock assimilation (AFC). Quantitative modelling indicates that a very high bulk distribution coefficient for Nd and low ratios of assimilation to crystallization are needed to produce the observed Nd isotope evolutionary trend.

Jurassic intraplate magmatism in southern Hunan-eastern Guangxi: 40 Ar/ 39 Ar dating, geochemistry, Sr-Nd isotopes and implications for the tectonic evolution of SE China

Abstract The Mesozoic geology of SE China is characterized by intensive and widespread magmatism. However, the tectonic regime that accounted for the Mesozoic magmatism has been an issue with little consensus. A comprehensive study of 40 Ar- 39 Ar dating, geochemistry and Sr-Nd isotopes has been conducted on basalts from southern Hunan and syenite intrusions from eastern Guangxi. Three episodes of Jurassic magmatism, i.e. alkaline basalts of c. 175 Ma in age, syenitic intrusions of c. 160 Ma and high-Mg basalts of c. 150 Ma, are identified. The older, c. 175 Ma alkaline basalts are characterized by low Sr ( I Sr = 0.7035–0.7040) and high Nd ( ε Nd ( T ) = 5 to 6) isotopic compositions and OIB-like trace-element patterns (e.g. Nb/La &gt; 1). In contrast, the younger, c. 150 Ma high-Mg basalts have high Sr ( I Sr c. 0.7054) and low Nd ( ε Nd ( T ) c. -2) isotopic compositions and incompatible trace-element patterns of arc affinity. The c. 160 Ma syenitic intrusions display a relatively large range of Sr and Nd isotopic compositions ( I Sr = 0.7032–0.7082, ε Nd ( T ) = 5.5 to −4.1), with the Qinghu syenites having the lowest I Sr , highest ε Nd ( T ) and OIB-type incompatible trace-element patterns analogous to the c. 175 Ma alkaline basalts. Such a secular variation in rock types and geochemical and isotopic characteristics reveals changes in melt segregation depth and mantle sources, which are inferred to have resulted from the post-Indosinian orogenic lithosphere extension and thinning. The c. 175 Ma alkaline basalts are suggested to have formed by small degrees of decompression melting of the asthenosphere or an enriched lithospheric mantle source accreted by asthenosphere-derived melts during the initial extension. The c. 160 Ma syenitic and c. 150 Ma high-Mg basaltic rocks mainly originated from the enriched lithospheric mantle that melted owing to a raised geotherm caused by lithosphere thinning. This interpretation is at odds with the active continental margin related to the subduction of palaeo-Pacific plate, but consistent with continental rifting and extension for the Mesozoic of SE China.

Evolution of the North Qôroq centre nepheline syenites, South Greenland: alkali-mafic silicates and the role of metasomatism

AbstractThe North Qôroq centre comprises a series of concentric nepheline syenite intrusions and forms part of the Igaliko Nepheline Syenite Complex, in the rift-related Gardar Province of South Greenland. The North Qôroq syenites range from mildly undersaturated augite syenite to strongly peralkaline agpaitic nepheline syenite. Extensivein situfractional crystallization has been postulated for the chemical variation both within units and throughout the centre. Many of the rocks have been affected by metasomatic fluids associated with the emplacement of younger syenite units, and this complicates their interpretation. In this study, the trends and compositions exhibited by pyroxene and amphibole from North Qôroq are examined and related to either primary crystallization or metasomatic activity (e.g. controls offO2, peralkalinity). Implications thus drawn are used to interpret the chemical processes inherent in the chemical and fluid evolution of alkaline magmas, and, in particular, the transition from miaskitic to agpaitic magmatism. In general, the major phases of the North Qôroq syenites records the increasing evolution of the units by crystal fractionation, towards peralkaline compositions. The composition of olivine, in the least evolved syenites, also points to a relatively high state of fractionation of the parent magma, whilst pyroxene and amphibole record an overall decrease in Mg/Mg+Fe), and a general increase in Fe3+and alkali content, with increased fractionation. The increasing peralkalinity of the magma also governs the evolution of pyroxene and, to a lesser degree, amphibole towards higher Zr and Ti contents in the more Na-rich compositions. The trends for pyroxene from metasomatized syenite show similar patterns, but lower Fe2+enrichment, suggesting the source of the metasomatic fluids is similar to the evolving syenites. The presence of amphiboles in metasomatic rocks, and high F contents attest to the F-rich nature of the metasomatic fluids, which is in agreement with results previously reported for metasomatic fluorapatite.

Enriched Subcontinental Lithospheric Mantle in the Northern Part of the South Indian Granulite Terrain: Evidence from Yelagiri and Sevattur Syenite Plutons, Tamil Nadu, South India

In the southern part of the Indian Peninsula, there are a number of alkaline plutons of Proterozoic age. In the northern part of the South Indian granulite terrain, the Yelagiri (syenite, pyroxenite) and Sevattur (syenite, pyroxenite, carbonatite) plutons intrude Archaean epidote-hornblende gneisses. Geochemical and isotopic characteristics of the Yelagiri and Sevattur plutons indicate that the syenitic magmas formed from highly differentiated mantle-derived alkali basalts. The Yelagiri and Sevattur syenites are characterized by evolved Sr and Nd isotopic compositions, pronounced enrichment in LILE and large negative Nb anomalies. Trace element and Sr-Nd isotope characteristics of the Yelagiri and Sevattur syenites are similar to those of the subduction-related alkaline rocks. The scarcity of geological evidence for subduction activity at the time of syenite intrusion during Neoproterozoic does not support a link between the alkaline magmatism and subduction. However, our data are consistent with the model of derivation of the Yelagiri and Sevattur syenites from the subcontinental lithospheric mantle, which was previously enriched by slab derived component. The geochemical and isotopic signatures of other mantle-derived intrusive rocks in the northern part of the South Indian granulite terrain with ages ranging from ca. 2.5 Ga to 0.75 Ga also support the above idea.

TEMPESTITOS COM INFLUÊNCIA GLACIAL DA FORMAÇÃO PLAYA HERMOSA (NEOPROTEROZÓICO), PIRIÁPOLIS, URUGUAI

Storm-wave influenced glacimarine deposits of the Playa Hermosa Formation (Neoproterozoic) crop out along the beaches of Playa Grande and Playa Hermosa (town of Piriapolis, southern Uruguay). These deposits are characterized by fining-upward cycles composed, from the base to the top of successions of: (i) conglomerates and sandy-rhythmites in decimetric layers showing similar processes of rain-out, which pass to, (ii) finely laminated and rippled centimetric sandstones with deformed beds and outsized clast up to 71cm of size, with associated bending and rucking structures, (iii) well-sorted fine centimetric sandstones with swaley and hummocky cross-stratification and wave ripples interpreted as sandy tempestites which are recovered by (iv) finely laminated rhythmtes affected by intrusions of trachytes, syenites and rhyolites of the Sierra de Animas Suite (570-550 Ma). The main evidences for a glaciomarine origin includes: (i) the confirmed presence of a dropstone in fine-grained sediments indicating a hydrodynamic paradox, (ii) the presence of massive conglomeratic sandstone facies formed by rain-out processes and (iii) according to specialized literature the paleomagnetic data indicative of a low to intermediate position for the Rio de La Plata Craton in the Neoproterozoic. The loneclast was interpreted as a dropstone due to the anomalous size, the hydrodynamic paradox and the composition, farther that other forms of transport are rejected here ( e . g . organic transport).

First report of early Triassic A-type granite and syenite intrusions from Taimyr: product of the northern Eurasian superplume?

Ion-microprobe U–Th–Pb analyses of zircon from three high-level syenite–granite stocks in the western part of the Taimyr fold-and-thrust belt have yielded early Triassic ages of 249–241 Ma. Those syenite–granite bodies intrude unmetamorphosed late Paleozoic to early Mesozoic terrigenous and volcanic supracrustal rocks, including the early Triassic Siberian traps. 40Ar– 39Ar isotopic ages of 245–233 Ma correlate well with the ion-microprobe data and define the time of closure for the K–Ar isotopic system. Limited geochemical data for the early Triassic syenite–granite plutons show that they have metaluminous compositions, high potassium, high REE and high LIL concentrations, and 87Sr/ 86Sr and ε Nd ratios intermediate between crust and mantle, suggesting a hybrid mantle–crustal origin. We tentatively suggest that they formed in an anorogenic setting as a result of the Permo-Triassic Euroasian superplume.

Petrogenesis of the Mesozoic Sistefjell syenite intrusion, Dronning Maud Land, Antarctica and surrounding low- 18O lavas

The Sistefjell intrusion is approximately 10km in diameter and is the largest Mesozoic pluton in western Dronning Maud Land. The main intrusive phase is a syenite consisting of >78 modal % alkali feldspar, quartz, sodic-calcic amphibole, aegirine-augite, and minor fayalite, aegirine and aenigmatite. Associated sills and dykes include microsyenite, quartz porphyry and microgranite. The variation in chemical composition of these rocks is largely consistent with fractional crystallization. Oxygen isotope ratios of mineral separates from the quartz syenite indicate an original magma δ 18 O value of 6.7‰, whereas the more evolved minor intrusions crystallized from magmas with slightly lower δ 18 O values. The only exposed country rock, the undeformed late-Precambrian (?) Sistenup lavas, have whole-rock δ 18 O values which range from +4.4 to −1.8‰ (mean +1.0‰, n=8), and δD values which range from −96 to −167‰ (mean −129‰, n=7). The low δ 18 O values of the lavas, coupled with an abundance of secondary hornblende, are consistent with alteration at elevated temperatures (450°C) and high water/rock ratios. Several lines of evidence suggest that fluid/rock interaction occurred prior to the intrusion of the syenite, and was possibly related to influx of meteoric water during regional metamorphism. The low δD values of the lavas indicate interaction with water with abnormally low δD value typically found at high latitudes and/or high altitude. The freshest syenite sample gave a three point (feldspar, amphibole whole-rock) Rb-Sr isochron (MSWD = 0.12) equivalent to an age of 173 ± 2 Ma, whereas 40 Ar- 39 Ar step-heating of amphibole from lavas close to the contact suggest a maximum age of intrusion of ~182 Ma. Older high-temperature 40 Ar- 39 Ar ages are consistent with a much older age for the Sistenup lavas. Although the syenite and related minor intrusions have low Zr/Nb ratios (mean value 7.2) and high Ce/Y ratios, which are characteristic of rift zone magmas, their ∈ Nd values of −1.8 to −4.2 indicate crustal contamination. Two stages of contamination are required involving, firstly, the dominant metamorphic rocks in the area and, secondly, the Sistenup lavas.

U-Pb ages of plutonism, wollastonite formation, and deformation in the central part of the Lac-Saint-Jean anorthosite suite

The Lac-Saint-Jean anorthosite suite (LSJAS) is the largest ensemble of plutons in the Grenville Province. Zircons from a pluton in the central part of the LSJAS yield an age of 1140+10–5 Ma, which accords with existing 1157–1142 Ma ages determined from the southern part of the LSJAS. The Du Bras granite is a linear pluton emplaced into the dominantly dextral Pipmuacan shear zone (PSZ). The U–Pb zircon age of 1148 ± 2 Ma for the Du Bras granite confirms earlier proposals that it is a granophyre, formed at greater depths by partial melting of paragneiss by the heat of the crystallizing anorthosite. Parts of the pluton are strongly deformed by the shear zone, and other parts cut the shear zone. This suggests that the pluton dates one phase of movement of the PSZ. The Canton-St-Onge wollastonite deposit was developed in paragneisses preserved within the PSZ. Titanite from this deposit gives an age of 1163 ± 18 Ma, identical within error to that of the Du Bras granite, indicating that this pluton may have provided the skarn fluids. A second plutonic cycle comprises the post-tectonic Astra granite, which stitches the PSZ, and an amazonite-bearing pegmatite dyke. Zircons gave an age of 1028 ± 2 Ma for the Astra granite. The amazonite-bearing dyke only yielded monazite, one fraction of which gave an age similar to that of the Astra granite. Other fractions record the effects of a thermal event at 1020–1017 Ma that may be related to a widespread magmatic event. The undeformed Venus de Milot syenitic intrusion yields a U–Pb age of 988 ± 2 Ma. It was emplaced along the northwest-trending Lac Rioux fault, indicating that one phase of movement occurred before 988 Ma. Another strand of the same fault system also cuts the intrusion, showing later reactivation that may have been associated with the formation of the Saguenay Graben.

Tectonic evolution of the southern Kaoko belt, Namibia

The tectonic evolution at the junction of the Panafrican Kaoko and Damara belts is well recorded in the siliciclastic and carbonate successions of the Neoproterozoic Zerrissene turbidite system, metamorphosed to the biotite zone of the greenschist facies. The structures in the turbidites are attributed to two main deformational events. The older one generated two continuous folding phases, D1 and D2, and the younger one resulted in D3 deformation. D1, of dominant E–W shortening, caused upright kilometre-scale folds with well-developed axial planar cleavage, N–S trending axial planes and subhorizontal axes. This phase graded into D2 that refolded the first folds coaxialy and developed a crenulation cleavage at a high angle to the first cleavage. D2 is interpreted as a phase associated with shear movement to the north and minor continuing E–W shortening. The third phase, D3, of apparently sinistral transpression caused localised fold trains on a metre to kilometre scale with NE to NNE trending subvertical axial planes. The first two phases predate intrusion of a 530 Ma syenite and probably correlate with the main deformation in both the Kaoko and Damara belts. D2 is likely coeval with motion of the sinistral strike-slip Purros lineament in the central part of the Kaoko belt. D3 postdates the syenite intrusion and is restricted in occurrence to the Damara and southern Kaoko belts. Deformation in the Damara belt therefore outlasted that in the central and northern parts of the Kaoko belt.