2D thermomechanical modelling of continent–arc–continent collision

Arc–continent collision is a key process of continental growth through accretion of newly grown magmatic arc crust to older continental margin. We present 2D petrological–thermo-mechanical models of arc–continent collision and investigate geodynamic regimes of this process. The model includes spontaneous slab bending, dehydration of subducted crust, aqueous fluid transport, partial melting of the crustal and mantle rocks and magmatic crustal growth stemming from melt extraction processes. Results point to two end-member types of subsequent arc–continent collisional orogens: (I) orogens with remnants of accretion prism, detached fragments of the overriding plate and magmatic rocks formed from molten subducted sediments; and (II) orogens mainly consisting of the closed back-arc basin suture, detached fragments of the overriding plate with leftovers of the accretion prism and quasi insignificant amount of sediment-derived magmatic rocks. Transitional orogens between these two endmembers include both the suture of the collapsed back-arc basin and variable amounts of magmatic production. The orogenic variability mainly reflects the age of the subducting oceanic plate. Older, therefore colder and denser oceanic plates trigger subduction retreat, which in turn triggers necking of the overriding plate and opening of a backarc basin in which new oceanic lithosphere is formed from voluminous decompression melting of the rising hot asthenosphere. In this case, subducted sediments are not heated enough to melt and generate magmatic plumes. On the other hand, young and less dense slabs do not retreat, which hampers opening of a backarc basin in the overriding plate while subducted sediments may reach their melting temperature and develop trans-lithospheric plumes. We have also investigated the influences of convergence rate and volcanic/plutonic rocks' ratio in newly forming lithosphere. The predicted gross-scale orogenic structures find similarities with some natural orogens, in particular with deeply eroded orogens such as the Variscides in the Bohemian Massif.     

U–Pb and Th–Pb dating of apatite by LA-ICPMS

Apatite is a common U- and Th-bearing accessory mineral in igneous and metamorphic rocks, and a minor but widespread detrital component in clastic sedimentary rocks. U–Pb and Th–Pb dating of apatite has potential application in sedimentary provenance studies, as it likely represents first cycle detritus compared to the polycyclic behavior of zircon. However, low U, Th and radiogenic Pb concentrations, elevated common Pb and the lack of a U–Th–Pb apatite standard remain significant challenges in dating apatite by LA-ICPMS, and consequently in developing the chronometer as a provenance tool.This study has determined U–Pb and Th–Pb ages for seven well known apatite occurrences (Durango, Emerald Lake, Kovdor, Mineville, Mud Tank, Otter Lake and Slyudyanka) by LA-ICPMS. Analytical procedures involved rastering a 10 μm spot over a 40 × 40 μm square to a depth of 10 μm using a Geolas 193 nm ArF excimer laser coupled to a Thermo ElementXR single-collector ICPMS. These raster conditions minimized laser-induced inter-element fractionation, which was corrected for using the back-calculated intercept of the time-resolved signal. A Tl–U–Bi–Np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias. External standards (Ple?ovice and 91500 zircon, NIST SRM 610 and 612 silicate glasses and STDP5 phosphate glass) along with Kovdor apatite were analyzed to monitor U–Pb, Th–Pb, U–Th and Pb–Pb ratiosCommon Pb correction employed the ²⁰⁷Pb method, and also a ²⁰⁸Pb correction method for samples with low Th/U. The ²⁰⁷Pb and ²⁰⁸Pb corrections employed either the initial Pb isotopic composition or the Stacey and Kramers model and propagated conservative uncertainties in the initial Pb isotopic composition. Common Pb correction using the Stacey and Kramers (1975) model employed an initial Pb isotopic composition calculated from either the estimated U–Pb age of the sample or an iterative approach. The age difference between these two methods is typically less than 2%, suggesting that the iterative approach works well for samples where there are no constraints on the initial Pb composition, such as a detrital sample. No ²⁰⁴Pb correction was undertaken because of low ²⁰⁴Pb counts on single collector instruments and ²⁰⁴Pb interference by ²⁰⁴Hg in the argon gas supply.Age calculations employed between 11 and 33 analyses per sample and used a weighted average of the common Pb-corrected ages, a Tera–Wasserburg Concordia intercept age and a Tera–Wasserburg Concordia intercept age anchored through common Pb. The samples in general yield ages consistent (at the 2σ level) with independent estimates of the U–Pb apatite age, which demonstrates the suitability of the analytical protocol employed. Weighted mean age uncertainties are as low as 1–2% for U- and/or Th-rich Palaeozoic–Neoproterozoic samples; the uncertainty on the youngest sample, the Cenozoic (31.44 Ma) Durango apatite, ranges from 3.7–7.6% according to the common Pb correction method employed. The accurate and relatively precise common Pb-corrected ages demonstrate the U–Pb and Th–Pb apatite chronometers are suitable as sedimentary provenance tools. The Kovdor carbonatite apatite is recommended as a potential U–Pb and Th–Pb apatite standard as it yields precise and reproducible ²⁰⁷Pb-corrected, ²³²Th–²⁰⁸Pb, and common Pb-anchored Tera–Wasserburg Concordia intercept ages.     

U–Pb dating of granites in the Neoproterozoic Sergipano Belt,NE-Brazil: Implications for the timing and duration of continental collision and extrusion tectonics in the Borborema Province

The Sergipano Belt is the outcome of collision between the Pernambuco–Alagoas Massif and the São Francisco Craton during Neoproterozoic assembly of West Gondwana. Field relationships and U–Pb geochronology of granites intruded in garnet micaschists of the Macururé Domain are used to constrain the main collisional event (D₂) in the belt. The granites are divided into two groups, the pre-collisional granites (pre- to early-D₂) and the syn-collisional granites (syn- to tardi-D₂), the latter were emplaced as sheets along the S₂ axial plane foliation or they were collected at the hinge zones of F₂ folds. A U–Pb SHRIMP zircon age of 628 ± 12 Ma was obtained for the pre-collisional Camará tonalite. Two U–Pb TIMS titanite ages were obtained for the syn-collisional granites, 584 ± 10 Ma for the Angico granite and 571 ± 9 Ma for the Pedra Furada granite, and these ages are close to the garnet-whole rock Sm–Nd isochron of 570 Ma found for the peak of metamorphism in the Sergipano Belt. The ages of the Camará tonalite (628 Ma) and the Pedra Furada granite (571 Ma) mark respectively the maximum age for beginning of the D₂ event and minimum age for the end in the Macururé Domain. Using these ages, the main Neoproterozoic D₂ collisional event has been in operation in the Sergipano Belt for at least 57 million years. Correlation with coeval granitoids farther north in the Borborema Province indicate that while in the Sergipano Belt the syn-D₂ granites (ca. 590–570 Ma) were emplaced under compression, in the Borborema Province they emplaced under extensional conditions related to regional strike-slip shear zones. These contrasting emplacement settings for contemporaneous Neoproterozoic granitoids are explained by a combination of continent–continent collision and extrusion tectonics.     

Reconnaissance-style EPMA chemical U–Th–Pb dating of uraninite

EPMA chemical U-Th-Pb uraninite analysis has been used to constrain the age of the granite-related, Rössing South uranium prospect in Namibia and the Kintyre unconformity-related uranium deposit in Western Australia. Uraninite from the Rössing South prospect has an age of 496.1 ± 4.1 Ma, which is similar to the age of other uranium deposits in the region at Rössing and Goanikontes. Uraninite grains analysed from the Kintyre deposit have an age of 837 +35/-31 Ma suggesting that the uranium mineralisation occurred during or after the latest period of sedimentation in the Yeneena Basin during the ca 850 to ca 800 Ma Miles Orogeny.     

Thermal history of Australian passive margin cover sequences accreted to Timor during Late Neogene arc–continent collision,Indonesia

Paleotemperature indicators and apatite fission track analysis of Australian continental margin cover sequences accreted to the active Banda arc–continent collision indicate little to no heating during rapid late Neogene uplift and exhumation. Thermal maturation patterns of vitrinite reflectance, conodont alteration and illite crystallinity show that peak paleotemperatures (PPT) increase with stratigraphic and structural burial. The highest PPT is found in the northern hinterland of the accretionary wedge, which was beneath progressively thicker parts of the upper plate towards the north. Major discontinuities in the pattern of PPT are associated with the position of major thrust ramps such as those forming the Ramelau/Kekneno Arch (RKA). PPT for Upper Triassic to Neogene strata south of the RKA are 60–80°C, which are similar to, and in many cases lower than, correlative and age equivalent units drilled on the NW Australian Shelf. Permian to Lower Triassic sedimentary strata thrust over younger units within and north of the RKA have PPT of 100–220°C. Thrust sheets accreted beneath the upper plate have PPT approximately 90°C higher than those frontally accreted. Metamorphism of the northernmost units of these sequences yield PPT of >300°C. Thrust stacking yields an inverted thermal profile of PPT decreasing discontinuously downward and to the south (towards the foreland). The timing of PPT is constrained by apatite fission track ages from mostly Triassic continental margin cover sequences. Ages of Upper Triassic units are primarily coeval with deposition and show little evidence of thermal annealing, whereas those of Lower Triassic units are almost completely annealed and range from 1.8±0.5–19.2±9.7 Ma. The clustering of apatite fission track ages into two distinct groups indicates that the upper boundary of the partial annealing zone has remained for some time at a Triassic stratigraphic interval in the slope and rise of the NW Australian continental margin. The position of this zone on the present shelf is higher in the stratigraphic column due to the greater thickness of post-breakup shelf facies units. Thrust stacking of rise, slope and shelf units produces an inverted vertical profile of increasing apatite fission track age with depth. Lack of any long confined track lengths in apatite from all of the units requires rapid and recent exhumation of the thrust stack, which is coincident with rapid phases of Pliocene–Pleistocene exhumation documented throughout Timor. These data preclude pre-Late Miocene tectonic burial or pre-Pliocene exhumation of the NW Australian continental margin.     

The final collision of the CAOB: Constraint from the zircon U–Pb dating of the Linxi Formation, Inner Mongolia

The Linxi Formation occupies an extensive area in the eastern Inner Mongolia in the Central Asian Orogenic Belt(CAOB).The Linxi Formation is composed of slate,siltstone,sandstone and plant,lamellibranch microfossils in the associated strata.Major and trace element data(including REE) for sandstones from the formation indicate that these rocks have a greywacke protolith and have been deposited during a strong tectonic activity.LA-ICPMS U—Pb dating of detrital zircons yield ages of 1801 to 238 Ma for four samples from the Linxi Formation.425—585 Ma,together with the ~500 Ma age for the metamorphism event previously determined for Northeast China,indicates that their provenance is the metamorphic rocks of Pan-African age that have a tectonic affinity to NE China.A few older zircons with U-Pb ages at 1689-1801 Ma,1307-1414 Ma,593-978 Ma are also present,revealing the Neoproterozoic history of NE China.The youngest population shows a peak at ca.252 Ma,suggesting that the main deposition of the Linxi Formation was at late Permain.Moreover,the ca.250 Ma zircon grains of all four samples yield weighted mean ~(206)Pb/~(238)U ages of 250 ± 3 Ma,248 ± 3 Ma,249 ± 3 Ma,and 250 ± 2 Ma,respectively.These ages,together with the youngest zircon age in the sample ZJB-28(ca.238 Ma),suggest that the deposition of the Linxi Formation extended to the early Triassic.Combining with previous results,we suggest that the final collision of the Central Asian Orogenic Belt(CAOB) in the southern of Linxi Formation,which located in the Solonker-Xra Moron-Changchun suture,and the timing for final collision should be at early Triassic.     

Rapid magmatic processes accompany arc–continent collision: the Western Bismarck arc, Papua New Guinea

New U–Th–Ra, major and trace element, and Sr–Nd–Pb isotope data are presented for young lavas from the New Britain and Western Bismarck arcs in Papua New Guinea. New Britain is an oceanic arc, whereas the latter is the site of an arc–continent collision. Building on a recent study of the Manus Basin, contrasts between the two arcs are used to evaluate the processes and timescales of magma generation accompanying arc–continent collision and possible slab detachment. All three suites share many attributes characteristic of arc lavas that can be ascribed to the addition of a regionally uniform subduction component derived from the subducting altered oceanic crust and sediment followed by dynamic melting of the modified mantle. However, the Western Bismarck arc lavas diverge from the Pb isotope mixing array formed by the New Britain and the Manus Basin lavas toward elevated ²⁰⁸Pb/²⁰⁴Pb. We interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision. ²³⁸U and ²²⁶Ra excesses are preserved in all of the lavas and are greatest in the Western Bismarck arc. High-Mg andesites with high Sr/Y ratios in the westernmost arc are attributed to recent shallow mantle flux melting at the slab edge. Data for two historical rhyolites are also presented. Although these rhyolites formed in quite different tectonic settings and display different geochemical and isotopic compositions, both formed from mafic parents within millennia.     

Modal metasomatism in the Kaapvaal craton lithosphere: constraints on timing and genesis from U–Pb zircon dating of metasomatized peridotites and MARID-type xenoliths

Modal metasomatism in the Kaapvaal craton lithosphere is well documented in upper mantle xenoliths sampled by both group I (mainly late Cretaceous) and group II (mainly early Cretaceous to late Jurassic) kimberlites in the Kimberley area. The metasomatic style is characterized by introduction of K, H and large ion lithophile/high field strength (LIL/HFS) elements into the lithospheric mantle leading to the crystallization of hydrous potassic phases such as phlogopite and/or K-amphibole. Textures indicate that the hydrous phases either replace pre-existing assemblages in peridotites, forming the metasomatized peridotite suite (phlogopite–K-richterite–peridotites: PKPs) or crystallize from K-rich melts, forming the mica–amphibole–rutile–ilmenite–diopside (MARID) suite of xenoliths. These K-rich assemblages become potential low melting source components for alkaline incompatible trace element enriched magmas. The timing of metasomatism and its temporal and possible genetic relation to kimberlite magmatism is poorly constrained because of the rarity of phases in the metasomatic assemblages suitable for precise dating. Here we present precise sensitive high resolution ion microprobe (SHRIMP) U–Pb formation ages of 88 ± 2 (1σ=1 standard deviation) and 82 ± 3 Ma data for zircons from a K-richterite–phlogopite-bearing metasomatized peridotite (PKP) and a MARID xenolith respectively, sampled by a group I kimberlite. Both average PKP and MARID zircon ages are indistinguishable from emplacement ages of group I kimberlites in the Kimberley area dated at 83 ± 4 (2σ) and 84 ± 0.9 Ma. One exceptionally old age spot of 102 ± 5 Ma from a PKP zircon provides evidence for modal metasomatism predating group I kimberlite emplacement by several millions of years with minor resetting of the U–Pb isotopic system of most analyzed PKP zircons to a group I emplacement age. Detailed textural and mineral chemical analysis, including high energy X-ray mapping and analysis of fluid inclusion daughter crystals, indicates a complex reaction history for both PKPs and MARIDs. U–Pb zircon ages from this study combined with literature data and experimentally derived models for MARID formation are used to suggest that MARID-formation is concurrent and genetically related to both group I and II kimberlite magmatism in the Kimberley area. MARID and PKP zircon ages are also consistent with the idea first proposed by Dawson and Smith (Geochim Cosmochim Acta 41: 309–323, 1977) that metasomatized peridotites may form from interaction of hydrous fluids expelled by solidifying MARID-type melts with peridotitic wall rocks. Received: 13 December 1999 / Accepted: 13 April 2000     

The first U–Pb isotope age data of island arc and continental–margin magmatism in the central part of the Koryak Highlands and U–Pb age of ore-controlling granitoids of the talyayigin ore field

Precision U–Pb (SHRIMP-II) isotope geochronological data, obtained for the first time, make it possible to suggest that sediments of the Neocomian primitive island arc sequence are missed or poorly developed in the South-Western part of the Mainitskii terrane of the Koryak Highlands. However, Late Albian mature island arc tuff and tuffaceous–turbidite formations are common. This enables us to extend the age range of the Mainitskii island arc from the Early Neocomian to the Late Albian and to suggest a two-stage pattern of its development. The isotope-geochronological data obtained for plagiogranite and moderately acid subvolcanics, previously attributed to the Koryak–Western Kamchatka volcanoplutonic belt, indicate that it is possible to combine them into the Middle Miocene postsubduction? polygenic complex. In addition, owing to modern high-precision isotope–geochronological methods, it has become possible to determine the age of gold–sulfide mineralization of the Talyaigin ore field, paragenetically related to the manifestations of the Middle Miocene Vilyuneiveem complex.     

Temporal association of arc–continent collision,progressive magma contamination in arc volcanism and formation of gold-rich massive sulphide deposits on Wetar Island (Banda arc)

Whole-rock ⁸⁷Sr/⁸⁶Sr and δ¹⁸O analyses of volcanic rocks and ³He/⁴He analyses of sulphides and sulphates from mineralized rocks on Wetar, Indonesia indicate a variable contribution of assimilated crustal material or sediment sourced from the subducted Australian craton to the south. These new data support the idea of progressive source contamination with precisely dated events showing that Wetar Island hosts the most extreme examples of crustal assimilation in the region. The increased continental contamination occurs during the Pliocene (Zanclian to Piacenzian) during distinct magmatic events between 5 and 4 Ma, and at 2.4 Ma when ⁸⁷Sr/⁸⁶Sr ratios in unaltered lavas, with whole-rock δ¹⁸O values between 5.7 and 9.6‰, increase from 0.707484 to extreme radiogenic values of 0.711656.The earlier of these magmatic events is important in the generation of the hydrothermal systems responsible for the mineralization recorded on Wetar. Samples from this yield radiogenic ³He/⁴He ratios between 0.5 and 1.4 R/R_A, similar to the data from volcanic rocks on nearby Romang. The later magmatic event coincides with the arrival of the Australian Continental Margin at the subduction zone along the Banda arc. Progressive incorporation of continental-sourced components into the source region below the Wetar Island edifice coincides with the formation of gold-rich volcanogenic massive sulphide deposits hosted within the contaminated volcanic pile.     

Juvenile granite in the Sanandaj–Sirjan Zone,NW Iran: Late Jurassic–Early Cretaceous arc–continent collision

The Sanandaj–Sirjan Zone (SSZ) of western Iran is characterized by numerous granitoids of mainly calc-alkaline affinities. Several leucogranite and monzonite bodies crop out in the eastern Sanandaj. Whole-rock Rb–Sr isochrons demonstrate that the Mobarak Abad monzonite (MAM) formed in two phases at 185 and 131 Ma. Low ⁸⁷Sr/⁸⁶Sr(i) (i represents initial) and high ¹⁴³Nd/¹⁴⁴Nd(i) ratios, resulting in positive ?^t _Nd, imply that the source magma originated from a depleted mantle; large ion lithophile element (LILE) and light rare earth element (LREE) enrichments imply that slab fluid was involved in the evolution of the parent magma. Geochemical characteristics of the MAM rocks show an affinity with I- and A-type granites, and the positive values of ?^t _Nd (+2 to +6), confirm that the MAM represents juvenile granite. Therefore, the MAM rocks are different from Himalayan, Hercynian, and Caledonian granites. Based on the geology of granitic host rocks that form the protoliths of metamorphic rocks, it is likely that the mafic part of the MAM formed in an island arc setting on Neo-Tethyan oceanic crust during Early to Middle Jurassic time. Subsequent collision of the island arc with the western part of the SSZ occurred in the Late Jurassic to Early Cretaceous. Metamorphism, accompanied by partial melting, occurred during collision. Finally, leucogranite magmas of the young Mobarak Abad dikes and the Suffi Abad body were generated in this collision zone. This new model suggests a Late Jurassic–Early Cretaceous arc–continental collision before final closing of the Neo-Tethys.     

First results of U–Pb dating of detrital zircons from the Ordovician clastic sequences of the Sol-Iletsk Block,East European Platform

The first LA–ICP–MS U–Pb isotopic ages of detrital zircons from the Ordovician sandstones of the Sol–Iletsk Block (well 2–Ordovician), located at junction of the East European Platform with the Pre-Caspian Basin and the Pre-Uralian foredeep, are presented. Two detrital zircons with well-defined ages of 561 ± 4 and 570 ± 5 Ma were found in sample K15–501. They confirm the Ordovician age of the sandstones, which earlier had been defined on the basis of seismic–stratigraphic and lithological correlations. The age distribution of the detrital zircons indicates the significant role of Late Precambrian rocks as provenance sources. However, those rocks still remain unknown in the Early Precambrian basement of the Volga–Ural part of the EEP.     

Chemical U–Th–total Pb dating and structural disorder of monazite-(Ce) from granitoids of the Aduiskii massif,Middle Urals

Using the methods of electron probe microanalysis and Raman spectroscopy, the zoning, chemical composition, and disorder in the matrix of accessory monazite extracted from a synplutonic quartz dioritic dyke intruding migmatite, diorite, and fine-granular granite of the Aduiskii massif were studied. It was established that monazite grains contained inner and outer zones. The contributions of chemical and radiation factors to mineral disorder were estimated. The results of chemical U-Th-total Pb dating of mineral are reported. The age 252 ± 4 Ma corresponds to the second maximum of granite formation.     

Molybdenite Re–Os and U–Pb zircon dating and genesis of the Dayana W-Mo deposit in eastern Ujumchin,Inner Mongolia

The Dayana W-Mo deposit in eastern Ujumchin of Inner Mongolia is a quartz-vein type deposit in the mid-western part of the Central Asian Orogenic Belt (CAOB). Biotite monzogranite, quartz porphyry and hornfels host W-Mo in quartz veins. Based on spatial relationships, molybdenite was deposited first followed by wolframite. This contribution presents precise laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) U–Pb zircon dating and geochemical analysis of the biotite monzogranite. The U–Pb dating shows that the monzogranite is 134 ± 1 Ma. Major and trace element geochemistry shows that the monzogranite is characterized by high SiO₂ and K₂O contents, a “Right-inclined” shape of the chondrite normalized REE patterns, enrichment of large ion lithophile elements (LILEs), and depletion of high field strength elements (HFSEs) such as Nb, P, Ba. The monzogranite is high-K calc-alkaline, has a strong negative Eu anomaly (Eu/Eu* = 0.04–0.45), low P₂O₅ content, high A/CNK of > 1.2, enriched in large-ion lithophile elements (LILEs; such as Rb, Th, U, Nd, and Hf), and notably depleted in Ba, Sr, P, Ti, and Nb. These characteristics define the Dayana monzogranite as a highly fractionated peraluminous granite. Re–Os isotopic analysis of seven molybdenite samples from the deposit yield an isochron age of 133 ± 3 Ma (MSWD = 2.2), which indicates that the monzogranite and ore have the same age within error, are probably genetically related, and related to a major Early Cretaceous mineralizing event in China known as the Yanshanian.     

U–Pb zircon ages,geochemical and Sr–Nd–Pb isotopic constraints on the dating and origin of intrusive complexes in the Sulu orogen,eastern China

TPost-orogenic intrusive complexes from the Sulu belt of eastern China consist of pyroxene monzonites and dioritic porphyrites. We report new U–Pb zircon ages, geochemical data, and Sr–Nd–Pb isotopic data for these rocks. Laser ablation-inductively coupled plasma-mass spectrometry U–Pb zircon analyses yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 127.4 ± 1.2 Ma for dioritic porphyrites, consistent with crystallization ages (126 Ma) of the associated pyroxene monzonites. The intrusive complexes are characterized by enrichment in light rare earth elements and large ion lithophile elements (i.e. Rb, Ba, Pb, and Th) and depletion in heavy rare earth elements and high field strength elements (i.e. Nb, Ta, P, and Ti), high (⁸⁷Sr/⁸⁶Sr)_i ranging from 0.7083 to 0.7093, low ?_Nd(t) values from ?14.6 to ? 19.2, ²⁰⁶Pb/²⁰⁴Pb = 16.65–17.18, ²⁰⁷Pb/²⁰⁴Pb = 15.33–15.54, and ²⁰⁸Pb/²⁰⁴Pb = 36.83–38.29. Results suggest that these intermediate plutons were derived from different sources. The primary magma-derived pyroxene monzonites resulted from partial melting of enriched mantle hybridized by melts of foundered lower crustal eclogitic materials before magma generation. In contrast, the parental magma of the dioritic porphyrites was derived from partial melting of mafic lower crust beneath the Wulian region induced by the underplating of basaltic magmas. The intrusive complexes may have been generated by subsequent fractionation of clinopyroxene, potassium feldspar, plagioclase, biotite, hornblende, ilmenite, and rutile. Neither was affected by crustal contamination. Combined with previous studies, these findings provide evidence that a Neoproterozoic batholith lies beneath the Wulian region.     

Evaluating igneous sources of the Taveyannaz formation in the Central Alps by detrital zircon U–Pb age dating and geochemistry

Late Palaeogene syn-tectonic volcanic products have been found in the Northern Alpine foreland basin and in the South Alpine hemipelagic basin. The source of abundant volcanic fragments is still in debate. We analyzed the geochronology and geochemistry of detrital zircons, and evaluated their temporal and genetic relationships with potential volcanic sources. The study shows that the detrital zircon U–Pb age patterns have two major age groups: a dominance (ca. 90%) of pre-Alpine zircons was found, as commonly observed in other Alpine flysch formations. These zircons apparently derived from erosion of the early Alpine nappe stack in South Alpine and Austroalpine units. Furthermore, a few Neo-Alpine zircons (ca. 10%) have ages ranging from Late Eocene to Early Oligocene (~ 41–29 Ma). Both source materials were mixed during long riverine transport to the basin margins before being re-deposited by gravity flows. These Palaeogene ages match with the activity of Peri-Adriatic magmatism, including the Biella volcanic suite as well as the Northern Adamello and Bergell intrusions. The values of REE and ¹⁷⁶Hf/¹⁷⁷Hf_(t) ratios of the Alpine detrital zircons are in line with the magmatic signatures. We observe an in time and space variable supply of syn-sedimentary zircons. From late Middle Eocene to Late Eocene, basin influx into the South Alpine and Glarus (A) basins from the Northern Adamello source is documented. At about 34 Ma, a complete reorganisation is recorded by (1) input of Bergell sources into the later Glarus (B) basin, and (2) the coeval volcaniclastic supply of the Haute-Savoie basin from the Biella magmatic system. The Adamello source vanished in the foreland basin. The marked modification of the basin sources at ~ 34 Ma is interpreted to be initiated by a northwestern shift of the early Alpine drainage divide into the position of the modern Insubric Line.     

Optimization of the in-situ U–Pb age dating method via LA-Quadrupole-ICP-MS with applications to the timing of U–Zr–Mo mineralization in the Poços de Caldas Alkaline Complex,SE Brazil

The high spatial resolution of the LA-ICP-MS systems allows rapid extraction of vital isotopic information from individual growth zones of minerals. This paper describes in detail the optimization of a relatively inexpensive LA-ICP-MS system consisting of a UV 213 Laser Ablation and a Quadrupole ICP-MS. The results of optimization take into account laser energy, beam diameter, frequency and ICP-MS gas conditions. The optimized conditions were tested for precision and accuracy on a number of well-characterized zircons, commonly used as primary and secondary quality control standards. The acquisition of the U–Pb data is carried out in automated mode (pre-set points) for up to 12 h/day with only minimal operator presence. Individual U–Pb zircon analysis lasts 80 s. The 2σ uncertainties of the standards ranged between 1.4 and 8.2%, and overall their relative deviations ranged from 0.02 to 0.87%. The results are comparable to techniques that use more complex and time-consuming approaches such as LA-MC-ICP-MS and ion-microprobe. We have applied this method to obtain ages of numerous granitoid rocks from the Southern São Francisco Craton and a well-known Archean granitoid of the Kaapvaal Craton, South Africa. We furthermore provide the first results of U–Pb age dating of U–Zr–Mo mineralization in the Poços de Caldas Alkaline Complex, SE Brazil, with a U–Pb age of 85 ± 3 Ma for zircon-bearing hydrothermal veins.     

Neoproterozoic island arcs in East Sayan: duration of magmatism (from U–Pb zircon dating of volcanic clastics)

Two island arcs of different ages have been reconstructed in the Neoproterozoic history of southeastern East Sayan: Dunzhugur and Shishkhid. According to earlier concepts, the Dunzhugur arc formed at ～1020 Ma and underwent collision with the Siberian(?) continent at ～810 Ma. The Shishkhid arc formed somewhat earlier than 800 Ma and existed till the end of the Late Baikalian (～600 Ma, from indirect data). This primitive geologic history, when each arc existed for 200 Myr, was suggested because of the deficit of direct data, and its reconstruction cast doubt. In this work we present results of preliminary dating of detrital zircons separated from the volcaniclastic rocks composing the above arcs. We analyzed 12 zircon crystals from the Dunzhugur volcanic clastics, whose ²⁰⁶Pb/²³⁸U age varies from 844 ± 8 to 1048 ± 12 Ma (1σ). Five most ancient zircons form a concordant cluster with an age of 1034 ± 9 Ma (2σ). Hence, the arc formed earlier than it was assumed and existed for a long time, most likely, till its collision with the continent. We also studied two zircon samples from the volcaniclastic rocks of the Oka accretionary prism, which probably formed in the Shishkhid arc. All ten crystals of the first sample form a concordant cluster with an age of 813 ± 7 Ma (2σ). The analyzed zircons of the second sample arrange in two clusters, with an age of 775 ± 8 Ma (2σ, nine crystals) and 819 ± 17 Ma (three crystals). Thus, the Shishkhid arc formed earlier than it was assumed, at the end of the Early Baikalian, and underwent active volcanism at least till 775 Ma. Dating of detrital zircons from the volcaniclastics generated at the mature stage of the Shishkhid-arc evolution will help to reconstruct partly or completely its history in the period 775–600 Ma.