Allanite-group minerals are known to incorporate not only U and Th but also initial, non-radiogenic Pb. Allanite can therefore be analyzed in order to assess its crystallization age as well as the ambient Pb composition at the time of crystallization. Whereas allanite age dating has been the focus of many studies, constraining its initial Pb composition has received much less attention. We collected a series of Phanerozoic, allanite-bearing magmatic rock samples (volcanic, plutonic, pegmatite) and measured both the age and initial Pb composition of allanite by laser ablation-multi collector-inductively coupled plasma-mass spectrometry. We show that allanite data can be corrected for mass bias and fractionation using zircon (for U/Pb and Th/Pb ratios) and glass (for Pb/Pb ratios) as reference material as long as allanite is not metamict. A lower intercept age and y-axis intercept Pb composition can be determined by linearly regressing U-Pb data in a Tera-Wasserburg diagram, and a 230Th disequilibrium correction is highly recommended. We find a good agreement between our allanite U-Pb dates and published U-Pb zircon ages for the same localities. Our initial Pb compositions are validated by a fair agreement with Pb isotopic data measured on co-genetic feldspars from the same samples. The initial Pb composition of samples ranging from ca. 530 to 18 Ma reveals fluctuations in initial 207Pb/206Pb ratio, which points to different degrees of crustal (elevated μ=238U/204Pb) contribution. These variations could be due to post-magmatic deformation, weathering or metamorphism, but we believe that they rather reflect differences in initial magma composition. We thus emphasize the usefulness of allanite initial Pb compositions to discuss the source of igneous rocks.
Heavy mineral sands (HMS) are a resource of Ti minerals, zircon, monazite and critical elements such as Hf and the rare earth elements (REE). Portable field techniques capable of measuring the geochemical composition, such as portable XRF (pXRF), may assist in identifying and quantifying the multiple minerals present in HMS. In this study, we assess the application of pXRF to the characterization of garnet–magnetite mineral sands from the Namibian coastline. Forty-six HMS samples from the northern Namibian coast were analysed by pXRF for SiO 2 , Ti, Mn, Fe, Zr, Th, Y, V and Hf. The HMS were analysed as powdered and unground samples to test whether minimal sample preparation yields reliable results. Our tests show that the pXRF results of unground sands scatter widely due to sample heterogeneity. Powdered samples, however, can be measured precisely and can be corrected with conventional laboratory analyses. Some spectral interference specific to this material occurs, e.g. Th on Pb and Bi and Hf on Cu. Selectively corrected pXRF data for powdered HMS were compared to XRD Rietveld data to determine proportions of heavy minerals (Ti minerals, Fe oxides, zircon, garnet). Geochemical and mineralogical data correlated well for garnet and moderately for Fe oxides and Ti minerals. The presence of zircon and monazite is also indicated by Zr, Hf and Th concentrations, which demonstrates that pXRF is a useful tool for HMS characterization. Supplementary material: Tables of raw data (Supplementary Table S1), statistical parameters (Supplementary Table S2) and corrected pXRF values (Supplementary Table S3) are available at https://doi.org/10.6084/m9.figshare.c.4536992
<p>This contribution reports LA-ICP-MS zircon ages and Rb-Sr biotite ages from the Troiseck-Floning Nappe, forming the northeasternmost extension of the Silvretta-Seckau Nappe System in the Eastern Alps. The Troiseck-Floning Nappe comprises a basement formed by the Troiseck Complex and a Permo-Triassic cover sequence. The basement consists of paragneiss with intercalations of micaschist, amphibolite and different types of orthogneiss, which was affected by a Variscan (Late Devonian) amphibolite facies metamorphic overprint. The cover sequence includes Permian clastic metasediments and metavolcanics, as well as Triassic quartzite, rauhwacke, calcitic marble and dolomite. During the Eoalpine (Cretaceous) tectonothermal event the nappe experienced deformation at lower greenschist facies conditions.</p> <p>Detrital zircon grains from paragneiss are in the range of 530-590 Ma, indicating an Ediacarian to earliest Cambrian source and a Cambrian to Ordovizian deposition age of the protolith. Late Cambrian to Ordovician crystallization ages from leucogranitic intrusions represent the earliest magmatic event of the Troiseck Complex. The amphibolite bodies derived from basalt with a calc-alkaline to island arc tholeiitic signature.</p> <p>Leucocratic orthogneiss with K-feldspar porphyroclasts and a calc-alkaline granitic composition plots in the field of volcanic arc granite. The youngest zircon grains indicate a Late Devonian crystallization. Two pegmatite gneisses with a calc-alkaline composition are early Mississippian in age.</p> <p>Mylonitic orthogneiss with a pronounced stretching lineation appears as irregularly shaped layers. It is leucocratic, very fine grained and contains scattered feldspar porphyroclasts with a round shape and a diameter of about 1 mm. Its chemical composition is granitic/rhyolitic with an alkali-calcic signature. In classification diagrams it plots in the field of syn-collision granite. Zircon ages of about 270 Ma indicate a Permian crystallization. Similar rocks interpreted as Permian rhyolitic metavolcanics appear in the cover sequence. They share a similar chemical composition and crystallization age of 270 Ma. Associated intermediate metavolcanics developed from calc-alkaline basaltic andesite.</p> <p>According to Rb-Sr biotite ages cooling of the Troiseck-Floning Nappe below c. 300&#176;C occurred at about 85 Ma in the west and 75 Ma in the east.</p> <p>In summary, the Troiseck Complex developed from Cambrian to Ordovizian clastic metasediments and granitic and basaltic magmatic rocks emplaced in the same time range. During the Late Devonian, it was affected by the Variscan collisional event, causing deformation at amphibolite facies conditions and intrusion of calc-alkaline granites. In early Mississippian time pegmatite dikes intruded, maybe induced by decompression and exhumation. The deposition of clastic sediments and (sub)volcanic rocks (rhyolite and basaltic andesite) constrains a surface position of the Troiseck Complex during the Permian. Based on regional considerations an extensional environment is assumed. In Triassic times carbonate platform sediments were deposited. During the Eo-Alpine collision in the Cretaceous the unit was part of the tectonic lower plate and subducted to shallow crustal levels, indicated by a lower greenschist facies metamorphic overprint. The Troiseck-Floning Nappe was formed and exhumed since about 85 Ma. Rb-Sr as well as apatite fission track data from the literature indicate tilting with more pronounced exhumation and erosion in the eastern part during Miocene lateral extrusion of the Eastern Alps.</p>
Abstract The low-grade metamorphic early Paleozoic basement of the Veitsch area presents a wide variety of sedimentary facies domains. The first domain consists of thick metadacites of Middle Ordovician age (Blasseneck Porphyroid), overlain by fine-grained metaclastics of the Rad Formation (Late Ordovician to Silurian) and Devonian limestones and calcitic marbles (Kaiserstein and Kaskögerl Formation, respectively). Rhyolitic to dacitic magmatism initiated at ca. 479 Ma (LAMC-ICP-MS U-Pb zircon data) and lasted until ca. 444 Ma. The second domain comprises metaclastics of the Stocker Formation (Early Ordovician to Silurian), characterized by thin volcanics and volcaniclastics of andesitic and rhyolitic composition. U-Pb zircon data give Middle Ordovician age (463 Ma – 468 Ma). The third domain, exposed northwest of Veitsch, consists of thick metadacites (Blasseneck Porphyroid, ca. 478 Ma), followed by (siliceous) phyllites which grade into turbiditic metasediments (Sommerauer Formation, Late Ordovician to Devonian?). Clastic sediments of the Stocker and Sommerauer Formations were sourced from northern Gondwana showing a prominent Pan-African detrital zircon peak at ca. 640 Ma. Middle to Upper Ordovician volcanics (ca. 462 Ma – 448 Ma) represent the second source. Tectonic reconstruction leads us to the arrangement of three facies domains. A shallow marine shelf facies is located in the present days southwest. A marginal basin with volcanic islands on a sloping continent, and a deep-water environment containing turbidites are situated further to the northwest. The present arrangement of these facies domains is explained by eo-Alpine and Variscan thrust tectonics.
This study examines three granitoid plutons, intruded during the latest Triassic - earliest Jurassic, that are exposed in south-western Vietnam and south-eastern Cambodia. On geochemical and petrological grounds the two plutons in Vietnam (Hon Da Bac and Hon Khoai island) are composed of slightly peraluminous I-type granitoids, with a high-K calc-alkaline affinity. The Cambodian pluton (Tamao) consists of peraluminous S-type granitoids, with high-K calc-alkaline and ferroan affinity. Based on zircon U-Pb geochronology the Hon Da Bac granitoids were intruded between 200.7 ± 2.7 and 196.2 ± 3.0 Ma, those from Hon Khoai between 199.6 ± 2.7 and 197.6 ± 2.7 Ma and the Tamao granitoids at 187.8 ± 3.2 Ma. These ages are consistent with intrusion during and soon after the Indosinian II Event of the Indosinian Orogeny, which was driven by docking of the Sibumasu and Indochina Blocks. Although these granitoids have some geochemical signatures similar to those of arc-related magmas, we conclude that they are more likely derived from syn to post-collisional magmatism. In terms of timing of their emplacement and geochemistry, all three plutons can be convincingly correlated with the Late Triassic-Early Jurassic Eastern Province granitoids in the Loei Fold Belt, which stretches along the western margin of the Indochina Block, extending through eastern Thailand and southern Cambodia. By this correlation we show that in fact, the Eastern Province granitoid belt extends further southeast than previously thought, occurring in south-eastern Cambodia and south-western Vietnam.
Abstract The Golden Quadrilateral of the Apuseni Mountains (Romania) represents the richest Au(-Cu-Te) porphyry and epithermal district of Europe and the Western Tethyan metallogenic belt. The Au(-Cu-Te) mineralization is associated with Neogene calc-alkaline magmatism along graben structures growing during the late stages of the Alpine-Carpathian orogeny. We use zircon petrochronology to study the time-space distribution, sources, composition, and timescales of the Au(-Cu-Te)-mineralizing magmatism and explore its link to regional tectonics. Our own and published U-Pb zircon ages document ore-forming magmatic activity between ~13.61 and 7.24 Ma. In combination with available paleomagnetic data, the new zircon ages corroborate the hypothesis that the magmatism in the Golden Quadrilateral evolved in a tectonic environment dominated by major (up to 70°) crustal block rotation. Hafnium isotope composition of Neogene zircon (εHf between –2 and 10) supports the predominant origin of the magmas from a heterogeneous lithospheric mantle, which may have been fertilized during an earlier Cretaceous subduction event and possibly by concurrent Miocene subduction. Xenocrystic zircon shows involvement of crustal sources resembling European continental basement. Fertility indicators, including Eu/Eu* and oxygen fugacity based on zircon composition, show no systematic correlation with the mineralizing events and/or age. High-precision (isotope dilution-thermal ionization mass spectrometry) U-Pb zircon geochronology demonstrates that the magmatic systems exposed at district scale evolved over less than ~100 k.y. and that durations of hydrothermal mineralization pulses were even shorter.
The Inthanon metamorphic core complex in northern Thailand comprises gneisses, schists, migmatites and calc-silicates which are intruded by a variety of granitoids of different age. New P-T estimates, U-Th-Pb total age and U-Pb age data of monazite indicate a multi stage history of the core complex. Garnet mica-schists and gneisses are exposed in the western part of the Inthanon metamorphic core complex. Garnets show two episodes of growth, with some displaying garnet breakdown and corona formation containing plagioclase, quartz, biotite, and muscovite. The garnet core (grt1) records medium to high-grade metamorphism in the Late Triassic to the Early Jurassic. The second garnet growth (grt2) can be distinguished by a significantly lower Ca content and formed during a high-grade metamorphic event in the Late Cretaceous. Both garnet generations contain abundant inclusions of biotite, muscovite, and monazite, which are used for the reconstruction of the P-T-t history. The monazite included in grt1 yields an age of ~230 Ma. Metamorphic conditions during the first episode of garnet crystallization are 0.7–0.8 GPa at 530–570 °C. The second garnet growth occurred in the upper amphibolite facies at pressures of 0.4–0.5 GPa and temperatures of 640–670 °C. The monazite inclusions in grt2 and monazite within the garnet coronae yield an age of about 80 Ma. The monazite in the matrix, which exhibits complex chemical zoning indicative of recrystallization and re-precipitation during multiple stages of metamorphism, yields a main population at 230 Ma and a typical lead loss trend to a few clusters of 80 Ma dates. Orthogneiss domains are characterized by a mylonitic texture with large K-feldspar augen and porphyroclasts surrounded by a fine-grained, foliated matrix of quartz and feldspar. This domain is mainly exposed in the core zone of the Inthanon complex and tends to become more mylonitic towards the east. Temperature conditions of 650–700 °C and 0.4 to 0.7 GPa were calculated by pseudosection modelling and using the Ti-in biotite-geothermometer. The monazite texture from the orthogneiss domain displays patchy zoning indicative of several resorptions and reprecipitations. The monazite exhibits an older age range of 230–210 Ma, with younger clusters yielding ca. 75 Ma, and shows a lead loss trend to ca. 30 Ma. The orthogneiss domain is extensively injected by concordant foliated biotite-garnet leucogranite with a monazite U-Pb age around 40 Ma. This age is considered as the upper age limit for the early stages of ductile shearing in the core complex. Our data indicate that the Western Gneiss Belt in Thailand underwent several tectono-metamorphic events: (1) a medium P-T regional metamorphic phase in the Late Triassic to the Early Jurassic related to Sukhothai-Sibumasu collision (~200 Ma), followed by (2) a widespread younger overprint at upper amphibolite facies in the Late Cretaceous connected with local plutonic activity 75‑65 Ma. (3) Local Late Eocene–Oligocene magmatism and Chiang Mai basin development within deformation zones led to the emplacement of orthogneisses and local metamorphic overprint as seen e.g. in the Mae Ping and Three Pagoda shear zones.
