Sm-Nd Dating Research Papers - Academia.edu (original) (raw)

The earliest Svecofennian magmatism in southern Finland has been dated at 1.90-1.88Ga. As an example of this, the Orijärvi (ca. 1.89Ga) and Enklinge (ca. 1.88Ga) volcanic centres comprise bimodal plutonic batholiths surrounded by volcanic rocks of comparable ages and chemical compositions. Here, we report geochemical and Sm-Nd isotope data from intrusive and extrusive samples, combined with zircon U-Pb and Lu-Hf isotopes for granodiorites from both study areas. The samples range from gabbros to granites and indicate a subduction-related continental margin setting. The zircons from the Orijärvi granodiorite define an age of 1892±4Ma whereas the Enklinge granodiorite yields an age of 1882±6Ma. Several inherited ages of 2.25-1.95Ga as well as younger ages of 1.86-1.80Ga were found in the Enklinge granodiorite. The initial εNd values from the mafic rocks from both locations fall in the range +1.1 to +2.9 whereas the felsic rocks exhibit initial εNd values of -0.4 to +1.2. The magmatic zircons from the Orijärvi and Enklinge granodiorites show average initial εHf values of -1.1 (at 1892Ma) and zero (at 1882Ma), respectively, each with a spread of about 7 ε-units. The initial εHf values for the inherited zircons from Enklinge range from +3.5 to +7.6 with increasing age. The Sm-Nd data indicate that the mafic rocks were derived from a “mildly depleted” mantle source while the felsic rocks show larger crustal contribution. Also, the variation in εHf values indicates minor mixing between mildly depleted mantle derived magmas and crustal sources. U-Pb ages and Hf isotopes for inherited zircons in the Enklinge granodiorite suggest the presence of juvenile Svecofennian “proto-crust” at depth.

- by and +1
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- Geology, Geochemistry, Volcanology, U-Pb Dating

In this study, we report the first discovery of microdiamond inclusions in kyanite–garnet schists from the Central Rhodope Mountains in Bulgaria. These inclusions occur in garnets from metapelites that are part of a meta-igneous and meta-sedimentary mélange hosted by Variscan (Hercynian) orthogneiss. Ultra-high-pressure (UHP) conditions are further supported by the presence of exsolved needles of quartz and rutile in the garnet and by geothermobarometry estimates that suggest peak metamorphic temperatures of 750–800 °C and pressures in excess of 4 GPa. The discovery of UHP conditions in the Central Rhodopes of Bulgaria compliments the well-documented evidence for such conditions in the southernmost (Greek) part of the Rhodope Massif. Dating of garnets from these UHP metapelites (Chepelare Shear Zone) using Sm–Nd geochronology indicates a Late Cretaceous age (70.5–92.7 Ma) for the UHP metamorphic event. This is significantly younger than previously reported ages and suggests that the UHP conditions are associated with the Late Mesozoic subduction of the Vardar Ocean northward beneath the Moesian platform (Europe). The present-day structure of the RM is the result of a series of subduction–exhumation events that span the Cenozoic, alongside subsequent post-orogenic extension and metamorphic core complex formation.

- by David Collings and +2
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- Garnets, UHP Metamorphism, Rhodope geology, Sm-Nd Dating

Nineteen single-grain detrital garnet ages from a tributary to the French Broad River (North Carolina, USA) establish a novel approach to Sm-Nd detrital garnet geochronology wherein the equilibrium bulk-rock composition lost during weathering and transport is replaced with the composition of inclusions leached from within each garnet grain. Detrital garnet ages were compared to published detrital zircon and monazite ages from the same river tributary system. Results show that 87% of the zircons have inherited Proterozoic ages; only zircon rims give Paleozoic ages. Monazites are exclusively Ordovician (weighted average: 460.9 ± 3.0 Ma). Our new detrital garnet ages (and the detrital zircon rims) record younger ages spanning the Late Ordovician to Early Devonian, likely reflecting prolonged metamorphic heating. The weighted average age of the detrital garnet population is Silu-rian (430.1 ± 7.2 Ma). Statistical tests confirm that the garnet population is younger than the monazite. The new detrital garnet ages illuminate a previously uninterpreted Silurian tectonometamorphic signal in this region. INTRODUCTION Garnet's resilience as a moderately to highly stable phase during weathering, transport, and deposition makes it a common component of clastic sediments and sedimentary rocks with metamorphic source terranes (Morton and Halls-worth, 1999). Detrital garnet has been used as a provenance tool but is limited in its full application potential without a robust method to determine the detrital garnet's age. Garnet's common growth as a rock-forming mineral across a wide range of pressures and temperatures, clear ties to prograde metamorphism, and utility in geo-thermobarometry (Baxter et al., 2017) also make determination of detrital garnet ages desirable, especially if primary metamorphic source out-crops are inaccessible or eroded. Furthermore, single-grain detrital garnet ages provide a method for investigating regional, catchment-scale metamorphic timing using a rock-forming index mineral without laborious outcrop sampling.

Accurate dating of eclogite-facies metamorphism is of paramount importance in order to understand the tectonic evolution of an orogen. An eclogite sample from the Najac Massif (French Massif Central, Variscan belt) displays a zircon-bearing garnet-omphacite-amphibole-rutile-quartz peak assemblage. Pseudosection modeling suggests peak pressure conditions of 15-20 kbar, 560-630 °C. Eclogite-facies garnet displays Lu-enriched cores and Sm-rich rims and yields a Lu-Hf age of 382.8 ± 1.0 Ma and a Sm-Nd age of 376.7 ± 3.3 Ma. The ages are interpreted as marking the beginning of the prograde garnet growth during the initial stages of the eclogite-facies metamor-phism, and the high-pressure (and temperature) peak reached by the rock, respectively. Zircon grains display chondrite-normalized REE spectra with variably negative, positive or no Eu anomalies and are characterized by either enriched or flat HREE patterns. However, they yield a well constrained in situ LA-ICP-MS U-Pb age of 385.5 ± 2.3 Ma, despite this REE pattern variability. Zr zonation in garnet, Y content in zircon and the diversity of zircon HREE spectra may suggest that zircon crystallized prior to and during incipient garnet growth on the prograde P-T path, recording the initial stages of the eclogite-facies conditions. Consequently, the zircon age of 385.5 ± 2.3 Ma, comparable within error with the Lu-Hf age obtained on garnet, is interpreted as dating the beginning of the eclogite-facies metamorphism. Accordingly, the duration of the prograde part of the eclogite-facies event is estimated at 6.1 ± 4.3 Myr. Subsequent exhumation is constrained by an apatite U-Pb age at 369 ± 13 Ma.