9:00am - 9:30amSession Keynote
Mineral inclusions in detrital garnet – An excellent petrogenetic tool
Georg-August-University Göttingen, Germany
Cold and deep subduction is a characteristic feature of modern-style plate tectonics (MSPT) and a prerequisite for the formation of low-temperature/high-pressure (LT/HP) and ultrahigh-pressure (UHP) rocks. Although having strong implications for processes like the movement of materials between surface and mantle, mantle convection, thermal regimes, or the crustal growth rate, the onset of MSPT is poorly understood and controversially debated. One argument for an early onset are local occurrences of Paleoproterozoic retrogressed rocks suggested to have initially formed at LT/HP conditions (e.g., Weller & St-Onge, 2017, Nature Geoscience 10, 305-311). In contrast, a major argument for a late onset is the absence of mineralogical indicators for LT/HP and UHP metamorphism, like glaucophane and coesite, from the pre-Neoproterozoic crystalline rock record (e.g., Stern, 2005, Geology 33, 557-560). A limiting factor for the exploration of LT/HP and UHP rocks through time is the decreasing preservation of crystalline rocks with increasing age. Thus, the sedimentary record represents an important archive that preserves information on ancient continental crust lost due to erosion (e.g., Dhuime et al., 2017, Sedimentary Geology 357, 16-32). Nevertheless, reconstructing metamorphic conditions from a sedimentary perspective is challenging, mainly because we face sand-sized mineral grains that lost their paragenetic context. Here we present a selection of case studies that highlight the applicability of mineral inclusion assemblages in detrital garnet as an excellent petrogenetic indicator. This technique enables to partially reconstruct the paragenetic context and to screen the underexplored sedimentary record for mineralogical evidence supporting the operation of MSPT in deep time.
9:30am - 9:45am
The European continental crust through detrital zircons from modern rivers: biasing effects in the detrital zircon record
1Institut für Geologie und Paläontologie, University of Münster, Germany; 2Departamento de Geología, Universidad de Chile, Chile; 3The Australian National University, Australia; 4Institut für Mineralogie, University of Münster, Germany
We present U-Pb, Lu-Hf, and O isotopic data, as well as size-shape data for approximately 3700 detrital zircons from 15 European rivers. In combination with geomorphological information for each river basin (area, drainage length, and hypsometric curves), we evaluate the representativeness and biases affecting such datasets. The new data allow us to demonstrate that the detrital zircon record from major rivers represents all relevant geological events in Europe at the continental scale, with detrital zircon ages ranging from Cenozoic to Archean. Several age peaks can be linked to different orogenic cycles and the formation of supercontinents such as Pangea and the Variscan Orogen, the largest episode of crustal reworking in Europe. These Variscan detrital zircons occur in all rivers, but Permian post-Variscan were only found in the Po (with significant crustal contamination) and Glomma rivers (with radiogenic and mantle-like signatures). Other important age clusters are the Alpine and post-Alpine Cenozoic 25-40 Ma and juvenile 0.2-10 Ma, the Caledonian 400-490 Ma, and the Avalonian-Cadomian 540-650 Ma. Detrital zircons of 930-1170 Ma and 1400-1700 Ma are significant in Scandinavia, as well as ca. 1850 and 2500-2900 Ma in east Europe. Despite the good representation of the different geological events in Europe, this does not occur when analyzing detrital zircon at smaller scales (i.e. the basin scale). The presence, importance, and proportions of peaks are strongly dominated by factors such as fertility, zircon-size-shape, and other geomorphological aspects. Excluding fertility, these factors alone can bias the proportion of peaks by up to 10%.
9:45am - 10:00am
Granulometric and lithologic control on apatite and zircon concentrations in Alpine fluvial sediment
1Technical University of Darmstadt, Institute of Applied Geosciences, Schnittspahnstraße 9, 64287 Darmstadt; 2University of Tübingen, Department of Geosciences, Schnarrenbergstraße 94-96, 72076 Tübingen
Detrital heavy mineral compositions are controlled by many factors such as mineral fertility in the source rocks and hydraulic sorting. Quantifying and understanding the resulting bias is crucial especially for the correct interpretation of single-grain analyses such as apatite or zircon geochronology in provenance studies.
In this study, an inter- and intrasample comparison of apatite and zircon concentrations is conducted on modern Alpine fluvial sands from five mono-lithological catchments draining granitoid, ophiolitic, metamorphic and sedimentary sources. The distribution of these minerals was quantified and compared within narrow grain size windows of each sample using point counting in strewn slides, XRF analysis of P2O5 and Zr as proxies for apatite and zircon, respectively and modelling based on the size shift. In addition, those results have been compared with other complementary monolitholigical catchments from the Alps.
While in line with published fertility values in the Alps, the apatite and zircon concentrations vary over three orders of magnitude. The intra-sample comparison shows highest zircon and apatite concentrations in the finer grain size fractions (63-250 µm), which is expected from the settling-equivalence principle. Furthermore, the apatite and zircon concentrations derived from point counting correlate well with those estimated from P2O5 and Zr concentrations through XRF analysis. However, the XRF analysis also reveals a significant amount of P2O5 and Zr contained in the grain sizes smaller than 63 µm. This is especially important, since many single-grain provenance studies do not consider the silt and clay fractions.
10:00am - 10:15am
Application of in-situ U-Pb-He double-dating on detrital zircons – an example of Alpine sediments from the Inn river and its tributaries
Georg-August-Universität Göttingen, Geoscience Center, Germany
In-situ U-Pb-He double-dating of detrital zircon provides information regarding formation and cooling ages of source rocks. Its advantage over conventional dating methods is the unique ability to obtain a high quantity of precise single zircon data with constraints for both low- and high-temperature chronology. In provenance studies the information collected by double-dating can be used to reconstruct more complex tectonic settings and geologic evolution of individual zircons. Here, it is used to distinguish the components and tentative mixing ratios of sediments from multi-source catchments with the aim of assigning them to their respective sources.
The European Alps provide a great natural laboratory to test the capabilities of the method. Considering their well-studied and complex tectonic evolution, i.e. the combination of Proterozoic to Tertiary magmatism and high-grade metamorphism with spatially strongly contrasting Late Mesozoic to Neogene exhumation, none of the conventional chronological tools can draw a nearly as accurate picture.
Detrital zircons from modern river sands taken in the catchments of the Alpine Inn and its tributaries were dated: Ages derived from the Zillertal (dewatering mostly the Tauern window) and the Ötztal (incising the Austroalpine basement nappes) correspond to published data and can explain the complex age distribution of zircons collected near the end of the Alpine Inn. Double-dating is extraordinarily well suited to decipher such in-depth information, especially in complex orogens where different structural units experienced different overprinting of isotopic systems, leading to high spatial contrasts in low- and high-temperature chronological data.
10:15am - 10:30am
Source-normalized α-dose: discrimination of first- and multi-cycle detrital zircon
Timescales of Mineral Systems Group, School of Earth and Planetary Sciences, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
Detrital zircon (DZ) U-Pb geochronology is widely applied in the geosciences to address a very wide range of questions. However, zircon is refractory and discrimination of first- versus multi-cycle origin is challenging which blurs source-to-sink relationships. We performed DZ U-Pb geochronology of modern sediments in fluvial and littoral environments on the Scott Coastal Plain in Western Australia. Principal age modes are at c. 730-500 Ma and c. 1100-880 Ma (Pinjarra Orogen), c. 1240-1120 Ma and c. 1700-1600 Ma (Albany-Fraser-Wilkes Orogen), and c. 2710-2580 Ma (Yilgarn Craton), corresponding to ultimate derivation from local crystalline basement rocks. The DZ U-Pb age spectra show a mismatch to the areal extent of source rocks in the catchment area. Here, we propose the application of a novel approach – source-normalized α-dose – to quantify active time of DZ grains in the sedimentary system and thus identify sedimentary recycling of DZ. This metric compares the α-dose (a measure of metamictization using U and Th content) of DZ and the values of crystals from their source crystalline basement. We show that source-normalized α-dose records the selective removal of labile (high α-dose) grains and is able to discriminate (i) first-cycle and (ii) multi-cycle DZ populations that experienced progressive sedimentary recycling and/or transport. Source-normalized α-dose provides an internal measure to address sedimentary recycling of DZ, i.e., it does not necessitate comparison with other mineral systems. Consequently, this tool aids in the identification of first- and multi-cycle DZ origin and ultimately strengthens source-to-sink correlations improving interpretation of DZ grain histories.