H. von Eynatten

Precise tracing of exhumation and provenance using 40 Ar/39 Ar geochronology of detrital white mica: the example of the Central Alps

Abstract Single-grain 40Ar/39Ar dating of detrital white mica from Oligocene to Miocene (31–13 Ma) sediments of the North Alpine Foreland Basin in Switzerland reveals three prominent age clusters indicating cooling of the source rocks below 350–420°C in Carboniferous, Early Permian, and Tertiary times. Precise calibration of sedimentation age throughout the study area enables the thermal evolution of the hinterland in space and time to be precisely traced. Palaeozoic mica ages are documented in all samples and are used as additional provenance indicators. Tertiary mica ages are restricted to sediments younger than 21 Ma, and are only found in central and western drainage systems. Tertiary micas document progressively increasing average cooling rates up to 34–41°C/Ma in the source area (Lepontine Dome), between 21 Ma and 14 Ma. The observed cooling rates and the time-span for rapid cooling in the source area (between 19 and 14 Ma) agree with thermal models derived from currently exposed rocks of the Lepontine metamorphic dome. This study proves that detrital mica geochronology is a robust tool for deciphering the thermal histories of ancient orogens which are no longer exposed today.

Metamorphic evolution of the Tethyan Himalayan flysch in SE Tibet

Abstract The metamorphic conditions and the age of thermal overprint were determined in metapelites, metaarenites and metabasites of the Tethyan Himalayan Sequence (THS) in SE Tibet using Kübler Index and vitrinite reflectance data and applying thermobarometrical (Thermocalc and PERPLEX) and geochronological methods (illite/muscovite K–Ar and zircon and apatite (U–Th)/He chronology). The multiple folded thrust pile experienced a thermal overprint reaching locally peak conditions between the diagenetic stage (c. 170 °C) and the amphibolite facies (c. 600 °C at 10 kbar). Burial diagenesis and heating due to Early Cretaceous dyke emplacement triggered the growth of illite in the metapelites. Eocene collision-related peak metamorphic conditions have been reached at c. 44 Ma. During collision the different tectonic blocks of the THS were tectonically buried to different structural levels so that they experienced maximum greenschist to amphibolite facies metamorphism. Later, during Oligocene to Miocene times the entire THS underwent anchi- to epizonal metamorphic conditions, probably associated to continuous deformation in the flysch fold-thrust-system. This period terminated at c. 24–22 Ma. Adjacent to the north Himalayan metamorphic domes, the base of the THS was metamorphosed during Miocene times (c. 13 Ma). Post-metamorphic cooling below c. 180 °C lasted until Late Miocene and took place at different times.

Simplifying compositional multiple regression: Application to grain size controls on sediment geochemistry

Modern geochemical data sets have typically around 20-30 compositional variables measured on some tens or hundreds of samples. A statistical analysis of data sets with so many variables should take as a priority the reduction of dimensionality of the model, in order to increase its reliability and enhance its interpretation. In the framework of compositional data analysis with multiple regression, such simplification can be achieved taking some geometric concepts into account. First, the sample space of compositions, the simplex, is given an Euclidean space structure by the compositional operations of perturbation, powering and Aitchison inner product. Then, given some qualitative information on which subcompositions might depend on each explanatory variable, one can decompose the simplex in a set of orthogonal subspaces, in such a way that the composition projected onto each subspace is independent of a subset of the explanatory variables. This is achieved with a series of singular value decomposition computations. The method is applied to a data set of 88 observations of six major oxides in molar proportions, from modern glacial and fluvio-glacial sediments, with grain size ranging from coarse sand to clay. The goal is to assess the influence of chemical weathering processes (expected to impose a linear relation of composition and grain size) against purely physical processes (expected to show step-wise functions following the largest characteristic crystal sizes of specific minerals in the source rock). We exhaustively explore all patterns of uncorrelation of the composition with three explanatory variables: grain size in @f scale, and two step functions for the silt and clay domains. The best pattern, chosen with a likelihood ratio test, has only a smooth trend of (Mg,Fe) vs. (Al,K,Ca+Na) enrichment towards finer grain sizes-explained as differential mechanical behaviour of phyllosilicates vs. feldspar-and coefficients for the two step functions related to the sharp decrease of quartz in silt fractions, and the sudden enrichment of mafic accessory minerals, alteration products and mechanically unstable phyllosilicates in the clay fraction. We could thus be confident that weathering is almost absent in this data set.

Constructing modal mineralogy from geochemical composition: A geometric-Bayesian approach

Modal mineralogical composition is known to carry more information than major element geochemistry, though the latter is far easier to determine in the lab. Constructing mineral compositions from geochemistry can be seen as a typical end-member problem, where one assumes that some multivariate observations are generated by a convex linear mixture of a few pure end-members: these end-member characteristics as well as the coefficients of the linear mixture for the observations can be then estimated from geochemical data. We propose a mixed geometric-probabilistic solution to this problem. First, we assume known end-members, in number and properties, and study the set of solutions from a purely geometric perspective. Second, we discuss how to select representative solutions from this space, in particular under some distributional assumptions. Third, we allow the end-member properties to randomly vary in a controlled, interpretable fashion. Finally we build a Bayesian model, with a parsimonious parametrization characterizing each of these three steps, that can be treated by conventional Markov-Chain Monte Carlo techniques. In the illustration case study, we apply the method to reconstruct the mineralogy of a set of fluvio-glacial monomictic sediments from an Alpine granitoid massif. Results suggest a trend of enrichment in chlorite, muscovite and Ti-bearing minerals, along with depletion in quartz and feldspar. This is tentatively interpreted as an effect of comminution combined with differential mechanical properties. Moreover, mineral chemistry is estimated to exhibit very low Na in muscovite, Fe-rich garnet, Na-rich plagioclase, K-feldspar with up to 10% Na-component (albite), and biotite with Mg>Fe. The reconstructed modal mineralogy stays in a reasonable agreement with quantitative XRD phase analyses.

The Rhodope Zone as a primary sediment source of the southern Thrace basin (NE Greece and NW Turkey): evidence from detrital heavy minerals and implications for central-eastern Mediterranean palaeogeography

Detrital heavy mineral analysis coupled with a regional geological review provide key elements to re-evaluate the distribution of the Rhodope metamorphic zone (SE Europe) in the region and its role in determining the evolution of the Thrace basin. We focus on the Eocene–Oligocene sedimentary successions exposed in the southern Thrace basin margin to determine the dispersal pathways of eroded crustal elements, of both oceanic and continental origins, as well as their different contributions through time. Lithological aspects and tectonic data coupled with geochemistry and geochronology of metamorphic terranes exposed in the area point to a common origin of tectonic units exposed in NW Turkey (Biga Peninsula) with those of NE Greece and SE Bulgaria (Rhodope region). The entire region displays (1) common extensional signatures, consisting of comparable granitoid intrusion ages, and a NE-SW sense of shear (2) matching zircon age populations between the metapelitic and metamafic rocks of the Circum-Rhodope Belt (NE Greece) and those of the Çamlica–Kemer complex and Çetmi mélange exposed in NW Turkey. Detrital heavy mineral abundances from Eocene–Oligocene sandstones of the southern Thrace basin demonstrate the influence of two main sediment sources mostly of ultramafic/ophiolitic and low- to medium-grade metamorphic lithologies, plus a third, volcanic source limited to the late Eocene–Oligocene. Detrital Cr-spinel chemistry is used to understand the origin of the ultramafic material and to discriminate the numerous ultramafic sources exposed in the region. Compositional and stratigraphic data indicate a major influence of the metapelitic source in the eastern part (Gallipoli Peninsula) during the initial stages of sedimentation with increasing contributions from metamafic sources through time. On the other hand, the western and more external part of the southern Thrace margin (Gökçeada, Samothraki and Limnos) displays compositional signatures according to a mixed provenance from the metapelitic and metamafic sources of the Circum-Rhodope Belt (Çamlıca–Kemer complex and Çetmi mélange). Tectonic restoration and compositional signatures provide constraints on the Palaeogene palaeogeography of this sector of the central-eastern Mediterranean region.

Comment on Maravelis et al. “Accretionary prism–forearc interactions as reflected in the sedimentary fill of southern Thrace Basin (Lemnos Island, NE Greece)”

Furthermore, it was a surprise to recognise that Maravelis et al. (2015) did not even mention the Reply of Caracciolo et al. (2013) to their discussion (Maravelis and Zelilidis 2013a, b, cited in Maravelis et al. 2015) where key problems about the provenance of the Thrace Basin sediments were debated. How should this issue be evaluated objectively, when Maravelis et al. (2015) use the evidences provided by Caracciolo et al. (2013) to support part of the provenance aspects included in their 2015 paper without referring to the original sources? Here we like to point out that Caracciolo et al. (2013) highlighted that Maravelis and Zelilidis (2010) have never recognised the occurrence of (1) glaucophane schists, nor of (2) picotite, a term used in d’Atri et al. (2012) for detrital Cr-spinel. Moreover, Maravelis and Zelilidis (2010) mentioned only a minor influx of volcanic material, but volcanic detritus is widely documented by other authors (Caracciolo et al. 2011, 2012; d’Atri et al. 2012; Cavazza et al. 2014). These data are mingled in the Maravelis et al. (2015) “Provenance” section, where the authors, for instance, falsely cite Caracciolo et al. (2011) as evidence for the scarcity of volcanic material. Conversely, the latter authors have demonstrated the dominance of volcanic lithic fragments, as shown in an Lm–Lv–Ls ternary diagram included in the respective paper (Fig. 6c in Caracciolo et al. 2011). The incorrect use of all these informations and the insufficient credit given to the results from other authors lead to a manipulation of the conclusions provided by Caracciolo et al. (2011, 2013), d’Atri et al. (2012) and Cavazza et al. (2014). Another point of discussion is related to both the analytical techniques and the “provenance” diagrams used by Maravelis et al. (2015). In particular, we refer to the use of hand-held XRF to determine the chemical data used for their interpretation. It is well known that measurement accuracy of hand-held XRF has limitations and needs a This contribution is intended to shed light on the content of the paper by Maravelis et al. (2015) about the evolution of the island of Limnos, a small portion of the Cenozoic Thrace Basin (Turkey, Greece and Bulgaria). Although we appreciate their approach to use sediment provenance for paleotectonic reconstructions, we have concerns how this subject has been dealt with and how it is presented to the scientific community. Most of our criticism is about the attempt of Maravelis et al. to provide a review of the evolution of the Thrace Basin using (1) an inadequate dataset, (2) an inadequate area (Limnos is <5 % of the Thrace Basin) and (3) the interpretation provided by different authors in previous papers without properly citing the source of the information. In their introduction section, Maravelis et al. (2015) summarise the results from previous provenance studies done in the whole Thrace Basin (extending through Greece, Bulgaria and Turkey). Interestingly, they do not mention the work by Meinhold and BouDagher-Fadel (2010) and Caracciolo et al. (2012) for the Eocene–Oligocene successions exposed on the island of Samothraki (NE Greece) and in Southern Bulgaria, respectively.