Detrital Garnets Research Articles

Deep subduction of felsic rocks hosting UHP lenses in the central Saxonian Erzgebirge: Implications for UHP terrane exhumation

Contrasting metamorphic conditions determined by chemical geothermobarometric investigations of ultrahigh-pressure (UHP) lenses surrounded by high-pressure (HP) and medium-pressure (MP) felsic country rocks are an enigmatic feature of UHP terranes. One of the major questions arising is whether the UHP lenses and the country rocks are a product of different peak metamorphic conditions corresponding to different maximum depth or whether country rocks also experienced UHP conditions but equilibrated and/or re-equilibrated at a different metamorphic stage. Here we address this question to the central Saxonian Erzgebirge in the northwestern Bohemian Massif, Germany. In order to screen the variety of garnet from lithologies occurring in the study area, we analyzed the detrital garnet record from seven modern stream sands. In addition to 700 inclusion-bearing garnet grains previously studied from the 125–250 μm grain-size fraction, we analyzed the 63–125 and 250–500 μm fractions and extended the dataset to overall 2100 inclusion-bearing grains. The new findings of coesite and diamond inclusions in several garnet grains, which are in compositional contrast to garnet of the known UHP lenses but match with those of the felsic country rocks, show that considerable parts of the country rocks underwent UHP metamorphism. Melt inclusions containing cristobalite, kokchetavite, and kumdykolite in garnet derived from the country rocks point to partial melting and re-equilibration during exhumation at HP/HT conditions. Although an amalgamation of rocks which reached different maximum depth may be responsible for some of the contrasting peak metamorphic conditions, the mineralogical evidence for UHP conditions in the felsic country rocks surrounding the UHP lenses proves a largely coherent slab subducted to UHP conditions. Furthermore, the presence of coesite in the subducting voluminous felsic crust and its transformation to quartz during exhumation have great implications for buoyancy development during the metamorphic cycle, which may explain the high exhumation rates of UHP terranes.

Middle Miocene denudation of the Higher Himalayan Crystallines revealed by chemical composition of detrital garnets from the Siwalik foreland basin sediments, western and central Nepal

The Middle Miocene to Early Pleistocene Siwalik Group is composed of sediments deposited in the Himalayan foreland basin. In this study, the heavy mineral assemblage and chemical composition of detrital garnets from the sandstones in the Lower Siwalik sub-group, which are exposed along the Karnali River in western Nepal and the Tinau Khola in central Nepal, are studied and the provenance shift during the Middle Miocene is discussed. The heavy mineral assemblage in the lower section of the Lower Siwalik sub-group is characterized as predominantly zircon-tourmaline-rutile. Chemical analysis of the detrital garnets showed a monomodal almandine-rich composition with low pyrope (Prp<10) and moderate grossular (Grs10-20) content. The composition pattern subsequently changed to a bimodal distribution with an increase in pyrope (Prp10-25) and low grossular (Grs<10) content with the appearance of detrital staurolite. These results indicate that the Middle Miocene erosion of the Higher Himalayan Crystallines (HHC) are the source of the deposits in the Siwalik Group. Changes in the heavy mineral assemblage and garnet composition are recognized in the Karnali River section at 14–12 Ma and in the Tinau Khola section at 11–10 Ma, which suggests that the denudation of the HHC progressed from west to east in the western and central Nepal Himalayas.

The use of heavy mineral chemistry in reconstructing provenance: A case study from Mesozoic sandstones of Kutch Basin, India

Petrography and geochemistry of the Mesozoic sedimentary rocks of the Kutch Basin in western India reveal the predominance of felsic source rocks. Single‐grain chemistry of the detrital heavy minerals in these sediments refines the existing provenance interpretations by incorporating additional data related to source lithology. The study presents information on type, composition, and metamorphic grade of the source rocks for the Mesozoics of the Kutch Basin, which comprises Jhurio, Jhumara, Jhuran, and Bhuj formations. Chemistry of rutile, garnet, and tourmaline supports multiple sources of sediments. The ratio between Cr and Nb in rutile suggests the dominance of sediment input from metapelites. The relationship between (Fe + Mn)–Mg–Ca in garnets and Ca–Fetotal–Mg in tourmalines indicates the dominance of Ca‐poor felsic metamorphic source along with intermediate‐acidic plutonic rocks. The multivariate discrimination of the detrital garnets reveals the dominance of granulite and amphibolite facies source rocks. Rutile compositions indicate subordinate inputs from mafic sources across the investigated sedimentary succession. The single‐grain analysis of heavy minerals, therefore, infers a wide variation in source rock types.

Garnet trace element geochemistry as a sediment provenance indicator: An example from the Qaidam basin, northern Tibet

Abstract Major element geochemistry of detrital garnet is a widely-used approach for sedimentary provenance analysis, in particular for unravelling parent-rock lithology of siliciclastic sediments. However, garnets from felsic crystalline rocks (e.g., intermediate–acidic igneous rocks and metapelites) often have similar major element composition. This results in ambiguity of provenance interpretation in most cases. Here, we collect trace element data of garnets from different rock types to explore the feasibility of detrital garnet trace element geochemistry in sediment provenance analysis. The Qaidam basin in northern Tibet is taken as an example and provenance of Cenozoic sedimentary rocks therein is interpreted based on detrital garnet trace element composition. We find that garnets in intermediate–acidic igneous rocks have much higher heavy rare earth element (ΣHREE > 600 ppm) and yttrium (Y > 800 ppm) abundances than garnets in metapelites (ΣHREE 4 ppm) and zinc (Zn > 150 ppm) abundances than garnets in low-, medium-grade metapelites (up to amphibolite facies). Based on these findings, we suggest that Fe- and Mn-rich detrital garnets in the Cenozoic sedimentary rocks from the northern Qaidam basin were probably derived from low-, medium-grade metapelites and Mg-rich, Ca-poor detrital garnets therein were most likely derived from granulite-facies metapelites. This provenance interpretation supports that the Qilian Mountains to the north was the major source for the northern Qaidam basin and major deformation style along the Altyn Tagh Fault to west have been dominated by large-amplitude lateral offset and extrusion, rather than crustal thickening and uplift during the early Cenozoic. This study emphasizes high potential of detrital garnet trace element geochemistry in provenance analysis and reconstruction of tectono-sedimentary evolution of clastic sedimentary basins.

Detrital garnet geochronology: Application in tributaries of the French Broad River, Southern Appalachian Mountains, USA

Abstract 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 Silurian (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.

Miocene basement exhumation in the Central Alps recorded by detrital garnet geochemistry in foreland basin deposits

Abstract. The Neogene evolution of the European Alps was characterized by the exhumation of crystalline basement, the so-called external crystalline massifs. Their exhumation presumably controlled the evolution of relief, distribution of drainage networks, and generation of sediment in the Central Alps. However, due to the absence of suitable proxies, the timing of their surficial exposure and thus the initiation of sediment supply from these areas are poorly constrained. The northern Alpine foreland basin preserves the Oligocene to Miocene sedimentary record of tectonic and climatic adjustments in the hinterland. This contribution analyses the provenance of 25 to 14 Myr old alluvial fan deposits by means of detrital garnet chemistry. Unusually grossular- and spessartine-rich garnet is found (1) to be a unique proxy for identifying detritus from the external crystalline massifs and (2) to occur abundantly in ca. 14 Myr old deposits of the foreland basin. In contrast to previous assumptions, we therefore propose that the external massifs were already exposed to the surface ca. 14 Myr ago.

Chatham Schist

ABSTRACTNew structural, petrographic, whole rock chemical and mineral chemical data from the Chatham Schist, the easternmost part of the Haast Schist, are presented. In combination with existing geochronological data, these bring knowledge of the basic geological framework of the Chatham Schist to the same standard as the rest of the Haast Schist. Chatham Schist is dominated by greyschist whose protoliths are correlated with the Permian-Triassic Rakaia Terrane of the Torlesse Composite Terrane. Graded bedding can be recognised in a few places in the northern Chatham Schist, but no fossils have been found. Metamorphic grade on Chatham Island is pumpellyite-actinolite facies and textural grade varies from TZ2A-3. Transitional greenschist-amphibolite facies, garnet-biotite zone Chatham Schist has been sampled in a xenolith from a Pliocene volcano and in a Southeast Chatham Terrace dredge. The age of Chatham Schist prograde metamorphism is Middle to Late Jurassic. At least two phases of penetrative deformation, and a third, brittle faulting phase, are recognised. Chatham Schist was rapidly exhumed in the late Early Cretaceous, possibly as a result of collision of the Hikurangi Plateau against the Gondwana supercontinent. Detrital garnets and spinels now found on Chatham Islands beaches were not sourced from Chatham Schist.

TETGAR_C: a novel three-dimensional (3D) provenance plot and calculation tool for detrital garnets

TETGAR_C: a novel three-dimensional (3D) provenance plot and calculation tool for detrital garnets

Diamond and coesite inclusions in detrital garnet of the Saxonian Erzgebirge, Germany

Abstract Local occurrences of coesite- and diamond-bearing rocks in the central Erzgebirge (northwestern Bohemian Massif, Germany) reveal ultrahigh-pressure (UHP) metamorphic conditions during the Variscan orogeny. Although UHP metamorphism supposedly affected a wider area, implying that rocks that equilibrated under UHP conditions occur dispersed in large volumes of high-pressure country-rock gneisses, mineralogical evidence is scarce. Here we have applied the new concept of capturing the distribution and characteristics of UHP rocks by analyzing inclusions in detrital garnet. Out of 700 inclusion-bearing garnets from seven modern sand samples from creeks draining the UHP area around the Saidenbach reservoir, we detected 26 garnets containing 46 mainly monomineralic coesite inclusions and 22 garnets containing 41 diamond inclusions. Combining these results with geochemical classification of the host garnets, we show (1) that coesite-bearing rocks are common and comprise eclogites as well as felsic gneisses, (2) that small inclusion size is a necessary precondition for the preservation of monomineralic coesite, and (3) for the first time, that diamond-bearing crustal rocks can be detected by analyzing the detrital record. Our results highlight the potential of this novel application of sedimentary provenance tools to UHP research, and the necessity of looking at the micrometer scale to find evidence in the form of preserved UHP minerals.

Trace and Rare Earth Elements chemistry of detrital garnets in the SE Alps and Outer Dinarides flysch basins: An important tool to better define the source areas of sandstones

Abstract Garnet is one of the most abundant heavy minerals present in the Cretaceous – Eocene flysch deposits of the Southeastern Alps and Outer Dinarides (Julian, Brkini and Istrian basins). About 300 detrital garnets from the Cretaceous-Eocene flysch deposits of the Southeastern Alps have been analysed by means of electron microprobe and LA-ICP-MS. In the Julian and Istrian basins, supplies are from amphibolite-facies rocks and mafic and ultramafic metamorphic rocks, while in the Brkini basin the latter are almost missing. Moreover, in the Julian and Istrian basins, supplies from skarns, very low-grade metabasites, or from ultra-high temperature metamorphosed calc-silicate granulites are present. Among these different groups, LA-ICP-MS analyses showed that trace element content can be very different in almandine-rich garnets from the different sources. In particular, the source that supplied the Julian Basin is significantly different from that of the Istrian Basin. From the Cretaceous to the Palaeocene the main supplies of Bi-type garnets derived from an area where feldspar-free garnet-bearing rocks were exposed. Successively, Bi-type garnets were supplied from an area where feldspar-garnet-bearing rocks were exposed. The presence of garnets from feldspar-free garnet-bearing rocks in the Brkini and Istrian basins can be ascribed to both recycling of material from the Julian Basin as well as direct input from the same areas that supplied the Julian Basin.

Provenance information recorded by mineral inclusions in detrital garnet

Garnet single-grain analysis is an often used and well established tool in sedimentary provenance studies, especially when metamorphic source rocks are involved. So far, however, solely the geochemical composition of detrital garnets is considered to draw conclusions concerning probable source rocks. The gained information is often limited by (i) geochemical overlap of garnets derived from different lithology and metamorphic grade, (ii) similar probabilities of belonging to more than one source rock type, and (iii) the limitations of discriminating different protolith compositions. Here we present the first attempt of using mineral inclusions in detrital garnet as a provenance tool. We analyzed the inclusions of ~300 fine to medium sand-sized detrital garnets from two proximal modern sand samples taken in the HP/UHP Western Gneiss Region of SW Norway. All mineral inclusions ≥2 μm were identified by Raman spectroscopy, showing that (i) most garnets from HP/UHP metamorphic source rocks contain mineral inclusions ≥2 μm, (ii) Raman spectroscopy is a very powerful tool to characterize the inclusion types, and (iii) less stable mineral phases like kyanite, omphacite, diopside, enstatite, coesite, amphibole group, and epidote group minerals occur as inclusions in garnet. These minerals, which are important for provenance studies, can thus be preserved in the sedimentary record as long as garnet is stable. The combination of inclusion types in garnet and geochemical garnet classification shows that (i) inclusions well reflect the geological characteristics of the sampled catchments, implying that they are useful indicators for HP/UHP provenance, and (ii) inclusions in garnet can be used to support and enhance the provenance information obtained by garnet geochemistry.

Provenance study of detrital garnets and rutiles from basaltic pyroclastic rocks of Southern Slovakia (Western Carpathians)

AbstractDetrital garnets and rutiles have been recovered from basaltic pyroclastic rocks in the northern part of the Pannonian Basin and characterized using electron probe microanalysis and imaging. All garnets are dominated by the almandine component, except for one sample dominated by spessartine. A total of three garnet groups have been distinguished according to the increased contents of grossular (Group I), pyrope (Group II) and spessartine components (Group III). Compositions of the group I and II garnets with fluctuating Ca- and relatively low Mg contents are consistent with low- to medium-grade metasediments and/or metabasites. Locally increased Mg contents could indicate higherP–Tmetamorphic overprint. The dominantly metamorphic origin of the Group I and II garnets (composed of &gt;99 % of samples) is also corroborated by chlorite, tourmaline, staurolite, ilmenite and andalusite inclusions. Spessartine-rich garnets (Group III composed of &lt;1 % of samples) could be genetically linked with granitoids. Detrital rutiles invariably plot within the field of metasediments metamorphosed under amphibolite-facies conditions. Possible proximal (subjacent basement sampled by ascending lava) or distal sources (catchment sediments from uplifted Central Carpathian basement) of heavy mineral assemblages are discussed.

Provenance of a Permian accretionary complex (Nishiki Group) of the Akiyoshi Belt in Southwest Japan and Paleogeographic implications

The Akiyoshi Belt in the Inner Zone of Southwest Japan is made up of a Permian accretionary complex, the main component of which is the Late Permian Nishiki Group that mainly consists of sandstone, mudstone, felsic tuff and a minor amount of chert and conglomerate. We employ multiple methods, which includes sandstone petrography, detrital garnet composition and detrital zircon U-Pb dating to investigate the likely sources of these terrigenous deposits and to reconstruct the paleogeographic link between the proto-Japan and the East Asian continent. The highly immature Late Permian sandstones are interpreted to derive from multi-type source rocks that include felsic igneous rocks, basalts, sedimentary rocks and low to medium-grade metamorphic rocks in proximal locations. The detrital zircon U-Pb results show that all samples contain a dominant Early to Late Permian zircon population (294–254 Ma) and these zircons are interpreted to be derived from an active volcanic arc, which was most likely caused by subduction of the Paleo-Pacific plate beneath the South China Block. We found that spessartine-rich almandine garnets and almandine garnets dominated assemblages in the lower unit changed to grossular-andradite garnets dominated assemblage in the upper unit, which was caused by a progressive uplifting and denudation of the Permian volcanic arc.

Heavy minerals and exotic pebbles from the Eocene flysch deposits of the Magura Nappe (Outer Western Carpathians, eastern Slovakia): their composition and implications on the provenance

The study aims to reconstruct the crystalline parent rock assemblages of the Eocene Strihovce Formation (Krynica Unit) and Mrazovce Member (Raca Unit) deposits, based on the heavy mineral suites, their corrosive features, geochemistry of garnet and tourmaline, zircon cathodoluminescence (CL) images, and exotic pebble composition. Both units are an integral part of the Magura Nappe belonging to the Flysch Belt (Outer Western Carpathians). Corrosion signs observable on heavy minerals point to different burial conditions and/or diverse sources. The compositions of the detrital garnets and tourmalines as well as the CL study of zircons indicate their origin in gneisses, mica schists, amphibolites, and granites in the source area. According to observed petrographic and mineralogical characteristics, palaeoflow data and palaeogeographical situation during the Eocene may show that the Tisza Mega-Unit crystalline complexes including a segment of the flysch substratum could represent the lateral (southern) input of detritus for the Krynica Unit. The Raca Unit might have been fed from the northern source formed by the unpreserved Silesian Ridge. The Marmarosh Massif (coupled with the Fore-Marmarosh Suture Zone) is promoted to be a longitudinal source.

Insights into the provenance of the Chinese Loess Plateau from joint zircon U-Pb and garnet geochemical analysis of last glacial loess

AbstractThe Chinese Loess Plateau, the world’s largest and oldest loess record, preserves evidence of Asia’s long-term dust source dynamics, but there is uncertainty over the source of the deposits. Recent single-grain detrital zircon U-Pb age analysis has progressed this issue, but debates remain about source changes, and the generation and interpretation of zircon data. To address this, we analyze different groupings of new and existing datasets from the Loess Plateau and potential sources. We also present the results of a first high resolution sampling, multi-proxy provenance analysis of Beiguoyuan loess using U-Pb dating of detrital zircons and detrital garnet geochemistry. The data shows that some small source differences seem to exist between different areas on the Loess Plateau. However, sediment source appears to be unchanging between loess and palaeosols, supporting a recent material recycling hypothesis. Our zircon and garnet data demonstrates, however, that Beiguoyuan experienced a temporary, abrupt source shift during the last glacial maximum, implying that local dust sources became periodically active during the Quaternary. Our results highlight that grouping data to achieve bigger datasets could cause identification of misleading trends. Additionally, we suggest that multi-proxy single-grain approaches are required to gain further insight into Chinese Loess Plateau dust sources.

A multivariate discrimination scheme of detrital garnet chemistry for use in sedimentary provenance analysis

Garnet chemistry provides a well-established tool in the discrimination and interpretation of sediment provenance. Current discrimination approaches, however, (i) suffer from using less variables than available, (ii) subjective determination of discrimination fields with strict boundaries suggesting clear separations where in fact probabilities are converging, and (iii) significant overlap of compositional fields of garnet from different host-rock groups. The new multivariate discrimination scheme is based on a large database, a hierarchical discrimination approach involving three steps, linear discriminant analysis at each step, and the five major host-rock groups to be discriminated: eclogite- (A), amphibolite- (B) and granulite- (C) facies metamorphic rocks as well as ultramafic (D) and igneous rocks (E). The successful application of statistical discrimination approaches requires consideration of the a priori knowledge of the respective geologic setting. This is accounted for by the use of prior probabilities. Three sets of prior probabilities (priors) are introduced and their advantages and disadvantages are discussed. The user is free to choose among these priors, which can be further modified according to the specific geologic problem and the level of a priori knowledge. The discrimination results are provided as integrated probabilities of belonging to the five major host-rock groups. For performing calculations and results a supplementary Excel® spreadsheet is provided.The discrimination scheme has been tested for a large variety of examples of crystalline rocks covering all of the five major groups and several subgroups from various geologic settings. In most cases, garnets are assigned correctly to the respective group. Exceptions typically reflect the peculiarities of the regional geologic situation. Evaluation of detrital garnets from modern and ancient sedimentary settings of the Western Gneiss Region (Norway), Eastern Alps (Austria) and Albertine Rift (Uganda) demonstrates the power to reflect the respective geologic situations and corroborates previous results. As most garnet is derived from metamorphic rocks and many provenance studies aim at reconstructing the tectonic and geodynamic evolution in the source area, the approach and the examples emphasize discrimination of metamorphic facies (i.e., temperature-pressure conditions) rather than protolith composition.

Cenozoic evolution of the Altyn Tagh and East Kunlun fault zones inferred from detrital garnet, tourmaline and rutile in southwestern Qaidam Basin (Northern Tibetan Plateau)

AbstractThis study focuses on the Cenozoic provenance and tectonic evolution of the southwestern Qaidam Basin through geochemical analysis of detrital garnet, tourmaline and rutile. The variation of detrital mineral compositions indicates that the Cenozoic evolution can be divided into three stages: (i) before the deposition of the upper Xiaganchaigou Formation (before 37.8 Ma); (ii) between the deposition of the upper Xiaganchaigou Formation and the Shangganchaigou Formation (from 37.8 to 22 Ma); (iii) since the deposition of the Xiayoushashan Formation (since 22 Ma). In the first stage, abundant garnets from high‐grade meta‐basic and ultramafic rocks in the sediments from the Ganchaigou area support a provenance from the South Altyn Tagh HP/UHP metamorphic zone. The low percentage of tourmalines from granitoid rocks in the sediments in the Kunbei‐Lücaotan area suggests a provenance from the East Kunlun fault zone, indicating that the Qimen Tagh Shan was not high enough to prevent the transport of sediments from the southern Qaidam Basin. The sediments in the Qigequan area were derived from both the Altyn Tagh fault zone and the East Kunlun fault zone. In the second stage, the tectonic activity consisted in the rapid uplift of the Altyn Shan. Changes in garnet composition indicate a lower detrital contribution from high‐grade metamorphic rocks. In the third stage, the disappearance of garnets from high‐grade metamorphic rocks and scattered temperatures of rutiles in the Ganchaigou area suggest that the source area shifted from the South Altyn Tagh HP/UHP metamorphic rocks to weakly metamorphosed Meso‐Neoproterozoic sedimentary rocks. The increase in granitoid‐derived tourmalines in the Kunbei‐Lücaotan area is indicative of the rapid uplift of the Qimen Tagh Shan. The provenance evolution in the southwestern Qaidam Basin indicates that the tectonic activity along the Altyn Tagh fault zone can be divided into an early stage of Altyn Shan uplift and a later stage of left‐lateral slip. At the same time, tectonic movement along the East Kunlun fault zone initiated.

Late Quaternary tectonic switching of siliciclastic provenance in the strike-slip-dominated foreland of the Western Carpathians; Upper Morava Basin, Bohemian Massif

This study is focused on the lithology and provenance of late Quaternary fluvial deposits of the Upper Morava Basin a pull-apart basin situated at the contact of the Bohemian Massif and Western Carpathians. Late Cenozoic tectonic convergence between these two units caused differential subsidence along strike-slip faults of the Elbe-Odra zone, leading to a distinct horst-and-graben morphology of the Upper Morava Basin. The Pleistocene fluvial deposits are preserved in several terrace levels and partly buried under the present-day floodplain of the Morava River. This study is based on four cores (11–25m deep) drilled in the floodplain of two major depocentres of the basin, the Lutín Graben, and the Upper Morava Basin sensu stricto. The drill cores were analysed for grain size, pebble- and heavy-mineral composition, chemical composition of detrital garnets, bulk magnetic susceptibility, sediment colour (visible-light spectral reflectance) and bulk element geochemistry. Age interpretations are based on eight optically stimulated luminescence (OSL) analyses. The Upper Pleistocene sediments were deposited in a gravelly braided to transitional braided-meandering river in both the Upper Morava Basin s.s. and the Lutín Graben (the oldest OSL age is 161.5ka, corresponding to the late Saalian). Between the end of the Saalian and late Weichselian glaciations, the Morava River abandoned the Lutín Graben for the Upper Morava Basin s.s. where it flows up to the present day. The Pleistocene fluvial style contrasts with the present-day meandering to anastomosing fluvial style of the Morava River. The Pleistocene deposits were sourced from areas corresponding to the present-day Morava River catchment including crystalline units of the eastern Bohemian Massif and the Moravo-Silesian Carboniferous Basin. They also contain a considerable input from the Bohemian Cretaceous Basin. The composition of late Weichselian deposits from the Dub nad Moravou core (34.53±3.42ka and younger) differs from the older fluvial deposits representing the other cores. It indicates that a major provenance change occurred between the latest Saalian and the late Weichselian. In the late Weichselian, the Morava River started to recycle loess deposits, which cover large areas of its catchment. Based on OSL dating, it may be assumed that the Morava River turned from degrading (between 92.6±8.03 and 34.53±3.42ka) to aggrading in style (from 34.53±3.42ka to the present day) due to coeval tectonic movements in the UMB, which are indirectly indicated by present-day seismicity, geomorphic faults and palaeoseismic evidence. Both the tectonic context and fluvial deposition styles of the Upper Morava Basin show similar features to the Upper Rhine Graben of the Alpine foreland.

Geochemistry and sedimentary provenance of the Upper Cretaceous Uberaba Formation (Southeastern Triângulo Mineiro, MG, Brazil)

ABSTRACT: The campanian Uberaba Formation, which crops out around the homonymous town, is composed of greenish lithic sandstones and conglomerates with possible volcanoclastic contribution. In this unit, heavy minerals such as ilmenite, garnet, perovskite and magnetite, besides clinopyroxene, plagioclase, quartz, calcite and apatite, are found. The Uberaba Formation geochemistry is marked by high grades of Ba, Ta, La, Nb and Th, with flat (no Eu anomaly) and highly fractionated (LaN/YbN = ca. 128) chondrite-normalized rare earth elements. Mineral chemistry analyses of detrital garnets from the Uberaba Formation indicate the predominance of chorlomite with an affinity to crustal garnets (G3). The sedimentary provenance of the Uberaba Formation corresponds to a mixing of materials from the alkaline districts of the Alto Paranaíba and from the erosion of the Serra Geral Formation and the Canastra and Araxá groups. Diamond-producing conglomerates which crop out nearby, in Romaria, were considered by some authors as correlated to the Uberaba Formation. However, the composition of garnets recovered from those conglomerates is different, plotting in the field of mantle garnets (G9/G10). We consider, then, that those units are not correlated, and that the Uberaba Formation is not a possible source for the alluvial diamonds found in the homonymous river.

The potential of detrital garnet as a provenance proxy in the Central Swiss Alps

Detrital garnet is a promising candidate to reliably fingerprint sediment sources in the Alps, which has so far been complicated by the wide range and similarity of some of the lithologies. Garnet is present in most Alpine sediments, is easy to identify, is fairly stable and, most importantly, reflects the type and the metamorphic grade of its source rock in its chemical composition. This study aims to establish fingerprints based on detrital garnet composition for the most important tectonic units of the Central Alps, including European, Penninic and Adriatic basement rocks and their respective metasedimentary covers. Sediments collected from modern rivers, which drain representative portions of the individual tectonic units, contain a natural mixture of the various garnet populations present in each unit. We selected six catchments in southwestern Switzerland draining the External massifs, Helvetic sediments and the Penninic nappe stack at the transition of Alpine greenschist- to amphibolite-facies metamorphism in order to test the variability of Alpine garnets and the role of inherited (pre-Alpine) garnets. Extraordinary grossular- and spessartine-rich garnets of the External massifs, which experienced greenschist facies metamorphism, are clearly distinguishable from generally almandine-rich garnets supplied by the higher-grade metamorphic Penninic nappe stack. The variable pyrope, grossular and spessartine components of these almandine-rich garnets can be used to further distinguish pre-Alpine, Alpine eclogite-facies and low-grade metasedimentary garnets. This provenance proxy has the potential to be used for reconstructing sediment sources, transport and dispersal patterns in a variety of settings throughout the Alpine sedimentary record.