Raman Spectroscopy Of Carbonaceous Material Research Articles

Raman Spectroscopy of carbonaceous material thermometry in the Carnic Alps (Austria, Italy)

Abstract The Carnic Alps (eastern Southern Alps) provide a classical area to study polyphase very-low- to low-grade metamorphism within the Variscan belt of Europe. Temperature indicators collected during the past three decades map the general metamorphic structure of a mountain chain affected by three major tectonic events (Variscan and Alpine thrusting, Oligocene transpression). Thermometric data obtained by Raman Spectroscopy of carbonaceous material (RSCM) described in this study extend the already published database, provide a map of metamorphic isotherms, and are interpreted in the view of current tectonic concepts. The RSCM temperatures of this study describe a gradient between ca. 460 °C in the tectonically deepest segments, bordered by the Periadriatic Fault System, and temperatures of ca. 200 °C in Permian- Triassic boundary strata of the Gartnerkofel-1 core. Mapped isotherms indicate three domains with different thermal histories, characterized by Variscan imbrication of an accretionary wedge, Permo-Mesozoic burial, and Oligocene contact metamorphism.

Structural and metamorphic evolution of a subducted passive margin: insights from the Briançonnais nappes of the Western Alps (Ubaye–Maira valleys, France–Italy)

This contribution analyses the structural architecture and tectono-metamorphic evolution of Briançonnais units in the southern French-Italian Western Alps. The studied area extends from a virtually non-metamorphic area adjacent to the Helvetic-Dauphinois External Domain in the west to the Monviso-Queyras ocean-derived units in the east, where metamorphism increases up to eclogite-facies. Mapping at the scale 1:10.000 of a mountainous part of the Ubaye-Maira transect was performed, which portrays in detail the Briançonnais units over ~ 100 km2. The lithologies include meta-andesite, meta-siliciclastics, marbles, turbiditic calcschists and flysch whose age spans from Late Carboniferous to Eocene. Metamorphism is low-grade greenschist-facies in the west but reaches the blueschist-facies to the east. Structures related to four phases of deformation are identified. The first two generations of structures (D1–D2), related to the original top-to-the-west nappe stacking, are associated with conditions close to the peak of Alpine metamorphism. Previous tectonic surfaces are transposed by the dominant deformation structures (D3), developed under retrograde/decompression conditions. This D3 corresponds to a backfolding and back-thrusting event with a top-to-the-E transport. A fourth phase (D4) developed during late low-grade metamorphic conditions and deforms the previous surfaces by locally developing a crenulation cleavage, followed by brittle tectonics. An updated metamorphic map is presented, backed on published estimates and new thermometric data obtained by Raman Spectroscopy of Carbonaceous Material (RSCM). The TRSCM values range from ~ 295 °C to > 350 °C, moving from the most external Briançonnais unit to the internal Queyras “Schistes Lustrés” units. Suspected Upper Cretaceous palaeofaults have been documented, allowing us to group the classic Aiguille de Mary and Ceillac (sub-) units into a single tectonic unit, here referred to as Maurin unit. The palaeogeographical reconstruction of the evolution of the studied transect highlights the importance of the Marinet and Maurin axial units in the feeding of the Late Cretaceous-Eocene breccias deposited on the more internal Acceglio-type units.

Metamorphic reactions and their implication for the fluid budget in metapelites at seismogenic depths in subduction zones

In order to determine fluid-mineral reactions and the fluid budget in subduction zones, we have examined rocks from two accretionary complexes, the Kodiak (Alaska) and Shimanto (Japan), that were buried at the temperature conditions of the seismogenic zone. Maximum burial temperatures based on Raman spectroscopy of carbonaceous material in both examples range from 230 to 350 °C, whereas pressures reached up to ⁓3 and ⁓4kbars (based on multi-equilibrium modelling) in the higher-grade units of the Japanese and Alaskan complexes, respectively. At 230–260 °C, chlorite is interpreted as a coproduct of the lower-grade smectite-to-illite transformation. Chlorite content increases as temperature increases to 330–350 °C, as a result of illite-to-chlorite transformation. Other reactions include pyrite oxidation, dissolution of titanite and precipitation of anatase. Whole-rock chemistry points to the absence of systematic chemical variations in the temperature range between 250 and 350 °C and only local transport of quartz and albite from the matrix, to veins nearby, is observed. Qualitative analyses of the composition of fluid inclusions using decrepitation method show that the composition of the fluid is significantly different from the original seawater trapped in pores. In addition, the salinity of fluid inclusions in the quartz veins at 230–260 °C, analyzed using the Raman spectrum of water, is lower than seawater and interpreted as dilution of the original seawater by the fresh water released by the smectite-to-illite transformation. In contrast, veins formed at 330–350 °C contain fluid inclusions with a salinity on the order of or higher than that of seawater. The increase in salinity for temperatures of 250–350 °C reflects hydration reactions, which are mostly controlled by chlorite crystallization. This study demonstrates an uptake of pore fluid by mineral reactions at temperature corresponding to the base of the seismogenic zone, which is in contrast to the general trend of dehydration reactions occurring along rock burial in subduction zones.

Pattern and source of unusually high‐temperature metamorphism in an Eocene forearc recorded by the Pacific Rim Terrane, British Columbia, Canada

AbstractThe Pacific Rim Terrane is of forearc affinity and one of the most recent crustal elements accreted to the North American Cordillera in western Canada. Two units, the Leech River Complex and Pandora Peak Unit, within the terrane were subject to high‐temperature, medium‐pressure metamorphism. Biotite, garnet and staurolite isograds occur concentrically in the Leech River Complex, centred on the Leech River shear zone at its southern boundary. A local thermal overprint in the Pandora Peak Unit is characterized by replacement of prehnite‐pumpellyite and lawsonite‐bearing assemblages with muscovite + chlorite. Pseudosection models (Perple_X), and thermometry using garnet‐biotite Fe‐Mg exchange and Raman spectroscopy of carbonaceous material (RSCM) show a thermal gradient at ~3.8 kbar from ~230°C in the north to ~600°C in the south. Isotherms are continuous across the Leech River–Pandora Peak boundary. The small‐volume, interfoliated intrusions of Eocene age occurring throughout the terrane show no spatial relation to the isotherms. Elevated forearc metamorphism is due to the subcretion at ~51 Ma of nascent oceanic crust (and related spreading ridge or hotspot) of the underlying Siletz‐Crescent terrane along the south‐bounding Leech River shear zone. Our re‐evaluation of the metamorphic history requires revision of the role of magmatism as a source of heat transport in forearc metamorphism and the tectonic assembly in this setting.

Spatial and vertical patterns of peak temperature in the Delaware Basin from Raman spectroscopy of carbonaceous material

Spatial and vertical patterns of peak temperature in the Delaware Basin from Raman spectroscopy of carbonaceous material

A Thermal Profile across the Idaho-Montana Fold-Thrust Belt Reveals a Low-Relief Orogenic Wedge That Developed atop a Pre-Orogenic Basement High

Abstract Growing orogenic wedges cool rocks during exhumation of thrust hanging walls and heat them during burial of footwalls, leaving behind a resilient thermal record of earlier deformation in fold-thrust belts. In order to investigate early burial of deformed strata within the retroarc Idaho-Montana fold-thrust belt, we use Raman spectroscopy of carbonaceous material to construct a maximum temperature profile that constrains the thicknesses of eroded rocks structurally above the Lemhi arch, a pre-thrusting basement high. In the eastern portion of the study area, a sharp maximum temperature change of ~120°C occurs across the Johnson thrust, signifying that regional burial and heating predated late-stage faulting. West of here, cumulative exhumation is irregular, varying by up to 5 km over large (~75 km) wavelength folds; however, maximum temperatures in this same region are consistently ~200°C higher than correlative stratigraphic units in the adjacent foreland. The pre-thrusting, low-relief unconformity above the Lemhi arch, which served as the early décollement to the fold-thrust belt, was everywhere buried to at least ~6.5 km depth, which is ~1.5-5.0 km deeper than can be explained by stratigraphic burial. We hypothesize that between ~145 and 80 Ma, a combination of Cretaceous deposition and folding and thrusting at higher structural levels buried the décollement of the Medicine Lodge-McKenzie thrust system to this depth. These results suggest that the early orogenic wedge had exceptionally low taper. We propose that thin strata over the low-relief Lemhi arch limited the availability of potential décollements, which restricted the maximum surface slope that could be constructed in a thin-skinned system. Subsequent growth of the orogenic wedge required activation of a much deeper décollement and a switch to a thick-skinned structural style, promoting a shift from burial to exhumation of the former décollement and the underlying Lemhi arch. This suggests that the growth of an orogenic wedge is dependent on the thicknesses of the preexisting strata and the availability of potential décollements, with sedimentation and burial heating potentially playing a key role.

Raman thermometry and (U-Th)/He thermochronometry reveal Neogene transpressional exhumation in the Nacimiento block of central California, USA

Abstract We present a novel approach for mapping vertical uplifts in exhumed metasedimentary rocks by coupling Raman spectroscopy of carbonaceous material with (U-Th)/He thermochronometry on apatite and zircon. We apply this approach to carbonaceous metasedimentary rocks of the Franciscan subduction complex, exposed in the Nacimiento block of central California, USA, an area that records high-pressure–low-temperature metamorphism prior to entrainment within the present-day transform plate boundary. We reveal the extent and magnitude of previously unrecognized exhumation gradients, which, combined with regional structural observations, can be used to quantify vertical crustal motion associated with localized transpression. We propose that the Nacimiento block was affected by a kilometer-scale, post-subduction thermal anomaly linked to a localized transpressive regime since ca. 25 Ma, with an uplift rate of ∼0.3 mm/yr.

The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolution of the Briançonnais distal margin

Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of the Briançonnais distal margin units from the south Western Alps. The studied area extends southwest of the Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and was associated with back-folding and -thrusting in the units overlying the Dora-Maira massif and with exhumation related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations, seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and correlates with the “Prepiemonte units” known from the Ligurian Alps to the Swiss Prealps. Three major shear zones involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dronero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present a new metamorphic map based on published and new petrological data, including new thermometric data obtained by Raman spectroscopy of carbonaceous material (RSCM). The TRSCM values range from ~ 400 °C to > 500 °C, going from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the Late Triassic, the width of the Briançonnais s.l. domain can be restored at ~ 100 km, whereas it reached ~ 150 km after the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.

The impact of melt versus mechanical wear on the formation of pseudotachylyte veins in accretionary complexes

Whether seismic rupture propagates over large distances to generate mega-earthquakes or is rapidly aborted mainly depends on the slip processes within the fault core, including particularly frictional melting or intense grain-size reduction and amorphization. The record of seismic slip in exhumed fault zones consists in many instances in Black Faults Rocks, dark and glass-like-filled aphanitic veins that have been interpreted as resulting from the quenching of frictional melts, i.e. pseudotachylytes. Such interpretation has nevertheless been questioned as similar macro to nano-microstructures have been observed either on intensely comminuted natural fault rocks or on slow creep experiments conducted on crustal rocks, where melting is absent. Here, we report a new dataset of Raman Spectroscopy of Carbonaceous Material analyses, aimed at discriminating the slip weakening processes operating in the fault core during slip. Using high spatial resolution profiles on natural Black Fault Rocks from exhumed accretionary complexes and an experimentally calibrated modelling of Raman intensity ratio evolution with temperature, we assessed different scenarios of temperature evolution during fault slip. None of them is able to account for the distribution of Raman signal, so that in the three studied Black Fault Rocks interpreted so far as natural pseudotachylytes, Raman Spectroscopy of Carbonaceous Material rather reflects the effect of intense and localized strain during fault slip. Furthermore, the absence of thermal imprint on Raman signal puts upper bounds on the temperature reached within the fault zone. If one cannot rule out the occurrence of high and short-lived temperature increase due to friction, the latter was not high enough as to melt the large quartz fraction of the fault zone rocks.

Metamorphic evolution of graphite in the Paleoproterozoic Savo Schist Belt (SSB), Central Finland: Constraints from geothermetric modeling

Flake graphite deposits of the Savo Schist Belt (SSB), Central Finland are mainly hosted by quartz-mica schist, hornblende biotite gneiss, and to a minor extent in amphibolite. These rocks represent high-grade metamorphic conditions of the SSB. Raman spectroscopy measurements on graphite flakes provide quantitative information on the degree of graphitization, which can be correlated to the grade of metamorphism. The metamorphic conditions of the graphite-bearing metapelites have been determined by P-T pseudosection modelling and three types of geothermometers: garnet-biotite (GB), Ti-in biotite and Raman spectroscopy of carbonaceous material (RSCM) geothermometry. The calculated P-T pseudosections indicate peak P-T conditions of ca. 5–6 kbar and 650 °C for two garnet-bearing hornblende biotite gneiss samples, and slightly higher conditions of ca. 5–6 kbar and 700 °C for a quartz mica schist sample. The GB thermobarometer yielded a temperature and pressure range of 600–725 °C and 5.2–8.0 kbar. The Ti-in-biotite applied to the same samples produced temperature ranging between 570 and 735 °C, and the Raman (RSCM) geothermometers indicated temperatures between 550 and 670 °C. For the garnet-bearing hornblende biotite gneiss samples there is no significant (<10–50 °C) difference between temperatures determined using the P-T pseudosections and mean temperatures calculated using RSCM, GB and Ti-in-biotite methods. Although the peak temperature in some highly crystalline graphite flakes of studied samples calculated by (RSCM) geothermometer yield temperatures of 641 °C with the Beyssac et al. 2002 method and 737 °C with the Rahl et al. (2005) method, which are the highest temperature for this calibration. Raman spectroscopy is now part of a wide selection of tools that can be used to identify the ordering of the graphite crystallinity, which is including the number of layers, quality of layers, doping level and confinement in graphene structures. The results of this work show that Raman spectroscopy is a simple and fast way to determine the peak metamorphic temperature of carbonaceous material CM in amphibolite facies metasedimentary rocks.

Exhumation of the Western Alpine collisional wedge: New thermochronological data

Collisional shortening in the external Western Alps was first accommodated by internal (distributed) deformation in the External Crystalline Massifs (ECM) and then on frontal crustal ramps (localized deformation). However, the timing of transition between these two periods is still under-constrained, mainly because the available dataset is incomplete in the Western Alps. We here provide new zircon and apatite fission-track (ZFT and AFT) and zircon (U-Th-Sm)/He (ZHe) data that constrain the early stages of cooling hence exhumation of the external Alpine wedge, as well as new Raman Spectroscopy of Carbonaceous Material (RSCM) data from the Belledonne massif. ZFT ages mainly range between 15 and 20 Ma, ZHe ages between 5 and 12 Ma, AFT ages between 2 and 10 Ma. Those data are integrated within inverse and forward thermal history modelling (HeFTy) along with literature data to constrain the late Oligocene-Miocene cooling history and suggest that exhumation of the Belledonne and the Pelvoux massifs may have started as early as ca. 27 Ma. This early exhumation was rather slow (~ 50 m.Myrs−1 ± 2 m.Myrs−1) and may date the transition between the distributed and the localized mode of shortening, i.e., the initiation of the crustal ramps below these massifs. Further north, in the Mont Blanc and Aiguilles Rouges massifs, exhumation was active around 18 Ma, and started possibly earlier, around 20–25 Ma. From this time on (18 Ma), exhumation rates increased in all external massifs (~ 500 ± 40 m.Myrs−1, both North and South). This age most likely corresponds to the end of the transition period between distributed and localized shortening with localisation along the frontal crustal ramps and the rapid associated exhumation, then cooling of the hangingwall (even considering that cooling may start a few Myrs later than exhumation if isoterms are advected). This timing notably corresponds to a transition between the two molasse mega-sequences in the foreland basin (Lower Marine/Freshwater Molasse and Upper Marine/Freshwater Molasse).

Investigating the application of Raman spectroscopy of carbonaceous material in sedimentary lithic artifacts for archaeological provenancing applications in the Canadian Rockies

AbstractThe chemical compositions of toolstones composed of clastic and chemical sedimentary rock are often not distinct among procurement locations. However, sedimentary toolstone sources may show variations in the structural characteristics of the included carbonaceous material (CM) related to differences in their postdepositional histories, providing a potential proxy by which procurement locations, rather than individual rock units, can be assessed as raw material sources for archaeological lithics. Here, we apply a well‐established method for investigating the nature of CM in sedimentary rocks, Raman spectroscopy of carbonaceous material (RSCM), to an archaeological case study. We test if the technique can be used to differentiate among toolstones from two procurement areas along the eastern slopes of the Canadian Rocky Mountains potentially used by pre‐European contact Indigenous inhabitants of the Hummingbird Creek Site (FaPx‐1). Artifacts found during excavations at Hummingbird Creek visually appear to match toolstone‐grade rocks in the site's vicinity. We analyzed source material from two localities: (1) the local toolstone around FaPx‐1 and (2) a toolstone procurement area ~45 km away associated with many precontact workshop sites. To test if the artifacts have compositions consistent with the local toolstones, we analyzed artifacts and potential source materials with RSCM as well as their major, minor, and trace element composition using X‐ray fluorescence (XRF). Our work is guided by the elimination of source locations using indirect geochemical proxies in accordance with the exclusionary Provenance Hypothesis. Both the structural and the chemical characteristics of the artifacts are similar to those of geologic samples from the nonlocal source location and are not consistent with samples collected locally, permitting the exclusion of local materials and suggesting that the inhabitants of FaPx‐1 used a variety of toolstones to manufacture the artifacts from materials that do not appear to be readily available near the site.

Progressive Low-Grade Metamorphism Reconstructed from the Raman Spectroscopy of Carbonaceous Material and an EBSD Analysis of Quartz in the Sanbagawa Metamorphic Event, Central Japan

Low-grade metamorphic temperature conditions associated with the Sanbagawa metamorphic event were estimated by the Raman spectroscopy of carbonaceous material (RSCM) in pelitic rocks and an electron backscatter diffraction (EBSD) analysis of the quartz in siliceous rocks. Analytical samples were collected from the Sanbagawa metamorphic complex, the Mikabu greenstones, and the Chichibu accretionary complex in the eastern Kanto Mountains, central Japan. Previously, low-grade Sanbagawa metamorphism was only broadly recognized as pumpellyite–actinolite facies assigned to the chlorite zone. The RSCM results indicate metamorphic temperatures of 358 °C and 368 °C for the chlorite zone and 387 °C for the garnet zone of the Sanbagawa metamorphic complex, 315 °C for the Mikabu greenstones, and 234–266 °C for the Chichibu accretionary complex. From the EBSD analyses, the diameter of the quartz grains calculated by the root mean square (RMS) approximation ranges from 55.9 to 69.0 μm for the Sanbagawa metamorphic complex, 9.5 to 23.5 μm for the Mikabu greenstones, and 2.9 to 7.3 μm for the Chichibu accretionary complex. The opening angles of the c-axis fabric approximate 40–50°, presenting temperatures of 324–393 °C for the Sanbagawa metamorphic complex and the Mikabu greenstones. The temperature conditions show a continuous increase with no apparent gaps from these low-grade metamorphosed rocks. In addition, there exists an empirical exponential relationship between the estimated metamorphic temperatures and the RMS values of the quartz grains. In this study, integrated analyses of multiple rock types provided valuable information on progressive low-grade metamorphism and a similar approach may be applied to study other metamorphic complexes.

Bulk inclusion micro‐zircon U–Pb geochronology: A new tool to date low‐grade metamorphism

AbstractDating low‐grade metamorphism is challenging since such rocks commonly lack suitable target minerals for acquiring pressure–temperature–time–deformation (P–T–t–d) data. Herein a new geochronological method termed ‘bulk inclusion dating’ is applied to a chloritoid‐bearing schist from the Staufen‐Höllengebirge Nappe (SHN, Austroalpine Unit, Eastern Alps, Austria) for which Cretaceous metamorphism is imprecisely constrained. Thermodynamic modelling of the phase relations and mineral chemistry predicts the stability of the equilibrium assemblage in a P–T field between 450–490℃ and 0.5–0.7 GPa, which agrees with peak temperature constraints ~490℃ derived from Raman spectroscopy of carbonaceous material. Chemical zoning of, and the zonation of inclusions within, chloritoid confirm porphyroblast growth at these conditions. High‐resolution imaging reveals thousands of minute (length: 0.1–3 µm), euhedral micro‐zircon crystals included in chloritoid porphyroblasts and in the matrix. The morphological and microstructural characteristics of micro‐zircon as well as the crystal size distributions indicate that it nucleated and grew at greenschist facies conditions most likely from a Zr‐saturated fluid. In situ laser ablation inductively coupled plasma mass spectroscopy bulk inclusion dating of metamorphic zircon in the chloritoid rim using a laser spot diameter of 120 μm yields a U–Pb age of 116.7 ± 9.1 Ma (MSWD: 1.5, n: 79). We interpret zircon precipitation and progressive coarsening coeval with chloritoid growth during prograde metamorphism and thus link the age to the late prograde part of the P–T evolution. The contribution of other U‐bearing phases (apatite, epidote, rutile) does not significantly disturb the U–Pb age. The data provide clear evidence for Early Cretaceous metamorphism in the SHN and indicates that metamorphism started at least 20 m.y. before the formation of eclogites in the Austroalpine Unit. The method introduced here allows integration between metamorphic conditions and age constraints in low‐grade metamorphic rocks and opens up new potential applications in petrochronology.

The thermo-tectonic evolution of the actively exhuming Mai'iu Fault footwall – Suckling-Dayman metamorphic core complex – in the Woodlark Rift of Papua New Guinea

The Mai'iu Fault in the Woodlark Rift of southeastern Papua New Guinea is an active north-dipping low-angle normal fault (LANF) that has exhumed the spectacular Suckling-Dayman metamorphic core complex (SDMCC) in its footwall. While its youthful domal geomorphology suggests recent tectonic exhumation of the SDMCC, the cooling and exhumation history of this active metamorphic core complex have not been studied in detail before. Here we provide the first zircon and apatite fission-track (FT) and (U-Th)/He thermochronometric data on the SDMCC based on a suite of samples from slip-parallel footwall transects and syn-extensional alluvial deposits shed from the emerging dome. In addition, we double dated zircon using U–Pb LA-ICP-MS. These chronometric techniques are complemented by temperature and pressure estimates from Raman spectroscopy of carbonaceous material (RSCM) and Al-in-amphibole and Al-in-biotite barometry on footwall phyllites/schists and granitoids, respectively. The data indicate that slip on the Mai'iu Fault had initiated by ~4 Ma. We show that slip on the Mai'iu Fault has persisted at cm/year rates since the onset of extension, making it one of Earth's most rapidly slipping continental LANF. Integrating temperature estimates from FT and (U-Th)/He thermochronometry and RSCM, we calculated a down-dip, fault-parallel paleo-field temperature gradient along the now-exhumed part of the Mai'iu Fault of ~11 °C/km. For a likely range of pre-extensional geothermal gradients in the Woodlark Rift (10–20 °C/km), this paleo-field temperature gradient implies an average initial dip of the Mai'iu Fault of ~46°. Given the present dip of ~21° at the surface, the thermal data reinforce other evidence that the footwall of the Mai'iu Fault has been back-rotated to the south by ≥20° during progressive unroofing since fault inception, consistent with a rolling hinge style evolution of the SDMCC.

Szenes anyagok maximális átalakulási hőmérsékletének meghatározása Raman-spektroszkópiai mérésekkel: standardizálás és esettanulmány az Aggtelek–Rudabányai-hegységből

A mély diagenetikus – kisfokú metamorf üledékes kőzetek csúcshőmérsékletének meghatározását illit (Küblerindex) és klorit „kristályossági”, valamint vitrinit reflexiós értékek alapján nagy bizonytalanság terheli. A hőmérséklet pontosabb meghatározására az elmúlt időszakban egyre elterjedtebbé vált a szenes anyag szemcsék mérésén alapuló Raman-spektroszkópia. Jelen munka célja, hogy a szenes anyag Raman-spektroszkópia (RSCM) egyik módszerének alapjairól, valamint hazai alkalmazási lehetőségeiről adjon részletes, magyar nyelvű összefoglalást. A bemutatott módszer alkalmazásához elengedhetetlen egy ismert metamorf csúcshőmérsékletű referencia kőzetsorozat elemzése, amely az ismeretlen minták metamorf csúcshőmérsékletének meghatározásához szükséges kalibrációs görbe alapjául szolgál. Ez a referencia kőzetsorozat teszi lehetővé az egyes laboratóriumokban született eredmények összehasonlíthatóságát is. Munkánkban ismertetjük az ELTE TTK KKIC Raman-laboratóriumában mért referencia kőzetsorozat eredményeit, valamint az ez alapján létrehozott kalibrációs görbét. A módszer alkalmazhatóságát az Aggtelek–Rudabányai-hegység Telekesoldali Formációjának példáján mutatjuk be. Az öt mintán (négy fúrás, egy felszíni) elvégzett mérések alapján az összletre meghatározott metamorf csúcshőmérséklet 273 ± 10 °C-nak adódott, amely adat jelentősen pontosabb a korábban rendelkezésre álló illit- és klorit kristályossági, valamint vitrinit reflexiós értékek alapján becsült metamorf csúcshőmérsékleti értékeknél. A módszer kitűnően kiegészíti, valamint részben helyettesítheti a szénhidrogén-, illetve szén geológiában alkalmazott vitrinit reflexió méréseket.

Late Paleozoic Gondwanide deformation in the Central Andes: Insights from RSCM thermometry and thermal modeling, southern Bolivia

A widespread unconformity between Ordovician metasedimentary rocks and nonmetamorphosed Mesozoic sedimentary rocks across the Eastern Cordillera of Bolivia provides evidence for significant pre-Late Cretaceous deformation and erosion. In this paper, we refine the middle to late Paleozoic tectonic history of the central Andean segment of Gondwana's western margin. We combine Raman spectroscopy of carbonaceous material (RSCM) thermometry with semi-quantitative deformation temperature estimates from quartz recrystallization microstructures and published illite crystallinity values from Cambrian-Devonian sedimentary rocks that span the Eastern Cordillera of southern Bolivia at ~21°S. Estimates of peak temperature and deformation temperature range primarily between ~220 and ~ 400 °C and show a general increase with depth that defines a metamorphic field gradient between ~37 and ~ 47 °C/km. Temperatures obtained from Ordovician rocks directly below the unconformity require erosion of a > ~5 km overburden prior to the Late Cretaceous, which was likely accomplished by a combination of distributed cleavage development and Paleozoic folding and thrust faulting revealed from cross-section reconstructions. The peak temperature data are incorporated into thermal models along with published illite KAr, zircon (UTh)/He, and zircon fission-track cooling ages that refine the timing of Paleozoic orogenesis in the central Andes. We interpret shortening-related flexural subsidence driven by cratonward growth of a pre-Andean orogenic wedge along the western margin of Gondwana, with peak temperature conditions attained during eastward advance of a foreland basin between ~420 and ~ 318 Ma during the Devonian-Carboniferous Gondwanide Orogeny. Erosional exhumation between ~352 and ~ 294 Ma was the result of continued growth and eastward expansion of the associated Transpampean tectonic highland across the Eastern Cordillera. The growth of this pre-Andean contractional orogen coincided with global plate reorganization and the reestablishment of subduction and arc magmatism along the western margin of Gondwana, as indicated by a significant influx of Carboniferous arc-related detritus into the foreland basin system.

Structural and thermal evolution of the eastern Aar Massif: insights from structural field work and Raman thermometry

The thermo-kinematic evolution of the eastern Aar Massif, Swiss Alps, was investigated using peak temperature data estimated from Raman spectroscopy of carbonaceous material and detailed field analyses. New and compiled temperature-time constraints along the deformed and exhumed basement-cover contact allow us to (i) establish the timing of metamorphism and deformation, (ii) track long-term horizontal and vertical orogenic movements and (iii) assess the influence of temperature and structural inheritance on the kinematic evolution. We present a new shear zone map, structural cross sections and a step-wise retrodeformation. From text{ca.;26,Ma} onwards, basement-involved deformation started with the formation of relatively discrete NNW-directed thrusts. Peak metamorphic isograds are weakly deformed by these thrusts, suggesting that they initiated before or during the metamorphic peak under ongoing burial in the footwall to the basal Helvetic roof thrust. Subsequent peak- to post-metamorphic deformation was dominated by steep, mostly NNW-vergent reverse faults (text{ca.} 22–14 Ma). Field investigations demonstrate that these shear zones were steeper than 50^{circ} already at inception. This produced the massif-internal structural relief and was associated with large vertical displacements (7 km shortening vs. up to 11 km exhumation). From 14 Ma onwards, the eastern Aar massif exhumed “en bloc” (i.e., without significant differential massif-internal exhumation) in the hanging wall of frontal thrusts, which is consistent with the transition to strike-slip dominated deformation observed within the massif. Our results indicate 13 km shortening and 9 km exhumation between 14 Ma and present. Inherited normal faults were not significantly reactivated. Instead, new thrusts/reverse faults developed in the basement below syn-rift basins, and can be traced into overturned fold limbs in the overlying sediment, producing tight synclines and broad anticlines along the basement-cover contact. The sediments were not detached from their crystalline substratum and formed disharmonic folds. Our results highlight decreasing rheological contrasts between (i) relatively strong basement and (ii) relatively weak cover units and inherited faults at higher temperature conditions. Both the timing of basement-involved deformation and the structural style (shear zone dip) appear to be controlled by evolving temperature conditions.

Paleotemperature investigation of the Variscan southern external domain: the case of the Montagne Noire (France)

The Montagne Noire located in the southern part of the French Massif Central represents the northern part of the South-Variscan Foreland. It is subdivided into three parts. The granite-migmatite Axial Zone dome is surrounded by non- or weakly metamorphosed Paleozoic sedimentary series. Both northern and southern flanks of the Montagne Noire dome are deformed by km-scale, south to southeast facing recumbent folds and thrusts sheets. The Raman Spectroscopy of Carbonaceous Material (RSCM) method, carried out in the low-grade metamorphic rocks of the southern flank of the Montagne Noire, yielded temperatures comprised between 400 °C near the dome, and 230 °C in the southern domain. Three Raman geothermometers were used to cover this temperature range. RSCM temperatures comply qualitatively with previous estimates based on illite crystallinity, conodont color alteration, and fluid inclusions carried out in the same area, which document a metamorphic temperature increase towards the dome. The isotherms cut across the different nappe contacts and are oriented parallel to the southern margin of the Axial Zone. This temperature distribution supports the idea that the thermal structure was acquired during the Axial Zone dome emplacement. The thermal structure acquired during the recumbent folds emplacement and burial of the sedimentary series is totally overprinted by the doming event. In addition, in a domain relatively remote from the Axial Zone dome, the RSCM measurements yielded significantly higher temperatures than illite crystallinity. This discrepancy points to a higher sensitivity of RSCM to short-lived thermal events than illite crystallinity, possibly because of more efficient kinetics of the carbonization reaction. On the other hand, high RSCM temperatures analysed far from the Axial Zone, between 300 °C and 360 °C could be explained by the presence of granitic plutons under the foreland basin.

Distribution and intensity of High-Temperature Low-Pressure metamorphism across the Pyrenean-Cantabrian belt: constraints on the thermal record of the pre-orogenic hyperextension rifting

Whereas a straightforward link between crustal thinning and geothermal gradients during rifting is now well established, the thermal structure of sedimentary basins within hyperextended domains remains poorly documented. For this purpose, we investigate the spatial distribution of rift-related High-Temperature Low-Pressure (HT/LP) metamorphism recorded in the preserved hyperextended rift basins inverted and integrated in the Pyrenean-Cantabrian belt. Based on Vitrinite Reflectance (Ro) data measured in 169 boreholes and more than 200 peak-metamorphic temperatures (Tmax) data obtained by Raman Spectroscopy of Carbonaceous Material (RSCM) added to ∼425 previously publishedTmaxdata, we propose a new map depicting the spatial distribution of the HT/LPmetamorphism of the Pyrenean-Cantabrian belt. We also provide three regional-scale geological cross-sections associated with RoandTmaxdata to constrain the distribution of paleo-isograds at depth. Based on these results, we show that the impact of rift-related metamorphism is restricted to the pre- and syn-rift sequence suggested by the depth profiles of Rovalues measured in different tectonostratigraphic intervals (pre-, syn- and post-rift and syn-convergence sediments). However, a small strip of early orogenic sediments (Santonian in age) appears also affected by high temperatures along the North Pyrenean Frontal Thrust and above the Grand Rieu ridge, which we attribute to the percolation of hot hydrothermal fluids sourced from the dehydration of underthrust basement and/or sedimentary rocks at depth during the early orogenic stage. The map shows that the HT/LPmetamorphism (reaching ∼500 °C) is recorded with similar intensity along the Pyrenean-Cantabrian belt from the west in the Basque-Cantabrian Basin to the east in the Boucheville and Bas-Agly basins, for similar burial and rift-related structural settings. This thermal peak is also recorded underneath the northern border of the Mauléon Basin (calibrated by wells). It suggests that the high temperatures were recorded at the basement-sediment interface underneath the most distal part of the hyperextended domain. At basin-scale, we observe in the Basque-Cantabrian, Mauléon-Arzacq and Tarascon rift segments an asymmetry of the thermal structure revealed by different horizontal thermal gradients, supporting an asymmetry of the former hyperextended rift system. Using our results, we compare the Pyrénées to the Alps that also recorded hyperextension but no HT/LPmetamorphic event and suggest that the high-temperature record within the basins depends on high sedimentation rate promoting a thermal blanketing effect and circulation of hydrothermal fluids.