Thermal Overprint Research Articles

Enhanced surface flatness of vitrinite particles by broad ion beam polishing and implications for reflectance measurements

Standardized measurements of vitrinite reflectance vary significantly between different measuring institutions. The only systematically adjustable influence factor on these differences is the polishing method. We present a recently developed preparation method using broad ion beam polishing, which, applied as additional polishing step or replacement of the standardized sample preparation, provides a dramatically improved surface flatness, and allows to remove scratches and debris of traditional polishing. Mechanical, near-surface damage of the sample, as known from traditional polishing, is not appearing in broad ion beam polished samples. Moreover, its water-free polishing allows excellent preparation of swelling clay-rich samples or granular solid bitumen surfaces.Using broad ion beam polishing the vitrinite reflectance increases by 0.18–0.24% VRr (relative increase: 12–28%) in coal samples (including an ICCP standard sample) and by 4.76% VRmax (relative increase: 90%) at a measured graphite without causing thermal overprinting of the sample based on our preliminary results. A subsequent traditional polishing step after broad ion beam polishing reverses this increase in reflectivity. The presented method has the potential to decrease the range of reflectance-based calculated maximum burial temperatures by up to 50°C if a reduction of 0.2 in the range of the measured vitrinite reflectance standard deviation can be achieved.

Lead and Mg isotopic age constraints on the evolution of the HED parent body

AbstractThe large collection of howardite‐eucrite‐diogenite (HED) meteorites allows us to study the initial magmatic differentiation of a planetesimal. We report Pb‐Pb ages of the unequilibrated North West Africa (NWA) 4215 and Dhofar 700 diogenite meteorites and their mass‐independent 26Mg isotope compositions (26Mg*) to better understand the timing of differentiation and crystallization of their source reservoir(s). NWA 4215 defines a Pb‐Pb age of 4484.5 ± 7.9 Myr and has a 26Mg* excess of +2.3 ± 1.6 ppm whereas Dhofar 700 has a Pb‐Pb age of 4546.4 ± 4.7 Myr and a 26Mg* excess of +25.5 ± 1.9 ppm. We interpret the young age of NWA 4215 as a thermal overprint, but the age of Dhofar 700 is interpreted to represent a primary crystallization age. Combining our new data with published Mg isotope and trace element data suggests that approximately half of the diogenites for which such data are available crystallized within the first 1–2 Myr of our solar system, consistent with a short‐lived, early‐formed magma ocean undergoing convective cooling. The other half of the diogenites, including both NWA 4215 and Dhofar 700, are best explained by their crystallization in slowly cooled isolated magma chambers lasting over at least ~20 Myr.

Northern Provenance of the Gondwana Formation in the Lesser Himalayan Sequence: Constraints From 40Ar/39Ar Dating of Detrital Muscovite in Darjeeling-Sikkim Himalaya

Single-grain 40Ar/39Ar dates are reported for detrital white mica from low-grade meta-arkose samples from the Gondwana Formation in the Darjeeling-Sikkim Himalaya. The majority (61%) of single grains from five samples yielded dates of ~480 Ma. The remaining grains yielded dates between ~590 and 1700 Ma, and a few grains were younger than 480 Ma. These data suggest that the source of the Gondwana sedimentary rocks in the Himalaya lay to the north, in contrast to the previously held view that it was in the south. This also indicates that the Cambro-Ordovician granites, extensively present in the Himalayan metamorphic core, the Greater Himalayan Sequence, were exposed at the surface in the Permian in a mountain range to the north of the Permian continental rifts. Preservation of the original muscovite ages and the absence of evidence of a thermal overprint during the Himalayan orogenesis indicate that the burial temperatures of the Gondwanan sedimentary rocks, located at the base of the Lesser Himalayan Sequence, did not exceed 400 °C.

Trans-Amazonian U-Th-Pb monazite ages and P-T-d exhumation paths of garnet-bearing leucogranite and migmatitic country rock of the southeastern Tandilia belt, Rio de la Plata craton in Argentina

A garnet-bearing leucogranite and two country rocks from the Transamazonian Tandilia belt of the Rio de la Plata craton were studied in detail. The leucogranite contains garnet with homogeneous composition of pyr6(gros+andr)2spes5alm87. In a garnet-biotite migmatite, the core and rim compositions of garnet are pyr1.7(gros+andr)5spes5.6alm87.7 and pyr1.2(gros+andr)5.5spes6.7alm86.6, respectively. These compositions in a sillimanite-garnet-muscovite migmatite are pyr4(gros+andr)2.7spes2.7alm90.6 and pyr2.7(gros+andr)4spes3.2alm90.1, respectively. We used this information to decipher the P-T evolution of the rocks applying P-T and T-H2O pseudosections with the PERPLE_X computer software package taking into consideration deformational microstructures. The leucogranite records an isothermal decompression from 5.3 to 3.8kbar at 665°C. The garnet-biotite migmatite was exhumed from 5.5kbar at 630°C to 4.3kbar at 615°C and the sillimanite-garnet-muscovite migmatite from supersolidus conditions of 670°C and 3.6kbar to 625°C at 2.4kbar. Late andalusite formed in this rock.Seventy four analyses of 28 monazite grains of the country rocks yielded three groups of U-Th-Pb ages which were related to a collisional event (I: ca. 2.13–2.14Ga.), a postcollisional thermal overprint (II: ca. 2.01Ga) and slow cooling of the orogen (III: 1.80–1.90Ga). Inherited ages of 2.28 and 2.25Ga could refer to an early accretionary stage of the orogen. An age of 2.41Ga indicates the presence of recycled Siderian continental crust. Synkinematic crystallization of melts and the subsolidus development of an S2-foliation, demonstrated by deformational microstructures, occurred during the exhumation of the studied area from depths of ~18km to ~8km in the time interval 2.01–1.90Ga.

Late Permian to Triassic isotope composition of sulfates in the Eastern Alps: palaeogeographic implications

AbstractLate Permian to Triassic phases from the evaporite deposits of the Northern Calcareous Alps (NCA) and Central Alpine Mesozoic (CAM) were analysed for sulfur and oxygen isotope compositions. For the Upper Permian, most of the δ34S values are in the 11 to 12‰ range. Röt-type sulfates of Early Triassic age are characterized by a heavy sulfur isotopic composition of c. 26‰. The spatial compilation of the available data concerning the isotopic composition of Röt-type sulfates demonstrates that these evaporites are distributed over the entire area of the NCA. Their occurrences are associated with Early Triassic high-temperature conditions of the seawater and a widespread anoxia. The development of sulfates of Carnian–Norian age situated in the CAM is more modest; sulfates are characterized by a δ34S value of c. 15‰. The δ18O values show a broader distribution from 9 to 22‰, related to several factors such as type of deposit, recrystallization processes and bacterial sulfate reduction. The sulfate–sulfide thermometer applied to samples from NCA deposits indicates a thermal overprint of between 215 and 315°C. Microbeam measurements support zonation of minor elements in sphalerite. Sphalerite microstructure indicates thermal overprinting, with no microbial structure being preserved.

Middle and Late Jurassic radiolarians from the Neotethys suture in the Eastern Alps

AbstractCherty limestones, marls and radiolarites are widespread in the Hallstatt Mélange of the Northern Calcareous Alps. The mélange was formed during the Neotethyan orogeny in a series of deep-water basins that progressively developed in front of the advancing nappe front. The low thermal overprint of these rocks favors the good preservation of radiolarians (polycystines) that have been used for dating and reconstructing the Jurassic tectonostratigraphy of the area. This paper describes rich Middle–Late Jurassic radiolarian faunas from four localities in the Hallstatt Mélange near Bad Mitterndorf in Austria. Two different successions, both spanning from the Bathonian to the Oxfordian, are dated. In the first succession, the radiolarites are intercalated between or occur as matrix in mass-flow deposits originating from the accretionary wedge. The second succession is nearly 100 m thick but is devoid of mass-flow deposits and documents a continuous radiolarite deposition in greater distance from the nappe stack. Both successions are ascribed to the Sandlingalm Basin, which evolved on a relatively distal continental margin during early stages of the orogeny.The highly diverse and well-preserved radiolarian assemblages have been used for a detailed taxonomic study. Two new families are described: Minocapsidaen. fam. andXitomitridaen. fam.; six new genera are describedDoliocapsan. gen.,Crococapsan. gen.,Parvimitrellan. gen.,Xitomitran. gen.,Campanomitran. gen., andMizukidellan. gen. In addition, one new replacement name,Takemuraella(proTriversus), is introduced, the diagnoses of 6 genera are emended, and two new species are described:Hemicryptocapsa nonagintan. sp. andMizukidella mokaensisn. sp.

In search of early life: Carbonate veins in Archean metamorphic rocks as potential hosts of biomarkers

The detection of early life signatures using hydrocarbon biomarkers in Precambrian rocks struggles with contamination issues, unspecific biomarkers and the lack of suitable sedimentary rocks due to extensive thermal overprints. Importantly, host rocks must not have been exposed to temperatures above 250 °C as at these temperatures biomarkers are destroyed. Here we show that Archean sedimentary rocks from the Jeerinah Formation (2.63 billion yrs) and Carawine Dolomite (2.55 billion yrs) of the Pilbara Craton (Western Australia) drilled by the Agouron Institute in 2012, which previously were suggested to be suitable for biomarker studies, were metamorphosed to the greenschist facies. This is higher than previously reported. Both the mineral assemblages (carbonate, quartz, Fe-chlorite, muscovite, microcline, rutile, and pyrite with absence of illite) and chlorite geothermometry suggest that the rocks were exposed to temperatures higher than 300 °C and probably ∼400 °C, consistent with greenschist-facies metamorphism. This facies leads to the destruction of any biomarkers and explains why the extraction of hydrocarbon biomarkers from pristine drill cores has not been successful. However, we show that the rocks are cut by younger formation-specific carbonate veins containing primary oil-bearing fluid inclusions and solid bitumens. Type 1 veins in the Carawine Dolomite consist of dolomite, quartz and solid bitumen, whereas type 2 veins in the Jeerinah Formation consist of calcite. Within the veins fluid inclusion homogenisation temperatures and calcite twinning geothermometry indicate maximum temperatures of ∼200 °C for type 1 veins and ∼180 °C for type 2 veins. Type 1 veins have typical isotopic values for reprecipitated Archean sea-water carbonates, with δ13CVPDB ranging from −3‰ to 0‰ and δ18OVPDB ranging from −13‰ to −7‰, while type 2 veins have isotopic values that are similar to hydrothermal carbonates, with δ13CVPDB ranging from −18‰ to −4‰ and δ18OVPDB ranging from −18‰ to −12‰. Evidently, the migration and entrapment of hydrocarbons occurred after peak metamorphism under temperatures congruous with late catagenesis and from fluids of different compositions. The relatively high temperatures of vein formation and the known geotectonic history of the rocks analysed suggest a probable minimum age of 1.8 billion yrs (Paleoproterozoic). Our results demonstrate that post peak-metamorphic veins provide an exciting opportunity in the search for evidence of early life. The integration of petrological and organic geochemical techniques is crucial for any future studies that use biomarkers to reconstruct the early biosphere.

Reappraising the P–T evolution of the Rogaland–Vest Agder Sector, southwestern Norway

The Rogaland–Vest Agder Sector of southwestern Norway comprises high-grade metamorphic rocks intruded by voluminous plutonic bodies that include the ∼1000 km2 Rogaland Igneous Complex (RIC). New petrographic observations and thermodynamic phase equilibria modelling of three metapelitic samples collected at various distances (30 km, 10 km and ∼10 m) from one of the main bodies of RIC anorthosite were undertaken to assess two alternative P–T–t models for the metamorphic evolution of the area. The results are consistent with a revised two-phase evolution. Regional metamorphism followed a clockwise P–T path reaching peak conditions of ∼850–950 °C and ∼7–8 kbar at ∼1035 Ma followed by high-temperature decompression to ∼5 kbar at ∼950 Ma, and resulted in extensive anatexis and melt loss to produce highly residual rocks. Subsequent emplacement of the RIC at ∼930 Ma caused regional-scale contact metamorphism that affected country rocks 10 km or more from their contact with the anorthosite. This thermal overprint is expressed in the sample proximal to the anorthosite by replacement of sillimanite by coarse intergrowths of cordierite plus spinel and growth of a second generation of garnet, and in the intermediate (10 km) sample by replacement of sapphirine by coarse intergrowths of cordierite, spinel and biotite. The formation of late biotite in the intermediate sample may suggest the rocks retained small quantities of melt produced by regional metamorphism and remained at temperatures above the solidus for up to 100 Ma. Our results are more consistent with an accretionary rather than a collisional model for the Sveconorwegian Orogen.

An Upper Cretaceous paleo-aquifer system in the Eromanga Basin of the central Gawler Craton, South Australia: evidence from apatite fission track thermochronology

ABSTRACTAn apatite fission track (AFT) study of crystalline basement in the central Gawler Craton reveals apparent ages in the range of ca 430–58 Ma. The majority of samples underwent protracted monotonic cooling related to regional Paleozoic exhumation, consistent with long-term crustal stability as expected for cratonic interiors. However, multiple samples show evidence of Late Cretaceous–early Paleogene reheating, indicating a more dynamic low-temperature history. Inverse time–temperature modelling of AFT data indicates varying degrees of thermal overprinting between ∼60 and 110°C, with substantially overprinted and negligibly overprinted samples in close proximity (<1 km). Time–temperature histories for samples that experienced thermal overprinting reveal localised Late Cretaceous–early Paleogene (ca 100–50 Ma) heating that is significantly younger than the Paleozoic–early Mesozoic exhumation recorded regionally. The highly localised nature and non-systematic patterns of overprinting combined with the lack of major Mesozoic or Cenozoic fault structures are not consistent with a regional thermal event associated with substantial reburial and later exhumation. Rather, localised reheating was most likely caused by heated groundwater from the once-overlying Mesozoic Eromanga Basin aquifer system, whose modern discharge margin (∼400 km north of the study area) is marked by thermal mound springs that produce fluids with temperatures up to 100°C. Only basement rocks in close proximity to fluid pathways in the overlying aquifer would have recorded reheating, resulting in the observed sporadic distribution of partially overprinted samples. Thermal history modelling indicates rejuvenated apatite grains cooled to near-surface temperatures in the latest Cretaceous–Paleogene. This was likely in response to local removal of the overlying Eromanga Basin aquifer unit due to a relatively minor degree of exhumation (≤1 km) recorded regionally, which consequently disrupted the anomalous heating mechanism. These results show that the flow of heated groundwater is a feasible reheating mechanism for low-temperature thermochronometers, resulting in cooling patterns that may become decoupled from exhumation in cratonic interiors.

Grain size matters: Implications for element and isotopic mobility in titanite

The U–Pb isotopic signature of titanite collected across an exhumed refractory lower crustal block within the Albany–Fraser Orogen, Australia, records thermal overprints not apparent in a suite of other U–Pb chronometers. This helps to reconcile a dichotomy within the geochronological record of two adjacent zones within the orogen. The zircon U–Pb record for the older Biranup Zone preserves widespread overprinting at 1225–1140Ma (Stage II), whereas the younger Fraser Zone records an older 1330–1260Ma (Stage I) tectonothermal event. Titanite in the Fraser Zone also predominantly records a U–Pb age of 1299±14Ma, reflecting the interval of closure to radiogenic Pb mobility. Nonetheless, small titanite grains reveal subsequent overprinting with a mean reset age of 1205±16Ma. By contrast, titanite from metasedimentary rocks within the adjacent Biranup Zone principally record U–Pb ages of 1200–1150Ma, interpreted as dating cooling after prolonged Stage II metamorphism. Interestingly, titanite also preserves domains with old apparent ages. These domains have a statistically significant association with lower U content and also indicate reduced Sm/Yb ratios and are interpreted to have lost U but acquired HREE (e.g. Yb) more rapidly than MREE (e.g. Sm). The old apparent ages are interpreted as artefacts of a Stage II U redistribution process, leading to unsupported radiogenic Pb. In addition, titanite grain size has a strong effect on the preservation or resetting of metamorphic U–Pb ages. Thermochronological modelling based on apparent age versus grain size relationships indicates that complete resetting of small titanite grains requires overprinting temperatures of 695–725°C during Stage II in the Fraser Zone. This result is similar to estimates from the Biranup Zone based on phase equilibrium modelling that indicates pressures and temperatures of 6.5–8.5kbar and 675–725°C. An in situ U–Pb analysis strategy for titanite that targets a range of grain sizes has the potential to reveal differential resetting and place important controls on thermal history.

Evolution of the Chilka Lake granulite complex, northern Eastern Ghats Belt, India: First evidence of ~ 780 Ma decompression of the deep crust and its implication on the India–Antarctica correlation

High-grade para- and orthogneissic rocks near the Chilka Lake granulite complex, northern part of the Eastern Ghats Belt show complex structural and petrological history. Based on field and petrographic characters, five (M1–M5) metamorphic events could be identified. The earliest metamorphic event (M1) produced amphibolite grade mineral assemblage which produced the peak granulite (M2) assemblages at 900–950°C, 8.5–9.0kbar. The third metamorphic event caused decompression of the deeper crust up to 700–800°C, 6.0–6.5kbar. This was followed by cooling (M4) and subsequent thermal overprinting (M5). Fluid-composition during M3 was dominated by high-density CO2 and changed to low-density mixed CO2–H2O during the M3. Zircon U–Pb SHRIMP data suggest 781±9Ma age for M3 event. Texturally constrained monazite U–Th–Pb EPMA data, on the other hand, yield a group age of 988±23Ma from grain interior, which can signifies the age of M2 event. Few spots with younger dates in the range of 550–500Ma are also noted. This interpretation changes the existing tectonothermal history of northern Eastern Ghats Belt. Our data show that the two adjacent crustal domains of the Eastern Ghats Belt show distinctly contrasting Neoproterozoic histories. While the central Domain 2 evolved through early anticlockwise P–T path culminating in ultrahigh temperature, the northern Domain 3 evolved through a clockwise P–T path. It appears that the Domain 3 was contiguous to East Antarctica and became part of the Eastern Ghats Belt during the assembly of Gondwana. The ca. 780Ma decompression event in the northern Eastern Ghats Belt opens up new possibilities for interpreting the breakup of Rodinia.

Thermal history of the westernmost Eastern Alps (Penninic Rhenodanubian Flysch nappes, Helvetic nappes, and Subalpine Molasse thrust sheets)

The frontal part of the westernmost Eastern Alps comprises from top to bottom of the Austroalpine and Penninic nappes, Ultrahelvetic slices, and two Helvetic thrust sheets, thrust upon the northern Alpine Molasse Basin. The thermal evolution of the Penninic Rhenodanubian Flysch nappes, the Helvetic nappes, and the allochthonous part of the Alpine Molasse Basin is constrained by vitrinite reflectance measurements and apatite fission track dating and implemented in a tectonic evolution scheme. Within the Helvetic nappes, vitrinite reflectance increases regionally from north to south and stratigraphically from the Campanian–Maastrichtian Wang Formation to the Toarcian Mols Member. Apatite fission track ages from Penninic and Subalpine Molasse units are consistently younger than the deposition age. They indicate therefore a post-depositional thermal overprint exceeding approximately 120 °C, the upper temperature limit of the apatite partial annealing zone. 1D thermal modelling suggests that the Penninic nappes attained deepest burial between the latest Cretaceous and Early Palaeocene with the Penninic basal thrust being located at approximately 8 km in the north compared to approximately 12 km in the south. Deepest burial of the upper Helvetic nappe occurred between the latest Eocene and Early Miocene. Its base was buried down to approximately 10.5 km in the north compared to 11.5 km in the south. Exhumation of the entire nappe stack started in the Early to Middle Miocene. For both Penninic and Helvetic models, a heatflow minimum during the Cenozoic deformation (max. 27–32 mW/m2), followed by an increase from the Middle Miocene onwards (up to 60 mW/m2), was assumed.

Fission track and U–Pb zircon ages of psammitic rocks from the Harushinai unit, Kamuikotan metamorphic rocks, central Hokkaido, Japan: constraints on metamorphic histories

AbstractAccurate pressure–temperature–time (P–T–t) paths of rocks from sedimentation through maximum burial to exhumation are needed to determine the processes and mechanisms that form high‐pressure and low‐temperature type metamorphic rocks. Here, we present a new method combining laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb with fission track (FT) dates for detrital zircons from two psammitic rock samples collected from the Harushinai unit of the Kamuikotan metamorphic rocks. The concordant zircon U–Pb ages for these samples vary markedly, from 1980 to 95 Ma, with the youngest age clusters in both samples yielding Albian‐Cenomanian weighted mean ages of 100.8 ± 1.1 and 99.3 ± 1.0 Ma (2σ uncertainties). The zircon U–Pb ages were not reset by high‐P/T type metamorphism, because there is no indication of overgrowth within the zircons with igneous oscillatory zoning. Therefore, these weighted mean ages are indicative of the maximum age of deposition of protolithic material. By comparison, the zircon FT data yield a pooled age of ca. 90 Ma, which is almost the same as the weighted mean age of the youngest U–Pb age cluster. This indicates that the zircon FT ages were reset at ca. 90 Ma while still at their source, but have not been reset since. This conclusion is supported by recorded temperature conditions of less than about 300 °C (the closure temperature of zircon FTs), as estimated from microstructures in the deformed detrital quartz grains in psammitic rocks, and no shortening of fission track lengths in the zircon. Combining these new data with previously reported white mica K–Ar ages indicates that the Harushinai unit was deposited after ca. 100 Ma, and underwent burial to its maximum depth before being subjected to a localized thermal overprint during exhumation at ca. 58 Ma.

The Age of the Latest Thermal Overprint of Tin and Polymetallic Deposits in the Erzgebirge, Germany: Constraints from Fluorite (U-Th-Sm)/He Thermochronology

The Erzgebirge region of Germany records two major episodes of hydrothermal activity, which includes a Late Carboniferous to Early Permian event associated with significant Sn-W mineralization that is related to late Variscan granite magmatism, and a Mesozoic episode of polymetallic vein mineralization. In contrast to the first event, the age of the younger hydrothermal activity is poorly constrained. For the latter, various geochronological methods yielded a wide age range from Permian to early Tertiary. Here we apply fluorite (U-Th-Sm)/He thermochronology (FHe) on both types of mineralization with a twofold goal: (1) to investigate the sensitivity and applicability of the new FHe method (Evans et al., 2005), and (2) to constrain the thermal history of ore deposits in the Erzgebirge region. Two hundred and thirty-three aliquots from seven mineralization localities have been dated. Fluorite from six deposits yielded Cretaceous FHe ages between 112 to 79 Ma, which are independent of their paragenesis. In contrast, fluorite from the Sadisdorf Sn-W deposit yielded an age of 234 Ma. The younger ages are interpreted as cooling ages indicating the time of the last thermal overprint, including possible hydrothermal activity, in the Erzgebirge. The oldest, Triassic FHe ages at Sadisdorf indicate that the Mesozoic thermal overprint only partially reset the (U-Th-Sm)/He system of the late Variscan mineralization. Thermal modeling based on FHe ages and He diffusion parameters in fluorite results in thermal histories comparable to the results from the well-established apatite-based thermal modeling. This study emphasizes the applicability of fluorite (U-Th-Sm)/He thermochronology, which is especially suited for ore deposits where apatite is lacking.

Taconian retrograde eclogite from northwest Connecticut, USA, and its petrotectonic implications

Mafic lenses hosted by felsic paragneiss in a Taconic thrust slice (Canaan Mountain Formation) from northwest Connecticut (New England, USA) contain relict mineral assemblages and decompression textures indicative of high pressure (HP) precursors. Symplectic intergrowths consisting mostly of diopside+plagioclase or biotite+plagioclase are pseudomorphous after omphacite and phengite, respectively. Pseudosection analysis and thermobarometry demonstrate that the inferred peak assemblage of garnet+clinopyroxene+phengite formed at >14kbar and ~710°C in the eclogite facies. Bulk-rock geochemistry and field relations indicate that the protoliths of the mafic gneisses were likely rifting-related mafic intrusions. Zircon U–Pb dating by ion probe yields a 456±4.6Ma (2σ) metamorphic age for the mafic gneiss. The zircons from the felsic host rocks have an identical 456±11Ma metamorphic rim age and Grenvillian detrital cores. The HP metamorphism and the coeval arc magmatism reflect the collision between the Laurentian passive margin and a Taconic arc complex over an east-dipping subduction zone that was active until ~456Ma in Connecticut (southern New England). The P–T path is characterized by post-peak-T compression, suggesting that the eclogite-facies metamorphism was associated with the deformation of the collision zone after the initial continent–arc collision. After the culmination of collision the subduction polarity switched; metamorphic ages decrease southward along the orogen suggesting that this reversal occurred ~10Myr later in New England than in Newfoundland. Nearly all mafic rocks in the study area crop out as fairly ordinary-looking amphibolites, so it is reasonable to speculate that HP metamorphism was more extensive than currently recognized in New England but has been obscured by thermal overprinting and retrogression.

Fluid generation in the Boucheville Basin as a consequence of the North Pyrenean metamorphism

Metamorphic fluids transport heat and matter. In the Boucheville Basin (North Pyrenean Zone), the circulation of metamorphic fluids is attested by abundant synmetamorphic quartz – calcite veins. The Boucheville Basin formed during the Albian extensional regime and was filled by the so-called “Albian flysch”. The basin underwent a thermal overprint (the North Pyrenean Metamorphism) related to the exhumation of nearby mantle rocks that advected large amounts of heat to the upper crustal levels. The oxygen and carbon isotope compositions of quartz – calcite veins and their host rocks show strong buffering of the composition of the fluids by the Albian metamorphic host rocks. Some host rocks are depleted in calcite near vein contacts showing that some of the elements implicated in crystal growth in veins were derived from the local host rock. The Albian rocks display a range of oxygen and carbon isotope compositions potentially related to closed-system processes of decarbonation–dehydration. We argue then that the fluids that circulated throughout the Boucheville Basin were generated within the basin itself. Their upward migration led to heat advection in the metamorphic pile, a consequence of which was some homogenization of the recrystallization temperatures in the basin.

Thermal maturity of Carboniferous to Eocene Sediments of the Alpine–Dinaric Transition Zone (Slovenia)

The Alpine–Dinaric Transition Zone in Slovenia comprises the fold and thrust belt of the Southern Alps (South Karawanken Range, Julian Alps), Slovenian Basin and the Dinarides. The Slovenian Basin located between the Julian and Dinaric carbonate platforms evolved during Middle Triassic time and remained in a deep marine setting till the Late Cretaceous. The thermal history of Carboniferous to Eocene rocks in that area was investigated using vitrinite reflectance (VR) data, apatite fission track analysis and numeric 1D basin modeling. The study shows that maturity patterns are mainly controlled by the thickness of Upper Cretaceous to Eocene flysch deposits, filling the accommodation space. Therefore the thermal overprint reaches a maximum (>4%Rr) in Triassic to Cretaceous sediments of the Slovenian Basin and decreases towards the north and south. Minor sedimentary burial of the Adriatic Carbonate Platform and the Julian Alps results in a lower thermal overprint (<1.5%Rr). The thickness of flysch sediments was about 5km in the area of the Sava Folds, but significantly higher in the central part of the Slovenian Basin. Heat flow during maximum burial in Eocene time was in the order of the global average (60mW/m2). Cooling of Paleozoic and Mesozoic sediments below 110°C occurred between Late Eocene and Early Oligocene times in different parts of the study area. Nappe stacking due to Early (Dinaric) and Late Cenozoic (Alpine) compressional tectonics did not influence the thermal maturity of the sediments.

Superposition of burial and hydrothermal events: post‐Variscan thermal evolution of the Erzgebirge, Germany

AbstractThe post‐Variscan thermal history of the Erzgebirge (Germany) is the result of periods of sedimentary burial, exhumation and superimposed hydrothermal activity. The timing and degree of thermal overprint have been analysed by zircon and apatite (U–Th)/He and apatite fission track thermochronology. The present‐day surface of the Erzgebirge was exhumed to a near‐surface position after the Variscan orogeny. Thermal modelling reveals Permo‐Mesozoic burial to temperatures of up to 80–100 °C, although the sedimentary cover thins out towards the north resulting in maximum burial temperatures of less than 40 °C. This thermal pattern was locally modified by Cretaceous hydrothermal activity that reset the zircon (U–Th)/He thermochronometer along ore veins. The thermal models show no significant regional exhumation during Cenozoic times, indicating that the peneplain‐like morphology of the basement is a Late Cretaceous feature.

Molybdenite Re-Os dating of Mo-Th-Nb-REE rich marbles: pre-Variscan processes in Moldanubian Variegated Group (Czech Republic)

Abstract In an effort to contribute to the discussion concerning the age of rocks of the Moldanubian Variegated Group, we have undertaken Re-Os dating of molybdenite of banded carbonatite-like marbles (CLM) from the graphite mine Václav at Bližná (Southern Bohemia), which belong to the metamorphic sequence of this group. The Re-Os model ages for the molybdenites range between 493 and 497 Ma and apparently correspond to the early stages of metamorphism connected with pre-Variscan rift-related tectono-metamorphic events, which affected and recrystallized sedimentary CLM material rich in Mo-Th-Nb-REE. The molybdenite bearing carbonatite like marbles situated in the footwall of Bližná graphite mine have been interpreted as carbonates with a large share of volcano-detritic material derived from contemporaneous primitive alkaline (carbonatite-like) volcanism deposited in a shallow marine lagoonal environment. There is no geological evidence for the participation of fluids mobilized from host rocks in the formation of the CLM. Because the Re-Os chronometer in molybdenite is demonstrably stable through later Variscan facies metamorphism, the molybdenite chronometer has not been affected by subsequent thermal overprints associated with the Variscan orogeny.

Syn- and post-tectonic granite plutonism in the Sausar Fold Belt, central India: Age constraints and tectonic implications

Sausar Fold Belt (SFB) in central India forms the southern part of the Central Indian Tectonic Zone (CITZ) – a crustal scale Proterozoic mobile belt dissecting the Indian craton, whose tectonothermal history and age is important for understanding the Proterozoic crustal history of the Indian craton. SFB comprises a gneissic basement (TBG: Tirodi Biotite Gneiss) overlain by a supracrustal sequence of quartzite–pelite–carbonate (SSG: Sausar Group). SSG and TBG are deformed and metamorphosed in greenschist to amphibolite facies. Two phases of granite intrusion are observed in the SSG – a syntectonic foliated granite and a post-tectonic massive granite, with clear structural relationship with the host rocks. Monazite chemical dating (U–Th–total Pb) of the foliated and massive granites yield Neoproterozoic (ca. 945–928Ma) ages that contradict many earlier geochronological interpretations. Foliated granites and the immediately adjacent TBG show monazite grains with ca. 945Ma mean age, interpreted as the timing of D2 deformation and amphibolite facies metamorphism of SSG. The post tectonic granites intruded these rocks around 928Ma, and were largely undeformed. A terminal thermal overprint is found in some monazite grain rims at ca. 785Ma age. The younger Sausar tectonothermal events have overprinted the adjacent high-grade granulites of Ramakona–Katangi Granulite (RKG) belt, and should not be considered as parts of the same tectonothermal event representing different depth sections only.