Preservation of diagenetic products of β-carotene in sedimentary rocks from the Lopare Basin (Bosnia and Herzegovina)

Sedimentary rocks from the saline formation of the Lopare Basin were investigated. Sediments contain a moderate amount of immature to marginally mature algal organic matter deposited under slightly reducing to anoxic and slightly saline to hypersaline conditions. Almost all of the samples contain β-carotane in a relatively high quantity, and in some, it represents the most abundant compound in the total distribution of hydrocarbons. The objective of the study was to determine the conditions that are favourable to precursors of β-carotene and/or the preservation of the carotenoid hydrocarbon skeleton. Moreover, the dominant transformation pathways of β-carotene under different redox and salinity conditions, which lead to the formation of aromatic carotenoids were defined.     

Neogene–Quaternary sedimentary and paleovegetation history of the eastern Solimões Basin,central Amazon region

Palynological and stratigraphical analyses were carried out on the outcrops of the Solimões River to present new information about freshwater paleoenvironments of a fluvial–deltaic and meandering river system, evaluate the vegetation changes in the upper Solimões and Içá Formations in the eastern Solimões Basin, and the role of the Purus Arc in the evolution of central Amazon during the upper Neogene. The upper Miocene to Pliocene Solimões Formation is related to a fluvial–deltaic system, with fine-grained sediments of the prodelta–lacustrine environment supplied by meandering distributaries and delta front environment. The lake and distributaries were surrounded by extensive deltaic and floodplains colonized by lowland freshwater forests under wet climate conditions, persisting until the Pliocene. The Içá Formation started to deposit unconformably on the Solimões Formation during the Pleistocene, following the development of extensive meandering channels surrounded by floodplains of an essentially fluvial system linked to development of the present eastward direction of the Amazon River until the Atlantic coast. These floodplains were colonized by few palm species and pteridophytes of lowland freshwater forests during the Pleistocene. Additionally, no algae and fungi were observed, which may be related to drier climate conditions and/or different morphological conditions than the upper Solimões Formation.     

Evaluating the provenance of Archean sedimentary rocks of the Diemals Formation (central Yilgarn Craton) using whole-rock chemistry and precise U – Pb zircon chronology

Whole-rock chemistry and precise U – Pb zircon chronology have been used to determine the provenance of Archean greenschist-facies siliciclastic sedimentary rocks of the Diemals Formation in the Marda – Diemals area of the central Yilgarn Craton, Western Australia. Field evidence shows that these siliciclastic rocks are, at least in part, derived from uplift and erosion of underlying greenstones, and this is borne out by the similar La/Sc, Cr/Th and REE chemistry of Diemals Formation siltstones and some sandstones to mafic volcanic rocks of the underlying greenstones. The higher Cr/V and lower Y/Ni of some siltstones is consistent with input from ultramafic and mafic rocks. Diemals Formation sandstones and siltstones cannot be separated in terms of ratios such as Zr/La, and siliciclastic rock chemistry reflects provenance rather than the effects of transport and depositional processes, such as sorting. Chemistry does not support input to Diemals Formation sedimentary rocks from the Marda volcanic complex despite both units being close to each other, and having overlapping maximum depositional and crystallisation ages, respectively. Instead, it is likely that detritus for the two units was deposited in adjacent, physically discrete basins. Some Diemals Formation sandstones are geochemically similar to felsic rocks intruding the underlying greenstone succession, with higher La/Sc and lower Cr/Th, and LREE-enriched patterns with negative Eu anomalies. Support for a genetic relationship is shown by the overlap in the maximum depositional age of these sandstones with the crystallisation age of the geochemically identical Pigeon Rocks Monzogranite. Combined whole-rock chemistry and precise U – Pb zircon chronology indicates that Diemals Formation sedimentary rocks were in large part derived from the underlying mafic volcanic rocks, with progressive unroofing of this succession leading to erosion of felsic intrusive rocks, now represented by sandstones found at various levels in the Diemals Formation.     

U-Pb zircon and Sm—Nd geochronology of mafic and ultramafic rocks from the central part of the Tauern Window (eastern Alps)

Geochronological and geochemical analyses were carried out in order to identify the pre-Variscan basement of the Tauern Window (eastern Alps). Maficultramafic rocks from the central part of the Tauern window have been studied by REE-analysis and U-Pb and Sm-Nd isotopic analyses on whole rock, zircons, garnets and sphene. U-Pb and Sm—Nd zircon dating define both magmatic Pan-African and Cambro-Ordovician events from 650 Ma to 486 Ma within the Alpine fold belt. This indicates a time span of 150 Ma for magmatic activities in the Tauern Window of the eastern Alps. The ages of 657 Ma (U-Pb zircon) and 644 (Sm—Nd zireon) obtained from an amphibolite are the oldest dates of the Eastern Alps; they may be related to the Pan-African orogeny, and imply an early cycle of magmatic intrusion before major activity started at around 500 Ma. Sm-Nd whole rock analyses of the Precambrian rocks do not define an isochron, reflecting heterogenities within the mantle source. The initial _Nd values (+1.2 to +4.7) are very low, implying an enrichment of the magma source. The second main phase of magmatic activity (539 486 Ma) is characterized by the emplacement of mafic/ultramafic rock sequences. As no ophiolitic relies are observed in these domains, the Early Paleozoic magmatism was likely associated with extensional tectonics. Obtained ages of 301±3 and 314+4/-3 Ma point to a Variscan metamorphism. The first combined U-Pb zircon/Sm—Nd zircon data for an amphibolite from the Basal Amphibolite Formation (BAF) favoured the Sm-Nd zircon isochron age as a magmatic age, whereas the low initial _Nd value point to an enriched magma source as well as to heterogenities within the magma source. The obtained ages suggest that parts of the pre-Variscan basement within the Alpine fold belt were formed during the Pan-Africa cycle. The detection of Pre-Variscan ages within the Alpine basement must reffect a complex history involving significant pre-Variscan activity.     

Provenance and paleogeography of Carboniferous–Permian strata in the Bayanhot Basin: Constraints from sedimentary records and detrital zircon geochronology

The Bayanhot Basin is a superimposed basin that experienced multiple-staged tectonic movements; it is in the eastern Alxa Block, adjacent to the North China Craton(NCC) and the North Qilian Orogenic Belt(NQOB).There are well-developed Paleozoic–Cenozoic strata in this basin, and these provide a crucial window to a greater understanding of the amalgamation process and source-to-sink relationships between the Alxa Block and surrounding tectonic units.However, due to intensive post-depositional modification, and lack of subsurface data,several fundamental issues—including the distribution and evolution of the depositional systems, provenance supplies and source-to-sink relationships during the Carboniferous– Permian remain unclear and thus hinder hydrocarbon exploration and limit the geological understanding of this basin.Employing integrated outcrop surveys, new drilling data, and detrital zircon dating, this study examines the paleogeographic distribution and evolution, and provenance characteristics of the Carboniferous–Permian strata in the Bayanhot Basin.Our results show that the Bayanhot Basin experienced a long-term depositional evolution process from transgression to retrogression during the Carboniferous–late Permian.The transgression extent could reach the central basin in the early Carboniferous.The maximum regional transgression occurred in the early Permian and might connect the Qilian and North China seas with each other.Subsequently, a gradual regression followed until the end of the Permian.The northwestern NCC appeared as a paleo-uplift area and served as a sediments provenance area for the Alxa Block at that time.The NCC, Bayanwula Mountain, and NQOB jointly served as major provenances during the Carboniferous–Permian.There was no ocean separation, nor was there an orogenic belt between the Alxa Block and the NCC that provided sediments for both sides during the Carboniferous–Permian.The accretion of the Alxa and North China blocks should have been completed before the Carboniferous period.     

The provenance of northern Kalahari Basin sediments and growth history of the southern Congo Craton reconstructed by U–Pb ages of zircons from recent river sands

The southern Congo Craton is widely overlain by Meso- to Cenozoic sediments of the northern Kalahari Basin, which hamper any correlation of basement units. The latter are represented by the Archaean Angola and Kasai Blocks, while the southern cratonic margin is framed by several Meso- to Neoproterozoic orogenic belts. For provenance analysis of the sedimentary cover and reconstruction of the main zircon-forming events, we studied zircons from recent sediments of the largest rivers at the southern margin of the Congo Craton. U–Pb zircon ages suggest a major amount of the sediments to originate from E Lufilian and Kibaran Belts, while input from the S Damara Belt seems to increase to the W. Ages related to the Angola Block were only noticed in the westernmost parts of the working area, which is not in accordance with the SE-trending drainage pattern, proposed to have been onset during the Cretaceous. Thus, it is assumed that the Meso- to Cenozoic sedimentary cover extended further west than today prior to the Mesozoic to Neogene uplift of the Angola Block and that subsequent erosion exhumed the basement stepwise from west to east. A recurrent destabilisation of the southern margin of the Congo Craton at ~2.7, 1.9, 1.0 and 0.6 Ga is supposed to be represented by major peaks in the age distribution pattern of the total amount of concordant zircons. This is in accordance with similar studies in adjacent areas. Additionally, the obtained data fit well to several hypothesised major events during the supercontinent cycle.     

Detrital zircon U-Pb ages of Middle–Late Permian sedimentary rocks from the southwestern margin of the North China Craton: Implications for provenance and tectonic evolution

The southwestern margin of the North China Craton (NCC) is located between the Alxa Terrane to the northwest, the North Qilian Orogen to the west and the North Qinling Orogen to the south. However, the paleogeographic and tectonic evolution for the southwestern part of the NCC in the Late Paleozoic is still poorly constrained. In order to constrain the Late Paleozoic tectonic evolution of the southwestern NCC, we carried out detailed field work and detrital zircon U-Pb geochronological research on Middle–Late Permian sedimentary rocks at the southwestern margin of the NCC. The U-Pb age spectra of detrital zircons from six samples are similar, showing four populations of 2.6–2.4 Ga, 2.0–1.7 Ga, 500–360 Ma and 350–250 Ma. Moreover, on the basis of the weighted-mean age of the youngest detrital zircons (257 ± 4 Ma), combined with the published results and volcanic interlayers, we propose that the Shangshihezi Formation formed during the Middle–Late Permian. Our results and published data indicate that the detrital zircons with age groups of 2.6–2.4 Ga and 2.0–1.7 Ga were likely derived from the Khondalite Belt and Yinshan Block in the northwestern NCC. The junction part between the North Qinling and North Qilian Orogen may provide the 500–360 Ma detrital zircons for the study area. The 350–250 Ma detrital zircons were probably derived from the northwestern part of the NCC. The majority of materials from Shangshihezi Formation within the study area were derived from the northwestern part of the NCC, indicating that the northwestern part of the NCC was strongly uplifted possibly resulting from the progressive subduction and closure of the Paleo-Asian Ocean. A small amount of materials were sourced from southwestern part of the NCC, indicating that the North Qinling Orogen experienced a minor uplift resulting from the northward subduction of the South Qinling terrane.     

The tectonics and stages of the geological history of the Yenisei–Khatanga Basin and the conjugate Taimyr Orogen

A new interpretation of the seismic profile series for the Taimyr Orogen and the Yenisei–Khatanga Basin is given in terms of their tectonics and geological history. The tectonics and tectonostratigraphy of the Yenisei–Khatanga and the Khatanga–Lena basins are considered. In the Late Vendian and Early Paleozoic, a passive continental margin and postrift shelf basin existed in Taimyr and the Yenisei–Khatanga Basin. From the Early Carboniferous to the Mid-Permian, the North and Central Taimyr zones were involved in orogeny. The Late Paleozoic foredeep was formed in the contemporary South Taimyr Zone. In the Middle to Late Triassic, a new orogeny took place in the large territory of Taimyr and the Noril’sk district of the Siberian Platform. A synorogenic foredeep has been recognized for the first time close to the Yenisei–Khatanga Basin. In the Jurassic and Early Cretaceous, this basin was subsided under transpressional conditions. Thereby, anticlinal swells were formed from the Callovian to the Aptian. Their growth continued in the Cenozoic. The Taimyr Orogen underwent tectonic reactivation and apparently right-lateral transpression from Carboniferous to Cenozoic.     

Geochemical Constraints of Sediments on the Provenance, Depositional Environment and Tectonic Setting of the Songliao Prototype Basin

The geochemistry of sediments is primarily controlled by their provenances, and different tectonic settings have distinctive provenance characteristics and sedimentary processes. So, it is possible to discriminate provenances, depositional environments and tectonic settings in the development of a sedimentary basin with the geochemistry of the clastic rocks. The analytical results of the present paper demonstrate that sediments in the Songliao prototype basin are enriched in silica (SiO2=66.48-80.51 %), and their ΣREE are 30-130 dmes of that of chondrite with remarkable Eu anomalies. In discriminating diagrams of Eu/Eu vs eeeeeREE and (La/Yb)N vs ΣREE, most samples locate above the line Eu/ Eu=l, on the right of the line Eu/Eu/ΣREE=1 and under the line La/Yb)N/eeeeeREE=1/8, which indicates that the depositional environment of sediments in the basin was oxidizing. In addition, variations of MgO, TiO2, A12O3, FeO+Fe2O3, Na2O and CaO vs SiO2 reflect a tendency of increasing mineral maturity of sediments     

LA-ICPMS U–Pb zircon age constraints on the provenance of Cretaceous sediments in the Yichang area of the Jianghan Basin,central China

Detrital zircon U–Pb ages have been shown to be a powerful tool for provenance analysis and determining the exhumation of sediment source areas. This paper presents the results of detrital zircon LA-ICPMS U–Pb ages for Cretaceous sediments from the Yichang area of the Jianghan Basin, central China. The results provide new information on the provenance of these sediments and the detailed exhumation process of the Huangling Dome. Zircons with different age populations have been derived from the strata of the Huangling Dome. The Liantuo, Gucheng and Nantuo formations and the Kongling complex were exhumed, leading to deposition of the early Cretaceous Wulong Formation, which provides the sources of zircons with age peaks at 3.1–3.0, 2.5 and 1.8 Ga. Exhumation of the Huangling granitoid and contemporary volcanics provided the source of the late Cretaceous Luojingtan Formation, which contains zircons with age peaks at 1.1–0.95 and 0.83–0.74 Ga. The Qinling-Dabie orogeny supplied zircons with an age cluster of 0.27 to 0.18 Ga. These results indicate the timing of initial exhumation for the Huangling granite. They also show how overlying strata was first uplifted and eroded, followed by exposure of underlying strata at the surface during continued exhumation.     

The tectonic evolution of the Neoproterozoic Brasília Belt,central Brazil,based on SHRIMP and LA-ICPMS U–Pb sedimentary provenance data: A review

The Brasília Belt is a Neoproterozoic orogenic belt in central Brazil, developed between the Amazon, São Francisco-Congo and Paranapanema cratons. It consists of a thick sedimentary pile, made up of several stratigraphic units, which have been deformed and metamorphosed along the western margin of the São Francisco Craton during the Brasiliano orogenic cycle. In the western part of the belt, a large, juvenile magmatic arc is exposed (the Goiás Magmatic Arc), consisting of calc-alkaline plutonic suites as well as volcano-sedimentary sequences, ranging in age between ca. 860 and 650 Ma. Regional-scale, west-dipping thrusts and reverse faults normally mark the limits between the main stratigraphic units, and clearly indicate tectonic transport towards the east. The age of deposition and tectonic significance of the sedimentary units comprising the Brasília Belt have been a matter of continuous debate over the last three decades. In the present paper, recent provenance data based on LA-ICPMS U–Pb ages of detrital zircon grains from several of these units, are reviewed and their significance for the age of deposition of the original sediments and tectonic evolution of the Brasília Belt are discussed.The Paranoá, Canastra and the Vazante groups, in the central part of the Belt, have detrital zircon grains with ages older than ca. 900 Ma and are interpreted as representative of the passive margin sequence deposited on the western margin of the São Francisco Craton. On the other hand, samples from the Araxá and Ibiá groups have a much younger population of Neoproterozoic zircon grains, as young as 650 Ma, and have been interpreted as syn-orogenic (fore-arc?) deposits. The Bambuí Group, exposed in the easternmost part of the belt and covering large areas of the São Francisco Craton also has young zircon grains and is interpreted, at least in part, as the foreland basin of the Brasília Belt.     

Geochemistry of sedimentary rocks from Permian–Triassic boundary sections of Tethys Himalaya: implications for paleo-weathering,provenance, and tectonic setting

The geochemical characteristics of two sections—the Permian–Triassic boundary (PTB) Guryul Ravine section, Kashmir Valley, Jammu and Kashmir, India; and the Attargoo section, Spiti Valley, Himachal Pradesh, India—have been studied in the context of provenance, paleo-weathering, and plate tectonic setting. These sections represent the siliciclastic sedimentary sequence from the Tethys Himalaya. The PTB siliciclastic sedimentary sequence in these regions primarily consists of sandstones and shales with variable thickness. Present studied sandstones and shales of both sections had chemical index of alteration values between 65 and 74; such values reveal low-to-moderate degree of chemical weathering. The chemical index of weathering in studied samples ranged from 71 to 94, suggesting a minor K-metasomatism effect on these samples. Plagioclase index of alteration in studied sections ranged from 68 to 92, indicating a moderate degree of weathering of plagioclase feldspars. The provenance discriminant function diagram suggests that the detritus involved in the formation of present studied siliciclastic sedimentary rocks fall in quartzose sedimentary and felsic igneous provenances. These sediments were deposited in a passive continental margin plate tectonic setting according to their location on a Si₂O versus K₂O/Na₂O tectonic setting diagram.     

Chronological and Geochemical Characterisitics of Volcanic Rocks of the Yingcheng Formation in the Southern Songliao Basin: Constraints on the Early Cretaceous Evolution of the Songliao Basin

The discoveries of oil and gas reservoirs in the volcanic rocks of the Songliao Basin(SB) have attracted the attention of many researchers. However, the lack of studies on the genesis of the volcanic rocks has led to different opinions being presented for the genesis of the SB. In order to solve this problem, this study selected the volcanic rocks of the Yingcheng Formation in the Southern Songliao Basin(SSB) as the research object, and determined the genesis and tectonic setting of the volcanic rocks by using LA-ICP-MS zircon U-Pb dating and a geochemical analysis method(major elements, trace elements, and Hf isotopes). The volcanic rocks of the Yingcheng Formation are mainly composed of rhyolites with minor dacites and pyroclastic rocks. Our new zircon U-Pb dating results show that these volcanic rocks were erupted in the Early Cretaceous(113–118 Ma). The primary zircons from the rhyolites have εHf(t) values of +4.70 to +12.46 and twostage model age(TDM2) of 876–374 Ma. The geochemical data presented in this study allow these rhyolites to be divided into I-type rhyolites and A-type rhyolites, both of which were formed by the partial melting of the crust. They have SiO2 contents of 71.62 wt.%–75.76 wt.% and Al2 O3 contentsof 10.88 wt.% to 12.92 wt.%. The rhyolites have distinctively higher REE contents than those of ordinary granites, with obvious negative Eu anomalies. The light to heavy REE fractionation is not obvious, and the LaN/YbN(average value = 9.78) is less than 10. The A-type rhyolites depleted in Ba, Sr, P, and Ti, with relatively low Nb/Ta, indicating that the rocks belong A2 subtype granites formed in an extensional environment. The adakitic dacites are characterized by high Sr contents(624 to 1,082 ppm), low Y contents(10.6 to 12.6 ppm), high Sr/Y and Sr/Yb ratios, and low Mg# values(14.77 to 36.46), indicating that they belong to "C" type adakites. The adakitic dacite with high Sr and low Yb were likely generated by partial melting of the lower crust under high pressure conditions at least 40 km depth. The I-type rhyolites with low Sr and high Yb, and the A-type rhyolites with very low Sr and high Yb, were formed in the middle and upper crust under low pressure conditions, respectively. In addition, the formation depths of the former were approximately 30 km, whereas those of the latter were less than 30 km. The geochemical characteristics reveal that the volcanic rocks of Yingcheng Formation were formed in an extensional environment which was related to the retreat of subducted Paleo-Pacific Plate. At the late Early Cretaceous Period, the upwelling of the asthenosphere mantle and the lithosphere delamination caused by the retreat of the subducted Paleo-Pacific Plate, had resulted in lithosheric extension in the eastern part of China. Subsequently, a large area of volcanic rocks had formed. The SB has also been confirmed to be a product of the tectonic stress field in that region.     

Whole-rock Nd–Hf isotopic study of I-type and peraluminous granitic rocks from the Chinese Altai: Constraints on the nature of the lower crust and tectonic setting

The nature of the lower crust and tectonic setting of the Chinese Altai in the early to middle Paleozoic are still hotly debated. Decoupling between zircon Hf and whole-rock Nd isotopic systems for granites results in different interpretations for the above issues. In order to solve the problem, whole-rock Nd–Hf isotopic analyses were conducted on representative early to middle Paleozoic I-type granite and strongly peraluminous granites and rhyolites from the Chinese Altai. The I-type granites show metaluminous to weakly peraluminous feature and have ε_Nd(t) values ranging from − 2.2 to + 0.8 and ε_Hf(t) from + 3.9 to + 12.9, respectively. The strongly peraluminous granites and rhyolites have similar ε_Nd(t) and ε_Hf(t) values ranging from − 3.0 to + 1.7 and from + 2.1 to + 10.4, respectively. All samples plot above the Terrestrial Array on Nd–Hf isotopic diagram, indicating significant Nd–Hf isotopic decoupling in the magma sources. These samples show flatten HREE pattern and have Lu/Hf ratios similar to the average crust, suggesting that Nd–Hf isotopic decoupling was not originated from an ancient basement with elevated Lu/Hf ratios. The observed isotopic decoupling is similar to those modern island arcs, such as the Lesser Antilles and Sunda, where Nd selectively enriched over Hf due to metasomatism in the mantle wedge and consequently resulted in decoupling between the Sm–Nd and Lu–Hf isotopic systems. Our results, combined with the available data, show that prolonged subduction and crust–mantle interaction caused the Nd–Hf isotopic decoupling in the lithospheric mantle beneath the Chinese Altai. The crust of the Chinese Altai was extracted from the lithospheric mantle and inherited the Nd–Hf isotopic decoupling feature. Therefore, the Hf, rather than Nd, isotopic data more faithfully reflect the nature of the lower crust that was quite juvenile in the Paleozoic, and the Chinese Altai represents an early Paleozoic magmatic arc possibly built near western Mongolia.     

Paleo-environment and provenance in a lacustrine shallow-water delta-meandering river sedimentary system: insights from the Middle–Upper Jurassic formations of the Fukang Sag of Junggar Basin,NW China

The Middle–Upper Jurassic Shishugou Group in the central Junggar Basin was deposited in a lacustrine shallow-water delta-meandering river sedimentary system. The integrated petrological (thin-section, granularity and heavy minerals analysis), geochemical (trace elements and rare earth elements analysis) and geophysical analyses (well logging and 3D-seismic slice analysis) are used to determine redox conditions, paleoclimate, paleosalinity, provenance and sedimentary evolution extant during deposition of the Shishugou Group: (1) the redox condition changed from a weak anoxic/oxic condition to a strongly oxic condition; (2) the climate changed from humid to hot and arid in the Middle–Late Jurassic, which may have resulted in the lake water having slight–medium salinity; (3) the relatively distant northeastern provenance from the Kelameili Mountain is the most important sediment source; and (4) the south provenance from the Tianshan Mountains (Bogeda Shan) decreases with the development of the sag piedmont, which supplies sediments to the southeastern Fukang Sag. The sedimentary environment changed from a lacustrine shallow-water delta to a meandering river during the deposition of the Shishugou Group. The shallow-water meandering river delta was characterised by pervasive mudstones with oxide colours, thin single-layer sand bodies (1–15?m, mean 3?m), relatively low sand–strata ratios (0.2–0.5) and the absence of progradation, mouth bars and reverse rhythms. The gentle slope is the primary condition necessary for the formation of a shallow-water meandering river delta. Paleo-environment (climate change from warm-humid to hot-arid) and the stable and remote Kelameili Mountain provenance played critical roles in the development and evolution of lacustrine–delta-meandering river sedimentary systems.     

The age and geochemistry of volcanic rocks on the eastern flank of the Umlekan–Ogodzha volcanoplutonic belt (Amur region)

⁴⁰Ar/³⁹Ar dating yielded the reliable ages of andesite from the Unerikan complex (102.1 ± 1.4 Ma) and basaltic andesite from the Burunda complex (107.3 ± 2.4 Ma). The established age of volcanism is close to one of the stages of formation of the Khingan–Okhotsk volcanoplutonic belt. The petrography and geochemistry of basic, normal-basic, and normal rocks point to their dual character: They combine features specific for tholeiitic and calc-alkalic rocks. Most likely, these rocks formed in the setting of transform continental margin.     

U-Pb garnet chronometry in high-grade rocks—case studies from the central Damara orogen (Namibia) and implications for the interpretation of Sm-Nd garnet ages and the role of high U-Th inclusions

Garnets from different migmatites and granites from the Damara orogen (Namibia) were dated with the U-Pb technique after bulk dissolution of the material. Measured ²⁰⁶Pb/²⁰⁴Pb ratios are highly variable and range from ca. 21 to 613. Variations in isotope (²⁰⁸Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb) and trace element (Th/U, U/Nd, Sm/Nd) ratios of the different garnets show that some garnets contain significant amounts of monazite and zircon inclusions. Due to their very low ²⁰⁶Pb/²⁰⁴Pb ratios, garnets from pelitic migmatites from the Khan area yield Pb-Pb ages with large errors precluding a detailed evaluation. However, the ²⁰⁷Pb/²⁰⁶Pb ages (ca. 550–500 Ma) appear to be similar to or older than U-Pb monazite ages (530±1–517±1 Ma) and Sm-Nd garnet ages (523±4–512±3 Ma) from the same sample. It is reasonable to assume that the Pb-Pb garnet ages define growth ages because previous studies are consistent with a higher closure temperature for the U-Pb system in garnet relative to the U-Pb system in monazite and the Sm-Nd system in garnet. For igneous migmatites from Oetmoed, Pb-Pb garnet ages (483±15–492±16 Ma) and one Sm-Nd garnet whole rock age (487±8 Ma) are similar whereas the monazite from the same sample is ca. 30–40 Ma older (528±1 Ma). These monazite ages are, however, similar to monazite ages from nearby unmigmatized granite samples and constrain precisely the intrusion of the precursor granite in this area. Although there is a notable difference in closure temperature for the U-Pb and Sm-Nd system in garnet, the similarity of both ages indicate that both garnet ages record garnet growth in a migmatitic environment. Restitic garnet from an unmigmatized granite from Omaruru yields similar U-Pb (493±30–506±30 Ma) and Sm-Nd (493±6–488±7 Ma) garnet ages whereas the monazite from this rock is ca. 15–25 Ma older (516±1–514±1 Ma). Whereas the monazite ages define probably the peak of regional metamorphism in the source of the granite, the garnet ages may indicate the time of melt extraction. For igneous garnets from granites at Oetmoed, the similarity between Pb-Pb (483±34–474±17 Ma) and Sm-Nd (492±5–484±13 Ma) garnet ages is consistent with fast cooling rates of granitic dykes in the lower crust. Differences between garnet and monazite U-Pb ages can be explained by different reactions that produced these minerals at different times and by the empirical observation that monazite seems resistant to later thermal re-equilibration in the temperature range between 750 and 900 °C (e.g. Braun et al. 1998). For garnet analyses that have low ²⁰⁶Pb/²⁰⁴Pb ratios, the influence of high- inclusions is small. However, the relatively large errors preclude a detailed evaluation of the relationship between the different chronometers. For garnet with higher ²⁰⁶Pb/²⁰⁴Pb ratios, the overall similarity between the Pb-Pb and Sm-Nd garnet ages implies that the inclusions are not significantly older than the garnet and therefore do not induce a premetamorphic Pb signature upon the garnet. The results presented here show that garnet with low ²³⁸U/²⁰⁴Pb ratios together with Sm-Nd garnet data and U-Pb monazite ages from the same rock can be used to extract geologically meaningful ages that can help to better understand tectonometamorphic processes in high-grade terranes.Editorial responsibility: J. Hoefs     

New data on chemical composition of Jurassic–Lower Cretaceous sedimentary rocks of the Bureya basin (Far East of Russia)

The first data on the whole-rock chemical composition of Jurassic–Lower Cretaceous sedimentary rocks cropping out in the Soloni–Umal’ta river interfluve (Bureya sedimentary basin) are used for revealing the distribution of their rock-forming elements. It is shown that the clastic material originated mostly from acid igneous rocks, while their intermediate varieties, as well as quartz-rich sedimentary and metamorphic rocks, played a subordinate role. It is assumed that the bulk of the clastic material was transported from the west and southwest (Bureya massif) and a smaller share from the east. The most significant differences between the Lower–Middle Jurassic and Upper Jurassic–Lower Cretaceous rocks mark a break in sedimentation.     

Exhumation of the Neuquén Basin in the southern Central Andes (Malargüe fold and thrust belt) from field data and low-temperature thermochronology

Apatite fission-track analysis performed on eighteen Mesozoic sediment samples of the Neuquén Basin from the Southern Central Andes orogenic front between 35°30′ and 37°S has revealed Campanian-Paleocene (75-55 Ma), late Eocene-early Oligocene (35-30 Ma) and middle Miocene (15-10 Ma) cooling episodes. Each cooling episode corresponds closely with major unconformities observed in the preserved sedimentary sequences, and is associated with kilometer-scale additional burial and subsequent exhumation. A similar degree of cooling is inferred from associated vitrinite reflectance data. Late Eocene-early Oligocene exhumation is recognized only near the eastern orogenic front adjacent to the foreland in the southernmost part of the study area and may be related partly to within-plate magmatism and associated extension in the Palaoco Basin. The Campanian-Paleocene and middle Miocene cooling episodes are recognized more widely in the fold and thrust belt and appear to coincide with periods of eastward arc expansion and mountain building processes.     

The Pfitsch-Mörchner Basin,an example of the post-Variscan sedimentary evolution in the Tauern Window (Eastern Alps)

A review of post-Variscan metasedimentary and metavolcanic successions in the western Tauern Window is presented. U/Pb – datations of zircons in metavolcanic rocks reveal ages between 309 and 280 Ma. Deposition of grey conglomerates and black pelites started before 309 Ma in the northernmost basin of the Tauern, the Riffler-Schönach basin. In the more central Pfitsch-Mörchner basin, the onset of conglomerate sedimentation can be dated into the time span between 293 and 280 Ma. The Pfitsch and Windtal Formations are newly defined. The basins were filled with up to 1 km of mainly continental clastics until Early Triassic. Short marine ingressions in Middle- and Late Triassic times flooded only basinal parts of the area where we suppose a more or less continuous sedimentation until the Late Jurassic. Only the Hochstegen Marble documents a nearly complete submergence in the area of the Tauern Window. In spite of the metamorphic overprint, the tentative interpretations of the sedimentary facies give a reasonable picture and allow correlations to nonmetamorphic areas in South Germany or the External Massifs of Eastern Switzerland.