Sedimentation of submarine fan deposits in the Pindos foreland basin,from late Eocene to early Oligocene,west Peloponnesus peninsula,SW Greece

Turbidite facies distribution and palaeocurrent analysis of submarine fan evolution in the Pindos foreland basin of west Peloponnesus peninsula (SW Greece) indicate that this part of the foreland was developed during Late Eocene to Early Oligocene in three linear sub‐basins (Tritea, Hrisovitsi and Finikounda). The basin fill conditions, with a multiple feeder system, which is characterized by axial transport of sediments and asymmetric stratigraphic thickness of the studied sediments, indicate that the Pindos Foreland Basin in this area was an underfilled foreland basin. Sediments are dominated by conglomerates, sandstones and mudstones. The flow types that controlled the depositional processes of the submarine fans were grain flows, debris flows and low‐ and high‐density turbidity currents. The sedimentary model that we propose for the depositional mechanisms and geometrical distribution of the turbidite units in the Tritea sub‐basin is a mixed sand‐mud submarine fan with a sequential interaction of progradation and retrogradation for the submarine fan development and shows a WNW main palaeocurrent direction. The Hrisovitsi sub‐basin turbidite system characterized by small‐scale channels was sediment starved, and the erosion during deposition was greater than the two other studied areas, indicating a more restricted basin topography with a NW main palaeocurrent direction. The Finikounda sub‐basin exhibits sand‐rich submarine fans, is characterized by the presence of distinct, small‐scale, thickening‐upward cycles and by the covering of a distal fan by a proximal fan. It was constructed under the simultaneous interaction of progradation and aggradation, where the main palaeocurrent direction was from NNW to SSE. Copyright © 2012 John Wiley & Sons, Ltd.     

Unravelling provenance from Eocene–Oligocene sandstones of the Thrace Basin,North‐east Greece

New sandstone petrology and petrostratigraphy provide insights on Palaeogene (Middle Eocene to Oligocene) clastics of the Thrace Basin in Greece, which developed synchronously with post‐Cretaceous collision and subsequent Tertiary extension. Sandstone petrofacies are used as a tool to unravel complex geodynamic changes that occurred at the southern continental margin of the European plate, identifying detrital signals of the accretionary processes of the Rhodope orogen, as well as subsequent partitioning related to extension of the Rhodope area, followed by Oligocene to present Aegean extension and wide magmatic activity starting during the Early Oligocene. Sandstone detrital modes include three distinctive petrofacies: quartzolithic, quartzofeldspathic and feldspatholithic. Major contributions are from metamorphic basement units, represented mostly by low to medium‐grade lithic fragments for the quartzolithic petrofacies and high‐grade metamorphic rock fragments for the quartzofeldspathic petrofacies. Volcaniclastic sandstones were derived from different volcanic areas, with a composition varying from dominantly silicic to subordinate intermediate products (mainly rhyolitic glass, spherulites and felsitic lithics). Evolution of detrital modes documents contributions from three key source areas corresponding to the two main crystalline tectonic units: (i) the Variegated Complex (ultramafic complex), in the initial stage of accretion (quartzolithic petrofacies); (ii) the Gneiss–Migmatite Complex (quartzofeldspathic petrofacies); and (iii) the Circum‐Rhodope Belt. The volcaniclastic petrofacies is interbedded with quartzofeldspathic petrofacies, reflecting superposition of active volcanic activity on regional erosion. The three key petrofacies reflect complex provenance from different tectonic settings, from collisional orogenic terranes to local basement uplift and volcanic activity. The composition and stratigraphic relations of sandstones derived from erosion of the Rhodope orogenic belt and superposed magmatism after the extensional phase in northern Greece provide constraints for palaeogeographic and palaeotectonic models of the Eocene to Oligocene western portions of the Thrace Basin. Clastic detritus in the following sedimentary assemblages was derived mainly from provenance terranes of the Palaeozoic section within the strongly deformed Rhodope Massif of northern Greece and south‐east Bulgaria, from the epimetamorphic units of the Circum‐Rhodope Belt and from superposed Late Eocene to Early Oligocene magmatism related to orogenic collapse of the Rhodope orogen. The sedimentary provenance of the Rhodope Palaeogene sandstones documents the changing nature of this orogenic belt through time, and may contribute to a general understanding of similar geodynamic settings.     

Petrography and geochemistry of Jumara Dome sediments,Kachchh Basin: Implications for provenance,tectonic setting and weathering intensity

In the Kachchh Mainland, the Jumara Dome mixed carbonate-siliciclastic succession is represented by the Jhurio and Patcham formations and siliciclastic-dominating Chari Formation （Bathonian to Oxfordian）. The Ju- mara Dome sediments were deposited during sea-level fluctuating, and were interrupted by storms in the shallow marine environment. The sandstones are generally medium-grained, moderately sorted, subangular to subrounded and of low sphericity. The sandstones are mineralogically mature and mainly composed of quartzarenite and subar- kose. The plots of petrofacies in the Qt-F-L, Qm-F-Lt, Qp-Lv-Ls and Qm-P-K ternary diagrams suggest mainly the basement uplift source （craton interior） in rifted continental margin basin setting. The sandstones were cemented by carbonate, iron oxide and silica overgrowth. The Chemical Index of Alteration values （73% sandstone and 81% shale） indicate high weathering conditions in the source area. Overall study suggests that such strong chemical weathering conditions are of unconformity with worldwide humid and warm climates during the Jurassic period. Positive correlations between A1203 and Fe203, TiO2, Na20, MgO, K20 are evident. A high correlation coefficient between A1203 and K20 in shale samples suggests that clay minerals control the major oxides, The analogous con- tents of Si, A1, Ti, LREE and TTE in the shale to PAAS with slightly depleted values of other elements ascribe a PAAS like source （granitic gneiss and minor mafics） to the present study. The petrographic and geochemical data strongly suggest that the studied sandstones/shales were deposited on a passive margin of the stable intracratonic basin.     

海南岛北西部邦溪地区奥陶纪火山-碎屑沉积岩岩石学、矿物学和地球化学:源区及构造环境暗示

海南岛奥陶纪火山-碎屑沉积岩矿物、地球化学成分与源区岩性质和沉积构造环境密切相关.岩相学观察和矿物化学分析表明,所研究的岩石主要碎屑矿物由石英、绿泥石(Si:5.8～6.2 a.p.f.u.、Mg/(Mg Fe):～0.45)和自色云母(Si:6.2～6.8 a.p.f.u.、Al:4.5～5.7 a.p.f.u.)组成,含少量的长石(主要为钾长石)、黑云母(Mg/(Mg Fe)=0.43、TiO2=0.01%)和磷灰石等矿物.地球化学特征上,这些岩石普遍具有高SiO2含量(Al2O3/SiO2=0.07～0.19)、相对高的K2O/Na2O比值(1.25～14.71)和高的FeOtotal MgO含量(普遍大于5%,最高达11.5%),但微量元素(含REE)含量普遍低于PAAS(NASC),且REE分布模式显示LREE富集((La/Yb)N=6.1～12.5)、Eu负异常或弱负异常(δEu=0.57～0.86).高的前交代CIA(～84)和PIA(～85)指数、A-CN-K分布模式以及可变的Th/U(2.9～25)比值还说明源区已经历强烈的化学和机械风化作用.低的Ce/Pb比值(2.1～2.8)则反映海南岛早古生代地壳已经历过Pb的富集事件.结合矿物化学成分和区域地质对比,研究区奥陶纪火山-碎屑沉积岩源区主要由变质的岩浆弧花岗闪长岩和弧长英质火山岩组成,少量的铁镁质成分不能排除,因而暗示一个从活动大陆边缘到被动大陆边缘的过渡性沉积构造环境.同时,推测海南岛在早古生代和晚古生代-早中生代可能分别存在一次俯冲增生-碰撞造山事件;海南岛(可能包括华夏地块)应于晚奥陶世后才脱离东冈瓦纳大陆的澳大利亚边缘.     

Petrography and geochemistry of Eocene sandstones from eastern Pontides (NE TURKEY): Implications for source area weathering,provenance and tectonic setting

Subaerial weathering level, source area and tectonic environments were interpreted by using petrographic and geochemical characteristics of Eocene age sandstones found in the eastern Pontides. The thickness of Eocene age clastic rocks in the eastern Pontides ranges from 195 to 400 m. Mineralogical components of sandstones were mainly quartz, feldspar, rock fragments, and opaque and accessory minerals. Depending on their matrix and mineralogical content, Eocene age sandstones are identified as arkosic arenite-lithic arenite and feldspathic wacke-lithic wacke. CIA (Chemical Index of Alteration) values observed in the Eocene age sandstones (43–55) suggest that the source terrain of the sandstones was not affected by intense chemical weathering. Low CIW/CIA (Chemical Index of Weathering/Chemical Index of Alteration) values of the sandstones studied here suggest only slightly decomposed material and having undergone little transport until final deposition. Zr/Hf, Th/Sc, La/Sc and CIA ratios are low and demonstrate a mafic source; on the other hand, high LREE/HREE ratios and a slightly negative Eu anomaly indicate a subordinate fclsic source. Modal mineralogical and SiO₂/Al₂O₃ and K₂O/Na₂O and Th, Zr, Co, Sc of Eocene age sandstone contents indicate that they are probably magmatic arc originated and deposited in the back arc basin.     

Petrology and geochemistry of sandstones in the southern Benue Trough of Nigeria： Implications for provenance and tectonic setting

Petrographic and geochemical analyses of three Cretaceous lithostratigraphic sandstone units were undertaken to constrain their provenance and tectonic setting. Petrographic analysis showed that there are differences in composition between the three sandstone bodies, which can be attributed to differences in provenance relief, transport distance and geology of the terrain. Composition of the three lithostratigraphic sandstone bodies fall within the craton interior field.
Framework mode and chemical features indicated their derivation from basaltic volcanics, source rocks during the early rifting stage, and felsic, intermediate and mafic igneous source rocks located at the southeast basement complex terrain, with minor sedimentary components from the uplifted and folded older Cretaceous strata.
The chemical composition of the sandstones is mainly related to source rocks, chemical weathering conditions and transport agents. The source rocks were derived mainly from the southeastern Precambrian basement of Nigeria. Through examination of the sandstones, the tectonic setting was modeled. The Benue Trough belongs to a continental sedimentary basin of the passive margin type.
The tectonic evolution from Albian to Maastrichtain of the trough is contributed to the difference in framework mode and chemical composition of the sandstones. The evolution of the basin was reconstructed in terms of sandstone petrology and geochemistry. The tectonic evolution can be subdivided into three stages from the petrology and geochemistry data. The first stage covers Albian; the second stage the Turonian-Coniacian, and the third stage the Campanian-Maastrichtain. These are the three mega discontinuities in the sandstone composition among these three stages. These three discontinuities signify the influence of tectonism.     

Petrography and geochemistry of the Middle Miocene Gebel El Rusas sandstones,Eastern Desert,Egypt: Implications for provenance and tectonic setting

Petrography and bulk rock geochemistry of the Middle Miocene sandstones of the lower and upper members of Gebel El Rusas Formation along the Egyptian Red Sea Coastal plain, have been investigated to determine the provenance, tectonic setting, and weathering condition of this formation. The Lower Member is formed mainly of sandstones and conglomerates with clay interbeds. The Upper Member is more calcareous and formed mainly of sandstones and limestones with marls and clays intercalations. Petrographically, the Lower Member sandstones are mostly immature and classified as arkoses with an average framework composition of \(\hbox {Q}_{66}\hbox {F}_{29}\hbox {R}_{5}\), and the Upper Member sandstones are partly submature (more quartzose, less feldspathic) and classified as subarkoses with an average framework composition of \(\hbox {Q}_{80}\hbox {F}_{17}\hbox {R}_{3}\). The Gebel El Rusas sandstones are enriched in Sr, Ba, Zr and Rb and depleted in Co and U, as compared to UCC. The chemical index of alteration (CIA) values suggest moderate weathering conditions. The geochemistry results revealed that the Gebel El Rusas sandstones were derived from felsic-granitic source rocks and deposited in a passive margin of a synrift basin. The inferred tectonic setting for Middle Miocene Gebel El Rusas sandstones in the study area is consistent with the regional geology of the Eastern Desert of Egypt during Middle Miocene.     

Petrography and geochemistry of the Late Cretaceous redbeds in the Gan-Hang Belt,southeast China: implications for provenance,source weathering,and tectonic setting

ABSTRACT

The distinct basin and range tectonics in southeast China were generated in a crustal extension setting during the late Mesozoic. Compared with the adjacent granitoids of the ranges, the redbeds of the basins have not been well characterized. In this article, provenance, source weathering, and tectonic setting of the redbeds are investigated by petrographic and geochemical studies of sandstone samples from the Late Cretaceous Guifeng Group of the Yongchong Basin in the Gan-Hang Belt, southeast China. Detrital grains are commonly subangular to subrounded, poorly sorted, and are rich in lithic fragments. The variable pre-metasomatic Chemical Index of Alternation (CIA* = 62–85), Chemical Index of Weathering (CIW = 70.90–98.76, avg. 85.62), Plagioclase Index of Alteration (PIA = 60.23–98.35, avg. 79.91), and high Index of Compositional Variability (ICV = 0.67–3.08, avg. 1.40) values collectively suggest an overall intermediate degree of chemical weathering and intense physical erosion of the source rocks, but a relatively decreased degree of chemical weathering during the late stage (Lianhe Formation) of the Guifeng Group is observed. Several chemical ratios (e.g. Al₂O₃/TiO₂, La/Th, Cr/Th, Th/Sc, Zr/Sc) also suggest a dominant felsic source nature, significant first-cycle sediment supply, and low sedimentary recycling. Such features are consistent with active extension tectonic setting. Sandstone framework models and geochemical characteristics suggest the provenance is related to passive margin (PM), active continental margin (ACM), and continental island arc (CIA) tectonic settings. Sediment derivation from the Neoproterozoic metamorphic rocks and Silurian–Devonian granites indicates a PM provenance, whereas sediments derived from the Early Cretaceous volcanic-intrusive complexes suggest an ACM and CIA nature. Therefore, the Late Cretaceous redbeds were deposited in a dustpan-like half-graben under the back-arc extension regime when southeast China was possibly influenced by northwestward subduction of the Palaeo-Pacific plate beneath East Asia.     

Petrographic and geochemical characteristics of Paleogene turbidite deposits in the southern Aegean (Karpathos Island,SE Greece): Implications for provenance and tectonic setting

The provenance and depositional setting of Paleogene turbidite sediments from the southern Aegean are investigated using petrography and whole-rock geochemistry. Petrography indicates that Karpathos Island turbidites are consisting of compositionally immature sandstones (graywackes–litharenites) derived from igneous (plutonic–volcanic), sedimentary, low-grade metamorphic and ophiolitic sources. The studied sediments probably reflect a mixing from an eroded magmatic arc and from quartzose, recycled sources. Major and trace element data are compatible with an acidic to mixed felsic/basic source along with input of ultramafic detritus and recycling of older sedimentary components. Geochemical data also reveal that the sediments have undergone a minor degree of weathering and no significant sediment recycling. Chondrite-normalized REE plots show a light REE enrichment (LaN/YbN ca. 7) and absence of significant negative Eu anomalies, indicating provenance from young undifferentiated arc material with contribution from an old upper continental crust source. Turbidite sedimentation probably took place in a continental island arc depositional setting as a result of subduction of a branch of Neotethys beneath a continental fragment of the Anatolide domain in Early Tertiary times. The relation of Karpathos turbidites with the Pindos foreland basin (Gavrovo and Ionian Zones of western Greece) remains problematic.     

Petrography, geochemistry, and geochronology of granitoid rocks in the Neoproterozoic-Paleozoic Lufilian–Zambezi belt, Zambia: Implications for tectonic setting and regional correlation

There are several pre-orogenic Neoproterozoic granitoid and metavolcanic rocks in the Lufilian–Zambezi belt in Zambia and Zimbabwe that are interpreted to have been emplaced in a continental-rift setting that is linked to the break-up of the Rodinia supercontinent. However, no geochemical data were previously available for these rocks in the Zambian part of the belt to support this model. We conducted petrographic and whole-rock chemical analyses of the Neoproterozoic Nchanga Granite, Lusaka Granite, Ngoma Gneiss and felsic metavolcanic rocks from the Lufilian–Zambezi belt in Zambian, in order to evaluate their chemical characteristics and tectonic settings. Other magmatic rocks of importance for understanding the evolution of the belt in Zambia, included in this study, are the Mesoproterozoic Munali Hills Granite and associated amphibolites and the Mpande Gneiss. The Neoproterozoic rocks have monzogranitic compositions, aluminum-saturation indices (ASI) < 1.1, and high contents of high field strength elements (HFSE) and rare earth elements (REE). The chondrite-normalised spider diagrams are similar to those of A-type granites from the Lachlan fold belt and show negative Sr, P, and Ti anomalies. On various tectonic discrimination diagrams the Neoproterozoic rocks plot mainly in A-type granite fields. These petrographic and trace element compositions indicate that these rocks are A-type felsic rocks, but they do not have features of granites and rhyolites emplaced in true continental-rift settings, as previously suggested. On the basis of the A-type features and independent regional geological and geochronological data, we suggest that the Neoproterozoic granitoid and felsic metavolcanic rocks were emplaced during the earliest extensional stages of continental rifting in the Lufilian–Zambezi belt. The apparent continental-arc like chemistry of the granitoid and felsic metavolcanic rocks is thus inferred to be inherited from calcalkaline sources. The Mesoproterozoic Munali Hills Granite and Mpande Gneiss have trace element features e.g., Nb–Ta depletions, which indicate that that these gneisses were emplaced in a convergent-margin setting. The MORB-normalised spider diagram of co-magmatic amphibolites exhibit a fractionated LILE/HFSE pattern recognized in subduction zones. This inference is consistent with remnants of ocean crust, juvenile Island arcs and ophiolites elsewhere in the Mesoproterozoic Irumide belt in Zambia and Zimbabwe. In addition, we report the first U–Pb zircon age of 1090.1 ± 1.3 Ma for the Munali Hills Granite. The age for the Munali Hills Granite provides new constraints on correlation and tectono-thermal activity in the Lufilian–Zambezi belt. The age of the Munali Hills Granite indicates that some supracrustal rocks in the Zambezi belt of Zambia, which were previously thought to be Neoproterozoic and correlated with the Katanga Supergroup in the Lufilian belt, are Mesoproterozoic or older. Consequently, previous regional lithostratigraphic correlations in the Lufilian–Zambezi belt would require revision.     

Potential source rocks, organic geochemistry and thermal maturation in the southern depocenter (Kipourio–Grevena) of the Mesohellenic Basin, central Greece

The study area is the southern depocenter (depth > 4200 m) of the Mesohellenic Basin which extends between Kipourio and Grevena, central Greece. The Mesohellenic Basin is a Middle-Tertiary intramontane basin developed within the Hellenide orogen. Previous studies have focused on the depositional environments, configuration and hydrocarbon potential of the basin. In this paper we present additional geochemical and petrographic data from outcrop samples of the basin's southern depocenter, which is considered the most promising area, in terms of hydrocarbon prospectivity. A total number of thirty six samples were analysed: Rock-Eval pyrolysis, maceral analysis, vitrinite reflectance and thermal alteration index, bitumens extraction, liquid chromatography, and GC-MS. The samples were collected from deltaic deposits and submarine fan sediments of Late Eocene to Late Oligocene age. The TOC values of the analysed samples range between rich and very rich and the organic matter consists mainly of type III kerogen and the organic matter consider to be predominately gas prone. The thermal maturity assessed from T_max and vitrinite reflectance shows an immature stage of the organic matter along with the presence of layers having reached the very early mature stage. Vitrinite reflectance measurements and maturity calculations (applying the Lopatin modeling), reveal that the lower part of the depocenter sediments falls within the ‘oil window’. The extractable organic matter (EOM) (mg bitumens/g TOC) indicate the existence of samples (from deltaic deposits) with high ratio of transformation (EOM) (> 100 mg bitumen/g TOC). The GC and GC-MS analyses of the biomarkers indicate mainly the occurrence of terrestrial organic matter reflecting oxidizing conditions and both immature and very early mature stages. The results of the Rock-Eval pyrolysis and the distribution of the isoprenoids support the assumption of the input of an organic matter mixture.     

Mesoproterozoic–Neoproterozoic transition: Geochemistry, provenance and tectonic setting of clastic sedimentary rocks on the SE margin of the Yangtze Block, South China

The SE margin of the Yangtze Block, South China is composed of the Mesoproterozoic Lengjiaxi Group and the Neoproterozoic Banxi Group, with Sinian- and post-Sinian-cover. A geochemical study was undertaken on the Mesoproterozoic–Neoproterozoic clastic sediments in order to delineate the characteristics of the sediment source and to constrain the tectonic development and crustal evolution of South China.Our results show that the Mesoproterozoic clastic sediments have a dominant component derived from a metavolcanic-plutonic terrane, with a large of mafic component. There is a minor contribution of mafic rocks and older upper crustal rocks to the provenance. Strong chemical weathering in the source area occurred before transport and deposition. The provenance for the Neoproterozoic clastic sediments was most likely old upper continental crust composed of tonalite–granodiorite-dominated, tonalite–granodiorite–granite source rocks, which had undergone strong weathering and/or recycling. A minor component of older K-rich granitic plutonic rocks and younger volcanogenic bimodal rocks is also indicated.Based on the regional geology, the geochemical data and the inferred provenance, the Mesoproterozoic Group is interpreted as a successive sedimentary sequence, deposited in an extensional/rifting back-arc basin, adjacent to a >1.80 Ga continental margin arc-terrane. The progressive extension/rifting of the back-arc basin was followed by increasing subsidence and regional uplift during continental marginal arc-continent (the Cathaysian Block) collision at 1.0 Ga caused the deposition of the Neoproterozoic Group into back-arc to retro-arc foreland basin. Therefore, the depositional setting of the Proterozoic clastic sediments and associated volcanic rocks within the back-arc basin reflected basin development from an active continental margin (back-arc basin), with extension or rifting of the back-arc basin, to a passive continental margin.     

Geochemistry and zircon U-Pb geochronology of the rhyolitic tuff on Port Island,Hong Kong: Implications for early Cretaceous tectonic setting

Early Cretaceous rhyolitic tuffs, widely distributed on Port Island, provide insights into the volcanism and tectonic setting of Hong Kong. In this paper we present petrological, geochronological and geochemical data of the rhyolitic tuff to constrain the diagenesis age and petrogenesis of the rocks, tectonic setting and early Cretaceous volcanism of Hong Kong. The first geochronological data show that the zircons in the volcanic rocks have U-Pb age of 141.1–139.5 Ma, which reveals that the rhyolitic tuff on Port Island was formed in the early Cretaceous (K₁). Geochemically, these acid rocks, which are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs), belong to the high K calc-alkaline to shoshonite series with strongly-peraluminous characteristic. The geochemical analyses suggest that the volcanic rocks were derived from deep melting in the continental crust caused by basaltic magma underplating. Based on the geochemical analysis and previous studies, we concluded that the rhyolitic tuffs on Port Island were formed in a back-arc extension setting in response to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.     

Detrital mode, mineralogy and geochemistry of the Sidi Aïch Formation (Early Cretaceous) in central and southwestern Tunisia: Implications for provenance, tectonic setting and paleoenvironment

The chemical and mineralogical composition of the Sidi Aïch Formation sandstones in central and southwestern Tunisia has been investigated in order to infer the provenance and tectonic setting, as well as to appraise the influence of weathering. The sixteen studied samples are mainly composed of quartz, feldspar, kaolinite and/or illite. Sidi Aïch sandstones are mainly arkosic, potassic feldspar-rich and immature. Much of the feldspar was transformed to kaolinite. Concerning the relation between sandstone detrital composition and their depositional setting, the Sidi Aïch Formation sandstone in the major studied localities, probably accumulated in relatively proximal small basins within the continental interior. However, for the Khanguet El Ouara study site, sandstones may have been deposited in a foreland basin which received recycled sediments from an adjacent orogenic belt.The source area may have included quartzose sedimentary rocks. The dominance of quartz and enrichment in immobile elements suggest that the depositional basins were associated with a passive margin. The petrography and geochemistry reflect a stable continental margin and sediments were derived from granitic and pegmatitic sources located in the southern parts of the Gafsa basin. High values for the chemical index of alteration (CIA) indicate that recycling processes might have been important. Particularly high CIA values in the Garet Hadid locality indicate more intense chemical alteration, either due to weathering processes or tectonic control.     

Sedimentary petrology and geochemistry of siliciclastic rocks from the upper Jurassic Tordillo Formation (Neuquén Basin, western Argentina): Implications for provenance and tectonic setting

The Upper Jurassic Tordillo Formation is exposed along the western edge of the Neuquén Basin (west central Argentina) and consists of fluvial strata deposited under arid/semiarid conditions. The pebble composition of conglomerates, mineralogical composition of sandstones and pelitic rocks, and major- and trace-element geochemistry of sandstones, mudstones, and primary pyroclastic deposits are evaluated to determine the provenance and tectonic setting of the sedimentary basin. Conglomerates and sandstones derived almost exclusively from volcanic sources. The stratigraphic sections to the south show a clast population of conglomerates dominated by silicic volcanic fragments and a predominance of feldspathic litharenites. This framework composition records erosion of Triassic–Jurassic synrift volcaniclastic rocks and basement rocks from the Huincul arch, which was exhumed as a result of Late Jurassic inversion. In the northwestern part of the study area, conglomerates show a large proportion of mafic and acidic volcanic rock fragments, and sandstones are characterised by a high content of mafic volcanic rock fragments and plagioclase. These data suggest that the source of the sandstones and conglomerates was primarily the Andean magmatic arc, located west of the Neuquén Basin. The clay mineral assemblage is interpreted as the result of a complex set of factors, including source rock, climate, transport, and diagenesis. Postdepositional processes produced significant variations in the original compositions, especially the fine-grained deposits. The Tordillo sediments are characterised by moderate SiO₂ contents, variable abundances of K₂O and Na₂O, and a relatively high proportion of ferromagnesian elements. The degree of chemical weathering in the source area, expressed as the chemical index of alteration, is low to moderate. The major element geochemistry and Th/Sc, K/Rb, Co/Th, La/Sc, and Cr/Th values point to a significant input of detrital volcanic material of calcalkaline felsic and intermediate composition. However, major element geochemistry is not useful for interpreting the tectonic setting. Discrimination plots based on immobile trace elements, such as Ti, Zr, La, Sc, and Th, show that most data lie in the active continental margin field. Geochemical information is not sufficiently sensitive to differentiate the two different source areas recognized by petrographic and modal analyses of conglomerates and sandstones.     

The upper Viséan–Serpukhovian in the type area for the Serpukhovian Stage (Moscow Basin,Russia): Part 2. bulk geochemistry and magnetic susceptibility

This study is the summary analysis of bulk XRF geochemistry (233 samples from three sections) of the Oka and Zaborie groups of the type Serpukhovian succession in the Moscow Basin. The siliciclastic wedges in the limestone‐dominated Oka Group are two to three times enriched in Fe, Ti, and Zr compared to Clarke values. Bulk iron strongly correlates with magnetic susceptibility. Iron tends to form ferruginized horizons (original siderites) in finer grained siliciclastic beds associated with coal seams. These beds also tend to be enriched in Cu, Ni, Pb, Zn, and other trace metals (metal enrichment horizons or MEHs). MEHs formed in ponded conditions of coastal low‐pH marshlands vegetated by mangrove‐like lycopsid bushes. Well‐drained environments of palaeokarst formation and alkaline everglades (Akulshino palustrine event) on the other hand did not accumulate Fe and trace metals. The thin shale seam (found close to the Viséan–Serpukhovian boundary in Polotnyanyi Zavod) has unusually high Rb and Sr values, which may contain volcanigenic material useful for absolute dating. The Gurovo Formation (Steshevian Substage of the Serpukhovian) is less enriched in Fe and Ti. In the Gurovo Formation, the transition from the lower montmorillonitic shale (Glazechnya Member) to the upper palygorskitic shale (Dashkovka Member) is expressed by a five‐fold increase in background MgO values, which indicates progressive shoaling and climatic aridization. Phosphorus remains close to 0% in the Oka Group and tends to increase in the Zaborie Group, in agreement with a dramatic increase of conodont numbers and other signatures of a lower Serpukhovian marine transgression. The lower half of the Glazechnya Member exhibits fluctuating enrichment in Fe, Cu, Ni, Pb, Zn, V, Cr, and Co. These fluctuations are mostly inverse to fluctuations of Mn. This pattern has been interpreted as a signature of seafloor oxygen deficiency, where Mn‐rich samples record oxygen‐poor environments (redox barrier level with the sediment surface) and Mn‐poor samples enriched in Fe and trace metals record transitions to anoxic setting. This interval is interpreted as the Lower Serpukhovian highstand. Enrichment in Fe, Ti, and Zr of Oka siliciclastic units of Polotnyanyi Zavod indicates provenance from the ore‐rich Voronezh Land, south of the Moscow Basin. The westerly flux regarded as a possible provenance in previous palaeogeographic reconstructions is discarded for the studied sections. The Gurovo Shale is also linked to the Voronezh province, although Fe, Ti, and Zr concentrations are lower than in the Oka shales. Copyright © 2014 John Wiley & Sons, Ltd.     

Elemental and Sr–Nd–Pb isotopic geochemistry of Late Paleozoic volcanic rocks beneath the Junggar basin, NW China: Implications for the formation and evolution of the basin basement

The basement beneath the Junggar basin has been interpreted either as a micro-continent of Precambrian age or as a fragment of Paleozoic oceanic crust. Elemental and Sr–Nd–Pb isotopic compositions and zircon Pb–Pb ages of volcanic rocks from drill cores through the paleo-weathered crust show that the basement is composed mainly of late Paleozoic volcanic rock with minor shale and tuff. The volcanic rocks are mostly subalkaline with some minor low-K rocks in the western Kexia area. Some alkaline lavas occur in the central Luliang uplift and northeastern Wulungu depression. The lavas range in composition from basalts to rhyolites and fractional crystallization played an important role in magma evolution. Except for a few samples from Kexia, the basalts have low La/Nb (<1.4), typical for oceanic crust derived from asthenospheric melts. Zircon Pb–Pb ages indicate that the Kexia andesite, with a volcanic arc affinity, formed in the early Carboniferous (345 Ma), whereas the Luliang rhyolite and the Wucaiwan dacite, with syn-collisional to within-plate affinities, formed in the early Devonian (395 and 405 Ma, respectively). Positive ε_Nd(t) values (up to +7.4) and low initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of the intermediate-silicic rocks suggest that the entire Junggar terrain may be underlain by oceanic crust, an interpretation consistent with the juvenile isotopic signatures of many granitoid plutons in other parts of the Central Asia Orogenic Belt. Variation in zircon ages for the silicic rocks, different Ba, P, Ti, Nb or Th anomalies in the mafic rocks, and variable Nb/Y and La/Nb ratios across the basin, suggest that the basement is compositionally heterogeneous. The heterogeneity is believed to reflect amalgamation of different oceanic blocks representing either different evolution stages within a single terrane or possibly derivation from different terranes.