Late Cretaceous redbeds from the Gan-Hang Belt in Southeast China: petrography and geochemistry implications for provenance,source weathering,and tectonic setting

ABSTRACT

The distinct basin and range tectonics in Southeast China were generated by crustal extension associated with subduction of the Palaeo-Pacific plate during the late Mesozoic. Compared with 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 sandstones from the Late Cretaceous Guifeng Group in the Yongchong Basin, Southeast China. Detrital grains are subangular to subrounded, poorly sorted, and rich in lithic fragments. Variable Chemical Index of Alternation values (59.55–79.82, avg. 66.79) and high Index of Compositional Variability (ICV) values (0.67–3.08, avg. 1.40) indicate an overall low degree of chemical weathering and rapid physical erosion of source rocks. Such features are consistent with an active extension tectonic setting. Other chemical indices (e.g. Al₂O₃/TiO₂, Th/U, Cr/Th, Th/Sc, Zr/Sc) also suggest significant first-cycle sediment input to the basin and a dominant felsic source nature. Thus, the Guifeng Group possibly underwent moderate to low degrees of weathering upwards. Sandstone framework models and geochemical characteristics suggest the provenance was likely a combination of passive margin (PM) and active continental margin (ACM) with minor continental island arc (CIA) tectonic settings. Sediment derivation from Neoproterozoic metamorphic rocks and Cambrian to Triassic granitoids indicates PM provenance, whereas sediments derived from Jurassic to Cretaceous granitoids suggest ACM and CIA nature. Therefore, the Late Cretaceous redbeds were deposited in a dustpan-like half-graben basin under the back-arc extension regime when Southeast China was possibly influenced by northwestward subduction of the Palaeo-Pacific plate beneath East Asia.     

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.     

Geochemical characteristics and depositional environment of the Middle Permian mudstones from central Qiangtang Basin,northern Tibet

Geochemical characteristics of the Middle Permian mudstones from the Longge Formation in the Jiaomuri area, central uplift zone of Qiangtang Basin have been analysed to constrain their provenance, tectonic setting, depositional redox condition and palaeoclimate. Based on the geochemical parameters like CIA, PIA, ICV values and Th/U, K/Rb ratios indicate that the source rocks were affected by low degree weathering conditions. The U/Th, V/Cr, Ni/Co, V/Sc ratios and U_au, C‐values of samples suggest that the mudstones were formed in an oxic environment, with arid palaeoclimate. Bulk‐rock geochemistry analysis indicates that they are mainly arkose and subarkose, derived from dominantly andesitic rocks, probably a mixing of felsic and mafic provenance. The CIA, PIA and ICV values of these samples suggest that the Longge mudstones are compositionally immature and related to moderate weathering. The tectonic discrimination diagrams, as well as critical trace and REE characteristic parameters imply an oceanic arc or continental arc setting for the depositional basin of mudstones from the Longge Formation, probably back‐arc basin. This study supports an occurrence of archipelagic oceanic basins in the Jiaomuri area in the Middle Permian, which provides evidence for the research of the Longmu Co–Shuanghu–Lancangjiang Suture. Copyright © 2015 John Wiley & Sons, Ltd.     

Provenance of Middle Jurassic clastic rocks from the Bogda area of Eastern Tianshan and its implications

ABSTRACT

The siliciclastic rocks in the Xishanyao (XSY) and Toutunhe (TTH) formations recorded the Middle Jurassic palaeogeographic evolution processes of the Bogda Mountains. To explore their provenance, the geochronological, geochemical, and petrological characteristics of the Middle Jurassic sandstones from the west Bogda area are analyzed in this study. The detrital grains of sandstones are generally poorly to moderately sorted and subangular to subrounded. Low to moderate CIA* (chemical index of alteration), PIA (plagioclase index of alteration), CIW (chemical index of weathering) and relatively high ICV (index of compositional variability) values indicate the overall low to moderate weathering intensity, low sediment recycling, and abundant first-cycle siliciclastics supply. The abundant acid volcanic lithic fragments and element contents/ratios of the sandstones indicate source areas dominated by felsic rocks. The detrital zircon U-Pb age spectrum of the lower XSY Formation is dominated by a single Carboniferous age group. Multiple Palaeozoic zircon U-Pb age groups exist in the middle-upper XSY Formation. The TTH Formation has a typical age of late Middle Jurassic. Combined with the sedimentological characteristics, it reveals that (1) the Eastern Bogda Mountains (EBM) were the only provenance for the lower XSY Formation; (2) both the Yemaquan arc and the EBM provided detritus for the middle XSY Formation; (3) the Yemaquan arc became a minor provenance for the TTH and upper XSY formations. Middle Jurassic palaeogeographic evolution in the EBM is inferred as follows: (1) during the deposition of the lower XSY Formation, the EBM suffered a rapid uplift and became an obvious positive relief; (2) the palaeorelief became high in the Yemaquan arc area and low in the EBM during the deposition of the middle XSY Formation; (3) the Yemaquan arc was considerably eroded, which caused the palaeotopography to decline sharply during the deposition of the upper XSY Formation; (4) the EBM uplifted again during the TTH Formation period, accompanied by volcanic activities at ~165Ma.     

Geochemistry of lower Jurassic shales of the Shemshak Formation, Kerman Province, Central Iran: Provenance, source weathering and tectonic setting

Lower Jurassic shales of the Shemshak Formation of Kerman Province, Central Iran, were analyzed for major and selected trace elements to infer their provenance, intensity of palaeoweathering of the source rocks and tectonic setting. Plots of shales on Al₂O₃ wt.% versus TiO₂ wt.% diagram and Cr (ppm) versus Ni (ppm) diagram indicate that acidic (granitic) rocks constitute the source rocks in the provenance. Average CIA, PIA and CIW values (84%, 92%, 93%, respectively) imply intense weathering of the source material. Plots of shales on bivariate discriminant function diagram reveal an active continental margin setting for the provenance. The inferred tectonic setting for the Lower Jurassic shales of the Shemshak Formation of Kerman Province is in agreement with the tectonic evolutionary history of the Central Iran during the Jurassic period.     

Detrital-zircon record of major Middle Triassic–Early Cretaceous provenance shift,central Mexico: demise of Gondwanan continental fluvial systems and onset of back-arc volcanism and sedimentation

New stratigraphic and petrographic data and zircon U–Pb geochronology from sandstones and volcanic rocks in the states of Queretaro and Guanajuato in central Mexico indicate an important provenance change between Late Triassic and latest Jurassic–Early Cretaceous time. The Upper Triassic El Chilar Complex consists of pervasively deformed, deep-marine olistostromes, and debris-flow deposits of arkosic and subarkosic composition. Detrital-zircon populations range from latest Palaeoproterozoic (1.65 Ga) to Middle Triassic (240 Ma), all predating the depositional age of the strata. The detrital-zircon populations are similar to those previously reported from turbidites of the Potosi fan complex of north-central Mexico and interpreted as derived from Grenville and Pan-African (Maya block) basement and Permo-Triassic arc of continental Mexico directly to the east of the basin. A single sample with a dominant Proterozoic population at ～1.65–1.30 Ga was likely derived either from the Rio Negro-Juruena province of the Amazonian craton or from a local source in the Huiznopala Gneiss, and indicates that El Chilar strata were likely deposited in the proximal part of a submarine-fan system separate from the Potosi fan.

Uppermost Jurassic–Lower Cretaceous strata of the San Juan de la Rosa Formation unconformably overlie the El Chilar Complex and likewise consist of deep-marine olistostromes, slump deposits, debris-flow deposits, and proximal fan-channel fills, but are volcanogenic litharenites with abundant felsic and vitric volcanic lithic fragments. Detrital-zircon populations are dominated by Early Cretaceous grains (150–132 Ma) with no known sources in eastern Mexico. Abundant young grains indicate a maximum depositional age of ～134 Ma (Valanginian–Hauterivian). The San Juan de la Rosa Formation is overlain by deepwater carbonates with interbedded siliciclastic beds of the Peña Azul Formation, which contains detrital-zircon ages as young as ～130 Ma, indicating possible equivalence with similar strata of the Las Trancas Formation, with a maximum depositional age of ～127 Ma and lying to the east in the Zimapan Basin, now part of the Sierra Madre Oriental fold and thrust belt. Decreasing content of volcaniclastic strata eastward indicates a volcanic source to the west. Upper Cretaceous marine strata in the Mineral de Pozos area to the northwest in the state of Guanajuato contain litharenites with a maximum depositional age near 92 Ma, and are thus part of a younger depositional system.

Composition and detrital-zircon content of the Upper Triassic and Lower Cretaceous successions in central Mexico indicates an important shift from Gondwanan continental sediment sources in the Triassic to western volcanic sources, probably on the edge of the newly opened Arperos basin, by the end of the Jurassic. This important sediment-dispersal change records the break-up of Pangea and concomitant development of arc-related sedimentary basins on the western edge of Mexico.     

Geochemistry,provenance and tectonic setting of the Late Cambrian–Early Ordovician Seydişehir Formation in the Çaltepe and Fele areas,SE Turkey

The major, trace and rare earth element (REE) contents of metapelite (MPL), metapsammite (MPS) and metamarl (MM) samples from the Cambro-Ordovician Seydi?ehir Formation were analyzed to investigate their provenance and tectonic setting. The MPS, MPL, and MM samples have variable SiO₂ concentrations, with average values of 72.36, 55.54, and 20.95 wt%, moderate SiO₂/Al₂O₃ ratios (means of 6.88, 3.23, and 3.80), moderate to high Fe₂O₃ + MgO contents (means of 5.14, 9.55, 3.56 wt%), and high K₂O/Na₂O ratios (means of 3.26, 3.64, 2.90), respectively. On average, the chemical index of alteration (CIA) values of the MPS and the MPL are 65.87 and 71.96, respectively, while the chemical index of weathering (CIW) values are 74.54 and 85.09, respectively. These data record an intermediate to high degree of alteration (weathering) of plagioclase to illite/kaolinite in the samples’ provenance. The chondrite-normalized REE patterns of all the sample groups are similar and are characterized by subparallel light rare earth elements (LREE)-enriched, relatively flat heavy rare earth elements (HREE) patterns with pronounced Eu anomalies (mean of 0.69) and moderate fractionation [average (La/Yb)_N = 8.7]. Plots of sediments in ternary diagrams of La, Th, Sc and elemental ratios (La/Sc, Th/Sc, Cr/Th, Eu/Eu*, La/Lu, Co/Th, La/Sc and Sc/Th), which are critical for determining provenance, and REE patterns indicate that the metaclastic units of the Seydi?ehir Formation were derived dominantly from felsic to intermediate magmatic rocks and not from a mafic source. The La–Sc–Th and Th–Sc–Zr/10 ternary diagrams of the Seydi?ehir Formation are typical of continental island arc/active continental margin tectonic settings. The geologic location and geochemistry of the Seydi?ehir Formation suggest that it was deposited in an Andean-type retroarc foreland basin during the Late Cambrian–Early Ordovician period. The Neoproterozoic intermediate to felsic magmatic rocks and metaclastic sediments with felsic origins of the Sand?kl?–Afyon Basement Complex (SBC) and their equivalent units, which are thought to be overlain by the younger units in the study area, may be the dominant source rocks for the Seydi?ehir Formation.     

川西南地区五峰—龙马溪组黑色页岩古气候及物源特征: 来自新地2井地球化学记录*

晚奥陶世—早志留世之交是地球历史时期中的关键时段,该时段伴有全球生物大灭绝、冈瓦纳冰川以及火山事件。晚奥陶世末期的冰川事件历时相对较短,在中国尚未找到冰川存在的直接证据。为了更好地研究奥陶—志留纪交替时期是否存在古气候的变化,选取了川西南地区新地2井岩心中新鲜的上奥陶统五峰组—下志留统龙马溪组泥页岩样品,通过其主量、微量及稀土元素特征分析,采用多种化学风化指数判定源区风化作用强度及古气候条件。新地2井五峰—龙马溪组所有岩石样品成分变异指数(ICV)均大于1,表明源岩成分成熟度低,属构造活动时期的首次沉积。稀土元素表现为轻稀土富集、重稀土平坦并伴有明显的负Eu 异常,配分模式与花岗岩相似,表明源岩以亲花岗岩、长英质为主;A-CN-K图解、主量元素Al₂O₃/TiO₂值、微量元素Cr/Zr值和Th/Sc值也体现长英质成分为主的物源特征。化学蚀变指数(CIA)、化学风化指数(CIW)和斜长石蚀变指数(PIA)等多种化学风化作用指标均指示,晚奥陶世—早志留世古风化作用强度经历中等—低等—中等、古气候温暖湿润—寒冷干燥—温暖湿润的变化过程。CIA指数的低值指示的五峰组顶部(平均值64.14)、观音桥组(平均值61.7)和龙马溪组底部(平均值64.61)即赫南特期存在短暂的寒冷气候,间接证实了冈瓦纳冰川作用在中国华南地区存在相应的地球化学记录。     

Geochemistry of the Jurassic and Upper Cretaceous shales from the Molango Region,Hidalgo, eastern Mexico: Implications for source-area weathering,provenance, and tectonic setting

This study focuses on the Jurassic (Huayacocotla and Pimienta Formations) and Upper Cretaceous (Méndez Formation) shales from the Molango Region, Hidalgo, Mexico. In this article, we discuss the mineralogy, major, and trace element geochemistry of the Mesozoic shales of Mexico. The goal of this study is to constrain the provenance of the shales, which belong to two different periods of the Mesozoic Era and to understand the weathering conditions and tectonic environments of the source region.     

Sandstone petrology and mudstone geochemistry of the Peruc–Korycany Formation (Bohemian Cretaceous Basin,Czech Republic)

We have studied the petrography and the bulk-rock geochemistry of arenites and mudstones of the Cenomanian Peruc–Korycany Formation to characterize their provenance and sedimentary history, as well as the influence of weathering, hydraulic sorting, and recycling of the source rocks. The Peruc–Korycany Formation contains sedimentary facies reflecting both meandering- and braided-river systems and shallow-marine systems. Differences in the three depositional settings did not cause distinctly different modifications of the framework compositions of the arenites. The sand from the fluvial systems is very mature (Qm₉₈F₀Lt₂). These fluvial arenites were subsequently modified by shallow-marine processes; reworking produced very slight decreases in the abundance of lithic fragments and polycrystalline quartz grains. The Cenomanian strata of the Bohemian Cretaceous Basin were derived dominantly from metasedimentary and crystalline rocks of the Palaeozoic Teplà-Barrandian and Cadomian Moldanubian units, respectively. Periods of low tectonic activity resulted in the deposition of arenites with quartzose framework compositions, indicating that climatic and/or transport/depositional-environmental controls overwhelmed factors such as source-rock compositions. Ultrastable dense minerals are useful indicators of sedimentary recycling within the Peruc–Korycanytarenites. Mudstone samples are characterized by abundant kaolinite, illite, chlorite, and quartz but by negligible amounts of goethite and gypsum. Concentrations normalized to the post-Archaean Australian shale (PAAS) show that the sediments are strongly depleted of Na, K, Ca, Sr, and Ba, probably because of the mobility of these elements during weathering. Chemical indices of alteration (CIA, CIW, and PIA) show that the degree of weathering of the source area was high. The data fall closer to the compositional fields of highly weathered minerals such as kaolinite, gibbsite, and chlorite on an A-CN-K diagram. The indices of compositional variability of the studied samples are much less than 1, suggesting that the samples are compositionally mature and were likely dominated by recycling. The elemental ratios critical of provenance (La/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th) are similar to fine fractions derived from the weathering of mostly granitoids rather than mafic rocks.     

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.     

Stratigraphy and palynostratigraphy, Karoo Supergroup (Permian and Triassic), mid-Zambezi Valley, southern Zambia

The Karoo Supergroup outcropst in the mid-Zambezi Valley, southern Zambia. It is underlain by the Sinakumbe Group of Ordovician to Devonian age. The Lower Karoo Group (Late Carboniferous to Permian age) consists of the basal Siankondobo Sandstone Formation, which comprises three facies, overlain by the Gwembe Coal Formation with its economically important coal deposits, in turn overlain by the Madumabisa Mudstone Formation which consists of lacustrine mudstone, calcilutite, sandstone, and concretionary calcareous beds. The Upper Karoo Group (Triassic to Early Jurassic) is sub-divided into the coarsely arenaceous Escarpment Grit, overlain by the fining upwards Interbedded Sandstone and Mudstone, Red Sandstone; and Batoka Basalt Formations.Palynomorph assemblages suggest that the Siankondobo Sandstone Formation is Late Carboniferous (Gzhelian) to Early Permian (Asselian to Early Sakmarian) in age, the Gwembe Coal Formation Early Permian (Artinskian to Kungurian), the Madumabisa Mudstone Late Permian (Tatarian), and the Interbedded Sandstone and Mudstone Early or Middle Triassic (Late Scythian or Anisian). The marked quantitative variations in the assemblages are due partly to age differences, but they also reflect vegetational differences resulting from different paleoclimates and different facies.The low thermal maturity of the formations (Thermal Alteration Index 2) suggests that the rocks are oil prone. However, the general scarcity of amorphous kerogen, such as the alga Botryococcus sp., and the low proportion of exinous material, indicates a low potential for liquid hydrocarbons. Gas may have been generated, particularly in the coal seams of the Gwembe Coal Formation, that are more deeply buried.     

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.     

Middle Jurassic–Early Cretaceous tectonic evolution of the Bayanhushuo area,southern Great Xing’an Range,NE China: constraints from zircon U–Pb geochronological and geochemical data of volcanic and subvolcanic rocks

This study presents new zircon U–Pb geochronology, geochemistry, and zircon Hf isotopic data of volcanic and subvolcanic rocks that crop out in the Bayanhushuo area of the southern Great Xing’an Range (GXR) of NE China. These data provide insights into the tectonic evolution of this area during the late Mesozoic and constrain the evolution of the Mongol–Okhotsk Ocean. Combining these new ages with previously published data suggests that the late Mesozoic volcanism occurred in two distinct episodes: Early–Middle Jurassic (176–173 Ma) and Late Jurassic–Early Cretaceous (151–138 Ma). The Early–Middle Jurassic dacite porphyry belongs to high-K calc-alkaline series, showing the features of I-type igneous rock. This unit has zircon ε_Hf(t) values from +4.06 to +11.62 that yield two-stage model ages (T_DM2) from 959 to 481 Ma. The geochemistry of the dacite porphyry is indicative of formation in a volcanic arc tectonic setting, and it is derived from a primary magma generated by the partial melting of juvenile mafic crustal material. The Late Jurassic–Early Cretaceous volcanic rocks belong to high-K calc-alkaline or shoshonite series and have A₂-type affinities. These volcanics have ε_Hf(t) and T_DM2 values from +5.00 to +8.93 and from 879 to 627 Ma, respectively. The geochemistry of these Late Jurassic–Early Cretaceous volcanic rocks is indicative of formation in a post-collisional extensional environment, and they formed from primary magmas generated by the partial melting of juvenile mafic lower crust. The discovery of late Mesozoic volcanic and subvolcanic rocks within the southern GXR indicates that this region was in volcanic arc and extensional tectonic settings during the Early–Middle Jurassic and the Late Jurassic–Early Cretaceous, respectively. This indicates that the Mongol–Okhotsk oceanic plate was undergoing subduction during the Early–Middle Jurassic, and this ocean adjacent to the GXR may have closed by the Late Middle Jurassic–Early Late Jurassic.     

Petrography and geochemistry of the Upper Cretaceous Nubia sandstones,Eastern Desert,Egypt: implications for provenance,paleoweathering, and tectonic setting

The provenance of the Upper Cretaceous Nubia sandstones from four vertical sections along Qena-Safaga and Qena-Quseir roads in central Eastern Desert of Egypt was investigated based on their modal composition and geochemical data. The Nubia sandstone samples are abundant in quartz content with low feldspar and lithic fragments. Their average modal composition (Q_94.2F_1.3R_4.5) classifies them as quartz arenites with subordinate quartz wackes which is consistent with geochemistry data. The average CIA, CIW, PIA, and Th/U ratio values revealed that the intensity of weathering in the studied areas was similar, varying from moderate to intensive weathering, and may reflect low-relief and warm humid climatic conditions in the source area. The ICV (<?1) and SiO₂/Al₂O₃ ratio (>?5) indicate that the Nubia sandstones are texturally and mineralogically mature. The petrographical and geochemical analyses suggest that the Nubia sandstones were mainly derived from felsic (granitic) and/or recycled sand sources. The major element-based multidimensional tectonic discrimination diagrams suggested the Nubia sandstones were deposited in a passive continental margin of a syn-rift basin. This result agreed with the general geology of central Eastern Desert of Egypt during the Upper Cretaceous.     

http://www.sciencedirect.com/science/article/pii/S1674987113000352

The Yidun Group extends from the Shangri-La region to the south and the Changtai region to the north,and is an important component of the Triassic Yidun arc in the eastern Tibetan plateau.It is composed of the Lieyi,Qugasi,Tumugou and Lanashan Formations from the base upward.Both the Lieyi and Lanashan Formations consist dominantly of black or gray slate and sandstone,whereas the Qugasi and Tumugou Formations have variable amounts of mafic to felsic volcanic rocks and turfs accompanied with gray slate and sandstone.Sandstone from the Yidun Group has variable CIA values from 55 to 76,indicative of mild to moderate weathering condition for the source rocks.All the sandstones define a general weathering trend nearly parallel to the A-CN boundary in the A-CN-K triangular diagram,implying limited effect of diagenetic and post-depositional K-metasomatism.Dominant detrital quartz and feldspar grains of the sandstones suggest predominantly felsic sources.Relatively high Y/Ni and low Cr/V ratios of sandstones from the Yidun Group indicate more contribution from felsic than mafic sources.Similarly,the Yidun sandstones have Co/Th and La/Sc ratios generally similar to upper continental crust （UCC） and cluster between UCC and felsic sources,indicating felsic rocks as primary sources.Granodiorite represents the average chemical composition of sources as evaluated by extending the predicted weathering trend back to the feldspar join in A-CN-K diagram.Prominently high Zr/Sc ratio or Hf concentration and Paleoproterozoic Nd modal ages （1.94-2.21 Ga）point to input of recycling components derived from old sedimentary source in a relatively stable tectonic setting.     

Geochemistry of Lower Jurassic sandstones of Shemshak Formation,Kerman basin,Central Iran: Provenance,source weathering and tectonic setting

Lower Jurassic sandstones of Shemshak Formation of Kerman basin, central Iran were analyzed for major and select trace elements to infer their provenance, palaeoweathering of source rocks and tectonic setting. Average modal framework components (Qt: F: L= 67.25: 2.41: 30.48) and chemical composition of the sandstones classify them as litharenites. The sandstones are quartz-rich (～ 67% quartz; 75.34 wt.% SiO₂) and derived from a recycled orogen composed of quartzose sedimentary rocks. Average CIA, PIA and CIW values (69%, 76% and 80%, respectively) indicate moderate to intense chemical weathering of the source material. The inferred index of weathering/alteration is the sum total of intensities of weathering witnessed by the lithocomponents during atleast two cycles of sedimentation involving (1) chemical weathering of the source rocks («ultimate» granodiorite source and «proximal» quartzose sedimentary source), (2) chemical weathering during fluvial transport of the detritus, (3) chemical weathering of the detritus in depocenters, and (4) chemical weathering during diagenesis. Sandstones exhibit moderate maturity and were deposited under humid climatic conditions. Plots of the chemical analyses data on tectonic setting discrimination diagrams indicate active continental margin setting, which is in agreement with the tectonic evolutionary history of the Central Iran during Jurassic period.     

准噶尔盆地玛湖凹陷晚二叠世至中三叠世古气候、物源及构造背景

二叠纪-三叠纪之交重大地质转折期,海相地质记录指示全球发生了一系列显著的生物和环境事件, 但是,该时期陆相古气候、古风化作用等方面的研究还很薄弱,争议较大.为了恢复新疆准噶尔盆地玛湖凹陷上二叠统乌尔禾组至中三叠统克拉玛依组的古气候、物源特征等, 本研究对玛湖凹陷钻井岩心中的泥岩样品开展了全岩主量和微量元素测试,采用多种化学风化指数判定源区风化程度及古气候条件,通过多种地球化学比值及图解来恢复源岩岩性及其构造背景.化学蚀变指数(Chemical Index of Alteration, CIA)、化学风化指数(Chemical Index of Weathering, CIW)、Parker风化指数(Weathering Index of Parker, WIP)和斜长石蚀变指数(Plagioclase Index of Alteration, PIA)等多种化学风化作用指标均指示,玛湖凹陷自晚二叠世至早三叠世发生显著的风化作用变化,由低等程度的化学风化作用转变为中等程度的化学风化作用,某些季节可能会较为温暖湿润,且在整个早三叠世保持大致稳定,在早三叠世晚期稍减弱,这与锶同位素反映的全球风化作用变化趋势一致.中三叠世时的化学风化作用与早三叠世相比,并未降低,反而稍有增加,这可能代表了地区性事件.早三叠世化学风化作用显著增强的原因可能在于全球变暖、植被破坏及季节性降雨增加等.此外,上二叠统乌尔禾组至中三叠统克拉玛依组的物源岩性主要为长英质火成岩,源岩形成时的构造背景可能为大洋岛弧环境.      

松辽盆地东南隆起区上白垩统嫩江组泥页岩段地球化学特征与物源分析

嫩江组泥岩的地球化学分析结果表明:泥岩中含有一定量的铁镁质组分和较多的稳定组分。微量元素和稀土元素分析结果显示,嫩江组含油页岩段沉积时期存在多个物源。CIA和PIA值没有随着埋深的减少而增加,说明嫩江组泥岩受后期风化影响很小。CIA值为70.5~77.0,Th/U值为2.67~3.98,说明泥岩源区经历了中等强度风化作用。Zr/Sc—Th/Sc图解表明泥岩的化学成分受源岩成分控制。通过源岩和构造背景判别图以及前人资料可知,松辽盆地嫩江组母岩主要为海西期花岗岩,源岩的构造背景为大陆边缘造山带火山岛弧岩系。物源中输入较多的铁镁及含钙物质,促进油页岩的形成。     

满洲里地区灵泉盆地中生代火山岩的锆石U-Pb年代学、地球化学及其地质意义

本文对满洲里地区灵泉盆地中生代火山岩进行了锆石LA-ICP-MS U-Pb年代学和岩石地球化学研究,以便对其岩石成因和区域构造演化给予制约。研究区内9个代表性火山岩中的锆石均呈自形-半自形晶,显示出典型的岩浆生长环带或条痕状吸收,结合其高的Th/U比值(0.25~7.78),暗示其岩浆成因。测年结果表明,研究区中生代火山岩可分为3期,分别以中侏罗世(约166Ma)塔木兰沟组玄武岩类、早白垩世早期(约142Ma)吉祥峰组流纹岩类和早白垩世晚期(约125Ma)上库力组流纹岩类和伊列克得组玄武岩类为代表。中侏罗世(塔木兰沟组)火山岩具有富碱、高钾和高的La/Yb、La/Nb、Hf/Sm、Nb/U和Ce/Pb比值,以及富集大离子亲石元素(LILEs)、轻稀土元素(LREEs)和亏损Nb等特征,暗示其形成应与陆壳加厚之后的岩石圈伸展作用有关;早白垩世早期(吉祥峰组)火山岩具有高硅、富碱、富K、Rb、Th及低Al、Mg、Ca、Ni、Cr、Ti等特点,类似于A型流纹岩,揭示其形成于陆内伸展环境;早白垩世晚期火山岩具双峰式组合特点,基性端元具有富碱、高钾、高度富集LILEs和LREEs,类似于钾玄质玄武岩,而酸性端元显示A型流纹岩特点,揭示该期火山岩应形成于一种与裂谷发育过程相似的强烈引张环境。结合区域中生代火山岩的空间展布特征,认为中-晚侏罗世和早白垩世早期火山岩的形成应与蒙古-鄂霍茨克缝合带的演化有关,而早白垩世晚期双峰式火山岩的形成应与太平洋板块向欧亚大陆的俯冲作用相联系。