The northern part of the Kedougou Kenieba Inlier (Eastern Senegal) is composed of Palaeoproterozoic volcano‐plutonic and volcano‐sediment areas cross‐cut by differentiated plutonic bodies of gabbroic to granitic composition. They are emplaced in an arc‐type environment and accreted during the Eburnean Orogeny. We present here new K–Ar ages obtained on several plutons from different mineralogical phases, each characterized by a different closure temperature for the K–Ar system. Ages from hornblende, between 2013 ± 29 and 2141 ± 30 Ma, constrain the cooling history of the magma bodies through the 550 °C isotherm. They are slightly younger than U–Pb ages previously obtained from the same units, attesting of a rapid cooling, tectonically controlled, of about 7 °C/Myr. Ages obtained on biotite minerals range from 1955 ± 28 to 1893 ± 27 Ma, suggesting that from 550 to 325 °C, cooling rate decreased to about 3 °C/Myr. Significantly younger ages ranging from 1704 ± 24 to 1615 ± 23 Ma have been obtained on plagioclase and microcline. They can be interpreted in terms of either a slow cooling with a rate of only 1 °C/Myr, from about 1.9 to 1.6 Ga, or either reflecting the occurrence of a rather limited thermal event younger than 1.7 Ga. Since no age younger than 2.0 Ga has previously been obtained for plutonic rocks from the early Birimian in the West African Craton, ages of 1.7–1.6 Ga obtained here from feldspars might be the first evidence for a regional re‐heating event having occurred in the East–North Kedougou Kenieba inlier. Copyright © 2015 John Wiley & Sons, Ltd.
The Mangxiang Formation black shales are the most important hydrocarbon source rocks in the Wuyu Basin. Trace and rare earth element (REEs) of the black shales from the Wuyu Basin were studied in order to understand their depositional environments and palaeoclimate. Thirty one black shale samples from the Wuyu Basin were analysed by inductively coupled plasma–mass spectrometry and X‐ray fluorescence. The black shales are characterized by moderate SiO2 (52.55–58.73%) contents and medium K2O/Na2O (2.21–4.83) ratio values but relatively high MgO + Fe2O3 (5.8–6.91%) and Al2O3 (15.1–17.5%) contents. The Chemical Weathering Index of Alternation (CIA) ranges from 57 to 70, together with medium Th/U (2.66–4.97) ratio values, reflecting a weak to moderate degree of chemical weathering of the source area. The palaeoredox condition of black shale was slightly oxic (or dysoxic) during black shale deposition as evidenced by slightly Ce anomalies (0.92–0.99) and Mn enrichment (EF = 1.6). The moderate palaeosalinity values (9.32‰–17.75‰), together with medium B/Ga (3.90–5.57) ratio values, indicate a brackish water environment. The palaeoclimate index ∑(Fe + Mn + Cr + Ni + V + Co)/∑(Ca + Mg + Sr + Ba + K + Na) ranges from 0.28 to 0.50 and low Sr/Ba (0.15–1.22) ratio values, indicating a semiarid to semimoist climatic condition during the sedimentation of the black shale. The high ω(La)N/ω(Yb)N ratio values (1.02–1.09) indicate a fast sedimentary rate during black shale deposition. In this study, a preservation model of the Mangxiang Formation black shale was established. The model indicates that excellent preservation may be the major controlling factor for the accumulation of organic matter. Copyright © 2015 John Wiley & Sons, Ltd.
In order to evaluate rare earth elements (REEs) as a potential proxy for solid bitumen classification, we employed traditional correlation approaches, such as carbon isotopes and V/(V + Ni) ratios, to infer the source rocks of Sinian–Cambrian reservoir solid bitumens in the Sichuan Basin and analyse solid bitumens for their REE compositions by inductively coupled plasma–mass spectrometry. Our data suggested that Sinian–Cambrian reservoir solid bitumens were primarily sourced from the lower Cambrian shales deposited under anoxic–euxinic conditions. REE and their associated parameters seemed to be more sensitive to classify solid bitumens than traditional correlation approaches. REE concentrations played a dominant role and fractionation degree between light and heavy REE and played a secondary role in solid bitumen classification. REE concentration and pattern in solid bitumens might be controlled by two processes, including inheritance from source rocks and water–rock interaction. The major factor controlling REE concentrations in solid bitumens was the type of organic matter of source rocks. Authigenic minerals in solid bitumens formed by water–rock interaction appeared to have little influence on REE compositions of solid bitumens because of their trace contents. Overall, REE could be used as a novel complementary approach to solid bitumen classification in complicated petroleum systems. Copyright © 2015 John Wiley & Sons, Ltd.
Late Viséan brachiopods from the eastern Tafilalt (Morocco), and more precisely from the area comprised between the Jebel Begaa to the southwest, and Gara El Itima to the northeast (close to the Algerian border), are described systematically for the first time. Despite the limited material available, 18 species belonging to 17 genera have been recognized within the limestone beds of the Merdani and Zrigat formations, in which the productides (Productidina) and spiriferides are the most diverse. Representatives of the subfamily Gigantoproductidinae, which are close, if not conspecific, to those present in contemporaneous rocks of the nearby Béchar Basin (Algeria), occur in the late Viséan Zrigat Formation. Additional research based on more abundant material is necessary to investigate thoroughly the relationships existing between the Béchar Basin and the Tafilalt, which may have been part of the former during the Carboniferous. Copyright © 2015 John Wiley & Sons, Ltd.
Combined stratigraphic and bio‐magnetostratigraphic analyses on a fluvial sequence from the southern margin of the Alessandria Basin (at the junction between the Alps and the Apennines) provide evidence for the first documentation of a late Piacenzian–Gelasian continental record in northern Italy. The basin experienced general growth at the margins and ongoing subsidence in the depocentre, during overthrusting onto the Po Foreland Basin. The studied fluvial succession is 25–30 m thick and can be divided into three units (MRZ1, MRZ2 and MRZ3) delimited by erosional unconformities (S1, S2 and S3). The MRZ1 and MRZ2 units (shallow to confined braided systems) are chronologically restricted between 2.8 and 2.58 Ma based on bio‐magnetostratigraphic analyses. The overall stratigraphic framework indicates deposition and erosion during the progressive uplift of the Alps/Apennines system. The frequency of unconformities and sedimentary changes, compared to other coeval successions of other syn‐orogenic basins of northern and central Italy, suggests the interaction between high‐amplitude global cold stages and tectonic deformation. Tectonic climax occurred in the Gelasian, as revealed by the 450‐ to 600‐ka‐long hiatus associated with the S3 unconformity, whereas the subsequent deposition of the MRZ3 unit (meandering alluvial system) including a Gelasian magnetic reversal points to the fading of tectonic activity. The stratigraphic reconstruction provided in this study enriches the current knowledge of the European continental biota: the absence of thermophilous elements in the MRZ1 carpoflora reflects the late Piacenzian cooling phase after 2.8 Ma, whereas the persistence of several extra‐European taxa in MRZ3 (e.g. Ampelopsis cf. ludwigii, Boehmeria lithuanica, Cryptomeria rhenana) rules out an age younger than 1.4 Ma. The freshwater mollusc assemblage of MRZ2 shows the occurrence of Pomatias elegans and Tournouerina belnensis, identified for the first time in the Pliocene of northern Italy. Copyright © 2015 John Wiley & Sons, Ltd.
The layered gabbro‐anorthosite suite in the eastern Indian shield is ideal for a texture based study of the crystallization history of a mafic magma in a static magma chamber. The magmatic suite comprises 40–150 m thick persistent layers of different lithounits, which have been divided into two cycles. The lower cycle (I) is represented from bottom to top by gabbro, gabbronorite, leucogabbronorite, anorthositic gabbronorite and anorthositic gabbro. The upper cycle (II) is similar to cycle I, except that the anorthositic gabbronorite and anorthositic gabbro units are not present there. The characteristic textures of different lithounits in each cycle repeats with repetition in lithology. Distinctive textures observed in the suite have been classified into three categories, Types I, II and III. Adcumulus growth was the principle crystallization mechanism for Types I and III textures, which are characteristically present in the lower layers of cycle I and cycle II. The upper layers of cycle I show Type II texture, typically with bimodal size distribution of plagioclase, and pyroxene oikocrysts. Thus, Type II texture indicates a possible interplay between adcumulus growth and either polybaric crystallization or textural coarsening processes. Electron microprobe analysis (EPMA) data indicate uniform composition of plagioclase (An76–85), both from inclusions in pyroxene oikocrysts and from groundmass grains, hence ruling out the possibility of involvement of the polybaric crystallization in the layers with Type II texture. Crystal size distribution (CSD) analysis of plagioclase grains stands for textural coarsening process, and the graphical plots have been utilized to quantify the process. P‐T calculation using EPMA data from selected pyroxene grains suggests that the textural coarsening took place at about 13.3 kb pressure and 990 °C temperature. We propose that adcumulus growth operating in a static magma chamber for a long period of time might have created conditions conducive for textural coarsening process to take place. Copyright © 2015 John Wiley & Sons, Ltd.
Abundant Cu and Mo deposits are spatially related to granitic complexes in the Nuomin area, eastern Da Hinggan Mountains, China. The petrogenesis of these deposits is investigated using data from zircon U–Pb dating, whole‐rock major and trace element geochemistry, and Sr–Nd–Pb isotopic compositions. The results show that the fine‐grained biotite monzogranite and medium‐grained biotite syenogranite formed in a Late Jurassic (155.7 ± 3.6 Ma) collision environment, characterized by fluid–melt interaction and plagioclase differentiation. These rocks belong to the high‐K calc‐alkaline, K‐peraluminous series, indicating that the parent magma evolved from the low‐temperature partial melting of lower crust, with geochemistry consistent with Neoproterozoic oceanic crust. The monzonitic diorite porphyrite, monzonite porphyry (with a crystallization age of 130.64 ± 0.74 Ma) and quartz monzonite porphyry belong to the high‐K calc‐alkaline, sodic, quasi‐aluminous series. The results indicate petrogenesis in a post‐collisional or late‐orogenic tectonic environment, characterized by interactions with CO2‐rich fluid and the fractional crystallization of amphibole, biotite and apatite. The parent magma formed from the partial melting of residual Neoproterozoic oceanic crust. The circulation of mineralized fluids was related to the petrogenesis of the monzonitic diorite porphyrite, monzonite porphyry and quartz monzonite porphyry. Copyright © 2015 John Wiley & Sons, Ltd.