Stratigraphic evolution of a fluvial–eolian succession: The example of the Upper Jurassic—Lower Cretaceous Guará and Botucatu formations, Paraná Basin, Southernmost Brazil

The Guará and Botucatu formations comprise an 80 to 120 m thick continental succession that crops out on the western portion of the Rio Grande do Sul State (Southernmost Brazil). The Guará Formation (Upper Jurassic) displays a well-defined facies shift along its outcrop belt. On its northern portion it is characterised by coarse-grained to conglomeratic sandstones with trough and planar cross-bedding, as well as low-angle lamination, which are interpreted to represent braided river deposits. Southwards these fluvial facies thin out and interfinger with fine- to medium-grained sandstones with large-scale cross-stratification and horizontal lamination, interpreted as eolian dune and eolian sand sheets deposits, respectively. The Botucatu Formation is characterised by large-scale cross-strata formed by successive climbing of eolian dunes, without interdune and/or fluvial accumulation (dry eolian system). The contact between the Guará and the Botucatu formations is delineated by a basin-wide deflation surface (supersurface). The abrupt change in the depositional conditions that took place across this supersurface suggests a major climate change, from semi-arid (Upper Jurassic) to hyper-arid (Lower Cretaceous) conditions. A rearrangement of the Paraná Basin depocenters is contemporaneous to this climate change, which seems to have changed from a more restrict accumulation area in the Guará Formation to a wider sedimentary context in the Botucatu Formation.     

Petrogenesis of Cretaceous adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province (eastern China): Implications for geodynamics and Cu–Au mineralization

Both adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province, eastern China are associated with Cretaceous Cu–Au mineralization. The Shaxi quartz diorite porphyrites exhibit adakite-like geochemical features, such as light rare earth element (LREE) enrichment, heavy REE (HREE) depletion, high Al₂O₃, MgO, Sr, Sr / Y and La / Yb values, and low Y and Yb contents. They have low ε_Nd(t) values (− 3.46 to − 6.28) and high (⁸⁷Sr / ⁸⁶Sr)_i ratios (0.7051–0.7057). Sensitive High-Resolution Ion Microprobe (SHRIMP) zircon analyses indicate a crystallization age of 136 ± 3 Ma for the adakitic rocks. Most volcanic rocks and the majority of monzonites and syenites in the Luzong area are K-rich (or shoshonitic) and were also produced during the Cretaceous (140–125 Ma). They are enriched in LREE and large-ion lithophile elements, and depleted in Ti, and Nb and Ba and exhibit relatively lower ε_Nd(t) values ranging from − 4.65 to − 7.03 and relatively higher (⁸⁷Sr / ⁸⁶Sr)_i ratios varying between 0.7057 and 0.7062. The shoshonitic and adakitic rocks in the Luzong area have similar Pb isotopic compositions (²⁰⁶Pb / ²⁰⁴Pb = 17.90–18.83, ²⁰⁷Pb / ²⁰⁴Pb = 15.45–15.62 and ²⁰⁸Pb / ²⁰⁴Pb = 38.07–38.80). Geological data from the Luzong area suggest that the Cretaceous igneous rocks are distributed along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China and were likely formed in an extensional setting within the Yangtze Block. The Shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of > 50 km (i.e., 1.5 GPa), possibly leaving rutile-bearing eclogitic residue. The shoshonitic magmas, in contrast, originated mainly from an enriched mantle metasomatized by subducted oceanic sediments. They underwent early high-pressure (> 1.5 GPa) fractional crystallization at the boundary between thickened (> 50 km) lower crust and lithospheric mantle and late low-pressure (< 1.5 GPa) fractional crystallization in the shallow (< 50 km) crust. The adakitic and shoshonitic rocks appear to be linked to an intra-continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China. Both the shoshonitic and adakitic magmas were fertile with respect to Cu–Au mineralization.     

Aeschnidiid nymphs from the Jehol biota (latest Jurassic–Early Cretaceous), China, with a discussion of the family Aeschnidiidae (Insecta, Odonata)

Undoubted fossil nymphs of the family Aeschnidiidae within the Aeschnidioidea of the Anisoptera, Odonata are identified and described and their phylogenetic relationship and bioecological characteristics are deduced. The nymphs discovered previously in China, Mongolia and Russia and considered to be aeschnidiids are, in fact, unrelated to this group. All nine Chinese species in six genera can be merged into one alone, Aeschnidium heishankowense. It is the first animal from the ‘Jehol biota’ of East Asia to be linked with the species known from the Lower Tithonian of Germany. The geological age of the oldest strata bearing the fossil nymphs is debatable; it could be latest Jurassic rather than Early Cretaceous.     

Geochemical Characterization of Magnetite and Geological Setting of the Iron Oxide ± Iron Silicate ± Iron Carbonate Rich Pb–Zn Sulfides from the La Unión and Mazarrón Stratabound Deposits (SE Spain)

From a mineralogical point of view the La Unión ore field (SE Spain) can be regarded as an oddity as some of its hydrothermal, stratabound type deposits display an extremely unusual paragenesis comprising magnetite ± greenalite ± minnesotaite ± siderite and galena ± sphalerite (IOSC – LZS). Recent mineralogical studies have shown that this paragenesis is also present at the neighboring Mazarrón ore field. These ore fields share a similar geologic setting, involving metamorphic and sedimentary rocks (Paleozoic s.l. to Permian) hosting late Miocene high‐K calc‐alkaline volcanic and subvolcanic rocks. The latter have andesitic to dacitic composition, and triggered hydrothermal activity and ore deposition. This study discusses the detailed mineral chemistry of magnetite samples from Mazarrón and La Unión and provides some hints for the origin of the IOSC – LZS paragenesis. We performed electron microprobe (EPMA) analyses in magnetite samples from La Unión and Mazarrón to determine the contents of minor and trace elements (Zn, Ni, Mn, Cr, V, K, Ca, Ti, Al, Si, Mg). Given that some results fell below the detection limit for the EPMA instrumental conditions we used robust regression on order statistics (robust ROS), with the NADA package in R to deal with these data sets. The Ca + Al + Mn contents in magnetites from San Cristóbal and Emilia are equivalent to the mean contents of those of IOCG, Kiruna, BIF, Cu porphyry, skarn, VMS, hydrothermal and clastic Pb–Zn deposits, but they are low in Ti + V and Ni + Cr. The principal components analysis indicates that Zn, Ni, Cr, V, K, Ca, Ti, and Mg are roughly collinear, therefore correlated, being also independent from Fe. Besides, the function discriminant analysis of data shows that the magnetites from Emilia and San Cristóbal cluster in differentiated groups, thus probably reflecting some differences related to the distance to the magmatic source that triggered the hydrothermal system: proximal in the case of San Cristóbal and distal in Emilia.     

Seismotectonics of strike–slip earthquakes within the deep crust of southern Italy: Geometry, kinematics, stress field and crustal rheology of the Potenza 1990–1991 seismic sequences (Mmax 5.7)

We present a revision and a seismotectonic interpretation of deep crust strike–slip earthquake sequences that occurred in 1990–1991 in the Southern Apennines (Potenza area). The revision is motivated by: i) the striking similarity to a seismic sequence that occurred in 2002  140 km NNW, in an analogous tectonic context (Molise area), suggesting a common seismotectonic environment of regional importance; ii) the close proximity of such deep strike–slip seismicity with shallow extensional seismicity (Apennine area); and iii) the lack of knowledge about the mechanical properties of the crust that might justify the observed crustal seismicity. A comparison between the revised 1990–1991 earthquakes and the 2002 earthquakes, as well as the integration of seismological data with a rheological analysis offer new constraints on the regional seismotectonic context of crustal seismicity in the Southern Apennines. The seismological revision consists of a relocation of the aftershock sequences based on newly constrained velocity models. New focal mechanisms of the aftershocks are computed and the active state of stress is constrained via the use of a stress inversion technique. The relationships among the observed seismicity, the crustal structure of the Southern Apennines, and the rheological layering are analysed along a crustal section crossing southern Italy, by computing geotherms and two-mechanism (brittle frictional vs. ductile plastic strength) rheological profiles. The 1990–1991 seismicity is concentrated in a well-defined depth range (mostly between 15 and 23 km depths). This depth range corresponds to the upper pat of the middle crust underlying the Apulian sedimentary cover, in the footwall of the easternmost Apennine thrust system. The 3D distribution of the aftershocks, the fault kinematics, and the stress inversion indicate the activation of a right-lateral strike–slip fault striking N100°E under a stress field characterized by a sub-horizontal N142°-trending σ1 and a sub-horizontal N232°-trending σ3, very similar to the known stress field of the Gargano seismic zone in the Apulian foreland. The apparent anomalous depths of the earthquakes (> 15 km) and the confinement within a relatively narrow depth range are explained by the crustal rheology, which consists of a strong brittle layer at mid crustal depths sandwiched between two plastic horizons. This articulated rheological stratification is typical of the central part of the Southern Apennine crust, where the Apulian crust is overthrusted by Apennine units. Both the Potenza 1990–1991 and the Molise 2002 seismic sequences can be interpreted to be due to crustal E–W fault zones within the Apulian crust inherited from previous tectonic phases and overthrusted by Apennine units during the Late Pliocene–Middle Pleistocene. The present strike–slip tectonic regime reactivated these fault zones and caused them to move with an uneven mechanical behaviour; brittle seismogenic faulting is confined to the strong brittle part of the middle crust. This strong brittle layer might also act as a stress guide able to laterally transmit the deviatoric stresses responsible for the strike–slip regime in the Apulian crust and may explain the close proximity (nearly overlapping) of the strike–slip and normal faulting regimes in the Southern Apennines. From a methodological point of view, it seems that rather simple two-mechanism rheological profiles, though affected by uncertainties, are still a useful tool for estimating the rheological properties and likely seismogenic behaviour of the crust.     

Provenance of the Neoproterozoic Maricá Formation (Sul-rio-grandense Shield, Southern Brazil): Petrographic and Sm–Nd isotopic constraints

This paper reports the integrated application of petrographic and Sm–Nd isotopic analyses for studying the provenance of the Neoproterozoic Maricá Formation, southern Brazil. This unit encompasses sedimentary rocks of fluvial and marine affiliations. In the lower fluvial succession, sandstones plot in the “craton interior” and “transitional continental” fields of the QFL diagram. Chemical weathering probably caused the decrease of the ¹⁴⁷Sm/¹⁴⁴Nd ratios to 0.0826 and 0.0960, consequently lowering originally > 2.0 Ga T_DM ages to 1.76 and 1.81 Ga. ¹⁴³Nd/¹⁴⁴Nd ratios are also low (0.511521 to 0.511633), corresponding to negative ε_Nd present-day values (− 21.8 and − 19.6). In the intermediate marine succession, sandstones plot in the “dissected arc” field, reflecting the input of andesitic clasts. Siltstones and shales reveal low ¹⁴³Nd/¹⁴⁴Nd ratios (0.511429 to 0.511710), ε_Nd values of − 18.1 and − 23.6, and T_DM ages of 2.16 and 2.37 Ga. Sandstones of the upper fluvial succession have “dissected arc” and “recycled orogen” provenance. ¹⁴³Nd/¹⁴⁴Nd isotopic ratios are also relatively low, from 0.511487 to 0.511560, corresponding to ε_Nd values of − 22.4 and − 21.0 and T_DM of 2.07 Ga. A uniform granite–gneissic basement block of Paleoproterozoic age, with subordinate volcanic rocks, is suggested as the main sediment source of the Maricá Formation.     

Cretaceous evolution of a metamorphic core complex,the Veporic unit,Western Carpathians (Slovakia): P–T conditions and in situ40Ar/39Ar UV laser probe dating of metapelites

Alpine metamorphism, related to the development of a metamorphic core complex during Cretaceous orogenic events, has been recognized in the Veporic unit, Western Carpathians (Slovakia). Three metamorphic zones have been distinguished in the metapelites: 1, chloritoid + chlorite + garnet; 2, garnet + staurolite + chlorite; 3, staurolite + biotite + kyanite. The isograds separating the metamorphic zones have been modelled by discontinuous reactions in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH). The isograds are roughly parallel to the north‐east‐dipping foliation related to extensional updoming along low‐angle normal faults. Thermobarometric data document increasing P–T conditions from c. 500 °C and 7–8 kbar to c. 620 °C and 9–10 kbar, reflecting a coherent metamorphic field gradient from greenschist to middle amphibolite facies. ⁴⁰Ar/³⁹Ar data obtained by high spatial resolution in situ ultraviolet (UV) laser ablation of white micas from the rock slabs constrain the timing of cooling and exhumation in the Late Cretaceous. Mean dates are between 77 and 72 Ma; however, individual white mica grains record a range of apparent ⁴⁰Ar/³⁹Ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous. The reconstructed metamorphic P–T–t path is ‘clockwise’, reflecting post‐burial decompression and cooling during a single Alpine orogenic cycle. The presented data suggest that the Veporic unit evolved as a metamorphic core complex during the Cretaceous growth of the Western Carpathian orogenic wedge. Metamorphism was related to collisional crustal shortening and stacking, following closure of the Meliata Ocean. Exhumation was accomplished by synorogenic (orogen‐parallel) extension and unroofing in an overall compressive regime.