The Early Carboniferous Xiaomiaogou granite porphyry dykes in the northern margin of the North China Craton: implication for crust–mantle interaction and intraplate magmatism

The age, petrogenesis and geodynamic implication of Xiaomiaogou granite porphyry dykes in the northern margin of the North China Craton were investigated using geochronology, geochemistry and Sr–Nd–Pb–Hf isotopic data. LA‐MC‐ICP‐MS zircon U–Pb dating yielded the concordant 206Pb/238U weighted mean age of 350.4 ± 1.4 Ma for the dykes, suggesting they were the productions of the Early Carboniferous magmatism in the northern margin of the North China Craton. The granite porphyries display the pronounced A‐type affinities: apart from higher zircon saturation temperature, they are high‐K calc‐alkaline and metaluminous, with high concentrations of K2O + Na2O varying from 7.93 to 8.55 wt.%, incompatible elements (e.g. Zr, Y, Nb and Ce), REEs and 10 000 Ga/Al ratios but less obvious LREE and HREE fractionation. With the lower initial 87Sr/86Sr ratios (0.7055 to 0.7070), negative values of εNd(t) (−7.1 to −4.9) and εHf(t) (−11.2 to −8.0) and old Nd (1505 to 1681 Ma) and Hf (1861 to 2061 Ma) model ages, all the rocks exhibit the similar geochemical compositions to the melts obtained in experimental studies by dehydration melting of meta‐basalts, implying a lower continental crust origin. However, the high Mg# values (47.53 to 53.60) and subcontinental lithospheric mantle‐like εNd(t), εHf(t) and Pb isotopic compositions ((206Pb/204Pb)i = 17.296–17.766, (207Pb/204Pb)i = 15.460–15.490 and (208Pb/204Pb)i = 37.097–37.568) indicate the involvement of melts from subcontinental lithospheric mantle. Combined with the special features of mafic microgranular enclaves occurring within the granite porphyries, mixing of the lower crust and subcontinental lithospheric mantle of the North China Craton is proposed for the genesis of granite porphyry dykes in this study. During the hybrid magma ascending, pronounced fractional crystallization and crustal assimilation may have also occurred. Based on geochemical criteria, the Xiaomiaogou granite porphyries can be classified into A1 group. Characterized by the compositional features of anorogenic granites, an intraplate setting is envisaged for the formation of Xiaomiaogou granite porphyry dykes. Such a tectonic environment may be related to the northward drift of the North China Craton during rifting away from East Gondwana. Copyright © 2016 John Wiley & Sons, Ltd.

Identification of A‐type granite in the southeastern Kunlun Orogen, Qinghai Province, China: implications for the tectonic framework of the Eastern Kunlun Orogen

The Dalijigetang syenogranite in the eastern section of the Eastern Kunlun Orogen (EKO), China, is rich in SiO2 and alkali elements, has high FeOT/MgO ratios and is strongly depleted in CaO and MgO. Chondrite‐normalized REE patterns show light REEs enrichment, heavy REEs depletion and negative Eu anomalies (δEu = 0.43–0.53). The 104Ga/Al values range from 2.69 to 10.31. The combined total concentrations of Zr, Nb, Ce and Y (mean = 420.04 ppm) indicate that the Dalijigetang syenogranite is an A‐type granite. The zircon εHf(t) values relatively are all moderately negative (−6.1 to −10.7), and the model ages (tDM2) range from 1666 to 1960 Ma (mean = 1801 Ma), which is much older than zircon U–Pb ages (259 Ma). Based on integrated isotopic and geochemical data, we infer that the syenogranite formed by the partial melting of lower crustal material under low‐pressure and high‐temperature conditions. Based on the above data, combined with the regional geological background, we conclude that the Dalijigetang A‐type syenogranite formed during an end‐early Permian period of extension between two episodic subduction events within the EKO. This suggests that the onset of northward subduction of the Palaeo‐Tethyan Ocean plate might have commenced as early as the early Permian. Copyright © 2016 John Wiley & Sons, Ltd.

Trace fossils from the Middle and Upper Eocene (Bartonian–Priabonian) molasse deposits of the Pamplona Basin (Navarre, western Pyrenees): palaeoenvironmental implications

An ichnological study was carried out in the Middle–Upper Eocene sedimentary succession of the Pamplona Basin. This succession represents the molasse stage of the western part of the South Pyrenean peripheral foreland basin and extends from deep‐marine turbiditic (Ezkaba Sandstone Formation) to deltaic (Pamplona Marl, Ardanatz Sandstone and Ilundain Marl formations) and marginal marine deposits (Liédena Sandstone Member, Gendulain Formation). The tectonosedimentary evolution of the basin can also be recognized by means of the ichnoassemblages preserved throughout the succession and by the ichnofacies analysis. A total of 23 ichnogenera and at least 28 ichnospecies have been identified. The assemblage of the Ezkaba Sandstone is characteristic of the deep‐sea Nereites ichnofacies. The Ophiomorpha rudis and the Paleodictyon ichnosubfacies can be differentiated in this unit, which are typical of channel axis and off‐axis environments. Trace fossils in the Ardanatz Sandstone and Ilundain Marl formations, with dominance of Thalassinoides and especially Ophiomorpha, could be related to the Cruziana ichnofacies, developed in relatively low‐energy environments, in semicohesive muddy substrates with intercalated silt and sand. The ichnofauna of the Liédena Sandstone Member is composed of bird tracks, and invertebrate horizontal and vertical structures. Microbial mat impressions also occur. The evidence suggests that microbial mats acted as a taphonomic bias favouring the preservation of vertebrate tracks and conditioning the presence of other trace fossils. Consequently, the assignation of this ichnofauna to a specific ichnofacies is tentative, most likely a combination of the Mermia and Scoyenia ichnofacies. Copyright © 2016 John Wiley & Sons, Ltd.

New conodont taxa and biozones from the Lower Ordovician of the Cordillera Oriental, NW Argentina

The upper strata of the Santa Rosita and the Acoite formations (Lower Ordovician) exposed at Chulpíos Creek in the Santa Victoria study area of the Cordillera Oriental, NW Argentina, are analysed by means of conodont biostratigraphy. Four conodont zones are analysed for the referred formations, i.e. the Acodus apex, Acodus triangularis, Gothodus vetus and Gothodus andinus zones, on the basis of a collection with ca. 2800 conodont specimens that includes new conodont taxa. Acodus apex Albanesi and Zeballo, Acodus triangularis (Ding), Acodus zeballus n. sp., Gothodus andinus (Rao et al.), Gothodus vetus n. sp., and Paltodus cf. inaequalis (Pander) are described herein. The conodont genus Zentagnathus is diagnosed including two previously known species: Zentagnathus argentinensis (Rao et al.) and Zentagnathus primitivus (Voldman et al.). The Tetragraptus akzharensis and Baltograptus cf. deflexus graptolite zones were identified as well in the ∼1800‐m‐thick stratigraphic section. The incertae sedis Phosphannulus universalis Müller et al. is discussed for the Cordillera Oriental for the first time. The lower part of the Chulpíos section that involves the contact between the Santa Rosita and the Acoite formations is not able to be dated with precision either as upper Tremadocian or lower Floian because the ranges of the index taxa recorded in that interval of the study section span the stage boundary. Copyright © 2016 John Wiley & Sons, Ltd.