Xiangshan Group Research Articles

Sedimentary Paleoenvironment of the Eastern Hexi Corridor, NW China: Constraints from Chert Geochemistry and Sedimentary Analysis of Early Paleozoic Strata

AbstractThe eastern Hexi Corridor, northwest China, is located at the tectonic junction of the Alxa Block, the North China Craton, and the Qinling‐Qilian Orogen. The early Paleozoic Xiangshan Group record critical information regarding paleoenvironment, paleoclimate and paleotectonic setting, from which we here present a focused study on the chert beds within the Xiangshan Group. Through field mapping, microstructural observation, whole‐rock geochemistry analyses and detrital zircon dating, we suggest that the Xiangshan Group chert was deposited along a passive continental margin, formed primarily through biological activity with minor hydrothermal influence and terrestrial input. The characteristics of the chert support a low latitude sedimentary paleoenvironmental origin, and reveal the fact that the Alxa Block was separated from the North China craton, while emerged some paleogeographic affinity with the Qilian region in the Middle‐Late Cambrian.

First palynological data from the Jurassic South Xiangshan Formation (Nanjing area, China)

The outcrops of the Xiangshan Group have been studied since the beginning of the twentieth century, yielding a rich macroflora (the Xiangshan Flora), mainly from the lower part of the Xiangshan Group (South Xiangshan Formation). Nevertheless, no palynological data have been published so far from the South Xiangshan Formation. The present study provides the first palynostratigraphic data of the South Xiangshan Formation. More than 50 fossil taxa from 30 fossil genera have been identified, allowing for a more accurate dating. The most characteristic taxa are Polycingulatisporites triangularis, Quadraeculina anellaeformis, Manumia delcourtii, Ischyosporites variegatus, Callialasporites turbatus, C. trilobatus, C. minus, and Sestrosporites pseudoalveolatus, suggesting a late Toarcian-late Aalenian age for the South Xiangshan Formation. These results are consistent with previous studies suggesting a middle-late Early Jurassic age for this formation; consequently, the most probable age for the South Xiangshan Formation is late Toarcian (late Early Jurassic).

Petrology, Geochemistry and Zircon U‐Pb Geochronology of the Xiangshan Group in the Eastern Hexi Corridor Belt: Implications for Provenance and Tectonic Evolution

AbstractThe eastern Hexi Corridor Belt (HCB) is located in the transitional belt among the Alxa Block, the Qilian Orogenic Belt and the North China Block. Because of its unique tectonic location, the tectonic setting, provenance, and even the age of the sedimentary strata in the eastern HCB during the Early Paleozoic remain controversial. This study analyzes the provenance of the poorly studied Xiangshan Group, discusses its age of development and tectonic setting in the eastern HCB using a combination of petrological, geochemical and LA‐ICP‐MS U‐Pb zircon dating methods. Based on the youngest age peaks and the fossil evidence, we suggest that the Xiangshan Group is Middle Cambrian to Late Ordovician in age. The complexity of the geochemical characteristics and associated diagrams suggests that the early stage of the Xiangshan Group developed in a passive continental margin environment, late in the back‐arc basin of the eastern HCB. Based on the sandstone detrital composition, whole‐rock geochemistry and detrital zircon ages, we conclude that the Xiangshan Group had an early provenance that was mainly from the Qilian Block and a late provenance from the Qilian Block and the western Alxa Block. The eastern HCB and its northern and southern blocks have similar palaeontology, lithology and basement age characteristics to the South China Block. This indicates that the eastern HCB might not have formed in the intra‐continental aulacogen of the North China Block during the Early Paleozoic but has a close affinity to eastern Gondwana.

Late Palaeozoic tectonic setting of the southern Alxa Block, NW China: constrained by age and composition of diabase

ABSTRACTThe ages of diabase outcropping on the southern Alxa Block and in related tectonic setting are not well constrained, but are important for exploring the tectonic evolution of the southern Alxa Block. The ages of the zircons from the diabase intruding the Middle–Late Cambrian Xiangshan Group were measured using sensitive high-resolution ion microprobe II (SHRIMPII) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and the weighted mean ages of 276.8 ± 7.9 Ma and 276.2 ± 9.7 Ma were obtained, respectively. The major elements in the diabase indicate that it may belong to the tholeiitic series. With specific features of the rare earth elements (REEs) and trace elements, potentially affected by the continental crust based on the observed strong positive Pb anomaly, the diabase was produced in an intra-plate extension environment. Similar diabase/basalts were also found along the southern margin of the Alxa Block. Combined with sedimentary, palaeo-current, mafic, and felsic rocks in the southern Alxa Block, a back-arc extension environment potentially occurred in the southern Alxa Block during the Early–Middle Permian that resulted from the convergence between the North China Block (NCB) and the Yangtze Block.

Geochronology and Tectonic Implications of Diabase Intruded Into Xiangshan Group in the Southeastern Alxa Block, NW China

Geochronology and Tectonic Implications of Diabase Intruded Into Xiangshan Group in the Southeastern Alxa Block, NW China

Tectonic affinity of the Alxa Block, Northwest China: Constrained by detrital zircon U–Pb ages from the early Paleozoic strata on its southern and eastern margins

The tectonic affinity of the Alxa Block is important in the reconstruction of the paleogeographical evolution of China. The early Paleozoic strata (the Dahuangshan Formation and Xiangshan Group) of the southern and eastern Alxa Block have consistent rock compositions, similar depositional ages, paleocurrents, detrital zircon age distributions, and cumulative probability curves of crystallization ages for detrital zircon grains relative to the depositional ages, and were deposited in similar slope basins. All these data indicate that the early Paleozoic strata of the Alxa Block were sourced predominantly from Neoproterozoic orogenic belts in the eastern Gondwana continent, instead of the Alxa Block to the north, the North China Block to the east and the North Qilian Orogenic Belt to the south. During the early Paleozoic, the Alxa Block was an independent block with the South China Block (SCB) to the west and the North China Block (NCB) to the east and situated to the northwest of the eastern Gondwana with its long axis trending north northwest–south southeast, and it belonged to a passive continental margin dipping to the north northwest, with the eastern part of the margin located closer to the eastern Gondwana. The Hexi Corridor is part of the Alxa Block and a part of the same slope basin during the early Paleozoic. The southern boundary of the Alxa Block is now the front thrust of the North Qilian Shan.

Linking the Alxa Terrane to the eastern Gondwana during the Early Paleozoic: Constraints from detrital zircon U–Pb ages and Cambrian sedimentary records

The detrital zircon ages of the Early Paleozoic Xiangshan Group in the southeastern Alxa Terrane show that the lower age limit for the Xiangshan group is 512Ma. The Xiangshan Group is older than the Middle Ordovician strata in the study region and is the Middle–Late Cambrian in age, which is the same age as the fossils found in it. The detrital zircon ages record the Grenville and Pan-African Orogenic Events, and the major age peaks are 963Ma, 1179Ma, 551Ma and 2517Ma. The North China Terrane, the Alxa Terrane and the Qilian Orogenic Belt were not the provenances of the Xiangshan Group. The age spectrum of the Xiangshan Group is similar to terranes associated with eastern Gondwana. The sandstone composition modes of the Xiangshan Group indicate that the source areas were the recycled orogenic belts. The zircons in the Xiangshan Group with an age peak at ca. 963Ma may come from the Eastern Ghats–Rayner orogenic belt or other terranes located to the north of eastern Gondwana. The zircons with a peak at ca. 1179Ma may come from the Wilkes–Albany–Fraser orogenic belt. Those with a peak at ca. 551Ma may come from the Prydz–Darling orogenic belt, and the zircons with a peak at ca. 2517Ma may be from northern India and the Yilgarn Craton in Western Australia. Similar paleontology, paleomagnetic data and detrital zircon age spectra indicate that the Alxa Terrane was located to the east of the South China Terrane and to the north of the eastern Gondwanan continent (present coordinates).

Reply to the Discussion of He et al. (2011, Geo-Marine Letters) Evidence of internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China

A point-by-point reply is here made to the discussion put forward by G. Shanmugam of our recent paper on internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China. We readily acknowledge that some issues raised by that author are pertinent. However, others are inconsequential, marginal, disputable, or redundant. The available evidence clearly supports our interpretation that the Middle Ordovician Xujiajuan Formation was deposited in a deep-water setting. Furthermore, we uphold that an internal-wave and internal-tide interpretation for these deposits is plausible in view of the occurrence of bidirectional cross-bedding and unidirectional cross-bedding dipping upslope in a deep-water environment. Also, we consider the request to provide conclusive evidence for the existence of a pycnocline in our stratigraphic record as unreasonable—it is self-evident that these data are currently not available, and the absence of proof is not proof of the contrary.

Discussion of He et al. (2011, Geo-Marine Letters) Evidence of internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China

This discussion of He et al. (Geo-Mar Lett (2011). doi:10.1007/s00367-011-0253-z, 2011) Evidence of internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China, in Geo-Marine Letters, is placed within the broader context of what is known and what is unknown about the oceanographic phenomenon of internal tides and their deposits.

Sedimentary Types of Internal-wave and Internal-tide Deposits of Middle Ordovician, Xujiajuan Formation, Xiangshan Group, Ningxia Autonomous Region, China

The study of internal-wave and internal-tide deposits is a very young research field with twenty years of history in the deep-water deposition,and there is no study on its sedimentary types.Here,we forcus on the internal-wave and internal-tide deposits developed in Xujiajuan Formation,Xiangshan Group,Ningxia Autonomous Region and describe related sedimentary structures in detail.We interpret combined-flow-ripple lamination and the cross beddings with undulatory laminations,offshooting laminations and cross-laminated lenses as combined-flow and oscillatory flow generated by short-period internal waves respectively.According to these new interpretations and the interpretations of deep-water bidirectional cross-beddings and unidirectional cross-beddings in literatures,we attribute internal-wave and internal-tide deposits to short-period internal-wave deposits,internal-tide deposits and interaction internal-wave deposits.Here,we use internal-wave and internal-tide deposits instead of internal-wave deposits because of the special origin and research status of internal tides.

Evidence of internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China

The Xujiajuan Formation of the Lower Xiangshan Group in Ningxia, China, is composed of grayish-green to yellowish-green, fine- to medium-grained sandstone, calcareous sandstone, siltstone, and shale. The upper part is thin-bedded limestone. At the top of the second and third members of the formation, a number of beds intercalated between turbidites and deep-water shale show well-developed cross-bedding. These beds are composed mainly of thin- to medium-bedded calcareous siltstone, fine-grained sandstone, fine-grained calcisiltitic limestone, and silty shale. All bedding types reflect traction-current action. The laminae of the bidirectional and unidirectional cross-bedded units tend to dip either opposite to or at a large angle to the regional slope. The units vary in shape and orientation in both upslope and downslope directions. A comprehensive evaluation of the sedimentary structures and inferred paleocurrents suggests that the cross-bedded intervals were not formed by contour currents or turbidity currents, but most probably represent internal-wave and internal-tide deposits.

Geochemistry of Apatite from the Apatite‐rich Iron Deposits in the Ningwu Region, East Central China

Abstract: Four types of apatite have been identified in the Ningwu region. The first type of apatite is widely distributed in the middle dark colored zones (i.e. iron ores) of individual deposits. The assemblage includes magnetite, apatite and actinolite (or diopside). The second type occurs within magnetite‐apatite veins in the iron ores. The third type is seen in magnetite‐apatite veins and (or) nodules in host rocks (i.e. gabbro‐diorite porphyry or gabbro‐diorite or pyroxene diorite). The fourth type occurs within apatite‐pyrite‐quartz veins filling fractures in the Xiangshan Group. Rare earth elements (REE) geochemistry of apatite of the four occurrences in porphyry iron deposits is presented. The REE distribution patterns of apatite are generally similar to those of apatites in the Kiruna‐type iron ores, nelsonites. They are enriched in light REE, with pronounced negative Eu anomalies. The similarity of REE distribution patterns in apatites from various deposits in different locations in the world indicates a common process of formation for various ore types, e.g. immiscibility. Early magmatic apatites contain 3031.48–12080 times 10−6 REE. Later hydrothermal apatite contains 1958 times 10−6 REE, indicating that the later hydrothermal ore‐forming solution contains lower REE. Although gabbro‐diorite porphyry and apatite show similar REE patterns, gabbro‐diorite porphyries have no europium anomalies or feeble positive or feeble negative europium anomalies, caused both by reduction environment of mantle source region and by fractionation and crystallization (immiscibility) under a high oxygen fugacity condition. Negative Eu anomalies of apatites were formed possibly due to acquisition of Eu2+ by earlier diopsite during ore magma cooling.The apatites in the Aoshan and Taishan iron deposits yield a narrow variation range of 87Sr/86Sr values from 0.7071 to 0.7073, similar to those of the volcanic and subvolcanic rocks, indicating that apatites were formed by liquid immiscibility and differentiation of intermediate and basic magmas.

Additional data on Todites (Osmundaceae) from the Lower Jurassic—with special references to the paleogeographical and stratigraphical distributions in China

Additional material of Todites (Osmundaceae) from the Lower Jurassic Xiangshan Group in Jiangsu and Anhui Provinces, Eastern China is investigated. Five species of Todites are described based on collections (including sterile and fertile pinnae) from eight localities along the Yangtze River, including Todites denticulatus (Brongniart) Krasser, Todites cf. goeppertianus (Muenster) Krasser, Todites nanjingensis sp. nov., Todites princeps (Presl) Gothan and Todites sp. Material of T. denticulatus and T. nanjingensis sp. nov. contains further data of the fertile pinnae. The stratigraphical ranges and the geographical distribution of the genus Todites in China are discussed and reviewed. Available data show that Todites mostly occurs in the Late Triassic to Middle Jurassic of China. The genus was widely distributed in both the Northern and Southern Floristic Provinces of China, and especially abundant and diverse in the Southern Floristic Province.