Geoheritages in the Qinling Orogenic Belt of China: Features and comparative analyses

The Qinling Orogenic Belt is a typical composite continental orogenic belt in China. The Qinling Mountains is not only a natural demarcation that separates China into a southern part and a northern part, in terms of Qinling Orogenic Belt’s geography, climate, organisms, and river systems, but also forms a cultural boundary between the Yellow River Civilization to the north and the Yangtze River Civilization to the south. The Xi’an Qinling Zhongnanshan World Geopark is situated in the major portion of the collision zone between the South China and North China plates, which is also a typical section of the Qinling Orogenic Belt. It has been the focus of great attention for its long history of geological evolution, intensive tectonism, various rock types, unique strata, frequent magmatism, and abundant geological heritage sites. This paper clarifies the regional geological background of the Qinling Zhongnanshan World Geopark in 3 aspects, that is, the regional geological setting, regional evolution, and regional strata. With an integrated analysis and classification, the paper describes the main features of geological relics and geological remains of scientific significance in the geopark, on the basis of an investigation and systematic analysis for the geological relic resources of the world geopark. Compared with the geosites in other world geoparks, the sites in the Zhongnanshan area are particularly distinctive, even unique.

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Origin and influence of a Late Mesozoic multistage I- and A-type granitic complex in northern Fujian Province, South China

The Late Mesozoic Pucheng granitic complex in northern Fujian Province is composed of the Chengbu and Shipi peralkaline A-type granitoids, the Pucheng A-type granitoid, and the Yongxing I-type granitoid. The Chengbu and Shipi peralkaline granitoids were dated at 160 and 130 Ma, respectively, and are explained by the melting of meta-igneous rocks with TDM2 of 1.70 Ga and 2.01 to 2.05 Ga. The Pucheng A-type granitoid was emplaced at 110 and 102 Ma and is divided into two groups. Group 1 comprises the extremely felsic A-type granite (SiO2 > 73 wt.%), high FeOT/MgO (>16), and low Ga/Al ratios and Zr + Nb + Ce + Y content. Group 2 contains a less-evolved A-type granite with high Zr + Nb + Ce + Y content. Both groups can be explained by dehydration melting of meta-igneous rocks at low pressure. The diversity between the two groups was caused by different physicochemical environments. The Yongxing I-type granite, which is abundant in mafic microgranular enclaves, was emplaced at 106 ± 1 Ma. It can be explained by the mingling of mantle-derived and crustal-derived magmas. Our data along with previously published data demonstrate that the northern Fujian Province was under an extensional environment from 160 to 100 Ma. This was caused by the NW-trending subduction of the palaeo-Pacific Plate. The Jurassic peralkaline granitoids were probably generated in a rift environment as a result of the reactivation of pre-existing faults caused by the initial subduction of the palaeo-Pacific Plate. The Early Cretaceous peralkaline granitoids may represent a rift environment as a tectonic response from low-angle subduction to an increasing subduction dip angle. The 100 Ma granitoids derive from an intraplate extensional environment caused by the roll-back of the palaeo-Pacific Plate.

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Linking the Qinling Orogeny with the Chang 7 shale (Triassic Yanchang Formation) deposition: Evidence from major, trace, and rare earth element geochemistry

The lacustrine organic-rich shale of Triassic Chang 7 Member is an unconventional shale oil reservoir that is a major hydrocarbon target in the Ordos Basin, northern China. This study uses major, trace, and rare earth element (REE) abundance documented through a 115-m-thick core obtained from the western part of the Ordos Basin to assess dominant controls on temporal variations in total organic carbon in the Chang 7 shale. Our results suggest that total organic carbon trends in the Chang 7 can be directly linked with the tectonic and magmatic evolution of Qinling orogen. The Chang 73 and Chang 72 submembers of the lower part of the Chang 7 were likely deposited in association with heighted tectonic and magmatic activity of the Qinling Orogeny. Sediments derived from these submembers display light REE and trace element compositions similar to the Tianshui rhyolites suggesting deposition contemporaneous with magmatic and/or volcanic activity of the Qinling Orogeny. Higher nutrient concentrations introduced by volcanic eruptions would have stimulated primary productivity in the surface water enhancing the organic carbon flux to the lake bottom. In addition, higher rates of mountain building in the Qinling Orogeny may have been responsible for increasing subsidence rate of the southern Ordos Basin, also favourable for the establishment of anoxic bottom-water conditions. In contrast, lower light REE and trace element abundances of the Chang 71 submember at the top of the Chang 7 suggest deposition during a tectonically and magmatic quiescent period of the Qinling Orogeny. The diminished volcanic flux could have lowered the rate of primary productivity. Moreover, a reduced rate of mountain building would have decreased the rate of foredeep subsidence favouring shallower water and oxic/suboxic bottom-water conditions. Thus, the hydrography of the lake during accumulation of the Chang 71 was not conducive for organic carbon accumulation. Together, the correlation of the Qinling mountain building and magmatism and deposition of the Chang 7 shale provide evidence for the role of tectonism in the accumulation of organic carbon enriched sediment.

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Detrital zircon U–Pb ages, Hf isotopic constraints, and trace element analysis of Upper Cretaceous–Neogene sedimentary units in the Western Nepal Himalaya: Implications for provenance changes and India–Asia collision

The present study integrates detrital zircon U–Pb–Hf isotopic analysis from 13 sandstone samples from an Upper Cretaceous–Miocene sedimentary sequence in Nepal Himalaya to determine their provenance. These sequences constrain a shifting of provenance from south to north. The U–Pb ages from Upper Cretaceous–Palaeocene strata (Amile Formation) mainly cluster between ~1,860 and 1,400 Ma with a peak at ~1,630 Ma and an absence of grains younger than the Palaeoproterozoic (1,400 Ma) age. The detritus yielded positive detrital zircon ƐHf (t) values (as high as +10). However, the detrital zircon U–Pb ages from Eocene–Miocene sequence cluster at ~500–650, ~700–900, ~1,600–1,850, and ~2,500 Ma, and in addition, they have both positive and negative ƐHf (t) values (+11 to −25). This finding further elucidates that the detritus in the Amile Formation was entirely sourced from India, which changed following the time of the Bhainskati Formation deposition, to a mixture of both Asian and Indian affinities (the Himalayan region). This change in source region marks the possible time of the India–Asia collision during this transition phase, that is, Late Palaeocene–Earliest Eocene.

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78 seconds of Earthquake Early Warning

Late on Friday afternoon, February 16, during the Chinese New Year street fair in la Ciudad de México, the tight hold between the North America and Cocos plates failed, the fault slipped, and the Pacific coast of Oaxaca lurched around a meter out toward the ocean. Within six seconds, the profound ripple this let loose through the crust heaved the Huazolotitlán seismic recording station westward as well, followed shortly by …

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Enigmatic nodule-bearing rocks in the mafic to ultramafic Diana Mills pluton, Piedmont Province, Virginia

The Silurian Diana Mills pluton is a metamorphosed mafic–ultramafic body in the Piedmont Province of Virginia. Major rock types in the pluton include metadiorite and chlorite-amphibole rock, with lesser amounts of hornblendite. However, the most visually striking rocks, found at 4 locations, consist of tan-weathering nodules in a dark green matrix. The purpose of this study was to investigate the origin of these nodular rocks. All rocks show evidence of greenschist-facies metamorphism, but relict igneous textures are preserved locally. Apart from the nodules, Ca-amphibole is ubiquitous in all rock types. Nodules are dominated by serpentine or talc (+magnetite ± chromite), and many show a thin (≤5 mm) radially oriented shell of serpentine against adjacent matrix. The nodules are ultramafic (Mg# ~80–90, up to7,563 ppm Cr, up to 2,038 ppm Ni), and their normative mineralogy is dominated by olivine and orthopyroxene. Thus, they represent metamorphosed harzburgites and pyroxenites. Matrix minerals are dominantly amphibole + chlorite, along with variable amounts of talc + magnetite. Some matrix samples are chlorite-rich, probably reflecting metasomatic reaction with nodules (i.e., they are small-scale “blackwalls”). Matrix samples are also ultramafic (high Mg#, Cr, and Ni). The matrix of the nodule-bearing outcrops is essentially the same as other chlorite-amphibole rocks elsewhere in the pluton. We consider these rocks to represent emplacement of an original hornblende peridotite crystal mush (a mixture of crystals and hydrous melt), which locally carried harzburgitic nodules. The nodules most likely represent the earliest-formed cumulates from the Diana Mills parent magma.

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