Spectral and chemical characterization of gypsum-phyllosilicate association in Tiruchirapalli, South India, and its implications

Here, we present the detailed chemical and spectral characteristics of gypsum-phyllosilicate association of Karai Shale Formation in Tiruchirapalli region of the Cauvery Basin in South India. The Karai Shale Formation comprises Odiyam sandy clay and gypsiferous clay, well exposed in Karai village of Tiruchirapalli area, Tamil Nadu in South India. Gypsum is fibrous to crystalline and translucent/transparent type with fluid inclusions preserved in it. Along some cleavage planes, alteration features have been observed. Visible and near infrared (VNIR), Raman, and Fourier transform infrared techniques were used to obtain the excitation/vibration bands of mineral phases. VNIR spectroscopic analysis of the gypsum samples has shown absorption features at 560, 650, 900, 1,000, 1,200, 1,445, 1,750, 1,900, 2,200, and 2,280 nm in the electrical and vibrational range of electromagnetic radiation. VNIR results of phyllosilicate samples have shown absorption features at 1,400, 1,900, and 2,200 nm. Further, we have identified the prominent Raman bands at 417.11, 496.06, 619.85, 673.46, 1,006.75, 1,009.75, ∼1,137.44, ∼3,403, and 3,494.38 cm−1 for gypsum due to sulphate and hydroxyl ion vibrations. We propose that gypsum veins in Karai may have precipitated in the fractures formed due to pressure/forces generated by crystal growth. The combined results of chemical and spectral studies have shown that these techniques have significant potential to identify the pure/mineral associates/similar chemical compositions elsewhere. Our results definitely provide the database from a range of spectroscopic techniques to better identify similar minerals and/or mineral-associations in an extraterrestrial scenario. This study has significant implications in understanding various geological processes such as fluid-rock interactions and alteration processes involving water on the planets such as Mars.

Petrogenesis of pillow basalts in West Junggar, NW China: Constraints from geochronology, geochemistry, and Sr–Nd–Pb isotopes

West Junggar is located at the southwest margin of the Central Asian Orogenic Belt and includes Silurian pillow basalts of the Mayilashan Formation. The petrogenesis and tectonic setting of these pillow basalts are important for the understanding of the tectonic evolution and metallogeny of the West Junggar area. This paper presents geochronological, geochemical, and whole-rock Sr–Nd–Pb isotope data from the pillow basalts of the Mayilashan Formation. Zircon LA-ICP-MS U–Pb dating of a pillow basalt, which is in conformable contact with the chert, suggests that they were erupted at 437.2 ± 2.2 Ma marking the timing of generation of these rocks as Middle Silurian. Geochemically, all the pillow basalts bear the signature of ocean island basalt (OIB), and are characterized by alkaline affinity with high concentrations of TiO2 (3.28–4.12 wt.%), LILE and LREE enrichment and HREE depletion ((La/Yb)N = 5.5–7.3), with very weak Eu anomalies (Eu/Eu* = 0.96–1.06), and no obvious Nb, Ta, or Ti negative anomalies. Their Sr–Nd–Pb isotopic compositions ((87Sr/86Sr)I = 0.7037–0.7051, εNd(t) = 1.9–2.9, 206Pb/204Pbi = 17.74–18.22, 207Pb/204Pbi = 15.48–15.52, and 208Pb/204Pbi = 36.49–37.86) show Dupal-like isotopic signature of ophiolites in the southern Paleo-Asian Ocean. These characteristics indicate that the magmas were derived from a deep OIB reservoir, that is, a depleted but slightly heterogeneous asthenospheric mantle source with ~5–15% partial melting of garnet and spinel lherzolite. Our obtained results, in conjunction with previous published data, allow us to suggest that the alkaline pillow basalts formed in a seamount within an intraoceanic setting, where a larger number of seamounts with different ages occurred in the Paleo-Asian Ocean.

Tectonic setting of the Late Triassic magmatism in the Qinling Orogen: New constraints from the interplay between granite emplacement and shear zone deformation in the Shagou area

Multidisciplinary studies of the Baliping granite pluton and the Shagou shear zone (East Qinling Orogen, Central China) have been conducted to reconstruct the emplacement and deformation processes of the Baliping pluton, to explore the relationship between granite pluton and shear zone deformation, and further to constrain the tectonic setting of the Late Triassic magmatism in the Qinling Orogen which remains controversial. Zircon U–Pb geochronology studies confirmed that the Baliping granites and the protolithes of the Shagou granitic mylonites crystallized from the same magmas at ca. 221 Ma since the two nearly have the same zircon U–Pb ages and Ti–in–zircon temperatures. Meanwhile, the 210 Ma of metamorphic zircons and the 201 Ma of syntectonic granitic vein provide precise limits on the timing of synkinematic deformation in response to Late Triassic orogeny. Anisotropy of magnetic susceptibility, microstructural observation, and shape preferred orientation were combined to study the internal structures. New fabrics datasets display a high degree of coupling between the Baliping pluton and the Shagou shear zone, indicating a syntectonic emplacement of the pluton. Microstructural data suggest that the magmatic to high-temperature solid-state deformation is a continuum process in which the fabrics were mainly acquired. We propose an integrated syntectonic emplacement model of the Baliping pluton, in which the shear zone deformation (sinistral transpression) play a significant role on the emplacement of the magma by creating rooms. Emplacement of the pluton, in turn, triggers the nucleation of Shagou shear zone by providing the protoliths and the heat. On the basis of this study, we argued that the Qinling Orogen was still under the convergent setting during 221–201 Ma.

Spatial and temporal change of fractional vegetation cover in North-western China from 2000 to 2010

Northwestern China is located at the core area of The Belt and The Road at present and it once has been an essential transportation hub of the ancient Silk Road. However, the north-western part belongs to arid and semi-arid region where water resource and vegetation cover are scarce, leading to soil erosion, desertification, and environmental degradation. So it is meaningful to study fractional vegetation cover in North-western China. Vegetation MODIS data of northwestern China from 2000 to 2010 were collected, and then, the pixel dichotomy model and simple linear regression model were applied in this research. The results were as follows. First, the fractional vegetation cover was decreasing from the margin to the middle and western part and the higher value shown in the north-east part of Inner Mongolia, the south part of Shaanxi, the north-west part of Ningxia, the south-east part of Gansu, and the north-west part of Xinjiang. Second, the overall trend of fractional vegetation cover was rising during 2000 to 2010, which is clearly shown in northern Shaanxi, south-eastern Qinghai and Gansu, and also south-western Inner Mongolia. Third, fractional vegetation cover varied differently with season changes. To be more specific, the index in spring and winter was considerably low; in contrast, the index in summer and autumn was improved dramatically.