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Yang S.-Y.,Wuhan University | Jiang S.-Y.,Wuhan University | Jiang S.-Y.,Nanjing University | Jiang S.-Y.,State Key Laboratory for Mineral Deposits Research | Palmer M.R.,University of Southampton
Chemical Geology | Year: 2015

The Miocene Nyalam leucogranites from the South Tibetan Himalaya consist of two-mica leucogranite and tourmaline leucogranite, in which tourmaline is an important constituent in nodular tourmaline-quartz segregations. Within the two-mica leucogranite these nodules display two basic morphologies: rounded (2mg-ro type) and radial or dendritic (2mg-ra type). The nodules in the tourmaline leucogranite share similar textural features with the 2mg-ro type tourmaline nodules, but without the leucocratic halo. Tourmaline crystals in the tourmaline leucogranite display overgrowth textures - most commonly in the form of homogeneous cores (Tg-I type) surrounded by oscillatory-zoned rims (Tg-II type). Based on the petrography, chemical and boron isotopic compositions of the various types of tourmaline, it is hypothesized that the 2mg-ro type tourmaline nodules crystallized during the transition from magmatic to hydrothermal evolution of the granite, while the 2mg-ra type tourmaline nodules likely formed during syn-magmatic crystallization from boron-rich melts. In this hypothesis, the Tg-I type tourmalines also formed during the transitional stage from late magmatic to early hydrothermal process. Tg-II type tourmalines were obviously formed after the Tg-I type tourmaline, and can be attributed to the mixing of juvenile fluids and volatiles coming from the wall rocks to the nodule in a post-magmatic hydrothermal environment where Tg-I type tourmaline has already crystallized. Thus, the origin of the tourmaline nodules is related to the different stages of syn-magmatic crystallization from a B-rich melt, or the subsequent transition from magmatic to hydrothermal environment, or crystallized in the post-magmatic hydrothermal fluids. The average δ11B values of the 2mg-ra type and 2mg-ro type tourmaline are -15.1‰ and -14.4‰, respectively. The tourmalines in tourmaline leucogranite have slightly higher δ11B values, with the Tg-I type tourmalines averaging -12.5‰ and the Tg-II type tourmalines averaging -12.1‰. The boron isotopic composition of the tourmalines has implications for the origin of the leucogranites. We suggest that the distinction of the boron isotopic compositions between the source rocks of the two types of leucogranite could account for this boron isotope distinction in tourmalines. © 2015 Elsevier B.V. Source


Li Y.X.,State Key Laboratory for Mineral Deposits Research | Jiao W.J.,State Key Laboratory for Mineral Deposits Research | Liu Z.H.,University of Hong Kong | Jin J.H.,Sun Yat Sen University | And 3 more authors.
Climate of the Past | Year: 2016

The Paleogene sedimentary records from southern China hold important clues to the impacts of the Cenozoic climate changes on low latitudes. However, although there are extensive Paleogene terrestrial archives and some contain abundant fossils in this region, few are accurately dated or have a temporal resolution adequate to decipher climate changes. Here, we present a detailed stratigraphic and paleomagnetic study of a fossiliferous late Paleogene succession in the Maoming Basin, Guangdong Province. The succession consists of oil shale of the Youganwo Formation (Fm) in the lower part and the overlying sandstone-dominated Huangniuling Fm in the upper part. Fossil records indicate that the age of the succession possibly spans the late Eocene to the Oligocene. Both the Youganwo Fm and the overlying Huangniuling Fm exhibit striking sedimentary rhythms, and spectral analysis of the depth series of magnetic susceptibility of the Youganwo Fm reveals dominant sedimentary cycles at orbital frequency bands. The transition from the Youganwo oil shale to the overlying Huangniuling sandstones is conformable and represents a major depositional environmental change from a lacustrine to a deltaic environment. Integrating the magnetostratigraphic, lithologic, and fossil data allows establishing a substantially refined chronostratigraphic framework that places the major depositional environmental change at 33.88 Ma, coinciding with the Eocene-Oligocene climate transition (EOT) at ∼ 33.7 to ∼ 33.9 Ma. We suggest that the transition from a lacustrine to deltaic environment in the Maoming Basin represents terrestrial responses to the EOT and indicates prevailing drying conditions in low-latitude regions during the global cooling at EOT. © Author(s) 2016. Source


Satkoski A.M.,University of Wisconsin - Madison | Satkoski A.M.,NASA | Beukes N.J.,University of Johannesburg | Li W.,State Key Laboratory for Mineral Deposits Research | And 4 more authors.
Earth and Planetary Science Letters | Year: 2015

Before the Great Oxidation Event (GOE) 2.4-2.2 billion years ago it has been traditionally thought that oceanic water columns were uniformly anoxic due to a lack of oxygen-producing microorganisms. Recently, however, it has been proposed that transient oxygenation of shallow seawater occurred between 2.8 and 3.0 billion years ago. Here, we present a novel combination of stable Fe and radiogenic U-Th-Pb isotope data that demonstrate significant oxygen contents in the shallow oceans at 3.2 Ga, based on analysis of the Manzimnyama Banded Iron Formation (BIF), Fig Tree Group, South Africa. This unit is exceptional in that proximal, shallow-water and distal, deep-water facies are preserved. When compared to the distal, deep-water facies, the proximal samples show elevated U concentrations and moderately positive δ56Fe values, indicating vertical stratification in dissolved oxygen contents. Confirmation of oxidizing conditions using U abundances is robustly constrained using samples that have been closed to U and Pb mobility using U-Th-Pb geochronology. Although redox-sensitive elements have been commonly used in ancient rocks to infer redox conditions, post-depositional element mobility has been rarely tested, and U-Th-Pb geochronology can constrain open- or closed-system behavior. The U abundances and δ56Fe values of the Manzimnyama BIF suggest the proximal, shallow-water samples record precipitation under stronger oxidizing conditions compared to the distal deeper-water facies, which in turn indicates the existence of a discrete redox boundary between deep and shallow ocean waters at this time; this work, therefore, documents the oldest known preserved marine redox gradient in the rock record. The relative enrichment of O2 in the upper water column is likely due to the existence of oxygen-producing microorganisms such as cyanobacteria. These results provide a new approach for identifying free oxygen in Earth's ancient oceans, including confirming the age of redox proxies, and indicate that cyanobacteria evolved prior to 3.2 Ga. © 2015 Elsevier B.V. Source


Yao C.,State Key Laboratory for Mineral Deposits Research | Ma D.,State Key Laboratory for Mineral Deposits Research | Ding H.,State Key Laboratory for Mineral Deposits Research | Zhang X.,Jiangsu Environment Monitoring Center
Chinese Journal of Geochemistry | Year: 2011

Carbon isotope chemostratigraphy has been used worldwide for stratigraphic correlation. In this study, δ13Ccarb values are estimated for the Early Cambrian Sugaitebulake section in the Tarim Basin, Xinjiang Autonomous Region, China. As a result, one positive and two negative carbon isotope excursions in the studied section were identified. The δ13Ccarb values reached the maximum negative excursion (N1: -12.39‰) at the basal of the Yuertusi Formation, and then increased to P1. After P1, δ13Ccarb values sharply decreased to about -7.06‰ (N2) in the studied section. The pattern of δ13Ccarb in the Early Cambrian is comparable to the synchronous records of other sections, such as the Laolin section, the Xiaotan section and the Anjiahe section of the Yangtze Platform. It is concluded that the Early Cambrian Yuertusi Formation from the Tarim Basin is within the Nemakit-Daldynian stage, and the lower strata of the Yuertusi Formation may belong to the Zhujiaqing Formation (Meishucun Formation) of the Yangtze Platform. The Ediacaran/Cambrian boundary of the Tarim Block should be located in cherts and phosphorites successions at the basal of the Yuertusi Formation. The δ13Ccarb negative excursion N1 is just across the PC/C boundary, and may be related to certain biomass extinction due to anoxic sedimentary environment, transgression and/or the oceanic overturn. The second δ13Ccarb negative excursion N2 may account for the sea-level falling in the Early Cambrian. © 2011 Science Press, Institute of Geochemistry, CAS and Springer-Verlag Berlin Heidelberg. Source

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