Search results for "Base"
showing 10 items of 8362 documents
Archean-Paleoproterozoic crustal evolution of the Ordos Block in the North China Craton: Constraints from zircon U–Pb geochronology and Hf isotopes f…
2015
Abstract The Ordos Block has been considered as an Archean micro-block in North China Craton. However, its formation and evolution are poorly understood for a long time due to lack of available basement rocks. Our LA-ICPMS and SIMS zircon U–Pb dating of gneissic granitoids from the Ordos Block basement identified two distinct periods of granitic magmatism at ∼2.5 Ma and 2.2–2.0 Ga, and two phases of metamorphisms at ∼1.95 Ga and ∼1.85 Ga, respectively. Of which most zircons of ∼2.5 Ga granitoids show positive ɛ Hf ( t ) values ranging from +0.1 to +4.1 with a peak T DM of ∼2.7 Ga. Minor zircons have negative ɛ Hf ( t ) values from −3.8 to −0.02 and T Hf C of 2.8–3.1 Ga. Moreover, the 3.40 G…
Archaean to Neoproterozoic magmatic events in the Kaoko belt of NW Namibia and their geodynamic significance
1998
Abstract Age relationships in the N–S trending Neoproterozoic (Pan-African) Kaoko belt of northwestern Namibia are still poorly constrained. U–Pb and Pb–Pb zircon age determinations by single grain evaporation, conventional multigrain fraction analyses and ion microprobe (SHRIMP) from a profile along the E–W Hoanib River reveal various episodes of zircon growth, ranging from late Archaean to late Neoproterozoic. From the eastern part of the profile we report crystallization ages of gneiss protoliths from 2645 to 2585 Ma, the oldest ages so far found in Namibia. Sm–Nd isotope data suggest the involvement of still older crustal material in the generation of these gneisses. A thermal event at …
Zircon REE patterns and geochemical characteristics of Paleoproterozoic anatectic granite in the northern Tarim Craton, NW China: Implications for th…
2012
Abstract Because Archean basement rocks are sparsely distributed around the Tarim Basin, little is known of the relationship between the Tarim Craton and the Paleo- to Mesoproterozoic Columbia supercontinent. Zircon U–Pb dating of a Paleoproterozoic gneissic granite in the northern Tarim Craton yielded a crystallization age of 1915 ± 13 Ma, consistent with global Paleoproterozoic collisional events (ca. 2.1–1.8 Ga) recorded in most cratons. Despite the fact that some zircons display discordant U–Pb ages with a distinct loss of radiogenetic Pb, all grains have similar 176Lu/177Hf and 176Hf/177Hf(t) values. Zircons from this granite have high Th/U ratios (0.15–0.95), but contrasting rare eart…
Mesoproterozoic (Grenville-age) terranes in the Kyrgyz North Tianshan: Zircon ages and Nd–Hf isotopic constraints on the origin and evolution of base…
2013
Abstract The North Tianshan orogenic belt in Kyrgyzstan consists predominantly of Neoproterozoic to early Paleozoic assemblages and tectonically interlayered older Precambrian crystalline complexes and formed during early Paleozoic accretionary and collisional events. One of the oldest continental fragments of late Mesoproterozoic (Grenvillian) age occurs within the southern part of the Kyrgyz North Tianshan. Using SHRIMP zircon ages, we document two magmatic events at ~ 1.1 and ~ 1.3 Ga. The younger event is characterized by voluminous granitoid magmatism between 1150 and 1050 Ma and is associated with deformation and metamorphism. The older event is documented by ~ 1.3 Ga felsic volcanism…
Late Quaternary Variations in the South American Monsoon System as Inferred by Speleothems – New Perspectives Using the SISAL Database
2018
Here we present an overview of speleothem δ18O records from South America, most of which are available in the Speleothem Isotopes Synthesis and Analysis (SISAL_v1) database. South American tropical and subtropical speleothem δ18O time series are primarily interpreted to reflect changes in precipitation amount, the amount effect, and consequently history of convection intensity variability of convergence zones such as the Intertropical Convergence Zone (ITCZ) and the South America Monsoon System (SAMS). We investigate past hydroclimate scenarios in South America related to the South American Monsoon System in three different time periods: Late Pleistocene, Holocene, and the last two millenni…
Semi-natural Grassland Vegetation Database of Latvia
2012
The Semi-natural Grassland Vegetation Database of Latvia (GIVD ID EU-LV-001) was first developed in 1999 as part of the PhD Thesis on syntaxonomy and diversity of dry and mesic grasslands of Latvia. The database includes the original releves established by the author as early as 1997. The majority of releves belongs to the Filipendulo-Helictotrichion (Festuco-Brometea) and Arrhenatheretalia communities, but Molinietalia, Nardetalia, Caricion nigrae, Koelerion glaucae and Plantagini-Festucion are covered, as well. The total number of releves stored is 2,500 and there are approximately 700 releves to be digitised. Plot size varies from 1 to 125 m, the majority of releves are 4-9 m large. Rele…
EnvDB, a database for describing the environmental distribution of prokaryotic taxa.
2009
Summary EnvDB is a database that classifies the environmental samples and their associated 16S rDNA sequences currently stored in GenBank. The samples were cat- egorized in a three-level, hierarchical classification of media: the five upper levels (terrestrial, aquatic, thermal, host-associated and other) are further decomposed in 20 intermediate (such as marine, marine sediments, freshwater, soil, gut, etc.) and 47 lower levels (for instance, soil is further decomposed in forest, agricultural, wetlands, grasslands, tropical, arid, etc.). Each sample was also characterized with nine environmental features: polluted, diseased (for clinical samples), acidic, alkaline, hot environment, cold en…
Soil CO2 degassing along tectonic structures of Mount Etna (Sicily): the Pernicana fault
1997
Abstract Carbon dioxide emissions from the soil have been investigated along lines of equally spaced sampling points perpendicular to the Pernicana fault on Mt Etna. Anomalous values of soil CO2 have been found not only along the fault plane, but also along directions parallel to it, both to the N and to the S of the main fault. The acquired data seem to reveal a shallow step-like geometry of the Pernicana fault system with parallel faults being generally not deeper than the interface between Etna's volcanic cover and its sedimentary basement (a few hundred meters). The distribution of the anomalous CO2 emissions has also revealed that the Pernicana fault continues at least as far as the Io…
Pan-African high-pressure metamorphism in the Precambrian basement of the Menderes Massif, western Anatolia, Turkey
2001
The Menderes Massif is made up of Pan-African basement and a Paleozoic to Early Tertiary cover sequence imbricated by Late Alpine deformation. The Precambrian basement comprises primarily medium- to high-grade schists, paragneisses, migmatites, orthogneisses, metagranites, charnockites, and metagabbros. High-pressure relies in the Pan-African basement are divided into two groups: eclogites and eclogitic metagabbros. The mineral assemblage in the eclogites is omphacite (Jd 44)-garnet-clinozoisite-rutile. The eclogites occur as pods and boudinaged layers in the basement schists and paragneisses. Inclusions found in the cores of the garnets indicate a medium-pressure protolith. The eclogitic m…
Geology of the Bozdag area, central Menderes massif, SW Turkey: Pan-African basement and Alpine deformation
1998
The Menderes massif consists of a Precambrian Core Series that preserves evidence for a polymetamorphic history and a Paleozoic/Mesozoic Cover Series that experienced only the Alpine tectonometamorphic evolution. Structural, petrographic, and geochronologic investigations in the central Menderes massif demonstrate that (a) part of the metamorphic and structural evolution of the Precambrian basement is older than the undeformed 551+/-1.4-Ma-old Birgi metagranite, and (b) inferred Alpine fabrics overprinting the Cover Series largely have the same attitudes as the old structures in the much older Core Series. The inferred Alpine fabrics include both contractional and extensional structures. Co…