Search results for "Zircon"
showing 10 items of 693 documents
Age and provenance constraints on seismically-determined crustal layers beneath the Paleozoic southern Central Asian Orogen, Inner Mongolia, China
2016
Abstract We present 110 ages and 51 in-situ δ18O values for zircon xenocrysts from a post-99 Ma intraplate basaltic rock suite hosted in a subduction–accretion complex of the southern Central Asian Orogenic Belt in order to constrain a seismic profile across the Paleozoic Southern Orogen of Inner Mongolia and the northern margin of the North China Craton. Two zircon populations are recognized, namely a Phanerozoic group of 70 zircons comprising granitoid-derived (ca. 431–99 Ma; n = 31; peak at 256 Ma), meta-granitoid-derived (ca. 449–113 Ma; n = 24; peak at 251 Ma) and gabbro-derived (436–242 Ma; n = 15; peaks at 264 and 244 Ma) grains. Each textural type is characterized by a distinct zirc…
Neoproterozoic-Early Paleozoic Peri-Pacific Accretionary Evolution of the Mongolian Collage System: Insights From Geochemical and U-Pb Zircon Data Fr…
2017
Neoproterozoic to early Paleozoic accretionary processes of the Central Asian Orogenic Belt have been evaluated so far mainly using the geology of ophiolites and/or magmatic arcs. Thus, the knowledge of the nature and evolution of associated sedimentary prisms remains fragmentary. We carried out an integrated geological, geochemical and zircon U–Pb geochronological study on a giant Ordovician metasedimentary succession of the Mongolian Altai Mts. This succession is characterized by dominant terrigenous components mixed with volcanogenic material. It is chemically immature, compositionally analogous to graywacke and marked by significant input of felsic to intermediate arc components, pointi…
The complexities of zircon crystllazition and overprinting during metamorphism and anatexis: An example from the late Archean TTG terrane of western …
2017
Abstract There are different viewpoints on metamorphic and anatectic zircons recording ages of 2.45–2.48 Ga or even younger in some areas of the North China Craton where both late Neoarchean and late Paleoproterozoic tectono-thermal events are well developed. These are: 1) partial resetting of the U-Pb isotopic system in the late Neoarchean zircons, 2) metamorphism lasting from the late Neoarchean to the earliest Paleoproterozoic, and 3) earliest Paleoproterozoic metamorphism as separate different event. Western Shandong Province is an area where the late Neoarchean tectono-thermal event is widely developed but the late Paleoproterozoic event has not been identified. This provides an opport…
Age and isotopic evidence for the origin of the Archæan granitoid intrusives of the Johannesburg Dome, South Africa
1999
Abstract Results of RbSr, PbPb and SmNd whole rock, Rbr biotite and PbPb zircon evaporation analyses are presented for certain granitoid rocks from the Johannesburg Dome. These data indicate that the granodiorite, granite and leucosome from migmatite were emplaced ∼ 3090 Ma ago, were genetically related and were derived primarily from a source between ∼ 3300 and ∼ 3500 Ma old. A portion of the granodiorite and granite might have been derived from a source between ∼ 4000 and ∼ 4300 Ma old. The tonalite was emplaced ∼ 3170 Ma ago and was derived from a source between 3.3 and 3.5 Ga old. RbSr biotite-whole rock ages, ranging between about ∼ 2614 and ∼ 2080 Ma, probably reflect complete r…
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 geochronology and metamorphic evolution of mafic dykes in the Hengshan Complex of northern China: Evidence for late Palaeoproterozoic extensio…
2006
Abstract Magmatic and metamorphic zircons have been dated from ductilely deformed gabbroic dykes defining a dyke swarm and signifying crustal extension in the northern part of the Hengshan Complex of the North China Craton. These dykes now occur as boudins and deformed sheets within migmatitic tonalitic, trondhjemitic, granodioritic and granitic gneisses and are conspicuous due to relics of high-pressure granulite or even former eclogite facies garnet + pyroxene-bearing assemblages. SHRIMP ages for magmatic zircons from two dykes reflect the time of dyke emplacement at ∼1915 Ma, whereas metamorphic zircons dated by both SHRIMP and evaporation techniques are consistently in the range 1848–18…
Age and evolution of a late Archean to Paleoproterozoic upper to lower crustal section in the Wutaishan/Hengshan/Fuping terrain of northern China
2005
Abstract The Taihangshan–Wutaishan area forms part of the central North China Craton and consists of three main components: the Hengshan and Fuping complexes, containing predominantly ductilely deformed late Archean to Paleoproterozoic high-grade granitoid orthogneisses, intruded by mafic dykes of gabbroic composition, and the low-grade late Archean Wutai greenstone belt, developed between the high-grade terrains and consisting of bimodal volcanic rocks and metasediments, associated with coeval granitoids. Zircon dating shows that both the Hengshan and Fuping complexes were intruded by major granitoid bodies between 2.52 and 2.48 Ga, with rare occurrences of 2.7 Ga gneisses. Wutai granitoid…
New geochemical and combined zircon U–Pb and Lu–Hf isotopic data of orthogneisses in the northern Altyn Tagh, northern margin of the Tibetan plateau:…
2014
Abstract Dunhuang Block is previously considered to be an eastern part of the Tarim Craton, but now it is proposed to be the western extension of the Alxa Block of the NCC as a result of displacement along the Altyn Tagh fault. The oldest basement rocks of the Dunhuang Block, named Aketashitage Complex, were mainly exposed in the northern Altyn Tagh. Migmatites of the Aketashitage Complex are sodic and subalkaline in composition. Zircon U–Pb dating of the tonalitic and monzogranitic migmatites indicates that these rocks were produced by strong deformation of earlier formed tonalites with innumerous granitic veins. The tonalitic melanosome of the migmatites was generated approximately at 2.7…
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…