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Age Constraints (age + constraint)
Selected AbstractsCosmogenic 10BE Age Constraints for The Wester Ross Readvance Moraine: Insights Into British Ice-Sheet BehaviourGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 1 2006Jeremy D. Everest This study presents the first absoluteage constraints from a palaeo-ice-sheet margin in western Scotland. Cosmogenic 10Be from four Lewisian gneiss boulders on the Gairloch Moraine in NW Scotland have yielded reliable exposure ages. Three of these dates, taken from a single moraine ridge, cluster around c. 15.5,18 ka BP, with a weighted mean of 16.3 ± 1.6 ka BP. These findings indicate that the last British Ice Sheet had retreated to the present-day coastline in NW Scotland by this time. It is suggested that the Wester Ross Readvance represents an ice-sheet oscillation during, or in the immediate aftermath of, Heinrich Event 1 (c. 17,18 ka BP). [source] Mineralogy, Geochemistry, and Age Constraints on the Beni Bou Ifrour Skarn Type Magnetite Deposit, Northeastern MoroccoRESOURCE GEOLOGY, Issue 1 2002Mohammed EL RHAZI Abstract: The Beni Bou Ifrour deposit of northeastern Morocco is a skarn type magnetite deposit. K-Ar age determination suggests that the mineralization occurred at 7.040.47 Ma. The spatial relationship between skarn and dikes of microgran-odiorite derived from the batholith of Wiksane Granodiorite, and the similarity of age (8.020.22 Ma), confirms that the Wiksane Granodiorite is the igneous rock most probably related to mineralization. The skarn is distributed asymmetrically in the limestone, and magnetite ore was developed just below the calc-silicate skarn as two parallel beds separated by 100 m of barren limestone and schist. The mineralization can be divided into three stages. The early stage is characterized by the formation of calc-silicate minerals, mainly clinopyroxene (80,70 % diopside) and garnet (early almost pure andradite to the late 60 % andradite). The main stage is characterized by the formation of a large amount of magnetite. Epidote and quartz formed simultaneously with magnetite. Fluid temperatures exceeded 500 C during the early to main stages. Fluid with very high salinity (50,75 wt% NaCl equiv.) was responsible for the formation of the magnetite ore. The oxygen isotope composition, together with the fluid inclusion data, suggests that magmatic fluid was significant for the formation of calc-silicate skarn minerals and magnetite. Low temperature (-230C) and low salinity (-10 % NaCl equiv.) hydrothermal fluids dominated by meteoric water were responsible for the late stage quartz and calcite formation. [source] Genesis and Age Constraints on Gold Deposits of the Daerae Mine, Sangju Area, Central-Northern Sobaegsan Massif, KoreaRESOURCE GEOLOGY, Issue 3 2001Seong, Taek YUN Abstract: Gold mineralization of the Daerae mine represents the first recognized example of the Jurassic gold mineralization in the Sangju area, Korea. It occurs as a single stage of quartz veins that fill fault fractures in Precambrian gneiss of the central-northern Sobaegsan Massif. The mineralogical characteristics of quartz veins, such as the simple mineralogy and relatively gold-rich (65,72 atomic % Au) nature of electrum, as well as the CO2,rich and low salinity nature of fluid inclusions, are consistent with the ,mesothermal-type' gold deposits previously recognized in the Youngdong area (about 50 km southwest of the Sangju area). Ore fluids were evolved mainly through CO2 immiscibility at temperatures between about 250 and 325 C. Vein sulfides characteristically have negative sulfur isotopic values (,1.9 to +0.2 %), which have been very rarely reported in South Korea, and possibly indicate the derivation of sulfur from an ilmenite-series granite melt. The calculated O and H isotopic compositions of hydrothermal fluids at Daerae (,18Owater = +5.2 to +5.9 %; ,Dwater = ,59 to ,67 %) are very similar to those from the Youngdong area, and indicate the important role of magmatic water in gold mineralization. The 40Ar,39Ar age dating of a pure alteration sericite sample yields a high-temperature plateau age of 188.3 0.1 Ma, indicating an early Jurassic age for the gold mineralization at Daerae. The lower temperature Ar-Ar plateau defines an age of 158.4 2.0 Ma (middle Jurassic), interpreted as reset by a subsequent thermal effect after quartz vein formation. The younger plateau age is the same as the previously reported K-Ar ages (145,171 Ma) for the other ,mesothermal,type' gold deposits in the Youngdong and Jungwon areas, Korea, which are too young in view of the new Jurassic Ar-Ar plateau age (around 188 Ma). [source] Quaternary reactivation of Tertiary faults in the southeastern Korean Peninsula: Age constraint by optically stimulated luminescence datingISLAND ARC, Issue 1 2003Jin-Han Ree Abstract Two groups of Quaternary faults occur in the southeastern Korean Peninsula. The first group is north-northeast-striking, high-angle dextral strike,slip faults. The second group is north-northeast-striking, low-angle reverse faults that represent the reactivation of the pre-existing normal faults. Optically stimulated luminescence dating of Quaternary sediments cut by one of the reverse faults constrains the faulting age to post-32 Ka. These faults seem to be capable of further slip under the current tectonic stress regime, as determined by recent earthquake events in northeast Asia. Therefore, the traditional concept that the southeastern Korean Peninsula is seismically stable should be reappraised. [source] Probing the nature of multiple lobe-like emissions in 3C 84ASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009H. Nagai Abstract The bright radio source 3C 84 has a CSO-like feature in the central 10-pc scale with extended emissions on scale up to kpc. Our kinematic and synchrotron age measurements support that the innermost feature is a CSO formed by recent restarted activity before the radio emission that originated in the earlier activities has completely faded. The average rate of the jet power of the innermost feature over the source age is at least ,1044 erg s,1. Synchrotron age constraint for an outer lobe-like feature beyond the central CSO indicates that this feature originates in the outburst from ,1.3×105 years ago. Inferred duty cycle of the jet ejection is much shorter than the time scale expected from microquasars (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Not so old Archaea , the antiquity of biogeochemical processes in the archaeal domain of lifeGEOBIOLOGY, Issue 5 2009CARRINE E. BLANK Since the archaeal domain of life was first recognized, it has often been assumed that Archaea are ancient, and harbor primitive traits. In fact, the names of the major archaeal lineages reflect our assumptions regarding the antiquity of their traits. Ancestral state reconstruction and relaxed molecular clock analyses using newly articulated oxygen age constraints show that although the archaeal domain itself is old, tracing back to the Archean eon, many clades and traits within the domain are not ancient or primitive. Indeed many clades and traits, particularly in the Euryarchaeota, were inferred to be Neoproterozoic or Phanerozoic in age. Both Eury- and Crenarchaeota show increasing metabolic and physiological diversity through time. Early archaeal microbial communities were likely limited to sulfur reduction and hydrogenotrophic methanogenesis, and were confined to high-temperature geothermal environments. However, after the appearance of atmospheric oxygen, nodes containing a wide variety of traits (sulfate and thiosulfate reduction, sulfur oxidation, sulfide oxidation, aerobic respiration, nitrate reduction, mesophilic methanogenesis in sedimentary environments) appear, first in environments containing terrestrial Crenarchaeota in the Meso/Neoproterozoic followed by environments containing marine Euryarchaeota in the Neoproterozoic and Phanerozoic. This provides phylogenetic evidence for increasing complexity in the biogeochemical cycling of C, N, and S through geologic time, likely as a consequence of microbial evolution and the gradual oxygenation of various compartments within the biosphere. This work has implications not only for the large-scale evolution of microbial communities and biogeochemical processes, but also for the interpretation of microbial biosignatures in the ancient rock record. [source] An unconformity in the early Miocene syn-rifting succession, northern Noto Peninsula, Japan: Evidence for short-term uplifting precedent to the rapid opening of the Japan SeaISLAND ARC, Issue 3 2002Kazuhiko Kano Abstract The present paper describes the newly discovered early Miocene unconformity in the northern Noto Peninsula, on the Japan Sea side, central Japan. The unconformity marks the boundary between an early Miocene non-marine to marine succession and a more extensive, late early to early middle Miocene marine succession, and contains a time gap of an order of 1 million years or less from 18 Ma or earlier to 17 Ma. The early Miocene succession likely represents an early phase of marine transgression and initial slow rifting. The overlying early to early middle Miocene succession records the climax of the opening of the Japan Sea at ca 16 Ma with widespread, rapid subsidence of the Japan Arc. The unconformity between the two transgressive successions may represent a global sealevel fall or, more likely, crustal uplifting because no upward-shallowing or regressive facies remains between the two successions. Early Miocene unconformities that are thought to be correlative with this unconformity in the northern Noto Peninsula occur in places along the Japan Sea coast of Sakhalin and Japan. They are likely to have been produced during rifting in response to upwelling of asthenospheric mantle, although more accurate age constraints are necessary to evaluate this idea. [source] Amphibolite facies retrograde metamorphism of the Zhujiachong eclogite, SE Dabieshan: 40Ar/39Ar age constraints from argon extraction using UV-laser microprobe, in vacuo crushing and stepwise heatingJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2010H.-N. QIU Abstract The Zhujiachong eclogite in the south-eastern Dabieshan ultra-high- P terrane has been overprinted during retrograde metamorphism, with the development of garnet-amphibolite mineral assemblages in most rocks in the outcrop. This study is focused on providing age constraints for the retrograde amphibolite facies and greenschist facies mineralogy by 40Ar/39Ar dating. By applying a novel approach of combining three different techniques for extracting argon: laser stepwise heating of single grains and small separates, a spot fusion technique by UV-laser ablation microprobe on polished sections and an in vacuo crushing technique for liberating radiogenic argon from fluid inclusions, it is demonstrated that an internally consistent thermal history can be derived. The 40Ar/39Ar ages indicate that phengite formed before 265 Ma, probably during the ultra-high- P event. Ages associated with amphibolite facies retrograde metamorphism range from 242 to 217 Ma by the analyses of amphibole. Ages of c. 230 Ma were found for the symplectite matrix that formed during retrogression from eclogite pyroxene. Late stage hydrothermal activity leading to the formation of coarse-grained paragonite and fluid inclusions in vein amphibole was dated at c. 200 Ma. These age results agree well with the mineral crystallization sequence observed from thin-sections of the retrograded eclogite: phengite , paragonite and amphibole in matrix , amphibole in the corona. [source] | ||||||||||