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  • 1. Ledneva, Galina V.
    et al.
    Pease, Victoria L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sokolov, Sergey D.
    Permo"Triassic hypabyssal mafic intrusions and associated tholeiitic basalts of the Kolyuchinskaya Bay, Chukotka (NE Russia): Links to the Siberian LIP2011In: Journal of Asian Earth Sciences, ISSN 1367-9120, E-ISSN 1878-5786, Vol. 40, no 3, p. 737-745Article in journal (Refereed)
    Abstract [en]

    In order to test tectonic hypotheses regarding the evolution of the Arctic Alaska-Chukotka microplate prior to the opening of the Amerasian basin, we investigated rocks exposed near Kolyuchinskaya Bay, eastern Chukotka. Hypabyssal mafic rocks and associated basaltic flows enclose terrigenous sediments, minor cherts and limestones in pillow interstices. The hypabyssal mafic rock yields a U-Pb zircon age of 252 +/- 4 Ma and indicates intrusion of basic magma at the Permo-Triassic boundary, contemporaneous with voluminous magmatism of the Siberian large igneous province (LIP). The lava flows and hypabyssal mafic rocks of the Kolyuchinskaya Bay region have trace elements, Sm-Nd and Rb-Sr isotope compositions identical to the tholeiitic flood basalts of the main plateau stage of the Siberian LIP, but differ from the latter in the major-element variations. We conclude that compositional variations in the hypabyssal rocks studied reflect their generation in an extensional environment that might be related to the Siberian super-plume activity at the time. Although the genetic and temporal links between intrusive mafic rocks and lavas are not well proved, compositional variations of the eruptive rocks still indicate their generation in an extensional environment.

  • 2.
    Löwemark, Ludvig
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chen, H. -F
    Yang, T. -N
    Kylander, Malin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Yu, E. -F
    Hsu, Y. -W
    Lee, T. -Q
    Song, S. -R
    Jarvis, S.
    Normalizing XRF-scanner data: A cautionary note on the interpretation of high-resolution records from organic-rich lakes2011In: Journal of Asian Earth Sciences, ISSN 1367-9120, E-ISSN 1878-5786, Vol. 40, no 6, p. 1250-1256Article in journal (Refereed)
    Abstract [en]

    X-ray fluorescence (XRF) scanning of unlithified, untreated sediment cores is becoming an increasingly common method used to obtain paleoproxy data from lake records. XRF-scanning is fast and delivers high-resolution records of relative variations in the elemental composition of the sediment. However, lake sediments display extreme variations in their organic matter content, which can vary from just a few percent to well over 50%. As XRF scanners are largely insensitive to organic material in the sediment, increasing levels of organic material effectively dilute those components that can be measured, such as the lithogenic material (the closed-sum effect). Consequently, in sediments with large variations in organic material, the measured variations in an element will to a large extent mirror the changes in organic material. It is therefore necessary to normalize the elements in the lithogenic component of the sediment against a conservative element to allow changes in the input of the elements to be addressed. In this study we show that Al, which is the lightest element that can be measured using the Wax XRF-scanner, can be used to effectively normalize the elements of the lithogenic fraction of the sediment against variations in organic content. We also show that care must be taken when choosing resolution and exposure time to ensure optimal output from the measurements.

  • 3. Löwemark, Ludvig
    et al.
    Chen, Tzu-Tung
    Gunnarson, Björn
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Su, Chih-Chieh
    Yang, Tien-Nan
    Huang, Jyh-Jaan
    Lan, Yung-Hsiang
    Burr, George
    The Tienchi Pond on Lanyu Island (Western Pacific): Lake formation and potential as environmental archive2015In: Journal of Asian Earth Sciences, ISSN 1367-9120, E-ISSN 1878-5786, Vol. 114, p. 435-446Article in journal (Refereed)
    Abstract [en]

    The formation, evolution, and potential as a climate archive of Tienchi Pond was evaluated using geomor-phology, sedimentology, geochemistry, and AMS radiocarbon measurements of tree ring cores. Tienchi Pond is a small ephemeral lake situated on the subtropical Lanyu Island in the Western Pacific Ocean about 90 km east of southern Taiwan. The lake is situated on a mountain ridge and is today characterized by numerous dead trees and stumps. The position at the boundary between the two main components of the SE Asian monsoon system, the Asian landmass and the Western Pacific, makes this lake particularly interesting as a potential natural climate archive. Since previous studies have shown that the bedrock underlying the lake formed over 2 my ago, a volcanic crater origin was ruled out. Rather, element ratios of K/Ti and Ca/Fe together with Pb-210 dating suggest a recent origin for the lake, probably as the result of a natural damming event in the southern, narrower, part of the lake basin, or as a result of increased precipitation at the end of the Little Ice Age (or a combination of both). Radiocarbon measurements on a dead tree near the lake perimeter show C-14 levels of more than 1.7% modern radiocarbon near the bark, suggesting that the tree died in the early 1970s. In contrast, by correlating a large number of radiocarbon measurements to a northern hemisphere reference curve, it was shown that trees closer to the center of the lake died in the early 20th century. This agrees with a scenario where a small lake formed by initial damming, and gradually grew larger through erosion of the surrounding shore lines caused by rapid lake level oscillations.

  • 4. Meng, Qingpeng
    et al.
    He, Yuankai
    Zhang, Wen
    Stockholm University, Faculty of Science, Department of Geological Sciences. Peking University.
    Zheng, Rongguo
    Xu, Cao
    Zhang, Zhaoyu
    Wu, Tairan
    Time constraints on the inversion of the tectonic regime in the northern margin of the North China Craton: Evidence from the Daqingshan granites2014In: Journal of Asian Earth Sciences, ISSN 1367-9120, E-ISSN 1878-5786, Vol. 79, p. 246-259Article in journal (Refereed)
    Abstract [en]

    The Daqingshan granites are located in a late Mesozoic tectono-magmatic belt at the northern margin of the North China Craton (NCC), and include the Deshengying, Xinisubei, Gulouban, and Kuisu plutons. Ion probe U-Pb zircon dating indicates that the granites were emplaced at 131 +/- 1, 140 +/- 4, 145 +/- 1, and 142 +/- 2 Ma, respectively. All of the granites are alkali- and potassium-rich, with high SiO2 (73.2-76.7 wt.%), K2O (4.50-5.57 wt.%), Na2O (3.60-4.93 wt.%), and K2O/Na2O (0.99-1.49), and low Al2O3 (12.3-14.5 wt.%), CaO (0.45-0.79 wt.%), and MgO (0.12 wt.%). The granites are light rare earth element enriched a La/YbIN = 5.6-48.7). The Xinisubei and Gulouban monzogranites and the Kuisu mylonitic monzogranite have small Eu anomalies (8Eu = 0.65-1.23), low Zr + Nb + Ce + Y (132-321 ppm), and exhibit a negative correlation between P2O5 and SiO2 contents, which are characteristic of highly fractionated I-type granites with a post-collisional origin. The Deshengying monzogranite is distinctive in being an aluminous A-type granite as evidenced by high 10,000 x Ga/AI (>2.6) and Zr + Nb + Ce + Y (312-532 ppm), low Ba and Sr, marked negative Eu anomalies (8Eu = 0.08-0.20), strong Ba, Sr, P, and Ti depletions, and an absence of alkali minerals. This granite was probably produced by partial melting of continental crust heated by hot mantle-derived magmas during crustal extension. The Deshengying monzogranite represents a post-kinematic pluton emplaced into the Daqingshan fold-and-thrust belt, whereas the Kuisu mylonitic monzogranite is a syn-kinematic pluton intruded along the Hohhot detachment fault. It is evident that the Daqingshan area experienced a change from a compressional to an extensional tectonic regime during 145-140 Ma. The post-orogenic collapse may have resulted in extension of the upper continental crust. Subsequently, as the thrust-detachment system became inactive, the lower crust of the NCC underwent modification and melting from 131 Ma. We conclude that the Early Cretaceous tectonic evolution of the Daqingshan area was caused by post-orogenic collapse and melting of the lower crust of the NCC. Delamination of the lower crust in the northern NCC resulted in crustal extension and asthenospheric upwelling, which produced A-type granites. As such, melting of the lower crust in the northwestern part of the NCC took place as early as the late Mesozoic.

  • 5.
    Zhang, Wen
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wu, Tairan
    Zheng, Rongguo
    Feng, Jicheng
    Luo, Hongling
    He, Yuankai
    Xu, Cao
    Post-collisional Southeastern Beishan granites: Geochemistry, geochronology, Sr-Nd-Hf isotopes and their implications for tectonic evolution2012In: Journal of Asian Earth Sciences, ISSN 1367-9120, E-ISSN 1878-5786, Vol. 58, p. 51-63Article in journal (Refereed)
    Abstract [en]

    Bandaoshan granites are exposed in the Southeastern Beishan area, in the central part of the Central Asia Orogenic Belt (CAOB). LA-ICP-MS U-Pb zircon dating indicates that Bandaoshan granites were emplaced into Precambrian basement at 285 +/- 4 Ma and their geochemistry indicates that they are alkali-rich potassium-high granites. Initial epsilon(Nd) values (-4.3 to -2.7) and epsilon(Hf) values (-2.7 to +0.7) suggest that Bandaoshan granites were derived from mantle-derived melt and an upper continental crustal or sedimentary component. The latter plays a significant role in their genesis. In combination with regional geology, the Early Permian Bandaoshan pluton is interpreted to form in a post-collisional environment. In the Southeastern Beishan area Late Carboniferous Qiaowan granites, Early Permian Yin'aoxia granites and Middle Permian Xijianquanzi granites are also considered as post-collisional granites, and together with Bandaoshan granites indicate that the region was in a post-collisional stage from the end of Late Carboniferous to Middle Permian.

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