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Zircon U-Pb Age, Geochemistry and Geological Implication of the 255 Ma Alkali-Rich Dykes from Ulungur Area, North Xinjiang

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Journal of Earth Science 2013年第4期24卷 519-528页

作者：陈言飞王玉往王京彬丁汝福袁月蕾石煜 School of Gemmology China University of Geosciences Beijing Institute ofGeology for Mineral Resources Beijing Institute of Geology for Mineral Resources School of Earth Sciences and Mineral Resources China University of Geosciences

Alkali-rich dykes of the Late Permian in Ulungur （乌伦古） area are representative prod- ucts of granitic magmatism in the evolution of the Paleozoic orogenic belt in the East Junggar （准噶尔）, North Xinjiang （新疆）. We selected two representative samples for geochemical analysis （major and trace elements）, and twenty-two zircon grains for zircon dating. Isoplot （ver3.0） was used to calculate isotopic age and make concordia diagrams. This study shows that the trachy porphyry dykes, featuring low concentration of A1203, CaO, MgO and high alkali contents, are metaluminous alkaline rock and belong to A-type granitoids. The dykes have low concentration of the REE and incompatible elements, and the REE patterns show clear negative Eu anomalies （6Eu=0.74-0.58）, enriched LREE （LREE/HREE〉4, （La/Yb）N=5.97-4.63） and undifferentiated HREEs. Similar normalized REE and in- compatible element patterns are also showed in the dykes from Yemaquan （野马泉） and granites from Ulungur, suggesting that they are possibly originated from the same source and formed in the same tectonic environment, but the trachy porphyry dykes are more evolved. The age of the trachy porphyry dykes is 255.3 - 2.4 Ma, which is probably the crystallization time of the trachy porphyry. The dykes formed in late-orogenic phase of post-collision.process or within-plate environment, which suggested that the trachy porphyry dykes possibly crystallized in the transition period during which the tectonic setting changed from post-collision.to with- in-plate environment. So we consider that the age when the post-collision.ended and the crustal cratonization begun in the East Junggar is 255 Ma, Late Permian.

关键词： Ulungur alkali-rich dyke zircon U-Pb age petrology and geochemistry tectonic setting post-collision.

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Geochemistry and Tectonic Setting of the Eshan Granites in the Southwestern Margin of the Yangtze Plate, Yunnan

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Journal of Earth Science 2018年第1期29卷 130-143页

作者： Jin Hu Shitao Zhang Guangzheng Zhang Siyu Tao Ying Zhang Faculty of Land Resource Engineering Kunming University of Science and Technology

The extensive Eshan granites of Yunnan are made up of three intrusive units distin- guished by their field contact relations; in descending order of age they are the Pojiao Unit, the Liizicun Unit and the Mokela Unit. The Pojiao Unit and Luzicun Unit contain petrographically and geochemically similar rocks but contact relationships show that the latter is younger. The Mokela Unit mainly consists of dykes intruding the other two and has petrographic and geochemical differ-ences. Zircon U/Pb dating and zircon crystallization temperature measurements confirm the se- quence of intrusions. Major and trace element analyses suggest that the magmas of the Pojiao Unit granites derived by partial melting of a clay-poor source from the upper crust; the magmas of the Luzicun Unit granites derived by partial melting of upper crust with a small proportion of middle crust accompanied by crystallization of albite which triggered strength reduction. Both magmas mixed and underwent with crustal contamination, assimilation and fractional crystallization. The magmas of the Mokela Unit derived from residual melts and assimilation of argillaceous rocks. A time sequence of melting, intrusion and deformation events is derived from these results and com- pared with other published tectonic models for the evolution of the SW margin of the Yangtze Plate. Magmatism was initiated by exhumation of upper continental crust during which strongly peralu- minous porphyritic biotite monzogranite granites were produced at ca. 854-852 Ma, and the genesis of two-mica granite reflected a later batch of exhumed melts with crustal contamination, assimilation and fractional crystallization at ca. 842 Ma. Finally biotite alkali-feldspar granite and tourmaline granite magmas experienced strong fractional crystallization, emplaced in the cooling stage at ca. 823 Ma, indicating the end of exhumation.

关键词： Yangtze Plate Eshan granite tectonic evolution continental exhumation post-collision.

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