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Timing of metamorphism in the Paleoproterozoic Jiao-Liao-Ji Belt New SHRIMP U-Pb zircon dating of granulites, gneisses and marbles of the Jiaobei massif in the North China Craton [查看] PuiYukTam、GuochunZhao、FulaiLiu、XiwenZhou、MinSun、SanzhongLi

The Paleoproterozoic Jiao-Liao-Ji Belt lies in the Eastern Block of the North China Craton, with its southern segment extending across the Bohai Sea into the Jiaobei massif. High-pressure pelitic and mafic granulites have been recently recognized in the Paleoproterozoic Jingshan Group (Jiaobei massif). New SHRIMP U–Th–Pb geochronology combined with cathodoluminescence (CL) imaging of zircon has been applied to the determination of the timing of the metamorphism of the high-temperature and high-pressure granulites and associated gneisses and marbles. Metamorphic zircons in these high-pressure granulites, gneisses and marbles occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores. Metamorphic zircons are all characterized by nebulous zoning or being structureless, with high luminescence and relatively low Th/U values. Metamorphic zircons from two high-pressure mafic granulites yielded 207Pb/206Pb ages of 1956±41 Ma and 1884±24 Ma. One metamorphic zircon from a garnet–sillimanite gneiss also gave an apparent 207Pb/206Pb age of 1939±15 Ma. These results are consistent with interval of ages of c. 1.93–1.90 Ga already obtained by previous studies for the North and South Liaohe Groups and the Laoling Group in the northern segment of the Jiao-Liao-Ji Belt. Metamorphic zircons from a high-pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207Pb/206Pb ages of 1837±8 Ma, 1821±8 Ma and 1836±8 Ma respectively. Two diopside–olivine–phlogopite marbles yielded weighted mean 207Pb/206Pb ages of 1817±9 Ma and 1790±6 Ma. These Paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c. 1.85 Ga produced from the Ji'an Group in the northern segment of the Jiao-Liao-Ji Belt and c. 1.86–1.80 Ga obtained for the high-pressure pelitic granulites from the Jingshan Group in the southern segment. As this metamorphic event was coeval with the emplacement of A-type granites in the Jiao-Liao-Ji Belt and its adjacent areas, it is interpreted as having resulted from a post-orogenic or anorogenic extensional event.

SHRIMP zircon U-Pb ages of eclogite and orthogneiss from Sulu ultrahigh-pressure zone in Yangkou area, eastern China [查看] A.Katsube、Y.Hayasaka、M.Santosh、SanzhongLi、K.Terada

We report SHRIMP U–Pb age of zircons in four samples of eclogite and one sample of orthogneiss from Sulu ultrahigh-pressure (UHP) zone in Yangkou area, eastern China. UHP rocks are distributed along the Sulu orogenic belt suturing North China Block with South China Block. In Yangkou area, UHP unit is well exposed for about 200 m along Yangkou beach section and consists mainly of blocks or lenses of ultramafic rocks and eclogite together with para- and orthogneiss which are highly sheared partly. Zircon grains examined in this study from eclogite show oscillatory zoning and overgrowth texture in CL images, and most of the grains have high Th/U ratio ranging from 0.8 to 2.1 indicating an igneous origin. The weighted mean 206Pb/238U ages of zircons from the four samples range from 690 to 734 Ma. These ages can be correlated to the magmatic stage of the protoliths. In rare cases, zircon grains possess a narrow rim with very low Th/U ratio (b0.02). EPMA U–Th total Pb dating of such rim yields younger ages that range from 240 to 405 Ma marking the metamorphic stage.On the other hand, zircons from the orthogneiss show irregular shape and zoning with inclusion-rich core and inclusion-free rim. These grains of zircon yield U–Pb discordia intercept ages of 226±63 Ma and 714±110 Ma (MSWD 0.78). Bulk of the areas of the rims rim of the zircons demonstrate younger 206Pb/238U ages close to the upper intercept, with low Th/U ratio (b0.20) indicating their metamorphic origin. In contrast, the cores show older 206Pb/238U ages close to lower intercept and high Th/U ratio of (0.14–5.25) indicating their igneous origin. The upper intercept age is also commonly noted in zircons from eclogite. Our results suggest a bimodal igneous activity along this zone during the Neoproterozoic, probably related to the rifting of the Rodinia supercontinent.

SHRIMP U–Pb zircon ages of granitoid rocks in the Luliang Complex Implications for the accretion and evolution of the Trans-North China Orogen [查看] GuochunZhaoa、SimonA.Wilde、MinSun、SanzhongLi、XupingLi、JianZhang

The lvliang Complex is situated in the central segment of the Trans-North China Orogen (TNCO), a continent-continent collisional belt along which the discrete Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The complex consists of supracrustal and granitoid rocks, of which the latter can be subdivided into pre-tectonic TTG gneisses, including the Yunzhongshan, Guojiazhuang and Chijianling–Guandishan gneisses; syn-tectonic gneissic granites represented by the Huijiazhuang gneissic granite; and post-tectonic granites,including the Luyashan charnockite, Luchaogou porphyritic granite and Tangershang/Guandishan massive granite. The pre-tectonic TTG gneisses are mostly calc-alkaline and considered to have formed in a magmatic arc environment. SHRIMP U–Pb analyses reveal that the Yunzhongshan gneisses were emplaced at ∼2499±9 Ma, representing the earliest arc-related magmatic event in the Luliang Complex. This was followed by the intrusion of the Guojiazhuang gneisses at 2375±10 Ma. The most widespread arc-related magmatic event in the region was the emplacement of the Chijianling–Guandishan granitoid gneisses, of which the tonalitic, granodioritic and monzogranitic gneisses were emplaced at 2199±11 Ma,2180±7Ma and 2173±7 Ma, respectively. Metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207Pb/206Pb age of 1872±7 Ma, consistent with the metamorphic age range of 1880–1820 Ma defined by metamorphic zircons from various high-grade rocks in the TNCO. Thus, the evolution of the lvliang Complex involved emplacement of the Yunzhongshan granitoids at ∼2499 Ma, the Guajiazhuang granitoids at ∼2375 Ma and the Chijianling–Guandishan granitoids at 2199–2173 Ma, with the final collision between the Eastern and Western Blocks in this area occurring at 1872±7 Ma, the whole series of magmatic events lasting nearly 650 Ma. This suggests that the Trans-North China Orogen represents a long-lived magmatic arc. The new SHRIMP zircon data, combined with structural and petrological studies, also reveal the existence of both syn- and post-collisional granites in the Luliang Complex. The former is represented by the Huijiazhuang gneissic granite that was emplaced at 1832±11 Ma, whereas the latter include the 1815±5Ma Luyashan charnockite, 1807±10 Ma Luchaogou porphyritic granite,1798±11 Ma Guandishan massive granite and 1790±14 Ma Tangershan massive granite.

SHRIMP U-Pb zircon geochronology of the Liaoji granitoidsConstraints on the evolution of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton [查看] SanzhongLi、GuochunZhao

The Paleoproterozoic Jiao-Liao-Ji belt lies at the eastern margin of the Eastern Block of the North China Craton and is composed mainly of the Liaohe Group and the Liaoji granitoids. The Liaoji granitoids are divisible into pre-tectonic magnetite and hornblende/ biotite monzogranitic gneisses and post-tectonic or anorogenic porphyritic monzogranites and alkaline syenites. SHRIMP U–Pb zircon geochronology, combined with cathodoluminescence (CL) imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the history of the Paleoproterozoic Jiao-Liao-Ji belt. SHRIMP U–P zircon analyses reveal that the pre-tectonic magnetite monzogranitic gneisses were emplaced in the period 2176–2166 Ma and metamorphosed at 1914 Ma, and the pre-tectonic hornblende/biotite monzogranitic gneisses were emplaced at 2150–2143 Ma. These data demonstrate that the pre-tectonic monzogranitic gneisses of the Liaoji granitoids were not younger than the deposition of the Liaohe Group, as previously considered, since the latter contains a large amount of 2100–2000 Ma detrital zircons. In contrast, the pre-tectonic monzogranitic gneisses of the Liaoji granitoids or similar-aged granitoids may have been an important component of the provenance for the Liaohe Group. SHRIMP U–Pb zircon analyses also reveal that the post-tectonic porphyritic monzogranites and granites in the Jiao-Liao-Ji belt were emplaced at 1875±10 and 1856±31 Ma, respectively, simultaneously with the emplacement of the alkaline syenites that was dated at 1857±20 and 1843±23 Ma. Similar post-tecotnic or anorogenic granites are not only limited to the Jiao-Liao-Ji belt, but are also found in the North and South Korea, suggesting that the ∼1.85 Ga post-tectonic or anorogenic granitic magmatic event may have occurred in both the Korean Peninsula and the Eastern Block of the North China Craton. The SHRIMP zircon ages of this study combined with lithological, structural, metamorphic and geochemical data is consistent with a rift closure model which suggests that the Archean complexes on the northern and southern sides of the Jiao-Liao-Ji belt were originally situated on a single continental block that underwent Paleoproterozoic rifting, associated with the formation of the Liaohe Group and Liaoji granitoids, and closed upon itself at ∼1.9 Ga.

Implications based on the first SHRIMP U-Pb zircon dating on Precambrian granitoid rocks in North Korea [查看] GuochunZhao、LinCao、SimonA.Wilde、MinSun、WonJongChoe、SanzhongLi

Precambrian basement rocks in North Korea consist of supracrustals and granitoid rocks, of which the former comprise the late Archean Nangrim Group and the Paleoproterozoic Jungsan andMachollyong Groups, whereas the latter include four major types: grey TTG gneisses, garnet/sillimanite-bearing (S-type) granites,massive porphyritic granites, and hornblende-bearing granites. SHRIMP U–Pb zircon geochronology, combined with cathodoluminescence (CL) imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the Precambrian history of North Korea and its relations toNorth China. SHRIMP U–Pb zircon analyses on two grey gneiss samples (PO501 and K14) reveal that they were emplaced at 2538±14 Ma and 2636±49 Ma,respectively, and metamorphosed at 2462–2433 Ma. These data, for the first time, confirm the existence of late Archean rocks in the Korean Peninsula.Moreover, one grey gneiss sample (K14) contains several zircons with ages of 3.1–3.4Ga, suggesting possiblemiddle Archean crust in North Korea. Rocks with such ages are widely distributed in North China, implying that North China and North Korea have similarArchean basement.One grey gneiss sample (R53) yielded a SHRIMP zircon age of 1933±12Ma, indicating that some grey gneisses in North Korea formed during the Paleoproterozoic. Two garnet- and/or sillimanite-bearing S-type granite samples gave 207Pb/206Pb ages of 1908±31Ma and 1903±49Ma, interpreted to reflect the time ofmetamorphismof the Paleoproterozoic Jungsan and Machollyong Groups since these S-type granites are considered to be derived fromthe partialmelting of pelitic rocks fromthese groups. The∼1.9Gametamorphic event has also been revealed in Paleoproterozoic rocks in South Korea and NorthChina. SHRIMP zircon data also show that massive porphyritic monzogranites inNorth Korea evolved over a protracted interval from1865Ma to 1843Ma. Similaraged porphyritic monzogranites are also found in North China and South Korea, and their origin is considered to be related to a posttectonic extensional event of regional extent. The last phase of Precambrian magmatism recognized in North Korea is Grenvillian-aged,represented by the hornblende-bearing granite that yields a SHRIMP zircon age of 1195±4Ma, and is considered to be the source rock of the adjacent Yushulazi Group (North China) that contains numerous 1.1–1.2 Ga old detrital zircons.

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