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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.

Prolonged magmatism, juvenile nature and tectonic evolution of the Chinese Altai,NW China Evidence from zircon U–Pb and Hf isotopic study of Paleozoic granitoids [查看] KedaCai、MinSun、ChaoYuan、GuochunZhao、WenjiaoXiao、XiaopingLong、FuyuanWu

Paleozoic granitoid magmatism played an important role in the tectonic evolution of the Chinese Altai,and zircon U–Pb and Hf isotopic compositions have been determined for samples from eleven granitic plutons/batholiths. The Jiadengyu gneissic granitic pluton yielded a zircon U–Pb age of 479 Ma, and thus it does not represent Precambrian basement as suggested previously. Our results and published data demonstrate that voluminous granitoids were continuously emplaced over more than 30% area of the Chinese Altai during the period from 447 Ma to 368 Ma with a climax at ca. 400 Ma. Ages for zircon overgrowth rims demonstrate additional thermal events at ca. 360 and 280 Ma, respectively. Positive eHf(t) values (0 to +9) of normal magmatic zircons suggest that the granitoid magmas were derived from juvenile sources. Xenocrystic zircon cores are 543–421 Ma old and also give positive eHf(t) values (+2.5 to +12), suggesting their origin as early crystallized minerals in the magma chambers or as inherited cores from newly-accreted meterials. The strong magmatism at ca. 400 Ma significantly changed the Hf isotopic composition of the magma source by substantial input of juvenile material in a relatively short period.Geophysical, geological and geochemical data support that ridge subduction was a possible mechanism for the strong magmatism ca. 400 Ma and the above mentioned change of Hf isotopic composition in the magma source.

Geochronology, petrogenesis and tectonic significance of peraluminous granites from the Chinese Altai, NW China [查看] KedaCai、MinSun、ChaoYuan、GuochunZhao、WenjiaoXiao、XiaopingLong、FuyuanWu

The Paleozoic granites in the Chinese Altai are important for the study of tectonic evolution and crustal growth in the Central Asian Orogenic Belt (CAOB). Four representative peraluminous granitic intrusions were selected for systematic studies of zircon U–Pb and Hf isotopic compositions and whole-rock geochemical and Nd–Sr isotopic analyses. These rocks have high ASI (Alumina Saturation Index, Al2O3/(CaO+Na2O+K2O)=1.01–1.46 molecular ratios), with 0.6–5.6 wt.% of normative corundum, and are characterized by moderately negative Eu anomalies (Eu/Eu*=0.38–0.98) and strong depletion in Ba, Nb and Sr elements. Our data suggest that these intrusions were emplaced from 419 to 393 Ma, consistent with a period of intensive magmatic activities and high temperaturemetamorphismin the Chinese Altai.While in situ zircon Hf isotopic analyses for these granites give predominantly positive εHf(t) values (+0.8 to+12.8), a few inherited zircons yield negative εHf(t) values from −12.5 to −1.53. The U–Pb age and Hf isotopic data of these inherited zircons are similar to that of the widespread metasediments. In addition, the peraluminous granitic rocks have near-zero or negative εNd(t) values(−3.3 to−0.5) and relatively high initial 87Sr/86Sr ratios (0.7079–0.7266), distinct fromthose of the I-type granites in the study region, but similar to the Early Paleozoic Habahe sediments. These isotopic compositions suggest that the newly accreted metasediments of Habahe Group may be the major source rock of the peraluminous granites. The geochemical compositions indicate that their precursor magmas were derived from a relatively shallow crustal level (P≤5 kbar) and zircon saturation temperatures suggest that these granitic intrusions were emplaced at 672–861 °C. The peraluminous granitic magmas were generated by dehydration melting of newly accreted materials, which were possibly brought to at least middle crustal depth by subduction-related processes in an active margin, and were subsequently molten by strikingly high ambient temperature probably caused by upwelling of the hot asthenosphere associated with ridge subduction in the Paleozoic.

Geochronological and Geochemical study of Palaeoproterozoic gneissic granites and clinopyroxenite xenoliths from NW Fujian, SE China Implications for the crustal evolution of the Cathaysia Block [查看] Long-MingLi、MinSun、YuejunWang、GuangfuXing、GuochunZhao、KedaCai、YuzhiZhang

U–Pb zircon dating of gneissic granite samples in northwestern Fujian Province, Southeast China, gave Neoarchean to Palaeoproterozoic ages (2.6–1.9 Ga) for the rounded zircon cores and Palaeoproterozoic upper intercept ages (1851 ± 21 Ma, 1857 ± 29 Ma, respectively) for zoned rims that have oscillatory growth zoning and yield eHf(t) values between 8.8 and +3.7 and TDM model ages between 2.1 and 2.6 Ga. These gneissic granites are peraluminous (A/CNK = 0.94–1.28), characterized by high SiO2 (68–72 wt.%), Al2O3 (14–15 wt.%) and low CaO, MgO, FeOt, TiO2 and P2O5 contents. They also possess relatively low REE contents and moderate LREE/HREE fractionation and display pronounced negative anomalies in Nb, Ta, Sr, P and Ti. All these features indicate that the precursor magmas were likely formed by partial melting of sedimentary rocks.Clinopyroxenite xenoliths with gneissosity structures are found in these gneissic granites and they have high MgO, Ni and Cr, but low contents of TiO2, and are characterized by high CaO/Al2O3 (4.1–5.3),low Al2O3/TiO2 ratios (8.7–9.5) and HREE depletion. These features are similar to the typical Al-depleted type komatiites. Their geochemical characteristics, such as high (Gd/Yb)N and low HREE, Y, Zr and Hf suggest partial melting of the upper mantle source with residual garnet. The negative Nb, Ta and Ti anomalies indicate a strong arc-related affinity.Our results and recently published data for granitic and mafic basement rocks in this region, reveal that 1.85 Ga was an important period for the evolution of the Cathaysia Block, possibly related to the Columbia supercontinent assembly. Large amount of granitic magma was emplaced, probably as a result of partial melting of old Neoarchean to Palaeoproterozoic materials.

Geochemical and geochronological study of early Carboniferous volcanic rocks from the West Junggar Petrogenesis and tectonic implications [查看] HongyanGeng、MinSun、ChaoYuan、GuochunZhao、WenjiaoXiao

A series of early Carboniferous volcanic rocks, i.e., basalt, andesite, and dacite, occur in the West Junggar.Zircon U–Pb isotopic data indicate that these rocks coevally erupted at 331–344 Ma. These volcanic rocks have comparable trace element patterns, characterized by the enrichment of large ion lithophile elements (LILE) and the depletion of high field strength elements (HFSE). Isotopically, they display consistently depleted Sr–Nd isotopic compositions (initial 87Sr/86Sr ratios = 0.7034–0.7054, eNd(t) = 4.2–7.7).These rocks are interpreted to be derived from a slightly metasomatized depleted mantle in a normal subduction environment. In the context of a previously proposed ridge subduction model, this study constrains that ridge subduction, if it indeed occurred, commenced between 331 and 310 Ma in the central West Junggar.

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.

What Happened in the Trans-North China Orogen in the Period 2560-1850 Ma [查看] GuochunZHAO、LIUShuwen、MinSUN、LISanzhong、SimonWILDE

Abstract: The Trans-North China Orogen (TNCO) was a Paleoproterozic continent-continentcollisional belt along which the Eastern and Western Blocks amalgamated to form a coherent North China Craton (NCC). Recent geological, structural, geochemical and isotopic data show that the orogen was a continental margin or Japan-type arc along the western margin of the Eastern Block, which was separated from the Western Block by an old ocean, with eastward-directed subduction of the oceanic lithosphere beneath the western margin of the Eastern Block. At 2550-2520 Ma, the deep subduction caused partial melting of the medium-lower crust, producing copious granitoid magma that was intruded into the upper levels of the crust to form granitoid plutons in the low- to medium-grade granite-greenstone terranes. At 2530-2520 Ma, subduction of the oceanic lithosphere caused partial melting of the mantle wedge, which led to underplating of mafic magma in the lower crust and widespread mafic and minor felsic volcanism in the arc, forming part of the greenstone assemblages. Extension driven by widespread mafic to felsic volcanism led to the development of back-arc and/or intm-arc basins in the orogen. At 2520-2475 Ma, the subduction caused further partial melting of the lower crust to form large amounts of tonalitic-trondhjedtic-granodioritic (TTG) magmatism. At this time following further extension of back-arc basins, episodic granitoid magmatism occurred, resulting in the emplacement of 2360 Ma, -2250 Ma 2110-21760 Ma and -2050 Ma granites in the orogen.Contemporary volcano-sedimentary rocks developed in the back-arc or intra-arc basins. At 2150-1920Ma, the orogen underwent several extensional events, possibly due to subduction of an oceanic ridge,leading to emplacement of mafic dykes that were subsequently metamorphosed to amphibolites and medium- to high-pressure mafic granulites. At 1880-1820 Ma, the ocean between the Eastern and Western Blocks was completely consumed by subduction, and the closing of the ocean led to the continent-arc-continent collision, which caused large-scale thrusting and isoclinal folds and transported some of the rocks into the lower crustal levels or upper mantle to form granulites or eclogites. Peak metamorphism was followed by exhumatioduplift, resulting in widespread development of asymmetric folds and symplectic textures in the rocks.

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.

EPMA U-TH-Pb MONAZITE AND SHRIMP U-Pb ZIRCON GEOCHRONOLOGY OF HIGH-PRESSURE PELITIC GRANULITES IN THE JIAOBEI MASSIF OF THE NORTH CHINA CRATON [查看] XIWENZHOU、GUOCHUNZHAO、CHUNJINGWEI、YUANSHENGGENG、MINSUN

ABSTRACT. The Jiaobei massif constitutes the southern segment of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton, and contains high-pressure (HP) mafic and pelitic granulites. The HP pelitic granulites contain four distinct mineral assemblages: garnet kyanite biotite muscovite rutile (M1),garnet kyanite ternary-feldspar biotite muscovite rutile (M2), garnet sillimanite biotite perthite anti-perthite (M3), and biotite muscovite sillimanite plagioclase (M4), of which M2 is a typical HP pelitic assemblage. The origin of these HP granulites is important for understanding the tectonic evolution of the Jiao-Liao-Ji Belt, but the timing of the HP event still remains unknown. This study applied the EPMA U-Th-Pb monazite and SHRIMP U-Pb zircon dating techniques to determine the metamorphic ages of the HP pelitic granulites in the Jiaobei massif. CL images and Th/U ratios show that all monazite and zircon grains in the analyzed samples are of metamorphic origin. Eighty-two EPMA analyses on 16 monazite grains from two samples yielded three age groups at 1895 to 1882 Ma, 1814 to 1813 Ma and 1706 to 1692 Ma, and the ages of 16 SHRIMP analyses on 14 zircon grains from one sample range from 1864 to 1803 Ma. The oldest ages of 1895 to 1882 Ma were yielded from high-Y monazite grains enclosed in garnet and sillimanite pseudomorph after kyanite, and thus interpreted as the time of the HP (M2) granulite-facies metamorphism.The EPMA monazite ages of 1814 to 1813 Ma and SHRIMP zircon ages of 1864 to 1803 Ma are interpreted as the approximate ages of the post-HP granulite-facies metamorphism (M3), most probably resulting from the exhumation of the HP granulites to the medium-pressure granulite-facies levels. The youngest age group of 1706 to 1692 Ma obtained from low-Y monazite grains in the matrix can be interpreted as the age of late cooling and retrograde metamorphism (M4) that occurred when the HP pelitic granulites were exhumed to the upper crust. The presence of Paleoproterozoic HP mafic and pelitic granulites in the southern segment of the Jiao-Liao-Ji Belt suggests that the evolution of the Jiao-Liao-Ji Belt in the Eastern Block of the North China Craton must have been involved in subduction- or collision-related tectonic processes.

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