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    Zircon SHRIMP U-Pb and in-situ Lu–Hf isotope analyses of a tuff from Western Beijing Evidence for missing Late Paleozoic arc volcano eruptions at the northern margin of the North China block [查看] Shuan-HongZhang、YueZhao、BiaoSong、Yue-HengYang
    Zircon SHRIMP U–Pb and in-situ Lu–Hf isotopic analyses via laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) of a tuff within the Upper Paleozoic from Western Beijing were carried out to give new constraints on volcano eruption ages and source area of the tuffs within the North China block (NCB). SHRIMP U–Pb zircon dating of the tuff yielded a 206Pb/238U weighted mean age of 296±4 Ma (95% confidence, MSWD=3.3), which is very similar to the emplacement age of the newly discovered Carboniferous calc-alkaline,I-type continental arc granitoid plutons in the Inner Mongolia Paleo-uplift (IMPU) on the northern margin of the NCB. In-situ Lu–Hf analysis results of most zircons from the tuff yielded initial 176Hf/177Hf ratios from 0.282142 to 0.282284 and εHf(t) values from −15.9 to −10.7. These Lu–Hf isotopic compositions are very similar to those of the Late Carboniferous granitoids in the IMPU, but are very different to those of the Central Asian Orogenic Belt (CAOB). Together with the sedimentary and tectonic analyses results, we inferred that the source area of the tuffs within the NCB is the IMPU instead of the CAOB. Therefore, some arc volcanoes once existed in the IMPU on northern margin of the NCB during the Late Carboniferous, but they were entirely eroded due to strong exhumation and erosion of the IMPU during the Late Carboniferous to Early Jurassic.
    The 1.75–1.68 Ga anorthosite-mangerite-alkali granitoid-rapakivi granite suite from the northern North China Craton Magmatism related to a Paleoproterozoic orogen [查看] Shuan-HongZhang、Shu-WenLiu、YueZhaoa、Jin-HuiYang、BiaoSong、Xiao-MingLiu
    The AMGRS is a suite of anorthosite-mangerite-alkali granitoid-rapakivi granite in the northern North China Craton (NCC).New zircon SHRIMP and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U–Pb dating of the Damiao anorthosite, Changsaoying K-feldspar granite, Lanying anorthosite and quartz syenite and the Gubeikou K-feldspar granite yielded emplacement ages of 1726±9 Ma, 1753±23 Ma, 1739±43 Ma, 1712±15 Ma and 1692±19 Ma, respectively. Combined with previous geochronological data from the Damiao norite and mangerite and Shachang rapakivi granite, it indicates that the whole AMGRS was intruded between 1750 Ma and 1680 Ma. The Damiao anorthosites and norites exhibit a high Mg#, and this decreases from mafic (up to 63) to felsic (<10) end members. The alkali granitoids of the AMGRS are characterized by high contents of alkali (K2O+Na2O), HSFE, TFeO/MgO, molecular (K2O+Na2O)/Al2O3 and Ga/Al values, that are typical features of A-type granites. All of the diverse components of the AMGRS have similar Sr–Nd–Hf isotopic compositions with low whole-rock initial 87Sr/86Sr ratios of 0.703–0.704, εNd(T) values of −5.0 to −6.3 (with TDM model ages of 2.34 to 2.58 Ga), and zircon εHf(T) values ranging from −4.1 to −7.5 (with Hf isotopic model TDM and TCDM ages ranging from 2.32 Ga to 2.43 Ga and 2.70 Ga to 2.88 Ga,respectively).This suggests that these rocks are the fractional crystallization products of one parental magma. Zircon Hf and wholerock Sr–Nd–Pb isotopic systematics combined with high La/Yb (>10) and intermediate Th/Ta ratios, suggest that the parental magma of the AMGRS could have been derived from EM-I lithosphere mantle enriched by recycled Archean continent crustal materials, accompanied by some assimilation of lower crustal components. The mafic to felsic compositions of the AMGRS are different from typical bimodal rocks formed in an anorogenic intra-continent rift setting, but resemble those emplaced during postcollisional/post-orogenic extensional tectonic setting. This is compatible with their age of <1.85 Ga, indicating they postdate the continent–continent collision between the Western and Eastern blocks of the NCC at ∼1.85 Ga.
    Petrochemistry, oxygen isotopes and U-Pb SHRIMP geochronology of mafic-ultramafic bodies from the Sulu UHP terrane, China [查看] R.ZHAO、R.Y.ZHANG、J.G.LIOU、A.L.BOOTH、E.C.POPE、C.P.CHAMBERLAIN
    Two Rongcheng eclogite-bearing peridotite bodies (Chijiadian and Macaokuang) occur as lenses within the country rock gneiss of the northern Sulu terrane. The Chijiadian ultramafic body consists of garnet lherzolite, whereas the Macaokuang body is mainly meta-dunite. Both ultramafics are characterized by high MgO contents, low fertile element concentrations and total REE contents, which suggests that they were derived from depleted, residual mantle. High FeO contents, an LREE-enriched pattern and traceelement contents indicate that the bulk-rock compositions of these ultramafic rocks were modified by metasomatism. Oxygen-isotope compositions of analysed garnet, olivine, clinopyroxene and orthopyroxene from these two ultramafic bodies are between +5.2‰ and +6.2‰(δ18O), in the range of typical mantle values (+5.1 to +6.6‰). The eclogite enclosed within the Chijiadian lherzolite shows an LREEenriched pattern and was formed by melts derived from variable degrees (0.005–0.05) of partial melting of peridotite. It has higher d18O values (+7.6‰ for garnet and +7.7‰ for omphacite) than those of lherzolite. Small O-isotope fractionations (DCpx-Ol: 0.4‰, DCpx-Grt: 0.1‰, DGrt-Ol: 0.3–0.4‰) in both eclogite and ultramafic rocks suggest isotopic equilibrium at high temperature. The P–T estimates suggest that these rocks experienced subduction-zone ultrahigh-pressure (UHP) metamorphism at~700–800℃, 5 GPa, with a low geothermal gradient. Zircon from the Macaokuang eclogite contains inclusions of garnet and diopside. The 225 ± 2 Ma U/Pb age obtained from these zircon may date either the prograde conditions just before peak metamorphism or the UHP metamorphic event, and therefore constrains the timing of subduction-related UHP metamorphism for the Rongcheng mafic–ultramafic bodies.
    Early history of the eastern Sibao Orogen (South China) during the assembly of Rodinia New mica 40Ar39Ar dating andSHRIMP U–Pb detrital zircon provenance constraints [查看] Zheng-XiangLi、Jo-AnneWartho、SandraOcchipinti、Chuan-LinZhang、Xian-HuaLi、JianWang、ChaominBao
    The Sibao Orogen in South China is one of the poorest known Grenville-aged orogenic belts through which the Neoproterozoic supercontinent Rodinia assembled. We report here the first UV laser spot 40Ar/39Ar mica and SHRIMP U–Pb zircon ages from a rare Grenville-aged metamorphic complex, the Tianli Schists, in the eastern Sibao Orogen. Our U–Pb zircon provenance ages indicate that the protolith of the Tianli Schists was a clastic sedimentary succession most likely derived from the Yangtze Block.The depositional age of the protolith is younger than 1530 Ma, as constrained by the youngest detrital zircon grains, but is older than 1040 Ma as constrained by the oldest 40Ar/39Ar muscovite ages. The Yangtze Block provenance for the Tianli Schists suggests that the Sibaoan ophiolitic complexes in northeastern Jiangxi, the ca. 970 Ma Xiwan adakitic granite intrusions, and the ca. 900 Ma(?) Xiwan blueschists, all to the northwest of the study region, were likely formed during the closure of a back-arc basin along the margin of the Yangtze Block. Our in situ UV laser 40Ar/39Ar results from S1 and S2 muscovites suggest that the Tianli Schists underwent metamorphism and deformation at 1042±7Ma to 1015±4 Ma, the oldest known metamorphic event in the easternSibao Orogen. Muscovite/biotite cooling ages of ca. 968±4 and 942±8Ma are recorded by deformed and recrystallised muscovite and biotite, respectively, indicating tectonic reactivation before 900 Ma, during the later stages of the Sibao Orogeny. Together with previous results from the western Sibao Orogen, our work suggests that the closure of the ocean between the Yangtze and Cathaysia Blocks during the assembly of Rodinia was diachronous: ≥1000 Ma at the western Sibao Orogen and ca. 900 Ma at the eastern Sibao Orogen.
    A cold Early Palaeozoic subduction zone in the North Qilian Mountains, NW China petrological and U-Pb geochronological constraints [查看] J.X.ZHANG、F.C.MENG、Y.S.WAN
    The north Qilian high-pressure (HP)/low-temperature (LT) metamorphic belt is composed mainly of blueschists, eclogites and greenschist facies rocks. It formed within an Early Palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved HP/LT metamorphic belts in China. Here we report new lawsonite-bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the North Qilian Mountains. Five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet, indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks. Peak pressure conditions estimated from lawsonite omphacite-phengitegarnet assemblages were 2.1–2.4 GPa at temperatures of 420–510℃, in or near the stability field of lawsonite eclogite, and implying formation under an apparent geothermal gradient of 6–8 ℃ km)1,consistent with metamorphism in a cold subduction zone. SHRIMP U-Pb dating of zircon from two lawsonite-bearing eclogitic metabasites yields ages of 489 ± 7 Ma and 477 ± 16 Ma, respectively. CL images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism. An age of 502 ± 16 Ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph-bearing eclogite, which is in agreement with the formation age of Early Ordovician for some ophiolite sequences in the North Qilian Mountains, and may be associated with a period of oceanic crust formation. The petrological and chronological data demonstrate the existence of a cold Early Palaeozoic subduction zone in the North Qilian Mountains.
    U-Pb zircon SHRIMP ages, geochemical and Sr-Nd-Pb isotopic compositions of intrusive rocks from the Longshan–Tianshui area in the southeast corner of the Qilian orogenic belt, China Constraints on petrogenesis and t [查看] Hong-FeiZhang、Ben-RenZhang、NigelHarris、LiZhang、Yue-LongChen、Neng-SongChen、Zhi-DanZhao
    The Longshan-Tianshui area is located in the southeast corner of the Qilian orogenic belt. Three intrusions (the Yanjiadian diorite, the Caochuanpu and the Guanshan granites) in this area provide insights into regional tectono-magmatic events, petrogenesis and tectonic evolution. Based on U–Pb zircon SHRIMP dating, the magma emplacement ages of the Yanjiadian diorite, the Caochuanpu and the Guanshan granites are 441G10, 434G10 and 229G7 Ma, respectively. The Yanjiadian diorites have geochemical characteristics similar to island-arctype rocks, but their formation resulted from northward subduction of the Huluhe (Erlanping) back-arc basin. The Caochuanpu granites with K2O/Na2O＜0.9, Y of ＜6.9 ppm, Yb of ＜0.39 ppm, Sr/Y of 36–97 and (La/Yb)N of 34.8–80.0 exhibit adakitic geochemical signatures. Combined with Pb–Sr–Nd isotope data, the magma for the Caochuanpu granites was suggested to be derived from partial melting of previously underplated Neoproterozoic mafic rocks in the Qinling (or Longshan–Tianshui) island-arc lower crust. The partial melting resulted from island-arc lower crustal thickening due to northward subduction of the Qinling ocean and closure of the Huluhe back-arc. The Guanshan granites with (La/Yb)N of 17.5–42.4, Sr/Y of 15.8–50.5, Y of 7.8–15.7 ppm and Yb of 0.64–1.31 ppm also display some adakitic geochemical signatures. However, the geochemical and Pb–Sr–Nd isotopic compositions of the Guanshan granites are quite distinct from those of the Caochuanpu granites. It is proposed that magma for the Guanshan granites was derived from partial melting of subducted South Qinling–West Qinling continental crust during Triassic continental collision between the North China and the South China plates. The result provides an example that the adakitic rocks can be generated from partial melting of subducted continental crust. The tectono-magmatic events and petrogenesis of intrusive rocks in the Longshan–Tianshui area are in agreement with those of the North Qinling tectonic unit in the Qinling orogenic belt. Therefore, the Qilian orogenic belt corresponds to the North Qinling tectonic unit in a regional tectonic framework. Both the Qilian and the Qinling belts had a similar tectonic evolution.
    SHRIMP zircon U-Pb dating for volcanic rocks of the Dasi Formation in southeast Hubei Province, middle-lower reaches of the Yangtze River and its implications [查看] XIEGuiqing、MAOJingwen、LIRuiling、ZHOUShaodong、YEHuishou、YANQuanren、ZHANGZusong
    Abstract The Jinniu Basin in southeast Hubei,located at the westernmost part of middle-lower valley of the Yangtze River, is one of the important volcanic basins in East China. Volcanic rocks in the Jinniu Basin are distributed mainly in the Majiashan Formation, the Lingxiang Formation and the Dasi Formation, consisting of rhyolite, basalt and basaltic andesite, (trachy)-basalt and basaltic trachy-andesite and (trachy)-andesite and (trachy)-dacite and rhyolite respectively, in which the Dasi volcanism is volumetrically dominant and widespread. The Dasi volcanic rocks were selected for SHRIMP zircon U-Pb dating to confirm the timing of volcanism. The results indicate that there exist a large amount of magmatic zircons characterized by high U and Th contents in the volcanic rocks. The concordia ages for 13 points are 128 ± 1Ma (MSWD = 3.0). On account of the shape of zircons and Th/U ratios, this age is considered to represent the crystallization time of the Dasi volcanism.The volcanic rocks in the Dasi, Majiashan and Lingxiang Formations share similar trace element and REE partition patterns as well as Sr-Nd isotopic compositions. In combination with the regional geology,it is proposed that the southeast Hubei volcanic rocks were formed mainly during the Early Cretaceous, just like other volcanic basins in middle-lower Yangtze valley. A lithospheric extension is also suggested for tectonic regime in this region in the Cretaceous Period.
    Paleoproterozoic Potassic Granitoids in the Sushui Complex from the Zhongtiao Mountains,Northern China Geochronology,Geochemistry and Petrogenesis [查看] TIANWei1、LIUShuwen、ZHANGHuafeng
    Abstract: Paleoproterozoic potassic granitoids in the southern Sushui Complex from the Zhongtiao Mountains yielded SHRIMP zircon U-Pb ages of 1968–1944 Ma. Lithologically, the potassic granitoid series consists chiefly of monzodiorite, quartz monzonite and syenogranite. Their trace elements and Sm-Nd isotope characteristics indicate that they were derived from partial melting of Archean TTG rocks in an overthickened continental crust. Petrogenesis of this potassic granitoid series implies a collisional environment within the Trans-North China Orogen in the Paleoproterozoic, which supports a tectonic model of Eastern and Western Continental Blocks being amalgamated in the Paleoproterozoic.
    Mesozoic Multi-phase Magmatism and Gold Mineralization in the Early Precambrian North China Craton, Eastern Hebei Province, China SHRIMP Zircon U-Pb Evidence [查看] LAICHENGMIAO、YUMINQIU、WEIMINGFAN、ANDFUQINZHANG
    The early Precambrian North China craton (NCC) in eastern Hebei Province (also known as the Jidong area) was intruded by granitic batholiths and plutons spatially associated with gold deposits. No consensus has been reached regarding timing and tectonic setting of the gold deposits, chiefly due to the lack of reliable geochronological data. The gold deposits in the district are localized by NE-striking faults within granite plutons and/or nearby Archean amphibolites, as well as in Proterozoic sedimentary rocks. Gold mineralizations in the area are characterized by quartz (± albite) veinand sulfide-disseminated styles; both types of ores have relatively low sulfide contents (<10 vol%) and similar sulfide mineral assemblages dominated by pyrite + chalcopyrite + pyrrhotite + galena + sphalerite, locally with molybdenite, tellurides, and bismuthinoids. The alterations around the gold lodes include K-feldspathization, sericitization, silicification, chloritization, and sulfidation. Previous fluid inclusion data from all the deposits of the district show that the mineralizing fluids are characterized by relatively high salinities (3 to 17 wt% NaCl equiv.), H2O-CO2 ± CH4, N2 solutions, with CO2 contents in the inclusions ranging from 5 to 40 mol%. Fluid inclusion homogenization temperatures are between 240 and 400°C, and estimates of the trapping pressures vary significantly from 0.5 to 3.7 kbar. Stable isotope (O, H, S, C, and Pb) data from these deposits indicate a major magmatic component in the mineralizing fluids and the ore-forming materials, with a partial contribution by Archean host rocks, suggesting that these deposits are basically intrusion-related. SHRIMP zircon U-Pb geochronology of the gold-hosting granitic intrusions, in combination with previous Ar-Ar and Rb-Sr dates on hydrothermal minerals (e.g., sericite), indicates that there was no Archean gold mineralization, but instead suggests that at least three episodes of granitic magmatism and associated gold mineralization took place during the Mesozoic. The first episode occurred in the Late Triassic at ~222 Ma, with emplacement of the Dushan granite batholith (223 ± 2 Ma), Sanjia granite porphyry (222 ± 4 Ma), and the Baizhangzi granite (222 ± 3 Ma). This episode of magmatism and gold mineralization was coeval with collision of the NCC with the South Mongolian block in the north along the Solonker suture and with the Yangtze craton in the south along the Dabie-Sulu suture. The second episode took place in the Early Jurassic, with emplacement of the Qingshankou granite (199 ± 2 Ma), and the third episode occurred in the Middle Jurassic, with intrusion of the Yuerya (~ 175 Ma) and Niuxinshan (172 ± 2 Ma) and granites. These two events are about 25 and 45 Ma later than the collision of the NCC with other continental blocks in a post-collisional environment.
    Zircon U-Pb age and Hf-O isotope evidence for Paleoproterozoic metamorphic event in South China [查看] Shao-BingZhang、Yong-FeiZheng、Yuan-BaoWu、Zi-FuZhaoa、ShanGaob、Fu-YuanWu
    To understand the connection between continental cratonization and global tectonothermal event is essential for recognizing the formation and evolution of continental crust. Paleoproterozoic is an important era with occurrence of megascale tectonomagmatism in the world, but it has been intriguing whether they also influenced the oldest continent in South China. In order to decipher the nature of Paleoproterozoic event in South China, a combined study of zircon U-Pb dating, Hf and O isotope analyses was carried out for metasediments and amphibolite from the Kongling terrane, the only Archean microcontinent outcropped in South China. U-Pb ages of 1.97±0.03 Ga were obtained with low Th/U ratios of 0.01–0.14, indicating that the ages are a record of Paleoproterozoic metamorphic event.18O values of ∼11‰ and ∼8‰ were measured for quartz from the metasediments and garnet from the amphibolite, respectively, suggesting that their sources experienced supracrustal recycling. εHf(t) values of about −6.5 and model Hf ages of about 3.0 Ga were acquired for zircons from the metapelites, suggesting an Archean source. Thus a response to the Paleoproterozoic global tectonothermal event in South China is reworking of Archean continental nucleus. Compared with Archean rocks at Kongling, abrupt changes in K2O/Na2O, REE and other trace elements are observed in the Paleoproterozoic metasedimentary rocks. This is interpreted to reflect a change in upper crustal composition at the Archean–Proterozoic boundary.A survey of Paleoproterozoic ages throughout the Yangtze Block suggests that metamorphic event and subsequent magmatic activity occurred in the north, but only magmatic activity in the south. Both metamorphic and magmatic activities are associated with formation of a unified basement responsible for cratonization of the Yangtze Block. This provides a geodynamic connection between the formation of this craton and the global tectonomagmatism in the Paleoproterozoic, marking continental accretion by arc-continent collision orogeny during assembly of the supercontinent Columbia.
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