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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.
    Timing of Paleoproterozoic ultrahigh-temperature metamorphism in the North China Craton Evidence from SHRIMP U-Pb zircon geochronology [查看] M.Santosh、S.A.Wilde、J.H.Li
    We report SHRIMP U–Pb zircon ages from two samples of Mg–Al granulite and one of normal khondalite from the Khondalite belt of Inner Mongolia, North China Craton. These data constrain the timing of extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions in the region. TheUHTindicator assemblages in these rocks include equilibrium sapphirine + quartz, high alumina orthopyroxene + sillimanite + quartz and high temperature perthites. The surrounding khondalites also carry spinel + quartz equilibrium assemblage. Based on the stability of sapphirine + quartz and orthopyroxene + sillimanite + quartz in the matrix assemblages, T > 1000 ◦C and P > 10 kbar for peak metamorphism was estimated in a previous study, with initial isobaric cooling and subsequent isothermal decompression along a broad anticlockwise trajectory. Metamorphic zircons from the UHT rocks are characterized by ovoid shapes with numerous high-order crystal faces and low luminescence in cathodoluminescence (CL) images. As metamorphic temperatures increase, based on the presence of UHT indicator minerals, relict oscillatory zones are progressively destroyed and the grains become uniformly dark in CL. The oldest detrital cores reveal 207Pb/206Pb ages up to 2090±22 Ma, but most have ages of ∼1970 Ma. Importantly, the highest grade sample with sapphirine-bearing UHT assemblage, contains only a single zircon population with a weighted mean 207Pb/206Pb age of 1919±10 Ma. This is the same age, within error, as the youngest populations in the other two samples and is interpreted to record the time of UHT metamorphism in the North China Craton. The zircons in the highest-grade sample have unusually high Th/U ratios (average 1.44) for metamorphic zircon, which may prove to be a feature of UHT rocks. The Khondalite belt defines a collisional orogen between the Yinshan block to the north and the Ordos block to the south, that together form theWestern Block of the North China Craton. The ∼1.92 Ga UHT metamorphic event in the North China Craton preceded the collision that amalgamated the North China Craton along the Trans-North China Orogen at ∼1.85 Ga, but both coincided with the assembly of the Paleoproterozoic supercontinent Columbia.
    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.
    Tectonic driving of Neoproterozoic glaciations Evidence from extreme oxygen isotope signature of meteoric water in granite [查看] Yong-FeiZheng、Yuan-BaoWu、BingGong、Ren-XuChen、JunTang、Zi-FuZhao
    The global context of glaciation in the Neoproterozoic has been hypothesized to result from the massive reorganization of the Earth's land and ocean systems due to breakup and dispersal of the supercontinent Rodinia at about 750 Ma. This hypothesis gains support from unusually light O isotope records of hydrothermally altered rocks in a rift tectonic zone at that time, which is indicative of interaction with meteoric water of low mean annual temperature, and thus tectonic driving of cold paleoclimate at the time of hydrothermal alteration. Very negative δ18O values of −14.4 to −10.0‰ are found for garnet from Neoproterozoic granite in South China, which are the lightest O isotope record so far reported for minerals from igneous rocks. Negative δD values of −129 to −109‰ are obtained for garnet, magnetite and zircon. Thus high-T meteoric-hydrothermal alteration occurred during magma emplacement. SHRIMP U–Pb dating for magmatic and hydrothermal zircons yields two groups of ages at 782±3 Ma and 748±3 Ma, respectively, responsible for granite crystallization and hydrothermal alteration. The garnet is in O isotope disequilibrium with zircon, indicating differential effects of subsolidus hydrothermal alteration on the minerals of different O diffusion rates. Occurrence of the unusually negative δ18O granite at mid-low paleolatitudes provides a geochemical proxy for a cold paleoclimate at 748±3 Ma, possibly a continental glaciation corresponding to the Kaigas iceage. It suggests a tectonic link to the climatic effect of uplifted rift flanks due to the Rodinia breakup at about 750 Ma, and thus the ice–fire interaction by syn-rift magmatism of low δ18O imprints in association with the mantle event of asthenospheric upwelling. Hence the tectonic driving is evident for regional glaciations in supercontinental rift basins.
    SHRIMP zircon U-Pb geochronological and whole-rock geochemical evidence for an early Neoproterozoic Sibaoan magmatic arc along the southeastern margin of the Yangtze Block [查看] Mei-FangYe、Xian-HuaLi、Wu-XianLi、YingLiu、Zheng-XiangLi
    It has been generally accepted that the South China Block was formed through amalgamation of the Yangtze and Cathaysia Blocks during the Proterozoic Sibaoan orogenesis, but the timing and kinematics of the Sibao orogeny are still not well constrained. We report here SHRIMP U–Pb zircon geochronological and geochemical data for the Taohong and Xiqiu tonalite–granodiorite stocks from northeastern Zhejiang, southeastern margin of the Yangtze Block. Our data demonstrate that these rocks, dated at 913±15 Ma and 905±14 Ma, are typical amphibole-rich calcalkaline granitoids formed in an active continental margin. Combined with previously reported isotopic dates for the ∼1.0 Ga ophiolites and ∼0.97 Ga adakitic rocks from northeastern Jiangxi, the timing of the Sibao orogenesis is thus believed to be between ∼1.0 and ∼0.9 Ga in its eastern segment. It is noted that the Sibao orogeny in South China is in general contemporaneous with some other early Neoproterozoic (1.0–0.9 Ga) orogenic belts such as the Eastern Ghats Belt of India and the Rayner Province in East Antarctica, indicating that the assembly of Rodinia was not finally completed until ∼0.9 Ga.
    Shrimp U-Pb zircon geochronology, geochemistry, and Nd-Sr isotopic study of contrasting granites in the Emeishan large igneous province, SW China [查看] HongZhong、Wei-GuangZhu、Zhu-YinChu、De-FengHe、Xie-YanSong
    The Cida A-type granitic stock (∼4 km2) and Ailanghe I-type granite batholith (∼100 km2) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261±4 Ma) was emplaced shortly before the Ailanghe granites (251±6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites.The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios (.εNd(.t)=−0.25 to +1.35 and (87Sr/86Sr)i=0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic.εNd(.t) ratios (−6.34 to −6.26) and high (87Sr/86Sr)i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida Atype granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe Itype granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks. In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal materialinvolved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.
    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.
    SHRIMP U–Pb Zircon geochronology of the Altai No. 3 Pegmatite, NW China, and its implications for the origin and tectonic setting of the pegmatite [查看] TaoWang、YingTong、Bor-mingJahn、Tian-renZou、Yan-binWang、Da-weiHong、Bao-fuHan
    The “Altai No. 3 granitic pegmatite” in NW China is the largest pegmatite in the world that has been found in an orogenic belt; it forms one of the largest muscovite deposits in Asia and is most famous for its concentric-ring structure. Although it has been mined for more than 70 years, the age of formation and mineralization of the pegmatite has not been well determined, despite numerous previous geochronological studies. Here we present new results of SHRIMP U–Pb zircon dating. Three weighted mean 206Pb/238U ages of 220±9 Ma, 198±7 Ma and 213±6 Ma were obtained for pegmatite zones 1, 5, and 7, respectively. The analyses were made on clear,recrystallized domains and overgrowth rims of large, opaque high-U zircon grains. We interpret the age of 220±9 Ma for zone 1 to approximately represent the initial time of formation for No. 3 pegmatite. An additional date of 213±2 Ma was determined on zircon from a small subordinate vein formed in the late-stage phase of the No. 3 pegmatite. This age set the late age of emplacement of the pegmatite. The time span of 213 to 198 Ma may reflect late stage hydrothermal alteration. In addition, a weighted mean 206Pb/238U age of 277±6 Ma was obtained from small, clear, transparent low-U zircon grains which are more probably xenocrysts. Our new age data indicate that the pegmatite has no genetic relationship with the wall granitic rocks (409 Ma), but is likely related to the early Mesozoic (220 Ma) granitic magmatism that was just recognized. The No. 3 pegmatite was formed in an early Mesozoic anorogenic extensional regime. A stable tectonic setting was undoubtedly required for the formation of the large pegmatite.
    SHRIMP U-Pb zircon geochronology and geochemistr y of metavolcanic and metasedimentary rocks in Northwestern Fujian, Cathaysia block, ChinaTectonic implicatio ns and the need to redefine lithostratigraphic units [查看] YushengWan、DunyiLiu、MeihuiXu、JianminZhuang、BiaoSong、YuruoShi、LilinDu
    Northwestern Fujian Province is one of the most important Pre-Palaeozoic areas in the Cathaysia Block of South China. Metavolcano sedimentary and metasedimentary rocks of different types, ages and metamorphic grades (granulite to upper greenschist facies) are present, and previously were divided into several Formations and Groups. Tectonic contacts occur between some units, whereas(deformed) unconformities have been reported between others. New SHRIMP U–Pb zircon ages presented here indicate that the original lithostratigraphy and the old “Group” and “Formation” terminology should be abandoned. Thus the “Tianjingping Formation” was not formed in the Archaean or Palaeoproterozoic, as previously considered, but must be younger than its youngest detrital zircons (1790 Ma) but older than regional metamorphism (460 Ma). Besides magmatic zircon ages of 807 Ma obtained from metavolcano-sedimentary rocks of the “Nanshan Formation”and 751–728 Ma for the “Mamianshan Group”, many inherited and detrital zircons with ages ranging from 1.0 to 0.8 Ga were also found in them. These ages indicate that the geological evolution of the study area may be related to the assembly and subsequent break-up of the Rodinia supercontinent. The new zircon results poorly constrain the age of the “Mayuan Group” as Neoproterozoic to early Palaeozoic (728–458 Ma),and not Palaeoproterozoic as previously thought. Many older inherited and detrital zircons with ages of 3.6, 2.8, 2.7, 2.6–2.5, 2.0–1.8 and 1.6 Ga were found in this study. A 3.6 Ga detrital grain is the oldest one so far identified in northwestern Fujian Province as well as throughout the Cathaysia Block. Nd isotope tDM values of eight volcano-sedimentary and clastic sedimentary rock samples centre on 2.73–1.68 Ga, being much older than the formation ages of their protoliths and thus showing that the recycling of older crust played an important role in their formation.These rocks underwent high grade metamorphism in the early Palaeozoic (458–425 Ma) during an important tectono-thermal event in the Cathaysia Block.
    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.
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