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Provenance of Meso- to Neoproterozoic cover sediments at the Ming Tombs, Beijing,North China Craton An integrated study of U–Pb dating and Hf isotopic measurement of detrital zircons and whole-rock geochemistry [查看] YushengWan、DunyiLiu、WeiWang、TianruiSong、AlfredKröner、ChunyanDong、HongyinZhou、XiaoyanYin

The North China Craton (NCC)was subjected to an extensional regime after the Lüliangmovement at ~1.8 Ga and then was covered by an extensive Meso- to Neoproterozoic sedimentary succession, namely the Changcheng,Jixian and Qingbaikou Groups in ascending order. We report age spectra for detrital zircons and monazites, Hf isotopic systematics of detrital zircons, and whole-rock chemical and Nd isotopic compositions for sediments from the succession in the Ming Tombs area, Beijing, one of the typicalMeso- to Neoproterozoic areas in the NCC.Detrital zircons of six sedimentary samples have two distinct age peaks at ~2.52 Ga and ~1.85 Ga. There are some detrital zircons at 2.4–2.0 Ga but none at 2.3 Ga and only a few N2.7 Ga. The detrital zircon age spectra change with time. Sediments in the lower succession (Changcheng Group) and in the upper successions (Jixian and Qinbaikou Groups) are dominated by significant detrital zircon populations of late Neoarchean and late Paleoproterozoic ages, respectively. The ~2.5 Ga detrital zircons of the ChangchengGroup have εHf(2.5 Ga) values and tDM(Hf) model ages mainly ranging from −2 to +7 and 2.8 to 2.7 Ga, respectively. Detrital monazites of a sample fromthe JixianGroup exhibit amajor age peak between 1.95 and 1.80 Ga with somedata between 2.0 and 1.95 Ga. The sedimentary rocks of the Changcheng Group are characterized by high K2O contents (mostly 7.09–15.20%) and insignificant Eu anomalies (Eu/Eu*=0.71–1.16). They have tDM(Nd) model ages ranging from 2.70 to 2.43 Ga, being older than the tDM(Nd) ages (2.11 and 1.99 Ga) of sedimentary samples from the Qingbaikou Group. Based on a comparison with ages for the early Precambrian (N1.8 Ga) basement of the NCC, it can be concluded that (1) the sediments of theMeso- to Neoproterozoic coverwere undoubtedly derived fromthe NCC itself or once neighboring terranes; (2) variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late Paleoproterozoic rockswere exposed at the surface after the upper crustal late Neoarchean rocks had already been eroded.

Combined U-Pb, hafnium and oxygen isotope analysis of zircons from meta-igneous rocks in the southern North China Craton reveal multiple events in the Late Mesoarchean-Early Neoarchean [查看] DunyiLiu、SimonA.Wilde、YushengWan、ShiyanWang、JohnW.Valley、NorikoKita、ChunyanDong、HangqiangXie、ChangxiuYang、YixinZhang、LinzhiGao

We report the results of a comprehensive isotopic investigation of zircons from metamorphosed tonalites and amphibolites from Lushan in the far south of the Trans-North China Orogen of the North China Craton that reveal the oldest rocks yet dated from the orogen. A detailed investigation of the internal structures of zircons, combined with targeted SIMS U–Pb and oxygen isotope analyses and ICP-MS Hf zircon analyses, has resulted in the discovery of previously unknown geological events in the North China Craton. Magmatic zircon from the tonalite and amphibolite was found to be essentially coeval with a small spread of ages from 2829±18Ma to 2832±11 Ma in the tonalites and from2838±35 Ma to 2845±23Ma in amphibolite. Hafnium isotope data suggest derivation of both tonalite and amphibolite from a depleted mantle source with minor crustal contamination. Oxygen isotope ratios in zircons range from 5.1 to 7.3‰ VSMOW, with high δ18O results recorded only from metamorphic domains. The earliest stage of metamorphic zircon growth, present in all four samples analyzed in this study, occurred between 2772±17/22 in the tonalites and 2776±20 and 2792±12 Ma in the amphibolites. A younger metamorphic event at 2638±61 in the tonalite and between 2671±13 and 2651±13 Ma in the amphibolites is recognized in zircons from three of the four samples investigated. This is the first time that metamorphic events at either of these times have been recorded in the North China Craton. Importantly, there is no evidence for the 1.8 Ga high-grade metamorphic event that is manifest in the northern and central parts of the Trans-North China Orogen.

THE SIGNIFICANCE OF PALEOPROTEROZOIC ZIRCON IN CARBONATITE DIKES ASSOCIATED WITH THE BAYAN OBO REE-Nb-Fe [查看] YULONGLIU、IANS.WILLIAMS、JIANGFENGCHEN、YUSHENGWAN、WEIDONGSUN

Sensitive high resolution secondary ion mass spectrometry (SHRIMP II), laser ablation inductively coupled plasma mass spectrometry (ICP-MS) and laser Raman spectroscopy have been used to determine the ages in zircons from carbonatite dikes associated with the Bayan Obo giant REE-Nb-Fe deposit, Inner Mongolia, China.Analyses of small amounts of zircon extracted from large samples of three carbonatite dikes yield late Palaeoproterozoic 207Pb/206Pb ages of 1894±27, 1944 ±20 and 1956±9 Ma. One sample also contained inherited grains with ages up to ~2.55 Ga.Zircon grains, even from a single rock, have a wide range of REE patterns, suggesting that they are probably inherited zircons trapped during magma genesis, not zircons crystallized from a single magma. None of the zircon grains has the extremely high Th/U characteristic of the Bayan Obo ore deposit or of the associated carbonatites. Further, mineral inclusions in the zircon identified by Raman spectroscopy are all silicate minerals (quartz and feldspars), consistent with crystallization of the zircon from silicate, not from carbonate, magmas. The Palaeoproterozoic zircons in the Bayan Obo carbonatite dikes did not crystallize from carbonatite magma at the time of dike emplacement, but were derived from wall rock contamination. The ages obtained from the zircons are consistent with ages measured on basement igneous rocks elsewhere in the Bayan Obo region.

PALEOPROTEROZOIC CRUSTALLY DERIVED CARBONATE-RICH MAGMATIC ROCKS FROM THE DAQINSHAN AREA, NORTH CHINA CRATON GEOLOGICAL, PETROGRAPHICAL, GEOCHRONOLOGICAL AND GEOCHEMICAL (Hf, Nd, O AND C) EVIDENCE [查看] YUSHENGWAN、DUNYILIU、ZHONGYUANXU、CHUNYANDONG、ZHEJIUWANG、HONGYINGZHOU、ZHENSHENGYANG、ZHENGHONGLIU、JIASHANWU

ABSTRACT. Most carbonate-rich magmatic rocks are mantle-derived, namely car-bonatite(s), with a minority of them being contaminated by crustal rocks. It is debated whether there are also carbonate-rich magmatic rocks derived solely from crustal sources. In this contribution, we report crustally derived carbonate-rich magmaticrocks, named herecrustal carbonatite(s) , in the Daqingshan area, Western Block of the North China Craton. The Daqingshan crustal carbonatites were previously considered to be metasedimentary marbles. However, they cut adjacent rocks and contain some enclaves that are irregular in shape and show unoriented distribution of lithologies that cannot be found in the local wall rocks. Zircons from the crustal carbonatites show clear oscillatory zoning, and contain calcite, quartz, feldspar, diopside and CO2 inclusions. The zircons have correlated U and Th abundances and give a U/Pb age of 1951±5 Ma, being the same age as metamorphic zircon cores and rims from a diopsidite enclave (1954±27 Ma of core and 1944±40 Ma of rim). The zircon from the crustal carbonatite has tDM(Hf) and dHf(t) of 2353 to 2457 Ma and –3.2 to 0.7, whereas the tDM(Hf) and dHf(t) of the core and rim zircons from diopsidite range from 2228 to 2160 Ma and 3.0 to 4.8 and 2219 to 2057 Ma and 3.2 to 7.4, respectively. The δ18O(‰)Zircon-V-PDB of zircon from the crustal carbonatite ranges from –21.5 to –19.6, with O isotope equilibrium temperature being 555°C to 635 °C. The crustal carbonatite shows a large variation in chemical composition, with SiO2 δ10.2–37.3% and Total REE δ48 –267 ppm. The tDM(Nd) age and dNd(t) are 2.5 Ga and –2.9 to –2.4. Theδ18O(‰)Rock-V-PDB andδ13C(‰)Rock-V -PDB vary from –19.5 to –15.2 and from –5.2 to –2.4, being distributed between primary carbonatite field and the field of dolomitic marble from the study area inδ18O(‰) vs.δ13C(‰) diagram. Combined with previous studies, we drew the conclusion that some of the carbonate-rich rocks in the study area are magmatic in origin, by anatexis of impure marble plus common contamination by calc-silicates and other materials. This is consistent with the high-P-T experiments of CaO-CO2-H2O system (Wyllie and Tuttle, 1960) and MgO-CaO-CO2-H2O system (Fanelli and others, 1986), which indicate that partial melting of limestone will happen when temperature is> 700°C and when water is also present.

NEW U-Pb AND Hf ISOTOPIC DATA CONFIRM ANSHAN AS THE OLDEST PRESERVED SEGMENT OF THE NORTH CHINA CRATON [查看] DUNYILIU、SIMONA.WILDE、YUSHENGWAN、JIASHANWU、HONGYINZHOU、CHUNYANDONG、XIAOYANYIN

ABSTRACT. The Anshan area in the northeastern part of the North China Craton contains the oldest known rocks in China. Rocks with ages of 3.8 Ga are present in three trondhjemite-dominated gneiss complexes, referred to as the Baijiafen, Dongs-han and Shengoushi complexes. The oldest material identified in these complexes has weighted mean 207Pb/206Pb ages of 3800 5 Ma, 3794 4 Ma and 3777 13 Ma,respectively. In addition, younger geological events at 3680 19 Ma, 3620 23 Ma, and 3573 21 Ma have been identified, recording the emplacement of several generations of trondhjemite. A biotite schist of possible supracrustal origin contains a zircon population with a weighted mean 207Pb/206Pb age of 3723 17 Ma and is cut by a 3620 Ma trondhjemite, implying formation prior to3.62 Ga. TDM(Hf) model ages for the zircons range from 3256 to 4264 Ma, but are mostly 3.9 Ga, indicating that significantly older crust was not present in the source region of the trondhjemitic magmas.

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.

Metamorphism in the Central Zone of the North China Craton SHRIMP U-Pb dating of zircon from metamorphic rocks in the Lushan area, Henan Province [查看] YushengWan、SimonA.Wilde、DunyiLiu、ChangxiuYang、BiaoSong、XiaoyanYin

This paper reports SHRIMP zircon U–Pb dating of Precambrian supracrustal and granitic rocks from the Lushan area, Henan Province, in the southern portion of the Central Zone (also referred to as the Trans-North China Orogen) of the North China Craton. A graphite–garnet–sillimanite gneiss (Sample TW0006/1) of the Shangtaihua F Group gives a range of inherited zircon ages from 2.73 to 2.26 Ga and a metamorphic zircon age of 1.84±0.07 Ga. A garnet-bearing gneissic granitoid (Sample TWJ358/1), which is considered to intrude the Shangtaihua F Group, gives a magmatic zircon age of 2.14±0.02 Ga and a metamorphic zircon age of ¨1.87 Ga. The metamorphic zircon ages of 1.87–1.84 Ga obtained in this study indicate that an important tectonothermal event occurred at the end of the Paleoproterozoic in the Lushan area. This supports the southern continuation of a Central Zone in the North China Craton that workers have recently considered to result from continent–continent collision. It is also evident that the Shangtaihua FGroup_ was formed during the Paleoproterozoic (between 2.26 and 2.14 Ga), and not during the Archean, as previously considered.

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