南太行山闪长岩的SHRIMP锆石U-Pb年龄及岩石成因研究

南太行山符山和东冶角闪闪长岩岩体的精细SHRIMP锆石U-Pb定年和元素-同位素地球化学研究表明:符山角闪闪长岩体形成于126.7±1.1Ma,东冶角闪闪长岩体形成于125.9±0.9Ma,与区内基性侵入岩和北太行地区侵入杂岩具有相似的形成年龄,表明晚中生代(±130Ma左右)太行山地区经历了与华北陆块同期的重要构造岩浆事件。区内闪长质岩石SiO_2=54.84％～65.75％,MgO=1.31％～3.89％,K_2O Na_2O=6.53％～11.40％,mg值=0.36～0.58,(La/Yb)_(cn)=9.86～22.77,(Gd/Yb)_(cn)=1.51～2.00;Eu/Eu=1.00～1.23,以富集LREE、LILE元素和明显亏损Nb-Ta、Zr-Hf-Ti等高场强元素为特征。~(87)Sr/~(86)Sr(t)=0.705363～0.706165,ε_(Nd)(t)=-13.8～-16.8,源自于EMI型富集岩石圈地幔,可解释为新生地幔底侵物质熔融后经过结晶分异作用的产物。与华北克拉通内部其他地区一样,早白垩世南太行山地区处于软流圈上涌的岩石圈伸展构造背景。     

U-Pb Zircon Calendar for Namaquan (Grenville) Crustal Events in the Granulite-facies Terrane of the O'okiep Copper District of South Africa

The O'okiep Copper District is underlain by voluminous 1035–1210Ma granite gneiss and granite with remnants of metamorphosedsupracrustal rocks. This assemblage was intruded by the 1030Ma copper-bearing Koperberg Suite that includes jotunite, anorthosite,biotite diorite and hypersthene-bearing rocks ranging from leuconoriteto hypersthenite. New sensitive high-resolution ion microprobeage data demonstrate the presence of 1700–2000 Ma zirconas xenocrysts in all of the intrusive rocks, and as detritalzircon in the metasediments of the Khurisberg Subgroup. Thesedata are consistent with published Sm–Nd model ages ofc. 1700 Ma (T_CHUR) and c. 2000 Ma (T_DM) of many of the intrusivesthat support a major crust-forming event in Eburnian (Hudsonian)times. In addition, U–Th–Pb analyses of zirconsfrom all major rock units define two tectono-magmatic episodesof the Namaquan Orogeny: (1) the O'okiepian Episode (1180–1210Ma), represented by regional granite plutonism, notably theNababeep and Modderfontein Granite Gneisses and the Concordiaand Kweekfontein Granites that accompanied and outlasted (e.g.Kweekfontein Granite) regional tectonism [F₂(D₂)] and granulite-faciesmetamorphism (M₂); (2) the Klondikean Episode (1020–1040Ma), which includes the intrusion of the porphyritic RietbergGranite and of the Koperberg Suite that are devoid of regionalplanar or linear fabrics. Klondikean tectonism (D₃) is reflectedby major east–west-trending open folds [F₃(D_3a)], andby localized east–west-trending near-vertical ductilefolds [‘steep structures’; F₄(D_3b)] whose formationwas broadly coeval with the intrusion of the Koperberg Suite.A regional, largely thermal, amphibolite- to granulite-faciesmetamorphism (M₃) accompanied D₃. This study demonstrates, interalia, that the complete spectrum of rock-types of the KoperbergSuite, together with the Rietberg Granite, was intruded in ashort time-interval (<10 Myr) at c. 1030 Ma, and that therewere lengthy periods of about 150 Myr of tectonic quiescencewithin the Namaquan Orogeny: (1) between the O'okiepian andKlondikean Episodes; (2) from the end of the latter to the formalend of Namaquan Orogenesis 800–850 Ma ago. KEY WORDS: U–Pb, zircon; O'okiep, Namaqualand; granite plutonism; granulite facies; Koperberg Suite; Namaquan (Grenville) Orogeny     

A Sequence of Pan-African and Hercynian Events Recorded in Zircons from an Orthogneiss from the Hercynian Belt of Western Central Iberia--an Ion Microprobe U-Pb Study

Sensitive high-resolution ion microprobe U–Pb dating showsthat a biotite orthogneiss from the Hercynian belt of westerncentral Iberia contains 1000–300 Ma zircon. Older, 1000–570Ma ages within this range represent inherited, detrital materialamong which four age components may be recognized:     

Accurate U-Pb Age and Trace Element Determinations of Zircon by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry

Various zircons of Proterozoic to Oligocene ages (1060-31 Ma) were analysed by laser ablation-inductively coupled plasma-mass spectrometry. Calibration was performed using Harvard reference zircon 91500 or Australian National University reference zircon TEMORA 1 as external calibrant. The results agree with those obtained by SIMS within 2s error. Twenty-four trace and rare earth elements (P, Ti, Cr, Y, Nb, fourteen REE, Hf, Ta, Pb, Th and U) were analysed on four fragments of zircon 91500. NIST SRM 610 was used as the reference material and ²⁹Si was used as internal calibrant. Based on determinations of four fragments, this zircon shows significant intra-and inter-fragment variations in the range from 10% to 85% on a scale of 120 μm, with the variation of REE concentrations up to 38.7%, although the chondrite-normalised REE distributions are very similar. In contrast, the determined age values for zircon 91500 agree with TIMS data and are homogeneous within 8.7 Ma (2 s ). A two-stage ablation strategy was developed for optimising U-Pb age determinations with satisfactory trace element and REE results. The first cycle of ablation was used to collect data for age determination only, which was followed by continuous ablation on the same spot to determine REE and trace element concentrations. Based on this procedure, it was possible to measure zircon ages as low as 30.37 0.39 Ma (MSWD = 1.4; 2 s ). Other examples for older zircons are also given.     

U–Pb geochronology of the southern Brasília belt (SE-Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana

The Brasília belt borders the western margin of the São Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U–Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the São Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana.Ages of detrital zircons (by ID–TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive margin of the São Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxá Group in the Áraxá and Passos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperança Metasedimentary Sequence and Andrelândia Group) and the autochthonous or Cratonic Domain (Andrelândia Group). The patterns of provenance ages for these units are uniform and are characterised as follows: Archean–Paleoproterozoic ages (3.4–3.3, 3.1–2.7, and 2.5–2.4 Ga); Paleoproterozoic ages attributed to the Transamazonian event (2.3–1.9 Ga, with a peak at ca. 2.15 Ga) and to the ca. 1.75 Ga Espinhaço rifting of the São Francisco Craton; ages between 1.6 and 1.2 Ga, with a peak at 1.3 Ga, revealing an unexpected variety of Mesoproterozoic sources, still undetected in the São Francisco Craton; and ages between 0.9 and 1.0 Ga related to the rifting event that led to the individualisation of the São Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in the Araxá Group yields a rutile age of ca. 0.9 Ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin.Detrital zircons from the autochthonous and parautochthonous Andrelândia Group, deposited on the southern margin of the São Francisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that 1.6–1.2 Ga source rocks must be present in the São Francisco Craton. They could be located either in the cratonic area, which is mostly covered by the Neoproterozoic epicontinental deposits of the Bambuí Group, or in the outer paleo-continental margin, buried under the allochthonous units of the Brasília belt.Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630 Ma mark the orogenic event that started with westward subduction of the São Francisco plate and ended with continental collision against the Paraná block (and Goiás terrane). Continuing collision led to the exhumation and cooling of the Araxá and Passos metamorphic nappes, as indicated by monazite ages of ca. 605 Ma and mark the final stages of tectonometamorphic activity in the southern Brasília belt.Whilst continent–continent collision was proceeding on the western margin of the São Francisco Craton along the southern Brasília belt, eastward subduction in the East was generating the 634–599 Ma Rio Negro magmatic arc which collided with the eastern São Francisco margin at 595–560 Ma, much later than in the Brasília belt. Thus, the tectonic effects of the Ribeira belt reached the southernmost sector of the Brasília belt creating a zone of superposition. The thermal front of this event affected the proximal Andrelândia Group at ca. 588 Ma, as indicated by monazite age.The participation of the Amazonian craton in the assembly of western Gondwana occurred at 545–500 Ma in the Paraguay belt and ca. 500 Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collision between the Paraná block and Goiás terrane with the São Francisco Craton along the Brasília belt preceded the accretion of the Amazonian craton by 50–100 million years.     

Volcanic resurfacing and the early terrestrial crust: Zircon U-Pb and REE constraints from the Isua Greenstone Belt, southern West Greenland

The oldest known bona fide succession of clastic metasediments occurs in the Isua Greenstone Belt, SW Greenland and consists of a variety of mica schists and rare metaconglomerates. The metasediments are in direct contact with a felsic metavolcanic lithology that has previously been dated to 3.71 Ga. Based on trace element geochemical data for > 30 metasediments, we selected the six samples with highest Zr concentrations for zircon extraction. These samples all yielded very few or no zircon. Those extracted from mica schists yielded ion probe U/Pb ages between 3.70 and 3.71 Ga. One metaconglomerate sample yielded just a single zircon of 3.74 Ga age.The mica schist hosted zircons have U/Pb ages, Th / U ratios, REE patterns and Eu anomalies indistinguishable from zircon in the adjacent 3.71 Ga felsic metavolcanic unit. Trace element modelling requires the bulk of material in the metasediments to be derived from variably weathered mafic lithologies but some metasediments contain substantial contribution from more evolved source lithologies. The paucity of zircon in the mica schists is thus explained by incorporation of material from largely zircon-free volcanic lithologies. The absence of older zircon in the mica schists and the preponderance of mafic source material imply intense, mainly basaltic resurfacing of the early Earth. The implications of this process are discussed.Thermal considerations suggest that horizontal growth of Hadean crust by addition of mafic-ultramafic lavas must have triggered self-reorganisation of the protocrust by remelting. Reworking of Hadean crust may have been aided by burial of hydrated (weathered) metabasalt due to semi-continuous addition of new voluminous basalt outpourings. This process causes a bias towards eruption of Zr-saturated partial melts at the surface with O-isotope compositions potentially different from the mantle. The oldest zircons hosted in sediments would have been buried to substantial depth or formed in plutons that crystallised at some depth, from which it took hundreds of millions of years for them to be exhumed and incorporated into much younger sediments.     

新疆阿尔泰造山带构造作用的锆石裂变径迹分析

在新疆阿尔泰造山带所获得的19个锆石裂变径迹年龄变化于155-243Ma之间，明显地分为2组，分别对应于2个构造活动期，早期为155-189Ma，晚期为189-243Ma。这与磷灰石裂变径迹年龄反映的62-100Ma和100-160Ma两个构造期完全一致。早期和晚期构造活动期持续的时间分别为54-60Ma和34-38Ma，而这两个构造期之间的间隔时间，则从早到晚由83-89Ma变为89-93Ma。同时，锆石裂变径迹年龄与距特斯巴汗断裂和巴寨断裂的距离有关，反映这两条断裂带对区域构造演化的控制作用。     

Experimental determination of REE partition coefficients between zircon,garnet and melt: a key to understanding high‐T crustal processes

The partitioning of rare earth elements (REE) between zircon, garnet and silicate melt was determined using synthetic compositions designed to represent partial melts formed in the lower crust during anatexis. The experiments, performed using internally heated gas pressure vessels at 7 kbar and 900–1000 °C, represent equilibrium partitioning of the middle to heavy REE between zircon and garnet during high‐grade metamorphism in the mid to lower crust. The D_REE (zircon/garnet) values show a clear partitioning signature close to unity from Gd to Lu. Because the light REE have low concentrations in both minerals, values are calculated from strain modelling of the middle to heavy REE experimental data; these results show that zircon is favoured over garnet by up to two orders of magnitude. The resulting general concave‐up shape to the partitioning pattern across the REE reflects the preferential incorporation of middle REE into garnet, with D_Gd (zircon/garnet) ranging from 0.7 to 1.1, D_Ho (zircon/garnet) from 0.4 to 0.7 and D_Lu (zircon/garnet) from 0.6 to 1.3. There is no significant temperature dependence in the zircon–garnet REE partitioning at 7 kbar and 900–1000 °C, suggesting that these values can be applied to the interpretation of zircon–garnet equilibrium and timing relationships in the ultrahigh‐T metamorphism of low‐Ca pelitic and aluminous granulites.     

Zircon U–Pb ages and Hf isotope compositions of migmatites from the North Qinling terrane and their geological implications

Migmatites are predominant in the North Qinling (NQ) orogen, but their formation ages are poorly constrained. This paper presents a combined study of cathodoluminescence imaging, U–Pb age, trace element and Hf isotopes of zircon in migmatites from the NQ unit. In the migmatites, most zircon grains occur as new, homogeneous crystals, while some are present as overgrowth rims around inherited cores. Morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting. The inherited cores have oscillatory zoning and yield U–Pb ages of c. 900 Ma, representing their protolith ages. The early Neoproterozoic protoliths probably formed in an active continental margin, being a response to the assembly of the supercontinent Rodinia. The migmatite zircon yields Hf model ages of 1911 ± 20 to 990 ± 22 Ma, indicating that the protoliths were derived from reworking of Palaeoproterozoic to Neoproterozoic crustal materials. The anatexis zircon yields formation ages ranging from 455 ± 5 to 420 ± 4 Ma, with a peak at c. 435 Ma. Combined with previous results, we suggest that the migmatization of the NQ terrane occurred at c. 455–400 Ma. The migmatization was c. 50 Ma later than the c. 490 Ma ultra‐high‐P (UHP) metamorphism, indicating that they occurred in two independent tectonic events. By contrast, the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the NQ unit, which collectively argue for their formation due to the northward subduction of the Shangdan Ocean. UHP rocks were distributed mainly along the northern margin and occasionally in the inner part of the NQ unit, indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process.     

Zircon in amphibolites from Naxos,Aegean Sea,Greece: origin,significance and tectonic setting

We report U–Pb zircon ages of c. 700–550 Ma, 262–220 Ma, 47–38 Ma and 15–14 Ma from amphibolites on Naxos Island in the Aegean extensional province of Greece. The zircon has complex internal structures. Based on cathodoluminescence response, zoning and crosscutting relationships a minimum of four zircon growth stages are identified: inherited core, magmatic core, inner metamorphic (?) rim and an outer metamorphic rim. Trace element compositions of the amphibolites suggest igneous differentiation and crustal assimilation. Zircon solubility as a function of saturation temperatures, Zr content and melt composition indicates that the zircon did not originally crystallize in the mafic bodies but was inherited from felsic precursor rocks, and subsequently assimilated into the mafic intrusives during emplacement. Zircon inheritance is corroborated by the complex, xenocrystic nature of the zircon in one sample. Ages of c. 700–550 Ma and 262–220 Ma are assigned to inherited zircon. Available geochemical data suggest that the 15–14 Ma metamorphic rims grew in situ in the amphibolites, corresponding to a high‐grade metamorphic event at this time. However, the geochemical data cannot conclusively establish if the c. 40 Ma zircon rims also grew in situ, or whether they were inherited along with the xenocrystic cores. Two scenarios for emplacement of the mafic intrusives are discussed: (i) Intrusion during late‐Triassic to Jurassic ocean basin development of the Aegean realm, in which case the 40 Ma zircon rims would have grown in situ, and (ii) emplacement in the Miocene as a result mafic underplating during large‐scale extension. In this case, only the 15–14 Ma metamorphic outer rims would have formed in situ in the amphibolitic host rocks.