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    U-Pb SHRIMP and Sm-Nd geochronology of granite-gneiss complexes and implications for the evolution of the Central Brazil Archean Terrain [查看] Cla´udiaLimaQueiroz、HardyJost、LuizCarlosdaSilva、NealJ.McNaughton
    The paper is a first attempt to unravel the Archean multi-stage metaplutonic assemblage of the Meso/Neoarchean terrane of the State of Goia´s, Central Brazil, by means of the U–Pb SHRIMP zircon and Sm–Nd techniques. Two stages of granitic plutonism,spanning ca. 140 m.y., were precisely established for the accretion of the gneiss protoliths. The earliest stage embraces tonalitic to granodioritic and minor granitic orthogneisses with Nd juvenile signature, emplaced from ca. 2845 to ca. 2785 Ma, interpreted as the roots of an early arc. Inherited zircon xenocrysts and Nd isotopic data indicate that the juvenile magmas underwent contamination from a sialic crust as old as 3.3 Ga, from which there are, so far, no recognizable exposures. The second stage comprises granodioritic to granitic gneisses and lasted from ca. 2711 to 2707 Ma. Based on their Nd isotopic signatures and on inherited zircon crystals, their protoliths are interpreted as dominantly crustal-derived. The SHRIMP data from zircon crystals did not depict a Paleoproterozoic overprinting on the Archean gneisses, which is due to geological processes with prevailing temperatures below the isotopic stability of the U/Pb/Th system in the mineral. These processes comprise crustal extension and intrusion of a mafic dike swarm at ca. 2.3 Ga, followed by low grade events mostly related to shear zones between ca. 2.15 and 2.0 Ga. The study also revealed the extent of the Pan- African tectono-thermal overprinting on the Archean orthogneisses. Most of the zircon populations show morphological evidence of metamorphic peripheral recrystallization dated between ca. 750 and 550 Ma. One of the banded gneisses with a crystallization age of ca. 2700 Ma (2r) has a more complex zircon population including magmatic new grains, which yielded a precise 206Pb/238U crystallization age of 590 ± 10 Ma (2r). These new grains are interpreted to have grown in anatectic veins injected within strongly sheared gneiss.The data characterize a widespread Pan-African-aged metamorphic overprinting, culminating with localized anatexis of the Archean orthogneisses.
    Provenance and age delimitation of Quadrila´tero Ferrı´fero sandstones based on zircon U–Pb isotopes [查看] Le´oA.Hartmann、IssamuEndo、MarcosTadeuF.Suita、Joa˜oOrestesS.Santos、Jose´CarlosFrantz、Maurı´cioA.Carneiro、NealJ.McNaughton、MarkE.Barley
    The Quadrila´tero Ferrı´fero has some of the largest iron and gold deposits in the world and is a major geotectonic unit of the Sa˜o Francisco Craton in Brazil. U–Pb zircon SHRIMP geochronology of six detrital sedimentary and metasedimentary rocks (114 zircon crystals, 118 spot analyses) has improved the understanding of the sedimentary processes and provenance ages of both rocks and the associated iron formation. The age of deposition of the iron formation is constrained between 2.58 and 2.42 Ga. The presence of an old Paleoarchean crust is dated in detrital zircon crystals, including the oldest zircon in South America (3809G3 Ma). Only high-Th/U, magmatic zircon crystals are present in the dated sedimentary rocks, and these indicate that the crust of the region was formed mostly during the Jequie´ cycle (six age peaks between 3055 and 2635 Ma). This time span ofw420 m.y. is similar to the duration of a long-lived Wilson cycle. Most of the Rio das Velhas Basin was filled during approximately 30 m.y. between 2746 and 2717 Ma, though volcanism probably started earlier. The youngest detrital zircon age from the Minas Supergroup indicates that the intracratonic basin fill, including the iron formation, was deposited after 2580 Ma. Therefore, the crust was cratonized shortly after the intrusion of minor granitic bodies at around 2.62–2.58 Ga. A large gap in orogenic activity is indicated by the absence of zircon ages of 2580–2182 Ma.
    Age constraints on felsic intrusions,metamorphism and gold mineralisation in the Palaeoproterozoic Rio Itapicuru greenstone belt, NE Bahia State, Brazil [查看] EdsonF.Mello、RobertoP.Xavier、NealJ.McNaughton、SteffenG.Hagemann、IanFletcher、LarrySnee
    U–Pb sensitive high resolution ion microprobe mass spectrometer (SHRIMP) ages of zircon, monazite and xenotime crystals from felsic intrusive rocks from the Rio Itapicuru greenstone belt show two development stages between 2,152 and 2,130 Ma, and between 2,130 and 2,080 Ma. The older intrusions yielded ages of 2,152±6 Ma in monazite crystals and 2,155±9 Ma in zircon crystals derived from the Trilhado granodiorite, and ages of 2,130±7 Ma and 2,128±8 Ma in zircon crystals derived from the Teofilândia tonalite. The emplacement age of the syntectonic Ambrósio dome as indicated by a 2,080±2-Ma xenotime age for a granite dyke probably marks the end of the felsic magmatism. This age shows good agreement with the Ar–Ar plateau age of 2,080±5 Ma obtained in hornblendes from an amphibolite and with a U–Pb SHRIMP age of 2,076±10 Ma in detrital zircon crystals from a quartzite, interpreted as the age of the peak of the metamorphism. The predominance of inherited zircons in the syntectonic Ambrósio dome suggests that the basement of the supracrustal rocks was composed of Archaean continental crust with components of 2,937±16, 3,111±13 and 3,162±13 Ma. Ar–Ar plateau ages of 2,050±4 Ma and 2,054±2 Ma on hydrothermal muscovite samples from the Fazenda Brasileiro gold deposit are interpreted as minimum ages for gold mineralisation and close to the true age of gold deposition. The Ar–Ar data indicate that the mineralisation must have occurred less than 30 million years after the peak of the metamorphism, or episodically between 2,080 Ma and 2,050 Ma, during uplift and exhumation of the orogen.
    大别山英山和熊店榴辉岩单颗粒锆石SHRIMP U-Pb年代学研究 [查看] 高山、YuminQiu、凌文黎、NealJ.McNaughton、张本仁
    报道了对大别山高压-超高压带河南罗山熊店和湖北英山蜜蜂尖两地榴辉岩单颗粒锆石SHRIMP U-Pb原位（in-situ）定年结果。英山超高压榴辉岩年龄有4组：（773±20）Ma，（730±16）Ma，（461±7）Ma 和（262±6）Ma，其中以（461±7）Ma占绝对优势。熊店高压榴辉岩有3组年龄：（449±14）Ma，（307±14）Ma 和（216±4）Ma。两地榴辉岩中均存在约450Ma的早古生代奥陶纪年龄，它们均具有极低的Zr含量（9×10-6～19×10-6）结合熊店榴辉岩样品具有类似于MORB的微量元素特征和εNd值（450Ma时为+6.56），说明该古生代年龄应代表古生代洋壳和岛弧物质俯冲经高压-超高压变质作用的产物。由于奥陶纪高压-超高压事件的年龄同时出现在英山和熊店榴辉岩中，且熊店榴辉岩中清楚地记录了三叠纪高压-超高压事件，表明大别高压-超高压带北界无论是在奥陶纪或三叠纪时均应在熊店榴辉岩以北的桐柏-商城断裂。
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