Discordant RbSr and PbPb whole rock isochron ages for the Archaean basement of Sierra Leone

Field mapping and structural studies in northern Sierra Leone by an I.G.S. team have established a stratigraphic sequence in this part of the Archaean of the West African Craton. An older “Leonian” granite-greenstone terrain is identified which experienced a tectonic-metamorphic event before the formation of the granite-greenstone terrain which ended with the Liberian tectonic-metamorphic event. Granite gneisses in the Fadugu district with Leonian structures yield statistically acceptable but discordant Pb-Pb and Rb-Sr whole-rock isochron ages of 2959±50 Ma and 2753±61 Ma, respectively (2 σ errors). These ages may be correlated with radiometric ages for the Leonian and Liberian structures elsewhere in Sierra Leone, and it is concluded that the Fadugu Rb-Sr whole-rock isochron has been reset by the Liberian event. The Pb-Pb whole-rock isochron for the Fadugu gneisses and a previously determined (but recomputed and partially checked) Rb-Sr whole-rock isochron age of 2980+80 Ma for granite gneisses from southeastern Sierra Leone provide a definitive age for the Leonian tectonic-metamorphic event at about 2970 Ma. Both the initial ⁸⁷Sr/⁸⁶Sr ratios and present-day first-stage model ²³⁸U/²⁰⁴Pb value for the Leonian granitoids are indistinguishable from mantle values, but do not preclude the possibility that these granitoids were derived from parental material with a short history in the crust or lower crust. The Rb-Sr whole-rock isochron age of 2753+61 Ma for the Fadugu granite gneiss provides a definitive age for the Liberian event in northern Sierra Leone. A succession of rocks older than the Leonian (i.e., older than 2970 Ma) has been identified in the field but not yet dated.     

Lead ages,reset rubidium-strontium ages and implications for the Archaean crustal evolution of the Diemals area,Central Yilgarn Block,Western Australia

The Pb-Pb whole-rock geochronology of Archaean granitic and gneissic rocks from the Diemals area in the Central Yilgarn granite-greenstone terrain provides important constraints on crustal evolution. The regionally extensive banded gneisses, previously considered as candidates for basement to the greenstones give a Pb-Pb whole-rock age of 2700 ± 97 Ma (2σ errors). This is within error of previously published Rb-Sr and Sm-Nd gneiss ages and also within error of the Sm-Nd ages on the greenstones in the Eastern Goldfields Province. Two synkinematic plutons give Pb-Pb whole-rock ages (2737 ± 62 Ma and 2700 ± 100 Ma) and Pb isotopic compositions consistent with the hypothesis, based on field and geochemical relations, that these plutons were derived by partial melting of the precursors to the banded gneisses. Assuming this, the combined data date the melting event at 2723 ± 25 Ma with a model source μ value of 8.18 ± 0.02. This source μ value is close to the range postulated for mantle values and restricts the crustal history of the precursors to less than ~200 Ma. A post-kinematic pluton with a whole-rock Pb-Pb age of 2685 ± 26 Ma and μ value of 8.26 ± 0.02 puts a younger limit on this relatively short lived crustal accretion-differentiation event.Comparison of Pb-Pb and Rb-Sr whole-rock dates for the plutons suggests that the latter became closed systems up to 200 Ma after the Pb-Pb ages, and that the plutons gained or lost Rb or Sr at this time.     

胶东谭格庄地区奥长花岗质片麻岩和斜长角闪岩的野外地质和锆石SHRIMP定年

对胶东谭格庄地区一地质剖面进行了详细野外观察和锆石SHRIMP年代学研究.剖面主要是由奥长花岗质片麻岩和斜长角闪岩组成,两者互层产出并一起发生褶皱变形,在强变形域两者完全平行化.所以,在强变形地区,不同成分岩性的薄层状互层产出不能作为变质沉积岩的识别标志.奥长花岗质片麻岩(S1105)形成于2496±10Ma,遭受了古元古代早期和晚期变质.两个斜长角闪岩样品(S1238和S0812)都只发育变质锆石,年龄分别为1842±3Ma和1833±13Ma.该剖面岩石记录的两期变质作用在胶东地区具有普遍意义.结合前人研究,作者指出该地区太古宙末存在区域性变质深熔作用,早期地壳受到了强烈的再造;古元古代末的变质作用具有带状分布特点,与造山作用有关.     

赣南矽卡岩型钨矿成岩成矿年代学及地质意义——以焦里和宝山矿床为例

矽卡岩型钨矿是赣南地区的一种重要钨矿床类型。本文以焦里和宝山两个典型的矽卡岩型钨多金属矿床为例,开展了系统的成岩成矿年代学和岩石地球化学研究,结合区域钨锡矿床最新年代学研究成果,探讨了赣南地区钨矿的成岩成矿时代及形成构造环境。结果表明,焦里矽卡岩型钨多金属矿区斑状花岗闪长岩SHRIMP锆石U-Pb年龄为164.4±1.1Ma,辉钼矿Re-Os等时线年龄为170.6±4.6Ma;宝山矽卡岩型钨矿区花岗岩SHRIMP锆石U-Pb年龄为156.6±3.9Ma,辉钼矿Re-Os模式年龄加权平均值为161.0±1.9Ma,厘定了它们的成岩成矿年龄为中-晚侏罗世,同一矿区的成岩和成矿年龄在误差范围内基本一致。结合赣南崇(义)-(大)余-(上)犹钨锡矿集区已有的年代学资料,指出本区的成岩成矿年龄介于170～150Ma,集中在160～150Ma,认为赣南钨矿形成于华南中生代岩石圈伸展-减薄时期的侏罗纪板内拉张的地球动力学背景,系同一成岩成矿系统的产物。     

吕梁地区2.2~2.1Ga岩浆事件及其构造意义

吕梁地区古元古代岩浆作用非常强烈,其成因与构造环境对客观恢复华北克拉通早期地质演化具有重要意义。本文选择了杜家沟长石斑岩、吕梁群近周峪组中基性火山岩和恶虎滩片麻岩进行了较系统研究。2个杜家沟长石斑岩的SHRIMP锆石U-Pb年龄分别为2189±6Ma和2186±3Ma,恶虎滩闪长质片麻岩的SHRIMP锆石U-Pb年龄为2182±5Ma。杜家沟长石斑岩为A型花岗岩,具有造山后花岗岩特征;吕梁群近周峪组中基性火山岩具有岛弧火山岩的地球化学特征;恶虎滩片麻岩也具有岛弧岩浆岩特征。综合分析华北克拉通2.2～2.0Ga的岩浆岩构造性质认为,吕梁地区岩浆性质的复杂性可能与所处的构造位置有关。古元古代吕梁地区可能位于华北克拉通古陆块边缘,其2.2～2.1Ga的岩浆岩可能经历岛弧和裂谷两种体制的共同制约。     

New SHRIMP U-Pb zircon ages of the metapelitic granulites from NW of Madurai,Southern India

Zircon cathodoluminescent imaging and SHRIMP U-Pb dating were carried out for metapelitic rocks (sapphirine-bearing granulites and garnet-cordierite gneisses) from the NW of Madurai, Southern India. The cathodoluminescence images reveal the complex, inhomogeneous internal structure having irregular-shaped core and overgrowths. Zircon grains have obliterated oscillatory zoning. SHRIMP U-Pb chronological results yield ages of 550±15 Ma and 530±50 Ma as a time of metamorphic overprint, and the age of 2509±12 Ma and 2509±30 Ma corresponding to a timing of protolith formation for sapphirine-bearing granulites and garnet-cordierite gneisses respectively. Zircon ages reflect that continental crust in the NW of Madurai region resulted from the recycling of Archaean protolith of an igneous origin similar to the preserved crust in the southern part of Dharwar craton. The present SHRIMP U-Pb zircon ages are in close agreement with earlier published Nd isotopic data which suggest an extended precrustal history of their protoliths. The abraded zircon grains indicate multiple recycling and repeated metamorphism that has ultimately resulted in present day continental crust exposed in Madurai region. These SHRIMP U-Pb zircon ages from metapelitic UHT granulites are also significant to understanding the architecture of the SGT during the amalgamation of Gondwana in Neoproterozoic time.     

Ages of internal granitoids in the Southern Cross region,Yilgarn Craton,Western Australia,and their crustal evolution and tectonic implications

Southern Cross, where gold deposits are sited in narrow greenstone belts surrounding granitoid domes, was one of the earliest gold mining centres in Western Australia. SHRIMP U–Pb zircon and Pb‐isotope studies of the largest granitoid dome, the Ghooli Dome (80 × 40 km), provide important constraints on the crustal evolution and structural history of the central part of the Archaean Yilgarn Craton, Western Australia, which includes Southern Cross. The north‐northwest‐south‐southeast‐oriented ovoid Ghooli Dome has a broadly concentric foliation that is subhorizontal or gently dipping in its central parts and subvertical along its margins. Foliated granitoids in the dome are dated at ca 2724 ± 5 and 2688 ± 3 Ma using the SHRIMP U–Pb zircon and Pb–Pb isochron methods, respectively. These new data, together with the published SHRIMP U–Pb zircon age of 2691 ± 7 Ma at another locality, 20 km from the centre of the Koolyanobbing Shear Zone, suggest that the Ghooli Dome was emplaced at ca 2.72–2.69 Ga. Because the Ghooli Dome and the other domes, which are enveloped by narrow greenstone belts, are cut by the >650 km‐long and 6–15 km‐wide Koolyanobbing Shear Zone, the ca 2.69 Ga age is interpreted as the maximum age of the last major movement on this structure. The pre‐2.69 Ga history, if any, of the shear zone remains unknown. The shear zone is intruded by an undeformed porphyritic granitoid which has a SHRIMP U–Pb zircon age of 2656 ± 4 Ma. This age is, thus, the minimum age of major movement along this shear zone. Post‐gold mineralisation pegmatitic‐leucogranite from the Nevoria gold mine has a SHRIMP U–Pb zircon age of 2634 ± 4 Ma, with xenocrystic zircon cores of ca 2893 ± 6 Ma, constraining the minimum age of gold mineralisation there to ca 2.63 Ga. The ca 2.72–2.69 Ga granitoids also contain ca 2.98 and 2.78 Ga xenocrystic zircon cores, suggesting an extensive crustal prehistory for their source. Whereas there is a general temporal relationship between the periods of older (ca 3.0 Ga) and younger (ca 2.80 and 2.73 Ga) volcanism and the older (2.98, 2.78 and 2.72–2.69 Ga) granitoid intrusions, there is no known volcanism temporally associated with the 2.65–2.63 Ga granitoid intrusions in the Yilgarn Craton. Other heat sources and/or tectonic processes, required for the generation of these intrusions, are interpreted to be related to a lithospheric delamination event related to continental collision.     

Time Constraints for the Formation of the Archean Rio Maria Crust,Southeastern Amazonian Craton,Brazil

The Rio Maria area (southeastern Carajás metallogenic province, Brazil) consists of a typical Archean Au-bearing granite-greenstone association intruded by Paleoproterozoic anorogenic granites. U-Pb analyses were performed on single zircon grains from the main Archean units of this area in order to elucidate the history of formation and evolution of the continental crust in this part of the southeastern Amazonian Craton. The Arco Verde metatonalite yielded one of the oldest U-Pb zircon ages from the Amazonian Craton (2957 +25/?21 Ma). Zircons from a meta-rhyodacite from the greenstone belt of the Lagoa Seca Group (Andorinhas Supergroup) are dated at 2904 +29/?21 Ma. This sequence is crosscut by Archean granitoids, such as the Rio Maria granodiorite, emplaced at 2874 +9/?10 Ma. These results suggest a short time interval (～100 Ma) for crustal evolution of the Rio Maria area.     

内蒙古贺兰山地区古元古代晚期的花岗岩浆事件及其地质意义: 同位素年代学的证据

位于华北克拉通西缘的贺兰山杂岩主要由孔兹岩系和变形花岗岩(正片麻岩)所组成,前者主要由夕线石榴片麻岩、石榴二长片麻岩、变粒岩和少量的大理岩及麻粒岩所组成,后者主要包括黑云斜长片麻岩、石榴子石花岗岩、斑状花岗岩和片麻状变质闪长岩.本文报道了该区变形花岗岩的锆石SHRIMP U-Pb定年结果.黑云二长片麻岩和石榴子石花岗岩分别形成于2053±58Ma和2047±42Ma,斑状花岗岩和片麻状闪长岩分别在1955Ma和1920Ma侵位.大量的年代学资料表明,在华北克拉通北缘存在一条古元古代晚期的花岗杂岩带,该带中的花岗杂岩主要形成于三个阶段,第一阶段大于2.0Ga,第二阶段主要出现在2.0～1.87Ga期间,第三阶段的花岗杂岩在1.85～1.80Ga期间侵位.年代学研究还表明,古元古代晚期的花岗岩浆作用常常与变质事件紧密相关.     

塔里木盆地东南缘阿克塔什塔格地区新太古代陆壳增生:米兰岩群和TTG片麻岩的地球化学及年代学约束

塔里木盆地东南缘的阿克塔什塔格地区,保存有较为完好的早前寒武纪基底变质岩——阿克塔什塔格杂岩,主要由米兰岩群、新太古代TTG花岗片麻岩和侵入其中的各类古元古代花岗片麻岩构成。其中米兰岩群和TTG片麻岩发育塑性流变褶皱和高角闪岩相-麻粒岩相变质,具有强烈的混合岩化,并遭受后期的角闪岩相变质改造。米兰岩群中的长英质片麻岩和TTG岩系的锆石SHRIMP U-Pb年龄分别为(2 567±32)Ma和(2 592±15)Ma,二者普遍低Si高Al、富Na贫K、富Sr贫Mg、富集LILE和LREE,亏损HSFE和HREE、轻重稀土分馏强烈、Eu异常不明显,具有类似于埃达克岩的岩石地球化学特征,表明它们形成于俯冲带的岛弧环境,为岛弧玄武岩俯冲至下地壳部分熔融的产物,指示了塔里木盆地东南缘新太古代晚期古老克拉通的大陆地壳水平增生。在此基础上,文章还探讨了塔里木盆地周缘早前寒武纪基底岩系的年代格架问题,认为塔里木盆地具有统一的早前寒武纪变质基底。     

The time length of formation of the Angara-Vitim batholite: Results of U-Pb geochronological studies

This paper describes the results of geochronological studies (U-Pb method over micro lots and single grains of zircon) of autochtonous and allochtonous granitoids of the Barguzinskii complex of the Angara-Vitim batolite of the petrotypical area in the basin of the Dzhirga and Kovyli rivers (tributaries of the Barguzin River). The age of crystallization of gneissose granitoids is 297 ± 5 Ma, and that of intrusive leucocratic biotite granites is 291 ± 1 Ma. The estimates of the age finalize the discussion on the age of granitoids of the Barguzin complex and cannot be considered as ??rejuvenated.?? The analyses of the geochronological data that have been obtained up to the present for granitoids of the Angara-Vitim batolite with the SHRIMP and U-Pb methods for large samples of zircons show that in the majority of cases they cannot be used for precise estimation of the age of their crystallization. The geochronological data obtained with use of the U-Pb method over micro samples and single grains of zircon allow one to make a conclusion on the formation of granitoids of the described complexes of the Angara-Vitim batholite that occurred within 303 ± 7?C281 ± 1 Ma. Thus, the time length of formation of the largest in the eastern segment of the Central Asian belt of the Angara-Vitim batholite is not more than 22 Ma (minimum 6 Ma), which allows us to consider it as a large granitic province and is a boundary condition for development of the geodynamic models of its formation.     

南岭诸广山北体复式花岗岩锆石SHRIMP U-Pb年龄及多期岩浆活动

诸广山复式花岗岩基位于湘、赣、粤三省交界处,是南岭花岗岩的重要组成部分,由南体和北体组成,出露面积大于5000 km2,形成于加里东期、印支期、燕山期3个时代。通过对诸广山北体3期花岗岩锆石SHRIMP U-Pb定年,结果表明,加里东期花岗闪长岩锆石SHRIMP U-Pb年龄为(414.5±4.5)Ma(n=13,MSWD=0.52)、(417.3±4.8)Ma(n=12,MSWD=0.84),形成于晚志留世末。印支期黑云母二长花岗岩锆石SHRIMP U-Pb年龄(207.5±2.7)Ma(n=10,MSWD=1.6),形成于晚三叠世末。燕山期二云母二长花岗岩锆石SHRIMP U-Pb年龄(148.2±1.7)Ma(n=13,MSWD=0.93),形成于晚侏罗世。该成果为南岭地区构造-岩浆演化及成矿作用研究,提供了重要的花岗岩年代学依据。     

湖南锡田云英岩-石英脉型钨锡矿的形成时代及其 赋矿花岗岩锆石SHRIMP U-Pb 定年

[摘 要] 具超大型规模远景的湖南锡田钨锡多金属矿床是1999 年开始的新一轮国土资源大调查的重大发现之一。矿床产于锡田复式花岗岩体与碳酸盐岩接触带及岩体内部。矿床类型主要为矽卡岩型,其次为破碎带蚀变岩型和云英岩-石英脉型。采用石英流体包裹体Rb-Sr 法和锆石SHRIMP U-Pb法分别获得:云英岩-石英脉型钨锡矿形成年龄为153±12Ma(MSWD=0.80),其赋矿花岗岩形成年龄为147±3Ma(MSWD= 0.90),加权平均年龄为147郾0依3郾5 Ma(MSWD =0郾23),两者在误差范围内一致;并进一步证实,锡田钨锡多金属矿的主成矿期是燕山早期。     

花岗岩锆石U-Pb年龄与全岩Rb-Sr等时线年龄对比研究及其地球化学意义

对国内外32个花岗岩体的锆石U-Pb年龄与全岩Rb-Sr等时线年龄之间差值（Δt）进行的频数统计分析表明：Δt呈对称正态分布（偏度系数CSK=0.36；峰度系数CKU=2.99）；年龄差（Δt）既呈正值又有负值，其均值为2.08Ma；相对年龄差（Rt）小于5%。采用最小二乘法计算，花岗岩体锆石U-Pb年龄（tZr）对全岩Rb-Sr等时线年龄（tRb）拟合出相关系数很高（r=0.998），回归系数接近l（α=1.003）的线性回归方程（tRb =1.003tZr +1.258）。这些统计特征表明，从总体来看，花岗岩体的Rb-Sr等时线定年测定结果与锆石U-Pb定年测定结果是一致的，花岗岩全岩Rb-Sr等时线定年方法是成熟、可信的，同时也为花岗岩锆石U-Pb年龄代表结晶年龄而不代表花岗岩侵位年龄提供了依据。     

Diversity of Archean crust in the eastern Tula Mountains,Napier Complex,East Antarctica

The Napier Complex of Enderby and Kemp Lands forms the north-western part of the East Antarctic Shield and consists predominantly of gneisses and granulites metamorphosed during a ca. 2.8 Ga high-grade and a ca. 2.5 Ga ultra-high temperature event. The western segment of the Napier Complex includes coastal outcrops, islands and nunataks around Amundsen and Casey Bays, and the Tula Mountains. This region records some of the highest metamorphic temperatures measured on Earth, affecting a variety of gneisses as old as ca. 3.8 Ga. Five samples of orthogneiss from the less-studied eastern Tula Mountains, including three granitic, one trondhjemitic and one dioritic gneiss, were dated by zircon U-Pb Secondary Ion Mass Spectrometry (SIMS). The three orthogneisses yield protolith ages of 3750 ± 35 Ma (granitic), 3733 ± 21 (trondhjemitic) Ma and 3560 ± 42 Ma (dioritic), whereas the two other granitic orthogneisses record ages of 2903 ± 14 Ma and 2788 ± 24 Ma. Zircon growth during metamorphism occurred at 2826 ± 10 Ma, and also between 2530 Ma and 2480 Ma. Samples from the Tula Mountains can be geochemically subdivided into Y-HREE-Nb-Ta depleted and undepleted groups. Eoarchean granitoids are included in both geochemical groups, as are Meso- and Neoarchean granitoids. The Y-HREE-Nb-Ta depleted granitoids can be generated by medium- to high-pressure melting of mafic crust, whereas undepleted granitoids can be generated by low-pressure melting. However, relatively high potassium contents in most samples, and the presence of xenocrystic/inherited zircon in some, reflect the likely involvement of felsic crustal sources. This diversity in granitoid composition occurs across the Napier Complex. The lack of a simple correlation between protolith age and geochemical type is an indication that magmatism during the Eoarchean (and later) involved diverse sources and processes, including re-melting and recycling of various crustal components, rather than just the formation of juvenile crust.     

RbSr isotope systematics at amphibolite facies conditions,Uppsala Region,Eastern Sweden

On the basis of U-Pb, Rb-Sr and K-Ar isotope analyses of Proterozoic rocks and minerals, a chronology has been established for the tectonic, intrusive and metamorphic evolution of the Svecokarelian orogeny 1750–1950 Ma ago in the Uppsala Region, Eastern Sweden. It is suggested that when synkinematic granitoids intruded the orogenic belt, at a stage of general subsidence and at medium metamorphic conditions (600°C and 3.5–4 kbar), the U-Pb isotope system in zircons closed earlier than the Rb-Sr whole-rock system. The zircon age (1886 Ma) reflects the intrusion and crystallization of the rock melt and the Rb-Sr whole-rock age (1830 Ma) the time when the temperature had decreased below the threshold for ⁸⁷Sr migration. The Rb-Sr whole rock age (1898 Ma) determined for metaandesites and metadacites reflects a recrystallization related to the intrusion of the granitoids. On the contrary, the more silicic metarhyodacitic volcanic rocks have a Rb-Sr whole rock age (1830 Ma) reflecting the cessation of the synkinematic metamorphism. The difference in the way the Rb-Sr isotope system responds in subsilicic or silicic metavolcanics is probably dependent on the amount of radiogenic ⁸⁷Sr and on the fixation of ⁸⁷Sr in Ca-rich minerals. Subsequent, late-kinematic, low amphibolite facies metamorphism has not altered the Rb-Sr ages of the granitoids and the recrystallized metavolcanics.     

Constraints on Archaean crustal evolution of the Zimbabwe craton: a U-Pb zircon,Sm-Nd and Pb-Pb whole-rock isotope study

 The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and Pb-Pb and Sm-Nd whole-rock isotope data, constrain the timespan of development of the Harare-Shamva granite-greenstone terrain and establish the relative involvement of juvenile mantle-derived and reworked crustal material. Basement-cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3.2–2.8 Ga basement gneisses, in ensialic, continental basins. Geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre-existing crustal nucleus consistent with our interpretations are rift-related: (1) intracontinental rifting related to mantle plume activity or; (2) rifting in a back-arc environment related to a marginal volcanic arc. The data, in conjunction with field evidence, do not indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone sequence in the Harare part of the greenstone belt, as was recently postulated on the basis of SHRIMP zircon ages. Zircon ages for basal felsic volcanics (2715±15 Ma) and a subvolcanic porphyry (2672±12 Ma) constrain the initiation and termination of deposition of the greenstone sequence. The timespan of deposition of the Upper Bulawayan part of the greenstone sequence corresponds well with radiometric ages for Upper Bulawayan greenstones in the central and southern part of the craton and supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic gneiss (2667±4 Ma) and a late syn-tectonic intrusive granodiorite (2664±15 Ma) pinpoint the age of deformation of the greenstone sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659±13 Ma) associated with the latter intrusive phase. The intimate timing relation of greenstone deformation and granitoid emplacement, but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts, and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths, imply that the intrusion of the granitoids had a significant influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic activity is indicated by the zircon ages of small, post-tectonic plutons intrusive into the greenstone belt, with a mineralized granodiorite dated at 2649±6 Ma and an unmineralized tonalite at 2618± 6 Ma. The 2601±14 Ma crystallization age of an “external” Chilimanzi-type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton, demonstrating a craton-wide event and heralding cratonization. The similarity between U-Pb zircon ages and T_DM model ages (2.65–2.62 Ga) and the positive ɛNd_T values (+3 to +2) for the late syn-tectonic and post-tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and minor contamination with older crust. The rather high model μ₁ values (8.2–8.6) are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older T_DM ages for the felsic volcanics (3.0–2.8 Ga) and the porphyries (2.8–2.7 Ga) suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction between depleted mantle-derived melts and older crustal material. Received: 15 August 1995 / Accepted: 12 January 1996     

吉林中部晚三叠世和早侏罗世两期铝质A型花岗岩的厘定及对吉黑东部构造格局的制约

高精度同位素年代学和岩石学、元素地球化学研究结果表明,吉林省中部地区存在晚三叠世和早侏罗世两期铝质A型花岗岩。其中三道河正长花岗岩的锆石LA ICPMS年龄为(216±3) Ma,形成于晚三叠世,受控于华北板块和其北侧板块在晚二叠世—早三叠世沿西拉木伦河—长春—延吉缝合带碰撞拼合后的岩石圈伸展作用,标志古亚洲洋构造域的演化结束。天桥岗碱长花岗岩的锆石SHRIMP和TIMS年龄分别为(182±3) Ma和(188±4) Ma,全岩Rb Sr等时线年龄为(185±4) Ma,形成于早侏罗世,可能是与佳木斯板块和松嫩—张广才岭板块在早侏罗世早期沿嘉荫—牡丹江缝合带碰撞拼合有关的伸展作用的产物。这次板块碰撞作用很有可能标志着东北地区东部此时已经开始进入滨太平洋构造域的演化阶段。更详细的研究显示,两期A型花岗岩岩浆都来源于年轻的基性玄武质下地壳的部分熔融,岩浆经历了分离结晶作用。     

辽宁鞍山地区东山杂岩带3.3~3.1 Ga期间的岩浆作用——锆石SHRIMP U-Pb定年

报道了鞍山地区东山杂岩带奥长花岗岩和二长花岗岩的锆石SHRIMP U-Pb年龄。中粗粒奥长花岗岩中岩浆锆石的年龄为3329 Ma ± 22 Ma (MSWD=9.6),存在年龄为3687~3784 Ma的残余锆石。细粒奥长花岗岩和二长花岗岩中岩浆锆石的年龄分别为3141 Ma ± 8 Ma (MSWD=1.5)和3142 Ma ± 5 Ma (MSWD=0.35)。研究表明,约~3.3 Ga和3.1 Ga是鞍山地区2个重要的地壳演化阶段。     

Late Neoarchean synchronous TTG gneisses and potassic granitoids in southwestern Liaoning Province,North China Craton: Zircon U-Pb-Hf isotopes,geochemistry and tectonic implications

Abundant late Neoarchean granitoids occur in southwestern Liaoning Province, part of the Eastern Ancient Terrane of the North China Craton. These rocks include intermediate gneiss, TTG gneisses and potassic granitoids, and we report on the geochemistry and zircon SHRIMP ages as well as Hf-in-zircon isotopes of these granitoids in order to determine their petrogenesis. Field relationships suggest that most of these granitoids experienced widespread metamorphism and deformation, associated with anatexis at some localities. The intermediate gneisses, TTG gneisses and potassic granitoids were all emplaced at the end of the Neoarchean (2.50–2.53 Ga), and CL images document widespread recrystallization in the zircons. The intermediate and TTG gneisses yielded similar Hf isotopic systematics (ε_Hf(t) = −3.73 to +6.42) as the associated potassic granitoids (ε_Hf(t) = −2.44 to +7.80), and both rock types yielded mean Hf crustal model ages of 2.8–2.9 Ga. Combined with the geochemistry, we propose that the formation of the intermediate and TTG gneisses was related to partial melting of mafic rocks at different depth, whereas the potassic granitoids have variable petrogenesis. The nearly coeval TTG gneisses and potassic granitoids and their widespread metamorphism, deformation and zircon recrystallization suggest that a large-scale heat source must have been present at or near the base of the crust in southwestern Liaoning Province at the end of the Neoarchean. We propose that collision and post-collisional extension is the most likely tectonic environment for generation of the above granitoids, and the formation of widespread potassic granitoids played an important role in the maturation of continental crust in the North China Craton.