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1.
U–Pb zircon analyses from a series of orthogneisses sampled in drill core in the northern Gawler Craton provide crystallisation ages at ca 1775–1750 Ma, which is an uncommon age in the Gawler Craton. Metamorphic zircon and monazite give ages of ca 1730–1710 Ma indicating that the igneous protoliths underwent metamorphism during the craton-wide Kimban Orogeny. Isotopic Hf zircon data show that 1780–1750 Ma zircons are somewhat evolved with initial εHf values –4 to +0.9, and model ages of ca 2.3 to 2.2 Ga. Isotopic whole rock Sm–Nd values from most samples have relatively evolved initial εNd values of –3.7 to –1.4. In contrast, a mafic unit from drill hole Middle Bore 1 has a juvenile isotopic signature with initial εHf zircon values of ca +5.2 to +8.2, and initial εNd values of ～+3.5 to +3.8. The presence of 1775–1750 Ma zircon forming magmatic rocks in the northern Gawler Craton provides a possible source for similarly aged detrital zircons in Paleoproterozoic basin systems of the Gawler Craton and adjacent Curnamona Province. Previous provenance studies on these Paleoproterozoic basins have appealed to the Arunta Region of the North Australian Craton to provide 1780–1750 Ma detrital zircons, and isotopically and geochemically similar basin fill. The orthogneisses in the northern Gawler Craton also match the source criteria and display geochemical similarities between coeval magmatism in the Arunta Region of the North Australian Craton, providing further support for paleogeographic reconstructions that link the Gawler Craton and North Australian Craton during the Paleoproterozoic.  相似文献   

2.
滇西哀牢山变质岩系锆石U-Pb定年及其地质意义   总被引：11，自引：7，他引：4  
哀牢山-红河构造带是滇西地区最著名的带状变质带之一,其主体由哀牢山深变质岩系(哀牢山岩群)组成,一直被认为是扬子陆块古元古代结晶基底.本文选取哀牢山深变质岩系内的花岗片麻岩(11 ALl7-1和11AL09-1)和石英岩(11AL08-1),以及邻区的花岗岩(11ALl2-1)进行LA-ICP-MS锆石U-Pb定年.结果显示,花岗片麻岩11 ALl7-1有岩浆和变质两类锆石,两者的206Pb/238U年龄加权平均值分别为700±6Ma(MSWD=1.4,n=14)和27.4±1.2Ma(MSWD=1.9,n=3),代表原岩形成时代和变质年龄.花岗片麻岩llAL09-1岩浆锆石206 pb/238U年龄为220±3Ma(MSWD=3.1,n=14),变质锆石年龄为31.2±2.3Ma(MSWD =6,n=5),分别代表原岩结晶时代和后期变质年龄.石英岩11AL08-1中所有锆石具有核-边结构,92颗锆石核部年龄集中分布在6组,分别为493～528Ma(n=42)、635 ～ 640Ma(n=2)、701～784Ma(n=44)、976 ～980Ma(n=2)、1839Ma(n=1)和2487Ma(n=1).92个核部分析点具有高的Th/U比值(＞0.23),指示岩浆来源.最年轻一组的42个核部年龄加权平均值为509Ma,代表石英岩原岩的最大沉积时代.7颗锆石变质边年龄为26～ 75 Ma内,代表变质年龄.花岗岩11 ALl2-1锆石206pb/238U年龄加权平均值为750±4Ma(MSWD =0.6),代表岩石形成时代.这些年龄表明哀牢山变质岩系是一个原岩复杂的变质杂岩带,它的原始物质至少包含新元古代～ 700Ma岩浆岩、～509 Ma沉积地层及220 ～ 240Ma的岩浆岩和地层,而不是以往认为的古元古代结晶基底.现今所见的哀牢山岩群“古老”岩石面貌主要是由地质历史上的浅变质或未变质的地层和岩浆岩在新生代26～31Ma发生变质变形作用改造的结果.哀牢山变质带的源区物质特征和主要岩浆事件与扬子陆块西缘十分相似,具有亲扬子的构造属性.  相似文献   

3.
The Dashiqiao Formation on the Liaodong Peninsula constitutes an important component within the Jiao–Liao–Ji Belt, North China Craton. It is composed dominantly of dolomitic marbles intercalated with minor carbonaceous slates and mica schists, hosting one of the largest magnesite deposits on Earth. This study presents zircon cathodoluminescence (CL) images and U–Pb–Hf isotope data, as well as single-mineral geochemical data for the staurolite–garnet–mica schist from the Dashiqiao Formation, in order to constrain its protolith age and provenance, and further to discuss the early Precambrian tectono-thermal events of the North China Craton. U–Pb isotopic dating using the LA–ICP–MS method on detrital zircons from the schist preserves at least three age populations ranging in age from 2.99 to 2.02 Ga, and grains as old as ca 4087 Ma. The dominant Neoarchean detrital zircons were most probably sourced from the basement within the Longgang and Nangrim blocks, while the minor Mesoarchean zircons were only sourced from the Longgang Block. The subordinate middle Paleoproterozoic zircons are consistent with ages of the regionally distributed coeval Liaoji granites and volcanics within the Jiao–Liao–Ji Belt. Zircon U–Pb dating yields a metamorphic age of 1930 Ma for the sample, interpreted to represent the peak stage of epidote amphibolite facies metamorphism. Thus, the depositional age for the protolith of the schist was proposed in the period between 2.01 and 1.93 Ga. LA–MC–ICP–MS Lu–Hf isotopic data show that all Archean (2.45–2.55) detrital zircons possess positive εHf(t) values from +?0.7 to +?7.5 with juvenile depleted mantle model ages, suggesting a significant crustal growth event during the Neoarchean in the North China Craton. The Paleoproterozoic detrital zircons possess variable εHf(t) values (??5.5–+?8.3) and depleted mantle model ages from Mesoarchean to Paleoproterozoic. The zircons with negative εHf(t) values implies the Mesoarchean to Neoarchean crust undergoing a recycling event in the period 2.40–2.01 Ga, while those with positive εHf(t) value suggest some indication of juvenile addition to the crust during the Paleoproterozoic. Using regional geological and new detrital zricon U–Pb–Hf isotopic data, the early Precambrian tectono-thermal events can be subdivided into the following episodes: Mesoarchean, late Neoarchean, middle Paleoproterozoic, and late Paleoproterozoic times.  相似文献   

4.
本文报道了辽东半岛古元古代胶—辽—吉活动带内辽河群变质火山岩和辽吉花岗岩的锆石LA-ICP-MS U-Pb年代学和地球化学数据。变质安山岩的锆石具典型的岩浆振荡环带结构和较高的Th/U值(0.5),锆石U-Pb年龄为(2 182±6)Ma和(2 229±22)Ma,该年龄可代表安山岩的形成年龄。辽吉花岗岩的锆石同样具有典型的岩浆振荡环带结构和较高的Th/U值(0.3),锆石U-Pb年龄为(2 199±10)Ma,代表花岗岩的侵位时代,在误差范围内与辽河群火山岩喷发时代一致,表明辽吉花岗岩并不是辽河群的基底,二者可能为同一次岩浆作用过程的产物。辽东半岛~2.2Ga岩浆事件的识别及性质,对于正确认识古元古代胶—辽—吉活动带的属性至关重要。结合前人有关辽东半岛前寒武纪岩石的研究成果,本文研究认为胶—辽—吉活动带的形成演化可能与弧-陆碰撞有关。  相似文献   

5.
The Huangtuling hypersthene-garnet-biotite gneiss at Luotian County, Hubei Provine, is a typicalgranulite-facies rock of the Dabie Group Complex in the Dabie orogenic belt. Investigations on the morphology andoccurrence of zircons and their internal structures shown in the thin sections lead to the recognition of three types ofzircons, which are in good agreement with the types identified on the basis of morphology, colour and external fea-tures from the related zircon concentrates. The observation of zircons in the rock reveals that part of type 1 zirconsshow signs of a double-layered structure. The interval part existed in the protolith prior to the granulite-facies meta-morphism. Type 2, the prismatic zircons which mainly occur in garnet and hypersthene are metamorphic minerals ofthe granulite-facies metamorphism. Type 3, the round multifaceted zircons in felsic minerals and biotite, are proba-bly attributed to a later geological event related to migmatization. The ~(207)Pb/~(206)Pb zircon dating by direct evaporationon (thermal evaporation ion mass spectrometer) yields ages ranging from 2814 Ma to 1992 Ma. The age discrepancyamong these different zircon types is conspicuous. The yellow-brown(type 1) zircons give ages of 2814±29 Ma to2527±6 Ma, the prismatic euhedral zircons (type 2), 2456±7 Ma to 2254±4 Ma, and the round multifaceted zircons(type 3), 1992±10 Ma. The results are geologically interpreted in consideration of the complicated behaviours of zir-cons during Precambrian geological evolution of the Dabie area. (1) If the protolith of the gneiss is a sedimentaryrock, then type 1 zircons are clastic ones and the ages 2814±29 Ma and 2811±27 Ma may reflect the minimum age ofthe rocks of its source region. also the first geological event in the area. Sedimentation of the protolith occurred be-tween 2814 Ma and 2527 Ma, probably close to 2814 Ma. If the protolith is a volcanic rock, then the formation age ofthe supracrustal rocks of the Dabie Group Complex is around 2814 Ma. The age 2456±7 Ma reflects the time whenthe granulite-facies metamorphism took place. The later migmatization event is dated at aboat 1992±10 Ma, and isprobably the latest early Precambrian event in the area. The present work provides geochronological evidence for the existence of the Dabie Archaean craton, whichhad probably experienced 3 or 4 geological events during its early Precambrian evolution.  相似文献   

6.
The Archean continental crusts account for ca.20% of the present volume,but the thermal history of the Earths' mantle suggests much more continental crusts were formed in the early Archean.Because the Archean continental crust underwent severe metamorphism,it is important to avoid influence by the later thermal events.We carried out a comprehensive geochronological work of Cathodoluminescence(CL) observation and U-Pb dating of zircons from orthogneisses and supracrustal rocks over the Saglek Block to obtain their protolith ages.The zircons were classified into three domains of core,mantle and rims,and the cores were further classified into three groups of inherited,altered and zoned cores based on the zonation on the CL images.We estimated the protolith ages from Pb-Pb ages of the zoned-cores of zircons with low U contents.We made a detailed sketch of a small outcrop in St.John's Harbour South(SJHS) area,and classified the orthogneisses and mafic enclaves into seven generations based on the geologic occurrence.The first and second generations comprise mafic rocks and lack magmatic zircons.We conducted CL imaging and U-Pb dating of zircons from the third,sixth and seventh generation of the orthogneisses to estimate the protolith ages at 3902 L 25,3892 ± 33 and 3897 ± 33 Ma for each,supporting the presence of the over 3.9 Ca Iqaluk Gneiss.The geological occurrence that the mafic rocks occur as enclaves within the 3.9 Ga Iqaluk Gneiss indicates that they are the oldest supracrustal rocks in the world.Our geochronological and geological studies show the Uivak Gneiss is quite varied in lithology and age from 3.6 to 3.9 Ga,and tentatively classified into six groups based on their ages.The oldest Uivak Gneiss components including the Iqaluk Gneiss are present around the SJHS area,and the orthogneisses become young as it is away.The lines of evidence of overprinting of younger granitoid on older granitoid in small outcrops and geological-map scale as well as presence of inherited zircons even in the oldest suite suggests that crustal reworking played an important role on erasing the ancient crusts.  相似文献   

7.
The Variscan Erzgebirge represents an antiform with a core of gneisses and mica schists, surrounded by a phyllitic mantle. The Gneiss-Eclogite Unit (GEU), in the central part, is a composite tectonometamorphic assemblage characterized by a HP-HT imprint and comprises migmatitic para- and orthogneisses, HT mylonites, HP granulites, eclogites and garnet peridotites. It is tectonically sandwiched between two major units with distinctly lower PT histories. The GEU experienced a characteristic “kinked” retrograde PT path after HP-HT equilibration with: (1) strong near-isothermal decompression at high temperatures; (2) extensive re-equilibration at medium pressures, followed (3) by rapid cooling during continued uplift. We dated zircons (Pb-Pb evaporation) from granitoid orthogneisses and metapelites of the GEU. The orthogneisses contain euhedral, long-prismatic zircons of igneous origin that provided protolith ages between 470 and 524 Ma. Metapelites retain well-preserved granulite-facies mineral assemblages and contain spherical, multifaceted metamorphic zircons that grew near the peak of HP/HT metamorphism. Inclusions of prograde HP phengite (∼15 kbar) and rutile are included in one such zircon. Metamorphic zircons of three samples from different localities yielded identical 207Pb/206Pb ages of 340.5 ± 0.7 Ma, 341.2 ± 0.5 Ma and 341.6 ± 0.5 Ma respectively. Consideration of these zircon ages with published 39Ar/40Ar white mica ages suggests fast cooling and uplift rates in excess of 50 °C/Ma and 4 km/Ma. This is typical for large-scale extensional tectonic unroofing of the ultra-deep part of a fossil, thickened Variscan continental crust (>60 km) during continuing continental collision and orogenic collapse. Received: 5 June 1997 / Accepted: 7 January 1998  相似文献   

8.
The Suretta nappe of eastern Switzerland contains a series of meta-igneous rocks, with the Rofna Porphyry Complex (RPC) being the most prominent member. We present LA-ICP-MS U–Pb zircon data from 12 samples representing a broad spectrum of meta-igneous rocks within the Suretta nappe, in order to unravel the pre-Alpine magmatic history of this basement unit. Fine-grained porphyries and coarse-grained augengneisses from the RPC give crystallization ages between 284 and 271 Ma, which either represent distinct magma pulses or long-lasting magmatic activity in a complex magma chamber. There is also evidence for an earlier Variscan magmatic event at ~320–310 Ma. Mylonites at the base of the Suretta nappe are probably derived from either the RPC augengneisses or another unknown Carboniferous–Permian magmatic protolith with a crystallization age between 320 and 290 Ma. Two polymetamorphic orthogneisses from the southern Suretta nappe yield crystallization ages of ~490 Ma. Inherited zircon cores are mainly of late Neoproterozoic age, with minor Neo- to Paleoproterozoic sources. We interpret the Suretta nappe as mainly representing a Gondwana-derived crustal unit, which was subsequently intruded by minor Cambrian–Ordovician and major Carboniferous–Permian magmatic rocks. Finally, the Suretta nappe was thrust into its present position during the Alpine orogeny, which hardly affected the U–Pb system in zircon.  相似文献   

9.
ABSTRACT

The Tiantang Cu–Pb–Zn polymetallic deposit in western Guangdong, South China, is hosted in the contact zone between the monzogranite porphyry and limestone of the Devonian Tianziling Formation. Orebodies occur in the skarn and skarnized marble as bedded, lenses, and irregular shapes. In this study, we performed LA-ICP-MS zircon U–Pb dating, zircon trace elements, and Hf isotopic analyses on the Tiantang monzogranite porphyry closely related to Cu–Pb–Zn mineralization. Twenty-two zircons from the sample yield excellent concordia results with a weighted mean 206Pb/238U age of 104.5 ± 0.7 Ma, which shows that the emplacement of the monzogranite porphyry in the Tiantang deposit occurred in the Early Cretaceous. The zircon U–Pb age is largely consistent with the sulphide Rb–Sr isochron ages, indicating that both the intrusion and Cu–Pb–Zn mineralization were formed during the Early Cretaceous in South China. The εHf(t) values of three inherited zircons from the monzogranite porphyry are 13.1, 11.9, and 12.9, respectively, and the two-stage Hf model ages are 1096 Ma, 1087 Ma, and 1055 Ma, respectively. Except for the three inherited zircons, all εHf(t) values of zircons are negative and have a range of ?7.6 to ?3.4, with the two-stage model ages (TDM2) of 1380–1643 Ma, which indicates the rock-forming materials were mainly derived from the partial melting of Mesoproterozoic to Neoproterozoic crust rocks, and probably included some Neoproterozoic arc-related volcanic-sedimentary materials. In this study, the monzogranite porphyry from the Tiantang deposit has calculated Ce4+/Ce3+ ratios of zircon ranging from 91 to 359, indicative of a more oxidized signature and significant prospecting potential for ore-related magmatism. Based on ore deposit geology, isotope geochemistry, and geochronology of the Tiantang Cu–Pb–Zn deposit and regional geodynamic evolution, the formation of Early Cretaceous magmatism and associated polymetallic mineralization in South China is believed to be related to large-scale continental extension and subsequent upwelling of the asthenosphere.  相似文献   

10.
大别山东部花岗片麻岩的锆石U-Pb年龄   总被引：29，自引：0，他引：29  
花岗片麻岩是大别山区除超高压变质杂岩外的另一种重要岩石类型,本文测得南大别二长花岗片麻岩中单颗粒锆石U-Pb不一致线的上交点年龄为789±43 Ma.位于五河一水吼韧性剪切带南缘的糜棱岩化二长花岗片麻岩中锆石的207Pb/206Pb表面年龄接近一致年龄,为715～777 Ma,平均757 Ma,U-Pb不一致线的上交点年龄为815±70 Ma,下交点年龄为482±167 Ma.北大别石英二长片麻岩中锆石207Pb/206表面年龄的变化范围较广,介于594～700 Ma,平均为649 Ma,U-Pb不一致线的上、下交点年龄分别为814±97 Ma和477±77 Ma.这说明这些正片麻岩的侵位时间范围大致为750～850 Ma的晚元古代;南、北大别正片麻岩中的锆石年代学显示它们可能具有相似的形成与演化历史;约480 Ma前后的加里东运动对大别山地区可能也有影响.  相似文献   

11.
胶东地区的荆山群呈近东西向环绕太古宙TTG花岗质片麻岩展布,主要由成熟度高的含石墨变泥砂质岩石、钙硅酸岩和大理岩组成,变质程度达高角闪岩相-麻粒岩相,具孔兹岩系性质。变质中-基性岩侵入到荆山群。它们的侵位时代对于探讨华北克拉通东部元古宙构造演化以及对荆山群沉积时代的制约,都有重要意义。锆石SHRIMP U-Pb定年结果表明,遭受低级变质的闪长岩(S0835)岩浆锆石年龄为1852±9Ma (MSWD=2.1),遭受中高级变质的辉长岩(S0816)变质锆石年龄为1865±11Ma (MSWD=0.76)。结合区域资料,可得出如下结论:(1)荆山群孔兹岩系形成于古元古代晚期(2.2~1.9Ga);(2)古元古代期间,胶东地区从挤压体制转入伸展体制的时间在1.87Ga之前。在华北克拉通中西部的恒山、大青山地区,存在1.97~1.92Ga辉长岩,在1.92~1.83Ga期间发生变质,与本文研究结果类似。这表明华北克拉通中-西部和东部具有类似的古元古代演化历史。  相似文献   

12.
U–Pb zircon analyses from three meta-igneous and two metasedimentary rocks from the Siviez-Mischabel nappe in the western Swiss Alps are presented, and are used to derive an evolutionary history spanning from Paleoarchean crustal growth to Permian magmatism. The oldest components are preserved in zircons from metasedimentary albitic schists. The oldest zircon core in these schists is 3.4 Ga old. Detrital zircons reveal episodes of crustal growth in the Neoarchean (2.7–2.5 Ga), Paleoproterozoic (2.2–1.9 Ma) and Neoproterozoic (800–550 Ma, Pan-African event). The maximum age of deposition for the metasedimentary rocks is given by the youngest detrital zircons within both metasedimentary samples dated at ~490 Ma (Cambrian-Ordovician boundary). This is in the age range of two granitoid samples dated at 505 ± 4 and 482 ± 7 Ma, and indicates sedimentation and magmatism in an extensional setting preceding an Ordovician orogeny. The third felsic meta-igneous rock gives a Permian age of intrusion, and is part of a long-lasting Variscan to post-Variscan magmatic activity. The zircons record only minor disturbance of the U–Pb system during the Alpine orogeny.  相似文献   

13.
Zircon ages are reported for three Moldanubian amphibolite grade orthogneisses from the southern Bohemian Massif obtained by conventional U/Pb analyses. For two of these orthogneisses, conventional U/Pb data are supported by ion microprobe single zircon ages or single grain evaporation data. The amphibolite grade orthogneisses, occurring in three small tectonic lenses within the Varied Group close to the South Bohemian Main Thrust, are of tonalitic, granodioritic or quartz dioritic composition.Conventional bulk size fraction and ion microprobe analyses of nearly euhedral zircons from a metatonalite, erroneously interpreted as a metagreywacke in a previous study, yielded an upper Concordia intercept age of 2048 ± 12 Ma. The well preserved euhedral grain shapes of the zircons suggest crystallization from a magmatic phase, and the upper Concordia intercept age is now interpreted as reflecting a magmatic event at that time. The age of this rock is compatible with the conventional zircon data and the (207Pb/206Pb)* single grain evaporation result from two further orthogneisses providing intrusion ages of 2 060 ± 12, 2 104 ± 1 and 2 061 ± 6 Ma, respectively. For one sample a concordant U/Pb age for sphene of 355 ± 2 Ma defines the age of amphibolite facies metamorphism. The upper Concordia intercept ages of three orthogneisses constitute the first direct evidence for the presence of early Proterozoic crust under the supracrustal cover in the southern part of the Bohemian Massif. Correspondence to: J. I. Wendt  相似文献   

14.
陈正  李俊建 《地质通报》2016,35(4):578-582
对蒙古国南戈壁省东部的西南欧玉岩体黑云母花岗岩锆石进行了阴极发光照相,结果显示,其具有明显的振荡环带,为典型的岩浆锆石特征。对22颗锆石进行了LA-MC-ICP-MS U-Pb年龄测定,结果显示,21个测点的~(206)Pb/~(238)U年龄介于334±2~335±2Ma之间,年龄加权平均值为335±1Ma,表明西南欧玉岩体形成于早石炭世。  相似文献   

15.
Here we present the first results of LA—ICP—MS Lu—Hf and U—Pb isotope analyses (Geoanalytic Center for Common Use, Ural Branch, Russian Academy of Sciences, and Center for Isotope Research, Karpinskii All-Russia Geological Research Institute, St. Petersburg (CCU UB RAS)) of zircons from finegrained granite of the Neplyuev Pluton which is the final phase earlier dated to 278 ± 1 Ma by the Rb–Sr method, along with new SIMS U—Pb results (SHRIMP, Center for Isotope Research, Karpinskii All-Russia Geological Research Institute, St. Petersburg (CIR VSEGEI)). The U—Pb results of similar zircon grain domains obtained using the two techniques agree well with each other. A difference in the age of zircon of two different morphologies has been found: the first one with a pronounced concentric oscillatory zoning yielded the concordia U—Pb age of 360 ± 2 Ma (SHRIMP) and 368 ± 6 Ma (LA—ISP—MS); the second type (“amoeba-shaped cores” with patchy zoning) gave 430 ± 9 Ma as a result. Considering the εHf vs. εNd systematics, the zircon from the granite is assumed to be of xenogenic but not magmatic nature.  相似文献   

16.
1 Introduction Since the Guilaizhuang gold deposit was discovered in the 1980s, the Tongshi magmatic complex has attracted attention since it exhibits a spatial-temporal relationship to gold mineralization. In the past ten years, a number of detailed fundamental researches have been carried out on the complex, Guilaizhuang gold deposit and their relationships (Qiu et al., 1994; Lin et al., 1996; Yan et al., 1996; Xu et al., 1999; Kong et al., 2001; Shun et al., 2001). Up to date, some res…  相似文献   

17.
松嫩地块位于兴蒙造山带东部,其内及周边地区早前寒武纪的岩石记录报道极少。笔者对黑龙江省龙江县大泉子屯碎裂岩化花岗岩进行年代学和地球化学研究表明,该岩石的锆石LA ICP MS U-Pb年龄为(2 579±15)Ma(n=20,MSWD=0.49),代表了花岗岩的结晶年龄,为新太古代晚期,地球化学特征显示该花岗岩为富钠花岗岩,具TTG中奥长花岗岩特征。本套新太古代花岗岩为松嫩地块及其周边地区的首次报道,证实了龙江地区存在早前寒武纪地质记录,为本区新太古代TTG岩石组合的寻找提供依据,也说明松嫩地块及其周边地区不仅存在古元古代岩浆活动记录、也存在新太古代岩浆活动记录,同时为松嫩地块及其周边大量的新太古代岩浆成因碎屑锆石的来源研究提供依据,为兴蒙造山带中残留的早期地质记录研究及兴蒙造山带的构造格局划分提供新材料。  相似文献   

18.
1 Introduction According to recent researches, the North China Craton consists of three parts: the eastern block, western block and central zone (Zhao, 2001; Wilde et al., 2002). Paleoarchean continental blocks and zircon residuals have only been found in a few regions, such as Anshan, East Liaoning (Liu et al., 1992; Song et al., 1996; Wan et al., 2002, 2005), Caozhuang, East Hebei (Liu et al., 1992) and Xinyang, West Henan (Zheng et al., 2004), which are mainly distributed in the east…  相似文献   

19.
U–Pb zircon dating of three metagranitoids, situated within a tilted crustal section at the northwestern border of the Teplá Barrandian unit (Teplá crystalline complex, TCC), yields similar Cambrian ages. The U–Pb data of zircons of the Teplá orthogneiss define an upper intercept age of 513 +7/–6?Ma. The 207Pb/206Pb ages of 516±10 and 511±10?Ma of nearly concordant zircons of the Hanov orthogneiss and the Lestkov granite are interpreted to be close to the formation age of the granitoid protolith. Similar to the Cambrian granitoids of the southwestern part of the Teplá Barrandian unit (Doma?lice crystalline complex, DCC) the Middle Cambrian emplacement of the TCC granitoids postdates Cadomian deformation and metamorphism of the Upper Proterozoic country rocks, but predates Variscan tectonometamorphic imprints. Structural data as well as sedimentological criteria suggest a dextral transtensional setting during the Cambrian plutonism, related to the Early Paleozoic break-up of northern Gondwana. Due to strong Variscan crustal tilting, the degree of Variscan tectonometamorphic overprint is strikingly different in the dated granitoids. It is lowest in the weakly or undeformed Lestkov granite, located in the greenschist-facies domain. The Teplá orthogneiss in the north underwent pervasive top-to-NW mylonitic shearing under amphibolite-facies conditions. There is no indication for a resetting of the U–Pb isotopic system of the Teplá orthogneiss zircons that could be attributed to this imprint. Radiation damages accumulated until recent have probably caused lead loss.  相似文献   

20.
冀西北怀安地体高级变质表壳岩的锆石年代学研究   总被引：4，自引：3，他引：1  
蔡佳  刘平华  冀磊  施建荣 《岩石学报》2017,33(9):2811-2826
位于华北克拉通中部造山带中北段的怀安地体与内蒙孔兹岩带相接,出露高压麻粒岩和退变榴辉岩等多种高级变质岩,是洞悉华北克拉通古元古代构造演化历史的重要窗口。研究区变质表壳岩包括夕线石榴长英质片麻岩、石榴长英质粒状岩石以及紫苏黑云二长片麻岩。阴极发光图像特征显示研究区样品的锆石主要包括碎屑锆石和变质锆石,其中碎屑锆石具有岩浆结晶环带,而变质锆石为单颗粒或围绕着继承性碎屑锆石边部生长,内部结构均匀,Th/U比值较低。锆石LAICP-MS U-Pb定年结果与前人研究结果综合表明该区变质表壳岩石的碎屑锆石的207Pb/206Pb年龄主要集中在~2040Ma,其原岩形成时代与孔兹岩带变泥质岩石相近,均为~2.0Ga。变质锆石记录其变质时代为1957~1804Ma,结合前人对怀安地区变泥质岩和变基性岩变质作用和年代学研究结果,推测得出1.95~1.92Ga代表了峰期(高压)麻粒岩相变质时代,1.90~1.85Ga代表峰后减压阶段变质时代,而1.85~1.80Ga代表退变质晚期的时代。怀安地区变质岩石可能卷入了阴山陆块、鄂尔多斯陆块以及东部陆块间的先后碰撞造山过程,并持续较长时间(1.95~1.80Ga),最终拼贴为统一的整体。  相似文献   

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