S型花岗岩中锆石U—Pb同位素体系的多阶段演化及其年代学意义：以桂北…

Ｓ型花岗岩中含有残留锆石并经历过后期Ｐｂ丢失事件是一个较普遍的现象，这使得其锆石Ｕ－Ｐｂ体系呈现出复杂的多阶段演化特征，虽然离子探针对各种复杂锆石Ｕ－Ｐｂ体系定年最为有效，但因其昂贵的成本使其应用受到限制，目前较为常用的仍是常规化学－质谱分析方法，在现有实验Ｐｂ本底水平上恰当地选择单颗或多颗粒锆石进行分析，是获得高质量Ｕ－Ｐｂ年龄数据的关键，桂北三防岩体含有残留锆石并经历过期Ｐｂ丢失事件，使其锆石     

河南焦作云台山早前寒武纪变质基底锆石SHRIMP U-Pb年龄

应用离子探针技术,对华北太行山南缘钾长石化长英质副片麻岩和钾长石化片麻状奥长花岗岩进行了锆石年龄测定。钾长石化长英质副片麻岩的变质原岩为泥砂质碎屑沉积岩,其形成时代很可能为新太古代。碎屑锆石普遍存在强烈铅丢失,靠近上交点的5个数据点207Pb/206Pb加权平均年龄为3399Ma±8Ma,代表了物源区组成的时代。钾长石化片麻状奥长花岗岩2组锆石207Pb/206Pb加权平均年龄分别为2511Ma±13Ma(岩体形成时代)和2735Ma±16Ma(残余锆石年龄),分别代表该岩体形成时代和残余锆石年龄。新的资料支持了华北克拉通中部造山带太古宙地质体与其东部陆块存在亲缘关系的认识。     

新疆霍什布拉克地区花岗岩锆石U-Pb年龄

对塔木、霍什布拉克两个碱长花岗岩地质、地球化学特点,锆石的特征进行了详细的研究和单颗粒锆石U-Pb同位素年龄测定.塔木放射成因铅丢失最少的2个数据点锆石的206Pb/238表面年龄统计权重平均值为(229.2±25)Ma,6个锆石颗粒组成的不一致线上交点年龄为(235±29)Ma.霍什布拉克放射成因铅丢失最少的1号数据点的206Pb/238U表面年龄为(2615±2.7)Ma,6个锆石颗粒构成的不一致线上交点年龄为(267±51)Ma.综合各方面因素认为这两个岩体侵位时代为晚二叠世(海西晚期).     

秦岭造山带商南-西峡地区富水杂岩的变辉长岩中斜锆石与锆石U-Pb同位素年龄的差异

秦岭造山带富水杂岩体的一些变辉长岩含有粒度较大且U、Pb含量较高的斜锆石和锆石,是U-Pb同位素测年的极好矿物.对该杂岩体的中粗粒角闪黑云辉长岩中的斜锆石和锆石分别进行了SHRIMP法和TIMS法U-Pb同位素年龄测定,获得斜锆石和锆石的U-Pb同位素年龄分别为501.4 Ma±1.2 Ma和480.0 Ma±3.4Ma,两者相差约20 Ma.对该岩石中部分斜锆石向锆石转化的现象及锆石的成因进行了初步研究,认为斜锆石的U-Pb同位素年龄应解释为秦岭富水杂岩中基性岩石的形成时代,而锆石的成因及其U-Pb同位素系统在变质作用过程中的变化比较复杂,对其U-Pb同位素年龄地质意义的合理解释需做进一步的研究.     

连山关-高家沟花岗岩体LA—ICP—MS锆石U—Pb年龄及其地质意义

笔者采用颗粒锆石LA—ICP—MS法获得了连山关-高家沟花岗岩体U—Pb不一致线上交点年龄为2198±31．5Ma和2162±31Ma。通过与辽东地区U—Pb法、Sm—Nd法和颗粒锆石年龄比对,揭示了辽东地区广泛发育的一期古元古代花岗岩浆事件的精确年代。根据岩石地球化学特征和岩体与地层接触关系,笔者认为该期花岗岩是在拉伸状态下早期基底岩石重熔的产物。年代学资料表明,连山关-高家沟岩体为壳源岩浆多次重熔侵位的花岗杂岩体。     

朝鲜甑山“群”变质岩中锆石-榍石-金红石U-Pb体系:古元古代-中生代构造-热事件记录

朝鲜甑山地区甑山"群"(杂岩)的主要岩石类型包括石榴云母片麻岩、石榴角闪岩,并有少量大理岩。其原岩为泥质岩为主的碎屑岩,并含有少量火山岩和碳酸盐岩,经历了角闪岩相到麻粒岩相变质作用。本文对甑山"群"2件石榴云母片麻岩和1件石榴角闪岩样品进行了锆石-榍石-金红石U-Pb体系年代学研究。石榴云母片麻岩中分选出的锆石均为变质新生锆石,Pb-Pb加权平均年龄为1850±5Ma,没有发现继承锆石组分,暗示原岩为缺乏碎屑锆石的泥质岩。石榴角闪岩中的榍石根据成因不同,可分为两期,即早期麻粒岩相变质生长的榍石和后期热事件生长榍石。早期榍石具有高U、高Th/U比值特点,U-Pb分析构成一条不一致线,上交点年龄1831±5Ma代表了早期变质热事件冷却到榍石U-Pb体系封闭温度时代,下交点年龄155±3Ma代表后期热事件造成Pb丢失时代;另一种榍石具有低U低Th/U比值特点,U-Pb年龄为单一的153±3Ma,为中生代构造热事件的新生榍石。两类样品中的金红石少量颗粒保存有早期U-Pb年龄信息,绝大多数在~155Ma发生完全重置,并直到~110Ma一直处于U-Pb体系开放状态。以上数据说明,甑山"群"在~1850Ma前发生高级变质,1850~1830Ma开始抬升出露,并成为中新元古界沉积岩物源与沉积基底,中生代受到朝鲜广泛发育的岩浆-热事件的影响。     

北大别片麻岩的锆石U—Pb年龄离子探针测定及其地质意义

本文对北大别西部的湖北英山万家老屋、罗田七道河尹家店和安徽金寨丁埠乡李集3个片麻岩样品中锆石进行了BSE显微结构分析，并在此基础上对锆石进行了离子探针U－Pb年龄测定。结果表明北大别存在两种类型的片麻岩，它们的原岩分别形成于晋宁期和燕山期，其中英山万家老屋片麻岩的形成年龄为794Ma，后期地质事件对其锆石U－Pb年龄造成的影响较小；七道河尹家店片麻岩的形成年龄为730Ma，并可能受到燕山期岩浆事件的影响，得到了140Ma左右的变质年龄；丁埠乡李集片麻岩的形成年龄为140Ma。结合已有的年代学结果，北大别地体广泛存在晋宁期的岩浆事件，表明其可能为扬子板块的北缘。     

陕西宽坪岩群变基性熔岩锆石U-Pb年龄

宽坪岩群广东坪岩组绿片岩岩石学、岩石化学、稀土、微量元素、副矿物等研究表明其原岩为基性火山岩。对商州市板桥地区基性熔岩变质而成的绿片岩区域地质、地球化学特点、锆石特征进行了详细研究和单颗粒锆石U-Pb同位素年龄测定。放射性成因铅丢失最少的岩浆成因锆石的206Pb/238U表面年龄为1154Ma,变质成因锆石的206Pb/238U表面年龄为444Ma。5个锆石颗粒构成的不一致线上交点年龄为1827±11Ma,代表基性熔岩形成年龄;下交点年龄为418±8Ma,代表加里东期变质年龄。综合认为宽坪岩群形成时代为早元古代。     

大兴安岭海拉尔地区奈吉岩体锆石U-Pb年龄 及其地质意义

研究大兴安岭中生代花岗岩对探讨兴蒙造山带的形成演化具有十分重要意义。对大兴安岭中北段海拉尔地区奈吉岩体进行了锆石LA-ICP-MS U-Pb测年。结果显示二长花岗岩锆石~(206)Pb/~(238)U加权平均年龄为(227.6±1.2)Ma(n=28,MSWD=1.3),正长花岗岩锆石~(206)Pb/~(238)U加权平均年龄为(232.5±1.4)Ma(n=28,MSWD=0.74),岩体的形成时代应为晚三叠世。该岩体的形成可能与蒙古—鄂霍茨克洋演化相关。     

碧溪岭岩体中石榴橄榄岩的锆石显微结构及离子探针定年

对碧溪岭岩体中石榴橄榄岩的锆石进行了BSE和CL显微结构分析，并在此基础上用离子探针进行了锆石微区U－Pb定年。锆石CL图像显示其有明显的核边结构，核部具岩浆锆石，边部是变质增生锆石的特征。结合锆石的外形特征认为，边部的变质锆石是在超高压变质作用中重结晶形成的。离子探针分析结果在一致曲线图中构成不一致线，得到上下交点 年龄分别为824±39Ma和254±38Ma。结合CL图像认为上交点824±39Ma的年龄代表其原岩形成年龄，下交点254±38Ma为基超高压变质作用的年龄。另有少量较年龄的捕虏晶锆石，可能来自岩浆上升过程中同化混染的围岩。     

扬子板块西缘黄草山和下索子花岗岩体锆石U-Pb年代学研究

对扬子板块西缘泸定—石棉地区黄草山花岗岩体和康定地区下索子花岗岩体中自形锆石,采用205Pb-235U混合同位素稀释法测定了锆石U-Pb年龄。测试结果表明,黄草山岩体中锆石结晶年龄为786±36Ma,下索子岩体中锆石结晶年龄为805±15Ma,表明它们都是晋宁期岩浆活动产物。     

Late Archaean—early proterozoic source ages of zircons in rocks from the Paleozoic orogen of western Galicia,NW Spain

U-Pb data are reported for nine suites of zircons and three monazites from the Paleozoic orogen in western Galicia (NW Spain): one paragneiss and six orthogneisses from the early Paleozoic basement, and two Carboniferous (ca. 310 Ma old) intrusions of two-mica granite. New whole-rock Rb-Sr analyses, along with earlier data, indicate an age of ca. 470-440 Ma (Ordovician) for the emplacement of the granitic precursors of the orthogneisses. Monazite from the paragneiss also yields an U-Pb age of ca. 470 Ma. For all nine investigated suites of zircons the U-Pb systematics signal the presence of a minor proportion of Precambrian zircon. The zircon data from two orthogneisses and the paragneiss display roughly linear arrays with upper intercepts between about 3.0 Ga and 2.0 Ga, and lower intercepts between ca. 480 and 460 Ma; the former are interpreted as approaching the age of the old zircon component, and the latter as reflecting either the time of crystallization of new zircons from the magma (orthogneisses), or the time of radiogenic lead loss from the old zircons (paragneiss). The suites of zircons from all other investigated orthogneisses suffered isotopic disturbance posterior to the granite emplacement 470-440 Ma ago, in most cases leading to ‘false’ discordias without geochronological significance. Similarly, the zircons of the two investigated two-mica granites do not produce meaningful discordias because of post-Paleozoic disturbance. The monazite U-Pb systems of the latter granites indicate (sub)recent lead loss.From the high initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios an involvement of Precambrian continental crust material is evident in the generation of the early Paleozoic suite of granites, while the zircon U-Pb data give evidence of the presence of about 3.0-2.0 Ga old (late Archaean—early Proterozoic) components in the source material. Zircons from the oldest sedimentary rocks in the area, now present as catazonal paragneisses and a likely source for the granites, likewise reveal a provenance age of 3.0-2.0 Ga. The late Archaean—early Proterozoic source rocks in western Galicia probably formed part of an Archaean continental crust underlying much of western Europe, but mostly modified beyond recognition by younger events.     

Late Jurassic-Early Cretaceous Plutonism in the Northern Part of the Precambrian North China Craton： SHRIMP Zircon U-Pb Dating of Diorites and Granites from the Yunmengshan Geopark, Beijing

The Yunmengshan Geopark in northern Beijing is located within the Yanshan range.It contains the Yunmengshan batholith,which is dominated by two plutons:the Yunmengshan gneissic granite and the Shicheng gneissic diorite.Four samples of the Yunmengshan gneissic granite give SHRIMP zircon U-Pb ages from 145 to 141 Ma,whereas four samples of the Shicheng gneissic diorite have ages from 159 Ma to 151 Ma.Dikes that cut the Yunmengshan diorite record SHRIMP zircon UPb age of 162±2 and 156±4 Ma.The cumulative pl...     

广西钦甲花岗岩体单颗粒锆石LA-ICP-MS U-Pb定年及其地质意义

本文对钦甲花岗岩体进行了详细的LA-ICP-MS锆石U-Pb同位素年龄测定.这些锆石的阴极发光图像发育环带结构,Th/U值较高(大于0.1),具有岩浆成因锆石的特征.研究结果表明,钦甲花岗岩体为一复式岩体,其不同单元锆石的206Pb/238U加权平均年龄分别为412.4±1.8 Ma、434.8±1.7 Ma、442....     

西南天山科桑溶洞新元古代-早奥陶世花岗岩锆石U-Pb年代学研究

在新疆西南天山科桑溶洞地区,新厘定出一套斜长角闪岩-花岗岩地质单元:侵入斜长角闪岩中的新元古代白云母花岗岩(片麻状构造)、以及侵入上述古老岩石单元的早奥陶世花岗岩(块状构造)。片麻状白云母花岗岩中锆石具有热液锆石边、岩浆锆石幔和碎屑锆石核(边-幔-核结构),剔除被热液锆石和碎屑锆石混染的SHRIMP测点,获得岩浆锆石幔的加权平均年龄752.3±5.1Ma(MSWD=0.95),代表岩浆的结晶年龄。块状花岗岩的锆石具有边-核结构,热液锆石边的U-Pb年龄(419.5±5.7Ma)明显偏低。剔除热液锆石和碎屑锆石,获得岩浆锆石的平均U-Pb年龄481.1±4.4Ma(MSWD=0.88),代表花岗岩的结晶年龄。早奥陶世早期,岩浆侵入新元古代片麻状白云母花岗岩中。在晚志留世或者更晚时期,两类花岗岩共同经受了变质热液改造,变质流体交代岩浆锆石,导致锆石溶蚀再生长。     

伊舒地堑基底花岗岩的锆石U-Pb年代学及其构造意义

为了探讨伊舒地堑与松辽盆地基底的构造演化历史, 对位于伊舒地堑中的2个基底花岗岩进行了锆石LA-ICP-MS U-Pb定年研究, 并与松辽盆地基底花岗岩进行了对比.研究结果表明, 伊舒地堑基底花岗岩中的锆石呈自形或半自形, 并具有典型的岩浆生长环带, 个别发育有核边结构, 锆石的Th/U比值介于0.32 ~ 2.76, 暗示其岩浆成因.CH11井花岗岩中锆石18个测点206Pb/238U加权平均年龄为162±2 Ma, X13井花岗岩中锆石13个测点206Pb/238U加权平均年龄为159 ± 3 Ma, 其核部存在176 ±1 Ma的谐和年龄.上述结果表明, 伊舒地堑中基底花岗岩形成于中侏罗世晚期或晚侏罗世早期, 它们与松辽盆地基底中最晚期花岗岩的形成时间相同.这暗示伊舒地堑与松辽盆地自燕山期以来具有类似的基底演化历史. 176 ±1 Ma继承锆石的存在也证明了上述认识.      

Late Jurassic–Early Cretaceous Plutonism in the Northern Part of the Precambrian North China Craton: SHRIMP Zircon U–Pb Dating of Diorites and Granites from the Yunmengshan Geopark, Beijing

The Yunmengshan Geopark in northern Beijing is located within the Yanshan range. It contains the Yunmengshan batholith, which is dominated by two plutons: the Yunmengshan gneissic granite and the Shicheng gneissic diorite. Four samples of the Yunmengshan gneissic granite give SHRIMP zircon U–Pb ages from 145 to 141 Ma, whereas four samples of the Shicheng gneissic diorite have ages from 159 Ma to 151 Ma. Dikes that cut the Yunmengshan diorite record SHRIMP zircon U–Pb age of 162±2 and 156±4 Ma. The cumulative plots of zircons from the diorites show a peak age of 155 Ma, without inherited zircon cores, and the peak age of 142 Ma for granite is interpreted as the emplacement age of the Yunmengshan granitic pluton, whose igneous zircons contain inherited zircon cores. The data presented here show that there were two pulses of magmatism: early diorites, followed c13 Ma later by true granites, which incorporated material from an older continental crust.     

西藏拉萨地块过铝质花岗岩中继承锆石的物源区示踪及其古地理意义

富含继承锆石的过铝质花岗岩一般来源于富铝质岩石(如变泥质岩)的部分熔融,因而分析这些继承锆石的U-Pb年龄可以像分析沉积岩碎屑锆石的U-Pb年龄一样,提供过铝质花岗岩源区物质中碎屑沉积物物源区的丰富信息。本文报道了中部拉萨地块早侏罗世过铝质花岗岩的全岩地球化学和锆石U-Pb年代学数据,结合拉萨地块已有二叠纪和晚三叠世过铝质花岗岩的继承锆石年代学数据,总结了目前已有的拉萨地块过铝质花岗岩的继承锆石U-Pb年龄特征(共199个谐和测点)。这些过铝质花岗岩属强过铝质S型花岗岩,其中的继承锆石定义了1250~1100Ma(峰值1181±14Ma)和550~450Ma(峰值494±7Ma)2个最突出的年龄群,分别可比于拉萨地块古生代沉积岩的碎屑锆石年龄峰值(约1170Ma)和寒武纪火山岩的侵位时代,明显不同于西羌塘、安多和特提斯喜马拉雅新元古代-古生代沉积岩中的碎屑锆石年龄频谱。拉萨地块过铝质花岗岩中约1181Ma的继承锆石,可能与拉萨地块古生代沉积岩中的同期碎屑锆石一样,都来自澳大利亚南西部Albany-Fraser造山带和东南极Wilkes等地,而约494的继承锆石,既可能来自澳大利亚西部,也可能来自拉萨地块本地。本文提供了拉萨地块与澳大利亚大陆北缘具有古地理联系的过铝质花岗岩继承锆石U-Pb年龄证据。拉萨地块的研究实践表明,采用过铝质花岗岩继承锆石和古生代沉积岩碎屑锆石相结合的锆石U-Pb年代学方法,可为重建冈瓦纳大陆北缘其它微陆块的古地理和构造岩浆演化提供重要约束。     

Late-Variscan magmatism revisited: new implications from Pb-evaporation zircon ages on the emplacement of redwitzites and granites in NE Bavaria

Pb-evaporation zircon analyses coupled with a detailed cathodoluminescence (CL) study on the complete series of granitoids from the northern Oberpfalz, NE Bavaria, provide new evidence for the commencement and timing of late-Variscan magmatism. All granitoids analysed in this study were dated before by Rb-Sr and/or K-Ar methods. Investigated samples comprise medium-grained, I-type dioritic rocks (redwitzites), I/S-type granites (Leuchtenberg, Marktredwitz (G1), Zainhammer) and S-type granites (Falkenberg, Liebenstein, Mitterteich, Friedenfels, Steinwald, Flossenbürg, Bärnau). The zircon evaporation technique reveals three groups of ²⁰⁷Pb/²⁰⁶Pb ages which are interpreted to represent magmatic crystallisation: (1) ages of 324-321 Ma are found in all analysed redwitzites and in almost all I/S-type granites; (2) the granites of Falkenberg and Liebenstein yield ages of ~315 Ma; (3) ages between 312 and 310 Ma are recorded in the Mitterteich, Friedenfels, Steinwald and Flossenbürg granites. Titanite crystals from different redwitzite bodies yield conventional U-Pb ages of 325-322 Ma, identical to the Pb-evaporation zircon data of these rocks. The S-type granites of groups 2 and 3 contain zircons with relict cores but only a small number of them yield older ages, indicating that some of the cores must have lost their radiogenic Pb. From the geochronological data, we infer that metamorphic conditions of the Variscan crust produced different granite types at different times. The data support a model involving an early period of mantle-related magmatism which postdates the final convergence stage of the Variscan orogen. This magmatic activity was at the same time as the thermal peak of regional metamorphism and is followed by a late period of crustal anatexis, which is probably related to post-collisional extension of the thickened Variscan crust.     

http://dx.doi.org/10.1016/j.gsf.2016.10.001

The Francevillian Group in Gabonese Republic was recently established as a typical sedimentary sequence for the Paleoproterozoic.However,its age is rather poorly constrained,mainly based on Rb-Sr and Nd-Sm datings.This study reports new zircon data obtained from Chaillu massif and N'goutou complex,which constrain the protolith age of the basement orthogneisses and the igneous age of an intrusive granite,respectively.Most zircons from the orthogneisses are blue and exhibit oscillatory zoning in cathode-luminescence images.Zircons with lower common lead abundances tend to be distributed close to the concordia curve.Two age clusters around 2860 Ma and 2910 Ma are found in zircons plotted on the concordia curve.Based on the Th/U ratios of zircons,these ages correspond to the protolith ages of the orthogneisses,and the zircons are not metamorphic in origin.Syenites and granites were collected from the N'goutou complex that intrudes into the FA and FB units of the Francevillian Group.The granitoids exhibit chemical composition of A-type granite affinity.Half of zircons separated from the granite are non-luminous,and the remaining half exhibit obscure internal textures under cathode-luminescence observation.All zircon grains contain significant amounts of common lead;the lead isotopic variability is probably attributed to the mixing of two components in the zircons.The zircon radiogenic ~(207)Pb/~(206)Pb ratio is 0.13707 ± 0.0010.corresponding to a ~(207)Pb/~(206)Pb age of 2191 ± 13 Ma.This constrains the minimum depositional age of the FA and FB units.Furthermore,the FB unit consists of manganese-rich carbonate rocks and organic carbon-rich black shales with macroscopic fossils.Based on our age constraints,these organisms appeared in the study area just after the last Paleoproterozoic Snowball Earth event,in concert with global scale oxidation event encompassing the Snowball Earth.