首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 43 毫秒
1.
万渝生  耿元生  沈其韩  刘敦一  宋彪  唐索寒  王进辉 《地质科学》2002,37(2):143-151,164,T001,T002
鞍山地区中太古代铁架山花岗岩中存在多种类型的表壳岩包体。本文对其中 类变泥砂质岩石进行了研究。一类包括云母石英片岩、黑云变粒岩、浅粒岩, 它们高钾低钠, 具有强的负铕异常(Eu/Eu* = 0.10~0.23) 和负钡异常(Ba/Ba*＝*＝0.09～0.23),tDM和εNd(3.0 Ga)变化范围分别为3.36～3.51 Ga和-3.89～-1.59.岩石高的成熟度和长的地壳滞留时间表明鞍山地区中太古代以前陆壳基底就有了相当的规模,并具有很长的地壳演化历史.它们在元素组成上与铁架山花岗岩十分相似,相互间存在物源上的成因联系.铁架山花岗岩的tDM和εNd(3.0 Ga)变化范围分别为3.39～3.70 Ga和-4.99～-2.36,Sm、Nd同位素组成与变质泥砂质岩石存在一定差异,看来主要与前者形成过程中源区选择性部分熔融和稀土组成发生变化有关.另一类为黑云斜长片麻岩,组成上与第一类变泥砂质岩石存在较大区别,而与古太古代陈台沟花岗岩较为相似,相互间可能存在物源上的成因联系.它们对铁架山岩体部分轻、重稀土强烈分离花岗质岩石的形成可能起了一定作用.物源区的复杂性是铁架山花岗岩组成上存在较大变化的主要原因.  相似文献   

2.
鄂尔多斯地块基底岩芯岩石组合、花岗片麻岩的锆石U-Pb年龄、Hf同位素和全岩Nd同位素组成以及航磁异常和地壳速度结构特征表明,该地块中部沿大同-华池基底断裂带存在一航磁异常梯度带,沿该带南缘发育一条2.2~2.0Ga古元古代弧岩浆活动带,并经历了1.96~1.85Ga的变质作用改造,与东缘中部带古元古代中晚期构造演化过程一样,可能代表该地块内部古元古代中晚期形成的一条构造活动带。该活动带以北为东西向展布的正航磁异常带,基底由混合岩化的变泥质和杂砂质片麻岩、片岩及大理岩和石英岩等变质表壳岩和一些花岗片麻岩类构成,记录了新太古代末~2.50Ga和古元古代中期2.2~2.0Ga两期岩浆活动及古元古代末期1.95~1.85Ga变质作用等地质事件,为一主要由新太古代陆壳物质构成的陆块。活动带南部为北东向不规则正、负相间航磁异常带,基底除少量副变质岩外,多为花岗片麻岩类,记录了古元古代中期2.2~2.0Ga岩浆热事件以及1.95~1.85Ga的变质作用改造,除新太古代物质外,还存在古、中太古代陆壳物质,是由一定规模中太古代和部分古太古代陆壳物质以及新太古代新生陆壳添加构成的古老地块。因此,鄂尔多斯地块中部沿大同-华池断裂带成为分割该地块一条重要分界带,其北部地块与其北缘孔兹岩带构成同一构造带,可能代表新太古代陆壳基底之上于古元古代晚期发育的一条大陆边缘活动带。南部则为古、中太古代陆壳与新太古新生陆壳增生而成的古老地块,代表鄂尔多斯地块古老基底陆壳的主体。  相似文献   

3.
Fulai Liu  Zhiqin Xu  Huaimin Xue 《Lithos》2004,78(4):411-429
Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20–0.39), and negative Ba anomalies (Ba/Ba*=0.25–0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U–Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P–T data is estimated to be 5.6–11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle.  相似文献   

4.
藏东羌塘地块变质基底的时代归属和构造演化一直以来存在较大争议,沿龙木错-双湖缝合带东段出露的由酉西群和吉塘群组成的吉塘变质杂岩体成为解决这一问题的理想研究对象。本文对酉西群变质岩和吉塘群变质岩进行了系统的岩石学、岩相学、地球化学及锆石年代学研究。吉塘变质杂岩体的基底主要由中-新元古代的变沉积岩组成,其中酉西群岩性主要为云母绿泥钠长片岩,吉塘群则以黑云母二长片麻岩和黑云母斜长片麻岩为主,它们具有高Si O2、高Al2O3、富Fe OT、贫碱和贫钙的特征。微量元素均显示富集K、Rb、Th等大离子亲石元素(LILE),亏损Ti、Nb、Ta等高场强元素(HFSE),而Zr、Hf、Th、U等轻度富集则说明其源区更富集长英质组分而贫镁铁组分。碎屑锆石均为岩浆成因,年龄峰值范围为560～600Ma、808～865Ma、925～997Ma和1071～1119Ma,代表变质基底的年龄主要由泛非期和格林威尔造山期的构造-岩浆热事件所形成,在560Ma的沉积时代之前,源区可能发生有4次岩浆事件。地球化学特征和碎屑锆石组成指示酉西群和吉塘群原岩分别以泥砂质沉积岩和长英质粘土岩及砂砾岩等副变质岩为主。吉塘变质杂岩体所在的南羌塘地块与喜马拉雅地块和拉萨地块具有相近的物源区,故而认为南羌塘地块在中-新元古代期间可能属于冈瓦纳大陆的一部分。  相似文献   

5.
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   

6.
This work presents geochemical data for granitic plutons that intruded the Alto Pajeu terrane, Borborema Province, northeastern Brazil, during several distinct tectonic cycles. The Alto Pajeu terrane is represented in the study area by metasedimentary (São Caetano Formation) and metavolcanosedimentary (Irajai Formation) sequences overlying a gneissic-migmatitic basement. Several granitic plutons of different ages, mineralogical proportions, and chemical compositions have invaded the supracrustal and basement rocks. Two deformational events, one tangential (older) and the other transcurrent (younger), were identified. The metamorphic grade determined for the metasedimentary rocks varies from amphibolite to high amphibolite facies, with temperatures ranging from 600 to 680°C and pressures from 6.0 to 7.5 kbar. Three magmatic events with distinct ages were identified for the tectonic evolution of the Alto Pajeu terrane in the study area. The older magmatic event is related to the Transamazonico orogenic cycle (～ 2.0 Ga), the intermediary magmatic event to the Cariris Velhos cycle (～0.97 Ga), and the younger magmatic event to the Brasiliano orogenic cycle (～0.55 Ga). The Cariris Velhos- and Brasiliano-age granites are geochemically classified as peraluminous, subalkalic granites of the calc-alkalic series, whereas the Transamazônico-age granite is metaluminous and alkaline. Geochemical data presented and discussed here suggest that the magmatism related to the three tectonic events had distinctly different sources and petrogenetic evolutions.  相似文献   

7.
李猛  王超  李荣社  彭岩 《地质通报》2015,34(8):1438-1446
祁连山西段吊大坂新元古代花岗质片麻岩出露于北大河岩群二云母片麻岩中。地球化学特征显示,吊大坂新元古代花岗质片麻岩主要为碱性、弱过铝质花岗岩(A/CNK=1.04~1.12)。岩石主体Rb含量较低,Sr和Ba含量高,K/Rb值介于179~225之间,说明吊大坂新元古代花岗质片麻岩不是高分异花岗岩,但明显的负Eu异常(Eu/Eu*=0.25~0.35),说明该岩浆经历了弱的分离结晶作用。在(Al_2O_3+TFe O+Mg O+Ti O_2)-Al_2O_3/(TFe O+Mg O+Ti O_2)图解中,未经历强烈分异的花岗质岩石样品点落入地壳杂砂岩或中性岩浆岩源区。未经历强烈分异的样品富集强不相容元素,并且在Y-Nb和(Y+Nb)-Rb图解中,主体落入板内花岗岩区域。吊大坂花岗质片麻岩LA-ICP-MS锆石206Pb/238U年龄为736±5Ma,该年龄被解释为花岗质片麻岩的形成时代,说明祁连山存在750~730Ma的岩浆作用记录,可能为新元古代晚期伸展背景下的产物。祁连山新元古代岩浆活动记录了新元古代早期汇聚、新元古代晚期伸展到裂解的长期演化过程。  相似文献   

8.
戚学祥  齐金忠 《现代地质》2006,20(4):555-563
二云花岗片麻岩是组成苏鲁地体南缘超高压变质带朐山杂岩体的重要岩石类型,虽然经历了超高压变质作用,但仍保留了花岗岩的岩石学特征。常量元素和微量元素分析结果表明该片麻岩具有高w(K2O)、低w(CaO)、高w(TFeO)/w(MgO)比值、铝饱和指数偏高的A型花岗岩的特征,岩石类型为高钾碱性过铝质A型花岗岩。稀土元素中轻稀土富集、分馏程度高、Eu强烈亏损,微量元素中Ba、P、Ti、Sc具有明显的负异常,w(Sr)/ w(Y)、w(La)/ w(Yb)和w(Rb)/ w(Sr)、w(Rb)/ w(Ba)较高,尤其是w(TiO2)＜0.2％和w(Y)/ w(Nb)＞1.2的特征,以及在w(Rb)-w(Yb+Ta）和w(Rb)-w(Y+Nb）判别图解上样品投点位于板内环境区等,表明该片麻岩的原岩形成于板内与裂谷有关的非造山环境。锆石SHRIMP U-Pb测年结果揭示其侵位时代为新元古代中晚期((722±32）Ma）,与杂岩体中早期侵位的二长花岗片麻岩及杂岩体上覆地层中的变质火山岩同为Rodinia大陆裂解、扬子地块陆内裂谷形成过程中伴随的岩浆活动的产物。  相似文献   

9.
北苏鲁超高压变质带前寒武纪基底研究新进展   总被引：1，自引：0，他引：1  
北苏鲁超高压变质带位于胶东牟平-即墨断裂以东的地区,其前寒武纪基底以出露新元古代的双峰式火成岩为主要特征,其主要岩石组合包括大量花岗片麻岩以及少量变（超）基性岩（榴辉岩）、变沉积岩。对花岗片麻岩、变质（超）基性岩的岩石组合、地球化学、锆石U-Pb和Lu-Hf同位素的研究表明,北苏鲁存在新太古代-古元古代的残留地壳,经历了1.8~2.2 Ga期间的岩浆-火山-变质事件;新元古代中期（0.72~0.80 Ga）与Rodinia超大陆的裂解相关的岩浆活动促使地壳的生长和再造,形成了北苏鲁的前寒武纪基底的主体;北苏鲁和苏鲁-大别造山带一样,其前寒武纪基底是扬子板块北缘的一部分,苏鲁造山带的西北边界是五莲-烟台断裂带。  相似文献   

10.
ABSTRACT

The Tibetan Plateau is located in the eastern Himalayan–Alpine orogen, an area where previous research has focused on ophiolites and a high-pressure metamorphic belt, whereas comparatively little research has been undertaken on the Tibetan basement. Cambrian granitic gneiss crops out in the Duguer area of the South Qiangtang terrane in northern Tibet and yields zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages of 502–492 Ma, providing insight into the possible existence of basement rocks within the South Qiangtang terrane. The granitic gneisses are geochemically similar to high-K, calc-alkaline S-type granites, and Hf isotopic analysis of zircons within the gneisses yields negative εHf(t) values (–7.4 to – 1.1) and old zircon Hf model ages (TDMC = 1757–1406 Ma). These granitic gneisses were generated by partial melting of ancient pelitic rocks, and the resulting melts were contaminated by a small amount of mantle-derived material. Combining our new data with previous research, we conclude that these Cambrian granitic gneisses developed in a post-collisional tectonic setting after Pan-African tectonism. This suggests that the South Qiangtang terrane might have the same early Palaeozoic crystalline basement as the Lhasa, Himalaya, Baoshan, Gongshan, and Tengchong terranes.  相似文献   

11.
武当地块耀岭河群中两类不同性质的酸性火山岩研究   总被引：1，自引：0，他引：1  
耀岭河群火山岩中可划分出两类不同性质的酸性火山岩:酸性岩A主要属碱流岩-白碱流岩类,酸性岩B主要属流纹英安岩/英安岩类。总体上看二者主量元素组成差异不大:SiO2含量为65.00%～79.85%,(Na2O+K2O)为5.26%～9.68%。强烈富集Th、Nb、Ta、La、Ce、Nd、Zr、Sm和Hf等微量元素,强烈亏损Ba、K、Sr、P、Ti等元素,稀土元素特征表现为∑REE很高(339.94～1152.85μg/g),具有强烈的铕负异常(δEu=0.18～0.32),与典型的大陆裂谷流纹岩的稀土曲线一致。酸性火山岩B显示K、Rb、Ba、Th和LREE的正异常和Sr、P、Ti和Nb的负异常,Eu具弱负异常(δEu=0.64～0.88),表现出与酸性火山岩A迥然不同的微量元素及稀土元素特征,而与耀岭河群基性火山岩相似,显示其成因可能与之有相同源区。  相似文献   

12.
松潘-甘孜地体内花岗岩锆石SHRIMP U-Pb定年及其构造意义   总被引：36，自引：17，他引：36  
松潘-甘孜地体总体上是一个三角状褶皱带,其北侧、东南侧及东侧分别与东昆仑-西秦岭构造带、金沙江构造带及龙门山构造带相邻。地体内几乎全部被三叠系浊积岩所覆盖,其中侵位了很多花岗岩体。显然,这些花岗岩的岩浆特征、来源与侵位时代,对研究松潘-甘孜地体基底性质、构造演化等问题具有重要意义。本文报道利用SHRIMP定年技术对一些花岗岩岩体锆石所进行的精确测年数据。测试结果表明:(1)松潘-甘孜地体内的花岗岩体主要形成于晚三叠世,但岩浆活动可延续到早侏罗世晚期;(2)早期花岗岩浆活动与三叠纪系褶皱变形大致同期,指示这一时期的花岗岩浆的形成可能与三叠系下部大型拆离滑脱构造相关;(3)花岗岩结晶锆石普遍具有浑圆状或不规则状较老的继承锆石核,这些核部继承锆石可能代表花岗岩浆在上升过程中从不同地层内捕获的碎屑锆石,或者是下地壳岩石深熔残留锆石。根据SHRING U-Pb定年,这些继承锆石的年龄分别为二叠纪、加里东期、晚元古代和早元古代。元古代碎屑锆石的存在也可能指示松潘-甘孜地体具前寒武系基底。  相似文献   

13.
Xiba granitic pluton is located in South Qinling tectonic domain of the Qinling orogenic belt and consists mainly of granodiorite and monzogranite with significant number of microgranular quartz dioritic enclaves. SHRIMP zircon U–Pb isotopic dating reveals that the quartz dioritic enclaves formed at 214±3 Ma, which is similar to the age of their host monzogranite (218±1 Ma). The granitoids belong to high-K calc-alkaline series, and are characterized by enriched LILEs relative to HFSEs with negative Nb, Ta and Ti anomalies, and right-declined REE patterns with (La/Yb)N ratios ranging from 15.83 to 26.47 and δEu values from 0.78 to 1.22 (mean= 0.97). Most of these samples from Xiba granitic pluton exhibit εNd(t) values of ?8.79 to ?5.38, depleted mantle Nd model ages (TDM) between 1.1 Ga and 1.7 Ga, and initial Sr isotopic ratios (87Sr/86Sr)i from 0.7061 to 0.7082, indicating a possible Meso- to Paleoproterozoic lower crust source region, with exception of samples XB01-2-1 and XB10-1 displaying higher (87Sr/86Sr)i values of 0.779 and 0.735, respectively, which suggests a contamination of the upper crustal materials. Quartz dioritic enclaves are interpreted as the result of rapid crystallization fractionation during the parent magmatic emplacement, as evidenced by similar age, texture, geochemical, and Sr-Nd isotopic features with their host rocks. Characteristics of the petrological and geochemical data reveal that the parent magma of Xiba granitoids was produced by a magma mingling process. The upwelling asthenosphere caused a high heat flow and the mafic magma was underplated into the bottom of the lower continent crust, which caused the partial melting of the lower continent crustal materials. This geodynamic process generated the mixing parent magma between mafic magma from depleted mantle and felsic magma derived from the lower continent crust. Integrated petrogenesis and tectonic discrimination with regional tectonic evolution of the Qinling orogen, it is suggested that the granitoids are most likely products in a post-collision tectonic setting.  相似文献   

14.
The granitic gneisses from the ultrahigh-pressure (UHP) metamorphic terrain of the southeastern Dabie Mountains encompass two types: monzonitic granitic gneiss and alkali-feldspar granitic gneiss, which are characterized by rich alkalis, poor CaO, high FeO/MgO, particularly high Ba, Rb, Th, Ta, REE (except Eu), Ga, Nb and Zn, and low Sr, Eu, Cr, Co and Ni. The gneisses, particularly the alkali-feldspar granitic gneiss, have typical chemical characteristics of A-type granites. They resulted from partial melting of crustal materials existing in the rift zone along the northern margin of the South China block during the Neoproterozoic. These gneisses might not have undergone UHP metamorphism during the late Triassic, but were involved into UHP rocks by the tectonic mixing process and kept the exhumation message of the UHP rocks from the middle and upper crust.  相似文献   

15.
FROM BACK-ARC BASIN TO BACK-ARC FORELAND BASIN—THE SEDIMENTARY BASIN AND TECTONIC EVOLUTION OF THE LATE CALEDONIAN—EARLY HERCYNIAN STAGES IN CORRIDOR AND NORTH QILIAN MTSthenationalNaturalScienceFoundationofChina(No.4 9972 0 78)  相似文献   

16.
安徽北淮阳构造带基底变质岩的构造属性   总被引：3，自引：2，他引：1  
安徽北淮阳构造带的基底由一套变火山沉积岩建造 (即原称庐镇关群 )所组成。岩类学、岩石地球化学、年代学的研究表明 ,这套火山岩属碱性玄武岩系列、拉斑玄武岩系列和钙碱性玄武岩系列 ,分别形成于中元古宙陆内裂解 (扩张 )带和晚元古宙岛弧两种构造环境 ,其中以岛弧环境火山岩为主要部分 ,由此可以证明北淮阳构造带是在中元古宙陆内裂解带基础上发展起来的古弧系 ,具有大陆型基底性质  相似文献   

17.
中祁连东段晋宁期碰撞型花岗岩及其地质意义   总被引：30，自引：0，他引：30  
对中祁连地块东段西宁西部近S-N向展布的元古宙花岗岩带地质地球化学和年代学研究表明:它侵位于元古宙基底中浅变质岩系湟源群中,具有同碰撞S型花岗岩的性质;(917±12)Ma的单颗粒锆石U-Pb年龄记录了其侵位的时代为新元古宙晋宁期,这一新元古宙晋宁期碰撞型花岗岩带具有重要的地质意义.  相似文献   

18.
安徽的地壳演化:Sr,Nd同位素证据   总被引：13，自引：0，他引：13  
陈江峰  谢智  张巽  周泰禧 《安徽地质》2001,11(2):123-130
在地壳(幔)演化和板块构遣的框架内，评述了有关安徽南部(扬子地块东部，包括大别遣山带和江南遣山带)的同位素地质年代学和Nd，Sr同位素地球化学示踪研究的成果。该地区出露地表的中元古界溪口群浅变质岩代表皖南的基底，沿江地区和大别山区的基底包舍太古宇或／和古元古界古老岩石。此格局还影响到从震旦纪到古生代沉积岩的物源区，江南深断裂以北的沉积岩中有古老岩石的贡献，而以南的物源主要来自出露的中元古界岩石。扬子陆块南北缘(大别和江南遣山带)的晋宁期演化可能与罗迪尼亚超大陆演化有密切关系，但有关研究开展很少。三叠纪大陆深俯冲和超高压变质作用研究已成为国际地球科学的热点。晚中生代(120-140Ma)本区发生强烈的岩浆活动，并伴有重要矿床的形成。中酸性岩的形成是一种壳幔物质混合的过程。沿江地区陆下地幔具有富集特征，为扬子型岩石圈地幔与软流圈地幔混合的产物。从晚中生代到第四纪，基性岩指示其源区的地球化学性质有随时间变得越来越亏损的趋势。  相似文献   

19.
The middle segment of the northern margin of the North China Craton （NCC） consists mainly of metamorphosed Archean Dantazi Complex, Paleoproterozoic Hongqiyingzi Complex and unmetamorphosed gabbro-anorthosite-meta-alkaline granite, as well as metamorphosed Late Paleozoic mafic to granitoid rocks in the Damiao-Changshaoying area. The -2.49 Ga Dantazi Complex comprises dioritic-trondhjemitic-granodoritic-monzogranitic gneisses metamorphosed in amphibolite to granulite facies. Petrochemical characteristics reveal that most of the rocks belong to a medium- to high-potassium calc-alkaline series, and display Mg^# less than 40, right-declined REE patterns with no to obviously positive Eu anomalies, evidently negative Th, Nb, Ta and Ti anomalies in primitive mantlenormalized spider diagrams, εNd（t）=＋0.65 to -0.03, and depleted mantle model ages TDM=2.78-2.71 Ga. Study in petrogenesis indicates that the rocks were formed from magmatic mixing between mafic magma from the depleted mantle and granitoid magma from partial melting of recycled crustal mafic rocks in a continental margin setting. The 2.44-2.41 Ga Hongqiyingzi Complex is dominated by metamorphic mafic-granodioritic-monzogranitic gneisses, displaying similar petrochemical features to the Dantazi Complex, namely medium to high potassium calc-alkaline series, and the mafic rocks show evident change in LILEs, negative Th, Nb, Ta, Zr anomalies and positive P anomalies. And the other granitiod samples also exhibit negative Th, Nb, Ta, P and Ti anomalies. All rocks in the Hongqiyingzi Complex show right-declined REE patterns without Eu anomaly. The metamorphic mafic rocks with εNd（t） = -1.64 may not be an identical magmatic evolution series with granitoids that have εNd（t） values of ＋3.19 to ＋1.94 and TDM ages of 2.55-2.52 Ga. These granitic rocks originated from hybrid between mafic magma from the depleted mantle and magma from partial melting of juvenile crustal mafic rocks in an island arc setting. All the -311 Ma Late Paleozoic metamorphic mafic rocks and related granitic rocks show a medium-potassium calc-alkaline magmatic evolution series, characterized by high Mg^#, obviously negative Th, Nb, Ta anomalies and positive Sr anomalies, from no to strongly negative Ti anomalies and flat REE patterns with εNd（t） = ＋8.42, implying that the maflc magma was derived from the depleted mantle. However the other granitic rocks are characterized by right-declined REE patterns with no to evidently positive Eu anomalies, significantly low εNd（t） = -13.37 to -14.04, and TDM=1.97-1.96 Ga, revealing that the granitoid magma was derived from hybrid between maflc magma that came from -311 Ma depleted mantle and granitoid magma from Archean to Early Paleoproterozoic ancient crustal recycling. The geochemistry and Nd isotopic characteristics as well as the above geological and geochronological results indicate that the middle segment of the northern margin of the NCC mainly experienced four crustal growth episodes from Archean to Late Paleozoic, which were dominated by three continental marginal arc accretions （-2.49, -2.44 and 311 Ma）, except the 1.76-1.68 Ga episode related to post-collisional extension, revealing that the crustal accretion of this segment was chiefly generated from arc accretion and amalgamation to the NCC continental block.  相似文献   

20.
Abstract: In the arc (basin)–back area of the Yidun arc belt in the north segment of the Sanjiang tectonic zone, southwestern China, there occurs a post‐orogenic granite belt extending for more than 300 km in NNW direction. It strides across two different tectonic units of the arc (basin)–back area and the subduction area, and is accompanied by extensive Ag‐Sn polymetal–lic mineralizations. More than ten granite bodies have very similar geochemical characteristics: high SiO2 (73.8–76.3 wt%) and K2O+Na2O (7.16‐8.41 %), and low Al2O3 (11.9–13.6 %), CaO (0.46‐1.54 %) and MgO (0.16‐0.61 %), as well as high enrichment of Nb, Ta, Ga and Y, and strong depletion of Sr and Eu. Most of these features are peculiar to A‐type granite. Rb‐Sr and 40Ar/39Ar isotopic dating results indicate that the formation ages of the granites decrease from 103.7 Ma of the north end to 75.2 Ma near the south end, and that the magmatism became younger from north to south. The tectonic environment analysis clearly reveals that they were formed in post‐orogenic within–plate extension settings. The magma genesis was controlled by a united crustal extension regime after the arc‐continent collision. The granites have low Nd values ranging from –4.96 to –8.40, whereas the Sr values vary greatly ranging from –31.7 to 296, reflecting that the source composition transited from mantle – differentiated igneous rocks in the north to basement – dominated metamorphosed sedimentary rocks in the south. Under high temperature and water‐absent conditions, the anatexes of the crustal rocks made a great amount of plagioclase separated from melts and left in magma sources. Through this mechanism, the post‐orogenic granites took geo‐chemical characteristics such as low Al2O3 and CaO, and strong depletion of Sr and Eu.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号