扬子块体南缘四堡群Sm－Nd同位素体系及其他壳演化意义

详细的主元素和Sm-Nd同位素体系研究表明,扬子块体南缘元宝山地区四堡群中镁铁质-超镁铁质岩可能来源于Al亏损地幔;而宝坛地区镁铁质-超镁铁质岩的源区则可能包括了A1未亏损和A1亏损两种地幔端元组分,部分样品可能受到围岩混染。镁铁质-超镁铁质岩的Sm-Nd数据构成了一条无地质意义的假等时线,由其斜率获得的年龄约2.2Ga明显偏老。四堡群浅变质沉积岩的Nd模式年龄限定了镁铁质-超镁铁质岩和四堡群的地层年龄应小于1.8Ga.扬子南缘最老的基底四堡群(及相应地层)主要是由地壳存留年龄为1.8-1.9Ga的未成熟陆壳再循环物质组成,明显不同于华南块体(华夏古陆)的早-中元古代变质基底。迄今为止获得的沉积岩和花岗岩的Sm-Nd同位素资料都不支持扬子南缘存在早元古代-晚太古代基底。     

湖南益阳早前寒武纪镁铁质-超镁铁质火山岩的Sm-Nd同位素年龄

益阳地区出露的似层状 -层状镁铁质 -超镁铁质火山岩的时代归属有不同的观点。研究认为其主要岩石类型为玄武质科马提岩和拉斑玄武岩。岩石地球化学和同位素年龄显示 :玄武质科马提岩 Sm-Nd全岩等时线年龄为 ( 3 0 2 8± 47) Ma,属中、新太古代洋中脊 -岛弧火山岩 ;拉斑玄武岩年龄为 ( 2 2 1 6± 3 8) Ma,属古元古代。这一结果揭示了早前寒武纪在古扬子微板块(川中微板块 )东南有多岛弧洋盆 ,湖南深部地壳有绿岩基底。     

四顶黑山镁铁质—超镁铁质岩Sm-Nd等时线年龄及其意义

四顶黑山岩体主要由超镁铁质岩和镁铁质岩组成。超镁铁质岩由橄榄岩、角闪橄榄岩、辉橄岩、辉石岩等组成;镁铁质岩由辉长岩、角闪辉长岩、橄榄辉长岩等组成。本次对辉长岩进行了全岩Sm-Nd同位素分析,得到的等时线年龄为327±9.0Ma,初始εNd（t）=＋3.97,表明镁铁质—超镁铁质岩中辉长岩形成于早石炭世末。属于富集地幔向亏损地幔过渡类型。这一年龄的确定,为探讨北山—天山结合部晚古生代镁铁质—超镁铁质岩的成因机制和该区的区域地质演化提供了有效的年代学依据。     

赣北元古代变质沉积岩的铅钕同位素特征

赣北地区元古代变质沉积岩的铅、钕同位素研究结果表明,它们与扬子古陆南缘其他元古代地层非常相似,表明它们具有相似的成岩物质来源及构造演化历史。研究结果还显示,双桥山群变质泥质岩的Pb-Pb等时线年龄为（1 490±68）Ma,而障公山群、双桥山群、程浪群和修水群的钕模式年龄分别为1.57 Ga、1.89Ga、1.77Ga和1.60 Ga,均分布在1.5～1.9 Ga之间,表明这些地层的形成年龄应小于1.9 Ga,是中—晚元古代的产物,同时表明扬子古陆南缘存在古元古代的基底。     

江南古陆南缘四堡群钐钕同位素年龄研究

本文利用钐钕同位素方法测试了四堡群文通组8件镁铁质-超镁铁质火山岩样品,获得成岩等时线年龄2 219±111(2δ)Ma的数据。证明了江南古陆南缘最古老地层单元四堡群为早元古代而不是中晚元古代的产物。     

吉林永吉县头道沟地区镁铁—超镁铁质岩U-Pb年代学研究及地质意义

吉林省永吉县头道沟地区出露许多与头道沟岩组相伴产出的镁铁—超镁铁质岩,鉴于其处于长春-延吉构造带附近而受到业内关注,但由于缺少高精度年代学资料,制约了对区域大地构造的深入研究。本文采用锆石U-Pb(LA-ICP-MS)方法,对镁铁—超镁铁质岩进行了年代学研究。变质辉绿岩年龄为270±5 Ma,变质橄榄岩中捕获锆石最小年龄为297 Ma,考虑岩石组合及二者紧密相伴产出,认为二者均形成于中二叠世。镁铁—超镁铁质岩中捕获的锆石记录了华北克拉通及其北缘多次重要的构造热事件。其中,变辉绿岩中获得446±6 Ma的年龄与变质橄榄岩中获得的不一致线下交点434±240 Ma年龄共同对应了华北克拉通北缘早古生代的重要构造岩浆热事件;大量的1.8~2.4Ga年龄对应古元古代辽吉造山带热事件;1377 Ma、1542 Ma与蓟县系建造时代对应;869~997 Ma与青白口系建造时代对应;在变质辉绿岩中还存在众多3.0~3.2Ga锆石年龄。分析上述年龄结构及龙岗陆块北缘古生代地质体分布特征,推测研究区深部可能存在古老的变质基底,同时也表明研究区出露的镁铁—超镁铁质岩形成于陆内构造环境,而非蛇绿岩的组成成分,这对深化区域大地构造研究具有重要意义。     

宝坛地区透闪石化镁铁质岩石成因的地质地球化学证据

宝坛地区呈岩脉状或岩席状侵入于四堡群变质地层之中的透闪石化镁铁质岩石包括堆积的超镁铁质岩和分异的闪长岩,其岩石类型为透闪石化辉石岩、辉橄岵、辉长岵、辉绿岵及闪长岵等。该镁铁质－闪长质岩石以富集轻稀土和大离子新石元素、亏损高场强元素（Ｎｂ、Ｔａ）为特征;除堆积成因的超镁铁质岩石外,透闪石化镁铁质岩石及其分异的闪长岵的ＭｇＯ为４．４５％￣７．９６％,是镁铁质岩浆经结晶分异（辉石、橄榄石）作用的产物。     

武夷地块中古元古代镁铁质-超镁铁质岩石的发现——剖面介绍及岩石学、岩相学、年代学特征

通过1:250 000衢州区域地质调查,在武夷地块浙西南地区发现古元古代镁铁质-超镁铁质岩石。该套岩石在金华、龙游-带呈面状分布,金华张村出露最为完整,主要由辉石角闪石岩、角闪石岩、斜长阳起石岩、斜长辉石岩和斜长角闪岩等组成。对辉石角闪石岩与阳起石岩开展了LA-ICP-MS锆石U-Pb测年分析,分别获得其成岩年龄为1 834±14 Ma(MSWD=0.23,N=16)、1 839±17 Ma(MSWD=0.22,N=11),暗示这套镁铁质-超镁铁质岩石为古元古代岩浆活动的产物。这一发现表明,约1 830 Ma武夷地块处于板内伸展构造环境,同时该时期武夷地块已经具备足够的刚性,以致产生大规模的脆性破裂以及可能由地幔对流作用引发的基性岩浆活动,推测在约1.83 Ga武夷地块已经具有克拉通的性质。     

龙胜地区镁铁质侵入体:年龄及其地质意义

对镁铁质侵入体进行了单颗粒锆石U-Pb年代学和Sm-Nd同位素示踪研究。结果表明:镁铁质侵入体是起源于长期亏损地幔源区的岩浆在760Ma侵位结晶的产物,其侵位过程中遭受地壳物质的混染。该年龄结果限定了丹洲群地层的沉积上限年龄,并为龙胜地区不存在蛇绿岩套的认识提供重要的年代学制约。镁铁质岩浆的形成、侵位可能与Rodinia超大陆～825Ma裂解之后的另一次裂解事件相联系。     

江南造山带西南段梵净山地区镁铁质-超镁铁质岩:形成时代、地球化学特征与构造环境

梵净山地区位于江南造山带的西南缘,这里新元古代的镁铁质-超镁铁质岩浆岩广泛发育,岩性包括枕状熔岩、超镁铁质-镁铁质岩床群以及浅成侵入的辉长岩,成分属拉斑玄武岩系列。其中枕状熔岩以富集轻稀土元素和Rb、Ba、Th、U等强不相容元素,亏损高场强元素Nb和Ta,低的εNd(t)值为特征,明显不同于洋脊玄武岩,推测其成因可能与富集型地幔的部分熔融有关,形成于与俯冲有关的弧后小洋盆环境。超镁铁质-镁铁质岩床群主要由辉绿岩和碳酸辉橄岩组成,其中超镁铁质岩床群中出现大量的原生碳酸盐矿物,指示它们形成于拉张(甚至裂谷)的构造环境。辉长岩可能是区内最晚形成的岩浆岩,其SHRIMP锆石U-Pb年龄为821±4Ma。由枕状熔岩经超镁铁质-镁铁质岩床群到辉长岩,高场强元素Nb和Ta的亏损程度减弱、轻稀土元素的富集程度降低、εNd(t)值由负值变为正值,指示随时间的由早到晚,来自亏损地幔的物质不断增加。推测梵净山地区新元古代岩浆作用的顺序大致为:枕状熔岩(～840Ma)→白云母花岗岩(～838Ma)→碳酸超镁铁质岩床群→镁铁质岩床群→辉长岩(～821Ma),构造环境由俯冲-碰撞到拉张-裂谷。     

中国西秦岭碎屑锆石U-Pb年龄及其构造意义

西秦岭是北接华北克拉通、西接祁连与柴达木、南接松潘—甘孜地块的东秦岭造山带的西延。文中研究了该区从前寒武纪到三叠纪的碎屑沉积岩。这些碎屑沉积岩中分离出的锆石由LA-ICPMS(激光剥蚀等离子体质谱)进行了U-Pb定年。全岩Nd亏损地幔模式年龄类似于扬子克拉通年龄,主要分布于1.55~1.98Ga,峰值为1.81Ga,而与华北克拉通主要为古元古代与太古宙的模式年龄形成明显的对比。泥盆系中的碎屑锆石930~730Ma的U-Pb年龄指示其与扬子克拉通具亲缘性。930~730Ma是源区地壳的强烈增长阶段。二叠系—三叠系的碎屑沉积岩主体以含老于1600Ma的碎屑锆石为特征。碎屑锆石U-Pb年龄与Sm-Nd同位素组成指示此时华北克拉通南缘的基底岩石成为二叠系—三叠系碎屑沉积岩的重要物源。扬子克拉通在三叠纪时与华北克拉通拼接。西秦岭二叠系—三叠系碎屑沉积岩含有高达50%的华北克拉通南缘的基底岩石。     

安徽的地壳演化:Sr,Nd同位素证据

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。     

扬子地块东南缘沉积岩的Nd同位素研究

扬子地块东南缘上溪群分布区及其周边沉积岩的Ｎｄ同位素研究结果,支持存在一条苏浙皖古生代裂陷槽（或江南深断裂）的观点。上溪群以北直至长江边所分布的震旦系－古生代的盖层沉积岩,其Ｎｄ模式年龄有两组,表明物源区不同。裂陷槽以北,沉积岩的物源区为Ｎｄ模式年龄约２.０～２.１Ｇａ的扬子物源区;以南的沉积岩表现出明显的幔源物质混染,显示出元古代岩浆活动的影响,而上溪群分布区以南直到江绍断裂附近主要表现上溪物源区的影响,华夏地块古老基底岩石则无显著贡献。     

海南岛地壳生长和基底性质的Nd同位素制约

通过对海南岛花岗岩、变质沉积岩等地壳岩石SmNd同位素的详细剖析,探讨了海南岛的基底性质及其地壳生长史。研究表明:海南岛九所—陵水断裂的南、北经历了不同的构造演化史。断裂以北地壳岩石的ε_Nd(t)分布在3.3至-17.2之间,Nd模式年龄分布在1.2～3.2Ga之间,并在1.4～1.6Ga和2.0Ga形成统计峰值。断裂以北地壳主要以幕式生长形成,太古代时期形成若干陆核(距今3.1～3.2Ga或更早?),1.4～1.6Ga和1.8～2.0Ga为地壳生长的主要时期。断裂以北地壳岩石的SmNd同位素特征和地壳生长史与华夏古陆的相似,其基底是华夏古陆的一部分,断裂以南可能是亲东冈瓦纳(澳大利亚)的裂解块体。     

康定杂岩Rb-Sr、Sm-Nd同位素系统及其意义

通过对康定—冕宁地区出露的英云闪长岩、黑云角闪斜长片麻岩、角闪变粒岩全岩及其中所分离出的角闪石、黑云母、斜长石、钾长石的Rb-Sr、Sm-Nd同位素的系统测定,结合岩石的锆石U-Pb年龄结果,确定这些变质杂岩由于经历了复杂的形成过程与变质历史,Rb-Sr、Sm-Nd同位素体系难以确定其结晶年龄。由单矿物与全岩Rb-Sr、Sm-Nd体系拟合的~700 M a的等时线年龄反映了角闪岩相-高角闪岩相的变质作用年龄。Sm-Nd同位素体系由于在变质作用过程中的部分开放性,很容易给出无意义的较老的混合年龄。康定杂岩结晶后并没有经历麻粒岩相变质作用,区域上所含的麻粒岩透镜体可能是新元古代(773~721 M a)期间由Rod in ia超大陆裂解产生的新生洋壳向扬子克拉通陆块俯冲消减过程的变质产物。俯冲到一定深度后,由于板片被拉断,软流圈上涌导致变质洋壳板片岩石、先前底侵变质的镁铁质岩石及扬子陆块长英质基底岩石发生部分熔融,以镁铁质岩石熔融产生的熔浆为主(>70%),与长英质基底岩石熔融产生的熔浆混合形成w(Na2O)/w(K2O)>1的TTG组合。     

Nd isotopic stratigraphy of Paleoproterozoic to late Paleozoic sedimentary strata of the southwestern Yangtze Block and implications for its tectonic evolution

ABSTRACT

A systematic Sm-Nd isotopic study is reported for Paleoproterozoic to Late Paleozoic strata from the Dongchuan area in the southwestern Yangtze Block. The results, combined with the available detrital zircon U-Pb ages and Hf isotope data, constrain the provenances of these sedimentary rocks and further identify three important tectono-magmatic activities. Most of the Paleo-Mesoproterozoic samples (Dongchuan Group) display a wide T_DM2 range of 1.92–2.52 Ga with corresponding ε_Nd(t) values of +4.0 to ?3.5, suggesting Paleoproterozoic-dominated provenances mixed with mantle-derived materials. This corresponds to the ~1.7–1.5 Ga mafic magmatic activities commonly occurred in the southwestern Yangtze Block, which are related to the early breakup of the Columbia supercontinent. The obvious vale of T_DM2 and apex of ε_Nd(t) occurred in the Neoproterozoic strata (~0.8 Ga) of the southwestern margin over the whole Yangtze Block. This is consistent with the widely recognized mantle-derived magmatism around the Yangtze Block related to the breakup of Rodinia. However, the decreases in Nd model ages are different among various regions, indicating that the Neoproterozoic mantle inputs are more profound in the southwestern and central Yangtze Block than the southeastern Yangtze and the Jiangnan orogenic belt. The late Ediacaran to early Cambrian strata from the southwestern Yangtze exhibit a decrease in T_DM2 (from 2.00 to 1.67 Ga) and increase in ε_Nd(t) (from ?9.0 to ?5.2). This is in accordance with the coeval juvenile crustal materials discovered in the northwestern Yangtze, which were probably derived from the assembly of the Gondwana continent. Thus, a Gondwanan affinity is suggested for the southwestern Yangtze Block. Overall, the Nd isotopic studies of the Paleoproterozoic to Late Paleozoic sedimentary strata from the southwestern Yangtze Block identified three major episodes of magmatic activities, late Paleoproterozoic (~1.7 Ga), Neoproterozoic (~0.8 Ga) and late Neoproterozoic-early Cambrian (~0.55 Ga) in the context of Columbia, Rodinia and the subsequent Gondwana supercontinents.     

SHRIMP U-Pb zircon geochronology and Nd-Sr isotopic study of the Mamianshan Group: implications for the Neoproterozoic tectonic development of southeast China

Northwestern Fujian contains abundant well-studied Precambrian basement, and was a composite terrane in Cathaysia during the Neoproterozoic; however, its magmatic activity, petrogenesis, and tectonic evolution remain controversial. This article focuses on the geochronology and geochemistry of the Neoproterozoic Group in order to resolve the above problems. We provide new SHRIMP U-Pb zircon dating for the Mamianshan Group: 851.9 ± 9.2 to 825.5 ± 9.8 Ma for the Longbeixi Formation, 796.5 ± 9.3 Ma for the Dongyan Formation, and 756.2 ± 7.2 Ma for the Daling Formation. These ages document the existence of Neoproterozoic magmatism in the northwestern Cathaysia Block. Dongyan Nd-Sr isotopic data show that mafic amphibolite schists, mafic greenschists, and quartzofeldspathic schists were derived from a more depleted mantle (initial ε_Nd ? +5.5 and ⁸⁷Sr/⁸⁶Sr ratio 0.703409643), a mixture of depleted mantle and crustal components (initial ε_Nd ? ?1 and ⁸⁷Sr/⁸⁶Sr ratio 0.702045282–0.704147714), and late Palaeoproterozoic continental crustal materials (initial ε_Nd < ?1 and ⁸⁷Sr/⁸⁶Sr ratio 0.71083603), respectively. These new data, together with previous studies, suggest a bi-subduction-collision orogenic model for the Neoproterozoic evolution of the Yangtze and Cathaysia blocks. Our plate tectonic scenario involves earlier NW-dipping subduction during 1.0 Ga–860 Ma along the southeastern margin of the Yangtze Block and later NW-dipping subduction near the northwestern margin of the Cathaysia Block starting at ca. 850 Ma. The 796.5 ± 9.3 Ma age of the volcanic Dongyan Formation suggests that the final assembly of the Yangtze and Cathaysia blocks probably occurred after ca. 800 Ma. The 756.2 ± 7.2 Ma age of the Daling Formation indicates that post-orogenic extensional magmatism took place after 800 Ma along the northwestern margin of Cathaysia.     

海南岛古元宙变质基底性质和地壳增生的Nd、Pb同位素制约

基于海南地壳各类型岩石的63个样品Nd和Pb同位素分析数据,研究了海南地块元古宙地壳变质基底的时代、特征和演化。研究结果表明,海南岛元古宙变质基底成熟度低,基底变质岩系的母岩物质来源于长期亏损的地幔源区,主要形成时代为古元古宙晚期－新元古宙;不同时代花岗岩具有较高的εNd(t)值和较低的Nd模式年龄,主要形成于幔源物质参与下的或含地幔成分较多的初生地壳再循环。地壳增生具幕式增生的特点,并在2．0Ga、1.7Ga、1.2Ga出现高峰;Pb同位素组成既不同于扬子地块又不同于华夏地块,介于两地块之间,和Nd同位素特征具有一致或耦合关系。结合海南岛地质特征,初步认为不能单纯地将海南岛基底理解为华南地块统一南延部分或是华夏古陆的部分,可能为不同的构造块体。     

Geochronological (Rb-Sr and Sm-Nd) studies on intrusive gabbros and dolerite dykes from parts of Northern and Central Indian cratons: Implications for the age of onset of sedimentation in Bijawar and Chattisgarh basins and uranium mineralisation

The Dargawan gabbros intrusive into the Moli Subgroup of Bijawar Group, yielded Rb-Sr whole rock isochron age of 1967 ± 140 Ma. Based on the oldest age from overlying Lower Vindhyan (1.6Ga) and the underlying youngest basement ages (2.2 Ga), the time range of Bijawar sedimentation may be assigned as 2.1–1.6 Ga (Paleoproterozoic). Sm-Nd Model ages (T_DM), obtained, for Dargawan gabbros, is c. 2876–3145 Ma. High initial ⁸⁷Sr/ ⁸⁶Sr ratio of 0.70451 (higher than the contemporary mantle) and negative ɛ_Ndi (at 1.9 Ga) value of −1.5 to − 4.5, indicate assimilation of Archaean lower crustal component by the enriched mantle source magma at the time of gabbroic intrusion. The dolerite, from Damdama area, which is intrusive into the basement and overlying sediments of Chandrapur Group in the central Indian craton, yielded Rb-Sr internal isochron age of 1641 ± 120 Ma. The high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7098 and ɛ_Ndi value of −3.5 to −3.7 (at 1.6 Ga) is due to contamination of the mantle source magma with the overlying sediments. These dolerites have younger Sm-Nd Model ages (T_DM) than Dargawan gabbros as c. 2462–2675 Ma, which is similar to the age of the Sambalpur granite, from which probably sediments to this part of Chattisgarh basin are derived. Hence mixing of sediments with the Damdama dyke during its emplacement, gives rise to high initial ⁸⁷Sr/⁸⁶Sr and low initial ¹⁴³Nd/¹⁴⁴ ratios for these dykes. The c. 1600 Ma age indicates minimum age of onset of the sedimentation in the Chandrapur Group of Chattisgarh basin. Both the above mafic intrusions might have taken place in an intracratonic rift related (anorogenic) tectonic setting. This study is the first reliable age report on the onset of sedimentation in the Chandrapur Group. The total minimum time span of Chandrapur and Raipur Group may be 1.6 Ga to 1.0 Ga (Mesoproterozoic). The unconformably underlying Shingora Group of rocks of Chhattisgarh Supergroup thus indicates Paleoproterozoic age (older than 1.6 Ga). Most part of the recently classified Chattisgarh Supergroup and Bijawar-Vindhyan sequence are of Mesoproterozoic-Paleoproterozoic age and not of Neoproterozoic-Mesoproterozoic age as considered earlier. Petrographic study of basic dykes from Damdama area (eastern margin of Chattisgarh Supergroup) indicated presence of primary uranium mineral brannerite associated with goethite. This is the evidence of mafic intrusive providing geotherm and helping in scavenging the uranium from the surrounding and later alterations causing remobilisation and reconcentration of pre-existing uranium in host rocks as well as in mafic dyke itself otherwise mafic rocks are poor source of uranium and can not have primary uranium minerals initially. It can be concluded that mafic dykes have role in uranium mineralisation although indirectly.